Research

Failure Analysis

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Since the inception of the DBEC retrieval library in 1976,our understanding of the complexities of joint replacement technology hasimproved substantially. Retrieval laboratories around the world, includingDartmouth’s, have greatly facilitated design optimization and materials fororthopaedic implants, which in turn produces profound quality-of-lifeimprovements for patients worldwide. The laboratory has evolved scientificallyand financially to weather dramatic changes in the orthopaedic industry, makingsignificant impacts on the care of patients worldwide. In the last fourdecades, Thayer School's implant retrieval analyses played a key role inidentifying failure modes and relating them to various designs and materials usedin industry. Indeed, in 2000, NIH's Consensus Development Program produced a technologyassessment statement acknowledging the value of implant retrieval programs. Thestatement drew the following conclusions:Implant retrieval and analysis is ofcritical importance in the process of improving care of patients in need ofimplants.Attention needs directed toward reducing obstacles to implantretrieval/analysis, particularly legal and economic disincentives.Failure toappreciate the value of implant retrieval/analysis is a serious impediment toresearch in devices.A focused educational program will provide the informationnecessary for improving the quality of future devices.While most peopleunderstand medical implants improve quality of life, few recognize theimportance of retrieving/analyzing failed implants when they fail or are nolonger in use.

The History of the Value of Retrieval Analyses

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A) 1970’s-1980’s: Porous coating for bone ingrowth is a successful fixation technique

  • Additional studies show porous coating needs specific metallurgy, structure, & adhesion to orthopaedic components
  • Industry/Clinical Change: Porous coatings are widely used and the primary fixation technique for total hip arthroplasty
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B) 1980’s-1990’s: Corrosion as a potential problem in metal components, particularly in modular devices

  • Industry/Clinical Change: Manufacturers changed metallurgical processing and minimized modularity in high-stress areas of devices

C) 1990’s: Illumination of challenges and benefits of modularity in patellar components

  • Industry/Clinical Change: Metal-baked patellae no longer include thin polyethylene or snap-together components
  • Industry/Clinical Change: 4mm is established as a minimum thickness
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D) 1990’s: Discovery of benefits and drawbacks of design strategies in the knee and hip

  • Industry/Clinical Change: Hydroxyapatite proved to be a successful fixation system
  • Industry/Clinical Change: Rough titanium trays are becoming less common in modular total knee systems
  • Industry/Clinical Change: Thin polyethylene leads to fracture
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E) 1990’s: Gamma in air oxidation leads to fatigue failure and increased wear of orthopaedic devices

  • Industry/Clinical Change: All polyethylene in the US are sterilized in a gamma-barrier package or using a non-ionizing radiation source
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F) 2000’s: Biomechanics contributes to wear and performance of devices

  • Industry/Clinical Change: All polyethylene in the US are sterilized in a gamma-barrier package or using a non-ionizing radiation source
  • Industry/Clinical Change: All polyethylene in the US are sterilized in a gamma-barrier package or using a non-ionizing radiation source
  • Industry/Clinical Change: All polyethylene in the US are sterilized in a gamma-barrier package or using a non-ionizing radiation source
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G) 2000’s: Material type contributes to performance of bearings

  • Industry/Clinical Change: Switch away from calcium-stearate containing polymers in knee and hip

H) 2000’s: Novel materials in hip and knee applications

  • Industry/Clinical Change: Move to highly crosslinked materials in knee and hip, with lower crosslink doses in knee
  • Industry/Clinical Chance: Surgeons advised to minimize stress in acetabular cups with higher crosslinking doses
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I) 2000’s:  In-vivo oxidation documented in gamma barrier components

  • Industry/Clinical Change: Many companies move away from gamma barrier devices, some institute antioxidant technologies
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J) 2010’s:  In-vivo oxidation documented in highly crosslinked devices

  • Industry/Clinical Change: Companies are moving to antioxidant technologies.
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K) 2010’s:  Retrieval studies document squeaking phenomenon in ceramic hips

