title: Acetabular shell deformation as a function of shell stiffness and bone strength.
abstract: Press-fit acetabular shells used for hip replacement rely upon an interference fit with the bone to provide initial stability. This process may result in deformation of the shell. This study aimed to model shell deformation as a process of shell stiffness and bone strength. A cohort of 32 shells with two different wall thicknesses (3 and 4 mm) and 10 different shell sizes (44- to 62-mm outer diameter) were implanted into eight cadavers. Shell deformation was then measured in the cadavers using a previously validated ATOS Triple Scan III optical system. The shell-bone interface was then considered as a spring system according to Hooke's law and from this the force exerted on the shell by the bone was calculated using a combined stiffness consisting of the measured shell stiffness and a calculated bone stiffness. The median radial stiffness for the 3-mm wall thickness was 4192 N/mm (range, 2920-6257 N/mm), while for the 4-mm wall thickness the median was 9633 N/mm (range, 6875-14,341 N/mm). The median deformation was 48 µm (range, 3-187 µm), while the median force was 256 N (range, 26-916 N). No statistically significant correlation was found between shell stiffness and deformation. Deformation was also found to be not fully symmetric (centres 180° apart), with a median angle discrepancy of 11.5° between the two maximum positive points of deformation. Further work is still required to understand how the bone influences acetabular shell deformation.
authors.affiliation: Medical Products Division, CeramTec GmbH, Plochingen, Germany p.dold@ceramtec.de.
authors.forename: Philipp
authors.initials: P
authors.lastname: Dold
authors.affiliation: Medical Products Division, CeramTec GmbH, Plochingen, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Pandorf
authors.affiliation: Medical Products Division, CeramTec GmbH, Plochingen, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Flohr
authors.affiliation: Medical Products Division, CeramTec GmbH, Plochingen, Germany.
authors.forename: Roman
authors.initials: R
authors.lastname: Preuss
authors.affiliation: School of Mechanical Engineering, Newcastle University, Newcastle upon Tyne, UK.
authors.forename: Martin C
authors.initials: MC
authors.lastname: Bone
authors.affiliation: School of Mechanical Engineering, Newcastle University, Newcastle upon Tyne, UK.
authors.forename: Tom J
authors.initials: TJ
authors.lastname: Joyce
authors.affiliation: Newcastle Surgical Training Centre, Freeman Hospital, Newcastle upon Tyne, UK.
authors.forename: James
authors.initials: J
authors.lastname: Holland
authors.affiliation: Newcastle Surgical Training Centre, Freeman Hospital, Newcastle upon Tyne, UK.
authors.forename: David
authors.initials: D
authors.lastname: Deehan
mesh_terms: D000077:Acetabulum / Q000502:physiology* / Q000601:surgery*; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip; D001696:Biomechanical Phenomena / Q000502:physiology; D019544:Equipment Failure Analysis; D005260:Female; D006622:Hip Prosthesis; D006801:Humans; D008297:Male
keywords: Biomechanical testing/analysis; acetabular shell deformation; acetabular shell stiffness; combined stiffness; hip prostheses