In Vitro Tensile Fatigue of Human Flexor Digitorum Profundus and Superficialis Tendons

Abstract

Distal upper extremity work-related musculoskeletal disorders (DUE WMSDs) such as carpal tunnel syndrome (CTS) and stenosing tenosynovitis (trigger finger) present a high cost on both society and industry. CTS in particular can result in a large number of days away from work (median=30 days). There is mounting evidence that DUE WMSDs (such as CTS) result from a fatigue failure process. This dissertation presents three experiments that contribute evidence regarding the fatigue failure of tendons associated with CTS and trigger finger; specifically, the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS). The ‘S-N’ equation for FDP and FDS tendons was derived (S = 35.178-1.86*ln(N)), relating imposed peak stress (S) to expected number of cycles until macroscopic failure (N). A Morrow power model was also derived (y = 2.1864x-3.198), empowering future studies to estimate characteristic life without destructively testing the material. DC level was not a significant predictor of fatigue life (p=0.563) nor work per cycle (p=0.44). When comparing the FDP/FDS ‘S-N’ equation with a previously reported ‘S-N’ equation of EDL tendons, the FDP/FDS equation reported a higher OR for only 1 of 15 DUE outcome and repetition definition combinations using an epidemiological database. The FDP/FDS ‘S-N’ equation is more liberal than that of the EDL data, meaning more jobs were considered safe. This ultimately caused a higher accuracy for most DUE outcomes and repetition definition combinations. The probability functions for all DUE outcomes versus log CD for FDP/FDS data were more gradual functions, inferring a weaker dose-response relationship. Future research on the fatigue properties of FDP/FDS should focus on cyclic stresses less than 40% ultimate tensile stress (UTS) and also on cyclic compression, shear, and vibration forces.