DOI: 10.1148/rg.284075111
RadioGraphics 2008;28:1043-1059
© RSNA, 2008
MR Imaging of Cartilage Repair in the Knee and Ankle1
Yun Sun Choi, MD,
Hollis G. Potter, MD, and
Tong Jin Chun, MD
1 From the Department of Radiology, Eulji Hospital, Eulji University School of Medicine, 280-1 Hagye 1-dong, Nowon-gu, Seoul 139-711, South Korea (Y.S.C., T.J.C.); and Department of Radiology, Hospital for Special Surgery, Weill Medical College of Cornell University, New York, NY (H.G.P.). Recipient of a Certificate of Merit award for an education exhibit at the 2006 RSNA Annual Meeting. Received May 14, 2007; revision requested June 15 and received July 24; accepted August 3. H.G.P. received research support from General Electric and is a consultant with Histogenics; all remaining authors have no financial relationships to disclose.
Address correspondence to Y.S.C. (e-mail: cys0128{at}eulji.ac.kr).
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Abstract
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Because of the relative avascularity of articular cartilage, lesions that are caused by trauma or degeneration of the cartilage do not heal spontaneously and must be repaired surgically. The interventional procedures that have been developed for the repair of such lesions include abrasion, microfracture, autologous osteochondral transplantation, allograft transplantation, and autologous chondrocyte implantation. An accurate imaging assessment of the repair tissue is necessary in order to objectively evaluate the postoperative outcome. Magnetic resonance (MR) imaging and arthroscopy provide complementary information and are especially useful for follow-up evaluation of cartilage repair in the knee and ankle. Standard MR imaging techniques may be used postoperatively to evaluate the success of implantation and the state of cartilage healing. Newer matrix assessment techniques, which include delayed gadolinium-enhanced MR imaging and mapping of T1
and T2 values, may provide useful supplemental information about the histologic and biochemical contents of reparative tissue. The normal postoperative appearance of the joints after cartilage repair varies according to the surgical technique used and the stage of healing. To identify potential complications, it is important to be familiar with the various repair procedures and the characteristic MR imaging features of the repair tissue at various postoperative intervals.
© RSNA, 2008
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Introduction
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The advent of new procedures for repairing cartilage in knee and ankle joints has increased the need for accurate noninvasive methods to objectively evaluate the success of repair. The combined use of standard and newer magnetic resonance (MR) imaging techniques makes it possible to evaluate both the morphologic status and the biochemical contents of the repair tissue (1–9). The articular cartilage imaging group of the International Cartilage Repair Society has issued detailed recommendations with regard to appropriate MR acquisition protocols for cartilage imaging (10). Intermediate-weighted fast spin-echo (SE) and three-dimensional (3D) fat-suppressed T1-weighted gradient-echo (GRE) sequences are the standard techniques most commonly used for this purpose (11–13). However, high-spatial-resolution MR imaging and quantitative techniques also may be needed to detect subtle cartilage abnormalities (3,4,7,14–16).
The article reviews the surgical procedures that may be used for cartilage repair and describes the MR techniques that are most useful for a systematic postoperative assessment. The MR imaging features that may be observed after cartilage repair in the knee and ankle are described in detail, and the usefulness of mapping of relaxation times (T1 and T2) for evaluating the status of repair tissue is demonstrated.
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Articular Cartilage Repair Techniques
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Bone Marrow Stimulation
Several techniques may be used to stimulate the growth of new fibrocartilage from bone marrow stem cells. These techniques include abrasion, subchondral drilling, and microfracture. Abrasion arthroplasty involves the removal of subchondral bone to a depth of 1–3 mm beneath the cartilage defect, a procedure that results in the formation of a fibrin clot (17). In the microfracture procedure, an awl or a pick is used to create multiple 4-mm-deep pits in the subchondral bone beneath the cartilage defect (Fig 1) (18–20). Subchondral drilling involves puncturing the subchondral bone by using a drill. Multipotential stem cells migrate from marrow beneath these pits to the cartilage defect. Each of these techniques eventually leads to the formation of fibrocartilaginous repair tissue.
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Figure 1. Schematic of the microfracture technique shows the use of an awl (arrow), which is introduced through an arthroscopic portal, to bore small holes in the subchondral bone. This procedure is performed to encourage the production of fibrocartilage from multipotential bone marrow stem cells.
