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Biomedical subjects

A H R W Simpson

Publications and source records attributed to A H R W Simpson.

14 recordsLinked to original sources
PubMed2006

Finite element modelling of the pelvis: inclusion of muscular and ligamentous boundary conditions.

Previous finite element studies of the pelvis, including subject-specific studies have made extensive simplifications with regards to the boundary conditions used during analysis. Fixed boundary conditions are generally utilised at the pubis and superior part of the ilium. While it can be demonstrated that these models provide a close match for certain in vitro experiments that use similar boundary conditions, the resulting stress-strain fields in the cortex in particular are unlikely to be those found in vivo. This study presents a finite element analysis in which the pelvis is supported by muscular and ligamentous boundary conditions, applied using spring elements distributed over realistic attachment sites. The analysis is compared to an analysis in which the pelvis is restrained by fixed boundary conditions applied at the sacro-iliac joints. Striking differences in the stress-strain fields observed in cortical bone in particular, are found between the two analyses. The inclusion of muscular and ligamentous boundary conditions is found to lower the occurrence of stress concentrations within the cortex.

Computer Simulation↗
PubMed2006

3D non-linear analysis of the acetabular construct following impaction grafting.

The study investigates the short-term behaviour of the acetabular construct following revision hip arthroplasty, carried out using the Slooff-Ling impaction grafting technique; using 3D finite element analyses. An elasto-plastic material model is used to describe the constitutive behaviour of morsellised cortico-cancellous bone (MCB) graft, since it has been shown that MCB undergoes significant plastic deformation under normal physiological loads. Based on previous experimental studies carried out by the authors and others, MCB is modelled using non-linear elasticity and Drucker Prager Cap (DPC) plasticity. Loading associated with walking, sitting down, and standing up is applied to the acetabular cup through a femoral head using smooth sliding surfaces. The analyses yield distinctive patterns of migration and rotation due to different activities. These are found to be similar to those observed in the clinical setting.

Acetabulum↗
PubMed2006

Assessment of a self-administration protocol for extended subcutaneous thromboprophylaxis in lower limb arthroplasty.

The risk of venous thromboembolism in patients following arthroplasty may be reduced by continuing chemical thromboprophylaxis for up to 35 days post-operatively. This prospective cohort study investigated the compliance of 40 consecutive consenting patients undergoing lower limb arthroplasty with self-administration of a recommended subcutaneous chemotherapeutic agent for six weeks after surgery. Compliance was assessed by examination of the patient for signs of injection, number of syringes used, and a self-report diary at the end of the six-week period. A total of 40 patients, 15 men and 25 women, were recruited. One woman was excluded because immediate post-operative complications prevented her participation. Self-administration was considered feasible in 87% of patients (95% confidence interval (CI) 76 to 98) at the time of discharge. Among this group of 34 patients, 29 (85%) were compliant (95% CI 73 to 97). Patients can learn to self-administer subcutaneous injections of thromboprophylaxis, and compliance with extended prophylaxis to six weeks is good.

Anticoagulants↗
PubMed2005

Response of the tendon during limb lengthening.

Little is known about the increase in length of tendons in postnatal life or of their response to limb lengthening procedures. A study was carried out in ten young and nine adult rabbits in which the tibia was lengthened by 20% at two rates 0.8 mm/day and 1.6 mm/day. The tendon of the flexor digitorum longus (FDL) muscle showed a significant increase in length in response to lengthening of the tibia. The young rabbits exhibited a significantly higher increase in length in the FDL tendon compared with the adults. There was no difference in the amount of lengthening of the FDL tendon at the different rates. Of the increase in length which occurred, 77% was in the proximal half of the tendon. This investigation demonstrated that tendons have the ability to lengthen during limb distraction. This occurred to a greater extent in the young who showed a higher proliferative response, suggesting that there may be less need for formal tendon lengthening in young children.

Aging↗
PubMed2004

Distraction osteogenesis in the Cbfa-1+/- mouse.

