Total hip arthroplasty (THA) is the gold standard in the treatment of degenerative hip disease, especially in the older patient. Concerns regarding the higher levels of failure of traditional implants in younger, more active patients [1] have led to a search for alternative arthroplasty techniques. Hip resurfacing (HR) is one of these alternatives, and has been around in a recognizable form since the 1970s [2]. When compared with THA, HR has some theoretical advantages that stem from preservation of the patient’s normal proximal femoral anatomy and the use of a large diameter metal on metal bearing. This has the potential to more accurately replicate physiological hip function, reduce the risk of dislocation and allow higher levels of activity with minimal wear of the articulating surface. In addition, the preservation of proximal femoral bone stock offers the potential for easier revision options as would inevitably be required in younger patients. In order to be considered a suitable alternative, HR would need to demonstrate improvements or at least equivalence in functional outcomes and survivorship along with evidence of successful preservation of bone stock leading to good outcomes from future revision surgery. Whilst the recent expansion of data both in the orthopaedic literature and the mainstream media concerning the potentially devastating problems from large metal-on-metal (MoM) bearings in some settings carries some salient lessons for the development, marketing and uptake of new orthopaedic implants, it should be put in the context of the resurfacing literature as a whole. Whilst a systematic review would allow the formation of more definite conclusions, it is hard to evaluate more than one or two outcomes. The choice of implant is a significant factor in the outcomes of HR especially; the proliferation onto the market of inferior implants had an effect on available evidence [3,4]. The results from even high quality studies evaluating the results of recalled or discontinued implants would skew the conclusions. In this paper we therefore chose to review the broader evidence base for HR compared with THA covering multiple different aspects between the two procedures. The aim is to provide a comprehensive overview to help surgeons understand the current areas of controversy.

The structure of this article was designed to review the relevant published evidence for HR and THA under the following sub-sections: Biomechanics, Clinical Function, Patient Reported Outcomes, Implant Survivorship, Adverse Events and Implant Revision. Articles for each section were identified using a broad range of search terms to identify comparative studies between HR and THA or descriptive studies for either technique relevant to each section heading. We included only studies published in the English language after searching the MEDLINE, PubMed, EMBASE and Scopus electronic databases. The numbers and references for each set of articles reviewed for each section are summarized in Appendix 1. Each has been graded for a level of evidence according to the system used by the JBJS (Am) since 2003 [5]

Biomechanics

From a biomechanical point of view, the minimal bone resection on the femoral side and the preservation of proximal femoral anatomy in HR has the potential to better replicate the normal hip physiology of the patient (Figure 1). Although accurate pre-operative templating may reduce inaccuracies in offset and leg length in THA, inevitably the ability to completely restore these factors will be limited by the modularity and flexibility of the implant system used. Two retrospective studies by Ahmed et al and Silva et al demonstrated that HR more accurately restores femoral offset and leg length compared with THA [6,7]. Girard et al. showed similar findings in a prospectively randomized study [8]. Notably however, the study by Silva et al. also suggested that if the pre-operative leg length discrepancy is greater than 1 cm, a THA may be required in order to restore this variable [7]. A single surgeon trial by Loughead et al. showed in contrast a more accurate restoration of offset and leg length in a THA group compared with HR, although the measured differences of this study (mean offset changes of 4.5mm and length increase of 3.1mm), may not be clinically significant.

Figure 1: Right total hip arthroplasty and left hip resurfacing.

A number of studies have looked at gait and postural balance after THA and HR, with conflicting results. In a retrospective study, matched for gender, Mont et al. found better gait parameters and walking speed in HR patients compared with THA [9]. In an age-matched cohort of high functioning HR and THA patients with asymptomatic controls, Shimmin et al could find no demonstrable differences, whilst Lavigne et al. looked at gait speed and postural balance between HR and largediameter THA in a prospective, randomized, double-blind trial out to 12 months and again found no significant differences between the two groups [10]. Aqil et al. used a small cohort of 9 patients with one HR and one THA, showing that the HR side demonstrated gait parameters which more closely approached those of normal control subjects [11]. From a more subjective point of view, Collins et al. evaluated the patient’s perception of their artificial joint relative to a normal native joint, and did not find any significant differences between groups of HR and THA patients [12].

