Knee Extensor Mechanism Strength and Its Relationship to Patellofemoral Kinematics in Individuals With Arthrofibrosis Within 6 Months After Anterior Cruciate Ligament Reconstruction

in Journal of Sport Rehabilitation
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Context: Performance in strength and assessment of patellar tracking is important for patients with arthrofibrosis after anterior cruciate ligament (ACL) reconstruction. Objective: The study was to examine the difference of patellofemoral kinematics between the affected and the contralateral limb and to evaluate the relationship between knee extensor strength and patellofemoral kinematics in patients with arthrofibrosis after ACL reconstruction. Design: Cohort study (diagnosis); level of evidence, 3. Setting: Laboratory. Patients: A prospective cohort of 20 patients with arthrofibrosis after ACL reconstruction was recruited. Interventions: A total of 20 patients who underwent arthroscopic reconstruction of the double-bundle ACL with a hamstring tendon autograft received standardized patellofemoral kinematics testing and knee extensor strength testing within 6 months after primary ACL reconstruction. Computed tomography and dual fluoroscopic imaging were used to evaluate in vivo patellofemoral kinematics of affected and contralateral knees during a lunge task. Knee extensor mechanism strength was measured using a handheld dynamometer. Main Outcome Measures: A limb symmetry index of knee strength and patellar mobility was calculated and satisfactory performance defined as ≥90%. Results: There was a statistically significant decrease in the range of patellar inferior shift (P = .020; d = 0.81), flexion (P = .026; d = 0.95), lateral tilt (P = .001; d = 1.04), and lateral rotation (P < .001; d = 0.89) in the affected knee compared with the contralateral knee from 15° to 75° of knee flexion. There was a strong positive linear correlation between knee extensor strength and patellar inferior shift (r = .747; P = .008). A knee extensor strength limb symmetry index <90% was 89% sensitive and 9% specific for limited patellar inferior shift. Conclusions: Patients with arthrofibrosis after ACL reconstruction presented decreased patellar mobility in the arthrofibrotic knee compared with the contralateral knee. The strong correlation between knee extensor strength and patellar inferior shift of the arthrofibrotic knee demonstrates the importance of knee extensor strength in the diagnosis and treatment of patients with knee arthrofibrosis. The knee extensor mechanism strength has high sensitivity but low specificity in identifying a decrease in patellar inferior shift in patients with arthrofibrosis after ACL reconstruction.

Arthrofibrosis is a devastating complication after anterior cruciate ligament (ACL) reconstruction, and commonly involves the patellofemoral joint (PFJ).1 Patients with knee arthrofibrosis often present with the following findings: decreased patellar mobility, anterior knee pain, and quadriceps weakness or atrophy.2 The current treatment is mainly to improve knee range of motion (ROM), and the literature mostly describes a relevant improvement in knee motion after various treatment interventions, whereas the findings of physical examination include persistent muscle weakness and abnormal patella excursion even after the intervention.3 Performance in patellar tracking and strength commonly involves the effective management of this condition and an assessment of readiness for return to sport.4 Patellar mobility is of particular interest in knee arthrofibrosis, as it is well recognized to be impaired in the knee with arthrofibrosis. It is known that patellofemoral kinematics changes with quadriceps activation.5

Physical examination of patellar mobility and excursion was previously performed in the supine position, which is confined to quasi-static conditions and not available to quantification of knee motion.4 With methodological advances, magnetic resonance images technique and the dual fluoroscopic imaging system could be used to investigate patellar tracking under physiological loading conditions.6,7 The limitations of these advanced methodologies are that they are time- and resource-intensive and require specialized equipment.8 They are not available to many patients with arthrofibrosis after ACL reconstruction during the rehabilitation process.

Strength testing using a handheld dynamometer is easily performed in most outpatient settings.9 It is quick to perform and can be conducted by physiotherapists. Knee motion during functional activities has also been reported to be related to knee extensor strength after ACL reconstruction.10 Wang et al11 hypothesized that PFJ cartilage damage might be related to the recovery of quadriceps strength after ACL reconstruction, and concluded that greater quadriceps strength is associated with less severe PFJ cartilage damage. This suggests that the restoration of knee extensor strength has a strong relationship with the improvement of knee joint function.4 Still, little is known about the relationship between knee extensor strength and knee motion in patients with arthrofibrosis of the knee.

