Incumplimiento del deber de información en los tratos preliminares

Since the use of PMMA bone cements was first established, various different techniques have been used for the mixing of the two components. Methods have been refined over this time to produce the techniques that are used in modern surgery.

When first used, cement was mixed in a bowl before being manually packed into the bony cavity as a paste. Problems were quickly identified with this procedure as it proved difficult to obtain a perfect mantle free of large pores. Pores are known to act as sites of stress concentration within the cement mantle from which cracks can initiate and grow.

Syringes were introduced as an insertion method in order to reduce the entrapment of air within the cavity. This method works very well in certain situations such as inserting cement in long cavities, like the femoral component of total hip replacement. However, many joint replacements require the cement to be moulded around or on top of much wider and flatter or even convex geometry. For these procedures, manual insertion is still preferred.

Following the recognition of pores as sites of crack initiation in the cement mantle, a number of methods for mixing the cement were developed to minimise the production of pores. When mixing in air, the agitation required to mix the cement entraps air into the cement. By removing the air it is possible to drastically reduce the amount of porosity. Vacuum mixing methods have been developed by many different cement manufacturers. Many have been shown to improve the quality of the bone cement by reducing the number of pores [60].

Other methods have also been developed in attempts to further reduce the porosity in the cement. By spinning the cement in a centrifuge after mixing it is possible to further reduce the porosity in the cement, although the effect of additionally centrifuging the cement following vacuum mixing on porosity has been shown by Macaulay et al. to be insignificant [70].

The most popular techniques used in modern surgery generally involve mixing the cement within the barrel of a cement syringe. A vacuum is applied to the barrel and the two components are inserted through a funnel. The barrel is then sealed creating a vacuum in the syringe. The cement is then mixed under vacuum using an agitator which runs inside the sealed syringe barrel. Once mixed thoroughly the vacuum is removed and the syringe barrel is inserted into a cement insertion gun. The cement can then be used by injection (Figure 3-14). The CEMVAC© system, manufactured by DePuy CMW™, operates on this principle and is shown in Figure 3-13.

Figure 3-13 - Photograph of Cem-Vac™ cement mixing system. Syringe with funnel, sealing plunger, nozzle cutter, cement delivery gun, sachet of cement powder and vial of liquid monomer are shown.

Many other manufacturers use slightly different techniques but the methods are generally similar. However, the quality of the cement that results can vary from one manufacturer to another. Dunne et al. showed the variation between the porosity and subsequent fatigue performance of 6 different vacuum mixing kits to be of statistical significance (porosity ranging from 1.44% using Mitab Optivac™ and 10.3% using Zimmer Osteobond™ when Palacos R™ cement was mixed in each, according to the manufacturers instructions [26].)

Figure 3-14 – Typical procedure for vacuum mixing cement [21].

There are still ongoing studies into techniques for mixing of bone cement, in an attempt to improve not only the resulting mechanical performance of the cement but also the heat produced during cure (discussed in 3.3.2 Aseptic Loosening). Wang et al. investigated changes to the thermal behaviour of cement as a result of vacuum mixing. Little change in polymerisation temperature was observed due to changes in pressure at which the cement was mixed [123]. McCullough et al. looked into the effects of initial temperature of the cement and also the effects of using an automated cement mixing machine on the homogeneity and the porosity of the final product [77]. It was seen that colder cement could be mixed for longer as the reaction ran more slowly (discussed in 3.2.3) and that this created more homogeneous cement as did automated mixing when compared to hand mixed cement. Conversely Lewis investigated the fatigue performance of cements that had been stored at different temperatures and found no significant difference between cement stored at room temperature (21°C) and cement stored at 4°C [60]. The effect of sterilisation method has been investigated by Graham et al. [34]. Et-O sterilisation produced little effect on the fatigue performance of the cement. Radiation sterilisation at increasing

intensities caused a degradation of the cements’ fatigue performance. This deleterious effect upon the mechanical performance seen in radiation sterilised, vacuum mixed bone cement is attributed to a decrease in molecular weight caused by scission of the polymer chain during the sterilisation process. An additional parameter, investigated by Bettencourt et al. is the liberation of methylmethacrylate monomer from the cement and the effect that vacuum mixing has on it [10]. It was seen that after low pressure (0.15bar) vacuum mixing, the liberation of potentially harmful methylmethacrylate to the atmosphere was reduced significantly. This is due to mixing the cement in a closed system with filtration on the vacuum line.