Limbs

Humerus: The humerus attaches proximally to the scapula (shoulderblade) at the humeral head and distally with the radius and ulna (lower-arm bones) at the trochlea and capitulum, respectively.

The proximal portion of the humerus can be divided into three parts.

1. The rounded humeral head projects medially and articulates with the glenoid cavity of the scapula.

2. Immediately adjacent to the head is the narrower anatomical neck, which allows for a wider range of movements of the head within the shoulder joint.

3. Finally, the greater and lesser tubercles are found at the most superior end of the main shaft of the humerus.

The four rotator cuff muscles attach to these tubercles, strengthening and maintaining the shoulder joint. Between the two tubercles lies a deep grove called the intertubercular sulcus, through which the tendon of the long head of the biceps brachii runs.

Below this proximal region lies the shaft, which is separated from the proximal region by the surgical neck, so termed as this in an area of frequent fracture. A major feature of the shaft is the deltoid tuberosity located laterally to which the deltoid muscle attaches.

The deltoid, corocobrachialis, brachialis, and brachioradialis muscles attach to the anterior surface, with the triceps brachii attaching to the posterior.

Distally, the humerus flattens to articulate with the ulna and radius at the elbow joint. The medially located trochlea articulates with the ulna. Located laterally to this is the capitulum that articulates with the radius.

Several muscles of the forearm responsible for extension at the wrist attach to the humerus immediately above the capitulum and trochlea.

Ulna and Radius (The Forearm)

The forearm contains two bones, the radius and the ulna.

LEARNING OBJECTIVES

Discuss the radius and ulna

KEY TAKEAWAYS

Key Points

• The radius and the ulna are long, slightly curved bones that lie parallel from the elbow, where they articulate with the humerus, to the wrist, where they articulate with the carpals.

• The radius is located laterally, near the thumb, and the ulna medially, near the little finger. The radius and the ulna have a styloid process at the distal end; they are also attachment sites for many muscles.

• The radius is smaller than the ulna.

Key Terms

• radius: One of two forearm bones, it is located laterally to the ulna.

• ulna: One of two forearm bones, it is located medially to the radius.

The forearm: The radius and ulna are the bones of the forearm.

The forearm contains two bones—the radius and the ulna—that extend in parallel from the elbow, where they articulate with the humerus to the wrist, where they articulate with the carpals. The space between the two bones is spanned by the interosseous membrane.

The Ulna

Anatomically, the ulna is located medially to the radius, placing it near the little finger. The ulna is slightly larger than the radius.

Proximally, there are five key regions of the ulna:

1. The olecranon is a projection of bone that extends proximally from the ulna.

2. The triceps brachii muscle attaches to the ulna superiorly.

3. The cornoid process, together with the olecranon, forms the trochlear notch where it articulates with the trochlea of the humerus.

4. Laterally to the trochlear notch lies the radial notch, which articulates with the head of the radius to form the proximal radioulnar joint.

5. Immediately distal to the coronoid process is the tuberosity of ulna, to which the brachialis muscle attaches.

The shaft of the ulna is triangular and numerus muscles involved in pronation and flexion of the forearm attach to its surface.

Distally, the ulna is much smaller and terminates with a rounded head that articulates with the ulnar notch of the radius to form the distal radioulnar joint. The styloid process of the ulna extends distally and is the site of attachment for ligaments found in the wrist.

The Radius

Anatomically, the radius is located laterally to the ulna placing it near the thumb. The radius is slightly smaller than the ulna and pivots around the ulna to produce movement at the proximal and distal radioulnar joints.

Proximally, the radius terminates with a disk-shaped head that articulates with the capitulum of the humerus and the radial notch of the ulna. Immediately below the head lies the radial tuberosity to which the biceps brachii attaches. As with the ulna, the shaft of the radius is triangular in shape and numerous muscles, including the protonator teres, attach to it.

Distally the radius expands, medially the ulnar notch articulates with the head of the ulnar. Immediately adjacent to the ulnar notch, the radius articulates with the scaphoid and lunate carpal bones to form part of the wrist.

Carpals, Metacarpals, and Phalanges (The Hand)

Each hand consists of 27 bones, divided between the wrist bones (carpals), the palm bones (metacarpals), and the finger bones (phalanges).

