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.
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.
