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Research

                                 Failure Analysis

                                                                                   

Since  the  inception  of  the  DBEC retrieval library in 1976, our  understanding of the complexities of joint  replacement  technology  has improved  substantially. Retrieval laboratories around the world, including Dartmouth’s, have greatly facilitated design optimization  and materials for orthopaedic implants, which in turn produces profound quality-of-life improvements for patients worldwide. 

The  laboratory  has  evolved  scientifically and financially to weather dramatic changes in  the  orthopaedic  industry,  making  significant impacts  on  the  care  of  patients  worldwide.  In the last four decades, Thayer School's implant retrieval  analyses  played  a  key  role  in identifying  failure  modes  and  relating them to  various  designs  and  materials  used  in  industry.  Indeed, in  2000,  NIH's  Consensus  Development  Program  produced  a  technology  assessment  statement acknowledging the  value  of  implant  retrieval  programs.  The statement drew the following conclusions:

  1. Implant retrieval and analysis is of critical importance in the process of improving care of patients in need of implants.
  2. Attention needs directed toward reducing obstacles to implant retrieval/analysis, particularly legal and economic disincentives.
  3. Failure to appreciate the value of implant retrieval/analysis is a serious impediment to research in devices.
  4. A focused educational program will provide the information necessary for improving the quality of future devices.

While most people understand medical implants improve quality of life, few recognize the importance of retrieving/analyzing failed implants when they fail or are no longer in use.

           The History of the Value of Retrieval Analyses

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

                                                                                

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

                                                                                    

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

                                                                           

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

                                                                                  

F) 2000’s: Biomechanics contributes to wear and performance of devices

  • Industry/Clinical Change: Rotating platform knees resurge
  • Industry/Clinical Change: Highly conforming knees are redesigned to reduce backside wear and accommodate femoral torques
  • Industry/Clinical Change: Surgeons cautioned against using femoral stems in poor bone stock

                                                     

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

                                                                  

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

                           

J) 2010’s:  In-vivo oxidation documented in highly crosslinked devices

  • Industry/Clinical Change: Companies are moving to antioxidant technologies.

                                                  

K) 2010’s:  Retrieval studies document squeaking phenomenon in ceramic hips

  • Industry/Clinical Change: Acoustic considerations are weighted heavily in bearing selection

               

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.

                                        

                                Bearing Function

                                                                                                   

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

An  important  early  outcome  from  this  effort  is  the  distinction  between  damage and wear of knee bearing inserts.  Damage of joint arthroplasty  bearings  can  be visually striking,  can be described semi-quantitatively according to published techniques,  and can impact mechanical  performance  and  kinematics  of  the  implant.  Wear  that occurs by abrasive/adhesive processes can be very challenging to discern   and   quantify,   yet   can   produce   large   volumes  of  small  debris  particles  that  can  lead  to  osteolysis  (see  figure  below). Damage and wear are important but distinct phenomena that can have different impacts on clinical performance.

                                                                                       

Highly  cross-linked  (HXL)  polyethylene  has  proven  to  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 HXL polyethylene,  as compared to conventional,  non-HXL bearings.   To   date,   radiographic   studies   of   head   penetration   represent   the   state-of-the-art   in   determining   clinical   wear  of polyethylene hip liners. However, as the amount of wear drops to very low levels, it becomes important to develop a precise and reliable method  for  measuring wear,  facilitating a comparison of clinical results to laboratory expectations.   Fixed-magnitude errors associated with  digital  imaging  necessitate  increasingly large studies to statistically elucidate the low wear rates.  Retrieval analysis provides much better precision, but is subject to different sources of error.

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

                                                      

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

                              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 most successful procedures of the last century, issues of wear, oxidation,   and   fatigue   failure   remain   obstacles  to  the  longevity  of  joint  replacements.   As  a  failure  mode,  wear  is  biologically compounded,  because wear debris can trigger a series of reactions leading to osteolysis, a condition resulting in long term resorption of the bone around the implant.