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Microfracture remains the most commonly performed cartilage repair procedure. No absolute contraindications or unique risks to the microfracture technique have been established (21). Despite the popularity of the technique, few prospective studies have been performed and only limited information is available about clinical outcomes (22,23). Mithoefer el al (22) reported that the microfracture of articular cartilage lesions in the knee results in significant functional improvement after 2 years. The best short-term results are observed in the presence of a good fill grade, a low patient body-mass index, and a short duration of preoperative symptoms (22). However, the new fibrocartilage has insufficient structural, biomechanical, and biochemical properties to sustain normal joint function over the long term (2,7,8,18,20,21).
Tissue-based Cartilage Repair
Autologous Osteochondral Autograft Transplantation (Mosaicplasty).—
In this procedure, osteochondral plugs are harvested from non–weight-bearing areas such as the lateral femoral condyle or the trochlea and transplanted in an articular defect in the same person (Fig 2) (8, 19,20,24). 
Osteochondral autograft transplantation or mosaicplasty provides autologous hyaline articular cartilage. This procedure is performed most frequently in the knee and ankle joints and is indicated for the repair of cartilage defects of 1–4 cm2, osteochondritis dissecans, and osteonecrosis (20,24,25). Osteochondral autografts are less likely than allografts to evoke an immune response leading to graft rejection and are associated with a higher rate of graft incorporation. However, autograft transplantation is limited by the availability of donor-graft tissue, the age dependence of donor cartilage, and the potential risk of donor site morbidity (21). Link et al (25) reported that there was no consistent correlation between the clinical findings and the MR findings in osteochondral autograft transplants in the knee and ankle over a follow-up period of 3 years, particularly in patients with autograft necrosis.
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Figure 2. Intraoperative photograph shows autologous osteochondral autografts: four osteochondral plugs harvested from the non–weight-bearing lateral femoral condyle have been implanted into a chondral defect in the medial femoral condyle (arrows).
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Osteochondral Allograft Transplantation.—
A fresh shell or plug of bone and cartilage harvested from a cadaver is transplanted to fill a full-thickness chondral or osteochondral defect. The transplanted tissue provides the hyaline cartilage. This type of graft is useful for repairing large defects, and there is no donor-site morbidity (2,7,19,21,26). However, unlike the grafts used in most other cartilage repair techniques, fresh shell osteochondral allografts are associated with the risks of immunologic rejection and of disease transmission from donor to recipient (21). Williams et al (27) reported that fresh osteochondral allografts stored hypothermically for 17–42 days were effective in the short term, both structurally and functionally, when used to repair symptomatic chondral and osteochondral defects in the knee.
Cell-based Cartilage Repair: Autologous Chondrocyte Implantation
The classic procedure of autologous chondrocyte implantation, which was first described in the mid-1990s, traditionally consists of two stages. In the first stage, healthy chondrocytes harvested from a non–weight-bearing cartilaginous surface are cultured in vitro for 3–5 weeks. In the second stage, approximately 1 month later, the cultured chondrocytes are implanted via an arthrotomy and covered with a periosteal flap, the edges of which are secured in place with fibrin glue or sutures. This technique may be used to treat cartilage defects of 2–12 cm2 in high-demand patients and osteochondritis dissecans lesions. If successful, it results in the formation of fibrocartilage that is similar to natural hyaline cartilage (28–33). Peterson et al (30) reported that autologous chondrocyte implantation resulted in well-integrated reparative tissue and successful clinical results in 54 of 58 patients with osteochondritis dissecans of the knee. Brown et al (34) reported that, at follow-up MR imaging of 180 knees (112 patients) in which cartilage defects had been repaired with either the microfracture procedure or autologous chondrocyte implantation, filling of the defect was consistently better with autologous chondrocyte implantation than with the microfracture technique. However, autologous chondrocyte implantation may be complicated by graft hypertrophy, which usually has been observed within 6 months after the procedure (34). Most of the complications of autologous chrondrocyte implantation are directly related to the periosteal flap; revision arthroscopy rates of 4.8%–60% have been reported and attributed to problems with the periosteal flap, such as hypertrophy, delamination, and arthrofibrosis (8,28–34).
