Patellofemoral pain syndrome (PFPS) is a common source of anterior knee pain affecting young active individuals.1 2 It is believed to account for between 25–40% of all knee problems presenting to sports medicine centres3^–5 while McConnell reports that it affects one in four of the total population.6 In a study carried out by Fairbank et al,7 30% of randomly selected students aged between 13–19 years had experienced anterior knee pain in the year before the study. Patellofemoral related problems occur with an incidence of two to one in females versus males, while men have been shown to have a greater incidence when athletes alone were studied.8 9

PFPS is a very common injury encountered during running.10 In a review of 2002 patients with running related injuries, Taunton et al11 reported that the knee joint was the most commonly injured body site (42.1%) with 46% of these injuries being due to PFPS. Commonly used synonymous terms include anterior knee pain, chondromalacia patella, patellofemoral arthralgia, patellar pain, patellar pain syndrome and patellofemoral pain.12 13

Symptoms usually consist of diffuse pain originating from the anterior aspect of the knee, most commonly along the medial aspect of the patella; however, retro-patellar and lateral patellar pain are not uncommon.9 13 Pain is typically reproduced on functions that increase patellofemoral compressive forces such as running, ascending and descending stairs, inclined walking, squatting and prolonged sitting.8 9 14 It therefore has a debilitating effect on sufferers’ daily lives by reducing their ability to perform sporting and work related activities pain-free.

Hip joint musculature strength in PFPS

The association between proximal weakness of the hip joint musculature and subsequent distal pathology has been documented rather extensively in the literature. Dating back to the 1980s, research has demonstrated the need to consider lower extremity kinetic chain factors when assessing and treating distal conditions,15^–17 with Carson et al16 suggesting that anterior and peripatellar knee pain are a result of lower extremity torsional malalignment. The results of a study by Lee et al18 on human cadaveric knees showed that with a 30° medial or lateral rotation deformity, increased patellofemoral contact pressure is produced on the lateral facet and medial facet of the patella respectively, further supporting this theory.

While these studies contribute valuable information on the possible aetiology of PFPS, more recent studies have concentrated on the direct relationship between PFPS patients and hip strength.

The first of such studies on symptomatic PFPS patients carried out by Ireland et al19 found that female patients with PFPS had 26% weaker hip abductors and 36% weaker hip external rotators than a cohort of age matched female control subjects. In contrast to this, however, Piva et al20 were unable to associate significant hip muscle weakness with PFPS. When data were analysed, they found that patients with PFPS in their study displayed 4% less external rotation strength and 14% less abduction strength when compared to age and gender matched controls, which was considered insignificant. The authors suggest that differences in studies possibly occurred due to variations in positioning for tests and inclusion criteria of subjects. They also allude to the fact that given a larger sample size, the abduction strength measurement may have proved to be significantly weaker compared to the control group, thus indicating that their study may have been under powered to detect differences between the groups.

Within the last year, two studies have been published, which conclude that hip muscle weakness is related to PFPS. The first study, carried out by Cichanowski et al,21 found that the hip abductors and external rotators of the injured limb were significantly weaker than the non-injured side in female PFPS patients. The study also found global hip muscle weakness in the female athletes with PFPS when compared with age and sports matched asymptomatic controls. The authors accept that the general weakness may be attributable to disuse atrophy from reduced training; however, they are quick to point out that the difference in hip abduction and external rotation strength between the affected and unaffected side in the symptomatic group could not be related to this. Their conclusions about the crucial role of hip abductors and external rotators in the development of PFPS are therefore in line with the results previously obtained by Ireland et al.19 A more recent study by Robinson et al,22 investigating the association between hip strength and PFPS, reports similar results to those observed by Cichanowski et al.21 The authors found that female subjects with unilateral PFPS displayed hip abductor, extensor and external rotator strength values that were only between 71–79% of the strength of the uninvolved side. Their results also indicate that PFPS patients have reduced hip muscle strength when compared to the weaker limbs of asymptomatic subjects.

Quadriceps muscle imbalances in PFPS

Maltracking of the patella on the femur as a result of an imbalance in the activity of the vastus medialis oblique (VMO) relative to the vastus lateralis (VL) as a possible cause of PFPS has received much attention in the literature.23^–25 It has been suggested that delayed onset and decreased activation magnitude of the VMO in relation to the VL leads to abnormal lateral tracking, increased patellofemoral contact pressure and resultant pathology of the articular cartilage.24 26 A reduction in the force producing capabilities of the VMO muscle27 or altered motor control of the VMO and VL24 28 have been proposed as possible causes of this imbalance. Neptune et al29 concluded that a 5 ms VMO timing delay is associated with a significant increase in lateral patellofemoral joint loading.

