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Document Title: Chandrashekar-JB-Dec06.shtml
Article Title: Sex-based differences in the tensile properties of the human anterior cruciate ligament
Authors: Naveen Chandrashekar, Hossein Mansouri, James Slauterbeck, Javad Hashemi
Publication: Journal of Biomechanics
Date: December 2006
Volume 39, Issue: 16, pages 2943-2950
Keywords: Sex/Gender, ACL injury, structural/mechanical properties, ACL cross-sectional area, cadaveric study
(Reference-denoting numbers appear in the same font and point size as the document text. As with all Knee Library documents, this article is provided in full-text form, complete with all figures and tables.)
Comments: This brilliant and penetratingly insightful study shows that the mechanical properties of the female ACL are distinctly inferior to those of the male ACL. Chandrashekar notes that the female ACL has an 8.3% lower strain at failure (i.e. it deforms less prior to disintegrating, so there is less warning), 14.3% lower stress at failure (i.e. less applied loading, hence less force, is required to get it to tear), 9.43% lower strain energy density at failure (in practice, this means less force is required to deform a given unit of the female ACL, i.e. strength per unit volume, and therefore strength per unit cross-sectional area, is lower), and, most important, 22.49% lower modulus of elasticity (i.e. less stretchiness, therefore more brittle, so it stretches less and fails sooner -- and less stretching means less of a timeframe in which the ligament's embedded tension-sensitive nerve endings can signal pain and trigger the ACL-protective hamstring reflex). So, from a materials-science point of view, the female ACL is of poorer quality. Comparing these attributes of a male and female ACL is analogous to comparing a cotton rope to a high-strength nylon one. This study builds on the equally important findings of its predecessor, which showed that the female ACL has a markedly smaller cross-section than that of its male counterpart. This structural difference, combined with the aforementioned differences in mechanical properties, helps explain why females have a 2-8 times higher ACL-injury incidence.
Abstract
After immense amounts of research, the root cause for the significantly higher rates of anterior cruciate ligament (ACL) failure incidents in females as compared to males still remains unknown and the existing sex-based disparity has not diminished. To date, the possibility that the female ACL is mechanically weaker than the male ACL has not been directly investigated. Although it has been established in the literature that the female ACL is smaller in size, the differences in the structural and material properties of the ACL between sexes have not been studied. The aim of this cadaveric study was to determine if any sex-based differences in the tensile properties of the human ACL exist when considering age as well as ACL and body anthropometric measurements as covariates. Ten male and 10 female unpaired cadaveric knees (mean age 36.75 years) were used for this study. The geometry of the ACL (including length, cross-sectional area, and volume) was analyzed using a 3-D scanning system. The femur-ACL tibia complex was tested to failure along the longitudinal axis of the ligament in a tensile testing machine. The structural properties of the ACL as well as its mechanical properties were determined. Analysis of covariance was performed to assess the effect of sex on tensile properties. The female ACL was found to have a lower mechanical properties (8.3% lower strain at failure; 14.3% lower stress at failure, 9.43% lower strain energy density at failure, and most importantly, 22.49% lower modulus of elasticity) when considering age, ACL, and body anthropometric measurements as covariates.
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