In vivo knee joint contact mechanics may provide insight into the initiation and progression of knee osteoarthritis following sports injuries. Acquiring these data in vivo are difficult to obtain in the clinic. In vivo cartilage deformation patterns have been widely investigated using MRI to assess contact mechanics^1,2, but the relationship between cartilage deformation and the actual contact mechanics, specifically the contact stress and contact area, has not been studied. The objective of this study was to evaluate in vivo cartilage deformation as a surrogate for in vivo contact mechanics. To achieve our objective, we developed an MRI compatible loading device and an intra-operative loading protocol to measure in vivo cartilage deformation and contact stresses and areas, respectively.Five patients (2M/3F, age: 21±4 years) undergoing meniscal allograft transplantation (MAT) surgery were enrolled following IRB approval with informed consent. One patient had a concomitant anterior cruciate ligament (ACL) reconstruction, one had ACL-revision, and two had osteochondral allograft transplantation in the femoral condyle. At the time of this study, all patients had undergone prior total meniscectomy. All patients showed no osteoarthritic findings on radiographs. Pre-operative MRI scans were acquired with the knee in unloaded and loaded configurations to quantify contact deformation (Fig. 1A) using an MRI compatible loading device ³. Patients were placed in a wheelchair for 30 minutes prior to scan to unload the limb. All scanning was performed on a clinical 3T scanner (GE Healthcare, Waukesha, WI) using an 8 channel phased array knee coil (Invivo, Gainesville, FL): 3D SPGR with frequency selective fat suppression, TE/TE = 3.2/15.4 ms, FOV = 14 cm, matrix = 512 × 512, voxel dimensions = 0.27 × 0.27 × 1.5 mm. Following the unloaded scan, a load of 50% of body weight was applied and held for 12 minutes before starting the loaded scan ³. During the subsequent MAT surgery, contact stress on the tibial plateau surface was measured using a thin-electronic sensor (Tekscan Inc., Boston, MA). The sensor was calibrated and trimmed to accommodate to the shape of the tibial plateau, then sterilized. The sensor was passed through a small arthrotomy from anterior to posterior, and its position was adjusted arthroscopically to cover the whole tibial plateau (Fig. 1B). The anterior edge of the sensor was flush with the contour of the tibial plateau, which was used for registration of the sensor position. A custom designed boot augmented with a load cell was used to manually apply an axial force of 50% body weight to the foot with the knee in extension, while the joint contact stresses were synchronously recorded. Contact stress and contact area were evaluated before allograft placement. Cartilage thickness was calculated from segmented MR images as the shortest distance between the subchondral and articular surfaces. Cartilage deformation within the contact region, determined by loaded/unloaded cartilage overlap, was calculated as the difference in thickness at the same location before and after joint loading.³ To compare the corresponding patterns of calculated cartilage deformation with measured contact stress, the 3-D cartilage surface mesh was projected to the transverse plane and resampled to mesh density similar to the sensing element of the electronic sensor (1.9x 1.9mm²). Custom software (Mathworks, Natic, MA) was used to assess the similarity between the two maps by normalized 2-D cross correlation. The range of the correlation was 0 to 1, indicating none or complete correlation.Cartilage deformation maps varied among patients (Fig. 2), with peak compressive strains from 17% to 24% and contact areas from 89 mm² to 214 mm². The peak contact stresses ranged from 1.7 MPa to 2.6 MPa and the contact areas were from 120 mm² to 225 mm². The calculated contact area from loaded-MRI was similar to that from direct measurement (paired t-test). The contact stress exhibited a pattern similar to the cartilage deformation map within each patient, with an average normalized cross correlation value of 0.72 (range: 0.56 to 0.85).This study compared indirect measurements of in vivo cartilage deformation with corresponding direct measurements of contact stresses. Good correlations were found between these measurements, indicating that a loaded MRI examination may be considered a surrogate for in vivo contact mechanics. A limitation of the study is that patients were evaluated in the loaded configuration with the knee fully extended, and this position may not be representative of a common activity of daily living. Cartilage deformation assessed by loaded MRI may be used as a surrogate measurement of in vivo knee joint contact mechanics and may be used to assess the potential chondro-protective effect of MAT.