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Musculoskeletal Imaging |
1 From the Department of Medical Imaging, Mount Sinai Hospital and the University Health Network, University of Toronto, 600 University Ave, Toronto, Ontario, Canada M5G 1X5 (D.A.E., L.M.W.); and the Department of Orthopedic Surgery, Southern California Permanente Medical Group, University of California, San Diego (D.C.F.). From the 2001 RSNA scientific assembly. Received September 27, 2001; revision requested November 13; final revision received April 4, 2002; accepted May 22. Address correspondence to L.M.W. (e-mail: lwhite@mtsinai.on.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Knee MR images obtained within 8 weeks after LPD were evaluated for medial retinacular and medial patellofemoral ligament (MPFL) disruption, vastus medialis obliquus (VMO) edema and/or elevation, and other derangements. One hundred patients with no evidence of prior LPD were evaluated as controls. The Student t test was used for statistical comparisons.
RESULTS: Eighty-two examinations were performed in 81 patients with LPD (mean age, 20 years; age range, 9–57 years). Seventy-six percent (62 of 82 examinations) showed medial retinacular disruption at its patellar insertion; 30% (25 of 82), at its midsubstance. The MPFL femoral origin was identified in 87% (71 of 82); of these, 49% (35 of 71) showed injury. Forty-eight percent (39 of 82) showed more than one site of injury to the medial stabilizers; 45% (37 of 82) showed edema or hemorrhage at the inferior VMO. Mean VMO elevation in the coronal plane of the adductor tendon was 2.2 cm, with a range of 0.6–4.5 cm (in control subjects, 0.9 cm; range, 0.1–2.5 cm; P < .001). At the inferomedial patella, 70% (57 of 82) of LPD examinations showed osteochondral injury and 44% (36 of 82) showed concave impaction deformity (0 of 100 control subjects). Other examination findings in LPDs included contusions of the lateral femoral condyle (66 [80%] of 82 examinations) or medial patella (50 [61%] of 82), intraarticular bodies (12 [15%] of 82), effusion (45 [55%] of 82), medial collateral injury (nine [11%] of 82), and meniscal tear (nine [11%] of 82).
CONCLUSION: Injury to the medial retinaculum, MPFL, and VMO may be identified at MR imaging after acute LPD. Concave impaction deformity of the inferomedial patella is a specific sign of prior LPD.
© RSNA, 2002
Index terms: Joints, injuries, 452.42, 453.42 • Joints, MR, 452.121411, 452.121412, 452.121419 • Patella, 453.42
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Investigators in previous studies (2–5) have described a constellation of magnetic resonance (MR) imaging findings that are characteristic of patellar dislocation and may be helpful for diagnosis. These findings include joint effusion and contusion or osteochondral injury of the anterolateral portion of the lateral femoral condyle and medial patella.In these studies, injury to the medial stabilizers of the patella was also identified as disruption of the medial retinaculum at its patellar attachment or midsubstance. However, investigators in biomechanical studies of fresh cadaveric specimens (6–9) have identified the medial patellofemoral ligament (MPFL) as the major ligamentous restraint preventing lateral patellar subluxation. In addition, the inferior portion of the vastus medialis muscle, the vastus medialis obliquus (VMO), appears to act as a dynamic stabilizer, neutralizing the lateralizing forces on the patella exerted by the vastus lateralis during quadriceps contraction. The authors of surgical studies (10,11) have identified disruption of these structures in patients with prior LPD.
The aim of our study, therefore, was to assess MR imaging findings following acute LPD with emphasis on the medial patellar restraints and to describe the imaging features of medial patellar impaction deformity associated with LPD.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Institutional review board approval was obtained for the study, and informed consent was obtained from all patients.
