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Four Popular Donor Muscle Insertion Sites for Opposition Tendon Transfers Produce Similarly Mis-directed Grasp Force Vectors
Joseph D. Towles, PhD1; Maciej Krolikowski2; Sean Hannon2; Mark McDaniel, MD3; Michael Bednar, MD3; Avinash Patwardhan, PhD3; (1)University of Wisconsin Madison, (2)Edward Hines Jr. VA Hospital, (3)Loyola University Chicago
Edward Hines Jr VA Hospital, Hines, IL, USA

Introduction. The optimal insertion site of a donor muscle in an opposition tendon transfer is debatable partly because the effects of different site locations on thumb movement and thumb-tip force during grasp are complex and cannot be thoroughly resolved without surgical simulation. The goal of this work was to rank popular opposition tendon transfer surgeries, each having a different insertion site, according to their capacity to produce thumb-tip forces that promote sustained (i.e., slip-free) grasp contact with an object. Methods. Four opposition tendon transfer surgeries were simulated in 8 cadaveric specimens. Donor muscle insertions sites were implemented according to prescriptions by Bunnell, Riordan, Camitz and Royale (Hentz et al. (2001), Hand Surgery: A Clinical Atlas). Fifty percent of maximum isometric force of the recipient muscle was applied to the insertion site and the resulting 3D thumb-tip force was measured. Force orientation relative to the direction perpendicular to the thumb pad was computed, and compared across tendon transfer surgeries (1-way ANOVA, alpha = 0.05) and to the force directional requirement (Student t-test, 2-tail, alpha = 0.05) for sustained grasp contact with object surfaces of varying degrees of slipperiness (Buchholz et al. (1988), Ergonomics 31(3): 317-325). Results.The Royale tendon transfer produced a statistically less obliquely directed force (78 (2.3)o) than those of Bunnell (82 (4.1)o) and Riordan (83(3.8)o), but the angular deviation across the 4 tendon transfers differed only by 5o (i.e., 78o to 83o). Further, we found that the orientation of the thumb-tip force produced by each tendon transfer was statistically greater than the force directional requirement for the least slippery object surface (20-grit sand paper: 30o). Conclusion. The virtual similarity in functional outcome among the insertion sites challenges the functional basis for having more than one proposed insertion site. The observation that no tendon transfer could produce a suitably directed force for the broad range of object surface properties considered suggests that neuromuscular control of other thumb muscles is required to complement the action of the donor muscle. It is unclear from the surgical literature that this neural interaction between intact thumb muscles and the transferred muscle is a consideration in the design of tendon transfers. In summary, this work illustrates that surgical simulation can help to establish a stronger biomechanical basis for the design of tendon transfers and illuminates another potentially important aspect of the design process.


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