Testing of the pHEMA hydrogel as an implantation material for replacement of osteochondral defects in animals

Objective: to evaluate the features of reparative chondrogenesis and osteogenesis in animal experiments with the implantation of porous poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel into osteochondral defects. Materials and methods. Cylindrical pHEMA implants (5 mm in diameter) were synthesized by radical polymerization. The implants were subjected to light microscopy and mechanical tests to characterize the structure and viscoelastic properties of the material. In experimental group #1, four pHEMA specimens were implanted into formed defects in the distal femoral epiphysis of rabbits. In experimental group #2, allogeneic chondrocytes were applied to the surface of four specimens before implantation. In the control series, four defects were not replaced with implants. Tissue regeneration was investigated by morphological and morphometric methods 30 days after operation. Results. The pHEMA implants were heterogeneous specimens with irregularly shaped pores – up to 30 × 10 μm at the surface and 300 × 120 μm inside. With >10% static compressive stress, the Young’s modulus was 54.7 kPa. For dynamic stress, increased frequency of compression-relaxation cycles from 0.01 Hz to 20.0 Hz led to increased storage modulus from 20 kPa to 38 kPa on average, and increased loss modulus from 2 kPa to 10 kPa. Indicators of semi-quantitative assessment of local inflammatory response to pHEMA implantation had the following values in points: pHEMA, 4.7 ± 0.3; pHEMA with allogeneic chondrocytes, 6.0 ± 1.0; control, 4.3 ± 0.3. The ratio of connective, bone, and cartilage tissues proper in the regenerates had the following respective values: pHEMA, 79%, 20%, 1%; pHEMA with chondrocytes, 82%, 16%, 2%; control, 9%, 74%, 17%. Conclusion. In a short-term experiment, pHEMA implants did not trigger a pronounced inflammatory response in the surrounding tissues and can be classified as biocompatible materials. However, the tested implants had low conductivity with respect to bone and cartilage cells, which can be improved by stabilizing the pore size and increasing the rigidity when synthesizing the material.