Experts from around the world gathered in Baltimore for the first-ever PFPS research retreat, in search of clues to the mechanism underlying this under-appreciated injury. The proceedings will be published later this year, but our exclusive report offers a sneak peek at what transpired.
If the last 10 years have been the ACL decade, the next 10 just might be the decade of patellofemoral pain. The exact mechanism by which anterior cruciate ligament injuries occur remains something of a mystery, but sports medicine researchers have been able to identify key predictive variables and verify the effectiveness of several training interventions for preventing such injuries. Perhaps as important, ACL injuries and the associated gender issues are now part of the sports world’s collective consciousness – something that was not the case a decade ago.
Now sports medicine experts are hoping to do the same for patellofemoral pain syndrome, which has spent far less time in the spotlight but can be just as devastating to an athlete.
“It’s been a very big problem, and one that we don’t have a lot of good ideas about,” said Irene Davis, PT, PhD, professor of physical therapy at the University of Delaware.
In many ways, an overuse injury like PFPS must be treated differently from an acute injury like an ACL tear, both scientifically and clinically. But to raise the profile and advance the field of PFPS, researchers have borrowed a strategy that has been used with considerable success by the ACL camp: the research retreat.
Four times since 2001, ACL injury experts have gathered for an intimate and intense two-day conference, each culminating in a consensus statement summarizing the current state of the science and suggesting future directions. In April, for the first time, a similar research retreat was convened in Baltimore to explore the topic of patellofemoral pain syndrome.
The resulting consensus statement, as well as abstracts of the more than 40 papers and posters presented, will be published this fall in the Journal of Orthopaedic & Sports Physical Therapy. Somewhat surprisingly, several themes that emerged from the Baltimore discussion bore an uncanny resemblance to themes of ACL research retreats past, suggesting that perhaps the two areas of study have more in common than one might think.
The field of ACL injury prevention is notable for having developed effective training interventions in the absence of a full understanding of the injury mechanism. The patellofemoral pain literature, including research presented in Baltimore, suggests a similar phenomenon may be unfolding with regard to PFPS.
“In the ACL, we’ve actually gained a lot about the mechanism from looking at the interventions,” said Christopher M. Powers, PT, PhD, associate professor of biokinesiology and physical therapy at the University of Southern California and a co-organizer of the research retreat along with Davis. “It may be that with patellofemoral pain as well, we should look at some of the mechanical variables before and after the interventions. Otherwise we’re just guessing as to what the mechanism is.”
Although placebo-controlled studies of patellofemoral interventions are rare, some of the existing evidence does suggest that exercise, foot orthoses, bracing, and taping can help relieve patellofemoral pain, at least in the short term.1-3 Unfortunately, evidence also indicates that at least 70% of patients treated for anterior knee pain report persistent or recurrent symptoms five to 20 years later.4-7 In fact, researchers are now starting to suspect that PFPS is a precursor to patellofemoral osteoarthritis (see sidebar, facing page),8 a possibility that underscores the need for effective early intervention.
Exercise as a potential intervention has become a popular topic of study, particularly in light of the realization that patellar tracking during weight bearing is significantly affected by proximal kinematics as well as—and possibly more so than—distal factors.9 With PFPS patients demonstrating abnormal increases in hip internal rotation, researchers reasoned, strengthening the muscles that work to restrain that motion should improve alignment and decrease symptoms. This theory was bolstered by a January JOSPT study that confirmed decreased hip abductor and extensor strength in patients with patellofemoral pain.10 And at the research retreat, a University of Calgary study found that PFPS patients had significantly less pain and decreased stride-to-stride variability following a three-week hip abductor strengthening protocol.11
Most exercise interventions, however, are not so specific. Many also include quadriceps strengthening, to address the deficits in vastus medialis oblique activity that have been reported in patellofemoral pain patients.12 Balance, flexibility, coordination, and neuromuscular control have been incorporated to varying degrees. All of which makes it very difficult to determine, when an intervention does appear to be effective, exactly why that might be.
