Introduction 1 1 2 3 4 5 Articular cartilage repair techniques Marrow stimulation 6 Osteochondral grafting 7 8 9 10 2 8 Autologous chondrocyte implantation (ACI) 2 11 12 14 15 15 16 17 18 11 15 19 20 Matrix-associated ACT (MACT) 21 Second generation ACI 22 23 Third generation of ACI 23 26 MR techniques for cartilage and cartilage repair imaging 27 33 34 37 29 35 27 29 31 35 40 28 31 27 31 33 35 38 27 30 39 3 41 42 42 44 20 29 20 29 Quantitative MRI of articular cartilage 45 46 High-resolution MRI Most MRI studies on articular cartilage have tried to optimise pulse sequences that accentuate the contrast-to-noise ratio for cartilage, but less attention has been paid to image resolution. 47 27 31 33 In order to obtain a sufficient resolution for cartilage repair imaging a 1.5-T MR scanner with a high performance gradient system and a dedicated extremity coil (quadrature/phased array coil) is a minimum requirement. Three-Tesla clinical MR systems are becoming more widespread and can generate images with both a high signal-to-noise ratio and a high resolution. 2 48 MR evaluation of the biochemical and biomechanical status of cartilage 49 50 51 2− 2− 52 53 54 55 56 MR classification systems of cartilage implants 37 57 37 48 37 1 48 48 58 58 Table 1 The MOCART scoring system Variable Classes Degree of defect repair and defect filling Complete (on a level with adjacent cartilage) Hypertrophy (over the level of the adjacent cartilage) Incomplete (under the level of the adjacent cartilage; underfilling)         >50% of the adjacent cartilage         <50% of the adjacent cartilage Subchondral bone exposed (complete delamination or dislocation and/or loose body) Integration to border zone Complete (complete integration with adjacent cartilage) Incomplete (incomplete integration with adjacent cartilage) demarcating border visible (split-like) Defect visible  <50% of the length of the repair tissue         >50% of the length of the repair tissue Surface of the repair tissue Surface intact (lamina splendens intact) Surface damaged (fibrillations, fissures and ulcerations)         <50% of repair tissue depth         >50% of repair tissue depth or total degeneration Structure of the repair tissue Homogeneous Inhomogeneous or cleft formation Signal intensity of the repair tissue Dual T2-FSE         Isointense         Moderately hyperintense         Markedly hyperintense 3D GE-FS         Isointense         Moderately hypointense         Markedly hypointense Subchondral lamina Intact Not intact Subchondral bone Intact Oedema, granulation tissue, cysts, sclerosis Adhesions No Yes Effusion No effusion Effusion MR findings following osteochondral autografting 59 60 61 An MRI evaluation of osteochondral grafts should include: the number and size of the grafts; bone and cartilage integration, the cartilage surface contour; the contour of the cartilage bone interface; an assessment of the signal in the graft, the adjacent bone marrow and at the donor site; details of any soft tissue abnormalities and an assessment of the contrast enhancement patterns. Cartilage and bone integration and congruity 59 1 59 2 Fig. 1 Normal cartilage integration of osteochondral autografts in the weight bearing region of the femoral condyle in a patient 2 years after osteochondral autografts  Fig. 2 arrow  61 59 60 59 Cartilage and bone signal intensity 59 3 59 61 9 Fig. 3 a b c  59 61 Graft and adjacent bone 4 60 Fig. 4a, b a arrows b arrows  Normal findings The results of these earlier follow-up studies have helped to define normal and abnormal MR findings following osteochondral autografting and help in the identification of possible complications. “Normal” MR findings associated with after osteochondral autografting include bone marrow oedema in and around the grafts in approximately 50% of the subjects during the first 12 months, with a gradual reduction thereafter. However persistent oedema may be seen in a small number of cases for up to 3 years post-operatively. Joint effusion and synovitis appear to follow a similar trend. Incongruities at the bone-bone interface occur frequently, while incongruities at the cartilage-cartilage interface occur in approximately 15% of grafts. The sometimes substantial incongruities of the bone-bone interface should not be interpreted as an abnormal finding or complication. Differences occur because the cartilage thickness of the donor site commonly differs in thickness from the implant site, but the cylinder is inserted to a depth to ensure a smooth cartilage surface. Abnormal findings 5 6 Fig. 5 Incongruity at the cartilage-cartilage interface on a medial femoral condyle 12 months after osteochondral autografting  Fig. 6 arrows  Future studies in this field should provide better understanding of the pathophysiology of transplanted hyaline cartilage and its function, which is important for the long-term prognosis of these patients. MR findings following ACI and MACT Assessment and interpretation of MR examinations for ACI and MACT patients should be performed in a systematic fashion. Careful attention should be paid to the degree of defect filling, the integration of the graft to adjacent cartilage and underlying bone, the graft’s internal structure and surface, its signal intensity and any changes in the subchondral bone. Last but not least, the presence of adhesions to the graft or joint effusion should be evaluated. Defect filling 7 20 29 34 36 Fig. 7 arrows  62 63 37 64 15 18 65 8 Fig. 8 Severe hypertrophy of cartilage implant 104 weeks after MACT surgery on a sagittal T2-weighted FSE image  22 66 64 18 67 19 64 Integration 20 34 36 9 15 18 20 34 36 67 Fig. 9 Arrows right-hand arrow  36 64 10 20 34 15 18 Fig. 10 A sagittal FSE image of incomplete delamination of the repair tissue 24 weeks after MACT surgery. Fluid partially demarcates the bone interface  Structure and surface 19 20 34 68 69 70 11 62 Fig. 11 a b  71 62 Signal intensity 62 72 12 19 62 65 Fig. 12a–e a b c d e  34 19 20 34 68 69 17 19 64 Subchondral lamina and bone 62 63 17 19 29 34 63 68 20 34 73 20 34 34 19 63 64 19 64 Adhesions 13 15 18 67 15 67 73 18 Fig. 13 Arrows  Effusion 64 Maturation of cartilage repair tissue 62 14 Fig. 14a, b a b  Histology 74 63 63 Functional outcome 58 Recommendations From reviewing our studies and those of other centres we recommend that follow-up MR studies should be performed at 3 months and 1 year. The initial imaging at 3 months allows the volume and adherence of repair tissue to be assessed. Imaging at 1 year demonstrates the maturation of the graft and allows complications to be identified both non-invasively and at a sufficiently early stage. Conclusion 15 Fig. 15a, b a b