There are many possible variations of the implantation procedure. In second-generation autologous chondrocyte implantation, a bilayer collagen membrane is used instead of a periosteal flap. In third-generation autologous chondrocyte implantation, a one-step arthrotomy (mini arthrotomy) is performed to allow the implantation of a 3D biologic scaffold optimized for the culture of seeded chondrocytes (matrix-associated autologous chondrocyte transplantation). The biologic scaffold material can be trimmed to fit a débrided cartilage ulcer and glued in place; no periosteal cover or sutures are needed (Fig 3) (8,35,36). Handl et al (37) reported that among five patients who received autologous chondrocyte implants (solid chondral grafts fixed with fibrin glue) for treatment of talar dome chondral defects, the procedure resulted in a significant improvement of ankle joint function in three patients but no clinical change in one patient during the follow-up period (6–24 months).
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Figure 3a. Third-generation autologous chondrocyte transplantation. (a) Photomicrograph (original magnification, x100; hematoxylin-eosin stain) shows chondrocytes seeded in a chondroid matrix. (b) Intraoperative radiograph shows injection of the seeded chondrocytes into a débrided osteochondral defect in the talus (arrows). No periosteal cover or sutures are needed for fixation.
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Figure 3b. Third-generation autologous chondrocyte transplantation. (a) Photomicrograph (original magnification, x100; hematoxylin-eosin stain) shows chondrocytes seeded in a chondroid matrix. (b) Intraoperative radiograph shows injection of the seeded chondrocytes into a débrided osteochondral defect in the talus (arrows). No periosteal cover or sutures are needed for fixation.
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Fixation with Biodegradable Pins
Biodegradable pins made of polydioxanone may be used to stabilize osteochondral fractures, chondral flap tears, and osteochondritis dissecans lesions. These pins generally resorb within 6–24 months, with the resultant synthetic debris being cleared predominantly by macrophages (21).
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MR Imaging Techniques
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Postoperative imaging is recommended for assessing the technical success of the procedure and the state of cartilage healing, as well as for identifying potential complications. MR imaging is a less invasive method than arthroscopy, and it allows a more comprehensive evaluation of repair tissue, from the articular surface of the joint to the bone-cartilage interface. MR imaging techniques also can be used to depict the components of the extracellular matrix and help assess the biochemical status of the reparative cartilage (7,8,13–16).
Repair cartilage may be evaluated by using the same acquisition techniques that are used for evaluating the native cartilage, as recommended by the International Cartilage Repair Society (10). Intermediate-weighted fast SE and 3D fat-suppressed T1-weighted GRE sequences are most commonly used for this purpose (Table 1). The accuracy of the MR imaging assessment of repair tissue depends on the signal-to-noise ratio obtained with a particular sequence and imaging system, the size of the lesion, and the contrast resolution between the lesion and adjacent tissues. In our opinion, the minimum requirements for obtaining images with sufficient resolution to evaluate cartilage repair are an MR unit with a 1.5-T magnet, a high-performance gradient system, and an extremity coil (quadrature or phased-array coil). A 3-T MR system also is capable of generating images with both a high signal-to-noise ratio and high spatial resolution. Investigators who used direct MR arthrography to evaluate autologous chondrocyte implants reported that the technique facilitated differentiation between delamination of the base of the graft and normal high-signal-intensity repair tissue in the immediate postoperative period (9).
Tissue Matrix Assessment
Various tissue parameters that may be evaluated with MR imaging are indicative of the biochemical properties of articular cartilage. The T1 and T2 relaxation times and apparent diffusion coefficients change during the cultivation of cartilage implants, which suggests that the biochemical properties of a cartilage implant change as the graft matures. Delayed gadolinium-enhanced MR imaging of cartilage or T1 (for glycosaminoglycan content) and T2 mapping (for collagen content) allow evaluation of the components of the repair matrix (7,8,13,16). The stratification of T2 relaxation times across the articular cartilage reflects changes in the orientation of collagen fibers in the extracellular matrix (7). The disorganized tissue found in cartilage repair sites generally has greater water mobility and therefore more prolonged relaxation times.
