Accurate orientation of the acetabular component during a total hip replacement is critical for optimising patient function, increasing the longevity of components, and reducing the risk of complications. This study aimed to determine the validity of a novel VR platform (AescularVR) in assessing acetabular component orientation in a simulated model used in surgical training. The AescularVR platform was developed using the HTC Vive® VR system hardware, including wireless trackers attached to the surgical instruments and pelvic sawbone. Following calibration, data on the relative position of both trackers are used to determine the acetabular cup orientation (version and inclination). The acetabular cup was manually implanted across a range of orientations representative of those expected intra-operatively. Simultaneous readings from the Vicon® optical motion capture system were used as the ‘gold standard’ for comparison. Correlation and agreement between these two methods was determined using Bland-Altman plots, Pearson's correlation co-efficient, and linear regression modelling.Abstract
Objectives
Methods
Primary mechanical stability is important with uncemented THR because early migration is reduced, leading to more rapid osseointegration between the implant and bone. Such primary mechanical stability is provided by the design features of the device. The aim of this study was to compare the migration patterns of two uncemented hip stems, the Furlong Active and the Furlong HAC stem; the study was designed as a randomised control trial. The implants were the Furlong HAC, which is an established implant with good long term results, and the Furlong Active, which is a modified version of the Furlong HAC designed to minimise stress concentrations between the implant and bone, and thus to improve fixation. The migration of 43 uncemented femoral components for total hip replacement was measured in a randomised control trial using Roentgen Stereophotogrammetric Analysis (RSA) over two years. Twenty-three Furlong HAC and twenty Furlong Active stems were implanted into 43 patients. RSA examinations were carried out post-operatively, and at six months, 12 months and 24 months post-operatively. The patients stood in-front of a purpose made calibration frame which contained accurately positioned radio-opaque markers. From the obtained images, the 3-D positions of the prosthesis and the host bone were reconstructed. Geometrical algorithms were used to identify the components of the implant. These algorithms allowed the femoral component to be studied without the need to attach markers to the prosthesis. The migration was calculated relative to the femoral coordinate system representing the anterior-posterior (A-P), medial-lateral (M-L) and proximal-distal (P-D) directions respectively. Distal migration was termed subsidence.Introduction
Materials and methods
The results of the original mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing because of high dislocation rates (11%). This original implant used a flat bearing articulation on the tibial tray. To address the issue of dislocation a new implant (domed tibia with biconcave bearing to increase entrapment) was introduced with a modified surgical technique. The aim of this study was to compare the risk of dislocation between a domed and flat lateral UKR. Separate geometric computer models of an Oxford mobile bearing lateral UKR were generated for the two types of articulation between the tibial component and the meniscal bearing: Flat-on-flat (flat) and Concave-on-convex (domed). Each type of mobile bearing was used to investigate three distinct dislocation modes observed clinically: lateral to medial dislocation, with the bearing resting on the tray wall (L-M-Wall); medial to lateral dislocation, out of the joint space (M-L); anterior to posterior dislocation, out of the joint space (A-P). A size C tray and a medium femoral component and bearing were used in all models. The femoral component, tibial tray and bearing were first aligned in a neutral position. For each dislocation the tibial tray was restrained in all degrees of freedom. The femoral component was restrained from moving in the anterior-posterior directions and in the medial-lateral directions. The femoral component was also restrained from rotating about the anterior-posterior, medial-lateral and superior-inferior directions. This meant that the femoral component was only able to move in the superior-inferior direction. Different bearing sizes were inserted into the model and the effect that moving the femoral component medially and laterally had on the amount of distraction required to cause bearing dislocation was investigated.Introduction
Methods
Unicompartmental Knee Replacement (UKR) is an appealing alternative to Total Knee Replacement (TKR) when the patient has isolated compartment osteoarthritis (OA). A common observation post-operatively is radiolucency between the tibial tray wall and the bone. In addition, some patients complain of persistent pain following implantation with a UKR; this may be related to elevated bone strains in the tibia. The aim of this study was to investigate the mechanical environment of the tibia bone adjacent to the tray wall, following UKR, to determine whether this region of bone resorbs, and how altering the mechanical environment affects tibia strains. A finite element (FE) model of a cadaver tibia implanted with an Oxford UKR was used in this study, based on a validated model. A single static load, measured in-vivo during a step-up activity was used. There was a 1 mm layer of cement surrounding the keel in the cemented UKR, and the cement filled the cement pocket. In accordance with the operating procedure, no cement was used between the tray wall and bone. For the cementless UKR a layer of titanium filled the cement pocket. An intact tibia was used to compare to the cemented and cementless UKR implanted tibiae. The tibia was sectioned by the tray wall, defining the radiolucency zone (parallel to the vertical tray wall, 2 mm wide with a volume of 782.5 mm3), corresponding to the region on screened x-rays where radiolucencies are observed. Contact mechanics algorithms were used between all contacting surfaces; bonded contact was also introduced between the tray wall and adjacent bone, simulating a mechanical tie between them. Strain energy density (SED), was compared between the intact and implanted tibia for the radiolucency zone. Equivalent strains were compared on the proximal tibia between the intact and implanted tibia models. Forty patients (20 cemented, 20 cementless) who had undergone UKR were randomly selected from a database, and assessed for radiolucency.Introduction
Materials and methods