Clinically, however, the results are encouraging. As several research retreat presentations illustrate, something about these multi-faceted regimens is having a positive effect. Investigators from the University of Wisconsin-Milwaukee found that a combination of strengthening and dynamic control exercises significantly improved pain and function after eight weeks.13 In a rare trial featuring a “control” group, Dutch researchers found that exercise therapy (balance, flexibility, coordination and strength training) was associated with less pain and better function at three and 12 months than patients’ “usual care.”14 And a University of Delaware study found that eight sessions of gait retraining, in which subjects were instructed to contract their gluteal muscles and maintain a target hip angle using visual feedback, significantly improved pain and function.15
But even if exercise-based therapy can help relieve symptoms of patellofemoral pain, it remains to be seen whether that symptom relief is associated with a change in biomechanics that would result in long-term improvement. The University of Wisconsin-Milwaukee’s rehabilitation protocol significantly reduced knee abduction moment, and the Delaware gait retraining protocol significantly reduced hip adduction angle. But the Delaware study also found no significant effect on hip internal rotation, and the Calgary strengthening protocol had no significant effect on peak genu valgum angle.
“If the pain waxes and wanes but the tracking stays constant, then what have you learned?” Powers said. “Is it the muscle or is it the geometry? Or is it both?”
Taping, bracing, and tracking
One might think that the forces applied by patellofemoral taping and bracing might be more effective for correcting patellar tracking, but research suggests this may not be the case.16,17 Consequently, many practitioners use taping and bracing primarily for short-term pain relief and as adjuncts to exercise therapy. A study from Bloomsburg (PA) University presented at the research retreat found that eight weeks of exercise alone did not significantly improve patellofemoral pain, but that exercise combined with patellar taping did.18
“I tape the patella to relieve patients’ pain for everyday activities. I might tape to help them do the exercises I want them to do,” said Kay Crossley, PhD, PT, a senior research fellow in physiotherapy at the University of Melbourne, and a keynote speaker in Baltimore.
A research retreat presentation from the NIH suggested that patellar taping does result in a patellar shift, but not necessarily in the way one might expect.19 They divided their patients into those who demonstrated lateral patellofemoral displacement at baseline and those who had medial displacement, and found that medial taping shifted the patella inferiorly across the population but medialized the patella only in the first group. In the so-called “non-lateral maltrackers,” medial taping actually lateralized the patella. The results support the idea, popular among practitioners who treat back pain and gaining traction in the ACL community, that treatment should be tailored to specific subgroups of patients (see “Solving for Variables,” page 25).
Looking at a more proximal intervention, a USC study presented in Baltimore found that the SERF femoral strapping device was associated with significant decreases in peak hip internal rotation and adduction during three dynamic weightbearing tasks.20 The findings support the concept of using the device as an adjunct to exercise therapy in patients whose patellofemoral pain is linked to abnormal hip mechanics.
However, another research retreat presentation from the University of Central Lancashire in the U.K. suggests that the kinematic effects of tape may not be the result of the force being applied.21 To measure the step descent kinematic effects of the Tru-Pull Advance sleeve knee brace, investigators compared a braced condition with no intervention and with neutral patellar taping as a control. Although the most notable kinematic effects were seen with the brace, the neutral taping condition was also associated with significant changes in the coronal plane, suggesting proprioceptive or neuromuscular factors may be involved. (For more on the Tru-Pull Advance and SERF products, see Market Mechanics, page 66).
Although current trends in PFPS research are focusing more on proximal variables, the possible effects of distal mechanics on patellofemoral pain still constitute a popular topic of discussion. In fact, the use of foot orthoses in this patient population resurfaced in the medical literature with the publication of a randomized clinical trial from the Melbourne group,22,23 presented by Crossley in Baltimore. The study found that foot orthoses were as effective as a multimodal physical therapy program (which included stretching, taping, and retraining of the hip and quadriceps) for the short term management of patellofemoral pain, and that combining the two interventions did not provide any additional benefit.
