Session I - Foot & Ankle
Fri., 10/8/04 Foot & Ankle, Paper #5, 9:46 am
Real-Time Transient and Whole-Stance Phase-Contact Stress and Contact-Stress Gradient Changes in an Ankle Incongruity Model
Purpose: Intraarticular fractures initiate a cascade of pathomechanical events that can cause posttraumatic arthritis (PTA). Recent technologic advances have enabled real-time articular-contact testing throughout an entire range of motion (ROM) under quasi-physiologic loads. Such testing can measure localized transient peak stresses and contact-stress gradients. Whole-cycle testing also allows pathologic stress and stress gradients that accumulate through the entire motion cycle to be measured. In this series, we measured changes in real-time contact stress and stress gradients and calculated corresponding whole-cycle values in an ankle-stepoff incongruity model.
Methods: Ten fresh-frozen cadaveric ankles were dissected free of soft tissues and were secured into a custom-loading fixture. The fixture allowed the ankle to plantar/dorsiflex its normal range of motion, while physiologic compressive and anteroposterior loads were applied by an MTS machine and a pneumatic actuator. Specimens were initially loaded intact. A fragment consisting of the anterolateral 25% of the distal tibia was cut, rigidly secured in its anatomic position, and testing was repeated. The fragment was then displaced proximally, in 1-mm increments, to a maximum stepoff of 4 mm, and testing was repeated with each step. Contact stresses were measured during motion with use of a custom-designed real-time stress transducer (Tekscan). Contact stresses were sampled at 132 Hz at 1472 separate sites (sensels) on the distal tibia. Peak contact stresses were measured for the entire ROM. Whole-cycle contact stresses were calculated by integrating stresses at each sensel for the entire motion cycle. Contact stress gradients were calculated by applying a LaGrange 4-point central-difference formula to local stress data. Peak transient-stress gradients were measured, and whole-cycle contact-stress gradient values were calculated.
Results: Minimal changes in peak-time variant and whole-cycle contact stress occurred with anatomic reduction. In contrast, stepoff configurations showed substantial increases in peak-time variant-contact stress (2 H to 3 H intact values) and whole-cycle integrated stresses (up to 2.5 H intact values). Intact specimens consistently demonstrated low-magnitude, randomly oriented contact-stress gradients. In contrast, stepoff specimens had two regions of high-magnitude preferentially oriented stress gradients. Peak whole-cycle stress gradients increased up to 2.5 H intact values in stepoff configurations.
Discussion: Previous static tests of articular incongruities have consistently measured seemingly modest increases in contact stress even with substantial incongruities. Static tests usually load samples in a single position, cannot detect transient stresses that occur during motion, and cannot measure pathomechanical stresses that may accumulate through the entire ROM. In this study, transient peak stresses and stress gradients were nearly 3 H intact values, and whole-cycle stress and stress gradients were 2.5 H intact values. Such transient peaks may be important determinants of PTA. Whole-cycle stresses account for potential pathologic changes that accumulate throughout the entire ROM.