Typically females have a higher risk of sustaining a periprosthetic femoral fracture [4]. This is partly due to the fact that osteoporosis and osteolysis increase the prevalence of these fractures. It is unclear to what extent anterior femoral notching during primary TKR has a significant influence on the occurrence of supracondylar fractures. Biomechanical studies demonstrated high stresses at the notch site and the authors believe that under some circumstances notching could increase the risk of a periprosthetic fracture [5]. Large clinical studies have been unable to demonstrate a clear relationship between notching and occurrence of a fracture [6, 7]. Modern condylar TKR design often has an anteriorly sloped flange, which makes the TKR instrumentation less prone for anterior notching. It will be interesting to see if such designs lead to overall reduced rates of supracondylar fractures. Most fractures occur after a low energy trauma such as a simple fall from a standing height [8]. Intraoperative periprosthetic fractures usually happen whilst preparing the box or seating the implant in posterior stabilised TKR components.

Periprosthetic tibial fractures are also more prevalent in females [9]. In contrast to femoral fractures, they are more frequently associated with a loose prosthesis. Osteolysis is often associated with fractures around the tibial baseplate [10]. Tibial shaft fractures are usually seen in patients with long-stemmed or poorly aligned implants and in association with tibial tubercle osteotomy [10].

Patellar fractures are one of the rarest but most complicating forms of periprosthetic fractures after TKR. Fractures predominantly occur in resurfaced patellae. One study found a rate of only 0.05 % in non-resurfaced patellae as compared to the literature rates of 0.5–3.6 % for resurfaced patellae. In addition, intraoperative fractures of the patella are very rare except in the revision situation [4]. The risk factors for patellar fracture are shown in Table 13.1. A defined event causing the fracture is also unusual, with most fractures being recognised on routine follow-up radiographs [11, 12].

Malalignment of TKR increases the risk of patellar fracture, probably due to the increased patellofemoral contact forces. The resulting clinical outcome following treatment of a patellar fracture in malaligned TKR is clearly inferior to those with perfectly aligned TKR [13]. Fractures are more prevalent in males than females [12] Excessive resection of the patella increases the risk of occurrence of a patellar fracture; the thickness of the remaining patella should be at least 12–15 mm to reduce this risk.

The blood supply of the patella is another important factor to consider. It has been shown that the patella has an extraosseous and an intraosseous blood supply. The extraosseous blood supply comes from a vascular anastomosis that encircles the patella and consists of the supreme geniculate artery, the medial superior geniculate artery, the medial inferior geniculate artery, lateral superior geniculate artery, lateral inferior geniculate artery and the anterior recurrent tibial arteries. The intraosseous blood supply comes from two sources, the mid-patellar vessels that enter through foraminae in the middle third of the anterior surface of the patellar and the infrapatellar anastomosis behind the patellar tendon [14]. A lateral release may compromise the blood supply, therefore increasing the risk of patellar fracture. This is particularly true when combined with a medial parapatellar approach [13]. An excessive fat pad resection may also damage the blood supply to the patella [11].

Classification systems are useful for the surgeon if they guide further treatment and give information on the prognosis after fracture treatment.

The factors important in any classification are:

The location of the fracture with regard to the prosthesis, whether the component is well fixed or loose and consideration of the available bone stock. No classification is perfect and currently there is no recognised classification for inter-prosthetic fractures.

A brief overview about the most commonly used classifications is given below.