Introduction

Total hip arthroplasty (THA) has been shown to successfully and predictably relieve pain and improve function in patients with symptomatic coxarthrosis. As a direct result of its success, hip replacement surgery is increasingly being considered for the management of younger patients with symptomatic osteoarthritis of the hip. These patients have higher functional demands and greater life expectancy ^1-3 than traditional THA patients, posing a challenge to arthroplasty surgeons.

The conventional approach to treating this younger group of patients has been THA with cemented or uncemented components, or a combination of these components, with good results reported for the combination. Likewise, good results have been reported with two alternative surgical options: custom Computer Assisted Design – Computer Assisted Manufacture (CAD CAM) THA ^4-6 and metal on metal hip resurfacing arthroplasty (MoM HR), although a great deal of controversy surrounds routine use of MoM prostheses. This brief review focuses on primary custom CAD CAM prostheses as well as MoM HR.

Custom CAD CAM Prostheses

These components are designed based on endosteal geometry in order to optimise fit and fill of the patient’s proximal femur with removal of as little bone as possible.⁴ Information on the shape of the proximal femur is provided by plain X-ray ⁴ or computed tomography images.⁵ The rationale behind the use of these implants lies with the notion that by conforming the shape and design of the prosthesis to the existing proximal femur, one could recreate a hip joint that is biomechanically and physiologically as close as possible to the patient’s normal hip. Specifically for uncemented total hip replacement designs, optimal results rely on obtaining initial stability, contact over the entire ingrowth surface, and uniform stress transfer to the proximal femur.⁷ Custom prostheses have produced excellent long-term results in young, active patients in the primary setting and good long-term results when used for revision THA.⁸

The medium- to long-term results of uncemented custom femoral components have been encouraging:

• Muirhead-Allwood et al ⁴ recently reported a worst-case survival of 98.2% at 13.2 years for a cohort of 112 CAD CAM femoral components in patients with a mean age of 46.2 years.
• Flecher et al ⁵ described improved function at up to 15 years, but survival of the femoral component was only 87%.
• Akbar et al ⁶ reported no femoral failures at a mean of 14 years postoperatively in a population of patients younger than 40 years.
• Benum et al ⁹ reported promising medium-term clinical results and a revision rate for the femoral component of 1.1% at 7 years in a cohort of 191 femurs of normal and abnormal shape and dimension.
• Similarly, encouraging results have been reported in patients with osteoarthritis secondary to hip dysplasia.^10-12

While these results are encouraging, the outcome measures and design processes have varied among papers, and not all authors measured activity levels. This makes comparison difficult. It is important to also note that these results are comparable to the best published results for conventional components with any method of fixation.

Hip Resurfacing

First attempts at hip resurfacing with Teflon components by Charnley in the 1960s and Wagner in the 1970s led to catastrophic failures. Gerard and Muller ¹³ pioneered MoM HR in the 1970s with only limited success. These failures were mainly due to poor engineering and material choice.

The modern era was heralded by McMinn in the mid-1980s after he recognised the low long-term wear and clinical success of the Ring and McKee Farrar prostheses in his region (Derek McMinn, personal communication). He re-introduced the concept of MoM HR using third-generation implants that consisted of a cemented metal femoral component and a press-fit metal acetabular component (designs varied among manufacturers).¹⁴ Proposed advantages of modern MoM HR include:

• Conservation of femoral bone stock
• Low wear rates
• Improved proprioception and stability,
• Increased range of motion
• Relative ease of revision (particularly on the femoral side)

These advantages have made MoM implants attractive for use with younger, high-demand patients.^15-17

Few subjects, however, are as controversial as contemporary MoM HR in current orthopaedic literature. Results are varied and seem to be design related. This has led to at least one prosthesis being withdrawn.¹⁸

Numerous case series have reported results of modern resurfacing implants.^15-17 Recent reviews have also addressed the evidence for the surgical technique, outcomes, complications and modes of failure of the hip resurfacing prostheses.^19-20 Furthermore, van der Wegen et al ²¹ recently demonstrated that none of the hip resurfacing implants used to date meets the full 10-year National Institute of Clinical Excellence (NICE) benchmark for choosing a primary total hip replacement, which is a survival rate of at least 90% at a 10-year follow up.

While good short-term results have been reported,¹⁷ potentially serious complications have also emerged including elevated metal ion levels, aseptic lymphocytic vasculitis associated lesions (ALVAL), and soft tissue destructive lesions (pseudotumors).²²

Hip Resurfacing vs. Custom Prostheses

Unfortunately, there is a paucity of literature on the subject of hip resurfaces versus custom prostheses, with many authors focusing on the outcome between conventional THA and hip resurfacing. A recent review ²³ found that while some authors report earlier return to sporting activities, functional results and patient satisfaction levels are comparable between the two groups.

Sandiford et al ²⁴ recently reported results of a prospective study comparing two well-matched cohorts of young patients treated with hip resurfacing and custom uncemented (CAD CAM) prostheses. They found no significant clinical difference or difference in level of function between the two groups in the short term. To the authors’ knowledge, this is the only study directly comparing these groups of patients.

National Joint Registry, England and Wales, 2010

The recent publication of the 7th annual report of the National Joint Registry revealed an overall revision rate of 2.9% at 5 years following primary hip replacement. The revision rates varied according to the type of prosthesis. The 5-year revision rate was lowest in patients who received a cemented prosthesis (2.0%). Patients who received a resurfacing prosthesis had a 5-year revision rate of 6.3%, worse than cemented, cementless, and hybrid prostheses.

Furthermore, the impact of prosthesis type depended on the age and gender of the patient:

• In men younger than age 55 years, the 5-year revision rates were lowest in those with hybrid prostheses (2.6%) and as high as 5.6% in those with hip resurfacing.
• In women younger than age 55 years, the lowest 5-year rates were seen in those with a cemented prosthesis (3.6%), whilst it was up to 8.3% in those with resurfacing.
• A similar pattern was found for patients aged 55 to 64 years, with 5-year revision rates being consistently worse for patients with resurfacing prostheses.

References

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