Journal of Applied Biomechanics, 2014, 30, 493-500 http://dx.doi.org/10.1123/jab.2011-0250 © 2014 Human Kinetics, Inc.

An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH

Comparison of Three-Dimensional Patellofemoral Joint Reaction Forces in Persons With and Without Patellofemoral Pain Yu-Jen Chen1,2 and Christopher M. Powers1 1University

of Southern California; 2National Taiwan University

The purpose of this study was to determine if persons with patellofemoral pain (PFP) exhibit differences in patellofemoral joint reaction forces (PFJRFs) during functional activities. Forty females (20 PFP, 20 controls) underwent two phases of data collection: (1) magnetic resonance imaging (MRI) and (2) biomechanical analysis during walking, running, stair ascent, and stair descent. A previously described three-dimensional model was used to estimate PFJRFs. Resultant PFJRFs and the orthogonal components were reported. The PFP group demonstrated lower peak resultant PFJRFs and posterior component and superior component of the PFJRFs compared with the control group across all conditions. However, the PFP group had a higher peak lateral component of the PFJRF in three out of the four conditions evaluated. The lower resultant PFJRFs suggested that individuals with PFP may employ strategies to minimize patellofemoral joint loading, but it did not result in diminished lateral forces acting on the patella. Keywords: patellofemoral joint, patellofemoral pain, modeling Patellofemoral pain (PFP) is one of the most common conditions affecting the lower extremity.1-3 Despite its high incidence, however, the pathomechanics underlying this disorder remains uncertain. A commonly cited theory states that PFP is the result of elevated patellofemoral joint reaction forces (PFJRFs), which contribute to excessive joint stress and articular cartilage pathology.4 Studies comparing joint reaction forces between persons with PFP and pain-free controls are few and have reported inconsistent results. For example, Heino Brechter and Powers reported a 22% reduction in peak PFJRF in females with PFP compared with a control group during walking; however, no significant group differences were found during fast walking.5 In a separate study, the same authors reported a 32% reduction in peak PFJRFs in a group of individuals with PFP during stair ascent when compared with a control group, but no significant group differences were found during stair descent.6 In both studies, Heino Brechter and Powers5 and Brechter and Powers6 attributed the lower PFJRFs in their PFP group to lower knee extensor moments and smaller knee joint excursions during the stance phase of gait. It was proposed that persons with PFP may have been employing compensatory strategies to minimize PFJRFs and subsequent pain. This premise is supported by the work of Salsich et al,7 who reported that subjects with PFP demonstrated reduced peak knee extensor moments during both stair ascent and descent.

Yu-Jen Chen and Christopher M. Powers are with the Jacquelin Perry Musculoskeletal Biomechanics Research Laboratory, Division of Biokinesiology and Physical Therapy at the University of Southern California, Los Angeles, CA. Yu-Jen Chen is also with the School and Graduate Institute of Physical Therapy, College of Medicine, at National Taiwan University, Taipei, Taiwan (R.O.C.). Address author correspondence to Christopher M. Powers at [email protected].

Although previous authors have reported decreased PFJRFs in persons with PFP, these studies have been limited by two-dimensional modeling approaches. As the magnitude of the medial/lateral component of the PFJRF has been implicated in this disorder, it would appear that a more physiologic representation of the patellofemoral joint is needed to determine whether joint forces are altered in the PFP population. In a previous publication, we described a subject-specific, three-dimensional (3D) model to assess PFJRFs.8 This model incorporates subject-specific anatomical parameters (as obtained through MRI) and lower extremity biomechanics to estimate patellofemoral joint loading during dynamic activities. The purpose of the current study was to determine if persons with PFP demonstrate differences in PFJRFs compared with painfree controls during walking, running, stair ascent, and stair descent. Based on the above noted limitations of previous work in this area, as well as the long-standing belief that PFP is the result of excessive joint loading, we hypothesized that persons with PFP would demonstrate greater resultant PFJRFs, as well as greater components of the PFJRF (ie, greater anterior/posterior, superior/inferior, and medial/lateral forces).