LEARNING OBJECTIVES

Describe the types of bones in the hand

KEY TAKEAWAYS

Key Points

• There are eight carpal bones in each wrist.

• There are five metacarpal bones in each hand.

• There are proximal, intermediate, and distal phalanges in each digit except for the thumb, which lacks an intermediate phalange.

Key Terms

• metacarpal: Any of the bones of the palm.

• carpal: Any of the eight bones of the wrist.

• phalange: One of the bones of the digits.

Carpals of the left hand: There are eight carpal bones in each wrist: scaphoid, lunate, triquetral, pisiform, trapezium, trapezoid, capitate, and hamate.

The hand contains 27 bones. Each one belongs to one of three regions: the carpals, (wrist), the metacarpals, (the palm), and the phalanges (the digits).

Carpals

The eight, irregularly shaped carpals are the most proximal bones of the hand. The carpals are often split into two rows, the proximal row containing the scaphoid, lunate, triquetrum, and pisiform, moving lateral to medial.

The scaphoid and lunate articulate with the radius, and the lunate and triquetrum articulate with the articular disk of the wrist. The pisiform carpal is a sesamoid bone, located within a tendon and is not involved in movement at the wrist.

The distal row contains the trapezium, trapezoid, capitate, and hamate, moving lateral to medial. The trapezium articulates with the scaphoid proximally and the first, thumb, and second metacarpal distally. The trapezoid articulates with the scaphoid proximally and the second metacarpal distally.

Metacarpal bones of the left hand: The metacarpals connect the carpal bones of the wrist with the phalanges (finger bones).

The capitate articulates with the scaphoid and lunate proximally and the third and fourth metacarpal. Finally, the hamate articulates with the lunate and triquetral proximally and the fourth and fifth, little finger, metacarpals distally.

Metacarpals

The hand contains five metacarpal bones that articulate proximally with the carpals and distally with the proximal phalanges. They are numbered moving lateral to medial, and start with the thumb, which is metacarpal I, and end with metacarpal V, the little finger.

Each metacarpal consists of a base, shaft, and head, with the concave lateral and medial borders of the shaft allowing attachment of the interossei muscles.

Phalanges

The digits are named in a similar fashion to the metacarpals, moving lateral to medial, and starting at the thumb. With the exception of the thumb, each digit contains a proximal, intermediate, and distal phalange; the thumb lacks an intermediate phalange. The length of the phalanges decreases distally.

Femur: The anterior surface of the femur with parts labeled

Proximally, the femur exhibits four key regions. The femoral head projects medially and superiorly and articulates with the acetabulum of the pelvis to form the hip joint. Immediately lateral to the head is the neck that connects the head with the shaft. It is narrower than the head to permit a greater range of movement at the hip joint.

Located superiorly on the main shaft, lateral to the joining of the neck, the greater trochanter is a projection to which the abductor and lateral rotator muscles of the leg attach.

Also located on the main shaft, but inferiorly to the neck joint, is the lesser trochanter. A much smaller projection than the greater trochanter, the psoas major and iliacus muscles attach here.

Shaft

The shaft descends in a slightly medial direction that is designed to bring the knees closer to the body’s center of gravity, increasing stability. Due to the widening of the female pelvis this angle is greater in women and can lead to increased knee instability.

Two key features of the shaft are the proximal gluteal tuberosity to which the gluteus maximus attaches, and the distal adductor tubercle to which the adductor magnus attaches.

Distal

Distally, the femur exhibits five key regions. Two rounded regions, termed the medial and lateral condyles, articulate with the tibia at the most anterior projection of the patella.

Between the two condyles lies the intercondylar fossa, a depression in which key knee ligaments attach; this significantly strengthens the knee joint and protects it against torsional damage.

Finally, the two epicondyles, the medial and lateral, lie immediately proximal to the condyles; they are also regions where key internal knee ligaments attach.

Patella (The Knee)

The patella (knee cap) is the bone between the fibula and femur.

LEARNING OBJECTIVES

Identify the purpose of the patella

KEY TAKEAWAYS

Key Points

• The primary functions of the patella are to enhance leg extension and protect the joint of the knee.

• The patella is a bone embedded within a tendon. This means it is a sesamoid bone.

Key Terms

• patella: A sesamoid bone found in the knee, commonly known as the knee cap.

