Heterotopic ossification (HO) is a common post-surgery condition where bone abnormally forms within soft tissues. A new study out of Mass General Brigham assesses the viability of a simple blood test to detect HO long before it can be picked up by imaging techniques.
Q: What challenges or unmet needs make this study important?
Heterotopic ossification (HO), or abnormal bone formation within soft tissues, is a significant complication for nearly 1 in 3 patients undergoing hip replacement surgery. This condition not only leads to chronic pain but also restricts mobility. Current imaging methods such as X-rays and CT scans detect HO only 6–8 weeks after surgery—well past the window when preventive treatments are effective. Moreover, surgical removal of HO fails to restore function in more than 75% of cases.
The lack of early diagnostics also prevents the implementation of precision medicine strategies. Preventative treatments, including radiation and anti-inflammatory drugs, must be given within the first week following surgery, however, these interventions carry serious risks such as complications with wound healing or bone non-union (where a fractured bone fails to heal properly). Currently, all hip replacement patients receive these treatments, meaning that 3 out of 4 patients who would never develop HO are unnecessarily exposed to significant risks. There is a pressing need for early diagnostic capabilities to identify patients at risk and optimize the timing of their treatment.
Q: What central question(s) were you investigating?
Our investigation focused on whether we could detect heterotopic ossification weeks before it becomes visible on X-rays by analyzing rare cells found in a patient's blood. We hypothesized that mesenchymal progenitor cells (MPCs), the precursors to bone cells, are released into the bloodstream from the site of injury during the early stages of abnormal bone formation. If we could successfully capture and analyze these rare circulating MPCs (cMPCs) from a simple blood draw, we could develop a "liquid biopsy" that predicts which patients are likely to develop HO long before current radiographic detection is possible.
Q: What methods or approach did you use?
We collected blood samples from 22 patients undergoing hip arthroplasty before their surgery and on days one and 14 post-surgery, closely monitoring them with X-rays to determine if any developed HO. Using our previously developed iChip device—capable of isolating extremely rare cells from whole blood —we successfully isolated cMPCs and performed RNA sequencing to identify HO-specific gene expression patterns.
We validated these findings in a mouse HO model, using genetic lineage tracing to confirm that the circulating cells originated from the injury site. Further, we developed a prediction model using machine learning algorithms based on 32 genes associated with HO.
Q: What did you find?
Our findings revealed that cMPCs are released into the bloodstream as early as six hours following an injury that induces HO in mice—41 days before HO can be detected via X-ray imaging. This early release suggests that blood-based testing could offer a six-week head start for intervention in patients at risk. These cMPCs exhibit a unique 32-gene signature that is closely associated with bone formation.
The machine learning model achieved accuracy with up to 90% sensitivity and 100% specificity in predicting which hip arthroplasty patients would develop HO. This means we can accurately identify 9 out of 10 at-risk patients while avoiding false positives.
Additionally, we demonstrated that this liquid biopsy can effectively monitor treatment responses. In mice treated with an HO inhibitor, we detected significant decreases in key cMPC gene expression, correlating with reduced HO formation.
Lineage tracing confirmed these circulating cells originate from Gli1-positive periosteal cells at the injury site, the same type of cells responsible for forming heterotopic bone, providing direct evidence that blood-based cMPCs reflect tissue-level changes.
Q: What are the real-world implications, particularly for patients?
The adoption of this technology has the potential to transform HO prevention and treatment. A simple blood test 1-3 days after surgery can identify around 30% of patients truly at risk, helping to avoid unnecessary treatment and complications for the 70-75% of patients who are unlikely to develop HO. Additionally, our approach allows for real-time tracking of the effectiveness of preventative treatments, enabling healthcare providers to tailor therapy durations to individual patients.
By detecting changes associated with HO weeks before irreversible bone formation occurs, we can initiate treatment during the critical early window when interventions are more likely to be effective. Further, this methodology could extend to other medical conditions involving abnormal fates of MPCs such as osteoarthritis, chronic tendinopathy and fibrodysplasia ossificans progressiva, opening the door for broader implications in regenerative medicine and the management of various musculoskeletal disorders.
Q: Tell us about any follow-up studies you have planned to validate or build on these findings.
We're pursuing multiple validation steps toward clinical implementation. First, we are developing a droplet digital PCR (ddPCR) assay to replace RNA sequencing for the 32-gene signature, as ddPCR is more sensitive, faster, less expensive and better suited for clinical laboratories. Further, we plan to enroll a larger cohort of hip arthroplasty patients and low-risk controls to rigorously validate our predictive model in diverse populations.
We're also exploring whether measuring cMPC concentration alone—rather than analyzing gene expression—might suffice for HO prediction, making the test even faster and more cost-effective.
Lastly, we will be pursuing CLIA certification for the complete assay . Then, we aim to launch a biomarker-stratified clinical trial where the liquid biopsy guides patient enrollment and treatment decisions.
Authorship: In addition to Karabacak, Mass General Brigham authors include Matilda Holtz, Hanil Kang, Florence Lin, Hannah Stowe, Saeed Nazemidashtarjandi, Cenk Ayata and Mehmet Toner.
Paper cited: Nunez, Johanna, et al. "Early detection of aberrant cell fate and repair using circulating progenitor cells in patients with heterotopic ossification." Nature Communications. DOI: https://doi.org/10.1038/s41467-026-68857-8
Funding: This work was supported by funding from MGH ECOR (2022A009258), an Orphan Disease Center Million Dollar Bike Ride Grant (MDBR-22-116-FOP) and Shriners Children's.
Disclosures: Mass General Brigham has filed a patent application for the use of rare cell isolation technology in heterotopic ossification as described in this publication (PCT Application No. PCT/US2024/055005).
