Finite element modeling of retinal prosthesis mechanics.

Epiretinal prostheses used to treat degenerative retina diseases apply stimulus via an electrode array fixed to the ganglion cell side of the retina. Mechanical pressure applied by these arrays to the retina, both during initial insertion and throughout chronic use, could cause sufficient retinal damage to reduce the device's effectiveness. In order to understand and minimize potential mechanical damage, we have used finite element analysis to model mechanical interactions between an electrode array and the retina in both acute and chronic loading configurations. Modeling indicates that an acute tacking force distributes stress primarily underneath the tack site and heel edge of the array, while more moderate chronic stresses are distributed more evenly underneath the array. Retinal damage in a canine model chronically implanted with a similar array occurred in correlating locations, and model predictions correlate well with benchtop eyewall compression tests. This model provides retinal prosthesis researchers with a tool to optimize the mechanical electrode array design, but the techniques used here represent a unique effort to combine a modifiable device and soft biological tissues in the same model and those techniques could be extended to other devices that come into mechanical contact with soft neural tissues.

Inguinal herniorrhaphy in the continuous ambulatory peritoneal dialysis patient.

Inguinal hernia repair in the patient on continuous ambulatory peritoneal dialysis (CAPD) is complicated in theory by an increased potential for recurrence. In addition to the constant increased intraabdominal pressure, chronic renal failure has been shown to impair tissue healing. Controversy exists regarding the waiting period before resuming CAPD postoperatively. A retrospective review of all CAPD patients undergoing inguinal herniorrhaphy was performed. The patient's age, type of repair, duration of renal failure preoperatively, length of time on CAPD postoperatively, and date of resumption of CAPD were recorded. An inpatient and outpatient chart review was performed on all patients. Telephone follow-up was performed on surviving patients. From April 1981 to June 1989, 30 patients underwent 36 inguinal herniorrhaphies while on CAPD. One immediate postoperative death occurred due to underlying cardiac disease. The mean follow-up for surviving patients was 34 months (range, 16 to 91) and for those deceased was 25 months (range, 1 to 60). No recurrent hernias were identified either by extensive inpatient and outpatient chart review, or by direct patient telephone contact in all surviving patients. We conclude that inguinal herniorrhaphy can be safely performed in CAPD patients. Peritoneal dialysis can be initiated immediately after repair in this high-risk group of patients. There is a low risk of recurrence; however, long-term patient survival is not expected due to concurrent underlying medical problems.