Effects of Loading Duration and Short Rest Insertion on Cancellous and Cortical Bone Adaptation in the Mouse Tibia

Abstract

The skeleton’s osteogenic response to mechanical loading can be affected by loading duration and rest insertion during a series of loading events. Prior animal loading studies have shown that the cortical bone response saturates quickly and short rest insertions between load cycles can enhance cortical bone formation. However, it remains unknown how loading duration and short rest insertion affect load-induced osteogenesis in the mouse tibial compressive loading model, and particularly in cancellous bone. To address this issue, we applied cyclic loading (-9 N peak load; 4 Hz) to the tibiae of three groups of 16 week-old female C57BL/6 mice for two weeks, with a different number of continuous load cycles applied daily to each group (36, 216 and 1200). A fourth group was loaded under 216 daily load cycles with a 10 s rest insertion after every fourth cycle. We found that as few as 36 load cycles per day were able to induce osteogenic responses in both cancellous and cortical bone. Furthermore, while cortical bone area and thickness continued to increase through 1200 cycles, the incremental increase in the osteogenic response decreased as load number increased, indicating a reduced benefit of the increasing number of load cycles. In the proximal metaphyseal cancellous bone, trabecular thickness increased with load up to 216 cycles. We also found that insertion of a 10 s rest between load cycles did not improve the osteogenic response of the cortical or cancellous tissues compared to continuous loading in this model given the age and sex of the mice and the loading parameters used here. These results suggest that relatively few load cycles (e.g. 36) are sufficient to induce osteogenic responses in both cortical and cancellous bone in the mouse tibial loading model. Mechanistic studies using the mouse tibial loading model to examine bone formation and skeletal mechanobiology could be accomplished with relatively few load cycles.

Protective Effects of Controlled Mechanical Loading of Bone in C57BL6/J Mice Subject to Disuse

“…Trabecular bone has long been known to show a higher response to changes in the loading environment than cortical bone. This is a suggested effect of trabeculae having the ability to optimize load transfer because of the freedom to rearrange accordingly, and because of the more rapid remodeling rate of trabecular bone …”

Examining tissue composition, whole-bone morphology and mechanical behavior of GorabPrx1 mice tibiae: A mouse model of premature aging

“…There were originally 10 Gorab Prx1 mice, but two mice died while under anesthesia during in vivo microCT scanning and therefore were excluded from the study. The proximal metaphyseal volume of interest (VOI) started 100 μm below the distal-most point of the growth plate and extended distally 10% of the tibial length (Willie et al, 2013; Yang et al, 2017). The cancellous and cortical bone in the metaphyseal VOI were segmented manually in the scans and analyzed separately.…”

“…The cancellous and cortical bone in the metaphyseal VOI were segmented manually in the scans and analyzed separately. The cortical midshaft VOI was centered at the midpoint of the tibia and extended 5% of the tibial length (Willie et al, 2013; Yang et al, 2017). A threshold of 671 mg HA/ccm was used to segment both diaphyseal and metaphyseal cortical bone.…”

Evaluation of loading parameters for murine axial tibial loading: Stimulating cortical bone formation while reducing loading duration

“…Both tibiae were harvested and embedded in plastic. Diaphyseal transverse sections were cut at the site of strain gauge analysis (5 mm proximal to the distal TFJ), which corresponds approximately to a site 37% of the tibial length from the proximal end as described by others . (Unpublished data from our lab show that loading induces a similar bone formation response across the 25–50% tibial region.)…”

“…Protocols that use shorter daily loading bouts, fewer days/week, and shorter study length may be advantageous provided that they elicit a sufficient anabolic response. For example, the findings of Yang et al provide rationale for briefer daily loading (fewer cycles, no rest insertion). Regarding study length, initial studies of mouse axial loading used 2‐week (or longer) loading durations, yet more recent studies have reported significant loading effects in mice using only 3 or 5 days of loading .…”