Getting small is the key for the Strata cerebral spinal fluid valve. The white circule within the valve houses the magnetically adjustable pressure-control rotor. The design required development of a new linear spring configuration, such as this 0.14-inch diameter pressure/flow control spring, photo-etched from stainless steel (smaller photo).
Goleta, CA—Patients with the condition
hydrocephalus typically have a build-up in cerebral spinal fluid in the brain.
Unchecked, pressure on the brain restricts blood flow, leading to brain damage
and even death. Standard treatment is via a neuro shunt, consisting of an
implantable valve to regulate the pressure and a catheter from the valve that
ducts the fluid to the abdominal cavity for absorption by the body. Until now,
shunt valves operated at a fixed opening pressure, which the physician had to
match to the patient's physiology. If a mismatch occurs, then the original valve
must be surgically removed and replaced, causing additional trauma and surgical
risk, and possibly costing more than $20,000 to the healthcare system.
To cut the trauma, risk, and cost, design engineers at Medtronic PS Medical have developed the Strata valve, which non-invasively allows adjustment of the opening pressure level. The adjustment is done quickly, using a handheld magnetic tool. Jeff Bertrand, manager of applied research and project leader on the Strata valve, says that two technologies are key to realizing the device—keeping its profile low to minimize tissue abrasion and for cosmetic reasons; and development of a compact, low-rate spring that exhibits linear behavior from its free length down to completely compressed.
"We designed and built the valve in-house and pushed the molding process to its lower limits," Bertrand says. Limited to FDA approved polymer materials for the housing, "wall thickness and stack heights pushed the envelope," he adds. Another benefit was that the Strata was not much bigger and the same shape as the company's existing Delta fixed valve. Surgeons can thus handle it the same way they are used to—often to the extent that the valves are folded in half, inserted into a small incision, and then allowed to "pop open" to their full length inside the patient, where the inlet and outlet catheters are attached.
Spring fever. Bertrand notes that on the Strata, "Spring design is the main thing. It's not a coil but a flat spring that compresses to only the 0.001-inch thickness of the material itself." He adds that coil springs become non-linear as they flatten, leading to binding coils in the spring. Thus coil springs have to be made "taller" to stay in the linear range. Non-linearity in a medical valve is not desirable for the precision needed in servicing a patient. And a "taller" spring worked against keeping the size small.
The Strata valve also includes the Delta chamber that prevents siphoning of fluid from teh patient's brain. The membranes over the occluders can be pushed by the surgeon to check fluid flow in the catheters, as well as proper valve functioning.
The Medtronics team decided to design their own spring and thus approached the task without any preconceptions in spring design. The result: a unique flat spring, which is photo-etched en masse, from thin, flat stainless steel into a spiral pattern. There are two such springs in the Strata valve, one (0.14-inch dia.) to hold a ruby ball in the flow orifice, and another (0.23-inch dia.) which locks a rotor, on which the valve spring and ball sit, in one of five adjustable angular positions. These positions raise the rotor 0.005-inch each to put a different compression on the valve spring, governing flow rate.
The ball valve is used, says Bertrand, because a polymer membrane-type valve (as used in its fixed-rate Delta valve and in the Strata to prevent siphoning action) would exhibit material creep under the preloading needed to control and adjust flow rates.
In order to raise the Strata's rotor to adjust the valve, a small samarium cobalt magnet is encased within the polysulphone rotor when it is molded. With a circular tool, about two inches in diameter, placed on the patient's scalp, the doctor locates the valve and pinpoints the rotor. An indicator tool (actually a compass made by the maker of Boy Scout compasses) is then mounted on the locator to determine the angular position of the rotor. The indicator, which functions in both vertical and horizontal
The adjustable rotor features inner and outer support feet. These allow nearly 360 degrees of rotation for adjustment since the five step levels can run nearly the full circumference of the rotor. If edge steps were used, only 180 degrees of turning for adjustment would be available.
orientations, is then replaced by a adjustment tool containing a high-strength (3,000 gauss) magnet which lifts the rotor against its retention spring. When the doctor turns the tool to another angular position (usually one step away), the rotor follows.
Bertrand says the team balanced rotor retention spring strength (while having it function in all patient orientations) against environmental magnetic fields a patient might encounter, such as from telephones and headphones (roughly 40 gauss, he notes). Adjustments had to be effected through scalp thicknesses up to 3/4 inch.
The Strata valve is in use in Europe and Japan. Trials are underway in the U.S.
Contact: Jeff Bertrand, Medtronic PS Medical, 125 Cremona Dr., Goleta, CA 93117-5500; Tel: (805) 968-1546; Fax: (805) 968-7027; E-mail: jeff.bertrand@ medtronic.com.
Precise control of low pressure fluids
Springs for space-limited devices
Jeff Bertrand led the design team for the Strata adjustable shunt-valve project where his primary contribution was the basic concept and design of the valve. Many team members were responsible for making the design a reality, however—significant refinements came from Medtronic PS Medical's in-house machine shop, technologists, and computer-modeling, marketing, product-introduction, quality, and process-engineering groups. Hauser Inc., an outside consulting firm, was tapped to help with assembly and spring design and provide the primary design assistance for the handheld tools doctors use to adjust valve pressure.