1. Department of Orthopaedics, University of Duisburg-Essen, Pattbergstrasse 1-3, 45239 Essen, Germany
2. Central Animal Laboratory, Clinical Centre, University of Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany
Objective: Mechanotransduction is the mechanism that due to reacting chondrocytes on biomechanical loading of body mass. Higher biomechanical loading lead to increased degeneration of chondrocytes, whereas moderate loading is protecting. This suggests that body fat regulates bone metabolism first by means of hormonal factors and second that the effects of muscle and loading are signaling factors in mechanotransduction. Leptin, a peptide hormone produced predominantly by white fat cells, is one of these hormonal factors. The aim of this study was to investigate and measure the different effects of weight-bearing on trabecular bone formation in mice without the stimulation of leptin and with or without osteoarthritis. Materials and methods: 40 C57BL/ 6J ob/ob-mice in the age of 20 weeks have been devided into two groups with an ad-libitum-diet and with reduced diet. The hip- and knee-joints have been examinated in micro-CT-scan and histomorphologically. Results: Animals with an ad-libitum-diet were found to increase body weight significantly at the age of six weeks in comparison with lean mice. At the age of twenty weeks the obese mice were almost twice as heavy as the lean mice. Significant statistical differences are shown between the two groups for body weight and bone mineral density. Examination of trabecular bone in micro-CT revealed that the only statistically significant difference between the two groups was the trabecular number for the proximal femur. High weight-bearing insignificantly improved all trabecular bone parameters in the obese mice. Correlation was found between trabecular number and bone mineral density on the one hand and body weight on the other hand. The correlation between body weight and osteoarthritis shows a significant increase in grade of osteoarthritis as body weight increases in hip-joint and knee-joint but not in osteoarthritis-positive (OP) versus osteoarthritis-negative (ON) mices. The correlation of the hip-joint between micro-CT data and body weight shows an increase in these data as body weight increases in OP mices. The correlation of the hip-joint between micro-CT data and osteoarthritis shows a decrease in these data as osteoarthritis increases in OP mices. The correlation of the knee-joint between micro-CT data and body weight shows differencies between ON and OP mices. The correlation of the knee-joint between micro-CT data and osteoarthritis shows an increase in these data as osteoarthritis increases in OP mices. Conclusion: biomechanical loading led to decreased bone mineral density by a decrease in the number of trabeculae. Trabecular thickness was not increased by biomechanical loading in growing mice. Decreased body weight in leptin-deficient mice protects against bone loss. This finding is consistent with the principle of light-weight construction of bone. Differences in osteoarthritis-positive and osteoarthritis-negative mices show the eventual importance of diet in leptin-deficience. It is not possible to conclude that these results also apply to human beings.
Keywords: Bone mineral density, leptin, biomechanical loading, micro-CT, mice