Effect of load reduction on the calcification of rat tibial tuberosity: Focus on calcification factors and chondrocyte mechanosensors

The tibial tuberosity has a superficial patellar tendon-embedded portion and a deep uncalcified cartilage portion. Suppressed calcification of the tibial tuberosity leads to Osgood-Schlatter disease. The tibial tuberosity calcifies with age; load reduction degrades the cartilage matrix and promotes calcification, suggesting that reduced mechanical stimulation of the tibial tuberosity promotes calcification. However, this is yet to be clarified. Therefore, in this study, we aimed to investigate the effects of mechanical stimulation reduction on the tibial tuberosity tissue structure and calcification mechanism. Specifically, we examined the effect of load reduction on tibial tuberosity calcification in 20 male 7-week-old Wistar rats classified into two groups: hind-limb suspension (HS, n = 10) and control (CO, n = 10). We observed superficial and deep tibial tuberosities in both groups. The tibial tuberosity in the HS group had narrower areas of deep portions than did those in the CO group (p = 0.000539), and immature bone tissue and cartilage tissue were observed in the HS group. Enpp1 expression did not significantly differ between the groups (p = 0.804). In contrast, Alpl (p = 0.001) and Mmp3 (p = 0.006) expression increased whereas Timp3 expression decreased (p = 0.002) in the HS group. Thus, these results showed a maturing of bone ossification, and this gene expression trend was similar to that observed in a murine join instability model of osteoarthritis with articular cartilage calcification and ossification. The HS tibial tuberosity also showed immature bone tissue. In conclusion, reduced mechanical stimulation caused tibial tuberosity calcification and pathological changes. These findings highlight the importance of optimal exercise to avoid premature pathological structural changes in bones and joints.