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Why does an individual develop PFP?

It is the million-dollar question that almost all patients ask: why did my PFP start? Most clinicians would love to be able to answer this question, as it likely holds the key that can unlock the treatment box.

Back in 2018, we attempted to answer this question by conducting a systematic review that was then published in the British Journal of Sports Medicine, which is available open access here. We searched the prospective literature with two specific objectives in mind:

1. How common are new cases of PFP amongst specific cohorts and;

2. What variables are associated with these new cases of PFP?

So, what did we find?

Firstly, PFP is common. Whilst incidence varied, on average one in ten (10%) military recruits and adolescents will go on to develop PFP. This figure was lower for recreational runners (6%).

Secondly, we know little about what variables are associated with these new incident cases. Amongst military recruits, those with weaker quadriceps were at a greater risk of developing PFP during basic training, irrespective of measurement method. For adolescents, those with stronger hip abductors (yes, you read that correctly) were at a greater risk. For recreational runners, no variables were found that were associated with a risk of future PFP development.

We can be more confident about the variables that were not associated with a future risk of developing PFP. Sex, age, height, body mass, BMI and Q-angle were all identified to be unassociated with future PFP risk. Interestingly, our experience is that these variables are often colloquially suggested to increase an individual's’ risk of developing PFP, but there is no level one evidence to support this stance.

What do these data mean?

For us, these data reflex the complexity of why individuals develop PFP. What do weak quadriceps at baseline indicate? Whilst hypothetical, do they perhaps reflect poor conditioning at baseline of the recruits that went on to develop PFP? The same could be said for stronger hip abductors in the adolescent group - could this perhaps reflect a high level of activity at baseline?

There is certainly evidence that support this notion for adolescents. High activity levels are common in this group [1] and adolescents do not demonstrate the muscle deficits that are observed in adults with PFP [2]. Quadriceps strengthening is also effective at reducing the incidence of PFP amongst military recruits [3].

We attempted to reflect this complexity using 'causal inference diagrams', which are designed to demonstrate how specific variables may fit together to result in an end outcome. Perhaps, as Elliot Eisner is reported to have said, not everything that matters can be measured, and not everything that can be measured, matters. Our job is to try and piece it all together.

Can we make it simpler?

Yes we can. For military recruits (or arguably any other cohort who are about to increase their activity levels), quadriceps strength appears to matter. Whether weak quadriceps result in altered patellofemoral joint loading [4], or reflect a poor level of baseline conditioning, arguably doesn’t matter in clinical practice. It makes plausible sense to screen quadriceps strength and consider a period of strengthening for individuals that demonstrate weakness. Clinical measurement of quadriceps strength with a hand-held dynamometer is valid and reliable (in the right hands) [5] and > 50% of bodyweight when measured open chain at 90˚ reflects adequate strength [6].

We think similarly about adolescents, in that if during your examination you find evidence of very high activity levels, you should consider whether this may be why that patient developed PFP. This can provide a positive treatment target and adolescents have demonstrated positive outcomes after education interventions centered on activity levels [7]. There is suggestion of such a sub-group existing amongst adults with PFP too [8].

Take home messages

We understand little about why people develop PFP and each case is likely to be individual and multifaceted. A skilled clinician can conduct a quality subjective and objective examination, gaining important pieces of information that may help to determine why each patient may have developed their symptoms. From here, you can set about putting together an evidence-informed, individualised management plan.

We will follow this up with an article about how we go about assessing someone with PFP, but for now, happy reading!

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