Biomechanics and Histology of Intact and Repaired Digital Nerves: An In Vitro Study Steven H. Goldberg, MD, Charles M. Jobin, MD, Austin G. Hayes, BS, Tom Gardner, MCE, Melvin P. Rosenwasser, MD, Robert J. Strauch, MD From the Department of Orthopaedic Surgery, Columbia University, College of Physicians and Surgeons, New York, NY. Purpose: To investigate the biomechanical properties of intact and repaired cadaver digital nerves. Methods: Ultimate tensile failure strength and stiffness were determined in 67 human cadaver digital nerves. Total nerve area, fascicular area, and nonfascicular (connective tissue) area were determined from the metacarpophalangeal to the distal interphalangeal joint in another 35 axial nerve sections to determine regional anatomic variation. Thirty-eight additional digital nerves were transected, and epineural repairs were performed using simple, inter- rupted sutures. Suture number (2 vs 4), gauge (8-0 vs 9-0), and purchase length (1 mm vs 2 mm) were used in various combinations, and then the repaired nerves were pulled to failure. The mechanism of repair-site failure was determined for each suture. In situ tension of the intact digital nerves was measured during passive metacarpophalangeal and proximal inter- phalangeal joint motion in another 19 intact digital nerves. Results: There were no significant differences in failure load or stiffness with respect to the radial or ulnar nerves within a finger or between fingers. The primary tactile side of the finger tended to have a larger diameter digital nerve. Digital nerve failure was more common proximally than distally. Intact digital nerves failed at 6 N with a stiffness of 1 N/mm. Histologic analysis showed that fascicular area and total area decreased from proximal to distal, whereas the nonfascicular-to-fascicular area ratio increased. Four epineural sutures were statistically stronger than 2 sutures. Suture purchase length and gauge did not affect repair strength. The 8-0 nylon sutures failed primarily by cut-out, whereas the 9-0 sutures failed by cut-out or breakage. Repaired nerves failed at 1 to 2 N. Maximal metacarpophalangeal joint hyperextension resulted in 4 N of digital nerve tension. When the metacarpophalangeal joint was not hyperextended, proximal inter- phalangeal joint motion did not generate tension. Conclusions: Similar to flexor tendons, the number of suture strands crossing the repair site was the most important variable affecting digital nerve repair strength in this cadaveric model. (J Hand Surg 2007;32A:474 – 482. Copyright © 2007 by the American Society for Surgery of the Hand.) Key words: Biomechanics, digital nerve, histology, rehabilitation, repair, epineural, tension. D igital nerve lacerations are common, and they can occur in isolation or combined with flexor tendon lacerations. Biomechanical characteristics of flexor tendons and factors that af- fect tendon healing and repair strength have been extensively studied. 1–14 Relatively little is known, however, about the biomechanical properties of dig- ital nerves, nerve healing, and repair strength. This is a 4-part study performed on fresh-frozen cadavers to investigate digital nerve biomechanics and technical factors affecting nerve repair. First, digital nerve biomechanical properties were determined. Sec- ond, histologic analysis of normal digital nerve sequen- tial sections was performed to analyze regional ana- tomic differences along the proximal– distal length. Third, variations in epineural repair technique (suture caliber, number of sutures, suture bite purchase length) were studied to determine factors that might affect nerve repair strength and stiffness. Finally, in situ ten- sion of intact digital nerves was measured during pas- 474 The Journal of Hand Surgery