Trapeziometacarpal joint

Articular surfaces

Being a saddle joint, the articular surfaces of the trapeziometacarpal joint are reciprocally concavo-convex. More precisely, these are the concave distal surface of the trapezium bone and accordingly convex base of the first metacarpal bone. Both surfaces are covered by hyaline cartilage.

Ligaments and joint capsule

The trapeziometacarpal joint is enclosed by a loose fibrous capsule that attaches to the margins of the articular surfaces. The inner surface of the capsule is lined by a synovial membrane which bounds the joint cavity and secretes the synovial fluid.

The joint is reinforced by the two sets of ligaments;

• Radial carpometacarpal ligament, which attaches between the lateral (radial) sides of the trapezium and the base of the first metacarpal bone.
• Anterior and posterior oblique ligaments, that originate from the anterior and posterior surfaces of trapezium, respectively. They both take an oblique medial course and attach to the medial side of the first metacarpal bone.

Further stability to the joint is provided by the muscles that cross its sides; thenar muscles anteriorly, the tendon of flexor pollicis longus medially, and the tendons of extensor pollicis longus and brevis laterally.

Innervation

The trapeziometacarpal joint is innervated by the anterior and posterior interosseous nerves (C7, C8). The former is a branch of the median nerve, while the latter is a continuation of the deep branch of the radial nerve.

Blood supply

The blood supply to the trapeziometacarpal joint comes from the branches of the palmar and dorsal carpal arterial arches, which are the anastomotic networks between the terminal branches of the radial and ulnar arteries.

Movements

The trapeziometacarpal joint is a multiaxial joint, allowing the movements of the thumb in three degrees of freedom;

• Flexion-extension (total range of motion 40°-50°)
• Abduction-adduction (total RoM 80°)
• Axial rotation (total RoM 70°-110°)

The movements of the thumb often bring in confusion, due to the thumb being rotated at about 90° in respect of the plane of the hand. So to clarify the concept of thumb mobility, remember that the flexion and extension of thumb happens in a coronal plane, in which the remaining digits abduct and adduct. The axis for these movements passes through the centers of the articular surfaces in the coronal plane so that in extension, the thumb is carried laterally away from the palm, while during flexion it is pulled medially towards the palm.

The abduction and adduction of thumb happen in the sagittal plane, in which the rest of the digits flex and extend. The axis for these movements passes through the center of the trapeziometacarpal joint and is perpendicular to the axis relevant to flexion and extension. During abduction, the thumb is pulled anteriorly away from the palm, while in adduction it is pulled back posteriorly towards the palm.

Due to the shape of the articular surfaces and the looseness of the joint capsule, the trapeziometacarpal joint allows the external and internal rotation. The axial rotation occurs around an axis that passes through the length of the thumb.

When combined together, these movements of the thumb result in a complex movement called the opposition. The opposition is a movement by which the tip of the thumb is brought in contact with the tip of any other of the remaining four digits. The sequence of movements that result in opposition is as follows;

1. Simultaneous flexion and abduction of the thumb on the trapeziometacarpal joint
2. The tension of the posterior oblique ligaments causes the passive internal rotation of the first metacarpal bone. Subsequently, the opponens pollicis muscle actively finishes the internal rotation of the thumb.
3. The adductor pollicis adducts the thumb back towards the palm, bringing its tip towards the desired digit.

Muscles acting on the trapeziometacarpal joint

The flexion and extension at the trapeziometacarpal joint are the secondary movements produced by the muscles that act on the interphalangeal and metacarpophalangeal joints of the thumb. Precisely, flexion is caused by the flexor pollicis longus and flexor pollicis brevis, while extension is produced by the extensor pollicis longus and extensor pollicis brevis muscles.

The adduction is produced by the adductor pollicis muscle. The abduction, however, is a secondary movement. It is a result of the action of the abductor pollicis brevis on the metacarpophalangeal joint of the thumb.

The axial rotation of the thumb happens passively at the extremes of flexion and extension as a result of tension in the oblique ligaments. More precisely, the posterior oblique ligament becomes taut in full flexion, causing the thumb to rotate internally. Conversely, full extension tenses the anterior oblique ligament, resulting in external rotation of the thumb. The full range of motion is carried by the opponens pollicis muscle.