Hip strength asymmetry has been observed in patients with existing patellofemoral pain syndrome (PFPS) and potentially could be used to screen for at-risk individuals. However, new findings suggest this type of asymmetry does not appear to be associated with early stage PFPS.
By Franklin Caldera, DO, MBA; and Christopher Plastaras, MD
Patellofemoral pain syndrome (PFPS) is one of the most common overuse knee injuries seen in sports medicine and orthopedic clinics. Here, the term “overuse” injury refers to the effects of repeated minor trauma to the knee joint. The trauma is usually from recreational or competitive jumping or running as a sport or part of a sport. The injury occurs gradually and symptoms may not at first interfere with participation.1
The most common symptom of PFPS is diffuse anterior knee pain around the patella. Individuals have difficulty describing the location of the pain and when doing so may place their hands over the anterior patella in a circular motion (the “circle sign”).2 Pain usually occurs when the knee extensor muscles experience load, such as when ascending or descending stairs or when doing slopes, squatting, kneeling, or running; it can also occur following prolonged sitting with flexed knees (the “theater sign”).3
PFPS can be seen in both men and women who participate in running and running-related sports. It accounts for approximately 25% of all knee injuries.4 Reliable clinical diagnostic tests have not been developed yet, so diagnosis is typically made based on a clinical exam and exclusion of other causes, such as intraarticular pathologies, patellar tendinopathy, and plica syndrome.5
PFPS and hip abduction in women
Many studies have shown that women are at higher risk of developing PFPS than men.6 One study reported the ratio of female-to-male patients with PFPS is as high as three-to-one.7 Many factors have been suggested as possible causes of PFPS, including an increased Q angle, patella alta, abnormal or excessive foot pronation, vastus medialis muscle weakness, hip abductor weakness, decreased flexibility of the hamstring muscles, and malalignment of the femur.8,9 It has also been suggested that anatomic, hormonal, and neuromuscular factors contribute to women’s greater risk of developing PFPS.10
Several studies have reported that hip kinematics during running differ between healthy men and women, and these differences may help explain the gender differences in PFPS incidence. In their landmark paper on the condition, Powers et al noted that women exhibit greater internal rotation of the femur and larger hip adduction during running than men.11 Ferber et al reported significantly greater peak hip adduction and hip internal rotation during the stance phase of running in female recreational runners compared with their male counterparts.12 Willson et al also found female runners had greater hip adduction than male runners both at initial contact and at the time of peak vertical ground reaction force.13
Studies of female runners have also reported that hip kinematics differ between runners with PFPS and healthy individuals. Noehren et al found greater peak hip adduction and hip internal rotation during running in women with PFPS than in healthy controls;14 in a prospective study, the same group found female runners with greater hip adduction at baseline were the ones most likely to develop PFPS.15 Souza and Powers also found greater peak hip internal rotation in female runners with PFPS during multiple activities than in healthy controls.16
Cichanowski17 and Ireland18 theorized the kinematics of the lower extremity associated with PFPS might change as a result of deficits in hip muscle strength. Gluteal weakness has been implicated in other knee pain syndromes, such as iliotibial band syndrome.19 In female athletes with unilateral PFPS, Cichanowski et al found less hip abduction strength and hip external rotation strength in the affected leg compared with the unaffected leg and compared with the corresponding limb of healthy controls. Ireland et al also reported that young women with PFPS were more likely than controls to demonstrate weak hip abductors and external rotators. In their aforementioned study, Souza and Powers found that female runners with PFPS had less hip abductor strength and hip extensor strength than healthy controls, as well as altered kinematics.16
The success of hip-strengthening programs in individuals with PFPS also supports the theory that deficits in hip muscle strength contribute to PFPS development. Dolak et al found that improvement in hip abductor strength was associated with improvement in symptoms in women with PFPS.20 Khayambashi et al found similar symptom improvement associated with increased hip strength in women with PFPS, and that the symptom improvement was sustained six months after the strengthening intervention.21 Ferber et al observed that increased hip strength was associated with reduced pain in individuals with PFPS. Those improvements were not associated with any change in lower extremity kinematics, but the study included both men and women with PFPS.22 Those findings contrast with those of Baldon Rde et al, who found that improvement in gluteus medius strength had a mediating effect on frontal plane trunk and hip kinematics in female athletes with PFPS.23
It would be clinically useful to be able to identify hip strength deficits in the early stages of PFPS so that strengthening interventions could be implemented before symptoms become severe. The most convenient way to do this would be to compare hip strength in the affected and unaffected limbs, based on the assumption that hip strength would be more symmetrical in healthy individuals than in individuals with or at risk for PFPS. We conducted a study to address this issue.
