TIBIAL GROWTH PLATE THICKNESS DYNAMICS IN COLLAGENASE INDUCED RAT KNEE JOINT OSTEOARTHRITIS

Longitudinal bone growth occurs in growth plate (GP) via endochondral ossification, where hyaline cartilage is converted to bone. While it is known that glucocorticoids may prevent bone growth, role of OA in GP morphology is not known. Here we developed an automatic method for quantifying the GP thickness (GP.Th) in rat tibias, in order to investigate how this behaves in collagenase induced OA model.

To quantify the effect of collagenase induced OA to the GP.Th of rat tibia.

Male Wistar rats (N=50) were split into unoperated (NAIVE, n=10), sham injected (SHAM, n=20) and collagenase-injected (COL, n=20) groups, where OA was induced via collagenase-injection into the left hind-limb knee. SHAM group had their knee capsule punctured without injection. Unoperated contralateral joints (COL_CL, SHAM_CL) served as internal controls. The joints were harvested 10 and 60 days after the operation and the tibias were imaged with µCT (Skyscan 1272 Brüker MicroCT, 2.8 µm voxel size) after 22h phosphotungstic-acid (PTA) staining. The µCT-volumes were then down sampled to 11.2 µm resolution. The subsequent analyses are described in Figure 1. AC and subchondral bone (SB) were segmented using U-Net based model. The SB masks were used to select 504 µm deep region below the bone for GP analysis, starting at 56 µm above the bottom edge of the SB. Bone was removed from the GP region using thresholding and denoising, followed by 3D sphere fitting for GP.Th measurement. Maximum projections of GP.Th maps were obtained under manually drawn lateral and medial regions of interest (ROI). Mean GP.Th was calculated for each ROI and the effect of operation was measured using linear mixed effect regression (LMER) RESULTS: The effect of injection and time with 95% confidence intervals (95% CI) to the GP.Th are shown in Figure 2. At the 10-day mark, COL joints have increased GP.Th compared to the COL_CL joints in both lateral (14.8 µm, 95% CI: 4.63 µm to 24.9 µm) and medial (23.6 µm, 95% CI: 14.2 µm to 33.0 µm) regions. Medial GP.Th of COL animals is also increased (12.0 µm, 95% CI: 3.60 µm to 20.3 µm) compared to the NAIVE joints. At 60-day mark, COL joints have decreased GP.Th compared COL_CL joints (-4.10 µm, 95% CI: -7.87 µm to -0.33 µm) and NAIVE joints (-8.36 µm, 95% CI: -14.2 µm to -2.57 µm) on the lateral side. The SHAM animals have significantly lower GP.Th compared to NAIVE joints (-10.1 µm, 95% CI: -18.8 µm to -1.30 µm for SHAM and -11.2 µm, 95% CI: -20.2 µm to -2.15 µm for SHAM_CL) at 10 days on the lateral side, but not on the medial side. The SHAM operated joints show no significant differences in GP.Th compared to the SHAM_CL joints. GP.Th decreases over time, as expected (-34.9 µm 95% CI: -39.2 µm to -30.4 µm on lateral side and -32.9 µm, 95% CI: -37.3 µm to -28.4 µm on medial side).

Collagenase induced OA triggers complex changes in the GP.Th. Collagenase injection appears to increase the GP.Th initially compared to contralateral and naive joints, which then decreases significantly over time. Whether the lower GP.Th is a result of overall thinning of the zone or increased bone bridging will be investigated in a future study.