Influence of assembly procedure and material combination on the strength of the taper connection at the head-neck junction of modular hip endoprostheses

Background: A stable fixation between femoral head and endoprosthesis taper is necessary to prevent relative motions and corrosion at the taper junction. Although the importance of the component assembly has been recognised, no definitive instructions are available. The purpose of this study was to assess the influence of assembly force, assembly tool and number of hammer strokes on the taper junction strength of various material combinations. Methods: Co-Cr29-Mo (n=10) and Ti-6Al-4V (n=10) neck tapers were assembled with Co-Cr and Al2O3 ceramic heads either by push-on or by impaction with single or multiple hammer blows. The strength of the taper-head connection was evaluated by measuring the head pull-off forces according to ISO 7206-10 and the turn-off moment capacity. Findings: The taper strength linearly increased with assembly forces (P<0.001). Co-Cr heads combined with Co-Cr tapers showed significantly lower pull-off forces and turn-off moments than the combination with Ti tapers (0.001<P<0.025). Multiple impaction did not increase taper strength (0.063<P<0.995). Ceramic and Co-Cr heads showed similar fixation patterns on Ti tapers. Turn-off moments varied between 6 Nm and 19 Nm, dependent on material combination and assembly force. Interpretation: It is suggested that sufficient head-taper junction strength in all bearing conditions is achieved by impaction forces of at least 4 kN. A single impact is sufficient to achieve fixation. Special attention should be paid to the assembly of Co-Cr heads on Co-Cr tapers. (C) 2011 Elsevier Ltd. All rights reserved.