Patient reported outcomes

Perhaps the most important outcome of all is whether hip function is ultimately improved for the patient. There is much debate about how this should be assessed, with many authors commenting on the potential ceiling effect of common hip function scores masking the potential benefits of one procedure over another [13-15]. A number of retrospective and non-randomized studies comparing HR and THA have produced conflicting evidence: Some demonstrate higher clinical scores [16,17] and others no difference [18-22]. Some of these latter studies did show higher activity levels for the HR group [18,19,21] but this was not universal [20,21]. The lack of randomization in all of these reports does however mean there is a significant risk of bias. Two prospective randomized studies by Garbuz et al. and Lavigne et al. showed similar clinical outcomes and UCLA scores comparing groups of HR patients and large diameter THRs [23,24]. A recent pragmatic randomized trial by Costa el al. showed no difference in clinical function at 12 months between hip resurfacings and standard THRs [25]. Penny et al. reported on their randomized trial at 2yrs followup, again demonstrating no significant differences in clinical function scores, UCLA or EQ-5D scores [26]. The longest running randomized trial by Vendittoli et al. recently published follow-up of 6 to 9 years and did not find any significant differences in clinical function scores, although patients in their HR group did demonstrate significantly higher UCLA activity scores 5 years after surgery [15].

Implant survivorship

One proposed advantage of HR is the potential longevity afforded by reliable modern implant fixation and the minimal wear properties of a metal on metal bearing. Overall, registry data demonstrates poorer results when HR is used in wider populations. The UK registry has a revision rate of 12% at 9 years for all HR procedures, and the Australian registry has 11% revision at 12 years. There are a number of confounding factors here. Many different companies developed resurfacing products after initial promising results from other designs, but with varying success. The same UK registry report has a revision rate for the now recalled ASR component at 36% at 9 years and problems with this particular prosthesis are reported by multiple authors in the literature [27-29]. Revision rates for HR in the setting of these problems and their appearance in the mainstream media and in large scale legal battles may well escalate further as the threshold for revision is likely to decrease in the light of the potential concerns regarding metal debris and the consequences of abnormal high wear. This factor should be taken into account when looking at registry and other data that is not implant specific, as the inclusion of dis-continued devices may skew the interpretation. This fact is well demonstrated in a recent systematic review of outcomes, where the authors demonstrated that revisions and re-operations were more frequent for HR, unless devices which have been withdrawn from the market are excluded [30]. The only randomised study comparing HR and THA with published mid-term outcomes did not demonstrate differences in revision rates at a mean of 8 years [15]. An interesting feature of survivorship studies in which the data is analyzed in more detail is the demonstration that survivorship of HR implants within certain cohorts of patients is better than others. In a review of multiple registry data, Corten el al. showed that HR component sizes greater than 50mm in diameter had much improved survival rates, and male patients younger than 65yrs had comparable or even slightly improved survivorship with HR compared with THA [31]. The latest published data analysis from the UK National Joint Registry shows that in men with a femoral head size greater than 54mm, revision rates are comparable to the best performing THAs [32,33]. This analysis is supported by long-term data from high volume and designer centers using the best performing implants, demonstrating excellent results in selected cohorts. In young male patients, 10-year survivorship has been reported in the order of 93-99% [2,34-38]. Many of these studies confirmed that the size of the implant and the sex of the patient were significant predictors of failure during this period and this has been confirmed by further large cohort analysis [29-40] suggesting that revision rates are higher with smaller resurfacing implants and female patients.