Utility of MR Imaging
The information gained from MR imaging is complementary to a subjective clinical assessment of the postoperative outcome. The parameters that can be evaluated with MR imaging at microfracture sites and osteochondral autograft transplant sites include the degree of defect filling, the extent of integration of repair tissue with adjacent tissues, the presence or absence of proud subchondral bone formation (extension of repair tissue beyond the adjacent subchondral plate to include new bone formation), the characteristics of the graft substance and surface, and the appearance of the underlying bone (Table 2). Some authors have recommended an additional assessment of adjacent cartilage surfaces (7,34). The MOCART (MR observations of cartilage repair tissue) system has excellent interobserver reproducibility for scoring of the defined variables, and it is an effective method for standardized reporting of the imaging features of autologous chondrocyte implants (Table 3). MOCART scores may be helpful in long-term follow-up of cartilage repair, as they may enable prospective multicenter studies in which outcomes with different cartilage repair techniques are compared (3,5,8,9). After microfracture repair, osteochondral autograft transplantation, or autologous chondrocyte implantation, the repair tissue ideally should have the same thickness as the adjacent native cartilage, the articular surface should be smooth, and the margins of the repair tissue should be continuous with the adjacent native articular cartilage (3,21). The capability of MR imaging to directly depict the subchondral bone and bone marrow gives the modality an advantage over arthroscopy. However, the true dimensions of a cartilage defect might be underestimated at MR imaging if the defect is not imaged in all three planes or if the intersection gap is too large (7–9,38).
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Table 2. Parameters of MR Imaging Assessment after Microfracture Procedures and Autologous Autograft Transplantations
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MR Imaging Features
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Microfractures
The MR imaging appearance of chondral lesions treated with the microfracture technique evolves over time. In the early postoperative period, the repair tissue may appear thin and indistinct. By 1–2 years after surgery, the reparative tissue should have filled the defect and should appear smooth and well defined (21). The signal intensity of the cartilage varies after a microfracture procedure. In general, the signal in the reparative fibrocartilage that forms over a microfracture is hyperintense compared with the signal in native cartilage on MR images obtained with a fast SE sequence with a moderate echo time (Fig 4). This signal characteristic is consistent with a less organized matrix and increased mobility of water. The signal intensity of the reparative cartilage decreases as the tissue matures (21,22,34). The signal intensity of subchondral bone marrow edema likewise decreases. Bone overgrowth has been observed in 25%–49% of patients (Fig 5) (22,34). However, the presence of bone overgrowth does not have a negative effect on clinical outcome scores (22). By contrast, persistent bone marrow edema and incomplete filling of the defect with thin and irregular repair tissue are indicative of treatment failure (Fig 6) (21).
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Figure 4a. Sagittal inversion-recovery fast SE (a) and sagittal intermediate-weighted fast SE (b) images acquired 6 months after a microfracture procedure in a 29-year-old man show hyperintense signal in the repair cartilage with good fill over the lateral femoral condyle (arrow).
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Figure 4b. Sagittal inversion-recovery fast SE (a) and sagittal intermediate-weighted fast SE (b) images acquired 6 months after a microfracture procedure in a 29-year-old man show hyperintense signal in the repair cartilage with good fill over the lateral femoral condyle (arrow).
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Figure 5. Sagittal intermediate-weighted fast SE image acquired 12 months after a microfracture procedure in an 18-year-old man shows an overgrowth of subchondral bone (arrow), thin overlying reparative fibrocartilage, and hyperintense signal in native cartilage at the anterior interface and across the opposite tibial plateau.
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Figure 6a. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired 6 months after a microfracture procedure in a 50-year-old man show hyperintense signal and superficial irregularity in the reparative cartilage over the medial femoral condyle (arrow). (c) Coronal intermediate-weighted fast SE image acquired at follow-up 4 years later shows superficial irregularity of the reparative fibrocartilage over part of the microfracture site (arrow), but no exposed subchondral bone.
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Figure 6b. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired 6 months after a microfracture procedure in a 50-year-old man show hyperintense signal and superficial irregularity in the reparative cartilage over the medial femoral condyle (arrow). (c) Coronal intermediate-weighted fast SE image acquired at follow-up 4 years later shows superficial irregularity of the reparative fibrocartilage over part of the microfracture site (arrow), but no exposed subchondral bone.
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Figure 6c. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired 6 months after a microfracture procedure in a 50-year-old man show hyperintense signal and superficial irregularity in the reparative cartilage over the medial femoral condyle (arrow). (c) Coronal intermediate-weighted fast SE image acquired at follow-up 4 years later shows superficial irregularity of the reparative fibrocartilage over part of the microfracture site (arrow), but no exposed subchondral bone.
