Department of Medical Engineering and Physics Royal Perth Hospital March 2006

Implant Retrieval and AnalysisSince the introduction of implant tracking in 2002, the number ofimplants referred to the Implant Retrieval Service has more thandoubled from 200 to over 500 in 2005. The benefit of moreretrievals is the improved capture rate of devices, leading to abetter understanding of failure trends.

There is however a need to rationalise the retrieval program toeffectively manage the increase in workload. To this end, thefocus has shifted to evaluating series of implants with commonmodes of failure. Equally, positive performance indicators suchas low wear rates or extensive bone ingrowth in implant seriesare of interest.

So whilst significant individual cases will always be reportedback to the surgeon, we see that an overview of the retrievalcollection and a focus on common success or failure modes of aseries of implants will be of most value.To manage this change we propose the following triage forretrieved devices:

1. Surgeon report – a report to be provided only on surgeonrequest as specified on the retrieval form.

2. Laboratory report – no specific request from the surgeon,but warrants preliminary or even full investigation.Reporting back to the surgeon will depend on anysignificant findings.

3. Storage – devices with no significant adverse features willbe stockpiled and may be used in series evaluation.

A number of implant series requiring further study have beenidentified including: Exeter (HNSS), Omnifit, Butel and Margronstems. Also, evaluation of PFN nails, LISS plates, HG shell/liners, Birmingham Resurfacing and Duracon and Preservationknees are continuing.

Welcome to our 5th Bulletin. As you will notice, 2005 was a year focussed on implant retrieval studies. Wehope that a snapshot of our work over the past year may be of interest to you.

Some interesting statistics from our retrieval collection overthe past 5 years:

Most common hips Charnley, Omnifit, PCAMost common knees Duracon, MG, OrtholocFractured stems 4 Exeter, 4 Omnifit, 3 Butel,

2 Charnley, 1CPT, 1 C-stem.Fractured knees 5 OrtholocLiner dissociation 15 MG -patella, 8 HG linersFractured nails/plates 5 PFN, 3 LISS, 1 IMHS

Implant Tracking UpdateAlthough implant tracking has only been fully operational inthe three major WA public hospitals over the past few years, inreality, implant tracking information via the theatre managementsystem has been captured over a much longer period.

Royal Perth Hospital main theatres had its first implant insertionevent recorded almost 8 years ago. In 1999, SPC, SCGH andFremantle theatres came on line. Since that time, thousands ofimplant insertions have been recorded. Implant removals werefirst recorded in RPH in late 1999, in Fremantle in 2001 andSCGH in 2002, thereby finally enabling the tracking of all devicesfrom insertion to removal.

Retrieval FormsThe recent introduction of electronic retrieval forms in publichospitals has only been mildly successful. Our experience isthat the paper retrieval forms appear to be better utilised andare more comprehensive in terms of data provided.

As such, all theatres have been sent updated paper forms andsurgeons are encouraged to complete them when referring theirimplants.

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Implant Retrieval Lab

Worn Through kneesOver the last 5 years an increased number of knee arthroplastieshave been received where the tibial polyethylene inserts haveworn-through, resulting in metal-on-metal wear. In some casesboth the tibial insert and tibial tray have been destroyed. Inboth these scenarios, significant implant debris is noted in theretrieval documentation with any or all of the following; grosstissue staining, metallic fragments, implant loosening, bonereaction, resorption, and osteolysis.

In these cases, arthroplasty revision is more demanding andthere is the potential of decreasing the life of the revisioncomponents due to extraneous wear debris. In several cases itwas evident that had the tibial insert been replaced prior todestruction, revision of the tibial and femoral components maynot have been necessary. The most commonly worn throughinserts we have received are summarised below:

After scrutinising the retrieval database, it was found that ~ 92% of worn-through knees have a polyethylene thicknessof <8mm (actual thickness, not labelled thickness). Commonly,the failure mechanisms are delamination as a result of sterilisationin air and high contact stresses, as well as severe pitting, andfracture. Most bearings worn to a thickness <2mm are fractured.Wear life predictions are difficult, as wear rates are non-linear,

From 1960 till the mid-90’s, UHMWPE components for jointreplacements were gamma sterilised in air. It has been establishedthat oxidation of free radicals persist for years after irradiation inair, even during shelf storage.

