Bone-particle-impregnated bone cement: an in vitro study

Bone-particle-impregnated bone cement specimens (up to 30% by weight) were characterized by various test methods. The experimental bone cement showed decreased crack propagation rates and increased Young's modulus, while the ultimate tensile strength and impact strength were decreased. The viscosity could be adjusted by adding initiators lost when substituting the PMMA powder with bone particles. The present study warranted further in vivo experiments on the possibility of tissue ingrowth for which the new bone cement was developed.

Pyrolytic carbon, porous implants, and the fibrin adhesive system

The bone implant interface for Pyrolite and titanium alloy was evaluated. The effects of a fibrin adhesive system (FAS) on the interface was also compared. Fixation of the Pyrolite and porous titanium implants was observed to occur in the 4-week period of the study with minimal fibrous tissue encapsulation. The ultimate interfascial shear strength for bone-Pyrolite interface was attained at 4 weeks. No adverse effect with utilization of the FAS could be identified. Further investigation into the use of Pyrolite and the FAS in foot surgery is anticipated.

Molecular and mechanical property changes during aging of bone cement in vitro and in vivo

The changes in mechanical properties and free radical concentration of curing Simplex P Radiopaque Bone Cement in vivo and in vitro conditions were studied. Samples were prepared so that each in vivo sample that cured and aged in the canine femoral intramedullary cavities had an in vitro counterpart that was cured and aged in a constant-temperature saline bath at 37 degrees C. An electron paramagnetic resonance (EPR) spectrometer was used to measure the growth and decay (curing) of polymerization radicals. The results of EPR measurements showed that the curing (disappearance of free radicals) of in vivo samples takes a much longer time (more than 4 weeks) than in vitro curing (less than 2 weeks). The mechanical tests indicate that, whether aged in vivo or in vitro, the strength increased rapidly for the first 1-2 weeks and then slight increases were seen for up to 6 months.

Acrylic bone cement: in vitro and in vivo property-structure relationship--a selective review

The nature and curing characteristics of acrylic bone cement are presented to give some basic understanding of the key to improving its performance in in vivo clinical use. The use of electron paramagnetic resonance (EPR) spectroscopy in the (polymerization) setting and curing/aging of bone cement under in vitro and in vivo conditions is presented. The current research effort to improve the implant fixation by precoating the prosthesis with acrylic polymers is also discussed.

The effect of initial temperature on free radical decay in PMMA bone cement

The effect of varying the initial temperature of the components of poly(methyl methacrylate) (PMMA) bone cement has been investigated. Electron paramagnetic resonance (EPR) spectroscopy was used to monitor free radical decay during the curing of the cement. Samples cured in saline at 37 degrees C were found to exhibit first-order decay kinetics for the polymerization radicals for approximately one week after mixing. This indicates that the decay did not take place by combination or disproportionation and was probably due to a transfer process. Decreasing the temperature of the bone cement components prior to mixing resulted in smaller decay rates, but still with first-order kinetics. This decrease in decay rate with lower initial temperature may be due to decreased porosity of the cement, possibly due to decreased monomer evaporation. SEM micrographs of the samples were consistent with this change in porosity.

Intramedullary fixation of artificial hip joints with bone cement-precoated implants. II. Density and histological study

Bilateral coxofemoral hemiarthroplasties were performed in dogs using experimental and control implants, which were fixed with bone cement. The stem of the experimental implant was precoated with bone cement, about 2 mm thick. After 1, 3, and 6 months the femora with implant specimens were harvested and sectioned for mechanical and histological evaluation. Histological observations on the implant-bone interface and density measurements of the bone cement are reported. The density of the precoated bone cement was higher than the same cement used for implant fixation at the time of implantation (1.202 vs. 1.188 g/mL). The precoating also resulted in milder histological reactions, including thinner fibrous tissue capsule and smaller gap between bone and cement. The present results and the previously reported mechanical findings strongly support our hypothesis that a better and longer lasting prosthesis fixation can be achieved using cement-precoated prosthesis combined with the customary cement fixation technique.

Intramedullary fixation of artificial hip joints with bone cement-precoated implants. I. Interfacial strengths

In order to minimize the problems associated with implant fixation using acrylic bone cement, a new technique has been investigated. Canine hip prostheses were precoated with self-curing acrylic bone cement and implanted in random source dogs using the same cement for fixation, a precoated prosthesis on one side and an uncoated (control) on the other. After 1, 3, and 6 months, both femora were excised and sectioned for mechanical assessment of the interfaces among bone, cement, and implant. It was found that the precoated implants had much higher interfacial shear strengths than the uncoated ones (average 14.2 and 6.8 MPa for implant-cement interface; 2.0 and 1.2 MPa for the cement-bone interface for all implant periods). The precoated "old" cement and the "new" cement's interfacial shear strength was the strongest with an average of 15.1 MPa for all implant periods. The present results indicate that the precoated hemiarthroplastic implants provide a firmer intramedullary fixation than the traditional, uncoated implants.