Distraction osteogenesis involves division of a bone and gradually pulling the bone ends apart. This delivers mechanical stimulation to mesenchymal cells in the distraction gap, where new bone is regenerated predominantly by intramembranous ossification. The transcription factor Cbfa1 has been reported to be essential for the differentiation of mesenchymal cells to osteoblasts. In homozygous Cbfa1 knockout mice, both intramembranous and endochondral ossification mechanisms are blocked and no bone formation occurs. In heterozygous Cbfa1 knockout mice, only the cranial bones and the clavicles, which form through intramembranous ossification, fail to develop properly as in the human condition of cleidocranialdysostosis. It has been suggested, therefore, that intramembranous ossification is affected by the absence of one of the paired Cbfa1 genes. We have assessed the potential for intramembranous ossification following distraction osteogenesis in heterozygous Cbfa1 knockout mice. Fourteen skeletally mature male heterozygous mice were used, together with 10 wild-type controls. The tibia was distracted by 0.25 mm twice a day (0.5 mm/day) for 10 days using the half-ring type fixator. Nine mice were kept for a further 28 days to observe the consolidation phase. In four out of five mice of the heterozygous group and in all three wild-type mice, bony fusion within the distraction gap was observed on radiographs. Histological findings were almost the same in the two groups at various stages of the procedure and intramembranous ossification was predominant in both the groups. Despite the inhibition of intramembranous ossification during the foetal development of Cbfa1+/- mice, distraction osteogenesis was as successful as in control mice.

Animals↗
PubMed2004

Does the type of flooring affect the risk of hip fracture?

BACKGROUND: The number of hip fractures occurring worldwide in 1990 was estimated at 1.7 million and is predicted to rise to 6.3 million by 2050. The vast majority occur as a result of simple falls and the impact of the femoral trochanter with the floor. Previous studies have addressed the problem from the patient's side of the impact. Little research has been carried out on the other surface involved in the impact, the floor. STUDY LOCATION: 34 residential care homes. METHODS: (1) The mechanical properties of the floor were measured with force transducers. (2) The number and location of falls and fractures on the various floors were recorded prospectively for 2 years. The threshold for reporting falls in different care homes was assessed using a standardised set of scenarios. RESULTS: A total of 6,641 falls and 222 fractures were recorded. Wooden carpeted floors were associated with the lowest number of fractures per 100 falls. The risk of fracture resulting from a fall was significantly lower compared to all other floor types (odds ratio 1.78, 95% CI 1.33-2.35). The mean impact force was significantly lower on wooden carpeted floors: 11.9 kN compared to the other floor types. DISCUSSION: The possible implications of our findings are considerable. Residents of homes are typically frail and many have a propensity to falls. In designing safer environments for older people, the type of floor should be chosen to minimise the risk of fracture. This may result in a major reduction in fractures in the elderly.

Accidental Falls↗
PubMed2004

Severity of disease and risk of malignant change in hereditary multiple exostoses. A genotype-phenotype study.

We performed a prospective genotype-phenotype study using molecular screening and clinical assessment to compare the severity of disease and the risk of sarcoma in 172 individuals (78 families) with hereditary multiple exostoses. We calculated the severity of disease including stature, number of exostoses, number of surgical procedures that were necessary, deformity and functional parameters and used molecular techniques to identify the genetic mutations in affected individuals. Each arm of the genotype-phenotype study was blind to the outcome of the other. Mutations EXT1 and EXT2 were almost equally common, and were identified in 83% of individuals. Non-parametric statistical tests were used. There was a wide variation in the severity of disease. Children under ten years of age had fewer exostoses, consistent with the known age-related penetrance of this condition. The severity of the disease did not differ significantly with gender and was very variable within any given family. The sites of mutation affected the severity of disease with patients with EXT1 mutations having a significantly worse condition than those with EXT2 mutations in three of five parameters of severity (stature, deformity and functional parameters). A single sarcoma developed in an EXT2 mutation carrier, compared with seven in EXT1 mutation carriers. There was no evidence that sarcomas arose more commonly in families in whom the disease was more severe. The sarcoma risk in EXT1 carriers is similar to the risk of breast cancer in an older population subjected to breast-screening, suggesting that a role for regular screening in patients with hereditary multiple exostoses is justifiable.

Adolescent↗
PubMed2004

Recovery of function after closed femoral shortening.

