Implant Technology And Biomaterials Bulletin No. 2 December 2002

Welcome to the second Bulletin!We were encouraged by the positive feedback from the Hospital Execu-tive and surgeons from our first bulletin and with this issue we hope tocontinue to provide items of orthopaedic interest.

The Smith & Nephew Genesis IIOxidized Zirconium knee has recentlybeen released onto the market. The kneeincorporates an innovative materialdesign, combining a zirconia (ceramic)surface layer with a zirconium alloy metalsubstrate. The ceramic surface isexpected to have excellent abrasionresistance and good lubricity withtoughness provided by the metalsubstrate. An evaluation wasconducted to investigate the material andto test the manufacturer’s claims.

Cyclic fatigue tests on the medial condyleat loads of 450 kg for 10 million cyclesshowed no signs ofcracking ordeformation. Thefemoral componentproved to haveexcellent fatiguestrength under arange of simulatedconditions.Corrosion performance was adequate,without being outstanding and themicrostructure was acceptable.

The zirconium femoral componentcomplied with all the relevant implantquality standards and on the basis of thetests performed would appear to be anacceptable and possibly superioralternative to similar designs usingconventional materials. Long term follow-up results are keenly awaited.

Recently, a fractured Exeter hip prosthesis was referred for failure analysis.Nothing unusual about that, until further investigation revealed someinteresting anomalies. The implant was manufactured from high nitrogenstainless steel, a high strength alloy rarely associated with stem fracture.

The fracture initiated at a region of surface corrosion on the superiorsurface of the neck. Electrochemical corrosion studies revealedsusceptibility to pitting and crevice corrosion. Several Exeter retrievalsalso showed surface corrosion damage, an unusual feature in a highlycorrosion resistant alloy.What was most disturbing howeverwas the stem microstructure. A cross-section revealed variable grain sizeand section hardness with large grainsnear the surface and regions ofbanding.These features are oftenassociated with reduced fatiguestrength. Again, other Exeter stemsthat had not failed, showed similarcharacteristics.The stem was insitu for 8 years and appeared well fixed. Distinct “beachmarks” on the fracture surface indicated a fatigue failure mechanism.

Oxidized ZirconiumThe Next Step!!

Although only two fractured Exeter stems of this alloy have been seenlocally, there remains concern with the variable microstructure and surfacecorrosion. A visiting emeritus Professor and fatigue specialist from theUK, Prof. Keith Miller, confirmed our findings and warned of the increasedfatigue failure risk.These concerns have been relayed to the manufacturers who haveresponded by organising a series of independent tests. In the meantimewe would appreciate receiving all retrieved Exeter stems for investigation.

Exeter Hip - Fatigue Risk?

Variable grain size

Fractured Exeter Stem Fracture Surface - beach marks

Device Evaluation - Cost Effective??In 1994 we evaluated Boneloc cement, a novel system which integrated the packaging, mixing and cementdelivery. Its appeal centered on its simple “touch free” mixing. Our initial enthusiasm was dampened when wefound it to have significantly inferior mechanical properties and particularly poor penetration. Based upon ourexperience with bone cement, it was fairly obvious the cement formulation had been changed considerably toachieve the mixing characteristics. We recommended against its use and since then many clinical studies havereported high failure rates from 20%-65% for up to 5 years follow up. Bad press caused Boneloc to be with-drawn in 1995.Consider an interesting hypothetical:

There have been approx. 8000 cement procedures at RPH since ‘94• 15% uptake of Boneloc = 1200 procedures• 65% Boneloc failure rate/5 yrs= 700 failures• 10-15% typical revision rate = 150 failures• Additional revisions attributed = 550 failures to Boneloc

Total additional cost to RPH could have been $16.5 million( $30,000/revision)

Shoulder InnovationLate last year, A/Prof Allan Skirvingapproached us with a patented ideafor a new shoulder hemiarthroplasty. Together with MikeEdwards, Bioengineering hascollaborated in several aspects ofthe development. We have focusedon the accurate measurement ofcadaveric humeri using a portableCoordinate Measuring Machine(CMM, accuracy better than ±0.2mm) and using mathematicalprocesses to describe the shapes ofthe articular surface and articularmargin.

In addition, 3D drafting of aprototype was done including ananimation sequence whichdemonstrated the modularity of theprosthesis. The findings andanimations were presented at theAOA conference in Melbourne.Thanks to Rio Tinto for the loan ofthe CMM.

The use of the new Stryker Tridenthip arthroplasty system is widespreadin WA with many surgeons using thepolymer and/or ceramic system.There have however been somereports of problems with the ceramicsystem. The major clinical problemto date, stems from difficulties ininserting the ceramic acetabular linerinto the shell.

Following requests from Mr DermotCollopy and Mr Dick Beaver, arecent laboratory study concludedthat under certain conditions the shellcan incur a diametral deflection dueprincipally to its ‘thin’ wall design. Thedesign rationale appears to have beento maximise the thickness of the poly-ethylene liner and therefore minimise

the shell wall thickness, especially asthe ceramic liner is a sandwichconstruction. The idea in principal issound, especially in light of recentfindings of the susceptibility for mis-alignment and potential for chippingand fracture where the ceramic lineris placed directly into the shell.The Stryker literature suggests that if‘hard bone’ is encountered, thesurgeon should over-ream theacetabulum to reduce the 1.8mm builtin interference fit, acknowledging theproblem that the shells may deformin relation to the bone quality.The difficult surgical decision whetheror not to over-ream the acetabulumcan be alleviated by an engineeringdesign change. Alternate designs onthe market, tested for comparison,do not present this complication withsubstantially thicker shells.At present, if one is aware that theshells are susceptible to deflectionthen an appropriate clinical decisioncan be made intra-operatively.Laboratory work and ongoingdiscussion with Stryker are continuingwith view to assisting surgical staffusing this technology.

