High amplitude direct compressive strain enhances mechanical properties of scaffold-free tissue-engineered cartilage

Adult cartilage has a limited healing capacity. Damages resulting from disease or injury increase over time and cause severe pain. One approach to reinstate the cartilage function is tissue engineering (TE). However, the generation of TE cartilage is time consuming and expensive and its properties are so far suboptimal. As in vivo cartilage is subject to loading, it is assumed that mechanical stimulation may enhance the quality of TE cartilage. In this study the short-term influence of variable compressive strain amplitudes on mechanical and biochemical properties of scaffold-free TE cartilage was investigated. Primary porcine chondrocytes were isolated, proliferated, redifferentiated, and transferred onto hydroxyapatite carriers, resulting in scaffold-free cartilage-carrier constructs. These constructs were placed in a custom-made bioreactor. Compression amplitudes of 5%, 10%, and 20% were applied. In each experiment four constructs were loaded with dynamic compression (3000 cycles/day, 1 Hz) for 14 days and four constructs served as unloaded control. The cartilage was evaluated biochemically, histological, and mechanically. No difference in glycosaminoglycan or collagen content between the loaded and the control groups was found. However, a positive correlation between compression amplitude and normalized Young's modulus was detected (R(2)=0.59, p<0.001). The highest compression amplitude of 20% had the strongest positive effect on the mechanical properties of the TE cartilage (Young's modulus increase of 241±28% compared to unloaded control). The data presented suggest that preconditioning with higher load amplitudes might be an attractive way of generating stiffer tissue and may help accelerating the cultivation of mechanically competent TE cartilage.