Reliability and Validity of an Ultrasound-Derived Measure for Axial Patellofemoral Alignment

Abstract

Background: Axial extensor mechanism alignment is routinely assessed in patients with patellofemoral instability.Although many of these assessments are described using magnetic resonance imaging, it is plausible that ultrasound (US) imaging could be utilized to provide similar information in a more cost-effective and time-efficient manner. Purpose: To (1) describe and assess the reliability of a novel measure of extensor mechanism alignment of the patellofemoral joint using musculoskeletal US and (2) establish the construct validity of this measure through comparison of patients with and without patellar instability. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: Patients with (n = 24; 14.2 ± 3.1 years; 83% female) and without (n = 26; 14.7 ± 2.8 years; 69% female) a clinical history of patellofemoral instability (PFI) participated. Extensor mechanism alignment was assessed bilaterally on US, measuring the distance between the midpoint of the patellar tendon to the lateral trochlear ridge (MPT-LTR). Interrater reliability of the measurements was assessed using the intraclass correlation coefficient (ICC), with a minimum of 1 week between measurements for test-retest reliability. Differences between limbs were assessed using paired-samples t tests, and between-group differences were compared using independent-samples t tests. Results: Patients with PFI demonstrated a significantly smaller MPT-LTR distance than healthy controls on both their involved (8.1 ± 3.6 vs 12.6 ± 2.6 mm) and uninvolved (8.9 ± 3.4 vs 12.9 ± 2.4 mm) limbs ( P < .001 for both), indicating greater lateralization of the patellar tendon relative to the trochlea. There were no differences found between limbs for either patients with PFI ( P = .26) or controls ( P = .46). Interrater reliability was good (ICC = 0.785; 95% confidence interval [CI], 0.579-0.890), and test-retest reliability (n = 8) was excellent (ICC = 0.958; 95% CI, 0.790-0.992). Conclusion: The US-based MPT-LTR distance demonstrated good-to-excellent reliability. When compared with controls, the MPT-LTR distance was smaller in patients with PFI, indicating greater lateralization of the extensor mechanism.