Implant Technology and Biomaterials Bulletin No. 4 March 2005

2004 was a very busy year in Bioengineering, hence the edition in March rather than December. We hopethat the articles presented may be of interest and feedback is always welcome.

Over 50 000 hip and knee replacements were performedin Australia in 2003. With an abundance of hip and kneedesigns currently in use in Australia; 141 hip stems, 124acetabular components and 47 knee prostheses, there is amyriad of choice for the surgeon.

In an attempt to rationalise the number of suppliers anddevices available to the public hospitals, a tender was calledin late 2004 for the selection of hip and knee prosthesesfor use over the next five years. Categories included, fullycemented, uncemented, hemi and hybrid hip and knee ar-throplasty for primary arthroplasty with a patient cohortof 80% of the 60 plus age group. As well as cost, theselection was based on 50% qualitative criteria, includingdemonstrated supply performance, local reps, training,warehousing etc, and 50% quantitative criteria, being clini-cal evidence (40%) and laboratory quality assurance test-ing (10%). The rationale was to improve on the originalcontract, allowing senior surgeons to select prostheses thatreflect best practice in arthroplasty surgery. The selectionof devices was based upon a minimum of 5 years clinicalfollow-up, with the clinical data rated on a scale of 1 to 5,1 being registry information and 5, company monographs.

Bioengineering was contracted by Health Supply WA tohelp in the process of collating the clinical evidence foreach device tendered and to perform a quality check ondevices selected by the surgical panel. This was to ensurethat devices conformed to relevant standards coveringthings such as packaging and labelling, microstructure,hardness, composition, dimensions and mechanical prop-erties etc.

The collating of the clinical evidence proved to be a diffi-cult task as many of the companies tendered shopping listsof items for each of the categories and some were unableor unwilling to provide a preferred device for each of thecategories. More than 30 system combinations (stem, headand acetabular components) were tendered for one hip cat-egory by one of the companies! In addition it was perhapssurprising to see minimal clinical evidence/support formany devices. Additional literature was sourced via re-sources such as Pub Med, FDA Website, Registrys’ etc asagreed by the surgical panel to aid in decision making. Interms of quality evaluation, it is of concern that some ran-dom samples sent to Bioengineering for evaluation con-

Supply of Primary Orthopaedic Hip and Knee Implants

tained a hair in the packaging, machining marks, liquidstaining, chipped coatings, poor laser etching, and metalfragments in polyethylene components, to name a few. In-terestingly, most of these problems would not likely beobserved in a busy theatre environment. Some microstruc-tures were also considered unacceptable with respect toimplant standards. It is hoped that identification of thesedefects will lead to improved quality.

At present, the surgical committee is in the process of per-forming the final evaluation and recommendation processwith a hope that the details may be finalised soon.

Custom Devices2004 was another busy year for custom devices. Thereappears to be an unprecedented need for cranioplasty plates(13 plates, 2 bifrontal plates). Other devices including hipspacers, shoulder arthrodesis plates and compression nailswere provided in addition to surgical models and a customacetabular cage.

After many years ofdesign and programm-ing, implant tracking is areality!! The currenttracking module,available to all the majorpublic hospitals in WA.

and monitored in Bioengineering, is designed toaccess all Theatre Management System (TMS)information relating to insertion and removal ofimplants, particularly orthopaedic devices. AlthoughTMS data on inserted devices has been available fora number of years, retrieved devices have never beentracked. The tracking of devices from insertion toremoval, will address the TGA need for post marketsurveillance and provides important information onimplant longevity, usage and failure rates. It will alsoprovide an improved ability to track patients in theevent of a device recall.

Bioengineering’s interest in the tracking facilityoriginates from its implant retrieval service that hasbeen operating for almost 30 years with more than

Small Punch Testing: A novel way to characterisethe mechanical behaviour of UHMWPE components !

Implant Tracking: Up and Running!!

IontophoresisThe iontophoresis projectcontinues within theBioengineering Division.Project Manager, KasiaMichalak finished work oniontophoresis of cancellousbone graft and is currentlycontracting through UWAto Australian OrthopaedicInnovations (AOI), acompany that has licensedthe rights to commercialiseiontophoresis of bone. Hermain tasks as ProjectManager will involveassisting the company ingaining approval from theTGA for Australian bonebanks to produce andsupply iontophoresed bonegrafts for transplant. Workin the laboratory continuesas part of this program. Itis proposed that a multi-siteinternational clinical trialusing iontophoresed bonegraft will be undertaken.

4000 devices referred to our labs. Despite excellentclinical support, we have never had a mechanism toensure that all failed devices were referred forinvestigation. The tracking module will facilitateorthopaedic devices retrieved in WA public hospitalsto be recorded and then referred to Bioengineeringfor analysis and archiving. This is important whenrecognising failure trends and in providing a promptresponse to surgeons, manufacturers and the TGA.

