Periprosthetic patellar fractures are classified on a temporal basis, intraoperative or postoperative. With direct fractures of the patella, the retinaculum and extensor mechanism often remain intact despite the substantial fracture comminution and cartilage damage. The indirect eccentric tension through the extensor mechanism often results in avulsion fractures of the inferior pole or transverse fractures extending into the extensor retinaculum, causing fracture displacement. If rapid knee flexion occurs during active contraction of the quadriceps, the 3-force bending forces may cause the patella to fail in compression. Additionally, the patella experiences compression and 3-point bending forces exerted by the quadriceps and patellar tendons during knee flexion. Conversely, flexing the knee places compressive forces along the posterior patella. With the knee in extension, the pull of the quadriceps places the patella in tension. The patella experiences complex, dynamic loading patterns. Periprosthetic fractures are classified by timing (intraoperative or postoperative) and subclassified by displacement, adequacy of bone stock, the stability of the patella component, and competence of the extensor mechanism. Revision total knee arthroplasty with patellar component substitution has twice the rate of postoperative periprosthetic patella fracture (1.8%) and nine times the rate of intraoperative patella fracture (0.2%) compared to primary total knee arthroplasty. Postoperative periprosthetic patella fractures are more common than intraoperative fractures. The Mayo Joint registry analyzed 12,000 primary total knee arthroplasties over thirteen years and found the prevalence of periprosthetic patellar fractures to be 0.68%. Periprosthetic fractures of the patella are devastating injuries and are the most frequent complication following total knee arthroplasty. In 50% of patients with this condition, the bipartite patella is found in both knees. On radiographs, they may be mistaken for a fracture. In a small percentage of the population, a secondary ossification center in the patella, most commonly at the superolateral patellar edge, may fail to fuse, resulting in a bipartite patella. Open injuries account for 6% to 9% of patellar fractures and are commonly associated with other injuries given the high energy mechanism. įractures of the patella account for approximately 1% of all fractures. Similar to native patella fractures, periprosthetic fractures are caused by direct trauma or eccentric load via the patella or quadriceps tendon. Periprosthetic fractures are complex problems and requiring a comprehensive history, thorough preoperative discussion with the patient and family members, and extensive surgical planning. The combination of increased life expectancy, increased activity demands of patients, along with an increasing need for total knee arthroplasty, has led to a simultaneous increase in periprosthetic fractures. With the demand for primary total knee arthroplasty projected to grow to 3.48 million procedures performed in 2030, a 673% increase from 2005, a rise in periprosthetic patella fractures is expected to increase. Due to the patella’s subcutaneous position, a direct injury may result from a blow to the anterior knee, such as from a fall or impact from the dashboard in a motor vehicle accident. Eccentric forces from the extensor mechanism may overpower the mechanical properties of the bone. The patella most commonly fails indirectly under tension. The most important blood supply to the patella penetrates the inferior pole along the fat pad below the patella.įractures of the patella may be due to direct or indirect forces, and the mechanism of injury often determines the fracture pattern. The patella receives centripetal blood supply from the geniculate arteries, with the superior vessels lying anterior to the quadriceps tendon and the inferior vessels passing posterior to the patellar tendon. The patellar retinaculum is formed by contributions from the fascia lata, vastus medialis, and vastus lateralis. The inferior pole of the patella attaches to the patella tendon. The patella protects the anterior aspect of the knee joint, serves as the insertion for the quadriceps tendon, and functions as a fulcrum to maximize the efficiency of the extensor mechanism. A vertical ridge separates the medial and lateral facets of the articular surface and articulates with the femoral trochlea. The posterior aspect of the patella contains a thick cartilage layer, which is the thickest cartilage in the body. The patella is the largest sesamoid bone in the human body.
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