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    Patellar kinematics, Part II Patellar kinematics, Part II Document Transcript

    • Research Report Patellar Kinematics, Part II: The Influence of the Depth of the Trochlear Groove in Subjects With and Without Patellofemoral Pain Background and Purpose. A shallow intercondylar groove has been implicated as being contributory to abnormal patellar alignment. The purpose of this study was to assess the influence of the depth of the intercondylar groove on patellar kinematics. Subjects. Twenty-three women (mean age 26.8 years, SD 8.5, range 14 – 46) with a diagno- sis of patellofemoral pain and 12 women (mean age 29.1 years, SD 5.0, range 24 –38) without patellofemoral pain participated. Only female subjects were studied because of potential biomechanical differences between sexes. Methods. Patellar kinematics were assessed during resisted knee extension using kinematic magnetic resonance imaging. Measurements of medial and lateral patellar displacement and tilt were correlated with the depth of the trochlear groove (sulcus angle) at 45, 36, 27, 18, 9, and 0 degrees of knee flexion using regression analysis. Results. The depth of the trochlear groove was found to be correlated with patellar kinematics, with increased shal- lowness being predictive of lateral patellar tilt at 27, 18, 9, and 0 degrees of flexion and of lateral patellar displacement at 9 and 0 degrees of flexion (r .51–.76). Conclusions and Discussion. The results of this study indicate that bony structure is an important determinant of patellar kinematics at end-range knee extension (0°–30°). [Powers CM. Patellar kinematics, part II: the influence of the depth of the trochlear groove in subjects with and without patello- femoral pain. Phys Ther. 2000;80:965–973.] Key Words: Magnetic resonance imaging, Patellar kinematics, Patellofemoral joint. Christopher M Powers Physical Therapy . Volume 80 . Number 10 . October 2000 965
    • P atellar malalignment is thought to be among depth of the intercondylar groove on patellar kinemat- the etiological factors contributing to patello- ics. I hypothesized that subjects with PFP would exhibit femoral pain (PFP).1 The cause of PFP appears greater amounts of lateral patellar displacement and to be multifaceted, with components being lateral patellar tilt compared with subjects without PFPdefined by 2 distinct categories: structural and dynamic. and that the magnitude of lateral patellar displacementStructural considerations include abnormal bony config- and lateral patellar tilt would be associated with theuration1– 6 or tightness of noncontractile elements.7–9 depth of the trochlear groove. For results and discussionDynamic components have been hypothesized as involv- concerning the influence of vastus muscle activity ining unequal activity of the different heads of the quad- patellar kinematics, the reader is referred to the articlericeps femoris muscle10,11; however, evidence to support by Powers titled “Patellar Kinematics, Part I: The Influ-this premise has not been consistent.12,13 ence of Vastus Muscle Activity in Subjects With and Without Patellofemoral Pain” in this issue.Brattstrom2 reported that dysplasia of the femoral troch-lea is the most important etiological factor in recurrent Methodpatellar subluxation. Because the lateral femoral condyleis larger and projects farther anteriorly than the medial Subjectscondyle, the trochlear groove is thought to provide bony Twenty-three women with a diagnosis of PFP and 12stability resisting laterally directed forces.7 Although women without PFP participated in this study. Onlysome authors2,14 have reported that the decreased depth female subjects were studied because of potential biome-of the intercondylar sulcus is a primary cause of lateral- chanical differences between sexes. Both groups wereization of the patella, other authors15–18 have hypothe- similar in age, height, and weight (Tab. 1). Age, height,sized that abnormal patellar kinematics are the result of and weight were found to be normally distributed withinthe patella resting above the trochlear groove. Recent each group and when data from both groups werework by Farahmand and colleagues,19,20 however, sug- combined. No attempt was made to match each subjectgests that stability of the patella is more a function of the specifically for age, height, and weight, as there is noincreased tension of the patellar tendon and quadriceps evidence in the literature to suggest that individuals oftendon as the knee flexes, and not necessarily a function different ages, heights, and weights will demonstrateof the depth of the trochlear groove. differences in patellar kinematics.Although bony abnormalities have been implicated as The subjects with PFP were patients of the Southernbeing contributory to abnormal patellar alignment, the California Orthopaedic Institute who were deemed to berelationship of these factors to patellar tracking patterns appropriate candidates by the treating physician. Priorhas not been established. With the advent of kinematic to participation, all subjects with PFP were screened tomagnetic resonance imaging (KMRI) and cine phase rule out