Alli Gokeler

An injury of the anterior cruciate ligament (ACL) is one of the most common injuries in sports. In particular, young female athletes are at risk to sustain this devastating injury. It has become standard practice since the early nineties of the past millennium to perform an ACL reconstruction (ACLR) with the ultimate goal to allow athletes to resume their sports as prior to the injury. The two most common ACLR procedures are either the use of an autologous graft from the patellar tendon or the semintendinosis-gracilis tendon, of which the latter has become the most popular since the last decade. Despite an ACLR, there is no guarantee that the knee function will restore to the pre-injury level. It has been shown that for months and even years after ACLR deficits in 1) gait, 2) running, 3) balance, 4) muscle strength, 10,11 5) proprioception 12,13 and 6) jumping and landing 14-21 persist.

The aim of this dissertation was to determine the effect of an ACL injury and subsequent ACLR on various motor skills and seek to find an explanation of the encountered observations.

In Chapter 2 the effect of quadriceps strength and anterior laxity on gait in patients six months after ACLR was analyzed. These two variables were postulated as causes for abnormal gait patterns in patients with ACL deficient (ACLD) knees. The contention is that patients with ACLD avoid activity of the quadriceps muscle as this may induce an anterior translation of the tibia, which may not be well controlled in case of an ACL injury. Hence, patients with ACLD limit the degrees of extension of the injured knee by keeping the knee in a stiff pattern of knee flexion. It was of interest to determine how restored laxity and quadriceps strength had an effect on gait patterns in patients after ACLR. Sagittal knee angles and knee moments during the stance phase were calculated. The most important findings were a reduced knee flexion and extension range of motion of the ACLR leg. In addition, the knee extension moment of the ACLR leg was significantly reduced. Most importantly, the results indicated that kinematic and kinetic gait parameters in ACLR knees were not related to quadriceps strength and laxity.

If biomechanical deficits are present during a relative low load activity like gait, it is reasonable to assume that deficits are even more pronounced during high demanding activities like hopping. Hop tests are used as indicators of predetermined objective guidelines to determine a safe return to sports after ACLR. 23,24 Typically, return to sports is often allowed as soon as six months after surgery.

The purpose of the study presented in Chapter 3 was to conduct a comprehensive assessment that included kinematic, kinetic and EMG-analysis during a single leg hop test for distance at six months after the surgical procedure. The results showed that in ACLR legs, significantly earlier onset times were found for all muscles, except the vastus medialis, compared with the uninvolved side. The involved legs had significantly reduced knee flexion during the take-off. In addition, increased plantarflexion angles of the ankle in the involved leg at initial contact during landing was observed. The knee extension moment was significantly lower in the involved leg. In the control group, significantly earlier onset times were found for the semitendinosus, vastus lateralis and medial gastrocnemius muscles of the non-dominant side compared with the dominant side. Differences in EMG onset times between the involved and the uninvolved leg in the patient group were significantly larger than differences between the dominant and the non-dominant side in healthy subjects, except for the semitendinosus, vastus lateralis and vastus medialis muscles.

The studies carried out in Chapter 2 and 3 were merely descriptive in nature. A classical experimental scientific approach was employed in terms of performing an experiment and determine if there are statistical significant differences for the various dependent and independent variables. Subsequently, the data were then compared to data available in the literature to seek any similarities or discrepancies between our findings and those of others. Scientists then propose hypotheses as explanations of phenomena, and design experimental studies to test these hypotheses via predictions which can be derived from them. Research may become more valuable if a theory is available prior to conducting experiments. Theories that encompass wider domains of inquiry may bind independently derived hypotheses together in a coherent, supportive structure. Theories, in turn, may help form new hypotheses or place groups of hypotheses into context. Looking at science from that perspective, led to a different approach for the remaining chapters of this dissertation. It was decided that the focus of research should shift towards an understanding of the role of sensorimotor control after ACL injury. Sensorimotor control encompasses all the afferent, efferent, and central integration and processing components in human movement. The peripheral mechanoreceptors, and in particular the ‘’proprioceptors” have received the most attention from a clinical orthopaedic perspective. Proprioception is the afferent pathway relaying the acquisition of stimuli by articular, cutaneous and muscular and tendinous receptors to the central nervous system. As the ACL contains various receptors, it is plausible that a rupture of the ACL may result in altered motor control. Several studies have claimed that these proprioceptive deficits adversely affect activity level, 25-27 balance, 9,28 re-establishment of 29 quadriceps strength and increase the risk of further injury. 30

