Use of finite element analysis in presurgical planning: treatment of mandibular fractures.

BACKGROUND: The current clinical procedure for mandible fracture fixation is plate application. 3D reconstructions are used to validate procedures numerically preceding experimental analysis. This study outlines the methods used to reconstruct a numerical model of the mandible. METHODS: A CT scan from a 22-year-old male patient with a healthy unfractured mandible was obtained. A 3D reconstruction was carried out using Mimics via thresholding and segmentation techniques. Boundary conditions and muscle forces were applied, and simulations were performed using ABAQUS. RESULTS: 3D reconstruction allows for precise anatomical dimensions, which can be used for further engineering analysis. Using the surgical Champy technique as an example, results showed that the mandible model returned to normal function post-plating. CONCLUSIONS: The study shows the clinical relevance of 3D reconstructions to plan surgical procedures. Results illustrate the benefit of carrying out numerical validations as a prerequisite to experimental modelling and as a method of pre-validating surgical procedures.

Autoregulation of nitrogenase expression in Klebsiella pneumoniae.

An investigation into the influence of N2 on the expression of Klebsiella pneumoniae nitrogenase has led to a reassessment of the role of the nitrogenase MoFe protein in autoregulation. Anaerobic derepression of nitrogenase (C2H2-reducing) activity, of NifD and K polypeptides, and of nifH-lac expression, following the removal of excess NH+4, were greater under N2 than Ar. This enhancement occurred in Nif+ but not in Nif- strains, and in Nif+ strains was prevented by C2H2, an inhibitor of N2 fixation. Thus N2 fixation is important for maintaining derepression. Derepression of nifH-lac under Ar in various Nif+ and Nif- strains (including NifH-, NifD-, NifB- and NifL- mutants) and of wild-type lac under N2 or Ar in a Nif+ strain were measured to investigate the regulation. The mechanism regulating the enhancement under N2 neither involved the MoFe protein of nitrogenase, as proposed by Dixon et al. (1980, Nature 286, 128-132), nor the nifL product, but was probably due to a general upgrading of the N status. Moreover, during batch growth limited by a non-repressing fixed N source, the levels of nifH-lac expression in the Nif+ and Nif- strains suggested that the nifH gene product (or Fe protein) may have a positive autoregulatory function.

Oxygen inhibition of nitrogenase activity in Klebsiella pneumoniae.

A purpose-built oxystat has been used to study reversible inhibition of nitrogenase by O2 in the facultative anaerobe Klebsiella pneumoniae. C2H2-reducing activity in samples from either an anaerobic glucose-limited or an O2-limited diazotrophic chemostat culture was completely inhibited by exposure to a dissolved O2 concentration (DOC) of 1.5 microM or above. Subsequently, under anaerobic conditions, C2H2-reducing activity returned in the absence of de novo protein synthesis. The amount of activity returning never reached 100% of the initial anaerobic activity before O2 treatment. The degree of reversibility was inversely proportional to the log of DOC during exposure and was decreased by increasing the time of exposure to O2 (about 60% reversibility occurred after a 20 min exposure to 6 microM-O2). The failure to obtain complete recovery of activity was apparently not due to inactivation of the very O2-sensitive pyruvate-flavodoxin oxidoreductase (nifJ product) which provides electrons for nitrogenase activity in vivo. Samples from the O2-limited culture behaved similarly to those limited by glucose. Thus, 'training' of the organism to use O2 during growth does not influence the tolerance of nitrogenase to O2. Since the behaviour towards O2 reported here for K. pneumoniae differs from that known to occur in Azotobacter, the mechanism of protection of nitrogenase from O2 damage may differ in these organisms.

Roles of nifF and nifJ gene products in electron transport to nitrogenase in Klebsiella pneumoniae.

Crude extracts of the wild-type Klebsiella pneumoniae reduced C2H2 with either pyruvate or formate as reductant (specific activity, 3 nmol min-1 mg of protein-1), whereas crude extracts of nifF mutant were almost inactive (specific activity, 0.05). However, activity in the latter extracts was stimulated by adding Azotobacter chroococcum flavodoxin (specific activity, 10). Thus, nifF mutants may lack an electron transport factor. Crude extracts of nifJ mutants had about 20% of the wild-type level of active MoFe protein, and thus nifJ has a presumptive role in maintaining active MoFe protein. Studies on pyruvate or formate as reductants for nitrogenase in extracts of the nifJ mutants suggest in addition a role in electron input to nitrogenase for the following reasons. (i) Nitrogenase activity with these reductants was very low (specific activity, 0.06) and was not stimulated by extra MoFe protein or the flavodoxin. (ii) Activity was increased by adding a crude extract of a mutant lacking the structural nif genes (specific activity, 1) or a crude extract of the nifF mutant (specific activity, 4).