Arthrogenic muscle inhibition is a modifiable but systematically underassessed barrier to successful anterior cruciate ligament surgery.
Despite technical advances in ACL (anterior cruciate ligament) reconstruction, a substantial proportion of patients still present with persistent extension deficits and quadriceps inhibition post-operatively. The evidence increasingly points to AMI (arthrogenic muscle inhibition) as a critical, underaddressed variable in this failure pattern.
This article is grounded in the peer-reviewed editorial Rethinking arthrogenic muscle inhibition in anterior cruciate ligament injury and surgery, published in Knee Surgery, Sports Traumatology, Arthroscopy (2025, DOI: 10.1002/ksa.12790), co-authored by Professor Étienne Cavaignac, Bertrand Sonnery-Cottet, Alexandre Le Guen, and the SANTI (Scientific Anterior cruciate ligament Network International) Study Group.
Key Figures about Arthrogenic Muscle Inhibition from the Study
| Metric | Finding |
| AMI prevalence in acute ACL injury | 56.7% out of 300 patients across multiple institutions |
| AMI resolution with bedside exercises | ~80% of affected patients |
| Post-operative AMI recurrence | Up to 48%, even after pre-operative resolution |
AMI in ACL-Injured Knees: A Three-Level Neurophysiological Mechanism
AMI in ACL-injured knees operates across three neurophysiological levels that compound each other, making isolated treatment at any single level insufficient.
The three levels of AMI
| Level | Mechanism | Clinical consequence |
| Local (peripheral) | Joint effusion and mechanoreceptor disruption alter afferent signalling | Impaired proprioception, abnormal reflex input to spinal cord |
| Spinal | Inhibitory reflex loops suppress extensor motor neuron pools | Voluntary quadriceps activation unreliable despite effort |
| Supraspinal | Altered cortical excitability in supplementary motor area, thalamus, putamen (confirmed by neuroimaging) | Motor planning compromised; central reorganisation persists beyond structural healing |
AMI is frequently compounded by hamstring hypertonicity, driving flexion contracture and masking extension deficits that are often misattributed to pain or poor compliance. The documented clinical sequelae include:
- Quadriceps atrophy: progressive loss of VMO (vastus medialis obliquus) mass and force output
- Proprioceptive degradation: reduced joint position sense, increasing reinjury risk
- Gait compensation patterns: biomechanical adaptations that overload adjacent structures
- Post-operative complications: arthrofibrosis (excessive intra-articular scar tissue formation) and cyclops syndrome (fibrous nodule causing a mechanical block to full extension)
The neuroimaging data repositions ACL injury as a neurophysiological event, not merely a structural one. This has direct implications for how pre-operative readiness should be defined and assessed.
The SANTI AMI Classification: Validated Grading for Pre- and Post-Operative Assessment
The SANTI AMI classification was developed to address the absence of a standardised, equipment-free clinical tool for grading AMI severity. First proposed in 2022, it has now been validated across multiple institutions with robust inter- and intra-rater reliability.
SANTI AMI classification: grade summary
| Grade | VMO (vastus medialis obliquus) inhibition | Hamstring contracture | Reversibility |
| 0 | Absent | Absent | N/A — normal |
| 1 | Present | Absent | Fully reversible at bedside |
| 2 | Present | Present | Partially reversible |
| 3 | Present | Fixed contracture | Not immediately reversible |
Key advantages over previous AMI assessment approaches:
- No EMG (electromyography) required: fully applicable in a standard consultation room
- Bedside administration: VMO activation and prone hamstring fatigue tests executable in minutes
- Dual-phase validation: confirmed reliable for both pre- and post-operative assessment
- Actionable by grade: resolution exercises can be initiated immediately based on the assigned grade
In the multicentre study of 300 patients with acute ACL injury, 56.7% presented with AMI at initial consultation. In approximately 80% of these cases, AMI resolved following in-consultation exercises alone, without additional referral. This is a critical statistic: the majority of pre-operative AMI is both detectable and correctable within a single clinical encounter.