Molecular and macroscopic properties of PMMA bone cement: free-radical generation and temperature change versus mixing ratio

The molecular and macroscopic changes occurring during the polymerization of poly(methyl methacrylate) (PMMA) bone cement have been investigated. Electron paramagnetic resonance (EPR) spectroscopy was used to monitor free-radical generation and this was compared to temperature changes occurring in the cement for various ratios of polymer powder to liquid monomer (P/L ratio) used in the sample preparation. Both the concentration and the characteristic growth time of the free radicals associated with the polymerization of the bone cement depended on the P/L ratio used. Larger P/L ratio resulted in shorter characteristic growth time for the free radicals as well as a shorter time for the occurrence of the peak sample temperature. Smaller P/L ratios gave smaller maximum concentrations of free radicals and larger peak temperatures. These results are explained on the basis of (1) more initiators present at higher P/L ratios resulting in faster polymerization and (2) less initiators and more monomers present at smaller P/L ratios resulting in fewer radicals but more exothermic reactions. The free radicals present in the bone cement due to the manufacturer's sterilization process were found to be proportional to the fraction of powder used in the preparation, indicating negligible monomer loss during sample mixing.

Prevention of erythrocyte adhesion onto porous surfaces by fluid perfusion

This study was undertaken in order to investigate the use of fluid perfusion to prevent the adhesion of erythrocytes to a porous foreign surface. Three pore sizes each of two different commercially available polyethylene and aluminum oxide (AI2O3) tubes were investigated. Tubes were perfused with Ringer's solution while immersed in a bath of whole canine blood. Control tubes were prepared in an identical fashion but were not perfused. After testing, tubes were fixed in glutaraldehyde and dehydrated with graded ethanol. Samples were then sectioned and prepared for erythrocyte adhesion analysis by scanning electron microscopy (SEM). Results indicate that fluid perfusion can be used as a means to prevent erythrocyte adhesion in the range of pore sizes (10-60 micrometer diameter) studied. Critical values of the fluid perfusion rate are 0.027, 0.073 and 0.21 ml/min. cm2 for 10 micrometer, 35 micrometer, and 60 micrometer pore polyethylene respectively. The critical values for the ceramic samples are 0.22, 0.16 and 0.72 ml/min cm2 respectively for 7, 11 and 31 micrometer pore diameter respectively.

Piezoelectric ceramic implants: in vivo results

The suitability of barium titanate (BaTiO3) ceramic for direct substitution of hard tissues was evaluated using both electrically stimulated (piezoelectric) and inactive (nonpolarized) test implants. Textured cylindrical specimens, half of them made piezoelectric by polarization in a high electric field, were implanted into the cortex of the midshaft region of the femora of dogs for various periods of time. Interfacial healing and bio-compatibility of the implant material were studied using mechanical, microradiographical, and histological techniques. Our results indicate that barium titanate ceramic shows a very high degree of biocompatibility as evidenced by the absence of inflammatory or foreign body reactions at the implant-tissue interface. Furthermore, the material and its surface porosity allowed a high degree of bone ingrowth as evidenced by microradiography and a high degree of interfacial tensile strength. No difference was found between the piezoelectric and the electrically neutral implant-tissue interfaces. Possible reasons for this are discussed. The excellent mechanical properties of barium titanate, its superior biocompatibility, and the ability of bone to form a strong mechanical interfacial bond with it, makes this material a new candidate for further tests for hard tissue replacement.

The effect of direct electrical current stimulation on the bone/porous metallic implant interface

Cylindrical porous plugs (6.35 mm dia. 11 mm long, average pore size of 190 micron dia.) made of electrically conductive Co-Cr-Mo surgical alloy powders were implanted in the canine femur. An electrical stimulation device (mercury battery, 1.35 V, connected in series with a 150k omega resistor) was attached to all implants directly. The in vivo current was about 8 microA for the stimulated implants while no current was delivered for the control ones. After predetermined implant periods, tensile test specimens were made to measure the interfacial strength between bone and implants. Some samples were used for histological observations. The present results show that in vivo electrical stimulation substantially increased the strength of the union between porous implants and bone when compared to the controls up to 12 weeks. Histological observations show that the increased strength is mainly due to the increased new bone formation in the pores of implants. It was also observed that the fractional callus volume in the intramedullary canal for the stimulated samples retained more than the controls after reaching maximum at 3 weeks.