The end result is that the UHMWPE components show varyingdegrees of oxidative degradation, which has a detrimental effecton mechanical properties, often resulting in gross wear damage,osteolysis and ultimately component failure. This is most obviousin bearing surface delamination and yellowing (see below).

A major change insterilisation practicesoccurred between 1995-1998, when companiesbegan to sterilise theirp o l y e t h y l e n ecomponents in an inertatmosphere (low

oxygen). Gamma sterilised components were also barrierpackaged. Other sterilisation processes including gas plasmaand ethylene oxide sterilisation were also employed.

In the retrieval studies it has become apparent that componentssterilised by any of these new techniques have significantlyless oxidation damage than components produced prior to 1998.

Effect of UHMWPE Sterilisation Treatment: Stock control considerationsA recent retrievedDuracon insert has raiseda few concerns that maybe relevant to sometheatres. This componentwas inserted in 2001, atleast 3 years after thecompany had revised itssterilisation procedure.On examination the tibial insert displayed regions of delaminationtypical of UHMWPE components that have been gammasterilised in air. Further investigation revealed the insert was infact a pre-1998 component, obviously sterilised in air and storedon the shelf for at least 3 years.

This raises the possibility that some components sterilised priorto 1998 may still be in stock, particularly if no expiry date isdisplayed. This may not be relevant to consignment stock asmost companies regularly replace expired stock. It isrecommended that theatres check their non-consignment stockand replace all UHMWPE components that are sterilised in air(pre-98).

An increasing number of oxidation effected UHMWPE retrievalsare anticipated over the next 10 years, before the beneficial effectsof the new sterilisation methods are fully realised.

however a ballpark figure for these bearings is 0.4 - 0.6mm/yr. Tibial inserts are therefore expected to wearthrough in approximately 8-10 years. At present it isrecommended that patients with the identified knee arthroplastiesand with tibial inserts <10mm in thickness be followed upbetween 7-10 yrs with a CT scan to determine wear progression.A protocol has been formulated for CT measurement ofpolyethylene thickness and is available on request.

Duracon Insert

Knee LabledThickness mm

ActualThickness mm

Whiteside Ortholoc 8 5Tricon 6 6Genesis 10 7Howmedica PCA 7 4

Common worn though knees

A joint study has been undertaken with Piers Yates, currently inthe Avon Orthopaedic Centre, Bristol, on the failure of 14 modern,high nitrogen stainless steel (HNSS) stems. A control cohort of203 CPT stems was used for the study. Fractured stems included5 Exeter's, 8 CPT's and one C-stem. Interestingly, both the CPTand Exter stems showed variable microstructural grain-size.

In terms of fatigue resistance, the importance of a fine,homogeneous microstructure for high nitrogen stainless steelis quite a complex physical metallurgy argument. In summary,for fatigue crack propagation in materials that are sensitive tograin size, the grain boundaries act as microstructural barriers instage I of the fatigue fracture process. I.e. the more boundaries(finer grain size), the greater resistance to fatigue crackpropagation.

The clinical and radiological data suggested that the potentialfor a catastrophic complication is real but requires a combinationof factors including excessive BMI, poor proximal support andinadequate microstructural control. A comparison of the canalmorphology between the control cohort and the fracture grouprevealed that the fracture group had predominantly "Champagneglass" shaped canals. When the non-fractured group withsimilar shaped canals were considered, measurements revealed

a more substantial cement mantle in the calcar region, zone 1 andat the lesser trochanter. Despite this, the stem sizes were largerin the non-fractured group, which strongly suggests thataggressive removal of cancellous bone is better in order to allowmore complete cement mantles and the use of larger sized stems.

