A comprehensive testing protocol for macro-scale mechanical characterization of knee articular cartilage with documented experimental repeatability

Abstract: Articular cartilage mechanics has been extensively studied with various approaches and mechanical characterization strategies. However testing protocols can be highly varying and difficult to reproduce, particularly for specimen-specific analyses. Detailed knowledge of testing protocols is important for reliable use in concordant finite element analyses. This study presents a detailed, robust procedure for cartilage testing—with multiple regions and per specimen repeatability data. Samples were taken from femur, tibia and patella of a human cadaver knee and tested in unconfined compression, confined compression and uniaxial tension. Each test was repeated three times. The testing protocols provide elastic and time dependent characterization data. Results, for example equilibrium modulus of 0.28 (0.0024) MPa for patella under unconfined compression indicate that variability is well controlled and that protocol(s) presented here can generate repeatable specimen-specific data. As per the authors' knowledge this is the first study to report in-depth uncertainty assessment of the experimental procedures for multi-region knee cartilage characterization.

Testing equipment and testing environment All the tests were conducted on a Mach-1 mechanical testing machine (Biomomentum Inc, Laval, Québec, Canada), with a 10 kg load cell (resolution: 0.5 g). Three types of tests were conducted: confined compression (CC), unconfined compression (UC) and uniaxial tension (T) (Figure 2, middle panel). Each of the 6 compression samples was tested under both confined and unconfined compression. Custom test chambers were designed for each of the test type. For unconfined compression, the sample simply sat on an acrylic plate that attached to the bottom of the test chamber and a flat indenter (with a diameter larger than the sample) was used to compress the sample. For confined compression, a compression chamber made of two parallel acrylic plates with a cylindrical cavity on the top plate was designed. Once the sample was placed in the compression chamber, a stainless steel filter (5 mm diameter, 5 μm pore size, thickness 1 mm) was placed on top of the sample. A 3 mm diameter indenter rod was then used to compress the sample by pushing on the filter. Custom saw tooth stainless steel clamps were used to grip the tensile samples. Additionally, tissue adhesive was used to ensure that the samples would not slip out of the grips. All the tests were conducted at room temperature and the samples were immersed in PBS for the entire duration of the test. The tensile samples were marked with a marker pen and a ruler was placed in the bath parallel to the tensile sample for potential optical strain measurement once the images were captured using the camera system of Mach-1.

Keywords: Human knee, articular cartilage, experimental repeatability.