Joint Structure and Function

chapt09_lecture Joints

Structure, Type, and Location of Cartilage

Cartilage is an avascular, flexible connective tissue located throughout the body that provides support and cushioning for adjacent tissues.

LEARNING OBJECTIVE

Differentiate among the types of cartilage

KEY TAKEAWAYS

Key Points

Cartilage is a flexible connective tissue that differs from bone in several ways; it is avascular and its microarchitecture is less organized than bone.
Cartilage is not innervated and therefore relies on diffusion to obtain nutrients. This causes it to heal very slowly.
The main cell types in cartilage are chondrocytes, the ground substance is chondroitin sulfate, and the fibrous sheath is called perichondrium.
There are three types of cartilage: hyaline, fibrous, and elastic cartilage.
Hyaline cartilage is the most widespread type and resembles glass. In the embryo, bone begins as hyaline cartilage and later ossifies.
Fibrous cartilage has many collagen fibers and is found in the intervertebral discs and pubic symphysis.
Elastic cartilage is springy, yellow, and elastic and is found in the internal support of the external ear and in the epiglottis.

Key Terms

chondroitin sulfate: An important structural component of cartilage that provides much of its resistance to compression.

connective tissue: A type of tissue found in animals whose main function is to bind other tissue systems (such as muscle to skin) or organs. It consists of the following three elements: cells, fibers, and a ground substance (or extracellular matrix).

hyaline cartilage: A type of cartilage found on many joint surfaces; it contains no nerves or blood vessels, and its structure is relatively simple.

temporal mandibular joint: A joint of the jaw that connects it to the temporal bones of the skull.

Chondrocytes: Cells that form and maintain the cartilage.

What is Cartilage?

Cartilage is a flexible connective tissue that differs from bone in several ways. For one, the primary cell types are chondrocytes as opposed to osteocytes. Chondrocytes are first chondroblast cells that produce the collagen extracellular matrix (ECM) and then get caught in the matrix. They lie in spaces called lacunae with up to eight chondrocytes located in each.

Chondrocytes rely on diffusion to obtain nutrients as, unlike bone, cartilage is avascular, meaning there are no vessels to carry blood to cartilage tissue. This lack of blood supply causes cartilage to heal very slowly compared with bone.

The base substance of cartilage is chondroitin sulfate, and the microarchitecture is substantially less organized than in bone. The cartilage fibrous sheath is called the perichondrium. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself.

Articular cartilage function is dependent on the molecular composition of its ECM, which consists mainly of proteoglycans and collagens. The remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix, which responds to tensile and compressive forces experienced by the cartilage.

Cartilage types: Images of microscopic views of the different types of cartilage: elastic, hyaline, and fibrous. Elastic cartilage has the most ECM; hyaline a middle amount; and fibrous cartilage has the least amount of ECM.

Types of Cartilage

There are three major types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage.

Hyaline Cartilage

Hyaline cartilage is the most widespread cartilage type and, in adults, it forms the articular surfaces of long bones, the rib tips, the rings of the trachea, and parts of the skull. This type of cartilage is predominately collagen (yet with few collagen fibers), and its name refers to its glassy appearance.

In the embryo, bones form first as hyaline cartilage before ossifying as development progresses. Hyaline cartilage is covered externally by a fibrous membrane, called the perichondrium, except at the articular ends of bones; it also occurs under the skin (for instance, ears and nose).

Hyaline cartilage is found on many joint surfaces. It contains no nerves or blood vessels, and its structure is relatively simple.

If a thin slice of cartilage is examined under the microscope, it will be found to consist of cells of a rounded or bluntly angular form, lying in groups of two or more in a granular or almost homogeneous matrix. These cells have generally straight outlines where they are in contact with each other, with the rest of their circumference rounded.

They consist of translucent protoplasm in which fine interlacing filaments and minute granules are sometimes present. Embedded in this are one or two round nuclei with the usual intranuclear network.

Fibrocartilage

Fibrous cartilage has lots of collagen fibers (Type I and Type II), and it tends to grade into dense tendon and ligament tissue. White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions.

It owes its flexibility and toughness to the fibrous tissue, and its elasticity to the cartilaginous tissue. It is the only type of cartilage that contains type I collagen in addition to the normal type II.

Fibrocartilage is found in the pubic symphysis, the annulus fibrosus of intervertebral discs, menisci, and the temporal mandibular joint.

Elastic Cartilage

Elastic or yellow cartilage contains elastic fiber networks and collagen fibers. The principal protein is elastin.

Elastic cartilage is histologically similar to hyaline cartilage but contains many yellow elastic fibers lying in a solid matrix. These fibers form bundles that appear dark under a microscope. They give elastic cartilage great flexibility so it can withstand repeated bending.

Chondrocytes lie between the fibers. Elastic cartilage is found in the epiglottis (part of the larynx) and the pinnae (the external ear flaps of many mammals, including humans).

Cartilage Growth

Chondrification is the process by which cartilage is formed from condensed mesenchyme tissue.

LEARNING OBJECTIVE

Describe cartilage growth and repair

KEY TAKEAWAYS

Key Points

The division of cells within cartilage occurs very slowly.
Articular cartilage function is dependent on the molecular composition of its extracellular matrix (ECM).
The remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix in response to forces experienced by the cartilage.
Cartilage growth mainly refers to matrix deposition, but can include both growth and remodeling of the extracellular matrix.

Key Terms

collagen matrix: The most abundant protein in the human body and accounts for 90% of bone matrix protein content.
mesenchyme: Undifferentiated cells of the early embryo are able to develop into the different tisue types, including bone and cartilage.
chondrocyte: A cell that makes up the tissue of cartilage.
chondrification: The process by which cartilage is formed from condensed mesenchyme tissue.

Formation

Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue.

A chondrocyte: A chondrocyte, stained for calcium, showing its nucleus (N) and mitochondria (M).

Mesenchyme tissue differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix (ECM). Mesenchymal stem cells (MSCs) are undifferentiated, meaning they can give rise to different cell types. Under the appropriate conditions and at sites of cartilage formation, they are referred to as chondrogenic cells.

During cartilage formation, undifferentiated MSCs are highly proliferative and form dense aggregates of chondrogenic cells at the center of chondrification. These condrogenic cells then differentiate to chondroblasts, which will then synthesize the cartilage ECM.

Cartilage: Hyaline cartilage showing chondrocytes and organelles, lacunae and matrix.

The extracellular matrix consists of ground substance (proteoglycans and glycosaminoglycans) and associated fibers, such as collagen. The chondroblasts then trap themselves in lacunae, small spaces that are no longer in contact with the newly created matrix and contain extracellular fluid. The chondroblast is now a chondrocyte, which is usually inactive but can still secrete and degrade the matrix depending on the conditions.

