Propagation of Syndesmotic Injuries During Forced External Rotation in Flexed Cadaveric Ankles.

BACKGROUND: Forced external rotation of the foot is a mechanism of ankle injuries. Clinical observations include combinations of ligament and osseous injuries, with unclear links between causation and injury patterns. By observing the propagation sequence of ankle injuries during controlled experiments, insight necessary to understand risk factors and potential mitigation measures may be gained. HYPOTHESIS: Ankle flexion will alter the propagation sequence of ankle injuries during forced external rotation of the foot. STUDY DESIGN: Controlled laboratory study. METHODS: Matched-pair lower limbs from 9 male cadaveric specimens (mean age, 47.0 ± 11.3 years; mean height, 178.1 ± 5.9 cm; mean weight, 94.4 ± 30.9 kg) were disarticulated at the knee. Specimens were mounted in a test device with the proximal tibia fixed, the fibula unconstrained, and foot translation permitted. After adjusting the initial ankle position (neutral, n = 9; dorsiflexed, n = 4; plantar flexed, n = 4) and applying a compressive preload to the tibia, external rotation was applied by rotating the tibia internally while either lubricated anteromedial and posterolateral plates or calcaneal fixation constrained foot rotation. The timing of osteoligamentous injuries was determined from acoustic sensors, strain gauges, force/moment readings, and 3-dimensional bony kinematics. Posttest necropsies were performed to document injury patterns. RESULTS: A syndesmotic injury was observed in 5 of 9 (56%) specimens tested in a neutral initial posture, in 100% of the dorsiflexed specimens, and in none of the plantar flexed specimens. Superficial deltoid injuries were observed in all test modes. CONCLUSION: Plantar flexion decreased and dorsiflexion increased the incidence of syndesmotic injuries compared with neutral matched-pair ankles. Injury propagation was not identical in all ankles that sustained a syndesmotic injury, but a characteristic sequence initiated with injuries to the medial ligaments, particularly the superficial deltoid, followed by the propagation of injuries to either the syndesmotic or lateral ligaments (depending on ankle flexion), and finally to the interosseous membrane or the fibula. CLINICAL RELEVANCE: Superficial deltoid injuries may occur in any case of hyper-external rotation of the foot. A syndesmotic ankle injury is often concomitant with a superficial deltoid injury; however, based on the research detailed herein, a deep deltoid injury is then concomitant with a syndesmotic injury or offloads the syndesmosis altogether. A syndesmotic ankle injury more often occurs when external rotation is applied to a neutral or dorsiflexed ankle. Plantar flexion may shift the injury to other ankle ligaments, specifically lateral ligaments.

Origin of directionally tuned responses in lower limb muscles to unpredictable upper limb disturbances.

Unpredictable forces which perturb balance are frequently applied to the body through interaction between the upper limb and the environment. Lower limb muscles respond rapidly to these postural disturbances in a highly specific manner. We have shown that the muscle activation patterns of lower limb muscles are organized in a direction specific manner which changes with lower limb stability. Ankle muscles change their activity within 80 ms of the onset of a force perturbation applied to the hand which is earlier than the onset of changes in ground reaction force, ankle angle or head motion. The latency of the response is sensitive to the perturbation direction. However, neither the latency nor the magnitude of the response is affected by stiffening the arm even though this alters the magnitude and timing of motion of the body segments. Based on the short latency, insensitivity of the change in ankle muscle activation to motion of the body segments but sensitivity to perturbation direction we reason that changes in ankle muscle activation are most likely triggered by sensory signals originating from cutaneous receptors in the hand. Furthermore, evidence that the latency of changes in ankle muscle activation depends on the number of perturbation directions suggests that the neural pathway is not confined to the spinal cord.

Effects of amplitude and predictability of perturbations to the arm on anticipatory and reactionary muscle responses to maintain balance.

