summary TKA Heterotopic Ossification is a complication of TKA with bone formation in the soft tissues that develops after surgery with or without a precipitating event. Diagnosis can be made with plain radiographs of the knee. Treatment is observation in majority of cases as HO rarely impacts clinical outcome. Surgical excision is indicated in the presence of quadriceps muscle snapping, patellofemoral tracking difficulties, or patellofemoral instability. Epideomiology Incidence rates range from 1% to 42%, but less than 1% have any impact on clinical outcome much less frequent than after total hip arthroplasty (THA) Risk factors hypertrophic arthrosis male gender obesity periosteal injury/stripping off anterior femur post-op knee effusion or hematoma post-op forced manipulation for restricted motion Etiology Pathophysiology any bone formation in the soft tissues (other than periosteum) that develops after surgery with or without a precipitating event distinct from myositis ossificans, which evelops after traumatic muscle injury and inflammation Classification Furia and Pellegrini classification system IA Island of bone localized to suprapatellar soft tissues; less than or equal to 5 cm IB Island of bone localized to suprapatellar soft tissues; greater than 5 cm IIA bone organized into areas of ossification contiguous with the anterior distal femur; less than or equal to 5 cm IIB bone organized into areas of ossification contiguous with the anterior distal femur; greater than 5 cm Presentation Symptoms (rare) loss of motion pain quadriceps muscle snapping patellofemoral instability Imaging Radiographic evaluation recommended views AP lateral findings ossification along anterior distal aspect of femur and within quadriceps mechanism typically visible 3-4 weeks post-operatively and matures within 1-2 years Prevention High-risk patients, generally those undergoing THA and not TKA external beam radiation therapy pharmaceutical prophylaxis with NSAIDs no literature regarding these therapies in TKA patients Treatment Nonoperative observation indications in majority cases as HO rarely impacts clinical outcome Operative surgical excision indications quadriceps muscle snapping patellofemoral tracking difficulties patellofemoral instability