Causes / typical injury mechanism: Heterotopic ossification (HO), or the abnormal formation of bone in soft tissues, may occur following surgery such as total knee arthroplasty (TKA). HO is essentially an aberrant tissue repair process that usually develops within 3 months postoperatively, and full ossification maturation can be seen within 2 years.

Classic history and presentation: Following TKA, less than 1% of patients who develop HO will be symptomatic. If symptomatic, patients will typically present with localized pain, swelling, and restricted knee flexion that may self-resolve.

Prevalence: Between 1% and 42% of patients may develop radiographic signs of HO following TKA, with approximately 1% of patients becoming symptomatic.

• Age – Typical age of onset is during the second and third decades of life.
• Sex / gender – Male:Female = 3:2.

Risk factors: Risk factors for HO include TKA surgical techniques (ie, notching of the anterior femur, exposure of the distal femur for instrumentation placement with concomitant periosteal injury, and surgical trauma to the quadriceps mechanism), history of HO, hypertrophic arthrosis, increased lumbar bone mineral density, obesity, forced manipulation of knee postoperatively, and postoperative knee effusion or hematoma formation.

Pathophysiology: HO is thought to be the result of an aberrant tissue repair process in which primitive mesenchymal cells present in muscle and soft tissue differentiate into osteoblastic tissue. This causes subsequent calcification and, ultimately, the formation of mature lamellar bone with trabeculae within soft tissue.

Grade / classification system (if any): The Furia and Pellegrini classification system consists of 2 separate classes and grades, which are based on HO localization and radiographic size, respectively:

• Class I – Island of bone localized to suprapatellar soft tissues.
• Class II – Bone organized into areas of ossification contiguous with the anterior distal femur.
• Grade A – Less than or equal to 5 cm.
• Grade B – Greater than 5 cm.