Unresolved AMI as a Contraindication to ACL Reconstruction: Evidence-Based Surgical Planning
The editorial’s central clinical directive is explicit: no ACL reconstruction should be performed on a knee with unresolved AMI. Failure to address AMI preoperatively is strongly associated with post-operative stiffness in the knee and poor functional outcomes, an association now substantiated across multiple institutional cohorts.
Revised pre-operative checklist for ACL (anterior cruciate ligament) reconstruction
The standard pre-operative protocol must be extended to systematically include AMI assessment. Recommended additions:
- SANTI AMI grading at first consultation and at each pre-operative visit
- VMO activation exercises initiated immediately if Grade ≥ 1 is identified
- Prone hamstring fatigue protocol where hamstring contracture is present (Grade ≥ 2)
- Surgical delay until SANTI Grade 0 is confirmed, particularly for Grades 2–3
- Reassessment on the day of surgery given AMI can recur between consultation and the operative date
In cases where AMI persists through conservative prehabilitation, the risk-benefit of proceeding must be carefully weighed. Operating on a Grade 3 knee dramatically increases the probability of arthrofibrosis and may ultimately require posterior arthrolysis (a secondary surgical release of posterior capsular contracture), an invasive intervention for a preventable complication.
Professor Cavaignac integrates this AMI pre-operative protocol systematically, as part of a broader commitment to evidence-based, individualised surgical planning in ACL management.
Post-Operative AMI Recurrence: Implications for Rehabilitation Protocols
The most operationally significant finding for rehabilitation teams: AMI recurs post-operatively in up to 48% of patients, including those who achieved full pre-operative resolution. This rate fundamentally challenges the assumption that pre-operative AMI clearance provides durable protection throughout the recovery period.
Recommended post-operative AMI monitoring framework
| Post-op timepoint | AMI (arthrogenic muscle inhibition) risk | Recommended action |
| Day 1–7 | High: acute inflammation, effusion, pain | VMO activation protocol initiated; SANTI grade documented |
| Week 2–6 | High: graft remodelling, ongoing neural inhibition | Formal SANTI reassessment; hamstring stretching protocol if Grade ≥ 2 |
| Week 6–12 | Moderate: risk of consolidating compensatory patterns | Criteria-based progression conditional on SANTI Grade 0 |
| Month 3–6 | Lower but persistent in complex cases | Integration of SANTI reassessment into return-to-sport protocol |
Standard ACL rehabilitation criteria such as quadriceps strength ratios (LSI: limb symmetry index), pain scores and time elapsed are insufficient on their own. They must be supplemented with systematic SANTI AMI grading at key milestones. The validated post-operative application of the classification makes this feasible without additional infrastructure.
Remote follow-up via the Orthense digital platform enables Professor Cavaignac’s team to monitor AMI indicators between consultations, allowing early detection of recurrence before it consolidates into fixed deficits.
Conclusion & Findings regarding AMI
The evidence consolidated in Rethinking arthrogenic muscle inhibition in anterior cruciate ligament injury and surgery demands a substantive revision of how AMI is integrated into ACL surgical and rehabilitation practice. The key takeaways for clinicians:
- 56.7% of patients with acute ACL injury present with AMI: it is the rule, not the exception
- ~80% of pre-operative AMI resolves with simple bedside exercises: it is actionable within the consultation
- Unresolved AMI is a contraindication to ACL reconstruction: surgical timing must reflect this
- Up to 48% recurrence post-operatively: rehabilitation protocols must incorporate systematic SANTI reassessment throughout recovery
The SANTI AMI classification provides the field with the standardised, equipment-free and validated tool it has needed to make this integration routine. For further reading, explore Professor Cavaignac’s other scientific publications across ACL reconstruction, multiligament surgery, arthroplasty and sports traumatology.
For complex ACL cases, including patients with suspected AMI, prior reconstruction failure or persistent extension deficits, Professor Etienne Cavaignac, orthopaedic and sports surgeon in France, is available for specialist referral consultations.