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

Does the Effectiveness and Mechanical Strength of Kanamycin-Loaded Bone Cement in Musculoskeletal Tuberculosis Compare to Vancomycin-Loaded Bone Cement

BACKGROUND

Antibiotic-loaded bone cement (ALBC) is used to deliver antimycobacterial agents into the focal lesion of musculoskeletal tuberculosis. Although kanamycin is currently used as an antimycobacterial agent for the treatment of multidrug-resistant tuberculosis, there is no information about its suitability in ALBC.

METHODS

An in vitro experiment was conducted with cylindrical shape of 40 g of bone cement with 1, 2, and 3 g of kanamycin. Eluate (1 mL) was extracted from each specimen to measure the level of elution and antimycobacterial activity on days 1, 4, 7, 14, and 30. The quantity of kanamycin in eluates was evaluated by a liquid chromatography-mass spectrometry system, and the antimycobacterial activity of eluates against Mycobacterium tuberculosis H37Rv was calculated by comparing the minimal inhibitory concentration. The ultimate compression strength was conducted using a material testing system machine (Instron 3366; Instron, Norwood, MA) before and after elution.

RESULTS

Eluates from ALBC containing 2 and 3 g of kanamycin had effective antimycobacterial activity for 30 days, whereas eluates from ALBC containing 1 g of kanamycin were partially active until day 30. The pre-eluted compression strength of kanamycin-loaded cement and vancomycin-loaded cement was weaker as they contained a larger amount of antibiotics. There was no statistical difference between the strength of all kanamycin regimens and 1 g of vancomycin in the ultimate compression test. After 30 days of elution, the strength of all kanamycin-loaded cement and vancomycin-loaded cement cylinders was significantly lower than that of initial specimens (P < .05).

CONCLUSION

The antimycobacterial activity of ALBC containing more than 2 g of kanamycin was effective during a 30-day period. The ultimate compression strength of bone cement loaded with 1-3 g of kanamycin was comparable with 1 g of vancomycin while maintaining effective elution until day 30.

Elution and Mechanical Strength of Vancomycin-Loaded Bone Cement: In Vitro Study of the Influence of Brand Combination

Antibiotic-loaded bone cement (ALBC) is widely used in orthopaedic surgery for both prevention and treatment of infection. Little is known about the effect of different brand combinations of antibiotic and bone cement on the elution profile and mechanical strength of ALBC. Standardized specimens that consisted of one of the 4 brands of bone cement and one of the 3 brands of vancomycin were fashioned, producing 12 combinations of ALBC. Two dosages of vancomycin in 40g bone cement were used to represent the high (4g vancomycin) and low (1g vancomycin) dose groups. Concentrations of vancomycin elution from ALBC was measured for up to 336 hours. The ultimate compression strength was tested at axial compression using a material testing machine before and after elution. In both high-dose and low-dose groups, Lyo-Vancin in PALACOS bone cement resulted in the highest cumulative elution and Vanco in Simplex P bone cement resulted in the lowest elution (458% and 65% higher in high- and low-dose groups, respectively). The mechanical strength was not significantly compromised in all groups with low dose vancomycin (range: 70.31 ± 2.74 MPa to 87.28 ± 8.26MPa after elution). However, with the addition of high dose vancomycin, there was a mixed amount of reduction in the ultimate compression strength after cement aging, ranging from 5% (Vanco in Simplex P, 81.10 ± 0.48 MPa after elution) to 38% (Sterile vancomycin in CMW, 60.94 ± 5.74 MPa after elution). We concluded that the selection of brands of vancomycin and bone cement has a great impact on the release efficacy and mechanical strength of ALBC.

