Teaching Case

A 14-year-old girl presented with a history of persistent right knee pain.

Knee radiographs revealed a round, well-circumscribed, lucent lesion with sclerotic borders located in the superolateral aspect of the right patella. A follow-up radiograph a few months later demonstrated no changes in the lesion.

A computed tomography (CT) scan and a magnetic resonance imaging (MRI) of the knee were performed for further evaluation due to persistent pain. CT images of the lesion showed a rounded bony defect in the described location, surrounded by a sclerotic margin and abutting against articular surface of the patella. 3T MRI revealed the same findings, with intact overlying articular cartilage and no bone marrow oedema associated. No other injuries were identified.

The examination revealed a typical dorsal defect of the patella (DDP). The patient was managed conservatively.

Background

DDP is an infrequent anomaly of ossification of the patella, consisting of a delay or failure in its developmental process [1–4]. It appears in less than 1% of the general population [2,4–6], and it is most common in adolescents [3,4]. It is rarely seen in adult patients and its disappearance has been documented, suggesting its spontaneous resolution [3,4,7]. It occurs bilaterally in up to one-third of cases [3,5,8,9]. DDP and bipartite or multipartite patella can associate, and they share the same typical location, leading to believe in a common origin [4,6,10]. The aetiology of DDP is not completely known; it has been postulated that increased stress caused by chronic trauma, combined with a deficient vascular supply can contribute to the disorder [3,4].

Clinical Perspective

In most cases DDP is asymptomatic. It is usually seen as an incidental finding observed on radiographs performed for other problems [2,3,5]. However, occasionally it may cause chronic knee pain in the absence of other abnormalities [2,3,7,8,10,11]. Some studies have shown that symptomatic cases with disrupted or invaginated articular cartilage into the defect may improve with surgical treatment [9,12]. Nevertheless, in more than half of the symptomatic cases managed with surgery or arthrography, there were no injuries in the articular surface [1,3].

Imaging Perspective

On plain radiographs the defect appears as a well-circumscribed, radiolucent lesion with a peripheral sclerotic margin invariably located on the superolateral aspect of the patella, abutting against articular surface [2–4,6–8]. This is a characteristic appearance which allows the diagnosis by itself, without biopsy or other intervention [4,11]. For this reason DDP was considered by Clyde A. Helms one of the skeletal “do not touch” lesions [7,13]. However, MRI is an excellent method in those cases with inconclusive plain radiograph findings for assessing the articular cartilage and for excluding coexisting disorders [2,6,8,11]. On MRI fluid-sensitive sequences DDP is seen as a hyperintense defect [2], with sclerotic margins. Lesions in the overlying articular cartilage and bone marrow oedema should not be present.

Outcome

The treatment consists of conservative management, with reduction of physical activity [1,7,11]. DDP tends toward spontaneous, slow healing [3,4]. Surgical treatment is rarely indicated, but in cases in which radiologic findings are not conclusive and the symptoms persist, curettage of the lesion by arthroscopy has resulted in relief of symptoms [4,8].