The terminal tendon of the digital extensor mechanism: Part II, kinematic study.

PURPOSE To conduct kinematic analyses of both intact and sectioned terminal tendon (TT) of multiple fingers in the hand. METHODS The TTs of 36 fresh-frozen cadaveric digits were used in this study. TT excursion was assessed along with the influence on proximal joint motion. The influence of TT lengthening and shortening on distal interphalangeal (DIP) joint motion were investigated. RESULTS TT excursion averaged 1 mm at the DIP joint and was influenced by the proximal interphalangeal (PIP) joint but not the position of other joints in the hand and wrist. The greatest degree of DIP joint motion averaged 86 degrees when the PIP joint was in full flexion, whereas the least motion averaged 45 degrees when this joint was in neutral position. Lengthening of the TT resulted in angular deformity at the DIP joint. Average flexion deformities reached 25 degrees at 1 mm, 36 degrees at 2 mm, 49 degrees at 3 mm, and 63 degrees at 4 mm of lengthening. The middle finger showed the greatest flexion deformity, followed by the ring, small, and index fingers. Shortening the TT by as little as 1 mm resulted in difficult tendon repair because of excessive tension and minimal or no DIP joint flexion was obtained. CONCLUSION Only DIP and PIP joints affect TT excursion; hence these are the main joints to be immobilized to protect TT repair. The middle finger TT showed the least tolerance to lengthening with potential for mallet deformity. Joint flexion deformity is proportional to tendon lengthening. Only 1 mm of TT lengthening results in approximately 25 degrees of DIP joint extension lag, and 4 mm of TT lengthening results in DIP joint flexion deformity greater than 60 degrees . Even 1 mm of TT shortening will seriously restrict DIP joint flexion.

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