Hip resurfacing: The metal-on-metal bearing material is not the problem

To the Editor:

The success of total hip replacement (THR) in the 20th century has been tremendous with improvements in the durability of new designs, bearing materials and fixation techniques. However, the young and active patients have historically had high revision rates compared with older, more sedentary patients, notably when the etiology of the disease is osteonecrosis. Despite great improvements in cementless stem fixation, hip resurfacing arthroplasty (HRA) has the advantages of replicating leg length and offset, and maintaining proximal bone unlike THR. Moreover, dislocation in THR remains a problem when small femoral heads are used. Resurfacing patients also do not report thigh pain as it sometimes happens after THR.

When it comes time for revision surgery, as should be expected for most young and active patients, whether treated with a resurfacing or a primary THR, the preserved bone stock with hip resurfacing provides more favorable conditions for a successful surgery and the technical difficulty of the conversion is comparable to that of a primary THR. This enables patients seeking to restore their previous lifestyle to be more active than with a THR, and numerous authors have reported high levels of physical activity in patients after hip resurfacing.

Preserving bone and replacing only the affected articular surfaces has always been and remains a worthy treatment goal, but the first generation of hip resurfacing suffered from the use of ultra-high molecular weight polyethylene sterilized in air, a material unsuitable for a large bearing size in young and active patients. Unfortunately, and because of the poor results of the materials used during that era, many become detractors of the concept of resurfacing.

The current generation of resurfacing devices uses a metal-on-metal (MoM) bearing because the volumetric wear is low, even with large diameter components, and the material permits manufacturing of thin one-piece acetabular components with porous ingrowth for cementless fixation.

Recent long-term data show that certain currently available hip resurfacing devices can reach up to 99.7% survivorship at 10 years in patients with good bone quality and implanted with large component sizes. The importance of patient selection for hip resurfacing and the key role of component size have been highlighted by the reports of several large centers and the findings of hip registries. However, improvements in surgical technique have considerably reduced the rate of aseptic femoral failures (femoral neck fractures and femoral component loosening) associated with the learning curve of the surgeons who pioneered the procedure, even with patients who have risk factors.

In the past 3 years, the report of numerous adverse local tissue reactions (ALTR) has raised doubts in the orthopedic community over the benefits of MoM implants used for HRA and even more so for THR, probably because of the corrosion products generated at the ball-stem tapered interface. Often misunderstood, the hypersensitivity or allergic reactions to metal are a very rare occurrence. In nearly every case, ALTR are associated with increased wear from poorly positioned or poorly designed components or both. The recall of a particular MoM resurfacing device resulted in intense medico-legal solicitations accompanied by grossly unbalanced reporting in a media blitz and has brought intense scrutiny to all MoM devices. Anxiety has grown among patients and surgeons despite the fact that the vast most of MoM devices are functioning well with up to 20 years of follow-up. This was only aggravated by two reports of extremely rare cases of cobaltism in association with very high wear of a metal-on-metal bearing. Naysayers abound, but few have researched the literature where the answers to the problem can be found (i.e., proper acetabular component design and orientation).

The acetabular components with the largest femoral head coverage have been associated with low production of metal ions and a virtual absence of ALTR. The hypothesis tested in several centers is that a reduced coverage of the head coming from a combination of increased abduction and/or anteversion leads to more instances of edge loading, and in the case of the recalled device, a possible double edge loading, increasing the volumetric wear of the device.

The current focus of most debates about MoM bearings has so far been the wear properties of the material. I believe that the problems ALTR associated with MoM devices, unlike the first generation of resurfacing with polyethylene bearings, are not a bearing material issue per se, but one of device design and technique and can be prevented by proper component socket design and optimized socket orientation in both the coronal and sagittal planes.