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Osteochondral Autografts
The MR imaging evaluation of a transplanted osteochondral autograft should include assessments of the degree of defect filling by the osteochondral plug, the peripheral integration of the reparative cartilage and bone, the cartilage surface contour, and the morphologic characteristics of the autologous bone. The donor site also should be assessed. A well-incorporated plug is depicted at MR imaging as a region of uniform fatty signal intensity (Fig 7) (7). Gaps between cartilage plugs and adjacent native cartilage have been reported (Fig 8) (21,25,39). The presence of fluid signal intensity at the interface between the graft and the host bone is suggestive of incomplete graft incorporation and potential instability (21). An important element in osteochondral transfer is restoration of the normal radius of curvature, which helps maintain the biomechanical integrity of the osteochondral plug in the setting of osteochondritis dissecans (Fig 9) (7). Normal MR findings associated with osteochondral autograft transplantation include bone marrow edema in and around the graft, which occurs in approximately 50% of patients during the first 12 months, with a gradual reduction thereafter. Persistent edema has been observed in a small number of cases as late as 3 years after the procedure (8,25). Joint effusion and synovitis may persist for more than 2 years. The presence of subchondral cysts with the signal intensity of fluid, the persistence of signal intensity indicative of edema within the subchondral bone, or both are suggestive of poor integration of the graft with native bone (Fig 10) (2,25). Complications that may be detected at MR imaging include osteonecrosis of the graft (Fig 11) (2,8,25).
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Figure 7. Coronal intermediate-weighted fast SE image acquired 2 years after mosaicplasty in a 36-year-old woman shows good integration of osteochondral plugs into the medial femoral condyle, with a slight prominence of cartilage over the most medial plug (arrow). A high-signal-intensity fissure (arrowhead) is visible at the medial margin of the graft.
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Figure 8. Axial intermediate-weighted fast SE image acquired after mosaicplasty in a 17-year-old boy. The subchondral plate is flush, and there is only a slight (<2 mm wide) fissure at the medial margin of the articular surface (arrow). The plugs have largely been incorporated. The adjacent cartilage has slightly hyperintense signal, a finding suggestive of mild softening.
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Figure 9a. Sagittal intermediate-weighted fast SE (a) and 3D fat-suppressed T1-weighted GRE (flip angle, 40°) (b) images acquired 5 years after autologous osteochondral transplantation in a 23-year-old man show that the osseous portions of the plugs (arrows) are proud relative to the subchondral bone (ie, extend into the repair tissue), whereas the reparative cartilage lies relatively flush with the native cartilage.
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Figure 9b. Sagittal intermediate-weighted fast SE (a) and 3D fat-suppressed T1-weighted GRE (flip angle, 40°) (b) images acquired 5 years after autologous osteochondral transplantation in a 23-year-old man show that the osseous portions of the plugs (arrows) are proud relative to the subchondral bone (ie, extend into the repair tissue), whereas the reparative cartilage lies relatively flush with the native cartilage.
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Figure 10a. (a, b) Sagittal inversion-recovery fast SE (a) and coronal intermediate-weighted fast SE (b) images acquired 3 months after mosaicplasty in a 40-year-old man show good incorporation of the osteochondral plugs over the medial femoral condyle. Hyperintense signal in the adjacent cartilage in a is indicative of softening. The donor site, at the articular surface close to the notch (small arrow in b), is filled with high-signal-intensity fibrocartilaginous tissue. Focal collapse of a portion of the subchondral bone, central cystic changes (large arrow), and subchondral depression also are visible. (c) Color-coded map of T2 values (5–100 msec) shows longer values indicative of more disorganized repair tissue at the center of the plug and over the donor site (green and blue areas).
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Figure 10b. (a, b) Sagittal inversion-recovery fast SE (a) and coronal intermediate-weighted fast SE (b) images acquired 3 months after mosaicplasty in a 40-year-old man show good incorporation of the osteochondral plugs over the medial femoral condyle. Hyperintense signal in the adjacent cartilage in a is indicative of softening. The donor site, at the articular surface close to the notch (small arrow in b), is filled with high-signal-intensity fibrocartilaginous tissue. Focal collapse of a portion of the subchondral bone, central cystic changes (large arrow), and subchondral depression also are visible. (c) Color-coded map of T2 values (5–100 msec) shows longer values indicative of more disorganized repair tissue at the center of the plug and over the donor site (green and blue areas).
