Indentation Method to Map Mechanical Properties of Articular Surface to Identify Degenerated Regions

Materials and Methods: Complete articular surfaces from 8 distal femurs of human tissue donors aged 46 to 64 years were obtained from RTI Surgical. Articular lesions were graded with the visual ICRS classification system. Mechanical properties were mapped ex vivo, using a novel technique allowing for automated indentation mapping of articular surfaces (Mach-1 v500css, Biomomentum Inc.). Subsequently, the thickness was measured with an adapted version of the needle technique. The instantaneous modulus at each position was obtained by fitting the load-displacement curve (with corresponding thickness) to an elastic model in indentation. 72 osteochondral cores were harvested from healthy (ICRS Grade 0) and osteoarthritis (OA)-like regions (ICRS Grade > 0) and scored with the Mankin histological grading system in which higher scores indicate degraded tissue quality. From those cores, 21 were tested in unconfined compression where mechanical properties were extracted from the stress relaxation curve prior to histoprocessing.

Results: OA-like regions were identified mechanically and were wider than the regions identified by visual assessment (Blue-green regions in Fig. 1A). A strong correlation (r = 0.84, p<0.0001) was observed between the mechanical properties measured in indentation and in unconfined compression (Fig. 1B) while correlation with the Mankin score was lower (r = −0.39, p<0.0007).