The effect of isotonic saline on the strength of bone cement during the polymerisation period.

It is common practice to irrigate the wound in hip and knee replacement surgery during the working and setting phases of cement polymerisation. We sought to establish whether the addition of normal saline during the various stages of polymerisation would affect bone cement strength. Cement specimens were moulded to the dimensions defined in ISO 5833: 2002 and tested in an electro-mechanical test machine with a calibrated 50 kN load cell at a compression rate of 24 mm/min. The results suggest that the compressive strength of bone cement is not affected by the presence of saline following the initiation of polymerisation. However, saline added to the mix prior to the initiation of polymerisation increased its porosity and significantly decreased its strength. This may explain why vacuum mixing is important as it removes water vapour.