JOINTS
The Radioulnar Joints
• The radius and ulna articulate with each other at their proximal and distal ends at synovial joints, called the proximal and distal radioulnar joints. These articulations are the pivot type of synovial joint that produces pronation and supination.
• An interosseous membrane and a strong oblique cord connect the interosseous borders of the radius and ulna.
• The interosseous membrane is a strong, broad fibrous sheet that stretches between the interosseous borders of the radius and the ulna, commencing 2 to 3 cm distal to the tuberosity of the radius.
• In addition to providing a flexible and strong attachment between the forearm bones, it provides the proximal attachment for the deep layer of muscles of the forearm.
• A thin fibrous layer, called the quadrate ligament, extends between the radial notch of the ulna and the medial surface of the neck of the radius. This ligament covers the synovial membrane and probably supports it.

The Proximal Radioulnar Joint
• This is a pivot type of synovial joint.

The Articular Surfaces of the Proximal Radioulnar Joint
• The radial head articulates with the radial notch of the ulna.
• The head of the radius is held in position by the strong anular (annular) ligament, a U-shaped fibrous collar which is attached to the anterior and posterior margins of the radial notch.

The Articular Capsule of the Proximal Radioulnar Joint
• The fibrous capsule enclosing the joint is continuous with the fibrous capsule of the elbow joint.
• The synovial capsule, which lines the fibrous capsule, is an inferior prolongation of the synovial capsule of the elbow joint.
• The deep surface of the anular ligament is lined with synovial membrane, which continues distally as a sacciform recess on the neck of the radius.
• This arrangement allows the radius to rotate within the anular ligament without tearing the synovial capsule.
• The synovial cavities of the elbow and proximal radioulnar joints are in free communication with each other.
The Distal Radioulnar Joint
• This is also a pivot type of synovial joint.
• The radius moves around the relatively fixed inferior end of the ulna.

The Articular Surfaces of the Distal Radioulnar Joint
• The rounded side of the head of the ulna articulates with the ulnar notch in the distal end of the radius.
• A fibrocartilaginous articular disc binds the ends of the ulna and radius together and is the main uniting structure of the joint.
• The base of the articular disc is attached to the medial edge of the ulnar notch of the radius, and the apex of the disc is attached to the lateral side of the base of the styloid process of the ulna.
• The proximal surface of this triangular disc articulates with the distal aspect of the head of the ulna.
• Hence, the joint cavity is L-shaped in coronal section. The articular disc separates the cavity of the distal radioulnar joint from the cavity of the wrist joint.

The Articular Capsule of the Distal Radioulnar Joint
• The fibrous capsule encloses the joint. It is formed by relatively weak transverse bands that extend from the radius to the ulna across the anterior and posterior surfaces of the joint.
• The synovial membrane lines the fibrous capsule and the proximal surface of the articular disc. The synovial capsule extends proximally a short distance between the radius and ulna as the sacciform recess.
• The redundancy of the synovial capsule accommodates the twisting of the capsule that occurs when the distal end of the radius travels around the relatively fixed distal end of the ulna during pronation of the forearm.

Movements of the Distal Radioulnar Joints
• Movements at these joints make pronation and supination of the forearm and hand possible.
• Pronation is rotation that turns the palm posteriorly or inferiorly when the forearm is flexed.
• Supination carries the palm anteriorly or superiorly when the forearm is flexed.
• The axis for these movements passes proximally through the centre of the head of the radius and distally through the site of attachment of the apex of the articular disc to the head of the ulna.
• During pronation and supination, it is mainly the radius that rotates. Its head rotates within the cup-shaped ring formed by the anular ligament and the radial notch on the ulna. Distally the end of the radius rotates around the head of the ulna.

Blood Supply of the Radioulnar Joints
• The articular arteries supplying the proximal radioulnar joint are derived from the anastomoses around the elbow region, whereas those supplying the distal radioulnar joint are derived from the anterior and posterior interosseous arteries.

Nerve Supply of the Radioulnar Joints
• The articular nerves to the proximal radioulnar joint are derived mainly from the musculocutaneous, median, and radial nerve. The articular nerves to the distal radioulnar joint are derived from the anterior and posterior interosseous nerves.
The Wrist Joint
• The wrist or radiocarpal joint is between the distal end of the radius and carpus.
• It is a condyloid type of synovial joint.

