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00005768-201210000-0000100005768_2012_44_1827_pattyn_patellofemoral_10article< 110_0_17_5 >Medicine & Science in Sports & Exercise©2012The American College of Sports MedicineVolume 44(10)October 2012p 1827–1833What Predicts Functional Outcome after Treatment for Patellofemoral Pain?[CLINICAL SCIENCES]PATTYN, ELS1; MAHIEU, NELE1; SELFE, JAMES2; VERDONK, PETER3; STEYAERT, ADELHEID4; WITVROUW, ERIK11Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent University, De Pintelaan 185, 3B3, Ghent, BELGIUM; 2Division of Physiotherapy and Sports Therapy, School of Sport, Tourism and The Outdoors, University of Central Lancashire, Preston, Lancashire, England, UNITED KINGDOM; 3Department of Orthopaedic Surgery, Ghent University Hospital, De Pintelaan 185, Ghent, BELGIUM; and 4Department of Physical and Rehabilitation Medicine, Ghent University Hospital, De Pintelaan 185, Ghent, BELGIUMAddress for correspondence: Els Pattyn, PT, Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent University, De Pintelaan 185, 3B3, B9000 Ghent, Belgium; E-mail: e.pattyn@UGent.be.Submitted for publication October 2011.Accepted for publication May 2012.ABSTRACTPurpose: Although physical therapy is known to be effective in treating patellofemoral pain (PFP), there is considerable individual variation in the treatment response. It is unclear why some patients benefit from a specific treatment while others do not experience improvement. This study, using a prospective study design, aims to identify factors that could predict the short-term functional outcome and account for the variation frequently seen in the outcome after conservative treatment of PFP.Methods: Thirty-six patients (20 female and 16 male with a mean age of 23.8 ± 6.7 yr) followed a physical therapy rehabilitation program of 7 wk. Before this treatment, all patients were evaluated on subjective symptoms (pain on visual analog scales in millimeters) and functional performance (step test expressed as highest level, single-legged hop test in centimeters, and triple-hop test in centimeters). The concentric and eccentric knee extensor strength at 60°·s−1 and 240°·s−1 (N·m) were measured as well as the quadriceps muscle size by calculating the cross-sectional area (cm2) with magnetic resonance imaging. The success of the treatment was evaluated by the functional Kujala anterior knee pain scale. A linear regression model was used to identify predisposing factors for the functional outcome.Results: The total quadriceps cross-sectional area (P = 0.010), the eccentric average peak torque at 60°·s−1 (P = 0.015), and the frequency of pain at baseline (P = 0.012) have been indicated as predisposing variables in the short-term functional outcome after a physical therapy rehabilitation program for PFP (adjusted R2 = 0.46).Conclusion: Patients with a greater quadriceps muscle size, lower eccentric knee strength, and less pain have a better short-term functional outcome after conservative treatment for PFP.Patellofemoral pain (PFP) is known as a common and often chronic musculoskeletal condition, affecting young and physically active adults (12,25). PFP may account for almost 10% of all visits to outpatient sports clinics (20). The exact etiology is still unknown but has been proposed to be multifactorial (10,14). Because of the diverse origin of PFP, many rehabilitation programs with various emphases have been proposed to treat this disorder.Several studies have demonstrated that physical therapy is effective in treating PFP (8,9,28,38). According to the review of Crossley et al. (9), there appears to be a consistent improvement in short-term pain and function as a result of physical therapy treatment. However, a third to a quarter of patients with PFP persist in having PFP. Nimon et al. (29) reported that one in four of their patients with anterior knee pain continued to have symptoms such as pain and disability at a mean follow-up of 16 yr after nonoperative treatment. In a prospective study of military recruits, 35% of the affected knees were still mildly, moderately, or severely painful at 6 yr of follow-up (27).It seems unclear why some patients benefit from a specific treatment although others do not experience improvement. Therefore, several attempts have been made to determine which factors may predict the outcome and account for the variation often seen in the conservative treatment of PFP (7,21,28,33,39). These few prospective studies showed no consensus in identified prognostic factors. Collins et al. (7) and Witvrouw et al. (39) both indicated that patients with a long duration of knee pain had poorer prognosis after a treatment program. Natri et al. (28) demonstrated