Effects of taping on quadriceps electromyogram (EMG) activation in PFPS

The use of patellar taping techniques for the treatment of PFPS became popular following the publication of McConnell’s original article in 1986.6 It was proposed that when the patella undergoes medial glide taping, it changes position, resulting in better alignment between the patella and the troclear notch of the femur, there is a stretch on the lateral structures, as well as an increase in VMO activity and a decrease in pain, thereby facilitating an earlier initiation of strengthening exercises.

Since this original hypothesis, numerous studies have been carried out in an attempt to explain the mechanisms of action of patellar taping. Some studies suggest that taping produces a reduction in neural inhibition of the quadriceps as a result of proprioceptive feedback to A-β afferents, thereby modulating pain and resulting in increased quadriceps force.30 Consequently there would be greater articulation between the patella and the trochlear groove resulting in increased leverage and maximised mechanical advantage of the quadriceps muscle.31 32

The proposed effects of patellar taping on the neuromotor control of the patellofemoral joint are that it either improves the amplitude or timing of the VMO or decreases the amplitude or timing of VL, or both.6

Open versus closed kinetic chain exercises

Quadriceps strength deficits are a common clinical finding in patients with PFPS and therefore strengthening exercises either in the form of open kinetic chain (OKC) or closed kinetic chain (CKC) often form part of the rehabilitation process.32 Much debate exists in the literature as to the relative efficacy of open and closed kinetic chain exercises. OKC exercises are single joint movements performed in non-weight bearing positions with a free distal segment. CKC exercises in comparison are multi-joint movements performed in weight bearing or simulated weight bearing positions with a fixed distal segment.33 Selection for either exercise regimen is based upon the assumption that both strategies have different physiological effects. OKC leg extension exercises have been the traditional means of strengthening the quadriceps; however, in recent times, these exercises have been contraindicated in the treatment of PFPS with some authors suggesting that they aggravate symptoms.6 As a result, there has been a significant increase in the use of CKC exercises in the treatment of PFPS.34

A number of benefits associated with CKC have been suggested. As CKC exercises simulate and replicate the role of the lower limb muscles during activities of daily living, they are believed to be more functional and potentially task/sport specific.35 Proprioceptive feedback is also believed to differ between CKC and OKC tasks possibly due to the greater compressive force from the body’s mass36 and foot–ground contact37 in CKC. CKC exercises are also believed to result in lower patellofemoral joint stresses when compared with OKC exercises. In CKC exercises compressive forces are heightened with increasing knee flexion—for example, squatting; however, this force is distributed and compensated for by greater contact area between the patella and trochlear groove of the femur. During OKC exercises—for example, knee extension exercises—joint stress increases from 90° flexion as the knee extends. This suggests that patients may tolerate and comply better with CKC exercises resulting in more favourable outcomes.

AIMS OF THIS REVIEW

Traditionally, physiotherapy has focused on treating the structures surrounding the patellofemoral joint itself. This review, however, examines the relationship between PFPS and deficits in neuromuscular function around the knee and hip joints. Specific aims of the review are to identify:

• randomised controlled trials (RCTs) and intervention studies that evaluate the effectiveness of strengthening the hip joint musculature in subjects with PFPS

• RCTs and intervention studies that evaluate the effectiveness of physiotherapeutic interventions for quadriceps muscle imbalances in subjects with PFPS

• RCTs and intervention studies that evaluate the effectiveness of taping on the quadriceps muscle activation in subjects with PFPS

• RCTs and intervention studies that evaluate the effectiveness of open and closed kinetic chain exercises in subjects with PFPS.

METHODS

Search strategy for identification of studies

Relevant studies were identified using a computer based literature search of the databases Medline (1998–January 2006) and PEDro, using the following keywords: patellofemoral pain, patellofemoral pain syndrome, patellar taping, hip strength, vastus medialis oblique, vastus lateralis, open kinetic chain exercise, closed kinetic chain exercise. Limits were imposed in the search strategy to restrict the search to articles published in the English language. Reference lists of identified studies were also checked for additional sources of information.

Inclusion criteria

All studies generated from the initial search were screened by both authors, and studies deemed appropriate were grouped into one of four sections. Inclusion criteria were then applied to all studies in each of these four groups:

1. Hip joint musculature strength in PFPS

2. • studies involving patients with a diagnosis of PFPS were considered

   • studies which examined the effects of hip strengthening in subjects with PFPS

3. Quadriceps muscle imbalance in PFPS

4. • studies involving patients with a diagnosis of PFPS were considered

   • studies which examined the effects of physiotherapy treatment aimed at restoring muscle balance between the VMO and VL in subjects with PFPS

5. The effects of taping on quadriceps muscle activation in PFPS

6. • studies involving patients with a diagnosis of PFPS were considered

   • studies which examined the effect of taping on EMG muscle onset timing in subjects with PFPS

   • studies which examined the effect of taping on EMG muscle amplitude in subjects with PFPS

7. Open versus closed kinetic chain exercises in PFPS

8. • studies involving patients with a diagnosis of PFPS were considered

   • studies which compared the effects of open versus closed kinetic chain exercises in the treatment of subjects with PFPS.

