Objective: Myelomeningocele is a neural tube defect resulting in an exposed spinal cord, which leads to irreversible neurologic damage at birth. We proposed development of a fetal rabbit model of myelomeningocele to study in utero spinal cord injury and repair strategies. Methods: New Zealand white rabbits (n = 10) at 22 days of gestation (term = 31 days) underwent laparotomy to expose the gravid uterus; a hysterotomy exposed the fetal hindlimbs and back. A three to four level lumbar laminectomy was performed, and the dura over the posterior spinal cord was removed. At 30 days of gestation, the does underwent C-section for fetal harvest, and total fetal number, length, weight, and the presence or absence of a spinal defect were recorded for all viable fetuses. Results: All injured fetuses were smaller and weighed less than the nonoperated littermate controls, and histologic examination confirmed a spina bifida-like lesion of their spinal cords. Conclusions: We successfully created an exposed spinal cord defect in the fetal rabbit model similar to the lesion found in humans. Advantageous because of low animal cost, relatively large fetal size, multiple fetuses per pregnancy, and short total gestation, this model will allow us to study the mechanism of injury to the exposed spinal cord, and perhaps develop strategies to repair human myelomeningoceles.
The ideal intracorporeal lithotriper would comminute all types of calculi into small readily excreted particles. It would be small and flexible with an energy source safe for the uroepithelium. It should not break, should be inexpensive, and should not retropulse the stone up the urinary tract. This investigation was designed to quantify the last quality for the holmium:YAG laser. The mechanism of action of the pulsed Ho:YAG laser (wavelength 2100 nm) is the generation of a gas plasma at the stone-fluid interface causing a shockwave. The holmium laser was employed for lithotripsy of model stones composed of silicate with a ferrous coating. Stones were selected with a mass of 2 mg +/- 0.1 mg. We sequentially investigated three variables: energy (0.6, 0.8, and 1.0 J), frequency (10, 16, and 20 Hz), and fiber diameter (200, 365, 550, and 1000 microm). Ten stone trials were performed with each of the 36 possible combinations of energy, pulse frequency, and fiber diameter. Our model ureter consisted of a clear rigid polymer tube filled with 0.9% saline. The system was closed and permitted intertrial flushing of generated air bubbles. The laser fiber was maintained at constant extension from the ureteroscope, with stones positioned at the fiber tip before each trial. Laser energy was applied for 2 seconds, with maximum and net retropulsion recorded in millimeters. Each measurement series was recorded in a database for paired Student t-tests. Net retropulsion was then compared by statistically holding each of the three variables constant (fiber size constant with power and frequency varying; frequency constant with power and fiber size varying; and power constant with fiber size and frequency varying). Most retropulsion occurred with the 365-microm and 550-microm fibers. Most comminution was also noted with these fiber sizes. There was no statistical correlation between observed retropulsion and efficiency of comminution. This self-contained model for laser lithotripsy allowed us to measure retropulsion accurately. Silicate stones are not chemically similar to human uroliths but are of uniform composition. The irregular surface characteristics are similar to human stones, making them ideal for retropulsion investigations.
In patients with large optical zones, it may be preferable to calculate tissue ablation depth using the exact formula. Alternately, the Munnerlyn formula can be used to calculate ablation depth and then an adjustment factor can be added.
Myelomeningocele (MMC) is characterized by paraplegia and incontinence, often necessitating surgery. Current models of MMC in sheep and primates create a spinal defect long after anomalous neural tube closure ordinarily occurs. An ideal model of MMC would allow creation of the defect at the earliest age in a low-cost species with a short gestation. We present a method utilizing the holmium laser to create spinal defects in rabbits in utero for the study of the pathophysiology and repair of MMC. Pregnant rabbits of 22 to 23 days' gestational age were prepared and draped in sterile fashion for laparotomy under general anesthesia. The abdomen was opened, and both uterine horns were inspected. Double opposing pursestring sutures were placed to secure the chorioamniotic membranes over the fetal lumbar spine. Amniotic fluid was removed with a needle and saved. Electrocautery was used to open the uterus within the pursestring suture, exposing the fetal dorsum. The spine was exposed by laser dissection of the fetal dorsal musculature. Posterior laminectomy was accomplished with laser incisions of each side of the spinous process, leaving the underlying dura and cord exposed. The pursestring was then cinched, amniotic fluid was returned, and the uterus and trocar sites were closed. Cesarian section was performed at 30 to 31 gestational days, and the pups were examined and then humanely sacrificed for histologic evaluation of the lesion. The rabbit is an inexpensive species with a short gestation (33-35 days), and four or more fetuses may be operated on per litter, with the remainder serving as controls. Utilization of minimally invasive techniques including holmium:YAG laser dissection facilitates creation of spinal defects at an early age in this small-animal model.
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