We have investigated the role of the penetration of saline on the shear strength of the cement-stem interface for stems inserted at room temperature and those preheated to 37°C using a variety of commercial bone cements. Immersion in saline for two weeks at 37°C reduced interfacial strength by 56% to 88% after insertion at room temperature and by 28% to 49% after preheating of the stem. The reduction in porosity as a result of preheating ranged from 71% to 100%. Increased porosity correlated with a reduction in shear strength after immersion in saline (r = 0.839, p < 0.01) indicating that interfacial porosity may act as a fluid conduit.

The site of initiation of failure of a cemented femoral component is usually the prosthesis-cement interface. Strengthening this interface with porosity reduction may improve survivorship. Cement pores which propagate crack formation can be reduced by vacuum mixing or centrifugation, but this does not effect interface porosity.

Utilising simulated stems cemented into a “Sawbones” femur in a manner replicating surgery, we determined the effect of stem warming on various parameters. Maximum temperature and time of polymerisation, mechanical strength, porosity reduction and pore distribution in the cement mantle were measured with stems at room temperature (RT), 37, 44, and 50 degrees Celcius. Mechanical testing included initial “push-out” tests, tests after agiing in 37 degrees Celcius saline for two weeks,and fatigue testing (3 HZ at 90% initial failure load). Porosity distribution was measured by the percentage area of pores on the interface surfaces and the transverse plane.

Polymerisation time decreased as the stems were heated. The time decreased from 8.1 minutes at RT to 5.9 minutes at 50 degrees Celcius. The maximum temperature in the cement mantle rose from 50.2 to 56.4 degrees Celcius comparing stems at RT to those at 37 degrees Celcius, and did not elevate further as stems were preheated to 44 and 50 degrees Celcius. Similarly, static and fatigue interface strength improved by preheating stems, but no significant gain compared to RT stems was realised by heating above 37 degrees Celcius.

A dramatic reduction in porosity at the prosthesis-cement region was found with the heated stems, with no additional benefit to heating beyond 37 degrees Celcius. An increase in porosity at the cement-bone interface was noted as stems were heated. This may be due to the direction of polymerisation shrinkage in the cement mantle as influenced by stem temperature.