A limb of symmetry of index (LSI), where the involved knee’s strength or kinematics is expressed as a percentage of the contralateral limb, is commonly used to assess knee function after ACL reconsruction.10,12 It has been reported that 90% LSI was used as the cut point of “satisfactory performance on the knee function.”10,13 The purpose of the present study was (1) to examine the difference of patellofemoral kinematics between the affected and the contralateral limb in patients with arthrofibrosis after ACL reconstruction, (2) to examine the relationship between knee extensor strength and in vivo 3-dimensional (3D) patellofemoral kinematics in the arthrofibrotic knee during a single-leg lunge task, and (3) to investigate the diagnostic utility of knee extensor mechanism strength as a measure of patellar mobility. Furthermore, we hypothesized that greater knee extensor strength might be associated with greater patellar mobility at the arthrofibrotic knee under weight-bearing conditions.

Materials and Methods

Design

A prospective, cohort study (diagnosis) was used, in which all participants having suffered arthrofibrosis after ACL reconstruction participated in knee strength testing and evaluation of patellar tracking.

Patients

Twenty patients (12 women and 8 men) with a mean age of 35 (6.3) years who underwent primary unilateral ACL reconstruction and were identified with arthrofibrosis according to international consensus on the definition of fibrosis of the knee joint14 were enrolled in the study. Postoperative fibrosis of the knee was defined as a limited ROM in extension and/or flexion that is not attributable to ligament reconstruction, pain, infection, or other specific causes, but rather due to soft tissue fibrosis that was not present preoperatively.14 The diagnose of arthrofibrosis was confirmed with both MRI and physical examination by the same senior doctor. Inclusion criteria were (1) no other injuries or previous surgery to the affected knee other than no more than one-third of meniscal tears requiring partial meniscectomy, (2) knee extension restriction >5° or knee flexion range <100°, (3) within 6 to 24 weeks after ACL reconstruction, and (4) no injuries or surgeries to the contralateral knee. Exclusion criteria were injury to the hip joint or ankle joint for both limbs, malpositioned ligament reconstruction, and neuromuscular disorder of the lower extremity. Subjective evaluation included the International Knee Documentation Committee and Lysholm scores. Objective evaluation included knee ROM, knee extensor strength, and patellofemoral kinematics. The cutoff angle was defined as knee extension restriction greater than 5° and knee flexion range <100° to determine knee arthrofibrosis.14 Knee ROM in extension and flexion was measured from 6 weeks postoperatively to discriminate arthrofibrosis of the knee, and mean time from surgery to knee ROM assessment was 13.5 (4.1) weeks. The range of knee extension and flexion were measured by a plastic goniometer with 25-cm movable arms in 1° increments. With patients lying supine on an examination table, measurements of knee ROM were taken in active and maximum extension and flexion of the knee with the hip flexed. One arm of the goniometer was placed along the greater trochanter, and the lateral epicondyle of the femur and the other arm was aligned with the fibular head and the lateral malleolus of the fibula. All patients underwent strength testing and laboratory kinematic evaluation 1 to 2 days after knee ROM assessment. Demographic data for the included patients are summarized in Table 1.

Table 1

Patient Characteristics

Participants
Age, y35.2 (6.3)
Sex: male/female, n8/12
Height, cm169.3 (7.2)
Weight, kg62.2 (9.8)
Body mass index, kg/m221.7 (3.2)
IKDC score43.1 (5.6)
Lysholm score60.6 (4.8)

Abbreviation: IKDC, International Knee Documentation Committee. Note: Values are presented as mean (SD).

The study was approved by the University Review Board (SH9H-2019-T220-1) and is registered with Chinese Clinical Trial Registry (www.chictr.org.cn) with registration number ChiCTR1900025977. Each subject provided an informed written consent form at the time of recruitment.

Surgical Technique and Postoperative Rehabilitation

All patients underwent arthroscopic reconstruction of the double-bundle ACL with a hamstring tendon autograft. It was performed using suspensory fixation with an EndoButton (Smith & Nephew, London, England) on the femoral side and interference fixation with a screw (ConMed Linvatec, Utica, NY) on the tibial side. The tunnels for the anteromedial and posterolateral bundles were positioned at the center of the respective bundle footprints on both the tibial and femoral side. Sixteen patients required partial removal of the meniscus, and the remaining 4 patients had no significant damage to the meniscus documented during arthroscopic reconstruction.