Adverse events

The incidence of general complications of hip arthroplasty applicable to both procedures, such as venous thromboembolism, pulmonary embolism, infection, acetabular component malposition, nerve palsy or mortality does not differ between HR and THA, and due to the nature of resurfacing, it is perhaps not surprising that dislocation rates are lower for this procedure [15,30,41]. Native femoral neck fracture is unique to HR, and is often the main cause for revision [30,32,40]. However, this particular problem may relate to errors in surgical technique, and this is one of the reasons why the learning curve for HR is highlighted as an important factor in outcomes [42,43].

One complication that has had significant attention recently is adverse reactions to metal debris (ARMD), which is of significant interest in both HR and large diameter head THA (LDH THA) with metal-on-metal bearings. The release of metal ions appears to cause a spectrum of effects, from asymptomatic raised metal ions with wellfunctioning implants to host responses leading to the formation of pseudotumours, sometimes accompanied by significant soft tissue and bony destruction [44]. Protocols for accurate diagnosis are still evolving. Currently, a combination of patient symptoms, metal ion levels and cross sectional imaging is used to establish the extent of any problem [44-46]. Identification is important as the consequences, most notably in the setting of tissue loss, have the potential to be catastrophic for the patient. The full causation of this problem has not yet been fully elucidated. There is undoubtedly a contribution from excessive wear in poorly engineered implants, and wear problems will be exacerbated by malposition and smaller components [47,48]. This latter issue has been cited as one reason for the higher failure rates in female patients, but there also appears to be a additional and as yet undefined patient specific contribution [49,50] which means the occurrence of this potentially very serious complication cannot be fully predicted. It is also possible that a number of different pathological processes may occur in different situations, partly explaining the wide spectrum of noted effects [51]. Despite the concerns and increasing literature on the subject, however, in a recent review of 2773 HR performed by 11 Canadian centers, only 10 potential ARMD (6 confirmed) were reported (0.36%) which reflects the low occurrence rate of the problem in a group of different HR designs performed by multiple surgeons [40]. A meta-analysis of the literature identified a pooled incidence of adverse reactions of just 0.6%, from just fewer than 14,000 MoM THA or HR operations [52]. ARMD rates do appear to be higher in LDH THA where altered forces from the large articulation acting at the modular junction at the stem trunion and head taper have been incriminated as an additional source of metal ion release [53-55]. Interestingly, and possibly as a result of the problems with metal bearings, the diagnosis of ARMD is now increasingly made in the presence of more standard THA implants without metal articulating surfaces. Again, wear and corrosion at junction of the femoral trunion and head taper, potentially as a result of manufacturing changes and more widespread use of larger diameter heads in all bearing types, has been identified as a potential causative factor [56-58].

Implant revision

Many of the younger, more active patients at whom HR technology is targeted will eventually be facing revision surgery. Whilst hip resurfacing has potential benefits for preservation of femoral bone stock, and thus use of a straightforward THA femoral component at revision, there are concerns surrounding loss of bone stock after removal of a supposedly larger acetabular component. A retrospective study by Loughead et al. found larger acetabular sizes were used in an HR group compared with a hybrid THA group [59]. A similar study by Naal et al came to the same conclusion [60], whereas Moonot et al suggested that acetabular components in HR were either comparable to THA or even smaller in women [61]. Vendittoli et al. compared acetabular bone resection between HR and THA in a randomized trial [62]. Using the size of the last reamer as a surrogate measure of acetabular bone loss, there were no significant differences between the two groups, although in a small proportion of cases (6.8%) the acetabular component had to be upsized by 2mm to accommodate the selected femoral component size. Similarly, Brennan et al. showed no significant differences in dehydrated, defatted acetabular bone reaming weights between HR and THA [63].

In terms of the functional outcomes of revision, there are multiple small short-term reports suggesting that the HR revision may have similar functional outcomes to primary THA (Figure 2a and 2b). Some of these series report femoral revisions only [64], others a majority of single component revision [65-67] or a majority of both component revisions [68]. In contrast, Desloges et al. reported on a retrospective propensity matched series of HR revisions, and found HR revision outcomes similar to revision THA, but not to primary THA [69]. The outcome may depend on the reason for revision [70], with poor outcomes being reported in patients having HR revised for ARMD [71]. Direct comparison with primary THA is probably not warranted due to the risk of further surgery. Data from the Australian Registry suggests that femoral component only HR revision has a similar rerevision rate to both component revision, but in both cases this is higher than the revision risk for primary THA [72]. A review of data from multiple registries had similar findings, but did point out that subsequent re-revision rates are similar to that of revision THA [31].

