Introduction                       

The choice of prosthesis for total hip replacement has traditionally been made by the operating surgeon, based on a combination of personal experience, reported results and the individual needs of the patient. The femoral and acetabular components can be chosen independently, but the entire hip system, including the method of fixation, the type of cement and cementing technique, if used, as well as the bearing surfaces must also be considered.

More recently the issue of cost has become an increasing influence as budgets are cut, and central purchasing for individual hospitals and even cities or countries, has been introduced. Modern hard on hard bearings are relatively expensive, as are uncemented components, but in order for their use to be justified on an economical basis independent results would have to demonstrate a significantly improved survivorship, which has not been the case.

The national registry results have, with very few exceptions, demonstrated superior results for conventional cemented total hip replacements in all age groups and pathologies, which has seen the use of uncemented implants being abandoned in some Scandinavian countries and of note is the fact that countries with national registries have lower revision rates.

Other issues require consideration such as the surgical approach, minimally invasive techniques, the experience of the surgeon, computer assisted surgery, and the use of special implants such as resurfacing or short stems. Patient factors are important and correct patient selection remains paramount. Age, activity, bone quality and morphology must be evaluated and the effects of the internet and direct marketing have altered patient expectations and demands, whilst not always being realistic or based on sound evidence.

In broad terms, and ignoring the crucial issue of the bearing materials, hip arthroplasties can be categorized based on the method of fixation as cemented, uncemented or hybrid, but the question remains – how to choose the best for your patient?

 

How     to     Choose     an     Implant                 

Guidance with regard to choosing hip prostheses is available from many sources including the peer-reviewed Orthopedic literature, national joint registries, colleagues, conferences, professional bodies, manufacturers, laboratory studies, the Government and the internet. All of these sources are flawed, with the evidence base being of variable quality, but the use of several in conjunction with the surgeons personal experience is likely to offer the best opportunity for making a sound decision.

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Figure 42.1: Generations of the Charnley femoral stem

Figure 42.2: Generations of the Exeter stem

 

 

Conventional cemented total hip replacement, consisting of cemented acetabular and femoral components, a monobloc stem and a metal on high-density polyethylene bearing was rightly regarded as the gold standard for primary total hip replacement for many years. Such implants feature most frequently in the literature with the longest duration of follow-up and many studies reporting ten year survivorship in excess of 95%. With the passage of time implant design, material strength, manufacturing methods, surgical technique, theater environment and cementing techniques have all improved, leading to the likelihood of even better results in the future.

It is however essential to ensure that the implant has not been altered during the period of the study, for example with the use of the Charnley stem, which began as a polished taper, but was subsequently modified and became a composite beam (Fig. 42.1). During a similar period the Exeter stem went from a polished monobloc to a matt finish and then returned to the polished finish but also became modular (Fig. 42.2). The design rationale and philosophy of the implant must be understood and any changes whatsoever mean that a new implant has been created and that the heritage and results of previous generations cannot automatically be conferred upon it. In many cases by the time that the ten year results have been reported the implant may have become obsolete or may have been discontinued.

The philosophy of the implant must also be thoroughly understood as implants within the cemented and uncemented categories achieve fixation and function by differing methods. Cemented acetabular components may be manufactured from ultra-high molecular weight polyethylene (UHMWP) or may be highly cross-linked, and may feature flanges of different shapes and spacers to reduce the risk of bottoming out. Cemented femoral implants may function as either composite beams or by the taper-slip philosophy, with the choice of cement and the specifics of the cementing technique being critical to the success. Uncemented acetabular components may have different coatings and other features such as screws, spikes or fins to augment fixation, and the integrity of the locking mechanism is crucial to minimize backside (Type 4) wear. Uncemented femoral implants may aim to achieve fixation proximally in the metaphysis or distally in the diaphysis and a variety of surface modifications have been designed to enhance fixation such as the use of hydroxyapatite coating or porous or trabecular metals.

The results of cemented total hip replacement have stood the test of time, but less satisfying results have been reported in the high demand younger patients due to the development of osteolysis and loosening.1 The results even in this challenging group remain good2 and with

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Search for the Ideal Hip Implant—Looking for the Holy Grail!

 

Figure 42.3: The pyramid of evidence

 

 

a predictable mode of failure revision can be relatively straightforward if performed early. The less satisfying results have however driven the search for implants that will permit greater function and durability, although this has yet to be consistently achieved and may indeed represent the equivalent of a search for the Holy Grail.