Methods Subjects A total of 40 subjects were recruited for this study. Twenty females between the ages of 18 and 45 with PFP constituted the experimental group while 20 age and size-matched (height and weight) pain-free females served as a control group (Table 1). The determination of sample size was based on a prior sample size calculation using the patellofemoral joint reaction force data reported by Heino Brechter and Powers5 and Brechter and Powers.6 Based on this analysis it was estimated that a sample size of 40 would achieve 80% power to detect group differences across the varied conditions evaluated (using an alpha level of .05 and an effect size of 0.5). 493

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Table 1  Subject information Age (yrs)

Height (cm)

Weight (kg)

Control

26.1 (7.2)

165.3 (6.9)

59.1 (7.2)

PFP

27.9 (6.7)

168.1 (5.8)

62.4 (6.8)

Individuals over the age of 45 were excluded from the study to control for the possible effects of degenerative joint disease. The two groups were similar in terms of age, height, and weight (Table 1). PFP subjects were recruited from local orthopedic clinics and the student population at the University of Southern California. Subjects with PFP were screened (through physical examination) to rule out ligamentous instability, internal derangement, patella tendinitis, and large knee effusion. Only those subjects meeting the following criteria were admitted to the PFP group: (1) pain originating specifically from the patellofemoral articulation (vague or localized) and (2) readily reproducible pain (visual analog pain scale score of three or higher) with at least two of the following functional activities commonly associated with PFP; stair ascent or descent, squatting, kneeling, prolonged sitting, or isometric quadriceps contraction. PFP subjects were excluded from participation if they reported having any of the following: (1) previous history of knee surgery, (2) history of traumatic patella dislocation, (3) neurological involvement that would influence gait, or (4) implanted biological devices that could interact with the magnetic field (ie, pacemakers, cochlear implants, or ferromagnetic cerebral aneurysm clips). The control group was selected based on the same criteria as the experimental group except that subjects had none of the following: (1) history or diagnosis of knee pathology or trauma, (2) current knee pain or effusion, (3) knee pain with any of the activities described for the PFP group, (4) limitations that would influence gait, and (5) implanted biological devices, such as pacemakers, cochlear implants, or clips that are contraindicated for MRI. The purpose of the study, procedures, and risks were explained to each subject. Informed consent as approved by the institutional review board for the University of Southern California Health Sciences Campus was obtained. Subjects underwent two data collection sessions. The first session consisted of MRI assessment of the knee and thigh, while the second session consisted of instrumented gait analysis. Only the symptomatic limb of each PFP subject and the corresponding limb of the age and size matched control was tested. Before MRI and gait testing, the following anthropometric measurements were obtained: height, weight, leg length (from anterior superior iliac spine [ASIS] to the medial malleolus), and frontal plane patellar ligament orientation. The frontal plane orientation of the patellar ligament was defined by the quadriceps angle (Q-angle) which was quantified as the angle between the line from the midpoint of the patella to the ASIS and the line from the midpoint of the patella to the tibial tuberosity. The Q-angle was measured in standing using a goniometer.

Instrumentation and Procedures MRI Instrumentation.  Sagittal, frontal, and axial imaging of the

knee and thigh was performed using a 1.5T MR system (General Electric Medical Systems, Waukesha, Wisconsin). From these images, subject-specific input variables were obtained: extensor mechanism lever arm (sagittal), vasti muscle fiber orientation (sagittal and frontal), quadriceps muscle physiological crosssectional area proportions (axial), patellar flexion angle (sagittal), and patellar ligament orientation (frontal and sagittal).

