Little CP, Ruiz AL, Harding IJ, et al. Osteonecrosis in retrieved femoral heads after failed resurfacing arthroplasty of the hip. J Bone Joint Surg [Br] 2005;87-B:320-3.

Shimmin AJ, Back D. Femoral neck fractures following Birmingham hip resurfacing: a national review of 50 cases. J Bone Joint Surg [Br] 2005;87-B:463-4.

Dunstan E, Sanghrajka AP, Tilley s, et al. Metal ion levels after metal-on-metal proximal femoral replacements. J Bone Joint Surg [Br] 2005;87-B:628-31.

Granchi D, Savarino L, Ciapetti G, et al. Immunological changes in patients with primary osteoarthritis of the hip after total joint replacement. J Bone Joint Surg [Br] 2003;85-B:758-64.

The arrival of metal-on-metal hip resurfacing arthroplasty on the elective orthopaedic scene has made a major difference to the subspecialty of hip surgery. Although metal-on-metal is not a new concept, the increasing numbers being performed, combined with a more media-driven healthcare system, have served to make it appear as if the concept is new. However, the JBJS [Br] has recently published a series of papers on the topic, which has served to both further our understanding of metal-on-metal resurfacing as well as to put some of the wilder claims about the procedure into perspective.

Long awaited was the paper by one of the founders of the prosthesis, which was published in March 2004.¹ Here, a single surgeon performed a series of 446 hip resurfacings (384 patients) using cemented femoral and uncemented hydroxyapatite-coated acetabular components. The mean follow-up was 3.3 years (maximum 8.2 years) and only one revision was reported. This was despite a high level of occupational and leisure activities after surgery and a mean patient age of 48.3 years (26.8 to 54.9).

Clearly, such a report is of major significance to the modern-day hip surgeon. However, there are some shortcomings that need to be considered. First, only one surgeon was involved, the founder of the procedure, from whom one must expect good results. How this prosthesis will fare in the hands of others remains unknown. Secondly, a mean follow-up of 3.3 years is short by any standard, and would generally be regarded as of dubious value if applied to a total hip replacement. Of course, whether it is valid to compare a resurfacing with a total hip replacement at all is open to question, particularly when the former is generally regarded as a holding procedure before the latter. Thirdly, there was a minor alteration of design during the period of this study, certainly in respect of the acetabular hydroxyapatite coating. It is unlikely that this would have made a great difference to the results. However, taken as a whole, this paper shows that good surgeons tend to get good results and that the modern metal-on-metal hip resurfacing arthroplasty is perhaps the best of the resurfacing designs to be manufactured to date. The orthopaedic community awaits the longer-term results from this group, as well as the results of others, in order to put metal-on-metal resurfacing arthroplasty in its appropriate place in the orthopaedic armamentarium.

With all new techniques it is important that initial enthusiasms should be tempered by realities, so the JBJS [Br] has published a number of papers describing the complications of this procedure. Osteonecrosis is one such issue, featuring in the paper by Little et al,² who reported the histological findings of bone retrieved from beneath the femoral components of failed metal-on-metal hip resurfacing arthroplasties. Of 377 hip resurfacings, 15 had required revision, the mean time to failure being nine weeks (1 to 69). Failure was caused by a number of different factors: fracture of the femoral neck (eight hips), component loosening (five), culture-negative inflammation (one) and persistent pain (one). Histological specimens had been taken from the femoral head at the time of the initial implantation, as well as at revision, so this was an ideal opportunity to identify any changes that might have occurred. The authors’ finding was fascinating – osteonecrotic changes were identified in all but one of the revisions, leading to the conclusion that osteonecrosis is a common finding in failed hip resurfacings. The authors went on to suggest that the high incidence of osteonecrosis might explain the aetiology of fractures of the femoral neck after resurfacing surgery. This paper is clearly of great interest for a number of reasons. First, it originates from a unit other than that of the founding surgeon and reports a significant post-operative revision rate (15 of 377, 3.98%) at a mean of nine weeks after surgery. Secondly, it opens the debate about post-operative fracture of the femoral neck and whether this is indeed caused by osteonecrosis or whether, as some believe, it may be related to operative technique. Final impaction of the femoral component may place significant stresses through the head/neck junction, the results of which may only become apparent once the patient begins to put full, unsupported weight through their hip resurfacing some weeks after the procedure. This paper was not the first in this area, as others have previously considered the issue of osteonecrosis^3,4 although the association between osteonecrosis and fracture proposed by Little et al² is manifestly of concern.

In orthopaedic circles, a fracture of the femoral neck after hip resurfacing arthroplasty is now a hotly debated topic. Consequently, the paper by Shimmin and Back⁵ is also of interest. This reports on 3497 Birmingham hip resurfacings inserted over a five-year period by 89 surgeons. There were 50 fractures of the femoral neck, giving an overall incidence of 1.46%, although this figure was higher for women (1.91%) than men (0.98%). Again, however, the mean time to fracture was short at 15.4 weeks (0 to 56). Associated problems appeared to be varus placement of the femoral component, intra-operative notching of the femoral neck and technical issues at surgery. There was no relationship between the experience of the surgeon and the time in their individual series when fracture occurred. The earliest fracture in the surgical learning curve was with the first patient and the latest was patient number 224. However, whatever the cause of fracture, this paper, and that by Little et al,² have now identified the likelihood of a fractured femoral neck either at or after hip resurfacing surgery, as being slightly less than 2%.