This has led to the paradoxical situation whereby the youngest and most active patients are often subjected to the use of the newest and most expensive implants, based purely on hypothetical grounds instead of using tried and tested implants with a predictable and low rate of failure. The introduction of such implants has often led to poorer results, new types of complication and modes of failure, early catastrophic failure and complex revision scenarios as demonstrated by the 3M Capital Hip,3 Hylamer and more recently the Metal on Metal and Articular Surface Replacement (ASR) experiences.

The peer reviewed orthopedic literature generally has a low impact rating and as can be seen by reference to the pyramid of evidence (Fig. 42.3) the arthroplasty literature contains no metanalyses or prospective randomized controlled double-blind trials. The majority of papers are case series, usually without control groups, and typically report the result of a prosthesis in isolation, acetabulum or femur, without regard to other issues such as the performance of the entire hip system and the details of the bearing and head size used.

Many papers have limited duration of follow-up, relatively small numbers of patients, often with different pathologies and age groups, high rates of loss to follow-up and poor statistical analysis. The majority of studies reporting long-term follow-up of more than ten years duration tend to come from designer surgeons or specialist centers and the applicability of such results to the general orthopedic community is doubtful. Other issues such as potential conflicts of interest can also lead to a degree of scepticism when the same results cannot subsequently be reproduced in other centers.

Personal experience and that of senior colleagues is of value, but essentially anecdotal. A prosthesis must however be user friendly and not overly complicated in order to promote reproducibility and often the experience of an individual during specialty fellowship training will ultimately shape their choice of implant. It is therefore essential that during the training period surgeons are exposed to a broad spectrum of arthroplasty surgery to allow experience to be gained in the use of different surgical approaches and prostheses whilst regularly reviewing the published literature.

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Figure 42.4: The ‘Blue Book’

 

The effects of direct marketing and of the internet became prevalent with the introduction of modern resurfacing and the computer literate patient generation of the silver surfers. The major drawback is that the ‘research’ accessed on the internet is not peer-reviewed, has no scientific validity and is simply a means of marketing. In 2008 Saitha et al4 looked at the quality of information available on the internet with regards to resurfacing and analyzed the top 100 sites. Manufacturers hosted 50% of the sites and 80% had serious shortcomings. Only 47% quoted results, 40% mentioned possible complications and only 30% were found to be relevant. Many different methods of rating outcome had been used making it impossible to compare any of the ‘results’.

Guidance is available from professional societies such as the Royal Colleges of Surgeons, the British Orthopedic Association and the British Hip Society in the form of the ‘blue book’ entitled Primary Total Hip Replacement: A Guide to Good Practice. This was first produced in 1999 and revised in 20065 (Fig. 42.4) and contains a lot of useful information with regard to total hip replacement in general, but offers limited advice with specific regard to implant selection. The recommendation being that the choice of the prosthesis should be governed by evidence of the effective performance of that implant and if possible of the operating team using it.

Courses are often sponsored by industry and instead of being generic, purely educational and discussing principles, the lectures are often implant driven and once again are a method of marketing. The manufacturers do however produce helpful information on design rationale

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and surgical technique for their prostheses (IFU – information for use) based mainly on in-vitro testing, which should be read. Experience has however taught us that the performance of an implant or material in the human body cannot be predicted based solely on laboratory studies and unexpectedly poor results were seen with Boneloc cement, highly cross-linked Hylamer polyethylene and the 3M Capital hip.

Search for the Ideal Hip Implant—Looking for the Holy Grail!

 

A related issue is the rate of development and release of new prostheses in an uncontrolled manner, with no track record of clinical use. In an editorial in 1993 entitled ‘Designer hips: Don’t let your patient become a fashion victim’6 Bulstrode et al highlighted this issue and in an article the following year entitled ‘Which primary total hip replacement’7 Murray et al presented comparative information on all of the hip implants on the market in the United Kingdom in order to assist surgeons in the selection process.

Murray et al reported that in 1994 there were 19 distributors and manufacturers marketing 62 different total hip replacements in the UK market, with femoral and acetabular components being sold separately, thus allowing surgeons to use numerous different combinations. The number of implants being introduced each year was increasing, with 50% having been introduced during the preceding five years. Advertising for each of the implants claimed unique combinations of features and the price of the implants varied by a factor of ten. Only eight implants had published peer-reviewed 5-year results and 70% had none whatsoever. Only the Charnley prosthesis had a market share of more than 20% and published 20-year results.