Disturbances to balance arising from forces applied to the upper limb have received relatively little attention compared to disturbances arising from support surface perturbations. In this study we applied fast ramp perturbations to the hand in anterior, posterior, medial and lateral directions. The effects of perturbation predictability and amplitude on the postural response of upper limb, trunk and lower limb muscles were investigated. Perturbations were applied either in blocks of constant amplitude and direction (predictable) or with direction and amplitude varying randomly (random) from trial to trial. The spatial-temporal patterns of anticipatory muscle activation under the predictable condition and the reactionary responses following the perturbation under both conditions were similarly organized. The size of the response increased systematically with the perturbation magnitude for both anticipatory and reactionary changes in muscle activation. However, the slope of the relation between perturbation amplitude and the magnitude of the change in muscle activation was greater when perturbations were predictable than when they were randomly selected. The timing of both the anticipatory and reactionary increases in muscle activation was invariant across perturbation amplitudes. The characteristics of the reactionary responses have a similar organization to the long latency muscle responses to support surface perturbations.

Short-latency muscle response patterns to multi-directional, unpredictable perturbations to balance applied to the arm are context dependent.

A number of studies have shown that sensory inputs from the hand can have a profound effect in stabilizing upright posture. This suggests that the central nervous system can extract information about body motion and external forces acting on the body from cutaneous sensory signals. We have recently shown that the central nervous system determines the direction of an unpredictable force applied to the hand so rapidly that it is able to activate ankle muscles in advance of the perturbing effect that this force has at the ankles. In this study we investigate whether this rapid change in activation of lower limb muscles is an invariant response determined by the pattern of somatosensory information arising from sensory receptors in the hand or whether it adapts to changes in postural stability. We manipulated lateral stability of upright stance by changing stance width which had no effect on the activation of upper limb muscles or hand kinematics, but produced profound changes in the activation patterns of lower limb muscles when perturbations were in the medial/lateral direction without affecting the activation patterns of muscles when perturbations were in the anterior/posterior direction.

Postural response characterization of standing humans to multi-directional, predictable and unpredictable perturbations to the arm.

When the arm of a standing human is perturbed in an unpredictable direction, postural muscles are activated at latencies as short as 50-110ms. While the motion of the body clearly progresses in hand-to-leg sequence, there is no systematic muscle activation sequence from the arm to the leg muscles, suggesting that the activation of the muscles is not likely the result of local stretch reflexes. In fact, the lower limb muscles are activated before the upright posture is significantly disturbed. The short-latency activation amplitude and the activation probability are clearly tuned to the direction of the arm perturbation for both rostral and caudal muscles. The effect of central set on the short-latency response has been investigated by manipulating the predictability of the perturbations. Possible underlying neural mechanisms have been discussed.

Whole-body Response for Pedestrian Impact with a Generic Sedan Buck.

To serve as tools for assessing injury risk, the biofidelity of whole-body pedestrian impact dummies should be validated against reference data from full-scale pedestrian impact tests. To facilitate such evaluations, a simplified generic vehicle-buck has been recently developed that is designed to have characteristics representative of a generic small sedan. Three 40 km/h pedestrian-impact tests have been performed, wherein Post Mortem Human Surrogates (PMHS) were struck laterally in a mid-gait stance by the buck. Corridors for select trajectory measures derived from these tests have been published previously. The goal of this study is to act as a companion dataset to that study, describing the head velocities, body region accelerations (head, spine, pelvis, lower extremities), angular velocities, and buck interaction forces, and injuries observed during those tests. Scaled, transformed head accelerations exceeded 80 g prior to head contact with the windshield for two of the three tests. Head xaxis angular velocity exceeded 40 rad/s prior to head contact for all three tests. In all cases the peak resultant head velocity relative to the vehicle was greater than the initial impact speed of the vehicle. Corridors of resultant head velocity relative to the vehicle were also developed, bounded by the velocities observed in these tests combined with those predicted to occur if the PMHS necks were perfectly rigid. These results, along with the other kinematic and kinetic data presented, provide a resource for future pedestrian dummy development and evaluation.