Single focal grade IV cartilage lesion in the knee has a poor healing capacity. Instead these lesions often progress to severe and generalized osteoarthritis that may result in total knee replacement. Current treatment modalities aim at biological repair and, although theoretically appealing, the newly formed tissue is at the best cartilage-like, often fibrous or fibrocartilaginous. This at the expense of sophisticated laboratory resources, delicate surgery and strict compliance from patients. An alternative may be small implants of biomaterial inserted to replace the damaged cartilage. We investigated the response of the opposing tibia cartilage to a metallic implant inserted at different depth into the surrounding cartilage level. The medial femoral condyle of both knees of 12 sheep, 70–90kg, 2 year of age and from the same breeder, was operated. A metallic implant with an articulating surface of 316L stainless steel, diameter of 7mm, HA plasma sprayed press-fit peg and a tailored radius and contour to the sheep femoral condyle was placed at the most weight-bearing position. The level of the implant was aimed flush, 0,3 and 0,8 mm below surrounding cartilage. The animals were stabled indoors, allowed to move freely and euthanized after 6 and 12 weeks. Postoperatively the knees were high resolution photographed for macroscopic evaluation. The position and depth of the implant were analysed using a laser scan device. Tibial and femoral condyles specimen were decalcified and slices were prepared for microscopic evaluation. Implant position and cartilage damage was assessed from two independent observers using a macroscopic ICRS score and a modified histologic score according to Mankin. 22 tibia condyles showed a variety of cartilage damage ranging from severe damage down to subchondral bone to an almost pristine condition. There was a strong correlation between implant position and damage to opposing cartilage surface. Mankin score correlated significantly with implant position (p<0.001 regression analysis, r2=.45) as did the ICRS score (p<0.001, regression analysis, r2=.67). Implants sitting proud were associated with poor Mankin score. There was no difference between 6-week and 3-months knees.Methods
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