Growth

The majority of body cartilage is synthesized from chondroblasts that are largely inactive at later developmental stages compared to earlier years (pre-pubescence). The division of cells within cartilage occurs very slowly.

Therefore, growth in cartilage is usually not based on an increase in size or mass of the cartilage itself. Remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix, which responds to tensile and compressive forces experienced by the cartilage. Cartilage growth thus mainly refers to matrix deposition, but can include both growth and remodeling of the ECM.

Early in fetal development, the greater part of the skeleton is cartilaginous. This temporary cartilage is gradually replaced by bone (endochondral ossification), a process that ends at puberty. In contrast, the cartilage in the joints remains permanently unossified during life.

Repair

Once damaged, cartilage has limited repair capabilities because chondrocytes are bound in lacunae and cannot migrate to damaged areas. Also, because cartilage does not have a blood supply, the deposition of new matrix is slow.

Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue. Over the last few years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement.

These include marrow stimulation techniques, including surgeries, stem cell injections, and grafting of cartilage into damaged areas.

However, due to the extremely slow growth of cartilage and its avascular properties, regeneration and growth of cartilage post-injury is still very slow.

Joints connect the components of the skeletal system together. They give theskeletal system flexibility, and allow muscles to direct movements by moving bones in different directions.

Joints can be classified structurally or functionally.

There are three basic functional classifications for joints:

Synarthroses are immovable joints; these joints are common where protection of delicate internal structures (such as the brain and spinal cord) is important.
Amphiarthroses are slightly movable joints; these joints are common where protection of delicate internal structures (such as the brain and spinal cord) is important.
Diarthroses are freely movable joints; these joints dominate in the limbs and areas of the body where movement is important.There are three basic structural classifications for joints: fibrous, cartilaginous and synovial joints.

Fibrous joints allow very little movement, and are composed of fibrous (dense) connective tissue. The skull sutures and syndesmoses such as the connection between the tibia and fibula are fibrous joints.

Cartilaginous joints allow very little or no movement, and are characterized by a connection between adjoining bones made of cartilage. The pubic symphysis, intervertebral joints and connection between the first rib and sternum are slightly movable cartilaginous joints. The epiphyseal plate of growing bones is an immovable cartilaginous joint.

Synovial joints are the most complex of the joint types. They are characterized by articular (hyaline) cartilage covering the ends of bones, a fibrous articular capsule (composed of fibrous connective tissue) lined with synovial membrane, a joint cavity containing synovial fluid and reinforcing ligaments to hold the bones together. Synovial joints are found in between the bones of the limbs, and are freely movable.

Synovial joints are also associated with bursae, which are flattened fibrous sacs lined with synovial membrane that develop in areas of friction. Tendon sheaths are special bursae that wrap around tendons in areas of friction.

There are several types of synovial joints, each specialized in specific types of movement.

Plane joints connect two flat surfaces of bone to one another, and only allow side-to-side movement with no rotation. Since plane joints allow no rotation, they are called nonaxial joints. The short bones of the wrist move against one another with plane joints.
Hinge joints connect a cylindrical bone end to a concave (indented) portion of another bone. Rotation can occur in only one plane (or axis), much like a door hinge, and are thus called uniaxial joints. The elbow and ankle joints are hinge joints.
Pivot joints connect the rounded end of one bone to a ring or sheath formed by another bone. Pivot joints are uniaxial joints. The joint between the radius and ulna at the elbow is a pivot joint.
Condyloid joints fit the rounded convex articular surface of one bone into the rounded concave surface of another bone. Because both bone ends are rounded and fit closely together, condyloid joints allow side to side and forwards-backwards movements, but no rotation, similarly to saddle joints. Movement occurs in two planes, making condyloid joints biaxial joints. The joints of the knuckles are condyloid joints.
Saddle joints are characterized by concave and convex surfaces on both articular surfaces. Saddle joints allow side to side and forwards-backwards movements, but no rotation, similarly to condyloid joints. Movement occurs in two planes, making saddle joints biaxial joints. The joint between the carpal and metacarpal of the thumb is a saddle joint.
Ball-and-socket joints join the spherical end of one bone to the concave, rounded socket of another bone. These joints allow movement in all axes and rotation, and are therefore multiaxial joints. They are the most movable of all synovial joints, and are found in the shoulder and hip.

Structural Classification of Joints

There are three structural classifications of joints: fibrous, cartilaginous, and synovial.

LEARNING OBJECTIVES

Describe the three structural categories of joints

KEY TAKEAWAYS

Key Points

The type
and characteristics of a given joint determine the degree and type of movement.
Structural classification categorizes joints based on the type of
tissue involved in their formations.
There are three structural
classifications of joints: fibrous, cartilaginous, and synovial.
Of the three types of fibrous joints, syndesmoses are the most movable.
Cartilaginous
joints allow more movement than fibrous joints
but less than synovial joints.
Synovial joints ( diarthroses ) are the most movable joints of the body and contain synovial fluid.

Key Terms

periosteum: A membrane that covers the outer surface of all bones.
manubrium: The broad upper part of the sternum.
synovial fluid: A viscous fluid found in the cavities of synovial
joints that reduces friction between the articular cartilage during movement.

A joint, also known as an articulation or articular surface, is a connection that occurs between bones in the skeletal system. Joints provide the means for movement. The type and characteristics of a given joint determines its degree and type of movement. Joints can be classified based on structure and function.

Structural classification of joints categorizes them based on the type of tissue involved in formation. There are three structural classifications of joints: fibrous, cartilaginous, and synovial.

Fibrous Joints

Fibrous joints are connected by dense, tough connective tissue that is rich in collagen fibers. These fixed or immovable joints are typically interlocked with irregular edges. There are three types of fibrous joints.

Sutures are the types of joint found in the cranium (skull). The bones are connected by Sharpey’s fibres. The nature of cranial sutures allows for some movement in the fetus. However, they become mostly immovable as the individual ages, although very slight movement allows some necessary cranial elasticity. These rigid joints are referred to as synarthrodial.

Syndesmoses are found between long bones of the body, such as the radio-ulnar and tibio-fibular joints. These moveable fibrous joints are also termed amphiarthrodial. They have a lesser range of movement than synovial joints.

Gomphosis is a type of joint found at the articulation between teeth and the sockets of the maxilla or mandible (dental-alveolar joint). The fibrous tissue that connects the tooth and socket is called the periodontal ligament.

Fibrous joints: Image demonstrating the three types of fibrous joints. (a) Sutures (b) Syndesmosis (c) Gomphosis.

