The TM joint is a biconcave-convex saddle joint with minimal bony constraints, so ligamentous support is extremely important, especially considering the compressive forces transmitted across the joint during functional pinch. Eaton and Littler identified the anterior oblique “beak” ligament, so called for its attachment on the palmar beak of the TM, as the primary stabilizer of the TM joint.

With the assistance of TM joint arthroscopy, Bettinger et al¹ have further defined the anterior oblique ligament (AOL) into a superficial anterior oblique ligament (sAOL) and deep anterior oblique ligament (dAOL). The dAOL, which is intracapsular, is, in fact, the beak ligament. The dAOL plays an important role in the kinematics of thumb opposition. It acts as a pivot point and becomes tight during pronation, opposition, and palmar abduction. The dAOL limits pronation in flexion and both pronation and supination in extension.

In their comprehensive assessment of the ligamentous anatomy of the TM joint, Bettinger et al¹ described a total of 16 ligaments that stabilize the trapezium and TM. Seven of these ligaments, including the sAOL, dAOL beak ligament, dorsoradial (DRL), posterior oblique, ulnar collateral, intermetacarpal, and dorsal intermetacarpal, are responsible for directly stabilizing the TM joint.

The DRL’s role in joint stability has been debated, but Bettinger et al¹ showed that the DRL is an important joint stabilizer. The DRL, which covers a large percentage of the posterior aspect of the joint, is a wide, thick ligament that attaches to the trapezium and inserts on the dorsum of the metacarpal base. This ligament tightens with DRL and dorsal translational forces in all positions except full extension. It also tightens in supination and in pronation with joint flexion.

Functional incompetence of the basal joint’s AOL results in pathologic laxity, abnormal translation of the metacarpal on the trapezium, and generation of excessive shear forces between the joint surfaces, particularly within the palmar portion of the joint during grip and pinch activity. Histologic study has shown that attritional changes in the AOL at its attachment to the palmar lip of the metacarpal precede degeneration of cartilage.²

Because the AOL appears to be the primary stabilizer of the TM joint and because its detachment results in dorsal translation of the metacarpal, its reconstruction has been recommended to restore thumb stability not only in cases of end-stage osteoarthritis but also for early-stage disease.

Pellegrini et al⁶ were the first to evaluate the biomechanical efficacy of extension osteotomy. Palmar contact area was unloaded with a concomitant shift in contact more dorsally so long as arthrosis did not extend more dorsal than the midpoint of the trapezium.

Shrivastava et al⁷ studied the effect of a simulated osteotomy on TM joint laxity by flexing the metacarpal base 30 degrees, thus placing the joint in the relationship it would assume if an extension osteotomy was performed.

Basal joint arthritis may present with mild symptoms beneath the thenar eminence at the level of the TM joint, particularly during pinch and grip. Ultimately, the greatest functional impairment occurs with advanced disease—limiting breadth of grasp and forceful lateral pinch activities such as brushing teeth, turning a key, opening a jar, or picking up a book.

Complaints are directed toward the base of the thumb, and pain is frequently associated with a sensation of movement or “slipping” within the joint. An enlarging prominence, or “shoulder sign,” inevitably develops at the base as the clinical manifestation of dorsal metacarpal subluxation on the trapezium and metacarpal adduction.

Early presentation may result in only pain with TM stress and palpation beneath the thenar cone, without deformity, instability, subluxation, or crepitance.

Methods for examining the thumb CMC joint for hypermobility (stage 1 disease) include the following:

Radiographic evaluation includes a posteroanterior (PA) 30-degree oblique stress view, lateral view, and a Robert (pronated anteroposterior [AP]) view (FIG 1).

Osteoarthritis may be confined to the TM joint, or it may involve the pantrapezial joint complex. Indeed, the staging system described by Eaton and Littler describes four stages: