Background.

The articular cartilage of the femoral condyles is at the distal end of the femur and is a main load-bearing tissue of the knee. During skeletal growth, the condyles expand substantially, increasing in diameter in humans by ~54% from age 11 to skeletal maturity. During such growth, articular cartilage stretches biaxially, including in the anterior-posterior and medial-lateral directions. However, most previous studies of articular cartilage tensile properties have used a uniaxial configuration. Hypothesis & Aims. We hypothesised that a biaxial stretch device would reveal that the tensile properties of articular cartilage are affected by stretch in both of the orthogonal axes. Thus, the aims of this study were (1) to develop a biaxial test system, and (2) to use the device to test if biaxial and uniaxial tensile properties of articular cartilage are different.

Methods

 A biaxial test instrument was constructed to allow application of displacement and measurement of load along both vertical and horizontal axes. The system consisted of a scaffold system, pulleys, and two electronic scales. The scaffold provided a rigid vertical test frame with a horizontal cross-bar. Along the vertical axis, stretch was controlled by manual advancement of the cross bar along threaded rods, with load measurement via a scale underneath. Along the horizontal axis, stretch was controlled by manual movement of lines, redirected by pulleys, one of which pulled against a scale for load measurement. Cross-shaped samples were attached to the test instrument via four lines, two vertical and two horizontal, attached to clamps that secured the sample. An articular cartilage sample was tested in uniaxial and biaxial tension. A porcine knee joint was obtained from a local butcher. An articular cartilage specimen, with the articular surface intact, was isolated by dissecting out a cruciform shape and undercutting the articular cartilage to release a sample with an (8.1mm) 2 central region, four (8.1mm) 2 arms with ~2mm for grasping, and a thickness of 1 mm. Samples were subjected to (1,2) uniaxial tests, in the (1) vertical and (2) horizontal directions, and (3) a biaxial test, with strains of 0% or 18%, 36%, or 54% in along one or both axes according to the test. Photographs and loads were recorded after each test increment.

Results

The desired strains (±1%) were achieved by adjusting the loading lines. The uniaxial loads (in Newtons) at 18, 36, and 54% extension were 7.74±1.24, 21.07±0.69, and 45.57±7.62 in the stretch direction and much lower 1.67±0.42 (-78%), 5.38±0.54 (-74%), and 10.07±2.46 (-78%) in the orthogonal direction (held at 0 extension). The biaxial loads in the stretch direction were substantially larger, 8.38±0.62 (+8%), 24.31±0.42 (+15%), and 61.25±6.24 (+34%) N, respectively.

Discussion 

These results (1) establish a simple but functional biaxial tensile test system, and (2) confirm preliminarily that loads along the stretched axis are much greater with biaxial than uniaxial testing. Additional samples are currently being tested.