A Case for Ceramic-on-ceramic Total Hip Arthroplasty
Abstract
Abstract
Hip replacement surgery has dramatically improved the quality of life of patients with coxarthrosis. The prevalence of primary total hip arthroplasty (THA) has been steadily increasing over time, and the incidence is predicted to grow by 174% to approximately 572,000 cases by 2030.1,2 Over the years, advances in surgical techniques, implant design, and metallurgy and improvements in polyethylene liner properties have improved the fixation, performance, and durability of these implants. However, for young active patients with hip arthritis there is clearly a need for bearing surfaces with improved wear characteristics.3–5 Ceramics were recognized as an ideal bearing surfacefor THA in the 1970s.6 While earlier ceramic hip systems were limited by fixation issues, modern designs have solved many of the problems associated with its predecessors, making ceramic-on-ceramic THA the ideal alternative bearing surface for young patients undergoing hip replacement surgery.
The Need for Alternative Bearings
There is no question that conventional metal-on polyethylene THA can provide durable and reliable long-term results. Mullins et al.7 evaluated the long-term outcomes of the Charnley THA and found 10-year survivorship of 93% and a survival rate at 30 years of 73%. Other series with modern hybrid and cementless designs have demonstrated 10-year survivorship greater than 99%.8,9
While there are many studies that have shown that a metal-on-polyethylene bearing surface can provide good long-term results, they have also discovered that wear and osteolysis are the leading causes of component loosening and failure.10,11 Furthermore, in the US the trend has been for patients requiring hip replacement surgery to be younger and more active. According to National Institute of Surgeons (NIS) data from 2005, 35,200 patients who were between 45 and 54 years of age underwent THAs. This number is predicted to increase to 234,000 by 2030.12 Consequently, there is a need to consider bearing surfaces with improved wear characteristics for the young patient with hip arthritis.
Metal or Ceramic?
Current choices for alternative bearing surfaces in hip replacement surgery include ceramic-on-ceramic or metal-on-metal articulations. While the wear characteristics for both bearing couples are superior to traditional metal-onpolyethylene hips, there are significant differences between the two.
Metal-on-metal hip resurfacing and replacements have recently gained popularity among hip surgeons because of their low wear rates and the ability to maximize femoral head size, thereby reducing the risk for dislocation.13 Contrary to traditional bearing THA, where the surgeon needs to be cognizant of the size of the femoral head as it relates to acetabular component size and the polyethelyne thickness, metal-on-metal THAs encourage the surgeon to maximize femoral head size to improve the lubrication properties and decrease wear. As a consequence of increasing the head and neck ratio, this allows for a theoretically greater range of motion, decreased risk of impingement, and greater joint stability. However, there are several concerns with metal-on-metal articulations. Hip resurfacing procedures promise a bone-conserving procedure for the young patient with hip arthritis. Aumstutz et al.14,15 reported good longterm results in a series of 350 hips treated with metal-on-metal hip resurfacing, with a five-year survivorship of 97.8%; they also reported a 0.83% risk of femoral neck fractures with this procedure. Other concerns with metal-onmetal bearings are related to the issues of blood metal ion levels and metal hypersensitivity. Several studies have reported detectable levels of chromium and cobalt ions in the urine and blood of patients following implantation of metal-on-metal hip replacements.16,17 While chronic exposure to metals has been shown to be carcinogenic in laboratory animals, no human cases have been reported.18 Visuri et al. reported a slightly increased incidence of leukemia compared with the general population based on data from the Finnish registry.19 Finally, there have been several reports of immune hypersensitivity reaction to metal components leading to pain and early loosening following implantation.20–23 The mechanism remains unclear, and no true causal relationship between metal ion hypersensitivity and osteolysis has yet been proved; however, this remains a significant concern after metal-on-metal THA.
Ceramic-on-ceramic bearings have similar wear characteristics to metal-onmetal bearings. However, because ceramic is inert and has unique material properties, it has advantages over metal-on-metal bearings and is an ideal material for hip replacement in young active patients, in whom implant longevity becomes paramount.
Material Properties
Modern alumina ceramics are ideal materials for bearing surfaces. They are very hard substances and are virtually resistant to scratch, abrasion, and thirdbody
wear. Furthermore, ceramic is hydrophilic and wettable. This allows for decreased friction, high lubrication, and little or no adhesive wear. All of these properties confer significant wear resistance. In both in vivo and simulator studies, ceramic bearing surfaces exhibit significantly less volumetric wear than traditional metal-on-polyethelyne couples.24 In a biphasic wear model based on hip simulator studies, Clarke and his colleagues25 showed that the ceramic-onceramic implants had a >1,000-fold decrease in volumetric wear compared with metal-on-polyethylene at 10 years.
Further advances in ceramic manufacturing technology continue to improve its material properties, increase prosthetic options, and minimize the risk for fracture and/or other complications. Current ceramic-on-ceramic hip articulations are composed primarily of the Biolox® forte alumina (Ceramtec AG, Germany). While this newest generation of ceramics has improved the hardness and material properties compared with its predecessors, and allowed for increased head options (28 versus 32mm and various neck lengths), it currently does not allow for 36mm femoral heads to be used with acetabular shells less than 60mm in diameter. This is a drawback compared with its metalon- metal counterpart.
