Metal on Metal Hip Replacements

Metal on metal (MOM) hip replacements have been in clinical use since 1936. Several generations are recognised but all use both bearing surfaces made from cobalt-chromium-molybedanum alloy¹. Early devices such as the McKee-Farrar suffered from high friction, sometimes due to equatorial contact or “clutch-coning”. However, some survived 30 years of clinical use and showed very little wear². Improvements in manufacturing in the 1990s enabled a revival of this potentially low wearing hip by reliably manufacturing a polar bearing MOM hip. For the current generation of devices, laboratory studies have shown 100 fold less volumetric wear rates of MOM hips when compared to metal on polyethylene (MOP) hips^1,3,4. This has resulted in more than 500K implantations of current generation MOM hip replacements (figure 1) over the last 15 years. We are only now in a position to be able to assess the medium term performance of these bearings in large patient populations.

Figure 1. MOM hip resurfacing (AH, 2007) used to treat end-staged osteoarthritis in a 55 year old man.

There are reports from inventor-surgeons of the high success rates of these current generation devices^5-7. Perhaps more importantly, there are good results reported by non-inventor surgeons (table 1).

Table 1. Clinical outcome reports from non-inventor surgeons of current generation MOM hip resurfacing8-10

Our estimate that at least 500,000 current generation MOM hips have been implanted is based on the data in Table 2. It is more straightforward to calculate data from MOM hip resurfacings because these are usually coded separately from hip replacements with a stem (which usually have either MOM, MOP or ceramic on ceramic (COC) bearings). Some of the hip joint registries report the different bearing types (MOM, MOP, COC) used but do not distinguish between MOM as a resurfacing or as a modular / stemmed hip replacement.

Table 2. Reports used to estimate the number of implantations of current generation MOM hip replacements implanted over the last 15 years.

Table 2 includes revision rates from the most commonly cited hip replacement registries. Data of this type is usually reported after exclusion of failure due to infection and with osteoarthritis as the primary diagnosis. Therefore, even if all primary and revision operations were recorded and linked, the actual number of revisions is greater than reported. The linking of primary and revision operations can be difficult to validate and virtually impossible in the UK if the revisions are conducted in the independent sector. However, the UK NJR has linked 157K primaries over a 5.5 year period in which approximately 300K would have taken place.

Accepting the limitations regarding linkability, there are several patterns shared by these registries. First, the revision rate for MOM hip resurfacing is greater than for all types of hip replacements with a stem, regardless of bearing surface. It should be noted that some of the revision rates were calculated from a small sample size (sometimes 10

Figure 2. 60% of the 240 MOM hips collected by the LIRC are from females, yet the UK NJR reports that only 33% of hip resurfacings are in females.

revisions or less) and therefore may not be fully representative. Secondly, factors associated with failure include: female gender; small femoral head component size (<50mm); age > 65 years. Female gender and small size are associated so it is not yet clear which is the most important contributor to failure. Preliminary unpublished data from the London Implant Retrieval Centre (LIRC) supports the higher failure rate in females (Figure 2). The effect of age is probably not surprising given that bone quality decreases and that the area of purchase of the femoral component is less for hip resurfacing than for stemmed hip replacement.

Additional risk factors for failure of hip resurfacing are likely to include osteonecrosis, large femoral head cysts, abnormal anatomy, and previous surgery. Surprisingly, younger age and higher demand do not appear to increase the failure of hip resurfacing when compared to stemmed hip.

If the use of hip resurfacing was determined solely by its failure rate then it would probably not still be in clinical use. The higher failure rate may be a consequence of failure to avoid its use in patients that have risks factors for failure. In males, aged less than 65 years old, hip resurfacing has been shown to work very well, possibly better than any other device¹⁶. Furthermore, these patients have not sacrificed their: femoral head; femoral neck and intra-medullary portion of the proximal 15cm of femur.

MOM is the only bearing combination that allows resurfacing: no other bearing surface allows a relatively thin (3mm) femoral “cap” and acetabular “lining” to provide bone fixation on the non bearing surface and low wear rates. In terms of low wear rates, COC outperforms MOM¹⁷. Therefore the only advantage we accept for MOM bearings is their application to resurfacing. Perhaps a more pertinent question is: can we justify the use of MOM bearings on a stem when we can use lower wearing, large diameter COC bearings such as in figure 3?