Foot orthoses may be particularly useful in situations where compliance is an issue, Crossley said.
“Not everyone wants to come in and have physical therapy. We need to have other options,” she said. “If they’re not going to do the exercises at all, then you might as well just give them an orthotic.”
But questions remain as to why foot orthoses might decrease patellofemoral pain. There is limited evidence in the literature for an association between PFPS and pronation,24 which is thought to affect patellar tracking by altering tibial alignment and/or rotation. Several studies presented in Baltimore appear to support this theory.
Research from the University of Wales suggests that plantar pressure patterns in PFPS patients are similar to uninjured individuals with pronated foot postures, specifically a lower force at heel contact and delayed weight transfer at both the rearfoot and midfoot.25 A La Trobe University study presented at the research retreat also found that patients with patellofemoral pain were more likely to demonstrate a pronated foot posture than uninjured subjects, but only when assessed using certain foot posture measures.26 A University of Virginia study reported that navicular drop and rearfoot angle were among the factors affecting weight-bearing quadriceps angle (Q angle) in uninjured subjects, along with femoral anteversion and leg length discrepancy.27
Research from the University of North Carolina-Greensboro, however, found that while tibiofemoral angle was a significant predictor of Q angle in uninjured subjects—to a greater extent than femoral anteversion—navicular drop was not.28
Underlying this discussion as well is the fact that a 2007 prospective study from Ghent University found that military subjects who developed patellofemoral pain during basic training were less pronated than those who did not.29 Erik Witvrouw, PhD, PT, professor of rehabilitation sciences and physiotherapy at Ghent University and a keynote speaker in Baltimore, suggested that either excessive or insufficient pronation could be risk factors, perhaps by impairing shock absorption.
Solving for variables
The apparent contradiction between the Ghent findings and studies supporting pronation as a risk factor for patellofemoral pain offers yet another example of how such risk factors may vary between subgroups of patients. Other examples cropped up continually throughout the research retreat, as one investigator after another noted within-group variability. Some subjects in the SERF study20 actually reported increased pain while using the device, suggesting that excessive hip internal rotation may not be a contributing factor in all patients. In the Wisconsin-Milwaukee rehabilitation study,13 no significant changes in joint range of motion were seen across the group, but several individuals did demonstrate improved ROM—suggesting that something about that patient subgroup made them more likely to benefit more from the type of intervention used.
In fact, Powers noted that a strength of the Delaware gait retraining study15 was that the inclusion criteria were very specific, targeting runners with patellofemoral pain and increased hip adduction at initial screening. Even in that study, however, the standard deviation for improvement in hip internal rotation was relatively large, reflecting that not all subjects experienced similar magnitudes of change, according to Brian Noehren, PT, PhD, now an assistant professor of physical therapy at the University of Kentucky, who presented the findings.
Practitioners who treat low back pain have long been familiar with this phenomenon, and as a result have developed classification-based protocols that match therapies to specific sets of symptoms.30 ACL injury investigators noticed a similar trend at their most recent research retreat, in April 2008, specifically noting variations within gender classifications, despite the common assumption that gender itself is a risk factor.31
If the discussion in Baltimore is any indication, patellofemoral pain experts are now thinking they should embrace a similar approach.
“It’s just like in treating the back,” Davis said. “Not all back pain is created equal, and we’ve had to come up with different interventions for different etiologies.”
To Crossley, this type of philosophical shift can’t come soon enough.
“Identification of subgroups is far and away the most important thing we need to do,” she said. “We need to individualize treatment based on contributing factors, and also on patient response to treatment.”
In the meantime, clinicians should bear in mind that even if a study’s between-group comparisons don’t show an effect of a patellofemoral pain intervention, that doesn’t mean the intervention isn’t effective for some individual patients.
“If we only look for differences between groups, we might not be correct about the cause of the problem,” Witvrouw said. “We need statistics, but when we go to the clinic we need a tailored approach.”
