Imaging longitudinal changes in articular cartilage and bone following doxycycline treatment in a rabbit anterior cruciate ligament transection model of osteoarthritis

Abstract

Objective: The development of osteoarthritis following traumatic anterior cruciate ligament (ACL) injury is well established. However, few reliable indicators of early osteoarthritic changes have been established, which has limited the development of effective therapies. T(1ρ) and T(2) mapping techniques have the ability to provide highly accurate and quantitative measurements of articular cartilage degeneration in vivo. Relating these cartilaginous changes to high-resolution bone-densitometric evaluations of the late-stage osteoarthritic bone is crucial in elucidating the mechanisms of development of traumatic osteoarthritis (OA) and potential therapies for early- or late-stage intervention.

Methods: Twelve rabbits were monitored with in vivo magnetic resonance imaging (MRI) scans following ACL transection surgery with a contralateral leg sham operation. Six of the rabbits were treated with oral doxycycline for the duration of the experiment. At 12 weeks, the excised knees from three animals from each group (n=6 overall) were subjected to micro-computed tomography (CT) analysis.

Results: Consistent with previous studies, initial elevations in T(1ρ) and T(2) values in ACL-transected animals were observed with relative normalization towards values see in sham-operated legs over the 12-week study. This biphasic pattern could hold diagnostic potential to differentiate osteoarthritic cartilage by tracking the relative proportions of T(1ρ) and T(2) values as they rise with inflammation then fall as collagen and proteoglycan loss leads to further dehydration. The addition of doxycycline resulted in inconclusive, yet potentially interesting, cartilaginous changes in several compartments of the rabbit legs. Micro-CT studies demonstrated decreased bone densitometrics in ACL-transected knees. Correlation studies suggest that the cartilaginous changes may be associated with some aspects of bony change and the development of OA.

Conclusion: We conclude that there are definite relationships between cartilaginous changes as seen on MRI and late-stage microstructural bony changes after traumatic ACL injury in rabbits. In addition, doxycycline may show promise in mitigating early-stage cartilage damage that may serve to lessen late-stage osteoarthritic changes. This study demonstrates the ability to track OA progression and therapeutic efficacy with imaging modalities in vivo.

Fig. 1

Representative T_1ρ (A) and T₂ (B) maps of ACL-transected knee and the sham-operated knee in the same rabbit. Maps are overlaid on SPGR images. Images were acquired 3 weeks after surgery. An increase in both T_1ρ and T₂ values on the weight-bearing portions of the femoral condyles in ACL-transected knees as compared to sham-operated knees highlights the typical changes in MRI signal found with osteoarthritic changes.

Fig. 2

Example of images obtained using ex vivo µCT from the femur of an ACL-transected rabbit (A) and sham-operated rabbit (B), and the tibia of an ACL-transected rabbit (C) and sham-operated rabbit (D). Some macroscopic bony changes in ACL-transected rabbits can be appreciated, such as osteophytes and thickened, roughened cortical layers, as indicated by arrowheads. Sample imaged volumetric regions of interest of the femur (E) and tibia (F) with the medial compartments colored red and green and the lateral compartments shaded blue and orange are shown for illustration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

µCT results normalized to the average of each parameter across all compartments and groups. Only parameters that showed significant bone quality differences between ACL-transected and sham-operated knees in either the femur or tibia are shown for the LFC (A), MFC (B), LT (C) and MT (D) of untreated animals. Data represent three untreated rabbits from a group of six in which half were treated with doxycycline. Each animal had an ACL transection surgery (ACLX) on the left knee and a sham operation on the right knee. Error bars represent the 95% confidence interval of the normalized means. There is no significant difference in any of the bone quality values when considering only doxycycline-treated animals (data not shown). *P<.05 (specific values listed in Results).