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Research Reports |
CM Powers, PT, PhD, is Director, Musculoskeletal Biomechanics Research Laboratory, and Assistant Professor, Department of Biokinesiology and Physical Therapy, University of Southern California, 1540 E Alcazar St, CHP-155, Los Angeles, CA 90033 (USA) (powers{at}hsc.usc.edu)
Submitted December 28, 1999;
Accepted May 29, 2000
 | Abstract |
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Key Words: Magnetic resonance imaging • Patellar kinematics • Patellofemoral joint
| Introduction |
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 TopAbstract Introduction MethodResultsDiscussionConclusionsReferences |
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Brattstrom2 reported that dysplasia of the femoral trochlea is the most important etiological factor in recurrent patellar subluxation. Because the lateral femoral condyle is larger and projects farther anteriorly than the medial condyle, the trochlear groove is thought to provide bony stability resisting laterally directed forces.7 Although some authors2,14 have reported that the decreased depth of the intercondylar sulcus is a primary cause of lateralization of the patella, other authors15–18 have hypothesized that abnormal patellar kinematics are the result of the patella resting above the trochlear groove. Recent work by Farahmand and colleagues,19,20 however, suggests that stability of the patella is more a function of the increased tension of the patellar tendon and quadriceps tendon as the knee flexes, and not necessarily a function of the depth of the trochlear groove.
Although bony abnormalities have been implicated as being contributory to abnormal patellar alignment, the relationship of these factors to patellar tracking patterns has not been established. With the advent of kinematic magnetic resonance imaging (KMRI) and cine phase contrast imaging techniques,21 quantification of patellar movement throughout an arc of resisted knee extension is possible.22–24 These diagnostic techniques have a distinct advantage over imaging procedures used without allowing for knee movement because contributions of the extensor mechanism to patellofemoral joint kinematics can be assessed.25
The purposes of this investigation were to compare patellar tracking patterns between subjects with PFP and subjects without PFP and to assess the influence of the depth of the intercondylar groove on patellar kinematics. I hypothesized that subjects with PFP would exhibit greater amounts of lateral patellar displacement and lateral patellar tilt compared with subjects without PFP and that the magnitude of lateral patellar displacement and lateral patellar tilt would be associated with the depth of the trochlear groove. For results and discussion concerning the influence of vastus muscle activity in patellar kinematics, the reader is referred to the article by Powers titled "Patellar Kinematics, Part I: The Influence of Vastus Muscle Activity in Subjects With and Without Patellofemoral Pain" in this issue.
| Method |
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TopAbstractIntroductionMethodResultsDiscussionConclusionsReferences |
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Individuals comprising the comparison group were recruited by word of mouth and were either employees of Rancho Los Amigos Medical Center (Downey, Calif) or students from the University of Southern California. Subjects had to have no history or diagnosis of knee pathology or trauma and they had to be free of knee pain at the time of the study. In addition, these subjects did not report pain with any of the activities listed earlier. The kinematic data from the comparison group were previously described in an article discussing the use of magnetic resonance imaging (MRI) for assessing patellar tracking.23
Instrumentation
Kinematic magnetic resonance imaging of the patellofemoral joint was assessed with the transmit and receive quadrature body coil of a 1.5T magnetic resonance system* using a pulse sequence that allowed fast imaging times with the best possible temporal resolution (fast-spoiled gradient recall acquisition in the steady state). Axial-plane imaging was performed using the following parameters: time to repeat=6.5 milliseconds, time to echo=2.1 milliseconds, number of excitations=1.0, matrix size=256 x 128, field of view=38 cm, flip angle=30 degrees, and a 7-mm section thickness with an interslice spacing of 0.5 mm.23 Acquisition time was 6 seconds to obtain 6 images (ie, 1 image per second).
All imaging was performed using a specially constructed, nonferromagnetic positioning device
that permitted bilateral knee extension against resistance (in the prone position) from 45 degrees of flexion to full extension (see Fig. 1 in the companion article by Powers in this issue). The device was designed to allow uninhibited movement of the patellofemoral joint and normal rotation of the lower extremities. I believe that these design features are important because patellar tracking may be influenced by tibial rotation.26
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supplied the resistive force for this maneuver. These plates were placed on a movable carriage that was attached to the pulley apparatus (see Fig. 1 in the companion article by Powers in this issue).
