Three dimensional finite element analysis of the pediatric lumbar spine. Part II: biomechanical change as the initiating factor for pediatric isthmic spondylolisthesis at the growth plate

A non-linear 3-dimensional finite element pediatric lumbar spine model with vertebral growth plate and apophyseal bony ring was developed. Lumbar spondylolysis was simulated in the model. The Von Mises stresses in the structures surrounding the vertebral growth plate, including apophyseal bony ring and osseous endplate were calculated in various loading modes. Instantaneous axis of rotation (IAR) path from flexion to extension was also analyzed. The results were compared with those of the intact model and the literature. The IAR path was at the posterior disc-endplate space of the lower vertebra in the intact spine, and moved cranially towards the upper-posterior disc space in the lytic spine. This was in agreement with in vivo radiological data by Sakamaki et al. [19]. During various loading modes, stresses in the spondylolytic pediatric model were higher than that of the intact model; ranging from 1.1 to 6.0 times, with the highest value in extension at the growth plate. In conclusion, FE models indicate that stress concentrations in the lytic model increase at the growth plate which may lead to physis stress fracture leading to spondylolisthesis.Part I of this article can be found at http://dx.doi.org/ 10.1007/s00586-005-1026-z     

Comparison of retinal outcomes after scleral buckle or lens-sparing vitrectomy for stage 4 retinopathy of prematurity

PURPOSE: To compare anatomic outcomes after lens-sparing vitrectomy (LSV) or scleral buckle (SB) for stage 4 retinopathy of prematurity (ROP). METHODS: Nonrandomized, retrospective study of infants consecutively treated for stage 4 ROP by LSV or SB. Outcomes were retinal attachment at 1 month of initial surgery and at the end of follow-up (6 months) and number of procedures to achieve retinal attachment. Exact chi2 methods determined significance, and student's t-test compared mean postmenstrual age and birthweight between the groups. RESULTS: After one procedure, LSV (72%) was associated with retinal attachment more often than was SB (31%). At the end of follow-up, after one or more procedures, there was no difference in retinal reattachment rate between LSV or SB as the first procedure. There were no differences between the surgical groups by mean postmenstrual age and birthweight or severity of ROP determined by zone, clock hours of ridge elevation, or quadrants of plus disease. CONCLUSION: This study supports the hypothesis that vitrectomy by LSV stops progressive stage 4 ROP. As an initial procedure, LSV was associated with retinal attachment more often than SB. Future prospective studies can determine the effects of LSV and SB on visual development in progressive stage 4 ROP.     

Coronary vasospasm and the regulation of coronary blood flow

Under physiologic conditions, epicardial arteries contribute minimally to coronary vascular resistance. However, in the presence of endothelial dysfunction, stimuli that normally produce vasodilation may instead cause constriction. Examples include neural release of acetylcholine or norepinephrine, platelet activation and production of serotonin and thrombin, and release of local factors such as bradykinin. This shift from a primary endothelial-mediated vasodilator influence to one of endothelial dysfunction and unchecked vasoconstriction is precisely the milieu in which coronary vasospasm is observed. This condition, which typically occurs during periods of relatively sedentary activity, is associated with focal and transient obstruction of an epicardial arterial segment resulting in characteristic echocardiographic changes and symptoms of myocardial ischemia. This review highlights the current understanding of mechanisms regulating the coronary circulation during health and examines the pathophysiologic changes that occur with coronary spasm. Genetic and other predisposing conditions are addressed, as well as novel therapies based on recent mechanistic insights of the coronary contractile dysfunction associated with coronary spasm.