Bone-loading response varies with strain magnitude and cycle number

Abstract

Mechanical loading stimulates bone formation and regulates bone size, shape, and strength. It is recognized that strain magnitude, strain rate, and frequency are variables that explain bone stimulation. Early loading studies have shown that a low number (36) of cycles/day (cyc) induced maximal bone formation when strains were high (2,000 microepsilon) (Rubin CT and Lanyon LE. J Bone Joint Surg Am 66: 397-402, 1984). This study examines whether cycle number directly affects the bone response to loading and whether cycle number for activation of formation varies with load magnitude at low frequency. The adult rat tibiae were loaded in four-point bending at 25 (-800 microepsilon) or 30 N (-1,000 microepsilon) for 0, 40, 120, or 400 cyc at 2 Hz for 3 wk. Differences in periosteal and endocortical formation were examined by histomorphometry. Loading did not stimulate bone formation at 40 cyc. Compared with control tibiae, tibiae loaded at -800 microepsilon showed 2.8-fold greater periosteal bone formation rate at 400 cyc but no differences in endocortical formation. Tibiae loaded at -1,000 microepsilon and 120 or 400 cyc had 8- to 10-fold greater periosteal formation rate, 2- to 3-fold greater formation surface, and 1-fold greater endocortical formation surface than control. As applied load or strain magnitude decreased, the number of cyc required for activation of formation increased. We conclude that, at constant frequency, the number of cyc required to activate formation is dependent on strain and that, as number of cyc increases, the bone response increases.

Trabecular bone adaptation to loading in a rabbit model is not magnitude‐dependent

“…The relationship between load magnitude and bone formation is further complicated due to the interaction that between strain magnitude and other loading parameters such as cycle number, 15,28 strain rate, frequency, and strain distribution that have only partially been investigated for cortical bone and largely unexamined for trabecular bone. This complexity is exemplified in studies loading at high frequencies and low loading magnitudes through whole body vibration, which have reported a non-dose-dependent response of trabecular bone volume to load magnitude.…”

Low‐level accelerations applied in the absence of weight bearing can enhance trabecular bone formation

“…[7][8][9] It has further been suggested that strain thresholds must be exceeded to achieve a quantifiable response, both at the tissue 10 and the cell 11 levels. Mechanical treatments for low bone mass may appear advantageous when compared to pharmaceutical interventions that bear the risk of side effects.…”

Finite element analysis of the mouse tibia: Estimating endocortical strain during three‐point bending in SAMP6 osteoporotic mice

“…In rats and mice, two widely used noninvasive methods for skeletal overloading are tibial bending (Turner et al, 1991;Akhter et al, 1998) and forelimb compression (Torrance et al, 1994;Lee et al, 2002). Peak strain magnitude is typically used as an index of the severity of loading in these models, and the magnitude of bone formation has been shown to correlate with strain magnitude (Turner et al, 1994;Forwood and Turner, 1995;Mosley et al, 1997;Cullen et al, 2001;Lee et al, 2002). Using these models, investigators have focused primarily (but not exclusively) on periosteal responses.…”