Abstract
INTRODUCTION
Ceramic heads are used in hip revision surgery to mitigate corrosion concerns. Manufacturers recommend using a pristine titanium sleeve in conjunction with a well-fixed metal stem to prevent early failure of the ceramic head. However, the influence of impact force, head size, and sleeve offset on pull-off strength and seating displacement of a revision head assembly is not fully understood. Therefore, the purpose of this study was to investigate the pull-off strength and displacement of commercially available revision ceramic heads and titanium taper sleeve offsets (BIOLOX OPTION, CeramTec GmbH, Plochingen, Germany) while covering a range of clinically relevant impaction forces.
METHODS
Two head sizes (28 mm, n = 12 and 36 mm, n = 12) and two taper adapter sleeve offsets (small, n = 12 and extra-large, n =12) were tested in this study. A dynamic impaction rig was constructed to seat the head, sleeve, and stem assembly. Consistent impaction forces were achieved by a dropping a hammer fixed to a lever arm from a pre-determined height onto a standard impactor instrumented with a piezoelectric force sensor (PCB Piezotronics Inc.). Axially applied forces of 2 kN and 6 kN were used to cover a range of typical impaction forces. Three non-contact differential variable reluctance transducers (LORD Sensing Systems) were used to track the displacement of the head relative to the stem. Subsequently, samples were transferred a servo hydraulic testing machine, and a pull-off test was carried out per ISO 7206- 10 to measure the disassembly force.
RESULTS
For all head and sleeve combinations assembled at 6 kN, pull-off forces and assembly displacements were over two times the values measured at 2 kN. As expected, an increased assembly force resulted in increased pull-offs and displacements. Head size did not play a significant role on measured outcomes. Regarding sleeve offsets, at assembly of 6 kN mean pull-off forces for extra-large sleeves were reduced by approximately 25% relative to small sleeves. However, at a 2 kN assembly, sleeve offsets did not appear to influence pull-off forces.
DISCUSSION
This study assessed the effect of impact assembly force, head size, and sleeve offset on pull-off strength and seating displacement of revision ceramic heads. The data suggests assembly force and sleeve offset may influence the pull-off strength and seating displacement of modular heads used in revision hip arthroplasty. Mean pull-off forces for revision heads were comparable in magnitude and trend to previous studies assessing the linear relationship between assembly force and pull-off force in primary heads (Krull et al., 2017, Rehmer et al., 2012). Lower pull-off forces were observed for extra-large sleeves when compared to small sleeves, indicating, decreased contact at the sleeve and stem interface for extra-large offsets may play a role in reducing pull-off forces.