Procedure
Prior to testing, all procedures were explained to each subject and written informed consent was obtained. All imaging was performed at Tower Imaging Center in west Los Angeles, Calif. Subjects were placed prone on the positioning device in a position designed to allow for natural lower-extremity rotation. After this position was achieved, Velcro straps
were used to secure the subjects' thigh and tibia to the positioning device. Resistance on the device was then set at 15% of body weight.
After familiarization with the knee extension apparatus, subjects were instructed to practice extending their knees at a rate of approximately 9° /s. This rate ensured 6 evenly spaced images throughout the 45-degree arc of motion (including the 45° position) and permitted imaging at 45, 36, 27, 18, 9, and 0 degrees of knee flexion. Approximation of this rate was made by the principal investigator (CMP) with the use of a stopwatch.
Once the subject, in the opinion of the principal investigator, was able to reproduce the desired rate of motion in a smooth and even manner, imaging commenced. Subjects were instructed to initiate extension upon verbal command and continue until full extension had been reached. Imaging was done at 3 different image planes to assess the entire excursion of the patella in relation to the trochlear groove (ie, 3 slices were obtained for each angle of knee flexion). These procedures were repeated if I thought the rate of knee extension was too fast or too slow, or not performed in a smooth manner. In addition, the procedure was repeated if 6 adequate images were not obtained. An adequate image was one in which the medial and lateral borders of the midsection of the patella, the trochlear groove, and the posterior femoral condyles were well defined. Visualization of these landmarks was necessary for subsequent analysis.
Data Management
Prior to analysis, all images were screened by the principal investigator to ascertain the midsection of the patella (maximum patellar width) at each angle of knee flexion. Once the midsection of the patella was determined, measurements for these images were obtained. Only images containing a midpatella slice were analyzed.
To examine patellofemoral joint relationships at the various degrees of knee flexion, measures that were independent of the shape of the patella and the anterior femoral condyles were used.23 This was done in an effort to avoid measurement variability resulting from the continually changing contour of these structures when viewed at different angles of knee flexion and to allow assessment of patellar orientation when the intercondylar groove was not well visualized. All measurements were made with a computer-assisted program and included assessment of medial and lateral patellar displacement, medial and lateral patellar tilt, and the sulcus angle.
Medial and lateral patellar displacement were determined by the "bisect offset" measurement as described by Stanford et al27 and modified by Brossmann et al.22 The bisect offset was measured by drawing a line connecting the posterior femoral condyles and then projecting a perpendicular line anteriorly through the deepest point (apex) of the trochlear groove. This line intersected with the patellar width line, which connected the widest points of the patella (see Fig. 2 in the companion article by Powers in this issue).23 The perpendicular line was projected anteriorly from the bisection of the posterior condylar line to obtain data when the trochlear groove was flattened (see Fig. 2 in the companion article by Powers in this issue). All bisect offset data represented the extent of the patella lying lateral to the projected perpendicular line and were expressed as a percentage of total patellar width.
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The day-to-day reliability for obtaining the KMRI data using the procedures and measurements described was determined in a previous study to have intraclass correlation coefficients ranging from .66 to .82).23 Based on repeated testing, intraobserver measurement error (standard error of measurement) was determined to be 3.4% for the bisect offset measurement, 2.9 degrees for patellar tilt, and 2.0 degrees for the sulcus angle. Although anatomical landmarks were identified manually, all lines used for angle and displacement measurements were drawn by the computer software. Quantification of all angles and distances was performed by this same program. This procedure assisted in minimizing measurement error.
Data Analysis
All statistical procedures were performed with BMDP statistical software.|| Prior to analysis, descriptive statistics were calculated for all variables, and normality of distribution was assessed using the Wilk-Shapiro test. Based on the analysis of distribution, all data were analyzed using parametric tests. Significance levels were set at P<.05.