Figure 2: Hip resurfacing femoral component loosening 1 year after surgery (a) and revision surgery with a primary cemented stem preserving the acetabular component (b).

Again, the explosion of manufacturing development in the area of large metal bearings gives a number of confounding issues here. Until recently, it was assumed that femoral revision of HR with a well fixed and positioned acetabular component was a relatively straightforward procedure. However, emerging data from both published series [73], and the UK registry [74] suggests that large bearing MoM THRs have unacceptably high failure rates, although data from the Finnish Registry only found this to be the case in females over 55yrs old [75]. The British Orthopaedic Association now says that surgeons and hospitals ‘must not use such implants [76]. Although there is little data regarding the longer term outcome of conversion of a HR to a stemmed large head MoM THA, it would be reasonable to assume that the potential consequences are no different from using such an implant in the primary setting. However, each different design of LDH THA performs differently, and the decision to convert a HR to a LDH THA should be based on the results of the specific implant to be revised [77].

The explosion of interest in large bearing hip arthroplasty, both as HR and LDH THA, and the subsequent manufacturing and commercial rush for involvement has led to the rather unwelcome situation of the orthopaedic community discovering problems and complications in this area in large patient populations rather than controlled research cohorts. Undoubtedly, this story still has some way to play out, and there is a danger that the reports of the beneficial sides to HR technology will be lost as surgeons and implant companies aim to reclaim the trust of patients. Certainly if HR is to be used, then the current evidence base would suggest that the patient cohort in which it should be considered is shrinking. Arguably it should be limited to young, male patients of sufficient dimensions to allow a large bearing to be implanted. In addition, it ought to be performed by experienced surgeons in high volume centers. Surgeons wishing to perform the procedure should recognize the significant learning curve and seek appropriate initial training and supervision.

As the current evidence base stands however, despite the theoretical potential, numerous trials have failed to prove that HR provides significant functional benefit to patients over THA, although another way of looking at it would be to say that HR appears to perform equally well. This may well reflect ongoing developments in modularity and bearing surfaces in THA that allow accuracy of replication of hip anatomy without the bearing wear issues that initially prompted the search for an alternative in younger, more active patients. It might also be argued this is a reflection on how we measure our outcomes and whether we currently have the tools to realistically differentiate these gains clinically, but it should also be born in mind that any measurable differences should be relevant to the patient and it must be proven that there are clinically significant gains to be made before a conclusion of anything over equivalence is made. As the suitable population for HR narrows, it may be possible to perform more focused studies to further establish where such gains might be made.

Again within a young male cohort, however, it appears that one area in which hip resurfacing is performing as hoped is in its longevity. Even if functional comparisons are disregarded, this is of significant potential benefit to a young patient population. In addition, current data would appear to support the observation that revision of HR to THA in the absence of infection or ARMD is relatively straightforward and provides good outcomes. However, the recent concerns over large head MoM THA probable mean that for the foreseeable future, further replacement in the presence of a resurfacing is likely to involve a both component revision. Although this would appear to mitigate some of the proposed ease of resurfacing revision in the presence of a well fixed acetabular component, it does appear that revision of the acetabular component should not prove any more destructive than during revision of a THA, whilst femoral bone stock remains preserved, allowing the use of a standard primary THA stem.

Overall, although not all the proposed advantages of hip resurfacing appear to have been realized, the results in a selected patient population remain encouraging: However, despite the increasing numbers of higher quality trials, this particular conclusion remains largely based on level IV evidence. Further studies within more defined cohorts are required to elucidate the ideal option for young, active patients with degenerative hip disease.