Early Alterations of Subchondral Bone in the Rat Anterior Cruciate Ligament Transection Model of Osteoarthritis

Abstract

Objective: Advances in research have shown that the subchondral bone plays an important role in the propagation of cartilage loss and progression of osteoarthritis (OA), but whether the subchondral bone changes precede or lead to articular cartilage loss remains debatable. In order to elucidate the subchondral bone and cartilage changes that occur in early OA, an experiment using anterior cruciate ligament transection (ACLT) induced posttraumatic OA model of the rat knee was conducted.

Design: Forty-two Sprague Dawley rats were divided into 2 groups: the ACLT group and the nonoperated control group. Surgery was conducted on the ACLT group, and subsequently rats from both groups were sacrificed at 1, 2, and 3 weeks postsurgery. Subchondral bone was evaluated using a high-resolution peripheral quantitative computed tomography scanner, while cartilage was histologically evaluated and scored.

Results: A significant reduction in the subchondral trabecular bone thickness and spacing was found as early as 1 week postsurgery in ACLT rats compared with the nonoperated control. This was subsequently followed by a reduction in bone mineral density and bone fractional volume at week 2, and finally a decrease in the trabecular number at week 3. These changes occurred together with cartilage degeneration as reflected by an increasing Mankin score over all 3 weeks.

Conclusions: Significant changes in subchondral bone occur very early in OA concurrent with surface articular cartilage degenerative change suggest that factors affecting bone remodeling and resorption together with cartilage matrix degradation occur very early in the disease.

Keywords: ACLT; HR-pQCT; osteoarthritis; subchondral bone.

Figure 1.

Overview of high-resolution peripheral quantitative computed tomography (HR-pQCT) imaging acquired from a right rat knee. Knees are aligned with the vertical axis of the scanner to obtain sagittal slices cut from lateral to medial (from A to L). Scale bar = 5 mm.

Figure 2.

Manual contouring (green line) of the tibial subchondral trabecular bone is performed on each cut and 3D scans are generated. Scale bar = 5 mm.

Figure 3.

Comparison of subchondral trabecular bone microstructure parameters in ACLT and control groups across the time points. Error bars represent ± 1 standard error of the mean. ACLT = anterior cruciate ligament transection; vBMD = volumetric bone mineral density; BV/TV = bone volume fraction; Tb.N = trabecular number; Tb.Th = trabecular thickness; Tb.Sp = trabecular spacing; *P < 0.05 and **P < 0.005 compared to control; ^#P < 0.05 week 1 versus week 3 ACLT; ^##P < 0.05 week 2 versus week 3 ACLT.

Figure 4.

Representative comparison of HR-pQCT (high-resolution peripheral quantitative computed tomography) images acquired from ACLT (anterior cruciate ligament transection) and control groups across the time points. Changes between groups were hard to discern visually over the short time frame. There appears to be an area of increased radiolucency in the medial tibial epiphysis obvious in ACLT week 3 scans. Otherwise, no subchondral sclerosis, subchondral cysts, or osteophytes were seen. Scale bar = 5 mm.

Figure 5.

Histological assessment demonstrating articular cartilage changes in ACLT (anterior cruciate ligament transection) and control groups across the time points. Coronal sections from the proximal tibia are fixed and stained with Safranin-O and fast green. Small areas of surface proteoglycan loss (black arrowheads) can be seen at 1 week, which increased in surface area at 2 and 3 weeks. There is associated surface irregularities detected at week 2, which progress into superficial fissures (white arrow) by week 3. Scale bar = 200 µm.