Elevated In Vivo ACL Strain Is Associated With a Straight Knee in Both the Sagittal and the Coronal Planes

Abstract

Background: Noncontact anterior cruciate ligament (ACL) injuries typically occur during deceleration movements such as landing or cutting. However, conflicting data have left the kinematic mechanisms leading to these injuries unclear. Quantifying the influence of sagittal and coronal plane knee kinematics on in vivo ACL strain may help to elucidate noncontact ACL injury mechanisms. Purpose/Hypothesis: The purpose of this study was to measure in vivo sagittal and coronal plane knee kinematics and ACL strain during a single-leg jump. We hypothesized that ACL strain would be modulated primarily by motion in the sagittal plane and that limited coronal plane motion would be measured during this activity. Study Design: Descriptive laboratory study. Methods: Seventeen healthy participants (8 male/9 female) underwent magnetic resonance imaging (MRI) followed by high-speed biplanar radiography, obtained as participants performed a single-leg jump. Three-dimensional models of the femur, tibia, and associated ACL attachment site footprints were created from the MRIs and registered to the radiographs to reproduce the position of the knee during the jump. ACL strain, knee flexion/extension angles, and varus/valgus angles were measured throughout the jump. Spearman rank correlations were used to assess relationships between mean ACL strain and kinematic variables. Results: Mean ACL strain increased with decreasing knee flexion angle (ρ = −0.3; P = .002), and local maxima in ACL strain occurred with the knee in a straight position in both the sagittal and the coronal planes. In addition, limited coronal plane motion (varus/valgus angle) was measured during this activity (mean ± SD, −0.5°± 0.3°). Furthermore, we did not detect a statistically significant relationship between ACL strain and varus/valgus angle (ρ = −0.01; P = .9). Conclusion: ACL strain was maximized when the knee was in a straight position in both the sagittal and coronal planes. Participants remained in <1° of varus/valgus position on average throughout the jump. As a ligament under elevated strain is more vulnerable to injury, landing on a straight knee may be an important risk factor for ACL rupture. Clinical Relevance: These data may improve understanding of risk factors for noncontact ACL injury, which may be useful in designing ACL injury prevention programs.

The Predicted Position of the Knee Near the Time of ACL Rupture Is Similar Between 2 Commonly Observed Patterns of Bone Bruising on MRI: Response

“…We believe that our previous work using imaging techniques to quantify in vivo ACL elongation and joint motion supports the findings of the present study, in that the previous studies suggested that knee extension elevates ACL strain 2-6,8,9,22-24 and puts the ligament at increased risk for injury. As noted previously, these techniques have advantages over cadaveric studies, which may not accurately reproduce in vivo loading conditions, 7 and videographic techniques, which may be susceptible to errors due to the viewpoint of the camera 3 and skin motion artifact.…”

“…15 Specifically, this study suggested that the knee was in a straight position in both the sagittal (\20°of flexion) and coronal (\10°of valgus) planes and underwent large anterior tibial translation (.20 mm) near the time of ACL rupture for both bone bruise patterns. 15 We believe that our previous work using imaging techniques to quantify in vivo ACL elongation and joint motion supports the findings of the present study, in that the previous studies suggested that knee extension elevates ACL strain [2][3][4][5][6]8,9,[22][23][24] and puts the ligament at increased risk…”

Bone morphology features associated with knee kinematics may not be predictive of ACL elongation during high‐demand activities

“…Those studies found that the ACL elongation increased during knee extension and decreased during flexion. The same pattern of ACL elongation was observed in more demanding activities, such as a single‐leg hop test [12] and the double‐legged drop jump [9, 29]. However, in fast running, ACL relative elongation increased during knee flexion immediately after foot strike and decreased during knee extension [29].…”