Acrylic bone-cement. Influence of mixer design and unmixed powder

Accuracy Assessment of Molded, Patient-Specific Polymethylmethacrylate Craniofacial Implants Compared to Their 3D Printed Originals

The use of patient-specific implants (PSIs) in craniofacial surgery is often limited due to a lack of expertise and/or production costs. Therefore, a simple and cost-efficient template-based fabrication workflow has been developed to overcome these disadvantages. The aim of this study is to assess the accuracy of PSIs made from their original templates. For a representative cranial defect (CRD) and a temporo-orbital defect (TOD), ten PSIs were made from polymethylmethacrylate (PMMA) using computer-aided design (CAD) and three-dimensional (3D) printing technology. These customized implants were measured and compared with their original 3D printed templates. The implants for the CRD revealed a root mean square (RMS) value ranging from 1.128 to 0.469 mm with a median RMS (Q1 to Q3) of 0.574 (0.528 to 0.701) mm. Those for the TOD revealed an RMS value ranging from 1.079 to 0.630 mm with a median RMS (Q1 to Q3) of 0.843 (0.635 to 0.943) mm. This study demonstrates that a highly precise duplication of PSIs can be achieved using this template-molding workflow. Thus, virtually planned implants can be accurately transferred into haptic PSIs. This workflow appears to offer a sophisticated solution for craniofacial reconstruction and continues to prove itself in daily clinical practice.

Critical evaluation of pulse-echo ultrasonic test method for the determination of setting and mechanical properties of acrylic bone cement: influence of mixing technique

Currently there is no reliable objective method to quantify the setting properties of acrylic bone cements within an operating theatre environment. Ultrasonic technology can be used to determine the acoustic properties of the polymerising bone cement, which are linked to material properties and provide indications of the physical and chemical changes occurring within the cement. The focus of this study was the critical evaluation of pulse-echo ultrasonic test method in determining the setting and mechanical properties of three different acrylic bone cement when prepared under atmospheric and vacuum mixing conditions. Results indicated that the ultrasonic pulse-echo technique provided a highly reproducible and accurate method of monitoring the polymerisation reaction and indicating the principal setting parameters when compared to ISO 5833 standard, irrespective of the acrylic bone cement or mixing method used. However, applying the same test method to predict the final mechanical properties of acrylic bone cement did not prove a wholly accurate approach. Inhomogeneities within the cement microstructure and specimen geometry were found to have a significant influence on mechanical property predictions. Consideration of all the results suggests that the non-invasive and non-destructive pulse-echo ultrasonic test method is an effective and reliable method for following the full polymerisation reaction of acrylic bone cement in real-time and then determining the setting properties within a surgical theatre environment. However the application of similar technology for predicting the final mechanical properties of acrylic bone cement on a consistent basis may prove difficult.

The condition of the cement mantle in femoral hip prosthesis implantations--a post mortem retrieval study

Despite numerous studies demonstrating the characteristics of the optimal cement mantle in joint replacement, the clinical state of the cement mantle is rarely assessed. A random sample of 214 cemented implanted femoral hip components was retrieved post mortem from Hamburg, Germany, and sectioned to investigate the quality of the cement mantle. The most common observation made in at least one measured region per retrieval was debonding (82% of stems), followed by a thin cement mantle (74%), stem-bone contact (48%), soft tissue at the stem interface (44%), no cement-bone interdigitation (30%), a gap at the stem interface (28%), voids in the cement (22%) and cracks and blood in the cement mantle (<10%). 21% of stems demonstrated complete debonding of the interface. However, distributions of all other defects were local, with less than 10% of stems demonstrating any imperfection in more than 21% of the regions assessed. No progressive damage was observed with implantation duration. The results suggest that current implantation technique may be adequate for proper implant function over the service life in the older patient population. However, for younger and more active patients, perfection of the cementation technique is crucial, particularly in modern implant systems such as resurfacing. The frequency of almost all defects could be further reduced by careful implantation technique, providing the increased service life necessary for the ever younger, more physically demanding, patient population.

Comparison of various vacuum mixing systems and bone cements as regards reliability, porosity and bending strength

BACKGROUND

There are several vacuum mixing systems on the market which are arbitrarily used with various bone cements in clinical work. Hardly any studies have been done on the performance and handling of these systems in combination with different cement brands.

