Fibrodysplasia Ossificans Progressiva

Also known as: Generalised mysositis ossificans; Myositis Ossificans Progressiva; Stone man disease


Fibrodysplasia ossificans progressiva (FOP) is a rare, progressively disabling condition caused by formation of bony bars within skeletal muscles, ligaments and tendons (heterotopic bone formation). This bone formation occurs in FOP via an initial cartilage template in a process called endochondral ossification. Its incidence worldwide is about one person per two million and is independent of race and gender.


Medical text written in October 2019 by Professor J.T. Triffitt, Emeritus Professor of Bone Metabolism, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Oxford, UK.

Although great care has been taken in the compilation and preparation of all entries to ensure accuracy, we cannot accept responsibility for any errors or omissions. Any medical information provided is for education/information purposes and is not designed to replace medical advice by a qualified medical professional.

What are the symptoms?

Most people with FOP have short big toes and some have additional abnormalities of the fingers or toes apparent at birth.

The bone formation is usually first noticed in early childhood following isolated flare-ups of inflammatory swellings that resolve into hard lumps in the neck or along the spine. These lumps may be tender and gradually shrink as affected muscles are replaced by bone. The bony lumps in muscles appear usually spontaneously but can be provoked by injury to muscle by trauma or injection. The progressive physical disability is variable in the way it affects a person. There may be long periods of inactivity that can be several years in length. The bone formation eventually locks movement of the affected joints leading to profound immobility.

What are the causes?

FOP is caused by one of 14 different mutations in the activin A receptor, type I (ACVR1) gene on chromosome 2, with one genetic variant being predominant.

How is it diagnosed?

Definitive genetic testing early in life when malformed big toes or when unexplained swelling and hard lumps in muscles are observed can confirm a diagnosis of FOP.

How is it treated?

Currently there is no proven treatment for the condition. Anecdotal evidence suggests that a course of corticosteroids, begun within a short period of a flare-up, may help reduce the intense inflammation and tissue oedema seen in the early stages of the disease. Non-steroidal anti-inflammatory medications COX-2 Inhibitors and paracetamol may be useful in managing pain and chronic discomfort.

Discovery of the causative gene has led to current clinical trials of possible treatments internationally. These agents act at different levels of the specific bone formation process seen in FOP. A retinoic acid receptor gamma agonist, palovarotene, is in a Phase 3 trial by Clementia Pharmaceuticals; a neutralising antibody to activin A, REGN2477, is in a phase 2 trial by Regeneron Pharmaceuticals and a kinase inhibitor, saracatinib, is in a Phase 2 trial under the Horizon 2020 programme of the European Commission. Details of these clinical trials may be found at or at

Trauma and injury to the muscles often provokes bone formation at the injury site and speeds up progression of FOP. Intramuscular injections should be avoided if at all possible. Similarly, operations on the skeletal muscles to remove pieces of bone invariably result in increased bone formation, and should be avoided. Dental treatment may result in bone formation in the jaw muscles and individuals with FOP should inform their dentist. In medical emergencies general anaesthesia intubations should be avoided unless absolutely essential and even then only with expert anaesthesia assistance.

Inheritance patterns and prenatal diagnosis

Inheritance pattern
Most cases are sporadic and due to a new chromosome mutation in ACVR1. Children of affected individuals, however, have a 50% chance of inheriting FOP.

Prenatal diagnosis
For pregnancy in someone affected by FOP prenatal diagnosis is now possible.

Is there support?

FOP Friends


FOP Friends is a registered charity (no. 1147704) that provides information and support to those with FOP, their family and friends in the UK and those further afield. The Group actively raises awareness of the condition, and fundraises for research into treatments.

Group details last updated October 2019.

FOP UK Support Group

Tel: 01772 339 296

This is a small network, established in 2000 offering informal support to affected individuals and their families.

Group details last reviewed October 2019.

International FOP Association (IFOPA)

Tel: +001(407) 365-4194

A non-profit organisation that provides hope to individuals with FOP and their families through education and support programs while funding research to find a cure. The IFOPA gives support to 1,000 individuals worldwide in over 50 countries.

Group details added October 2019.

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