Sagittal plane images of the knee were acquired with two 5-in receive-only coils placed on each side of the knee joint and secured with tape. T1-weighted imaging was obtained with the following parameters: repetition time (TR): 300 ms; echo time (TE): 10 ms; field of view (FOV): 20 cm × 20 cm; slice thickness: 2 mm; matrix: 256 × 256. Frontal plane images of the thigh were acquired using T1-weighted pulse sequence with the following parameters: TR: 450 ms; TE: 12.6 ms; FOV: 48 cm × 48 cm; slice thickness: 10 mm; matrix: 512 × 512. Axial images of the thigh were acquired using a T1-weighted, fast spin echo pulse sequence with the following parameters: TR: 400 ms; TE: 22 ms; FOV: 24 cm × 24 cm; slice thickness: 7 mm; matrix: 256 × 256. For both the frontal and axial plane imaging, only the body coil was used. The total scan time for each subject was approximately 1 hour. MRI Procedures.  Imaging was performed at the LAC+USC Imaging Science Center and was performed under loaded and unloaded conditions. First, subjects were positioned supine in the MR system with the knee in 0° of knee flexion and the quadriceps unloaded. Axial, frontal, and sagittal plane images of the thigh (knee joint line to hip joint center) were then obtained. Following unloaded imaging of the thigh, loaded sagittal plane images of the knee and patellofemoral joint were obtained using a custom-made nonferromagnetic loading apparatus. The loaded images were used to obtain anatomic variables that would be expected to be influenced by quadriceps contraction (quadriceps lever arm, patella flexion angle, patella ligament orientation). Subjects were asked to lay supine on the loading device with their knee extended and foot placed on the footplate of the loading device. A 25% body weight resistance was provided through the pulley system to the footplate. Sagittal plane images were obtained at 20°, 40°, and 60° of knee flexion. Gait Analysis Instrumentation.  Three-dimensional motion analysis was performed using a computer aided video (VICON) motion analysis system (Oxford Metrics LTD, Oxford, England). Kinematic data were sampled at 120 Hz. Reflective markers (9-mm spheres) placed at specific anatomical landmarks (see “Gate Analysis Procedures” section for details) were used to determine sagittal plane motion of the pelvis, hip, knee, and ankle. As described in previous publications9,10 data for walking and running were acquired along a 10 m walkway. Analysis of stair ascent and descent was made on a portable four-step staircase which had a slope of 33.7°, a step height of 20.3 cm, and a tread depth of 30.5 cm. Ground reaction forces were collected at a rate of 2500 Hz using two AMTI force plates (Model #OR6-6-1; AMTI, Newton, Massachusetts). The force plates were situated within the middle of the 10 m walkway with the pattern of tile flooring camouflaging their location. For assessment of ground reaction forces during stair ambulation, the raised floor tiling was removed and portable steps were positioned such that the force plate became the second step of the staircase. The portable steps were designed to avoid contact with the force plate. Electromyographic (EMG) signals of the flexor muscles crossing the knee (medial/lateral hamstrings and medial/lateral gastrocnemius) were recorded at 2500 Hz, using preamplified bipolar, grounded, surface electrodes (Motion Laboratory, Salt Lake City, Utah). EMG data were telemetered to an analog to digital converter using a 14-channel unit. Differential amplifiers were used to reject the common noise and amplify the remaining signal (gain = 1000). EMG signals were band-pass filtered (20–500 Hz) and a 60 Hz notch filter was applied.

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3D Patellofemoral Joint Reaction Force  495 Gait Analysis Procedures.  Gait analysis was performed at the Musculoskeletal Biomechanics Research Laboratory at the University of Southern California. Subjects were appropriately clothed, allowing the pelvis, hip, thigh, and lower leg of the involved limb to be exposed. A triad of rigid reflective tracking markers was securely placed on the lateral surface of the subject’s right and left thigh, leg, and heel counter of the shoe. Tracking markers were placed on each ASIS and iliac crest, as well as on the L5/S1 junction. In addition to the tracking markers, calibration markers were placed bilaterally on the greater trochanters, medial and lateral femoral epicondyles, medial and lateral malleoli, and first and fifth metatarsal heads. Surface EMG electrodes were taped to the skin over the medial and lateral hamstrings, and the medial head of the gastrocnemius. Electrodes were connected to the hardwire unit, which was strapped around the subject’s back. To allow for comparison of EMG intensity between subjects and muscles, and to control for variability induced by electrode placement, EMG data were normalized to the EMG acquired during a maximum voluntary isometric contraction (MVIC). The MVIC of the hamstrings was performed with the subject lying supine on a treatment table with a custom-made box placed under the tibia to keep both the hip and knee flexed to 90°. A strap was used to secure the hip to the table. Subjects were asked to maximally flex their knee (isometrically) and raise their pelvis (such that that strap provided resistance). The gastrocnemius MVIC was performed with the subject standing with an adjustable strap looped from the bottom of the tested foot to both sides of the subject’s shoulders. Subjects were asked to perform a maximum heel raise such that the strap provided a resistance. Each MVIC test described above was held for 6 seconds. Following MVIC testing, a standing calibration trial was obtained. Sufficient practice trials of walking and running allowed subjects to become familiar with the instrumentation. Kinematic, kinetic, and EMG data were collected simultaneously (and synchronized) during walking (80 m/min), running (200 m/min), and ascending/descending stairs (50 steps/min). These velocities are considered to be normative values for these tasks.9,10 A trial was considered successful if the subject’s instrumented foot landed within either force plate (without targeting) at the required task velocity (± 5%). Task velocity was calculated immediately following each trial, and trials were repeated as necessary. Three acceptable trials of data were obtained for each condition and averaged for analysis.

