Munir A. Shah

Anatomy and pathomechanics of ring and small finger carpometacarpal joint injuries.

PURPOSE The purpose of this study was to detail the pathomechanics and pathoanatomy of fracture dislocations of the ring finger and small finger carpometacarpal (CMC) joint by duplicating the pathomechanics of the fist blow. METHODS A custom-made jig was used to position 20 fresh-frozen cadaver upper extremities in forearm neutral rotation, 90 degrees of elbow flexion, 20 degrees of wrist extension, and 20 degrees and 30 degrees of flexion at the ring and small finger CMC joint, respectively. First 7.7 kg of weight were dropped from a height of 0.76 m to 1.1 m to axially load the ring and small metacarpal (MC) heads through a custom-made apparatus. Fluoroscopic examination before and after loading, and detailed dissection after loading, were used to identify any osseous and/or ligamentous injuries. RESULTS The most common fractures were a dorsal capitate fracture and a middle MC dorsal base fracture. The most common combinations of fractures were the dorsal capitate and dorsal hamate fractures. Multiple fractures often were identified in a number of locations including dorsally: the capitate, hamate, and index through small metacarpal bases, and volarly: the hook of the hamate and the middle through the small MC bases. CONCLUSIONS The patterns of injuries encountered at the ring and small CMC joints can be explained by the direction and force of the applied load, position of the CMC joint at the time of loading, and the constraints imposed by specific CMC ligaments. A detailed analysis of the fracture patterns and associated ligament anatomy suggests that the typical ring and small carpometacarpal fracture dislocations are a more complex combination of fractures than identified by plain radiographs alone. The complexity of these injuries is greater than previously recognized and is most likely the result of a combination of axial load and shear stresses resulting in carpal fractures and ligament avulsions as well as fracture dislocations. This study suggests that computed tomography may be the preferred diagnostic imaging method for complete assessment of these injuries.

Motion and Morphology of the Thumb Metacarpophalangeal Joint

PURPOSE To determine the variability in the flexion/extension motion of the thumb metacarpophalangeal (MCP) joint in a normal volunteer population and to determine any correlation between the shape of the thumb metacarpal head, gender, age, thumb interphalangeal (IP) joint range of motion (ROM), and thumb MCP joint ROM in a population of fresh-frozen cadaver arms. METHODS The ROM of the thumb MCP joints of 100 volunteers (200 thumbs) was measured. The thumb MCP joints of 70 fresh-frozen cadaver arms were examined. Posteroanterior and lateral radiographs were taken of 64 MCP joints. Passive flexion and extension at the MCP and IP joints were measured with a standard goniometer starting at neutral (0 degrees ) with the metacarpal, proximal, and distal phalanges axially aligned. The distance from the volar to the dorsal edge of the articular surface (A) and the radius of curvature of the articular surface (r) of the metacarpal head were measured on the lateral view. The shape of the metacarpal head was given a value using the A/r ratio. Regression analysis was used to determine the correlation between the head shape and joint motion. The thumbs were categorized into a round group (A/r ratio > 1.7) or a flat group (A/r ratio of < 1.7) to facilitate statistical analysis. RESULTS The volunteer population mean thumb MCP maximum flexion was 77 degrees range, 40 degrees -126 degrees ). Range of motion in hyperextension varied from 0 degrees to 72 degrees, with a mean of 35 degrees. Total ROM was from 55 degrees to 176 degrees, with a mean of 110 degrees. In the cadaver population studied the average MCP joint ROM was 94 degrees (mean flexion, 70 degrees; mean extension, 24 degrees ). The A/r ratio had a range of 1.1 to 2.2. There were 37 round and 27 flat thumb metacarpal heads. Regression analysis showed a significant correlation between metacarpal head shape and ROM; MCP joints with rounder metacarpal heads had greater motion. Round and flat metacarpal heads had significantly different motion arcs averaging 106 degrees and 77 degrees, respectively. Female gender was associated both with significantly greater MCP joint ROM (99 degrees women/87 degrees men) and a significantly higher incidence of round metacarpal heads (66% of women/36% of men). No significant correlation existed between specimen age, MCP, and/or IP joint ROM. CONCLUSIONS There is a wide range in the magnitude of the thumb MCP joint ROM and the normal shape (round vs flat) of the thumb metacarpal head. A rounder thumb metacarpal head has greater thumb MCP joint ROM than a flatter thumb metacarpal head. Clinically we have found this information helpful in predicting posttraumatic recovery of thumb MCP joint ROM and selecting candidates for and predicting patient satisfaction with thumb MCP joint arthrodesis.

Effects of extensor pollicis longus transposition and extensor indicis proprius transfer to extensor pollicis longus on thumb mechanics.

PURPOSE The spatial relationship of the extensor pollicis longus (EPL) to the thumb carpometacarpal (CMC) joint may be altered by its transposition from the third dorsal wrist compartment and by subcutaneous extensor indicis proprius (EIP) to EPL tendon transfer. Changes in tendon position could alter thumb function. This study examined changes in the EPL adduction moment arm after EPL tendon transposition from its extensor compartment or EIP transfer. METHODS The EPL adduction moment arm at the thumb carpometacarpal joint was determined under 4 tendon conditions: (1) intact extensor pollicis longus, (2) transposed extensor pollicis longus, (3) extensor indicis proprius to extensor pollicis longus tendon transfer through an extensor retinacular pulley, and (4) extensor indicis proprius tendon transfer through a subcutaneous route. Each tendon condition was tested in 2 wrist positions: neutral and 40 degrees of flexion. RESULTS The wrist neutral/flexion moment arms for the 4 tendon conditions, in millimeters, were 9.2/7.3, 3.6/1.2, 8.3/5.1, and 4.8/1.0. CONCLUSIONS EPL transposition produces a significant decrease of its adduction moment arm at the thumb CMC joint, an effect exacerbated by wrist flexion. The moment arm mechanics of the pulley and subcutaneous EIP tendon transfer resemble those of the intact and transposed EPL, respectively. Diminution of the adduction moment arm could impair thumb function, especially adduction.