Piezoelectric ceramic implants: a feasibility study

A piezoelectric ceramic has been investigated as a direct substitute for hard tissues. Barium titanate (BaTiOz) power was slipcast and fired at 1430 degrees C for 2 hr, then made piezoelectric by polarizing. After 16 and 86 days of implantation in the cortex of the femoral midshafts, the femora with test specimens were sectioned into about 4-cm lengths. Their voltage outputs were measured under cyclic load at 1 Hz. The present results show that the voltage gradient at the implant surface is 0.15 mV/mm for the 16-day implantation with a 445-N (100-lbs.) load. This in turn can give rise to about 0.01 microA current flow in the adjacent area of the 16-day implant. The 86-day implant showed an order of magnitude higher voltage output compared to the 16-day implant with the same magnitude of loads. This is probably due to the "load-transfer" efficiency through the implants, since the voltage output is directly proportional to the actual load transferred to the implant. The more bone implant interface matures, the better the load transfer occurs through the implant, resulting in higher voltage output.

The mechanical stability of barium titanate (ceramic) implants in vitro

Dense, polycrystalline barium titanate (BaTiO3) specimens showed a modulus of rupture of 85.5 +/- 9.0 MPa and a compressive strength of 486 +/- 75 MPa. These values are considerable higher than those which had been previously reported. In addition, there was no significant drop in compressive strength after in vitro aging for 4 weeks in saline solution.

EPR study of free radicals in PMMA bone cement: a feasibility study

In order to determine the feasibility of using electron paramagnetic resonance (EPR) to monitor the molecular processes occurring in polymethylmethacrylate (PMMA) bone cement, a preliminary study has been conducted on the bone cement in vitro. It has been found that a sufficient concentration of free radicals is generated in the bone cement during the polymerization process so that both the polymerization and the curing can be monitored by EPR. The results of these measurements on both radiolucent and radiopaque Surgical SimplexR P bone cement are presented. It has also been found that free radicals are present in the radiopaque bone cement powder as received from the manufacturer. These radicals which apparently are produced in the radiation sterilization process have been found to be unaffected by the polymerization/curing process and to be stable for temperatures below about 100 degrees C. It has clearly been shown that EPR can used to monitor the molecular processes of bone cement in vitro. It is hoped that this technique can be utilized to monitor the in vivo ageing process of bone cement used to fix orthopedic implants.

Pre-coated orthopedic implants with bone cement

A study on the feasibility of implants pre-coated with an acrylic bone cement has been performed. Four types of implants, an actual canine femoral prosthesis, a polished steel rod (0.49 cm dia. x 13 cm long) with and without pre-coating, and a sandblasted steel rod with pre-coating were implanted into canine femurs in vitro and in vivo to evaluate the interfacial shear strengths in addition to the bench test. After serial sectioning the samples in discs, push-out tests were made to evaluate the interfacial strengths of cement-bone-implant. The weakest interfacial shear strength was exhibited by the polished rod/cement interface (0.5 MPa) while the strongest was the "old" and "new" cement interface (23.4 MPa). The bone/cement interfacial strength was in between for in vitro (1.17 MPa) and in vivo (1.68 MPa). The shear strength of rod/cement interface increased substantially by sandblasting (6.84 MPa). The microscopic observation of the interface showed somewhat smaller gaps developed for the pre-coated rod than the uncoated rod due to the shrinkage effect. In addition to the overall increase in interfacial strength, the pre-coating may furthermore reduce the setting temperature, the shrinkage, and the amount of monomer released during operation due to the reduced amount of cement at the time of implantation. The more gradual transmission of load from implant to bone and "auto-centering" of implants during operation by pre-coating, are believed to be advantageous over conventional cement fixation method.

Intramedullary fixation of implants pre-coated with bone cement: a preliminary study

A preliminary study on the feasibility of implants pre-coated with an acrylic bone cement has been performed. Four types of implants, an actual canine femoral prosthesis, a polished steel rod (0.49 cm dia. x 13 cm long) with and without pre-coating, and a sand-blasted steel rod with pre-coating were implanted into canine femurs in vitro to evaluate the interfacial shear strengths. After serial sectioning the samples in discs, push-out tests were made. The weakest interfacial shear strength was exhibited by the polished rod/cement interface (0.5 MPa) while the strongest was the "old" and "new" cement interface (23.4 MPa). The bone/cement interfacial strength was in between (1.17 MPa). The shear strength of rod/cement interface increased substantially by sand-blasting (6.84 MPa or 585% increase). The proposed method increases the modified implant's interfacial shear strength by 337% (from 1.17 to 3.84 MPa) over the conventional implants. It may furthermore reduce the setting temperature, the shrinkage, and the amount of monomer released during operation due to the reduced amount of cement at the time of implantation. The more gradual transmission of load from implant to bone and "auto-centering" of implants during operation by pre-coating, are thought to be advantageous over conventional cement fixation method.