The Articular Surfaces of the Wrist Joint
• The distal end of the radius and the articular disc of the distal radioulnar joint articulate with the proximal row of carpal bones.
• The convex surfaces formed by the carpal bones fit into the concave surfaces of the distal end of the radius and articular disc.

The Articular Capsule of the Wrist Joint
• The fibrous capsule encloses the joint and is attached proximally to the distal ends of the radius and ulna, and distally to the proximal row of carpal bones.
• It is strengthened by dorsal and palmar radiocarpal ligaments, which run obliquely distally and medially from the radius.
• Radial and ulnar collateral ligaments also strengthen the fibrous capsule.
• The synovial membrane lines the fibrous capsule and is attached to the margins of the articular surfaces of the wrist joint. It presents numerous folds, especially dorsally.

Movements of the Wrist Joint
• The movements of adduction, abduction, flexion, extension and circumduction are possible.
• Rotation of the wrist joint is impossible because the articular surfaces are ellipsoid in shape; however, pronation and supination of the hand compensate for the absence of this movement.

Blood Supply of the Wrist Joint
• The articular arteries are derived from the dorsal and palmar carpal arterial arches.

Nerve Supply of the Wrist Joint
• The articular nerves are derived from the anterior interosseous branch of the medial nerve, the posterior interosseous branch of the radial nerve, and the dorsal and deep branches of the ulnar nerve.

Wrist Fractures
• Fractures of the wrist (e.g., Colles' fracture) involving the distal end of the radius are the most common type of fracture in persons over 50 years of age.
• This fracture commonly results when the person slips or trips and, in attempting to break the fall, lands on the outstretched hand with the forearm pronated.
• There is usually a complete transverse fracture of the distal 2 to 3 cm of the radius, and the fragment is displaced proximally causing shortening of the radius. The fragment is usually tilted posteriorly, producing a characteristic hump described as the "silver fork" or "dinner fork" deformity.

The Carpometacarpal and Intermetacarpal Joints
• These are plane synovial joints that permit a gliding movement.
• They share a common joint cavity with the intercarpal joints.
• These articulating bones are united by dorsal, palmar, and interosseous ligaments.

The Carpometacarpal Joint of the Thumb
• This articulation is a saddle type of synovial joint.

Articular Surfaces of the Carpometacarpal Joint of the Thumb
• The trapezium articulates with the saddle-shaped base of the first metacarpal bone.

Articular Capsule of the Carpometacarpal Joint of the Thumb
• The fibrous capsule encloses the joint and is attached to the margins of the articular surfaces.
• The looseness of its capsule facilitates its movements.
• The synovial membrane lines the fibrous capsule and forms a separate joint cavity from the rest of the carpus.

Movements of the Carpometacarpal Joint of the Thumb
• The joint permits angular movements in any plane and a restricted amount of axial rotation. Only ball and socket joints are more mobile.
• The following thumb movements are possible: flexion, extension, abduction, adduction, and opposition. The functional importance of the thumb lies in its ability to be opposed to the other digits.

Blood Supply of the Carpometacarpal Joint of the Thumb
• The articular arteries are derived from the dorsal and palmar metacarpal arteries and from the dorsal carpal and deep palmar arterial arches.
• These vessels are branches of the ulnar and radial arteries.

Nerve Supply of the Carpometacarpal Joint of the Thumb
• The articular nerves are derived from the anterior interosseous nerve of the medial, the posterior interosseous nerve of the radial, and the dorsal and deep branches of the ulnar nerve.

The Metacarpophalangeal Joints
• These articulations are condyloid (knuckle-like) synovial joints that allow movement in two directions.

The Articular Surface of the Metacarpophalangeal Joints
• The following movements occur at these articulations: flexion, extension, abduction, adduction, and circumduction.

Blood Supply of the Metacarpophalangeal Joints
• The articular arteries are branches of the digital arteries that arise from the ulnar and medial nerves.

Nerve Supply of the Metacarpophalangeal Joints
• The articular nerves are derived from the digital nerves that arise from the ulnar and medial nerves.

The Interphalangeal Joints
• These articulations are uniaxial hinge joints, which permit only flexion and extension.
• They joint the head of one phalanx with the base of the more distal one.
• They are structurally similar to the metacarpophalangeal joints and are reinforced dorsally by the extensor expansions of the digits.
• The articular arteries and nerves are derived from the adjacent digital arteries and nerves.