Summary

Vacuum mixing of bone cement has been used in cemented hip-joint replacement procedures for over two decades. Although literature on bone cement is voluminous, relatively few studies have reported the effects of vacuum-mixing methods on the cemented hip replacement. While it has been proven that vacuum mixing improves the mechanical properties of bone cement, other effects need to be clarified. This chapter discusses the effects of vacuum mixing on cement quality, cement homogeneity, the cement-prosthesis interface and the operating room environment. It also discussed vacuum mixing in terms of bone-cement shrinkage and antibiotic release in total hip replacements.

Introduction

Until the 1980s, the composition and preparation of bone cement had not changed from the standards introduced by Charnley back in 1959. Then, in the 1980s, techniques for improving cement strength began to be investigated. Closed mixing under vacuum was developed initially for environment reasons, but its benefits in terms of producing homogenous cement, enhancing the mechanical properties of cement, improving cementing technique as well as safeguarding the operating room environment soon became evident.

In order to improve the mechanical properties of bone cement and thereby reduce the risk of cement failure in arthroplasties, substantial efforts have been invested into the development of new techniques for bone cement mixing and delivery applications with the objective of reducing macro- and microporosity. These techniques have the net effect of improving the overall quality of the cement.

Numerous studies have shown that, compared to other mixing methods, vacuum mixing reduces monomer evaporation and exposure in the operating room. Furthermore, it prevents air entrapment in cement, reduces cement porosity, decreases the number of unbounded particles in cement and increases the cement's mechanical strength. Clinical observations have revealed that vacuum mixing of bone cement improves mid- and long-term survival rates of total hip implants [41].

The Evolution of Cement Mixing Systems

When bone cement was first used in arthroplasty, it was hand-mixed in a bowl in the operating room and then inserted by hand or transferred and injected into the desired location. Because PMMA comes as a powder composed of prepolymerised particles to be mixed with the liquid monomer, monomer fumes release into the air. Furthermore with hand mixing, a certain amount of porosity in the final material, even in lower viscosity cements, is unavoidable owing to air entrapment.

During the 1980s different vibrating mixing techniques were introduced in the hope of improving mixing and thereby bone cement properties [37]. The results, however, were not convincing. While Burke et al. [9] reported an increase in the cement's fatigue life and ultimate tensile strength when centrifugation was used, a phenomenon they attributed to a reduction in the number and size of pores, Rimnace et al. [49] found no

improvement in the static or dynamic properties of several brands of bone cement when mixed by centrifugation. The results seemed to vary significantly depending on the type of centrifugation and cement used, and the cement was not consistently homogenous. Moreover, antibiotics and radiographic contrast media tended to gather in the periphery of the mix, and the upper volume of the cement often became more porous than the rest of the cement.

At about the same time as vibration and centrifuga-tion were being developed and tested, closed mixing under vacuum was introduced [4, 35, 36]. After some refining, it produced better results than centrifugation, which was soon thereafter abandoned in favour of vacuum mixing, particularly because of the ease of delivery when using cartridge mixing. Today, vacuum mixing is widely accepted as the method of choice for achieving homogenous cement, reducing porosity and increasing cement strength, which is why it is an integral part of Modern Cementing Technique [40].

Improvement of Environment

MMA is a toxic, highly volatile organic solvent. Cytotoxic effects on human fibroblasts in culture media have been described and attributed to MMA [57]. It is now known that the adverse effects of working with MMA include local mucosal irritation i.e., irritation of the respiratory system, eyes and skin contact sensitivity that may lead to toxic dermatitis [20, 31, 33, 47]. Indications are headache, nausea and lack of appetite. MMA is not thought to be carcinogenic to humans under normal conditions of use. However, techniques should be employed to reduce medical staff exposure to MMA during cemented implantation. Operating staff should avoid direct contact with MMA, and room ventilation should be optimised.

Monomer exposure is regulated by law in many countries. The exposure limits range from 50-100 ppm in different countries in Europe. The exposure of conventional mixing in open bowl is about 10 ppm in the breathing zone [8]. Vacuum mixing systems reduce the exposure by 50-70% [53] and eliminate contact with bone cement during delivery [6, 8, 12, 19, 53]. The working environment for the operating staff is improved, and the risk of fume-induced headaches, respiratory irritation and allergic reactions becomes minimal.

