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Clinical Observations |
1 National Musculoskeletal Imaging, 1930 N Commerce Pkwy., Suite 5, Weston, FL
33326.
2 Uniformed Services University, Bethesda, MD
3 Department of Radiology, Miller School of Medicine, University of Miami,
Miami, FL 33136.
Received August 21, 2005;
accepted after revision November 12, 2005.
Address correspondence to T. G. Sanders
(timothy.sanders{at}nationalrad.com).
Abstract
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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.
Keywords: cartilage • knee • MRI • musculoskeletal imaging • trauma
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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).
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, 140-149 mm; matrix range, 256-320 x 224 pixels; slice thickness, 4.0 mm; skip, 1.0 mm), coronal proton-density (2,000/15; field of view, 150 mm; matrix, 320 x 224 pixels; slice thickness, 4.0 mm; skip, 1.0 mm), or coronal T1 (500/16; field of view, 140 mm; matrix, 384 x 224 pixels; slice thickness, 4.0 mm; skip, 1.0 mm), sagittal proton-density (1,950-2,766/14; field of view, 140-150 mm; matrix range, 320-384 x 192-224 pixels; slice thickness, 3.0-3.5 mm; skip, 0.5-1.0 mm), sagittal fat-saturated proton-density (2,650-4,366/13-16; field of view, 140-150 mm; matrix range, 256-384 x 224-256; slice thickness, 3.0-3.5 mm; skip, 0.5-1.0 mm), sagittal T2 (3,700-4,766/80-87; field of view range, 140-150 mm; matrix range, 320-384 x 224; slice thickness, 3.0-3.5 mm; skip, 0.5 mm), and axial fat-saturated proton-density (3,116-5,400/25-35; field of view range, 140-160 mm; matrix range, 384-320 x 192-224; slice thickness, 4.0 mm; skip, 1.0 mm) sequences.
Routine MRI performed on the low-field-strength magnets consisted of axial fast spin-echo T2-weighted (2,720-3,040/80-90; field of view, 159 mm; matrix range, 192 x 160-184 pixels; slice thickness, 4.0-5.0 mm; skip, 0 mm), coronal T1-weighted (620-850/18-26; field of view range, 159-179 mm; matrix range, 256-192 x 160-192 pixels; slice thickness, 4.0-5.0 mm; skip, 0 mm), coronal inversion-recovery (1,560-2,120/16-28; inversion time, 20 msec; field of view range, 159-179 mm; matrix, 192 x 160 pixels; slice thickness, 4.0-6.0 mm; skip, 0 mm), sagittal T1-weighted (580-920/18-26; field of view range, 159-179 mm; matrix range, 192-256 x 192 pixels; slice thickness, 4.0-4.5 mm; skip, 0 mm), sagittal fast spin-echo T2-weighted (2,720-2,800/80-90; field of view range, 159-179 mm; matrix range, 192 x 160-184 pixels; slice thickness, 4.0-4.5 mm; skip, 0 mm), sagittal 3D volume gradient-echo (38-50/16; field of view range, 178-198 mm; matrix range, 192 x 160-192 pixels; slice thickness, 1.7-3.5 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.
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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 (Figs. 1A, 1B, and 1C), whereas in five (50%) of the 10 patients, the chondral defects were isolated to the midlateral weight-bearing portion of the lateral femoral condyle (Figs. 2A, 2B, 2C, 3A, 3B, and 3C). 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 located along the posterior margin of the subchondral edema (Figs. 1A, 1B, 1C, 2A, 2B, and 2C).
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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 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 soft-tissue 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 injuries [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] (Figs. 4A and 4B).
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. 1A, 1B, 1C, 2A, 2B, and 2C). 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.
Another limitation of this study is that MRI examinations were performed on different scanners, which could affect the ability to detect chondral abnormalities. Low-field-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 low-field-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.
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