Cartilaginous Joints

Cartilaginous joints are connected by fibrocartilage or hyaline cartilage. They allow more movement than fibrous joints but less than that of synovial joints. These types of joints are further subdivided into primary (synchondroses) and secondary (symphyses) cartilaginous joints. The epiphyseal (growth) plates are examples of synchondroses. Symphyses are found between the manubrium and sternum (manubriosternal joint), intervertebral discs, and the pubic symphysis.

Cartilaginous Joints: Image demonstrates a synchondrosis joint with epiphyseal plate (temporary hyaline cartilage joint) indicated (a) and a symphysis joint (b).

Synovial Joint: This diagram of a synovial joint delineates the articular cartilage, articular capsule, bone, synovial membrane, and joint cavity containing synovial fluid.

Synovial Joints

This is the most common and movable joint type in the body. These joints (also called diarthroses) have a synovial cavity. Their bones are connected by dense irregular connective tissue that forms an articular capsule surrounding the bones’ articulating surfaces.

A synovial joint connects bones with a fibrous joint capsule that is continuous with the bones’ periosteum. This joint capsule constitutes the outer boundary of a synovial cavity and surrounds the bones’ articulating surfaces.

Synovial cavities are filled with synovial fluid. The knees and elbows are examples of synovial joints.

Functional Classification of Joints

Functional classification of joints is based on the type and degree of movement permitted.

LEARNING OBJECTIVES

Describe the three functional categories of joints

KEY TAKEAWAYS

Key Points

Synarthrosis joints are immobile or have limited mobility and include fibrous joints.
Amphiarthrosis joints allow a small amount of mobility and include cartilaginous joints.
Diarthrosis joints are the freely movable synovial joints.
Synovial joints can also be classified as nonaxial, monoaxial, biaxial, and multiaxial.
The various movements permitted by synovial joints are abduction, adduction, extension, flexion, and rotation.

Key Terms

fibrous joints: Fixed or immobile joints that are connected by dense, tough connective tissue that
is rich in collagen fibers.
cartilaginous joints: Joints connected by fibrocartilage or hyaline cartilage. They allow more movement than fibrous joints but less than synovial joints.
gomphosis joints: Joints of very limited mobility. These are found at the articulation
between teeth and the sockets of maxilla or mandible (dental-alveolar joint).

Joints or articulations (connections between bones) can be classified in a number of ways. The primary classifications are structural and functional. Functional classification is based on the type and degree of movement permitted.

Types of Synovial Joints.jpg: Image of a skeleton and skematics of the different classes of synovial joints.

Three Categories of Functional Joints

Synarthrosis: These types of joints are immobile or allow limited mobility. This category includes fibrous joints such as suture joints (found in the cranium) and gomphosis joints (found between teeth and sockets of the maxilla and mandible).
Amphiarthrosis: These joints allow a small amount of mobility. Most joints in this category
include cartilaginous joints such as those found between vertebrae and the pubic symphysis.
Diarthrosis: These are the freely-movable synovial joints. Synovial joints are further classified based on the different types of movement they provide, including:
- Plane joint
- Ball and socket joint
- Hinge joint
- Pivot joint
- Condyloid joint
- Saddle joint

Movement of Synovial Joints

Joints can also be classified by the number of axes of movement they permit:

Nonaxial (gliding): Found between the proximal ends of the ulna and radius.
Monoaxial (uniaxial): Movement occurs in one plane. An example is the elbow joint.
Biaxial: Movement can occur in two planes. An example is the wrist.
Multiaxial: Includes the ball and socket joints. An example is the hip joint.

The movements possible with synovial joints are:

Abduction: movement away from the body’s midline
Adduction: movement toward the body’s midline
Extension: straightening limbs at a joint
Flexion: bending the limbs at a joint
Rotation: a circular movement around a fixed point

Fibrous Joints

Fibrous joints are also called fixed or immovable joints because they do not move.

LEARNING OBJECTIVES

Describe fibrous joints

KEY TAKEAWAYS

Key Points

A joint is the location at which two or more bones make contact.
Joints are classified based on structural and functional properties.
Fibrous joints, such as sutures, syndesmoses, and gomphoses, have no joint cavity.
Fibrous joints are connected by dense connective tissue consisting mainly of collagen.
Fibrous joints are called “fixed” or “immovable” joints because they do not move.

Key Terms

syndesmoses: Slightly movable articulations where the contiguous bony surfaces are united by an interosseous ligament, as in the inferior tibiofibular articulation.
suture: In anatomy, a suture is a fairly rigid joint between two or more hard elements such as the bony plates of the skull.
gomphoses: A joint that binds the teeth to bony sockets (dental alveoli) in the maxillary bone and mandible.

A joint is the location at which two or more bones make contact. They are constructed to allow movement (except for skull bones), provide mechanical support, and are classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications.

Fibrous Joints: The adult skull is normally made up of 22 bones. Except for the mandible, all are joined together by sutures, semi-rigid articulations formed by bony ossification. The presence of Sharpey’s fibers permit a little flexibility.

Fibrous joints are joined by dense irregular connective tissue that is rich in collagen fibers.

Characteristics of Fibrous Joints

Fibrous joints are connected by dense connective tissue consisting mainly of collagen. These joints are also called fixed or immovable joints because they do not move. Fibrous joints have no joint cavity and are connected via fibrous connective tissue. The skull bones are connected by fibrous joints called sutures. The skull bones of a fetus are unfused so that they can move over each other slightly to compress skull size during birth. After birth, the bones slowly begin to fuse to become fixed, making the skull bones immovable in order to protect the brain from impact.

Syndesmoses of long bones and gomphoses of teeth are also types of fibrous joints. The movement of the root within a gomphosis has a threefold effect. It lessens some of the impact between the upper and lower teeth in biting. It also pumps blood and lymph from the periodontal membrane into the dental veins and lymph channels and stimulates sensory nerve terminals in the membrane to send signals to the brain centers that control the muscles of mastication.

Sutures

A suture is a type of fibrous joint (synarthrosis) bound by Sharpey’s fibers that only occurs in the skull (cranium).

LEARNING OBJECTIVES

Describe a suture joint

KEY TAKEAWAYS

Key Points

A suture ‘s fibrous connective tissue helps protect the brain and form the face by strongly uniting the adjacent skull bones.
Sutures form a tight union that prevents most movement between the bones. Most sutures are named for the bones they articulate.
Skull sutures visible from the side (norma lateralis) include the frontal, parietal, temporal, occipital, sphenoid, and zygomatic bones, while skull sutures visible from the front (norma frontalis) and above (norma verticalis) include those related to the frontal and parietal bones.
Skull sutures visible from below (norma basalis) include the frontal, ethmoid, and sphenoid bones.

