Jeffrey P. Davies

The effect of centrifuging bone cement

We have tested the porosity and fatigue life of five commonly used bone cements: Simplex P, LVC, Zimmer regular, CMW and Palacos R. Tests were conducted with and without centrifugation and with the monomer at room temperature and, except for LVC, at 0 degrees C. We found that the fatigue life of different specimens varied by a factor of nearly 100. It did not depend on porosity alone, but was more influenced by the basic composition of the cement. Simplex P when mixed with monomer at 0 degrees C and centrifuged for 60 seconds had the highest fatigue life and was still sufficiently liquid to use easily.

Porosity of various preparations of acrylic bone cements.

The total porosity and mean pore sizes of various bone cement preparations were measured using image analysis. The porosity in different commercial bone cements varied from 5% to 16% when these cements were prepared in the usual fashion. Centrifugation for 30 seconds resulted in a substantial reduction in the overall porosity of Simplex P, AKZ, Zimmer Regular, and CMW bone cements by reducing both the mean pore size and the number of pores per unit area. In contrast, the porosity of LVC, Palacos R, and Palacos R with gentamicin bone cements was not significantly decreased by centrifugation. Chilling the monomer before mixing resulted in higher porosity of both the centrifuged and uncentrifuged Simplex P, Zimmer Regular, and CMW bone cements. Simplex P mixed with chilled monomer and centrifuged for 120 seconds has one of the lowest porosities of the various cements, while retaining good handling characteristics and excellent fatigue strength.

Optimization and comparison of three vacuum mixing systems for porosity reduction of simplex P cement

Simplex P bone cement was prepared in three commercially available vacuum mixing systems, the Enhancement Mixer, the Mixevac II High Vacuum System, and the Mitab Vacuum System, to determine the improvement in fatigue strength associated with porosity reduction of the cement in all three systems. The results of the fatigue tests of vacuum-mixed Simplex P were also compared to the fatigue strength of Simplex P prepared by centrifugation of the cement immediately after mixing. Vacuum mixing one pack of Simplex P per syringe in all three systems was not effective in complete removal of all the large voids from the cement. Fatigue failure occurred very early in those specimens containing the large voids. There was no significant difference in fatigue life between one pack of cement per syringe mixed under vacuum in the three systems and the control cement (no vacuum, uncentrifuged). Vacuum mixing two packs of cement per syringe was more effective than one pack per syringe, and all three systems significantly increased the cycles to failure of Simplex P over the control cement. However, the Enhancement and Mitab vacuum mixing systems still produced some very weak specimens in fatigue. Two packs of cement per syringe prepared in the Mixevac II vacuum mixing system were significantly stronger in fatigue than two packs mixed in either the Enhancement or Mitab vacuum system. The Mixevac II vacuum mixing system was the most effective technique of the three vacuum mixing systems tested. Centrifugation of one or two packs of Simplex P per syringe produced a more uniform cement that was free of large voids and thus eliminated the very weak specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro and in vivo studies of pressurization of femoral cement in total hip arthroplasty

Improvements in cementing techniques in the absence of pressurization of the cement have led to major increases in the long-term success rate of fixation of the femoral components of cemented total hip arthroplasty (THA). The strength of the cement-bone interface is strongly related to cement intrusion into the bone. The depth of cement intrusion, in turn, is correlated with the cement-intrusion pressure. Thus, adding cement pressurization to those current techniques that have already been validated may further increase the long-term durability of fixation of the femoral component of cemented THA. To assess cement pressurization in the proximal femur for THA, the authors compared in vitro the efficacy of three existing pressurization systems (the Johnson and Johnson system [New Brunswick, NJ], the Miller system [Zimmer, Warsaw, IN], and the Zimmer system [Zimmer]) in cadaver femurs using pressure transducers and evaluated their ease and optimization for clinical use. The authors then selected one (the Zimmer system) for use in studies in vivo to quantify the actual pressures achieved in the medullary canal in vivo under surgical conditions using pressure transducers placed throughout the femoral cortex. Each of the three commercially available femoral cement pressurization systems has its own advantages and disadvantages. All three systems were shown to produce average peak cement-intrusion pressures in vitro of over 21 N/cm2 (30 psi) throughout the cement mantle including, importantly, in the proximal portion of the femur.(ABSTRACT TRUNCATED AT 250 WORDS)

