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A Battery of CNC Machines Spins Out Surgical Implants on a Just-in-time Schedule.

Mechanical Engineering 123(08), 60-62 (Aug 01, 2001) (3 pages) doi:10.1115/1.2001-AUG-4

OMC Precision Products, a maker of spinal implant, first line of attack for smoothness is machining to minimize cusps or scallops left behind by machine tool cutters. In finishing passes, the cusps are diminished by minimizing stepovers, the distance the tool moves into the new path. OMC focuses on making implants for correcting spinal deformities. OMC has three lines of implants, all machined from stainless steel or titanium. It makes plates and screws for spines that are degenerating or have undergone trauma; hooks and rods for deformities such as scoliosis; and cages to replace damaged vertebrae. First-article inspections are done on a Primus coordinate measuring machine from Mycrona Inc. of Plymouth, Michigan. The unit is equipped with a touch probe system from Renishaw Inc. of Schaumburg, Illinois. OMC also produces 2 alternatives to plates, a spine straightening system based on titanium or stainless-steel rods several inches long. The rods are locked in place with diverse types of bone screws or spinal hooks.

It’s summer vacation, but not every child gets to kick back and relax. In Anytown, USA, third-grader Angela Smith is bracing for spinal surgery to correct scoliosis, a lateral curvature of the spine. Left untreated, young Angela could suffer severe and possibly crippling nerve damage.

At the same time, in Indianapolis, a manufacturing engineer at OMC Precision Products, a maker of spinal implants, is checking CNC tool paths one last time for a batch of surgical implants. Some of them will soon be used to straighten Angela’s spine, allowing her to lead a normal life. In OMC’s shop, a machinist downloads the programs, loads gently curved titanium strips into a fixture, makes one last check, and presses the start button.

Minutes later, an exquisitely finished set of spinal braces comes out of the machine tool. They are checked again visually, then launched into a complex series of steps: cleaning, polishing, anodizing, marking, labeling, and packaging. Every step is prevalidated by the customer, the DePuy AcroMed unit of Johnson & Johnson Inc. in Raynham, Mass., and monitored by the U.S. Food and Drug Administration.

Meanwhile, the surgeon takes another look at the girl’s spinal X-rays. She carefully measures, one more time, the vertebrae where the implants will be attached. Two days later in an operating room, she attaches the titanium plates to Angela’s spinal column with specially made surgical screws, also from OMC.

Soon, the girl is on her way to the recovery room and to an active, healthy life.

OMC has put processes in place that let the company operate lean. According to its owners, the company turns out products fast, as orders come in. Because the products are going to reside in the human body, they must be manufactured to meet rigorous medical standards.

This article was prepared by staff writers in collaboration with outside contributors.

“We are mechanical engineers for surgeons,” said Paul Beckwith, manager of engineering development and a co-owner of OMC.

According to Andrew Elsbury, OMC’s president, “Machining these implants and ensuring that their surfaces are absolutely smooth are constant challenges for us. Any burr whatever will result in the entire lot being rejected.”

Surface roughness or undue sharpness in an implant can irritate the patient’s tissue or organs, or cut a surgeon’s glove.

Beckwith explained, “We always walk a fine line. We have to machine the flutes on the bone screws sharp enough to self-tap, but not so sharp they could slice though a glove.” The difference may be just a few ten-thousandths of an inch. OMC’s only sharp-edged parts are bone staples.

OMC’s first line of attack for smoothness is machining to minimize cusps or scallops left behind by machine tool cutters. In finishing passes, the cusps are diminished by minimizing stepovers, the distance the tool moves into the new path.

“On the edges of the implant plates, we have to machine multiple blended radii,” Beckwith said. “Typically, they are 0.02, 0.06, and 0.08 inch.

“We could smooth the implants with buffing,” he added, “but that can throw off the finished dimensions. And buffing takes time we don’t have when our customers expect 24-hour delivery. We have to machine as precisely as possible and then tumble-finish.”

