7.14 Hip resurfacing

Hip resurfacing arthroplasty is an attractive concept, as it not only preserves proximal femoral bone stock at the time of surgery, but by optimizing stress transfer to the proximal femur it minimizes stress shielding in the medium to long term. The large diameter of the articulation offers inherent stability and optimal range of movement. Total hip replacement has a higher failure rate in younger patients, i.e. those under 55 years, compared to elderly patients. Hip resurfacing arthroplasty has emerged as an alternative to total hip replacement in younger patients (Box 7.14.1). In less than a decade, since its renaissance in the 1990s, resurfacing has become very popular with the number of implantations accounting for 8–10% of all primary hip replacements in countries such as the United Kingdom, Australia, and the Netherlands.

The first hip resurfacing arthroplasty was performed by Smith-Petersen in 1923. He temporarily interposed a thin hemispherical shell between the femoral head and acetabulum in patients with arthrosis and expected new cartilage to grow on the articular surfaces. Originally, the shells were made of glass, bakelite, or celluloid. In 1938, he first implanted metal shells made of vitallium (cobalt-chrome-molybdenum (Co-Cr-Mo) alloy). In the 1970s, the second generation of resurfacings were developed. They were total resurfacings with a metal-on-polyethylene bearing. The clinical results were disappointing. Their failures can be mainly attributed to osteolysis induced by wear particles from the thin-walled polyethylene acetabular component.

In the 1990s, recognizing this problem, the third generation of resurfacings with a metal-on-metal bearing was developed in Birmingham, United Kingdom, by McMinn and Treacy. After a few attempts of trial and error, they found that the best fixation was obtained with a cemented, stemmed femoral hemispherical Co-Cr-Mo cap and an uncemented hydroxyapatite (HA)-coated Co-Cr-Mo acetabular cup and in 1994, implantation of McMinn prosthesis was started. Now, at least 15 types of resurfacing prosthesis are available in the market. These prostheses differ in many details, such as shape, sizing, head coverage, clearance, metal alloy used, heat treatment, instrumentation, and so on. Just as with conventional hips, they exhibit differences in clinical outcome. The Swedish and Australian registries have listed a two- to threefold difference in revision rate between different makes.

There are several theoretical advantages of hip resurfacing over conventional hip replacement but the lack of long-term data and emerging new modes of failure are the main disadvantages and cause for concern (Table 7.14.1).

A man younger than 65 years with primary osteoarthritis with good bone quality and normal proximal femoral anatomy is the ideal candidate for hip resurfacing. Infection, osteoporosis, severe bone loss either of femoral head or acetabulum, and renal failure are the absolute contraindications for resurfacing. Patient selection for hip resurfacing is no less important than any other surgery. Important indications and contraindications are listed in Table 7.14.2.

The National Institute of Health and Clinical Excellence (NICE) recommend metal-on-metal hip resurfacing arthroplasty to be performed only by surgeons who have received training specifically in this technique. Resurfacing can be carried out through a posterior or a direct lateral approach. The lateral approach to the hip has been advocated as a way of preserving the deep branch of the medial femoral circumflex artery thus preserving blood supply to the anterosuperior quadrant of the femoral head but with the lateral approach there is significant risk of abductor dysfunction. Abductor dysfunction is very poorly tolerated in young patients who wish to regain an active lifestyle. The posterior approach is currently favoured by most surgeons. The approach is more extensile than traditional posterior approach. Circumferential capsulotomy and gluteus maximus tenotomy is required for good exposure of femoral head and creation of a superior pocket is required to accommodate the femoral head and neck while acetabulum is prepared.

Easy resurfacing arthroplasties do not exist; with a steep learning curve, it remains a difficult procedure even in experienced hands. In particular, component positioning and cementing technique are critical factors—and the main causes of failure in retrieval studies. The main reason for early revision is fracture of the neck arising from the edge of the implant, which can be initiated by uncovered reamed bone and notching. Early revisions are more common in older patients and females, and bone quality seems to be a critical factor. Late revisions are without fracture and most of them show signs of wear and rim loading, which can be related to malpositioning. Varus placement of the femoral component leads to higher levels of stress and increases the probability of failure. It is recommended that a surgeon should strive to achieve a relative valgus placement of 5–10 degrees while avoid to notch the superolateral cortex of the femoral neck.

Computer-assisted navigation systems have been used with success to avoid malpositioning of components, especially in early phase of learning curve.

Very good early and mid-term results have been reported from the centre of origin of Birmingham hip resurfacing prosthesis. They reported survival of 99.7% of 439 hips at a mean follow up of 3.3 years for patients under the age of 55 years, and 98% of 144 hips at a minimum follow up of 5 years for males under the age of 65 years and females under the age of 60 years. Their survival rates in multicentre studies have shown worse results. The National Joint Registry for England and Wales (2007) reported revision rates of 1.8% at 3 years; the Australian Orthopaedic Implant Register (2007) reported 3.7% at 5 years; and the Oswestry Hip Outcome Centre reported 4.6% at 7 years.

The main reasons for revision of a metal-on-metal hip resurfacing are femoral neck fracture, aseptic loosening, and pain (Box 7.14.2).

Femoral neck fracture is a unique complication of resurfacing occurring in 0–7%. The causes of fracture can be related to the patient, the surgical technique, or a combination of both. Patient-related factors include decreased bone mass, obesity, and inflammatory arthritis. Intraoperative characteristics that may lead to fracture if proceeding with resurfacing include cysts and/or exposed bone found in the femoral neck during preparation of femoral head. Surgical errors include notching of the femoral neck, preparing the femoral head and, therefore, tilting the prosthesis into excess varus (<130 degrees stem-shaft angle), and seating of the cup into retroversion.