To determine whether patellar indexes varied between groups or angles of knee flexion, a 2 x 6 (group x angle) analysis of variance for repeated measures on one variable (angle) was performed. This analysis was performed for each kinematic variable. A regression analysis was performed to determine whether the sulcus angle (independent variable) was predictive of patellar tilt or patellar displacement (dependent variables). This analysis was repeated for both dependent variables at each angle of knee flexion. To control for differences between the 2 groups of subjects, the grouping variable was included in all regression equations.
| Results |
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TopAbstractIntroductionMethodResultsDiscussionConclusionsReferences |
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In contrast, there was no difference in bisect offset between the 2 groups (no group effect or interaction) (Fig. 3). When the data were averaged across all knee flexion angles, the average bisect offset measurement for the subjects with PFP was 57.9% of the patella lateral to midline, as compared with 53.8% of the patella lateral to midline in the subjects without PFP.
Similarly, there was no difference in the sulcus angle between the subjects with PFP and the subjects without PFP (no group effect or interaction) (Fig. 4). When averaged across all angles of knee flexion, the mean sulcus angle was 149.4 degrees for the subjects with PFP, as compared with 144.6 degrees for the subjects without PFP.
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| Discussion |
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TopAbstractIntroductionMethodResultsDiscussionConclusionsReferences |
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The sulcus angle was found to be a predictor of lateral patellar tilt at 27, 18, 9, and, 0 degrees, as well as a predictor of lateral patellar displacement at 9 and 0 degrees. This finding underscores the importance of the bony anatomy in contributing to patellar stability and could theoretically explain the clinical manifestation of lateral patellar subluxation during terminal knee extension. The association between bony anatomy and patellar stability was evident in the PFP data, where it was observed that the point at which the sulcus angle began to deviate from the data obtained for the comparison group (approximately 27°) was at the same point at which the lateral displacement became more pronounced (Figs. 3 and 4). The finding that more than half of the variability in patellar tilt and displacement could be explained by the sulcus angle at 0 degrees supports the argument of Brattstrom2 that a shallow femoral sulcus is a predisposing factor with regard to abnormal patellar kinematics at terminal knee extension.
During knee extension, the sulcus angle of the subjects without PFP increased an average of 10 degrees, indicating that the patella was moving to a more shallow portion of the femoral trochlea. Because the patella migrates superiorly as the knee extends,31,32 this observation, in my opinion, suggests that the bony stability afforded by the cranial portion of the trochlear groove is less than that provided by the caudal portion. This hypothesis is supported by the findings of Malghem and Maldague,14 who reported that the depth of the proximal trochlear groove (as determined by lateral radiographs) was less than the depth of the middle portion in subjects who were pain-free.
In contrast, the finding of an increasing sulcus angle with knee extension in my investigation appears to contradict the data of Farahmand and colleagues,20 who reported that the geometry of the trochlear groove (as encountered by the sliding patella during knee flexion) changed very little. Their findings, however, were based on their analysis of cadaver specimens under low-level, static loading conditions. I contend it is likely that the conditions used in my investigation (active quadriceps femoris muscle contraction/shortening) pulled the patella farther superiorly in the trochlear groove, thereby accounting for the differences in the sulcus angles.
Although not significant, the average increase (flattening) of the sulcus angle during extension in the subjects with PFP (19°) was almost twice that of the subjects without PFP (10°). Although this increase in the sulcus angle is indicative of compromised patellar stability, the etiological factor underlying this finding is not entirely evident. For example, there are 2 possible explanations for the increase in the sulcus angle: (1) dysplasia of the cranial portion of the femoral trochlea and (2) patella alta (excessive superior migration of the patella with respect to the trochlear groove). Although both of these alternatives are possible, it is difficult to separate the effects of each with regard to patellar tracking. Hvid and colleagues33 reported data that suggest that both findings are typically found in conjunction with each other. Without knowing the vertical position of the patella within the femoral trochlea, however, it would be difficult to ascertain whether an increased sulcus angle was the result of dysplasia or of patella alta, or a combination of both. This determination would require further radiological evaluation, using lateral-view techniques that have been described for assessing trochlear dysplasia14,34 and patella alta35–37 or serial axial views to determine the exact position of the patella within the trochlear groove.38
Despite the fact that the KMRI data collected in this study were limited for assessing the exact vertical position of the patella, I contend that some qualitative information was gained. For example, in 22% of the subjects with PFP, it appeared that the patella was superior to the femoral trochlea, which would be suggestive of patella alta. As shown in Figure 7, the patella of patient 3 is situated on the shaft of the femur, well above the level of the femoral condyles. In contrast, patient 2 demonstrates a relatively shallow trochlear groove, although the posterior femoral condyles are still visible, suggesting that this image section was not above the level of the femoral trochlea. Therefore, an argument could be made that the diminished sulcus depth in this subject was more likely the result of trochlear dysplasia.