Implant device tracking, retrieval and analysis is thesubject of a submission currently before the HealthDepartment. The proposal was put forward to gainfunding for a State facility, covering all implanteddevices with the potential for inclusion of proceduresconducted in the private hospitals, if requested. Whenimplemented this will provide WA with a serviceemulating the highly regarded Scandinavian models.

The success of the implant tracking program isdependant on participating hospitals ensuring that allimplant retrievals are recorded, and surgeonscompleting the electronic retrieval forms. We verymuch appreciate your continued support!!

Previously, one of the major problems in the mechanical testing of UHMWPE com-ponents including acetabular liners and tibial inserts, was in obtaining representativesamples in sufficient numbers. Using the ‘small punch test’, sample disks (6.25mmDia and 0.5mm thick) are removed from areas of interest and indented by a hemi-spherical punch according to ASTM F2183. Parameters such as stiffness, peak loadand work to failure are obtained from the load-displacement data and can be used tocompare UHMWPE devices. Subtle differences in UHMWPE grades, in degraded/non-degraded UHMWPE, in processing and in sterilisation/irradiation treatments,can be determined. In a recent study, pristine UHMWPE from seven different manu-facturers, were compared. Of interest were the distinctly different behaviour of thecrosslinked samples. The crosslinked liners including Reflection (XLPE,Smith&Nephew), Trilogy (Longevity, Zimmer) and Trident (Crossfire, Stryker)showed marked geometric strain hardening and higher work to failure (WTF) valuescompared to noncrosslinked liners. WTF is an important measure and has been cor-related to wear rate. The thermally stabilised Duration (Stryker) samples, althoughnot highly crosslinked, also demonstrated superior mechanical properties to the con-ventional UHMWPE samples.

The small punch test provides a means of determining the mechanical properties andstatus of UHMWPE samples, both new and retrieved, and suitably complementsthe current thin section oxidation studies.

Vacuum Mixing Revisited !!It is well recognised that vacuummixing reduces the porosity in bonecement, thereby improvingmechanical properties such as fatigueand shear strength.Theatres (SPC) have been using theRPH VacuMix for nearly 7 years, butwith increasing requests for mixingthree batches of bone cement (120gpowder), it was decided to evaluate

three contemporary systems. The Stryker ACM (formally HiVac), the Smith & Nephew Vortex (identical to the HowmedicaArtisan) and the Biomet Optivac were evaluated.They were compared for design, ease of operation, cost and thequality and strength of the cement. Three mixes (120g) of

Tender Moments: Catheter EvaluationAs with the Hip and Knee tender, Bioengineering werecommissioned to perform the scientific component of anevaluation of urinary catheters. Quality was assessed usingthe catheter standard (AS 2696), and two in-house testscomparing rigidity and friction. The affect of soaking inartificial urine was determined for both Nelation (24 hrs)and Foley (14 days) catheter types. Packaging, flow ratesand balloon performance under load was consideredacceptable in all cases, however a number of defects wereapparent.

Numerous catheters did not comply with all the marking anddimension requirements. Surface irregularities, materialdebris, cracks near the eyelets (Fig.1), hole punch markingsand the presence of an eyelet blank inside one of the catheters(Fig.2), were disturbing features. In addition, 40% of theFoley catheters did not meet the balloon minimum volumerecovery requirements following soaking.

The evaluation that may have appeared unnecessary at theoutset highlighted a number of quality issues that have beenreferred to the manufacturers and the TGA.

Non-implantable Surgical ItemsOver the past year we have had three in-stances of fractured surgical instrumentsor drills that have either been retrieved orare currently in situ. Most surgical in-struments including drills are manufac-tured from 400 series stainless steel.These alloys are hardenable, have goodtoughness and high hardness for edge re-tention. The 400 series alloys are notbiocompatible and within a short periodin vivo will corrode.

Retrieval of two such items, one drill anda fractured end of a bone lever, whichwere in situ for ~ 9-12 months demon-strated significant corrosion and deterio-ration. Chromium corrosion products maystimulate macrophages and induce an in-flammatory response, which may furtheraggravate corrosion, thereby setting up acorrosion-inflammation-corrosion-inflam-mation feed back loop. 400 series stain-less steels can produced Cr6+ corrosionproducts which may penetrate cell mem-branes, show toxicity and result in boneresorption. Our recommendation is thatpieces of surgical instruments or fractureddrills should be removed ASAP, howeverat a clinically acceptable time.

Simplex P were tested in all systems. Thepercentage porosity and shear strength of bonecement are indicators of cement quality andwere measured for both an experienced andan inexperienced user. Mixer type had asignificant effect on shear strength, whilstacross operators there was no variation.Despite all mixers effectively reducingporosity compared to hand-mixed cement,they all fell well short of the manufacturers’claims (0.1% porosity). Overall, littleseparated the Stryker ACM and the Smith andNephew Vortex mixers, with costs possiblydictating the final choice.

