Hip Compliance

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Knee Compliance

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Department of Medical Engineering and Physics Royal Perth Hospital March 2008Welcome to the 2008 Bulletin. A snapshot of the work carried out in the Biomaterials Laboratory over thepast year is presented. We welcome any feedback or requests for further information.

In late 2007, a request was made to Bioengineering byHealth Corporate Network to look at public hospitalcompliance with the 2004 primary hip and knee contract.The Implant Tracking module, available in all major publichospitals in Western Australia, allows tracking of implantsby linking the unique patient medical record number to theimplant components, procedure, operation date andhospital, for both insertion and removal events. A usefulapplication is the monitoring of implant trends as illustratedby the plot for hip stem insertions at RPH/SPC.

By focusing on the period 2004 to present, performancein accord with the contract could be assessed. All hip andknee insertion events for RPH/SPC, SCGH and FremantleHospitals were accessed via an implant activity reportgenerated for the contract period. Data for the threehospitals is illustrated.

Implant Tracking and Contract Compliance

• Compliance with the hip and knee contractapproximates 70%. This appears a good resultespecially considering:

- this is the first time such a comprehensive tenderhas been conducted;

- there is a need to accommodate a patient’sspecific implant requirements outside the contractscope, and

- the competitive nature of implant marketing andthe large number of new designs/innovations beingoffered.

• The three hip stems that significantly effected contractcompliance were Accolade (RPH), Secur- Fit (RPH)and CPCS (SCGH). These items were not selectedin the 2004 tender because they either failed to meetthe 5 year clinical history requirement or hadinsufficient clinical data. Similarly, the PFC knee(RPH, FH) was not included in the contract.

• Surgeon preference and historical alignment betweenimplant type and hospitals appear to influence contractcompliance.

Implant tracking is not only beneficial in evaluating implanttrends and contract compliance but can also assist indetermining aspects of implant performance includinglongevity, failure rates and monitoring technology drivenchanges in arthroplasty usage.

“We can’t make any judgement on a joint until its been in use for a good 5 years”- Mr Roger Smith, secretary of the British Orthopaedic Association.

Hip Arthroplasties at RPH/SPC(total insertions >15)

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DePuy Corail

Stryker Accolade

Following on from a report in our last bulletin, the laboratoryevaluation of four proximal femoral nails has now beencompleted. The study initiated from a retrieval analysis of 8fractured proximal femoral nails (Synthes PFN), thatdemonstrated poor fatigue properties. With the interest of theDepartment of Orthopaedics (RPH), Synthes (PFNA), Stryker(Gamma 3) and Smith and Nephew (Trigen, Intertan) nails werecompared to the superseded PFN (Synthes) intramedullarydevice.

Testing was conducted in three areas:• cut out resistance, the most important clinical failure mode,• rotational stability, which is claimed to often initiate cut out,• fatigue, being the mechanism of failure in 8 PFN retrievals.

No single nail was superior across all tests compared to thePFN; however this does not necessarily equate to an inferiorclinical performance, as the new generation nails are based upona different clinical philosophy. All the new generation nails havesingle element femoral head/neck fixation, which has reportedadvantages including, simplification/ease of clinical use, lessbone removal/destruction, an integrated anti-rotation/slidingcapability with or without locking, and the potential forminimally invasive surgery.

Cutout resistance results show a large variation due tomanufacturers using test conditions favourable to their

Proximal Nail Study

F atig u e Stren g th T itan iu m Nails

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stainless steel. This was expected, as the cross-sectional areain the region of the proximal lag screw, where all PFN failureshad occurred, was smaller for all new generation nails. Theredoes not appear any scientific evidence to support this reductionin dimensions, other than claims of facilitating minimally invasivesurgery, minimisation of incision length, and less bone removal.Albeit, when used strictly in accordance to the companiesindications, and for patients who readily heal, (thus early loadsharing), all new nails will most likely have adequate fatiguestrength. For the more difficult cases, eg. large statue patients,prophylactic tumour treatment, or impaired/compromisedhealing, a design trial is currently under way in Bioengineeringto produce a higher strength custom device.

Rotational stability testing was conducted as it has beenreported that varus collapse may initiate via neck/head rotation.None of the new nails out performed the PFN with its 2-elementfixation. The Trigen Intertan with its interlocking screw designwas superior to the PFNA with the standard Gamma 3 designsignificantly poorer, however the Gamma 3 with U-blade, whichadds an anti-rotation element to the lag screw, performedsimilarly to the PFNA.