were calculated for each EMG channel sample by sample. This process is the mathematical equivalent to full wave rectification. Next, a moving average (5-ms window) was calculated to generate a linear envelope. The EMG data were then integrated over time periods corresponding to 2% of the activity cycle. EMG intensities were expressed as a percentage of the EMG obtained during the MVIC.

Model Description An overview of the model is illustrated in Figure 1. The methods used to quantify the MRI input variables have been described in a previous publication.8 The following input variables obtained from the biomechanical testing were used in the model: (1) 3D kinematics of the lower extremity, (2) net knee joint moment in the sagittal plane, and (3) hamstring and gastrocnemius EMG.

Model Algorithm Step 1: Creating a Subject-Specific Representation of the Extensor Mechanism.  Using the subject-specific MRI and

Q-angle measurement described above, individual subject models were created using SIMM modeling software (MusculoGraphics, Inc., Santa Rosa, California). The knee extensor model was created in reference to the midpoint of patella (ie, patella convention system: X-axis = anterior/posterior, Y-axis = superior/inferior, and Z-axis = medial/lateral).

Step 2: Estimation of Knee Extensor Moment.  While the net knee extensor moment gives a reasonable estimate of quadriceps demand, this value would be underestimated in the presence of muscle cocontraction. To account for cocontraction during the functional tasks evaluated, an estimate of the knee flexor moment (KFM) was required. The KFM was obtained from SIMM modeling software. The SIMM lower limb model contains musculotendon actuators

Data Analysis Visual3D software (C-Motion, Inc., Rockville, Maryland) was used to quantify 3D motion of the hip, knee, and ankle motion. In Visual3D, all lower extremity segments are modeled as frusta of cones while the pelvis is modeled as an ellipsoid. VICON Workstation software was used to digitize the kinematic data. The local coordinate systems of the pelvis, thigh, leg, and foot were derived from the standing calibration trial. In addition, the segment ends were identified from the standing calibration trial to locate the segment origins. Coordinate data were low-pass filtered using a fourth-order Butterworth filter with a 6 Hz cutoff frequency for the gait and ascend/descend stairs data and a 12 Hz cutoff frequency for the running data. Visual3D software was used to calculate 3D net joint moments using inverse dynamics equations. To facilitate comparisons between groups, kinetic data were normalized to body mass. Determination of the presence or absence of muscle activity was made using the EMG data. Initially, the root mean square values

Figure 1 — Patellofemoral joint model overview.

496  Chen and Powers

with information about peak isometric muscle force, optimal muscle-fiber length, pennation angle, and tendon slack length for the muscles of the lower extremity.11 In the SIMM software, muscles are represented as a series of 3D vectors that are constrained to wrap over underlying structures. Using a Hill-based model,11 the SIMM software estimated the KFM based on the individual’s lower extremity kinematics, velocity of movement, and flexor muscle EMG. The adjusted knee extensor moment (KEMadj) was calculated by adding the net knee joint moment (NJM) and the KFM (Eq. 1).

KEMadj = NJM + KFM

(1)

Step 3: Calculating Resultant Quadriceps Force (Sagittal Plane). 