that the isometric extensor strength of the affected knee was a significant predictor of the functional outcome scores: the smaller the strength difference between the affected and the unaffected knee, the better the outcome. Other predictors of functional outcome were age and reflex response time of the vastus medialis obliquus (VMO) muscle (21,39).Studies identifying predictors of outcome mainly investigated pain, function, muscle strength and length, and biomechanical and clinical factors. To our knowledge, no attention has been paid to physiological characteristics such as quadriceps muscle size. Because atrophy of the quadriceps muscle and, in particular, the VMO has been demonstrated in patients with PFP (5,13,19,22,32), it seems valuable to investigate if muscle size has any influence on the functional outcome after treatment. Moreover, because maximum force-generating capacity (strength) cannot be measured accurately and separately in muscles such as VMO, there is potential to use change in muscle size as an indicator of muscle strengthening and hypertrophy (6).The authors who included knee extensor muscle strength in their analyses have only measured strength isometrically or concentrically (28,33,39). However, it is generally accepted that patients with PFP usually have slight concentric strength deficits, whereas their eccentric strength is remarkably lower compared with healthy persons (3,34,37). Consequently, it might be useful to evaluate the extent to which eccentric knee extensor strength at baseline plays a role in the outcome after treatment. To the best of our knowledge, this has never been studied.More insight is required to identify factors that could predict the outcome of a conservative rehabilitation program. Therefore, this study uses a prospective study design to identify factors that could predict the short-term functional outcome and account for the variation frequently seen in the outcome after conservative treatment of PFP.METHODSParticipantsForty patients (22 female and 18 male) 14–46 yr of age and diagnosed with PFP by an orthopedic surgeon were recruited for this study. To be eligible for the study, patients had to experience PFP for more than 3 months and exhibit at least two of the following criteria on initial assessment (with a minimal visual analog scale of 3/10 during the assessment): pain on resisted knee extension, pain with isometric quadriceps contraction against suprapatellar resistance with the knee in slight flexion, pain on direct compression of the patella against the femoral condyles with the knee in full extension, or tenderness on palpation of the posterior edge at the medial and/or lateral border of the patella (18,21). The peripatellar or retropatellar pain that the patients experienced in everyday life had to be provoked by two or more of the following activities: prolonged sitting with flexed knees, stair climbing, squatting, running, kneeling, and jumping (18,21). Patients with knee problems other than PFP or with patellar instability were excluded from the study. None of the patients had lower limb surgery or trauma within the last year. The other exclusion criteria were magnetic resonance imaging (MRI) contraindications such as pregnancy, claustrophobia, and implanted metals.Thirty-six of the 40 patients completed both the pre- and the posttest (16 men and 20 women with a mean age of 23.8 ± 6.7 yr) (Table 1). Their activity level was measured using the Baecke questionnaire (2) and averaged a score of 8.7 ± 1.9. The duration of the symptoms before the start of the treatment averaged 32 months. PFP was observed bilaterally in 23 of the 36 patients. Only their more severely affected leg was used for the analysis (14 dominant and 9 nondominant legs). The dominant knee was involved in seven patients with unilateral pain and the nondominant in six patients. Lower limb dominancy was defined by asking patients which leg was their preferred kicking leg (11). None of the patients received any treatment for PFP before entering the study. All patients were properly informed about the study goals and the setup, and they signed an informed consent form. The study was approved by the local ethics committee of the Ghent University Hospital.TABLE 1 Demographic data for study participants (N = 36).TreatmentAfter the initial evaluation, all patients started a 7-wk treatment protocol. A 7-wk period was chosen because improvement in pain and function as well as increase in cross-sectional area (CSA) can be expected in PFP patients within 6 to 8 wk (8,9,15–17,36). The therapy was provided under the direct supervision of a trained physical therapist experienced in knee rehabilitation. The patients performed the