Tissue modification of the lateral compartment of the tibio-femoral joint following in vivo varus loading in the rat

Abstract

This study describes the first application of a varus loading device (VLD) to the rat hind limb to study the role of sustained altered compressive loading and its relationship to the initiation of degenerative changes to the tibio-femoral joint. The VLD applies decreased compressive load to the lateral compartment and increased compressive load to the medial compartment of the tibio-femoral joint in a controlled manner. Mature rats were randomized into one of three groups: unoperated control, 0% (sham), or 80% body weight (BW). Devices were attached to an animal's leg to deliver altered loads of 0% and 80% BW to the experimental knee for 12 weeks. Compartment-specific material properties of the tibial cartilage and subchondral bone were determined using indentation tests. Articular cartilage, calcified cartilage, and subchondral bone thicknesses, articular cartilage cellularity, and degeneration score were determined histologically. Joint tissues were sensitive to 12 weeks of decreased compressive loading in the lateral compartment with articular cartilage thickness decreased in the peripheral region, subchondral bone thickness increased, and cellularity of the midline region decreased in the 80% BW group as compared to the 0% BW group. The medial compartment revealed trends for diminished cellularity and aggregate modulus with increased loading. The rat-VLD model provides a new system to evaluate altered quantified levels of chronic in vivo loading without disruption of the joint capsule while maintaining full use of the knee. These results reveal a greater sensitivity of tissue parameters to decreased loading versus increased loading of 80% BW for 12 weeks in the rat. This model will allow future mechanistic studies that focus on the initiation and progression of degenerative changes with increased exposure in both magnitude and time to altered compressive loads.

Fig. 1

Application of the varus loading device (VLD) to the rat. (a) Lateral view of the VLD applied to rat femur and tibia via transcutaneous bone plates secured with bone screws. The axis of the bearing is aligned with the epicondylar axis of the femur utilizing fluoroscopy. Setting the torque of the torsion spring applies a varus moment to the distal tibia, which results in an increased compressive load (+ΔP) in the medial compartment (M) and decreased load (–ΔP) in the lateral compartment (L) of the tibio-femoral joint. (b) Lateral view of the VLD disengaged. To disengage the VLD, the offset link and a portion of the load link are removed and the remaining load link is rotated into alignment with the femur tube and secured, thus removing the varus moment. (c) Anterior-posterior view of rat femur and tibia with a VLD attached and engaged, illustrating the delivery of a compressive overload to the medial (M) compartment and a decrease in loading in the lateral (L) compartment. ((d) and (e)) Rat with VLD attached and engaged. (Adapted from Roemhildt et al. [11]).

Fig. 2

(a) Superior view of left tibia plateau illustrating locations of mechanical test sites (•) in the medial and lateral compartments of the tibial plateau. (b) Safranin-O- and Fast Green-stained section illustrating identification of the articular cartilage surface (black), tidemark (dark green), calcified cartilage/subchondral bone boundary (light green), and subchondral bone plate (red) at 61 points (gray vertical lines) across each compartment. Yellow vertical lines divide the compartment into three equal-width regions (peripheral, central, and midline).

Fig. 3

Representative H&E stained sections from 0% BW (a) and 100% BW (b) groups, suggesting diminished chondrocytes with increased loading of the medial compartment