Introduction

Traditionally, radiological union of fractures treated with an Ilizarov frame is confirmed by a period of dynamization - destabilisation of the frame for a period prior to removal. Reduced clinic availability during the COVID-19 pandemic caused a shift to selective dynamisation in our department, whereby lower risk patients had their frames removed on the same day as destabilisation. This study investigates the effects of this change in practice on outcomes and complication rates.

Despite of the high success of TKA, 20% of recipients remain dissatisfied with their surgery. There is an increasing discordance in the literature on what is an optimal goal for component alignment. Furthermore, the unique patient specific anatomical characteristics will also play a role. The dynamic characteristic of a TKR is a product of the complex interaction between a patient's individual anatomical characteristics and the specific alignment of the components in that patient knee joint. These interactions can be better understood with computational models. Our objective was to characterise ligament characteristics by measuring knee joint laxity with functional radiograph and with the aid of a computational model and an optimisation study to estimate the subject specific free length of the ligaments.

Pre-operative CT and functional radiographs, varus and valgus stressed X-rays assessing the collateral ligaments, were captured for 10 patients. CT scan was segmented and 3D–2D pose estimation was performed against the radiographs. Patient specific tibio-femoral joint computational model was created. The model was virtually positioned to the functional radiograph positions to simulate the boundary conditions when the knee is stressed. The model was simulated to achieve static equilibrium. Optimisation was done on ligament free length and a scaling coefficient, flexion factor, to consider the ligaments wrapping behaviour.

Our findings show the generic values for reference strain differ significantly from reference strains calculated from the optimised ligament parameters, up to 35% as percentage strain. There was also a wide variation in the reference strain values between subjects and ligaments, with a range of 37% strain between subjects. Additionally, the knee laxity recorded clinically shows a large variation between patients and it appears to be divorced from coronal alignment measured in CT. This suggests the ligaments characteristics vary widely between subjects and non-functional imaging is insufficient to determine its characteristics. These large variations necessitate a subject-specific approach when creating knee computational models and functional radiographs may be a viable method to characterise patient specific ligaments.

Introduction

Successful designs of total hip replacement need to be robust to surgery-related variability. Until recently, only simple parametric studies have explored the influence of surgical variability [1]. This study presents a systematic method for quantifying the effect of variability in positioning on the primary stability of femoral stems using finite element (FE) models.

Introduction

Pre-clinical testing of orthopaedic devices could be improved by comparing performance with established implants with known clinical histories. Corail and Summit (DePuy Synthes, Warsaw) are femoral stems with proven survivorship of 95.1% and 98.1% at 10 years [1], which makes them good candidates as benchmarks when evaluating new stem designs. Hence, the aim of this study was to establish benchmark data relating to the primary stability of Corail and Summit stems.

Introduction

Primary stability is essential for long-term performance of cementless femoral components. There is debate as to whether collars contribute to primary stability. The results from experimental studies and finite element (FE) analysis have been variable and contradictory. Subtle differences in performance are often swamped by variation between cadaveric specimens in vitro, whereas FE studies tend to be performed on a single femur. However, FE studies have the potential to make comparisons of implant designs within the same cohort of femurs, allowing for subtle performance differences to be identified if present. This study investigates the effect of a collar on primary stability of a femoral prosthesis across a representative cohort of femurs.

We sought to determine what dimensional changes occurred from wear testing of a total knee implant, as well as whether any changes developed within the polyethylene subsurface. Three fixed bearing implants underwent wear simulator testing to 6.1 million cycles. Gravimetric analysis and micro-CT scans were performed pre-test, mid-test, and post-test. Wear volume and surface deviations were greater during 0–3.2 million cycles (91±12 mm³) than from 3.2–6.1 million cycles (52±18 mm³). Deviations (wear and creep) occurred across all surfaces of the tibial inserts, including the articular surface, backside surface, sides, and locking mechanism. No subsurface changes were found. The micro-CT results were a useful adjunct to gravimetric analysis, better defining the dimensional changes that occurred with testing and ruling out subsurface fatigue.

Introduction:

Primary stability is crucial for long-term fixation of cementless tibial trays. Micromotion less than 50 μm is associated with stable bone ingrowth and greater than 150 μm causes the formation of fibrous tissue around the implant [1, 2]. Finite element (FE) analysis of complete activities of daily living (ADL's) have been used to assess primary stability, but these are computationally expensive. There is an increasing need to account for both patient and surgical variability when assessing the performance of total joint replacement. As a consequence, an implant should be evaluated over a spectrum of load cases. An alternative approach to running multiple FE models, is to perform a series of analyses and train a surrogate model which can then be used to predict micromotion in a fraction of the time. Surrogate models have been used to predict single metrics, such as peak micromotion. The aim of this work is to train a surrogate model capable of predicting micromotion over the entire bone-implant interface.

BACKGROUND:

Implant wear continues to be a limitation of total knee replacement (TKR). Wear simulator studies are a valuable screening tool in new implant development. The purpose of this study was to determine the ability of micro-CT to prospectively measure wear in TKR implants during a wear simulator trial.

Introduction

Cementless tibial fixation has been used for over 30 years. There are several potential advantages including preservation of bone stock and ease of revision. More importantly, for young active patients there is the potential for increased longevity of fixation. However, the clinical results have been variable, with reports of extensive radiolucent lines, rapid early migration and aseptic loosening. Problems appear to stem from a failure to become sufficiently osseointegrated, which in turn suggests a lack of primary stability. In order to achieve boney ingrowth, interface micromotions should be less than 50 microns, whereas fibrous tissue formation is known to occur if micrmotions are in excess of 150 microns. The degree of micromotion at the bone-implant interface are dependent on the kinematics and kinetics of the replaced joint. Finite element analyses has been used to assess primary stability, however, it is becoming increasing difficult to differentiate performance. The aim of this study was too examine the micromotion for a variety of different activities for three commercially available tibial tray designs.

Introduction

The number of total knee joint replacements has increased dramatically, from 28,000 in 2004 to over 73,000 in 2008 in the UK. This increase in procedures means that there is a need to assess the performance of an implant design in the general population. For younger, more active patients, cementless tibial fixation is an attractive alternative means of fixation and has been used for over 30 years. However, the clinical results with cementless fixation have been variable, with reports of extensive radiolucent lines, rapid early migration and aseptic loosening [1]. This study investigates the inter-patient variability of bone strain at the implant-bone interface of 130 implanted tibias over a full gait cycle.