An 8-year-old female patient presented with a spontaneous patellar dislocation that was manually reduced by paramedics. At the emergency department the patella was clinically reduced and there was soft-tissue swelling. Ten months later, a second episode occurred.
On conventional radiograph of the knee, performed at the second episode of patellar dislocation, the patella appears laterally dislocated and there is soft-tissue swelling (Fig. 1). Lateral radiograph shows a patella alta and signs of joint effusion at the suprapatellar recess (Fig. 2).
MR of the knee revealed hypersignal on DP FS sequences at the lateral aspect of lateral femoral condyle and medial facet of patella (Fig. 3). Patellar position was high and laterally displaced. Trochlear dysplasia with medial condyle hypoplasia was observed. Medial patellofemoral ligament was stretched and hyperintense. Joint effusion was present.
Patellofemoral instability is a clinical syndrome due to morphologic or dynamic changes in patellofemoral joint that lead to anterior knee pain and predispose to recurrent lateral patellar dislocation. It is more common in young females in the second decade of life due to ligament laxity [1].
Acute patellar dislocation can be spontaneous or traumatic and if risk factors are present can become chronic and lead to patellofemoral osteoarthrosis.
Anatomic shape of the trochlea predisposes to lateral dislocation of patella, that is held in place by active stabilisers (extensor muscles) and passive stabilisers, which include bone morphology (patellar, trochlear and condyle shape) and ligaments (medial retinaculum, patellar tendon and, most important, medial patellofemoral ligament(MPFL)) [2].
The main risk factors of instability are patella alta, trochlear dysplasia, lateralisation of tibial tubercle and insufficient MPFL. Other factors include patellar dysplasia, increased femoral anteversion, ligament laxity and abnormal muscle tone [3].
Patella alta is evaluated by Insall-Salvati index, which is abnormal if >1,2 (Fig. 2). Trochlear dysplasia includes shallow sulcus angle (>145º) (Fig. 4), decreased lateral trochlear inclination (<11º) and Dejour classification is used to describe trochlear morphology (Fig. 5). Patellar translation is measured by TT-GT distance (tibial tubercle to trochlear groove), being abnormal above 20mm (Fig. 6). MPFL rupture can be partial or complete. Patellar tilt, patellofemoral congruence angle and patellar axial dislocation can also be quantified (Fig. 7) [4].
Sagittal knee radiograph may show patella alta, crossing sign or a trochlear bump. Axial views may show a laterally displaced patella or signs of trochlear dysplasia. CT scans are important to calculate TT-GT distance. MRI is the most complete technique, allowing not only morphologic evaluation of trochlea and patella, but also evaluation of ligaments, osseous contusions and osteochondral lesions [5]. Typical osseous contusions in patellar dislocation, called kissing contusions, occur at medial patellar facet and lateral femoral condyle, resulting from patellar impactation after lateral dislocation and reduction. Osteochondral defects may produce free intraarticular bodies, which can cause joint block. MPFL ligament injuries are best seen in MRI, appearing disrupted and hyperintense on T2-WI [2].
Treatment can be conservative in acute phase or, if recurrent dislocation, require surgical correction of bone defects or ligament injury [1].
Take home lessons: PFI predisposes to recurrent patellar dislocation. Imaging evaluation includes conventional radiography, CT and MRI to assess bone morphology and alignment. MRI also allows evaluation of ligaments and osteochondral lesions. Identifying the associated risk factors is important for surgical planning.
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Recurrent patellofemoral dislocation due to patellofemoral instability
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Based on the provided knee X-rays and MRIs, the following characteristics can be observed:
Overall imaging findings indicate structural abnormalities of the patellofemoral joint and signs of soft tissue injury in the patient, consistent with patellofemoral instability and recurrent patellar dislocation.
Based on the patient’s age of 8 years, history of two patellar dislocations, and the imaging findings, the possible diagnoses include:
All of the above diagnoses can lead to recurrent patellar dislocation and related complications, requiring detailed differentiation in conjunction with clinical examination and medical history.
Considering the child’s age, symptoms (recurrent patellar dislocation), previous similar episodes, and imaging findings (patellofemoral structural abnormalities, possible MPFL injury, patella alta), the most likely diagnosis is:
Recurrent Patellar Dislocation (Patellofemoral Instability)
If the degree of articular cartilage damage is uncertain, further MRI, arthroscopic evaluation, or higher-resolution imaging may be considered to clarify.
The management strategy depends on the frequency of dislocation, the extent of cartilage damage, and individual patient factors. It typically includes the following:
Throughout rehabilitation, follow the “FITT-VP” principle (Frequency, Intensity, Time, Type, Progression, and Individualization):
Throughout rehabilitation, consider the patient’s developing skeleton and avoid excessive loading or high-impact activities. If significant pain or worsening swelling occurs, re-evaluation and treatment modifications are recommended.
Disclaimer: This analysis report is intended for clinical reference only and does not substitute an in-person consultation or a professional physician’s advice. Specific diagnosis and treatment should be determined by medical professionals based on the patient’s actual clinical scenario.
Recurrent patellofemoral dislocation due to patellofemoral instability
