Development and validation of a computational model of the knee joint for the evaluation of surgical treatments for osteoarthritis.

A three-dimensional (3D) knee joint computational model was developed and validated to predict knee joint contact forces and pressures for different degrees of malalignment. A 3D computational knee model was created from high-resolution radiological images to emulate passive sagittal rotation (full-extension to 65°-flexion) and weight acceptance. A cadaveric knee mounted on a six-degree-of-freedom robot was subjected to matching boundary and loading conditions. A ligament-tuning process minimised kinematic differences between the robotically loaded cadaver specimen and the finite element (FE) model. The model was validated by measured intra-articular force and pressure measurements. Percent full scale error between FE-predicted and in vitro-measured values in the medial and lateral compartments were 6.67% and 5.94%, respectively, for normalised peak pressure values, and 7.56% and 4.48%, respectively, for normalised force values. The knee model can accurately predict normalised intra-articular pressure and forces for different loading conditions and could be further developed for subject-specific surgical planning.

Stresses in cement mantles of hip replacements: effect of femoral implant sizes, body mass index and bone quality.

The effects of femoral prosthetic heads of diameters 22 and 28 mm were investigated on the stability of reconstructed hemi-pelves with cement mantles of thicknesses 1-4 mm and different bone qualities. Materialise medical imaging package and I-Deas finite element (FE) software were used to create accurate geometry of a hemi-pelvis from CT-scan images. Our FE results show an increase in cement mantle stresses associated with the larger femoral head. When a 22 mm femoral head is used on acetabulae of diameters 56 mm and above, the probability of survivorship can be increased by creating a cement mantle of at least 1 mm thick. However, when a 28 mm femoral head is used, a cement mantle thickness of at least 4 mm is needed. Poor bone quality resulted in an average 45% increase in the tensile stresses of the cement mantles, indicating resulting poor survivorship rate.

Configuration of anchorage holes affects fixation of the acetabular component in cemented total hip replacement--a finite element study.

Our survey of current practice among UK orthopaedic surgeons shows wide variations in fixation techniques. The aim of this study, is to investigate the effect of drilling different configurations of anchorage holes in the acetabulum on implant stability. To avoid variables that could incur during in vitro testing, we used commercially available COSMOS finite element analysis package to investigate the stress distributions, deformations, and strains on the cement mantle when drilling three large anchorage holes and six smaller ones, with straight and rounded cement pegs. The results, which are in line with our in vitro studies on simulated reconstructed acetabulae, indicate better stability of the acetabular component when three larger holes than six smaller holes are drilled and when the necks of the anchorage holes are rounded. The longevity of total hip replacements could be improved by drilling three large anchorage holes, rather than many smaller ones, as initially proposed by Charnley.

Total hip replacement. Results of a postal survey of current practice on the cement fixation of the acetabular cup in the uk.

Previous finite element studies and laboratory investigations on reconstructed acetabulum joints show that long-term fixation of the acetabular cup in total hip replacements (THRs) is influenced by surgical fixation techniques. The aim of this study is to determine and understand the reasons of current practice in the cement fixation of the acetabular cup in THRs in the UK. Following a pilot study, a postal survey was carried out among 1350 orthopaedic consultants. Response rate was 40% and data obtained from the returned questionnaires provided information about the current practice of 431 consultants with an average of 16.5 years of experience and who perform an average of 55 cemented THR operations annually. The survey showed wide variations in the fixation methods of the acetabular component. 95% of the respondents use cement to fix the acetabular cup, 46% maintain the subchondral bone and 63% use a flanged acetabular cup. The numbers of anchorage holes drilled vary from zero to thirty-six and drill diameters vary from 2 to 15 mm. Anchorage hole depths vary from 3 to 20 mm. Given the variability of surgical fixation methods, further studies need to be carried out to determine how fixation techniques could be improved to increase the longevity of the acetabular component in THRs. Further investigations could lead to a better understanding of the factors that contribute to the stability of THRs. (Hip International 2004; 14: 155-62).

Fixation of the acetabular cup in cemented total hip replacement: improving the anchorage hole profile using finite element method.

Long-term studies have shown that failure of the acetabular component in total hip replacement increases exponentially ten years following surgery and occurs mostly at the bone-cement interface. During the cemented fixation of the acetabular cup, straight anchorage holes, 3-15 mm diameter and 3-20 mm deep, are drilled in the acetabulum in order to increase torsional resistance at the bone-cement interface. The aim of this paper is to provide guidelines for improving the profile of anchorage holes. Results from our finite element models show that the efficiency of anchorage holes may be improved if they are drilled perpendicularly to the acetabulum floor and if they have chamfered necks. A 10 degree inclination of the anchorage hole increases Von Mises stress in the cement mantle by 6% while creating chamfered anchorage holes, instead of straight holes, decreases it by 14%. Increasing depth of anchorage holes does not improve efficiency.