Assessment of methodological quality and levels of evidence

All identified RCTs were rated for methodological quality using the PEDro scale.

Summary statements on the efficacy of four interventions—(a) hip joint musculature strengthening, (b) physiotherapy treatment aimed at addressing quadriceps muscle imbalances, (c) taping for quadriceps muscle activation, (d) open versus closed kinetic chain exercises—were based on a system described by van Tulder et al38 and previously used by Maher39 in a systematic review of workplace interventions to prevent low back pain. This system considers the quality, amount and consistency of evidence from RCTs:

• Strong evidence: >1 high quality RCT with consistent outcomes

• Moderate evidence: 1 high quality and 1 low quality RCT with consistent outcomes

• Limited evidence: 1 high quality or 1 low quality RCT with consistent outcomes

• No evidence: 1 low quality RCT, no RCTs or inconsistent outcomes.

Statistical pooling was not possible, because the trials often did not present sufficient data and, more importantly, there was not a common set of outcomes across trials. A trial was considered to be of high quality if it scored at least 5/10 on the PEDro quality scale.

RESULTS

Efficacy of hip joint musculature strengthening in subjects with PFPS

No RCTs were identified which investigated the effect of hip joint strengthening in the treatment of PFPS. Two case reports, a cohort study and an intervention study were identified.15 40 41 These studies are summarised in table 1. Thus, based on the system developed by van Tulder et al,38 there is no evidence to suggest that hip joint strengthening can improve symptoms in subjects with PFPS.

Efficacy of physiotherapeutic intervention aimed at addressing quadriceps muscle imbalances in subjects with PFPS

Two RCTs were identified which investigated the effect of physiotherapeutic interventions aimed at addressing quadriceps muscle imbalances in subjects with PFPS.23 32 These scored 5/10 and 6/10 on the PEDro scale, respectively. These RCTs are summarised in table 2. Based on the system developed by van Tulder et al38 there is strong evidence to suggest that physiotherapeutic intervention is efficacious in addressing quadriceps muscle imbalances in subjects with PFPS.

Efficacy of taping for quadriceps muscle activation in subjects with PFPS

Three RCTs were identified which examined the effect of taping of quadriceps muscle activation in subjects with PFPS.42^–44 These scored 3/10, 5/10 and 4/10 on the PEDro scale, respectively. These RCTs are summarised in table 3. Furthermore, four intervention studies were identified.45^–48 The results of these RCTs are as follows. The first RCT42 indicated that the use of therapeutic taping alters the onset timing of the VMO and VL during a stair stepping task. Before the application of therapeutic taping the EMG onset of the VMO occurred after the VL in the PFPS group during both the concentric and eccentric phases of a stair stepping task, in contrast to a prior and simultaneous onset in the control group. Following the application of therapeutic taping, onset of the VMO occurred before that of the VL and simultaneous to that of the VL during the concentric and eccentric phases of the stair stepping task, respectively, in subjects with PFPS, with no change in onset timing in the control group. The second identified RCT43 indicated that therapeutic taping does not produce a change in VMO or VL amplitude, with the authors concluding that the main effect of therapeutic taping is not the result of changes in EMG amplitude but may rather be the result of EMG onset timing.

The results of the third identified RCT44 indicated that the application of therapeutic taping produced a decrease in both VMO EMG amplitude and the VMO to VL ratio.

There were no consistent results to be found across the intervention studies. Thus, there is inconclusive evidence to suggest that therapeutic patellar taping has any beneficial effect on quadriceps muscle activation in subjects with PFPS.

Efficacy of open versus closed kinetic chain exercises in subjects with PFPS

Three RCTS were identified which investigated the use of OKC and CKC exercises in subjects with PFPS (table 4).33 34 49 These scored 6/10, 7/10 and 6/10 on the PEDro scale, respectively. The study by Witvrouw et al34 was a 5 year follow-up of the study originally conducted by the same author.33 Results of these RCTs indicted that there is strong evidence to suggest that both OKC and CKC exercises are beneficial in reducing symptoms associated with PFPS.