Is PFPS a precursor to patellofemoral osteoarthritis?
Adding urgency to the search for effective PFPS therapies is the increasingly popular theory that anterior knee pain is a precursor to patellofemoral osteoarthritis. A Temple University study presented at the research retreat in Baltimore adds support to this theory.
Researchers used motion analysis to analyze eight subjects with radiographically-confirmed patellofemoral OA and seven control subjects during sit-to-stand exercises. Tibial abduction (valgus) was significantly greater in the PFOA group than in the controls. In addition, within the PFOA group, the non-dominant limb was associated with greater tibial external rotation than the dominant limb.32
The findings are consistent with previous reports of static tibial malalignment in patients with patellofemoral OA.33,34 Given the body of research suggesting that tibial alignment also affects patellar tracking, the Temple authors propose that similar biomechanical factors may underlie both PFPS and PFOA. This theory is particularly attractive in light of a 2005 study from the U.K. in which patellofemoral arthroplasty patients were more than three times as likely as medial unicompartmental arthroplasty patients to report having suffered from anterior knee pain during adolesence or early adulthood.8
Although practitioners outside arthroplasty circles may be more familiar with tibiofemoral OA, patellofemoral OA may actually be more prevalent and more likely to cause pain. One study found that half of all hospital-based knee OA cases involved both the medial tibiofemoral and patellofemoral compartments, and 24% involved the patellofemoral aspect alone.35 And more recently, an MRI study reported that the presence of an osteophyte in the patellofemoral compartment was one of the few imaging findings associated with pain in 205 patients with various types of knee OA.36
“Most of the people we see with patellofemoral pain I think have early OA. We just don’t know it,” said keynote speaker Kay Crossley, PT, PhD, of the University of Melbourne. “We have an obligation to try to prevent progression in this group.”
- D’Hondt NE, Struijs PA, Kerkhoffs GM, et al. Orthotic devices for treating patellofemoral pain syndrome. Cochrane Database Syst Rev 2002;(2):CD002267.
- Heintjes E, Berger MY, Bierma-Zeinstra SM, et al. Exercise therapy for patellofemoral pain syndrome. Cochrane Database Syst Rev 2003;(4):CD003472.
- Bizzini M, Childs JD, Piva SR, Delitto A. Systematic review of the quality of randomized controlled trials for patellofemoral pain syndrome. J Orthop Sports Phys Ther 2003;22(1):4-20.
- Blond L, Hansen L. Patellofemoral pain syndrome in athletes: a 5.7-year retrospective follow-up study of 250 athletes. Acta Orthop Belg 1998;64(4):393-400.
- Stathopulu E, Baildam E. Anterior knee pain: a long-term follow-up. Rheumatology 2003;42(2):380-382.
- 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):118-122.
- Price AJ, Jones J, Allum R. Chronic traumatic anterior knee pain. Injury 2000;31(5):373-378.
- Utting MR, Davies G, Newman JH. Is anterior knee pain a predisposing factor to patellofemoral osteoarthritis? Knee 2005;12(5):362-365.
- Powers CM, Ward SR, Fredericson M, et al. Patellofemoral kinematics during weight-bearing and non-weight-bearing knee extension in persons with lateral subluxation of the patella: a preliminary study. J Orthop Sports Phys Ther 2003;33(11):677-685.
- Souza RB, Powers CM. Differences in hip kinematics, muscle strength, and muscle activation between subjects with and without patellofemoral pain. J Orthop Sports Phys Ther 2009;39(1):12-19.
- Ferber R, Farr L, Kendall K. The relationship between hip abductor muscle strength and knee genu valgum for PFPS patients following a strengthening protocol. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Chester R, Smith TO, Sweeting D, et al. The relative timing of VMO and VL in the aetiology of anterior knee pain: a systematic review and meta-analysis. BMC Musculoskelet Disord 2008;9:64.
