3 The technologies
3.1 In total hip replacement (THR) surgery, the acetabulum (hip socket) is replaced with either a single-piece cup made from 1 material (polyethylene, ceramic or metal) or a 2-piece (modular) cup made from a metal outer shell and a polyethylene, ceramic or metal liner. The head of the femur (thigh bone) is replaced with either a single-piece metal stem and head, or a modular component consisting of a metal stem (which may consist of more than 1 piece) with a metal, ceramic or ceramicised metal head.
3.2 THRs vary in what fixation method is used for each component of the prosthesis. In some THRs, all the components are fixed into position using cement (hereafter referred to as cemented THRs). Other types of THR are designed to be used without cement (hereafter referred to as cementless THRs); instead, they are inserted using press-fit fixation, and natural bone growth over time secures the prosthesis in place. Some prostheses are hybrid, in which the femoral component is cemented into place while the cup is fixed without cement, or reverse hybrid, in which the femoral component is fixed without cement while the cup is cemented into place. THRs may also vary by femoral head size, with a large head defined as being 36 mm or more in diameter.
3.3 Hip resurfacing arthroplasty involves removing and replacing the surface of the femoral head with a hollow metal hemisphere, which fits into a metal cup fixed into the acetabulum. All resurfacing arthroplasty prostheses currently on the market are metal-on-metal (MoM), and can be hybrid or cementless. As with THR prostheses, resurfacing arthroplasty prostheses may also vary by femoral head size.
3.4 Patient selection for THR or resurfacing arthroplasty depends on various factors, including but not limited to: patient characteristics (for example a patient's age, activity and underlying hip physiology); the surgeon's choice; and the surgeon's experience of using a particular class of prosthesis.
3.5 Adverse events associated with hip replacement surgery (THR or resurfacing arthroplasty) may occur because of complications at the time of surgery, or may occur years afterwards. Complications that may lead to hip replacement revision surgery include prosthesis instability, dislocation, aseptic loosening, osteolysis (bone reabsorption), infection and prosthesis failure.
3.6 Guidance on the selection of prostheses for primary total hip replacement (NICE technology appraisal guidance 2) recommends that the best prostheses should have a revision rate of 10% or less at 10 years or, as a minimum, a 3-year revision rate consistent with this. Guidance on the use of metal on metal hip resurfacing arthroplasty (NICE technology appraisal guidance 44) recommends MoM resurfacing arthroplasty as an option for people with advanced hip disease who would otherwise receive, and are likely to outlive, a conventional primary THR. The guidance recommends that the best prostheses should demonstrate the same revision rates as recommended in NICE technology appraisal guidance 2.
3.7 The Medicines and Healthcare products Regulatory Agency (MHRA) monitors the safety of devices used in clinical practice. In June 2010, the MHRA issued an alert on all MoM hip replacement prostheses (both THR and resurfacing arthroplasty) after reports of soft tissue reactions that may be associated with pain. In June 2012, the MHRA released an updated alert noting that MoM prostheses (THR and resurfacing arthroplasty) may wear at an accelerated rate. The MHRA stated that people with MoM prostheses may develop soft tissue damage caused by wear debris from these prostheses. It advised annual monitoring of the hip using imaging and measurement of metal levels in the blood to determine whether a revision is needed in people with MoM hip replacement prostheses who have symptoms, or who have a certain type of MoM hip replacement, including stemmed MoM THRs with a larger femoral head (36 mm diameter or more) or the recalled DePuy ASR hip replacements (THR and resurfacing arthroplasty).
3.8 Over 20 companies manufacture prostheses for hip replacement (THR and resurfacing arthroplasty), and some produce multiple brands of components. The NHS Supply Chain provided the average list price costs for 5 manufacturers of the 5 THR categories (varying by fixation method and bearing surface) identified by the Assessment Group and 3 manufacturers of resurfacing arthroplasty prostheses and associated accessories. The average list prices for THRs across the manufacturers were: £1557 for a cemented polyethylene cup plus a metal head; £3016 for a cementless metal cup with a polyethylene liner plus a metal head (cementless stem); £3869 for a cementless metal cup with a ceramic liner plus a ceramic head (cementless stem); £2650 for hybrid cementless metal cup with a polyethylene liner plus a metal head (cemented stem); and £1996 for cemented polyethylene cup with ceramic head (cemented stem). The average list price for resurfacing arthroplasty prostheses across the manufacturers was £2672. Typically, the price of hip replacement prostheses depends on the volume ordered and locally negotiated discounts, so the prices paid by the NHS are routinely lower than the list prices listed above.