protocol twice a week for 1 h and received a daily home exercise program. The rehabilitation program was created on the basis of previous research (8,9,15,16,24) and clinical expertise and was adapted to the patient’s needs. It consisted of a general quadriceps exercise program with the emphasis on a controlled performance and a high-quality execution of the exercises (Table 2). The use of painkillers and/or nonsteroidal anti-inflammatory drugs was not encouraged and only allowed in case of severe pain that prohibited the performance of the usual daily activities or the rehabilitation program.TABLE 2 Rehabilitation program.Thirty-six of the initial 40 patients completed the rehabilitation period. Of the four patients who were not evaluated at the posttest, one patient was unable to be contacted and three did not attend the end evaluation because of personal reasons. Separate sample size calculations for each category of the variables were performed. On average, a sample size of 35 patients was required using a probability level of 0.05 and a power of 80%.PredictorsSubjective assessmentPain during rest and activities was evaluated using 100-mm visual analog scales, where 0 = no pain and 100 = extremely intense pain. The patients scored each of the following items with a number between 0 and 100: frequency of pain (how often), intensity of worst pain, pain ascending stairs, pain descending stairs, pain during squatting/kneeling, pain during prolonged sitting, and pain during daily activities.MRIThe anatomical CSA of the quadriceps muscle was obtained through MRI using a 3-T magnet (MAGNETOM Trio-Tim System; Siemens AG, Erlangen, Germany) and two body matrix coils (32). The patients were positioned in supine with both legs extended and relaxed. Two plastic tubes, filled with water and fixed to the patient’s upper legs, indicated where the images should be taken. One axial slice was taken at 2 cm above the superior border of the patella. The second slice was located at the middle of the distance from the anterior superior iliac spine to the upper border of the patella. The T2-weighted images were taken with the following sequence parameters: field of view, 380 mm; repetition time, 3000 ms; echo time, 38.0 ms; voxel size, 1.1 × 0.8 × 5.0 mm; slice thickness, 5.0 mm.The image data were transferred to an independent workstation (Leonardo, Siemens AG) and postprocessed with standard software (Syngo VB13, Siemens AG). A blinded and trained person measured the CSAs (cm2) of the quadriceps muscles directly from the axial images by manually drawing contours around the muscle boundaries. Vascular and nerve bundles were excluded, although the intramuscular fat was not quantified. At the patellar level, the CSAs of the VMO and the vastus lateralis (VL) muscle were measured. The CSAs of the vastus medialis and intermedius (VMVI) were combined into a single measurement (because they were considered as one entity because they could not be fully distinguished from each other at thigh level). VL, the rectus femoris (RF) and the total quadriceps muscle (sum of all the quadriceps components) were also calculated at the middle of the thigh (Fig. 1).FIGURE 1. A, MRI image of a female PFP patient 2 cm above the superior border of the patella. VMO, vastus medialis obliquus; VL, vastus lateralis. B, MRI image of a male PFP patient at midthigh level. VMVI, vastus medialis and intermedius muscle; RF, rectus femoris.Muscle strength measurementThe patients were seated on an isokinetic dynamometer chair (Biodex system 4; Biodex Medical Systems, Shirley, NY), with the hip flexed to 90° and the knee joint aligned with the axis of rotation of the dynamometer. Straps were placed around the upper body and thigh for stabilization, and the lower leg was secured to the shin pad of the dynamometer. At each velocity, the patients performed three repetitions for familiarization. Maximal knee extensor strength was assessed in the concentric and eccentric mode at the angular velocities of 60°·s−1 (4 repetitions) and 240°·s−1 (10 repetitions) with a half-minute rest period between each speed. Each contraction was carried out over a knee range of motion of 100° of knee flexion with 0° corresponding to knee fully extended. The isokinetic testing was conducted by an independent researcher who is blinded to all other parameters and the treatment and who is experienced in isokinetic testing. Verbal encouragement was provided in a consistent manner during the tests. The average peak torque (N·m) of the most severely involved leg was used for analysis.Functional assessmentThe patients performed three functional tests in the same order. During the step test, the patients were asked to step successively up