THIRD GENERATION BONE CEMENT MIXING SYSTEMS, ARE THEY DOING THE JOB?

Abstract

Introduction: It has been shown that acrylic bone cement is weakened by its porosity, which enhances the formation of micro-cracks that contribute to major crack propagation. It has also been observed, that mixing procedures play a significant role in determining the quality of bone cement produced. A high degree of porosity is found to exist in cement that is inadequately mixed.

Currently mixing system allow for the preparation of the bone cement under the application of a vacuum in a closed, sealed chamber by means of a repeatable mixing action. These systems are perceived to be repeatable, reliable, and operator independent. The objective of this study is to evaluate the quality and consistency of acrylic bone cement prepared by scrub staff in an orthopaedic theatre using a commercially available third generation vacuum mixing syringe, in terms of the level of voids within the cement microsturcture.

Materials and Methods: The mixing devices were stored at 23°C ± 1°C for a minimum of 24 hours prior to mixing. The acrylic bone cement (Palacos R® with gentamicin, Biomet Merck, UK) was stored at 4°C ± 1°C for a minimum of 24 hours prior to mixing.

Bone cement was mixed using a commercially available third generation mixing device (vacuum = −550mmHg) at Musgrave Park Hospital, Belfast, Northern Ireland. The cement was mixed according to the device manufacturers’ instructions for use. Mixing was carried out during a joint replacement surgery by a number of experienced theatre scrub staff (n = 35). The cement remaining at the end of the procedure was allowed to cure within the delivery nozzle, made from linear low-density polyethylene (LLDPE) and having an internal diameter of 10mm. 205 nozzles were collected post-operatively and stored at 23°C ± 1°C prior to testing. The percentage porosities were determined by measuring the apparent densities based on Archimedes principle and, as a direct result; it was possible to calculate the mean percentage porosities.

Discussion: It can be observed that the majority of the theatre nurses, ie 46.8% prepared bone cement using the vacuum mixing system containing a porosity of between 2% to 4%. A cement porosity of this range would be the accepted optimum content for acrylic bone cement. However, 6.4% of the theatre nurses prepared cement demonstrating a porosity content ranging from 8–16%, which is highly unsatisfactory when you consider that the cement mixing system is perceived to be a consistent and reliable mixing device that is operator independent.

Figure 2 illustrates a bar chart representing the bone cement porosity as a function of which orthopaedic theatre the cement was prepared. There was no significance difference when comparing the quality of the cement mixed in terms of porosity with the different theatres. The mean porosity values of the cement mixed ranged between 2.5% and 5.2% depending on which theatre was used.

Conclusions: Bone cement mixed using the commercially available third generation device in theatre by 35 scrub staff was found to have a high degree of variability. Thus demonstrating that even an alleged reproducible mixing system is independent on mixing technique when used in a clinical situation by a number of users. Thus illustrating the system is not wholly user independent.

As a consequence of this investigation it is recommended that the key to ensuring high quality bone cement, with a good mechanical strength, that can be consistently prepared in theatre by scrub staff are two fold.

1. The orthopaedic staff must be aware of the significance of cement mixing and how it is affected by a number of factors including the type of mixing system, vacuum level applied, and mixing technique.

2. Education in the use of vacuum mixing systems should be ongoing and frequent. Practice mixing in non-clinical situations and feedback through quality measurements is particularly important.