**7. Surgical risk factors**

286 Recent Advances in Arthroplasty

*et al* compared the wear rates of UHMWPE produced using three combinations of polyethylene production and sterilization techniques79 and found the best wear rates were achieved in sterilization by radiation in an inert gas with molded polyethylene. Irradiation sterilization of ram extruded components in an inert gas and in air had 11% and 16% more

Highly cross-linked polyethylene has exhibited reduced wear rates clinically in short-term studies80,81, and thus their potential role in reducing the incidence of osteolysis is promising. Further developments in polyethylene modification techniques are currently being explored to further reduce oxidization in-vivo and optimize the wear performance of UHMWPE without compromising its other mechanical properties, and include doping with antioxidants such as vitamin E and cycling of annealing and irradiating. However, the macrophage response in osteolysis is influenced by the size, composition and number of wear particles82,83. Particle size and number vary with the extent of cross-linking within the material. Although cross-linking reduces the total amount of wear debris generated versus conventional UHMWPE, the particle size produced is smaller, and the number of particles is increased, which may enhance their osteolytic potential *in-vivo*. Also, whilst increased crosslinking results in enhanced wear resistance there is a reduction in fatigue strength

Although metal on polyethylene bearings have most commonly been used in THA, there is a long history of use of other bearing couples, including metal on metal, ceramic on ceramic,

Metal on metal bearings have reduced wear rates compared with metal on polyethylene. Jacobbson reported a 77% 20-year survivorship of the metal on metal McKee Farrar THA compared to 73% for the Charnley THA85. Metal on metal prostheses also have the advantages of allowing a larger bearing diameter, improving stability characteristics, and are self-polishing. Although the volumetric wear rate of metal on metal bearings is low, the particles generated are in the nanometer range and the number of particles is far greater86. These particles circulate widely within the body and their systemic effects remain unclear. At a local level metal release can cause an adverse surrounding tissue reaction, termed aseptic lymphocytic vasculitis associated lesions (ALVAL), and inflammatory masses87,88.

Ceramic on polyethylene and ceramic on ceramic bearing couples have lower wear and osteolysis rates versus metal on polyethylene bearings in some long-term studies89,90. Most ceramic wear particles are also in the nanometer range and wear volume is lower than that of metal on metal bearing couples. A prospective randomized multicenter study of 930 hips comparing alumina-on-alumina with cobalt chromium-on-polyethylene bearing couples reported an alumina-alumina survival rate of 96.8% at 10 years91. However, cases of osteolysis have also been reported in poorly functioning ceramic on ceramic prostheses. Yoon reported osteolysis rates of 22% in a series of patients with ceramic on ceramic prostheses92. Nam reported a case of alumina debris induced pelvic and femoral osteolysis in a well-functioning prosthesis93. Ceramics are also expensive, have a small fracture risk due to their brittleness, and are sensitive to component mal-positioning that may result in impingement damage and stripe wear. There are also some reports of squeaking associated

wear respectively.

potentially leading to mechanical failure84.

Metal hypersensitivity may also occur87.

with ceramic on ceramic bearing couples94.

**6.3 Alternate bearing couples** 

and ceramic on polyethylene.

Regardless of prosthesis design and bearing surface, surgical technique is an important factor that affects prosthesis survival. Data from large national joint registries has recently facilitated examination of these factors in relation to prosthesis survival.

### **7.1 Hospital type and surgeon operating volume**

Type of hospital and the surgeon undertaking the procedure can influence THA survival. Fowles *et al* showed that low operating volume is associated with increased risk of THA revision95. Similarly, Espehaug *et al,* using data from the Norwegian arthroplasty register, found the lowest revision rates amongst surgeons with the highest THA volume96. In the same study, university hospitals had higher revision rates than local and central hospitals. This may be attributed to the lower number of operations per surgeon at these hospitals or possible centralization of high-risk patients and more complex cases. Bordini *et al* found that prosthesis survival was negatively associated with lower surgeon skill38.

### **7.2 Prosthesis alignment and soft tissue balancing**

Malalignment of prostheses may alter the articulation of prosthesis components with the potential to increase contact stresses and increase wear, this increases the incidence of edge loading and results in stripe wear in hard on hard bearing couples. Despite the advantage of larger femoral head size, soft tissue balancing remains important in the reduction of dislocation of the femoral head. Subluxation of the femoral head during the swing phase of gait, especially in metal on polyethylene couples, causes socket edge contact resulting in wear97. Complete dislocation of the femoral head may damage the head during dislocationrelocation, and can increase wear rates.

### **7.3 Prosthesis dislocation and interface micromotion**

Prosthesis stability influences the development of aseptic loosening. Motion between the prosthesis and bone contributes to the formation of a fibrous membrane rather than bone98. Bechtold et al found that particulate wear debris prevents bone formation in the presence of prosthesis instability 99. In addition, prosthesis motion alters local joint fluid pressures and can transport particles along the periprosthetic space.