and down from a 10-cm step. If the patients did not experience pain, the height was increased by 5 cm until pain occurred. The highest pain-free level was recorded. The maximum height of the step was 40 cm. The patients performed the step up and down test first with the noninvolved or less involved leg and afterward with the (more severely) involved leg. The single-legged hop test was performed by having the patient stand on one leg, hop once as far as possible, and land on the same leg (4,30). In the triple-hop test, the patients undertook three consecutive forward hops in a straight line on one leg (4,30). For both hop tests, the distance was recorded from the tip of the patient’s great toe at the starting position to the rear of the heel at the landing position (cm). The hop tests were performed bilaterally, although only the mean of three trials of the most severely affected leg was used for analysis.Outcome AssessmentThe success of treatment was evaluated immediately after the 7-wk program. The combined subjective and functional evaluation was made by completing the Kujala anterior knee pain scale (23). This scale is specifically designed for patients with PFP. It ranges from 0 to 100 points with 100 indicating maximal function. It comprises the evaluation of pain during walking, ascending and descending the stairs, squatting, running, jumping, prolonged sitting with the knees flexed, and the presence of a limp, support for walking, swelling, subluxation, quadriceps atrophy, and flexion deficiency.At the final assessment, the patient’s perceived response to treatment was measured with a five-point scale (marked worsening, moderate worsening, same, moderate improvement, or marked improvement) (8).Statistical AnalysisAll analyses were performed with the SPSS statistical package (version 19.0; SPSS, Chicago, IL). To identify predisposing factors for the functional outcome (post–Kujala score), a linear regression model was used. One variable of each category (pain, CSA, strength and function), measured before the treatment, was successively added in the linear regression analysis until the best fitted model with the highest adjusted R2 was found. This method allowed us to use on the one hand the most clinically relevant variables and on the other hand independent variables belonging to different categories of parameters. A P value of less than 0.05 was considered to be statistically significant.RESULTSAccording to 17 patients (47%), their status improved markedly after the rehabilitation period. Twelve patients (33%) indicated a moderate improvement. The condition of three patients (8%) was unchanged. Two patients (6%) reported a moderate worsening and two other patients (6%) a marked worsening of their complaints after the treatment period. The Kujala score was significantly increased from 70 ± 12 to 79 ± 10, which means an overall improvement of 13% (P < 0.001).An oversight of all the predictor variables and dependent variable with their mean and SD is presented in Table 3. Table 4 reveals the best fitted model (adjusted R2 = 0.46) of the linear regression analysis with the posttreatment Kujala score as the dependent variable and one parameter of each category evaluated before the treatment as a predictor. The CSA of the total quadriceps at midthigh level (P = 0.010), the eccentric average peak torque at 60°·s−1 (P = 0.015), and the frequency of pain (P = 0.012) are identified as predisposing variables to the functional outcome after a rehabilitation program in patients with PFP. Inclusion of any other variable did not significantly improve the prediction obtained by linear regression.TABLE 3 Mean and SD for the posttreatment Kujala outcome score and the potential predictors of the short-term functional outcome after conservative treatment for PFP.TABLE 4 Linear regression model predicting changes in functional outcome in patients with PFP.DISCUSSIONThe purpose of this study, using a prospective research design, was to identify factors that could predict the short-term functional outcome and account for the variation frequently seen in the outcome after conservative treatment of PFP. The findings indicated that the lower the frequency of pain before treatment, the greater the quadriceps muscle size, and the smaller the average eccentric peak torque at 60°·s−1, the better the functional outcome after 7 wk of therapy. The results of the model suggest that PFP patients with a lower eccentric strength and a normal CSA will be likely to improve. Clinically, it is thought that the reason for this is that the patients, although weak, actually have the required normal basic muscle volume, which should be optimally used. Therefore, they should optimize the