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Reduction of the Thumb-tip Trajectory Area after Trapeziometacarpal Joint Fusion: a Cadaveric Study
Toshiyasu Nakamura, MD, PhD1; Yusuke Kawano, MD2; Koji Abe, MD1; Mitsunori Tada, MD, PhD3
1Clinical Research Center, International University of Health and Welfare, Tokyo, Japan; 2Department of orthopaedic surgery, Keio University, Tokyo, Japan; 3National Institute of Advanced Industrial Science and Technology, Digital Human Research Center, Tokyo, Japan

Introduction: There is no biomechanical study examining reduced area of the thumb-tip trajectory after the thumb trapeziometacarpal (TMC) joint fusion. We investigated how large area of the thumb-tip trajectory decreased after the TMC joint fusion.

Methods: 6 fresh-frozen cadavers were used in this study. Three optical markers were fixed directly to the bones by stainless steel wires drilled into the scaphoid, trapezium, metacarpal, proximal phalanx, and distal phalanx of the thumb and 6 optical markers were fixed to the base of the custom-built experimental apparatus. Four extrinsic tendons (FPL, EPL, EPB, APB) were pulled independently by computer-controlled serve motors. Tension by the servo motors was applied to the distal tendons of 4 extrinsic thumb muscles, while tension by static weight was applied to 4 intrinsic muscles (APB, FPB, AdD, OP) through Nylon stitches representing the direction of each muscle. Thumb motion was measured under six different intrinsic muscle tensions, 0.00N, 0.98N, 1.96N, 2.94N, 3.92N, and 4.90N, before and after the TMC fixation with pins. The motions of these markers were recorded by a motion capture system (OptiTrack Flex 13; Natural Point, Inc.)(Fig. 1). Surface geometries of the markers and bones created from the CT images were fit into the marker trajectories from the motion capture system to reconstruct the 3-dimensional bone motion.

Results: Fig. 2 shows the area of the thumb-tip trajectories for six different tensions of the APB, when the FPL was pulled before (Fig. 2A) and after (Fig. 2B) the TMC joint fixation. Figure 3 demonstrates the superimposed trajectories. This figure clearly indicates that the fingertip trajectory area was extremely smaller after the TMC fusion. Trajectory area was limited to approximately 30% of the original area after the TMC joint was fixed.

Discussion: TMC fusion provides stability of the thumb, while resulting in decrease of the range of motion of the thumb. Despite the marked decrease in motion, subjective functional complaints may be minimal. In this study, we revealed how the area of thumb-tip trajectory decreased after the TMC joint fixation compared with before fixation. The trajectory area decreased to approximately 30% of original area after the TMC joint fusion. We consider that this technique and result of this study will be useful for understanding thumb motion after the TMC arthrodesis.

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