There are many things that can derail the recovery following an ACL injury and ACL-reconstruction. The early rehab phase following an ACL reconstruction is a critical timeframe to pave the way for a successful outcome and rehab. Many patients are humbled, confused, and frustrated when they commonly encounter difficulty fully activating their quadriceps and performing a straight leg raise or walking without the involved knee buckling in the early post-op or post-injury period. For many patients it can seem as if the connection between the brain and their surgical knee is now “offline” and they have lost voluntary control of their knee. This phenomenon is commonly identified as arthrogenic muscular inhibition (AMI) in rehab research literature. When AMI persists, it can have significant detrimental long-term effects limiting return of quadriceps strength, normal gait patterns, and restoration of normal lower extremity biomechanics and muscle activation patterns for common sport tasks. These persistent effects may also contribute to increased reinjury risk or development of knee osteoarthritis.
Dr. Dave Sherman of Northeastern University joined us on the Rehab, Exercise, and Sports Therapy (REST) podcast to discuss his research findings on the topic of AMI and strategies to combat this issue in early ACL rehab.
What is AMI?
AMI is a disruption and alteration of the normal neural signaling between the joint receptors and the central nervous system (i.e. brain and spinal cord). This disruption and alteration of nerve signaling may result from damage to mechanoreceptors in the injured ACL as well as changes in knee joint homeostasis due to tissue damage, swelling/effusion, pain, inflammation, and joint laxity. Fortunately or unfortunately depending on your perspective, when these changes are not addressed and become more chronic, the nervous system finds ways to adapt and compensate for this lack of sensory input or information commonly called deafferentation and other related neural changes due to the interconnectedness of the nervous system. Regrettably, these changes may often result in maladaptive changes at a brain level that cause alterations in strength and movement patterns that may negatively impact athletic performance and increase risk of reinjury to the ACL or secondary injuries. These changes have been extensively researched by Drs. Dustin Grooms and Meredith Chaput, and are outlined in great detail in this paper by Drs. Adam and Lindsey Lepley.
What Can I Do To Limit or Address AMI?
Unfortunately, because AMI involves neural adaptations, it does not simply resolve with time and rest. “Hope is not a strategy” is especially relevant in this post-operative circumstance. Fortunately, there are some evidence-backed strategies you can implement to combat AMI in the acute post-operative period following ACL-reconstruction.
Aggressively Manage Swelling
Swelling in the initial post-operative period is inevitable following ACL surgery because it often involves drilling tunnels into bone and harvesting tissue for a graft to replace the damaged ACL. There are many commercially available cryocompression devices to assist in managing the resulting swelling in the early post-operative period. Many of these devices may be covered by insurance and some are available for short-term rentals. However, the traditional technology-free option of elevating the affected limb above the heart while applying ice or cold therapy is still a cost-effective alternative when these more glamorous devices are not available.
Additionally, a number of compressive garments or sleeves are available to assist in managing swelling in the affected lower extremity in the early to mid post-operative stages. Patients should be mindful of wound and incision care to limit infection risk in the acute period, but may receive significant benefits of compression to limit swelling and effusion accumulation at the involved knee joint.
It is imperative that the patient and rehab providers monitor swelling during the recovery process because it can have such a detrimental effect on muscle activation and restoring range of motion at the knee. A number of factors may contribute to swelling and reactivity of the knee joint including standing or weight bearing activity volume, presence of bone bruise from the time of injury and/or the extent of the original injury trauma, the patient’s individual biochemistry and inflammatory reaction, the extent of concomitant surgical procedures performed with the ACL reconstruction, and other factors that are beyond the scope of this post. While initiating weight bearing activities and gait training to facilitate normal movement patterns, muscle firing, and strength gains is essential, patients should know that periods of prolonged standing and walking in the early to mid stages of ACL reconstruction recovery may significantly increase joint swelling and effusion , which may delay recovery. Swelling should be monitored accordingly and the volume of weight bearing activities may need to be adjusted.
Control and Limit Pain and Inflammation
I am not going to downplay the addiction risks of narcotic medications or the unwanted side effects of both narcotic and anti-inflammatory or pain-relieving medications. All pharmaceuticals come with some element of risk and impact on the body. However, in the early post-operative period, forgoing all medications as a badge of honor and enduring high levels of pain that may limit compliance with rehabilitation exercises can have similarly damaging consequences and facilitate development of AMI. Controlling pain and inflammation through short term use of prescribed medications as directed may mitigate some of these effects. If you still prefer to avoid any medications, ice has overwhelmingly been shown to assist in pain control and can be routinely implemented in post-operative pain management strategies.
