Little research specifically examines the risk of ACL injury related to weather conditions. Orchard et al73 examined environmental variables as risk factors for noncontact ACL injuries in male Australian football players and found that they were more common during periods of low rainfall and high evaporation. This combination results in a dry playing surface, which, theoretically, increases friction forces between the shoe surface and the playing surface.73 Orchard et al75 also observed a relative risk of 2.5 for suffering an ACL injury in dry weather conditions, with high water evaporation in the month prior.
Furthermore, epidemiological data reveal that injury incidence increases during games.26,31,75-77 This could be due to the intensity of play increasing by the athlete and his or her competitors. It has been speculated that competition is more physically demanding, with greater risk taken by the participants, compared to practice activities.38,78,79
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Anatomical dimensions are hypothesized to influence the gender differences in ACL incidence. Lower-extremity alignment, intercondylar notch size, and posterior tibial slope may increase strain on the ACL and predispose individuals to injury. Many studies have looked at specific structures of the lower extremities and their influence on ACL injury rates.70
Lower-extremity alignment is recognized by the International Olympic Committees current concept statement as an internal risk factor for ACL injury. Examining the position of the hip, knee, and ankle and noting abnormal posture can cue a clinician into increased ACL strain.70 This being said, scarce literature supports alignment and its relationship to ACL injury, but it is still recommended as good clinical practice.
Intercondylar Notch Size
Geometric differences in the ACL have been examined heavily but are not well described. There is a lack of standardization in the methods used to obtain these data. Notch width of unilateral and bilateral ACL patients are smaller than notch widths of normal controls. Therefore, a strong relationship exists between small notch size and ACL injury.80 Furthermore, there is evidence that the notch width of patients who have suffered bilateral ACL injury is smaller than those with unilateral injury to the ACL.80 Notch size has also been shown to differ between genders, with females having a femoral intercondylar notch width smaller than that of males,81 but some argue that there is no conclusive correlation between ACL size and notch dimension in relation to risk of ACL injury.82 Comparatively, when normalizing for body mass index, women have a geometrically smaller ACL than men. The physical properties of the ACL may differ between the sexes, as well.70 The female ACL is smaller than the male ACL. The female ACL shows lower mechanical properties compared to the male ACL. These differences are not simply due to the size of the ACL, but to the sex-based differences in the ACL itself. The female ACL has been found to have an 8.3% lower strain (relative change in shape or size due to external force) at failure, 14.3% lower stress (internal force associated with strain) at failure, 9.43% lower strain energy (energy stored during deformation) density at failure, and 22.49% lower modulus of elasticity (resistance to being deformed elastically) compared to the male ACL and considering factors such as age and anthropometric measurements.83
Posterior Tibial Slope
Increased anterior translation has been linked to a posteroinferior tibial slope (Figure 8-1).80 Magnetic resonance imaging can be utilized to measure posterior tibial slope. One study revealed that subjects with ACL-deficient knees had a significantly greater slope of the lateral tibial plateau and a lower slope of the medial tibial plateau than the control group.84