Novel Technique to Map the Biomechanical Properties of Entire Articular Surfaces Using Indentation to Identify Degenerated (Osteoarthritis-like) Cartilage

Methods: The distal femurs of eight human tissue donors aged 46 to 64 years were obtained from RTI Surgical (FL,USA). Articular lesions were graded ex vivo with the visual ICRS classification system (Fig.1A). Mechanical properties were mapped ex vivo, using a novel technique allowing for automated alignment and mapping of articular surfaces (Mach-1 v500css, Biomomentum Inc., Laval, Canada). Subsequently, the thickness was measured with an adapted version of the needle technique. The instantaneous modulus (IM) at each position (Fig.1B) was obtained by fitting the load-displacement curve (with corresponding thickness) to an elastic model in indentation (Hayes,1972). 21 osteochondral cores (3.5mm diameter) were then harvested from healthy (ICRS Grade 0) and degenerated (OA-like) regions (ICRS Grade > 0) and tested in unconfined compression where mechanical properties were calculated from the stress relaxation curve.

Results: Average mappings of the IM (Fig.1C) were created from healthy regions (ICRS Grade 0). OA-like regions were identified mechanically and were wider than the regions identified by visual assessment (Blue-green regions in Fig.1D). Unpaired t-tests revealed a significant decrease of the IM in indentation (p=0.04) and of the fibril modulus in unconfined compression (p=0.02) in OA-like vs. healthy cartilage. A strong correlation (r=0.84, p<0.0001) was observed between the mechanical properties measured in indentation and in unconfined compression.