Joint Structure and Function

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.

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:

1. Synarthroses are immovable joints; these joints are common where protection of delicate internal structures (such as the brain and spinal cord) is important.

2. Amphiarthroses are slightly movable joints; these joints are common where protection of delicate internal structures (such as the brain and spinal cord) is important.

3. 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.

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

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: 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.

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.

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