chapter 8: Patellar Tilt‑Compression and the Excessive Lateral Pressure Syndrome (ELPS)
Indirect Signs of Excessive Pressure
With excessive lateral pressure related to chronic lateral patellar tilt, there will ultimately be changes in the osseous portions of the patellofemoral joint.
Increased density in the subchondral bone layer may be evident early in the syndrome, mainly under the lateral facet (Fig. 8.9). Also, there may be some decrease in medial facet subchondral density as it becomes relatively unloaded. The cancellous portions of bone are equally involved and serve as an indirect sign of excessive lateral compartment load. These are not inevitable changes, and cartilage softening has been encountered without them. Subchondral cysts are evidence of advanced cartilage changes, even though they may be present without significant joint narrowing.
LATERALIZATION OF CANCELLOUS TRABECULAE (FIG. 8.10)
It will be recalled that the trabeculae of the patellae normally run perpendicular to the equator of the patella. With tilt‑compression, there may be a change in this orientation as the trabeculae become more perpendicular to the lateral facet. As the lateral vector increases due either to lateral tethering or to increased Q angle, the resultant force (the summation of the resultant of flexion and the lateral tilt vector) is more perpendicular to the lateral facet. Both the sclerosis and lateralization would appear to be a simple expression of Wolff's law (8).
It is likely that excessive lateral pressure during growth and development will modify the shape of the patella, which ossifies rather late, as well as the trochlea. This would follow the law of Delpech (9) enunciated in 1829, concerning the influence of compression and tensile forces on an epiphysis. Hueter (10) and Volkmann (11) recognized these factors in 1862. Compression retards epiphyseal growth, whereas decreased pressure of traction stimulates epiphyseal growth. This has been referred to as the Hueter‑Volkmann law and has been confirmed experimentally by several authors (12, 13). Relative elongation and flattening of the lateral facet as seen in the Wiberg Type III patella and the alpine hunter's cap deformity are examples.
HYPOPLASIA OF THE LATERAL TROCHLEA
This is likely a result of excessive modulating force during growth, similar to what occurs on the patella side. Retardation of lateral trochlea development because of excessive lateral facet compression is most likely caused by chronic lateral tilt.
LATERAL MARGIN FRACTURE (FIG. 8.11)
These are different from bipartite patellae in that the margins are irregular and the separation is greater. However, if the fracture fragments are brought together on the radiographic view, one has the impression of a patellae too wide for the sulcus. These may also represent a fatigue fracture from excessive lateral pull on the patella (14), secondary to a shortened retinaculum. With repeated knee flexion, the "fixed" patella is unable to sublux and, eventually, a lateral margin fracture may occur.
LATERAL MARGIN OSTEOPHYTES
Macnab (15) has signaled the importance of traction in osteophyte production. The osteophyte may extend the lateral border of the patella, giving the impression of wrapping around the lateral condyle. As in the lateral margin fracture, such osteophytes are most likely a result of chronic excessive lateral traction from a secondarily shortened retinaculum that exerts pathologic pull on the patella upon flexing the knee.
The lateral retinaculum may become visible on a slightly underpenetrated view, testifying to the thickening of this fibrous tissue. CT gives a better view of the peripatellar retinaculum (Fig. 8.12). Ultimately, however, MRI gives the best view of retinacular and soft‑tissue structures. The indirect radiologic features are summarized in Figure 8.13.
Figure 8.1 1. A, Lateral marginal fracture indicating excessive lateral tension, in this case associated with a very minor injury. B, Operative view of the same case. The fracture margin is outlined by methylene blue. The distal portion of the lateral facet shows marked fibrillation (darkened area). The arrow marks the proximal portion of the patella at the median ridge.
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