Improvements in Cement Quality Porosity

Pores and voids of different sizes in cement are caused by air from the polymer powder. The air becomes trapped in the cement during mixing and transferring from mixing container to delivery system [11, 43, 62, 69]. Conventional mixing of bone cement produces porosity of 5-16%. Vacuum mixing produces porosity of 0.1-1% [38, 65].

Relationship Between Porosity and Fatigue Property of Cement

Porosity has been found to be the major cause of decreased mechanical strength and fatigue life of bone cement. To ensure its in vivo survival, the cement must be able to withstand the varying loads it endures. Thus fatigue property, which is directly affected by porosity, is as important in determining the long-term survival of a joint replacement as static strength.

Fatigue failure occurs when cement cracks are initiated as a result of defects in the cement mantle. It is the presence of large voids within the cement that leads to a rapid propagation of fracture (O Fig. 4.1a). Secondary cracks develop along the small pores (O Fig. 4.1b). Such fractures are commonly observed in vitro and in vivo [10, 27, 30, 45, 56]. Because of stress concentration, the initial crack is likely to start in an area of weakness or at a void in the material [29]. Evidence of this cracking has been found when examining retrieved cement [30, 56].

Fig. 4.1a,b. SEM micrograph of fatigue fracture surface. a A large pore causes stress concentration and initiates crack [27]. b Secondary crack associated with pores [56]

The reduction of porosity prevents or at the very least retards the initiation of fatigue propagation. It is known that vacuum mixing of cement increases mechanical properties [4, 5, 13, 35, 36, 38, 69] largely as a result of decreasing micro- and macropores [62, 66]. Numerous studies have confirmed that vacuum mixing enhances the fatigue life of the bone cement [17, 23, 34, 43, 45, 52,65, 67, 70].

Cement Homogeneity

Bulk form PMMA cement exhibits good biocompatibility when implanted in bone. However, in particulate form, PMMA can evoke a foreign body and chronic inflammatory reaction similar to that seen around loose cemented arthroplasties [1, 22, 32, 55, 68].

Incomplete mixing of the monomer and polymer may lead to partially united and, in some cases, free unbonded cement particles. Vacuum mixing of bone cement not only decreases the number of voids but also improves the microscopic homogeneity of Palacos R cement [64]. Electronic microscopy shows that voids located on the surface of cement as well as on the fracture surface of cement invariably contain partially polymerised PMMA particles and contrast media particles, such as zirconium dioxided particles (O Fig. 4.2). When cement fracture occurs, less homogeneous cement may release a larger number of PMMA spheres and contrast media particles to the bone-cement interface. These particles may evoke a foreign body response or stimulate osteoclast activity [50 ,51, 68], leading to osteolysis of the surrounding bone.

Cement-Prosthesis Interface

Studies have indicated that mechanical loosening of cemented implants originates at the stem-cement interface [28, 30, 60]. Loosening of the cemented stem further increases stress in the cement mantle [24, 59]. Extensive porosity at the cement-stem interface has been found in retrieved cement mantles and in laboratory-prepared specimens [7, 28] (Fig. 4.3).

This interface porosity is caused by entrapment of air at the stem surface during stem insertion and by residual porosity in the cement. A further cause is the cement's shrinkage away from the colder stem surface which produces pores [7]. Although cement curing is chemically initiated, polymerisation is thermally activated. Thus cement curing starts at the warmer bone surface and progresses towards the cooler stem. Resultant pores as well as residual pores in the cement are driven towards the last polymerising region on the stem.

When cement is mixed under vacuum, cement porosity is significantly reduced, producing less porosity at the cement-prosthesis interface [7, 61] (O Fig. 4.4). Various studies have shown that interface porosity affects the debonding energy of the interface [43], weakens the resistance of the cement to torsional load [15] and decreases fatigue life of the cement-metal interface [26]. Interface porosity has also been linked to the initiation of cement cracks [28, 30, 58]. The evidence is convincing that reduction of interface porosity improves the strength of the interface, thereby increasing the longevity of cemented implants.

