Functional Anatomy of the Thumb Sagittal Band

Abstract

To study the anatomy and kinematics of the thumb sagittal band (SB) and the influence of simulated injury on the stability of the extensor pollicis longus (EPL) tendon. Twenty-four below-elbow, cadaveric amputation specimens were used. The study had 3 components: SB anatomy, SB kinematics, and assessment of SB influence on extensor stability through simulated injury by serial sectioning. The radial and ulnar components of the SB originated on their respective sides of the thumb metacarpophalangeal joint from a soft tissue confluence and inserted into the margins of the EPL tendon. The radial component was much more transparent and thinner, but slightly wider (2.4 mm) and longer (0.5 mm) than the ulnar component. The SB fibers were oriented perpendicular to the long axis of the thumb and EPL tendon. Interphalangeal joint motion had the greatest influence on SB fiber angulation and excursion. Partial and complete sectioning of the ulnar SB had minimal influence on EPL tendon stability. Partial sectioning of the radial SB resulted in ulnar EPL tendon subluxation. Complete sectioning of the radial SB resulted in ulnar dislocation of the EPL tendon. The radial component of the thumb SB seems to be more important to EPL stabilization at the level of the metacarpophalangeal joint than its ulnar counterpart. EPL tendon instability was observed to be proportional to the degree of radial SB injury. Knowledge of the anatomy of the thumb SB and its functional relationship to the extensor apparatus provides an understanding of the repair and reconstructive considerations for conditions of EPL tendon instability associated with trauma and inflammatory and nonrheumatoid thumb deformities.