efficiency of exploiting their muscle volume. The linear regression model further appears to support this view, because in patients with a low eccentric force combined with a smaller quadriceps CSA, the model predicts a rather less successful outcome. This could be explained by the fact that it is probably easier to optimize the efficient use of muscle volume within a 7-wk rehabilitation program than to enlarge the CSA in the same period.The combination of normal muscle volume and a lack of eccentric strength may be explained as a neuromuscular coordination problem. Several authors stated that patients with extensor mechanism dysfunction demonstrate a neurophysiological motor control imbalance that may account for, or contribute to, their knee pain (1,3,31,35). Thomeé et al. (34) attributed the lower isokinetic knee extensor torque and electromyographic activity in patients with PFP to inhibition of quadriceps muscle activity. This reflex inhibition may be manifested in the inability to generate a controlled smooth eccentric quadriceps contraction under load (1,31). This phenomenon is often interpreted as a compensatory strategy of patients to decrease the level of stress on the patellofemoral joint and therefore the pain experienced (1).The frequency of pain before treatment is also identified as a predisposing variable to the functional outcome in patients with PFP. The less frequently a patient experiences pain before the start of the therapy, the better the functional outcome. This result is in agreement with previous research that has demonstrated that higher baseline pain severity in patients with musculoskeletal pain was associated with poor outcome at follow-up (26).Being aware of the factors determining the functional outcome after treatment is of important value for the patients as well as for physical therapists, physicians, and sport coaches. The results of this study suggest that rapid pain relief should be specifically targeted during the treatment of patients with PFP along with the advice to avoid pain-provoking activities. In addition, neuromuscular coordination training should be emphasized to restore the motor control imbalance that may contribute to the knee pain. The patients should be taught how to optimize the use of the present muscle volume. This can be achieved by neuromuscular coordination exercises, which intend to contract quadriceps and VMO in a controlled way.Comparing the results of the present study regarding the quadriceps CSA and eccentric strength with other research is difficult. To our knowledge, our study was the first to evaluate quadriceps muscle size or eccentric torque measurements as possible predictors for the outcome of PFP after exercise treatment. A few studies included knee extensor strength in their analysis, although not measured eccentrically. Natri et al. (28) evaluated the isometric extensor strength and used the value for the unaffected side as reference point. In contrast with our results, they showed that the smaller the muscle strength difference between the affected and unaffected knee was, the better the final outcome. The conflicting results may be clarified in different ways. On the one hand, eccentric measurement may be considered as more functional than isometric testing. On the other hand, previous research has demonstrated that the decreased force production of the knee extensors in patients with PFP is predominantly present during eccentric contractions (3,34,37). Moreover, taking one of the patients’ legs into account may be more reliable than comparing the affected with the unaffected leg because the presence of pathology in one leg may influence the parameters of the other leg. Witvrouw et al. (39) and Piva et al. (33) could not find any relationship between the pretreatment concentric and isometric extensor muscle strength and the functional outcome.The applied regression model explained around 46% of the variation in the functional outcome. Therefore, other factors of a physical or psychological nature may also contribute to the outcome of function in PFP patients. Longitudinal, prospective studies on larger samples are needed to get more insight in the main predictors of the functional outcome.A limitation of this study was the relative short-term follow-up period, which limits long-term interpretation. It might have been interesting to retest the patients a few months or years after the end of treatment. In spite of this limitation, this study added considerably to the body of knowledge in the area because it is the first study to evaluate both muscle size and eccentric strength as possible predictors of functional outcome after conservative treatment for PFP.In conclusion, the