Approximately 6,200 hip and 3,300 knee revision operations (National Joint Registry, 2006) are conducted every year in the UK with the average cost of a revision in Europe being £8,000 (Rasanen et al, 2007), which represents a significant burden on the National Health Service. Due to an ageing population and a growth in primary procedures, the number of revisions is anticipated to grow significantly over the next 10 years.
Preparation of the bone cement is one of the key steps in cemented joint arthroplasty. The perioperative practitioner needs not only to be familiar with the vacuum mixing device being used, but also must ensure that attention is given to the timing of each phase of the mixing process. This article sets out to demonstrate the importance of mixing technique and the implications of incorrect timing on the longevity of the prosthesis.
What is bone cement?
Polymethyl methacrylate (PMMA) bone cements are two-component systems, comprising a polymer powder and a liquid monomer. The polymer powder component is composed of PMMA and/or methacrylate copolymers. Furthermore, the polymer powder contains benzoyl peroxide (BPO), which acts as an initiator of the free radical polymerisation reaction. The BPO can form part of the polymer micro-beads or simply be incorporated into the powder. The powder also contains an X-ray contrast agent and possibly an antibiotic. In the liquid phase MMA is the main constituent but sometimes other methacrylates such as butyl methacrylate are also present.
In order for the MMA to be used for bone cements it must be polymerisable, therefore it must contain a carbon double bond which can be broken. As an activator of the formation of radicals the liquid contains an aromatic amine, such as N,N-dimethyl-p-toluidine (DmpT). Additionally, it contains an inhibitor (hydroquinone) to avoid premature polymerisation during storage and optionally a colouring agent such as chlorophyll.
The behaviour of the cement differs between manufacturer depending on the ratio of the polymer powder and liquid monomer components, the size, shape and weight of the molecules, the manufacturing process and the methods of sterilisation.
The function of bone cement
PMMA bone cements are primarily used for the fixation of joint prostheses. In the fixation of joint replacement the self-curing cement fills the free space between the prosthesis and the bone, and constitutes a very important interface (Figure 1). Due to their optimum elasticity, the bone cements can evenly cushion the forces acting against the bone. The close association between the cement and the bone leads to optimal distribution of the stresses and interface strain energy.
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| Figure 1: Schematic diagram of prostheses and PMMA bone cement in an acetabular socket and femur. |
The transfer of the force from bone-to-implant and implant-to-bone is the primary function of bone cement. The ability to do this reliably for a long time is crucial for long-term survival of the implant. Adequate cement interdigitation and reinforcement of the cancellous bone are of utmost importance. If the extremely generated stresses exceed the capability of the bone cement to transfer and absorb forces, a fatigue fracture is possible.
Antibiotic loaded bone cements are also used as drug-delivery systems. Artificial implants are more susceptible to bacterial colonisation on their surfaces because the germs can then hide from the natural protection via the body and cause periprosthetic infection. When loaded with antibiotics, bone cement functions as carrier matrix.
Choosing the right mixing technique
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 pre-polymerised particles to be mixed with the liquid monomer, monomer fumes are released into the air. Furthermore with hand mixing, a certain amount of porosity in the final material is unavoidable owing to air entrapment, even in lower viscosity cements.
During the 1980s different techniques were introduced in the hope of improving mixing and thereby bone cement properties (Burke et al., 1984, Lind