ligamentous instability, internal derangement,contrast imaging techniques,21 quantification of patellar and patellar tendinitis. Each subject’s pain originatedmovement throughout an arc of resisted knee extension from the patellofemoral joint, and only patients withis possible.22–24 These diagnostic techniques have a dis- histories relating to nontraumatic events were accepted.tinct advantage over imaging procedures used without In addition, pain had to be readily reproducible with atallowing for knee movement because contributions of least 2 of the following activities: stair ascent or descent,the extensor mechanism to patellofemoral joint kine- squatting, kneeling, prolonged sitting, or isometricmatics can be assessed.25 quadriceps femoris muscle contraction.1,19 Subjects were excluded from the study if they reported previous kneeThe purposes of this investigation were to compare surgery or a history compatible with acute traumaticpatellar tracking patterns between subjects with PFP and patellar dislocation.subjects without PFP and to assess the influence of theCM Powers, PT, PhD, is Director, Musculoskeletal Biomechanics Research Laboratory, and Assistant Professor, Department of Biokinesiology andPhysical Therapy, University of Southern California, 1540 E Alcazar St, CHP-155, Los Angeles, CA 90033 (USA) (powers@hsc.usc.edu).Dr Powers provided concept/research design, writing, data collection and analysis, subjects, project management, and fund procurement.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.This article was submitted December 28, 1999, and was accepted May 29, 2000.966 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000
    • Table 1.Subject Characteristics Subjects With Patellofemoral Pain Subjects Without Patellofemoral Pain (n 23) (n 12) X SD Range X SD Range Pa Age (y) 26.8 8.5 14–46 29.1 5.0 24–38 .38 Height (cm) 165.6 7.2 151.3–177.1 168.4 8.0 153.6–183.5 .29 Weight (kg) 62.2 9.1 42.0–82.7 61.2 8.0 48.7–74.1 .76a Probability values based on independent t tests.Individuals comprising the comparison group were device was such that the application of the force wasrecruited by word of mouth and were either employees always perpendicular to the tibia to ensure a constantof Rancho Los Amigos Medical Center (Downey, Calif) (isotonic) torque throughout the entire range ofor students from the University of Southern California. motion.23 Weights constructed of nonmagnetic, 316LSubjects had to have no history or diagnosis of knee series stainless steel‡ supplied the resistive force for thispathology or trauma and they had to be free of knee maneuver. These plates were placed on a movablepain at the time of the study. In addition, these subjects carriage that was attached to the pulley apparatus (seedid not report pain with any of the activities listed Fig. 1 in the companion article by Powers in this issue).earlier. The kinematic data from the comparison groupwere previously described in an article discussing the use Procedureof magnetic resonance imaging (MRI) for assessing Prior to testing, all procedures were explained to eachpatellar tracking.23 subject and written informed consent was obtained. All imaging was performed at Tower Imaging Center in westInstrumentation Los Angeles, Calif. Subjects were placed prone on theKinematic magnetic resonance imaging of the patello- positioning device in a position designed to allow forfemoral joint was assessed with the transmit and receive natural lower-extremity rotation. After this position wasquadrature body coil of a 1.5T magnetic resonance achieved, Velcro straps§ were used to secure the subjects’system* using a pulse sequence that allowed fast imaging thigh and tibia to the positioning device. Resistance ontimes with the best possible temporal resolution (fast- the device was then set at 15% of body weight.spoiled gradient recall acquisition in the steady state).Axial-plane imaging was performed using the following After familiarization with the knee extension apparatus,parameters: time to repeat 6.5 milliseconds, time to subjects were instructed to practice extending theirecho 2.1 milliseconds, number of excitations 1.0, knees at a rate of approximately 9°/s. This rate ensuredmatrix size 256 128, field of view 38 cm, flip 6 evenly spaced images throughout the 45-degree arc ofangle 30 degrees, and a 7-mm section thickness with an motion (including the 45° position) and permittedinterslice spacing of 0.5 mm.23 Acquisition time was 6 imaging at 45, 36, 27, 18, 9, and 0 degrees of kneeseconds to obtain 6 images (ie, 1 image per second). flexion. Approximation of this rate was made by the principal investigator (CMP) with the use of a stopwatch.All imaging was performed using a specially constructed,nonferromagnetic positioning device† that permitted Once the subject, in the opinion of the principal inves-bilateral knee extension against resistance (in the prone tigator, was able to reproduce the desired rate of motionposition) from 45 degrees of flexion to full extension in a smooth and even manner, imaging commenced.