CSA of the total quadriceps, the eccentric peak torque at 60°·s−1, and the frequency of pain at baseline are identified as predisposing variables to the short-term functional outcome after treatment in patients with PFP. On the basis of these findings, patients with less frequent pain and normal quadriceps muscle size and lower eccentric knee strength have a better short-term functional outcome. The results of this study are clinically very important because they broaden our understanding of the wide variety of clinical results after conservative treatment for PFP. Further research is needed to establish the influence of determining factors on the functional outcome after therapy for PFP.No outside funding or grants have been received that assisted the study.No conflicts of interests are reported.The results of the present study do not constitute endorsement by the American College of Sports Medicine.REFERENCES1. Anderson G, Herrington L. A comparison of eccentric isokinetic torque production and velocity of knee flexion angle during step down in patellofemoral pain syndrome patients and unaffected subjects. Clin Biomech (Bristol, Avon). 2003; 18 (6): 500–4. [Context Link]2. Baecke JA, Burema J, Frijters JE. A short questionnaire for the measurement of habitual physical activity in epidemiological studies. Am J Clin Nutr. 1982; 36 (5): 936–42. [Medline Link] [Context Link]3. Bennett JG, Stauber WT. Evaluation and treatment of anterior knee pain using eccentric exercise. Med Sci Sports Exerc. 1986; 18 (5): 526–30. [CrossRef] [Full Text] [Medline Link] [Context Link]4. Bolgla LA, Keskula DR. Reliability of lower extremity functional performance tests. J Orthop Sports Phys Ther. 1997; 26 (3): 138–42. [CrossRef] [Full Text] [Medline Link] [Context Link]5. Callaghan MJ, Oldham JA. Quadriceps atrophy: to what extent does it exist in patellofemoral pain syndrome? Br J Sports Med. 2004; 38 (3): 295–9. [Context Link]6. Calmels P, Berthouze S, Barral FG, Domenach M, Minaire P. A comparative study of the muscle strength and mass of the arm flexors and extensors in paraplegic and in non paraplegic basketball players. Paraplegia. 1992; 30 (7): 509–16. [CrossRef] [Medline Link] [Context Link]7. Collins NJ, Crossley KM, Darnell R, Vicenzino B. Predictors of short and long term outcome in patellofemoral pain syndrome: a prospective longitudinal study. BMC Musculoskelet Disord. 2010; 11: 11. [CrossRef] [Medline Link] [Context Link]8. Crossley K, Bennell K, Green S, Cowan S, McConnell J. Physical therapy for patellofemoral pain: a randomized, double-blinded, placebo-controlled trial. Am J Sports Med. 2002; 30 (6): 857–65. [Context Link]9. Crossley K, Bennell K, Green S, McConnell J. A systematic review of physical interventions for patellofemoral pain syndrome. Clin J Sport Med. 2001; 11 (2): 103–10. [CrossRef] [Full Text] [Medline Link] [Context Link]10. Davis IS, Powers CM. Patellofemoral pain syndrome: proximal, distal, and local factors, an international retreat, April 30–May 2, 2009, Fells Point, Baltimore, MD. J Orthop Sports Phys Ther. 2010; 40 (3): A1–16. [CrossRef] [Full Text] [Medline Link] [Context Link]11. de Ruiter CJ, de KA, Schreven S, de HA. Leg dominancy in relation to fast isometric torque production and squat jump height. Eur J Appl Physiol. 2010; 108 (2): 247–55. [Context Link]12. Devereaux MD, Lachmann SM. Patello-femoral arthralgia in athletes attending a Sports Injury Clinic. Br J Sports Med. 1984; 18 (1): 18–21. [CrossRef] [Medline Link] [Context Link]13. Doxey G. Assessing quadriceps femoris muscle bulk with-girth measurements in subjects with patellofemoral pain. J Orthop Sports Phys Ther. 1987; 9 (5): 177–83. [CrossRef] [Medline Link] [Context Link]14. Dye SF. The pathophysiology of patellofemoral pain: a tissue homeostasis perspective. Clin Orthop Relat Res. 2005; 436(7): 100–10. [CrossRef] [Full Text] [Medline Link] [Context Link]15. Fagan V, Delahunt E. Patellofemoral pain syndrome: a review on the associated neuromuscular deficits and current treatment options. Br J Sports Med. 2008; 42 (10): 489–95. [CrossRef] [Full Text] [Medline Link] [Context Link]16. Heintjes E, Berger MY, Bierma-Zeinstra SM, Bernsen RM, Verhaar JA, Koes BW. Exercise therapy for patellofemoral pain syndrome. Cochrane Database Syst Rev. 2003; (4): DOI: 10.1002/14651858. CD003472. [Context Link]17. Hespel P, Op’t EB, Van LM, et al.. Oral creatine supplementation facilitates the rehabilitation of disuse atrophy and alters the expression of muscle myogenic factors in humans. J Physiol. 