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Two years after their inaugural event, experts on patellofemoral pain syndrome congregated in Belgium to once again dissect and analyze the latest research on the mechanisms underlying PFP and the effectiveness of such interventions as bracing, foot orthoses, and exercise.
By Cary Groner
Two years after their initial 2009 conference, researchers and clinicians interested in patellofemoral pain (PFP) gathered in late August in Ghent, Belgium, to present and discuss their latest findings. The international research retreat offered promising insights but also highlighted the extent to which investigators remain baffled by PFP’s complex challenges.
“We are starting to understand that different sources may be responsible for the pain,” said conference director Erik Witvrouw, PT, PhD, who is on the medical faculty in the Department of Rehabilitation at Ghent University. “Different types of patellofemoral pain may exist, and these will require different treatment strategies.”
Although PFP was for some time an ugly stepchild among lower extremity pathologies, it has recently garnered more attention (see “Patellofemoral pain takes turn in spotlight”). One reason is because it affects so many people. It’s one of the most common presentations in sports medicine, accounting for about 19% of running injuries.1,2 Moreover, the pain—typically provoked by common activities such as stair negotiation, running, and squatting—is notorious for its chronicity. Four years after initial diagnosis, 94% of patients continue to experience pain; even after 20 years, a quarter of patients report pain.3
Subgroup mentality
Witvrouw provided an example of the new thinking regarding factors that may increase PFP risk.
“Malalignment—rotational factors of the femur—might be much more important in females than in males,” he said. “Males may have less maltracking or malrotational problems, and I think we’ll be focused on that in upcoming years. Some patients may have strength problems; others might have neuromotor control problems. Patellofemoral pain isn’t necessarily situated in the patellofemoral joint itself; it might be located elsewhere, even in the brain.”
As the saying goes, if you’ve got a hammer, every problem looks like a nail, and Witvrouw expressed a similar take on the PFP conundrum.
“Biomechanists tend to think that biomechanical structures are the source of the pain,” he said. “Motor-learning people say that neuromotor control is the problem. I think we need to identify each patient with a specific group and treat them accordingly.”
Patients might be profiled with a variety of diagnostic and risk-group analyses, some of which were discussed at the conference, but Witvrouw stressed the importance of finding practical evaluation methodologies that will be used by therapists on a day-to-day basis.
“We need to develop a test battery that can be used in the field, so each clinician is able to identify a patellofemoral pain patient in a specific subgroup type,” he explained. “But first, we need to determine whether different subtypes actually exist.”
Factors
Research presented at the conference sheds some light, at least, on which factors may be most closely associated with PFP. Investigators from Erasmus University in Rotterdam, the Netherlands, presented two systematic reviews—one of factors associated with PFP in patients who already have it, and one of risk factors associated with its development in others.
In the first review they included 37 studies of patients with existing PFP. Analysis of those studies, which together evaluated 380 variables, revealed that a larger Q-angle, less hip abduction strength, and lower knee extension strength (expressed as peak torque) were associated with PFP.4 The second review looked at studies conducted in high-risk populations such as female athletes and military recruits who did not yet have PFP, but some of whom developed it. The authors selected seven studies including 135 variables, and reported that female gender, decreased flexibility in the quadriceps and gastrocnemius muscles, and, most significantly, diminished knee extension strength, were risk factors for PFP.5
Of course, this raises the obvious question about cause and effect: do people develop PFP because of weaknesses, or do they get weaker because the syndrome’s pain inhibits their activities?
“It’s definitely a good question,” said Marienke van Middelkoop, PhD, a senior researcher at Erasmus and one of the papers’ coauthors. “We can’t really say what is a cause of the pain and what is a consequence of it. If we look at risk factors prospectively, though [i.e., in the second study], we get a better idea, and lower knee extension strength was a risk factor—but we still don’t know why, or what causes the lower strength.”
Measuring change
Both investigators and clinicians who need to evaluate patients may gain insight from Australian research into which functional measures of PFP are most responsive to change. The team initially published the results of a yearlong randomized trial of foot orthoses and physical therapy in the British Medical Journal;6 the data presented at the Ghent conference represented an evaluation of a subset of measures that had been part of that trial.7
The authors assessed 179 PFP patients in three physical tests (step up, step down, and squat) and three self-reported measures. Over the 12 months, the Patient-Specific Functional Scale (PSFS)8 was the patient-reported outcome measure most responsive to change, whereas the step-up test was the most responsive physical assessment.
