Measurement characteristics of a force-displacement curve for chronic patellar instability.

OBJECTIVE

The clinical diagnosis of patellar instability is subjective, depending on the patient's apprehension. We hypothesized that a subjective diagnosis could be supported by a numerical evaluation of the slope, or compliance, of the early phase of a force-displacement curve.

DESIGN

Cross-sectional control group comparison.

SETTING

A university-based sports clinic.

PARTICIPANTS

Healthy volunteers (n = 21; 16-40 years old) and patients (n = 21; 15-34 years old) who had a clearly definable diagnosis of unilateral dislocation, followed by chronic anterior knee pain, and positive apprehension signs for more than 6 months, without patella alta or abnormal limb alignment.

INTERVENTION

Lateral displacement and reactive forces were measured with a Patella Stability Tester.

MAIN OUTCOME MEASURES

Sensitivity in detecting injured knees was defined, with the 95% confidence interval of healthy subjects determined as the normal range.

RESULTS

Compliance at 5 mm in displacement showed the highest reproducibility (plot difference of 4.7%) and the highest sensitivity (95%). Among injured knees, compliance was significantly correlated with the activity-related symptoms of the Kujala score (correlation coefficient, -0.61; P = 0.004). Compliance at 5 mm of displacement showed the highest sensitivity (95%) and significant correlation with the activity-related symptoms of the Kujala score among injured knees (coefficient of determination, 0.37; P = 0.004).

CONCLUSIONS

It is a novel finding that early compliance at 5 mm of displacement showed a correlation with the subjective diagnosis and symptoms.