Mixer Evaluation SummaryParameter Stryker S& N BiometQuality & strength 1 2 3Mixer Operation 2 1 3Cost 2 1 1

Rating: 1 good - 3 not as good.

Mixer porosities

Fig. 1 Fig. 2

De Puy LCS KneesA common finding in our implant retrieval studies is the remarkablylow wear rates of retrieved LCS (De Puy) components for both theRotating Platform (RP) and Anterior/Posterior Glide (APG) designs.This led to a tribological study of retrieved knee components in 2004.Thirteen LCS retrieved components including 10 APG and 3 RPdesigns with an implantation period ranging from 9-60 months wereevaluated. Utilising a coordinate measuring machine (CMM) withan accuracy of 0.010 mm, we were able to determine the wear.Overall results indicated an average wear rate for the superior surfaceof the tibial insert of the APG bearings of 0.07mm/year and 0.09mm/year for the RP design. Given the complexity of makingmeasurements, there are few results cited in literature. Wear ratesbetween 0.025 and 0.35mm/year have been reported, withmeasurements tending towards the latter.

The most significant finding was the low wear and absence ofdelamination and surface deformation compared to typical fixedbearing inserts. Wear mechanisms including burnishing, pitting,scratching and abrasion were observed on both the superior andinferior tibial insert surfaces, however in contrast to 4 retreived fixedtibial insert designs , there were no signs of delamination or surfacedeformation. The APG design showed notable multi-directionalscratching on the inferior tibial insert surface compared with the RPdesign. Short term retrieval analysis of the APG design demonstratescomparable low wear rates to the RP designs and significantly lesssurface degradation in comparison to fixed bearing designs. Longer-term quantitative wear measurements are required to confirm thesuperior performance of the APG design.

Royal Perth Hospital 2005Not to be reproduced without written permission of the Dept. of Medical Engineering and Physics

At the 2004 Scientific Orthopaedic Meeting, an interesting case of adislocated oxinium head was presented. The manufacturer claims thatthe oxinium material has all the benefits of ceramic and cobalt chro-mium materials with none of the risks. Clearly, the retrieved headdemonstrated that no deviceis risk free. The concernswere reported to the TGA,who will continue to moni-tor the occurrence of dislo-cation in oxinium coated to-tal hip replacements.

From a materials viewpoint, interest centres on the head structure,being a surface oxide layer (0.005mm) with a microhardness of 1235Hv and substrate of 285Hv, which is relatively soft compared to com-mon acetabular alloys. Laboratory testing of oxinium and cobaltchromium heads contacting a simulated titanium cup rim at a load of700N resulted in more damage to the oxinium heads. Once the ex-tremely hard surface is penetrated, the substrate is readily deformed.This also has been observed in a recent study by Smith and Nephew,

Robin HiggsWe were pleased to welcome Prof RobinHiggs to our Department in 2004, who en-lightened us especially with respect to thelaw and medical devices. His participa-tion at the local AOA conference was alsowell received.

World Biomaterials CongressApproximately 1700 delegates, includingRPH Bioengineering personnel, attendedthe World Biomaterials Congress in Syd-ney in May 2004. Due to an overwhelm-ing response (2000 abstracts for 700 oralpresentations) scrutiny of abstracts wastough. Fortunately three of our papers wereaccepted. Particular areas of interest werepolyethylene, implant retrieval, orthopae-dic materials and arthroplasty tribology.Other peripheral interest areas included thehuge increase in biomaterials usage for cos-metic surgery, bioglass and tissue engi-neered products.

Bioengineering ScholarshipCongratulations to Noel Jones on beingawarded the inaugural Bioengineering Schol-arship. Noel, a 4th year Materials Engineer-ing student from UWA has been working onseveral projects including improving the pro-duction methods for titanium alloy used in theshoulder arthrodesis plate.

SNIPPETS

Oxinium Heads acknowledging that in certain rare clinical situ-ations, such as scraping against the acetabu-lar shell during dislocation, the oxide couldbe penetrated. However in using relativelylow indentation loads (445N laboratory simu-lation), the resultant damage was much lesssevere than in our retrieved head. Subsequentwear in a hip simulator showed similar resultsto undamaged cobalt chromium heads.Theoxinium material offers some unique proper-ties that may be beneficial in hip arthroplasty,however no material is ‘risk free’. If a patientdislocates and is closed reduced, scrutiny ofsubsequent x-rays for accelerated wear iswarranted to ensure that significant head dam-age has not occurred. A simple alloying modi-fication to increase the substrate hardness mayreduce the risk of dislocation damage.

Contact us:Department: 9224 2500Eric Swarts: 9224 2997Alan Kop: 9224 3228

Fixed Bearings (4 -5yrs) LCS (RP 5yrs) LCS (APG 4yrs)