FIRM-P: how it came about, which problems it solves and the principles it rests on
When the anterior cruciate ligament tears, the question always lands in the same place: reconstruct or rehabilitate. That question has been argued for decades, often with conviction and in good faith on both sides. FIRM-P grew out of a simple observation: very often, the two sides are not talking about the same thing.
How it came about
The origin is clinical, not theoretical. An MRI report reads "anterior cruciate ligament tear" and the discussion begins, the same everywhere in the world: the surgeon thinks of reconstruction, the physiotherapist of rehabilitation, the patient tries to work out who is right. Both positions defend the patient. And both use a single word, instability, as if it named one single thing.
It does not. A knee can be unstable in the way it moves under load, in the giving-way the person feels, in what imaging shows and in what laxity instruments measure. These are four different questions, each measurable with its own tools, and they can give discordant answers in the same knee. Hence the idea: separate the questions, answer each with the best available data, verify the discordances, and only then bring in what concerns the person, that is sport, work, expectations and age.
The framework was developed by Dario Giunchi and Matteo Denti, orthopaedic knee surgeons, together with Marc Barrera Uso, co-author with the SANTI Study Group in Lyon of studies on arthrogenic muscle inhibition. It was published in the Journal of Experimental Orthopaedics, the open access journal of ESSKA, as a conceptual framework (DOI 10.1002/jeo2.70914). The name is an acronym: F for functional control, I for individual perception, R for the radiological profile, M for mechanical laxity and P for patient context.
Which problems it solves
The first problem is language. As long as "unstable" means different things to different people, every discussion is flawed from the start. FIRM-P requires you to state which instability you are talking about and which data you are relying on.
The second is decision by opinion. Without an explicit structure, the choice between surgery and conservative treatment depends on one's school, personal experience and whoever speaks last. FIRM-P does not remove clinical judgement, it makes it legible: given the same four answers, the knee phenotype is the same for everyone, and the points where two clinicians may legitimately diverge become visible.
The third is the contamination of diagnosis by context. A high-level athlete and a sedentary person with the same knee deserve different proposals, but not different diagnoses. In the framework, patient context enters after the knee has been classified and modifies the treatment proposal, never the phenotype.
The fourth is discordance between tests, which is usually treated as noise to be rounded off. In FIRM-P it is the most informative moment: it triggers a verification loop that first rules out artefacts, for instance an MRI read without the clinical question or a laxity test performed in the acute phase, and then, if the discordance persists, recognises it as a real finding that defines the phenotype.
The fifth is timing. Arthrogenic muscle inhibition, the brake the nervous system places on the quadriceps after an injury, is frequent after ACL rupture and can distort both functional assessment and the patient's perception. FIRM-P treats it as a cross-domain modifier: it does not change the phenotype, but it changes how some answers are read and when any operation makes sense.
The sixth concerns roles. Functional assessment is the ground where physiotherapists and movement scientists have the deepest expertise, and the framework gives that expertise a formal channel into the decision. Interpreting imaging and measuring laxity with instruments remain medical acts, and the treatment decision remains with the treating physician, because whoever decides is accountable. The assessment is multi-professional, the prescription is not.
The principles it rests on
Four measured domains and a fifth element, context, which comes afterwards. One explicit, mutually exclusive classification rule: mechanical laxity is the necessary condition for attributing instability to the ligament; with confirmed laxity, function and perception distinguish a compensated knee from an overtly unstable one; without laxity, imaging distinguishes an occult or associated picture from a non-ligamentous one. This yields four phenotypes, labelled A to D, and thirty-two scenarios covering every possible combination in the two contexts of functional demand.
It is not a score. Nothing is summed or weighted and no probability is calculated: the domains are questions, not points, and the output is a reasoning sequence, not a number. Nor is it a medical device: the simulator processes no real patient data but explores predefined hypothetical scenarios, as an interactive textbook chapter would.
The best available data, not every instrument. The tests listed under each domain are examples in descending order of quality, not requirements. Those without a movement analysis laboratory can work with clinical surrogates for function, and say so. What cannot be done is to infer one domain from the others: if laxity has not been measured, the picture is incomplete and must be completed, not guessed.
An open, evolving framework. Texts and framework are released under a Creative Commons Attribution 4.0 licence, the simulator code under the MIT licence: anyone may use them, teach them and build on them, citing the authors. Authorship of the concept stays with the authors, progress belongs to everyone. The simulator already incorporates suggestions from physiotherapists, biomechanists and surgeons received after publication, and every change is recorded in a version history. FIRM-P is a conceptual framework awaiting formal validation: it is published precisely to be put to the test.
The simulator
The quickest way to understand FIRM-P is to use it. The simulator lets you describe a knee with the four questions, watch the phenotypes narrow down with every answer, read the reasoning step by step, browse the map of thirty-two scenarios with references verified on PubMed, and download the whole matrix in an open format. A fifteen-second guided demo and a quiz mode make it useful for teaching as well. It works on computers, tablets and phones with nothing to install.
Portal address: dariogiunchi.ch/en/post/firmp-en
If you are a patient
If you have an ACL injury, FIRM-P is not a test to take on your own and it does not decide in place of your doctor. It helps those treating you to ask the right questions in the right order, and to explain clearly why one path is being proposed rather than another. To learn how the operation works, you can read the page on anterior cruciate ligament reconstruction.
How to cite: Giunchi D, Denti M, Barrera Uso M. FIRM-P separates knee phenotype from patient context in ACL-related instability: a conceptual framework integrating arthrogenic muscle inhibition as a cross-domain modifier. Journal of Experimental Orthopaedics (ESSKA). 2026;13(3):e70914. doi:10.1002/jeo2.70914
Comments, criticism and proposals for collaboration or validation: studio.giunchi@hin.ch. Authors: Dario Giunchi, Matteo Denti, Marc Barrera Uso.
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