Within total hip replacement, articulation of the femoral head near the rim of the acetabular liner creates undesirable conditions leading to a propensity for dislocation[1], increased contact stresses[2], increased load and torque imparted on the acetabular component[3], and increased wear[4]. Propensity for rim loading is affected by prosthesis placement, as well as the kinematics and loading of the patient. The present study investigates these effects. CT scans from an average-sized patientwere segmented for the hemipelvis and femur of interest. DePuy Synthes implant models were aligned in a neutral position in Hypermesh. The acetabular liner was assigned deformable solid material properties, and the remainder of the model was assigned rigid properties. Joint reaction forces and kinematics of hip flexion were taken from the public Orthoload database to represent ADLs [5]: Active flexion lying on a table, gait, bending to lift and move a load, and sit-stand. The pelvis was fully constrained, while three-degree-of-freedom (3-DOF) forces were applied to the femur. Hip flexion was kinematically-prescribed while internal-external (I-E) and adduction-abduction (Ad-Ab) DOFs were constrained. Angles of acetabular implant positioning were based on published data by Rathod [6]. Femoral implant position was chosen based on cadaveric in vitro DePuy Synthes measurements of variation in femoral prosthesis position reported previously [7]. Acetabular and Femoral alignment angles were represented for nominal position, as well as positioning + 1σ and + 2σ from the mean in both anteversion and inclination for acetabular components, and both Varus/Valgus and Flexion (angle in sagittal plane) for the femoral component. The analyses were automated within Matlab to execute 68 finite element analyses in Abaqus Explicit and structured in a DOE style analysis with Cup inclination, Cup version, Stem Flexion, and Stem Varus/Valgus, and Activity as variables of interest (64 runs + 4 centerpoints = 68 analyses). From a previous study it was known that acetabular component inclination had the greatest effect on contact pressure location [7], so all data were analyzed relative to inclination, allowing other positioning variables to be represented as variation per inclination position. Results are presented as a percentage, with 0% being pole loading and 100% being rim loading, to normalize for head diameter.INTRODUCTION
METHODS
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. Patient specific finite element models were generated of two femurs, one male and one female. An automated algorithm positioned and sized a Corail stem (DePuy Synthes, Warsaw) into each of the femurs to achieve maximum fill of the medullary canal without breaching into the cortical bone boundaries.. Peak joint contact and muscle forces associated with level gait were applied[2] and scaled to the body mass of each subject, whilst the distal femur was rigidly constrained. The space prone to surgical variation was defined by the “gap” between the stem and the inner boundary of the cortical bone. The anterior/posterior and the varus/valgus alignment of the stem within this “gap” was controlled by varying the location of the points defining the shaft axis. The points were taken at 20% and 80% of the stem length (Figure 1). The anteversion angle as well as the vertical and the medial position of the stem were controlled by changing the location of the head centre within the femoral head radius. The location of these points was varied using Latin Hypercube sampling to generate 200 models per femur, each with a unique stem position. The risk of failure was evaluated based on stem micromotion, equivalent strains, and percentage of the bone-prosthesis contact area experiencing more than 7000 µstrains [3].Introduction
Methods
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. Finite Element (FE) simulations were run for 34 femurs (from the Melbourne femur collection) for a diverse patient cohort of joint replacement age (50 – 80 yrs). To account for the diversity in shape, the cohort included femurs with the maxima, minima and medians for 26 geometric parameters. Subject-specific FE models were generated from CT scans. An in-house developed algorithm positioned idealized versions of Corail and Summit (Figure 1) into each of the femur models so that the stem and femur shaft axes were aligned, and the vertical offset between the trunnion centre and the femoral head centre was minimised. For such a position, the algorithm selected the size that achieved maximum fill of the medullary canal without breaching the cortical bone boundaries. Joint contact and muscle forces were calculated for level gait and stair climbing[2] and scaled to the body mass of each subject. Femurs were rigidly constrained at the condyles. Risk of failure was assessed based on (i) stem micromotion, (ii) equivalent strains (iii) percentage of the bone-prosthesis contact area experiencing micromotions < 50 μm, micromotions > 150 μm and strains > 7000 μstrains [3].Introduction
Methods
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. FE models were generated from QCT scans of eight cadaveric femurs taken from the Melbourne Femur Collection (4 male and 4 female; BMI: 18.7 – 36.8 kg.m-2; age: 59 – 80 years) which were of joint replacement age. Heterogeneous bone material properties were assigned based on the CT greyscale information. Each femur was implanted with the collared and collarless version of Corail femoral stem (DePuy, Leeds, United Kingdom). The stems were sized and positioned so that the prosthesis filled the medullary canal with minimal gap between the prosthesis and the inner boundary of the cortical bone. The peak muscle and joint contact forces associated with level gait were applied and the distal femur was rigidly fixed. The forces were scaled based on the body weight for each subject. Micromotion, as well as microstrains at the bone-prosthesis interface were measured for each subject. Paired t-test was run to compare the micromotion and the microstrains measured for the collared and collarless prosthesis.Introduction
Materials and Methods
