What is Patellofemoral Pain?
PFP is characterised by diffuse pain of the anterior knee, either retropatellar or peripatellar in nature (Neal et al., 2019). It is most commonly aggravated by loaded knee flexion, such as squatting, ascending stairs or descending stairs. Numerous structures can contribute to nociception in PFP, including; subchondral bone, infrapatellar fat pad, retinaculum and ligamentous structures, however the exact cause is not known (Powers, Witvrouw, Davis, & Crossley, 2017). A widely cited paper by Dye (2005) introduced a pathophysiological model of PFP, focusing on a disruption of tissue homeostasis in the patellofemoral joint (PFJ) caused by factors such as inflammation. The contribution of central pain mechanisms and psychosocial factors must also be considered (Powers et al., 2017).
Risk Factors for PFP
A number of potential risk factors for PFP have been hypothesised and researched, however there is conflicting evidence for many of these factors.
Demographic and Anthropometric Factors:
Taunton et al. (2002) found a higher incidence of PFP in women compared to men. This has since been disputed in two high quality review articles finding no statistically significant association with PFP and sex, age, or body mass index (BMI) (Neal et al., 2016; Neal et al., 2019).
Changes in the strength and activation of muscles of the knee and hip have been long thought to be associated with PFP. Earlier electromyography (EMG) studies focussed on function of the vastus medialis oblique (VMO) compared with that of vastus lateralis (VL) in subjects with PFP. Multiple studies have found associations between PFP and delayed contraction and decreased amplitude of VMO versus VL (Cesarelli, Bifulco, & Bracale, 2000; Owings & Grabiner, 2002). However, a 2008 systematic review found no clear association between VMO-VL dysfunction and PFP based on the heterogeneity of study design, significant normal inter-subject variability and the potential for bias in many studies (Chester et al., 2008). This said, generalised quadriceps weakness may be a risk factor (Neal et al., 2019).
More recent literature has focused on the association of proximal muscle strength with PFP. Interestingly, results have been conflicting when comparing cross-sectional studies to prospective study designs. A cross-sectional study by Cowan, Crossley, and Bennell (2009) assessed the EMG activity of gluteus medius (GM), VMO and VL as well as hip abduction and trunk side flexor strength in participants with and without PFP. Their findings showed a delay in GM and VMO activation as well as reduced trunk side flexion strength in participants with a history of PFP. These findings are supported by baseline data measured by Ferber, Kendall, and Farr (2011) in their study investigating the effects of a hip abductor strengthening protocol for runners with PFP. Contrastingly, data from prospective studies has found hip strength to decrease from baseline levels in subjects who developed PFP during the follow-up period (Finnoff et al., 2011; Thijs, Pattyn, Van Tiggelen, Rombaut, & Witvrouw, 2011). Hip abduction strength was even found to be greater in young basketballers who developed PFP (Herbst et al., 2015). These findings suggest that weakness may occur secondary to PFP, rather than being causal as was previously thought. Pooled data from systematic reviews supports this suggestion (Neal et al., 2019; Rathleff, Rathleff, Crossley, & Barton, 2014).
Biomechanical and Spatiotemporal Characteristics of Running:
Despite the belief that biomechanical factors play a strong role in affecting risk of running-related injury (RRI), limited evidence exists to support these claims (Ceyssens, Vanelderen, Barton, Malliaras, & Dingenen, 2019). Numerous biomechanical differences have been studied in multiple populations, with the heterogeneity of variables studied, and study designs, contributing to the limited conclusive evidence available.