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The bisect offset data of the subjects with PFP demonstrated large variability at 18, 9, and 0 degrees of flexion. At these angles, the standard deviations were approximately 2 to 3 times those of the subjects without PFP, indicating that these subjects exhibited a wide range of horizontal patellar displacement (Fig. 3). At 0 degrees, for example, 22% of the subjects with PFP had a bisect offset value greater than 2 standard deviations of the comparison group, whereas 61% had a bisect offset value within 1 standard deviation of the control group. These findings support the work of Shellock et al,40 who reported that only 26% of their subjects demonstrated lateral subluxation of the patella. Although the data of Shellock and colleagues40 were based on qualitative MRI assessment, the results of these previous studies, as well as the data of my investigation, indicate that excessive lateral displacement of the patella is not a universal finding in this population. The role of abnormal patellar kinematics as a primary cause of PFP, in my view, may be questioned.
The patellar tilt data showed that the patella was laterally tilted throughout the range of motion in both groups, with the subjects with PFP demonstrating greater magnitudes compared with the subjects without PFP when the data were averaged across all knee flexion angles. These results suggest that excessive lateral tilt may be a more frequent radiological finding in PFP compared with lateral displacement or subluxation. A larger sample size (including male subjects), however, would be necessary to confirm this observation.
The subjects without PFP demonstrated an overall pattern of decreasing lateral tilt as the knee extended, which is consistent with findings obtained with cadaver specimens41,42 and cine phase contrast imaging techniques.21 The average tilt values for the subjects, with PFP, however, remained fairly consistent across all knee flexion angles. This finding is in contrast to the data of Brossmann and colleagues,22 which showed an overall tendency toward progressive lateral tilt as the knee extended. This pattern of movement was evident in only 27% of the subjects with PFP in my investigation, which suggests that this should not be considered the dominant motion pattern. This discrepancy could have been the result of the difference in subjects in the 2 studies, as well as the different measurement techniques used to determine patellar tilt.
The results of my study may have clinical implications for the treatment of people with patellar malalignment. For example, if patellar tracking is primarily dictated by bony structure, then treatment procedures that address only soft-tissue components (such quadriceps femoris muscle strengthening or a lateral retinacular release) may have limited success. Likewise, the long-term success of a procedure such as a distal realignment may depend on whether the patella can be relocated within the bony confines of the trochlea.
A limitation of my study was the fact that a relatively small comparison group was used to provide comparison data. Although differences were found with respect to patellar tilt, a larger sample size might have increased the ability to find group differences in the bisect offset and sulcus angle measurements. Additional study in this area should consider larger sample sizes, particularly given the large variability among individuals with PFP. A post hocpower analysis revealed that approximately 80 and 110 subjects would be required to find group effects (10% differences) for the sulcus angle and bisect offset, respectively.
As a result of the limitations imposed by the size of the MRI bore, the loading condition used in this study (non–weight bearing) was not consistent with the loading condition that would be evident with weight-bearing activities. Therefore, care should be taken in interpreting the results of this study until differences in patellar kinematics can be established between various loading conditions.
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TopAbstractIntroductionMethodResultsDiscussionConclusionsReferences |
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| Footnotes |
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This study was approved for human subjects by the Los Amigos Research and Education Institute Inc of Rancho Los Amigos Medical Center, Downey, Calif.
This study was partially funded by a grant from the Foundation for Physical Therapy.
* General Electric Medical Systems, 3200 N Grandview Ave, Waukesha, WI 54601. 
Captain Plastic, PO Box 27493, Seattle, WA 98125.
Esco Corp, 6415 E Corvette St, Los Angeles, CA 90242.
Velcro USA Inc, PO Box 5218, 406 Brown Ave, Manchester, NH 03108.
|| SPSS Inc, 444 N Michigan Ave, Chicago, IL 60611.
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