Postoperative Cartilage Repair

Chondral injuries are the most common abnormality detected at arthroscopy, identified in more than 60% of cases [60]. As traumatic cartilage injuries lead to premature degenerative disease, techniques to repair such defects have developed, although as yet there is no optimal technique or clear consensus as to the most effective method.

MRI, together with clinical assessment, is frequently used to assess the status of the repair site following surgery. The most commonly used pulse sequences for assessment of cartilage repair are fast spin echo (FSE) proton density or T2-weighted images with or without fat suppression, and T1-weighted fat-suppressed 3D spoiled gradient echo images (T1 3D-SPGR) [61-63]. On FSE imaging articular cartilage is visualized as intermediate signal intensity in contrast to joint fluid, which demonstrates high signal intensity. Filling of cartilage defects with high signal intensity fluid helps to delineate the area of abnormality and its conspicuity may be enhanced by use of concomitant fat suppression. FSE techniques have the added benefit of allowing accurate evaluation of the menisci and ligamentous structures. On T1 fat-suppressed 3D-SPGR sequences, in contrast to FSE sequences, hyaline cartilage demonstrates high signal intensity with joint fluid exhibiting low signal intensity. This technique permits acquisition of thin slices that can then be reformatted in other planes. However, SPGR sequences are prone to susceptibility artifacts in the postoperative patient, which may result in obscuration of the articular cartilage in some cases.

Cartilage repair procedures are composed of marrow stimulation, autolo-gous osteochondral transplantation, autologous chondrocyte implantation/ transplantation, and osteochondral allograft transplantation.

Different Stress Fractures
Fig. 17. Coronal intermediate-weighted MR images (TR/TE, 3600/35) in two different patients following surgical repair of complete tears of the proximal (A) and distal (B) MCL. There is localized susceptibility arifact but ligament continuity is displayed (arrows).

Marrow stimulation techniques include microfracture, abrasion arthroplasty, and subchondral drilling. All these procedures aim to fill the cartilage defect with fibrocartilaginous repair tissue by releasing pleuripotent stem cells from the subchondral bone. Following marrow stimulation techniques, intermediate signal intensity repair tissue may be seen filling the defect sites at MRI. The thickness of such repair tissue increases over time from an initial thin layer to full thickness by 1 to 2 years [64]. The subchondral marrow typically demonstrates increased signal intensity that diminishes with time but may persist over prolonged periods. An optimal result is considered as congruent articular surface without chondral flaps or fissures (Fig. 18).

Autologous osteochondral transplantation techniques consists of harvesting multiple small tubular plugs of subchondral bone and overlying articular cartilage from a non-weight bearing harvest site, typically the inferior aspect of the lateral trochlea or the intercondylar region, and transplanting these cylinders into prepared tunnels at the site of a cartilage defect being resurfaced. The aim of autologous osteochondral transplantation is to fill a preexisting cartilage defect with transplanted plugs producing a congruent articular surface consisting of multiple hyaline cartilage covered cylinders (Fig. 19). The gaps between the individual osteochondral cylinders gradually fill with fibrocartilage and the osseous component is incorporated into the host site. At MRI the transplanted hyaline cartilage demonstrates similar signal intensity to the normal articular cartilage on all sequences while the fibrocartilaginous component may display heterogeneity in comparison with the native cartilage. Contour irregularities with protuberance or depression at the articular surface can be visualized on MRI, often with a step deformity, either as a result of suboptimal positioning t

Mri Microfracture

Fig. 18. Sagittal T2-weighted fat-suppressed MR image (TR/TE, 3500/70) (A) demonstrating cartilage denudation of the femoral condyle (arrows), which was filled with pannus and synovitis at arthroscopy. The corresponding Tl-weighted fat-suppressed 3D-SPGR image (TR/TE, 50/10, 45° flip angle) following microfracture (B) illustrates fibrocartilage growth within the defect as characterized by a smooth area of lower signal intensity (arrows).

at surgery or as a result of subsidence of the osteochondral plugs [65] (Fig. 20). The osteochondral plugs demonstrate normal fatty marrow within the first 2 weeks followed by increased T2 signal and enhancement with IV contrast administration, beginning within 4 weeks postoperatively. Between 5 and 12 months postoperatively the graft plugs invariably demonstrate fatty marrow, although marrow edema adjacent to the plugs may be visualized up to 22 months after surgery [66] (Fig. 21). Areas of subchondral cystic cavities suggest poor graft incorporation, and potential complication [64]. Graft harvest sites are characterized by cylindrical defects of increased T2-weighted signal extending from the articular cartilage into the subchondral bone. Gradual stabilization of the signal changes at harvest sites are typically seen over 9 months after surgery with associated progressive filling of the resultant cartilage defects with fibrocar-tilage repair tissue.

