The radiographic features relating to complications or failure can be classified into three major categories, based on their radiographical appearance.
Aseptic loosening or osteolysis (Fig. 15) is a biological process that is initiated by macrophage phagocytosis of particulate debris, causing an aseptic foreign body granulomatosis . The implant becomes separated from the host bone, resulting in mechanical (aseptic) loosening. On a radiograph, this manifests as a periprosthetic zone of radiolucency around the bone-cement or the bone-prosthesis interface.
Aseptic loosening and osteolysis should be differentiated from other, nonpathological causes of periprosthetic lucencies. In a cemented arthroplasty, a < 2 mm lucency at the bone-cement interface indicates the formation of a fibrous membrane (representing the lucency), outlined by a thin, sclerotic demarcation line . This is thought to represent a stable fibrous reaction to cement. In a cementless arthroplasty, a similar < 2 mm lucency also outlined by a thin sclerotic line, along a polished segment where no bony ingrowth is expected, indicates fibrous bony ingrowth and is thought to provide sufficient stability (Fig. 16) .
As a rule of thumb, periprosthetic lucencies wider than 2 mm and/or progressive lucencies are signs of abnormality.
Infection remains a major and devastating long-term complication, occurring in 1–2 % . Similarly to aseptic loosening, plain radiography shows a periprosthetic zone of radiolucency around the bone-cement or the bone-prosthesis interface. The differential diagnosis between septic and aseptic loosening can be very challenging, especially when no previous radiographs are available. However, the presence of a femoral periosteal reaction  (Fig. 17) or rapid progressive disease  (Fig. 18) are indicative of septic rather than aseptic loosening.
Deposition of metallic wear particles in periprosthetic tissues (Fig. 19) may occur, particularly in metal-on-metal bearing arthroplasty. This process has been given the umbrella term ‘adverse reaction to metal debris’, including metallosis , aseptic lymphocytic vasculitis associated lesions  and pseudotumours . Radiographs usually show normal findings, but in longstanding cases there may be evidence of loosening or, in a resurfacing arthroplasty, pressure erosion on cortical bone .
Sclerosis and bone proliferation
Development of bone outside its normal location in the skeleton is termed heterotopic bone formation, occurring in up to half of patients; this rarely results in significant limitation of movement (Figs. 20 and 21) .
Spot welding consists of new bone formation originating from the endosteal surface and reaching the prosthesis. This is mostly seen in cementless femoral stems and is a strong indicator of stability (Fig. 22) .
Stress shielding refers to the transfer of the normal load from the femoral neck and intertrochanteric region to the proximal femoral diaphysis (the hip implant causes altered mechanical forces), causing bone resorption on the lateral side of the proximal femur, most commonly seen in Gruen zone 1, as well as bone hypertrophy at the medial side of the proximal femur (Fig. 23). This process implies stability and should not be misinterpreted as a complication .
Sclerosis at the tip of a cementless femoral component, bridging the medullary canal, is a bone pedestal (Fig. 24). The association of this often incidentally found entity with loosening remains unclear .
Component failure/ fracture
Linear wear occurs typically in hip arthroplasty with a polyethylene component (hard-on-soft or soft-on-soft bearing surface combination). An asymmetric position of the femoral head within the acetabular cup on radiographs is a definite sign of polyethylene wear (Fig. 25).
The reported rate of dislocation varies from 0.5 to 10 % after primary total hip arthroplasty . Most dislocations occur in the early postoperative period, during the initial weight bearing (Fig. 26) . Abnormal acetabular inclination, acetabular retroversion or an incorrect center of rotation, among others, increase the likelihood of dislocation.
Periprosthetic fractures occur more often around the femoral than the acetabular component, be it intra- or postoperative. The Vancouver classification divides the periprosthetic, postoperative fractures of the femur into three major types (Fig. 27) . Postoperative femoral fractures occur typically, but not exclusively, at the level of the tip of the femoral stem (Fig. 28).
Prosthetic fractures occur mostly in the femoral stem of the implant, representing a metal-fatigue stress fracture; this typically occurs in prostheses that are well fixed distally but are mobile proximally and result in fractures through the middle or proximal third of the stem (Fig. 29) . Patients with increased body mass index (BMI) are at greater risk of reaching an implant failure point due to fatigue loading . Varus malpositioning predisposes to fractures of the femoral stem.