Abstract
Background:
Patellar instability (PI), although rare, occurs most often in younger patients with underlying pathoanatomy. Patellar tilt is one of many identified risk factors for PI. Lateral patellar inclination (LPI) angle measures patellar tilt relative to the distal femur as viewed on the axial MRI with the widest cross-section of the patella. The LPI measurement has historically been described with reference to the posterior aspect of the femur at the same transverse level as the widest cross-section of the patella. Given the transitional anatomy of the distal femur and variable heights of the patella, the LPI may be better represented by referencing the axis of the fully formed posterior condyles. The posterior condyles represent a true axis of rotation that serves as an internal reference for knee motion and are clearly visible on MRI. Normative values for LPI have not been established and accepted in the literature; and, proposed maximum thresholds range from 10° to 20°. We hypothesized that a modified LPI measurement (LPI) referencing the posterior condylar axis would be different from the apparent LPI (ALPI) as measured historically in a pediatric and adolescent population. Given the clarity of fully formed posterior condyles as compared with the posterior femur transitioning from the shaft to the condyles, we also hypothesized that the modified LPI would have higher inter- and intra-rater reliability than the ALPI measurements.
Methods:
Patients aged 9 to 18 years and treated for PI at our tertiary referral center between January 2014 and August 2017 were identified. The ALPI measurement was made as previously described on axial MRI images (Figure 1). The LPI was measured with reference to the posterior condylar orientation (Figure 2 A-B). All measurements were performed by two independent observers. A cohort was randomly selected from these patients and measurements were performed for this cohort by three independent observers and re-performed after a minimum of two weeks. Inter- and intra-rater correlation coefficients were calculated for this subgroup and regression analysis was performed on the entire cohort.
Results:
Sixty-five patients met inclusion criteria for this study, and thirty patients were randomly selected for reliability analysis. The ALPI inter-rater reliability (ICC = 0.708, 95% CI: 0.47 – 0.85) showed good agreement while that of the LPI (ICC = 0.885, 95% CI: 0.77 – 0.95) also showed good agreement with less variability. Intra-rater reliability for ALPI (ICC = 0.975, 95% CI: 0.95-0.99) was similar to the intra-rater reliability for LPI (ICC = 0.975, 95% CI: 0.95-0.99). In the entire cohort of 65 patients, the average ALPI (14.6+/-9.9°) was 6.1+/-3.4° less than the average LPI (19.6+/-9.4°) (p = 0.037). Referencing a previously described maximum threshold of 13.5° for LPI, 46% of the patients in the cohort had normal tilt based on ALPI measurements while only 27% of patients in the cohort had normal tilt based on new LPI measurements.
Conclusion:
The LPI measurement has similar inter and intra rater reliability with less inter-rater variability compared to the historical measurement (ALPI). A significant difference in patellar tilt was found between the two techniques of measuring LPI with the historical technique underestimating the amount of tilt present. Furthermore, a larger percentage of this PI patient cohort would be classified as having normal patellar tilt based on the ALPI measurement than would be classified as abnormal based on the LPI measurement. When assessing children and adolescents with symptomatic PI, the LPI measurement referencing the posterior condylar axis provides more consistent and more representative measurements of the true patellar tilt. Previously described threshold values for patellar tilt should be re-examined using this new measurement technique to appropriately risk stratify patients with PI as historical measurements appear to underestimate this pathoanatomy.
Apparent lateral patellar inclination (ALPI) angle. The angle subtended between a line drawn through the widest portion of the patella and a line drawn along the posterior axis of the femur is measured on a single axial MRI image. In the case the ALPI is 18°.
Lateral patellar inclination (LPI) angle.