Autologous chondrocyte transplantation is a 2-stage process performed largely for medium sized-lesions or in patients failing marrow stimulation techniques. At the initial operation cartilage is harvested from a non-weight-bearing location and cultured ex vivo for 4 weeks. This is followed by an open operation with debridement of the articular cartilage defect repair site and injection of the cartilage cell suspension deep to a periosteal patch sewn over the defect site. MRI is useful in follow-up evaluation of the defect fill, assessment of the subchondral bone, and incorporation of the cartilage to the subchondral plate [67]. The cartilage implant demonstrates high signal intensity at MRI and enhancement with IV contrast administration on FSE sequences in the

Condyle Femoral Stress Fracture

Fig. 19. Coronal FSE intermediate-weighted (TR/TE, 3600/35) (A) and sagittal proton density (TR/TE, 2200/15) (B) images in a patient following autologous osteochondral transplantation. There is smooth cartilage covering at the host site within the medial femoral condyle (white arroW) with some residual signal change of the subchondral bone (black arrow) (A). There has been healing with cartilage cover growth at the harvest site (black arrow) (B).

Fig. 19. Coronal FSE intermediate-weighted (TR/TE, 3600/35) (A) and sagittal proton density (TR/TE, 2200/15) (B) images in a patient following autologous osteochondral transplantation. There is smooth cartilage covering at the host site within the medial femoral condyle (white arroW) with some residual signal change of the subchondral bone (black arrow) (A). There has been healing with cartilage cover growth at the harvest site (black arrow) (B).

initial postoperative period and may have a variable and heterogeneous appearance in the ensuing months with a more uniform appearance by 1 year [68]. The graft should ideally be flush with the adjacent native cartilage with no contour step deformity of the cartilage surface. Contour abnormalities

Fig. 20. Coronal FSE intermediate-weighted image (TR/TE, 3600/35) demonstrating collapse of the subchondral bone and a recurrent osteochondral defect (arrow) in a patient with failed autologous osteochondral transplantation.

Fig. 21. Sagittal T2-weighted MRI with fat saturation (TR/TE, 3500/70) demonstrating residual edema at the host site following autologous osteochondral transplantation (arrow).

may result from underfilling of repair tissue with depression of the articular cartilage or from periosteal hypertrophy at the implantation site causing focal prominence and incongruity of the articular surface. The interface between the repair tissue and native cartilage may be indistinct or may have a sharp margin with fluid signal intensity. The fluid signal intensity line may even be seen in normal cases but obliquity of such signal or extension of fluid deep to the repair tissue is abnormal and suggestive of a cartilage fissure [27]. Fluid between the implant and the subchondral bone may be seen in the setting of graft de-lamination, typically occurring within the first 6 months postoperatively, which may progress to graft dislodgement. Graft delamination and displacement may also be accurately displayed with MR arthrographic techniques. High T2-sig-nal intensity changes may be identified in the subchondral bone deep to the implantation site but this should subside and return to normal with time. However, progressive increasing marrow signal over time may be related to a poor result [68].

The above techniques may not be suitable in treatment of larger osteochon-dral lesions, necessitating the use of fresh osteochondral allografts to maintain congruity of the articular surface. Allograft fixation in such cases is achieved by cancellous screws, bioabsorbable pins, or press fit techniques. Modification of pulse sequences to minimize metal artifact may be required with the use of metallic screws [65]. High signal changes may be seen at the graft-host interface and the adjacent marrow during the remodeling phase. However progression of signal changes at these sites may herald development of graft rejection. As in the other techniques of cartilage repair, MRI can clearly demonstrate the integrity of graft cartilage cover and congruity/smoothness of the allograft contour relative to the native articular surface.

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