During the development of osteoarthritis (OA), subchondral bone undergoes substantial changes in structure and strength prior to the onset of cartilage damage. Consequently, bone anti-resorptive agents such as bisphosphonates have been investigated for possible mitigating effects on the progression of OA. Results thus far have shown promise, with prior studies presenting evidence of strong bone turnover and angiogenesis-inhibiting capabilities as well as chondroprotection. Alendronate, a commonly used bisphosphonate for osteoporosis, has specifically been shown to prevent osteophyte growth in a dose dependent manner during OA. However, prior work investigating alendronate in OA is limited in several aspects including invasive or non-physiologic injury methods, and relatively late time points of interest, which may miss important early stage events. We sought to address these limitations using a non-invasive ACL rupture model in mice to analyze the effect of alendronate on PTOA development as early as 1 week following injury. We hypothesized that alendronate treatment would inhibit early trabecular bone loss from the epiphysis, and would decrease long-term osteophyte formation and cartilage degeneration after joint injury.

ACL injury via tibial compression overload was performed on 54 mice as previously described. Another 36 mice were subjected to sham injury. Mice were treated with low-dose alendronate (0.08 mg/kg/week), high-dose alendronate (2.0 mg/kg/week), or vehicle, starting immediately after injury and lasting until sacrifice at 7, 14 or 56 days. Blood serum was analyzed for crosslinked C-terminal telopeptide of type I collagen (CTX-I), a biomarker of bone resoprtion, and procollagen type 1 N-terminal propeptide (P1NP), a biomarker of bone formation. Whole joints were scanned using micro-computed tomography (ȝCT), and structural analysis was performed at the trabecular bone of the femoral epiphysis. Osteophyte volume was also measured on 56- day samples. Whole-joint histology was performed to analyze changes in articular cartilage. MicroCT analysis revealed significant loss of trabecular bone from the femoral epiphysis due to injury by 7 and 14 days post-injury, consistent with our previous studies. This loss of epiphyseal trabecular bone was not inhibited by low-dose alendronate treatment, however high-dose alendronate treatment was able to fully prevent the initial loss of trabecular bone associated with knee injury. By 56 days post-injury, vehicle treated mice still exhibited a deficit in trabecular bone volume in the injured knee compared to the uninjured knee. However, both low-dose and high-dose alendronate treatment were able to restore trabecular bone volume at this time point. Contrary to previously reported results, alendronate treatment was not able to reduce osteophyte volume at 56 days post-injury. Whole-joint histology scores of tibial and femoral articular cartilage revealed considerable degeneration in all injured knees, with no differences in OA development due to alendronate treatment. Analysis of serum biomarkers revealed a nearly 5-fold increase in CTX-I (bone resorption) induced by joint injury by 14 days post-injury. Alendronate treatment was able to significantly reduce this bone resorption at early time points, although CTX-I levels remained higher than in uninjured mice. We observed no changes in P1NP levels (bone formation) due to injury or alendronate treatment.

Consistent with our hypothesis, alendronate was able to prevent early trabecular bone loss following non-invasive joint injury, particularly at high dose. However, contrary to our hypothesis, alendronate was not able to prevent osteophyte formation, nor was it able to affect long-term articular cartilage loss or joint degeneration. These results are in contrast to previous studies, which showed that alendronate treatment was chondroprotective and inhibited osteophyte formation. Our results also suggest that subchondral bone changes initiated by joint injury are due to increased bone resorption, with little change in bone formation at any of the time points quantified. Altogether, these data contribute to understanding the role of bone in osteoarthritis development, and call into question the efficacy of using anti-resorptive agents for slowing or preventing osteoarthritis.