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Figure 10c. (a, b) Sagittal inversion-recovery fast SE (a) and coronal intermediate-weighted fast SE (b) images acquired 3 months after mosaicplasty in a 40-year-old man show good incorporation of the osteochondral plugs over the medial femoral condyle. Hyperintense signal in the adjacent cartilage in a is indicative of softening. The donor site, at the articular surface close to the notch (small arrow in b), is filled with high-signal-intensity fibrocartilaginous tissue. Focal collapse of a portion of the subchondral bone, central cystic changes (large arrow), and subchondral depression also are visible. (c) Color-coded map of T2 values (5–100 msec) shows longer values indicative of more disorganized repair tissue at the center of the plug and over the donor site (green and blue areas).
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Figure 11a. Sagittal (a) and coronal (b) intermediate-weighted fast SE images acquired 5 months after transplantation of an osteochondral autograft over the lateral femoral condyle in a 14-year-old boy show a failure of repair. The plugs have largely resorbed, likely because of a failure of osseous integration (arrow). An area of depressed subchondral bone, severe fibrillation of the adjacent cartilage, and reactive secondary synovitis are visible.
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Figure 11b. Sagittal (a) and coronal (b) intermediate-weighted fast SE images acquired 5 months after transplantation of an osteochondral autograft over the lateral femoral condyle in a 14-year-old boy show a failure of repair. The plugs have largely resorbed, likely because of a failure of osseous integration (arrow). An area of depressed subchondral bone, severe fibrillation of the adjacent cartilage, and reactive secondary synovitis are visible.
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Osteochondral Allografts
During the early postoperative period (0–3 months), a bone marrow edema pattern is seen in the graft. During the late postoperative period (3–6 months), the bone marrow edema decreases. A graft bone marrow edema pattern that persists for more than 12 months, fluid signal intensity at the graft-host interface, or surface collapse may be indicative of graft rejection or incomplete incorporation (Fig 12) (2,7,21,26,27).
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Figure 12a. (a, b) Sagittal fast SE images acquired with (a) and without (b) fat suppression 20 months after a fresh osteochondral allograft in a 32-year-old woman show intense bone marrow edema in the graft (arrow in a). The graft is proud, with a severe anterior offset indicative of poor integration. (c) Sagittal fast SE image acquired 18 months later shows revision of the graft with a new autologous osteochondral plug (arrow).
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Figure 12b. (a, b) Sagittal fast SE images acquired with (a) and without (b) fat suppression 20 months after a fresh osteochondral allograft in a 32-year-old woman show intense bone marrow edema in the graft (arrow in a). The graft is proud, with a severe anterior offset indicative of poor integration. (c) Sagittal fast SE image acquired 18 months later shows revision of the graft with a new autologous osteochondral plug (arrow).
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Figure 12c. (a, b) Sagittal fast SE images acquired with (a) and without (b) fat suppression 20 months after a fresh osteochondral allograft in a 32-year-old woman show intense bone marrow edema in the graft (arrow in a). The graft is proud, with a severe anterior offset indicative of poor integration. (c) Sagittal fast SE image acquired 18 months later shows revision of the graft with a new autologous osteochondral plug (arrow).
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Autologous Chondrocyte Implants and Matrix-associated Transplants
MR imaging was reported to be effective for detecting the incomplete filling of a defect (Fig 13) (3,5–9). Underfilling may be described either as filling to more than 50% or as filling to less than 50% of the depth of the adjacent native cartilage. In approximately 2% of cases, underfilling is severe enough to require further surgery (32). Graft hypertrophy may occur 3–7 months after autologous chondrocyte implantation and has been reported as a complication in 10%–63% of cases (28,31,34). Integration between the repair tissue and the native cartilage is considered complete if the margin between the two is continuous, without any discernible interface; if there is a fissure, the peripheral integration is incomplete.
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Figure 13a. (a) Sagittal inversion-recovery fast SE image acquired 1 year after autologous chondrocyte implantation for an osteochondral lesion of the medial talar dome in a 34-year-woman shows persistent bone marrow edema within the talus. (b, c) Sagittal (b) and coronal (c) intermediate-weighted fast SE images show incomplete filling of the defect in comparison with the adjacent cartilage. The surface of the reparative tissue (arrow) is irregular, with signal that is hyperintense in comparison with that of native cartilage.
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Figure 13b. (a) Sagittal inversion-recovery fast SE image acquired 1 year after autologous chondrocyte implantation for an osteochondral lesion of the medial talar dome in a 34-year-woman shows persistent bone marrow edema within the talus. (b, c) Sagittal (b) and coronal (c) intermediate-weighted fast SE images show incomplete filling of the defect in comparison with the adjacent cartilage. The surface of the reparative tissue (arrow) is irregular, with signal that is hyperintense in comparison with that of native cartilage.
