The femur is the commonest site of periprosthetic fracture and is the most studied. Several classification systems exist, based either on the configuration of the fracture, the involvement of the prosthesis or a combination of the two. Two descriptive classification systems, by Di Giola and Rubash and Chen et al., classify fractures based on the degree of displacement and comminution; neither provides information about the stability of the implant, and both are of only limited usefulness in terms of management [25, 26].

Lewis and Rorabeck classify periprosthetic femoral fractures into three types: types I and II are non-displaced and displaced fractures, respectively, adjacent to a well-fixed prosthesis; type III is any fracture adjacent to a loose prosthesis [27]. The authors suggest that type I fractures be treated nonoperatively, type II fractures be treated with fixation and type III fractures be treated with revision surgery. A similar system is proposed by Kim et al.: type I fractures occur adjacent to well-fixed implants in good quality bone; the authors suggest either conservative management (in reducible fractures, which they term Ia) or fixation (in irreducible, type Ib fractures) [28]. Type II and III fractures involve loose implants, either with good quality bone (type II, which can be revised using a long-stem component) or with poor bone (type III, where an endoprosthesis is required). Su et al. describe a classification system based on fracture position: type I is proximal to the femoral component, type II is at the proximal part of the component, and type III is distal to the proximal part of the component [29]. They suggest that type I fractures should be treated with an intramedullary nail, type II with a fixed-angle plate and type III with revision arthroplasty.

Fractures of the tibia are less common than fractures in the femur but are more likely to occur intraoperatively [11]. The classification system of Felix et al. works on similar principles to the main classification systems for femoral fractures, classifying fractures by their position and the degree of involvement of the prosthesis [13]. Type I fractures are splits or depressions in the tibial plateau; type II fractures are adjacent to the tibial stem; and type III fractures are distal to the stem of the prosthesis. Fractures of the tibial tuberosity are termed type IV. In each case, the prosthesis can be classed as stable (A) or unstable (B); as with the femur, fractures adjacent to stable prostheses are best treated with fixation whilst if the implant is loose, revision is recommended. Fractures occurring during implantation are given the suffix C; in these cases, on-table revision to a stemmed prosthesis is recommended.

Periprosthetic patellar fractures overwhelmingly occur in patellae which have been resurfaced: a systematic review of 582 cases reported that only five cases (0.9 % of the total) were in unresurfaced patellae [10]. Unlike other fractures about the knee, periprosthetic patellar fractures are often asymptomatic: in the same systematic review, in 539 cases where the mechanism of injury was documented, 476 (88.3 %) were diagnosed during routine follow-up and had no history of trauma [10].

As with the femur, multiple classification systems have been proposed for the patella [30]. The most commonly used system is that of Ortiguera and Berry, which resulted from a study of 85 fractures in the Mayo Clinic database [10, 12]. Types I and II have a stable prosthesis and are classified according to the state of the extensor mechanism. Type I fractures have an intact extensor mechanism and may be managed nonoperatively. In type II fractures, the extensor mechanism is disrupted, and the authors recommend operative fixation or patellectomy. In type III fractures, the implant is loose. If the bone stock is good (type IIIa), fixation and implant revision can be attempted; if the bone stock is poor (type IIIb), the authors recommend removal of the patellar component and patelloplasty or complete patellectomy.

The Universal Classification System (UCS) is a classification system intended to apply to any periprosthetic fracture in any bone [30]. Similar to the Vancouver classification of periprosthetic fractures about the hip, the UCS classifies fractures into types A–C based on position within the bone, with the addition of types D, E and F (Table 19.1). It is straightforward and intuitive and has the advantage of accounting for fractures (such as inter-prosthetic fractures) not classifiable using the other systems discussed here. Unlike the other classification systems, it has been examined for intra- and interobserver reliability in both experts and trainees [31]. Interobserver reliability was substantial in both groups (Kappa 0.741 and 0.765, respectively), and intraobserver reliability was near perfect (Kappa 0.898 and 0.878). The UCS is in its infancy and has not been used widely in studies of periprosthetic fracture to date; in this chapter, we will use the existing classification systems outlined above.