Biocompatibility and biodegradation studies of PCL/β-TCP bone tissue scaffold fabricated by structural porogen method

Three-dimensional printer (3DP) (Z-Corp) is a solid freeform fabrication system capable of generating sub-millimeter physical features required for tissue engineering scaffolds. By using plaster composite materials, 3DP can fabricate a universal porogen which can be injected with a wide range of high melting temperature biomaterials. Here we report results toward the manufacture of either pure polycaprolactone (PCL) or homogeneous composites of 90/10 or 80/20 (w/w) PCL/beta-tricalcium phosphate (β-TCP) by injection molding into plaster composite porogens fabricated by 3DP. The resolution of printed plaster porogens and produced scaffolds was studied by scanning electron microscopy. Cytotoxicity test on scaffold extracts and biocompatibility test on the scaffolds as a matrix supporting murine osteoblast (7F2) and endothelial hybridoma (EAhy 926) cells growth for up to 4 days showed that the porogens removal process had only negligible effects on cell proliferation. The biodegradation tests of pure PCL and PCL/β-TCP composites were performed in DMEM with 10 % (v/v) FBS for up to 6 weeks. The PCL/β-TCP composites show faster degradation rate than that of pure PCL due to the addition of β-TCP, and the strength of 80/20 PCL/β-TCP composite is still suitable for human cancellous bone healing support after 6 weeks degradation. Combining precisely controlled porogen fabrication structure, good biocompatibility, and suitable mechanical properties after biodegradation, PCL/β-TCP scaffolds fabricated by 3DP porogen method provide essential capability for bone tissue engineering.

Factors affecting flexural strength in cement within cement revisions

Cement within cement revisions provide substantial benefits for conventional revision yet remains uncommon possibly because of the perceived weakness of the cement-cement interface. This study investigated the flexural strength of beams composed of 2 different cements, exploring the factors of pore size, fracture location, viscosity, and the surface roughness of the interface. We found no significant difference when comparing combinations of different cements (P = .30), varying pore sizes (P = .13), or surface roughness (P = .39). Differences in fracture locations and viscosity combinations approached statistical significance (P = .08 and .05, respectively). Our findings suggest strong bonding between cements at the interface, with other factors being more important causes of weakness. Thus, we recommend that the strength of the cement-cement interface should not be a factor when considering such revisions.

The long-term in vivo behavior of polymethyl methacrylate bone cement in total hip arthroplasty

BACKGROUND AND PURPOSE

The long-term success of cemented total hip arthroplasty (THA) has been well established. Improved outcomes, both radiographically and clinically, have resulted mainly from advances in stem design and improvements in operating techniques. However, there is concern about the durability of bone cement in vivo. We evaluated the physical and chemical properties of CMW1 bone cements retrieved from patients undergoing revision THA.

METHODS

CMW1 cements were retrieved from 14 patients who underwent acetabular revision because of aseptic loosening. The time in vivo before revision was 7-30 years. The bending properties of the retrieved bone cement were assessed using the three-point bending method. The molecular weight and chemical structure were analyzed by gel permeation chromatography and Fourier-transform infrared spectroscopy. The porosity of the bone cements was evaluated by 3-D microcomputer tomography.

RESULTS

The bending strength decreased with increasing time in vivo and depended on the density of the bone cement, which we assume to be determined by the porosity. There was no correlation between molecular weight and time in vivo. The infrared spectra were similar in the retrieved cements and in the control CMW1 cements.

INTERPRETATION

Our results indicate that polymer chain scission and significant hydrolysis do not occur in CMW1 cement after implantation in vivo, even in the long term. CMW1 cement was stable through long-term implantation and functional loading.

Eugenol derivatives immobilized in auto-polymerizing formulations as an approach to avoid inhibition interferences and improve biofunctionality in dental and orthopedic cements