As well as highlighting potential early problems associated with hip resurfacing arthroplasty, the JBJS [Br] has also focused on the longer term. The paper by Dunstan et al⁶ is of value in this respect. They determined the levels of cobalt, chromium, titanium and vanadium in urine and whole blood of patients who had either a metal-on-metal or metal-on-polyethylene hip articulation in place for a mean of 32 years (26 to 38). These values compared with those of a group of controls. Significantly elevated levels of titanium and vanadium were found in whole blood, as well as raised urinary chromium, in both arthroplasty groups. Whole blood and urinary cobalt levels were significantly elevated (50 to 300 times normal) when the articulations were loose. These findings are clearly of interest as there is intense current debate about the possible long-term effects of exposure to high metal ions after metal-on-metal hip resurfacing arthroplasty. The fact that high metal levels may fluctuate over time, but never disappear, and indeed may become worse as components loosen or shift, is an important finding. Some early reports have suggested that there may be an association between hip arthroplasty and tumour formation^7,8 while the Finnish registry⁹ highlighted a nearly four-fold increase in leukaemia, not statistically significant, with metal-on-metal as opposed to metal-on-polyethylene articulations. The paper by Dunstan et al⁶ cannot relate high metal levels to the development of a tumour. Indeed, malignant tumours had formed the surgical indication for many of the procedures they investigated. Group sizes (five groups in all) were also small, making statistical comparison between the groups largely irrelevant. Nevertheless, it is clear that the area of metal toxicity after hip resurfacing arthroplasty is still under intense scrutiny, and rightly so. A paper which may well enter the debate about metal toxicity, again published by the JBJS [Br], predates that by Dunstan et al⁶ and is by Granchi et al.¹⁰ This paper investigated immunological abnormalities in patients after a total hip replacement using either a metal-on-metal, metal-on-polyethylene, or ceramic-on-ceramic articulation. Patients with metal-on-metal or metal-on-polyethylene articulations showed a significant decrease in the number of T-lymphocytes and a significant increase in serum chromium and cobalt levels, although there was no correlation between these metal levels and the immunological changes. The ceramic-on-ceramic articulations did not demonstrate such features. Granchi et al¹⁰ concluded that a cell-mediated immune response might be present in patients with a well-fixed total hip replacement, whose immunological changes may be more evident if there is more than one metal component in the articular coupling. However their paper was unable to answer the question as to whether these immunological changes might compromise the body’s defence mechanisms against foreign organisms or deviant native cells. Time, and further research, will no doubt tell.

References:

1. Daniel J, Pynsent PB, McMinn DJW. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg [Br] 2004;86-B:177-84.
2. Little CP, Ruiz AL, Harding IJ, et al. Osteonecrosis in retrieved femoral heads after failed resurfacing arthroplasty of the hip. J Bone Joint Surg [Br] 2005;87-B:320-3.
3. Howie DW, Campbell D, McGee M, Cornish BL. Wagner resurfacing hip arthroplasty: the results of one hundred consecutive arthroplasties after eight to ten years. J Bone Joint Surg [Am] 1990;72-A:708-14.
4. Campbell P, Mirra J, Amstutz HC. Viability of femoral heads treated with resurfacing arthroplasty. J Arthroplasty 2000;15:120-2.
5. Shimmin AJ, Back D. Femoral neck fractures following Birmingham hip resurfacing: a national review of 50 cases. J Bone Joint Surg [Br] 2005;87-B:463-4.
6. Dunstan E, Sanghrajka AP, Tilley S, et al. Metal ion levels after metal-on-metal proximal femoral replacements. J Bone Joint Surg [Br] 2005;87-B:628-31.
7. Gillespie WJ, Frampton CM, Henderson RJ, Ryan PM. The incidence of cancer following total hip replacements. J Bone Joint Surg [Br] 1988;70-B:539-42.
8. Visuri T, Koskenvuo M. Cancer risk after McKee-Farrar total hip replacement. Orthopedics 1991;14:137-142.
9. Visuri T, Pukkala E, Paavolainen P, Pulkkinen P, Riska EB. Cancer risk after metal-on-metal and polyethylene-on-metal total hip arthroplasty. Clin Orthop 1996;329(Suppl):280-9.
10. Granchi D, Savarino L, Ciapetti G, et al. Immunological changes in patients with primary osteoarthritis of the hip after total joint replacement. J Bone Joint Surg [Br] 2003;85-B:758-64.

Mr Richard Villar, BSc MA MS FRCS, Consultant Orthopaedic Surgeon
The Wellington Hospital, Wellington Place, London, NW8 9LE

Mr Vijay Shetty, MS(Orth), Consultant Orthopaedic Surgeon
LH Hiranandani Hospital, Powai, Mumbai 400 076, India