(see Fig. 1 in the companion article by Powers in this Subjects were instructed to initiate extension upon ver-issue). The device was designed to allow uninhibited bal command and continue until full extension hadmovement of the patellofemoral joint and normal rota- been reached. Imaging was done at 3 different imagetion of the lower extremities. I believe that these design planes to assess the entire excursion of the patella infeatures are important because patellar tracking may be relation to the trochlear groove (ie, 3 slices wereinfluenced by tibial rotation.26 obtained for each angle of knee flexion). These proce- dures were repeated if I thought the rate of kneeResistance was accomplished through a pulley system extension was too fast or too slow, or not performed in awith a constant 30.5-cm lever arm. The design of the smooth manner. In addition, the procedure was ‡* General Electric Medical Systems, 3200 N Grandview Ave, Waukesha, WI 54601. Esco Corp, 6415 E Corvette St, Los Angeles, CA 90242.† § Captain Plastic, PO Box 27493, Seattle, WA 98125. Velcro USA Inc, PO Box 5218, 406 Brown Ave, Manchester, NH 03108.Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 967
    • repeated if 6 adequate images were not obtained. Anadequate image was one in which the medial and lateralborders of the midsection of the patella, the trochleargroove, and the posterior femoral condyles were welldefined. Visualization of these landmarks was necessaryfor subsequent analysis.Data ManagementPrior to analysis, all images were screened by the princi-pal 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. Onlyimages containing a midpatella slice were analyzed. Figure 1.To examine patellofemoral joint relationships at the Method used to measure the sulcus angle. This angle was defined byvarious degrees of knee flexion, measures that were lines joining the highest points of the medial and lateral condyles and the lowest point of the intercondylar sulcus (AB and CB) (left). In order toindependent of the shape of the patella and the anterior obtain data when the trochlear groove lacked discernible depth, thefemoral condyles were used.23 This was done in an effort center of the sulcus angle was defined by a perpendicular line that wasto avoid measurement variability resulting from the projected anteriorly from the bisection of the posterior condylar linecontinually changing contour of these structures when (right). All sulcus angle measurements were reported in degrees.viewed at different angles of knee flexion and to allow Reprinted by permission of Lippincott Williams & Wilkins from Powers CM, Shellock FG, Beering TV, et al. Effect of bracing on patellarassessment of patellar orientation when the intercondy- kinematics in patients with patellofemoral joint pain. Med Sci Sportslar groove was not well visualized. All measurements Exerc. 1999;31:1714 –1720.were made with a computer-assisted program andincluded assessment of medial and lateral patellar dis-placement, medial and lateral patellar tilt, and the sulcus The sulcus angle was described by Brattstrom2 as theangle. angle formed by the highest points of the medial and lateral femoral condyles and the lowest point of theMedial and lateral patellar displacement were deter- intercondylar sulcus (Fig. 1).23 To obtain data when themined by the “bisect offset” measurement as described trochlear groove lacked discernable depth, the center ofby Stanford et al27 and modified by Brossmann et al.22 the sulcus angle was defined by a perpendicular line thatThe bisect offset was measured by drawing a line con- was drawn anteriorly from the bisection of the posteriornecting the posterior femoral condyles and then project- condylar line (Fig. 1). The estimation of the center ofing a perpendicular line anteriorly through the deepest the sulcus angle was based on the evaluation of normalpoint (apex) of the trochlear groove. This line inter- images that showed that the deepest portion of thesected with the patellar width line, which connected the intercondylar groove typically overlies the midpoint ofwidest points of the patella (see Fig. 2 in the companion the posterior condyle interval. All sulcus angles werearticle by Powers in this issue).23 The perpendicular line reported in degrees.was projected anteriorly from the bisection of the poste-rior condylar line to obtain data when the trochlear The day-to-day reliability for obtaining the KMRI datagroove was flattened (see Fig. 2 in the companion article using the procedures and measurements described wasby Powers in this issue). All bisect offset data represented determined in a previous study to have intraclass corre-the extent of the patella lying lateral to the projected lation coefficients ranging from .66 to .82).23 Based onperpendicular line and were expressed as a percentage repeated testing, intraobserver measurement errorof total patellar width. (standard error of measurement) was determined to be 3.4% for the bisect offset measurement, 2.9 degrees forMedial and lateral patellar tilt were measured using a patellar tilt, and 2.0 degrees for the sulcus angle.modification of the technique described by Sasaki and Although anatomical landmarks were identified manu-Yagi.28 The patellar tilt angle was the angle formed by ally, all lines used for angle and displacement measure-the lines joining the maximum width of the patella and ments were drawn by the computer software. Quantifi-the line joining the posterior femoral condyles (see cation of all angles and distances was performed by thisFig. 3 in the companion article by Powers in this issue). same program. This procedure assisted in minimizingAll tilt measurements were reported in degrees. measurement error.968 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000
