MRI of Osteochondral Defects of the Lateral Femoral Condyle: Incidence and Pattern of Injury After Transient Lateral Dislocation of the Patella

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1 Sanders et al. MRI of Osteochond ral Defects of the Lateral Femoral Condyle Musculoskeletal Imaging Clinical Observations A C M E D E N T U R I C A L I M A G I N G AJR 2006; 187: X/06/ American Roentgen Ray Society Y Timothy G. Sanders 1,2 Narayan Babu Paruchuri 1,3 Michael B. Zlatkin 1,3 O Sanders TG, Paruchuri NB, Zlatkin MB F Keywords: cartilage, knee, MRI, musculoskeletal imaging, trauma DOI: /AJR Received August 21, 2005; accepted after revision November 12, National Musculoskeletal Imaging, 1930 N Commerce Pkwy., Suite 5, Weston, FL Address correspondence to T. G. Sanders (timothy.sanders@nationalrad.com). 2 Uniformed Services University, Bethesda, MD 3 Department of Radiology, Miller School of Medicine, University of Miami, Miami, FL MRI of Osteochondral Defects of the Lateral Femoral Condyle: Incidence and Pattern of Injury After Transient Lateral Dislocation of the Patella OBJECTIVE. The typical bone bruise pattern involving the anterolateral femoral condyle and inferomedial patella after transient lateral dislocation of the patella is a well-described MRI finding. In our study, however, we sought to determine the incidence and location of lateral femoral condyle osteochondral injuries after transient lateral dislocation of the patella. CONCLUSION. Osteochondral defects of the lateral femoral condyle are a common sequela after transient lateral patellar dislocation. A significant number of osteochondral injuries involve the midlateral weight-bearing portion of the lateral femoral condyle and are more posterior than would be expected after transient dislocation of the patella. ransient lateral patellar dislocation is a common injury that typi- T cally occurs in the young, athletic individual. The typical MRI findings after transient lateral dislocation of the patella have been well described and include a bone contusion pattern involving the inferomedial pole of the patella and the anterolateral aspect of the nonarticular portion of the lateral femoral condyle. Early reports of MRI findings after transient dislocation of the patella described chondral defects of the patella but made no mention of chondral injuries of the lateral femoral condyle [1 5]. More recently, there has been brief mention in both the radiology and orthopedic literature of osteochondral injuries involving not only the patellar articular surface but also the articular surface of the lateral femoral condyle [6 10]. We also have observed several patients with osteochondral injuries of the lateral femoral condyle after transient dislocation of the patella. Consequently, our objective was to determine the incidence of lateral femoral condyle osteochondral injuries and to better define the locations and patterns of chondral injury involving the lateral femoral condyle after transient dislocation of the patella based on MRI and follow-up arthroscopy. Materials and Methods Subjects The reports of 476 consecutive MRI examinations of the knee on patients under 36 years of age, which were performed at six referring outpatient imaging centers between January 1, 2005, and April 20, 2005, were retrospectively reviewed. Each of the original interpretations was performed by one of four experienced, fellowship-trained musculoskeletal radiologists. The reports were reviewed for evidence of prior lateral patellar dislocation. Findings suggestive of a prior transient lateral patellar dislocation included mention of marrow edema involving both the anterolateral femoral condyle and the inferomedial patella or mention in the report of transient dislocation of the patella. A total of 25 patients (male:female ratio, 9:16; age range, years; mean age, 17 years) were found to have MRI evidence of prior transient dislocation of the patella. The MR images of these 25 patients were then reviewed and 10 patients were found to have osteochondral defects involving the lateral femoral condyle. The final study group was composed of these 10 patients (male:female ratio, 7:3; age range, years; mean age, 18 years). MRI These MRI examinations were performed at six different outpatient imaging facilities. Examinations at three of the centers were obtained using high-field-strength (1.5 T) magnets, and examinations at the other three centers were obtained using low-field-strength (0.2 T) magnets. Two of the high-field-strength magnets were Horizon LX units (GE Healthcare) and the third was a Signa (GE Healthcare). Two centers with low-field-strength magnets used Lunar Escan units (GE Healthcare), and the third center used a Magnetom Jazz (Siemens Medical Solutions) AJR:187, November 2006