Undisplaced fracture can be successfully treated with a few weeks of toe-touch weight bearing. Displaced fractures or undisplaced fractures failing to unite with conservative treatment can be treated by conversion to standard total hip replacement. If the cup is in a good position and well fixed, the hip can be revised using a standard stem with a large modular femoral head to articulate with the cup, otherwise both components can be revised.

Aseptic loosening of the femoral component has been reported in some series, occurring in up to 2% of cases. Large areas of cystic degeneration in the head leaving less surface area available for fixation and also a lower stem shaft angle are more frequently associated with aseptic loosening.

Hip resurfacing performed with a posterior approach has been shown to cause a 60% decrease in oxygen concentration in the femoral head and component insertion results in a further 20% decrease with no significant improvement occurring after wound closure. This raises concern regarding viability of femoral head after resurfacing, but excellent short-term results of resurfacing have been reported in patients with established osteonecrosis and it remains to be determined with longer follow-up whether femoral head viability has any significant effect on the outcome of resurfacing. True incidence of avascular necrosis is difficult to determine in asymptomatic patients due to overshadowing of metal on plain x-ray. Avascular necrosis of femoral heads accounting for revision have been reported in 0.5–1% of all resurfaced hips and may be held responsible for some of the neck fractures in the absence of other risk factors.

The approach for hip resurfacing is usually extensile and involves more tissue dissection and handling than conventional hip replacement. Heterotopic ossification has been seen in 28–60% of patients after hip resurfacing but only 7–8% develop Brooker grade 3 or 4. High-risk patients should have prophylactic treatment.

Hip resurfacing is inherently more stable than conventional hip replacement because of larger diametric clearance of femoral components. The reported incidence is less than 1% as opposed to 3–6% in conventional hip replacement.

Some degree of narrowing of the femoral neck is seen in three-quarters of the patients who have had hip resurfacing but only 14–28% have more than 10% of narrowing. Female patients are 2.5 times more likely to develop neck narrowing and patients with higher (valgus) neck-shaft angle are also more at risk. Neck narrowing seems to stabilize with time with no adverse clinical or radiological features noted in cases followed-up for 6 years. No significant progression of narrowing was noted in these patients after 3 years.

Several other complications related to resurfacing arthroplasty have been reported including clicking, squeaking, soft tissue impingement, psoas tendinitis, nerve palsy, metallosis, osteolysis, raised metal ion levels, and aseptic lymphocytic vasculitis associated lesions (ALVAL) or pseudotumours (Figure 7.14.4).

The latter two have been investigated by many and remains focus of most research related to metal on metal bearing arthroplasty. The peak serum cobalt level and that of serum chromium level have been seen at 6 months and 9 months respectively. The serum levels of metal ions gradually decline as the bedding-in wear changes to steady-state wear but have remained higher than preoperative levels even at 2-year follow-up. Long-term impact of raised metal ion levels is unknown and remains a major concern against general acceptance of metal-on-metal hip resurfacing arthroplasty. The exact aetiology of aseptic periarticular soft tissue mass (pseudotumours) is unknown, but the two widely accepted explanations in the current period of time are T-lymphocyte-mediated delayed-type hypersensitivity reactions and/or direct toxic effect of a very high concentration of metal debris in the joint fluid. The individual biological response to the presence of wear debris seem to vary, and may be due to different toxic-effect threshold or immunological tolerance. They occur with an overall incidence of 1% in the first 5 years after implantation of metal-on-metal hip resurfacing and are commoner in females. Histological specimens harvested at the time of revision show inflammatory changes, vasculitis, and necrosis.

Periprosthetic osteolysis is a rare complication of metal-on-metal arthroplasty. The histological examination showing a perivascular accumulation of T lymphocytes and immunohistochemical analysis showing elevated levels of bone-resorbing cytokines such as IL-1 and TNF-α produced by infiltrating lymphocytes and activated macrophages suggest that early osteolysis is associated with delayed-type hypersensitivity to metal.

Speculation of future association of metal-on-metal bearing arthroplasty with malignancy or genotoxicity is based on the findings of research related to intracellular metabolism of chromium and cobalt, which has been found to release free radicals (reactive oxygen and nitrogen species). Free radicals are known to be involved in protein oxidation, leading to their degradation, lipid peroxidation, and DNA damage.

Lessons from the failure of previous generations of hip resurfacing have helped to select better materials, improve implant design and manufacture, and also upgrade instrumentation. Cementless acetabular components and cemented femoral components fixation provide better and more reliable results compared to alternative methods of fixation. Orientation of implants and surgical time improve with experience as surgical technique has a steep learning curve. Patient selection is of paramount importance and not all patients with osteoarthritis of hip under the age of 65 years are candidates for hip resurfacing. Female patients tend to have a higher incidence of complications and they must be carefully warned before offering hip resurfacing at any age. Recent studies demonstrating excellent short-term results of metal-on-metal resurfacing in active patients younger than 60 years of age have established it as an alternative to conventional hip replacement.

The importance of long-term clinical follow-up of patients undergoing metal on metal hip resurfacing arthroplasty cannot be overstressed. The short-term success of resurfacing arthroplasty should not overwhelm and the research into the molecular and cellular mechanism involved in the pathogenesis of periprosthetic biological adverse reactions caused by metal wear particle must continue.