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Figure 13c. (a) Sagittal inversion-recovery fast SE image acquired 1 year after autologous chondrocyte implantation for an osteochondral lesion of the medial talar dome in a 34-year-woman shows persistent bone marrow edema within the talus. (b, c) Sagittal (b) and coronal (c) intermediate-weighted fast SE images show incomplete filling of the defect in comparison with the adjacent cartilage. The surface of the reparative tissue (arrow) is irregular, with signal that is hyperintense in comparison with that of native cartilage.
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At fast SE MR imaging in the early postoperative period, the signal in the repair cartilage appears hyperintense when compared with that in the native cartilage and the periosteal cover (34). During the late postoperative period (approximately 12 months after implantation), the signal intensity of the repair cartilage decreases steadily until it approaches that of the adjacent native cartilage (Fig 14) (2,7,32,33,36,40).
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Figure 14a. Sagittal intermediate-weighted fast SE images acquired after implantation of an autologous chondrocyte graft over the medial femoral condyle in a 31-year-old man show gradual maturation of the graft. (a) Image acquired 6 weeks after surgery shows a high-signal-intensity graft matrix and hypointense signal in the intact overlying periosteum (arrow). (b) Image acquired 20 months after surgery shows the incorporation of reparative cartilage and periosteum (arrow), with the border between the graft and native tissue now appearing indistinct.
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Figure 14b. Sagittal intermediate-weighted fast SE images acquired after implantation of an autologous chondrocyte graft over the medial femoral condyle in a 31-year-old man show gradual maturation of the graft. (a) Image acquired 6 weeks after surgery shows a high-signal-intensity graft matrix and hypointense signal in the intact overlying periosteum (arrow). (b) Image acquired 20 months after surgery shows the incorporation of reparative cartilage and periosteum (arrow), with the border between the graft and native tissue now appearing indistinct.
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Three-dimensional T1-weighted fat-suppressed GRE images show low signal intensity in healthy repair tissue after autologous chondrocyte implantation (Fig 15). The signal intensity of the repair tissue increases over time until it resembles that of the native cartilage, 6–9 months after transplantation (41).
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Figure 15. Coronal fat-suppressed 3D spoiled GRE image (flip angle, 40°), acquired 3 months after autologous chondrocyte implantation in an osteochondral defect in the medial talar dome in a 54-year-old woman, shows hypertrophied repair cartilage (arrow) with signal that is hypointense compared with that of adjacent native cartilage.
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After autologous chondrocyte implantation, the edema in subchondral bone marrow typically decreases with time. Some authors have suggested that the persistence of bone marrow edema beyond 12 months, or an increase in the severity of edema, is abnormal and requires close clinical follow-up (21,32,33). By contrast, others have suggested that the persistence of a bone marrow edema pattern is a sign of undetermined importance (13,42,43). The presence of fluid signal intensity between the cartilage repair tissue and the subchondral bone is indicative of delamination, which occurs most commonly during the first 6 months after implantation (9,32). Formation of a subchondral cyst beneath the interface also suggests a failure of integration (33).
Observations of irregularities in the graft surface at follow-up MR imaging are relatively common (21,33) and accord with the findings at follow-up arthroscopy (Fig 16). However, the worsening of surface defects over time should be considered abnormal. Incongruence may be secondary to surface changes such as a fissure or an ulcer (9). Less common complications specific to autologous chondrocyte implantation are intraarticular adhesions and hypertrophic synovitis (8,9,31,32,37,44).
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Figure 16a. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired 1 year after autologous chondrocyte transplantation in an osteochondral defect in the medial talar dome in a 29-year-old man show complete filling of the defect by repair cartilage (arrow), which has an irregular surface and signal that is hyperintense compared with that of adjacent native cartilage. (c) Arthroscopic image obtained at follow-up shows fibrillation of the repair cartilage (arrows) over the talus.
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Figure 16b. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired 1 year after autologous chondrocyte transplantation in an osteochondral defect in the medial talar dome in a 29-year-old man show complete filling of the defect by repair cartilage (arrow), which has an irregular surface and signal that is hyperintense compared with that of adjacent native cartilage. (c) Arthroscopic image obtained at follow-up shows fibrillation of the repair cartilage (arrows) over the talus.
