A Comparison of Notch Width Index Differences When Measured on Axial and Coronal MRI.

Abstract

Background: There are several known risk factors for ACL injury including knee valgus moment, smaller notch width index (NWI), female sex, increased tibial slope, and meniscal deficiency. Among these, the NWI has typically been determined using either coronal or axial magnetic resonance imaging (MRI) cuts; however, it is unknown whether these measurements differ in each plane.

Purpose: To compare axial versus coronal NWI measurements in ACL-injured patients to establish a benchmark for calculation.

Study design: Cross-sectional study; Level of evidence, 3.

Methods: Patients ≥14 years of age with a confirmed ACL injury on MRI were included for review. Two orthopaedic resident physicians measured the NWI using both axial and coronal cuts using previously described measurement criteria as well as a modification utilizing the MRI cuts with the widest bicondylar width. The NWI was computed and a t test was performed to compare the axial and coronal measurements. Interobserver reliability was assessed using 2-way mixed intraclass correlation coefficients (ICCs).

Results: A total of 64 patients with ACL injuries were included. Modified axial and coronal NWIs for the cohort were a mean ± SD of 0.27 ± 0.02 and 0.23 ± 0.02, respectively. Modified axial NWIs were greater than coronal NWIs (P < .001). Standard axial and coronal NWIs for the cohort were 0.29 ± 0.03 and 0.25 ± 0.03, respectively. Standard axial NWIs were greater than coronal NWIs (P < .001). Modified axial and coronal NWIs for male and female patients were not different (P = .61 and P = .14, respectively). Axial notch width measurements were greater than coronal notch widths for both modified and standard measurements (P < .001). Axial intercondylar distances were not different from coronal intercondylar distances for both standard and modified measurements (P = .06 and P = .22, respectively). The ICCs for modified coronal and axial NWIs were 0.88 and 0.97, respectively. The ICCs for standard coronal and axial NWIs were 0.99 and 0.99, respectively.

Conclusion: NWIs were greater when measured on an axial MRI compared with the coronal plane for both measurement methods. This was because notch widths were measured wider in this plane since intercondylar distances were not statistically different.