There is scope to get really complicated under this heading. We could lose ourselves in the sensitivity and specificity of tests and positive/negative likelihood ratios. We could question a test's validity, or even more challenging still, delve into the realms of an individual’s perceptions and how these may or may not reflect the health of the tissues in the anterior knee. What we hope to do is keep it close to the science, acknowledging the limitations, whilst still giving you some important takeaways that can be used in clinical practice.
When an individual presents to your clinic in pain and is experiencing pain with certain activities, arriving at a diagnosis becomes one of the important outputs of your initial clinical assessment.
Because around the diagnosis discussion can develop. A discussion that centres around factors that may have contributed to the onset of symptoms and around factors that may be important to their persistence.
It also enables clinicians to communicate with each other about common clinical presentations. Something that we do, not only in the clinic corridor, but also on social media and within the scientific literature. The positive outcome of a commonly understood diagnosis therefore, is that we begin talking about a similar ‘thing’, allowing for education and shared knowledge – not just with other clinicians but with patients too.
So how do we arrive at a diagnosis of PFP?
We listen! When we look at all the studies conducted around PFP, its diagnosis and its treatment, there are common characteristics that patients describe and there are concomitant pathologies that we are looking to exclude. In short, PFP is a diagnosis of exclusion, with commonly reported signs and symptoms that indicate its presence.
So, what are we listening for from the patient’s subjective history?
1. Insidious onset:
I just noticed it getting worse … there was no one event where I started to get pain
2. Pain at, around or behind the knee cap:
it’s sort of under my knee cap … it's deep … I can’t get to it … I feel it more on the inside (medial) front of my knee
3. Aggravated by activities that load the anterior knee (e.g. running, squatting, stairs):
it’s worse after I run and for the next few hours … stairs are almost always painful
What are we looking for in our objective exam?
Even with the history presented above, it remains plausible that the diagnosis may not be PFP. Patellar tendinopathy can present in a similar way, as can some degenerative meniscal tears or a medial plica, as well as PCL deficient knees and quadriceps tendinopathy. Additionally, in the young, so can Sinding Larsen Johansson. To start our process of exclusion, we need to take the knee through a collection of tests that might indicate that a diagnosis of PFP is most appropriate for this individual.
Whilst accepting that all clinical tests have their limitations, the presence of focal tenderness at the inferior pole of the patella is more indicative of patellar tendinopathy. Deep flexion, a positive McMurray's and joint line tenderness are more indicative of meniscal pathology. Performing a Lachman ± observing for a sag sign will give you an indication of PCL deficiency with a fairly high degree of accuracy and reproducibility. Your index of suspicion should be elevated for SLJ in early adolescents who are presenting with patellar tendinopathy symptoms. Localised anteromedial pain ± a joint effusion may increase your index of suspicion of a medial plica.
A systematic review completed in 2013 by Nunes et al, summarised that 2 clinical tests, patellar tilt/compression and a single leg squat, had the highest value in demonstrating a trend towards a PFP diagnosis, out of 25 different tests examined across 5 studies. They concluded that their values did not, however, represent clear evidence of diagnostic accuracy in their own right.
When faced with this diagnostic challenge, it remains imperative that we apply a deductive approach to our assessment. The early listening is imperative. When an individual describes insidious onset of diffuse anterior knee pain aggravated by appropriate activities, you should start thinking about PFP as a plausible diagnosis.
You then examine. If the patellar tendon and joint lines are non-tender and deeper ranges of flexion are not provocative, but a single leg squat and patellar tilt test are positive, PFP represents the most plausible diagnosis. Other differential diagnoses should remain in your working hypothesis and can be revisited at a later date if the individual is not progressing as you expect they should.
Extracted from Nunes et al. as way of a little further reading.
Sensitivity measures a test’s capacity to identify individuals affected by a disease. It is calculated by dividing the true positive results by the true positive results plus the false negative results and is expressed as a percentage. The higher the value, the higher a test’s capacity to identify affected individuals (Deeks, 2001; Jaeschke et al., 1994).
Specificity indicates a test’s capacity to identify individuals who are not affected by the disease in question. It is calculated by dividing the true negative results by the true negative results plus the false positive results and is expressed as a percentage. The higher the value, the higher the chance that the test will identify individuals who are not affected by the disease (Deeks, 2001; Jaeschke et al., 1994).
Positive (LR+) and Negative (LR-) Likelihood Ratios refer to the discriminatory measurements of tests, indicating how many times more (LR+) or less (LR-) likely the test results will be in affected than non-affected individuals (Deeks, 2001; Hayden & Brown, 1999; Jaeschke et al., 1994).
LR+ is calculated as follows: sensitivity/1 - specificity. (Hayden & Brown, 1999)
LR- is calculated as follows: 1 - sensitivity/specificity. (Hayden & Brown, 1999).
LR+ results greater than 10 and LR- less than 0.1 indicate convincing diagnostic evidence.
Predictive Value refers to the percentage of times that a test will correctly diagnose the evaluated condition. Positive predictive value (PV+) refers to the proportion of affected individuals with positive results and negative predictive value (PV-) refers to the proportion of non-affected individuals with negative results (Fritz & Wainner, 2001). Predictive values generally are expressed as a percentage.
PV+ is the ratio of true positive results to all positive results obtained, whether true or false. PV- is the ratio of the true negative results to all negative results (Fritz & Wainner, 2001).