• sesamoid: A bone embedded within a tendon.

The patella or knee cap is the bone between the fibula and femur. Each leg has a patella to protect its knee joint. The patella serves two functions:

1. To protect the knee from physical trauma.

2. To enhance the leverage that the quadriceps tendon can exert on the femur, thereby increasing muscle efficiency.

The apex of the patella faces inferiorly and connects to the tibia tuberosity through the patella ligament that attaches to the anterior surface. The base of the patella faces superiorly and is the attachment point for the quadriceps tendon.

The posterior surface of the patella contains the medial and lateral facets that articulate with the condyles of the femur. The lower posterior region of the patella has vascular canaliculi, small channels within the bone, which form the infrapatellar fat pad.

Knee Joint: This image shows the position of the patella relative to the articulation of the femur and the tibia.

Tibia and Fibula (The Leg)

The tibia and the smaller fibula bones comprise the lower leg and articulate at the knee and ankle.

LEARNING OBJECTIVES

Describe the fibula and tibia

KEY TAKEAWAYS

Key Points

• The tibia is more commonly called the shinbone. It is located between the ankle and the patella.

• The fibula is a long, thin bone also located between the ankle and the patella. It runs parallel to the tibia.

• Like the femur, the tibia bears much of the body’s weight and plays an essential role in movement and locomotion. The fibula, along with the tibia and the tarsals, forms the ankle.

Key Terms

• tibia: The inner and usually the larger of the two bones of the lower leg.

• fibula: The smaller of the two bones in the lower leg, the calf bone.

The tibia and fibula are the two bones of the lower leg. The tibia is located medially to the fibula and is much larger. Both are bound together with the interosseous membrane.

The Tibia

The leg: Tibia and fibula in anatomical position with parts labeled.

The tibia, or shin bone, spans the lower leg, articulating proximally with the femur and patella at the knee joint, and distally with the tarsal bones, to form the ankle joint. It is the major weight-bearing bone of the lower leg.

Proximally, there are five key features of the tibia:

1. It widens and forms two condyles —the lateral and medial—that articulate with the condyles of the femur.

2. Between the two condyles is the intercondylar fossa, a small grove, into which two intercondylar tubercles sit. Numerous internal ligaments of the knee joint attach to these tubercles and strengthen it significantly.

3. On the anterior surface of the proximal region and inferiorly to the condyles is the tibial tuberosity to which the patella ligament attaches.

4. The shaft of the tibia is triangular and the soleus muscle, which gives the calf its characteristic shape, originates on the posterior surface.

5. Distally, the tibia also widens to aid with weight bearing and it displays two key features. The medial malleolus is a bony projection that articulates with the tarsal bones to form the ankle joint. Laterally, there is the fibular notch that articulates with the fibula.

The Fibula

The fibula also spans the lower leg, although proximally it does not articulate with the femur or patella. It serves more as an attachment point for muscles rather than a weight-bearing bone.

Proximally, the fibula head articulates with the lateral condyle of the tibia, and the biceps femoris attaches to the fibula head. As with the tibia, the shaft of the fibula is triangular and numerus muscles are involved in the extension and flexion of the foot. These muscles originate from the fibula’s surface and include the extensor digitorum longus, soleus, and flexor hallucis longus, among others.

Distally, the fibula forms the lateral malleolus, which is more prominent than the medial malleolus of the tibia. It also articulates with the tarsal bones to form the ankle joint.

Tarsals, Metatarsals, and Phalanges (The Foot)

The human ankle and foot bones include tarsals (ankle), metatarsals (middle bones), and phalanges (toes).

LEARNING OBJECTIVES

Describe the different types of bones in the foot

KEY TAKEAWAYS

Key Points

• The human foot contains 26 bones.

• The foot can be subdivided into the tarsals, metatarsals, and phalanges.

Key Terms

• phalange: A bone within the digit.

• metatarsal: A bone from the center of the foot that articulates with the tarsals and phalanges.

• tarsal: A bone forming part of the ankle or heel.

The foot contains 26 bones that are divided into three regions: the tarsals (or ankle and heel), the metatarsals (forming the sole of the foot), and the phalanges (forming the digits). While sharing a similar underlying structure with the hand, the foot is visibly and structurally different to account for its greater load-bearing and locomotive duties, and reduced fine movements.