Viscoelastic response of the rat loading model: Implications for studies of strain-adoptive bone formation

Studies of the adaptive skeletal response to mechanical loading require app ropriate animal models. Two new approaches involve the nonsurgical applicat ion of loads to either the ulna or tibia of rats. Both of these approaches require the loading of bone through adjacent soft tissues, and thus the tis sue viscoelasticity might affect the way load is transferred to the bone. T he objective of this study was to characterize the mechanical strain in the rat tibia or ulna during applied loading at different frequencies. For the rat ulna model, loading was applied to the ulnae of four adult, female rat s as a haversine waveform at frequencies of 1, 2, 5, 10, and 20 Hz and peak loads of 5, 10, 15, and 20 N. Mechanical strain was measured on the medial and lateral ulnar surfaces using single element strain gauges. For the rat tibia model, four-point bending loads were applied to the right tibiae of seven rats at frequencies of 0.5, 1, 2, 5, 10, and 20 Hz and peak loads of 30, 40, 50, and 60 N. Mechanical strain was measured on the lateral tibial surface at 5 mm proximal to the tibiofibular junction. We found that peak s trains were linearly proportional to applied load, but decreased logarithmi cally as loading frequency was increased, indicating a significant viscoela stic effect in the soft tissues surrounding the ulnocarpal joint and in the soft tissues surrounding the tibia shaft. The viscoelastic response of the ulna and tibia tends to "filter out" high-frequency loading components and , as a result, the rat loading systems act as a low-pass filter. Consequent ly, any experiment designed to test the effect of loading frequency on bone formation in the rat ulna and tibia should employ progressively larger loa ds at higher loading frequencies to guarantee a consistent peak strain magn itude in the bone. The filtering effect of the ulna loading system is illus trated by an analysis of the strain waveforms from the recent study by Mosl ey and Lanyon (Bone 23:313-318; 1998) that was designed to evaluate the eff ect of strain rate on bone formation. (C) 1999 by Elsevier Science Inc. All rights reserved.