chapter 6: Dysplasias
Global Dystrophy of the Trochlea
This exists whenever the opening angle of the sulcus is greater than 142 degrees. This, in fact, is not infrequent. It is not rare to encounter a trochlea that has no sulcus and presents a flat or even convex surface on the axial view. In these situations, if the patella is likewise flat or concave, one can hope at least that a certain amount of congruence exists. However, stability is completely dependent on retinacular and muscle balance, and it is not surprising that this form of dysplasia is frequently associated with dislocation of the patella. One must consider that chronic patellar instability early in life may give rise to trochlear dysplasia. Also, intercondylar fracture may lead to trochlear dysplasia (Fig. 6.17).
Soft tissue dysplasia is undoubtedly a factor in many patients with patellar instability. Most common, the peripatellar retinaculum becomes adaptively dysplastic in association with chronic malalignment. A patella that is aligned such that the lateral facet is chronically tilted downward will eventually manifest adaptively shortened lateral retinaculum. This form of dysplasia, secondary to malalignment, probably occurs early in life when congenital, structural malalignment exists. Similarly, one might think of the medial retinaculum as dysplastic if it is chronically stretched because of recurrent subluxation.
Hallisey et al (34) have noted considerable variation in the angles of insertion of the vastus lateralis obliquus (VLO) in cadaver dissections. This variability suggests that there may be angles of VLO insertion that are congenitally abnormal and thus predispose the patella to excessive lateralization. Also, there is variability in the angles of insertion of the vastus medialis obliquus. One might expect, therefore, that alteration of this muscle balance, either congenital or induced, might result in patellofemoral imbalance secondary to soft tissue dysplasia. Similarly, in establishing surgical congruence of the patellofemoral joint, soft tissue balance is important.
Inside Chapter 6: