Diagnosis
MPS diagnosis can be challenging. Often, clinical examination findings are subtle, and radiographs may not show significant pathology. The most accurate diagnosis is obtained by combining patient history, physical examination findings, imaging studies, and diagnostic arthroscopy.
Patient History
Patients with MPS report chronic pain localized to the inferior medial patella and anterior-medial joint line. Occasionally, they complain of crepitus and intermittent swelling. Other symptoms include pain with knee flexion activity, such as squatting and climbing or descending stairs. Some patients describe episodes of giving way and feelings of instability. Often, they are aware the direction of instability is medial. The pain typically is not relieved by medication, physical therapy, or bracing.
Physical Examination
MPS must be identified by clinical examination. Peripatellar tenderness is typically noted. There is often no effusion or crepitus, but the patella is unstable in early flexion. Active and passive ROM is painful through the first 30° of knee flexion. The patient may have a positive medial apprehension test7 in which he or she experiences apprehension of the patella being subluxated with a medially directed force on the lateral border of the patella.
The gravity subluxation test described by Nonweiler and DeLee6 is useful in detecting MPS after lateral release and indicates that the vastus lateralis muscle has been detached from the patella and that the lateral retinaculum is lax. In this test, the patient is positioned in the lateral decubitus position with the involved knee farthest from the table. In this position, gravity causes the patella to subluxate out of the trochlea. The test is positive for MPS when a voluntary contraction of the quadriceps does not center the patella into the trochlear groove. Patients with MPS without previous lateral release can have the patella subluxate medially in the lateral decubitus position, but it is pulled back into the trochlea with active quadriceps contraction (Figure 1).
Patients with MPS often have lateral patellar laxity (LPL), which allows the patella to rotate upward on the lateral side and skid across the medial facet of the femoral trochlea. A physical examination sign combining lateral patellar glide and tilt was described by Shneider24 to identify LPL. This “lateral patellar float” sign is present when the patella translates laterally and rotates or tilts upward with medial pressure on the patella (Figure 2). Another maneuver to test for subtle MPS involves manually centering the patella in the trochlea during active knee flexion and extension. The involved knee is examined in the seated position. The examiner attempts to center the patella in the trochlea with a laterally directed force from the examiner’s thumb on the medial border of the patella. This will usually provide immediate relief as the patient actively ranges the knee.
Imaging Studies
Diagnostic imaging is a crucial component of the evaluation and treatment decision process. Plain radiographs often are not helpful in diagnosing MPS but may provide additional information.5 A variety of radiographic measurements have been described as indicators of structural disease, but there is a lack of comprehensive information recommending radiographic evaluation and interpretation of patients with patellofemoral dysfunction. It is crucial that orthopedic surgeons have common and consistent radiographic views for plain radiographic assessment that can serve as a basis for accurate diagnosis and surgical decision-making.
Standard knee radiographs should include a standing anteroposterior view of bilateral knees, a standing lateral view of the symptomatic knee in 30° of flexion, a patellar axial view, and a tunnel view. These views, occasionally combined with magnetic resonance imaging (MRI), can yield information vital to surgical decision-making. Image quality is highly technique-dependent, and variability in patient positioning can substantially affect the ability to properly diagnose structural abnormalities. For improved diagnostic accuracy and disease classification, radiographs must be obtained with use of the same standardized imaging protocol.
Kinetic MRI was shown by Shellock and colleagues25 to provide diagnostic information related to patellar malalignment. As kinetic MRI can image the patellofemoral joint within the initial 20° to 30° of flexion, it is useful in detecting some of the more subtle patellar tracking problems. In their study of 43 knees (40 patients) with symptoms after lateral release, Shellock and colleagues25 found that 27 knees (63%) had medial subluxation of the patella as the knee moved from extension to flexion. Furthermore, MPS was noted on the contralateral, unoperated knee in 17 (43%) of the 40 patients.