chapter 6: Dysplasias

PATELLAR DYSPLASIAS

Aplasia
                                                        
Complete absence of one or both patellae (Fig. 6.1) was reported as early as 1897 by Little (1) and Mayer (2). In 1899, Wuth (3) reported bilateral patella absence in all the male members of a family for three generations. Since then, many others have reported small series or isolated cases (4‑6). The abnormality may exist as an isolated entity or in association with other congenital anomalies. Most commonly, it is part of the nail‑patella syndrome, in which patellar malformation is associated with ungual dysplasia and iliac horns (7). Most patients function without difficulty, although Stuart (8) reported one female patient who lacked full knee extension by 50 degrees bilaterally.

Hypoplasia

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.

Partial Hypoplasia

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).

TYPE I

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).

TYPE II

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.

TYPE III

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:

  1. An angle greater than 140 degrees gives a flattened, potentially unstable patella, a so‑called "pebble‑shaped" patella (Fig. 6.5).
  2. With an angle from 90 to 100 degrees, one sees the forminto which most Wiberg Type III patients will fall .
  3. By 90 degrees, we have a hemipatella with one articular facet. This has been re­ferred to as the alpine hunter's cap deformity (Fig. 6.6) and is frequently observed in association with lateral instability. Not uncommonly, hypoplasia of the vastus me­dialis and even the medial facet of the trochlea can be seen with this patella form.
  4. An acute angle with a single articular facet, and the patella in the form of a half­ moon as seen on the axial view, is often associated with marked permanent subluxation or dislocation of the patella (Fig. 6.7). The more important morphologic dysplasias of the patella are summarized in Figure 6.8.

 

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.

 

        

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