Periprosthetic Knee Fractures

Intraoperative

Intraoperative fractures can potentially occur at any stage of the TKA procedure, including exposure, bone preparation, placement of trial components, insertion of the final components, and insertion of the polyethylene insert.

Intraoperative femur fractures can occur in either the diaphyseal or metaphyseal region. Metaphyseal fractures are more common and can include the medial or lateral condyle, medial or lateral epicondyle, and the supracondylar metadiaphyseal region. These fractures occur most commonly during bone preparation, trialing, and implantation of final components. Technical errors that can lead to these fractures include inadequate preparation of the femoral box in posterior-stabilized (PS) designs and aggressive impaction of the femoral trial or final component in osteopenic patients. In one series, all intraoperative femoral fractures occurred in females with narrow femurs in which a PS implant was used. Another series reported 40 distal femur intercondylar fractures in 898 primary PS TKAs. The authors attributed this to a wide box cut in their implant. Their fracture incidence decreased once they changed their implant to one with a smaller box cut. Alden et al. reported a relative risk of 4.74 when using a PS design as opposed to a cruciate-retaining (CR) design. It is important that surgeons understand the subtleties of the implant that they are using and that meticulous attention be paid to deciding the medial-lateral placement of the box cut to avoid this complication.

Diaphyseal fractures are less common and typically involve penetration of the anterior or posterior cortex from a malpositioned intramedullary drill or guide during distal femoral preparation ( Fig. 11.1 ). These fractures are easily missed intraoperatively because they typically will not cause instability or deformity and the femoral diaphysis is not exposed during the procedure. ^,

Intraoperative tibia fractures are less common than femur fractures. These fractures can occur during any stage of the procedure and can include the medial or lateral plateau; the tibial tubercle; and the medial, lateral, anterior, or posterior cortex. ^, These fractures are more commonly encountered during revision surgery with forceful extraction of a well-fixed component, during cement removal, aggressive impaction of the tibial component, eccentric preparation or placement of a tibial stem, and cone or sleeve preparation and impaction. ^,

Postoperative

Supracondylar femoral fractures are the most common postoperative fractures around total knee implants. ^, These fractures are often the result of low-energy trauma such as ground-level falls, although they also occur after higher-energy trauma, seizures, and manipulation of a stiff knee.

Patella fractures are the second most common periprosthetic fracture after primary TKA. Their etiology can be either traumatic or atraumatic. They can occur after severe thinning of the native patella due to the disease process, and can be associated with avascular necrosis of the remaining bone. A systematic review reported that only 11.6% of fractures had a traumatic incident, with the remainder occurring without any significant trauma. Most of these fractures occur within 2 years of the index procedure. These can be very difficult fractures to treat, as the index procedure can decrease patellar blood supply and bone stock. It can also be difficult for patients to return to satisfactory function with treatment, especially if the extensor mechanism is disrupted or does not return to baseline.

Periprosthetic tibia fractures are the least frequent postoperative periprosthetic knee fracture. These fractures previously occurred more frequently with earlier total knee designs. However, these fractures occur more rarely now with the use of short-stemmed or keeled tibial baseplate designs. ^, Interestingly, a systematic review showed that nearly 80% of fractures of the tibial plateau did not have a traumatic etiology.

Classification

Various classification systems have been proposed for distal femur fractures. Neer and associates reported the first classification system in 1967, which is based on the amount and direction of displacement of the fracture. Rorabeck and Taylor proposed a classification system 30 years later that added the fixation of the prosthesis, with Types I and II being nondisplaced and displaced fractures, respectively, with a well-fixed component, while Type III fractures involve a loose component and are either displaced or nondisplaced ( Table 11.1 ). In 2006, Su and associates proposed a classification system based on the location of the fracture, with Type I proximal to the prosthesis, Type II starting at the anterior flange of the femoral component and extending proximally, and Type III fractures involving any other aspect of the femoral component distal to the most proximal aspect of the anterior flange ( Fig. 11.2 ). Kim et al. proposed a classification system that takes into account the stability of the implant, the volume and quality of bone stock available for fixation, and the reducibility of the fracture.

The most commonly used classification for patella fractures was described by Ortiguera and Berry. This classification system considers both the stability of the patellar implant and the integrity of the extensor mechanism. Types I and III fractures have an intact extensor mechanism with stable and loose patellar components, respectively. Type II fractures have a disrupted extensor mechanism ( Table 11.2 ).

With respect to periprosthetic tibia fractures, Felix et al. proposed a classification system for periprosthetic tibial fractures. This system divided fractures into Types I to IV based on anatomic location (I: tibial plateau; II: adjacent to stem; III: distal to prosthesis; IV: tibial tubercle) and into subcategories based on prosthesis fixation (A: well-fixed; B: loose; C: intraoperative; Fig. 11.3 ).

Risk Factors

Risk factors reported for distal femur periprosthetic fractures include female sex, osteoporosis, neuromuscular disease, cognitive disorders, chronic steroid use, inflammatory arthritis, infection, and polyethylene wear resulting in osteolysis. Increased constraint can predispose to distal femur fractures, as this can cause an increased stress on the bone-implant interface with the implant having a much higher modulus of elasticity. ^{, ,} Additionally, constrained components require a larger box cut, which results in a decreased segment of bone to bridge the medial or lateral condyle to the femoral diaphysis and to each other. Anterior femoral notching, which occurs when the anterior flange of the femoral component violates all or part of the anterior cortex of the femur, has an associated theoretical increased risk for distal femur periprosthetic fracture. Biomechanical studies have shown decreased bending and torsional loads to failure when notching is present. ^, However, clinical studies have not shown an increased rate of fracture with notching.

The risk of postoperative tibia fracture can be increased when the baseplate is placed in varus or with a loose component. Intraoperative fractures are typically due to technical error, most commonly when the baseplate is impacted too aggressively. Intraoperative fracture risk is also increased in revision surgery and when a tibial tubercle osteotomy is used to improve exposure. ^,

Periprosthetic patella fractures can occur in both resurfaced and nonresurfaced patella, and literature has shown a higher rate of fracture in resurfaced patellas. ^, Bourne and Burnett classified the risk factors for patella fractures into three groups. The first group relates to patient factors, the second relates to implant design issues, and the third includes intraoperative factors related to technical error. Patient factors include osteoporosis, male sex, inflammatory arthropathy, and excessive postoperative mobilization. Factors related to implant design include cementless implants and implants with a single large central peg. Technical errors are the most preventable and include aggressive use of patellar clamps, asymmetric or over-resection of the patella, and femoral implant malrotation. Femoral malrotation leads to patellar instability and insult, which can increase the risk for fracture. The surgeon should also take care to preserve the blood supply to the patella, as ischemia increases risk for fracture. It has been shown that routine exposure during TKA causes insult to the patellar blood supply. Some authors suggest that the surgeon should retract the patella laterally rather than everting. One study used Doppler flowmetry to quantify patellar ischemia and showed that eversion resulted in a much higher degree of ischemia than lateral retraction alone. Additional factors that can reduce patellar blood supply include aggressive resection of the fat pad and lateral retinacular release. Residual bone thickness is also a significant factor that can lead to fracture. The precise appropriate thickness is being debated in the literature. However, residual bone thickness of less than 15 mm may predispose toward a patella fracture.