Introduction 6 13 14 9 15 21 8 10 6 11 15 5 18 20 6 11 The objective of the present study was to investigate the biomechanical effectiveness of osteochondral transplantation. More specifically we assessed whether the treatment would decrease peak stresses at the boundaries of the original articular cartilage defect. Subsequently, we analysed if articular stresses were dependent on plug placement (bottomed versus unbottomed) and how this was affected by loads that represented (for instance unintended) early weight bearing. Materials and methods Materials 1 4 2 Fig. 1 2  Preparations 1 19 2 2 8 Fig. 2 a b c d e  Operation and testing 2 2 To investigate the consequences of a cartilage defect and subsequent mosaicplasty on the stress distribution under various conditions, Tekscan recordings were performed under five different conditions: (A) preoperatively, intact condition, (B) cartilage defect without any reconstruction, (C) following mosaicplasty, (D) after mosaicplasty and subsequent non-weight bearing motion of the knee and (E) after mosaicplasty with weight bearing motion (E). All surface measurements were performed with the knee in 0° extension, thereby ensuring loading of the (restored) defective sites, and an axial load of 350 N. The ‘preoperative’ measurement (A) was with intact condyles. After removing the sensors, both on the medial and lateral condyle a standardized subchondral defect, 8 mm deep was created by a circular drill of 16 mm diameter, the center being at exactly the same location as the center of the plastic marker that was measured previously. 2 2 The ‘mosaicplasty’ surface measurement (C) was executed after performing both mosaicplasties. The ‘non-weight bearing motion’ measurement (D) was performed following flexing and extending the knee 20 times, while no weight was attached to the test device. The axial load of 350 N was re-applied before the measurement was performed. The final ‘weight bearing motion’ measurement (E), was performed once the knee had been flexed and extended 20 times with 350 N of axial force applied to the knee while performing the flexion-extension movements. Evaluation of variables Each of the eight knees provided two paired standardized mosaicplasties of three plugs, one bottomed and one unbottomed. Thus, there were 16 mosaicplasties: eight bottomed and eight unbottomed. Three general groups were created for statistical evaluation: the whole group of all mosaicplasties (bottomed and un-bottomed combined), a group with only bottomed and one with only unbottomed mosaicplasties. The five measurements performed were: (A) preoperative, (B) defect, (C) mosaicplasty, (D) non-weight bearing motion, (E) and weight bearing motion. 3 3 Fig. 3 a b  Statistics Contact pressures were calculated at the boundaries of the cartilage defect (region 2) as well as for the defect/mosaicplasty (region 1) itself. This was done for five different conditions as described before (A–E). These values were expressed for three groups, namely bottomed, unbottomed and combined. P Results 4 Fig. 4 blue line green line grey line b a c c–e 1  5 Fig. 5 blue line green line grey line b c a c–e 1  Two out of eight unbottomed versus zero bottomed mosaicplasties showed a decreased mosaicplasty pressure and increased rim stress after weight bearing motion on individual Tekscan measurements, indicating that the mosaicplasty subsided below flush level. Five out of eight bottomed plugs were placed on the medial condyle and no significant differences in pressure or rim stress was found. Statistics 1 Table 1 Results of the surface contact pressure measurements of five conditions Measurement n = P n = n = Border A. Preoperative a  40.2 (8.3) 37.1 (6.0) B. Defect 74.3 (5.5) (B vs. A–C–D–E) 0.000 75.5 (3.9) 73.2 (6.8) C. Mosaicplasty 52.1 (9.3) (A vs. C) 0.001 54.6 (7.5) 49.7 (10.7) D. Motion without weight 53.2 (9.8) (A vs. D) 0.001 53.9 (8.4) 52.4 (11.6) E. Motion with weight 53.8 (9.6) (A vs. E) 0.000 55.1 (9.0) 52.5 (10.6) Mosaic A. Preoperative 35.7 (7.8)  33.6 (9.3) 37.9 (5.6) B. Defect 4.0 (4.0) (B vs. A–C–D–E) 0.000 3.7 (3.8) 4.3 (4.4) C. Mosaicplasty 23.8 (8.2) (A vs. C) 0.002 21.4 (7.4) 26.2 (8.9) D. Motion without weight 22.8 (8.7) (A vs. D) 0.001 22.1 (8.1) 23.5 (9.7) E. Motion with weight 22.0 (9.1) (A vs. E) 0.001 21.0 (8.4) 23.0 (10.1) a  P Effect of defect (A B) P P Effect of mosaicplasty (B C): P Effect of flexion-extension motion (C D E) P P Discussion In this biomechanical human cadaver study we clearly demonstrated that an osteochondral cartilage defect severely affects the contact pressure on the remaining intact joint surface. Obviously, there are some additional limitations to our study. The study had only a limited number of knees. During testing osteoarthritic changes in bone were found in some of these elderly knees. Clearly this is different to the bone quality of the typical patient who is relatively young and active. Another limitation was that during the preparations the knee was positioned in such a way that both condyles would be loaded with approximately the same force, which may be different from in vivo loads. This might have had an influence on the absence of medial/lateral differences. Contrary to other experiments we kept all collateral ligaments and intra-articular structures intact, which resembles the anatomical situation more closely and thus gives a greater translational value of pressure transfer. Clearly, no biological effects were taken into account such as the resorption of the plugs (thereby reducing the stability) or bony ingrowth (thereby enhancing stability). We also measured the contact patterns under static conditions with the knee in extension, whereas in reality shear forces at different flexion angles are also applied to the reconstruction. These limitations should be taken into account when interpreting the results. 3 9 4 2 16 12 7 14 13 17 9 15 8 10 11 15 Clinical relevance A cartilage defect results in increased stress levels at the articular cartilage boundaries of the defect. A multiple plug mosaicplasty has a positive effect in reducing these stress elevations, which will reduce the potential of cartilage degeneration, and thus may postpone secondary osteoarthritis. Post-operative non-weight bearing and weight bearing motions did not seem to influence the surface congruency of the mosaicplasty, although the unbottomed mosaicplasties showed a trend of subsidence below flush level after weight bearing motion. It appears feasible to allow limited weight bearing of the knee after osteochondral transplantation, especially when plugs have been bottomed.