Aim: Suction fit is a common phenomenon of metal on metal (MOM) or ceramic on ceramic hip joints, in which the head and the cup can be hold to each other in the presence of lubricant. The aim of this study was to measure the suction force of MOM hip joints with different clearances and fluid viscosities.

Materials and Methods: CoCrMo hip prostheses of Ø50 mm with two diametral clearances of 100 and 200 microns were tested. A range of fluids were made using different ratios of deionized water and carboxy-methyl cellulose (CMC). The fluid viscosities ranged from 0.0011 to 0.3 Pa s determined by a cone-on-plate viscometer (Physica Rheolab MC100). Before the measurement, about 1 ml fluid was placed in the cup and the bearing surfaces of both the head and the cup were fully wetted by rotating the femoral ball in the cup. A pre-loaded 3kN was applied to push the head into the cup by a servohydraulic test machine, the femoral head was then lifted out of the cup at a rate of 2 mm/s and the maximum suction force value was recorded.

Results: Generally, the suction force increased with the increase of viscosity for both 100 and 200 microns clearances. In the lower range of the viscosity, the suction force increased rapidly, but when the viscosity reached a certain value it leveled off to about 190 N for the 100 microns clearance and 150 N for the 200 microns clearance. In the whole range of viscosity, the suction forces of 100 microns clearance were higher than those of 200 microns clearance and the differences were statistically significant (t-test, P< 0.01). In the lower range of viscosity (< 0.04 Pa s), the suction force of 100 microns clearance was more than two times higher than that of 200 microns clearance.

Discussion: For a MOM hip joint, suction force can be explained by the vacuum within the bearings. When the clearance is small and the lubricant viscosity is high, the edge of the bearing surfaces can be easily sealed off so that certain degree of vacuum can be produced between the head and the cup. In this test, the maximum suction force for the Ø50 mm MOM hip joint can be calculated as: F = (projected area) x (atmospheric pressure) = 196 (N). The results confirmed that the suction force of Ø50 mm components can not be higher than 196 N.

Although suction force may have a benefit in reducing the risk of dislocation, it may prevent lubricant recovery between the bearings and will influence the sliding resistance. If the suction force is too high, the head and cup can be held together such that the recovery of synovial fluid is restricted or impossible, even when the hip is not loaded during the swing phase. Both the clearance and the viscosity have a significant effect to determine the suction force and the lubrication of MOM hip joints. It is concluded that suction force is a factor to be considered during the selection of MOM hip joint clearance.