Graft Thickness is Associated with Subchondral Cyst Formation in Patients After Osteochondral Allograft Transplantation in the Knee
Authors: Ackermann Jakob , Merkely Gergo , Shah Nehal , Gomoll Andreas H. 
Published: July 2019

Objectives: The purpose of this study was to determine potential predictive associations between preoperative patient characteristics or osteochondral allograft (OCA) morphology, and postoperative OCA appearance assessed by the osteochondral allograft magnetic resonance imaging scoring system (OCAMRISS) at 6-month follow-up. It was hypothesized that preoperative patient factors or OCA morphology are associated with postoperative OCAMRISS scores. Methods: This study evaluated 74 OCAs that were implanted in the femoral condyles of 63 patients for the treatment of symptomatic osteochondral defects in the knee.Postoperative MRI was obtained at an average follow-up of 5.5 ± 1.0 months. A musculoskeletal radiologist scored all grafts according to the OCAMRISS. Pearson’s correlation, Mann Whitney U test and chi-square test were used to distinguish associations between age, sex, smoker status, BMI, previous surgeries, concomitant surgeries, bone marrow augmentation, graft location, graft size, bony graft thickness, and OCAMRISS subscales. Results: At 6-month postoperative MRI evaluation, the mean OCAMRISS score was 3.9 ± 2 with 87.8% of OCAs presenting with crossing trabeculae indicating osseous integration, and 21.6% showing cystic formation of the graft and host-graft junction. When correlating patient and lesion characteristics with OCAMRISS subscales, following associations were identified: cartilage signal and age (p=0.021), subchondral bone plate congruity and bone marrow aspirate augmentation (p=0.046), cystic changes and bony graft thickness (p=0.019), opposing cartilage and prior surgery (p=0.045) and BMI (p=0.003), and synovitis and age (p=0.044) and positive smoking status (p=0.009). Osseous integration was not associated with any preoperative factor. Conversely, patient’s sex, OCA graft size and location did not correlate with any OCAMRISS subscale (p > 0.05). OCA bony thickness was the only plug-specific factor being associated with the OCAMRISS, and the only factor related to cystic formation at 6 months (p = 0.019). Grafts that presented with cystic formation were significantly thinner than grafts that did not show cystic changes at the host-graft junction (p = 0.008). Grafts with less than 5 mm bony thickness had an almost 5-fold increased risk of demonstrating cystic changes on MRI (odds ration [OR]= 4.9; 95% Confidence Interval [CI]= 1.5 - 16.1; p = 0.006). Conversely, OCAs thickness was not associated with osseous integration, except grafts with a bony thickness of more than 9 mm presented significantly more often with a discernible cleft than did shallower grafts (p = 0.049). Conclusion: Osteochondral allograft thickness is associated with subchondral cyst formation at short-term follow-up. Shallow grafts demonstrate a substantially increased risk of developing subchondral cysts at the graft-host junction after OCA transplantation. Conversely, thicker grafts may negatively affect osseous graft integration. Hence, the authors suggest a bony graft thickness of 5-9 mm for OCAs to mitigate the risk of cystic formation and delay of osseous integration after cartilage resurfacing. [Table: see text]