Does vacuum mixing affect diameter shrinkage of a PMMA cement mantle during in vitro cemented acetabulum implantation?

Radiolucent lines on immediate postoperative cemented acetabular component radiographs between the PMMA bone cement mantle and bone are an indicator of an increased risk of early loosening. The cause of these lines has yet to be identified. Thermal and chemical necrosis, fluid interposition and cement shrinkage have all been suggested in the literature. The aim of the study reported here was to take an engineering approach – eliminating confounding variables present during surgery – to quantify the size of the interstice created by cement shrinkage when a 50 mm diameter flanged acetabular cup is implanted in a model acetabulum with a 52 mm hemispherical bore under controlled conditions using vacuum and non-vacuum mixed cement. Irrespective of the mixing method used, a significant interstice was created between the bone cement and the mock acetabulum. When the cement was mixed under vacuum the interstice created between the mock acetabulum and the cement mantle was 0.60 mm ± 0.09 mm; when the cement was mixed under non-vacuum conditions the interstice created was 0.39 mm ± 0.15 mm. Possible explanations for radiolucent lines are discussed.

Spontaneous changes

To set the scene for the Second Law, this (non-mathematical) chapter explores our experience of spontaneous changes - such as the expansion of a gas into a vacuum, mixing, phase changes and many chemical reactions - demonstrating that they are all irreversible, and proceed unidirectionally from non-equilibrium states to equilibrium states. Furthermore, they all comply with the First Law. The First Law therefore cannot be used as a predictor of spontaneity. What, then, can be used as a predictor?