2 MRI of Osteochondral Defects of the Lateral Femoral Condyle TABLE 1: MRI and Surgical Findings of Lateral Femoral Condyle Chondral Defects Patient No. MRI Findings Chondral Defects Sex Age (y) Size (cm) Grade Location Arthroscopy Findings of Chondral Defects (cm) 1 M WB M WB Finding present but no measurements given 3 F T Finding present but no measurements given 4 F C M C Not mentioned 6 F WB Not mentioned 7 M T M T No surgery 9 M WB Finding present but no measurements given 10 M WB No surgery Note WB = weight-bearing articular surface of the lateral femoral condyle, T = trochlear groove articular surface, C = combined involvement of trochlear groove and lateral femoral condyle articular surfaces. Although individual parameters varied slightly from examination to examination, the routine MRI performed on the high-field-strength magnets consisted of coronal inversion-recovery (TR/TE range, 5,800 3,016/25 39; inversion time, 150 msec; field of view range, mm; matrix range, pixels; slice thickness, 4.0 mm; skip, 1.0 mm), coronal proton-density (2,000/15; field of view, 150 mm; matrix, pixels; slice thickness, 4.0 mm; skip, 1.0 mm), or coronal T1 (500/16; field of view, 140 mm; matrix, pixels; slice thickness, 4.0 mm; skip, 1.0 mm), sagittal proton-density (1,950 2,766/14; field of view, mm; matrix range, pixels; slice thickness, mm; skip, mm), sagittal fat-saturated proton-density (2,650 4,366/13 16; field of view, mm; matrix range, ; slice thickness, mm; skip, mm), sagittal T2 (3,700 4,766/80 87; field of view range, mm; matrix range, ; slice thickness, mm; skip, 0.5 mm), and axial fat-saturated proton-density (3,116 5,400/25 35; field of view range, mm; matrix range, ; slice thickness, 4.0 mm; skip, 1.0 mm) sequences. Routine MRI performed on the low-fieldstrength magnets consisted of axial fast spin-echo T2-weighted (2,720 3,040/80 90; field of view, 159 mm; matrix range, pixels; slice thickness, mm; skip, 0 mm), coronal T1- weighted ( /18 26; field of view range, mm; matrix range, pixels; slice thickness, mm; skip, 0 mm), coronal inversion-recovery (1,560 2,120/16 28; inversion time, 20 msec; field of view range, mm; matrix, pixels; slice thickness, mm; skip, 0 mm), sagittal T1- weighted ( /18 26; field of view range, mm; matrix range, pixels; slice thickness, mm; skip, 0 mm), sagittal fast spin-echo T2-weighted (2,720 2,800/80 90; field of view range, mm; matrix range, pixels; slice thickness, mm; skip, 0 mm), sagittal 3D volume gradient-echo (38 50/16; field of view range, mm; matrix range, pixels; slice thickness, mm; skip, 0 mm) sequences. Image Analysis The 25 cases were retrospectively reviewed in consensus by two musculoskeletal radiologists who had 7 and 18 years, respectively, of clinical experience in musculoskeletal radiology. Images were first evaluated for the presence of the typical bone contusion pattern involving the anterolateral femoral condyle and the inferomedial patella. Next, the images were evaluated for the presence of an osteochondral injury or defect involving the lateral femoral condyle, and if present, the osteochondral defect was graded according to a modified Outerbridge classification system [5]: grade 1, chondral softening or blistering with an otherwise intact surface; grade 2, shallow superficial fissuring or ulceration involving less than 50% of the depth of the articular surface; grade 3, deep ulceration, fissure, or flap that involved more than 50% of the depth of the articular cartilage without exposure of subchondral bone; grade 4, full-thickness chondral defect with exposure of subchondral bone; and grade 5, full-thickness chondral abnormalities with underlying cortical defect. Next, the size (using electronic calipers) and location of the lateral femoral condyle defect were recorded. Transverse dimensions were obtained from the coronal images, whereas anteroposterior dimensions were obtained from the sagittal images. Osteochondral defects located anterior to the anterior margin of the anterior horn of the lateral meniscus were designated as involving the articular surface of the trochlear groove. Chondral defects located posterior to the anterior margin of the anterior horn of the lateral meniscus were designated as involving the weight-bearing aspect of the lateral femoral condyle. Chondral defects overlapping the anterior margin of the anterior horn of the lateral meniscus were considered to be involving both chondral surfaces. The location of the lateral femoral condyle marrow edema was recorded relative to the position of the chondral defect. Surgical Correlation Operative reports were obtained on all patients who underwent follow-up arthroscopy. The operative reports were reviewed after MR image analysis was completed, and surgical findings were compared with the MRI results. The arthroscopic surgeries were performed by a number of orthopedic surgeons with varying levels of experience and expertise. Results Table 1 summarizes the patients profiles and the results of MRI and surgical findings. MRI Findings Of the 25 patients for whom there was MRI evidence of prior transient dislocation of the patella, 10 (40%) were found to have chondral defects