    • Figure 2. Figure 3.Comparison of patellar tilt between the subjects with patellofemoral pain Comparison of patellar displacement (bisect offset) between the subjects(PFP) and the subjects without PFP from 45 to 0 degrees of knee flexion. with patellofemoral pain (PFP) and the subjects without PFP from 45 to 0Positive values indicate lateral tilt. Lateral patellar tilt was greater for the degrees of knee flexion. Error bars indicate one standard deviation.subjects with PFP than for the subjects without PFP (P .05). Error bars Data for subjects with PFP previously reported by Powers et al.23indicate one standard deviation. Data for subjects without PFP previ-ously reported by Powers et al.23 subjects without PFP), which occurred at 27 degrees ofData Analysis knee flexion.All statistical procedures were performed with BMDPstatistical software. Prior to analysis, descriptive statistics In contrast, there was no difference in bisect offsetwere calculated for all variables, and normality of distri- between the 2 groups (no group effect or interaction)bution was assessed using the Wilk-Shapiro test. Based (Fig. 3). When the data were averaged across all kneeon the analysis of distribution, all data were analyzed flexion angles, the average bisect offset measurement forusing parametric tests. Significance levels were set at the subjects with PFP was 57.9% of the patella lateral toP .05. midline, as compared with 53.8% of the patella lateral to midline in the subjects without PFP.To determine whether patellar indexes varied betweengroups or angles of knee flexion, a 2 6 (group Similarly, there was no difference in the sulcus angleangle) analysis of variance for repeated measures on one between the subjects with PFP and the subjects without PFPvariable (angle) was performed. This analysis was per- (no group effect or interaction) (Fig. 4). When averagedformed for each kinematic variable. A regression analysis across all angles of knee flexion, the mean sulcus angle waswas performed to determine whether the sulcus angle 149.4 degrees for the subjects with PFP, as compared with(independent variable) was predictive of patellar tilt or 144.6 degrees for the subjects without PFP.patellar displacement (dependent variables). This anal-ysis was repeated for both dependent variables at each Relationship Between Sulcus Angle and Patellarangle of knee flexion. To control for differences Kinematicsbetween the 2 groups of subjects, the grouping variable The Pearson correlation coefficients obtained whenwas included in all regression equations. assessing the relationship between the sulcus angle and patellar displacement at the various knee flexion anglesResults ranged from .15 to .74 (Tab. 2). Similarly, the correla- tion coefficients obtained when assessing the relation-Patellar Kinematics ship between the sulcus angle and patellar tilt at theA difference was found in patellar tilt between the 2 various knee flexion angles ranged from .26 to .76groups. Compared with the comparison group, the (Tab. 2).subjects with PFP demonstrated a greater degree oflateral patellar tilt when the data were averaged across all The sulcus angle was a predictor of patellar displace-angles of knee flexion (10.7° versus 5.5°, P .02) (Fig. 2). ment at 9 degrees of knee flexion (r .46, R2 .21);The largest difference between the 2 groups was 7 however, it was a stronger predictor of patellar displace-degrees (11.7° in the subjects with PFP versus 4.7° in the ment at 0 degrees (r .74, R2 .55; Fig. 5). In general, as the sulcus angle increased (ie, became more shallow), the amount of lateral patellar displacement also SPSS Inc, 444 N Michigan Ave, Chicago, IL 60611. increased.Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 969
    • Table 2. Pearson Correlation Coefficients for Sulcus Angle and Kinematic Variables Knee Flexion Angle (°) Dependent Variable 45 36 27 18 9 0 Patellar displacement .15 .23 .16 .35 .46a .74a Patellar tilt .26 .34 .51a .54a .63a .76a a Significant at P .05. The sulcus angle also was a predictor of patellar tilt at 27 degrees (r .51, R2 .26), 18 degrees (r .54, R2 .29), 9 degrees (r .63, R2 .40), and 0 degrees ofFigure 4. knee flexion (r .76, R2 .58; Fig. 6). As with patellarComparison of sulcus angle between the subjects with patellofemoral displacement, an increase in the sulcus angle resulted inpain (PFP) and the subjects without PFP from 45 to 0 degrees of knee greater amounts of lateral patellar tilt.flexion. Error bars indicate one standard deviation. Data for subjectswith PFP previously reported by Powers et al.23 Discussion The sulcus angle, as measured in this study, was repre- sentative of the depth of the femoral trochlea at the midsection of the patella. In general, there was a trend toward a