MR Imaging Technique
All MR imaging was performed within 8 weeks after injury by using a 1.0-T imager (Magnetom; Siemens Medical Systems, Erlangen, Germany). Patients underwent imaging with a dedicated knee coil, with the knee positioned in full extension and with 10°–20° of external rotation of the hip. The following sequences were performed in all patients: Transverse gradient-echo imaging (repetition time msec/echo time msec, 450–540/10; flip angle, 30°), with a field of view of 210 x 210 mm, two signals acquired, a matrix of 192 x 256, section thickness of 4.0 mm, and an intersection gap of 0.8 mm; and sagittal T1-weighted spin-echo imaging (700/15) with a field of view of 200 x 200 mm, two signals acquired, a matrix of 256 x 256, section thickness of 3.0 mm, and an intersection gap of 0.6 mm. Sagittal three-dimensional gradient-echo imaging (47/15; flip angle, 40°), with a section thickness of 1.5 mm, and coronal dual-echo imaging (2,000/20 and 2,000/80), with a field of view of 200 x 200 mm, one signal acquired, a matrix of 200 x 256, a section thickness of 4.0 mm, and an intersection gap of 0.6 mm, was also performed.
Image Evaluation
MR images were evaluated by two musculoskeletal radiologists (L.M.W., D.A.E.) with 7 and 2 years experience, respectively, with agreement by consensus. Images were evaluated for effusion; disruption of the medial ligamentous stabilizers of the patella; edema and/or elevation of the VMO; bone contusions; osteochondral injuries; and injuries to the menisci, cruciate and collateral ligaments, and extensor tendons. Patellar tilt, subluxation, and height and trochlear dysplasia were also assessed.
The medial ligamentous stabilizers were assessed on transverse images and divided into three regions: the medial retinaculum at the level of its patellar insertion, the medial retinaculum at its midsubstance, and the MPFL at its femoral origin. The MPFL was considered to have been visualized if low-signal-intensity fibers were seen arising between the region of the adductor tubercle and the medial epicondyle of the femur, running just inferior to the inferior border of the VMO and passing forward and inferiorly toward the medial patella (Fig 1). The MPFL was determined to be partially disrupted when some fibers were identified but there was partial discontinuity, marked irregularity of fiber contour, and/or intraligamentous or extensive periligamentous edema. Disruption was defined as complete if fibers in the expected region of origin of the MPFL were completely discontinuous or appeared absent, with extensive surrounding edema. When no distinct fibers were identified at the expected MPFL origin and there was no appreciable surrounding edema, the ligament was designated as nonvisualized. By using similar criteria, the midsubstance and patellar insertion of the medial retinaculum were evaluated as normal or as partially or completely disrupted.
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On sagittal images, joint effusion was defined as anteroposterior fluid depth greater than 10 mm in the lateral recess or greater than 4 mm in the suprapatellar pouch (14). Hemarthrosis was diagnosed when a fluid-fluid level was identified within the effusion.
Bone contusions were identified as areas of increased signal intensity on T2-weighted images and as areas of decreased signal intensity in bone marrow on T1-weighted images. Osteochondral injuries were defined as impaction or avulsion injuries of the patella or femur that caused anatomically discernible irregularity of an osteochondral surface. Intraarticular bodies were defined as focal fragments within joint fluid that had low signal intensity with all imaging sequences. It was initially noted that some patients with patellar osteochondral injury showed a clear concave deformity in the contour of the inferomedial patella. All cases were evaluated for such a deformity. Examination findings were considered positive for the deformity when a concave defect was present on at least two consecutive transverse images obtained at the inferomedial patella.
Patellar tilt was assessed on transverse images by comparing a line drawn through the lateral patellar facet at the level of the midpatellar cartilage with a line crossing the anterior tips of the medial and lateral femoral condyles (15). The angle between the two lines was considered normal (negative for patellar tilt) if it opened laterally or abnormal (positive for patellar tilt) if the lines were parallel or the angle between them opened medially. Patellar subluxation was subjectively assessed on transverse images by deciding whether the patellar apex lay in the same parasagittal plane as the apex of the femoral trochlear groove. Subluxation was defined as the patellar apex lying lateral to the plane of the trochlear apex.
