Patellar dislocation with chondral fracture: a case report

Clinical History

A 49-year-old man was admitted to the ER following a twisting trauma to the left knee during amateur physical exercise. Clinical examination of the knee revealed swelling and anterior pain with any type of passive or active motion of the knee. Radiographic and Magnetic Resonance examinations of the knee were performed.

Imaging Findings

Radiographic examination did not show any sign of traumatic lesions (Fig. 1).

MR examination revealed joint effusion and post-traumatic bone marrow oedema of the medial side of the patella and of the lateral femoral condyle, due to previous patellar dislocation (Fig. 2); as a consequence of the lateral patellar dislocation we can also observe a severe signal change of the medial patellofemoral ligament at the site of his distal attachment (Fig. 2), in keeping with high-grade strain lesion. MR evaluation also showed a wide medial retropatellar chondral fracture (Fig. 3) and a free cartilaginous fragment (of about 16 mm) located in the medial synovial recess of the knee (Fig. 4).

Discussion

Patellar dislocation is defined by the loss of contact between the patellofemoral joint surfaces. The incidence of primary patellar dislocation is 5.8 per 100,000 [1] and instability of the patellofemoral joint is a multifactorial issue: patellar stability depends on limb alignment, on the soft-tissues structures integrity and on the role of the surrounding muscles [2]. The most common mechanism of patellar dislocation is an internal rotation of the knee in a flexed position [3].
Previous studies described a number of symptoms and magnetic resonance imaging findings, such as joint effusion, contusion or osteochondral injury of the anterolateral portion of the lateral femoral condyle and medial patella and disruption of the medial retinaculum at its sites of attachment; all these are characteristics of patellar dislocation and may be helpful for diagnosis [4]. Additionally, cadaveric studies have identified the medial patellofemoral ligament as the main ligamentous structure preventing lateral patellar dislocation [5].
MR imaging can be used to diagnose patellar dislocation, since there are typical injury patterns: bone marrow oedema of inferomedial patella and lateral femoral condyle, in combination with medial patellofemoral ligament disruption (which may occur at patellar or femoral attachment), is considered diagnostic for recent patellar dislocation. Bone marrow oedema resulting from contusion presents high signal intensity on T2-weighted images and low signal intensity on T1-weighted images. A tear of a medial stabiliser structure is seen at MR imaging as a complete disruption of the patellofemoral ligament and as a local soft-tissue oedema, which has high signal intensity on T2-weighted images [6]. Several patients show chondral or osteochondral lesions of the medial patella, with the classic finding of a concave deformity of the inferomedial patella, which is considered a highly specific sign of former patellar dislocation [5]. Knee effusion may be present.
Various surgical options are available to treat patients with patellar dislocation (medial patellofemoral ligament reconstruction, medial capsular plication, lateral capsular release, trochleoplasty, tibial tuberosity transfer). Diagnostic evaluation includes identification of underlying anatomical anomalies, in order to correct such anomalies and the damage caused by the trauma [2].
MR is the preferred imaging modality in patients with suspected patellar dislocation thanks to its ability to evaluate soft-tissues, ligamentous and tendinous lesions as well as bony injuries, while other modalities (i.e. x-rays or US) can't provide an equally wide and exhaustive evaluation. For these reasons MR is essential to select the most fitting treatment.

Differential Diagnosis List

Final Diagnosis

Lateral patellar dislocation and medial retropatellar chondral fracture

URL:	https://eurorad.org/case/9561
DOI:	10.1594/EURORAD/CASE.9561
ISSN:	1563-4086