Persisting side-to-side differences in bone mineral content, but not in muscle strength and tendon stiffness after anterior cruciate ligament reconstruction

Tendon stiffness may be involved in limiting peak musculoskeletal forces and thus may constitute an upper limit for bone strength. The patellar tendon bone (PTB) graft, which is harvested from the patellar tendon during surgical reconstruction of the anterior cruciate ligament (ACL), is an ideal scenario to test this hypothesis. Eleven participants were recruited who had undergone surgical reconstruction of the ACL with a PTB graft 1-10 years prior to study inclusion. As previously reported, there was no side-to-side difference in thigh muscle cross-sectional area, in maximum voluntary knee extension torque, or in patellar tendon stiffness, suggesting full recovery of musculature and tendon. However, in the present study bone mineral content (BMC), assessed by peripheral quantitative computed tomography, was lower on the operated side than on the control side in four regions studied (P = 0·0019). Differences were less pronounced in the two sites directly affected by the operation (patella and tibia epiphysis) when compared to the more remote sites. Moreover, significant side-to-side differences were found in BMC in the trabecular compartment in the femoral and tibial epiphysis (P = 0·004 and P = 0·047, respectively) with reductions on the operated side, but increased in the patella (P = 0·00016). Cortical BMC, by contrast, was lower on the operated side at all sites except the tibia epiphysis (P = 0·09). These findings suggest that impaired recovery of BMC following ACL reconstruction is not because of lack of recovery of knee extensor strength or patellar tendon stiffness. The responsible mechanisms still remain to be determined.