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. In order to determine the minimum clearance the following in vivo, in vitro and in silico tests were taken into account:
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. The stiffnesses of the metal shells (Cm) were experimentally determined within a two-point-loading frame acc. to ISO 7206-12. 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). 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).Introduction
Materials and Methods
Acetabular cup deformation is an important topic in today's THA and was investigated for a variety of metal cup designs (e.g. 1,2,3). Cup deformation caused by press-fit forces can have negative effects on the performance of such systems (e.g. high friction, metal ion release). When considering new materials for monolithic acetabular cups - such as ceramics - detailed knowledge about the deformation behaviour is essential to ensure successful performance. Therefore, the deformation behaviour of monolithic ceramic cups was investigated. Testing was conducted with monolithic ceramic cups (under development, not approved) of size 46mm and 64mm. One cup design of each size had a constant wall thickness of 3.0mm and an offset of 0.0mm (center of rotation on front face level), the other design was lateralized with an offset of 3.5mm (46mm) or 5.0mm (64mm), leading to an increased wall thickness. First, 3 cups of each design were impacted into 1.0mm underreamed Sawbones® blocks (pcf 30, geometry: see (2)). Second, all cups were quasi-statically assembled into the Sawbones® blocks of the same design using a material testing machine. Third, the cups were placed in a two-point-loading frame (acc. to ISO/DIS 7206–12:2014(E)) and a load of up to 1kN was applied. The inner diameter of all cups was measured under unloaded and loaded conditions for all scenarios using a coordinate measurement machine at 9 locations of each cup, 1.5mm below the front face (Fig.1). As the diametrical deformation (unloaded inner diameter – loaded inner diameter) was not normally distributed a Wilcoxon test was performed to statistically analyse the deformation differences of the different cup designs (p<0.05).Introduction
Materials and Methods
Recent literature demonstrates that the assembly load to connect ball head and femoral stem affects the taper junction fretting wear evolution in THR [1]. During assembly the surface profile peaks of the mostly threaded tapers are deformed. This contributes to the taper locking effect. Very little is known about this deformation process and its role in the evolution of fretting and wear. Therefore, this study aimed to experimentally determine the deformation of the profile peaks after the initial assembly process. 36 tapers of three different stem materials acc. to ISO5832-3 (titanium), ISO5832-9 (steel), ISO5832-12 (cobalt chromium) and 36 ceramic ball heads were tested under quasi-static (4kN) and dynamic (impaction) (3.7±0.3kN) axial assembly. Before and after loading 4 surface profiles in 90° offset were measured on each taper. Height differences of profile peaks and areas under profile curves were calculated and compared. Both parameters provide insights into the deformation behavior of the surface structure. Additionally, subsidence of tapers into ball heads was measured and subsidence rates were calculated with regard to varying impaction forces. Due to different thermal expansion coefficients tapers could be disconnected from ball heads by utilizing liquid nitrogen. Thus, further surface damage due to disassembly was avoided. Statistical analysis was performed using a Wilcoxon test (p<0.05).Introduction
Materials and Methods
