When synovial joints are loaded, the articular cartilage and the cells residing in it deform. Cartilage deformation has been related to structural tissue damage, and cell deformation has been associated with cell signalling and corresponding anabolic and catabolic responses. Despite the acknowledged importance of cartilage and cell deformation, there are no dynamic data on these measures from joints of live animals using muscular load application. Research in this area has typically been done using confined and unconfined loading configurations and indentation testing. These loading conditions can be well controlled and allow for accurate measurements of cartilage and cell deformations, but they have little to do with the contact mechanics occurring in a joint where non-congruent cartilage surfaces with different material and functional properties are pressed against each other by muscular forces. The aim of this study was to measure in vivo, real time articular cartilage deformations for precisely controlled static and dynamic muscular loading conditions in the knees of mice. Fifty and 80% of the maximal knee extensor muscular force (equivalent to approximately 0.4N and 0.6N) produced average peak articular cartilage strains of 10.5±1.0% and 18.3±1.3% (Mean ± SD), respectively, during 8s contractions.

A sequence of 15 repeat, isometric muscular contractions (0.5s on, 3. Tabledit Mac Serials. 5s off) of 50% and 80% of maximal muscular force produced cartilage strains of 3.0±1.1% and 9.6±1.5% (Mean ± SD) on the femoral condyles of the mouse knee. Cartilage thickness recovery following mechanical compression was highly viscoelastic and took almost 50s following force removal in the static tests. Introduction Articular cartilage is a hydrated fibre composite material that covers the articular surfaces of bones in synovial joints. It consists of cells (chondrocytes) that occupy 2–15% of the volumetric fraction, and an intercellular matrix (85–98% of total volumetric fraction) with 65–80% water content [;]. The primary functions of articular cartilage include the transmission and distribution of forces to minimize stress concentrations, and to provide smooth areas for the gliding of articulating joint surfaces. Mechanical loading of joints, and the associated cartilage deformations, have been implicated as primary reasons for the development and progression of osteoarthritis (OA) [].

The deformational behavior of articular cartilage from excised tissue samples has been investigated thoroughly in confined and unconfined compression with metallic indentation devices [–]. In addition, cartilage deformations have been calculated based on theoretical tissue models [–] in an attempt to understand joint biomechanics. In-vivo studies have been performed using MR imaging to describe changes in thickness of knee joint cartilage after activities such as bending, normal gait, and squatting [–]. These previous studies were limited to measuring cartilage deformations during steady-state conditions following a loading protocol and although they likely reflect the cartilage response to physiological loading conditions, they are not time-sensitive enough to measure the continuous cartilage deformations during the mechanical loading of a joint.

Cartilage deformation is known to cause deformations of the chondrocytes and their nuclei [;–], and these deformations, in turn, are known to affect the biological signaling response of chondrocytes that control the maintenance and adaptation of the tissue [–]. However, the pathways from joint loading, to global and local cartilage deformation, the associated cell deformations, and the corresponding cellular responses remain unexplored in intact joints, in part due to the difficulties of loading joints in a controlled, physiological manner and simultaneously measuring cell responses. Recently, we developed a novel in vivo testing system that allows for controlled loading of mouse knees through muscular contraction and permits the quantification of the associated chondrocyte deformations. This system has also been used for analyzing changes in synovial fluid composition following controlled loading of knees [;]. It can also be used to measure chondrocyte signaling responses associated with joint loading. Results from these studies showed that chondrocyte mechanics are different in joints compared to the traditional in situ and in vitro approaches. For example, cells deform quickly (within seconds) upon joint loading but take minutes to recover their original, pre-load shapes following load removal [].