The mechanical and microstructural bases of tendon fatigue, by which damage accumulates and contributes to degradation, are poorly understood. To investigate the tendon fatigue process, rat flexor digitorum longus tendons were cyclically loaded (1–16 N) until reaching one of three levels of fatigue damage, defined as peak clamp‐to‐clamp strain magnitudes representing key intervals in the fatigue life: i) Low (6.0%–7.0%); ii) Moderate (8.5%–9.5%); and iii) High (11.0%–12.0%). Stiffness, hysteresis, and clamp‐to‐clamp strain were assessed diagnostically (by cyclic loading at 1–8 N) before and after fatigue loading and following an unloaded recovery period to identify mechanical parameters as measures of damage. Results showed that tendon clamp‐to‐clamp strain increased from pre‐ to post‐fatigue loading significantly and progressively with the fatigue damage level (p ≤ 0.010). In contrast, changes in both stiffness and hysteresis were significant only at the High fatigue level (p ≤ 0.043). Correlative microstructural analyses showed that Low level of fatigue was characterized by isolated, transverse patterns of kinked fiber deformations. At higher fatigue levels, tendons exhibited fiber dissociation and localized ruptures of the fibers. Histomorphometric analysis showed that damage area fraction increased significantly with fatigue level (p ≤ 0.048). The current findings characterized the sequential, microstructural events that underlie the tendon fatigue process and indicate that tendon deformation can be used to accurately assess the progression of damage accumulation in tendons. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:264–273, 2009

Background: Radial transection of the peripheral fibers of the meniscus could render it nonfunctional; however, the biomechanical consequences of a complete lateral meniscal radial tear and repair in human specimens have not been elucidated. Hypothesis: A complete radial tear will exhibit knee contact mechanics approaching those of total meniscectomy. Repair of complete radial tears will re-create normal load transmission across the joint. Study Design: Controlled laboratory study. Methods: Five matched pairs of fresh-frozen human cadaveric knees were tested in axial compression (800 N) at 2 knee flexion angles (0° and 60°). Six meniscal conditions were sequentially tested: (1) intact lateral meniscus; radial width tears of (2) 50%, (3) 75%, and (4) 100%; (5) meniscal repair; and (6) total meniscectomy. Repairs were pair matched and used either an inside-out or all-inside technique. Tekscan sensors measured tibiofemoral contact pressure, peak contact force, and contact area in the lateral meniscus and medial meniscus. Results: Complete radial tears of the lateral meniscus produced significant increases in mean contact pressure (P = .0001) and decreased contact area (P < .0001) compared with the intact state. This effect was significantly less than that of total meniscectomy (P < .0023). Lesser degrees of radial tears were not significantly different from the intact state (P > .3619). Mean contact pressure after either repair technique was not significantly different from the intact state (P = .2595) or from each other (P = .4000). Meniscal repair produced an increase in contact area compared with a complete tear but was still significantly less than that of the intact meniscus (P < .0001). The medial compartment showed no significant difference between all testing conditions for 0° and 60° of flexion (P ≥ .0650). Conclusion: A complete radial meniscal tear of the lateral meniscus has a detrimental effect on load transmission. Repair improved contact area and pressure. Contact pressures for repaired menisci were not significantly different from the intact state, but contact area was significantly different. Biomechanical performance of repair constructs was equivalent. Clinical Relevance: Repair of complete radial tears improves joint mechanics, potentially decreasing the likelihood of cartilage degeneration.