chapter 10: Patellar Dislocation
ETIOLOGY OF PATELLAR DISLOCATION
Due to the "Q" angle, with its resultant lateral vector, there is normally some tendency for the patella to displace laterally. This tendency is balanced passively by the medial ligaments and by a buttress effect of the lateral femoral trochlea. The patella is balanced functionally by the orientation of the distal vastus medialis fibers. In the case of acute or recurrent dislocation, there is a breakdown of this equilibrium. Because there are numerous cases of recurrent dislocation in the complete absence of any morphologic bony abnormality, it is clear that, in certain cases at least, an imbalance in the extensor mechanism can be responsible for patellar dislocation.
Brattstrom (2), using a complicated radiologic technique for assessing the trochlear surface of the distal femur, determined in 131 patients with recurrent dislocation of the patella that, on the average, the trochlear facet (sulcus) angle was 10 degrees greater in dislocators than in a normal population (200 similarly studied controls). Vainionpaa et al (3) confirmed the presence of abnormally high sulcus angles in patients with patellar dislocation. Brattstrom (2) thought that this was the most important factor in patients with recurrent dislocation, even though fully half of the patients studied had a normal sulcus angle. His technique allowed him to conclude that the change in the trochlear sulcus angle is due to a rising depth of the sulcus rather than to a decrease in the prominence of the facets. Measurement of the angle is not subject to distortion by rotation as is measurement of the relative prominence of the lateral facet, vis‑á‑vis the medial facet.
Today, one may obtain an accurate appraisal of trochlear anatomy using computerized tomography (CT) or magnetic resonance imaging (MRI). By obtaining serial sections of the patellofemoral joint, centering the cut at the midtransverse patella, and obtaining tomographic slices at 10‑degree increments of knee flexion, one can achieve an excellent radiographic picture of trochlear anatomy as it relates to patellar tracking and trochlear damage from patellar dislocation. Nietosvaara et al (4) noted that the trochlear fracture from patellar dislocation occurs mostly on the edge of the articular surface at the middle third of the condylar arc. With CT, one can also define the extent of trochlear dysplasia preoperatively .
Not all series reporting recurrent dislocation have paid sufficient attention to patellar form. Baum and Bensahal (5), however, measured a patellar depth coefficient and found 35 of 59 patellae were abnormally shallow. As far as patellar form is concerned, these authors encountered 42 cases of Wiberg Type III patellae in their series of 59 patients. Similar percentages have been reported by Rohlederer (6) (76%) and Loff and Freidebold (7) (86%). Abnormal patellar morphology may be more effect than cause, however. If a patella is chronically malaligned, particularly during skeletal development, it is likely there will be secondary morphologic changes. Dislocation, therefore, is probably a result, in most cases, of chronic malalignment or secondary to trauma.
ABNORMAL BONY RELATIONSHIPS
External Tibial Torsion. Brattstrom (2) was not able to document any statistically significant difference in rotation or torsion between his patient and control groups. Heywood (8) reported two cases in his series of 54 patients.
Femoral Anteversion. When femoral anteversion presents as a toe‑in gait without compensatory external tibial torsion, patellar problems are not common. Some patients, however, who have excessive femoral anteversion and external tibial torsion, will show signs of patellar malalignment and may be at increased risk of dislocation.
Lateral Location of the Anterior Tibial Tubercle. This has been thought to be an important factor by many authors, particularly Trillat et al (9), based more on clinical impressions than on precise measurements. Dupuis (10), in his monograph on tibial torsion, measured the distance between a vertical line from the center of the patella to a vertical line from the tibial tubercle. However, these measurements are of little value because the patella may likewise be laterally situated with a laterally placed tuberosity. Because of tibial translation and rotation with knee flexion, there is no one measurement that will consistently indicate a proper relationship of the patella to the tibial tubercle.
Genu Valgum. Heywood (8) reported genu valgum in 7 of his 106 cases, and Hughston (11) found it in 1 of 111 cases. Bizou (12) concluded that genu valgum is not a prominent cause of patellar dislocation. The large number of young children with significant genu valgum without patellar problems supports the validity of Bizou's conclusion.
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