“We think that the PSFS was most responsive because it was specific to functional tasks that people were having trouble with, whereas the other questionnaires [Functional Index Questionnaire and Lower Extremity Functional Scale (LEFS)] were not specific to patellofemoral pain,” said Natalie Collins, PT, PhD, a postdoctoral research fellow and lecturer in physiotherapy at the University of Melbourne. “For the PSFS, they nominated things like squatting, stairs, and running, which are known to aggravate patellofemoral pain.”
The step-up test proved more responsive than the other two physical tests because it showed the biggest change over time, expressed as the number of pain-free steps the subjects could take.
Gender matters
Collins and her colleagues also presented a paper examining the reasons for changes in PFP pain severity in the course of the menstrual cycle.9 In a study of 45 women (mean age 29 years), they found that subjects experienced more knee pain during menstruation, possibly due to increases in pain sensitivity and decreases in tissue laxity and neuromuscular control.
“Healthy females have a decrease in tissue laxity during the menstrual phase, and because patellofemoral pain is often associated with tightening of the lateral structures of the patellofemoral joint, that could make the patella track more laterally and increase lateral loading,” Collins said. “We also know that neuromuscular control is reduced during the menstrual phase, and if you add that to the fact that women with patellofemoral pain already have impaired neuromuscular control, then potentially you’re compounding that impairment.”
Another study presented at the conference examined gender differences in hip and knee mechanics in PFP patients during running.10 Previously published research suggests that altered patellofemoral joint kinematics in women with PFP result from excessive femoral internal rotation. That rotation occurs as a result of weakness in the hip extensors, abductors, and external rotators.11
In the study presented at Ghent, authors Richard Willy, PT, PhD, and Irene Davis, PT, PhD, reported that women with PFP had more hip adduction and less knee adduction than their male counterparts. Men with PFP had greater knee adduction and knee external adduction moment than men without PFP, and both men and women with PFP had increased contralateral pelvic drop compared to male control subjects.
The study’s results indicated that PFP therapies may need to be gender-specific; specifically, women should focus on strengthening and neuromuscular reeducation to reduce excessive peak hip adduction and increase knee adduction. Men, by contrast, should focus on decreasing knee adduction and peak knee external adduction moment.
“Healthy females tend to move in valgus; they’re a little more knock-kneed than healthy men,” said Davis, who is director of the Spaulding National Running Center in the Department of Physical Medicine and Rehabilitation at Harvard Medical School in Boston. “We found that males with PFP tend to be more varus than healthy males, and that females with PFP tend to be more valgus than healthy females.”
Davis emphasized that these findings need to be validated in other studies, but if they prove reliable, the implication is that people of each gender may experience extremes of knee position that predispose them to PFP.
“And if that’s true, then it means you really need to treat them differently,” she concluded. “Those treatments might include gait retraining, but in terms of men, you wouldn’t want to teach them to pull their knees out, as you would with females—you’d want to teach them to bring their knees in.”
Exercise and gait
Gait retraining and other exercise-based approaches are gaining traction in PFP therapy, as recent research shows.12 Willy, an assistant professor of physical therapy at Ohio University in Athens, OH, and Davis reported the results of a study in which eight women with PFP received mirror-based gait retraining and verbal feedback on their lower extremity alignment during treadmill running.13 Researchers asked the runners to reduce their hip adduction, then progressively cut back on mirror and verbal feedback. Participants reported reduced pain and better LEFS scores at the study’s conclusion and three months later.
These interventions are typical of an emerging trend in which exercise and strength training for PFP patients is increasingly being aimed at the hips. One study presented in Ghent found that a program of isolated hip abductor and external rotator strengthening improved pain and health status in women with PFP compared to a control group that didn’t do the exercises—gains that were maintained six months later.14
“The big shift we’re seeing in rehabilitation for patellofemoral pain is more of a focus on the hip,” said Christopher Powers, PT, PhD, director of the program in biokinesiology, and codirector of the Musculoskeletal Biomechanics Research Lab, at the University of Southern California, and a presenter at the Ghent conference.
Irene Davis agrees on the importance of the hip, but adds that, from her perspective, the foot also plays an essential role that has yet to be adequately quantified.
“We haven’t learned much about how the foot affects the patellofemoral joint because we can’t measure foot mechanics easily,” she said. “The foot has twenty-six bones and thirty-three articulations and six degrees of freedom in each of those, so you can’t really evaluate it as a lump sum. But there aren’t a lot of good studies on foot mechanics because that’s how it has been modeled.”
Davis considers the midfoot to be potentially crucial to the function of lower extremities, but it’s been difficult to measure because its many small bones can’t be tracked accurately with external markers. Associated factors need to be assessed as well, she added.
