Neuromuscular Training After Anterior Cruciate Ligament Reconstruction

Introduction

Rehabilitation after anterior cruciate ligament (ACL) injury and surgery has evolved in the past 2 decades from programs that recommended 6 to 8 weeks of immobilization, restricted range of motion (ROM) and strengthening exercises, and a delay in return to sports activities for 9 to 12 months postoperatively. Current therapeutic programs implement immediate knee motion, weight bearing, and functional exercises, as well as proprioception and neuromuscular control drills aimed at improving the patient's overall outcome, thereby promoting an earlier restoration of functional dynamic stability and return to sports activities.

Although there have been numerous advances in surgery and rehabilitation after ACL injury and surgery, the rate of return to preinjury levels of activity is alarmingly low. Arden and associates reported in a meta-analysis of 5770 patients that 63% returned to preinjury sports levels and only 44% returned to competitive sports. The most common reason reported for not returning to preinjury sport levels was fear of reinjury. These investigators found that 19% of all patients had a fear of reinjury and 13% stated they had structural integrity issues with the knee. This concept of fear of reinjury is referred to as kinesophobia , as described by Chmielewski and colleagues. Lentz and coworkers recently reported that 45% of the patients with ACL reconstructions did not return to preinjury sport levels. Of those patients not returning to sports, 45% reported kinesophobia and 68% had a feeling of knee instability. We believe a rehabilitation program that has a significant focus on proprioception and neuromuscular control exercises and drills will assist in preventing the development of kinesophobia and will ensure that patients develop confidence in their knee and improved function.

Fitzgerald and associates reported improved outcomes in ACL-deficient patients who participated in a rehabilitation program that emphasized proprioception and perturbation training. Shelbourne and Nitz first reported improved clinical outcomes in patients who participated in earlier functional activities after ACL reconstruction. The results of these studies increased the emphasis on neuromuscular training in postoperative programs. In this chapter, proprioception and neuromuscular control are defined, the implications after ACL surgery are discussed, and techniques commonly used to restore functional dynamic stability to allow a return to a prior level of function are described.

Terminology

Proprioception , kinesthesia , and neuromuscular control are terms that are often used interchangeably to describe exercises and techniques implemented in treatment; however, these terms have distinctive definitions. Proprioception is the awareness of joint position, whereas kinesthesia is the cognizance of joint movement. Neuromuscular control has been defined as the efferent response to an afferent input, thus providing the functional component to movement and athletic activities that is referred to as dynamic stability.

Mechanoreceptors (pacinian corpuscles, Ruffini endings, muscle spindle, and Golgi tendon organs [GTO]) located in muscles, capsules, and ligaments provide sensory input into the central nervous system (CNS). These mechanoreceptors act together to give sensory awareness of joint position, movement, acceleration, and strain via afferent pathways to the CNS. The CNS provides an efferent response to the surrounding joint musculature to elicit the desired response.

Several proposed mechanisms have been described for the joint mechanoreceptors' influence for joint stability. Extrinsic muscle stiffness is mediated through two mechanisms: (1) force feedback, which is provided by the GTO, and (2) length feedback, which is stimulated by the muscle spindle pathway that facilitates motor activity. Additionally, the joint mechanoreceptors contribute to joint stability through two possible mechanisms. The direct ligamentous-muscular protective reflex, a reflex between the ACL and hamstring muscles, causes reflex inhibition of the quadriceps and facilitation of the hamstrings when stimulated that is aimed at minimizing strain on the ACL. Another proposed mechanism for added joint stability is the receptors' indirect contribution to dynamic joint stability through what is termed presetting . The receptors contribute to preparatory adjustments of muscle stiffness and dynamic stability that are essential in preparation for deceleration and cutting activities.

Effects of Injury on Proprioception

Normal recruitment and timing patterns of the surrounding musculature are impeded after an injury because of changes that occur within the joint. Two theories are proposed for this mechanism, including an alteration occurring in the ratio of muscle spindle-to-GTO activity that causes an interference of the proprioceptive pathway. Joint effusion has also been credited with a diminished ability of the musculature to contract, thereby decreasing proprioception. A significantly decreased activation of the vastus medialis and lateralis muscles during a single-leg drop landing has been demonstrated with a 30-mL knee joint effusion.