Postoperative rehabilitation followed a standardized criterion, as previously reported.15 In the first 3 weeks, elevation, compression, rest, and ice were used to reduce knee swelling. Patients were encouraged to walk with braces and crutches from the first day after surgery, and straight-leg raises, quadriceps sets, and prone leg hanging were commenced. From 3 weeks, wall squats, forward lunges, hamstring curls, and a stationary bicycle were introduced. At 5 weeks, a gymnasium program, including half squat, leg presses, a rowing machine, and calf raises were commenced. Closed kinetic-chain exercises were started at 6 weeks. From 10 to 16 weeks, landing dills and hopping were commenced. Patients progressed to running without support at 16 to 24 weeks. Patients were typically allowed to return to original sporting activities approximately 24 weeks after surgery.

Procedures

Strength Testing

Maximal isometric voluntary contraction was measured with a MicroFET2 handheld dynamometer (Hoggan Health Industries, Inc., West Jordan, UT). Maximal isometric muscle strength of knee extensors was measured. Each participant was instructed to perform 3 consecutive maximal contractions for knee extensors with 30-second intervals between contractions, preceded by 3 warm-up trials. Before the actual test, participants were shown the tested movement and then asked to perform it correctly, and finally did the warm-ups. The highest performance of the 3 measurements was taken for analysis. The contralateral leg was tested first, followed by the affected leg. Participants were asked to sit with thigh resting on a cushion, leg hanging, and hip and knee flexed at 90°, and the MircoFET2 was placed on the front of the skin. Participants gradually increased their muscle force to maximum effort and sustained it for 6 seconds. The tester provided standardized encouragement. The reliability and validity of the MicroFET2 has been proved previously.9

Evaluation of Patellar Tracking

Patellofemoral kinematics was assessed with a novel methodology based on the fusion of computed tomography and dual fluoroscopic system. High accuracy and repeatability of this method has been reported previously.16 First, the affected and the contralateral knees were imaged using a computed tomography scanner (General Electric Medical Systems, Milwaukee, WI) in the knee extension position. Computed tomography scans were captured in a 30-cm display field of view with thickness of 1 mm and resolution of 512 × 512 pixels. These images were then imported into a solid modeling software (3D Slicer, www.slicer.org) to create 3D anatomical models of the femur, tibia, and patella, and the coordinate systems of the femur, tibia, and patella were manually created, as previously described.17 Next, a dual fluoroscopic system (65 kvp, 50 mA, and an average dose rate of 0.08 mSv/100 frame) was used to image the PFJ motion during a single-leg lunge task. Dynamic fluoroscopy images were collected for 10 seconds at 100 Hz when subjects performed a single-leg lunge task (the extended knee position to maximum flexion). The testing procedure of this functional task has been described in previous studies.6,17 Afterward, the fluoroscopic images were imported into a custom MATLAB program (R2018a; MathWorks, Inc, Natick, MA) and were placed based on projection geometry of the fluoroscopes during the actual test. To measure the relative 3D motion of the patella throughout the tested ROM, the 3D anatomical models of the knee were then imported into this software and manipulated in 6 degrees of freedom until the projections of the knee model matched the outlines of fluoroscopy images taken during the lunge motion. Finally, when the 3D models best matched the fluoroscopic images, the positions of the models reproduced the relative positions of the knee at each flexion angle.6

A joint coordinate system was installed for each knee to describe the motion of the patella.6 The femoral coordinate system consisted of the long axis along the femoral shaft in sagittal plane and the transepicondylar axis (TEA), with the knee center at the midpoint of the TEA. An axis along the posterior wall of the tibial shaft was defined as the tibial long axis. The knee flexion angle was defined as the angle between the long axes of the femur and tibia in the sagittal plane. For the patellar coordinate system, a cuboid was fitted around the patella touching the contours of the patella in the anterior–posterior (AP), superior–inferior (SI), and medial–lateral (ML) directions. The geometric center of the cuboid was defined as the origin of the patella. The long axis of the patella was defined as the line along the SI direction. The ML axis of the patella was the line connecting the medial and lateral ridges of the patella. The AP axis was perpendicular to the other axes.