In summary, the laboratory testing elucidated the mechanicalperformance of the nails in vitro, highlighting advantages andweaknesses in design. None of the nails were superior in alltests when compared to the PFN, however a planned clinicalevaluation at RPH should ensure the most appropriate nail/sare selected for clinical use.

implant, as such, all nails were tested supported (complete neckfracture apposition) and unsupported (neck fracture gap - theworst case scenario). Both the Gamma 3 and Trigen Intertanperformed well when compared to the PFN, whilst the PFNA wasinferior when tested unsupported but similar to the TrigenIntertan and Gamma 3 in the supported test case. Of interest areseveral clinically noted instances of axial cut out of the PFNAblade, which were predicted by the unsupported test.

Fatigue studies confirmed that none of the nails were as strongwhen compared to the PFN even when manufactured from

The Margron implant was introduced in 1997with great interest due to its unique femoralstem with differential threads and modular neckcomponent. Clinical advantages of the proximalneck design are intraoperative adjustment ofleg length via the neck-head taper and femoralanteversion via the neck-stem taper with theaim of optimising the patients’ biomechanics.However, the take up of the arthroplasty hasbeen limited and Australian registry resultsshow 10.6% cumulative revision at 5 years, withrevision 3 times more likely than othercementless stems. Implants sent to RPH havepredominantly been revised due to pain andloosening; however recent observations of

Margron components have revealed another problem, namely extensivefretting and crevice corrosion at the stem-neck tapered joint. This is ofconcern as fretting and crevice corrosion can lead to the generation ofsoluble and particulate debris that migrate locally or systemically and maycontribute to periprosthetic osteolysis. The laboratory study demonstratedthat despite an optimal taper design, (being a large taper to maximiseconstruct stiffness and minimise fretting) and use of proven corrosionresistance alloys, that increased modularity can lead to fretting and crevicecorrosion. Furthermore this may result in significant metal ion generationand particulate debris.

The Margron StoryThe Bioengineering Division was back in theclassroom in 2007 with a series of 8 lecturescommissioned by Prof. Allan Skirving for theBone School. The topics ranged from the mostbasic (what is a force?) to cutting edgebiomechanics and biomaterials.

The course aimed to prepare Registrars fortheir exams while still providing broadbackground knowledge. This difficult task wasmade harder by the variation of experienceamong the Registrars. This should be less ofan issue when the new training scheme isintroduced, as everyone will take the courseat the same point in their training.

We hope to improve the course and make itmore relevant to the needs of the wholeorthopaedic community. One suggestion wasa series of refresher lectures for practisingsurgeons, to keep them abreast of recentdevelopments. If anyone is interested in thisidea, please contact the Division.

Bone School

Recently an increasing number of Preservation Uni-compartmentalknee arthroplasties (UKA) have been received for retrieval analysis.As a consequence, we decided to collate all the retrieval data(n=19) in order to determine if there were common failure modes.All the devices were cemented, with an average patient age of 78years and implantation time of 2.2 yrs. We found a high incidenceof component loosening, particularly femoral loosening, withapproximately equal numbers of mobile and fixed bearingPreservation UKA’s. In contrast, the AOA registry showed doublethe revision rate of the mobile bearing UKA’s compared to thefixed UKA’s, with implantation of the mobile knee significantlyreducing since 2002. Unfortunately, the Australian registry doesnot publish reasons for removal, however in a recent publicationby Mariani et al. (J Arthroplasty, vol 22, 6, 2007), there was a40% failure rate reported for fixed Preservation UKA’s due tofemoral loosening (n=39), which correlates with our retrievalfindings. Literature suggests that the extended femoral componentmay allow increased flexion, which in turn promotes edge loadingand pushes the femoral component off the end of the femur. Alateral X-ray in maximum flexion was found to be useful inascertaining femoral loosening. Given the usage of PreservationUKA’s (n=108) in the public system, we are likely to see moreretrievals.

