chapter 6: Dysplasias
Many diverse forms of patella fragmentation have been described by numerous authors. The basic configurations are summarized in Figure 6.9. The fragments would appear to result from an aberrant ossification center that ailed to fuse with the main mass of the patella. However, the pathogenesis is obscure, because it is known that the patella ossifies from a single ossific nucleus. It would be easier to understand if one accepted the pluricentric ossification theory of Schaer (18), but that does not, normally at least, appear to be the case.
Whatever the real ossification, one can pose the question of whether the real etiology might not be an increase in the constraint of muscle traction on a particular section of the patellar ossific nucleus at a critical time in its formation. This would perhaps better explain the overwhelming predominance for localization to the superolateral region of the patella. The lateral predominance of the patellofemoral joint, which plays such a significant role in the physiology and pathology of this articulation, is fully consistent with this concept.
Although many forms of patellar fragmentation exist, all except bipartite patella (Fig. 6.10) are extremely infrequent. Since it was originally described in 1882 by Wensel Grüber (19), many cases have been published worldwide by so many authors that interest in this abnormality as a curiosity has waned, only now to be reawakened with the expansion of insurance claims that pose the medicolegal problem of separating a traumatic form from a congenital malformation.
The frequency rate of bipartite patella has been reported to be as low as 0.05% (Stucke ) in 20,000 soldiers to as high as 1.66% (Blumensaat ) in 1378 patients. Anatomically, bipartite patella appears indistinguishable from a pseudarthrosis, with a cartilaginous bridge uniting the two fragments. In many respects, it is similar to a synchondrosis.
The clinical manifestation of bipartite patella presents several possibilities and some problems. Many, if not most, cases remain clinically silent, only to be fortuitously discovered at the time of radiograph for other reasons. Also, there are some cases that will show up only on axial view. Bipartite patella may become spontaneously symptomatic just as other synchondroses become symptomatic, for example, the costochondral junction. The bipartite fragment may also be the site of early degenerative changes, which lead to symptoms.
A history of trauma frequently antedates symptoms and poses several possibilities. One may be dealing with a fracture of the superolateral corner. Factors favoring fracture would be: (1) a history of significant direct trauma to this localized corner, (2) hematoma and swelling, (3) localized point tenderness and occasionally crepitus, and (4) a radiograph showing an irregular sharply outlined line of separation. Of course, if films before the accident are available, the problem can be resolved.
Even when one can resolve the question of fracture versus bipartite patella, the determination of the role of injury is not solved. The bipartite fragment may undergo symptomatic changes of post‑traumatic chondromalacia, as elsewhere in the patella. The synchondrosis itself may be disrupted or strained as manifested by localized direct tenderness and swelling, minimal effusion, painful restriction of motion, and possibly muscle atrophy. Those factors that support a previously existing bipartite fragment include: (1) clearly defined radiolucency with rounded margins separating the fragments, (2) sclerosis of the margins, and (3) bilateral lesions. The latter is very helpful, but unilateral bipartite patella is by no means rare. Certainly, the most difficult situation is when the patient is seen for the first time for examination of the reported trauma after considerable delay, because a true pseudarthrosis will have the same radiographic appearance as a bipartite patella.
All patients with bipartite patella who have become symptomatic for whatever reason should undergo an attempt at conservative management. Depending on the severity of the symptoms, this might include immobilization in a cylinder cast or knee immobilizer, anti‑inflammatory therapy, and occasionally, injection into an area of local tenderness, particularly if tenderness can be localized in the adjacent retinaculum. Surgical treatment is considered only after conservative management has failed. The smaller fragment is simply excised through a short lateral parapatellar skin incision. The lateral patellar retinaculum is not released. If the fragment is not mobile, and articular cartilage is intact, a lateral retinacular release, in which the surgeon detaches the vastus lateralis insertion into the fragment (22), may be preferable in selected cases (Fig. 6.11). Although excellent results and healing of the fragment are reported with this approach, I prefer to reattach the released vastus lateralis tendon into the adjacent central quadriceps tendon. At surgery, the cartilage covering the fragment is frequently found to show significant degenerative changes and may require limited debridement when this procedure is necessary.
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