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.
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).
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.
Lateral patellar dislocation and medial retropatellar chondral fracture
Based on the patient’s left knee X-ray and MRI images, the following main features are observed:
Based on the above imaging findings, along with the history of twisting injury and clinical symptoms of knee swelling and pain, the following diagnoses or differential diagnoses are proposed:
Combining the patient’s age (49 years), mechanism of injury (twisting trauma), clinical presentation (anterior knee pain, swelling), and the typical MRI findings of bone bruises and MPFL disruption, the most likely diagnosis is: Acute Patellar Dislocation (accompanied by partial or complete tear of the medial patellofemoral ligament).
If more detailed information on cartilage or ligament injury is required, knee arthroscopy may be considered for further evaluation.
The rehabilitation goal is to relieve acute pain and swelling, restore knee stability, and gradually regain muscle strength and mobility. A gradual approach is recommended, guided by the FITT-VP principle:
Throughout rehabilitation, closely monitor changes in knee stability and pain. If recurrent swelling, joint locking, or significantly increased pain occurs, prompt medical evaluation is necessary.
[Disclaimer] This report is a reference analysis based on imaging and patient history and cannot replace an in-person consultation or the opinion of a qualified physician. Patients should seek timely medical attention and undergo treatment and rehabilitation under professional guidance.
Lateral patellar dislocation and medial retropatellar chondral fracture
