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
Realistic in-vivo loads on knee implants from telemetric analyses were recently published. Impacting an implant, especially a ceramic one, will produce high peak stresses within the component. Data for loads occurring during implantation of a knee implant are scarce. To ensure a safe impaction of ceramic tibial trays the stresses caused by it need to be known. Impaction testing including force measurements (using Kistler piezo load cell 9351B) was performed on a ceramic tibial tray. The same test was simulated by computational analysis using FEM (Finite-Element-Method). Because the forces measured and those calculated by FEM were significantly different, an in vitro impaction study was performed to obtain realistic loads for a ceramic tibial tray. A surgeon was asked to perform heavy hammer blows which may occur during implantation. Using a high speed camera (phantom V7.2) the velocity of the hammer at the time of impaction was determined. Using this parameter instrumented ceramic tibial trays (BPK-S Knee, P. Brehm) were implanted into a biomechanical Sawbones® model. Linear strain gauges were attached to the four fins of the tibial tray as these are the regions of highest stresses. Simulating the surgeon's highest impacts measurements were conducted at a frequency of 1 MHz. The identical hammer was used in this in vitro study and the velocity of the hammer was measured by using the same high speed camera. To investigate the damping effect of bone cement Palacos®R bone cement was used. Only worst-case impacts within the range achieved by the surgeon were applied to evaluate the stress distribution within the ceramic tibial tray.Introduction
Materials and Methods