Key Terms

fontanelle: An anatomical feature of the infant human skull comprising the soft membranous gaps.
Sharpey’s fibres: A matrix of connective tissue consisting of bundles of strong collagenous fibers connecting periosteum to bone.
suture: A type of fibrous joint which only occurs in the skull (cranium).

A suture is a type of fibrous joint which only occurs in the cranium, where it holds bony plates together. Sutures are bound together by a matrix of connective tissues called Sharpey’s fibers, which grow from each bone into the adjoining one. A tiny amount of movement is permitted at sutures, which contributes to the compliance and elasticity of the skull. These joints are synarthroses (immovable joints).

Cranial Sutures

Cranial Sutures: Lateral view of skull showing the location of some of the cranial sutures.

Most sutures are named for the bones they articulate, but some have special names of their own. Sutures primarily visible from the side of the skull (norma lateralis) include:

Coronal suture: between the frontal and parietal bones
Lambdoid suture: between the parietal, temporal, and occipital bones
Occipitomastoid suture
Parietomastoid suture
Sphenofrontal suture
Sphenoparietal suture
Sphenosquamosal suture
Sphenozygomatic suture
Squamosal suture: between the parietal and the temporal bone
Zygomaticotemporal suture
Zygomaticofrontal suture

Sutures primarily visible from front of the skull (norma frontalis) or above the skull (norma verticalis) include:

Frontal suture / Metopic suture: between the two frontal bones, prior to the fusion of the two into a single bone
Sagittal suture: along the midline, between parietal bones.

Sutures primarily visible from below the skull (norma basalis) or inside the skull include:

Frontoethmoidal suture
Petrosquamous suture
Sphenoethmoidal suture
Sphenopetrosal suture

The fibrous connective tissue found at a suture (to bind or sew) strongly unites the adjacent skull bones and thus helps to protect the brain and form the face. In adults, the skull bones are closely opposed and fibrous connective tissue fills the narrow gap between the bones. The suture is frequently convoluted, forming a tight union that prevents most movement between the bones.

Fontanelles

Frontal suture top view: Drawing of human baby skull seen from the top. Cranial sutures are depicted with the frontal suture highlighted in blue.

It is normal for many of the bones of the skull to remain unfused at birth. The fusion of the skull’s bones at birth is known as craniosynostosis. The joint between the mandible and the cranium, the temporomandibular joint, forms the only non-sutured joint in the skull. In newborns and infants, the areas of connective tissue between the bones are much wider, especially in those areas on the top and sides of the skull that will become the sagittal, coronal, squamous, and lambdoid sutures.

These broad areas of connective tissue are called fontanelles. During birth, the fontanelles provide flexibility to the skull, allowing the bones to push closer together or to overlap slightly, thus aiding movement of the infant’s head through the birth canal. After birth, these expanded regions of connective tissue allow for rapid growth of the skull and enlargement of the brain. The fontanelles greatly decrease in width during the first year after birth as the skull bones enlarge. When the connective tissue between the adjacent bones is reduced to a narrow layer, these fibrous joints are now called sutures.

Synostosis

At some sutures, the connective tissue will ossify and be converted into bone, causing the adjacent bones to fuse to each other. This fusion between bones is called a synostosis (joined by bone). Examples of synostosis fusions between cranial bones are found both early and late in life. At the time of birth, the frontal and maxillary bones consist of right and left halves joined together by sutures, which disappear by the eighth year as the halves fuse together to form a single bone. Late in life, the sagittal, coronal, and lambdoid sutures of the skull will begin to ossify and fuse, causing the suture line to gradually disappear.

Syndesmoses

Syndesmoses are slightly movable joints formed where an interosseous ligament joins two bones.

LEARNING OBJECTIVES

Describe syndesmoses

KEY TAKEAWAYS

Key Points

A syndesmosis, a subcategory of fibrous joints, is a slightly movable (amphiarthrodial) articulation where the contiguous bony surfaces are united by an interosseous ligament, such as the tibiofibular articulation.
Tears in this joint are generally repaired with a syndesmotic screw.
Due to the lack of flexibility in these joint structures, ligament injuries in syndesmosis joints are common, particularly at the wrist and ankle.

Key Terms

symphysis: The cartilaginous material that adjoins and facilitates the junction of such bones, with or without synovia.
prime mover: A muscle that acts directly to bring about a desired movement.
diastasis: A separation between two parts of a bone, without fracture.

A syndesmosis is a type of articulation or joint in which two adjacent bones are joined by an interosseous membrane.

Interosseous Membrane

The interosseous membrane is a type of connective tissue found between certain bones, such as those in syndesmosis joints. The membrane is important in creating compartments to separate different structures, distributing the impact of forces and separating the joints. For example, the long bones of the lower arm and the leg both have attached interosseous membranes. In the leg, the interosseous membrane extends between the tibia and the fibula, running along the crests of the bones. The muscles in the leg are separated into sections in the front and back with this membrane. The strength of the membrane allows absorption and distribution of impacts to either bone.

The interosseous membrane in the lower arm extends between the radius and the ulna. It is involved in the elbow joint and helps to stabilize the lower arm bones for strength, durability, and flexibility. Like other joint tissue, it is designed to be able to deform and flex rather than shred or fracture on impact, allowing the joint to absorb considerable stress before damage occurs.

Syndesmoses and Amphiarthrosis Joints

Along with symphysis joints, syndesmoses are classified as amphiarthrosis joints in that they allow slight movement. Joints of this kind are found at several points in the human body, including the intermediate radioulnar joint where the radius and ulna meet above the wrist, in the spine between the spinous processes of various adjacent vertebra, and above the ankle joint where the tibia and fibula converge.

Fibrous Joints: Image of fibrous joints with the tibiofibular syndesmosis demonstration in figure (b).

Located directly above the ankle joint, which is a synovial hinge joint, the ankle syndesmosis is held together by four ligaments. The anterior inferior tibiofibular ligament crosses in front of the tibia and fibula bones. The posterior inferior tibiofibular ligament and the transverse ligament connect the two bones from behind, and the interosseous ligament runs between the contiguous bony surfaces of the two bones.

Due to the limited flexibility in these joint structures, ligament injuries in syndesmosis joints are common, particularly at the wrist and ankle. When the wrist or ankle joint is bent beyond its normal range of motion, a sprain or even a tear in these ligaments can occur. Mild syndesmosis injury may involve the sprain of a single ligament.