  • Industry/Clinical Change: Acoustic considerations are weighted heavily in bearing selection
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L) 2010’s:  Biomechanics, damage, and wear associated with metal on metal hips

  • Industry/Clinical Change: Studies are in progress. We expect failure analysis of all devices will inform future design of hard-on-hard hips.
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Bearing Function

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Retrieved knee devices sent to us by orthopaedicsurgeons are assessed for damage, and also quantitatively assessed for wear.Dimensions of retrievals are compared to design specifications or shorter in-vivoduration devices to calculate both articular and backside wear. Wear and wear rateare correlated with variables including polyethylene pedigree, articularbearing geometry, device fixation, and patient factors.

An important early outcome from this effort is thedistinction between damage and wear of knee bearing inserts. Damage of jointarthroplasty bearings can be visually striking, can be describedsemi-quantitatively according to published techniques, and can impactmechanical performance and kinematics of the implant. Wear that occurs byabrasive/adhesive processes can be very challenging to discern and quantify, yetcan produce large volumes of small debris particles that can lead to osteolysis(see figure below). Damage and wear are important but distinct phenomena thatcan have different impacts on clinical performance.

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Highly cross-linked (HXL) polyethylene has provento be a wear-resistant acetabular bearing material in total hip arthroplasty (THA).In vitro wear testing has predicted a tenfold reduction in the wear rate of HXLpolyethylene, as compared to conventional, non-HXL bearings. To date, radiographicstudies of head penetration represent the state-of-the-art in determining clinicalwear of polyethylene hip liners. However, as the amount of wear drops to verylow levels, it becomes important to develop a precise and reliable method for measuringwear, facilitating a comparison of clinical results to laboratory expectations.Fixed-magnitude errors associated with digital imaging necessitate increasinglylarge studies to statistically elucidate the low wear rates. Retrieval analysisprovides much better precision, but is subject to different sources of error.

Our current work focuses on locating and quantifyingthe maximum linear wear of retrieved acetabular liners using a coordinatemeasuring machine (CMM) and a reverse-engineering algorithm. Specifically, HXLliner wear can be assessed as a function of radiation dose and compared to abaseline of conventional, non-HXL bearings.

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Few retrievals studies have comparatively examined wear ofboth reverse and total shoulder arthroplasty. The lack of literature on thistopic prevents organizations from standardizing and publishing methods for weartesting of shoulder components. In order to create relevant wear testing standards,it is crucial to understand how components wear in vivo including the modes andlocations of wear. One goal of our current work is to examine series of reverseand total shoulders to determine the incidence of abrasive and adhesive wearand determine typical locations for these wear patterns on polyethylenecomponents.

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Material Behavior

UHMWPE Overview

Medical grade ultra-high molecular weight polyethylene(UHMWPE) is the current gold standard for joint bearing materials used in TJA.Although TJA involving UHMWPE as a bearing surface has been one of the mostsuccessful procedures of the last century, issues of wear, oxidation, and fatiguefailure remain obstacles to the longevity of joint replacements. As a failure mode,wear is biologically compounded, because wear debris can trigger a series ofreactions leading to osteolysis, a condition resulting in long term resorptionof the bone around the implant.

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Crosslinking and Heat Treatment of UHMWPE

Radiation crosslinking of UHMWPE significantly improves its wearresistance, as evidenced by in vivo clinical studies and in vitro hip simulatorstudies. During irradiation, crosslinks are formed between polymer chainsthrough homolytic cleavage of C-H and C-C bonds. However, ionizing radiation alsoproduces free radicals randomly throughout UHMWPE as part of the crosslinkingprocess. These long-lived species can react with oxygen, triggering a cyclic complexcascade of chemical reactions. While free radical oxidation involves a numberof possible pathways with different mechanisms and end products, the overalloutcome includes polymer chain scissions which reduce the molecular weight, andvarious oxidative products such as hydroperoxides, ketones, alcohols, andcarboxylic acids. Overall, this cascading oxidative reaction is responsible forprogressive embrittlement of the material. Oxidative degradation thus manifestsas a reduction of wear resistance and mechanical properties.