involving the articular surface of the lateral femoral condyle. The osteochondral defects were full-thickness (grade 4) chondral defects with exposure of the subchondral bone in seven (70%) of 10 patients and full-thickness chondral defects with underlying cortical abnormalities (grade 5) in three (30%) of 10 patients. The average size of the defects was 1.2 cm in the anteroposterior diameter and 1.0 cm in the transverse diameter. The osteochondral defects involved the articular surface of the trochlear groove in three (30%) of the 10 patients (Fig. 1), whereas in five (50%) of the 10 patients, the chondral defects were isolated to the midlateral weightbearing portion of the lateral femoral condyle (Figs. 2 and 3). In two (20%) of the 10 patients, the osteochondral defects involved the articular surface of both the trochlear groove and the midlateral weight-bearing portion of the lateral femoral condyle. Subchondral marrow edema was present underlying the chondral defects in 10 (100%) patients. In each case, the subchondral marrow edema was centered anterior to the chondral defect, with the chondral defect AJR:187, November

3 Sanders et al. located along the posterior margin of the subchondral edema (Figs. 1 and 2). Surgical Results At the time of this study, eight (80%) of the 10 patients with osteochondral injuries of the lateral femoral condyle identified on MRI had undergone follow-up arthroscopic surgery. Arthroscopy confirmed the presence of chondral defects of the lateral femoral condyle in six (75%) of the eight cases. The dimensions of the chondral defects were described in the operative report in only three of those six patients. Of the two lesions not identified at the time of arthroscopy, one was observed on a high-field-strength system and the other on a low-field-strength system. Discussion Lateral patellar dislocation most commonly occurs in young, active individuals as A B C Fig year-old boy with pain and swelling after fall. A, Axial T2-weighted image (TR/TE, 3,800/35) shows bone contusions involving both nonarticular surface of lateral femoral condyle (long arrow) and inferomedial aspect of patella (short arrow). B, Coronal STIR image (3,016/29) with fat saturation shows bone bruise involving non weight-bearing portion of lateral femoral condyle. An osteochondral fragment (long arrow) is located immediately adjacent to osteochondral defect (short arrow) involving marginal aspect of lateral trochlear groove articular cartilage. C, T2-weighted sagittal proton-density image (3,000/14) with fat saturation shows small osteochondral defect (short arrows) involving lower aspect of lateral trochlear groove. Small osteochondral fragment (long arrow) is noted immediately adjacent to defect. Note that osteochondral injury is located at posterior margin of lateral femoral condyle contusion. A B C Fig year-old man with persistent medial joint line tenderness after twisting injury of knee. A, Axial T2-weighted image (TR/TE, 3,766/35) with fat saturation shows classic bone contusion pattern involving inferomedial patella (short arrow) and nonarticular surface of lateral femoral condyle (long arrow). B, Coronal STIR image (5,766/39) shows full-thickness chondral defect (long arrows) involving midlateral weight-bearing portion of lateral femoral condyle with subjacent marrow edema (short arrow). C, Sagittal proton-density image (3,500/15) with fat saturation shows full-thickness chondral defect (long arrows) involving midlateral weight-bearing portion of lateral femoral condyle centered at posterior margin of lateral femoral condyle bone contusion (short arrows) AJR:187, November 2006

4 MRI of Osteochondral Defects of the Lateral Femoral Condyle the result of a twisting injury to the knee during participation in an athletic event. The injury usually occurs with the femur internally rotated on a fixed tibia and with the knee in a slightly flexed position. As the individual attempts to straighten or extend the knee, contraction of the quadriceps muscles, rather than extending the knee, places lateral force on the patella, which leads to the lateral subluxation or dislocation of the patella. The patient usually falls to the ground in pain, at which time the patella reduces spontaneously. These dislocations are often transient, and as a result, the patient and clinician may be unaware of the true nature of the injury. After the injury, the patient usually seeks medical attention because of persistent pain and swelling, which is typically located along the medial joint line, and the patient occasionally complains of a locking or catching sensation. A B C Fig year-old man evaluated with MRI on a low-field-strength unit after twisting injury to knee. A, Coronal STIR image (TR/TE, 1,560/16) shows typical bone contusion pattern (arrows) involving nonarticular portion of lateral femoral condyle, indicating recent transient dislocation of patella. B, Coronal STIR image (1,560/16) posterior to A shows full-thickness