Introduction: Adequate congruency and primary stability are vital for good long-term results after mosaicplasty. The strength of press-fit stability of the grafts depends upon the length and diameter of the graft, extent of dilation and bone quality. The aim of our study was to quantify the effect of graft diameter and dilation length on the primary stability of single osteochondral grafts against compression and compare the stability of single and multiple osteochondral grafts in an in vitro biomechanical animal model.

Methods: In the single graft series one osteochondral graft was transplanted from the trochlea of porcine femurs to the weight-bearing area of the lateral femoral condyle, while in the multiple graft series three grafts were transplanted in a row or in circular fashion in the same position. We used the MosaicPlasty instruments (Acufex, Smith & Nephew Inc. MA, USA). The specimen was installed on a testing machine (Computer controlled ZWICK FR005TH type tensile machine, Zwick GmbH Ulm, Germany) and the graft was first pushed in level with the surrounding cartilage surface, then it was pushed 3 mm deeper. The push-in forces were measured and the compression curve was registered.

Results: In the case of single 4.5-mm grafts, the mean level push-in force was 43.5 N, pushing 3 mm deeper needed a mean of 92.5 N (n=13). In the case of single 6.5-mm grafts, level push-in needed a mean of 76.2 N, while for pushing 3 mm deeper a mean of 122.2 N force had to be used (n=14). The length of the drill-hole and the dilation were both 20 mm in each setting. When using 20 mm long drill-holes and 15 mm dilation length, the values above were found to be 36.6 N and 122.5 N in the case of 4.5-mm grafts (n=12).

In case of multiple grafting level push-in needed a mean force of 31.8 N in the row series, while pushing 3 mm deeper needed a mean of 52.17 N (n=7). In the circle series level push-in needed a mean of 30.44 N, while for pushing 3 mm deeper a mean of 54.33 N force had to be used (n=9).

Conclusions: These results suggest that grafts of greater diameter are more stable in absolute values and the stability may be increased by shorter dilation length, while level push-in forces do not increase significantly. Multiple grafts may not be as stable as single grafts after transplantation and transplantation in a row or in circular fashion does not influence stability.