Monitoring the integrity of the cement—metal interface of total joint components in vitro using acoustic emission and ultrasound

Debonding of the cement-metal interface of cemented femoral components of total hip arthroplasty has been shown from clinical and autopsy material to be a common occurrence. Experimentally, debonding has been shown to increase markedly the strains in the adjacent cement mantle. Studies of autopsy-retrieved specimens demonstrate that debonding of the cement-metal interface is a key initiating event in loosening of cemented femoral components of total hip arthroplasty. However, both the radiographic and autopsy evidence of cement-metal interfacial debonding exist after the fact, that is, after debonding has occurred. The lack of prospective data showing that debonding does indeed occur under physiologic loading and occurs prior to other forms of failure of fixation leaves uncertain the issue of debonding and its role in initiating loosening of cemented femoral components. Knowing when, where, and to what extent the cement-metal interface debonds is critical information in understanding the process of loosening of cemented femoral components. Such information would contribute to improving the durability of stems and improving cementing techniques. In this study, the two nondestructive techniques of acoustic emission and ultrasonic evaluation of the cement-metal interface of cemented femoral stems of total hip arthroplasty were combined to investigate when, where, and to what extent cement-metal debonding occurred in vitro in simulated femurs loaded physiologically in fatigue in simulated single-leg stance. Debonding of the cement-metal interface of a cemented femoral component in this model was both an initiating event and a major mechanism of compromise of the cement-metal interface. Additional acoustic emission signals arose from cracks that developed in the cement.

The effect of centrifugation on the fatigue life of bone cement in the presence of surface irregularities.

Reduction of the porosity of bone cement by centrifugation significantly improves the fatigue life of the cement when smooth, waisted specimens are tested. However, bone cement in vivo has surface irregularities at the interdigitation of the cement with the trabecular bone. The effect of centrifugation on the fatigue life of Simplex P in specimens containing surface irregularities was investigated by examining both composite specimens of trabecular bone and bone cement and specimens containing a sharp, circumferential notch. For the specimens with the sharp notch, the bone cement that had been centrifuged lasted significantly longer in fatigue (47,039 +/- 40,277 cycles) than the uncentrifuged specimens (3103 +/- 1950 cycles). Eleven of 15 uncentrifuged specimens broke at the location of a void, rather than the notch. In contrast, when the porosity was reduced by centrifugation, 13 of the 15 specimens broke at the notch. For the specimens that were a composite of bone cement and trabecular bone, the centrifuged specimens had a significant increase in fatigue life compared to the uncentrifuged specimens when tested at both 7 MPA (641,056 +/- 444,131 cycles vs. 237,969 +/- 124,153 cycles) and 15 MPA (8800 +/- 4673 cycles vs. 1534 +/- 719 cycles). Reduction of porosity in bone cement by centrifugation significantly extends its fatigue life even in the presence of trabecular bone or sharp surface notches as used in total joint replacements. These data support the concept that reduction of porosity of bone cement by centrifugation may extend the duration of fixation of the components in cemented total joint arthroplasties.

Comparison and optimization of three centrifugation systems for reducing porosity of Simplex P bone cement.

Simplex P bone cement was spun for 30, 60, and 120 seconds in three different centrifugation systems (I.E.C. HN-S II, I.E.C. clinical, and Johnson & Johnson) to determine whether differences among the three systems also produce differences in the improvement of the fatigue strength of the cement. The fatigue properties of the cement after centrifugation were also assessed when it was mixed with monomer that had been chilled to 0 degrees C. There were important and statistically significant differences in the fatigue life of Simplex P spun for the same duration in the different centrifuges. Simplex P prepared as recommended by the manufacturer had an average fatigue life of 15,143 cycles in the test system used. Optimum centrifugation among the techniques studied increased the fatigue life nearly fivefold, to the range of 71,000 cycles. Taking into account both the fatigue strength and the viscosity of the cement, the optimum centrifugation system for improving the fatigue life of Simplex P bone cement is the Miller cartridge containing two packs of cement spun in the IEC-HN-S II centrifuge. The authors recommend 30 seconds of centrifugation if the monomer is not chilled prior to mixing and 60 seconds if the monomer is chilled.