FIRM-P separates knee phenotype from patient context in anterior cruciate ligament-related instability: A conceptual framework integrating arthrogenic muscle inhibition as a cross-domain modifier.

Purpose: Clinical reasoning after anterior cruciate ligament (ACL) injury is still often built on isolated proxies such as imaging, instrumented laxity, functional testing or patient-reported instability. These variables are clinically useful, but they are not interchangeable and do not answer the same question. The purpose of this work is to propose FIRM-P (Functional, Individual, Radiological and Mechanical instability plus patient context) as a practical conceptual framework that separates the ACL-related knee phenotype from patient-context indication, while integrating arthrogenic muscle inhibition (AMI) as a cross-domain modifier rather than as a separate instability pillar. Methods: FIRM-P has two layers. FIRM describes the knee through Functional, Individual, Radiological and Mechanical instability. P describes the patient context through performance demands, priorities and expectations, rehabilitation potential and personal modifiers. Each domain is interpreted as favourable, borderline or unfavourable. Discordant patterns do not directly produce treatment decisions; they trigger a verification loop. AMI is integrated as a mechanism that may alter the expression of Functional and Individual instability, may coexist with true ACL insufficiency and may influence the timing of elective surgery. Results: After verification, an explicit classification rule assigns the knee to one of four mutually exclusive phenotypes: low-instability/non-ACL, occult or associated-instability, compensated ACL-deficient or overt ACL-instability. Patient context is applied only after the knee has been phenotyped. A simplified decision pathway is provided in the main manuscript; the full operational 32-scenario matrix and an interactive HTML prototype that allows readers to simulate the algorithm across the 32 operational scenarios are provided as Supporting Information. Conclusions: FIRM-P is proposed as a conceptual framework, not a validated score. Its immediate value is to organize ACL-related instability reasoning into a transparent sequence: phenotype the knee, verify discordance and then apply patient context. Its future value will depend on content validation, reliability testing and prospective derivation work. Level of Evidence: Level V.