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In the late 1990’s, oxidation of UHMWPE was identifiedas a serious concern as it limits the overall lifetime and success of a jointreplacement. Hence, the elimination of free radicals in UHMWPE has been animportant focus of orthopaedic manufacturers ever since the industry’s responseto shelf storage oxidation occurring in gamma sterilized devices. In collaborationwith materials research laboratories, device manufacturers have developed a varietyof post-irradiation thermal treatments with the dual goals of promotingoxidative stability and enhancing the crosslink density of bearing materials.

One thermal treatment approach utilizes heatingabove the melting point of the crosslinked polymer following irradiation. Thismelts the crystalline regions and allows recombination of trapped free radicalsin these domains. After the polymer recrystallizes, the residual free radicals havebeen quenched and the material is both wear resistant and more oxidativelystable in shelf-aging and artificial aging according to current ASTM Standards.However, this improved wear and oxidation resistance comes at a cost becausepost-irradiation melting further decreases the fatigue strength of UHMWPE, alreadyreduced by radiation crosslinking; the melting step results in a decrease in crystallinityand ductility. Thus, upon cooling, the extent of recrystallization forcrosslinked UHMWPE is inferior to that of UHMWPE without crosslinks.

An alternative method of thermal treatment is post-irradiationannealing below the melting point of the crosslinked polymer. In the absence ofa recrystallization step, annealed materials possess superior mechanicalproperties in comparison to fully remelted materials with the same radiationdose. However, this approach reduces but does not completely eliminate freeradicals as achieved by melting. As a result, this material is stillsusceptible to in vivo oxidation. Our lab and others have reported oxidation inirradiated and annealed UHMWPE both in retrieval analysis and in vitroaccelerated aging studies (see below).

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Recent retrieval analyses conducted by our laboratory showedthat despite the absence of free radicals (prior to implantation), highlycrosslinked (HXL) remelted acetabular liners and tibial inserts showed signs ofoxidation occurring in vivo, with greater oxidation rates occurring in TKA in comparisonto THA. The crosslinking radiation dose significantly impacted the material’s oxidationpotential. Additionally, the in vivo oxidation rate significantly correlatedwith transvinylene bond concentration (also referred to as unsaturations), andpotentially to contact stress. Others have observed similar results, and havefurther shown potential connection between absorbed species and in-vivooxidation. There thus exist several potential oxidation mechanism pathways,including the following:

1. Free radical mediated mechanism (conventionally accepted)

2. Stress induced chain scission mechanism

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3. Absorbed pro-oxidative species mechanism

4. Irradiation-energy, chemical bond-related mechanism  

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Antioxidant Polyethylene

In an effort to improve oxidation resistance withoutcompromising mechanical properties through thermal treatments, manufacturershave examined the use of antioxidants to prevent oxidation of free radicals. Themost prevalent antioxidant available today is a liquid-based alpha-tocopherol (VitaminE). Two methods of adding vitamin E to the UHMWPE have been examined: combiningliquid antioxidant into UHMWPE resin powder prior to compression molding, anddiffusion of vitamin E into already cross-linked UHMWPE. Adding vitamin E intothe resin prior to crosslinking reduces the crosslink efficiency since theantioxidant scavenges free-radicals during irradiation, thereby reducing the effectiveamount of crosslinking. Diffusion of vitamin E into the polymer followingcrosslinking does not inhibit crosslinking. More recently, solid-statepentaerythritol tetrakis has been marketed in bearing materials for TKA. Due tothe novelty of these antioxidant materials, long duration clinical andretrieval studies haven’t yet been published, particularly with respect to invivo oxidation prevention.

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The documentation of unexpected in vivo oxidationof thermally stabilized UHMWPE may be of concern to clinicians, industry, andpatients, due to the potential for polymer oxidation to lead to increasedincidence of fatigue- and wear-related failures. Moreover, the shift inrecipient demographic to a younger, heavier, more active patient places greatermechanical demands on bearing surfaces. Thus, attaining a better understandingof the in vivo oxidation process and rate is crucial to ensure devicelongevity. With this understanding, it is important to develop a predictive capabilityto accelerate aging under controlled laboratory conditions to test new andexisting materials before they are employed in humans.

