Biomechanical Response of Bone to the Cessation of Exercise: Development ofa Mechanical Test Apparatus for Trabecular Bone

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It is crucial for clinical orthopaedic surgery to determine the response of bone to the cessation of exercise, because such a change in load condition normally happens in daily lives. It is also important for basic biomechanic's and engineering design to understand the mechanism of the response. Previous studies have shown that the mechanical properties of the tibia from immature rats were improved by running. In addition, the improved properties remained unchanged after the return of load to normal. The present study was performed to know whether the same phenomena were observed also in mature ratsSixteen-week-old mature Wistar female rats were divided into running (Run), running and sedentary(Run/Sed), and Control groups. The rats of Run group were subjected to 1.6 km/h, 1 hour-a-day, 6 days-a-week treadmill running program for up to 15 weeks. The rats of Run/Sed group,were subjected to the same running program for up to another 15 weeks. Control group rats were subjected to normal u … More nrestricted activities throughout experimental periods. After sacrifice, compression and hardness tests were performed on the specimens obtained from the midshaft of each tibia. The microstructure of the proximal tibia was also observed using an X-ray μCT system. Compressive load at failure was lower in Run group at 3 weeks as compared with that in Control group, although it increased to a control level at 15 weeks. Micro Vickers hardness in Run and Run/Sed groups was slightly higher than that in Control group. The areas of cortical and trabecular bones were smaller in Run group than those in Control group. After the cessation of running, the area of trabecular bone was remained smaller even at 30 weeks; however, the area of cortical bone increased to a control level at 30 weeks. For the evaluation of the mechanical property oftrabecular bones, a bending test apparatus was designed and developed. The edge parts of the apparatus which contact with the trabecular bone in bending test were consisted of stainless steel (SUS304), and a ring-shaped load transducer was consisted of aluminum alloy. A preliminary test for a pig tibia revealed that it was possible to perform three-point bending test using the developed apparatus. The elastic modulus of the trabecular bone was approximately 72 MPa in proximal areaIn summary, in mature rat tibiae the material properties were slightly increased, and the structural properties were slightly decreased by running for 15 weeks. The material properties were unchanged after the activity change to sedentary state. It is possible using the developed apparatus to perform the three-point bending test for the trabecular bones. This technique will be used in further studies on bone remodeling Less