In designing our study, we considered studies such as Magalhaes et al24 and Robinson and Nee.25 These studies investigated hip abduction strength symmetry in female patients with PFPS and reported 12% and 22% reduction, respectively, in hip strength of the leg with PFPS compared with the unaffected side. Both studies included control groups. Magalhaes et al reported that the hip abductors, lateral rotators, and flexors and extensors on the injured side of those with unilateral PFPS were significantly weaker than in those without the pain syndrome; only the hip abductors, however, were significantly weaker on the injured side than the uninjured side in those with PFPS (20%, p < .05).
In a secondary analysis, Robinson and Nee found that the symptomatic limbs of participants with PFPS had less hip extension strength (p < .001), less hip abduction strength (p = .007), and less hip external rotation strength (p = .04) compared with the control’s weaker limb. Cichanowski et al also showed less hip abduction, external rotation, and extension strength in female patients with PFPS than in healthy controls.17
Our study was prospective. We recruited 21 study volunteers (female runners with early PFPS) and 36 healthy controls using flyers and recruitment in the vendor section at local road races, half marathons, triathlons, and running club meetings in a metropolitan area. The participants were women aged between 18 and 45 years who ran at least 10 miles per week. We defined early PFPS with the following criteria: a report of unilateral anterior knee pain associated with running once per week for at least six weeks, pain with a single-leg squat, compression of the patella into the femoral condyles, and pain with palpation of the anterior surface of the patella.
After telephone prescreening, we did an assessment using four stations to gather further data needed to include or exclude patients. First-station participants filled out an autobiographical questionnaire. At the second station, participants underwent bilateral hip strength testing by a single trained examiner blinded to the groups. Hip strength was tested in a neutral position to recruit the tensor fasciae lata muscle and in an extended position to recruit the gluteus medius muscle.19 At the third station, an examiner blinded to the group measured participants’ height, weight, and leg length discrepancy. At the fourth station, participants underwent a focused physical examination of the knee by another examiner blinded to group assignment.
Analysis of baseline characteristics of the group of women with early unilateral PFPS and controls did not show significant differences between the two for height, weight, age, leg length discrepancy, or weekly running mileage.
We calculated hip abductor strength complex asymmetry using the Hip Strength Asymmetry Index (HSAI). P values greater than .05 were considered statistically significant. Female runners with early symptoms of PFPS did not appear to have clinically significant hip abduction strength asymmetry compared with healthy controls. HSAI values for the hip abductors with 95% confidence intervals did not differ significantly between the runners with PFPS and the controls (p = .2272 when tested in the neutral position, p = .6671 when tested in extension).
Our study found the affected PFPS limb was stronger than the weaker limb of the controls (in the neutral position) and not significantly different from the unaffected limb in the PFPS patients. Both findings were unexpected and in contrast with previous studies. The asymmetry may be theoretically explained by the fact that the tensor fascia lata and the gluteus medius are engaged during strength testing in the neutral position. The tensor fascia lata may be engaged more significantly in early stages of PFPS; this would help to unload stress on the knee, leading to stronger abductors on the affected side when tested in neutral. Our findings of no significant asymmetry in the PFPS patients could indicate that the imbalance takes time to develop, and therefore may not be measurable until the later stages of PFPS.
Studies have shown that women with PFPS have less hip abduction strength on the affected side compared with the hip strength in the asymptomatic controls.26 Dolak et al20 showed strengthening of hip abductor muscles during rehabilitation in early phases of PFPS decreases symptoms more quickly, so, if weakness or asymmetry associated with worsening of PFPS could be identified early, then interventions could be implemented before symptoms progress to clinical relevance. We performed our study to develop a prescreening tool for female athletes in the early stages of PFPS, but did not find any evidence of hip abduction strength asymmetry at this early stage.
Accepting that hip strength testing is difficult to quantify, we believe our methods were the best that are currently available. However, our results indicate that, unlike PFPS patients seeking medical care, women with early PFPS do not appear to have significant hip abduction strength asymmetry.
During our literature review we came across multiple studies that showed weakness in hip muscle strength in patients with PFPS. However, it remains unclear if the deficit in hip abduction strength is a cause or effect of PFPS; it’s the classic “chicken or egg” question—which came first, the symptoms or the underlying hip weakness? Herbst et al’s27 prospective study showed that female athletes who develop PFPS may actually have greater hip abduction strength at baseline than those who do not develop PFPS, which seems to suggest that any subsequent strength deficits are more likely to be an effect of PFPS than a cause.
Further studies are needed to dissect out which mechanism leads to PFPS. Furthermore, no studies have yet assessed hip muscle endurance and PFPS, which may be important as the syndrome occurs predominantly in endurance sports.
The cause of PFPS is still poorly understood and the mechanism of injury needs further study to facilitate development of clinically useful screening tests. Further study of the evolution of patellofemoral pain may help to prevent the occurrence of this pathology.
Franklin Caldera, DO, MBA, and Christopher Plastaras, MD, are assistant professors in the Department of Physical Medicine and Rehabilitation at the University of Pennsylvania Perelman School of Medicine in Philadelphia. Plastaras is also director of the Spine, Sports, & Musculoskeletal Medicine Fellowship at the University of Pennsylvania.
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