The conclusions were that choice should be based on sound clinical grounds; that modifying implants every few years without changing the name should cease, as modified implants should be regarded as new designs; that implants with no published data should only be used as part of trial or register and that the introduction of a CE mark would provide an important opportunity to rationalize the implant market in Europe.

In the United Kingdom the government established the National Institute for Clinical Excellence (NICE) in 1999, which produced its first guidelines for Total Hip Replacement in 2000.8 The main recommendation was that only implants with established 10 year results with greater than 90% survivorship should be used, but then completely fudged the issue by stating that it would also be reasonable to use implants which at three years appeared to be likely achieve the 10 year benchmark. It was also recommended that cemented implants should be used to the exclusion of all others.

Following the release of the NICE guidelines Roberts et al9 reported the changing rates of

implant usage in the Trent region over the following 5 years. They reported a marked increase in the use of uncemented (8 to 19%) and hybrid arthroplasties (9 to 22%), the introduction of modern resurfacing arthroplasty (0 to 7%) and a significant decrease in the use of cemented arthroplasties which fell from 84 to 59%. Clearly the government guidelines, or interference, had not been well received by the Orthopedic community.

In addition to NICE the Orthopedic Data Evaluation Panel of the NHS Purchasing and Supply Agency (ODEP)10 was established and consisted of surgeons, statisticians and representatives from industry, with a remit to provide an independent assessment of clinical evidence submitted by suppliers on the compliance of their implants for THR and resurfacing against NICE benchmarks for safety and effectiveness. The aim being to classify hip implants based upon their duration of follow-up, survivorship and the quality of the data submitted. However by 2009 only 38% of femoral implants and 44% of acetabular cups in use in England and Wales had been given ODEP ratings and no data had been submitted to ODEP for 46% of the femoral stems and 41% of the cups available. The full 10A rating had been given to 83% of the cemented stems for which data had been submitted, 62% of cementless

stems, 43% of cemented cups, 48% of resurfacing cups and only 7% of cementless cups.

Many believe that the most reliable data is now being obtained from the results of national joint registries, which are large population based studies, but the quality and completeness of the data does however vary, particularly in those countries where submission is not mandatory.

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Figure 42.5: The 7th Annual Report of the National Joint Registry for England and Wales

 

The Scandinavian joint registries11,12 have been collecting data and distributing results since 1975 as a result of which failing implants have been identified and withdrawn and surgical technique and results have improved. In the wake of the 3M Capital Hip ‘disaster’ there were renewed calls for the establishment of a National Joint Registry for England in Wales,13 which began collecting data on primary and revision total hip and knee replacement surgery in 2003 and produced its first annual report in September 2004 (Fig. 42.5).

The first Annual Report published in September 200414 looked at the period between 1st April 2003 and 31st December 2003 during which data had been collected on 46,798 procedures of which 22,672 were primary hip replacements. It was estimated that about 50% of all of the procedures performed had been entered, with 65% of patients having given specific consent.

The mean age of patients was 68 years, with 60% being female and osteoarthritis being the pathology in 94%. There were 72 brands of acetabular cups and 81 different brands of femoral stems, manufactured by 23 different companies. The total number of different combinations of these cups and stems was 369, of which 98 were entered just once. Sixty-three percent of primary total hip replacements were fully cemented, 16% were uncemented, 11% hybrid and 10% were resurfacings. Of particular concern was that with the use of so many different combinations of implant how can it be possible to establish robust long-term results.

By the time of the 7th Annual Report in September 2010,15 the registry for England and Wales had become the largest in the world containing information on more than 1 million operations. Data collection had been started for ankle replacement surgery and there are plans to start collecting data on elbow and shoulder arthroplasty as well as femoro-acetabular impingement surgery (FAI). In addition to this, data is to be collected on patient related outcome measures (PROMS) to assess the clinical outcomes and patients perspectives. Based on the results to date one design of knee replacement has been withdrawn and Research and Outlier sub-groups have also been established.

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Search for the Ideal Hip Implant—Looking for the Holy Grail!

 

The figures for primary hip replacement have however shown a dramatic expansion in the number of implants available and the decline in the use of cemented implants despite the superior results. Available acetabular designs have increased from 72 to 155, femoral designs from 81 to 176 and combinations of implant from 369 to 794. The presence of so many potential variations is undoubtedly heralding a return to anecdotal rather than evidence-based decision making. Uncemented fixation has now overtaken cemented for the first time in England and Wales, with the use of fully uncemented arthroplasties having risen from 16 to 39% and hybrid fixation from 11 to 16%, while fully cemented arthroplasties have fallen from 63 to 36% of cases.