“It’s important to look at tibial rotation and frontal-plane motion, because as the foot goes into more pronation, the inward motion of the tibia—tibial valgum—will increase the Q-angle and increase the tendency [in women] for patellofemoral joint malalignment,” she said. “It’s important to look at what happens in the foot, and how that relates to what happens in the tibia, and how that translates up to the knee.”
Davis hopes that better imaging techniques such as dual fluoroscopy will ultimately clarify some of these foot-related issues. In the meantime, she’s conducting research using the tools she has. For example, in a paper presented at the Ghent conference, Roy Cheung, PT, PhD, of Hong Kong Polytechnic University, and Davis trained three female runners with unilateral PFP to land with a forefoot strike rather than a rearfoot strike, based on the theory that this would diminish the sharp impact transient associated with the latter.15 In fact, the runners were able to modify their footstrike patterns and reduce the vertical impact peak and vertical loading rates. The changes were associated with reduced pain and better function.
“I think rate of loading is very important in PFP because skeletal structures are viscoelastic, and if loading rates are too high, tissues don’t adapt well and can be damaged,” Davis explained.
She also noted that the subjects shorten their strides in response to the change in foot strike pattern.
“We have preliminary data that suggests that shortening your stride results in a reduction in hip adduction,” she said. “This is an added benefit of adapting a more anterior strike pattern, as excessive hip adduction has been widely associated with PFP.”16
A Cochrane review of exercise interventions, conducted by Dutch researchers and presented at the conference, reported that exercise therapy was beneficial for reducing knee pain both short- and long-term and improving knee function short-term.17
“The exercise therapy had to include strengthening of the lower extremities, the quadriceps, or the abdomen and hips,” said Marienke van Middelkoop, one of the study’s coauthors.
Surprisingly, she and her colleagues found that the interventions were all about equally effective.
“I think that if you’re doing exercises of the lower extremities, most of the time many muscles are involved,” she said. “You’re strengthening quadriceps, hamstrings, hip muscles, and improving your balance. It’s difficult to train one particular group of muscles, and that may be why we didn’t find any difference.”
Bracing and taping
Investigators continue their research into bracing and taping for PFP. A product called the SERF strap (Stability through External Rotation of the Femur), developed by USC’s Chris Powers, is tensioned to pull the thigh into external rotation, thus limiting the extent to which the knee can collapse medially. In a study presented at the 2009 PFP retreat in Baltimore, subjects’ pain was reduced by half while wearing the strap, which also led to significantly reduced hip internal rotation during step down, running, and drop jump tasks.18 At this year’s Ghent retreat, a U.K. researcher reported that the difference between pre- and postbracing valgus values were more likely due to measurement error than brace effect.19
Another paper presented at Ghent,20 previously included in the proceedings of this year’s World Congress on Prevention of Illness & Injury in Sport, reported results of prophylactic use of a patellofemoral brace in 138 male military recruits. Just 16.3% of brace wearers developed PFP, whereas 33.7% of the controls did (see “PFP bracing finds focus,” April, page 15). One of the risk factors the researchers identified was, surprisingly, the expectation of sustaining an injury.
“We asked the subjects if they thought they’d be injured during training, and most of those who got the brace said no,” explained Erik Witvrouw, one of the study’s authors. “So the bracing may have had a mechanical effect, but I think there may be a psychological factor present in patellofemoral pain, as well.”
Another study compared a patellofemoral brace with a common elastic brace, and concluded that the former exerted a significant influence on patellar displacement versus the latter.21 Nevertheless, the brace didn’t appear effective under full loading in a squat test, according to one of the authors.
“In the fully loaded condition, we found no effect from the patellar brace,” said Kai Heinrich, a PhD candidate at the Institute of Biomechanics and Orthopedics at the German Sport University in Cologne. “For athletes or patients who want to participate in sports or go running, the brace may not be good enough to track the patella.”
Another paper reported that taping, sleeve-type bracing, and an elastic bandage all improved coronal plane range of motion during step descent, suggesting a proprioceptive effect, but transverse plane improvements associated with the brace alone suggest that it may have a mechanical effect as well.22
Presented studies of patellar taping alone suggested that it may also be effective for relieving PFP symptoms to some degree, possibly due to neuromuscular mechanisms.23,24
From the ground up
In keeping with Irene Davis’s interest in the foot, two studies presented in Ghent reported benefits from orthotic treatment for PFP.