Patellar flexion was defined as the rotation of the long axis of the patella around the TEA of the femur. Positive value refers to patellar flexion (Figure 1). Patellar tilt was defined as the rotation of the patella about its long axis, where lateral tilt followed the direction of external femoral rotation (Figure 1). Patellar rotation was the rotation of the patella about its AP axis, where lateral rotation followed the direction of valgus rotation in tibiofemoral motion (Figure 1). Patellar ML shift was defined as the medial or lateral translation of the center of the patella along the TEA of the femur. Patellar SI and AP shifts were the translations of the patellar center along the SI and AP axes relative to the knee center. Positive values refer to lateral, superior, and anterior patellar shifts (Figure 1). All included patients had extension and flexion restriction in the affected knee. To facilitate consistent comparisons between different limbs and patients, the in vivo 3D patellar motion was assessed with the knee flexion changing from 15° extension to 75° flexion (defined as the angle between the long axes of the femur and tibia in the sagittal plane).

Figure 1
Figure 1

Coordinate systems used to quantify the patella tracking. Patellar flexion, lateral shift, anterior shift, superior shift, lateral tilt, and lateral rotation are considered positive as shown in the figure.

Citation: Journal of Sport Rehabilitation 2021; 10.1123/jsr.2020-0468

Statistical Analysis

Continuous variables were described using means and standard deviations. The Pearson correlation coefficient was used to assess for linear associations between knee extensor strength and patellofemoral kinematics in the affected knee. The proportion of variance was determined by calculating the r2 value (coefficient of determination). A 2-tailed paired t test was used for a comparison of data of the affected and the contralateral limbs with the effect size measured using the Cohen d. An effect size of 0.2 was defined as small, 0.4 as medium, and ≥0.8 as large. Sensitivity and specificity were calculated from 2 × 2 tables with the knee extensor strength dichotomized as LSI <90% or ≥90% and patellofemoral kinematics dichotomized as LSI <90% or ≥90% (compared with patellofemoral kinematics testing LSI as the gold standard with a cutoff of <90%).12 All analyses were performed using SPSS Statistics (version 25.0.0.0; IBM Corp, Armonk, NY). A P value <.05 was considered significant, and LSI ≥90% was considered satisfactory, as previously reported.10,12

Results

Mean values for extensor mechanism concentric peak in the affected and contralateral limbs are listed in Table 2. There were significant differences in the mean extensor mechanism concentric peak value between the affected and the contralateral limbs (150.1 [64.6] vs 209.8 [80.7]; P = .008; d = 0.82).

Table 2

Knee Extensor Strength and Patellofemoral Kinematics

Affected limbContralateral limbLimb symmetry indexP valued
Knee extensor strength
 Concentric peak, N·m150.1 (64.6)209.8 (80.7)71.5 (20.8).0080.82
Patellar mobility
 AP shift, mm−20.9 (3.8)−24.3 (4.6)86.0 (14.5).7480.55
 SI shift, mm−26.9 (2.1)−28.6 (3.8)94.1 (21.4).0200.81
 ML shift, mm2.7 (1.0)3.0 (1.0)90.0 (28.9).4230.30
 Flexion, °31.4 (10.8)43.4 (14.3)72.4 (12.5).0260.95
 Tilt,°−0.6 (1.6)1.5 (2.4)28.6 (21.1).0011.04
 Rotation, °−1.7 (1.7)1.2 (4.3)58.6 (13.6)<.0010.89

Abbreviations: AP, anterior–posterior; ML, medial–lateral; SI, superior–inferior. Note: Data are presented as mean (SD).

The ranges of patellar motion in 6 degrees of freedom with knee flexion from 15° to 75° for the affected and contralateral limbs are also shown in Table 2. Arthrofibrosis of the knee decreased the range of patellar inferior shift compared with the contralateral knee (affected: −26.9 [2.1] mm, contralateral: −28.6 [3.8] mm; P = .020), with large effect size (d = 0.81). Arthrofibrosis of the knee decreased the range of patellar flexion (affected: 31.4° [10.8°], contralateral: 43.4° [14.3°]; P = .026; d = 0.95), lateral tilt (affected: −0.6° [1.6°], contralateral: 1.5° [2.4°]; P = .001; d = 1.04) and lateral rotation (affected: −1.7° [1.7°], contralateral: 1.2° [4.3°]; P < .001; d = 0.89) compared with the contralateral knee. There was no significant difference in the patellar AP shift and ML shift between the affected and the contralateral limbs (P > .05). There was a strong positive linear correlation between knee extensor strength and patellar inferior shift (r = .747; P = .008; Table 3).