Investigation suggests that care must be taken in "Champagne-glass" femurs to make sure that the proximal mantles are of thehighest standard. Lateral entry to the femur avoids varuspositioning of the stem and allows larger broaches to be used,which improve cement mantles in the critical areas and allow theuse of larger stems. In heavy patients, it is perhaps wise toconsider using higher strength alloys and/or uncemented stems.

Hip and Knee TenderIn the last bulletin it was reported that the evaluation committee was in the final stages of the selection of hip and kneearthroplasty for the public hospital system in WA. The tender is currently in a transition period of implementation into the majorpublic hospitals. The table below outlines the devices selected for use in primary arthroplasty cases.

The evaluation panel was aware that any selection of arthroplasty would create robust discussion and reduction in choice maycreate some difficulties. Confidence in the tender outcome however is based upon a rigorous assessment process with clinicalevidence being the primary consideration. Balancing the commercial aspects of the tender with laboratory assessment andqualitative criteria also contributed to the selection process. Ultimately the success of the tender will be determined by clinicaloutcomes, tender compliance, and cost savings.

Omnifit HipsSince May 2000 we have received 4 fractured Omnifit femoralstems for failure analysis. Of most interest is that all four stemshave fractured in the neck region below the modular femoralhead. The fracture surfaces are highly crystalline which raisedconcerns with respect to the implant microstructure and itspredisposition to failure. Investigations are continuing, howeverpreliminary findings reveal a very coarse grained material with apropensity for grain boundary carbide precipitation - certainlynot a preferred structure. Your contribution in continuing torefer all fractured devices is appreciated.

Fractured HNSS Stems

Hip Smith & Nephew

Zimmer De Puy Stryker Link/Orthotech

Monobloc hemi-arthroplasty Thompson Austin MooreFully-cemented THA Spectron &

ReflectionCPT &ZCA

Elite Plus &Elite Ogee

Exeter V40 &Contemporary

Lubinus SP II &Lubinus SP II Link

Cemented hemiarthroplasty Spectron CPT Charnley,Elite Plus

Exeter V40

Hybrid THA Spectron &Reflection

CPT &Trilogy

Charnley, ElitePlus & Duraloc

Exeter V40, Omnifit & ABG II

Fully-uncemented THA Spectron &Reflection

Alloclassic, CLS &Trilogy

Corail &Duraloc

ABG II &ABG II

Uncemented hemiarthroplasty Synergy Corail ABG II

Knee Total knee arthroplasty Genesis II Nex Gen LCS (Mobile) Duracon ExatechOrthotech

Unicompartmental SledLink

Stems Acetabular Components

Hip Smith & Nephew

Zimmer De Puy Stryker Link/Orthotech

Monobloc hemi-arthroplasty Thompson Austin MooreFully-cemented THA Spectron &

ReflectionCPT &ZCA

Elite Plus &Elite Ogee

Exeter V40 &Contemporary

Lubinus SP II &Lubinus SP II Link

Cemented hemiarthroplasty Spectron CPT Charnley,Elite Plus

Exeter V40

Hybrid THA Spectron &Reflection

CPT &Trilogy

Charnley, ElitePlus & Duraloc

Exeter V40, Omnifit & ABG II

Fully-uncemented THA Spectron &Reflection

Alloclassic, CLS &Trilogy

Corail &Duraloc

ABG II &ABG II

Uncemented hemiarthroplasty Synergy Corail ABG II

Knee Total knee arthroplasty Genesis II Nex Gen LCS (Mobile) Duracon ExatechOrthotech

Unicompartmental SledLink

Acetabular Components Stems

Hip Zimmer Smith & Nephew

De Puy Stryker Link/Orthotech

Monobloc hemi-arthroplasty Austin Moore ThompsonFully-cemented THA CPT &

ZCASpectron &Reflection

Elite Plus &Elite Ogee

Exeter V40 &Contemporary

Lubinus SP II &Lubinus SP II Link

Cemented hemiarthroplasty CPT Spectron Charnley,Elite Plus

Exeter V40

Hybrid THA CPT &Trilogy

Spectron &Reflection

Charnley, ElitePlus & Duraloc

Exeter V40, Omnifit & ABG II

Fully-uncemented THA Alloclassic, CLS &Trilogy

Spectron &Reflection

Corail &Duraloc

ABG II &ABG II

Uncemented hemiarthroplasty Synergy Corail ABG II

Knee Total knee arthroplasty Nex Gen Genesis II LCS (Mobile) Duracon ExatechOrthotech

