Lesions of the articular cartilage in the knee can cause serious reduction in quality of life of patients and if left untreated can be the cause of osteoarthritis of the joint.1 Autologous subchondral bone grafting combined with autologous chondrocyte implantation (sandwich procedure) is a well-accepted procedure for the treatment of osteochondral lesions of the knee. Sandwich procedure requires a different surgical technique and preoperative planning. Also pain from bone marrow donor site locations is expected and should be part of patient consent and expectations. However, the assessment of subchondral bone lesion and the need for using autologous bone graft is only done per-operatively. MRI imaging plays an important role in diagnosing cartilage defects and the follow-up. It is widely used as a pre-operative tool to estimate defect size.2 A systematic review by Blackman et al. showed a correlation between MRI findings and clinical outcome after cartilage repair, proving it could be a good way of non-invasive follow-up after surgery.3 However, there is no evidence about the use of MRI for defining the need for a sandwich procedure preoperatively. This study evaluates whether MRI can help in diagnosing these subchondral bone plate lesions and is able to predict the need for a sandwich procedure to help in optimising the preoperative planning and consent.A total of 109 preoperative MRI scans (PD and T2 sequences) of patients planned for ACI were included. Sagittal PD images were obtained using a TSE sequence with voxel size= 0.5x0.5x3mm, TR/TE= 4000/12 ms, FOV= 170x170 mm, FA= 90. The T2 images were obtained with fat suppression using SPAIR/SPIR, voxel size= 0.5x0.5x3mm, TR/TE= 3200/60 ms, FOV= 170x170mm, FA= 90.The integrity of the subchondral bone and lamina were scored by four different observers (3 experienced radiologists, experienced orthopaedic resident) as being intact or not. Also the depth of the defect was measured perpendicular from the subchondral layer to the bottom of the bony lesion. The area under the curve (AUC) for subchondral defect on MRI (i.e. lamina or bone defect) and eventual sandwich procedure were calculated. Also inter-observer Kappa values were determined using SPSS v21.0.The AUC’s (0.75-0.82 for lamina and 0.67-0.82 for bone defect) and inter-observer Kappa’s (ranging from 0.34-0.76) for lamina and bone defect were moderate indicating a large inter-observer variation and moderate prediction of the need for a sandwich procedure based on the presence of lamina and or subchondral bone defect on MRI. However, depth measurements resulted in an AUC of 0.92 (95%CI: 0.87-0.97, p=0.001) with an optimal cut-off point at 5.5 mm depth of the lesion (92% sensitivity, 80% specificity) to predict the need for a sandwich procedure.Our results show that the integrity of the subchondral layer on MRI moderately predicts the need for an eventual autologous bone graft to augment ACI whereas a depth of the lesion above 5.5 mm accurately predicts the need for a sandwich procedure.