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Using Equation 2, the sagittal quadriceps force (FQ) was calculated as the adjusted knee extensor moment (KEMadj) divided by the lever arm of the quadriceps (LA): FQ = KEMadj/LA (2) Step 4: Optimization.  The individual sagittal plane vasti forces

were estimated in MATLAB (Mathworks, Natik, Massachusetts) using a static optimization routine with the criteria of minimizing muscle stress cubed for each muscle (Eq. 3).12 Based on the 3D orientation and geometry of quadriceps elements from the SIMM model and the sagittal plane vasti forces, the 3D vasti forces with the sum of the projections of the forces at the sagittal plane equal to the resultant sagittal quad force (FQ) were obtained (Eq. 4). Minimize [∑ (Fvasti/PCSAvasti)3] (3) FQ = FVL + FVM + FRF + FVI (4) Step 5: Calculating 3D Resultant Quadriceps Force.  Using Equation 5, the 3D vasti force magnitudes multiplied by the unit vector of each muscle (as output from the SIMM modeling software) were used to provide the 3D vasti forces (magnitudes and direction).

Fvasti3D = Fvasti3D magnitude × [i j k] (5) Based on the magnitude and direction of the three vasti noted above, a resultant 3D quadriceps force vector (FQ3D) was calculated using Equation 6. FQ3D = FVM3D + FVL3D + FVI3D + FRF3D (6) Step 6: Calculation of 3D Patellar Ligament Force.  Because

of the lever action of the patella, the force in the patellar ligament (FPL) cannot be assumed to be equal to the force in the quadriceps tendon (FQ).13 Therefore, the patellar ligament force magnitude was estimated based on data obtained from a cadaveric study using a 3D muscle force application similar to what was employed in the current model.14 Using Equation 7, the magnitude of the patellar ligament force was obtained by multiplying the resultant quadriceps muscle force by the FPL/FQ ratio. FPL3D = FQ3D × FPL/FQ (7)

Step 7: Calculation of 3D PFJRFs.  Before calculating the sum

of 3D quadriceps muscle force vectors and patella ligament force, frontal and transverse plane motions were modeled in MATLAB software based on the frontal and transverse plane knee angles during the various functional tasks performed. All 3D quadriceps force/patellar ligament vectors were rotated in the frontal plane based on instantaneous angles between femur/tibia and vertical, to account for frontal plane motion of the knee. For transverse plane motion, the 3D patellar ligament force vector was rotated in the transverse plane based on instantaneous rotation angle to account for transverse plane motion of the knee. Using Equation 8, the 3D PFJRF was expressed as the resultant of the 3D quadriceps force vector (FQ3D) and the 3D patellar ligament force vector (FPL3D). The 3D orientation of the patellar ligament force vector was obtained from the SIMM lower extremity model and adjusted based on knee kinematics of each subject. 3DPFJRF = FQ3D + FPL3D (8) The reference point (origin) of the PFJRF was defined as the midpoint of patella and the resultant PFJRF was resolved into components along three orthogonal axes (anterior/posterior, medial/lateral, superior/inferior). Peak forces during each task were identified and used for statistical analysis.

Statistical Analysis To evaluate group differences in PFJRFs across tasks, 2 × 4 (group × task) analysis of variance tests (ANOVAs) were performed. Separate ANOVAs were employed for each dependent variable of interest (peak resultant PFJRF, peak anterior/posterior force, peak medial/ lateral force, peak superior/inferior force). For all ANOVA tests, significant main effects were reported if there were no significant interactions (P < .05). Tukey’s post hoc analysis was performed to identify the differences if a significant main effect or a significant interaction was found (P < .05).

Results With respect to the peak resultant PFJRF, a significant group effect was found (no interaction). When averaged across all conditions, the peak PFJRF was significantly lower in the PFP group compared with the control group (25.9 ± 2.1 N/kg-bwt vs 32.2 ± 2.3 N/kg-bwt, P = .032; Figure 2 and Table 2). When the resultant PFJRF was resolved into its three orthogonal components, forces throughout the range of knee motion achieved for each task were always in the posterior, superior, and lateral directions. For the peak posterior force (ie, compressive force), a significant group effect (no interaction) was found. When averaged across all conditions, the peak posterior force was significantly lower in the PFP group compared with the control group (24.2 ± 2.0 N/ kg-bwt vs 30.5 ± 2.1 N/kg-bwt, P = .036; Figure 3 and Table 2). For the peak superior force, a significant group effect (no interaction) was found. When averaged across all conditions, the peak superior force was significantly lower in the PFP group compared with the control group (7.9 ± 1.5 N/kg-bwt vs 10.4 ± 1.6 N/kg-bwt, P = .023; Figure 4 and Table 2). With respect to the peak lateral force, a significant group × task interaction was found. Post hoc analysis revealed that the PFP group demonstrated greater peak lateral forces during running (P = .031), stair ascent (P = .021), and stair descent (P = .023) when compared with the control group. No group difference in peak lateral force was found for walking (P = .241; Figure 5 and Table 2).