More severe injuries can involve damage to multiple ligaments at once or even the separating of the bones at the joint (known as diastasis). Players of rough sports such as football or rugby have an increased risk of fracturing their fibulas and tearing the interosseous ligament between it and the tibia. When that happens, the surgeon temporarily replaces the ligament with a syndesmotic screw.

Gomphoses

A gomphosis is a fibrous joint that binds the teeth to bony sockets in the bones of the maxilla mandible.

LEARNING OBJECTIVES

Describe a gomphosis joint

KEY TAKEAWAYS

Key Points

The gomphosis is the only joint type in which a bone does not join another bone, because teeth are not technically bone.
The motion of a gomphosis is minimal, though considerable movement can be achieved with pressure over time, which is why using braces can realign teeth.
One disorder that can affect the gomphosis is scurvy, which is a disease of connective tissue.

Key Terms

scurvy: A disease resulting from a lack of vitamin C.
synarthrosis: A type of joint in which two bones are connected rigidly by fibrous tissue.
gomphoses: A joint that binds the teeth to bony sockets (dental alveoli) in the maxillary bone and mandible.

A gomphosis is a joint that anchors a tooth to its socket. Gomphoses line the upper and lower jaw in each tooth socket and are also known as peg and socket joints. These joints have a very limited range of mobility so the teeth are held firmly in place. However, as illustrated with braces, it is possible to move them incrementally over time. Each tooth has bony protrusions or pegs that latch into the socket with the assistance of the gomphosis. Disorders of the mouth sometimes involve these joints.

Gomphoses: This image illustrates the gomphoses joints of teeth within the jaw.

This particular joint is an example of a synarthrosis, a joint with limited to no movement. Several other joints of this type can be found in the body, including the connections between the plates of the skull. The gomphosis is made up of fibrous tissue, a collection of tough ligaments that attach to the socket and base of the tooth. As people age and lose their initial set of baby teeth, the new teeth develop gomphoses to anchor them in the jaw.

One disorder that can affect the gomphosis is scurvy, a disease of connective tissue. Connective tissues such as the ligaments around the teeth start to dissolve. Patients with untreated scurvy develop loose teeth that may eventually fall out because the joints are too unstable. Periodontal infection and inflammation can also damage the joint, causing pain and erosion in the soft tissue. Chronic dental problems may loosen the ligaments and lead to tooth loss or instability.

Patients with braces and retainers take advantage of the limited range of movement offered by the gomphosis to pull teeth into new positions. This may be necessary for a variety of reasons. The goal is to align the teeth evenly to create a strong, healthy bite. Braces are adjusted incrementally over time to pull and push the teeth into place. Between each adjustment, the teeth and jaw have time to recover.

Cartilaginous Joints: Synchodroses

Cartilaginous joints connected by hyaline cartilage are termed synchondroses.

LEARNING OBJECTIVES

Describe synchondroses

KEY TAKEAWAYS

Key Points

The first sternocostal joint where the first rib meets the sternum is a synchondrosis.
The epiphyseal growth plate is a temporary cartilaginous joint formed as the cartilage is converted to bone during growth and development.
Cartilaginous joints are connected entirely by cartilage and allow more movement between bones than a fibrous joint, but less than the highly mobile synovial joint.

Key Terms

synchondrosis: A slightly moveable articulation between bones joined by hyaline cartilage.
apoptosis: A type of “cell suicide” called programmed cell death
that occurs in multicellular organisms.
epiphyseal plate: The epiphyseal plate is a hyaline cartilage plate where growth occurs in children and adolescents, located in the metaphysis at each end of a long bone.

Cartilaginous joints are connected entirely by cartilage (fibrocartilage or hyaline). Cartilaginous joints allow more movement between bones than a fibrous joint but less than the highly mobile synovial joint. The joint between the manubrium and the sternum is an example of a cartilaginous joint. This type of joint also forms the growth regions of immature long bones and the intervertebral discs of the spinal column.

Synchondroses: Section through occipitosphenoid synchondrosis of an infant, including the cartilage, perichrondrium, and periosteum.

Where the connecting medium is hyaline cartilage, a cartilaginous joint is termed a synchondrosis or primary cartilaginous joint. A synchondrosis joint is the first sternocostal joint (where the first rib meets the sternum). In this example, the rib articulates with the sternum via the costal cartilage. The rest of the sternocostal joints are synovial plane joints.

A temporary form of joint called an epiphyseal (growth) plate, is one where the cartilage is converted into bone before adult life. Such joints are found between the epiphyses and diaphyses of long bones, between the occipital and the sphenoid bones, and during the early years of life, between the petrous portion of the temporal and the jugular process of the occipital bone. The epiphyseal plate is a hyaline cartilage plate in the metaphysis at each end of a long bone.

The epiphyseal plate is found in children and adolescents. In puberty, increasing levels of estrogen, in both females and males, leads to increased apoptosis of chondrocytes in the epiphyseal plate. Depletion of chondrocytes due to apoptosis leads to less ossification, and growth slows down and later stops when the cartilage has been completely replaced by bone. This process leaves only a thin epiphyseal scar that later disappears. In adults who have stopped growing, the plate is replaced by an epiphyseal line.

Those with achondroplasia (a form of dwarfism) have premature closure of the epiphyseal growth plates, which results in shorter than average arms and legs.

Cartilaginous Joints: Symphyses

A symphysis is a secondary cartilaginous joint that is permanent and slightly movable.

LEARNING OBJECTIVES

Differentiate among the types of symphyses between two bones

KEY TAKEAWAYS

Key Points

Symphyses include the pubic symphysis and the intervertebral disc between two vertebrae, among others.
The pubic symphysis or symphysis pubis is the midline cartilaginous joint uniting the superior rami of the left and right pubic bones. It widens slightly whenever the legs are stretched far apart and can become dislocated.
Intervertebral discs lie between adjacent vertebrae in the spine. Each disc forms a cartilaginous joint to allow slight movement of the vertebrae and acts as a ligament to hold the vertebrae together.

Key Terms

annulus fibrosus: Fibrous ring of intervertebral disk.
nucleus pulposus: Inner gel-like center of the vertebral disc.
sciatica: Pain that travels down the leg from the lower back region.
symphysis: The cartilaginous material that adjoins and facilitates the junction of such bones, with or without synovia.
intervertebral disc: A cartilaginous joint that allows slight movement of the vertebrae by lying between adjacent vertebrae in the spine. It also acts as a ligament to hold the vertebrae together.

