Knee function and neuromuscular adaptations following ACL rupture and reconstruction

In order to alleviate symptoms associated with progressive knee dysfunction and deterioration following anterior cruciate ligament (ACL) injury, patients undergo either conservative non-operative rehabilitative regimens or early reconstructive surgery using the patella tendon (PT) or combined semitendinosus and gracilis tendon (STGT) grafts. Following treatment, ACL deficient (ACLD) and ACL reconstructed (ACLR) patients demonstrate varying levels of knee function with compensatory neuromuscular adaptations thought to be responsible for enhancing the dynamic restraint capabilities in more functional patients. Derivation of the neuromuscular factors that estimate participation restrictions could assist clinicians in developing prognoses and outcome measures for ACLD and ACLR patients. Therefore, the main aim of the present thesis was to identify neuromuscular variables, derived during open and closed kinetic chain tasks, that relate to and predict post ACL injury/ACLR functional outcome. To achieve this, 10 male ACLD subjects together with 27 matched-males who had undergone ACLR (14 PT graft and 13 STGT graft) and 22 matched-control subjects were recruited. In Experiment 1, the Cincinnati Knee Rating System was used to assess knee symptoms and limitations associated with activities of daily living and sports. Three single-leg tests designed to replicate athletic activities were also implemented. Subjective and objective scores were combined to provide an overall knee function score for each subject. The ACLD group was significantly more symptomatic and limited in activities of daily living and sports and they also demonstrated impaired jump and hop performance. Whilst the PT and STGT subjects rated significantly higher than their ACLD counterparts, their average subjective and overall knee function scores were significantly lower compared to the control group. Importantly, graft selection did not significantly influence average subjective, objective or overall knee function scores. In Experiment 2, the effect of ACL injury and ACLR on open kinetic chain isokinetic strength of the quadriceps and hamstrings was assessed in 10o intervals through their operational domain. Antagonist activity of the semitendinosus (ST) and biceps femoris (BF) muscles was also determined during knee extension in 10o intervals between 80 and 10o flexion. Conservatively managed subjects demonstrated significant quadriceps and hamstring weakness with involved limb quadriceps strength deficits transferred to the contralateral limb. Harvesting the central one-third of the PT as an ACL substitute did not inhibit quadriceps strength compared those ACL-insufficient knees in which the extensor mechanism was not used in the reconstruction technique (STGT graft). In contrast, harvesting the flexor mechanism for ACLR caused significant hamstring strength deficits that were not apparent in patients having undergone ACLR using the PT graft. Relatively large amounts of hamstring antagonist activity were evident during knee extension, although ST and BF electromyographic discharge was not influenced by ACL status. Hamstring antagonist activity increased and decreased widely as a function of joint angle with the BF significantly more active than the ST in order to control internal tibial rotation. Kinesthetic joint capsule receptors were thought to be the major source dictating hamstring muscle activity in such a manner that it varied nearly inversely relative to its moment arm. In Experiment 3, lower limb kinematics, kinetics and neuromuscular responses were assessed in ACLD and ACLR subjects during a closed kinetic chain task known to stress the ACL, namely abrupt deceleration when landing from a single-leg hop for distance. For the ACLD group, no significant alterations were evident in joint kinematic parameters. Biceps femoris of the involved limb of the ACLD group was activated significantly later compared to the non-involved limb, supporting the notion that after ACL injury, sensory feedback may be used to build a new internal model depicting the expected conditions during functional activities. The involved limb of the ACLD and ACLR groups demonstrated a significant reduction in vertical ground reaction force during the support phase of landing compared to the non-involved limb. Whilst the magnitude of peak tibial acceleration was not significantly different between test limbs or subject groups, it took significantly longer for the involved limb of the ACLD and ACLR groups to attain constant tibial motion compared to the non-involved limb. Subjects having undergone ACLR using the PT graft demonstrated a stiff knee strategy during landing and, whilst the STGT group also demonstrated trends towards decreased knee flexion during landing, no significant kinematic adaptations at the hip, knee or ankle were identified. Decreased knee flexion was found to significantly attenuate the mechanical advantage of the involved limb hamstrings of the ACLD, PT and STGT groups. Cumulative changes in involved limb hip and knee kinematics of the ACLD and ACLR subjects meant that the ST and BF muscles were significantly elongated when decelerating to improve dynamic restraint. Importantly, ACLR led to a restoration of normal quadriceps and hamstring electromyographic (EMG) synchrony in the involved and contralateral limbs and there was no evidence to suggest that the ACLD subjects adopted a pattern of quadriceps-avoidance. In Experiment 4, the strength of the associations among knee functionality of ACLD and ACLR subjects (Experiment 1) and neuromuscular variables derived from open (Experiment 2) and closed (Experiment 3) kinetic chain movements was determined. Numerous significant moderate to strong correlations were identified with determinants of knee functionality related to the type of ACL treatment and graft selection. Compensatory neuromuscular strategies that enhance function in the ACLD knee included amplified hamstring co-activation, increased hamstring preparatory activity and a greater ability to control tibial motion during dynamic deceleration. Following ACL replacement, the degree of residual strength deficit in the muscle from which the tendon graft was harvested (i.e. quadriceps or hamstrings) become an important prognosticator of knee functionality as did attenuated hamstring co-activation during knee extension within the range utilsed during single-limb deceleration. More functional PT subjects demonstrated enhanced tibial control whilst superior knee functionality in STGT subjects was associated with increased preparatory activity of the quadriceps when landing on the involved limb. Furthermore, by synchronising peak hamstring muscle activity at the time when the ACL graft is most vulnerable to injury, more functional STGT subjects enhanced dynamic restraint by increasing joint compression and posterior tibial drawer. By identifying neuromuscular factors that predict function in ACLD and ACLR subjects, the results of these studies will lead to the development of more specific and effective treatment strategies