Fig. 4.4. Samples from a cemented implant. The cement was mixed at atmospheric pressure (left), and under vacuum (right). M metal; BC bone cement

Shrinkage

Cement has 3-5% volumetric shrinkage after curing [11]. Concerns over this shrinkage have focused primarily on the stability of the implant. With vacuum mixing, the volumetric shrinkage may be increased from 3-5% to 5-7% in different cements [44]. In a cemented hip stem, for example, the cement grout is along the stem with a long cement mantle (150-200 mm) and thin cement layer (2-4 mm). The shrinkage occurs more along the longitudinal axis rather than diametrically [14], and it is the diametrical shrinkage that may influence the cement interface. Studies have been unable to find differences in diametrical shrinkage or gap formation between cement and prosthesis when a reduced porosity cement is used [14, 63]. Shrinkage, however, within the cancellous bone bed can be regarded as beneficial, as some interface gaps allow for re-vascularisation [16]. Vacuum mixing of cement has not been found to negatively affect interface strength between cement and prosthesis. RSA showed a stable cemented implant in early and middle term studies of vacuum-mixed cemented implants [2, 48].

Antibiotic Release

While vacuum mixing reduces the porosity of bone cement, it is thought that the process may adversely affect antibiotic release. Surgeons understandably are concerned about the extent to which the mixing procedure affects the release of antibiotics from the cement. Studies have shown that the concentrations of several antimicrobial agents from antibiotic-loaded bone cement exceed those obtained by systemic administration [21] as well as the minimal inhibitory concentrations of several pathogens according to the National Committee for Clinical Laboratory Standards breakpoints lasting from 3-38 days [3]. Vacuum mixing of gentamicin-loaded bone cements has been shown to effectively reduce the number and size of cement pores with only a minor reduction in antibiotic release [46]. The Norwegian Arthroplasty Register [18] documents the best results occurring when antibiotic prophylaxis is administered both systemically and with bone cement containing antibiotics prepared under vacuum.

Clinical Significance

The aetiology of aseptic loosening of total joint arthroplas-ties appears to be multifactorial, with surgical technique, cementing technique, cement quality, cement viscosity, prosthesis design, wear debris, joint fluid pressure and micromotion all being involved. Although improved surgical techniques have increased the probability of prosthesis survival, reducing or eliminating bone cement fracture by improving its material strength will further enhance the longevity of cemented prostheses.

The affects of porosity reduction on the longevity of the cement mantle and on the survival rate of a joint replacement are still debated. In favour of porosity reduction is the fact that no direct clinical evidence has ever shown an association between reduced survival rates and porosity reduction [39]. Janssen et al. [29] explain the apparent contradiction of porosity by means of a finite element model showing that when stresses are distributed homogenously in cement, pores act as crack initiators;

whereas under inhomogeneous stress conditions, crack formation is governed by local stress intensities. The study suggests that mechanical failure of cemented femoral components is initiated in areas where stress concentrations are generated. Therefore, the effect of reduced porosity on the failure mechanism in these areas will be limited and clinically detectable only in large studies.

It remains clear, however, that with pores located in areas of high stress concentrations, the cement mantle will fail rapidly. The Swedish Hip Register gives a risk ratio of vacuum mixing to revision of 0.74 after five years from implantation [41], suggesting that vacuum mixing improves the mid- to long-term survival rate of THA significantly as compared to hand mixing. With 25 years of clinical experience, Harris [25] indicated that in a cemented total hip replacement, bone cement can be made five times stronger just by porosity reduction. Various studies indicate that macropores increase the risk of fatigue failure, and the current opinion is that efforts should be made to minimise the number and size of macropores.

Take Home Messages

Vacuum mixing significantly reduces macro- and micropores in bone cement, thereby enhancing the cement's mechanical strength.

Vacuum mixing also improves cement homogeneity and strengthens the cement-prosthesis interface.

Closed vacuum-mixing systems reduce the risk of monomer exposure to operating room staff.

The slight bone cement shrinkage that occurs has not been shown to adversely affect prosthesis stability in any way. Nor does vacuum mixing have a significant effect on effective antibiotic release.

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