A finite element model for investigating the influence of keel design and position on unicompartmental knee replacement cementless tibial component fixation

OBJECTIVES

The cementless Oxford Unicompartmental Knee Replacement (OUKR) tibial component relies on an interference fit to achieve initial fixation. The behaviour at the implant-bone interface is not fully understood and hence modelling of implants using Finite Element (FE) software is challenging. With a goal of exploring alternative implant designs with lower fracture risk and adequate fixation, this study aims to investigate whether optimisation of FE model parameters could accurately reproduce experimental results of a pull-out test which assesses fixation.

MATERIALS AND METHODS

Finite element models of implants with three methods of fixation (standard keel, small keel, and peg) in a bone analogue foam block were created, in which implants were modelled using an analytical rigid definition and the foam block was modelled as a homogenous linear isotropic material. The total interference and elastic slip were varied in these models and optimised by comparing simulated and experimental results of pull-out tests for two (standard and peg) implant geometries. Then the optimised interference and elastic slip were validated by comparing simulated and experimental data of a third (small keel) implant geometry.

RESULTS

The optimisation of parameters established an interference of 0.16 mm and an elastic slip of 0.20 mm as most suitable for modelling the experimental force-displacement plots during pull-out. This combination of parameters accurately reproduced the experimental results of the small keel geometry. The maximum pull-out forces from the FE models were consistent with experimental data for each implant design.

CONCLUSIONS

This study shows that experimental pull-out tests can be accurately modelled using adjusted interference values and non-linear friction and outlines a method for determining these parameters. This study demonstrates that complex problems in modelling implant behaviour can be addressed with relatively simple models. This can potentially lead to the development of implants with reduced risk of failure.

High-sensitivity serum calcitonin assays applied to screening for thyroid C-cell disease in multiple endocrine neoplasia type 2A

Two serum calcitonin assays with sensitivities < or = 10 pg/mL were compared to our standard radioimmunoassay (sensitivity 100 pg/mL) in multiple endocrine neoplasia type 2A (MEN 2A) screening. Values from the Nichols displacement radioimmunoassay averaged 38% higher than values from the CIS immunoradiometric assay; values from both were highly correlated, r = 0.845. In three individuals, both of the newer assays revealed abnormalities in pentagastrin tests three to four years before abnormalities were detected by the standard assay. Pentagastrin tests after total thyroidectomy were assayed by the newer methods in patients with medullary thyroid carcinoma (MTC) diagnosed at initial testing (group I); in patients with early MTC diagnosed by prospective screening (group II); and in patients with pure C-cell hyperplasia detected by prospective screening (group III). At least 64% of group I, at least 25% of group II, but none of group III had detectable postoperative C-cell function.

CONCLUSIONS

1) The previous estimate of 12 years as median age of onset of C-cell disease in MEN 2A is probably three to four years too old. 2) Patients diagnosed with early MTC by screening had not necessarily skipped a preneoplastic phase of C-cell hyperplasias. At least some early disease was not detected by the standard assay. Higher sensitivity assay should improve screening for C-cell disease by earlier disease detection. 3) Biochemical cure by thyroidectomy after the development of MTC is not as frequent as previously thought, but the apparent cure rate of pure C-cell hyperplasia remains 100%.

Sheep-acquired severe Chlamydia psittaci infection in pregnancy

There have been five confirmed cases of severe Chlamydia psittaci infection during pregnancy, three having been treated in Edinburgh, Scotland. The most recent case is presented and previous experience is reviewed. The illness usually causes thrombocytopenia with disseminated intravascular coagulation, renal failure and hepatic dysfunction during the late second and early third trimester. The outcome for the fetus is usually fatal and the infection only resolves after delivery or abortion. The main hope is for education to prevent infection occurring in susceptible populations.

Continuous-flow enzymic determination of serum total and high density lipoprotein cholesterol using microlitre sample volumes

We determine serum total cholesterol and high-density lipoprotein cholesterol (HDL-C) using a modified Technicon Auto Analyzer II-BMC enzymic system. The method uses 25 microL of sample (serum or supernate) for cholesterol determinations. Pooled serum which was calibrated indirectly against CDC's Abell-Kendall method was used for standardization. Accuracy and precision for total cholesterol determinations are comparable to those obtained using a modified Technicon AAII method. Coefficients of variation for the determination of HDL-C prepared by heparin/Mn++ precipitation are 6.4% and 4.6% at concentration levels of 0.70 mmol/L and 0.94 mmol/L respectively. The interchangeable use of deionized-distilled water and 0.15 mol/L NaCl solution for dilution of samples analyzed by the micro method is shown to produce significantly different cholesterol estimates. The reduced reagent volumes significantly lower the cost of cholesterol determinations. The system is simple, inexpensive and yields reliable cholesterol and HDL-C results.

Chlamydia trachomatis and Ureaplasma urealyticum in men attending a sexually transmitted diseases clinic

Urethral specimens from 480 heterosexual patients were examined for Chlamydia trachomatis. Chlamydia were isolated from 32.7% of men with non-specific urethritis (NSU), from 16.1% of men with gonorrhoea, and from 4.1% of men without urethritis. Chlamydial isolation was not related to duration of symptoms, presence of discharge, or past history of attendance at the clinic. Urine from 176 heterosexual patients was examined for Ureaplasma urealyticum. Ureaplasmas were present in 53.8% of men with NSU, in 28% of men with gonorrhoea, and in 32.9% of men with no urethritis. Detection rates for ureaplasmas in patients with chlamydia-negative and chlamydia-positive NSU were similar, but ureaplasmas were present in significantly greater numbers in patients with chlamydia-negative NSU than in those with chlamydia-positive NSU.

Metabolism of 35S-labelled antithyroid drugs in man

Differences in the metabolic fate of antithyroid drugs influence the optimal frequency of administration and their therapeutic efficacy. (35)S propylthiouracil differed from the (35)S imidazoles (carbimazole and methimazole) in the more rapid absorption and excretion and the shorter biological half-life in the plasma of the former. Renal function may have a more important influence on the biological half-life of the drugs than thyroid status. Further work is required to determine the optimal frequency of administration for each compound.