Evaluation of cement stresses in finite element analyses of cemented orthopaedic implants

Stress analysis of the cement fixation of orthopaedic implants to bone is frequently? carried out using finite element analysis. However, the stress distribution in the cement laver is usually intricate, and it is difficult to report it in a way that facilitates comparison of implants for pre-clinical testing. To study this problem, and make recommendations for stress reporting, a finite element analysis of a hip prosthesis implanted into a synthetic composite femur is developed. Three cases are analyzed: a fully bonded implant, a debonded implant, and a debonded implant where the cement is removed distal to the stein tip. In addition to peak stresses, and contour and vector plots, a stressed volume and probability-of-failure analysis is reported. It is predicted that the peak stress is highest for the debonded stem, and that removal of the distal cement more than halves this peak stress. This would suggest that omission of the distal cement is good for polished prostheses (as practiced for the Exeter design). However; if the percentage of cement stressed above a certain threshold (say 3 MPa) is considered, then the removal of distal cement is shown to be disadvantageous because a higher volume of cement is stressed to above the threshold. Vector plots clearly demonstrate the different load transfer for bonded and debonded prostheses: A bonded stein generates maximum tensile stresses in the longitudinal direction, whereas a debonded stem generates most tensile stresses in the hoop direction, except near the tip where tensile longitudinal stresses occur due to subsidence of the stein. Removal of the cement distal to the tip allows greater subsidence but alleviates these large stresses at the tip, albeit at the expense of increased hoop stresses throughout the mantle. It is concluded that a thorough analysis of cemented implants should not report peak stress, which can be misleading, but rather stressed volume, and that vector plots should be reported if a precise analysis of the load transfer mechanism is required.

The experience of individuals with renal failure participating in home haemodialysis: An interpretative phenomenological analysis.

This study explored the experience of individuals with renal failure undertaking home haemodialysis (HHD). Semi-structured interviews were conducted with six participants who were active HHD users in a UK region. Participants’ accounts were transcribed verbatim and analysed using an interpretative phenomenological approach. Three main themes were identified: (1) embracing treatment and lifestyle freedom and flexibility; (2) re-establishing a sense of self and preferred self-identity; and (3) integrating aspects of active engagement and aspects of supported, life-sustaining dependence. A ‘good fit’ between the HHD user (an independent, self-determined health participant) and the healthcare provision (personalized, enabling) is proposed.

Estimation of survival rate and extinction probability for stage-structured populations with overlapping life stages

The development of methods providing reliable estimates of demographic parameters (e. g., survival rates, fecundity) for wild populations is essential to better understand the ecology and conservation requirements of individual species. A number of methods exist for estimating the demographics of stage-structured populations, but inherent mathematical complexity often limits their uptake by conservation practitioners. Estimating survival rates for pond-breeding amphibians is further complicated by their complex migratory and reproductive behaviours, often resulting in nonobservable states and successive cohorts of eggs and tadpoles. Here we used comprehensive data on 11 distinct breeding toad populations (Bufo calamita) to clarify and assess the suitability of a relatively simple method [the Kiritani-Nakasuji-Manly (KNM) method] to estimate the survival rates of stage-structured populations with overlapping life stages. The study shows that the KNM method is robust and provides realistic estimates of amphibian egg and larval survival rates for species in which breeding can occur as a single pulse or over a period of several weeks. The study also provides estimates of fecundity for seven distinct toad populations and indicates that it is essential to use reliable estimates of fecundity to limit the risk of under- or overestimating the survival rates when using the KNM method. Survival and fecundity rates for B. calamita populations were then used to define population matrices and make a limited exploration of their growth and viability. The findings of the study recently led to the implementation of practical conservation measures at the sites where populations were most vulnerable to extinction. © 2010 The Society of Population Ecology and Springer.

Scaling in the time-dependent failure of a fiber bundle with local load sharing

We study the scaling behaviors of a time-dependent fiber-bundle model with local load sharing. Upon approaching the complete failure of the bundle, the breaking rate of fibers diverges according to r(t)proportional to(T-f-t)(-xi) where T-f is the lifetime of the bundle and xi approximate to 1.0 is a universal scaling exponent. The average lifetime of the bundle [T-f] scales with the system size as N-delta, where delta depends on the distribution of individual fiber as well as the breakdown rule. [S1063-651X(99)13902-3].

Failure of fiber bundles with local load sharing

We develop a recursion-relation approach for calculating the failure probabilities of a fiber bundle with local load sharing. This recursion relation is exact, so it provides a way to test the validity of the various approximate methods. Applying the exact calculation to uniform and Weibull threshold distributions, we find that the most probable failure load coincides with the average strength as the size of the system N --> infinity.

Pharmaceutical care of patients with congestive heart failure: Interventions and outcomes

We evaluated a structured pharmaceutical care program for elderly patients (> 65 yrs) with congestive heart failure (CHF) based on objective measures of disease control, quality of life, and use of health care facilities in a randomized, controlled, longitudinal, prospective clinical trial. The 42 patients in group A received education from a pharmacist on the disease and its treatment, and lifestyle changes that could help control symptoms. Patients also were encouraged to monitor their symptoms and comply with prescribed drug therapy. If necessary, dosage regimens were simplified in liaison with hospital physicians. The 41 control patients (group B) received standard care. The following outcome measures were assessed in all patients at baseline (before the start of the trial) and at 3, 6, 9, and 12 months: 2-minute walk test, blood pressure, body weight, pulse, forced vital capacity, quality of life [disease-specific (Minnesota Living with Heart Failure questionnaire) and generic (SF-36)], knowledge of symptoms and drugs, compliance with therapy, and use of health care facilities (hospital admissions, visits to emergency room, emergency calls). Patients in group A showed improved compliance with drug therapy, which in turn improved their exercise capacity compared with those in group B; education on management of symptoms, lifestyle changes, and dietary recommendations were also of benefit. Group A patients significantly improved knowledge of their drug therapy over the 12-month study and had fewer hospital admissions compared with group B patients. They also had improved outcomes compared with group B, despite the small samples. An extension of this trial to other sites with pooling of results would provide additional evidence of the value of this structured program in elderly patients with CHF.