Patellar height ratio was evaluated on sagittal T1-weighted images by using the method of Miller et al (16), in which the length of the patellar tendon from the posterior aspect of its patellar attachment to the posterior aspect of its tibial attachment is measured and divided by the longest sagittal dimension of the patella, as measured on sagittal images obtained in the patellar midline.
Trochlear dysplasia was evaluated by measuring lateral trochlear inclination and trochlear facet asymmetry. Lateral trochlear inclination was determined according to the method of Carrillon et al (17), by measuring the angle between the subchondral bone of the lateral trochlear facet and a line joining the posterior borders of the femoral condyles on the most superior transverse image on which trochlear cartilage was identified. Trochlear facet asymmetry was measured according to the method described by Pfirrmann et al (18) on transverse images at six sections (ie, 2.88 cm) above the joint line. At this level, the ratio of the width of the medial facet to that of the lateral facet was calculated.
Control Group
To compose the control group, we evaluated images from 100 consecutive knee MR imaging examinations performed for various indications. Patients with clinical or MR imaging findings that were suspected to indicate prior patellar dislocation were excluded. Images were obtained with a 1.5-T unit (Signa MR; GE Medical Systems) at a hospital that did not have a 1.0-T imager. In each patient, we evaluated images obtained with a transverse gradient-echo sequence with parameters identical to those used in patients with LPD, as well as those obtained with a sagittal spin-echo intermediate-weighted sequence (1,000/20) with a field of view of 140 x 140 mm, two signals acquired, a matrix of 512 x 256, section thickness of 4 mm, and an intersection gap of 0 mm. Depiction of the MPFL, concave deformity of the inferomedial patella, VMO elevation, patellar height, lateral trochlear inclination, and trochlear facet asymmetry were assessed.
Statistical Evaluation
Images obtained in the patients with LPD and in the control subjects were compared for mean values of VMO elevation, patellar height, lateral trochlear inclination, and trochlear facet asymmetry by using the Student t test. Because there were differences in distributions of age and sex between patients and control subjects, linear regression analysis was used to assess the possibility of confounding by these variables. When the regression-adjusted difference in means changed by 10% or more as compared with the unadjusted values, the adjusted values, as well as the unadjusted values, were reported.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In the control group, 100 MR imaging examinations were performed in 47 right knees and 53 left knees in 98 patients (53 male patients and 45 female patients; mean age, 29 years; age range, 13–49 years).
Medial Patellar Restraints
The MPFL was visualized at its femoral attachment at 87% (71 of 82) of examinations performed in the patients with LPD and at 80% (80 of 100) of examinations performed in the control subjects. On the images obtained in the patients with LPD in which the MPFL was visualized, MPFL injury was identified in 49% (35 of 71 cases) (Fig 2). When MPFL disruption was identified, it was considered complete in 40% (14 of 35) of cases and partial in 60% (21 of 35).
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Contusion and Osteochondral Injury
Contusions were seen anterolaterally within the lateral femoral condyle at 80% (66 of 82) of the examinations performed in the patients with LPD and at the medial patella in 61% (50 of 82) (Fig 4). Images in two patients with LPD showed contusions of both the medial tibia and medial femoral condyles that were presumed to be related to a varus component of the initial injury, and images obtained in one patient with LPD showed a contusion at the lateral tibial plateau.
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Concave deformity of the inferomedial patella was identified at 44% (36 of 82) of the examinations performed in the patients with LPD (Fig 4). The deformity could not be identified at any (0 of 100) of the examinations of the control subjects. From these data, the defect had a sensitivity of 44% (36 of 82) and a specificity of 100% (100 of 100) for prior patellar dislocation.
Associated Internal Derangements
Effusions were noted at 55% (45 of 82) of examinations performed in the patients with LPD, and of these, 7% (three of 45) showed fluid layering consistent with a hemarthrosis (Fig 2b).
