Spinal fatigue strength
G. Huber, K. Nagel, D.M. Skrzypiec, A. Klein, K. Püschel, M.M. Morlock
Spinal in-vitro testing is commonly performed quasi-statically, but the slow loading velocity does not mimic in-vivo conditions well. During occupational activities subjects are exposed to vibrations with high numbers of loading cycles – e.g. during handling and driving of heavy machinery. The purpose of this study is to provide an estimation of fatigue failure strength of human functional spinal units using Wöhler equation.
41 lumbar specimens were loaded in axial compression for 300,000 cycles with different load levels and the results combined with data of 70 thoracic and lumbar specimens of Brinckmann et al. (1988) loaded with up to 5,000 cycles. Fatigue force of each specimen was normalized (Fnorm) by individual geometrical (endplate area) or material specific parameters (age, BMD) and used to derive a Wöhler equation with cycles to failure as independent variable.
The fatigue strength of spinal specimens after cyclic loading is significantly lower than their ultimate strength. Including the upper compressive peak, endplate area and age for normalization explained 28% of variation in fatigue force (p<0.001). Introducing BMD instead of age improved the prediction to 61% explained variance of Fnorm (p<0.001).
Superimposing movements (e.g. bending forward or twisting) might probably lead to smaller numbers of cycles to bone failure or also soft tissue failure. Since it is possible to determine those individual parameters for living subject, the risk of occupational activities can be estimated in combination with numerical models for the appraisal of occupational diseases or further the determination of duty cycles for spinal implants.
Funding of FIOSH, Germany (F2059, F2069) is kindly acknowledged.
Huber G., Nagel K., Skrzypiec D.M., Klein A., Püschel K., Morlock M.M. (2016) A description of Spinal Fatigue Strength. Journal of biomechanics, 49, 875-880
Huber G., Skrzypiec D.M., Klein A., Püschel K., Morlock M.M. (2010) High cycle fatigue behaviour of functional spinal units. Industrial Health, 48(5), 550-556