Because the need for proper socket orientation is so crucial, emphasis must be placed on using systems in which the instrumentation provides the surgeon with the tools to precisely implant the socket. Since all currently available designs have a reduced coverage in the smaller sizes, safe zones for socket orientation should be determined for all systems using an accurate measurement method for both cup abduction and anteversion (e.g. Einzel-Bild-Rontgen-Analyse; Innsbruck, Austria) in a large series with long-term results. After analysis of our 1350 Conserve Plus Hips (Wright Medical Technology), a safe zone was produced for all sizes suggesting 42° ± 10° of abduction angle and a 15· ± 10· of anteversion angle, which I believe represents a target well within reach for an experienced surgeon.

Although the study of navigation systems applied to hip resurfacing has so far essentially focused on the placement of the femoral component, a case could certainly be made for the use of navigation systems, which have shown efficacy in positioning accurately the acetabular component in THA, especially for inexperienced surgeons. Just as in THR, exposure of the acetabular cavity is essential for an accurate placement of the cup and, despite short-term reports of hip resurfacing performed with small incisions, the minimally invasive surgery technique can hardly be recommended for this procedure.

Unfortunately, the critical importance of these factors has not been well-understood until recently, the common belief being that a large diameter head solves the problem of dislocation in THR just as it does with HRA, while component orientation was less important because of the increased jump distance.

Every bearing material used in joint replacement has had its specific downsides, and this remains true today despite all the technological advances. For example, the necessity of proper acetabular component orientation for ceramic-on-ceramic bearings to prevent dislocation, chipping, runaway wear and squeaking was identified after clinical experience and resolved after in vitro testing. Similar to the ceramic-on-ceramic bearing wear, the MoM wear issues of design and component orientation became apparent after clinical experience, and there is tangible evidence that the problem is solvable. Clearly, this is highly desirable since MoM is the only proven and highly successful material for resurfacing at the present time permitting high levels of activity. The well-oriented and fixed components continue to perform at up to 15 years, with no increase in blood-ion concentration.

The porous fixation of the one-piece acetabular components used in the current designs of hip resurfacing is usually a cobalt chromium aggregate of beads with or without hydroxy apatite coating although several hip resurfacing systems now have titanium plasma spray porous sockets. This type of fixation, although reliable, may not match the performance of recently developed porous tantalum or titanium material mimicking trabecular bone. This new technology has already been adapted to hip resurfacing, although its availability is currently limited.

While most surgeons understand the increased difficulty of performing resurfacing compared with THR, there has been insufficient education for surgeons to gain supervised experience. Fellowships and resident training programs to learn the hip resurfacing techniques hopefully can produce more experienced surgeons ready to use with competence the full spectrum of reconstructive devices available today and in the future.

Improving the socket designs of existing devices to provide larger femoral head coverage by the socket will give the surgeon more flexibility of implantation and lessen the likelihood of abnormal wear. Modifications to socket designs are already in progress to address the weaknesses identified with certain devices. The manufacturing quality of MoM bearings is excellent nowadays in most designs, particularly the control of clearance and roundness of the components, and is certainly sufficient to produce safe and successful hip arthroplasty devices as long as conservative guidelines for cup implantation are followed. However, there will undoubtedly be further improvements and reduction in the wear properties of MoM bearings in the near future.

MoM hip resurfacing is not a new technology any more, and the devices that have been approved by the FDA after sufficient clinical trials have been available for more than a decade. It is important to avoid making statements about a procedure based on the poor results of subsequent designs which were still in their experimental (non-FDA approved) phase.

Many have written off MoM HRA, and this is unfortunate because superb results have been achieved with several devices in several centers for the young and active patient. The MoM bearing, unlike polyethylene of first generation devices, is not the problem. It is a treatment option for arthritis that is worth pursuing and improving. It just makes sense to save the head and neck, and adhere to a fundamental tenet of orthopedics espoused by our forebears — save bone.

It is my fervent desire to improve surgeon and patient education and emphasize that there are techniques to solve the problems associated with MoM hip resurfacing. Charles Kettering said, “The world hates change, yet it is the only thing that has brought progress.”

— Harlan C. Amstutz MD
Los Angeles
Disclosure: Amstutz has received research support from St. Vincent’s Medical Center, and research and royalities from Wright Medical Technology.

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