DISCUSSION

Hip joint musculature strength in PFPS

Currently no RCTs exist to suggest that hip joint musculature strengthening is an efficacious form of treatment for subjects with PFPS. A number of studies suggest that the hip joint musculature may play a role in the development of PFPS,19^–22 and from a biomechanical point of view there certainly seems to be some logic in this argument. However, caution must be taken when interpreting the results of such studies. A common limitation to all these studies lies with the inability of the authors to distinguish between cause and effect.19 Without prospective studies on the topic, it is not known whether hip weakness is a precursor of PFPS or whether it develops as a result of changes in motor control15 19 or disuse atrophy.19

There is a current need for further intensive research to be carried out to evaluate the effectiveness of hip joint musculature strengthening in reducing symptoms associated with PFPS.

Quadriceps muscle imbalances in PFPS

The use of physiotherapeutic intervention seems to be an efficacious form of treatment for the muscle imbalances thought to be present in subjects with PFPS, with two high quality RCTs reporting positive outcomes.23 32 Both of these studies had very similar types of interventions and indeed come from the same research group, which is an important consideration when interpreting the results of these studies. Thus, further studies are required to examine the most efficient form of intervention that can achieve the maximum effect. Also, neither study examined the effect of the intervention on pain in the recruited subjects as well as lacking a long term follow-up of subjects.

Effects of taping in PFPS

Currently there is conflicting evidence regarding the efficacy of therapeutic taping in changing quadriceps muscle activity. Limitations of identified studies must be considered when evaluating the results. Variations in sample size exist which reduces the ability of the authors to detect statistical significance, with many of the studies being under powered.44 46 48 A lack of adequate long term follow-up was a common limitation of most studies reviewed. Further studies should address not only one single application of tape but multiple applications over a longer time period.

Conflicting evidence on the effect of taping on muscle activity suggests that its use in a clinical setting should be based on an individual patient-to-patient basis with full attention given to the patient’s specific deficits. Having said this, much evidence, not reviewed in this study, exists on the pain relieving effects of patellar taping for PFPS.

Open versus closed kinetic chain exercises

PEDro scale scores for the three reviewed studies ranged from 6–7/10 suggesting high quality. The study by Herrington et al49 failed to assess the long term effects of OKC versus CKC exercises in PFPS patients, leaving Witrouw et al33 the only study group to have examined this; thus further studies are required to evaluate the long term effect of both types of intervention. It was found that at the 5 year follow-up period, 92% of the OKC group patients were involved in sport compared with only 60% of the CKC group patients.34 This factor may have contributed to the slightly better functionality of the OKC group compared with the CKC group observed during the 3 month and 5 year follow-up period. Despite the fact that only 20% of the patients were completely pain-free at the 5 year follow-up in one study, significant improvements in pain, strength and function were documented in all studies in both OKC and CKC exercise groups. The lack of statistically significant differences between the OKC and CKC groups in these studies highlights the beneficial effects of both regimens and therefore emphasises a combined approach in the treatment of PFPS. The results of these studies therefore reject the frequently encountered hypothesis that OKC exercises augment symptoms in PFPS patients.

The issue of long term follow-up of patients with PFPS

Previous research has suggested that the development of PFPS in childhood or adolescence may not be as self limiting as previously thought.50 51 A study by Stathopulu et al50 has indicated that 91% of respondents in their follow-up still had knee pain. Furthermore this affected daily life in 45% of cases. The results of this study are supported by the results of a study by Nimon et al,51 who indicated that about one in four of the patients originally diagnosed as having PFPS continued to have significant symptoms up to 20 years after presentation. Thus, it is vital that future studies evaluate outcome measures over an extended period of time.

Conclusions

Although no RCTs exist to support the use of hip joint musculature strengthening as an efficacious form of treatment for subjects with PFPS, the results of a number of case studies do support the use of hip joint musculature strengthening for the treatment of patients found to have hip strength deficits in association with PFPS.

Based on the results of reviewed studies, quadriceps retraining is, however, associated with good clinical outcomes in patients found to have VMO impairments and can therefore be recommended to therapists. Limited evidence exists supporting the use of patellar taping techniques in changing the amplitude or onset timing of the VMO relative to the VL. Refuting the common misconception that PFPS patients should not be treated with OKC exercises, the results of this study support the use of both OKC and CKC forms of exercise. Both have been shown to reduce pain, increase strength and ultimately improve function in subjects with PFPS. A combined approach is recommended.

In order for an accurate diagnosis, appropriate treatment plans and optimal clinical results, it is important to understand the neuromuscular mechanisms and causes of patients’ patellofemoral pain. Looking beyond the patellofemoral joint, at the rest of the lower extremity kinetic chain, may reveal the source of the patients’ problems. According to the current literature on this topic, the use of a combination of interventions based on the specific causes of the patients’ symptoms is recommended.