Spinal fixation using acrylic bone cement: mechanical property measurements

Following dorsal laminectomies (L2-L3), the resultant spinal instabilities were stabilized by placing four Steinmann pins and embedding them in acrylic bone cement. The cement was molded about the instability and incorporated the pins and articular process. Mechanical testing was performed to evaluate the strength of the resultant union between the two vertebrae. Three types of samples, in vivo, in vitro and normal were tested using a specially built apparatus. The results showed average maximum loads of 150, 180 and 180 Newtons respectively which represent 29, 42 and 37 MPa in calculated shear stresses. The results indicate that the use of bone cement with pins can stabilize the injured vertebral discs easily iwth the maximum load or shear strength equivalent to that of normal vertebral discs.

Dental implant fixation by electrically mediated process. I. Interfacial strength

In order to determine the effect of electrical stimulation on canine alveolar bone, porous PMMA dental implants with a solid core (on which a Pt-13% Rh electrode was wound) were implanted in the mesial socket of the canine mandibular fourth premolars bilaterally. The positive electrode was implanted into the distal socket. The power pack was placed over the masseteric fossa. The implants, wires and power packs were all implanted subcutaneously. Each animal had an experimental and control implant. Mechanical push-out samples were prepared by sectioning a 2mm thick section of the mandible with the implant in the middle. The samples were tested immediately and the load-deflection curves were obtained.

Dental implant fixation by electrically mediated process. II. Tissue ingrowth

The effect of electrical stimulation on the interfacial strength of the porous polymethylmethacrylate implant/oral tissue union and the amount of tissue growth was investigated in the fourth premolars of dogs. The study indicates the interfacial strength peaks at about three weeks and decreases thereafter for both control and the stimulated specimens. The stimulated side showed consistently higher strength than its paired control. There was a positive relationship between implant period and amount of tissue in the pores although the latter was not correlated with the interfacial strength. Microradiographs showed a different pattern of new bone formation on the stimulated side when compared to the control. On both sides, bone formation occurred upward from the bottom of the tooth socket while on the stimulated side, new bone also developed from the sides of the tooth socket which was minimal in the controls. It is proposed that the direction of oral tissue formation is responsible for the different results obtained in this study compared with a similar study on long bones.

Mechanical property changes of barium titanate (ceramic) after in vivo and in vitro aging

Since barium titanate (BaTi03) can be made piezoelectric, it may be used to substitute hard tissues directly. As a first step in testing this concept, a series of in vivo and in vitro aging and biocompatibility studies were performed. The mean compressive strength of samples implanted subcutaneously in the backs of rabbits decreased to 138 MPa after 20 weeks from a control value of 281 MPa. Similar, though less drastic losses of strength were seen when specimens were aged in distilled water (182 MPa at 28 weeks) and Ringer's solution (159 MPa at 28 weeks). The most rapid decrease of strength in all cases was seen prior to 4 weeks. Thereafter, the decrease was much slower. Histological evaluation of the tissue surrounding the implant revealed a thin fibrous capsule and no evidence of tissue inflammation.

Sequential enzymolysis of human aorta and resultant stress-strain behavior

The human aorta has five major components from which the aortic walls can be characterized: mucopolysaccharides, smooth muscle, collagen, micro-fibrilar glycoprotein (associated with the elastic fiber), and elastin. Enzymes were employed to remove four of the components sequentially without destroying the mechanical characteristics of the remaining components in order to elucidate the structure-property relationship in the human aorta. Before treatment the initial mechanical behavior was recorded on an Instron Tensile testing machine. After enzymolysis the samples and controls were again tested and these results compared to their prior characteristics. Stress-strain characteristics after a sequence of enzyme treatments indicate that two of the components share the major part of the stress in the circumferential direction. These components, elastin and collagen, contribute as if they were in parallel to each other with the collagen in a crimped state.

Effect of electrical stimulation on the interfacial tensile strength and amount of bone formation

The effect of electrical stimulation upon the direct tensile strength of the interface union between porous calcium aluminate implants and bone plus the amount of bone formation were investigated in the femurs of rabbits. The study indicates there is an increased tensile strength of the interface in proportion to the amount of bone formation into the pores and the amount of electrical stimulation used "in vivo."

Effect of electrical stimulation on the tensile strength of the porous implant and bone interface

The effect of electrical stimulation upon the direct tensile strength of the interfacial union between porous calcium aluminate implants (100 to 200 mu diameter pores) and bone was studied in the femurs of rabbits. After about 4 weeks of implantation the tnesile strength of the electrically stimulated specimens was approximately two times that of the nonstimulated ones. This indicates that electrical stimulation increased the rate of new bone formation under the experimental conditions.