One paper25 reported results of an Australian study comparing three footwear options in 23 subjects with patellofemoral joint osteoarthritis (which is hypothesized to evolve from PFP; see “Is PFPS a precursor to patellofemoral osteoarthritis?”, July 2009, page 23). Running sandals, sandals with prefabricated but customizable foot orthoses, and sandals with flat EVA (ethyl vinyl acetate) inserts were compared. The investigators found that both the orthoses and the EVA inserts produced immediate and significant reductions in perceived pain in walking and step-down tests, regardless of lower leg posture.
When asked how flat inserts could produce the same effects as a contoured insole, lead author Natalie Collins replied, “That’s the million-dollar question.
“We also found similar long-term effects between contoured orthoses and flat inserts in our RCT,6 and it raises the question of how orthotics have their therapeutic effect,” she continued. “It could be biomechanical, shock attenuation, or neuromuscular. The research is not strong enough at the moment, so we don’t really know enough about how they work. Our research suggests that it’s maybe not such a mechanical influence.”
Another study reported that in 39 patients with anterior knee pain, those given prefabricated foot orthoses improved significantly versus controls.26 Patients had a choice of soft, medium, or hard orthoses—whichever was most comfortable after jogging in each. After six weeks, 47% (9 of 19) of the orthosis group reported significant improvement versus just 5% (1 of 20) of those who were merely observed. Midfoot mobility—the difference in midfoot width between weightbearing and nonweightbearing—was another important predictor of success.
“A simple measurement of the foot is likely to give you a good indication of who might succeed with an orthosis and who might not,” said Bill Vicenzino, PT, PhD, head of the Division of Physiotherapy at the University of Queensland and a coauthor of that paper.
The future
As new directions for inquiry continue to evolve, future patellofemoral pain conferences will likely elucidate this complex affliction, as well as its multifaceted etiologies and treatment paradigms. Research will work to clarify the roles of rotational factors, muscle weakness, gender, proprioception, and the biomechanical and alignment issues that derive from them, and clinicians will add new tools to their arsenals.
Cary Groner is a freelance writer in the San Francisco Bay Area
References
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10. Willy R, Davis I. Gender differences in hip and knee mechanics of patellofemoral pain syndrome during running. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
11. Souza RB, Powers CM. Differences in kinematics, muscle strength, and muscle activation between subjects with and without patellofemoral pain. J Orthop Sports Phys Ther 2009;39(1):12-19.
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13. Willy R, Davis I. Mirror gait retraining for the treatment of patellofemoral pain syndrome in female runners. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
14. Lyle M, Khayambashi K, Mohammadkhani Z, et al. effects of isolated hip abductor and external rotator muscle strengthening on pain, health status, and hip strength in females with patellofemoral pain. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
15. Cheung R, Davis I. Effects of landing pattern modification in runners with patellofemoral pain: a case series with three months follow-up. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
16. Willson JD, Davis IS. Lower extremity strength and mechanics during jumping in women with patellofemoral pain. J Sport Rehab 2009;18(1):76-90.
17. Van Linschoten R, van Middelkoop M, Heintjes EM, et al. Exercise therapy for patellofemoral pain syndrome: a systematic review. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
18. Powers C. The influence of femoral strapping on pain response, hip rotation and gluteus maximus activation in persons with patellofemoral pain. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
19. Herrington L. The effect of a SERF strap on pain and knee valgus angle during unilateral squat and step landing in patellofemoral patients. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
20. Van Tiggelen D, Cowan S, Coorevits P, et al. Intrinsic risk factors and the effects of prophylactic bracing on the development of patellofemoral pain in male subjects. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
21.Heinrich K, Potthast W, Ellermann A, et al. The influence of two different braces on patellar alignment. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
22. Selfe J, Chohan A, Hill S, Richards J. A clinical study of the biomechanics of step to send using three treatment modalities for patellofemoral pain. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
23. Balachandar V, Barton C, Morrissey D. The efficacy of patellar taping in individuals with patellofemoral pain syndrome: a systematic review. PFP conference, Ghent, 2011.
24. Callaghan MJ. Patellar taping for patellofemoral pain syndrome in adults. Results from the Cochrane review. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
25. Collins N, Ozturk H, Schache A, et al. Shoe inserts produce immediate pain relief in individuals with patellofemoral joint osteoarthritis. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
26. Mills K, Blanch P, Dev P, et al. Greater treatment efficacy of orthoses compared to a wait-and-see approach in people with anterior knee pain and more mobile midfoot. Presented at 2nd International Research Retreat on Patellofemoral Pain Syndrome, Ghent, Belgium, September 2011.