A symphysis, a type of secondary cartilaginous joint, is a fibrocartilaginous fusion between two bones. It is an amphiarthrosis (slightly movable) joint, and an area where two parts or structures grow together. Unlike synchondroses, symphyses are permanent. The more prominent symphyses are the pubic symphysis; the symphyses between the bones of the skull, most notably the mandible (symphysis menti); sacrococcygeal symphysis; the intervertebral disc between two vertebrae; and in the sternum, between the manubrium and body, and between the body and xiphoid process.

Pubic Symphysis

The pubic symphysis or symphysis pubis is the midline cartilaginous joint (secondary cartilaginous) uniting the superior rami of the left and right pubic bones. It is a nonsynovial amphiarthrodial joint connected by fibrocartilage, and may contain a fluid-filled cavity. The ends of both pubic bones are covered by a thin layer of hyaline cartilage attached to the fibrocartilage.

Symphyses: Diagrammatic section of a symphysis including the ligament, disc of fibrocartilage, and articular cartilage.

The pubic symphysis is located anterior to the urinary bladder and superior to the external genitalia, above the vulva in females and above the penis in males. The suspensory ligament of the penis attaches to the pubic symphysis. In females, the pubic symphysis is intimately close to the clitoris. In normal adults, it can be moved roughly two mm and with one degree of rotation. Mobility of this joint increases for women at the time of childbirth.

During birth, the pubic symphysis of relaxes to slightly widen the birth canal. This movement is minimal, but along with the compression of the unfused fetal skull generally allows an infant to be born vaginally.

The pubic symphysis widens slightly whenever the legs are stretched far apart. In sports in which this movement is frequent, the risk of a pubic symphysis blockage is high. This injury occurs when the bones at the symphysis do not realign correctly after completion of the movement and get jammed in a dislocated position. The resulting pain can be quite severe, especially if further strain is put upon the affected joint. In most cases, the joint can only be successfully reduced into its normal position by a trained medical professional.

Pubic symphyses have importance in the field of forensic anthropology, as they can be used to estimate the age of adult skeletons. Throughout life, the surfaces become worn at a more or less predictable rate. By examining the wear of the pubic symphysis, it is possible to estimate the age of the person at death.

Mandible

The external surface of the mandible is marked in the median line by a faint ridge, indicating the symphysis menti, mandibular symphysis, or line of junction. This line delineates the two pieces of bone that compose the mandible during the first years of life.

Intervertebral Discs

Intervertebral discs (or intervertebral fibrocartilage) lie between adjacent vertebrae in the spine. Each disc forms a cartilaginous joint to allow slight movement of the vertebrae and acts as a ligament to hold the vertebrae together. The discs consist of an outer annulus fibrosus that surrounds the inner nucleus pulposus. The annulus fibrosus and the nucleus pulposus distribute pressure evenly across the disc.

The nucleus pulposus contains loose fibers suspended in a mucoprotein gel with the consistency of jelly. The nucleus of the disc acts as a shock absorber, absorbing the impact of the body’s daily activities and keeping the two vertebrae separated. The disc can be likened to a jelly doughnut with the annulus fibrosis as the dough and the nucleus pulposis as the jelly. If one presses down on the front of the doughnut, the jelly moves posteriorly. When one develops a prolapsed disc, the jelly (the nucleus pulposus) is forced out of the doughnut (the disc) and may put pressure on the nerve located near the disc, potentially causing symptoms of sciatica.

Diagram of Invertebral Disc: The lateral and superior view of an invertebral disc, including the vertebral body, intervertebral foramen, anulus fibrosis, and nucleus pulposus.

Aging causes disc degeneration, in which the nucleus pulposus begins to dehydrate and the concentration of proteoglycans in the matrix decreases, limiting the ability of the disc to absorb shock. This general shrinking of disc size is partially responsible for the common decrease in height as humans age.

Structure of Synovial Joints

A synovial joint or diarthrosis occurs at articulating bones to allow movement. It is distinguished by a surrounding synovial capsule.

LEARNING OBJECTIVES

Identify the structures of the synovial joint that allow it to move freely

KEY TAKEAWAYS

Key Points

The bones of a synovial joint are surrounded by a synovial capsule, which secretes synovial fluid to lubricate and nourish the joint while acting as a shock absorber.
The ends of the joint bones are covered with smooth, glass-like hyaline cartilage which reduces friction during movement.
A synovial joint contains a synovial cavity and dense, irregular connective tissue that forms the articular capsule normally associated with accessory ligaments.

Key Terms

articulation: A joint or the collection of joints at which something is articulated, or hinged, for bending.
synovial membrane: A thin membrane of joints comprised of smooth connective tissue and that secretes synovial fluid.
synovial fluid: A viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency, its principal role is to reduce friction between the articular cartilage of synovial joints during movement.
articular cartilage: A tough, elastic, fibrous connective tissue found in various parts of the body such as the joints, outer ear, and larynx. A major constituent of the embryonic and young vertebrate skeleton, converted largely to bone with maturation.
diarthrosis: A joint that can move freely in various planes.

A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in a mammal’s body. Diarthroses are freely movable articulations. In these joints, the contiguous bony surfaces are covered with articular cartilage and connected by ligaments lined by synovial membrane. The joint may be divided, completely or incompletely, by an articular disk or meniscus, the periphery of which is continuous with the fibrous capsule while its free surfaces are covered by synovial membrane.

The articular capsule is fibrous and continuous with the periosteum of articulating bones, surrounding the diarthrosis and uniting the articulating bones. The articular capsule also consists of two layers: (1) the outer fibrous membrane that may contain ligaments and (2) the inner synovial membrane that secretes the lubricating, shock-absorbing, and joint-nourishing synovial fluid. The bones of a synovial joint are covered by a layer of hyaline cartilage that lines the epiphyses of joint ends of bone with a smooth, slippery surface that does not bind them together. This articular cartilage functions to absorb shock and reduce friction during movement.

Synovial Membrane and Components

Synovial Joint: An illustration of the structure of a synovial joint.

A synovial membrane (or synovium) is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints. Synovial fluid is the clear, viscid, lubricating fluid secreted by synovial membranes. The morphology of synovial membranes may vary, but it often consists of two layers. The outer layer, or subintima, can be fibrous, fatty, or loosely areolar. The inner layer, or intima, consists of a sheet of cells thinner than a piece of paper.

Where the underlying subintima is loose, the intima sits on a pliable membrane called the synovial membrane. This membrane, together with the cells of the intima, acts like an inner tube, sealing the synovial fluid from the surrounding tissue and effectively stopping the joints from being squeezed dry when subjected to impact (such as when running). As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).