OMC focuses on making implants for correcting spinal deformities. “Our business strategy is to serve a few carefully selected and demanding customers very well,” said Elsbury. DePuy AcroMed and Medtronic Sofamor Danek in Memphis are his largest customers.

“We strive for business relationships that evolve into tightly linked mutual dependence,” he explained. “We want to be the sole or dominant supplier in each of its product lines, so we are very aggressive on quality, delivery, and price.”

Founded in 1997, the company usually has 25 people working three shifts five days a week. Operations extend to seven days a week beginning in late spring. Nearly all of the candidates for scoliosis surgery are school-age children. Since these surgeries are elective, they are done during summer vacations.

The market for spinal implants is still in its early, fast growth phase. Elsbury estimated that the market has a compound annual growth rate of perhaps 60 percent.

“Because the market for spinal implants is new, improvements in treatment methods are being made constantly,” Elsbury said. “We are set up to adapt fast.” He said OMC’s new implant and device designs are ready for prototyping in five days or less.

Before they started OMC Precision Products, Elsbury and Beckwith were principals in a firm called Operations Manufacturing Consulting (also in Indianapolis), which analyzed medical and other manufacturing practices. They attribute much of their business success to capitalizing on insights gained during their days as consultants.

OMC has three lines of implants, all machined from stainless steel or titanium. It makes plates and screws for spines that are degenerating or have undergone trauma; hooks and rods for deformities such as scoliosis; and cages to replace damaged vertebrae.

Smaller cervical plates for neck injuries are under development.

Almost all computer numerical control machining is programmed with EdgeCAM version 5.5 from Pathtrace Systems Inc. of Southfield, Mich., four postprocessors, and EdgeCAM’s CodeWizard.

OMC Precision uses Mechanical Desktop 2000 from Autodesk Inc. of San Rafael, Calif., as its CAD system. Autodesk works with Pathtrace Systems and other third-party developers under the Mechanical Applications Initiative, a program to foster compatibility of their software.

Machining of the implants is done on three vertical-spindle machining centers, all with indexers. They include a TC-22A marketed by Brother Industries Ltd. of Elk Grove Village, Ill., and a Frontier MII/40 from Mori Seiki USA Inc. of Irving, Texas.

Work envelopes run from about 4,200 to 7,400 cubic inches. The VMCs have Fanuc controls. Additional cutting is done with two FX-10 wire electric discharge machines distributed by Mitsubishi EDM MC Machinery Systems of Wood Dale, III.

The closeup view in the top image catches a spinal implant as it is machined by OMC Precision Products, but without the benefits of electropolishing. The lower image is the computer's rendering of the product as a sum of its toolpaths.

Grahic Jump LocationThe closeup view in the top image catches a spinal implant as it is machined by OMC Precision Products, but without the benefits of electropolishing. The lower image is the computer's rendering of the product as a sum of its toolpaths.

The company runs on lean manufacturing principles; that is, operations are tuned for flexibility and agility, so orders can be handled quickly and smoothly. Parts are run, packaged, and shipped in five days, but can be ready in 24 hours if they are needed that fast.

Elsbury and Beckwith focus on managing bottlenecks and ensuring that people are cross-trained. To comply with the FDA’s insistence on close process monitoring, work in process inventory is scanned at every workstation. All shop floor traveler documentation is bar-coded.

“We are always in a balancing act between stepovers and surface finishing,” Beckwith said. “We might try three different machining strategies.” The smaller the stepover, the better the surface finish—and the longer the program takes to run.

OMC makes 100 to 200 new programs a year. “A lot of this is repetitive,” Beckwith reported. Most plates ■ and all the screws have families varying in one-millimeter increments.

The company uses EdgeCAM’s NC Verify to predict the coarseness of the surface as it will be after machining. “Actually, we find the as-machined surface is always a little better than NC Verify leads us to expect,” Beckwith said.

Threads and surfaces must mate tightly, so they have to be machined slightly oversize. “This process is so sensitive that machining setups are not torn down until after the parts have been through electropolishing and inspection,” Beckwith said. In addition to electropolishing, the company has highly sophisticated cleaning, de-burring, and passivating operations.