After injury, the interactions within the neuromuscular system are disturbed as deafferentation of peripheral sensory receptors occurs, resulting in diminished proprioception and kinesthesia, abnormal patterns of muscle activity, and reduced joint dynamic stability. Changes in proprioception have been reported to occur within 24 hours following injury and may persist for as long as 6 years. Additionally, Hooks and coworkers reported that within 24 to 48 hours after ACL injury, the contralateral extremity had diminished proprioceptive sense. Therefore the rehabilitation program should consist of bilateral motor and proprioceptive training to minimize proprioceptive loss after injury.

A significant decrease in muscle activation timing and recruitment order of the quadriceps, hamstrings, and gastrocnemius in response to anterior tibial translation in the ACL-deficient knee compared with the intact-ACL knee has been reported. The delay in muscle recruitment may lead to decreased knee joint stability owing to the stabilization effect of the musculature. A reflexive activation of the hamstring musculature has been reported with the application of 100 N of anterior shear force on ACL-deficient knees. Paterno and colleagues reported a significant difference in force production 27 months after ACL reconstruction during a vertical jump in the involved knee compared with the contralateral knee.

Gait abnormalities are common after ACL injury because patients frequently demonstrate a “quadriceps avoidance gait pattern.” This altered pattern occurs because patients ambulate with a flexed knee gait as a result of increased hamstring activity and minimal or no quadriceps electromyography activity (EMG). This altered gait pattern can persist for several months because the patient will develop a protective neuromuscular adaptation if not addressed during rehabilitation.

The reported time frame varies regarding the length of time proprioceptive and neuromuscular control deficits persist after surgery. Harrison and coworkers found no significant differences in postural sway (eyes open and closed) of the involved and uninvolved lower extremity during single-leg stance 10 to 18 months after surgery. However, other examiners have reported more extended periods of deficits after ACL injury. Fremerey and coauthors measured joint position sense preoperatively and serially after surgery, and noted that restoration of joint position sense was virtually restored at near-end range of flexion and extension 6 months postoperatively. However, proprioception was impaired at the mid range of knee motion 6 months after surgery and persisted up to 3.5 years for some patients. Long-term deficits in proprioceptive awareness may influence the athlete's functional ability and/or likelihood of reinjury because dynamic stability is critical in the mid range position where the majority of activities occur.

Long-term follow-up studies after ACL injury have reported a 10-times greater incidence rate of knee osteoarthritis (OA), with a 40% to 90% occurrence 7 to 12 years after surgery. Carey and colleagues reported that although 80% of running backs and wide receivers returned to play in the National Football League, the performance was reduced by a third. Likewise, Busfield and associates reported a 44% decrease in the performance of National Basketball Association players who returned to play after ACL reconstruction as measured with statistical and efficiency ratings. Barber-Westin and Noyes conducted a systematic review with a minimum 2-year follow-up and reported reinjury rates of the ACL-reconstructed knee ranged from 0% to 24%, and injury to the contralateral knee ranged from 2% to 15%. Arden and coworkers reported that 63% of athletes were able to return to preinjury levels and only 44% were able to return to competitive sports. The authors reported a fear of reinjury (19%) as the number one reason athletes did not return to sports participation, with 13% reporting structural/functional problems. As discussed previously, the fear of movement and/or reinjury (kinesophobia) is a common problem and appropriate interventions to improve self-efficacy should be implemented to improve knee function.

Clinical Implications

The rehabilitation program after ACL injury should ensure the restoration of neuromuscular control beginning immediately after ACL reconstruction to prevent deafferentation of the joint. The therapist should safeguard against deleterious stresses that neuromuscular training may place on the healing structures, while ensuring a proper progression into advanced techniques. Therefore the clinician throughout the rehabilitation program monitors the progression of volume and advanced techniques.