Abstract
Summary
In a rabbit model of early osteoarthritis, structural changes in femoral condyle cartilage were severer in the lateral compartment and preceded alterations in the underlying bone. In the medial compartment, altered bone properties occurred together with structural changes in cartilage.
Introduction
Early osteoarthritic changes in cartilage have been previously studied through anterior cruciate ligament transection (ACLT) in rabbits. However, parallel changes in the structure of subchondral and trabecular bone at 4 weeks after ACLT are not known.
Methods
Skeletally mature 14-month old New Zealand white rabbits (n=8) underwent ACLT in the left knee, while right knees were used as controls (CTRL). Femoral condyles (FCs) were harvested at 4 weeks after ACLT.
INDENTATION TESTING
Stepwise stress-relaxation tests were performed on medial and lateral FC cartilage (100%/s ramp rate, 3×5% step, 15 min relaxation time). Sinusoidal loading was then applied (amplitude 4% of thickness, 1Hz, 4 cycles). Equilibrium (Eeq) and dynamic (Ed) moduli were derived from stress-relaxation and sinusoidal tests, respectively.
STRUCTURAL ANALYSIS OF CARTILAGE
Polarised light microscopy (PLM) and digital densitometry (DD) were used to analyze the collagen orientation angle (COA) and proteoglycan content in the cartilage samples.
STRUCTURAL ANALYSIS OF BONE
Distal compartments of FCs were scanned using a high-resolution µCT scanner (Skyscan 1172, Belgium) with an isotropic voxel size of 25 µm. µCT data were imported into Mimics (Materialise, Belgium) for segmentation. 2×2×4 mm3 volumes of interest (VOIs) were placed in weight-bearing regions of medial and lateral FCs. Subchondral bone plate thickness (Pt.Th), trabecular volume fraction (BV/TV), trabecular thickness (Tb.Th), structural model index (SMI) and trabecular separation (Tb.Sp) were calculated using the CTAnalyzer software (Skyscan) from the VOIs.
STATISTICAL TESTS
Mixed linear model for cartilage parameters and Wilcoxon signed-rank test for bone parameters were used to compare ACLT and CTRL groups (p < 0.05).
Results
In both lateral and medial FC compartments, Eeq was significantly smaller in ACLT than in CTRL cartilage. In the medial compartment, also Ed was significantly smaller in ACLT than in CTRL cartilage. As a result of ACLT, significant alterations in the COA extended deeper into cartilage in the lateral than medial compartment, while proteoglycan content was reduced significantly and similarly in both lateral and medial FC cartilages. After ACLT, Pt.Th was significantly reduced in the medial compartment, while no changes were observed in the lateral compartment. Furthermore, only in the medial compartment, both BV/TV and Tb.Th were significantly smaller in the ACLT compared to the CTRL group.
Discussion
The study showed that disruption of the collagen architecture in the ACLT joint cartilage extended into the middle zone only in the lateral FC compartment. Instead, thinning of the subchondral bone plate combined with resorption of trabecular bone was observed only in the medial FC compartment. The former finding reflects early osteoarthritic changes, while the latter finding may be indicative of a diminished loading in the medial FC compartment, as caused by ACLT .
