Variations in varus/valgus and internal/external rotational knee laxity and stiffness across the menstrual cycle

Abstract

Cyclic variations in genu recurvatum (GR), general joint laxity (GJL), varus-valgus (VV), and internal-external (IER) rotational laxities and stiffnesses were examined in 64 females and 43 males at two time points during the females' menstrual cycle [days of minimum (T1) and maximum (T2) anterior knee laxity (AKL)]. Cyclic increases in AKL (9.5%), GR (37.5%), and GJL (13.6%) were observed in females but not males from T1 to T2 (p < 0.001). Cyclic increases in VV and IER laxity were negligible (1.5-3.2%, p > 0.320). Females compared to males had lower overall VV stiffness at T2 (F 37% <M) vs. T1 (F 26.9% <M; p = 0.011), but no difference across time points for IER stiffness (p = 0.452). Across both time points, females had consistently greater VV (30.2%) and IER (20%) laxity and less VV (32.5%) and IER (24.3%) incremental stiffness (p < 0.001). Low-to-moderate associations were observed between AKL, GR, and GJL with VV and IER laxities and stiffnesses in females as measured at T1 and the change in values from T1 to T2. Whether these findings reflect ligament-specific responses to hormone changes, or implicate changes in injury risk potential across the menstrual cycle requires further study.

Figure 1

Exemplar load–displacement curve during the application of (a) 5 Nm external–internal and (b) 10 Nm varus–valgus rotation torques to the knee. Graphic representation of incremental stiffness and laxity calculations are provide for internal rotation and varus loadings. Each incremental stiffness is calculated as the slope (ΔNm/Δ°) of the line between each torque increment (K₀–₁ = 0 to 1 Nm loading, K_1–2 = 1 to 2 Nm loading, etc., for internal and external rotation; K_0–2 = 0 to 2 Nm loading, K_2–4 = 2 to 4 Nm loading, etc., for varus and valgus rotation). Laxity is calculated as the amount of rotation from 0 to 5 Nm (internal, external rotation) or 0–10 Nm (varus, valgus rotation).

Figure 2

Incremental varus (A), valgus (B), internal (C), and external (D) torsional stiffness (see Supplementary Table 1 for full ANOVA summary results).

Figure 3

Frequency distribution reporting the percentage of cases for each sex that experienced positive versus negative delta values for (a) VV and (b) IER laxities.