Meniscal injuries were identified at 11% (nine of 82) of the examinations performed in the patients with LPD. These included five medial meniscal tears (one in the anterior horn and four in the posterior horn) and four lateral meniscal tears (three in the anterior horn and one in the posterior horn). One examination showed a partial anterior cruciate ligament injury.
Medial collateral ligament injuries, all of which were partial, were identified at 11% (nine of 82) of examinations performed in the patients with LPD. All patients with MCL injury (nine of nine) showed extensive edema around the distal VMO, and 89% (eight of nine) showed MPFL injury (five partial and three complete). No lateral collateral ligament or extensor tendon injuries were identified.
Patellofemoral Alignment
Patellar tilt was identified at 43% (35 of 82) of the examinations of the patients with LPD, and 15% (12 of 82) of patellae illustrated lateral subluxation.
The mean patellar height ratio (patellar tendon length to patellar length) in the patients with LPD was 1.18 (range, 0.74–1.47). Twenty-one percent (17 of 82) of the examinations showed patellar alta (ratio of 1.30 or greater). The control group had a mean ratio of 1.09 ± 0.17 (SD) (range, 0.68–1.56), with 12% (12 of 100) of the examinations performed in the control subjects showing patellar alta. The ratio was significantly greater in the patients with LPD than in the control subjects (unadjusted difference in means, 0.086 [P < .001]; age- and sex–adjusted difference in means, 0.0593 [P = .025]).
The mean lateral trochlear inclination in the patients with LPD was 15° (range, -2° to 36°), and 28% (23 of 82) of cases were designated as abnormal (<11°), according to the criteria of Carrillon et al (17). The control group had a significantly higher mean lateral trochlear inclination, 23° (range, -5° to 40°) (unadjusted difference in means, 8.1671 [P < .001]; age- and sex–adjusted difference in means, 7.0686 [P < .001]), with 4% (four of 100) of findings in the control subjects designated as abnormal.
The mean ratio of the widths of the medial to the lateral trochlear facets in the patients with LPD (trochlear facet asymmetry) was 0.54 (range, 0.21–1.00). Thirteen percent (11 of 82) of examinations resulted in an abnormal ratio (<0.4) according to the criteria of Pfirrmann et al (18). The control group showed a significantly higher mean ratio of 0.67 (range, 0.33–1.00, P < .001; Student t test), with 1% (one of 100) of control subjects having abnormal findings. The 95% CI for the difference between the mean of the patients with LPD and that of the control subjects was 0.09 to 0.17.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Inability to identify the MPFL on some MR images may be due to partial-volume averaging through an obliquely orientated ligament. However, findings of anatomic studies (19,21) suggest that the MPFL may not always be present in normal knees, although research findings (6,9) suggest that it may be present in at least 90% of cases. Our finding of increased frequency of MPFL identification in the patients with LPD (87%), as compared with that in the control subjects (80%), could have been due to knee effusions or edema surrounding the MPFL in many patients with LPD, increasing the conspicuity of the ligament.
Investigators in previous studies of MR imaging (2–5) have identified injury to the medial patellar restraints in 82%–100% of cases of prior patellar dislocation, as compared with 87% in the current study. Our finding of injuries to the retinaculum at its patellar insertion and midsubstance are consistent with this literature. However, these studies did not involve evaluation of the MPFL.
The functional importance of the MPFL has been defined in biomechanical studies of cadaveric knees (6–9), in which it was shown that the ligament accounts for 50%–60% of the total restraining force to lateral patellar displacement. In surgical studies (10,11,22), MPFL injury was identified in 94%–100% of patients with LPD at open surgery. MR images obtained in 20 patients after LPD demonstrated MPFL injury in 18, with surgical confirmation in 17 (23). In a recent article (13), MPFL abnormalities were identified at MR imaging in 14 surgically proven cases of acute LPD with MPFL injury. In these studies (10,11,13,22,23), MPFL injury was almost invariably at or close to its femoral origin. This may have been due to reinforcement of the MPFL more anteriorly by the tendon of the VMO muscle and by the deep capsule. Fortunately, it is the femoral origin that is best visualized as a discrete structure on MR images, and this was the portion of the MPFL evaluated in our study. Several authors (6,11,22,24) have advocated the role of MPFL repair in patellar dislocation; therefore, identification of MPFL injury on MR images may be of clinical value. We found that in 48% of cases, there were multiple sites of injury to the medial ligamentous restraints (Table 1); this finding may also have importance in surgical planning.
