Causes / typical injury mechanism: Total knee arthroplasty (TKA) patellar prosthesis loosening occurs in 1%-5% of postoperative patients with total knee replacements, most often resulting in persistent knee pain and swelling. On average, patients with patellar prosthetic loosening begin experiencing symptoms around 2 years after surgery. In many of these cases, prosthetic loosening is a secondary pathology to other conditions such as infection, osteonecrosis, patellar maltracking, dislocation or lateral patella subluxation, fracture, poor cement technique, and poor bone stock.

Classic history and presentation: Pain and swelling are the result of the dissociation of the patellar component from the patella, causing improper positioning. While patellar prosthesis loosening can occur in up to 5% of individuals after TKA, many individuals may be asymptomatic. In symptomatic patients, revision or excision of the patellar component is indicated.

Risk factors: Risk factors for patellar prosthesis loosening are joint-line elevation leading to patella baja and increased patella contact pressures, lateral release, poor implant fixation, obesity, osteoporosis, patellar osteonecrosis, patellar fracture, and infection.