2001; 536 (Pt 2): 625–33. [Context Link]18. Insall J. Current concepts review: patellar pain. J Bone Joint Surg Am. 1982; 64 (1): 147–52. [Medline Link] [Context Link]19. Jan MH, Lin DH, Lin JJ, Lin CH, Cheng CK, Lin YF. Differences in sonographic characteristics of the vastus medialis obliquus between patients with patellofemoral pain syndrome and healthy adults. Am J Sports Med. 2009; 37 (9): 1743–9. [CrossRef] [Full Text] [Medline Link] [Context Link]20. Kannus P, Aho H, Jarvinen M, Niittymaki S. Computerized recording of visits to an outpatient sports clinic. Am J Sports Med. 1987; 15 (1): 79–85. [CrossRef] [Medline Link] [Context Link]21. Kannus P, Niittymaki S. Which factors predict outcome in the nonoperative treatment of patellofemoral pain syndrome? A prospective follow-up study. Med Sci Sports Exerc. 1994; 26 (3): 289–96. [CrossRef] [Full Text] [Medline Link] [Context Link]22. Kaya D, Citaker S, Kerimoglu U, et al.. Women with patellofemoral pain syndrome have quadriceps femoris volume and strength deficiency. Knee Surg Sports Traumatol Arthrosc. 2011; 19 (2): 242–7. [Context Link]23. Kujala UM, Jaakkola LH, Koskinen SK, Taimela S, Hurme M, Nelimarkka O. Scoring of patellofemoral disorders. Arthroscopy. 1993; 9 (2): 159–63. [CrossRef] [Medline Link] [Context Link]24. LaBella C. Patellofemoral pain syndrome: evaluation and treatment. Prim Care. 2004; 31 (4): 977–1003. [CrossRef] [Medline Link] [Context Link]25. Malek MM, Mangine RE. Patellofemoral pain syndromes: a comprehensive and conservative approach. J Orthop Sports Phys Ther. 1981; 2 (3): 108–16. [CrossRef] [Medline Link] [Context Link]26. Mallen CD, Peat G, Thomas E, Dunn KM, Croft PR. Prognostic factors for musculoskeletal pain in primary care: a systematic review. Br J Gen Pract. 2007; 57 (541): 655–61. [Context Link]27. Milgrom C, Finestone A, Shlamkovitch N, Giladi M, Radin E. Anterior knee pain caused by overactivity: a long term prospective followup. Clin Orthop Relat Res. 1996; 331(10): 256–60. [CrossRef] [Full Text] [Medline Link] [Context Link]28. Natri A, Kannus P, Jarvinen M. Which factors predict the long-term outcome in chronic patellofemoral pain syndrome? A 7-yr prospective follow-up study. Med Sci Sports Exerc. 1998; 30 (11): 1572–7. [CrossRef] [Full Text] [Medline Link] [Context Link]29. Nimon G, Murray D, Sandow M, Goodfellow J. Natural history of anterior knee pain: a 14- to 20-year follow-up of nonoperative management. J Pediatr Orthop. 1998; 18 (1): 118–22. [CrossRef] [Full Text] [Medline Link] [Context Link]30. Noyes FR, Barber SD, Mangine RE. Abnormal lower limb symmetry determined by function hop tests after anterior cruciate ligament rupture. Am J Sports Med. 1991; 19 (5): 513–8. [CrossRef] [Medline Link] [Context Link]31. Owings TM, Grabiner MD. Motor control of the vastus medialis oblique and vastus lateralis muscles is disrupted during eccentric contractions in subjects with patellofemoral pain. Am J Sports Med. 2002; 30 (4): 483–7. [Medline Link] [Context Link]32. Pattyn E, Verdonk P, Steyaert A, et al.. Vastus medialis obliquus atrophy: does it exist in patellofemoral pain syndrome? Am J Sports Med. 2011; 39 (7): 1450–5. [Context Link]33. Piva SR, Fitzgerald GK, Wisniewski S, Delitto A. Predictors of pain and function outcome after rehabilitation in patients with patellofemoral pain syndrome. J Rehabil Med. 2009; 41 (8): 604–12. [CrossRef] [Medline Link] [Context Link]34. Thomeé R, Renstrom P, Karlsson J, Grimby G. Patellofemoral pain syndrome in young women. II. Muscle function in patients and healthy controls. Scand J Med Sci Sports. 1995; 5 (4): 245–51. [CrossRef] [Medline Link] [Context Link]35. Voight ML, Wieder DL. Comparative reflex response times of vastus medialis obliquus and vastus lateralis in normal subjects and subjects with extensor mechanism dysfunction. An electromyographic study. Am J Sports Med. 1991; 19 (2): 131–7. [CrossRef] [Medline Link] [Context Link]36. Wernbom M, Augustsson J, Thomeé R. The influence of frequency, intensity, volume and mode of strength training on whole muscle cross-sectional area in humans. Sports Med. 2007; 37 (3): 225–64. [CrossRef] [Full Text] [Medline Link] [Context Link]37. Werner S. An evaluation of knee extensor and knee flexor torques and EMGs in patients with patellofemoral pain syndrome in comparison with matched controls. Knee Surg Sports Traumatol Arthrosc. 1995; 3 (2): 89–94. [CrossRef] [Medline Link] [Context Link]38. Witvrouw E, Danneels L, Van TD, Willems TM, Cambier D. Open versus closed kinetic chain exercises in patellofemoral pain: a 5-year prospective randomized study. Am J Sports Med. 2004; 32 (5): 1122–30. [CrossRef] [Full Text] [Medline Link] [Context Link]39. Witvrouw E, Lysens R, Bellemans J, et al.. Which factors predict outcome in the treatment program of anterior knee pain? Scand J Med Sci Sports. 2002; 12 (1): 40–6. 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