Auto-polymerizing formulations based on poly(ethyl methacrylate) (PEMA) and eugenol derivatives are reported for dental and orthopedic applications. Spherical beads of PEMA were used as the pre-polymer powder and mixed with combinations of ethyl methacrylate and eugenyl methacrylate (EgMA) or ethoxyeugenyl methacrylate (EEgMA). A range of concentrations from 10 to 30 wt.% of EgMA or EEgMA were used to impart bioactivity properties to the cements. Increasing concentrations of the eugenol derivatives decreased the maximum polymerization temperatures from 69 to 37 degrees C without altering the working or setting time. At concentrations of 10 and 15 wt.% of EgMA or EEgMA a noticeable increase in the compressive (8%), flexural (40%) and tensile (24%) strengths were recorded in comparison to the control cements containing PEMA/ethyl methacrylate only. In addition to the improvement in mechanical properties the cements yielded a slightly crosslinked network due to the participation of the allylic group present in the eugenol derivatives, the presence of which has an intrinsically bactericidal effect against Escherichia coli and Streptococcus mutans strains, as reported in a previous study, thus enhancing the properties of the cements.

In vitro evaluation of orthopedic composite cytotoxicity: assessing the potential for postsurgical production of hydroxyl radicals

Hydroxyl radical (*OH)-induced inflammation is a primary mode for in vivo cytotoxicity. A legitimate concern is whether particulate wear debris from orthopedic composites can stimulate inflammation via ferrous ion (Fe2+)-mediated production of *OH. The purpose of this research was to utilize electron paramagnetic resonance (EPR) spin trapping in investigating and comparing the potential for postsurgical cytotoxicity induced specifically by *OH in the presence of two composites: Simplex P and the novel, hybrid, CORTOSS. Cytotoxicity is evaluated based on the composites competitively chelating catalytic Fe2+ or readily reducing ferric ions (Fe3+), in facilitating the Fenton reaction (FR). *OH that are produced were then validated by a radical scavenger to confirm a genuine radical signal and mechanism. Spin adduct peak areas decreased in the presence of CORTOSS as opposed to increasing in the presence of Simplex P, evaluated against their respective controls. A plausible theory elucidating this finding is that CORTOSS may sequester the Fe form, by virtue of its monomers. Principally, direct comparison of composites indicated that Simplex P had greater tendency to produce *OH, yielding 25.6 and 48.7% greater spin adduct peak areas when chelated and non-chelated Fe2+ are used, respectively. Moreover, the rate of FR accelerated when chelated Fe2+ was used, leading to the formation of a ternary complex with the composites. This was more prominent in Simplex P, as coordination of chelated Fe2+ occurs on its surface via an electrostatic attraction to allow a seventh coordination site for ligand exchange in the ternary complex, stabilized by Ba2+. Conversely, the silica found in CORTOSS possesses radical quenching abilities that deactivate generated *OH in impeding the efficiency of FR. Neither composite demonstrated a capacity to readily reduce Fe3+ to the relevant Fe2+, as validated by a non-radical pathway. Instead, the artificial spin adduct signal attained when employing chelated Fe3+ was due to the nucleophilic addition of water onto DMPO. Simplex P may also serve as a template for surface catalysis of the nucleophilic addition of water onto DMPO involving chelated Fe3+. CORTOSS is thought not to induce cytotoxicity, whereas the propensity of Simplex P in promoting Fenton chemistry is a serious issue that must be addressed.

Histologic changes after vertebroplasty

BACKGROUND

Vertebroplasty with use of polymethylmethacrylate cement is gaining popularity in the treatment of some specific painful lesions of the spine. It remains unclear, however, what possible side effects this type of cement might have upon the vertebral body. We performed a histologic and radiographic analysis of the end plate and disc to determine whether there was a difference between vertebroplasty with polymethylmethacrylate cement and vertebroplasty with calcium phosphate cement in the surrounding tissue of the goat spine. Furthermore, we assessed whether a defect in the end plate, simulating end-plate fracture and allowing for direct contact of cement with disc tissue, had any effect on end-plate or disc degeneration.

METHODS

Twenty-four mature goats were divided between two follow-up periods (six weeks and six months). All animals underwent a bilateral transpedicular vertebroplasty at two lumbar levels, where one of the following treatments was applied: vertebroplasty with calcium phosphate cement with or without an end-plate defect, and vertebroplasty with polymethylmethacrylate cement with or without an end-plate defect. The effect of the various treatments on the integrity of the intervertebral disc, end plate, and surrounding tissue was examined with semiquantitative histologic analysis and radiography.