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Figure 2 — PFJRF during walking (top left), running (top right), stair ascent (bottom left), and stair descent (bottom right).

Figure 3 — Posterior component of the PFJRF during walking (top left), running (top right), stair ascent (bottom left), and stair descent (bottom right).

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Figure 4 — Superior component of the PFJRF during walking (top left), running (top right), stair ascent (bottom left), and stair descent (bottom right).

Table 2  Peak resultant PFJRF and orthogonal components (mean ± SD) Resultant PFJRF PFP

CTRL

Posterior Force PFP

Superior Force

Lateral Force

CTRL

PFP

CTRL

PFP

CTRL

Walking

7.8 (1.2)

9.8 (1.3)

6.4 (1.8)

8.0 (2.1)

5.1 (1.0)

6.7 (1.4)

1.8 (0.6)

1.6 (0.8)

Stair descent

21.9 (2.9)

28.4 (3.2)

20.9 (2.3)

27.7 (2.9)

5.1 (1.3)

6.9 (1.1)

7.2 (1.5)

3.3 (1.2)

Stair ascent

29.8 (3.0)

35.7 (3.1)

28.2 (3.1)

34.5 (4.1)

6.5 (1.7)

8.9 (2.1)

7.8 (1.6)

4.1 (1.3)

Running

44.2 (5.0)

54.8 (5.3)

41.2 (4.2)

51.6 (4.7)

15.0 (2.4)

18.9 (3.0)

8.0 (1.4)

3.8 (1.2)

Figure 5 — Lateral component of the PFJRF during walking (top left), running (top right), stair ascent (bottom left), and stair descent (bottom right). 498

3D Patellofemoral Joint Reaction Force  499

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Discussion The purpose of this study was to compare 3D PFJRFs between persons with PFP and pain-free controls using an imaging-based, subject-specific model. Contrary to our proposed hypothesis, persons with PFP demonstrated lower peak resultant PFJRFs across all tasks evaluated. Although similar trends were observed for the peak posterior and superior forces, subjects with PFP demonstrated significantly higher peak lateral forces in three out of the four conditions evaluated. When averaged across all conditions, the peak resultant PFJRFs in the PFP group were 20% lower than the control group. This finding is consistent with Heino Bretcher and Powers5 and Brechter and Powers6 who reported a 22% and 33% decrease in peak PFJRFs in persons with PFP during walking and stair ascent, respectively. In contrast, Bretcher and Powers did not report a significant group difference in PFJRFs during stair descent; however, we found a significant decrease in our PFP group (23%) during this task.6 When comparing the peak PFJRFs reported by Heino Brechter and Powers5 as well as Brechter and Powers6 to those observed in the current study, values were similar. For example, our peak PFJRFs for walking, stair ascent, and stair descent (8.8, 32.8, and 25.2 N/ kg-bwt, respectively) closely matched the walking values reported by Brechter and Powers5 (8.5 N/kg-bwt) and the stair ascent and descent values reported by Heino Brechter and Powers6 (31.2 and 26.6 N/kg-bwt, respectively). Our finding of lower peak resultant PFJRFs in the PFP group is consistent with the observation of previous authors who have suggested that individuals with PFP may adopt compensatory strategies to reduce loading across the patellofemoral joint to minimize symptoms. For example, Salsich et al reported that subjects with PFP exhibited reduced peak knee extensor moments during stair ambulation, suggesting that these individuals had adopted a quadriceps avoidance gait pattern.7 Brechter and Powers also reported that the reduction in PFJRFs in their subjects could be attributed to a decreased extensor moment.5,6 A post hoc analysis of the knee extensor moment in the current study revealed that our PFP subjects had significantly lower values compared with the control group when averaged across all tasks (1.96 ± 0.19 N·m/kg-bwt vs 2.26 ± 0.08 N·m/kg-bwt; P = .014). As noted above, when the resultant PFJRF was broken down into its orthogonal components, the forces were always in the posterior, superior, and lateral directions. This trend was consistent for each task evaluated. The posterior component of the PFJRF (ie, compressive force) always had the greatest magnitude, followed by the superior and lateral components (Table 2). The large posterior force reflects the fact that a majority of the resultant PFJRF would contribute to compression of the patella against the trochlear surface of the femur. The large posteriorly directed force is related to the fact that the vasti originate from the posterior aspect of the femur (linea aspera). As a result, the fibers of the vastus medialis and vastus lateralis are angled posteriorly relative to the femur. Therefore, a decrease in quadriceps force (as observed in the PFP group) would be expected to lead to a decrease in the posteriorly directed force acting on the patella. The superiorly directed PFJRF would be expected based on the fact that the quadriceps muscles insert into the proximal quadriceps tendon that originated from the anterior inferior iliac spine. Given as such, the quadriceps muscle contraction would create a superior pull on the patella during dynamic tasks.15 As noted with the posterior force, a decreased quadriceps force would lead to a