In electropolishing, the metal part is made the anode in an electric circuit. Atoms are pulled away from the part, leaving a shining, pure base metal surface. Any remaining cutter lines dissolve and disappear. Passivating is a chemical process to remove any reactive metals such as iron from machined surfaces.

First-article inspections are done on a Primus coordinate measuring machine from Mycrona Inc. of Plymouth, Mich. The unit is equipped with a touch probe system from Renishaw Inc. of Schaumburg, 111. It runs unattended. Customers use identical CMMs so results can be directly compared.

OMC is certified, and audited once or twice a year to assure conformance to standards.

Each of OMC’s spinal plates has six drilled holes, precisely angled so that screws will form an anchor in the vertebra.

Since the surgeon’s access to the patient’s spine may be restricted, the plates must accommodate screws that are initially misaligned. Surgeons access the spine through the back (where the spinal cord and nerves are) or from the chest or abdomen. The chest and abdomen offer easier access to the vertebra, but internal organs are in the way.

Prototypes of the small titanium cervical plates measuring about one by two inches illustrate a typical machining strategy. Corner radii are machined at 9,000 rpm for about two minutes per side, each of which is curved. Initial milling is done with a 0.25-inch ball-nosed cutter using parallel-lace machining.

The stepover is 1 percent of cutter diameter, or 0.003 inch. This leaves a cusp of just 0.0005 inch. Cusps and barely visible machining lines are removed during finishing. “We can modify the program with a single mouse-click,” Beckwith said.

The high-volume part of OMC’s business is surgical screws and plates for the vertebrae. As many as 20,000 screws a month may be produced, although part runs are usually under 200. Plate screws have two sets of threads. A standard machine screw thread for plates is one end. The other end has a long, special self-tapping thread for penetrating dense bone. For any part of the thread touching bone, tooth profiles are carefully rounded and smoothed to avoid any sharp edges.

Bone screws and other parts are produced on four Swiss-type turning centers; a widely used screw with a 7-mm diameter is made in lengths from 25 to 60 mm.

Grahic Jump LocationBone screws and other parts are produced on four Swiss-type turning centers; a widely used screw with a 7-mm diameter is made in lengths from 25 to 60 mm.

To prevent any motion from loosening the implants, OMC’s plate screws have a two-nut, dual-locking closure. An inner nut turns clockwise to lock the plate rigidly. The outer nut turns counterclockwise to prevent any loosening of the inner nut.

OMC also produces two alternatives to plates, a spine-straightening system based on titanium or stainless-steel rods several inches long. The rods are locked in place with different types of bone screws or spinal hooks. Unlike plate screws, these have oversize heads with large slots to hold the straightening rods. The slots have horseshoe-shaped cross-sections. The screws are produced in fixed-head and rotating-head variants. The latter are for rods not implanted in parallel and for surgeons working in awkward positions.

The spinal hooks are an alternative to the screws. Rods are held by a series of metal hooks that lock onto the spine’s many spurs.

Tooth profile specifications vary considerably—and often. The widely used 7-mm-diameter bone screws are offered in lengths from 25 to 60 mm. The screws are produced on OMC’s four single-spindle, Swiss-type turning centers. Three of them are SV20 models from Star CNC Machine Tool Co., while the fourth is a Citizen M20 CNC machine from Marubeni Citizen Cincom Inc. Both companies are in Elk Grove Village, Ill.

The cages are sliced from solid bar or titanium tubing roll-formed with an ovoid cross-section. The outside edges of the cages have teeth like the escapement gear of a watch. Once hammered into the bone during surgery, they won’t come back out, ever. The tooth profiles are cut on the Mitsubishi FX-10 wire EDMs—as are some slots and any angled cuts that surgeons may require to fit adjacent vertebrae.

Scoliosis appears in hundreds of thousands of U.S. children a year. Some 2 to 3 percent of them eventually require either surgery or a brace worn underneath clothing to avoid damage to nerves or internal organs. Advances in the skills of surgeons and engineers, meanwhile, are promising more of these children, like Angela, a brighter walk through life.

Copyright © 2001 by ASME
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