Abstract: The aim of this study was to analyze cartilage creep and creep recovery after static and cyclic loading. Up to now technical limitations have hindered the measurement of creep recovery. For this reason, we developed a closed loop micro creep and creep-recovery indentation test system with active force control. Bovine osteochondral explants were tested under static (110 kPa) and cyclic (100 cycles,
350 kPa/35 kPa, 1 Hz) loading conditions and subsequently creep recovery was measured. The cartilage thickness was determined using needle indentation. For static loading, the creep and creep-recovery rates were significantly different during the first 60 seconds (p < 0.05). Cyclic loading was assessed for the medial and lateral patella and resulted in a physiological patellar cartilage strain of 7.7% ± 2.6% and 8.2% ± 2.7%, respectively. We recorded a creep recovery of 97.8% ± 2.1% for the medial and 98.3% ± 2.4% for the lateral patella. The advantage of this study over earlier in vitro studies is that we recorded creep-recovery profiles with an actively controlled setup. This allowed us to analyze creep recovery immediately after removing the creep load, in contrast to MRI-based in vivo studies. In future, the presented method will enable us to quantify spatial variations within articular joints.

Key words: articular cartilage, indentation, creep recovery, arthritis, regenerative tissue