chondral defect (long arrows) involving midlateral weight-bearing portion of lateral femoral condyle with subjacent marrow edema (short arrow). C, Sagittal T2-weighted image (2,800/80) shows full-thickness chondral defect (arrows) involving midlateral weight-bearing portion of lateral femoral condyle. A The clinical history and physical findings may be inadequate to establish the exact nature of the injury, and as a result MRI can play a key role both in establishing the correct diagnosis and in delineating the extent of osseous and soft-tissue injury essential for directing appropriate surgical management [1]. Transient dislocation of the patella occurs in two separate stages. During the first stage, the patella translates laterally to lie along the lateral aspect of the lateral femoral condyle. During the second phase of the injury, the patella reduces to its normal position within the trochlear groove. It is during this stage of injury that the patella strikes against the nonarticular surface of the anterior aspect of the lateral femoral condyle as it attempts to reduce, thus giving rise to the classic bone contusion pattern. In addition to the bone contusion pattern, osteochondral injuries of the patella have also B Fig. 4 Artwork illustrates proposed mechanism of injury to patellar and femoral articular surfaces during two stages of transient dislocation of patella. A, Dislocation stage. During first stage of transient dislocation of patella, as patella dislocates laterally, shearing mechanism can result in damage to either patellar (short arrow) or lateral femoral condyle (long arrow) articular surfaces. B, Reduction stage. During second stage of transient dislocation of patella, as patella bounces back into normal position, impaction of medial patellar facet against nonarticular surface of lateral femoral condyle results in bone contusions of lateral femoral condyle and medial aspect of patella (long arrows). Osteochondral impaction injury can occur to medial patellar facet (short arrow), but concave configuration of trochlear groove protects femoral articular surface from impaction injury during reduction stage. been reported, and these range from mild articular cartilage surface irregularity to large displaced osteochondral fractures [1 4]. These osteochondral injuries typically involve the inferomedial pole, the median eminence of the patella [5], or both and can result either from a shearing injury at the time of dislocation or reduction or from an impaction injury as the patella strikes the nonarticular surface of the anterolateral femoral condyle. Potential softtissue injuries include a partial- or full-thickness tear of the medial soft-tissue restraints, including the medial patellofemoral ligament and medial retinaculum [7, 11, 12]. Hemarthrosis is common and loose intraarticular bodies are occasionally present [1 3]. In addition to the well-described osteochondral injuries of the lower pole of the patella, there have been a few reports in the literature regarding femoral condyle osteochondral inju- AJR:187, November

5 Sanders et al. ries [6 10]. One recent report describes seven patients in whom lateral femoral condyle shearing fractures were identified on conventional radiography after lateral patellar dislocation. In this report, the location of the donor sites was not specifically described, but review of the images in the report shows the donor sites to be located in the trochlear groove portion of the lateral femoral condyle [6]. Another recent article describing MRI findings of osteochondral injuries after acute lateral patellar dislocation reports four (5%) of 82 patients with osteochondral injuries involving the lateral trochlear groove [7]. There is no mention in this report of osteochondral injuries involving the weight-bearing aspect of the lateral femoral condyle. A third recent report in the orthopedic literature describes seven patients with osteochondral injuries involving the midlateral weight-bearing portion of the lateral femoral condyle after patellar dislocation. This report does not attempt to determine the frequency of this injury but simply describes the arthroscopic findings in seven patients [8]. Another recent report in the orthopedic literature describes osteochondral lesions of the lateral femoral condyle in 12 (31%) of 39 consecutive patients who underwent arthroscopy after transient dislocation of the patella [9]. Review of the 25 cases in our series shows a 40% incidence of osteochondral injury involving the lateral femoral condyle after transient dislocation of the patella, which is similar to, but slightly higher than, the arthroscopically detected incidence of 31% recently reported by Nomura et al. [9]. If only those patients with injuries isolated to the lateral trochlear groove are included, the incidence in our series is 12%, which is more in line with the 5% incidence reported by Elias et al. [7], who evaluated the knee using MRI. Each of the lateral femoral