Moving Patellar Apprehension Test | Patella Dislocation
Patellar dislocation can have several underlying anatomical factors including but not limited to abnormal bony anatomy of the trochlea and patella, mechanical misalignment of the lower limb as well as static and dynamic deficiency of soft tissue stabilizers. The traditional patellar apprehension test as well as CTs or MRI may fail to detect patients with patellar instability due to its static nature – especially outside of an acute event or if the patella failed to self-reduce.
Ahmad et al. (2009) proposed the moving patellar apprehension test to functionally mimic an actual dislocation or instability episode of the patella. In their diagnostic cohort study, they found an overall accuracy of 94.1% with a sensitivity of 100% and a specificity of 88.4%. The study has several limitations that for example exclude patients with symptomatic subluxations rather than frank dislocations and the test has not been validated in other studies which is why we give it a moderate clinical value.
To conduct the test, the patient is in supine lying position with the lower leg hanging over the edge of the bench. In the first part of the test, the leg is in full extension and the examiner uses the thumb of one hand to translate the patella laterally. Then the leg is moved into 90° of flexion and the examiner notes any signs of apprehension orally expressed by the patient or quadriceps contraction in an attempt to stop flexion and/or to dynamically reposition the patella. It is then brought back to full extension while the lateral translation is maintained.
In a second step, the knee is again in full extension and now the examiner uses the index finger to medially translate the patella as far as possible. The knee is then flexed to 90° and elimination of apprehension and quadriceps contraction is noted.
In order for the test to be considered positive, both steps of the test have to be positive.
Ahmad and colleagues state the following: In full knee extension, the patella is in its most unstable position and soft tissues and alignment are most responsible for the patella’s stability. As flexion begins, the patella must engage the trochlea, and then stability is shared by the soft tissues, overall alignment, and the bone geometry of the trochlea and patella. For a knee without patellar instability, as the knee flexes, the patella is guided into the trochlea by the MPFL and other medial soft tissue stabilizers. The patella subluxates most easily at 20° of knee flexion, and the MPFL seems to resist lateral patellar subluxation greatest when the knee is toward full extension. For patients with symptomatic instability, while the knee is flexing combined with an eccentric quadriceps force, the patella engages the trochlea in a lateral position and then dislocates or subluxates as the knee continues to flex. For the patient with patellar instability who is being evaluated with the MPAT, as the knee flexes during the first part of the test, the patella engages the trochlea in a lateral position and mimics a patellar dislocation episode, therefore reproducing the symptoms associated with a dislocation. The MPAT also actively assesses the MPFL since the MPFL is responsible for guiding the patella properly into the trochlea as the knee moves from full extension into flexion.
In the second part of the test, with medial force on the patella as the knee is flexed, the symptoms of impending dislocation are eliminated because the patella engages the trochlea in a normal fashion. The manual translation of the patella reproduces the function of the deficient and/or incompetent MPFL. The test is a combination of apprehension in part 1 and the reduction of apprehension in part 2. This dynamic, provocative test is similar in concept to the pivot-shift test for ACL insufficiency in the knee, the release test for anterior shoulder instability, and the moving valgus stress test for valgus instability of the elbow. All tests, which you can also find here on our channel.
21 OF THE MOST USEFUL ORTHOPAEDIC TESTS IN CLINICAL PRACTICE
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