In vivo deformation of articular cartilage during electrically stimulated contraction

Abstract

Purpose: In vivo movement of bones comprising synovial joints has been evaluated in various ways and has been calculated theoretically to understand joint biomechanics. In order to assess joint loading and bone movements accurately, the deformation behavior of articular cartilage has been investigated in confined and unconfined compression experiments and indentation tests (references), but to date there exist no data on dynamic in vivo deformation of articular cartilage in intact joints loaded through controlled muscular contraction. The objective of this study was to measure articular cartilage deformations as a function of load in an intact joint of live animals. Methods: In order to achieve the purpose of this study, we developed a novel in vivo testing system that allows for controlled loading of mouse knees through muscular contractions and quantification of the associated, cartilage and chondrocyte deformations. Mice were fixed in a custom-built jig onto the stage of a dual photon microscope. Controlled forces of the knee extensor muscles were produced through direct muscle stimulation using indwelling fine wire electrodes and a Grass (S8800) stimulator. The medial meniscus was excised to provide a direct view of the medial tibio-femoral joint (Figure 1). Imaging of articular cartilage deformation across the entire medial compartment of the knees was performed using a Zeiss 40×0.95 NA water-immersion objective coupled with a Coherent Chameleon IR laser tuned at 780 nm for two-photon excitation. Results: Increases in muscular loading of the knee caused an increase in articular cartilage deformation (Figure 2). Fifty and 80% of the maximal muscular forces produced average peak articular cartilage strains for an 8s contraction of 10 ± 1% to 19 ± 2% respectively (Figure 2). The average overall cartilage thickness was 32 ± 2 μm on the femoral condyles. For slow increasing forces (Figure 3a), the tibio-femoral joint space initially narrowed and reached zero (i.e., the two cartilage surfaces started to touch) when muscle forces reached approximately 40% of maximal isometric values (Figure 3b). Following cartilage contact, cartilage thickness decreased in an approximately exponential manner and reached the deformation peak approximately 5s after initial force application (Figure 3c). Cartilage tissue recovered to its original shape within approximately 30s following force removal (Figure 3c). Discussions And Conclusions: The results of this study suggest that tibio-femoral contact between the articulating surfaces is only achieved with relatively high muscular forces (about 40% of maximal). This may be an artifact of the current setup where the medial meniscus was removed, and physical contact between the two articulating surfaces may occur earlier (i.e., at lower forces) if the meniscus had been left intact. This will need to be investigated further. However, the finding seems to suggest, that at least in the presence of medial meniscectomy, high forces are required to obtain physical cartilage to cartilage contact in the medial tibio-femoral compartment of the mouse knee. Another surprising feature of this work was that cartilage behaves highly visco-elastically. Specifically, peak cartilage deformation is reached much after peak muscle forces have been reached. The delay of about 3s from peak force to peak cartilage deformation suggests that in a normal cyclic movement, such as walking (loading time approximately :800ms) peak steady-state deformation corresponding to the applied force is never reached. Similarly, full cartilage thickness recovery following load removal takes approximately 20s, suggesting that deformation recovery within cyclic movements such as walking, with an unloading time of about 1s, may be neglected. Finally, peak cartilage deformation under the present conditions seems to be maximal at muscle forces of about 70% of the maximal isometric force with further deformations with higher forces not detected by our system. This begs the question of stiffness changes in cartilage with increasing loads and the mechanisms underlying extreme deformation of cartilage under very high joint loading conditions.Figure 2Articular cartilage surface do not touch below approximately 40% of maximal muscle force, then cartilage strain increases exponentially between about 40 – 70% of the maximal force, and remains almost constant beyond about 70 – 80% of maximal force (n = 6).View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 3(a) Normalized (relative to maximum = 100%) knee extensor forces as a function of time. Muscles were stimulated for 8 seconds at a voltage and frequency producing approximately 60% and 35% of the maximal isometric force. (b) Medial tibio-femorol joint space as a function of time. Joint space decreased as muscular force increased and reached zero when force reached approximately 40% of maximum. (c) In vivo femoral cartilage thickness changes during loading with 60% of muscular force. No sign of cartilage deformation was noticed at 35% maximal force.View Large Image Figure ViewerDownload Hi-res image Download (PPT)