Unicompartmental SledLink

Acetabular Components Stems

Royal Perth Hospital 2006Not to be reproduced without written permission of the Dept. of Medical Engineering and Physics

SNIPPETSSingle use Devices (SUD’s)TGA has ruled that from March 2006 health carefacilities cease the re-use of critical SUD’s andDecember 2006 for semi-critical devices. As aconsequence of the new TGA rules,Bioengineering has been given the responsibilityof establishing a SUD re-manufacturing facilityat RPH for public hospitals in WA. Staff andspace have been allocated, and a fully operationalfacility will be up and running soon.

AOA Annual ScientificMeeting: Perth 2005A number of Bioengineering staff were fortunateto attend a well organised and particularly relevantnational meeting. Four Bioengineering paperswere presented including ‘Evidence basedselection of primary hip and knee replacements’,‘The use of iontophoresed allografts in revisionarthroplasty’, ‘Implant device tracking, retrievaland analysis’ and ‘Quantifying wear in mobilebearing knees’.

New Laboratory EquipmentA long awaited replacement Leica stereomicroscope with associated digital imaging hasbeen a welcome addition to our microscopefacilities.

We are also thankful to the Orthopaedic Dept(RPH) for assisting with the purchase of a “stateof the art” operating system (BlueHill) for ourInstron materials testing machine. Anyorthopaedic registrar or consultant wishing to usethe Instron for future projects will appreciate amore ‘user friendly’ machine.

Custom DevicesRoutine work in Bioengineering includes the provision of a custom deviceservice for Orthopaedics and other specialties. 2005 saw the completionof in excess of 15 cranioplasty plates and hip spacers, several arthrodesisnails and shoulder plates, compression nails, knee spacers and surgicalmodels.

In addition there were two extremely difficult metal removal cases. Of notewas the removal of an Austofix hip nail. In this case, the most proximalfixation screw had penetrated the lateral cortex, and become firmly ‘stuck’in the nail. Attempts to remove the screw had further deformed the head.

Bioengineering ScholarshipCongratulations toJoe Allen on beingawarded the 2005/2006B i o e n g i n e e r i n gScholarship. Joe, a UWA4th year Engineeringstudent, is working on thedevelopment and morei m p o r t a n t l y , t h eexperimental validation of afinite element model for thepre-clinical testing offemoral stem prosthesis.

BP AnkleWA has the largest number of BP ankle implants in Australia,(n =150), and during 2005 we received four explants. Upon investigation,there appears to be little published literature on the BP ankle other thanfrom its inventors Buechel and Pappas. The retrievals were characterisedby good or excellent boney ingrowth in the talar component, little ingrowthin the tibial component and loose bead impaction in the minimally wornpolyethylene bearing. It has been suggested that a review of patients whohave received a BP ankle may make an interesting clinical study.

The metal components arestriking in nature due to theirtitanium nitride ‘gold’appearance, which has beenreported to reduce wear.Titanium nitride has long beenused in metalworking especiallyon twist drills to maintain theircutting edge due to the superiorhardness of the diffusion layer.

New arthroplasties including theCCI knee and ACCIS hip(Munich) are coated withtitanium nitride.

Contact us:Trevor Jones: 9224 3226Eric Swarts: 9224 2997Alan Kop: 9224 3228trevor.jones@health.wa.gov.au

Upon referral, Dermott Collopy asked us to provide suitable removal tools.The removal device was based upon a diamond-tipped coring bit morecommonly used to drill through reinforced concrete. The proximal end ofthe nail was used as a guide and the high nitrogen stainless screw was cutthrough with ease and safely removed. Thanks to Dermot for his referraland successful removal.