Figure 3. A COC hip replacement (AH, 2008) used to treat end-staged osteoarthritis in a 57 year old female. A 50mm acetabular shell was used with a 36mm head and a 14mm diameter femoral trunion (head : neck ratio of 2.6 : 1).

We do not know whether the failure rate of identical MOM bearings is higher when used as a hip resurfacing rather than a stemmed version. Hip resurfacings are more susceptible to femoral neck fracture; however, the rate of this complication has dramatically reduced in recent years and may be equal to the rate of periprosthetic fracture around stemmed hips. The most common cause for revision of a hip resurfacing in the 5th annual report of the UK NJR was unexplained (“pain” plus “other”) in 43% of revisions. The proportion of unexplained revisions of stemmed hip replacements was approximately 14%.

There are clinical studies showing good results for stemmed / modular MOM hips¹⁸. In fact, the UK NJR reported a revision rate for MOM hips (excluding hip resurfacings) of 1.9% over the 2003 - 2008 surveyed period. This was comparable to MOP at 1.6% and better than COC at 2.2%. However, the number of MOM primary operations in the survey was only 1304.

One possible mechanism for unexplained failure of hip resurfacings may be malposition of the cup. The conservation of the femoral head during hip resurfacing results in difficulty achieving adequate access to the acetabulum. This is usually overcome by complete capsulotomy and release of the gluteus maximus and reflected head of rectus femoris tendons. These manoeuvres require special training and are more difficult in obese and muscular patients. Thus, hip resurfacing is more likely to be associated with malposition than stemmed hip replacement. Derek McMinn - pioneer of the Birmingham Hip Resurfacing (BHR)¹⁹ – probably realised that hip resurfacing was a technically demanding operation because he insisted that every surgeon who wanted to use the BHR prosthesis had to attend a dedicated course. I attended one of these courses in 2003: it was thorough and comprehensive. In Australia the first 3000 BHR implantations were supervised by surgeons experienced in the technique. This probably explains the relatively low failure rate of the prosthesis in the Australian Joint Registry.

It is accepted that a high inclination angle of the cup increases the chance of edge loading and therefore increases the wear rate of MOP hip replacements²⁰. The same is true for MOM hips from retrieval studies²¹ and studies that use blood metal ion levels as a surrogate marker for wear rate ^22-24. Both types of bearing produce wear debris that cause adverse biological reactions and therefore are likely to occur more frequently in hips with higher wear rates - although this has not yet been shown conclusively for MOM hips. However, the type of biological reaction depends on the bearing type, with MOP more likely to cause osteolysis²⁵ and MOM more likely to cause an inflammatory reaction of the hip capsule that has been labelled ALVAL²⁶ and pseudotumour²⁷. The ISO 10993 biocompatibility testing failed to predict this, presumably because the particulate form of the alloy was not tested.

Early analysis of LIRC data has revealed that approximately one third of failures have a cup inclination angle greater than 50 degrees. Many of these have high blood metal ion levels and inflammatory lesions on MARS MRI scan (figure 4). Yet we still do not know how sensitive these implants are to different positions and when to attach the label of malposition: is an inclination of 61 degrees malpositioned but 59 not?

Figure 4. A Metal Artefact Reduction Sequence (MARS) MRI of a patient with a fluid level within an inflammatory mass posterior to a MOM hip resurfacing.

There are a group of patients who have well positioned implants and suffer with pain, severe enough to require revision. Characterisation of these patients is essential²⁸ to ensure against unnecessary revision and to assist in understanding the mechanism of pain generation.

In the UK, surgeons rapidly adopted the use of hip resurfacing so that it became the most commonly used type of hip arthroplasty in men less than 50 years old²⁹. There has been a slight decline in its use in the last 3 years: 10% , 9% and 7% in the 4th, 5th and 6th annual reports of the UK National Joint Registry. This probably reflects a better understanding of which patients to select: males less than 65 years with good bone appear to do very well from this procedure.

Until we can explain the mechanism of failure in those patients with hip resurfacings revised for unknown causes then we should be cautious in adopting MOM bearings for stemmed hip replacement.

For further information please visit: www1.imperial.ac.uk/medicine/hipcentre

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