chondral defects in our series was a full-thickness chondral defect with or without underlying cortical abnormality, which is similar to the findings reported by Swischuk et al. [6]. The location of the lateral femoral condyle osteochondral injury was observed to maintain a constant relationship with regard to the location of the lateral femoral condyle bone contusion. In each case, the chondral defect was located at the posterior margin of the bone contusion. This proved to be a highly predictable pattern of injury and was observed in each of the 10 cases. Based on the known mechanism of injury and on the location and pattern of osteochondral injuries and marrow contusions recorded in our Results section, we propose the following mechanism of injury regarding lateral femoral condyle osteochondral injuries. The configuration of the articular surface of the patella is convex, whereas that of the trochlear groove is concave. This configuration allows for a potential shearing injury to involve the articular surface of either the patella or the femoral condyle during the first stage of dislocation. During reduction, the articular surface of the medial aspect of the lower pole of the patella first impacts the nonarticular portion of the lateral femoral condyle, resulting in the classic bone contusion. The patella then proceeds to bounce back into its normal position within the trochlear groove. The concave configuration of the trochlear groove protects its articular surface from injury during reduction of the patella, whereas the convex shape of the patella places its articular cartilage at risk for injury during the reduction stage as well. The fact that the patella is at risk for a shearing or impaction injury or both during both dislocation and reduction but the femoral articular surface is at risk only during dislocation is likely the reason for the higher incidence of articular cartilage lesions involving the patella [6] (Fig. 4). It is likely therefore that the lateral femoral condyle osteochondral injuries result from a shearing force that occurs during the first stage of injury. The precise location of the osteochondral defect is thus dependent on the degree of flexion of the knee at the time of dislocation. Osteochondral injuries involving the more posteriorly located midlateral weight-bearing portion of the lateral femoral condyle suggest that the patient s knee was in a greater degree of flexion at the time of dislocation, whereas a more anteriorly located osteochondral injury involving the trochlear groove suggests that the knee was more extended at the time of dislocation. The nonarticular marrow edema involving the lateral femoral condyle is always centered anterior to the chondral defect of the lateral femoral condyle, which suggests that the knee is likely more extended as the second stage of injury begins. The chondral injury is consistently located at the posterior margin of the nonarticular marrow edema (Figs. 1 and 2). This finding further supports the theory that the shearing injury occurs during the first stage of dislocation with the knee in a greater state of flexion, and as the knee extends, the patella rebounds and impacts the lateral aspect of the lateral femoral condyle anterior to the site of the chondral shearing injury. The significance of these findings is that when evaluating the MR images of a patient with bone contusions indicative of a prior transient dislocation of the patella, the radiologist should carefully evaluate the articular cartilage at the level of the posterior margin of the bone contusion because this is the most likely location for a femoral osteochondral injury to occur. Although osteochondral defects of the patella are usually best appreciated in the axial imaging plane, the lateral femoral condyle osteochondral injuries in this series were best depicted in either the coronal or the sagittal imaging plane. Multiple surgical options are now available for repairing osteochondral lesions, and this fact, combined with the fact that the majority of patients experiencing transient dislocation of the patella are young, increases the importance of accurately identifying these lesions [8, 12, 13]. High-grade osteochondral injuries involving the patella or the lateral femoral condyle are often treated with abrasion chondroplasty or autologous chondrocyte transplantation, and treatment of these lesions has been shown to improve functional outcome in adolescent athletes [14]. Limitations of this study include the small sample size. Nonetheless, both the MRI and surgical results suggest that lateral femoral condyle osteochondral lesions are common, and a search pattern should include an evaluation of the femoral condyle articular surface in both the sagittal and coronal imaging planes after transient dislocation of the patella. An additional limitation of this study is that multiple surgeons with varying skill levels operated on these patients, and there was no standardization