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Figure 16c. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired 1 year after autologous chondrocyte transplantation in an osteochondral defect in the medial talar dome in a 29-year-old man show complete filling of the defect by repair cartilage (arrow), which has an irregular surface and signal that is hyperintense compared with that of adjacent native cartilage. (c) Arthroscopic image obtained at follow-up shows fibrillation of the repair cartilage (arrows) over the talus.
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Biodegradable Pins
Biodegradable pins appear as linear tracts with low signal intensity on T1-weighted MR images acquired in the first 6 months after surgery. By the end of the 1st postoperative year, the pin sites are most conspicuous on T2-weighted images, where they appear as linear tracts of high signal intensity indicative of hydrolyzed debris or fluid. Finally, after 2 years, 80% of these pins are no longer visible at MR imaging (Fig 17) (2,21).
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Figure 17a. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired in a 16-year-old boy show features of unstable osteochondritis dissecans, with a focal fissure (arrow in a) that extends into the interface between the donor site and devitalized subchondral bone. Delamination is visible at the inter-condylar notch. (c, d) Coronal (c) and sagittal (d) intermediate-weighted fast SE images acquired 17 months after débridement and fixation with bioabsorbable pins show hypertrophy of the posteromedial subchondral matrix (arrows in d) and linear tracts extending deep to the repair site.
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Figure 17b. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired in a 16-year-old boy show features of unstable osteochondritis dissecans, with a focal fissure (arrow in a) that extends into the interface between the donor site and devitalized subchondral bone. Delamination is visible at the inter-condylar notch. (c, d) Coronal (c) and sagittal (d) intermediate-weighted fast SE images acquired 17 months after débridement and fixation with bioabsorbable pins show hypertrophy of the posteromedial subchondral matrix (arrows in d) and linear tracts extending deep to the repair site.
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Figure 17c. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired in a 16-year-old boy show features of unstable osteochondritis dissecans, with a focal fissure (arrow in a) that extends into the interface between the donor site and devitalized subchondral bone. Delamination is visible at the inter-condylar notch. (c, d) Coronal (c) and sagittal (d) intermediate-weighted fast SE images acquired 17 months after débridement and fixation with bioabsorbable pins show hypertrophy of the posteromedial subchondral matrix (arrows in d) and linear tracts extending deep to the repair site.
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Figure 17d. (a, b) Coronal (a) and sagittal (b) intermediate-weighted fast SE images acquired in a 16-year-old boy show features of unstable osteochondritis dissecans, with a focal fissure (arrow in a) that extends into the interface between the donor site and devitalized subchondral bone. Delamination is visible at the inter-condylar notch. (c, d) Coronal (c) and sagittal (d) intermediate-weighted fast SE images acquired 17 months after débridement and fixation with bioabsorbable pins show hypertrophy of the posteromedial subchondral matrix (arrows in d) and linear tracts extending deep to the repair site.
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Recommendations and Future Directions
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Follow-up MR imaging studies should be performed at 3–6 postoperative months and at 1 year. Initial follow-up imaging at 3–6 months allows an assessment of the volume and the integration of repair tissue. Subsequent imaging in the 1st postoperative year allows an evaluation of the maturation of the graft and identification of any complications (8). Whereas some authors suggest that MR arthrography is superior to MR imaging because it allows a more accurate characterization of the recipient site (9,21,39), others have reported the effective evaluation of repair tissue without the use of contrast agents (22,34). The correlation of matrix characteristics with glycosaminoglycan and collagen content information obtained with techniques such as delayed gadolinium-enhanced MR imaging and T2 mapping may be an interesting goal for future studies (7,8,13,14,16).
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Conclusions
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MR imaging with the use of appropriate acquisition protocols and matrix assessment techniques can provide detailed information about the natural history of cartilage repair in joints such as the knee and ankle as well as in the shoulder, elbow, and hip. Imaging of repair cartilage is needed to determine the extent of defect filling, the degree of peripheral integration with the host tissue, the morphologic structure and signal intensity of the repair tissue, and the integrity of the host cartilage. Familiarity with the various surgical techniques used and with their MR imaging appearances is important for the accurate assessment of repair tissue.
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Acknowledgments
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The authors thank Kyung Tai Lee, MD, and Nam-Hong Choi, MD, for providing the surgical information and Seok Hoon Lee, MD, for his help in preparing the digital images.
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Footnotes
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Abbreviations: GRE = gradient echo, SE = spin echo, 3D = three-dimensional
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References
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Abstract
Introduction
