Stem malalignment in total hip arthroplasty (THA) has been associated with poor long-term outcomes and increased complications (e.g. periprosthetic femoral fractures). Our understanding of the biomechanical impact of stem alignment in cemented and uncemented THA is still limited. This study aimed to investigate the effect of stem fixation method, stem positioning, and compromised bone stock in THA. Validated FE models of cemented (C-stem – stainless steel) and uncemented (Corail – titanium) THA were developed to match corresponding experimental model datasets; concordance correlation agreement of 0.78 & 0.88 for cemented & uncemented respectively. Comparison of the aforementioned stems was carried out reflecting decisions made in the current clinical practice. FE models of the implant positioned in varus, valgus, and neutral alignment were then developed and altered to represent five different bone defects according to the Paprosky classification (Type I – Type IIIb). Strain was measured on the femur at 0mm (B1), 40mm (B2), and 80mm (B3) from the lesser trochanter.Abstract
Objectives
Methods
Currently, total hip replacement surgery is an effective treatment for osteoarthritis, where the damaged hip joint is replaced with an artificial joint. Stress shielding is a mechanical phenomenon that refers to the reduction of bone density as a result of altered stresses acting on the host bone. Due to solid metallic nature and high stiffness of the current orthopaedic prostheses, surrounding bones undergo too much bone resorption secondary to stress shielding. With the use of 3D printing technology such as selective laser melting (SLM), it is now possible to produce porous graded microstructure hip stems to mimics the surrounding bone tissue properties. In this study we have compared the physical and mechanical properties of two triply periodic minimal surface (TPMS) lattice structure namely gyroid and diamond TPMS. Based on initial investigations, it was decided to design, and 3D print the gyroid and diamond scaffolds having pore size of 800 and 1100 um respectively. Scaffold of each type of structure were manufactured and were tested mechanically in compression (n=8), tension (n=5) and bending (n=1).Abstract
Objectives
Method
Ring breakage is a rare but significant complication requiring revision surgery and prolonging the course of treatment. We have encountered three cases with Taylor Spatial Frames (TSF) with breakage at the half ring junction of the distal ring. This experimental study examines the strains produced at different locations on the distal ring during loading and the effects of altering the construct in order to develop techniques to minimise the risk of breakage. We mounted different TSF constructs on tibia sawbone models. Construct 1 reproducing the configuration of cases where failure was seen, Construct 2 with different wire and half pin configuration and construct 3 with the distal ring rotated 60 degrees. Strain Gauges were attached to different locations and measurements were collected during loading. Statistical analysis was subsequently performed.Introduction
Materials and Methods
To quantify the micro-motion at the fracture gap in a tibial fracture model stabilised with an external fixator. A surrogate model of a tibia and a cadaver leg were fractured and stabilised using a two-ring hexapod external fixator. They were tested initially under static loading and then subjected to vibration.Aim
Method
There are a number of periprosthetic femoral fracture (PFF) fixation failures. In several cases the effect of fracture configuration on the performance of the chosen fixation method has been underestimated. As a result, fracture movement within the window that seems to promote callus formation has not been achieved and fixations ultimately failed. This study tested the hypothesis that: PFF configuration and the choice of plate fixation method can be detrimental to healing. A series of computational models were developed, corroborated against measurements from a series of instrumented laboratory models and in vivo case studies. The models were used to investigate the fixation of different fracture configurations and plate fixation parameters. Surface strain and fracture movement were compared between the constructs. A strong correlation between the computational and experimental models was found. Computational models showed that unstable fracture configurations increase the stress on the plate fixation. It was found that bridging length plays a pivotal role in the fracture movement. Rigid fixations, where there is clinical evidence of failure, showed low fracture movement in the models (<0.05mm); this could be increased with different screw and plate configurations to promote healing. In summary our results highlighted the role of fracture configuration in PFF fixations and showed that rigid fixations that suppress fracture movement could be detrimental to healing.
Periprosthetic femoral fractures can occur as a complication of total hip arthroplasty and are often challenging to treat as the mechanical scenario is influenced by the presence of the metal prosthesis within the bone. This research focuses on finding the optimum fixation for transverse, Vancouver type B1 periprosthetic fractures, stabilised using locking plates and secured using screws. The aim of this study was to experimentally validate a computer model of a human femur, develop that model to represent a periprosthetic femoral fracture fixation and show how the model could be used to indicate differences between plating techniques. In the first development stage, both a laboratory model and a finite element model were developed to evaluate the mechanical behaviour of an intact composite femur under axial loading. Axial strains were recorded along the medial length of the femur in both cases and compared to provide validation for the computational model predications. The computational intact femur model was then modified to include a cemented total hip replacement, and further adapted to include a periprosthetic fracture stabilised using a locking plate, with unicortical screws above, and bicortical screws below the transverse fracture. For the intact femur case, the experimental and computational strain patterns correlated well with an average difference of 16%. Following the inclusion of the stem, there was a reduction in the strain in the region of the prosthesis reducing by an average of 45%. There was also a large increase in bulk stiffness with the introduction of the prosthesis. When the fracture and plate fixation were included, there was little difference in the proximal strain where the stem dominated, and the strains in the distal region were found to be highly sensitive to the distribution of the screws. The results of this study indicate that screw configuration is an important factor in periprosthetic fracture fixation. A laboratory model of the periprosthetic facture case is now under development to further validate the computational models and the two approaches will then be used to determine optimum fixation methods for a range of clinical scenarios.
