Mappings of Thickness and Mechanical Properties of the Stifle (Knee) Joint Cartilage Reveal Low Intraspecies Variability

INTRODUCTION

Ex vivo assessment of an articular surface for detection of cartilage degeneration, focal defects or to evaluate the outcome of a repair treatment can be done by comparing the measured thickness and mechanical properties to normal tissue values at matched anatomical locations in similar animals. To do so, reference mappings created by averaging mapping data from OA-asymptomatic (normal) samples can be used. The purpose of this study was to present reference normal mappings of thickness and mechanical properties (instantaneous modulus) for different species and to highlight the low intraspecies variability of normal samples with the objective of providing normal reference values for the evaluation of articular cartilage health.

METHODS

Thickness and mechanical properties (instantaneous modulus using spherical indentation) of normal entire articular surfaces (femoral condyles and tibial plateau) of many samples from different species (rabbits, pigs, sheep, cows and human) were mapped ex vivo using a multiaxial mechanical tester (Mach-1 v500css, Biomomentum, Canada). For each species and articular surface, several samples were used to obtain average mapping values (Fig. 1). By interpolating and averaging the thickness and instantaneous modulus values obtained from all normal samples of each species, reference average mappings were generated. Since average mappings were generated using many samples, the intraspecies variability was calculated using the coefficient of variation (standard deviation/mean) for values obtained at anatomically matched position.

RESULTS

Reference average mappings of the thickness and instantaneous modulus for the femoral condyles and tibial plateaus of all species are presented in Figure 1 with average, minimum and maximum values of the parameters. Each species presents a unique distribution pattern of thickness and instantaneous modulus over the femoral condyles and tibial plateau. The studied parameters also present various large ranges of values from one species to another. In addition, both parameters (thickness and instantaneous modulus) vary independently between species, e.g. human presents an average cartilage thickness twice as large as pig but a similar range of instantaneous modulus. Mapping results and preliminary analyses demonstrate very uniform coefficients of variation for both surfaces in all species individually, although statistical results have yet to be calculated for all besides the sheep. By averaging values obtained over the entire articular surface for the sheep, a coefficient of variation of 0.22 ± 0.04 was obtained for the femoral condyles thickness and 0.22 ± 0.11 mm for the tibial plateau thickness. For the instantaneous modulus, an average coefficient of variation of 0.37 ± 0.08 and 0.34 ± 0.12 were obtained respectively for the femoral condyles and the tibial plateau.

DISCUSSION

Reference mappings of thickness and instantaneous modulus (Fig. 1) show that interspecies variability for both parameters is important. Nonetheless, according to the coefficient of variation results obtained from the sheep, intraspecies variability is very low, which indicates that generating valid reference maps requires only a small amount of control samples. Therefore, using average reference mappings of a species and articular surface of interest, as a way of comparing cartilage after a treatment, is a very efficient method. In this study, variability according to sample-specific characteristics such as age, gender, breed/ethnicity was not explored. Therefore, it is important to use reference average mappings that are representative of the treatment group used in a study to obtain valid results.

SIGNIFICANCE/CLINICAL RELEVANCE

Reference mappings provide a diagnostic tool for cartilage health essential in developing cartilage repair strategies. Reference mappings can be generated for any biological parameter characterizing cartilage health.