Analysis of 2 papers on tests of PFPS

Data regarding validity of clinical and radiographic findings in diagnosing patellofemoral pain syndrome are inconclusive.

1. 5 clinical tests in patellofemoral pain syndrome are:

1. Vastus medialis coordination test
2. Patellar apprehension test
3. Waldron's test
4. Clarke's test
5. Eccentric step test.

Nijs J et al examinied the validity of five clinical patellofemoral tests used in the diagnosis of patellofemoral pain syndrome (PFPS). Nijs J et al examinied above said 5 clinical tests in Likelihood ratio. (See below for interpretation of likelihood ratio)

Sample size:
45 knee patients were divided into either the PFPS or the non-PFPS group, based on the fulfilment of the diagnostic criteria for PFPS.

Focus points:
1. The positive likelihood ratio was 2.26 for both the vastus medialis coordination test and the patellar apprehension test.
2. For the eccentric step test, the positive likelihood ratio was 2.34.
Hence a positive outcome on either the vastus medialis coordination test, the patellar apprehension test, or the eccentric step test increases the probability of PFPS to a small, but sometimes important, degree.
3. For the remaining tests, the positive likelihood ratios were below the threshold value of 2, indicating that given a positive test result, the probability that the patient has PFPS is altered to a small, and rarely important degree.

However according to Nijs J et al negative likelihood ratios for all tests exceeded the threshold value of 0.5, suggestive of clinically irrelevant information. These data question the validity of clinical tests for the diagnosis of PFPS.

Information on likelihood ratios:
When we decide to order a diagnostic test, we want to know which test (or tests) will best help us rule-in or rule-out disease in our patient. In the language of clinical epidemiology, we take our initial assessment of the likelihood of disease (“pre-test probability”), do a test to help us shift our suspicion one way or the other, and then determine a final assessment of the likelihood of disease (“post-test probability”).
The “positive likelihood ratio” (LR+) tells us how much to increase the probability of disease if the test is positive, while the “negative likelihood ratio” (LR-) tells us how much to decrease it if the test is negative.

Likelihood ratio have unique properties that make them particularly relevant to clinicians:
• The LR+ corresponds to the clinical concept of "ruling-in disease"
• The LR- corresponds to the clinical concept of "ruling-out disease"
• The LR+ and LR- don't change as the underlying probability of disease changes (predictive values do change, as you just learned)
• LR's using multiple "levels" of positive (i.e. not just a simple yes/no or positive/negative result) provide much richer, more useful information to you as a clinician.
Interpreting likelihood ratios: general guidelines

The first thing to realize about LR’s is that an LR > 1 indicates an increased probability that the target disorder is present, and an LR < 1 indicates a decreased probability that the target disorder is present. The following are general guidelines, which must be correlated with the clinical scenario:
1. LR > 10: Large and often conclusive increase in the likelihood of disease
2. 5 – 10: Moderate increase in the likelihood of disease
3. 2 – 5: Small increase in the likelihood of disease
4. 1 - 2 : Minimal increase in the likelihood of disease
5. 1: No change in the likelihood of disease
6. 0.5 - 1.0 : Minimal decrease in the likelihood of disease
7. 0.2 - 0.5: Small decrease in the likelihood of disease
8. 0.1 - 0.2: Moderate decrease in the likelihood of disease
9. LR< 0.1: Large and often conclusive decrease in the likelihood of disease

2. Validity of clinical and radiological features

Haim A et al assessed how sensitive and specific key patellofemoral physical examination tests are, and evaluated the prevalence of physical examination and radiographic findings.
Sample size:
61 infantry soldiers with patellofemoral pain syndrome and 25 control subjects were evaluated.

Focus points:
1. The sensitivity of the patellar tilt, active instability, patella alta, and apprehension tests was low (less than 50%); specificity ranged between 72% and 100%.
2. Although the prevalence of positive patellar tilt and active instability tests was significantly greater in subjects with patellofemoral pain syndrome, there were no significant differences between the groups in the results of the other two tests.
3. Soldiers with patellofemoral pain syndrome presented with increased quadriceps angle, lateral and medial retinacular tenderness, patellofemoral crepitation, squinting patella, and reduced mobility of the patella.
4. There were no differences between the groups in the prevalence of lower limb and foot posture alignment and knee effusion.
5. Plain radiography showed increased patellar subluxation in soldiers with patellofemoral pain syndrome. Other radiographic measures (sulcus angle, Laurin angle, Merchant angle, and Insall-Salvati index) were similar in both groups.
6. Haim et al have also found that physical examinations were more useful than plain radiography.

Reference:
1. Nijs J et al, Man Ther. 2006 Feb;11(1):69-77. Epub 2005 Jun 13.
2. Haim A et al, Clin Orthop Relat Res. 2006 Oct;451:223-8
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