Philip L. Anderson

AVERAGE AND PEAK CONTACT STRESS DISTRIBUTION EVALUATION OF TOTAL KNEE ARTHROPLASTIES

Seven total knee arthroplasty systems were tested to determine contact stress patterns and contact areas using a calibrated Fuji film stress analysis technique. Knees were loaded to 2,000 N (204 kg) at 15 degrees, 60 degrees, 90 degrees, and 135 degrees flexion at 24 and 37 degrees C. Evaluation of stresses at 37 degrees C at 15 degrees and 60 degrees using an average contact stress assessment technique indicated that the LCS meniscal bearing knee system, (DePuy, Warsaw, IN), the AMK knee with a constrained insert (DePuy), and the PFC knee with a posterior-lipped insert (Johnson and Johnson, Raynham, MA) had the lowest average contact stresses (near or below 10 MPa). The PFC with a regular insert (Johnson and Johnson) the Ortholoc II (Dow Corning Wright, Arlington, TN), and the AMK with a regular insert (DePuy) had intermediate contact stresses. The AMK with a Hylamer-M insert (DePuy) and the MG II (Zimmer, Warsaw, IN) had the highest average contact stresses (near or above 20 MPa). A stress-calibrated Fuji film measurement technique has shown that an assessment of ranges of contact stress provides much more information about regions of expected wear than an assessment of average contact stresses. Testing of the tibiofemoral articulation of artificial knees revealed that all knees had some areas of contact with maximum stresses in excess of 15 MPa. As the yield strength of ultrahigh-molecular-weight polyethylene is approximately 15 MPa, all tibial inserts could wear to some extent. Peak contact stresses at four test angles of the AMK, Series 7000 (Osteonics, Allendale, NJ:) Genesis (Smith & Nephew Orthopaedics, Memphis, TN), and MG II patellofemoral articulations were high (above 30 MPa). Contact areas varied from line-shaped to bilateral circular or elliptical shapes. The LCS knee system experienced substantially lower patellofemoral contact stresses and larger contact areas. Changes in conformity of knee designs are warranted to overcome wear problems. Peak contact stresses measured from the LCS meniscal bearing tibiofemoral and patellofemoral joint were in excess of 30 MPa in some areas at low flexion angles. This design does create large areas of contact at very low contact pressures, however, and for this reason is expected to wear less than other designs.

Contact areas and pressures between native patellas and prosthetic femoral components

Contact areas and pressures between native patellas and a prosthetic condylar design femoral component were measured at flexion angles of 30 degrees, 60 degrees, and 90 degrees. These were compared to measurements obtained with a domed all-polyethylene patellar component. Mean native patellar contact areas were found to be fourfold greater than seen with the prosthetic patellar component. Contact stresses in the native patellas were below the yield strength of articular cartilage in 80% of the contact area. By contrast, stresses measured in the prosthetic patella exceeded the yield strength of ultrahigh molecular weight polyethylene in 64% of the measured contact area. Contact areas and stresses were not significantly effected by flexion angle. Although contact areas and stresses reflect only a part of the dynamics of the patellofemoral articulation this information would support the selective retention of the native patella in total knee arthroplasty.

An experimental method for the application of lateral muscle loading and its effect on femoral strain distributions.

Experimental models that have been used to evaluate hip loading and the effect of hip implants on bone often use only a head load and abductor load. Anatomic considerations and in vivo measurements have lead several investigators to suggest that these models are inaccurate because they do not incorporate the loads imposed by additional muscles. The aim of this study was to evaluate the strains in the proximal and mid diaphysis of the femur for five hip loading models, one with a head load and abductor load only and four which incorporated lateral muscle loads as well. Head load to body weight load ratios were used to evaluate the physiologic accuracy of these models and strains were compared to determine the extent of strain changes as a function of model complexity. All models which incorporated additional lateral muscle loads more accurately simulated head load to body-weight load ratios than the simple abductor-only model. The model which incorporated a coupled vastus lateralis and iliotibial band load in addition to the abductor load provided the simplest configuration with a reasonable body-weight to head-load ratio.

Evaluation of Factors Affecting Bonding Rate of Calcium Phosphate Ceramic Coatings for In Vivo Strain Gauge Attachment

The aim of this study was to compare the bone-bonding rates of eight calcium phosphate ceramic (CPC) coatings attached to strain gauges, alone and in conjunction with an OP1 device (Creative BioMolecules, Hopkinton, MA) and autologous concentrated pericyte cells. These coatings were studied to develop faster bone bonding to long-term in vivo strain sensors. Characterization of the CPC powders using electron microscopy and X-ray diffraction showed that they had shapes ranging from spherical to rocklike and properties ranging from highly crystalline to amorphous. CPC coated gauges were placed on the femora of young male dogs during aseptic surgery and were initially held in place using resorbable sutures. Test groups were euthanized after 3, 9, and 12 weeks. Both femora of the dogs were explanted and cantilever loaded. Response of the implanted hydroxyapatite (HA) coated gauges were compared to the response of bench-top glued sets of gauges (controls) attached to the contralateral femur and reported as a percentage of the control values. One CPC coating type showed an average response of 30% of controls after 3 weeks, four showed average responses higher than 75% after 9 weeks, and three showed averages higher than 82% after 12 weeks in vivo. Amorphous CPC coatings bonded more quickly than crystalline ones and particle shape had less effect than crystal structure on bonding rates. When either OP1 or autologous concentrated pericyte cells were placed on selected CPC coated gauge surfaces, the CPC5 coated gauges bonded best after 3 weeks with a response of 59%. After the same time period in vivo, CPC3 and CPC7 provided responses of 40 and 16%, respectively. Comparison of a soluble calcium-coated CPC with an uncoated one that had identical crystal structure and similar particle shape indicated that the calcium coating slowed bone bonding substantially in the young dog model. Optical microscopy of stained undecalcified bone sections and backscattered electron imaging indicated bone formation at all bone-HA interfaces and an increase in the number of areas of bone remodeling adjacent to the gauge at all time periods. Gross bone remodeling due to strain gauge placement was only observed near the distalmost cell-seeded strain gauges. Selection of the type of coating and enhancement system can accelerate bone bonding to strain sensors but must be tailored to the bone of the model in which it is being used. Augmentation of CPC coatings with cells or OP1 resulted in variable enhancement of the bonding rate and depended on the CPC and the enhancement system.

Bilateral symmetry of biomechanical properties in rat femora

In many studies, bone healing and remodeling have been examined in various animal models using one femur as a control for the contralateral femur based on the assumption that they are bilaterally symmetrical. Symmetry studies have been limited mainly to geometrical properties. The purpose of this study was to determine whether or not there is symmetry in the mechanical properties of rat femora. Two strain gauges were attached to the anterior surface parallel to the long axis of explanted femora of retired female breeder and 120-day-old male Sprague Dawley rats. Femora were mechanically tested in cantilever bending and the strain values were recorded. Moments of inertia, cortical areas, and moduli of elasticity were determined from strains and cross-sectional properties. Female femora showed a bilateral strain difference of less than 2.2% and an elastic modulus difference of less than 8.7%. Males had less than 2.0% and 7.9% differences for strain and elastic moduli, respectively. Statistical analysis showed no significant difference between left and right femoral strain values for the females, but modulus differences were significant different at the p = 0.05 level. There was no significant difference in strain and modulus values for the males, indicating mechanical and geometrical symmetry of their femora.

Bone bonding strength of calcium phosphate ceramic coated strain gauges.

Although strain transfer from bone to gauge has been used as an indication of the extent of bone bonding to calcium phosphate ceramic (CPC) coated strain gauges, interface strength measurements have not been reported. In order to develop bone-bonded gauges that remain attached to bone surfaces for long periods, the strength of the CPC-bone interface must be optimized. A shear test to assess the interface strength of the CPC-bone interface was developed using the femora of 120-day-old male rats. The mean interface strength of a blended CPC coating bonded to the femora of the rats for 6 weeks in vivo was 4.8+/-2.4 MPa, and one specimen achieved a strength of nearly 10 MPa. This mean strength value is higher then the CPC-gauge interface strength reported in early studies, but it is lower than recently developed heat treated CPC-gauge interfaces that have average strengths of approximately 7.0+/-2.0 MPa.

Polyethylene particle morphology in synovial fluid of failed knee arthroplasty.

Synovial fluid from the knees of 16 patients undergoing revision knee arthroplasty for aseptic failure was subjected to base digestion and ultrafiltration. Filtered particles were scanned using scanning electron microscopy and analyzed with an image program. Polyethylene particles were identified visually and confirmed with the use of electron diffraction spectroscopy. Averaging more than 1500 particles per patient sample, 25,148 particles were analyzed. This corresponded to a concentration of 3000 polyethylene particles per milliliter of synovial fluid. Three populations of wear debris were identified in the fluid. Small globular particles with a mean area of 75 mu 2 represented 94% of all particles observed. The particles averaged 10 mu in diameter and often were seen in clumps. Long fibrous particles with a mean area of 1164 mu 2 made up 4% of the particle population. Large rhomboidal particles with an area of 557 mu 2 were observed least commonly and comprised the remainder of the particles visualized. All three particle types were observed in each fluid sample regardless of the wear pattern of the retrieved polyethylene liner. There were no differences in absolute particle counts, particle morphologic characteristics, or particle size between patients with and without gross polyethylene wear.

Interface strength studies of calcium phosphate ceramic coated strain gauges

In vivo strain gauging has been used to understand physiological loading and bone remodeling. In early studies, a cyanoacrylate adhesive was used to bond gauges to bone, even though this adhesive is susceptible to biodegradation that results in rapid debonding. Calcium phosphate ceramic (CPC) coated gauges have been successfully bonded to bone for long periods. However, earlier studies noted occasional debonding of coatings from gauges. The goals of this project were to develop a technique to securely bond particles to gauge backings and develop an in vitro test and assess its accuracy in simulating in vivo degradation of this interface. Gauges were heated for different time intervals, roughened with carbide papers, and prepared using layered coatings of polysulfone and CPC particles that varied in size, shape, and crystallinity. They were soaked in solution or placed in muscle pouches of rats for up to 16 weeks. They were then epoxied to fixtures, mounted on an MTS machine, and loaded to failure. Heating and roughening gauge surfaces increased the interface strengths by up to 2000%. In vivo and in vitro testing showed an initial drop in the interface strength, which leveled off to approximately 7.0+/-2.0 MPa.

A comparison of in vitro and in vivo degradation of two CPC strain gauge coatings.

Calcium phosphate ceramic (CPC) coated strain gauges have been used to measure bone strain in animal models for up to 16 weeks and are being developed to collect measurements in patients for periods of 1 year or more. A published surface roughening and heat treating procedure produced improved dry strength and in vivo stability of CPC-gauge interfaces after 16 weeks. The long term bond strength of two CPC-gauge interfaces prepared using the roughening and heat treating process were evaluated after up to 1 year in vitro and in vivo using a lap shear test. The feasibility of using an in vitro test to predict long term in vivo interface changes was established. A blended tricalcium phosphate + hydroxyapatite had a CPC-gauge interface strength which decreased from 6.07 +/- 2.64 MPa at 16 weeks to 4.71 +/- 1.840 MPa after 1 year in Hanks Balanced Salts (HBS). The same coating had a strength that decreased from 8.51 +/- 2.63 MPa at 16 weeks to 5.35 +/- 1 MPa after 1 year in vivo. A soluble calcium enhanced hydroxyapatite had an interface strength of 4.83 +/- 1.106 MPa after 16 weeks and 4.51+/- 1.100 MPa after 1 year in HBS. The same coating had an interface strength of 8.34 +/- 2.40 MPa after 16 weeks and 5.20 +/- 2.00 MPa after 1 year in vivo. Although interface strengths decreased slightly with time in vivo, after 1 year they were in the same strength range as published CPC-bone interface strengths of 4.8 +/- 2.4 MPa. Comparison of in vitro with in vivo results indicated that in vitro results were a good predictor of strength change in the blended CPC coating, but a poorer predictor of strength changes in the soluble calcium-enhanced coating.

Microstructural evolution of low-dose separation by implanted oxygen materials implanted at 65 and 100 keV

Thin separation by implanted oxygen substrates are attractive candidates for low-power, low-voltage electronic devices and can be obtained by low-dose, low-energy oxygen-ion implantation. We report in this study a variation of the process parameters that have never been investigated before, particularly for implantation with a high current density implanter. Characterization of the sample sets by transmission electron microscopy, secondary ion mass spectroscopy (SIMS), and Rutherford backscattering spectrometry (RBS) shows an optimum dose of 3.0 to 3.5 × 10 1 7 O + /cm 2 at 100 keV for forming a continuous buried oxide (BOX) layer compared to 2.5 x 10 1 7 O + /cm 2 at 65 keV. At this optimum condition for 100 keV, the thickness of Si top layers and BOX layers is in the range of 175-185 nm and 70-80 nm, respectively. Analysis of the breakdown voltage of small area capacitors shows a breakdown field in the range of 6.0-7.0 MV/cm, which is adequate for low-power, low-voltage devices. SIMS analysis shows that the maximum oxygen concentration of as-implanted samples is located at depths of 160 and 240 nm for the implantation energy of 65 and 100 keV, respectively. A significant redistribution of oxygen occurs at temperatures above 1300 °C during the ramping process. RBS analysis showed that a high-quality crystalline Si layer was produced after annealing at 1350 °C for 4 h. The defect density determined by the chemical etching method was found to be very low (<300 defects per cm 2 ) for all samples with a dose range of 3.0 × 10 1 7 O + /cm 2 to 6.0 × 10 1 7 O + /cm 2 implanted at 100 keV. However, a 65 keV sample with a dose of 4.5 x 10 1 7 O + /cm 2 contains about 10 9 defects per cm 2 . The larger defect density in the 65-keV sample may be due to the shift of oxygen depth distribution toward the surface, resulting in easier defect extension during the annealing process. The oxide precipitates in the Si overlayer play a key role in defect reduction by blocking the extension of dislocations to the surface.