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

Limb length inequality results from many types of musculoskeletal disorders ranging from injury and illness to bone disease. Asymmetric weight bearing from a limb length discrepancy of less than 2% can have painful outcomes such as back problems and early onset osteoarthritis. Existing treatments include invasive surgeries and/or drug regimens that are often only partially effective. We previously developed a noninvasive, once daily unilateral heating model using targeted heat to increase limb length on one side of the body of growing mice. After two weeks, we demonstrated that tibial elongation rate increased >12% on the heat‐treated side with accompanying unilateral increases in femoral (1.3%) and tibial (1.5%) lengths. In this study, we applied heat for one 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. To test this, we measured tibial length and weight distribution before and after limb heating. If small changes in limb length cause imbalanced weight distribution as hypothesized, we expect to find a positive relationship between weight bearing asymmetry and limb length inequality after treatment. Methods Female 3‐week old C57BL/6 mice (N=12 total) were treated with targeted intermittent heat for 7 days (40C for 40 min/day). High‐resolution X‐ray images (N=6) and hindlimb weight bearing data (N=8) were acquired at the start and end of the experiments. Mice were given oxytetracycline (OTC, 7.5 mg/kg IP) to quantify growth rate, which was measured in unfixed proximal tibial slab sections on a fluorescence stereoscope. Tibial length was measured from live animal X‐rays and femoral length was measured from digitized bone images at the 4‐week endpoint. Statistical analyses were performed in SPSS using paired t‐tests and correlation. Significance was accepted at α = 0.05 (one‐tailed). Results There were no significant left‐right differences in starting tibial length or hindlimb weight bearing at 3‐weeks. After one week intermittent heat exposure, endpoint tibial (t = 7.7, p < 0.001) and femoral lengths (t = 11.5, p < 0.001) were ~1% and 1.3% greater, respectively, on the heat‐treated sides (40C) compared to the non‐treated contralateral sides (30C). Tibial elongation rate was over 6% greater on the heat‐treated side (t = 5.19, p < 0.001). Hindlimb weight bearing was over 20% greater (t = 11.9, p < 0.001) and significantly correlated with the increase in tibial elongation rate on the heat‐treated side (Pearson's R = 0.91, p < 0.01). Discussion These results support the hypothesis that even a small limb length discrepancy (less than 1.5%) can cause imbalanced weight distribution (over 20%) in healthy mice. This research has practical relevance for treating a broad spectrum of linear growth disorders in children. The increase in bone elongation rate generated by targeted heat could help equalize limb length and weight bearing asymmetry caused by disease or trauma. Results could lead to new approaches with better outcomes by using targeted heat therapy to reduce costs and side effects of surgeries and high‐dose pharmaceuticals. Support or Funding Information Supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health (1R15AR067451‐01). This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .