Anterior cruciate ligament (ACL) injury is one of the most common and potentially traumatic of all sports related injuries. Mechanisms underlying these injuries as a result of direct contact (eg., tackle) are typically evident from their clinical history. The mechanism/s underlying the 70% of sports related ACL injuries that occur as a result of a non-contact episode however remain elusive. A large percentage of non-contact injuries are reported to occur during execution of cutting or pivoting manoeuvres such as the sidestep cut. Despite this relationship however, there is limited quantitative data describing the knee joint movements associated with sidestepping. Further, the relationship between these movements and mechanical loading of the ACL is unknown. The purpose of this study was to first generate a detailed database describing the in vivo 3D knee rotations demonstrated during sidestepping and straight-line running. The second purpose was to develop a method capable of estimating in vivo ACL length changes during dynamic joint movements. Finally, this method was implemented to quantify in vivo ACL elongations occurring during the stance phase sidestepping and running. It was hoped that comparisons of the kinematic and elongation data between gait conditions would further elucidate the potential risks of non-contact ACL injury associated with sidestepping.
Knee joint (left and right) kinematic data were recorded during the stance phase of sidestepping (n = 5 trials) and running (n = 5 trials) for 20 male subjects (age = 21.7 ± 3.1 years) proficient in the sidestep cut. The three-dimensional coordinates of precisely attached skin markers were recorded via 4 high-speed (200Hz) video cameras, from which, descriptions of the three knee rotations (flexion-extension, adduction-abduction and external-internal rotation) were obtained from the relative positions of locally embedded segment coordinate systems. From these data, mean (±SD) maximum knee rotation values demonstrated during stance and the amount of variability (mean ± SD) displayed for each rotation between trials were calculated and compared between the two gait conditions. Sidestepping was found to produce significant (p<0.05) increases in three knee rotations (flexion, abduction and internal rotation) during stance compared to running. These increases however, were not deemed large enough to alone elicit noncontact ACL injury. A significant (p<0.05) increase in inter-trial variability was also observed in each of the three rotational degrees of freedom during sidestepping compared to running. Although the increased variability was felt to contribute to the potential risk of ACL injury, its direct impact could not be determined through an evaluation of rotational data only.
A method of estimating in vivo ACL length changes during dynamic joint motion was developed using combined high-speed video and magnetic resonance (MR) analyses. The 3D coordinates of skin markers, obtained via high-speed video cameras (200Hz) were again allowed knee joint motion to be quantified. A series of 2D MR scans enabled the 3D locations of ACL attachment sites, denoted by a single point, to be defined with respect to a reference marker (tibial tuberosity), with the position of this marker maintained during both series (video and MR) of analyses. Standard coordinate transformations were then used to define a mathematical relationship between knee motion and ligament attachment sites, allowing the straight-line length of a pseudo ligament fibre to be calculated. The method was validated for a series of static knee postures (0°, 10°, 15°, 20° and 30° flexion) and could successfully estimate ACL length changes demonstrated during a single sidestepping trial. Length estimations were also found not to be overly sensitive to errors arising from skin movement artifact and the identification of ligament attachment sites.
Changes in ACL length demonstrated during the stance phase of sidestepping (n = 5 trials) and running (n = 5 trials) were estimated for 7 male subjects (age = 22.6 ±1.4 years) using the above method. From these data, mean (±SD) maximum ACL length, maximum rate of length change and mean (±SD) variability displayed in maximum ACL length across trials were determined and compared between the two gait conditions. The mean maximum ACL length demonstrated during sidestepping (53.9 ± 2.3mm) was significantly (p<0.05) larger than that for running (49.0 ± 3.7mm), also occurring at a significantly (p<0.05) greater rate (cutting - 525.6 ± 100.9mm.sˉ¹; running - 256.1 ± 115.0mm.sˉ¹). Peak ACL lengths and rates of length change occurring during sidestepping are likely to subject the ligament to large strains and resultant stresses that may approach hazardous levels. Although increased variability in peak ACL length was not observed in sidestepping compared to running, this variability may still be significant in terms of injury potential. Considering the large stresses already imposed on the ACL during a normal sidestep, further small increases in ACL length are likely to be enough to compromise ligament integrity.
The results of this study suggest that ACL elongation demonstrated during typical sidestepping manoeuvres may place the ligament at an increased risk of injury, especially compared to more natural gait patterns such as running. Future studies would benefit from evaluating the potential causes of variation within and between individuals in ACL loading during sidestepping, such as the effects of gender, abnormal muscle strength and control and fatigue. Using the current method in conjunction with these analyses may enable the mechanism/s of non-contact ACL injury to be more readily transduced.