        

Crosslinking and Heat Treatment of UHMWPE

Radiation  crosslinking  of  UHMWPE  significantly  improves  its  wear  resistance,  as  evidenced by in vivo clinical studies and in vitro hip simulator  studies.  During irradiation,  crosslinks are formed between polymer chains through homolytic cleavage of C-H and C-C bonds. However,  ionizing  radiation  also produces free radicals randomly throughout UHMWPE as part of the crosslinking process.  These long-lived  species  can  react  with  oxygen,  triggering  a  cyclic  complex cascade of chemical reactions. While free radical oxidation involves a number of possible pathways with different mechanisms and end products,  the overall outcome includes polymer chain scissions which reduce  the molecular weight,  and various oxidative products such as hydroperoxides,  ketones,  alcohols,  and carboxylic acids.  Overall, this cascading oxidative reaction is responsible for progressive embrittlement of the material.  Oxidative degradation thus manifests as a reduction of wear resistance and mechanical properties.

                                                                   

                                                                             

In  the  late  1990’s,  oxidation  of  UHMWPE  was  identified  as  a  serious  concern  as  it  limits  the overall lifetime and success of a joint replacement.  Hence, the elimination of  free radicals in UHMWPE has been an important focus of orthopaedic manufacturers ever since the  industry’s  response  to  shelf  storage  oxidation  occurring  in  gamma  sterilized  devices.  In  collaboration  with materials research laboratories,  device  manufacturers  have  developed  a  variety  of post-irradiation thermal treatments with the dual goals of promoting oxidative stability and enhancing the crosslink density of bearing materials.

One thermal treatment approach utilizes heating above the melting point of the crosslinked polymer following irradiation. This melts the crystalline regions and allows recombination of trapped free radicals in these domains. After the polymer recrystallizes, the residual free radicals  have  been  quenched  and  the  material  is  both  wear  resistant  and  more oxidatively stable in shelf-aging and artificial aging according  to  current ASTM Standards.  However,  this improved wear and oxidation resistance comes at a cost because post-irradiation melting  further  decreases  the  fatigue  strength  of  UHMWPE,  already  reduced  by  radiation crosslinking; the melting step results in a decrease  in  crystallinity  and  ductility.  Thus,  upon cooling,  the extent of recrystallization for crosslinked UHMWPE is inferior to that of UHMWPE without crosslinks.

An  alternative  method  of  thermal  treatment  is  post-irradiation  annealing below the melting point of the crosslinked polymer.  In the absence of a recrystallization step, annealed materials possess superior mechanical properties in comparison to fully remelted materials with  the  same radiation dose.  However,  this approach reduces but does not completely eliminate free radicals as achieved by melting. As  a  result,  this  material is still susceptible to in vivo oxidation.  Our lab and others have reported oxidation in irradiated and annealed UHMWPE both in retrieval analysis and in vitro accelerated aging studies (see below).

                       

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

1. Free radical mediated mechanism (conventionally accepted)

 

2. Stress induced chain scission mechanism     

 

3. Absorbed pro-oxidative species mechanism

 

4. Irradiation-energy, chemical bond-related mechanism     

Antioxidant Polyethylene

In  an  effort  to improve oxidation resistance without compromising mechanical properties through thermal treatments,  manufacturers have  examined  the  use of antioxidants to prevent oxidation of free radicals.  The most prevalent antioxidant available today is a liquid-based   alpha-tocopherol   (Vitamin E).   Two   methods   of  adding  vitamin  E to  the  UHMWPE  have  been  examined:  combining  liquid antioxidant  into  UHMWPE  resin  powder  prior  to compression molding, and diffusion of vitamin E into already cross-linked UHMWPE.  Adding  vitamin  E into the resin prior to crosslinking reduces the crosslink efficiency since the antioxidant scavenges free-radicals during irradiation,  thereby  reducing  the  effective  amount  of  crosslinking.  Diffusion of vitamin E into the polymer following crosslinking does not  inhibit  crosslinking.  More  recently,  solid-state pentaerythritol tetrakis has been marketed in bearing materials for TKA.  Due to the novelty of these antioxidant materials, long duration clinical and retrieval studies haven’t yet been published, particularly with respect to in vivo oxidation prevention.

                  

The  documentation  of  unexpected  in  vivo  oxidation  of  thermally  stabilized UHMWPE may be of concern to clinicians,  industry,  and patients,  due  to  the  potential for polymer oxidation to lead to increased incidence of fatigue- and wear-related failures.  Moreover, the shift in recipient demographic to a younger, heavier, more active patient places greater mechanical demands on bearing surfaces.  Thus, attaining  a  better understanding of the in vivo oxidation process and rate is crucial to ensure device longevity.  With this understanding, it  is  important  to  develop  a  predictive  capability  to accelerate  aging  under controlled laboratory conditions to test new and existing materials before they are employed in humans.

