Kevin Cheah

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.

Stress in the Patella Following Autologous Chondrocyte Implantation - A Finite Element Study

Bovine patella cartilage shows signs of damage and cell death when subjected to a compressive cyclic load of 6 MPa, which results in a shear stress of 5.6 MPa. The aim of this research was to investigate the effect of activities of daily living (descending stairs, bicycling and deep flexion) on the contact stresses in the patellofemoral compartment following an articular chondrocyte implantation (ACI). A finite element (FE) model of the patellar femoral joint was created and dynamic non-linear analyses were carried out for this purpose. A shear stress of 5.6 MPa was used as the threshold that cartilage can tolerate without resulting in damage. The FE model was verified numerically. Our results show that, for a 70 kg individual at 50% recovery, (i) contact stress in the patella is 11% higher than that in the femur; (ii) shear stress in the host cartilage reaches 4.75 MPa at 50° of flexion; (iii) shear stress in the patella host cartilage is twice that in a healthy cartilage during deep flexion approaching 70°; (iv) maximum shear stress value was 2.75 MPa during cycling at 60% load; (v) stress shielding still occurs through the host cartilage even when the implanted cartilage reaches 97.5% the Young’s modulus of a healthy cartilage. Based on these results, (i) using an exercise bicycle is recommended for rehabilitation; (ii) deep knee flexion should be avoided; (iii) obese people with a BMI of over 42 kg/m2 should not undertake vigorous weight-bearing exercises involving deep knee flexion.

DYNAMIC CONTACT MECHANICS IN THE OVINE KNEE JOINT FOLLOWING PARTIAL MENISCECTOMY

Meniscus tears occur in up to one third of all sports injuries and 60% of patients over 65 years. Meniscus tears increase knee joint contact pressure and the risk of developing early osteoarthritis (OA). Integrating juxtaposed meniscus surfaces continues to be a challenge during meniscal repair. Partial meniscectomy is a common surgical procedure for meniscal tear, especially in the non-vascularized white-white zone. Most studies on the effects of size and location of partial meniscectomy on knee joint contact pressures simulated the standing posture and some static angles of flexion (30° and 60°). Information on the safe proportion and location of a partial meniscectomy that would maintain knee joint contact stresses close to those in the healthy joint during dynamic loading is not known. The aim of this study is to investigate the relationship between partial meniscectomy size and location with the corresponding knee joint contact pressures during dynamic loading, emulating activities of daily living. We hypothesize that partial meniscectomy size increases knee joint contact pressure.

KNEE JOINT CONTACT MECHANICS IN A MALALIGNED LIMB: AN INTEGRATED FINITE ELEMENT AND IN VITRO STUDY

Excessive joint stress, often caused by knee malalignment, contributes to osteoarthritis (OA) progression. High tibial osteotomy (HTO) is a conservative surgery that corrects lower limb malalignment to relieve damaged tissues from excessive loading. However, HTO outcome has been highly variable and the relationship between the degree of malalignment correction and knee joint contact stresses is not known. If this were known, HTO could be tailored to each patient to best restore joint stresses to normal levels. Therefore, the aim of this work is to create a three-dimensional (3D) finite element (FE) model of the knee joint to predict the effect of different malalignment corrections on knee joint contact stresses. In this study, we present the verification of our subject-specific 3D FE model.