Abstract
Introduction
The successful performance of ceramic on ceramic bearings in today's THA can mainly be addressed to the excellent tribological behaviour and the minimal wear of ceramic bearings. The clearance between head and shell plays a major role in this functionality of artificial hip joints. Knowledge about the deformation behaviour of the shell during implantation but also under daily loads is essential to be able to define a minimum clearance of the system. The aim of this work is to establish a tool for determining maximum ceramic shell deformation in order to predict minimum necessary clearance between heads and monolithic ceramic shells.
Materials and Methods
In order to determine the minimum clearance the following in vivo, in vitro and in silico tests were taken into account:
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Eight generic metal shells were implanted into cadaveric pelvises of good quality bone realizing an underreaming of 1 mm. Maximum deformation of the metal shells (um) after implantation were determined using an validated optical system. The deformations were measured 10 min. after implantation.
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The stiffnesses of the metal shells (Cm) were experimentally determined within a two-point-loading frame acc. to ISO 7206-12.
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The stiffness of a monolithic ceramic shell (Cc) representing common shell designs (outer diameter 46 mm, 3 mm constant wall thickness) were determined acc. to ISO 7206-12 using Finite-Element-Method (FEM).
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Maximum deformation for the ceramic shells (uc,dl) under daily loading, represented by jogging (5kN, Bergmann et. al), was determined applying FEM.
Press-fit forces (Fpf = umCm) can be calculated with the results of test 1 and 2 considering linear elastic material behaviour. Assuming force equilibrium and applying the evaluated stiffness from test 3 the deformation of the ceramic shell (uc) occurring after implantation can be estimated (uc = umCm/Cc). For minimum clearance calculation of a monolithic ceramic shell (uc,lt) in vivo deformation (uc,dl) has to be considered additionally (uc,lt = uc + uc,dl).
Results
An average deformation of 177 µm was measured for the metal shells (average shell stiffness of 4368 N/mm). From the FEM the stiffness of the monolithic ceramic shell was calculated to be 9510 N/mm (46 mm). Deformation of 13 µm need to be considered from in-vivo relevant loading. The calculation of the minimum clearance for a generic monolithic ceramic shell (46 mm; 3 mm constant wall thickness) would result in 94 µm.
Discussion
The above described method can be taken as a worst-case approach as long-term bone relaxation has deliberately been not taken into account, intra-operative and post-operative deformation has been superposed and a 1 mm underreaming represents an upper limit for good bone quality. More intra-operative shell deformation values would improve the power of the approach. The new tool can be used to define a necessary minimum clearance for a customer specific monolithic ceramic shell.