MPFL injury may be associated with stripping of the VMO from its attachment to the adductor tubercle (12,13). Sallay et al (11) noted increased signal intensity and variable retraction of the distal muscle belly of the VMO in 18 of 23 patients with acute LPD on MR images. This was noted to be correlated with hemorrhage in the area of the adductor tubercle and with cephalad displacement of the VMO in some patients at subsequent surgery. Recently, Sanders et al (13) reported fluid or edema between the distal VMO and the adductor tubercle in 12 of 14 acute LPD cases and found a mean VMO elevation of 1.7 cm above the adductor tubercle, as compared with elevation of 0.18 cm in control subjects. This correlates with our findings of VMO elevation in the patients with acute LPD, as compared with findings in the control subjects. Conlan et al (9) warned that the VMO origin may be solely from the distal adductor magnus tendon and not from the adductor tubercle in some normal knees; therefore, an origin lying proximal to the adductor tubercle is not necessarily evidence of avulsion. Also, muscle atrophy with increasing age may cause VMO elevation in the absence of prior LPD. The additional presence of fluid or edema between the VMO and the adductor tubercle may be firmer evidence of disruption. Edema was seen adjacent to the VMO in 45% of the acute cases of LPD in the current study.
We found no evidence of injury to the medial patellar restraints or VMO edema in 13% of cases in the current study. This may be a consequence of ligamentous laxity, which allows dislocation in some patients with little soft-tissue trauma (25).
The authors of previous studies of MR imaging in patients with LPD (2–5) have described osteochondral defects of the medial patellar margin. We found that a subset of patients with patellar osteochondral injury had a clear concave impaction deformity of the inferomedial patella that was not identified in the control group. Partial-volume averaging at the inferior patellar margin could produce the appearance of such a deformity on a single transverse image. Thus, we determined that the appearance should be present at the inferomedial patella on at least two consecutive transverse images for positive diagnosis of this deformity. We hypothesize that the defect is a consequence of impaction of the inferomedial patella on the lateral femoral condyle during dislocation, analogous to the Hill-Sachs lesion of the humeral head. Such osseous impaction defects of the medial patella have been described at conventional radiography (26–29), but their appearance and importance at MR imaging have not, to our knowledge, been described. We contend that concave deformity of the inferomedial patella at MR imaging is evidence of prior LPD, and in the current study the sign had a sensitivity of 44% and a specificity of 100%.
Contusions of the lateral femoral condyle and medial patella, osteochondral injuries to the medial patella, medial retinacular injuries, and effusions were the most frequent MR imaging signs of acute LPD in the current study. Bone contusions occur at characteristic locations, at the anterior portion of the lateral femoral condyle (anterior and superior to the typical site of contusion occurring with anterior cruciate ligament injury), and at the medial patellar margin inferiorly. The proportions of cases in the current study with contusions at the lateral femoral condyle or medial patella, osteochondral defects, intraarticular bodies, or other internal derangements were broadly within ranges defined in previous similar MR imaging studies (Table 2). We identified fewer effusions (55%), as compared with the number in previous studies (95%–100%), possibly because of the stringent criteria we used to diagnose effusions (14). The high rates of MPFL injury and VMO edema in cases of LPD with medial collateral ligament disruption reflect the close anatomic relationship of these structures. Because of this, we note that in some cases with extensive edema in this region, it may be difficult to definitively ascribe the edema to injury of one of these structures rather than to another.
