An anterior cruciate ligament (ACL) injury presents short and long-term challenges to the patient and the health system.
In Australia, ACL rupture is generally treated with a surgical reconstruction (ACLR), with an annual incidence of 77.4 reconstructions per 100,000 population (2015). This is a 43% increase since 2000 and, more importantly, a 74% increase in those under 25. Concerningly, while the peak in 2015 was in the 20-24 age bracket for men and 15-19 for women, the annual growth incidence was greatest in the 5-14-year-old bracket.
Short-term challenges are highlighted by return-to-sport rates, with only 82% of patients returning to some kind of sports participation following ACLR, and only 44% returning to competitive sport. Long-term, an increased risk of knee osteoarthritis (OA) (tibiofemoral and patellofemoral) after ACL injury is well established.
With rates of injury and reconstruction continuing to rise, the cost of ACL injury and reconstruction to the Australian health system is estimated at $315 million by 2030-2031 (excluding indirect costs of OA/persistent knee disability).
Exercise rehabilitation protocols significantly improve physical function following ACLR. However, even with protocols targeting muscle strength and function, quadriceps and hamstring strength impairments often persist.
Rehabilitation efforts tend to focus on the knee directly, minimising swelling, regaining range and quadriceps strength. Unfortunately, if the rehabilitation focus is solely on the injured joint, there is a risk ignoring a vital component of the rehabilitation journey, the brain.
Patients and rehabilitation providers need to understand the mechanisms by which the brain is affected by ACL injury and ACLR, how this can influence rehabilitation outcomes and the interventions available to address this. Transcranial direct current stimulation (tDCS), a form of non-invasive brain stimulation, may play a role in mitigating early maladaptive processes seen in ACL injury and ACLR.
Arthrogenic muscle inhibition
It is well established that a joint injury, such as a knee injury, can lead to arthrogenic muscle inhibition, which in part is a centrally mediated process. ACL rupture can also be considered a deafferentation injury, given the loss of joint proprioceptive information from the ACL.
Individuals with an ACL injury have been shown to have increased corticospinal inhibition, associated with decreased quadriceps strength. Not surprisingly an ACL rehabilitation program only addressing peripheral injury (i.e., knee effusion and range) and not central adaptations to an injury can see persisting impairments in quadriceps and hamstring strength.
Rehabbing the brain
Multiple rehabilitation techniques and interventions are available to address or mitigate deleterious central adaptation to injury and to maximise beneficial neuroplasticity during recovery. Eccentric cross-exercise (exercising the contralateral limb) may improve muscle recruitment and strength in the injured limb due to bilateral motor pathway changes.
Motor imagery (the mental practice of a motor action) can increase excitability and activation of motor cortex (and spinal) neurons. Utilising an external focus of attention can decrease motor cortex inhibition (i.e., “push the ground away forcefully” is an external focus cue, compared to “extend your knees and hips forcefully” for internal focus).
tDCS
Transcranial direct current stimulation (tDCS) is a safe, non-invasive form of brain stimulation that passes a weak direct electrical current (typically ≤2 mA, compared to 800-900mA for ECT) between an anodal and cathodal electrode placed on the scalp. Generally, cortical excitability is increased with anodal tDCS (anode applied to the target cortical region).
There is evidence that tDCS application to the primary motor cortex may promote an increase in muscular maximal voluntary contraction and possibly muscular endurance, improve balance performance, gait retraining, multi-task performance and motor learning of sport-specific skills and tasks.
The potential benefits of tDCS application align with ACL rehabilitation goals, namely preventing the loss of/regaining muscular strength, regaining, and improving neuromuscular control, and re-introducing sport-specific movements and tasks.
SportsMed Subiaco, in conjunction with Edith Cowan University, the Orthopaedic Research Fund of Western Australia (ORFWA) and researchers from Murdoch University and La Trobe University, have started the TACL Study (The impact of Transcranial direct current stimulation during rehabilitation following Anterior Cruciate Ligament (ACL) reconstruction on functional outcomes and return to play timelines. A double-blind, randomised controlled trial).
Suitable participants with an acute ACL rupture will undergo transcranial magnetic stimulation assessment pre-op, post-op and throughout rehabilitation to measure changes in corticospinal excitability, intracranial inhibition and facilitation. Sports physiotherapists at SportsMed Subiaco will provide free standardised ACL rehabilitation.
Participants will be randomised to either real or sham tDCS, to be applied three times a week from week two to week eight post-operatively. If tDCS application can mitigate the quadriceps inhibition seen post-injury/reconstruction, it may become another tool in the arsenal for rehabilitation providers. In ACL rehab, at least some of it is
in your head.
Key messages
- ACL injury induces central nervous system adaption, which may adversely influence rehabilitation outcomes, contributing to persistent quadriceps and hamstring weakness
- Rehabilitation should incorporate techniques that can mitigate maladaptive processes and enhance beneficial neuroplasticity
- Transcranial direct current stimulation (tDCS) may help overcome the central inhibition driving arthrogenic muscle inhibition.
The author acknowledges ECU (Dr Myles Murphy) and the Orthopaedic Research Foundation of WA (Dr Peter D’Alessandro).
Author competing interests – the author is involved in the research described