of the surgical reports to confirm the size or precise location of the osteochondral lesions. In addition, two of the patients have not undergone follow-up surgery. In two of the patients, the surgeon reported no evidence of femoral chondral injury. This could in part be related to the skill of the arthroscopist or could possibly be explained by the fact that what appeared to be an osteochondral defect on MRI was merely volume averaging with adjacent soft-tissue structures because these defects were often observed at the far lateral aspect of either the trochlear groove or the weight-bearing aspect of the lateral femoral condyle. It is also possible that a chondral defect was present but not considered by the surgeon to be significant enough to warrant treatment or mention in the operative report. That said, there were still six chondral defects confirmed by the arthroscopist, making this an important injury to be aware of AJR:187, November 2006

6 MRI of Osteochondral Defects of the Lateral Femoral Condyle Another limitation of this study is that MRI examinations were performed on different scanners, which could affect the ability to detect chondral abnormalities. Lowfield-strength magnets are limited in their ability to detect chondral abnormalities. The use of low-field-strength magnets may have actually resulted in underdetection of these lesions. Previous studies have shown that high-field-strength systems provide significantly better diagnostic performance than the low-field-strength systems when evaluating grade 2 or 3 chondral defects, but lowfield-strength systems can reliably evaluate high-grade chondral lesions, possibly because they imbibe joint fluid, thus providing adequate contrast [15, 16]. In summary, our results suggest that after transient lateral dislocation of the patella, osteochondral injuries of the lateral femoral condyle occur more commonly than has been previously reported in the MR literature. Although these lesions can occur in the region of the lateral trochlear groove as previously reported in the radiology literature, it is actually more common for this lesion to occur in the midlateral weight-bearing aspect of the lateral femoral condyle at the posterior margin of the lateral femoral condyle bone contusion. This area of the lateral femoral condyle should be closely evaluated for osteochondral injury on both sagittal and coronal MR images in all patients with evidence of prior transient dislocation of the patella. References 1. Sanders TG, Medynski MA, Feller JF, Lawhorn KW. Bone contusion patterns of the knee at MR imaging: footprint of the mechanism of injury. Radio- Graphics 2000; 20[spec no]:s135 S Kirsch MD, Fitzgerald SW, Friedman H, Rogers LF. Transient lateral patellar dislocation: diagnosis with MR imaging. AJR 1993; 161: Virolainen H, Visuri T, Kuusela T. Acute dislocation of the patella: MR findings. Radiology 1993; 189: Lance E, Deutsch AL, Mink JH. Prior lateral patellar dislocation: MR imaging findings. Radiology 1993; 189: Nomura E, Inoue M. Cartilage lesions of the patella in recurrent patellar dislocation. Am J Sports Med 2004; 32: Swischuk LE, Hernandez JA, Hendrick EP, Yngve DA, Carmichael KD. Lateral femoral condylar shearing fractures. Emerg Radiol 2003; 10: Elias DA, White LM, Fithian DC. Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft tissue restraints and osteochondral injuries of the inferomedial patella. Radiology 2002; 225: Mashoof AA, Scholl MD, Lahav A, Greis PE, Burks RT. Osteochondral injury to the mid-lateral weight-bearing portion of the lateral femoral condyle associated with patella dislocation. Arthroscopy 2005; 21: Nomura E, Inoue M, Kurimara M. Chondral and osteochondral injuries associated with acute patellar dislocation. Arthroscopy 2003; 19: Nietosvaara Y, Aalto K, Kallio PE. Acute patellar dislocation in children: incidence and associated osteochondral fractures. J Pediatr Orthop 1994; 14: Sanders TG, Morrison WB, Singleton BA, Miller MD, Cornum KG. Medial patellofemoral ligament injury following acute transient dislocation of the patella: MR findings with surgical correlation in 14 patients. J Comput Assist Tomogr 2001; 25: Arendt EA, Fithian DC, Cohen E. Current concepts of lateral patella dislocation. Clin Sports Med 2002; 21: Fithian DC, Paxton EW, Stone ML, et al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med 2004; 32: Mithofer K, Minas T, Peterson L, Yeon H, Micheli LJ. Functional outcome of knee articular cartilage repair in adolescent athletes. Am J Sports Med 2005; 33: Woertler K, Strothmann M, Tombach B, Reimer P. Detection of articular cartilage lesions: experimental evaluation of low- and high-field-strength MR imaging at 0.18 and 1.0 T. J Magn Reson Imaging 2000; 11: Ahn JM, Kwak SM, Kang HS, et al. Evaluation of patellar cartilage in cadavers with a low-fieldstrength extremity-only magnet: comparison of MR imaging sequences, with macroscopic findings as the standard. Radiology 1998; 208:57 62 AJR:187, November