Magnitude of peak hip adduction moment is associated with increased PFJ stress (John D Willson & Davis, 2008), therefore it is reasonable to suggest that hip adduction may be associated with PFP and subsequently, a number of studies have investigated this. In a prospective study of 400 female runners, those who developed PFP during the two year study period were found to exhibit significantly greater hip adduction angle during running (Brian Noehren, Hamill, & Davis, 2013). Whereas, in a cohort of collegiate cross country runners, increased hip adduction moment was not found to increase risk of RRI, including PFP (Dudley, Pamukoff, Lynn, Kersey, & Noffal, 2017). Inconsistencies between studies may be due to the small sample size in this paper (n=31) and relatively short follow up period (14 weeks). Peak rear-foot eversion was originally thought to be a risk factor for PFP, however this more recent literature does not support this (Neal et al., 2016). Biomechanical abnormalities appear in many individuals with PFP, however, these may occur secondary to PFP and do not necessarily worsen with increasingly difficult physical tasks (Willson & Davis, 2008).
Sagittal plane biomechanics related to landing strategies during running have been shown to effect PFJ loading, which may lead to PFP. Runners who fore-foot strike (FFS) were shown to have reduced PFJ loading compared with those who rear-foot strike (RFS) (Kulmala, Avela, Pasanen, & Parkkari, 2013).
The spatiotemporal characteristics of running are also important to consider. These include factors such as step length and cadence (step rate). A 10% increase in step length results in a 31% increase in PFJ stress per step, equating to a 14% increase in load per mile. Moreover, 10% decrease accounts for a 22% decrease in loading per step, 7.5% decrease per mile (Willson, Sharpee, Meardon, & Kernozek, 2014). A 2014 systematic review analysed the available literature regarding changes in step length during running and concluded that a shorter step length results in reduced ground reaction force and subsequently, reduced absorption of force at the hip, knee and ankle (Schubert, Kempf, & Heiderscheit, 2014). Despite these reductions in ground reaction force and PFJ stress, a correlation between increased cadence and PFP is yet to be shown in the literature (Luedke, Heiderscheit, Williams, & Rauh, 2016).
It is evident that there are a wide range of risk factors for PFP in runners that may combine in individuals causing pain to arise, however these risk factors rarely result in PFP in inactive individuals (Smith et al., 2018). It is often thought that a key component of PFP is simply an overload of tissues in the knee. This may occur as a result of beginning running as a novice, or significantly increasing training loads. There is strong evidence that excessive and rapid increases in training load can increase the overall risk of injury in sport (Gabbett, 2016). This is characterised by an increase in the acute:chronic workload ratio, whereby the total summation of training time and intensity ‘this week’ is significantly more than the average of the past three weeks. Sixty percent of all running injuries can be attributed to training errors of ‘too much, too soon’ (Hreljac, 2005). The idea of ‘too soon’ combined with ‘too much’ appears to be an important relationship. The evidence of ‘too much’ in isolation proved inconclusive in a mixed population of 748 high school runners where higher weekly mileage was associated with increased risk of RRI (including PFP, but not specified) in boys but not girls (Tenforde et al., 2011). The effect of rapid increase in training on increased RRI risk was supported by prospective data over a 1-year follow up period (Nielsen et al., 2014). Among a cohort of 874 healthy, novice runners, those who increased their running load by greater than 30% in a given week were at increased risk of distance related running injury, including PFP.
A popular method of maintaining safe progression of running training is the ‘10% rule’, where the maximum increase in training volume each week is 10%. Buist et al. (2007) carried out a randomised controlled trial (RCT) where novice runners preparing for a 4-mile (6.7 km) event were randomised into a ‘standard training group’ who carried out an 8 week training programme, or a ‘graded training group’ who performed a 13 week programme to reach the same point. Between groups, there was found to be no differences in the incidence of RRI (PFP not specified). While this paper had a large sample size and was methodologically sound, the selected parameters of the ‘standard training group’ were far from the extremes often seen clinically. Perhaps, a more excessive running protocol for the control group may have revealed a protective effect of the ‘10% rule’, in line with aforementioned prospective studies.
