ACL Size, but Not Signal Intensity, Is Influenced by Sex, Body Size, and Knee Anatomy

Abstract

Background: Little is known about sex-based differences in anterior cruciate ligament (ACL) tissue quality in vivo or the association of ACL size (ie, volume) and tissue quality (ie, normalized signal intensity on magnetic resonance imaging [MRI]) with knee anatomy.

Hypothesis: We hypothesized that (1) women have smaller ACLs and greater ACL normalized signal intensity compared with men, and (2) ACL size and normalized signal intensity are associated with age, activity levels, body mass index (BMI), bicondylar width, intercondylar notch width, and posterior slope of the lateral tibial plateau.

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

Methods: Knee MRI scans of 108 unique ACL-intact knees (19.7 ± 5.5 years, 62 women) were used to quantify the ACL signal intensity (normalized to cortical bone), ligament volume, mean cross-sectional area, and length. Independent t tests were used to compare the MRI-based ACL parameters between sexes. Univariate and multivariate linear regression analyses were used to investigate the associations between normalized signal intensity and size with age, activity levels, BMI, bicondylar width, notch width, and posterior slope of the lateral tibial plateau.

Results: Compared with men, women had significantly smaller mean ACL volume (men vs women: 2028 ± 472 vs 1591 ± 405 mm3), cross-sectional area (49.4 ± 9.6 vs 41.5 ± 8.6 mm2), and length (40.8 ± 2.8 vs 38.1 ± 3.1 mm) (P < .001 for all), even after adjusting for BMI and bicondylar width. There was no difference in MRI signal intensity between men and women (1.15 ± 0.24 vs 1.12 ± 0.24, respectively; P = .555). BMI, bicondylar width, and intercondylar notch width were independently associated with a larger ACL (R 2 > 0.16, P < .001). Younger age and steeper lateral tibial slope were independently associated with shorter ACL length (R 2 > 0.03, P < .04). The combination of BMI and bicondylar width was predictive of ACL volume and mean cross-sectional area (R 2 < 0.3). The combination of BMI, bicondylar width, and lateral tibial slope was predictive of ACL length (R 2 = 0.39). Neither quantified patient characteristics nor anatomic variables were associated with signal intensity.

Conclusion: Men had larger ACLs compared with women even after adjusting for BMI and knee size (bicondylar width). No sex difference was observed in signal intensity, suggesting no difference in tissue quality. The association of the intercondylar notch width and lateral tibial slope with ACL size suggests that the influence of these anatomic features on ACL injury risk may be partially explained by their effect on ACL size.

Registration: NCT02292004 and NCT02664545 (ClinicalTrials.gov identifier).

Keywords: ACL; anatomy; sex differences; signal intensity.

Conflict of interest statement

One or more of the authors has declared the following potential conflict of interest or source of funding: This study received funding support from the Translational Research Program at Boston Children’s Hospital, the Children’s Hospital Orthopaedic Surgery Foundation, the Children’s Hospital Sports Medicine Foundation, Boston Children’s Hospital Faculty Council, the Football Players Health Study at Harvard University (funded by a grant from the National Football League Players Association), and the National Institutes of Health and the National Institute of Arthritis and Musculoskeletal and Skin Diseases (grants R01-AR065462 and R01-AR056834). The content is solely the responsibility of the authors and does not necessarily represent the official views of Harvard Medical School, Harvard University or its affiliated academic healthcare centers, the National Football League Players Association, Boston Children’s Hospital, or the National Institutes of Health. M.M.M. is a founder, paid consultant, and equity holder in Miach Orthopaedics, which was formed to work on upscaling production of the BEAR scaffold. M.M.M. has also received honoraria from the Musculoskeletal Transplant Foundation. B.C.F. is a paid associate editor for The American Journal of Sports Medicine and the spouse of M.M.M. with the inherently same conflicts. A.M.K. is a paid consultant for Miach Orthopaedics. D.K. and Y.-M.Y. have received education payments from Kairos Surgical. B.P. has manufactured the scaffolds used in the trials at Boston Children’s Hospital and is a paid consultant and equity holder in Miach Orthopaedics. N.S. has manufactured scaffolds used in the trials at Boston Children’s Hospital and is a paid consultant for Miach Orthopaedics. M.M.M., A.M.K., B.P., and N.S. maintained a conflict-of-interest management plan that was approved by Boston Children’s Hospital and Harvard Medical School during the conduct of the trial, with oversight by both conflict-of-interest committees and the institutional review board of Boston Children’s Hospital. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

© The Author(s) 2021.

Figures

Figure 1.

(A) Three-dimensional segmentation of the anterior cruciate ligament (ACL) from magnetic resonance (MR) image stacks, and measurement techniques used to quantify (B) bicondylar and notch widths (BCW and NW) and (C) posterior slope of the lateral tibial plateau. The dashed red lines indicate the line passing through the inferior aspects of the femoral condyles (B) and the reference lines to measure the posterior slope of the lateral tibial plateau (C).

Figure 2.

Representative images of the anterior cruciate ligament for a (A) female and a (B) male patient.

Figure 3.

Sex-based differences in anterior cruciate ligament (ACL) size (volume, mean cross-sectional area, and length) and normalized signal intensity. Bars are median and P values are 2-sided.

Figure 4.

Univariate regression analysis demonstrating the linear relationship of anterior cruciate ligament (ACL) size (volume, mean cross-sectional area [CSA], and length) and signal intensity with age. Red dots represent women and blue dots men. The regression line (solid line) and corresponding 95%CI (dashed lines) are shown for each univariate regression analysis.

Figure 5.

Univariate regression analysis demonstrating the linear relationship of anterior cruciate ligament (ACL) size (volume, mean cross-sectional area [CSA], and length) and signal intensity with Marx Activity Score. Red dots represent women and blue dots men. The regression line (solid line) and corresponding 95%CI (dashed lines) are shown for each univariate regression analysis.

Figure 6.

Univariate regression analysis demonstrating the linear relationship of anterior cruciate ligament (ACL) size (volume, mean cross-sectional area [CSA], and length) and signal intensity with body mass index (BMI). Red dots represent women and blue dots men. The regression line (solid line) and corresponding 95%CI (dashed lines) are shown for each univariate regression analysis.

Figure 7.

Univariate regression analysis demonstrating the linear relationship of anterior cruciate ligament (ACL) size (volume, mean cross-sectional area [CSA], and length) and signal intensity with bicondylar width (BCW). Red dots represent women and blue dots men. The regression line (solid line) and corresponding 95%CI (dashed lines) are shown for each univariate regression analysis.

Figure 8.

Univariate regression analysis demonstrating the linear relationship of anterior cruciate ligament (ACL) size (volume, mean cross-sectional area [CSA], and length) and signal intensity with intercondylar notch width (NW). Red dots represent women and blue dots men. The regression line (solid line) and corresponding 95%CI (dashed lines) are shown for each univariate regression analysis.

Figure 9.

Univariate regression analysis demonstrating the inverse relationship of anterior cruciate ligament (ACL) size (volume, mean cross-sectional area [CSA], and length) and signal intensity with steeper lateral tibial slope (LTS). Red dots represent women and blue dots men. The regression line (solid line) and corresponding 95%CI (dashed lines) are shown for each univariate regression analysis.

Figure A1.

Age distribution at the time of surgery for the included patients.