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Biochemical Effects of Sagittal Band Continuity on Extension at the Metacarpophalangeal Joint
Somjade Jay Songcharoen, MD; Jason M. Clark, MD; Roberto Aru, BS; Randall Williamson, PhD; Michael Roach, PhD; Samantha Seals, PhD; Peter Arnold, MD, PhD1; Somprasong Songcharoen, MD
University of Mississippi Medical Center, Jackson, MS

Introduction: Subluxation of the extensor digitorum communis (EDC) tendon at the metacarpal phalangeal (MCP) joint is a result of both acute and chronic injuries to the sagittal bands. Although there are many published techniques to address chronic subluxation, none address the function and continuity of the sagittal bands. The purpose of this study is to determine the biomechanical impact of the sagittal bands on joint function. Our hypothesis is that any discontinuity in the sagittal bands will increase the effort required to extend the digit at the MCP joint.
Materials & Methods: 20 digits from 10 cadaver hands were studied in this experiment. Each hand was secured to a testing base with the extensor tendon of either the long or ring finger attached to a torsion frame to measure the force and tendon displacement. A digital camera was used to record high resolution video that was used to track the joint angles. Measurements for each digit were group by weight (0g, 50g, 100g, and 150g serially added at the proximal interphalangeal joint) as well as defect (control, ulnar, and bilaterally incised sagittal bands). Force, power, work, and the change in joint angle as a function of tendon displacement were calculated and expressed in each group a percent change from control. Variables were compared using Studentís t-test; p<0.05 is considered significant.
Results: In the experiments with no weight added (unloaded), the magnitude of force, power, and work decreased by 21%, 22%, and 28% respectively (p<0.0001) in the groups with defects in the sagittal bands in comparison to their respective controls. However, these values were increased by up to 13% once there was weight added, although not by a significant margin. In unloaded digits, the change in angle correlating to each unit change in linear tendon displacement was increased by 15% (p<0.0001) with a defect in the sagittal bands, but this was diminished once any load was introduced. There were no significant differences between the ulnar and bilateral defect groups.
Conclusions: Sagittal bands relate to the relationship between joint angle and tendon excursion in the unloaded digit. Although the amount of effort to extend the finger against load tends to increase when the sagittal bands are not intact, a higher powered data may be needed to find a significant difference if one exists. Nonetheless, the restoration of sagittal band continuity is an important consideration for centralization of the subluxed EDC tendon.

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