In Chapter 4, a systematic review was conducted to asses the role of proprioception on motor skills like balance, gait, hopping and strength in patients with ACLD or ACLR. In addition, we determined the association of proprioception and laxity of the knee as well as on patient reported outcome questionnaires. The correlation between proprioception and laxity, balance, hop tests and patient outcome was low. Only four studies reported a moderate correlation between proprioception, strength, balance, hop tests or gait. In conclusion, there is limited evidence that proprioceptive deficits as detected by commonly used tests adversely affect function in patients with ACLD and ACLR. Development of new tests to determine the relevant role of the sensorimotor system is needed. These tests should ideally be used as screening tests for primary and secondary prevention of ACL injury.

The literature search clearly demonstrated that proprioceptive deficits certainly can not fully explain why altered movement occurs after ACLR. As stated before, proprioception entails only the afferent pathway of a sub-component of the sensorimotor system. However, effective motor control after an ACL injury and subsequent ACLR calls for efficient information processing between the entire body, brain and environment.

In the study as presented in Chapter 5, a new theoretical construct was proposed in that cognitive changes in motor control may have occurred after injury of the ACL and play a role in preventing return of normal movement patterns. The biomechanical studies that have been carried out in Chapter 2 and 3 offer descriptions of the changed movement patterns in patients after ACLR. In the acute phase after ACL-injury or subsequent ACLR, it may be useful that patients use a motor control program that is aimed at protection of the knee. Patients may take small steps, reduce the range of motion of the knee, limit the amount of weight on the involved leg and look very cautiously where they place the foot. The movements are further dictated by pain early after ACLR and may therefore serve a useful purpose. The contention is that patients after ACLR may utilize an increased cognitive focus on movement even months after surgery, which inhibits the learning process of regaining normal movements. It was hypothesized that patients after ACLR move in a more natural way - causing them to extend the boundaries of their constrained movement patterns - when they are immersed in a life-like virtual reality setting compared to a normal lab environment.

It is difficult to meet these criteria for the practice of locomotor tasks in constrained indoor and outdoor settings. Virtual reality typically refers to the use of interactive simulations created with computer hardware and software to present users with opportunities to engage in environments that appear and feel similar to real world objects and events. 31 The technology, with the capacity of simulating environments, offers a new and safe way to offer the varied environments and controlled constraints needed to maximize learning. In a virtual environment, the simulated objects and events are not only sensed, but the user can anticipate and react to them as though they are real. Virtual reality enables researchers to analyze task performance in valid situations similar to real life, yet under experimentally controlled conditions. That was the main reason why an experiment using virtual reality was chosen, in order to control for attentional demands, but performed within a functional environment or context. The results confirm indeed that movement patterns in ACLR patients while immersed in virtual reality, approached those of healthy subjects.

The final experiment of this dissertation sheds a new light on the often reported movement aberrations. Starting from a theoretical construct derived from the field of motor control science, a hypothesis was formulated that increased cognitive attentional control would play a role in movement patterns in patients with an ACLR. As the study in Chapter 5 showed, possible changes in attentional focus had a greater impact in patients after ACLR compared to healthy controls. This rises a question as to whether there is an effective coupling between the surgical procedure and the functional outcome in terms of return of normal movement patterns.