This prospective, longitudinal study documents the muscle strength and baseline function of 18 patients undergoing closed femoral shortening for discrepancy in limb length. Patients were studied for two years following surgery. Function was measured by a self-reported questionnaire, timed tests of performance and measurements of muscle strength and power. After two years, the self-reported function and ability to complete timed functional tests had returned to or improved on the pre-operative values. Muscle strength remained slightly below the pre-operative value and was more marked in the quadriceps than the hamstrings. This study suggests that small decreases in muscle strength and power following closed femoral shortening do not adversely affect the patients' ability to perform everyday activities.

Adolescent↗
PubMed2004

Functional recovery in patients with nonunion treated with the Ilizarov technique.

There are few reports on function after limb salvage surgery using the Ilizarov technique, and none that document the pattern of recovery or predict when maximum function returns. This prospective, longitudinal study documents the baseline functional abilities of 40 consecutive patients with nonunion of a fracture in the lower limb. Patients were studied for at least two and a half years following the completion of surgery. Function was measured by timed tests of functional performance and by the Toronto Extremity Salvage Score self-reported patient questionnaire. Recovery was slowest in the early stages after removal of the frame and greatest between six months and one year. Statistically significant improvement continued up to, but not beyond two years. This observation has important implications for the length of follow-up incorporated into the rehabilitation programmes for patients, predictions of patient status in regard to compensation and for the design of future studies to evaluate functional outcome.

Adolescent↗
PubMed2003

Vascularity in a new model of atrophic nonunion.

Our aim was to develop a clinically relevant model of atrophic nonunion in the rat to test the hypothesis that the vessel density of atrophic nonunion reaches that of normal healing bone, but at a later time-point. Atrophic nonunion is usually attributed to impaired blood supply and is poorly understood. We determined the number of blood vessels at the site of an osteotomy using immunolocalisation techniques in both normally healing bones and in atrophic nonunion. At one week after operation there were significantly fewer blood vessels in the nonunion group than in the healing group. By eight weeks, the number in the atrophic nonunion group had reached the same level as that in the healing group. Our findings suggest that the number of blood vessels in atrophic nonunion reaches the same level as that in healing bone, but at a later time-point. Diminished vascularity within the first three weeks, but not at a later time-point, may prevent fractures from uniting.

Animals↗
PubMed2002

Histological assessment of the presence or absence of infection in fracture non-union.

BACKGROUND: Infection is a major cause of non-unions. Infection is not always evident clinically, nor on bacteriological analysis. If it is untreated, non-union treatment may fail. AIM: To establish whether histological analysis is of value as an adjunct to microbiology in establishing the diagnosis of infection. METHODS: Sixty patients who had both bacteriological and histological analysis of their non-unions were studied. Infection was indicated by an acute inflammatory response. In 45 of the 60 fractures, microbiological and clinical diagnoses were in agreement; in this subset the histology results were compared to the established diagnoses. RESULTS: The histological diagnosis for the 45 fractures in the group with a definite diagnosis gave four false negatives but no false positives. This represents a sensitivity of 87.1%, (95% CI, 70-96%) and a 100% specificity (95% CI, 77-100%). The overall accuracy was 91.1%. (95% CI, 79-98%) The predictive value of a positive test was 100% and of a negative test was 77.8%. In 25% of the series a definite diagnosis could not be made with purely clinical and microbiological information; with the additional histological information it was possible to determine the infection status of the non-union. CONCLUSION: Histology is of particular use when the microbiology is negative or equivocal.

Adolescent↗
PubMed2002

Human atrophic fracture non-unions are not avascular.

A small proportion of fractures progress to non-union. Non-unions are routinely classified into two groups either hypertrophic or atrophic according to their radiological appearance. It is a common preconception that non-unions with a hypertrophic appearance on X-ray are biologically active and vascular with potential to heal given the correct stable environment. Atrophic non-unions are considered to be avascular and inert and will not heal even under the correct stable environment. Non-unions are either infected or aseptic. In the present study, we tested the hypothesis that aseptic atrophic non-unions are less vascular than aseptic hypertrophic non-unions and healing fractures. Biopsies were taken from the fracture gap of patients with healing fractures, hypertrophic non-unions and atrophic non-unions. A dual labelling technique was used with antibodies against CD31 (JC70) and Collagen IV. Blood vessels were quantified using a Chalkley point eyepiece graticule. There was no statistically significant difference in the median vessel count between the three fracture groups. These findings do not support the hypothesis that established atrophic fracture non-union are less vascular than hypertrophic non-unions or healing fractures.

Adolescent↗