Growth Accommodation Research Articles

Expandable Valves, Annuloplasty Rings, Shunts, and Bands for Growing Children.

In congenital heart surgery, the surgeon must constantly consider how a palliative or corrective procedure could be impacted by the child's somatic growth. Within pediatric valve surgery, existing valve repair techniques lack growth-accommodating prostheses. Valve replacement options are fixed in size and unable to grow with the child, thus subjecting children to repeated valve reoperations. When creating a systemic-to-pulmonary artery shunt, replacing a branch pulmonary artery or conduit, creating an extracardiac Fontan pathway, or banding the pulmonary artery, the implant size must factor in both the child's current size and his or her anticipated growth. A variety of growth-accommodating technologies have been developed to fill this unmet need. Some devices have reached the clinical arena, while several are in preclinical development. The purpose of this review is to characterize the clinical need for growing device technology, and then review established and developing technologies for growth accommodation in congenital heart surgery.

A Growth-Accommodating Implant for Paediatric Applications.

Medical implants of fixed size cannot accommodate normal tissue growth in children, and often require eventual replacement or in some cases removal, leading to repeated interventions, increased complication rates and worse outcomes. Implants that can correct anatomic deformities and accommodate tissue growth remain an unmet need. Here, we report the design and use of a growth-accommodating device for paediatric applications that consists of a biodegradable core and a tubular braided sleeve, with inversely related sleeve length and diameter. The biodegradable core constrains the diameter of the sleeve, and gradual core degradation following implantation enables sleeve and overall device elongation in order to accommodate tissue growth. By using mathematical modeling and ex vivo experiments using harvested swine hearts, we demonstrate the predictability and tunability of the behavior of the device for disease- and patient-specific needs. We also used the rat tibia and the piglet heart valve as two models of tissue growth to demonstrate that polymer degradation enables device expansion and growth accommodation in vivo.

Urban Expansion or Clustered Deconcentration? An Applied Welfare Economic Analysis of Growth Controls and the Foundation of Satellites

How should urban containment and the diversion of households to nearby residential areas be evaluated from a welfare economic perspective? Assuming the existence of a negative externality of city size, we develop a concise general equilibrium model for a mother city and a satellite. This satellite should be founded if the gain in surplus exceeds the fixed costs of intercity infrastructure provision, and a Pigouvian tax on the conversion of land to urban use in both cities would then attain the first-best allocation. Rising incomes and falling transport costs enhance the surplus gain from clustered deconcentration, or the accommodation of growth in planned satellites, relative to expansion of the mother city. Nevertheless, plans by the Dutch government to uphold strict growth controls around Amsterdam, while fostering large-scale residential construction projects in the nearby satellite of Almere, are difficult to reconcile with the optimal policy in a calibrated version of our model.

A growth-management strategy for the Auckland region of New Zealand: pursuit of sustainability or mere growth accommodation?

Growth management as practised in the United States has demonstrated a decidedly pro-growth bias. The American management movement's historical focus on continued growth accommodation and its suggested form for future growth are portrayed as a backdrop to the consideration of the nature of a growth-management strategy adopted for the Auckland region of New Zealand. This article reveals that this principal, growth-management strategy adopted to date in New Zealand replicates the accommodative, growth-management model in the United States in spite of significant national, regional, and local initiatives intended to advance sustainability in New Zealand during the last decade.

Implications of replacement for reaction–transport modeling

Mineral replacement in rocks consists of growth of guest mineral and dissolution of host. The two reactions are coupled by the grain–grain stress generated by growth of the guest grain in a rigid or very viscous rock in which it has initially no available room. This stress self-adjusts to make the volumetric rates of guest growth and host dissolution equal to each other, which accounts for the volume conservation typical of replacement. Volume preservation during replacement is required by conservation of mass and momentum as well. The two reactions are also simultaneous and proceed by a sequence of many tiny alternating increments; where these increments are small enough, internal textural details of the host grain or grain aggregate are morphologically preserved (as ghost textures) by the replacement. The strong variation of mineral reaction rates with stress required by replacement is evidenced also experimentally. Reported widespread replacement in many rocks (laterites, diagenesis of siliciclastics and carbonates, metasomatic ores, metamorphic rocks, and hydrothermal alteration) warrants creating water–rock reactive transport models that predict when and where in a system replacement should take place. Current geochemical models implicitly view replacement as sequential, uncoupled dissolution of host and growth of guest; this sequence of reactions may also occur in rocks, but crucially differs from replacement. Implications of replacement for water–rock reaction modeling include: 1. balancing of replacement reactions on volume; 2. use of Helmholtz (rather than Gibbs) free energies of minerals involved in replacement; 3. taking account of the strong stress-dependence of mineral growth and dissolution rates in rocks; and 4. taking account of possible accommodation of growth by local deformation (as well as pressure solution) of the host rock, and of the effect of strain rate (=growth rate) on local rock viscosity.