Biomechanical studies of tendon transfers in the foot and ankle: implications for early rehabilitation

Abstract

Early weightbearing following tendon transfer in the foot and ankle is desirable, but is unproven and risks premature tendon pullout. The aims of this thesis were to determine whether early loading is safe following interference screw fixation (ISF). A cadaveric model simulated flexor digitorum longus tendon transfer, comparing tendon-to-tendon (TT) transfer versus ISF. There was no significant difference in tendon displacement after cyclic loading (50N-150N). Two premature failures occurred in the ISF group. Mean load to failure was significantly greater in the TT group (459N versus 327N, p=0.002). A similar cadaveric model simulated tibialis posterior tendon transfer for foot drop, comparing a Pulvertaft Weave (PW) technique versus ISF. One specimen failed in each group under cyclic loading. Mean load to failure was similar, measuring 419N and 499N in the PW and ISF groups respectively (p=0.06). A systematic review assessed early and standard rehabilitation protocols. In 494 patients treated using ISF, there were no early tendon failures, and both rehabilitation protocols led to significant improvements in function. However, due to high heterogeneity and bias, there is insufficient clinical evidence to support early rehabilitation. A sawbone model simulated normal and osteoporotic bone, using 5mm diameter rope fixed with interference screws (4.75mm to 7mm), in different sized pilot holes (5mm to 7mm). Screw insertion torque had a very strong correlation with pullout strength. ‘Osteoporotic bone’ had a statistically significantly lower load to failure and greater chance of premature failure during cyclic loading. In conclusion, biomechanical data suggest early rehabilitation is feasible, but that failure can occur at physiological loads. Increased fixation strength is associated with larger screw diameter, smaller pilot hole diameter, greater insertion torque, and less porous bone structure. Screw insertion torque might be a useful intraoperative indicator for secure fixation. In patients with osteoporotic bone, augmented fixation may be necessary.