more shallow groove in the subjects with PFP when the data were averaged across all knee flexion angles. It is evident from these data, however, that although the 2 groups had similar sulcus angles at 45, 36, and 27 degrees of flexion, a substantial increase (loss of depth) was observed in the subjects with PFP as the knee extended beyond 27 degrees. This increase in the sulcus angle is similar to the increases reported by Schutzer et al29 and Kujala et al30 and suggests that bony stability at the end-range of extension may be compromised in people with PFP.Figure 5.Relationship between the sulcus angle (in degrees) and bisect offset The sulcus angle was found to be a predictor of lateral(percentage of the patella width lateral to midline) for the subjects with patellar tilt at 27, 18, 9, and, 0 degrees, as well as apatellofemoral pain (PFP) and the subjects without PFP at 0 degrees ofknee flexion (r .74; F 19.3; df 2,33; P .05). 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 exten- sion. The association between bony anatomy and patel- lar 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 pro- nounced (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 kine-Figure 6. matics at terminal knee extension.Relationship between the sulcus angle (in degrees) and patellar tilt (indegrees) for the subjects with patellofemoral pain (PFP) and the subjectswithout PFP at 0 degrees of knee flexion (r .76; F 20.6; df 2,33; During knee extension, the sulcus angle of the subjectsP .05). Positive values of patellar tilt indicate lateral tilting. without PFP increased an average of 10 degrees, indicat-970 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000
    • changed very little. Their findings, how- ever, 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 indic- ative of compromised patellar stability, the etiological factor underlying this finding is not entirely evident. For example, there are 2 possible explana- tions 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 thatFigure 7. suggest that both findings are typicallyAxial-plane images obtained from a subject without patellofemoral pain (PFP) and 3 subjectswith PFP (patients 1–3). The subject without PFP and patient 1 demonstrate a centered patella found in conjunction with each other.within the trochlear groove. Patient 2 demonstrates a moderate degree of lateral displacement Without knowing the vertical position(lateral border of patella lateral to the anterior femoral condyle) and lateral tilting as well as a of the patella within the femoral troch-relatively shallow trochlear groove. In patient 3, the patella is positioned well above the lea, however, it would be difficult totrochlear groove, and there is extreme lateral displacement and lateral tilting of the patella. ascertain whether an increased sulcus angle was the result of dysplasia or of patella alta, or a combination of both.ing that the patella was moving to a more shallow portion This determination would require further radiologicalof the femoral trochlea. Because the patella migrates evaluation, using lateral-view techniques that have beensuperiorly as the knee extends,31,32 this observation, in my described for assessing trochlear dysplasia14,34 and patellaopinion, suggests that the bony stability afforded by the alta35–37 or serial axial views to determine the exact positioncranial portion of the trochlear groove is less than that of the patella within the trochlear groove.38provided by the caudal portion. This hypothesis is sup-ported by the findings of Malghem and Maldague,14 who Despite the fact that the KMRI data collected in thisreported that the depth of the proximal trochlear groove study were limited for assessing the exact vertical posi-(as determined by lateral radiographs) was less than the tion of the patella, I contend that some qualitativedepth of the middle portion in subjects who were pain-free. information was gained. For example, in 22% of the subjects with PFP, it appeared that the patella wasIn contrast, the finding of an increasing sulcus angle superior to the femoral trochlea, which would be sug-with knee extension in my investigation appears to gestive of patella alta. As shown in Figure 7, the patella ofcontradict the data of Farahmand and colleagues,20 who patient 3 is situated on the shaft of the femur, well abovereported that the geometry of the trochlear groove (as the level of the femoral condyles. In contrast, patient 2encountered by the sliding patella during knee flexion) demonstrates a relatively shallow trochlear groove,Physical Therapy . Volume 80 . Number 10 . October 2000 Powers . 971
