Arthroscopic treatment of first metacarpal base fractures

Ligament anatomy and biomechanics

Imaeda et al. dissected the trapeziometacarpal (TM) joint of 30 cadaver specimens and described the anatomy and properties of 3 major ligaments. Bettinger et al. revisited this and further described 16 ligaments stabilizing the TM joint. The AOL is a two-part ligament that consists of a superficial portion and an intraarticular deep portion ( Fig. 23.2 ). The superficial anterior oblique capsular ligament (sAOL) is immediately deep to the thenar musculature, which overlies the volar aspect of the TM joint and is superficial to the deep anterior oblique ligament (dAOL). The ligament originates 0.5 mm proximal to the articular surface at the volar tubercle of the trapezium and inserts broadly over the volar-ulnar tubercle of the thumb metacarpal base, 2 mm distal to the volar styloid process. The sAOL is lax throughout most of the TM range of motion and becomes taut at the extremes of thumb pronation and extension. In a biomechanical study of 17 cadaver hands, Colman et al. found that the broad, loose, and curtain-like superficial portion of the ligament plays only a minor role in joint stability, does not prevent dorsal metacarpal subluxation, and limits the joint’s motion only in pronation.

The dAOL, formerly known as the volar beak ligament, is an intraarticular ligament that lies deep to the sAOL. It originates from the volar central apex of the trapezium, ulnar to the ulnar edge of the trapezial ridge, and inserts into the articular margin ulnar to the volar styloid process (volar beak) of the thumb metacarpal base. The dAOL becomes taut with increasing thumb abduction, pronation, and extension. Colman et al. found the intraarticular dAOL to be a major stabilizer of the joint. Because it is the closest ligament to the center of the joint, it acts as a pivot point to guide the metacarpal during the pronation that occurs as a part of thumb opposition. Its intraarticular fibers run obliquely from distal-ulnar to proximal-radial; thus this ligament is positioned to prevent an ulnar shift of the metacarpal that would tighten the oblique fibers whereas a radial shift would slacken them. The sulcus between the sAOL and dAOL can often be palpated with an arthroscopic probe.

Edmunds emphasized the point that in the static resting position, the prominent volar beak of the thumb metacarpal is disengaged from its recess in the trapezium, the TM joint space is relatively large, and both the volar beak ligament and the dorsal ligament complex are lax. In the final phase of opposition during either active or passive screw-home torque rotation, the dorsal ligament complex tightens, the volar beak ligament becomes even more lax and redundant, the TM joint is compressed, and the volar beak of the thumb metacarpal is tightly compressed into its recess area in the trapezium. This dynamic force couple changes the TM joint from incongruity to congruity and from laxity to rigid stability. It changes a normally lax TM joint into a stable TM joint to support the powerful forces on the thumb in power pinch and grasp.

Biomechanical studies performed by Cullen et al. noted that 2 mm of residual displacement at the articular surface resulted in an overall increase in contact area at the TM joint, with a dorsal shift in contact pressures over the trapezial surface. In addition, no important increase in contact pressure was seen in the area of the articular step-off. The authors concluded that a 2-mm articular step-off is acceptable and should be well tolerated as long as the metacarpal was reduced. Such cadaveric studies are limited due to the constraints involved with use of contact-pressure film.