Transcutaneous Electrical Nerve Stimulation (TENS)
More good news for those who prefer the non-pharmacologic route for pain management. TENS units are an affordable and readily available option to manage pain that do NOT require a physician prescription or insurance approval. TENS application is most beneficial when applied for > 20 mins. prior to exercise in patients acutely after surgery or with swelling/effusion.
Pre-Cooling Joint
Further evidence in this study exists that pre-cooling the knee with ice for 20 mins. prior to exercise facilitated greater quadriceps strength gains in patients with AMI after ACL reconstruction versus exercise alone. This strategy can easily be coupled with a TENS unit prior to treatment to possibly augment the benefits and optimize the treatment effect. This is a low cost and low tech evidence-backed option to combat AMI.
Neuromuscular Electrical Stimulation (NMES)
Patients with AMI have increased difficulty voluntarily activating the quadriceps and maximizing force output to restore muscle function and limit any atrophy and weakness in the early post-operative period. NMES circumvents this issue by directly stimulating the muscle to facilitate motor unit recruitment and force output at the quadriceps. Some commercially available portable electrical stimulation units allow the user to apply the NMES treatment during home exercises to compound the beneficial effects. This clinical study and commentary reviews current evidence and some of the available home NMES units and ideal parameters and settings for application to optimize the benefits. Unlike some strictly passive modalities, NMES offers the patient the chance to actively participate with implementation during exercise, creating a more efficient treatment session from a time management perspective.
Eccentric Cross-Exercise and Cross Education
Following an ACL injury and subsequent surgery, many athletes mourn the loss of the ability to perform high intensity weight training activities. This type of high intensity weight training is not often an option at the surgical leg in the immediate early post-op period with concern for protecting the joint, the graft, and surrounding soft tissues. However, the benefits of performing high intensity weight training using eccentric exercise at the non-surgical leg following ACL-reconstruction are well-documented, and perhaps more importantly have been shown to have some benefits on strength and muscle recruitment on the surgical leg. Equally critical, the proposed mechanism of effectiveness of this strategy is via neural mechanisms at a supraspinal or brain level at the motor cortices to potentially offset or mitigate some of the adverse neural effects post-operatively. Research shows that the intensity and volume outlined for exercise dosage in this strategy is also critical. Performing at least 3 sets of < 8 reps at 80% or more of your 1 repetition maximum (1 RM) is suggested to facilitate the greatest benefits at the opposite leg.
Biofeedback
Using biofeedback at the quadriceps may help restore cortical drive to the quadriceps using a top-down (brain to knee) mechanism. Biofeedback uses visual and auditory real-time feedback to help restore voluntary cortically driven muscle control. A recent systematic review indicates that biofeedback may be a useful adjunct to ACL rehab to help restore and normalize neural function at the quadriceps and combat AMI.
Blood Flow Restriction (BFR) Training
Unlike some of the other techniques and strategies included on this list, BFR does not work primarily through a neural mechanism. Evidence is currently mixed on the efficacy of BFR and whether BFR improved post-operative outcomes, long term quadriceps strength gains, and other studied variables . Studies on BFR have varied significantly in how and when it is applied, dosage, duration, patient population and demographics, and what exercises are utilized in conjunction with BFR which may contribute to the mixed results of studies examining BFR use following ACL reconstruction. However, while BFR does not address AMI directly because it works by inducing muscle hypoxia mechanically and subsequent metabolic pathways, there is no dispute regarding its ability to preserve muscle mass or limit atrophy in the early post-surgical period. Limiting or preventing muscle atrophy and restoring normal muscular metabolic function may improve rehab effectiveness as neural efficiency improves and AMI effects resolve.
Additionally, another significant advantage of BFR is that it can be used in conjunction with low load exercises during the early post-operative period when the knee and surrounding soft tissues are not yet able to tolerate higher loads and intensity to address weakness. We previously discussed the use of BFR on a previous episode of the Rehab, Exercise, and Sports Therapy Podcast with Dr. Kyle Kimbrell.
Summary
There are many proactive options available to patients in the early rehab phase following ACL reconstruction to manage effects of the surgery to limit onset of AMI which may negatively impact rehabilitation outcomes. Swelling and pain control measures, TENS, pre-exercise focal joint cooling, NMES, biofeedback, eccentric exercise and cross-education, biofeedback, and blood flow restriction are all readily available options with varying cost that have been backed by research evidence to facilitate positive rehab outcomes in the early post-operative period following this surgery.
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