Table 3

Linear Correlation Between Knee Extensor Strength and Patellofemoral Kinematics in the Affected Knee

Knee extensor strength
rr2P value
AP shift−.374.140.257
SI or shift.747.558.008
ML shift−.323.104.195
Flexion.182.033.593
Tilt−.338.114.310
Rotation−.108.012.753

Abbreviations: AP, anterior–posterior; ML, medial–lateral; SI, superior–inferior.

In the present study, 5% of patients demonstrated satisfactory performance on knee extensor mechanism strength but had unsatisfactory patellar mobility (Table 4). The diagnostic utility of the knee extensor strength LSI in detecting restricted patellar SI shift was calculated (Table 4). Specificity was 9%, and sensitivity was 89%.

Table 4

Relationship Between Knee Extensor Strength and Patellar Inferior Shift

Knee extensor strength (LSI)
<90%≥90%Total
Inferior shift (LSI)<90%42547
≥90%48553
Total9010100

Abbreviation: LSI, limb symmetry index. Note: Data are presented as percentage.

Discussion

Arthrofibrosis following ACL reconstruction resulted in an alteration in patellar tracking. A strong correlation between extensor strength and patellar inferior shift of the arthrofibrotic knee was found in individuals with arthrofibrosis within 6 months after ACL reconstruction. Our findings demonstrate the importance of knee extensor strength in the clinical utility of diagnosing and treating patients with knee arthrofibrosis.

This investigation has demonstrated that arthrofibrosis of the knee decreased the range of patellar inferior shift and flexion during 15° to 75° of knee flexion with a large effect. These findings are consistent with the in-vitro study results obtained by Mikula et al,18 which showed that arthrofibrosis in the suprapatellar pouch resulted in decreases in patellar inferior shift and flexion. In the knee, arthrofibrosis involves the patellofemoral compartment, occurring in the suprapatellar pouch, the infrapatellar fat pad, and the anterior interval, which might restrict the patella from shifting inferiorly effectively during knee flexion.14,19 It has been reported that patellofemoral adhesions, particularly in the suprapatellar pouch, may lead to patellar flexion loss.20 This alternation is understandable because arthrofibrosis in the suprapatellar pouch resulted in a superiorly shifted position of the patella, suggesting that the patella does not adequately engage the trochlea during knee flexion, which is associated with patellar flexion loss. Furthermore, the patella in the arthrofibrotic knee tilted and rotated significantly less laterally from 15° to 75° knee flexion. Contracture of the medial retinaculum of the knee might decrease lateral tilt and lateral rotation of the patella.2 The decreased patellar tilt in the affected knee could be responsible for flexion contracture associated with arthrofibrosis after ACL reconstruction.18 Because of the high congruency of the PFJ, an alteration in patellar tracking would be expected to lead to persistent knee symptoms and changes in patellofemoral contact characteristics in individuals with arthrofibrosis after ACL reconstruction.21 Indeed, other investigators found that the more superiorly shifted position of the patella indicates decreased quadriceps-patellar tendon angle and increased patellofemoral contact force, resulting in anterior knee pain and degeneration of patellofemoral cartilage in the long term.1,18 In clinical practice, it is significant for the clinicians to restore normal patellofemoral kinematics at the arthrofibrotic knee.

In the current study, there was a strong and statistically significant linear correlation between knee extensor mechanism strength and patellar inferior shift. The quadriceps are critical for the stability of the patella gliding within the trochlear groove.22 Pietrosimone et al23 indicated that changes in voluntary quadriceps activation could predict knee biomechanical changes. Brossmann et al5 suggested that the influence of knee extensor mechanism is most important for patellofemoral congruence, which may be associated with patellar tracking patterns. With quadriceps contractions, the patella is pressed on the lateral femoral condyle, and there is enough muscular tension on the patellar ligament to produce a substantial inferior patellar shift.5,24 Whether the weakness in the extensor muscles is due to a maladaptive neuromuscular recruitment pattern or disuse is beyond the scope of our study; future studies using electromyography are needed to better understand the relationship between muscle strength, muscle activation, and patellar tracking.

