Acetabular shell deformation as a function of shell stiffness and bone strength.

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.

Shorter, rough trunnion surfaces are associated with higher taper wear rates than longer, smooth trunnion surfaces in a contemporary large head metal-on-metal total hip arthroplasty system.

Taper wear at the head-neck junction is a possible cause of early failure in large head metal-on-metal (LH-MoM) hip replacements. We hypothesized that: (i) taper wear may be more pronounced in certain product designs; and (ii) an increased abductor moment arm may be protective. The tapers of 104 explanted LH-MoM hip replacements revised for adverse reaction to metal debris (ARMD) from a single manufacturer were analyzed for linear and volumetric wear using a co-ordinate measuring machine. The mated stem was a shorter 12/14, threaded trunnion (n=72) or a longer, smooth 11/13 trunnion (n=32). The abductor moment arm was calculated from pre-revision radiographs. Independent predictors of linear and volumetric wear included taper angle, stem type, and the horizontal moment arm. Tapers mated with the threaded 12/14 trunnion had significantly higher rates of volumetric wear (0.402 mm3/yr vs. 0.123 mm3/yr [t=-2.145, p=0.035]). There was a trend to larger abductor moment arms being protective (p=0.055). Design variation appears to play an important role in taper-trunnion junction failure. We recommend that surgeons bear these findings in mind when considering the use of a short, threaded trunnion with a cobalt-chromium head.

A novel four-caster manual vehicle manoeuvring investigation: Higher loading-weights require larger turning spaces.

Patient-hoists, goods-trolleys and other omni-directional manually operated vehicles are ubiquitous. Yet no substantive, empirically based dynamic analysis has been made of these four-caster vehicles despite manual handling concerns. A relationship between loading-weight and turning space is indicated by theoretical analysis which further shows that this effect is represented by only 11 different manoeuvres. A qualitative account of the theory is presented. These 11 manoeuvres were implemented experimentally. A total of 17 subjects selected a maximum comfortable loading-weight for the four-caster vehicle for each of the 11 manoeuvres. Vehicle displacement and handle forces were measured for different centres of zero velocity. The median loading-weight of the manoeuvre with the highest loading-weight selections was 101% greater than the mean loading-weight of the three manoeuvres with the lowest loading-weight selections. The manoeuvre with the highest loading-weight selections required a larger vehicle turning space: one dimension increased by 37% (173 mm) compared with the three lowest loading-weight selection manoeuvres and the other dimension increased by 17% (130 mm) compared with one of the lowest loading-weight selection manoeuvres. Higher loading-weights require larger turning spaces. These results can contribute to building designs which facilitate safe manual manoeuvring of four-caster vehicles.

Changes in surface topography at the TKA backside articulation following in vivo service: a retrieval analysis.

PURPOSE: With the advent of modular total knee arthroplasty (TKA) systems, backside wear at the articulation between the ultra-high-molecular-weight-polyethylene (UHMWPE) component undersurface and the tibial baseplate has received increasing attention as a source of clinically significant polyethylene wear debris. The aim of this study was to investigate the reciprocating interface at the TKA undersurface articulation using profilometry after in vivo service. Our null hypothesis was that there would be no discernible pattern or relationship between the metal tibial baseplate and UHMWPE surface profile. METHODS: A nanoscale analysis of thirty retrieved fixed-bearing TKA explants was performed. Surface roughness (Sa) and skewness (Ssk) were measured on both the UHMWPE component undersurface and the tibial baseplate of explants using a non-contacting profilometer (1 nm resolution). Four pristine unimplanted components of two different designs (Stryker Kinemax and DePuy PFC) were examined for control purposes. RESULTS: Mean explant baseplate surface roughness was 1.24 µm (0.04-3.01 µm). Mean explant UHMWPE undersurface roughness was 1.16 µm (0.23-2.44 µm). Each explant had an individual roughness pattern with unique baseplate and undersurface UHMWPE surface roughness that was different from, but closely related to, surface topography observed in control implants of the same manufacturer and design. Following in vivo service, UHMWPE undersurface showed changes towards a negative skewness, demonstrating that wear is occurring at the backside interface. CONCLUSION: In vivo loading of the TKA prosthesis leads to measurable changes in surface profile at the backside articulation, which appear to be dependent on several factors including implant design and in vivo duration. These findings are consistent with wear occurring at this surface. Findings of this study would support the use of a polished tibial tray over an unpolished design in total knee arthroplasty with the goal of reducing PE wear by means of providing a smoother backside countersurface for the UHMWPE component.