Preservation Unicompartmental Knees

Preservation UKA’s N= 19 %Fixed bearing 9 47Mobile bearing 10 53Osteoarthritis (OA) 19 100Time In Situ (Ave) 2Reason for removalLoose 1 5Loose femur 11 58Loose tibia 3 16Total loose 15 79Pain 2 10# bone 1 5Excess weight 4No Clinical Details 1 5

a. Microstucture of material.

b. SEM image of fretting corrosion.

c. SEM image of corroded surface demonstrating inter and intragranular corrosion associated with chromium carbide precipitates.

d. & e. Macrostructure of tapered interface, note fretting corrosion.

f. Cyclic Corrosion plot of Margron material indicating good general corrosion resistance.

Royal Perth Hospital 2008Not to be reproduced without written permission of the Dept. of Medical Engineering and Physics

Conferences 2007/08EPSM (Engineering and Physical Sciences in Medicine - Fremantle)• Day, R. Complex Acetabular Reconstruction using SLM• Day, R. Iontophoresis• Swarts, E. Implant Surveillance• Kop, A. Biomechanical Evaluation of Intramedullary DevicesSLM Users Group (Germany)• Day, R. Complex Acetabular Reconstruction using SLMAOA Continuing Education (Sydney)• Day, R. Iontophoresis8th World Biomaterials Congress (The Netherlands)• Swarts,E. Evaluating Implant Performance using Implant

Tracking and Retrieval Analysis

Rob Day, David Wood & Roger Price(Medical Technology & Physics at SCGH)have received NH&MRC funding to extendthe iontophoresis technique developed byMegson, Day and Wood. Iontophoresisappears to protect cortical allografts fromperi-operative infection (JBJS Br 88(11) &88(9)). This project, as part of Rob’s PhDstudies, proposes a new way to loadantibiotics into bone that could be cheaperand easier than iontophoresis.

Grant Success:

A new Instron 8874 servo-hydraulicmaterials testing machine has beencommissioned, replacing the much largerInstron 1341, which after 20 years offaithful testing has finally surrendered tothe digital age. This servo-hydraulictesting machine will be ideal for axial-torsional cyclic and static testing ofbiomedical, advanced materials and

manufactured components. Thenew controller also facilitates theuse of ‘gait’ loading patterns. Anobvious application will be cut outtesting of intramedullary nailswhere both torsional and axialforces can be applied, which willbetter simulate the biomechanics invivo. The design of a high-strengthcustom proximal femoral nail iscurrently in progress.

New Staff:Ting Phila recently joined us as aLaboratory Technician to cover for SusanMiller who is currently on maternity leave.Ting has extensive research experience havingworked at the UWA School of Medicine &Pharmacology, Pathwest and ClinicalPhysics (RPH).Jessica Down is another addition to theBioengineering team. She is a graduateMechanical Engineer from UWA. Apart fromassisting in laboratory work, Jessica will beinvestigating the mechanical properties oftitanium alloys produced by the SelectiveLaser Melting technique. These alloys andproduction method are currently used incustom implant devices, principally customacetabular cages.

SNIPPETS

Bioengineering tested its smallest bones ever in 2007. Researchers atthe Molecular Bone Biology Laboratory (Concord Repatriation Hospital)and the Centre for Cancer Research & Cell Biology (Queen’s University,Belfast) have developed a transgenic mouse to study bone formation,particularly targeting osteoporosis. The genetic manipulations changedthe bone structure, but did it change the strength?Neither group had expertise in bone mechanics so RPH Bioengineeringwas asked to measure the strength of their mouse tibiae. These averaged15 mm long, 5mm shorter than the previous record rat tibiae, and assuch we had to develop a new 3 point bending rig for testing such tinybones.Even our small Instron, familiar togenerations of Registrars, produces10kN; much greater than the 10–15 Nbreaking strength of the tibiae. Justloading such small bones withoutbreaking them was a challenge. Theeffort proved worthwhile, with highlysignificant differences in both strengthand stiffness between the groups.In contrast, the strongest bones wehave ever tested were equine pasterns,which broke at over 80kN.

From the sublime to the ridiculous

New Instron Biaxial Testing Machine

Custom Devices & TGACompliance:Bioengineering is working towards fullcompliance with the TGA Regulations forcustom devices. The changes in paperworkare mainly internal, with a focus on Designand Failure Modes and Effects Analysis andmeeting the 14 essential principles formanufacture. However it is now also criticalto have a signed consultation form for allcustom devices prior to delivery.

Contact us: 9224 2500trevor.jones@health.wa.gov.auBioengineering Staff:Trevor Jones,Alan Kop, Eric SwartsRob Day, David Morrison,Cathie Keogh