Three rows of six evenly spaced 0.5 mm metal beads were implanted midsagittally into the discs of ten L4-5 human lumbar motion segments. The intradiscal bead displacements in response to compression, flexion, and extension loads were obtained by digitizing the bead positions from sagittal plane radiographs taken before and during the load application. Each disc was denucleated and the loading process was repeated. For the intact discs, in compression, the intradiscal bead displacements were predominantly anterior. In flexion, the beads in the center of the disc moved posteriorly whereas the beads closer to the periphery of the disc moved anteriorly. In extension, the central beads moved anteriorly and the beads closer to the periphery of the disc moved posteriorly. After denucleation, the bead displacements for compression and flexion implied an inward bulging of the inner wall of the annulus, despite outward bulging of the disc surface. We hypothesize that the inward bulging causes radial tensile stresses within the disc, leading to disruption of adjacent layers of annulus.
In the treatment of ulnar collateral ligament injuries in the metacarpophalangeal joint of the thumb, a cast or splint is often molded with the thumb and hand positioned so the patient can return quickly to a particular sport or activity. However, it is unknown whether a given position of immobilization or whether an early rehabilitation program will compromise ligament healing by causing undue tension in the ligament. To better define acceptable positions for thumb immobilization and a safe range of motion, this laboratory study measured strain in the ulnar collateral ligament at different degrees of metacarpophalangeal joint flexion. Ligament strain was found to increase with increasing joint flexion; the most significant increases in strain occurred as the joint moved from 0 degree to 25 degrees of flexion, with the dorsal portion of the ligament demonstrating greater strain than the palmar portion. Based on these results, immobilization of the metacarpophalangeal joint in excessive flexion and unrestricted early rehabilitation exercises should be done with caution and guided by the grade of injury or the quality of repair.
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