Long-Term Changes in a Non-Invasive Murine Model of Post-Traumatic Osteoarthritis

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Abstract

Current evidence indicates that after non-contact ACL injury, patients have an increased chance of developing post-traumatic osteoarthritis (PTOA). Non-contact ACL rupture is a common injury in athletes, and especially young women. Following ACL rupture OA will commonly develop 10-20 years after injury.

Animal models are useful tools for studying PTOA, since the disease process can be studied in a more controlled environment on a dramatically condensed time line. There have been a number of animal models developed for studying PTOA, but many of these still have significant drawbacks.

A new model developed by the Christiansen lab at U.C. Davis induces ACL injury in mice using a single non-invasive mechanical loading regiment. The knee of the mouse is subjected to a compressive load that anteriorly displaces the tibia, rupturing the ACL. This model closely mimics the injury process in humans, allowing us to study the entire course of the subsequent disease progression. An earlier study using the Christiansen model revealed that the model had some limitations, including creation of avulsion fractures, lack of biomechanical quantification, and only mild development of OA by 8 weeks post-injury. The current study addressed these limitations to further establish the mouse model.

The current study used 64 adult male C57BL/6N mice (10 weeks old at time of injury). 52 mice underwent ACL injury by either avulsion or midsubstance tear (n = 26 per injury mode). 12 mice underwent sham injury. Mice were sacrificed at day 0 (n=12), day 10 (n = 22), and 12 (n = 15) and 16 weeks (n = 15). µCT scans of limbs were performed at day 10, and 12 and 16 weeks. Bone parameters were evaluated for the distal femoral epiphysis. Joint laxity of limbs was measured at day 0, and 12 and 16 weeks. Safranin-O and Fast Green staining of cartilage was assessed using the Orthopedic Research Society International (OARSI) grading scale for 12 and 16 week limbs.

Results showed significantly lower trabecular bone fraction (BV/TV), reduced trabecular thickness (Tb.Th), and reduced apparent bone mineral density (BMD) in the 1 mm/s model compared to the 500 mm/s model at 10 days. This difference between the models is most likely due to the additional bone damage caused by avulsion fracture. By 12 and 16 weeks, there was no difference in bone parameters between the two models. In both models at 12 and 16 weeks there was significant osteophytosis surrounding both joints. Joint laxity results showed significantly increased anterior-posterior (AP) joint laxity at day 0 for both injury rate models compared to uninjured limbs. Joint laxity results at 12 and 16 weeks showed AP joint laxity reduced to control values. Restoration of joint laxity was most likely due to the significant osteophytosis surrounding the joint.

Safranin-O fast green results showed severe OA in injured joints at 12 and 16 weeks. On the posterior tibial plateau there was complete destruction of cartilage and subchondral bone, exposing the growth plate in many joints. Bone on bone contact and meniscal destruction was common. Uninjured control mice showed mild OA, most likely due to natural degeneration from age.

In this study we found significant differences in early trabecular bone changes induced by ACL injury with avulsion compared to ACL injury without avulsion in mice. We also found that the initial loss of knee stability due to ACL injury is restored 12 and 16 weeks post-injury due to considerable osteophyte formation, primarily on the medial aspect of the knee. These findings underline the role of bone turnover in PTOA progression, and support a biomechanical mechanism of osteophyte formation following injury

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