RESULTS

No sign of disc or end-plate degeneration was seen in any of the analyzed sections. The mean disc height did not decrease from the postoperative period to the time that the animals were killed in any group, thereby supporting the histologic findings. A mild inflammatory reaction was found in four vertebral bodies in the polymethylmethacrylate groups only.

CONCLUSIONS

Calcium phosphate cement and polymethylmethacrylate cement both seem to be adequate bone-void fillers in terms of biological behavior in the vertebral body.

Effect of physiological temperature on the mechanical properties and network structure of biodegradable poly(propylene fumarate)-based networks

Poly(propylene fumarate) (PPF)-based networks have exhibited increases in mechanical properties during their initial stages of degradation. This study was designed to investigate whether physiological temperatures are the source of this reinforcing behavior by influencing the formation of additional crosslinks within the network. Utilizing a model PPF network formed with the crosslinking agent poly(propylene fumarate)-diacrylate (PPF-DA), cylindrical specimens were stored in an inert environment and conditioned at -20 and 37 degrees C while their mechanical properties and network structure were monitored over a six week period. The PPF/PPF-DA specimens exposed to physiological temperatures showed an increase in compressive modulus from 1674 +/- 88 to 2059 +/- 75 MPa. The double bond conversion improved as well, from 64 +/- 1 to 70 +/- 1%, indicating that crosslinks were being formed in the network. The additional reactivity occurred exclusively with unreacted fumarate bonds. PPF/PPF-DA networks stored at -20 degrees C showed no changes in mechanical properties; however, they increased when subsequently conditioned at 37 degrees C. The results were used to explain that PPF-based networks undergo a biphasic degradation behavior due to the competing hydrolytic degradation and thermal induced crosslinking. In addition, heat treating the networks at higher temperatures can be utilized as a means to further reinforce PPF-based materials.

Effects of the initial temperature of acrylic bone cement liquid monomer on the properties of the stem-cement interface and cement polymerization

It has been shown that preheating the femoral stem prior to insertion minimizes interfacial porosity at the stem-cement interface. In this study, the effects of methylmethacrylate monomer temperature prior to mixing on the properties of stem-cement interface and cement polymerization were evaluated for 4 degrees C, room temperature, and 37 degrees C using a test model and cementing techniques that simulated a clinical situation. The nature and extent of interfacial porosity of stem-cement interface was quantified, the static shear strength of the stem-cement interface determined, and the time and temperature of polymerization at the cement-bone interface were measured. Compared to RT monomer, preheating monomer to 37 degrees C produced higher polymerization temperatures and greater initial interfacial shear strength with an unchanged amount of interfacial porosity. Precooling monomer to 4 degrees C produced lower polymerization temperatures and decreased initial interfacial shear strength, with the amount of interfacial porosity unchanged compared to the RT group. Although clinical techniques of preheating or precooling bone cement have some effects on the properties of the stem-cement interface and cement polymerization, they do not appear to enhance implant fixation.

Effects of preheating of hip prostheses on the stem-cement interface

BACKGROUND

Debonding of the cement from metal implants has been implicated in the loosening of cemented total hip prostheses. Strengthening of the stem-cement interface has been suggested as a way to prevent loosening of the component. Previously, it was reported that preheating the stem to 44 degrees C reduced the porosity of the cement at the stem-cement interface. The purpose of this study was to determine the effect of stem preheating on the characteristics of the stem-cement interface.

METHODS

The effects of stem preheating, at temperatures of 37 degrees C, 44 degrees C, and 50 degrees C, on the stem-cement interface were studied in a test model and a preparation that closely simulated the clinical situation. Static interface strength was determined initially and after the stems had been kept in isotonic saline solution at 37 degrees C for two weeks. Fatigue lifetimes were measured, and the nature and extent of porosity at the interface were quantified.

