Lateral Patellar Dislocation
A 15-year-old female Gymnastics injury. Diagnosis? • Radiology Case of the Week
Figure 1. A 15-year-old female Gymnastics injury. Diagnosis?
Figure 2. A. Cross-table lateral radiograph of the left knee. The patella (red arrows) is projected over the anterior femoral condyles (yellow arrow).
B. Anteroposterior radiograph of the left knee. The patella (red arrows) is displaced laterally and lies beside the lateral femoral condyle (yellow arrow).
C. Axial patellofemoral view of the left knee. The patella (red arrow) lies lateral to the lateral femoral condyle (yellow arrow), outside the trochlear groove (green arrow)..
Epidemiology
Lateral patellar dislocation is a common knee injury, particularly in children and adolescents. Acute patellar dislocation affects approximately 1 in 1000 healthy children aged 9–15 years, with the highest incidence occurring during adolescence. The injury is frequently associated with acute trauma or sporting activity [1,2].
Clinical Findings
Patients typically present with acute knee pain, swelling, and limited range of motion following trauma. The patella frequently reduces spontaneously during knee extension. When persistent, the patella remains displaced laterally. Joint effusion and tenderness along the medial retinaculum are common, and the patellar apprehension test may be positive [2]. In the present case, the patient is a 15-year-old female and presents with lateral patellar dislocation following trauma.
Pathology Lateral patellar dislocation results from displacement of the patella from the trochlear groove, with injury to the medial patellar stabilizers, particularly the medial patellofemoral ligament (MPFL) [3,5]. The patella may impact the lateral femoral condyle during dislocation, resulting in chondral or osteochondral injury. Predisposing anatomic factors include trochlear dysplasia, patella alta, lateral patellar tilt or subluxation, increased Q angle, femoral anteversion, and lateralization of the patellar tendon insertion [3,5].
Classification Patellar dislocations are classified according to the direction of displacement as lateral, medial, superior, or inferior. Lateral dislocation is the most common and typically occurs when the patella is displaced laterally from the trochlear groove. They may also be categorized as acute, recurrent, or chronic based on the clinical course [4].
Radiographic Features
The standard radiographic series includes anteroposterior (AP), true lateral, and axial (sunrise) views. In this case, the patella is laterally displaced on the AP view and lies outside the trochlear groove on the axial view, consistent with lateral patellar dislocation (Fig. 2 C). Radiographs may also demonstrate joint effusion, lipohemarthrosis, osteochondral fracture of the medial patella, lateral patellar tilt or subluxation, and a deep lateral sulcus sign resulting from impaction of the lateral femoral condyle [5].
MRI is useful for further evaluation of associated soft-tissue and osteochondral injuries. Typical MRI findings include injury or disruption of the medial patellofemoral ligament (MPFL), bone marrow edema or contusions involving the medial patella and lateral femoral condyle, and predisposing anatomic abnormalities such as trochlear dysplasia and patella alta [3,5].
Treatment and Prognosis
Initial management of a first-time lateral patellar dislocation generally involves reduction when necessary, followed by nonoperative treatment with bracing and rehabilitation focused on restoring range of motion and strengthening the quadriceps and medial stabilizers [6,7]. Surgical treatment may be considered in the presence of an associated osteochondral fracture, recurrent instability, or significant predisposing anatomic abnormalities [6,7]. Although many patients recover with conservative treatment, recurrent instability remains an important complication.
Conclusion
Lateral patellar dislocation is a common injury in adolescents and should be recognized promptly on radiographs. Assessment of patellar alignment, associated osteochondral injury, and underlying anatomic risk factors is important for appropriate management and prediction of recurrence.
References:
Sanders TL, Pareek A, Hewett TE, Stuart MJ, Dahm DL, Krych AJ. Incidence of First-Time Lateral Patellar Dislocation: A 21-Year Population-Based Study. Sports Health. 2018;10(2):146-151. DOI: 10.1177/1941738117725055
Gao B, Shi Y, Zhang F. Pediatric patellar dislocation. Minerva Pediatr. 2020;72(1):65-71. DOI: 10.23736/S0026-4946.17.04882-4
Earhart C, Patel DB, White EA, Gottsegen CJ, Forrester DM, Matcuk GR Jr. Transient lateral patellar dislocation: review of imaging findings, patellofemoral anatomy, and treatment options. Emerg Radiol. 2013;20(1):11-23. DOI: 10.1007/s10140-012-1073-9
Parikh SN, Lykissas MG. Classification of Lateral Patellar Instability in Children and Adolescents. Orthop Clin North Am. 2016;47(1):145-152. DOI: 10.1016/j.ocl.2015.08.016
5.Kim HK, Parikh S. Patellofemoral Instability in Children: Imaging Findings and Therapeutic Approaches. Korean J Radiol. 2022;23(6):674-687. DOI: 10.3348/kjr.2021.0577
Smith TO, Song F, Donell ST, Hing CB. Operative versus non-operative management of patellar dislocation. A meta-analysis. Knee Surg Sports Traumatol Arthrosc. 2011;19(6):988-998. DOI: 10.1007/s00167-010-1355-2
Parikh SN, Schlechter JA, Veerkamp MW, et al. Consensus-Based Guidelines for Management of First-Time Patellar Dislocation in Adolescents. J Pediatr Orthop. 2024;44(4):e369-e374. DOI: 10.1097/BPO.0000000000002616
Umaiza Ihsan, MD is a medical graduate of King Edward Medical University, Pakistan. She completed her internship at Mayo Hospital, Lahore, including a three month rotation in Radiology, which further strengthened her interest in diagnostic imaging. She has gained additional clinical exposure in Radiology through observerships at Mon Health Morgantown and the University of Maryland, Baltimore. She has a strong interest in Radiology, particularly diagnostic imaging and its role in clinical decision making, and is actively interested in clinical research, with experience in systematic reviews and medical research. All posts by Umaiza Ihsan, MD
Kevin M. Rice, MD is the president of Global Radiology CME and is a radiologist with Cape Radiology Group. He has held several leadership positions including Board Member and Chief of Staff at Valley Presbyterian Hospital in Los Angeles, California. Dr. Rice has made several media appearances as part of his ongoing commitment to public education. Dr. Rice's passion for state of the art radiology and teaching includes acting as a guest lecturer at UCLA. In 2015, Dr. Rice and Natalie Rice founded Global Radiology CME to provide innovative radiology education at exciting international destinations, with the world's foremost authorities in their field. In 2016, Dr. Rice was nominated and became a semifinalist for a "Minnie" Award for the Most Effective Radiology Educator. He was once again a semifinalist for a "Minnie" for 2021's Most Effective Radiology Educator by AuntMinnie.com. He has continued to teach by mentoring medical students interested in radiology. Everyone who he has mentored has been accepted into top programs across the country including Harvard, UC San Diego, Northwestern, Vanderbilt, and Thomas Jefferson.
Follow Dr. Rice on Twitter @KevinRiceMD
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