Traumatic patellar dislocation in childhood : late effects on knee function and cartilage quality

Abstract

Background and aim: Acute patellar dislocation affects approximately 1:1000 healthy children 9- 15 years of age, and up to 50% are at risk for recurrent dislocations. In adults the condition is associated with long-term complications, such as osteoarthritis and impairment of knee function. However, literature describing the outcome in a pediatric population is sparse. The aim of this thesis was to evaluate the long-term effects on knee function and cartilage quality after traumatic patellar dislocation in childhood, and also to evaluate the reliability of two clinical tests of medio-lateral knee position, in healthy children.

Patients and methods: In Study I, 246 healthy children were included to evaluate the Q-angle and the Single-limb mini squat test, reflecting static and dynamic medio-lateral knee position, respectively. In Study II, III and IV patients with a history of acute, unilateral, first-time traumatic patellar dislocation, 9-15 years of age at index injury, and with a follow-up time of ≥5 years were included. Recurrence rate and patient reported outcome were assessed. In Study II, n=52, the objective- and subjective knee function were evaluated in relation to type of obtained treatment. In Study III and IV the quality of the patellar cartilage was evaluated with quantitative MRI metrics, T2 mapping and delayed Gadolinium Enhanced Magnetic Resonance Imaging of Cartilage (dGEMRIC). In Study III 16 non-operatively treated patients with recurrent patellar dislocation, and in Study IV 17 patients surgically treated in childhood due to unilateral recurrent patellar dislocation, were evaluated.

Results: In Study I the reliability for the Single-limb mini squat test was determined moderate, the Q-angle measurement was found to have fair to moderate reliability. The Q-angle varied with age and sex; however, this finding may not be clinically relevant. Study II, III and IV reveal that the patients were affected KOOS quality of life and sports and recreation, with lower scores than normal for the age. 67% reported recurrence among the non-operatively treated patients. Despite regained stability in patients operated on due to recurrences, the subjective knee function was not restored. In both the non-operatively treated patients with recurrent patellar dislocations (Study III), and the surgically stabilized patients (Study IV), very early cartilage changes were detected in the patellar cartilage of the affected knee with dGEMRIC and T2 mapping. The shortening of T1(Gd) indicate loss of glycosaminoglycans. The localization of the findings were similar in Study III and IV, although, at different tissue depths; with changes in the superficial half of the cartilage in patients with recurrent dislocations (study III), and changes in the deep half in the operated patients (study IV). In Study III shorter T2 values were detected in superficial half of the cartilage in the peripheral parts of affected patella, whereas longer T2 was observed most medially in the deep cartilage of the operated group (Study IV).

Conclusion: The Single-limb mini squat test can be used to evaluate the medio-lateral knee position in a pediatric population, whereas the Q-angle only showed fair reliability. Acute traumatic patellar dislocation in childhood has a negative long-term impact on quality of life and ability to participate in physical activities. Traditional surgical methods reduced the recurrence rate, but the knee function was not restored.

Recurrent patellar dislocation, and patellar stabilizing surgery, seem to have a negative effect on cartilage quality; most likely through different biological mechanisms and at different depths of the cartilage. The results from quantitative MRI of the patellar cartilage indicate changes in both GAG content and collagen structure. These new findings show that dGEMRIC, in combination with T2 mapping, are feasible methods to detect early degenerative changes in vivo in this condition.