Synoviocytes

The intimal cells are termed synoviocytes and can be either fibroblastic (type B synoviocytes) and macrophagic (type A synoviocytes). Both types have differences from similar cells in other tissues. The type B synoviocytes manufacture a long-chain sugar polymer called hyaluronan, which combines with a molecule called lubricin to give the synovial fluid a stringy, egg-white consistency. The water component of synovial fluid is effectively trapped in the joint space by the hyaluronan due to its large, highly negatively charged moieties. The macrophages are responsible for the removal of undesirable substances from the synovial fluid.

Structure of Synovium

The surface of a synovium may be flat or covered with finger-like projections (villi) to allow the soft tissue to change shape as the joint surfaces move on one another. Just beneath the intima, most synovia have a dense net of small blood vessels that provide nutrients for the synovia and the avascular cartilage.

In any one position, much of the cartilage is close enough to get nutrition directly from the synovium. Some areas of cartilage have to obtain nutrients indirectly and may do so either from diffusion through cartilage or by the stirring of synovial fluid.

Synovial Bursa

The synovial bursa is a small, fluid-filled sac lined by synovial membrane containing synovial fluid. It provides a cushion between bones and tendons and/or muscles around a joint.

Nerve and Blood Supply

Synovial joints are highly innervated but vascularized indirectly by nearby tissues.

LEARNING OBJECTIVES

Identify the nerve and blood supply of synovial joints

KEY TAKEAWAYS

Key Points

Although the articular capsule is innervated with the nerves necessary for movement, it lacks blood vessels because the arteries wrap around the joint in an anastomosis, bypassing direct capillary contact with the capsule.
The articular and epiphyseal branches given off by the neighboring arteries form a periarticular arterial plexus.
Exchange of gases (oxygen and carbon dioxide) and nutrients is achieved, albeit slowly, via diffusion or more efficiently during exercise via convection.

Key Terms

convection: The movement of groups of molecules within fluids such as liquids or gases.
osteomyelitis: An infection of the bone and bone marrow characterized by inflammation.
anastomosis: A cross-connection between two blood vessels.

Elbow Joint: Diagram of the anastomosis around the elbow joint.

The blood supply of a synovial joint comes from the arteries sharing in anastomosis around the joint. The articular and epiphyseal branches of the neighboring arteries form a periarticular arterial plexus. The articular capsule is highly innervated but avascular (lacking blood and lymph vessels), and receives nutrition from the surrounding blood supply via either the slow process of diffusion or convection, a far more efficient process.

Numerous vessels from this plexus pierce the fibrous capsule and form a rich vascular plexus in the deeper part of the synovial membrane. The blood vessels of the synovial membrane terminate around the articular margins in a fringe of looped anastomoses termed the circulus vasculosus (circulus articularis vasculosus). It supplies the capsule, synovial membrane, and the epiphyses. After epiphyseal fusion in the growth of long bones, communication between the circulosus vasculosus and the end arteries of the metaphysis is established. This minimizes the chances of osteomyelitis in the metaphysis.

The synovial cartilage in the capsule acts somewhat like a sponge. A sponge will absorb fluid, but it will release little of that fluid unless it is squeezed. Exercising the joint, in effect, squeezes the synovial “sponge,” allowing gas exchange to occur and nutrients to flow into the cartilage. Flexing and extending the joint alternately squeezes the sponge and releases it to reabsorb more fluid.

Bursae and Tendon Sheaths

Joints are cushioned by small fluid-filled sacs called bursae and stabilized by tough bands of fibrous connective tissue called tendons.

LEARNING OBJECTIVES

List the components of a joint

KEY TAKEAWAYS

Key Points

Synovial joints are made up of five classes of tissues. These include bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments.
Tendons are tough bands of fibrous connective tissue that connect muscles to bones.
Bursae are sacs filled with synovial fluid that provide cushioning around a joint between the bones and the muscles and tendons crossing the joint.

Key Terms

retinacula: A band around tendons that holds them in place for stabilization.
tendon: A tough band of inelastic fibrous tissue that connects a muscle with its bony attachment.
synovial fluid: A viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency, its principal role is to reduce friction between the articular cartilage of synovial joints during movement.
connective tissue: A type of tissue found in animals that functions in binding other tissue systems (such as muscle to skin) or organs. It consists of the cells, fibers, and a ground substance or extracellular matrix.

Synovial joints are made up of five classes of tissues: bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments. The synovial lining in the bursae and tendon sheaths, similar to that within joints, is a slippery, non-adherent surface allowing movement between planes of tissue. Synovial tendon sheaths line tendons only where they pass through narrow passages or retinacula, as in the palm, at the wrist, and around the ankle. Elsewhere, the tendon lies in a bed of loose fibrous tissue.

Tendons: Illustration of the location of tendons in the hand

A tendon or sinew is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen, but a ligament joins one bone to another and fasciae connect muscles to other muscles. Tendons connect muscle to bone and move the bones or structures to which they are attached.

A bursa (plural bursae) is a small, fluid-filled sac lined by synovial membrane with an inner capillary layer of fluid (synovial fluid) with the consistency of raw egg white. It provides a cushion between bones and tendons or muscles around a joint. This helps to reduce friction between the bones and allows for free movement.

Bursae occur at sites of shearing in subcutaneous tissue or between deeper tissues such as muscle groups and fascia. Many bursae develop during growth, but new or adventitious bursae can occur at sites of occupational friction. Bursae are found around most major joints of the body, such as the shoulder and the knee. For example, to protect the knee and reduce friction from the various muscles, tendons, and ligaments that attach to and cross the knee joint, knees are cushioned by 14 different bursae: five in front, four laterally, and five medially.

Knee joint: Diagram of the knee joint.

Stability and Range of Motion at Synovial Joints

Tendons provide stability at joints.

LEARNING OBJECTIVES

Explain the roles of tendons in movement and flexibility

KEY TAKEAWAYS

Key Points

Although tendons have long been considered just a way to attach muscles to bones, research has shown that their springy properties also allow them to provide stability during locomotion with no active work.
The elasticity of tendons enables them to release stored energy during walking, allowing the muscles to generate greater force without changing length.
Many factors influence joint stability and range of motion.

Key Terms

pronation: The action of rotating the forearm so that the palm of the hand is turned down or back.
supination: The action of rotating the forearm so that the palm of the hand is turned up or forward
eversion: The condition of being turned outward.
plantarflexion: The movement that increases the approximate 90 degree angle between the front part of the foot and the shin.
dorsiflexion: The movement which decreases the angle between the dorsum (superior surface) of the foot and the leg, so that the toes are brought closer to the shin.

A tendon is a mechanism by which muscles connect to bone and that transmits force. However, over the past two decades, research has also characterized the elastic properties of tendons and their ability to function as springs. This characteristic allows tendons to passively modulate forces during locomotion, thus providing additional stability with no active work. It also allows tendons to store and recover energy with high efficiency.

