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Exploring the Rotational Instability Spectrum of ACL Deficiency: An Exploratory Phenotype-Based Classification
Horacio Rivarola1, Cristian Collazo1, Marcos Palanconi1
1Knee Surgery Unit, Department of Orthopaedic Surgery, Hospital Universitario Austral, Buenos Aires, Argentina.
Summary
Anterior cruciate ligament (ACL) deficiency can be classified into four distinct rotational instability phenotypes based on secondary stabilizer injury. Increased injury burden correlates with severe rotational instability, aiding in better knee injury characterization.
Area of Science:
- Orthopedic Surgery
- Sports Medicine
- Biomechanics
Background:
- Anterior cruciate ligament (ACL) deficiency is often viewed as a uniform condition regarding rotational instability.
- Secondary stabilizer injuries significantly impact knee rotational biomechanics, yet their role in phenotyping ACL deficiency is unclear.
Purpose of the Study:
- To determine if ACL-deficient knees can be categorized into distinct rotational instability phenotypes based on cumulative secondary stabilizer injury.
- To develop and validate a classification system for these phenotypes.
Main Methods:
- Retrospective cohort study of 300 primary ACL reconstruction patients.
- Recorded clinical, MRI, and arthroscopic data on lateral secondary stabilizers.
- Developed the Rotational Injury Burden Index (RIBI) and used hierarchical clustering to identify phenotypes.
Main Results:
- Identified four reproducible rotational instability phenotypes: Low-Burden (30.7%), Lateral Meniscal-Dominant (25.3%), Hyperlaxity-Dominant (20.3%), and High Rotational Burden (23.7%).
- Higher RIBI scores were independently associated with high-grade pivot shift (adjusted OR 1.47).
- Meniscal pathology strongly correlated with increased rotational burden and instability.
Conclusions:
- ACL-deficient knees exhibit distinct, reproducible patterns of secondary stabilizer injury, forming unique rotational instability phenotypes.
- Cumulative secondary stabilizer injury burden is directly linked to severe rotational instability.
- This framework offers enhanced structural characterization beyond simple injury burden, differentiating intermediate-risk knees.
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