 

Conclusion                        

There are many potential sources of advice to assist in choosing which implant to use, but all have potential or very obvious limitations.

The Orthopedic literature is full of papers describing the results of a femoral or acetabular implant in isolation, without clearly reporting the results of the other half of the prosthetic construct with which it has been paired and which on further scrutiny has inevitably performed less well.

Registries, if based purely on revision, do not take into account issues such as the competency with which the prostheses were originally inserted, the complexity of the procedure (case mix), the occurrence of clinical or radiological failure should the implant not come to revision, whether one or both of the implants has failed at the time of revision, the development of radiological features such as osteolysis, or selection bias when a surgeon chooses not to use a certain implant in certain circumstances.

The use of a specific implant, whether cemented or uncemented, does not however guarantee success. It is the process of implantation, with precise aseptic and surgical technique and the optimal use of the carefully selected implants, with or without cement, which will reduce the risk of complications.

Despite the population based registry results almost universally demonstrating the superiority of cemented hips the worldwide market has inexplicably moved towards the use of uncemented implants, which last year outnumbered cemented ones in England and Wales for the first time. When closely considered, the use of these more expensive implants is difficult to justify on clinical, scientific or economic grounds, particularly when used in conjunction with hard on hard bearings. In the United Kingdom some leading surgeons are predicting a return to the use of cemented implants based not on their superior results, but on economic grounds alone.

Ultimately it is the accuracy with which the implants are inserted that will have the greatest bearing on the function and longevity of the arthroplasty, and it is therefore the skill of the surgeon, which is the major determinant of the outcome. The old adage that ‘a bad workman blames his tools’ holds true, but the converse is that a good workman ensures that he chooses only the best tools for the job, using them expertly on each and every occasion and ideally this should be an evidence-based decision.

 

References                       

1. Sochart DH, Porter ML. The long-term results of Charnley low-friction arthroplasty in young patients who have congenital dislocation, degenerative osteoarthrosis or rheumatoid arthritis, The Journal of Bone and Joint Surgery (Am), 1997;79:1599-617.

2. Sochart DH. Relationship of acetabular wear to osteolysis and lossening in total hip arthroplasty. Clin Orthop Rel Res 1999;363:135-50.

3. Kasis AG, Farhan MJ, Elson R. A radiological study of 3M Capital femoral hip prostheses: an analysis of factors predisposing to failure. Hip International 2007;17(2):82-7.

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4. Saitha A, Ajayi OO, Davis ET. The quality of internet sites providing information relating to hip resurfacing. J RCS Ed 2008;6:85-7.

5. Primary total hip replacement: A guide to good practice. The British Orthopaedic Association 2006. http://www.boa.ac.uk/BOAhome.html.

6. Bulstrode CJK, Murray DW, Carr AJ, Pynsent PB, Carter SR. Designer hips: Don’t let your patient become a fashion victim. Br Med J 1993;306:732-3.

   Total Hip Arthroplasty

    

7. Murray DW, Carr AJ, Bulstode CJ. Which primary total hip replacement. J Bone Joint Surg (Br) 1995;77-B:520-7.

8. National Institute for Clinical Excellence (NICE): Technology Appraisal Guidance No. 2. Guidance on the Selection of Prostheses for Primary Total Hip Replacement. http://www.nice.org.uk

9. Roberts VI, Esler CN, Harper wm. What impact have the NICE guidelines had on the trends of hip arthroplasty since their publication. J Bone Joint Surg (Br) 2007;89-B:864-7.

10. NHS Purchasing and Supply Agency. ODEP Database. http://www.rcseng.ac.uk/services/ publications.

11. The Swedish National Hip Arthroplasty Register. http://www.jru.orthop.gu.se.

12. The Norwegian Arthroplasty Register. http://www.haukeland.no/nrl.

13. Sochart DH, Long AJ, Porter ML. Joint responsibility: the need for a national arthroplasty register. Br Med J 1996;313:66-7.

14. National Joint Registry for England and Wales, 1st Annual Report, September 2004. http:// www.njrcentre.org.uk.

15. National Joint Registry for England and Wales, 7th Annual Report, September 2010. http:// www.njrcentre.org.uk.