The effect of age and initial compression on the force relaxation response of the femur in elderly women

The effect of force amount, age, body weight and bone mineral density (BMD) on the femur's force relaxation response was analysed for 12 donors (age: 56-91 years). BMD and fracture load, F-L, were estimated from clinical CT images. The 30 min force relaxation was obtained using a constant compression generating an initial force F-0 between 7% and 78% of F-L. The stretched decay function (F(t) = A x e((-t/t)beta)) proposed earlier for bone tissue was fitted to the data and analysed using robust linear regression. The relaxation function fitted well to all the recordings (R-2 = 0.99). The relative initial force was bilinearly associated (R-2 = 0.83) to the shape factor, beta, and the characteristic time, t, when F-0/F-L was less than 0.4, although beta was no longer associated with F-0/F-L by pooling all the data. The characteristic time t increased with age (R-2 = 0.37, p = 0.03) explaining 35% of the variation of t in the entire dataset. In conclusion, the relative initial force mostly determines the femur's force relaxation response, although the early relaxation response under subcritical loading is variable, possibly due to damage occurring at subcritical loading levels.