The effects of tendon load and posture on carpal tunnel pressure

Two pressure measurement techniques (catheter and bulb) were used to decompose the effects of tendon loads on carpal tunnel pressure (CTP). The catheter technique measures true hydrostatic pressure, whereas the bulb technique is a estimate of contact force or pressure on the median nerve. Eight cadaveric wrists were moved through a range of flexion-extension (0 degrees, 10 degrees, 20 degrees, 30 degrees, and 45 degrees of each) and radioulnar deviation (10 degrees and 20 degrees radial and 0 degrees, 10 degrees, 20 degrees, and 30 degrees ulnar) while CTPs were measured under 4 muscle loading conditions with the thumb, index, and long finger in a pinch-grip posture. The first of these was zero load. Then a 1-kg mass was applied in turn to both flexors of the index and long fingers, the palmaris longus (PL); and the flexor pollicis longus. The hydrostatic pressure was found to be affected by both wrist posture and tendon load. With no load, highest pressures were seen in wrist extension. Muscular loading elevated CPT, particularly the loading of palmaris longus with the wrist in extension and the digital flexors with the wrist flexed. Bulb pressure measurements, related to local contact forces by the digital flexors, indicated the highest loads on the median nerve with the wrist flexed. Palmaris longus loading created the highest pressures in extension and only moderate pressure in flexion, indicating that it may alter the geometry of the transverse carpal ligament. In view of the data from this study, it is necessary to incorporate measures of hydrostatic pressure and local contact forces to describe possible trauma to the median nerve in the carpal tunnel, as neither appears sufficient when used independently.