RESULTS

Stem preheating had significant effects on the stem-cement interface. Stems preheated to 37 degrees C had greater interface shear strength than stems at room temperature both initially (53% greater strength) and after simulated aging (155% greater strength). Fatigue lifetimes were also improved, and there was a >99% decrease in interface porosity. The setting time of the cement decreased 12%, and the maximum temperature at the cement-bone interface increased 6 degrees C. Similar effects were found after preheating to 44 degrees C and 50 degrees C.

CONCLUSIONS

Stem preheating had significant effects on the stem-cement interface, with significant improvements in the shear strength and cement porosity of the interface. Also, polymerization temperatures at the cement-bone interface increased. The possible biological effects of these increased interface temperatures at the cement-bone interface require further study.

Noninvasive in vivo EPR monitoring of the methyl methacrylate polymerization during the bone cement formation

The curing of poly(methyl methacrylate) (PMMA) bone cement is done by a free radical polymerization. As the amount of free radicals present is a marker of the amount of unpolymerized chains present in the polymer, it is assumed that this could be related to the mechanical properties such as strength or density. In this study, the direct observation of the free radicals produced during the PMMA bone cement formation was obtained for the first time in vivo using low-frequency EPR spectrometers (1.2 GHz). Low frequency permits measurements in live animals due to the increased microwave penetration. The amount of polymerization radicals was carried out noninvasively over days on the same animals. The decay rates obtained in vitro and in vivo were compared: the decay rates were significantly lower when the curing process occurred in vivo compared to the situation in vitro. As the kinetics are rather different in vitro and in vivo, this emphasizes the value of the present method that permits the noninvasive monitoring of the curing process directly in vivo.

The science of bone cement: a historical review

The use of PMMA bone cement has been a key factor in the advent of joint replacement as a surgical option. Despite revolutionary changes in joint replacement technology for the treatment of hip and knee arthritis, the use of PMMA bone cement in its intraoperative application has not significantly changed since Harris' description of third generation cement technique. Future answers to questions regarding cemented implant longevity may lie in the further improvement of existing PMMA technology and standardization of the manufacturing of PMMA bone cement intraoperatively.

New acrylic bone cements conjugated to vitamin E: curing parameters, properties, and biocompatibility

Acrylic bone cement formulations with antioxidant character were prepared by incorporation of a methacrylic monomer derived from vitamin E (MVE). Increasing concentrations of this monomer provided decreasing peak temperature values, ranging from 62 to 36 degrees C, and increasing setting time with values between 17 and 25 min. Mechanical properties were evaluated by compression and tension tests. Compressive strength of the new formulations were superior to 70 MPa in all cases. The cement containing 25 wt % MVE, however, showed a significant decrease in tensile properties. Biocompatibility of the new formulations was studied in vitro. The analysis of the effect of leachables from cements into the media showed continued cell proliferation and cell viability with a significant increase for the cement containing 15 wt % MVE. This formulation also showed a significant increase in cellular proliferation over a period of 7 days as indicated by the Alamar Blue test. The cells were able to differentiate and express phenotypical markers in presence of all materials. A significant increase in alkaline phosphatase activity was observed on the cements prepared in presence of 15-25 wt % MVE compared with PMMA. Morphological assessment showed that the human osteoblast (HOB) cells were able to adhere, retain their morphology, and proliferate on all the cements.

Free radicals and side products released during methylmethacrylate polymerization are cytotoxic for osteoblastic cells

