Mechanics of Metacarpophalangeal Joint Extension.

PURPOSE: It is a common belief that extension of the metacarpophalangeal (MCP) joint of the finger is achieved via the sagittal bands acting as a sling or lasso to attach the extensor tendon to the base of the proximal phalanx. The aim of this study was to test the hypotheses that (1) division of the sagittal bands reduces extension force or torque of the MCP joint, and (2) division of the extensor tendon distal to the sagittal band will not affect the extension force or torque of the MCP joint. METHODS: Ten cadaver limbs were secured to a jig to allow for testing of the extension force of the MCP joints of the index, middle, and ring fingers. A 1-kg load was applied to the forearm extensor digitorum communis tendon and the extension force was measured with the MCP joint positioned at 0° (neutral extension) and again at 45° flexion. These measurements were repeated after the sagittal bands were divided in 15 specimens; in the other 15 specimens, the extensor tendon was divided just distal to the sagittal bands. RESULTS: After sagittal band division, extension force was similar in the 2 groups (0.11 N reduction after division with the MCP joints in neutral and 0.14 N in 45° flexion). There was significantly less extension force after division of the extensor tendon in both joint positions (0.95 N reduction after division in neutral extension and 0.66 N in 45° flexion). CONCLUSIONS: The sagittal bands do not primarily extend the MCP as a sling or lasso. The extensor tendon continuation to the extensor hood and middle phalanx is the major extension motor. The MCP joint is extended by the torque generated by the extensor tendon passing the joint carrying a force and possessing an extension moment arm. CLINICAL RELEVANCE: This principle should be correctly understood in the literature to ensure that clinical decisions related to injury and/or repair of the extensor tendon and sagittal bands are based on a sound understanding of their mechanics.

Low-intensity ultrasound stimulation in distraction osteogenesis in rabbits.

Low-intensity pulsed ultrasound has been shown to accelerate fracture healing. This experiment investigated its possible role in distraction. Thirty-four New Zealand White rabbits had distraction osteogenesis, followed by low-intensity pulsed ultrasound therapy. Seventeen animals had the ultrasound transducer switched off (controls). Four and 6 weeks postoperatively, tibiae were analyzed using quantitative computed tomography and four-point mechanical testing. Two tibiae from each group had histologic analysis at 4 weeks. No significant differences were identified between regenerates of ultrasound-treated and control groups with respect to bone mineral content, cross-sectional area, and strength. No significant reductions in osteopenia proximal and distal to the regenerate were observed. Histologic observation showed no differences in bone volume fraction, but ultrasound-treated regenerates appeared to have fewer trabeculae of increased thickness, and fewer osteoclasts. The modulation by ultrasound may occur by accelerating endochondral ossification through action on chondrocytes, yet distraction osteogenesis is largely intramembranous. Although ultrasound is proven to be effective in unconstrained systems such as plaster, the current study does not support the role of low-intensity pulsed ultrasound as an adjunct for patients having distraction osteogenesis in a rigid fixator. Additional research is needed to definitively support the use of low-intensity pulsed ultrasound in such situations.