In evaluating the utility of knee extensor mechanism strength to assess patellar mobility, extensor strength deficits had high sensitivity for decreased patellar inferior shift in patients with arthrofibrosis after ACL reconstruction. Only 5% of patients had satisfactory (LSI ≥ 90%) knee extensor strength but unsatisfactory patellar inferior shift. However, knee extensor strength had low specificity for identifying decreased patellar inferior shift. Almost half of the patients who had a satisfactory patellar inferior shift had extensor strength LSI deficits. The clinical implications of these results suggest that knee extensor strength deficits should be factored into the diagnosis and treatment considerations of patients with knee arthrofibrosis. Equally importantly, clinicians should be aware that there is a small proportion of patients who have restricted patellar inferior shift despite satisfactory extensor strength. Because of this, the knee extensor mechanism strength testing cannot be used to exclude the need for patellar tracking testing.

Strength testing using a handheld dynamometer has long been used in the assessment of patients after knee injuries or surgeries. It has the benefits of being quick and easy to administer, making it readily available to many patients during the diagnosis and rehabilitation process.9 Our study also found significant decrease in patellar flexion, tilt, and rotation in the affected knee in comparison to the contralateral knee. However, there was no significant linear correction between knee extensor strength and patellar motion in the rotated direction. In clinical practice, an extensor strength LSI of 90% did not totally discriminate between patients who had abnormal patellofemoral kinematics in 6 degrees of freedom.

The knee extensor strength testing may have clinical utility in screening patients with knee arthrofibrosis during the diagnosis and treatment process; extensor mechanism strength could be used as a screening test before quantitative assessment of patellar tracking to limit the number of knee kinematic tests. There are limitations with this study. One of the limitations of the current study was variations of assessment time point, which ranged from 6 to 24 weeks after ACL reconstruction. As the patellar mobility and knee extensor strength are most likely to improve during this period of time, and there may be differences in the improvement of these variables, future studies collecting data at the same time point following surgery are required. We only analyzed patellofemoral kinematics from 15° to 75° of knee flexion during a single-leg lunge. Qualitative factors such as balance between medial and lateral stabilizers, altered femoral rotation, and poor joint articulation were not assessed. The location and size of scarring in the knee was also not considered or analyzed, as intraarticular arthrofibrosis always involves patellofemoral and tibiofemoral compartment. These factors may supply additional information about patellar tracking. A limitation of the LSI has previously been reported.12 It assumes that the contralateral limb is “normal,” which may not be the case. In the present study, we have addressed some of these limitations by excluding patients with contralateral limb injury or surgery.

Conclusions

Decreased patellar mobility is common in patients with arthrofibrosis after ACL reconstruction. There is a strong linear correlation between knee extensor strength and patellar inferior shift in the arthrofibrotic knee. Knee extensor mechanism strength has high sensitivity but low specificity at identifying a decrease in patellar inferior shift in patients with arthrofibrosis after ACL reconstruction. These findings demonstrate the importance of knee extensor strength in the diagnosis and treatment of patients with knee arthrofibrosis.

Acknowledgments

The authors declare no potential conflicts of interest. All procedures in this study involving human participants were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The authors report no proprietary or commercial interest in any product mentioned or concept discussed in this article. This work was supported by the (China) State’s Key Project of Research and Development Plan (grant number 2018YFF 030050).

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    Pietrosimone B, Lepley A, Murray A, et al. . Changes in voluntary quadriceps activation predict changes in muscle strength and gait biomechanics following knee joint effusion. Clin Biomech. 2014;29(8):923929. doi:

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    Mariani S, La Marra A, Arrigoni F, et al. . Dynamic measurement of patello-femoral joint alignment using weight-bearing magnetic resonance imaging (WB-MRI). Eur J Radiol. 2015;84(12):25712578. PubMed ID: 26443639 doi:

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Zhang, Fan, Ye, and Cai are with the Department of Rehabilitation Medicine, Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China. Wang is with the Key Laboratory of Exercise and Health Science, Shanghai University of Sport, Shanghai, China.

Cai (shrehab@163.com) is corresponding author.
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    Coordinate systems used to quantify the patella tracking. Patellar flexion, lateral shift, anterior shift, superior shift, lateral tilt, and lateral rotation are considered positive as shown in the figure.

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    • PubMed
    • Search Google Scholar
    • Export Citation
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