Over the last decade, our understanding ofoxidation has advanced through systematic review of retrieval oxidation in thelaboratory. Specifically, we have observed exponential oxidation rates ingamma-air, gamma-barrier, annealed, and remelted materials. These rates appearto be influenced by several different factors, including stress, free radicalconcentration, absorbed species, and radiation source. We believe that thesefactors may lead to oxidation through a number of competing pathways, each ofwhich is a subject of research in our laboratory.

Bio-mechanics

Optical Motion Capture (MOCAP)

One gold standard method for capturing high - quality, accurate,and precise musculoskeletal biomechanics are optical motion capture (MOCAP)systems. These systems consist of cameras that emit infrared (IR) light thatcontacts reflective markers placed on the subject's bony anatomy (see picture below).That same IR light reflects back to the cameras that record the 3D position ofeach marker. Marker locations are then converted to semi-rigid bony segmentsand the relative orientation of distal-proximal segment pairs are quantified asjoint angles (e.g. knee flexion), also known as kinematics. In DBEC, weprimarily utilize our optical MOCAP system as a validation tool for othermethods. However, we are currently conducting numerous studies leveraging thehigh-fidelity kinematic information gleaned from optical MOCAP to establishperformance during a variety of tasks (e.g. ambulation, stairs, jumping, etc.)in a number of populations (e.g. total joint replacement, expectant mothers,etc.)

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Wearable Technology

Rehabilitation following joint arthroplasty is most often a ‘one-size-fits-all’approach: all patients receive the same physical therapy (PT). However, it is notwell established if this is the highest value approach to healthcare. It isconceivable that different patients would need variable levels of postoperativePT to achieve optimal recovery. In addition, postoperative progress is oftenonly gauged via single data points as measured in clinic / laboratory settings(e.g. passive range of motion via goniometer) whereas motion throughout the dayis only assessed anecdotally. Our laboratory has developed and implemented anovel method for monitoring continuous long term joint function using wearable devices.In collaboration with our orthopaedic surgery colleagues at Dartmouth HitchcockMedical Center, we have subsequently been issued a patent for parts of thissystem. Perhaps most critically, we have leveraged these wearable devices tocapture long duration, continuous range of motion in patients before and aftertotal joint replacement, lower extremity trauma, and expectant mothers to namea few. These data are be compared to a cohort of healthy individuals with nomovement related pathology.

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Shoulder Modeling

The number of total and reverse shoulderarthroplasty procedures performed annually in the United States has beengrowing steadily over the last decade. One aim of our current work is tounderstand the mechanical and tribological interactions of shoulderarthroplasty with the patient. The development of algorithms for analysis ofexplanted components, patient outcomes, in vitro wear testing, and finite elementanalysis will provide a better understanding of joint behavior and potentialimpacts to the patient. This knowledge may allow for device designs andimplantation methods to be adjusted to account for potential failure mechanisms.

Image of humeral model
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Computational research in the laboratory occurs at various scalesand couples the interaction between musculature and bone. This helps identify howdevice implantation can affect musculature using dynamic modeling (top left), andin return, result in changes to stresses and strains experienced by the bone(center, bottom). General or patient specific geometry and devices can beaccommodated by utilizing CT scans and solid modeling (center, top right).forpotential failure mechanisms.