MATERIAL AND METHODS

We therefore tested 6 vacuum mixing systems (Palamix, Summit, Cemvac, Optivac, Vacumix, MixOR) in combination with 6 cement brands (Palacos R, Simplex P, CWM 1, CWM 2000, Palamed G, VersaBond) concerning their reliability, user-friendliness, porosity and bending strength.

RESULTS

Our study indicated that each system has weak points. The preparation of the mixed cement for gun injection can present problems. If cement collection under vacuum fails, porosity is increased. Manual collection without a vacuum carries the risk of intermixing air. For comfortable and effective retrograde cement application, cement guns should have a stable connection with the cartridge and a high piston stroke. There are marked differences between the systems as regards overall porosity when all tested cements are considered (range 2-18%), and between the cements when all tested systems are considered (range 2-17%). All test samples exceeded the required bending strength of 50 MPa, according to ISO 5833. Palaces specimens showed excessive plastic deformation in the bending test.

INTERPRETATION

There are better and worse mixing system/cement combinations for a given system and a given cement. Systems with cement collection under vacuum reduce porosity best.

The relationship between porosity and fatigue characteristics of bone cements

In this study, the fatigue strengths of acrylic cement prepared by various commercially available reduced pressure mixing systems were compared with the fatigue strength of cement mixed by hand (control) under atmospheric conditions. The following observations were made from this investigation. The mean fatigue strength of reduced pressure mixed acrylic bone cement is double that of cement mixed by hand using an open bowl, 11,354+/-6,441 cycles to failure for reduced pressure mixing in comparison with 5,938+/-3,199 cycles for mixing under atmospheric conditions. However, the variability in mean fatigue strengths of reduced pressure mixed bone cement is greater for some mixing devices. The variation in fatigue strengths for the different mixing techniques is explained by the different porosity distributions. The design of the reduced pressure mixing system and the technique employed during mixing strongly contribute to the porosity distribution within the acrylic bone cement. The level of reduced pressure applied during cement mixing has an effect on the fatigue strength of bone cement, but the mixing mechanism is significantly more influential.

Mixing systems and the effects of vacuum mixing on bone cement

The aim of a good cement mix is to produce bone cement that has the best mechanical properties possible in order that it can carry out its load transfer role successfully over the lifetime of the implant. The data presented shows that vacuum mixing reduces the cement porosity, which results in improved strength, and resistance to creep deformation and fatigue failure in the bone cement. It seems, however, that eliminating a high degree or all of the cement porosity may be detrimental because it leads to greater shrinkage and cracking in the material. A moderate vacuum level will improve the mechanical properties, but reduce the risk of thermal shrinkage and cracking which is seen at higher vacuum levels. In addition, the mixer design has a significant influence on the quality of the cement produced, affecting porosity, unmixed powder and subsequently the mechanical properties of the material. In the next issue we will be concluding this series of articles by covering the importance of temperature and its effects on the phases of the cement polymerisation process. Bone cement training courses are being run within hospitals by Summit Medical as part of our commitment to enhancing the skills of the perioperative practitioner and to ensuring the best long-term outcome for the patient.

Effect of mixing technique on the properties of acrylic bone-cement: a comparison of syringe and bowl mixing systems

Syringe mixing systems have been introduced, but few data exist regarding the mechanical performance of cement they produce. We compared the properties of polymethyl methacrylate cement produced by these systems with that produced by a multiaxial bowl. Mixtures of cement were prepared using the Optivac, Cemvac, and Summit syringes and the Summit bowl. The mixtures were cured in molds to create casts that were radiographed and analyzed for void content, then cut into strips, weighed, measured, and tested to failure in 4-point bending. Syringe-mixed cement was of greater density, bending modulus, and bending strength than bowl-mixed cement (Mann-Whitney, P < .01) and contained fewer microvoids and macrovoids (Mann-Whitney, P < .01). No significant differences between the syringes were found for these variables (Kruskall-Wallis, P > .05).

Correlation between impact strength and fracture toughness of PMMA-based bone cements

The thrust of the work involved determining the impact strength, IS (in kJ m(-2)) [using non-ASTM-sized Charpy-type specimens and an in-house impact tester] and fracture toughness, K1c (in MPa square root(m)) [using ASTM-sized rectangular compact tension specimens] of Surgical Simplex P and three variants of Palacos R acrylic bone cements. The attractions and drawbacks of this method for determining IS are detailed. The best fit to the K1c and IS results yielded a power relationship K1c = 0.795(IS)(0.59). The usefulness and limitations of this relationship are detailed.