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Several factors may cause predisposition to patellofemoral malalignment, increasing the risk of patellar dislocation. We measured three indexes described in the MR imaging literature for which a range of normal was defined. Patellar alta is reliably measurable in full extension on MR images (16). Our control measurements of mean patellar height ratio (1.09 ± 0.17) compare closely with those of Miller et al (1.1 ± 0.1) (16). Patella alta was present in 21% of patients with LPD in the current study, similar to findings in a previous report in which patella alta was present in 23% of patients with LPD at conventional radiographic analysis (30).
We found a wide range of lateral trochlear inclination in patients with LPD and control subjects (-2° to 36° and -5° to 40°, respectively). Additionally, there was some discrepancy between our measurements (means in patients and control subjects were 15° and 23°, respectively) and those of Carrillon et al (6° and 17°, respectively) (17). Although we found a significant difference between patients and control subjects, we believe that the accuracy of this measurement may be limited, since it required identification of the most superior transverse image on which trochlear cartilage was present, and we found difficulty in consensus agreement as to which transverse section to choose.
Our mean measurements of trochlear facet asymmetry in the patients with LPD (0.54; range, 0.21–1.00) differed significantly from those in the control subjects (0.67; range, 0.33–1.00, P < .001). However, the 95% CI for the difference between the two means was small (0.09–0.17), and this difference was much smaller than that reported by Pfirrmann et al (0.12 for the patients with trochlear dysplasia, many with histories of LPD, vs 0.57 for the control subjects) (18). As an alternative to these measurements, trochlear sulcus depth may be a more accurate and reproducible method for identifying individuals predisposed to patellar dislocation, but this was not assessed in the current study.
We recognize several study limitations. Since the current study was performed as part of an evaluation of the natural history of LPD, surgical correlation was not available for the patients in the current study. With regard to imaging technique, the oblique orientation of the MPFL may mean that transverse oblique images, rather than the transverse images used in the current study, would be optimal for evaluating this ligament. Furthermore, gradient-echo imaging, as used in this study, may not have been optimal for evaluating the MPFL and, in particular, the edema surrounding it, and conspicuity may be improved with other sequences, such as T2-weighted fast spin-echo imaging with fat saturation. This may account for the higher rates of MPFL injury and VMO edema in other studies (13,23). With regard to imaging evaluation, observers could not be blinded as to whether images had been obtained in control subjects or in patients with LPD. It was not possible to perform blinded evaluation of each sign, as multiple findings of LPD were generally apparent on each image obtained in patients with LPD. Also, our assessments of patellar tilt and subluxation may have been suboptimal, since they were performed on fully extended knees (15).
In conclusion, we have described MR imaging of acute LPD in a large patient group. Contusions of the lateral femoral condyle and medial patella, osteochondral injuries of the medial patella, medial retinacular injuries, and effusions are the most common MR imaging signs of acute LPD. Injury to the medial patellar soft-tissue restraints may be identified along the medial retinaculum, MPFL, and distal VMO at MR imaging. A concave impaction deformity at the inferomedial patella is a specific sign of prior patellar dislocation previously unreported at MR imaging. These findings may be important in routine MR imaging assessment of internal derangement of the knee, since the diagnosis of prior patellar dislocation may be unsuspected from history or clinical examination findings. Also, evidence of prior LPD, along with precise delineation of injury patterns, may be important in patient care and surgical planning.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, L.M.W.; study concepts, D.A.E., L.M.W., D.C.F.; study design, D.A.E., L.M.W.; literature research, D.A.E., L.M.W.; clinical studies, D.C.F.; data acquisition, D.C.F.; data analysis/interpretation, D.A.E., L.M.W.; statistical analysis, D.A.E.; manuscript preparation, D.A.E.; manuscript definition of intellectual content, D.A.E., L.M.W., D.C.F.; manuscript editing and revision/review, D.A.E., L.M.W.; manuscript final version approval, D.A.E., L.M.W., D.C.F.
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REFERENCES |
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