Age / Sex : 23 / F

Chief complaint : Waxing and waning of the knee pain, no pain recently.

What is your diagnosis?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Ja-Young Choi, Seoul National University Hospital

Discussion

Answer:  Dorsal defect of the patella.

Findings:

X-ray: Circumscribed osteolytic lesion of the superolateral quadrant of the right patella.

CT: Subchondral osteolytic lesion of the lateral patellar facet with central oval ossification and peripheral uneven sclerotic rim.

MRI: Focal dimpling and thin fissure in the cartilage over the subchondral lesion is noted. The lesion shows hyperintensity around the central ossification on FS PD-weighted image.

Differential Diagnosis: Osteochondritis dissecans

Discussion:

A dorsal defect of the patella is a variant occurring in the ossification process of the patella and being part of the spectrum of bipartite or multipartite patella, and presents in 0.3 to 1 % of the population. It frequent occurs in the 2^nd and 3^rd decades of life. This lesion often is asymptomatic and an incidental finding, but may be a source of patellofemoral symptoms and knee pain. On X-ray, this manifests small circular radiolucent lesion with well-defined sclerotic rim in the typical superolateral aspect of the articular surface of the patella. On MRI, the cartilage over a dorsal defect is usually, but not always, intact. Sometimes, it is associated with a focal dimpling or slit-like defect in the overlying cartilage like this case. On T1-weighted images the signal of a dorsal defect is inhomogeneous and on T2- or PD-weighted images the signal is hyperintense. A dorsal defect of the patella is one of the skeletal "do not touch" lesions. Healing may occur spontaneously.

References:

1. van Holsbeeck M, Vandamme B, Marchal G, Martens M, Victor J, Baert AL. Dorsal defect of the patella: concept of its origin and relationship with bipartite and multipartite patella.

Skeletal Radiol 1987;16:304-11.

2. Ho VB, Kransdorf MJ, Jelinek JS, et al. Dorsal defect of the patella: MR features. J Comput Assist Tomogr 1991; 15:474.


3. Gerrie BJ, McCulloch PC, Labis JS, Lintner DM, Harris JD. Dorsal Defect of the Patella in a Teenage Male Football Player: A Case Report and Differential Diagnosis of Lytic Patellar Lesion. Orthop J Sports Med. 2016 Sep 9