Assessment of head displacement and disassembly force with increasing assembly load at the modular head/trunnion junction of a total hip replacement prosthesis

Introduction The aim of this study was to assess the effect of assembly force on the strength of head-trunnion interface and to measure the initial displacement of the head on the trunnion with different assembly forces. Methods Three assembly load levels were assessed (A:2kN, B:4kN, C:6kN). The stems were mounted in a custom rig and the respective assembly loads were applied to the head at a constant rate of 0.05kN/s (ISO7260-10:2003). Load levels were recorded during assembly. Head displacement was measured with a laser sensor. The disassembly force was determined by a standard pull-off test. Results The maximum head displacement on the trunnion was significantly different between the 2 kN group and the other two groups (4kN, 6kN, p=0.029), but not between the 4kN and 6kN groups (p=0.89). The disassembly forces between the three groups were significantly different (mean ± SD, A:1316±223kN; B:2224±151kN; C:3965±344kN; p= 0.007), with increasing assembly load leading to a higher pull-off force. For the 4kN and 6kN groups, a first peak of approximately 2.5kN was observed on the load recordings during assembly before the required assembly load was eventually reached corresponding to sudden increase in head displacement to approximately 150µm. Discussion An assembly force of 2kN may be too low to overcome the frictional forces needed to engage the head and achieve maximum displacement on the trunnion and thus an assembly load of greater than 2.5kN is recommended.