decrease in superior force in PFP individuals. The rectus femoris and vastus intermedius would have the largest contribution to the superior component of the PFJRF as their line of pull is parallel to the femur. All subjects in the current study exhibited a lateral force acting on the patella. A laterally directed force would be anticipated based on the fact that the patella ligament force vector and the resultant quadriceps force vector are not collinear. Despite the finding of lower posterior and superior forces, individuals with PFP demonstrated higher peak lateral forces when compared with the control group. This finding was observed in three of the four tasks evaluated (running, stair ascent, and stair descent). The observed increase in lateral force in the PFP group is consistent with the clinical findings of lateral patella subluxation in this population.16-18 Although the differences in the peak lateral force between groups was relatively small (3.0 N/bwt or 182 N), Jafari et al has reported that the force required to laterally displace the patella 10 mm is only 80 N.19 This would suggest that the observed increase in lateral force in our PFP group would be of sufficient magnitude to cause patella subluxation, particularly in the presence of inadequate soft tissue or bony restraints. Furthermore, the repetitive application of lateral forces would result in shear stresses across the patellofemoral joint which could explain the high incidence of lateral facet cartilage damage in persons with PFP and patellofemoral joint osteoarthritis.20 The higher laterally directed forces in the PFP group may be explained in part by the fact that the average static Q-angle in these subjects was significantly higher than the control group (19.1 ± 2.4 vs 14.2 ± 1.8°; P < .05). In light of the findings reported in this study, there are limitations that need to be acknowledged. First, our model has not been validated and did not account for contact geometry. However, it has been reported that reasonable estimates of the PFJRFs can be obtained using models that do not consider patellofemoral joint contact geometry.14 Furthermore, it has been reported that PFJRFs estimated by computational methods similar to what was used in the current paper closely agree with direct measurements obtained from an in vitro set-up.14 Second, passive structures such as the patella retinaculum were not modeled as part of this study. Previous work from our group indicates that the removal of the retinaculum increases patella tendon forces by 10% to 15%.21 Failure to account for soft tissues around the knee likely resulted in an overestimation of PFJRFs. However, we do not believe that this would influence the overall conclusions of our study as any error would be consistent across groups. Third, we did not perform a sensitivity analysis as part of this study. As such, it is not known how inaccuracies in measuring or estimating various modeling parameters would affect model predictions of PFJRFs. Last, care must be taken when translating our results to the general population, as our study design consisted of a relatively smaller sample size with female participants only. Our study is the first to compare 3D PFJRFs between PFP and pain-free individuals. The lower resultant PFJRFs observed in the PFP group support the premise that these individuals may be employing strategies to minimize patellofemoral joint forces (ie, quadriceps avoidance). The decrease in resultant force was reflected in a decrease in posterior and superior forces acting on the patella. However, the PFP group exhibit higher lateral forces, which is consistent with clinical observations of lateral patella subluxation in this population. Acknowledgments This manuscript was supported by a grant from the Whitaker Foundation.

500  Chen and Powers

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Comparison of three-dimensional patellofemoral joint reaction forces in persons with and without patellofemoral pain.

The purpose of this study was to determine if persons with patellofemoral pain (PFP) exhibit differences in patellofemoral joint reaction forces (PFJR...
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