Lisa Pruitt

Publications of Lisa Pruitt

• A fiber reinforced poroelastic model of nanoindentation of porcine costal cartilage: a combined experimental and finite element approach.

  Authors: Shikha Gupta, Jeremy Lin, Paul Ashby, Lisa Pruitt

  Journal of the mechanical behavior of biomedical materials. 09/2009; 2(4):326-37; discussion 337-8.

  Nanoindentation has shown promise as a mechanical characterization tool for orthopaedic biomaterials since it can probe the properties of small, heterogeneous, irregularly shaped tissue volumes inNanoindentation has shown promise as a mechanical characterization tool for orthopaedic biomaterials since it can probe the properties of small, heterogeneous, irregularly shaped tissue volumes in physiological environments. However, the majority of nanoindentation analyses have been limited to the determination of linear elastic and viscoelastic properties. Since biomaterials possess complex nonlinear, hydrated, time-dependent constitutive behavior, the objective of the present study is to explore the ability of nanoindentation to determine physiologically relevant material properties using a fibril reinforced poroelastic (FRPE) model. A further goal is to ascertain the sensitivity of nanoindentation load-displacement curves to different FRPE parameters, including the elastic properties of the nonfibrillar matrix, the composition and distribution of fibers, and nonlinearity in the fluid permeability. Porcine costal cartilage specimens are experimentally tested with nanoindentation load relaxation experiments at two different loading depths and loading rates. The FRPE material properties are extracted from comparisons to finite element simulations. The study demonstrates the behavior of the model in nanoindentation is distinct from bulk indentation; the static response of the nanoindentation is determined almost exclusively by the elastic properties of the nonfibrillar matrix and the volume fraction of fibers, while the transient response is dominated by the fluid permeability of the tissue. The FRPE model can accurately describe the time-dependent mechanical behavior of costal cartilage in nanoindentation, with good agreement between experimental and numerical curve fits (R(2)=0.98+/-0.01) at multiple indentation depths and indentation rates.

• Clinical fracture of cross-linked UHMWPE acetabular liners.

  Authors: Jevan Furmanski, Martin Anderson, Sonny Bal, A Seth Greenwald, David Halley, Brad Penenberg, Michael Ries, Lisa Pruitt

  Biomaterials. 08/2009;

  Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is increasingly used as a bearing material in total hip replacements. Cross-linking of UHMWPE has been shown to increase wearHighly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is increasingly used as a bearing material in total hip replacements. Cross-linking of UHMWPE has been shown to increase wear resistance but decrease its fracture resistance. We analyzed the clinical fracture failure of four cross-linked UHMWPE total hip replacement components of four different designs via microscopic observation of the fracture surfaces, and found that in all cases fractures initiated at stress concentrations in an unsupported region of the component (termed the elevated rim). Finite element analyses (FEA) of each individual implant design were then conducted. Results from this analysis demonstrated that the predicted magnitude and orientation of maximum principal stress due to mechanical loading of the elevated rim was sufficient to propagate initiated fatigue cracks in each case. FEA also predicted that cracks may arrest after some amount of growth due to a steep stress gradient near the initiation site. Further, while anatomical positioning of the implant and material properties affect the risk of fracture, we examined whether these failures are strongly related to the notched elevated rim design feature that is common to the four failed cases presented here. We believe that cross-linked UHMWPE remains an excellent bearing material for total hip replacements but that designs employing this material should mitigate stress concentrations or other design features that increase the risk of fracture.

• The tissue diagnostic instrument.

  Authors: Paul Hansma, Hongmei Yu, David Schultz, Azucena Rodriguez, Eugene A Yurtsev, Jessica Orr, Simon Tang, Jon Miller, Joseph Wallace, Frank Zok [......] Carol Chen, Mathilde Peters, David Kohn, Jenni Buckley, Xiaojuan Li, Lisa Pruitt, Adolfo Diez-Perez, Tamara Alliston, Valerie Weaver, Jeffrey Lotz

  The Review of scientific instruments. 06/2009; 80(5):054303.

  Tissue mechanical properties reflect extracellular matrix composition and organization, and as such, their changes can be a signature of disease. Examples of such diseases include intervertebral diskTissue mechanical properties reflect extracellular matrix composition and organization, and as such, their changes can be a signature of disease. Examples of such diseases include intervertebral disk degeneration, cancer, atherosclerosis, osteoarthritis, osteoporosis, and tooth decay. Here we introduce the tissue diagnostic instrument (TDI), a device designed to probe the mechanical properties of normal and diseased soft and hard tissues not only in the laboratory but also in patients. The TDI can distinguish between the nucleus and the annulus of spinal disks, between young and degenerated cartilage, and between normal and cancerous mammary glands. It can quantify the elastic modulus and hardness of the wet dentin left in a cavity after excavation. It can perform an indentation test of bone tissue, quantifying the indentation depth increase and other mechanical parameters. With local anesthesia and disposable, sterile, probe assemblies, there has been neither pain nor complications in tests on patients. We anticipate that this unique device will facilitate research on many tissue systems in living organisms, including plants, leading to new insights into disease mechanisms and methods for their early detection.

• The tissue diagnostic instrument

  Authors: Paul Hansma, Hongmei Yu, David Schultz, Eugene A. Yurtsev, Jessica Orr, Simon Tang, Jon Miller, Frank Zok, Richard Souza, Alexander Proctor, Xavier Nogues-Solan, M.Jesus Peña, Lisa Pruitt, Adolfo Diez-Perez, Tamara Alliston, Valerie Weaver, Jeff Lotz

  Review of Scientific Instruments. 05/2009;

  Tissue mechanical properties reflect extracellular matrix composition and organization, and as such, their changes can be a signature of disease. Examples of such diseases include intervertebralTissue mechanical properties reflect extracellular matrix composition and organization, and as such, their changes can be a signature of disease. Examples of such diseases include intervertebral disk degeneration, cancer, atherosclerosis, osteoarthritis, osteoporosis, and tooth decay. Here we introduce the tissue diagnostic instrument �TDI�, a device designed to probe the mechanical properties of normal and diseased soft and hard tissues not only in the laboratory but also in patients. The TDI can distinguish between the nucleus and the annulus of spinal disks, between young and degenerated cartilage, and between normal and cancerous mammary glands. It can quantify the elastic modulus and hardness of the wet dentin left in a cavity after excavation. It can perform an indentation test of bone tissue, quantifying the indentation depth increase and other mechanical parameters. With local anesthesia and disposable, sterile, probe assemblies, there has been neither pain nor complications in tests on patients. We anticipate that this unique device will facilitate research on many tissue systems in living organisms, including plants, leading to new insights into disease mechanisms and methods for their early detection. © 2009 American Institute of Physics. �DOI: 10.1063/1.3127602� I. INTRODUCTION The tissue diagnostic instrument �TDI� was redesigned from the bone diagnostic instrument1,2 so as to measure tissue mechanical properties subcutaneously and in vivo with additional probe assemblies and an adjustable compliance �Fig. 1�. It consists of a thin probe assembly that can penetrate skin and soft tissue to reach deep tissues. The disposable, sterilizable probe assembly consists of an outer reference probe made from a 23 gauge hypodermic needle and an inner test probe made from stainless steel wire ranging from 175 to 300 �m in diameter and from 2 to 90 mm in length. Since friction between the test probe and the reference probe increases with length, it is desirable to use only the length needed to access the desired tissue location. The test probe is held in a nickel tube that couples to a magnet, which in turn is linked to a force generator. During operation the force generator oscillates the probe within the tissue of interest and concurrently measures the force and displacement. The maximum values for force and displacement are 12 N and 600 �m. The probe is typically operated at a frequency of 4 Hz because this is rapid enough to allow hand holding yet sufficiently slow to allow easy decoupling of the elastic and viscous response of the tissue �see supplementary material3 for more details including force and displacement ranges.� II. MEASUREMENTS We first illustrate TDI use in human spinal disks that are composed of a thick outer ligament �annulus fibrosus� and a central swelling hydrogel �nucleus pulposus�. Spinal disk degeneration can be the underlying cause of back pain leading to significant morbidity and societal expense. Intervertebral disks are one of the most highly loaded tissues in the body, and consequently material property insufficiency can lead to damage accumulation, inflammation, and pain. Disk degen- REVIEW OF SCIENTIFIC INSTRUMENTS 80, 054303 �2009� 0034-6748/2009/80�5�/054303/6/$25.00 80, 054303-1 © 2009 American Institute of Physics Downloaded 28 May 2009 to 72.215.163.35. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp

• How do material properties influence wear and fracture mechanisms?

  Authors: Clare Rimnac, Lisa Pruitt

  The Journal of the American Academy of Orthopaedic Surgeons. 01/2008; 16 Suppl 1:S94-100.

  The wear and fracture mechanisms of ultra-high-molecular-weight polyethylene (UHMWPE) hip and knee implant components are of great interest. The material properties of UHMWPE are affected by ionizingThe wear and fracture mechanisms of ultra-high-molecular-weight polyethylene (UHMWPE) hip and knee implant components are of great interest. The material properties of UHMWPE are affected by ionizing radiation as used for sterilization and cross-linking. Cross-linking with high-dose irradiation has been shown to improve the wear resistance of UHMWPE. However, cross-linking leads to a loss in properties such as ductility and resistance to fatigue crack propagation. Highly cross-linked UHMWPE may be more susceptible than conventional UHMWPE to fracture under severe clinical conditions (eg, impingement). Contemporary hip and knee simulator studies provide good information with which new UHMWPE formulations can be screened for clinical wear performance. However, comparable methodologies are lacking for screening UHMWPEs for fracture resistance. Mechanical tests as well as computational material and structural models should be developed to evaluate the combined effect of material and geometry (structure) on fracture resistance under clinically relevant loading conditions.

• Osteolysis caused by tibial component debonding in total knee arthroplasty.

  Authors: Kevin Cheng, Lisa Pruitt, Charles Zaloudek, Michael D Ries

  Clinical orthopaedics and related research. 03/2006; 443:333-6.

  Late failure of total knee arthroplasties usually results from ultrahigh molecular weight polyethylene wear or implant loosening. Early failure from osteolysis is uncommon. However, we treated aLate failure of total knee arthroplasties usually results from ultrahigh molecular weight polyethylene wear or implant loosening. Early failure from osteolysis is uncommon. However, we treated a patient with a failed total knee arthroplasty from osteolysis that developed 2 years postoperatively. The failure was associated with tibial component debonding from the cement mantle with abundant cement and metal debris. Although there was some third-body debris in the ultrahigh molecular weight polyethylene insert surface, the insert wear was not extensive. Although abundant cement and metal debris were found in the periarticular soft tissues, no ultrahigh molecular weight polyethylene was seen in histologic specimens under polarized light. The osteolysis seems to have been caused primarily by debris generated from debonding and torsional motion at the tibial baseplate-cement interface rather than the bearing surface. Although this failure mechanism has been well recognized in cemented total hip arthroplasties, it has not been reported to be a substantial cause of failure in total knee arthroplasties.

• In vivo behavior of acrylic bone cement in total hip arthroplasty.

  Authors: Michael D Ries, Ernest Young, Laila Al-Marashi, Philip Goldstein, Alexander Hetherington, Timothy Petrie, Lisa Pruitt

  Biomaterials. 02/2006; 27(2):256-61.

  Polymethylmethacrylate (PMMA) bone cement serves as the primary fixation material between bone and the prosthetic component in cemented total hip arthroplasty. In vivo degradation of bone cement mayPolymethylmethacrylate (PMMA) bone cement serves as the primary fixation material between bone and the prosthetic component in cemented total hip arthroplasty. In vivo degradation of bone cement may lead to a decrease in mechanical properties of PMMA and result in aseptic loosening. However, other factors such as porosity and location of the cement relative to the bone implant interface may also contribute to mechanical behavior in vivo. This study investigated the mechanical properties of Simplex cement retrieved from 43 patients undergoing revision total hip arthroplasty. The time in vivo was between 1 month and 27 years. The variables studied included fracture toughness (KIC), porosity, molecular weight, time in vivo of the cement, and relative in vivo location of the cement with respect to the implant and bone. KIC did not correlate with time in vivo of the samples or with molecular weight. This suggests that time in vivo may not be the limiting factor in the mechanical integrity of the bone cement, A significant and inverse relationship was found between porosity and KIC. This implies that porosity is the most important factor in the mechanical behavior of bone cement during in vivo use.

• Effect of cross-linking on the microstructure and mechanical properties of ultra-high molecular weight polyethylene.

  Authors: Michael D Ries, Lisa Pruitt

  Clinical orthopaedics and related research. 12/2005; 440:149-56.

  Ultra-high molecular weight polyethylene is a semicrystalline polymer, which means that a portion of the molecules is in a solid crystalline phase and the remaining portion is in a rubbery amorphousUltra-high molecular weight polyethylene is a semicrystalline polymer, which means that a portion of the molecules is in a solid crystalline phase and the remaining portion is in a rubbery amorphous phase. Varying the polymer chemistry in the two phases can alter the mechanical properties of the material. When highly cross-linked polyethylene is formed, the cross-links occur in the amorphous but not the crystalline region. Remelting after irradiation-induced cross-linking neutralizes the free radicals that are caused by irradiation but also decreases the amount of crystallinity. Decreased crystallinity can contribute to a decrease in mechanical properties. Annealing below the melt temperature after irradiation retains a higher level of crystallinity. However, heating below the melt temperature does not neutralize irradiation-induced free radicals that can then react with oxygen, causing oxidative degradation. Newer "second-generation" highly cross-linked polyethylenes have been developed that are annealed below the melt temperature, but use either a pharmacologic antioxidant, mechanical deformation, or sequential low-dose irradiation and annealing treatments rather than heating above the melt point to neutralize residual free radicals. High-pressure treatment at elevated temperatures also can increase crystallinity. However, increased crystallinity is associated with an increase in modulus and contact stress, which can increase wear. Although cross-linking ultra-high molecular weight polyethylene can reduce wear, currently available highly cross-linked polyethylenes also decrease mechanical properties when compared with conventional ultra-high molecular weight polyethylene, so that use of these materials in total knee arthroplasty may contribute to mechanical failure of the bearing surface.

• Effect of bone porosity on the mechanical integrity of the bone-cement interface.

  Authors: Jove Graham, Michael Ries, Lisa Pruitt

  The Journal of bone and joint surgery. American volume. 10/2003; 85-A(10):1901-8.

  BACKGROUND: Osteopenia is one factor that may influence the decision about the type of implant fixation to use in total hip arthroplasty. However, clinical studies generally do not associate theBACKGROUND: Osteopenia is one factor that may influence the decision about the type of implant fixation to use in total hip arthroplasty. However, clinical studies generally do not associate the outcome of an arthroplasty with the degree of osteopenia. The mechanical integrity of the cement fixation of an implant may be affected by the relative degree of osteopenia, which could account for some of the variable long-term results after total hip arthroplasty performed with cement. The purpose of this study was to determine the effects of bone porosity, trabecular orientation, cement pressure, and cement penetration depth on fracture toughness at the bone-cement interface. METHODS: Trabecular bone from the proximal part of bovine femora was used with a single brand of commercial acrylic bone cement to form compact-tension interface specimens representing a range of bone porosities, orientations, and cement pressures within a clinically achievable range. All specimens were loaded to failure with use of a servohydraulic testing machine, and fracture toughness at the interface was calculated. After testing, images of a representative sample of specimens were made with use of computed tomography to measure the penetration depth of the cement into the bone. RESULTS: Significant correlations were found between fracture toughness and bone porosity, trabecular orientation, and cement pressure, with bone porosity having the strongest effect (p < 0.000015). Examination of the computed tomographic images also showed a significant correlation between fracture toughness and maximum cement penetration depth (p < 0.033), as well as significant partial correlations between maximum and mean penetration depth and bone porosity (p < 0.0037 and p < 0.0028). CONCLUSION: The fracture resistance of the bone-cement interface is greatly improved when the ability of the cement to flow into the intertrabecular spaces is enhanced.

• Retrieved glenoid components: a classification system for surface damage analysis.

  Authors: Stephen B Gunther, Jove Graham, Tom R Norris, Michael D Ries, Lisa Pruitt

  The Journal of arthroplasty. 02/2002; 17(1):95-100.

  There have been many reports describing modes of damage in retrieved total hip and total knee arthroplasty components. The most common mechanism in total hip arthroplasties has been shown to beThere have been many reports describing modes of damage in retrieved total hip and total knee arthroplasty components. The most common mechanism in total hip arthroplasties has been shown to be surface wear. Fatigue failure shown as pitting and delamination are observed more often in total knee components. There has been no previous analysis of retrieved polyethylene glenoid components. This study evaluated the wear mechanisms contributing to failure of total shoulder glenoid components. Polyethylene glenoid components from 10 consecutive total shoulder arthroplasties have been retrieved and analyzed. Wear mechanisms were analyzed under low-power magnification, and a classification system was designed for total shoulder arthroplasties. This classification system is an adaptation of previous models of hip and knee surface damage. The severity of each damage mode was graded in 4 separate quadrants. The most prevalent damage modes were abrasion, pitting, and delamination. These data show a combination of abrasive wear and fatigue in retrieved total shoulder specimens. Surface wear and subsurface fatigue failure mechanisms both contribute to glenoid implant failure.

• In vivo behavior of acrylic bone cement in total hip arthroplasty

  Authors: Michael D. Ries, Ernest Young, Laila Al-Marashi, Philip Goldstein, Alexander Hetherington, Timothy Petrie, Lisa Pruitt

  Biomaterials.

  Polymethylmethacrylate (PMMA) bone cement serves as the primary fixation material between bone and the prosthetic component in cemented total hip arthroplasty. In vivo degradation of bone cement mayPolymethylmethacrylate (PMMA) bone cement serves as the primary fixation material between bone and the prosthetic component in cemented total hip arthroplasty. In vivo degradation of bone cement may lead to a decrease in mechanical properties of PMMA and result in aseptic loosening. However, other factors such as porosity and location of the cement relative to the bone implant interface may also contribute to mechanical behavior in vivo. This study investigated the mechanical properties of Simplex® cement retrieved from 43 patients undergoing revision total hip arthroplasty. The time in vivo was between 1 month and 27 years. The variables studied included fracture toughness (KIC), porosity, molecular weight, time in vivo of the cement, and relative in vivo location of the cement with respect to the implant and bone. KIC did not correlate with time in vivo of the samples or with molecular weight. This suggests that time in vivo may not be the limiting factor in the mechanical integrity of the bone cement, A significant and inverse relationship was found between porosity and KIC. This implies that porosity is the most important factor in the mechanical behavior of bone cement during in vivo use.