Validation of an optical system to measure acetabular shell deformation in cadavers.

Deformation of the acetabular shell at the time of surgery can result in poor performance and early failure of the hip replacement. The study aim was to validate an ATOS III Triple Scan optical measurement system against a co-ordinate measuring machine using in vitro testing and to check repeatability under cadaver laboratory conditions. Two sizes of custom-made acetabular shells were deformed using a uniaxial/two-point loading frame and measured at different loads. Roundness measurements were performed using both the ATOS III Triple Scan optical system and a co-ordinate measuring machine and then compared. The repeatability was also tested by measuring shells pre- and post-insertion in a cadaver laboratory multiple times. The in vitro comparison with the co-ordinate measuring machine demonstrated a maximum difference of 5 µm at the rim and 9 µm at the measurement closest to the pole of the shell. Maximum repeatability was below 1 µm for the co-ordinate measuring machine and 3 µm for the ATOS III Triple Scan optical system. Repeatability was comparable between the pre-insertion (below 2 µm) and post-insertion (below 3 µm) measurements in the cadaver laboratory. This study supports the view that the ATOS III Triple Scan optical system fulfils the necessary requirements to accurately measure shell deformation in cadavers.

A novel method for measuring acetabular cup deformation in cadavers.

Press-fit acetabular shells are subject to compressive loading conditions upon insertion during surgery. These conditions may result in deformation of the shell, potentially adversely affecting the performance of any liner fitted within it. This technical note describes the method used to measure acetabular shell deformation in a cadaver study using an optical GOM ATOS Triple Scan system. Six custom-made titanium acetabular shells were scanned prior to and immediately following insertion into three cadavers (one shell per acetabulum). A three-dimensional colour map was created for each shell, and cross sections were taken at the rim to determine the maximum diametrical deformation. Values of between 30 and 150 µm were recorded, with the results suggesting a size-dependent response.

Comparative wear tests of ultra-high molecular weight polyethylene and cross-linked polyethylene.

Wear particle-induced osteolysis is a major concern in hip implant failure. Therefore, recent research work has focussed on wear-resistant materials, one of the most important of which is cross-linked polyethylene. In view of this, the objective of this study was to compare the in vitro wear performance of cross-linked polyethylene to traditional ultra-high molecular weight polyethylene. In order to mimic appropriate in vivo conditions, a novel high-capacity wear tester called a circularly translating pin-on-disc was used. The results of this in vitro study demonstrated that the wear rate for cross-linked polyethylene was about 80% lower than that of conventional ultra-high molecular weight polyethylene. This difference closely matches in vivo results reported in the literature for total hip replacements that use the two biopolymers. The in vitro results were also verified against ASTM F732-00 (standard test method for wear testing of polymeric materials for use in total joint prostheses). The 50-station circularly translating pin-on-disc proved to be a reliable device for in vitro wear studies of orthopaedic biopolymers.

A biomechanical study on variation of compressive force along the Acutrak 2 screw.

INTRODUCTION: Acutrak 2 screws are commonly used for scaphoid fracture fixation. To our knowledge, the variation in compressive force along the screw has not been investigated before. The objectives of our study were to measure variance in compression along the length of the standard Acutrak 2 screw, to identify the region of the screw which produces the greatest compression and to discuss the clinical relevance of this to the placement of the screw for scaphoid fractures. MATERIALS AND METHODS: A laboratory model was set up to test the compressive force at 2mm intervals along the screw, using solid polyurethane foam (Sawbone) blocks of varying width. The Acutrak 2 screws were introduced in the standard method. Forces were measured using a custom-made load cell washer introduced between the Sawbone blocks and were plotted as a graph along the whole length of the screw. RESULTS: Maximum compression was at the mid-point of the screw. Overall compressive forces were higher in the proximal half of the screw by 19% when compared with the distal half. Minimum compression was seen at 4mm or less from either end of the screw. CONCLUSIONS: There is variation in compression along the length of the standard Acutrak 2 screw and the maximum compression was obtained at the mid-point of the screw. From this study, we would recommend when using an Acutrak 2 screw for internal fixation of scaphoid fractures, to attain maximum compressive force, place the fracture at the mid-point of the Acutrak screw. If this is not possible, then place the fracture towards the proximal half of the screw.