New Materials

Angular Extrusion for Polymer Processing

Ultra high molecular weight polyethylene (UHMWPE), a common bearingsurface in total joint arthroplasty, is subject to material property tradeoffsassociated with conventional processing techniques. For orthopaedicapplications, radiation-induced cross-linking is used to enhance the wear resistanceof the material, but cross - linking also results in decreased relative chainmovement in the amorphous regions and hence decreased toughness. Equal Channel AngularExtrusion (ECAE) is employed as a novel mechanism by which entanglements can beintroduced to the polymer bulk during consolidation, imparting the same tribologicalbenefits of conventional processing without complete inhibition of chain motion.ECAE processing at temperatures near the crystalline melt for UHMWPE yields increasedentanglements over control materials, increasing entanglements with increasing temperature,and mechanical properties between never irradiated polyethylene and literaturevalues for cross-linked polyethylene. These results support our additionalresearch in ECAE-processed UHMWPE for joint arthroplasty and industrialapplications.

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Simulating In Vivo Behavior of Biomaterials

Total joint revisions due to infection pose significant burdensto the patients, hospitals, and the healthcare system. Transitioning from a two-stage infection treatment to a single stage procedure is one potentialsolution to these burdens. Off-label use of a resorbable calcium sulfate antibioticcarrier has been implemented in single stage and two-stage procedures in theUnited States. It is unknown if adverse effects of calcium sulfate on the jointspace during articulation exist. Current studies in our lab seek todetermine whether this new use of a biomaterial have the potential to changedamage patterns or wear rates of artificial joints.

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New Devices

Bearing Design

Total hip arthroplasty (THA) is an increasingly utilized andcost-effective treatment for osteoarthritis of the hip. An estimated 460,000hip replacement procedures are performed in the United States annually. Hipreplacement has been shown to have the lowest cost per quality adjusted lifeyear ($8,964 per QALY) compared to conservative treatments ($11,530 to $92,081per QALY). However, not all arthroplasty procedures have positive long termoutcomes. Overall survivorship for THA devices is less than 93% at 7.5 years,with significantly worse survivorship for younger, more active patients(<90%). Overall, approximately 13% of all hip arthroplasty procedures arerevisions, costing the U.S. health system approximately $3 billion in 2011. Revisionsurgeries present more risk and morbidity to the patient and require higherutilization of healthcare resources. Wear and/or failure of the bearingsurfaces is one of the leading causes of revision, either directly because ofpoor bearing articulation or through the detrimental effects of wear debris onperi-prosthetic tissues, device fixation, and the patient's immune system.ratesof artificial joints.

We are testing a new bi-material bearing to be employed in a THA device. It is possible that this innovative approach will reduce bearing surface damage and wear when compared to state of the art approaches in bearing design.

Algorithms to Determine Joint Alignment

Poor component alignment has the potential to increase the incidence of failure of total knee arthroplasty. Thus, surgeons are eager to validate their surgical cuts and corresponding component placement.

Currently, there are several validation methods available to orthopedic surgeons, however each has its limitations. At the most basic level, a surgeon utilizes a surgical cutting jig to select the location and orientation of his or her cuts in two planes (e.g. sagittal and frontal plane on the tibia in TKA). The only way for a surgeon to validate their cuts utilizing this method is with intraoperative radiography, computed tomography (CT) scans, or fluoroscopy, adding additional cost and time to each surgical procedure. Moreover, these traditional methods are inaccurate, allowing several degrees of variability in the frontal plane. To achieve greater levels of accuracy, some surgeons have turned to computer navigated TKA procedures. Despite the purported benefits of navigation, there have been mixed results with respect to the accuracy of component placement when compared to traditional validation methods . Although some surgeons have seen marked improvements when using navigation techniques, the added time to surgery and high cost to entry are may be barriers to widespread use and adoption.

There exists a distinct need for an intraoperative method for quantifying the orientation of the prosthetic components used in TKA that is efficient, easy to use, cost effective, and quick with respect to total surgical time. Recently, several companies have developed inertial measurement units (IMUs) to more effectively elucidate the orientation of surgical cuts. IMUs utilize gyroscopes, accelerometers, magnetometers, or some combination of all three to identify the orientation of the surgical cuts with respect to some known reference (e.g., gravity).

Current work in our laboratory centers on developing analytical and computational approaches to better measure surfaces cut by a surgeon. Our methods are derived from first principles, and are currently implemented in bench-top simulations and cadaveric models.