Patellar Dislocation Introduction and Diagnosis
Figure 1. The relationship of the MPFL to other structures.VML: vastus medialis longus, AM: adductor magnus, VMO: vastus medialis obliquus, PMC: posteromedial capsule;MPFL: medial patellofemoral ligament; SMCL: superficial band of medial collateral ligament. (Adopted from Amis et al. 2004)
On physical examination, the first step is to inspect the knee. An acutely dislocated patella is typically easy to see but there may be a large effusion or hemarthrosis that make it more difficult. The provider should also inspect for patella alta, tibial torsion, genu valgum or varum or general ligamentous laxity. General ligamentous laxity is assessed with the Beighton hypermobility score . There has been a term dubbed “miserable malalignment syndrome” for patients with excessive femoral anteversion, excessive tibial outward rotation, and genu valgum . The knee should be palpated including each pole of the patella, medial and lateral joint lines and retinaculum. The bulk of the VMO should be evaluated and quantified if possible. A palpable defect along the medial retinaculum or MPFL may be appreciable. Tenderness over the MPFL origin has been called the Bassett sign, which is consistent with ligamentous disruption [4,5]. The collateral ligaments should also be palpated and assessed along with the cruciate ligaments.
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Figure 3. Blumensaat line is drawn on the linear radiopacity of the roof of the intercondylar notch on the lateral knee radiograph. In this method, the patellar height is assessed according to the distance between the lower pole of the patella and the Blumensaat line in millimeters. The lower pole of the patella should normally lie on the Blumensaat line; if it is more than 10 mm above the line, it is classified as patella alta (adopted form Seyahi 2006)
Figure 4. Dejour classification of trochlear dysplasia 1. Type A indicates the presence of the crossing sign with a shallow trochlea of >145. Type B indicates the presence of a supratrochlear spur and a flat or convex trochlea. Type C indicates the presence of a double contour sign with a hypoplastic medial femoral condyle. Type D indicates the presence of a supratrochlear spur and a double contour sign with a cliff pattern between condyles. Adopted from Dejour et al. 2007.
Figure 5. Superimposed CT axial scans of the trochlear groove and the tibial tubercle demonstrating measurement of the TT-TG distance. Line A denotes the perpendicular to the deepest portion of the trochlear groove, and line B denotes the prominence of the tibial tubercle. The TT-TG measurement is the distance between lines A and B. Adopted from Weber et. al 2016.
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