chapter 6: Dysplasias
In patella parva (Fig. 6.2), the entire patella is small. Such is the case in congenital dislocation, but cases occur without any apparent abnormal function. This form of dysplasia is often associated with quadriceps hypoplasia. It has been seen with apophysitis of the inferior pole of the patella (Sinding‑Larsen‑Johansson disease) (9,10), where the decrease in overall patellar size may put additional strains on the ligamentous and tendinous attachments.
For the most part, this condition involves the medial facets. There appears to be much variation in patellar form consistent with normal function. Wiberg (11), in his classic and widely accepted article in 1941, proposed a three‑part classification to encompass the majority of patellae encountered. It is based upon the radiologic form on the axial view (Fig. 6.3, A‑C).
Both facets are gently concave, symmetrical, and roughly the same size, although slight lateral predominance is common. Theoretically, this would appear to be the ideal patellar form. It is, however, the least common, occurring in only 10% of the population, according to Hennsge (12).
There is a subtle flow from 1 to 11, where the medial facet is distinctly smaller than the lateral. The lateral facet remains concave, whereas the medial is either flat or slightly convex. The relative lateral facet predominance seems in accord with the general lateral predominance of the patellofemoral joint as a whole. This is the most common form, comprising 65% of Hennsge's series.
The medial facet is considerably smaller, with marked lateral predominance. Wiberg did not, in his original classification, specifically detail that the subchondral outline should be convex. However, examples he showed were convex, and subsequent authors have included medial facet convexity in the criteria for Type III. Even though this group comprises 25% of Hennsge's series, Wiberg regarded it as a frankly dysplastic form. Nevertheless, he was unable to show that it was associated with chondromalacia patellae with higher frequency than other forms. Also, other authors have noted no association between Type III and either chondromalacia (13, 14) or patellofemoral arthrosis (15).
Baumgartl (16) has further described a rare type that appears to be a variation of Type III. It is characterized by a convex projection of the medial facet (Fig. 6.4). The importance of this type according to Baumgartl would appear to be a frequent association with osteochondritis dissecans of the medial femoral condyle. A look at contact prints of the knee in full flexion shows that the odd facet normally articulates here.
Ficat (17) proposed another classification based upon the angle that the two major facets make with one another. With this method of measurement, there are four categories of abnormality:
Computerized tomography (CT) and magnetic resonance imaging (MRI) offer the alternative of defining articular cartilage contours as well as the subchondral bone. Staubli, in particular, pointed out the incongruity between osseous and chondral morphology( ). This is important in the interpretation of radiographic alignment, as the apparent congruity or incongruity of osseous structure relationships, such as the patella and the trochlea, may not adequately reflect the true articular relationship. Nonetheless, there are well-defined radiographic criteria of proper alignment based on osseous images (Merchant congruence angle is a good example, based on good control data), and the validity of these relationships remains important in clinical practice.
Inside Chapter 6: