Limb length inequality is a potentially disabling condition with few noninvasive treatment options. Our lab has previously shown that unilateral heat increases bone elongation rate, suggesting that temperature therapy could be a non‐surgical way to promote limb length equalization. Treatment age is important because we have shown that temperature effects occur primarily when growth is most rapid. We compared heat effects on limb lengthening during early (3‐5 weeks age) and late (5‐7 weeks age) phases of post‐weaning growth in mice (N=24). We tested the hypothesis that heat‐induced limb elongation varies with postnatal growth rate. Female mice (N=6 per age) were treated with 40C unilateral heat for 40 minutes per day for 14 days. Controls (N=6 per age) were not treated. Tibial elongation rate, measured by fluorochrome labeling, was over two‐fold greater during the early phase. Bone elongation rate reflected overall body growth rate, with lower absolute heat‐induced differences during the later phase. Relative increases in extremity length on the heat‐treated side were similar during early and late phases, suggesting that a heat‐induced growth response is still possible during later postnatal growth. These results are important for designing therapies to treat children with linear growth disorders at different stages of development by demonstrating that treatments may not be restricted to early growth phases. Supported by ASBMR GAP, WV‐NASA, UK‐CCTS (NIH UL1TR000117), and NIH/NIAMS R15AR067451‐01.

Our lab has previously demonstrated that daily unilateral limb heating permanently increases bone length in growing mice. We hypothesize that limb temperature remains elevated on the treated side due to a heat‐induced increase in vascular supply that accelerates bone lengthening. In order to test the temperature retention hypothesis, we conducted an extensive pilot study to determine the period of time in which limb temperature remains elevated on the heat‐treated side after each daily regimen. By collecting hourly and daily temperatures at scheduled time points throughout the trial, our goal was to determine optimal conditions for recording temperature data in our subsequent studies, as well as to identify potential technical problems that could interfere with outcomes of the heat‐treatment. Weanling female mice (N=6) were treated daily at 7am for 2‐weeks with 40C unilateral heat for 40 minutes per day following our published methods. Infrared thermal images were captured at 320 X 240 resolution using a FLIR E8 infrared camera (sensitivity <0.06C in temperature range of −20 to 250C). Surface temperatures of the heat‐treated (right) and non‐treated (left) hindlimbs were obtained from calibrated images using FLIR tools software. Statistical comparisons were made using ANOVA and paired t‐tests in SPSS. Data from multiple days were pooled at each time point since there were no significant differences among days analyzed. Common trends in temperature differences between heat‐treated and non‐treated limbs were seen throughout the day. At 6am baseline prior to treatment, the heat‐treated side was over 1% warmer than the non‐treated side (t=1.84, p=0.06). After the treatment at 8am, limb temperatures were nearly equivalent (t=1.13, ns), suggesting a systemic post‐anesthesia thermoregulatory response. By 11am, 4 hours after the treatment start, the heat‐treated side was over 2% warmer than the non‐treated side (t=8.06, p<0.001), indicating sustained heat retention. By 3pm, limb temperatures were again similar (t=0.13, ns), reflecting increased activity as mice approached the dark cycle. Excessive imaging (up to once per hour) negatively impacted daily gain in body mass, but this effect was mitigated by increasing intervals between imaging. These pilot results support our hypothesis that limb temperature remains elevated on the heated side up to 4 hours after treatment. This study is an important step toward understanding the mechanisms by which heat enhances limb elongation.Support or Funding InformationSupported by ASBMR Grants in Aid Program and the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health (1R15AR067451‐01).