Heat-Induced Limb Length Asymmetry Has Functional Impact on Weight Bearing in Mouse Hindlimbs

Abstract

Limb length inequality results from many types of musculoskeletal disorders. Asymmetric weight bearing from a limb length discrepancy of less than 2% can have debilitating consequences such as back problems and early-onset osteoarthritis. Existing treatments include invasive surgeries and/or drug regimens that are often only partially effective. As a noninvasive alternative, we previously developed a once daily limb-heating model using targeted heat on one side of the body for 2 weeks to unilaterally increase bone length by up to 1.5% in growing mice. In this study, we applied heat for 1 week to determine whether these small differences in limb length are functionally significant, assessed by changes in hindlimb weight bearing. We tested the hypothesis that heat-induced limb length asymmetry has a functional impact on weight bearing in mouse hindlimbs. Female 3-week-old C57BL/6 mice (N = 12 total) were treated with targeted intermittent heat for 7 days (40 C for 40 min/day). High-resolution x-ray (N = 6) and hindlimb weight bearing data (N = 8) were acquired at the start and end of the experiments. There were no significant left-right differences in starting tibial length or hindlimb weight bearing. After 1-week heat exposure, tibiae (t = 7.7, p < 0.001) and femora (t = 11.5, p < 0.001) were ~1 and 1.4% longer, respectively, on the heat-treated sides (40 C) compared to the non-treated contralateral sides (30 C). Tibial elongation rate was over 6% greater (t = 5.19, p < 0.001). Hindlimb weight bearing was nearly 20% greater (t = 11.9, p < 0.001) and significantly correlated with the increase in tibial elongation rate on the heat-treated side (R2 = 0.82, p < 0.01). These results support the hypothesis that even a small limb length discrepancy can cause imbalanced weight distribution in healthy mice. The increase in bone elongation rate generated by localized heat could be a way to equalize limb length and weight bearing asymmetry caused by disease or trauma, leading to new approaches with better outcomes by using heat to lengthen limbs and reduce costly side effects of more invasive interventions.

Anesthesia and analgesia for common research models of adult mice

“…In a model of heat-induced limb length asymmetry, the right sides of the body of the mice were positioned on a heating pad with a 40 °C constant temperature. For this purpose, isoflurane inhalation was used to hold the mice in lateral recumbency [ 67 ]. For Laser Doppler perfusion imaging of limbs, mice were lightly anesthetized by first inducing anesthesia in an induction chamber, and then by maintaining anesthesia by a nasal cannula delivering isoflurane/oxygen [ 63 ].Nerve transection on limbs may be performed on sciatic or femoral nerves.…”

The Skeletal Cellular and Molecular Underpinning of the Murine Hindlimb Unloading Model

“…Environmental temperature can affect bone remodeling, and exposure of growing mice to higher temperatures has shown to lengthen long bones (Racine et al, 2018 ). In a recent paper by Chevalier et al, warmth exposure (34°C) was reported to protect against ovariectomy-induced bone loss in mice.…”