    • although the posterior femoral condyles are still visible, ple size (including male subjects), however, would besuggesting that this image section was not above the level necessary to confirm this observation.of the femoral trochlea. Therefore, an argument couldbe made that the diminished sulcus depth in this subject The subjects without PFP demonstrated an overall pat-was more likely the result of trochlear dysplasia. tern of decreasing lateral tilt as the knee extended, which is consistent with findings obtained with cadaver speci-The bisect offset data obtained for both groups indi- mens41,42 and cine phase contrast imaging techniques.21cated that the patella was lateral to the midline through- The average tilt values for the subjects, with PFP, however,out the range of motion. On the average, the subjects remained fairly consistent across all knee flexion angles.with PFP demonstrated greater patellar lateralization at This finding is in contrast to the data of Brossmann andall angles of flexion. This finding, however, was not colleagues,22 which showed an overall tendency towardstatistically significant. The normal kinematic pattern for progressive lateral tilt as the knee extended. This pattern ofpatellar displacement was characterized by slight medial movement was evident in only 27% of the subjects with PFPdisplacement from 45 to 18 degrees of knee flexion, in my investigation, which suggests that this should not befollowed by subtle lateral displacement as the knee considered the dominant motion pattern. This discrepancyextended from 18 to 0 degrees (Fig. 3). This pattern of could have been the result of the difference in subjects inmovement is consistent with that previously described as the 2 studies, as well as the different measurement tech-a frontal-plane “C” curve.39 Although, the average patel- niques used to determine patellar tilt.lar displacement pattern of the subjects with PFP wassimilar to that of the subjects without PFP from 45 to 27 The results of my study may have clinical implications fordegrees of flexion, there was a reversal to a progressively the treatment of people with patellar malalignment. Formore lateral alignment as the knee continued to extend. example, if patellar tracking is primarily dictated by bonyThe largest difference between groups was evident at 0 structure, then treatment procedures that address onlydegrees (62% versus 54% of the patella lateral to the soft-tissue components (such quadriceps femoris musclemidline), which coincides with the contention of Fulk- strengthening or a lateral retinacular release) may haveerson and Hungerford1 that patellar subluxation typi- limited success. Likewise, the long-term success of acally occurs during terminal knee extension. procedure such as a distal realignment may depend on whether the patella can be relocated within the bonyThe bisect offset data of the subjects with PFP demon- confines of the trochlea.strated large variability at 18, 9, and 0 degrees of flexion.At these angles, the standard deviations were approxi- A limitation of my study was the fact that a relativelymately 2 to 3 times those of the subjects without PFP, small comparison group was used to provide comparisonindicating that these subjects exhibited a wide range of data. Although differences were found with respect tohorizontal patellar displacement (Fig. 3). At 0 degrees, patellar tilt, a larger sample size might have increasedfor example, 22% of the subjects with PFP had a bisect the ability to find group differences in the bisect offsetoffset value greater than 2 standard deviations of the and sulcus angle measurements. Additional study in thiscomparison group, whereas 61% had a bisect offset value area should consider larger sample sizes, particularlywithin 1 standard deviation of the control group. These given the large variability among individuals with PFP. Afindings support the work of Shellock et al,40 who post hoc power analysis revealed that approximately 80reported that only 26% of their subjects demonstrated and 110 subjects would be required to find group effectslateral subluxation of the patella. Although the data of (10% differences) for the sulcus angle and bisect offset,Shellock and colleagues40 were based on qualitative MRI respectively.assessment, the results of these previous studies, as well asthe data of my investigation, indicate that excessive lateral As a result of the limitations imposed by the size of thedisplacement of the patella is not a universal finding in this MRI bore, the loading condition used in this study (non–population. The role of abnormal patellar kinematics as a weight bearing) was not consistent with the loading condi-primary cause of PFP, in my view, may be questioned. tion that would be evident with weight-bearing activities. Therefore, care should be taken in interpreting the resultsThe patellar tilt data showed that the patella was laterally of this study until differences in patellar kinematics can betilted throughout the range of motion in both groups, established between various loading conditions.with the subjects with PFP demonstrating greater mag-nitudes compared with the subjects without PFP when Conclusionsthe data were averaged across all knee flexion angles. The results of this study indicate that the sulcus angle isThese results suggest that excessive lateral tilt may be a a predictor of both lateral patellar tilt and lateral patellarmore frequent radiological finding in PFP compared displacement during terminal knee extension. This findingwith lateral displacement or subluxation. A larger sam- suggests that bony structure is an important determinate of972 . Powers Physical Therapy . Volume 80 . Number 10 . October 2000
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