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

                                 Biomechanics

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 that contacts 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 of each marker.  Marker locations  are then converted to semi-rigid bony segments and the relative orientation of distal-proximal segment pairs are quantified as joint angles (e.g. knee flexion), also known as kinematics.  In DBEC,  we primarily utilize our optical MOCAP system as a validation tool for other methods. However, we are currently conducting numerous studies leveraging the high-fidelity kinematic information gleaned from optical MOCAP to establish performance during a variety of tasks (e.g. ambulation,  stairs,  jumping, etc.) in a number of populations (e.g. total joint replacement, expectant mothers, etc.).

                                         

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 not well established if this is the highest value approach to healthcare.  It is conceivable that different patients would need variable levels of postoperative PT to achieve optimal recovery.  In addition,  postoperative progress is often only gauged via single data points as measured in clinic / laboratory settings (e.g. passive range of motion via goniometer) whereas motion throughout the day is only assessed anecdotally. Our laboratory has developed and implemented a novel method for monitoring continuous long term joint function  using  wearable  devices.  In collaboration with our orthopaedic surgery colleagues at Dartmouth Hitchcock Medical Center, we have  subsequently  been issued a patent for parts of this system.  Perhaps most critically,  we have leveraged these wearable devices to capture  long  duration,  continuous  range of motion in patients before and after total joint  replacement,  lower extremity trauma,  and expectant mothers to name a few.  These data are be compared to a cohort of healthy individuals with no movement related pathology.

                         

Shoulder Modeling

The  number  of  total  and reverse shoulder arthroplasty procedures performed annually in the United States has been growing steadily over the last decade.  One aim of our current work is to understand the mechanical and tribological interactions of shoulder arthroplasty with  the  patient.  The  development  of  algorithms  for analysis of explanted components,  patient outcomes,  in vitro wear testing,  and finite  element  analysis  will  provide  a better understanding of joint behavior and potential impacts to the patient.  This knowledge may allow for device designs and implantation methods to be adjusted to account for potential failure mechanisms.

     

Computational  research  in  the  laboratory  occurs  at  various  scales and couples the interaction between musculature and bone.  This helps  identify  how  device  implantation  can  affect  musculature  using dynamic modeling (top left),  and in return,  result in changes to stresses and strains experienced by the  bone (center, bottom).  General or patient specific geometry and devices can be accommodated by utilizing CT scans and solid modeling (center, top right). 

                                   New Materials

Angular Extrusion for Polymer Processing

Ultra  high  molecular  weight  polyethylene  (UHMWPE),  a  common  bearing  surface  in  total  joint  arthroplasty,  is  subject  to material property  tradeoffs associated with conventional processing techniques.  For orthopaedic applications,  radiation-induced cross-linking is used  to  enhance  the  wear  resistance  of  the  material,  but  cross - linking  also  results  in  decreased  relative  chain movement in the amorphous  regions  and  hence  decreased  toughness.  Equal  Channel  Angular Extrusion (ECAE) is employed as a novel mechanism by which  entanglements  can  be  introduced  to  the  polymer  bulk  during  consolidation,  imparting   the   same   tribological   benefits  of conventional  processing  without  complete  inhibition  of  chain  motion.  ECAE  processing at temperatures near the crystalline melt for UHMWPE   yields   increased   entanglements   over   control   materials,   increasing   entanglements  with  increasing  temperature,  and mechanical properties between never irradiated polyethylene and literature values for cross-linked polyethylene.  These results support our additional research in ECAE-processed UHMWPE for joint arthroplasty and industrial applications.

                                                                                       

Simulating In Vivo Behavior of Biomaterials

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

New Devices

Bearing Design

Total hip arthroplasty (THA) is an increasingly utilized and cost-effective treatment for osteoarthritis of the hip. An estimated 460,000 hip replacement procedures are performed in the United States annually.  Hip replacement has been shown to have the lowest cost per quality adjusted life year ($8,964 per QALY) compared to conservative treatments ($11,530 to $92,081 per QALY).  However, not all arthroplasty procedures have positive long term outcomes. 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 are revisions, costing the U.S. health system approximately $3 billion in 2011.  Revision surgeries present more risk and morbidity to the patient and require higher utilization of healthcare resources.  Wear and/or failure of the bearing surfaces is one of the leading causes of revision, either directly because of poor bearing articulation or through the detrimental effects of wear debris on peri-prosthetic tissues, device fixation, and  the patient's immune system.

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.