Polymerization of orthopedic cements makes use of a peroxide initiator which is decomposed by an accelerator to provide free radicals. Free radicals which act on the monomer molecules are also known to induce cell lesions and cell death. We used an in vitro model of cement polymerization to study the effects of free radicals release on osteoblast-like cells. Initiation of methylmethacrylate was done with benzoyl peroxide and acceleration by N,N-dimethylaniline. Bulk polymerization was done in calibrated test tubes which were left aging until use. Polymers (aged from J1 to J31 days after completion of the polymerization process) were sawed to produce slices. Slices were rinsed in distilled water and free radical release was measured by spectrophotometric titration with p-iodonitrotetrazolium. Saos-2 osteoblast-like cells were cultured in parallel on the slices. Cells appeared to be round and were altered when grown on slices prepared freshly after polymerization. Cytomorphometric analysis of the cell shape (surface area and form-factor polyethylene confirmed that they spread and flatten on slices prepared a long time after polymerization. Free radical release from polymethylmethacrylate cements is a long-lasting event that can induce bone cells alterations in their neighborhood. Two cytotoxic mechanisms were evidenced: (a) polymer slices released a stable toxic component which could be removed by extensive washing; (b) they released free radicals which were still detectable several days after the end of polymerization. The titration curve was a negative exponential.

Detrimental effect of aging on the endurance of bone cement. An in vivo and in vitro study in rabbits

We investigated the effect of aging on the compressive strength of bone cement under in vivo and in vitro conditions. Cement molds were implanted in the dorsum of rabbits and other molds were kept under stable conditions and in darkness. Comparative measures taken at 15 days and 1, 3, 6, 12 and 24 months showed lower endurance of the implanted molds (p < 0.001). A reactive capsule surrounded the bone cement in vivo up to the 3rd month, its cellularity increased, and then almost disappeared by 1 year. Macrophages and foreign body cells reappeared at 2 years, indicating a "chemical aging" effect in the in vivo environment. Our findings suggest that aging may play an important role in the amelioration of the mechanical properties of bone cement.

Polymerization kinetics, glass transition temperature and creep of acrylic bone cements

Sulfix-6 and Zimmer LVC 60/30 bone cements were selected and the polymerization kinetics and resulting glass transition temperature Tg; creep behaviour in the dry or water-saturated state; and sorption and diffusion of water were studied. The calculation of conversion was based on a comparison of the residual polymerization heat measured by differential scanning calorimetry and the corresponding theoretical value. The conversion reached 99% after 90 min of quasi-adiabatic polymerization starting at 23 degrees C or after 10 min of isothermal polymerization at 37 degrees C. The Tgs of the cements prepared in the former way were about 82 and 100 degrees C, respectively. Creep rate of the bone cements at 37 degrees C decreased with the time of creeping. Sorbed water enhanced the compliance, but reduced the creep rate for long times so that water sorption during the service time may not have detrimental effects on the creep resistance of the cements. Both types of cements contained about 1% of low molar mass substances extractable by water. Measurements of the sorption kinetics of water showed that the diffusion coefficient is 0.14 x 10(-11) and 0.22 x 10(-11) m2/s and 1 yr sorption achieves 2.11% and 2.89% for Sulfix and Zimmer, respectively.

An in vitro study of the effect of environment and storage time on the fracture properties of bone cement

Changes either within the bone cement or at the cement-bone interface are known to contribute to loosening and hence failure of many cemented joint replacements. This study examines the in vitro changes in the fracture properties of bone cement as a result of storage, at both 21 and 37 degrees C, in air, water, Ringer's solution and lipid over a period of 2 years. Specimens stored in the fluid media were found to behave in a more ductile manner than those stored in air. Samples stored at 37 degrees C behaved in a more brittle manner than those stored at 21 degrees C. Although the work of fracture values measured for the samples stored in the water-based media increased during the first 18 months, this was followed by a decrease in the subsequent 6 months.

The influence of curing time and environment on the fracture properties of bone cement

Fracture of bone cement at the bone-cement interface is considered to be of significance in the aseptic loosening of orthopaedic implants. The characterisation of the fracture properties of bone cement is influenced by the time and environment in which it is cured. Cement samples stored in air and water at 21 and 37 degrees C for 7 and 21 days were tested using the 'Chevron' test to determine the work of fracture. It was found that the storage temperature and environment had important influences on the fracture resistance of bone cement. In a physiological environment cement appears to take longer to attain a fracture resistance equivalent to that of cement stored at room temperature.