Stryker Trident Cups: Ceramic Liner - Shell Fit

Peek Optima, based on polyetheretherketone resin, is a new advancedpolymer with excellent mechanical and chemical properties, and is nowavailable as an implant grade material. The material was evaluated to as-sess its potential for use in hip and knee spacer stems and intramedullaryflexible nailing systems. The average tensile strength and flexural strengthexceeded the values for polyacetyl (delrin) by 35% and 100% respectively.During bend strength tests of spacer stem assemblies without proximal ce-ment, the polyacetyl stem fractured at the proximal screw thread. In con-trast, the Peek Optima stem did not fail. Bending loads in excess of 200kgwere sustained in spacers with proximal cement.

Implant RetrievalThe implant retrieval service continuesto provide useful information ondevice performance:

• Surface corrosion damage onretrieved Howmedica Exeter hip stemshas been regularly observed. (see frontpage).

• HG (Zimmer) liner dissociationcontinues to be a major concern.Failure of the locking mechanism ofthe shell due to bent or fracturedlocking tines was highlighted in theprevious bulletin and at least 23retrieved dissociated liners have beenreferred to the Biomaterials lab to date.It is strongly recommended that forHG revisions, both the shell and linerbe replaced. (there may be someexceptions).

• Retrieved LCS (De Puy) kneecomponents show remarkably littleUHMWPE bearing surface wear. Thelow wear rates and minimal oxidationeffects are typically observed on therotating platform and AP glide designs.Most LCS knees were removedbecause of pain or loosening.

• Surface degradation is commonlyobserved in UHMWPE componentsthat have been sterilised in air. Sub-surface ‘white banding’ seen in thinsections is often associated withdelamination and cracking. ‘Whitebanding’ is particularly prominent intibial inserts that have been in situ for5 years or more. Those mostsusceptible appear to be WhitesideOrtholoc, Zimmer MG, HowmedicaPCA, Duracon and Richards Genesistibial inserts.

Most manufacturers have modifiedtheir sterilisation methods since 1995and no longer use gamma sterilisationin air. Regular monitoring of retrievedinserts and liners is important todetermine the success of the newsterilisation practices.

Best Wishes &Merry Christmas

from the Bioengineering Team

Eric Swarts, Ed Scull, Alan Kop, Trevor Jones, Rob Day,Cathie Keogh, Susan Miller, Manny Manolious.

ph: 9224 2500

Peek Optima is a significant advance in polymer technology with almostmetal–like mechanical properties. Given its implant grade status, it is ideallysuited as a replacement material for polyacetyl in most applications.

Stem samples following bend tests Axial loading of spacer with proximal cement

A ‘PEEK’ at a New Implant Polymer

Royal Perth Hospital 2002Not to be reproduced without written permission of the Bioengineering Division, Dept of Medical Physics, RPH

Implant trackingAn implant tracking module is beingintegrated into the Theatre Manage-ment System at RPH with the help ofInfoHealth and should be completednext year. The module will eventuallybe used at all major hospitals in W.A.with all devices referred to theBiomaterials lab for analysis. This willgreatly enhance the retrieval service,enabling a better understanding ofstate-wide practice.

2002 Registrar projectsDr McCormick - Gentamicin elutionDr Edwards - Morphology of the proximal humeral articular surfaceDr Clarke - Reference measurement of femurs for computer guided surgeryDr Erak - Combined lengthening of extensors in tennis elbowDr Brankov - Interfacial strength of ‘new’ PMMA on aged bone cementDr Lim - Reverse oblique fractures of the proximal femur

AO locking platesAn evaluation is currently in progressfollowing the release of the new AOlocking plates at RPH. In the labora-tory the plates performed well, al-though with uni-cortical fixation theyare not as stiff as the standard LCDCPplates. Fixing the plates off the bone(extra-periosteal) reduces the stiffnessfurther; however the construct is stillsuperior to comparable uniaxialexternal fixation. The lab study is con-tinuing.

Modification of AO obliqueangled platesSubsequent to a request from MrDavid Gill, a series of AO obliqueangled locking plates have been‘formed’ for plating the volar surfaceof the distal radius. The modifiedplates appear to be performing well.AO Mathys have been contacted tosee if they will produce the plates aspart of their kit.

Minimally invasive toolsOn request of Prof Wood in conjunc-tion with Smith and Nephew, an im-paction device for minimally invasivehip surgery has been designed andtested, with the manufacture of a sec-ond in progress.

Removal of devicesBioengineering has been called upon to assist in the removal of several de-vices including an original AO tibial nail (~1970’s, not seen in Australia) forwhich 5 removal attempts had previously been made. The nail was severelyshortened (20-25mm) due to each attempt and proved a challenge to remove.Successful removal was achieved by using a customised expanding bolt, alarge slap hammer and considerable effort by Mr John Croser.

Bioengineering NewsBioengineering News Bioengineering News

The take home message is that during surgery, if in doubt, photograph andmeasure the device and plan a definite attack. Other removals include a SmithPeterson nail and a Sherman plate insitu for 25 years. The retrieval collectionis certainly an asset in these cases.

Tissue analysis requestFor any metal sensitivity analysisplease supply at least 2g of fresh tissuefrom the implant site (please do notsend in formalin as it invalidates theanalysis).

This study was initiated after a recentfailure of a fully threaded titaniumscrew. A recommendation is ex-pected soon.

SCFE screwsA laboratory study is under way todetermine the strength (static and cy-clic) of fully threaded slipped capitalfemoral epiphysis screws, comparingvarious stainless steel and titaniumversions.