Effect of Tendon Elasticity

Achilles Tendon: The Achilles tendon, also called the calcaneus, provides stability and limits the range of motion at the ankle joint. It’s depicted in this diagram in relation to the tendo calcaneus.

During a human stride, the Achilles (calcaneal) tendon stretches as the ankle joint undergoes dorsiflexion. During the last portion of the stride, as the foot undergoes plantar flexion (pointing the toes downward), the stored elastic energy is released. Because the tendon stretches, the muscle is able to function with less or even no change in length, allowing it to generate greater force.

Joint Stability

Certain joints exhibit special movements including elevation, depression, protraction, retraction, inversion, eversion, dorsiflexion, plantar flexion, supination, pronation, and opposition. A number of factors influence joint stability. These include:

Shape of articular surfaces (how close they fit)
Strength and tension of capsule and ligaments (dependent on position)
Arrangement and tension of muscles
Contact with soft parts such as adipose tissue
Hormones
Disuse, causing decrease in synovial fluid, flexibility of ligaments and tendons, and muscle atrophy
Gravity and atmospheric pressure.

Typically, the more stable the joint is, the less is its range of motion and vice versa. Aging is another factor that influences motion due to decreased fluid, thinning of cartilage, shortening of ligaments, and loss of flexibility.

Synovial Joint Movements

Synovial joints allow an individual to achieve a wide range of movements.

LEARNING OBJECTIVES

Identify the different types of synovial joints

KEY TAKEAWAYS

Key Points

Synovial joints achieve movement at the point of contact of the articulating bones.
Synovial joints allow bones to slide past each other or to rotate around each other. This produces movements called abduction (away), adduction (towards), extension (open), flexion (close), and rotation.
There are six types of synovial joints. Some are relatively immobile but more stable than mobile joints.

Key Terms

synovial joint: Also known as a diarthrosis, the most common and most movable type of joint in the body of a mammal.
abduction: The movement that separates a limb or other part from the axis, or middle line, of the body.
flexion: The act of bending a joint. The counteraction of extension.
adduction: The action by which the parts of the body are drawn toward its axis.

A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. Synovial joints achieve movement at the point of contact of the articulating bones. Structural and functional differences distinguish synovial joints from cartilaginous joints (synchondroses and symphyses) and fibrous joints (sutures, gomphoses, and syndesmoses). The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).

Several movements may be performed by synovial joints. Abduction is the movement away from the midline of the body. Adduction is the movement toward the middle line of the body. Extension is the straightening of limbs (increase in angle) at a joint. Flexion is bending the limbs (reduction of angle) at a joint. Rotation is a circular movement around a fixed point.

Body Movements I: Image demonstrating the various joint movements.

There are six types of synovial joints. Some are relatively immobile but more stable than mobile joints. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The six types of joints include:

Gliding joints: only allow sliding movement
Hinge joints: allow flexion and extension in one plane
Pivot joints: allow bone rotation about another bone
Condyloid joints: perform flexion, extension, abduction, and adduction movements
Saddle joints: permit the same movement as condyloid joints and combine with them to form compound joints
Ball and socket joints: allow all movements except gliding

Six Types of Synovial Joints: Image demonstrating the six different types of synovial joints.

Types of Synovial Joints

There are six different types of synovial joint based on their shapes, each allowing a different kind of movement.

LEARNING OBJECTIVES

Describe the different types of synovial joints

KEY TAKEAWAYS

Key Points

Plane joints are flat and have slipping and gliding properties.
Hinge joints are formed between the cylindrical end of a bone and the trough-shaped surface of another bone, allowing flexion and extension in one plane.
Pivot joints are formed between the rounded end of the bone and a sleeve or ring of a bone, allowing up and down and side-to-side movement.
Condyloid joints occur where an egg-shaped surface of a bone fits into a concave space in another bone, allowing flexion, extension, abduction, and adduction movements ( circumduction ). The saddle joint resembles a saddle and permits the same movements as the condyloid joints.
Ball-and-socket joints occur where one bone ends in a spherical head and the other has a round socket, allowing all movements except gliding.
Saddle joints
are biaxial, and movements are the same as those for condyloid
joints; however, no axial
rotation is possible.

Key Terms

ball-and-socket joint: A joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone.
acromioclavicular joint: A joint at the top of the shoulder that is the junction between the acromion (a bony process on the scapula) and the clavicle.
circumduction: A conical movement of a body part consisting of a combination of flexion, extension, adduction, and
abduction.
condyle: A smooth prominence on a bone where it forms a joint with another bone.
Synovial joint: The most common and most movable type of joint in the body of a mammal.

There are six basic types of synovial joints. Anatomical joints may consist of a combination of two or more joint types. Some synovial joints are relatively immobile but stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The types of the synovial joints are based on their shapes and can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket. The following descriptions are in ascending order of mobility:

The articulating surfaces of the plane joint are usually flat to allow slipping and gliding properties. Examples include the carpals of the wrist and the acromioclavicular joint.
A hinge joint (ginglymus) is formed when the cylindrical end of a bone fits into a trough-shaped surface of another bone, like that of an elbow joint (between the humerus and the ulna). These joints act as a hinge, allowing flexion and extension in just one plane.
In a pivot joint, the rounded end of the bone fits into a sleeve or ring of bone. The atlanto-axial joint, proximal radioulnar joint, and distal radioulnar joint are examples of pivot joints.
The condyloid joint occurs where an egg-shaped surface of a bone fits into a concavity in another bone. Examples include the wrist joint (radiocarpal joint) and the temporomandibular joint. Some classifications make a distinction between condyloid and ellipsoid joints, but both allow flexion, extension, abduction, adduction, and circumduction movements.
The surface of a saddle joint has both convex and concave areas which resemble a saddle and permit the same movements as the condyloid joints. The carpometacarpal or trapeziometacarpal joint of the thumb (between the metacarpal and carpal, the trapezium) and the sternoclavicular joint are examples of saddle joints.
A ball-and-socket joint occurs where one bone ends in a spherical head and the other bone has a round socket. This joint creates the ball-and-socket movement found in such places as the hip and shoulder (glenohumeral). This type of joint allows for all movements except gliding.

The knee joint is an example of a compound joint/modified hinge joint where different types of joints combine. In this example, the condyles of the femur join with condyles of tibia and the saddle joint, where the lower end of the femur joins with the patella.

Plane Joint: The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula.

Ball and Socket Joint: Hip joint: the ball of the femur head fits in the socket of the acetabulum of the pelvis.

Saddle Joint: Sternoclavicular articulation. Anterior view.

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