Cyclic Testing of Flexor Tendon Repairs: An In Vitro Biomechanical Study David W. Sanders, MD, Andrew D. Milne, BEng, Anthony Dobravec, Joy MacDermid MSc, PT, JamesA. Johnson, PhD, Graham J. W. King, MD, FRCSC, London, Ontario, Canada Forty cadaveric flexor digitorum profundus tendons were repaired using the Tajima, Halsted, Silfverskiold, or Savage techniques. The tendons were cyclically loaded in sets of 4,000 cycles beginning at 25 N and increasing by 10-N increments until failure occurred. Gap for- mation was continuously monitored with an extensiometer. Only the Savage repair group consistently withstood 4,000 cycles of 25-N loading, with respect to 2-mm gap formation and repair rupture. Tendon repairs subjected to cyclic loading demonstrated gap formation and repair rupture at lower loads than with static testing. Of the methods of flexor tendon repair tested, only the Savage technique could withstand simulated early active motion. (J Hand Surg 1997;22A:1004-1010) Disability following flexor tendon injuries in the hand is common. Complications are frequent and include digital stiffness, repair rupture, and gap for- mation,~-3 despite appropriate surgical treatment and rehabilitation, including early passive mobilization. 4 Active postoperative motion may offer a number of potential advantages over passive mobilization. The ultimate range of motion, strength, DNA content, ten- don nutrition, and rate of healing can all be improved with mobilization.5-7 Active mobilization theoreti- cally allows an increase in tendon excursion relative to passive motion, which may result in reduced adhe- From the Musculoskeletal Research Laboratory, Hand and Upper Limb Centre,and the Departmentsof Surgery, Medical Biophysics, and Mechanical Engineering, St. Joseph's Health Centre, University of WesternOntario,London,Ontario,Canada. Supportedby the AmericanSocietyfor Surgeryof the Handresearch grant #1995-4. Received for publication Dec. 14, 1995; accepted in revised form July 9, 1997. No benefitsin any form havebeen receivedor will be receivedfrom a commercial party related directlyor indirectlyto the subject of this article. Reprint requests: Graham J. W. King, MD, Hand and Upper Limb Centre, St. Joseph's Health Centre, 268 Grosvenor Street, London, Ontario, CanadaN6A 4L6. sions between the flexor tendon repair site and the fibro-osseous canal. Recently, newer tendon repair techniques have been developed that produce increased strength, which may allow tendons to with- stand immediate postoperative active mobilization. Cyclic mechanical testing of tendon repairs offers important advantages over static testing. Pruitt et al. have shown that cyclic testing demonstrates gap for- mation at lower loads than does static load-to-failure testing. 8 In the clinical setting, tendon repairs are cyclically loaded at a relatively fixed load in keeping with the mode of rehabilitation (passive, active, or resisted motion). Because it is more physiologic, cyclic testing may provide more relevant information regarding the failure characteristics of a tendon repair. The tendon force encountered clinically during active and passive motion has been investigated by Schuind et al. In an in vivo study of patients under- going carpal tunnel surgery, these investigators determined that a mean force of 19 N (range, 11-30 N) is required to produce active distal interpha- langeal flexion and that a mean force of 25 N is required to produce active thumb flexion. 9 This study employed low-profile force transducers to avoid error induced by the angle of deflection of the tendon 1004 The Journal of Hand Surgery