- Earl JE, Hoch A, Labisch T, et al. Patient outcomes, strength and LE biomechanics following a proximal rehabiliation program in women with patellofemoral pain syndrome. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- van Linschoten R, van Middelkoop M, Berger M, et al. Exercise therapy vs ‘usual care’ for patellofemoral pain syndrome—the PEX study: A randomized clinical trial. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Noehren B, Davis I. Effect of gait retraining on hip mechanics, pain and function in runners with patellofemoral pain syndrome. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Powers CM, Ward SR, Chan LD, et al. The effect of bracing on patella alignment and patellofemoral joint contact area. Med Sci Sports Exerc 2004;36(7):1226-1232.
- Aminaka N, Gribble PA. A systematic review of the effects of therapeutic taping on patellofemoral pain syndrome. J Athl Train 2005;40(4):341-351.
- Miller M, Froehling L. An evaluation of the efficacy of two treatment protocols in the management of patellofemoral pain in females. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Sheehan FT, Derarsan A, Wilson NA, et al. McConnell taping shifts the patella inferiorly in the presence of patellofemoral pain. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Selkowitz DM, Souza RB, Powers CM. Effect of femoral strapping on lower extremity kinematics and pain response in females with patellofemoral pain. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Selfe J, Thewlis D, Hil S, et al. A clinical study of the biomechanics of step descent using different treatment modalities for patellofemoral pain. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Collins N, Crossley K, Beller E, et al. Foot orthoses and physiotherapy in the treatment of patellofemoral pain syndrome: randomised clinical trial. BMJ 2008;337:a1735.
- Collins N, Crossley K, Beller E, et al. Foot orthoses and physiotherapy in the treatment of patellofemoral pain syndrome: randomised clinical trial. Br J Sports Med 2009;43(3):163-168.
- Duffey MJ, Martin DF, Cannon DW, et al. Etiologic factors associated with anterior knee pain in distance runners. Med Sci Sports Exerc 2000;32(11):1825-1832.
- Curran S, Mathieson I, Upton D, Learmonth ID. Comparison between asymptomatic participants defined by a criterion categorization system in patients with patellofemoral joint pain using plantar pressure measurement. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Barton C, Bonanno D, Levinger P, Menz H. Determining the reliability and sensitivity of foot posture measures in individuals with patellofemoral pain syndrome. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Lee SY, Hertel J. Proximal and distal non-weight bearing lower extremity alignment affects weight bearing Q-angle. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Nguyen AD, Boling MC, Levine B, Shultz SJ. Relationship between lower extremity alignment and the quadriceps angle. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Thijs Y, Van Tiggelen D, Roosen P, et al. A prospective study on gait-related intrinsic risk factors for patellofemoral pain. Clin J Sports Med 2007;17(6):437-445.
- Foster, JB. Classification schemes take narrower approach to low back pain. BioMechanics 2007;14(7):49-54.
- Foster, JB. ACL injury experts shift focus to extend beyond gender bias. BioMechanics 2008;15(5):11.
- Hoglund L, Barbe M, Barr A, et al. Hip and knee kinematics are altered during sit-to-stand in persons with patellofemoral osteoarthritis. Presented at First International Patellofemoral Pain Syndrome Research Retreat, Baltimore, MD, April 2009.
- Elahi S, Cahue S, Felson DT, et al. The association between varus-valgus alignment and patellofemoral osteoarthritis. Arthritis Rheum 2000;43(8):1874-1880.
- Cahue S, Dunlop D, Hayes K, et al. Varus-valgus alignment in the progression of patellofemoral osteoarthritis. Arthritis Rheum 2004;50(7):2184-2190.
- Ledingham J, Regan M, Jones A, Doherty M. Radiographic patterns and associations of osteoarthritis of the knee in patients referred to hospital. Ann Rheum Dis 1992;52(7):520-526.
- Kornaat PR, Bloem JL, Ceulemans RY, et al. Osteoarthritis of the knee: association between clinical features and MR imaging findings. Radiology 2006;239(3):811-817.