Reducing Injury Risk
A number of RCTs investigating general PFP injury prevention interventions have been performed with mixed results. Many of these studies have selected populations of military recruits, as the control of potentially confounding variables is made easier in this context. Conflicting results were found between two large RCTs investigating the efficacy of a general lower limb strengthening and stretching programme on AKP in military recruits. Brushøj et al. (2008) randomised participants into an intervention group of lower limb strengthening and stretching three times per week, while the control group performed upper-limb strengthening exercises. At 12-week follow up, there was found to be no significant difference in the incidence of overuse knee injury between groups. Contrary to these findings, Coppack, Etherington, and Wills (2011) reported a 75% reduction in the risk of AKP in their intervention group. In this study, participants in the intervention group performed a lower limb strength and stretching programme seven times per week for 14 weeks. It is possible that the improved outcomes of this group compared with those studied by Brushøj et al. (2008) are a result of the higher frequency of exercise, longer duration of the intervention period, as well as an increased emphasis on unilateral strengthening exercises. Specific to runners, an 8-week strengthening programme targeting hip and core strength showed significant improvements in pain and functional ability in a cohort of female runners already exhibiting symptoms of PFP (Earl & Hoch, 2011).
With regard to interventions aimed at reducing injury risk factors of PFP other than strength, few studies have been performed in pain free populations. While a gap in the published literature remains, to guide clinical decision making, clinicians must make sensible extrapolations from interventional studies involving participants with PFP.
Regarding the biomechanical characteristics of running, evidence exists to support a link between increased hip adduction and PFP (Noehren et al., 2013). Two strategies exist to address this. Firstly, proximal muscle strengthening has been shown to reduce hip adduction moments during running (Earl & Hoch, 2011), secondly, there is limited evidence supporting gait retraining as an effective intervention for runners with PFP. Two studies investigated the efficacy of a two week (four sessions per week) gait retraining intervention focussing on frontal plane mechanics, both showing reduced peak hip adduction and pain levels following intervention, which was sustained at one month (Noehren, Scholz, & Davis, 2011) and three months (Willy, Scholz, & Davis, 2012), respectively.
Considering spatiotemporal factors, gait retraining interventions focussed on the sagittal plane have also proven effective at increasing cadence, reducing step length and improving landing strategies, transitioning towards FFS (Lenhart, Thelen, Wille, Chumanov, & Heiderscheit, 2014; Roper et al., 2016). All three of these factors have been shown to reduce PFJ stress during running. In a population of healthy adult runners, Lenhart et al. (2014) investigated the change in forces on the knee when step rate was increased to 110% of preferred step rate, finding a decrease in PFJ stress of 14%. Another interesting factor to consider, is the role of footwear on joint forces. Minimalist footwear has been shown to reduce PFJ stress during running, at preferred cadence, by 15%, and when combined with a 10% increase in step rate, joint forces were reduced by 29% (Bonacci et al., 2018).
As has been previously discussed, poorly managed training loads have the potential to contribute to running injuries regardless of whether the individual exhibits any of the risk factors evaluated above. Training plans should be carefully planned and individualised, with particular attention paid to the athlete’s training history. While the ‘10% rule’ provides a useful guide, there is evidence to suggest novice runners may tolerate higher than 10% increases, at least for a short period of time, therefore a strict adherence to the 10% rule may delay progression to full capacity. Contrastingly, for athletes with a high chronic training load, consecutive 10% weekly increases in training load may be excessive, and risk increased injury rates (Gabbett, 2018).
Potential risk factors for PFP in runners and interventions to address these identified in this review of literature, are summarised in Figure 1, below.
Over 100 factors have been investigated as potential risk factors for PFP, making it a confusing area for clinicians to gain a clear understanding of the literature and where emphasis for intervention should lie (Crossley, van Middelkoop, Barton, & Culvenor, 2019). It is important to consider that across a range of studies showing potential risk factors for PFP, very rarely did 100% of subjects exhibit a particular variable. Careful assessment of training history, strength and objective running evaluation will reveal potential contributing factors unique to the individual and allow the implementation of interventions targeting these factors.