chapter 6: Dysplasias
Marginal Patellar Defects
These defects are mentioned only for completeness of the chapter. They are extremely rare and of unknown clinical significance.
Double patella is rare and may become evident in one of two forms. Petty (23) described a case of bilateral horizontal duplication with a 2‑cm interval between the superior and inferior patella, the patellae lying in the center of the extensor apparatus. The interest here lies in the apparent similarity with lemurs and tarsiers, small tree‑living primates that are primarily climbers and jumpers (24, 25).
The second form, in which there is a reduplication in the frontal plane with the anterior patella more or less overlying the posterior one has been reported by both Haenisch (26) and Ficat (17). Ficat's case presented with moderate symptoms of degenerative arthritis. Figure 6.12 shows patellar reduplication in the coronal plane. Reduplication is also possible in the sagittal plane (Fig. 6.13).
Several factors can contribute to acquired patella magna. Infection and trauma with or without fracture are the most common causes. Calcification of the patellofemoral ligament may give the false impression of patellar enlargement. Also, medial and lateral osteophytes would have the same effect. Such osteophytes are covered on their contact surface with the femur by fibrocartilage, which enlarges the width of the patella as seen on the axial view.
A rare congenital form exists also, however, where the patella surpasses a maximal measurement of 57 mm in width, but more important, the patella surpasses the medial and lateral borders of the trochlear sulcus, thereby appearing too broad for its bed. The thickness is often increased as well, but may be within normal limits. Patella magna seems to be associated with a high incidence of degenerative arthritis, although the congenital form is seen too infrequently for this association to be a certainty. Bennett (27) has proposed an operation for reducing the volume by excising a vertical paramedian band in such a way to recreate approximately a normal‑sized facet.
Inside Chapter 6: