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[163] IMPROVED BONE CEMENT FATIGUE RESISTANCE VIA CONTROLLED STRENGTH INTERFACES
William G. Winter, MD; Donna Jo Blake, MD; Francis W.
Cooke, PhD; Elizabeth A. Friis, PhD; Charles D. Graber, BS;
Brijesh Kumar, MS; HK Yasuda, PhD
Department of Orthopaedic Surgery, University of
Colorado School of Medicine, Denver, CO 80202; Department of
Rehabilitation, University of Colorado Health Science
Center, Denver, CO 80202; VA Medical Center, Department of
Surgery, Orthopaedic Section and the Physical Medicine and
Rehabilitation Service, Denver CO 80220; Orthopaedic
Research Institute, Wichita KS 67214; Department of
Aerospace Engineering, Wichita State University, Wichita, KS
67208; Department of Chemical Engineering, University of
Missouri-Columbia, Columbia, MO 65211; email:
Francis_Cooke@via-christi.org;
yasuda@ecvax2.ecn.missouri.edu;
Lisa_Friis@via-christi.org
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #A796-RA)
PURPOSE--The problem addressed in this study is to find a means by which commercial bone cement can be modified so as to increase its fatigue resistance, especially its resistance to low stress, high-cycle fatigue, without impairing its intraoperative mixing and handling characteristics.
METHODOLOGY--The basic methodology of this study consists of adding thin fibers to commercial polymethyl methacrylate bone cement, with the objective of improving its resistance to fatigue crack growth and thereby increasing its fatigue life. The fibers impede crack growth by at least two mechanisms. First, the interface between the fiber and the cement matrix fails or debonds as the propagating crack approaches. This serves to blunt the crack locally and also absorbs some of the energy of propagation that is the driving force for crack growth. Secondly, stretching and pull out of the fibers from the matrix absorbs additional energy thus further impeding crack propagation. If sufficient fibers and interfaces are present, the blunting and energy absorbing interactions can slow the overall rate of crack growth and increase the observed fatigue life of the cement. The effectiveness of these mechanisms is evaluated by measuring the strength of the composite in the presence of cracks (fracture toughness testing) and by measuring the actual fatigue life (number of cycles to failure) of specimens prepared in the laboratory.
A secondary, but very important, concern is that the fiber additions can increase the viscosity of bone cement during mixing, which interferes with injection and intrusion of the cement into the bone during surgery. To circumvent this, the size, composition, and loading of the fibers must be manipulated.
PROGRESS--By trial and error, it has been established that both stiff carbon fibers and very flexible polyethylene terephthalate (PET) fibers can be added in small amounts without compromising the mixing and handling behavior of the cement. Milled carbon fibers, 8 µm in diameter with an average length of about 100 µm, have been successfully mixed with cement at loadings of one and two volume percent. Chopped PET fibers 12 µm in diameter with lengths of 0.5 and 1.0 mm have been added at a loading of one volume percent. These materials along with neat cement (controls) were subjected to extensive fatigue testing.
Tension-tension fatigue tests (R=0.1) were conducted on six MTS MiniBionix stations at 2 Hz in a isotonic saline at 37 °C. Intermediate fatigue testing was performed at a stress level of 15 Mpa. In addition, the two-point method was used to determine the endurance limit at 106 cycles for neat cement and for composites containing one volume percent each of carbon and PET fibers.
RESULTS--The results of these tests showed that all fiber additions significantly increased fatigue life at 15 MPa. Reducing the length of the PET fibers from 1.0 mm to 0.5 mm improved handling behavior slightly but slightly decreased fatigue resistance. At the same fiber loading (1 volume percent) the milled carbon fibers produced an 85 percent improvement in fatigue resistance compared to a 35 percent improvement with the PET fibers. Finally, a doubling of the carbon fiber loading to 2 volume percent was found to increase fatigue resistance by over 200 percent, compared to neat cement with only a slight increase in apparent viscosity.
The endurance limit for neat bone cement was established as 11.3 MPa, which was increased marginally to 11.8 MPa by the addition of PET fibers. The addition of 1 volume percent C fibers raised the endurance limit significantly to 12.6 MPa.
IMPLICATIONS--As anticipated, both tough, flexible fibers and stiff, brittle fibers can serve to increase the fatigue resistance of bone cement. It appears that milled carbon fibers are more effective than longer, more flexible PET fibers.
[164] DISUSE-INDUCED ARTICULAR CARTILAGE ATROPHY, OVER EXERCISE, AND ARTHRITIS
David S. Howell, MD; Lori Setton, PhD; Farsh Guilak,
PhD; Herman S. Cheung, PhD
University of Miami School of Medicine, Division of
Rheumatology and Immunology Miami, FL 33101; GRECC, VA
Medical Center, Miami, FL 33101; Departments of Biomedical
Engineering and Orthopaedic Surgery, Duke University,
Durham, NC 27708; email:
gjackson@mednet.med.miami.edu;
setton@duke.edu;
giulak@acpub.duke.edu
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #A739-2RA)
PURPOSE--The pathological reduction of cartilage volume during disuse is an important finding after fractures and other injuries where immobilization of the limb is required. To measure biomechanical parameters in surface, and deep layers of articular cartilage in disuse atrophy in a canine knee model caused by rigid immobilization for 6 weeks, as well as disuse followed by 3 weeks of conservative versus aggressive remobilization; and 2) to determine in the same animal preparation and articular cartilage layers metalloproteinase mRNAs and protein levels with special reference to correlative changes.
METHODOLOGY--We are investigating a model of articular cartilage breakdown caused by overexercise following disuse immobilization, as well as studying baseline immobilization. Biomechanical changes in disuse evaluated at Duke and Columbia universities were: 1) reduction of viscoelastic properties of pericellular matrix of chondrocyte isolated from articular surface zone using unique technologies; 2) increases in microscopic compressive strain in deep radial zone cartilage, and 3) surprisingly, normal tensile strains in middle and surface zones, also measured by novel methods.
RESULTS--The aggrecan gene expression and 35SO4 uptake were decreased significantly in all 4 regions of menisci tested, with similar decreases of these parameters in condylar hyaline cartilage. Similarly, metalloproteinases [MMP] were increased in cartilage, and aggrecan, but not link protein; ligands were increased in synovial fluid. It is likely that elevation of metalloproteinases in disuse cartilage was due to lymphatic stasis of disuse rather than increased local enzyme synthesis. No histologic changes were observed other than a reduced safranin 0 staining in the tangential zone, and reduction of cartilage diameter.
Following vigorous exercise of animals on a treadmill for 2 and 3 weeks, there was compared to baseline disuse; 1) cartilage levels (i.e., significant increase by immunobiochemical assay of MMP-1, 2, 3, and 9 activities); 2) Safranin 0 staining in the tangential zone; 3) ulceration and cleft formation with a Mankin scale of 10; 4) Heavy intense immunochemical staining for MMP-l and MMP-3 in the surface zone in the tangential zone, and in more deeply around ulceration was seen; 5) Some MMP-1 staining and increased capillary numbers in the adjacent calcified cartilage per unit area were observed.
In contrast, exercise only within the space of large cages, namely, conservative remobilization, resulted in no sign of histological abnormalities in the articular cartilage, and chemical changes were virtually restored to normal. However, these observations fit the hypothesis that cast immobilization of up to 6 weeks initiated significant change in the biochemical and biomechanical properties of cartilage and menisci.
IMPLICATIONS--We postulate that immobilization followed by aggressive rehabilitation accelerates the process of cartilage degeneration by up-regulating the synthesis of degrading metalloproteinases. Too rapid vigorous exercises after cast removal in humans may cause damage to articular cartilage. The present model will provide an unique opportunity to examining molecular mechanisms of biomechanical force transduction leading to articular degeneration is provided by this model system.
[165] BIOLOGICAL RESPONSES IN THE PATHOGENESIS OF ASEPTIC LOOSENING
Paul H. Wooley, PhD; Z. Song, MS; P. Vandevord, MS; Sam
Nasser, MD
VA Medical Center, Detroit, MI 48201; Department of
Orthopaedic Surgery, Wayne State University School of
Medicine, Detroit, MI 48201; email: ad8754@wayne.edu
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #A797-2DA)
PURPOSE--The objectives of this study are to evaluate whether the biological responses to biomaterials in vitro predict the inflammatory and immune activity of the pseudosynovial tissue surrounding failed joint prostheses, and to determine whether cytokine modulation represents a viable therapy to prolong the life of an orthopedic prosthesis. We continue to monitor the development of proliferative and cytokine responses to implant materials, and antibody responses to implant bound proteins in our established orthopedic patient population. We have established an animal model of the inflammatory response to orthopedic wear debris, and we are evaluating the cellular and cytokine responses to the various particulate components, in order to identify the key elements of the wear process that provoke an adverse biological response.
METHODOLOGY--We have established assays to measure in vitro responses to orthopedic biomaterials that are associated with the development of aseptic loosening, and we are monitoring a large patient database for changes in the inflammatory responses to biomaterials or immune responses to implant bound proteins. Changes in the status of the joint prosthesis are evaluated during the annual orthopedic follow-up examination, to determine if the patient shows clinical improvement, clinical worsening, or no change in prosthesis status. Concommitantly, the level of cell proliferation and cytokine gene activation in response to stimulation with PMMA, UHMWPE, Co-Cr, or Ti-4-6 particles, and the presence and titer of antibodies to prosthesis bound proteins, are determined. The data are analyzed to determine whether the development or increase in cellular or serological responses correspond with or predict worsening of the status of the prosthesis. Conversely, a reduction in the biological responses corresponding with joint stability and/or clinical improvement is examined.
We are also investigating the inflammatory responses to extracted wear debris in an animal model of the pseudosynovium. The murine air pouch model of inflammation has been adapted to determine the biological response to wear debris, and is being utilized to examine the influence of cytokine gene therapy on the cellular response to wear debris resulting from the deterioration of an artificial joint.
PROGRESS--During the current year, 123 patients were assessed for cellular responses to biomaterials, raising the current database to 647 assessments, 35 of which are follow-up evaluations of either primary or revision orthoplasties. During revision procedures, we have obtained 24 peri-prosthetic tissue samples from failed joint arthoplasties, and 18 have been evaluated for wear debris; such debris from 6 has completed evaluation in the murine air pouch model. We have assessed 21 failed prostheses for bound proteins and the presence of serum antibodies.
RESULTS--The data support the hypothesis that a reduction in the cellular response to biomaterials is associated with clinical improvement following revision arthoplasty. In revision patients where clinical improvement was noted, the response index to PMMA decreased (mean -0.369) and the response to Co-Cr essentially remained unchanged (mean -0.087). In contrast, primary arthroplasty patients showed an increase in both the response index to PMMA (mean 0.746) and the response index to Co-Cr (mean 0.386). The difference between these groups was statistically significant (p<0.01).
Orthopedic wear debris was extracted from the peri-prosthetic tissue associated with six failed total joint arthroplasties. The wear debris was separated into UHMWPE-rich fractions and metallic debris-rich fractions, and analyzed for size distribution. Particles were introduced into the murine air pouch model. There was reasonable agreement between the patient and animal model cellular and cytokine responses. Metal particles from the most cobalt-chrome-responsive patient (in vitro) were responsible for the highest degree of cellular and cytokine responses in the air pouch model. The murine membrane modelled early inflammatory responses to wear debris, with a high inflammatory cytokine (IL-1 and TNFa) activity. In general, metal particles provoked a stronger cellular and cytokine response than an equivalent number of UHMWPE particles. However, UHMWPE particles were predominant in actual numbers within the peri-prosthetic tissue, and the elevated response to metals must be considered in terms of the tissue dose response to the wear debris.
We have now examined 42 failed prosthetic implants for the presence of antibodies bound to the implant surface or reactive with other proteins bound to the implant. A minimum of one protein band and a maximum of 13 protein bands, with molecular weights ranging from 129 kilodalton (kd) to 13 kd were detected. Immunoblotting revealed the presence of Type I collagen in the majority of the explants (34/42), while aggrecan proteoglycans were detected in 8/42 samples. Immunoglobulin was also detected in most (33/42) extracts. Antibodies reactive against the ultra high molecular weight polyethylene bound proteins were detected in 26/42 implant recipients. Specificity assays revealed that these sera autoantibodies were reactive against Type I collagen present in the explant solutions. Our data thus support the concept that an immunological response toward connective tissue antigens bound to the implant surface may contribute to the pathology of aseptic loosening.
[166] DUPUYTREN'S DISEASE
Lawrence C. Hurst, MD; Marie A. Badalamente,
PhD
Department of Orthopaedics, S.U.N.Y. at Stony Brook,
H.S.C. T18-030, Stony Brook, New York 11794-8181; email:
ihurst@surg.som.sunysb.edu;
mbadal@surg.som.sunysb.edu
Sponsor: United States Food & Drug Administration, FDA Orphan Products Development Office, Rockville, MD 20857; web: http://www.fda.gov/orphan/
PURPOSE--Surgical fasciectomy is the current form of treatment for Dupuytren's disease. The purpose of this study was to test the clinical safety and efficacy of Clostridial collagenase (enzyme) injection as a nonsurgical therapy for Dupuytren's disease in a Phase 2, open label trial.
METHODOLOGY--Twenty-eight persons with Dupuytren's entered the study, 26 males and 2 females mean age 67.3±10 years. Using a dose escalation protocol, the first six study subjects received 300, 600, 1200, 2400, 4800, and 9600 units collagenase, respectively, injected into the cord causing contracture of the metacarpophalangeal (MP) joint. The remaining 22 patients had collagenase injections (27 MP joints and two PIP joints) at a dose level of 10,000 units followed by a 10- to 12-hr period of hand immobilization. Patients were seen the next morning, fitted with a nighttime extension splint to be worn for 4 mo, and instructed in at home extension exercises. One subject was lost to follow-up.
PROGRESS--In the first six subjects, treated under the dose escalation protocol, there was involvement of two ring and four little fingers with a mean degree of initial MP joint contracture of 49.2±11°. Collagenase injection had no effect in reducing the degree of MP joint contracture. In the remaining 22, the mean degree of initial MP joint contracture was 42.4±13.4°. Twenty-seven cords causing MP joint contracture were injected with 10,000 units collagenase (4 long, 13 ring, 10 little fingers, and one thumb web cord). The mean degree of initial MP joint contracture in two subjects was 45°, one ring and one little finger. Twenty-six of the 27 MP joint contractures corrected to normal (zero°) with full range of motion within 1 to 7 days of injection. The subject injected in a thumb web cord achieved only a 10° correction of an initial thumb web contracture of 45°. In those with PIP joint contractures, one joint corrected to normal (zero°) and the other PIP joint corrected to within 5° of normal (zero°), both within days of the injection. Minor adverse reactions included tenderness to pressure at the injection site with minimal palmar, and sometimes dorsal, edema that resolved in 1 to 2 weeks of the injection. Mean follow-up was 3.3 months; the range was 7 days to 1 year.
CONCLUSIONS--Clostridial collagenase injection of Dupuytren's cords causing MP and PIP joint contractures appears to have merit as a nonoperative treatment for this disorder. Disease recurrence in long-term follow-up is currently being assessed. Injection of increased numbers of PIP joint contractures is also planned. This study was designed as an open label investigation, in consultation with the US FDA, prior to initiating placebo, control, random double-blind studies. Pending further study, collagenase injection for Dupuytren's disease may be a safe and effective alternative to surgical fasciectomy as a cost-effective and noninvasive treatment for this disorder.
[167] A SURVEY OF PARENTS OF CHILDREN WHO USE ORTHOSES AND PROSTHESES
Claire B. Kopp, PhD; Toya Wyatt, PhD; Dani Hodge, MA;
Donald McNeal, PhD
PURPOSE--This study focuses on identifying parents' perceptions of health, motor skills, and the implications of orthopedic disabilities (OD) for children, and on identifying actions taken to secure services with respect to assistive devices.
METHODOLOGY--Information packets describing the study goals and inviting participation are distributed through local organizations. Parents of children with OD who agree to participate are interviewed in either English or Spanish at a time convenient to them.
PROGRESS--To date, 47 interviews have been completed (24 Caucasians, 12 Latinos, 8 African-Americans, 3 other) regarding children ranging in age from 2 to 17 yrs, 38 of whom were male. Diagnoses include, cerebral palsy, muscular dystrophy, limb deficiency, and traumatic brain injury. Approximately 18 of children have mild-to-moderate cognitive impairments, as reported by parents.
RESULTS--Preliminary analyses indicate a need for improving communication between parents and physicians. More than 50 percent of mothers indicate their child's health care could be improved and 39 percent of mothers specified issues related to parent-physician communication. Comments included: a need for physicians to listen to parents, to communicate more clearly, to be understanding of the parents' situation, and to be more optimistic about the child's future.
Interesting differences emerged across ethnic groups for interactions with physicians. Overall, African-American mothers reported more satisfactory interactions with physicians and the medical system in general. Parents whose children's orthopedic disability is accompanied by cognitive impairments are significantly less comfortable asking questions of medical professionals and less satisfied with responses to their question than parents of children without cognitive impairments.
Wheelchairs and braces are the most common devices used by these children; others were walkers, crutches, and lower-limb prostheses. Braces are a common source of discomfort; 60 percent of parents reported rubbing, blistering, and sweating. There were few complaints of discomfort related to other assistive devices. Suggestions for improving devices focus on comfort (addition of padding or ventilation) and convenience (more compact, lighter). Inconvenience and discomfort were both cited as reasons why devices were no longer in use.
FUTURE PLANS/IMPLICATIONS--Future plans include continued recruitment, coding of qualitative data, data entry, data analyses, and dissemination. Additional analyses will focus on the nature of orthopedically disabled children's social interactions, social supports offered to parents, and how each of these facilitates, or hampers access to, medical services. If preliminary findings are supported by additional analyses, a set of recommendations identifying needs, and possible methods for improving physician communication will be issued.
[168] FATIGUE STRENGTH OF COMPOSITE FEMORAL COMPONENTS FOR HIP ARTHROPLASTY
H. Del Schutte, MD; Robert A. Latour Jr., PhD; S.B.
Biggers Jr., PhD; J.M. Kennedy, PhD
Ralph H. Johnson Department of Veterans Affairs,
Charleston, SC; Department of Bioengineering and #Department
of Mechanical Engineering, Clemson University, Clemson, SC
29634; email: robert.latour@ces.clemson.edu
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #A670-3RA)
PURPOSE--The purpose of this research is to investigate how the design of fiber-reinforced, polymer composite femoral components influences component fatigue behavior, and how such components should be mechanically tested in vitro to closely simulate in vivo stress-states within the components. The goal of this research program is to develop the necessary technology base to enable low stiffness/high strength composite femoral components to be confidently designed and tested for clinical use with the potential for reducing stress shielding induced calcar bone resorption following hip arthroplasty.
METHODOLOGY--We must first develop models of the femur before and after hip joint replacement and develop 3-D composite fatigue analysis techniques and their application to design fatigue resistant composite femoral components. Using results from the fatigue analysis, we shall fabricate composite femoral components, and conduct modeling studies upon them to determine experimental fatigue test conditions that will induce a stress state within them equivalent to that predicted in the femoral models. Finally, we shall conduct fatigue testing of fabricated femoral components to verify the predictions of the 3-D composite fatigue analysis.
PROGRESS--The complex 3-D femoral and composite femoral component models have been completed, and 3-D computational techniques to model the fatigue behavior of thick composite structures are currently being finalized; they will next be applied in the 3-D femoral model to calculate how the laminated components should be designed for maximum fatigue strength. In preparation for component fabrication, manufacturing techniques have been assessed, including both hot-plate pressing and autoclave fabrication of thick composite plates. Additionally, CAD-CAM computer programming development has been completed to enable femoral components to be machined directly from the computerized solid models. Finally, preparations are presently being made to design the experimental fatigue test set-up, so that the fatigue test experiments can be undertaken as soon as femoral component fabrication is completed.
PRELIMINARY RESULTS--The preliminary results from the 3-D computer models of the femur under simulated joint and muscle forces show the expected trends in femoral stress distribution, thus indicating that the simulations are functioning properly. The 3-D fatigue damage initiation and growth modeling strategies have been developed and applied to simple test case structures. The results were found to predict expected damage development behavior, and thus the fatigue model is nearly ready for application in the actual hip joint model. Finally, results from studies into thick composite laminate fabrication and machining have demonstrated that femoral components will be capable of being machined as intended to match the computer-simulated designs.
FUTURE PLANS--It is planned to complete this research program over the upcoming year. Now that the preliminary work has nearly been completed, the fatigue behavior prediction model will be applied to the 3-D computer models of composite femoral components within the proximal femur, and analyses will be performed to predict how the laminated composite femoral components should be internally designed to maximize fatigue strength. Optimized femoral components will then be fabricated and fatigue tested (along with sub-optimally designed components as negative controls), and the fatigue behavior will be compared to the predicted behavior to verify the computer analyses.
IMPLICATIONS--The methods developed in this research program should have specific application for composite femoral component design for the development of improved femoral components for hip joint arthroplasty. Additionally, the developed methodology should have broad application for the design of composite structures for other implant applications, such as spinal fracture fixation devices and intervertebral disk designs.
[169] USE OF ULTRASONIC VIBRATION IN IMPROVING THE MECHANICAL PROPERTIES OF BONE CEMENT
Subrata Saha, PhD
Robert W. Christensen Biomechanics Laboratory,
Bioengineering Alliance of South Carolina, Department of
Bioengineering, Clemson University, Clemson, SC 29634-0905;
email: ssaha@clemson.edu
Sponsor: Clemson University, Clemson, SC 29634-0905
PURPOSE--Self-curing polymethylmethacrylate (PMMA) or acrylic bone cement is used extensively in total joint arthroplasty and in the repair of bone defects. However, the hand mixing of the methylmethacrylate polymer and the liquid monomer entraps air bubbles, making the cement porous, which adversely affects its mechanical properties. This has prompted several investigators to attempt to reduce the porosity by the use of centrifugation or by vacuum mixing. In this study, we investigated whether ultrasonic vibration during mixing could reduce the porosity and improve the mechanical properties of surgical grade polymethylmethacrylate.
METHODOLOGY--Radiopaque acrylic bone cement powder (Surgical Simplex-P, Howmedica, Inc.) was mixed with the liquid monomer by hand in plastic bowls. When the mixture reached a liquid state, the bowls were placed on the tray of an ultrasonic cleaner (Branson) for one minute. As air bubbles rose to the top during the ultrasonic vibration, they were removed by a spatula. The cement, in a dough state, was then poured into Teflon molds to prepare the cylindrical compression and the dog-bone-shaped tension specimens. A second batch of bone cement, not subjected to the ultrasonic vibration, was used to prepare a similar group of control samples. The specimens were radiographed. The amount of air bubbles (diameter >0.5 mm) was measured from these contact x-rays using an image analysis system. The specimens were then stored in water for at least 24 hrs before testing them mechanically using a servohydraulic mechanical testing machine (Instron).
RESULTS--Our earlier studies showed that both the ultimate tensile stress and the stiffness increased significantly (p<0.001) when vibrated ultrasonically. The ultrasonically vibrated samples also showed similar statistically significant (p<0.05) improvement for the maximum strain. The amount of porosity for the ultrasonically vibrated tension samples was 2.4±1.04% compared to 7.2±2.58% for the control group. The difference in these values were highly significant (p<0.0001).
Similar to the tensile behavior, the compressive strength of the bone cement samples also showed an increase for the ultrasonically vibrated samples. This increase of compressive strength was equivalent to an increase in the deformation rate from 1.2 to 12 mm/min. The compressive modules and the energy absorption capacity also showed an increase for the ultrasonically vibrated samples compared to the control group.
FUTURE PLANS--We are continuing our study to evaluate the effect of ultrasonic vibration during the mixing on the shear, fatigue, and fracture properties of bone cement. Our study indicates that ultrasonic vibration may be a practical way of reducing porosity and thus improving the mechanical properties of surgical grade PMMA.
[170] MODIFICATION OF POLYETHYLENE TO IMPROVE THE WEAR PERFORMANCE OF JOINT REPLACEMENT PROSTHESES
Myron Spector, PhD; Robert E. Cohen, PhD; Anuj Bellare,
PhD; Yot Boontongkong, MS; Natasha Chang, BS
Brockton/West Roxbury VA Medical Center, West Roxbury MA
02132; Brigham & Women's Hospital, Boston MA 02115;
Massachusetts Institute of Technology, Cambridge MA 02139;
email: spector@ortho.bwh.harvard.edu
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #A877-RA)
PURPOSE--This project was a study of the dependence of the wear behavior of ultra-high molecular weight polyethylene (UHMWPE) on molecular orientation. Our long-terms goals are to determine processing conditions and morphology of UHMWPE that optimize its wear and mechanical performance in total joint replacement prostheses.
METHODOLOGY--Commercially available rod stock of medical grade UHMWPE (GUR 4150 resin, Hoechst-Celanese, Inc.) was machined into rectangular slabs to snugly fit into a previously constructed channel-die apparatus equipped with a heater. The slabs were compressed to various compression ratios (CR) in the channel-die at a temperature of 90 °C. The oriented samples were slow cooled to room temperature prior to unloading, and then used for morphological and wear analyses.
Crystallographic and molecular orientation was determined using a combination of wide angle x-ray diffraction (WAXD) and small angle x-ray scattering (SAXS). WAXD and SAXS were performed on sections of 1-mm thickness, machined from the oriented slabs along three orthogonal directions with respect to the channel-die: loading direction (LD), constrained or transverse direction (CD) and flow direction (FD).
Selected slabs of a CR of 1.7 and 2.75 were used to conduct wear tests on three orthogonal surfaces (i.e., with surface normals along LD, CD, and FD directions). A four-station reciprocating (sliding) wear tester was employed for all wear tests. A load of 14.55 MPa was applied on each cylindrical cobalt chromium alloy counterface which slid on flat UHMWPE specimens. Wear measurements were taken after 14.8, 29.6, and 59.2 km of sliding, comparing the loss of weight of UHMWPE specimens to that of control specimens soaked in bovine serum. Finally, wear surfaces were examined using a low voltage high resolution scanning electron microscope.
PROGRESS--The project has been completed. All experiments were conducted successfully. The morphological and tribological studies have been submitted for publication.
RESULTS--SAXS and WAXD characterization showed a substantial degree of molecular orientation at CR above 1.4. The degree of orientation increased monotonically up to a CR of 6.0, after which brittle fracture occurred. There was a 5 percent reduction in the degree of crystallinity (or density) in samples that were compressed to a ratio above 2.0.
Wear tests of samples with a CR of 2.7 and 1.7 were compared to standard, uncompressed UHMWPE (CR=1.0). At a sliding distance of 14,813 m, the wear volume of standard UHMWPE normalized by applied load was (0.97±0.31)E-7 cubic mm/Newton. The volume of wear particles on LD, CD, and FD surfaces under identical conditions of wear was in the range of (1.27-0.79)E-7 cubic mm/Newton. This reveals that molecular orientation does not substantially affect wear rates in UHMWPE.
FUTURE PLANS/IMPLICATIONS--Our results suggest that molecular orientation does not have a substantial effect on wear performance. It is known that molecular orientation can reduce mechanical properties such as yield strength and modulus of elasticity along specific crystallographic directions. Therefore processing methods must be used that minimize molecular orientation in UHMWPE used for joint replacement prostheses. Further studies must be conducted on the effect of molecular weight and the formation of molecular networks of UHMWPE, which are known to strongly affect the wear performance of unoriented UHMWPE.
[171] DEVELOPMENT OF DATA CAPTURE SOFTWARE FOR AN ARTIFICIAL KNEE WITH SENSORS AND TELEMETRY
Harvinder Maan; John Szivek, PhD
Sponsor: DePuy-Dupont Orthopedics, Warsaw, IN 46581
PURPOSE--Development of a system to assess the amount of strain experienced by a polyethylene tibial insert of an implanted prosthetic knee would provide physicians with important information that would minimize the wear placed upon the knee joint, thereby helping to prevent premature failure. Additionally, this information could be utilized by research teams to develop more robust implant designs. The focus of this part of the study was to develop a telemetry system that would allow the real-time display of strain information from a prosthetic knee implant in vivo to a computer.
METHODOLOGY--The implantable telemetry system consists of a radio frequency (RF) digital transmitter, inductive coil power supply, RF receiver, and a Machintosh computer with custom software written in the C programming language. Located within the tibial stem extender, the transmitter collected information from strain gauges placed on the tibial insert of the prosthetic device and transmitted the strain information to a receiver. There was no battery-based power supply for this generation of transmitter; it was powered by an inductive coil around the prosthetic implant that created an alternating magnetic field. When a load was applied to the strain gauges, they deformed causing a resistance change. This measurable change in resistance was transmitted from the prosthetic device to a computer. Custom software displayed the strain experienced by the prosthetic device as the load on it varied.
PROGRESS--The artificial knee telemetry system hardware and software have been developed and are currently in a preclinical trial phase. To characterize the accuracy of the system, the information collected from the wireless system was compared with that from a hardwired system using an experimental apparatus consisting of an aluminum bar, with a known load/deformation response, fixed at one end and loaded on the other using an MTS servo hydraulic test machine. Strain gauges attached to the bar were wired to either the transmitter or signal conditioners and monitored. The MTS placed a sinusoidal varying load on the bar at one cycle per second. Initial results indicate that the data on strain changes and the strain change patterns collected by the wireless system are the same as those collected by the hardwired system. Signal drift was noted for the wireless system under some conditions.
FUTURE PLANS--In vivo strains collected from test animals will be evaluated next. Gauges will be placed on the femora of rats and strain information will be collected during a variety of activities, using both a hardwired and a telemetry system. Further refinements to the system are also planned, including software signal filtering enhancements and correction for signal drift.
[172] A MINIMAL EFFORT TEST FOR PATIENTS IN PAIN USING THE BACK ANALYSIS SYSTEM
Lars I.E. Oddsson, Dr Med Sc; Carlo J. De Luca, PhD;
Jonathan Bean, MD
NeuroMuscular Research Center, Boston University,
Boston, MA 02215; Physical Medicine and Rehabilitation
Service, Boston VA Medical Center, Boston, MA 02130; email:
loddsson@bu.edu
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #B872-2RA)
PURPOSE--Previous analysis of surface EMG signals of the lumbar back muscles assessed with the Back Analysis System (BAS), has been based on the concept of fatiguing the muscles by investigating the behavior of the initial median frequency and the rate of decrease of the median frequency during fatiguing contractions. The use of such procedures may not be well suited for subjects in acute phases of injury, due to obvious hesitation and fear of re-injury making the assessment of a reliable endurance effort unreliable. In this project, we are investigating new force insensitive parameters that reflect aspects of load-sharing by describing spectral interactions and imbalances between pairs of muscles of the lumbar back. The long-term objectives of this study are to provide clinicians and researchers with a procedure to objectively quantify muscle impairment associated with low back pain (LBP) in previously nontestable subjects with acute pain so that specific early interventions can be provided thus minimizing the risk that subjects reach a chronic injury state and with objective information regarding the transition of an injury from an acute to a sub-acute phase, thus assisting in decisions concerning treatment for the individual.
METHODOLOGY--Controls as well as LBP subjects in an acute phase of injury will be tested in the BAS. Test protocols to assess EMG parameters during symmetrical and asymmetrical efforts will be implemented. A separate study will address the repeatability of EMG-based parameters in both control and LBP subjects. Those with acute LBP will be monitored over time to address changes in EMG parameters during the sub-acute and chronic phases of injury. Surface EMG activity will be recorded from six muscle sites of the lumbar back while the subject performs brief isometric incrementing stair case contractions up to 80 percent of ideal body weight in the BAS. The behavior of ratios between spectral properties in contralateral as well as ipsilateral pairs of muscles of the lower back are being investigated. New parameters with clinical relevance describing segmental imbalances and bilateral "compensations" in spectral parameters have been introduced. A series of questionnaires will be administered to patients to assess fear-avoidance strategies in different phases of the injury.
PROGRESS--A BAS system was set up at the Boston VA Medical Center Spine Clinic. A research physical therapist has been conducting data collection in the patient population. We have collected data from 14 patients using the test protocol where subjects purposely develop a range of asymmetrical forces in the BAS system. In addition, data from 18 control subjects has been collected for the corresponding protocol. Twelve subjects in transition from acute to chronic injury are currently being monitored in the BAS, using the stair case protocol with loads based on their ideal body weight instead of a maximum voluntary contraction A database has been set up to accommodate the EMG data as well as clinical impairment measurements of pain and disability. associated with LBP.
PRELIMINARY RESULTS--Results thus far confirm that imbalances are present in both patients and controls. However, imbalances are to a larger extent compensated across lumbar levels for the controls as compared to LBP subjects.
FUTURE PLANS--Scheduling and testing of subjects continues. We predict to finish the asymmetry project in both types during the fall. Development of software for extraction of EMG-based parameters is progressing and should be finished by the end of the year.
[173] BACK EXERCISE PRESCRIPTION AND IMPLEMENTATION BY SURFACE ELECTROMYOGRAPHIC PROCEDURES
Serge H. Roy, ScD, PT; Carlo J. De Luca, PhD; Jonathan
Bean, MD
NeuroMuscular Research Center, Boston University,
Boston, MA 02215; Physical Medicine and Rehabilitation
Service, Boston VA Medical Center, Boston, MA 02130
Sponsor: Department of Veterans Affairs, VA
Rehabilitation Research and Development Service, Washington,
DC 20420
(Project #B2029-RA)
PURPOSE--The purpose of this project is to develop a low back pain (LBP) exercise procedure for restoring muscle impairments classified by the Back Analysis System (BAS). The BAS is a computerized diagnostic device that combines surface electromyography (EMG) with dynamometry and force-feedback to evaluate the fatigability, strength, and control of back extensor muscles. This study will considerably advance the BAS technique to include the prescription and implementation of muscle-specific, individualized exercises to improve strength and endurance.
METHODOLOGY--Phase I of the study focuses on modifying the BAS to implement isometric exercises for reconditioning impaired back extensor muscles. Isometric and isotonic exercises will be developed for use outside of the BAS to selectively recondition impaired muscle groups. A portable 2-channel EMG unit will be evaluated for its capabilities in enhancing muscle activation during the performance of the exercises outside of the BAS. Phase II of the study will evaluate the effectiveness of the exercise protocol by comparing treatment outcomes in subjects with LBP randomly assigned to either an experimental exercise group, a generalized strengthening exercise group, or a control group with supervised exercise.
PROGRESS--We have completed all of the Phase I objectives. We have implemented an isometric exercise protocol by modifying the force-feedback attributes of the BAS. A back exercise protocol using a portable EMG system has been developed for muscular enhancement of spinal stabilization. The exercises were developed based upon EMG results from 12 trunk muscles evaluated in 10 subjects during various floor exercises. Phase II of the study is proceeding well: we have completed nearly 100 BAS tests among the LBP patient population recruited from the Boston VA Medical Center Spine Clinic. A BAS system was set up on-site and a research physical therapist on the investigative team has been conducting the data collection. Clinical impairment measurements of pain, strength, and mobility are being collected from this population, as well as standardized instruments for disability associated with LBP. These variables will be related to the EMG findings and compared for the three treatment groups.
PRELIMINARY RESULTS--The modified BAS device functions well in enabling patients with LBP to activate specific muscle groups. Asymmetrical loading of the back extensor muscles can be produced by selecting appropriate targets on the feedback display of the BAS. The results from the preliminary floor exercise tests for spinal stabilization demonstrate the degree to which these exercises activate localized muscle groups. We are therefore able to use this information to select appropriate stabilization exercises to counteract muscle favoring or inhibition of specific muscle groups.
FUTURE PLANS--Analysis of the results from the longitudinal randomized clinical trials will commence shortly upon completion of the data collection.
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