After the fall: fasteners and orthopedic implants.

Perhaps it can be argued that the world's oldest profession is "bonesetter." After all, what did man likely do after standing fully upright, but fall over a tree root?

As hard as bones seem to be, they are not impervious to the effects of traumatic forces. If the patient is an adult, a cast or a splint is insufficient to treat fractures where the bone has completely broken in two or more pieces. Without surgical intervention, the broken ends cannot knit together properly, which can leave the patient disabled for life.

Intramedullary (IM) nails are long rods made of surgical-grade cobalt-chrome alloys, MP-35N, or titanium-based alloys. To repair fractures in arms and legs, the nail is implanted through the canal of the bone to secure pieces together and allow for proper healing. The nail is then secured to the bone on either end by cortical screws that pass through the bone and into threaded holes in the nail.

This technique is state-of-the-art fracture treatment, and has become the standard method of treating serious fractures in developed nations, making such fractures little more than painful (and temporary) inconveniences. IM nails are manufactured all over the world, providing an array of specialized products with a range of characteristics such as corrosion resistance, surface roughness, mechanical strength and bend properties, and dimensional variations, for orthopedic surgeons to use in routine or emergency situations.

Treatment and post-treatment

In an emergency situation, a surgeon may use whatever treatment tool is on-hand at the time. Implanted devices like IM nails could be the nail-of-the-day, a sample that was dropped off by a sales rep for a new manufacturer, or one of the clinic's regular suppliers' newer designs, expected to replace an older model. The orthopedic group could have recently changed suppliers as well, to get a better cost, a higher quality nail, or just to try out another product.

The largest problem arose when the time came to extract the supporting nail. Due to multiple IM nail manufacturers working on a non-standardized product, orthopedic surgeons often needed to have multiple rod and screw extraction systems on hand. This problem was especially the case because these surgeons didn't often know which manufacturers' product was used until the patient was prepped. A universal tool was desperately needed.

According to Dr. Kenneth Koval, an orthopaedic surgeon at Dartmouth-Hitchcock Medical Center in Lebanon, NH, "It's not only what devices your own orthopedic staff uses when they operate, you also have to consider what else is on the market." Dr. Koval goes on to explain that they often need to extract nails from people who are not the groups' original patients. "Another surgeon installed the pins and someone at our group has to remove them. Who knows what design or material the original orthopedic surgeon used? This sort of situation is what necessitates the purchase of multiple removal systems," says Dr. Koval. "And here's the scary part: if a surgeon opens you up and only then finds that they have the wrong removal system, you'll have to be sewed back up until they can locate a rep with the proper tools."

Solving the challenge

"That's where the Universal Maxi Ratcheting Screwdriver Set comes in handy," says Dr. Koval. Suitable for bone screw insertion and extraction, the system was created by engineers at SS White, Piscataway, NJ, specifically for orthopedic surgeons who need to remove IM nails from previously completed arm and leg fractures. The MaxiRatcheting System is a condensed version of the company's full-blown Winquist Universal Intramedullary Rod Removal System, which includes the added tools for actually installing and extracting the nails--a precision surgical procedure that involves the use of a 5 lb mallet. Both systems are distributed under the Xtract-All label.

Dr. Koval's group typically uses only three manufacturers' nails for insertion procedures, but often ran into up to ten additional manufacturers' products as they were performing extractions. Having a single insertion/extraction system available made their operation run much more smoothly than if they needed to contact multiple reps 'just in case' a different removal system was needed. Considerable time is saved in the operating room because the surgeons have a comprehensive set available for their extraction needs.

"I first ran into the Maxi System when I worked with a staff of sixty orthopedic specialists some years ago," explains Dr. Koval. "There were at least ten staff members who needed to use the snap-together insertion extraction system. We owned one set, but that was enough to handle our needs at the time. The group worked under a strict scheduling routine for all removals, which occurred on a limited basis."

Configuration of the tool

Encased in an anodized aluminum sterilization case, the set consists of twelve of the most frequently used bone-screw driver shafts, and includes a fully autoclavable ratcheting screwdriver handle. The handle, molded from medical grade silicone over a stainless steel 26-tooth mechanism, features the right surface texture and shape to provide the strongest and most comfortable grip possible to the orthopedic surgeon. The screwdriver functions in three modes: clockwise, counterclockwise, and a locked position. A 1/4-in. square drive allows for quick interchangeability of drive shafts, and the handle is cannulated to aid in fluid extraction during a procedure.

The collection of interchangeable stainless steel driver shafts allow the surgeon to remove virtually all cancellous and corticle bone screws commonly used today, reducing possible delays resulting from mismatched bone-screw and insertion systems. Among the most commonly used screw head styles include the flat, cruciform, Pozi-drive, and hex head tip configurations, accommodating IM nails currently employed by such companies as Acumed, Biomet, Depuy ACE, Zimmer M/DN, Synthes, and Encore True/Flex and True/Fix, as well as others. As SS White becomes aware of new hardware on the market, the company creates adapters for the redesigned products.

Observes Dr. Koval, "Working with one company that supplies a universal kit such as the Maxi Ratcheting Screwdriver Set has helped to keep our overhead costs down, surgical complications to a minimum, and confidence levels high."

Fasteners and knees

Biomet Inc., based in Warsaw, IN, developed an artificial knee implant that closely fit the individual patient's anatomy, creating a wide range of motion and superior joint functioning compared to previously available implants. Splitting the tibial component of the implant into two pieces fastened with a single screw made possible the ability to build the knee in a modular fashion, so that each component's size could be selected by the surgeon based on the patient's specific anatomy. However, the implant was highly dependent on a threaded fastener that would guarantee repetitive loading without loosening.

Knee implant surgery is intended to be permanent--re-tightening the implant in a second or third operation is not an option. Studies have shown that the human knee is subject to 1,000,000 or more loading cycles per year. Since conventional fastening methods such as add-on locking components, adhesives, patches and inserts are often rejected by the body, they are not acceptable methods for implant use.

The Biomet engineers examined a snap-fit-type locking fastener, but selected a self-locking fastener manufactured by Spiralock Corp. of Madison Heights, MI. The Spiralock component uses a 30-degree "wedge" ramp cut at the root of the female thread (while traditional fasteners use a 60-degree thread). Under clamp load, the crests of the threads on any standard male bolt are drawn tightly against the Spiralock fastener's wedge ramp. This configuration not only eliminates sideways motion that causes vibrational loosening, but also distributes the threaded joint's load throughout all engaged threads. The load percentage on the first engaged thread is significantly lower than traditional thread forms, which further reduces possible bolt failure and improves product performance. Extensive fatigue tests showed that the snap-fit fasteners tended to work loose under extensive loading, while Spiralock did not fail, regardless of the length and severity of the tests.

The modular tibial system consists of: a titanium alloy tray with fins to resist rotational forces, a polyethylene bearing, and a titanium modular tibial stem which attaches to the tibial tray for improved stability. A standard titanium screw attaches to a Spiralock-tapped hole in the stem to integrate the tray, polyethylene bearing and stem into a single unit using a Morse taper fit between the tray and stem.

And elbows

Though many implants are designed for permanent function without revision, some ultimately require revision because of unavoidable wear over time or the temporary nature of the implant itself. In these cases, the implant must nevertheless function flawlessly while inside the patient, while keeping the healing, revision, or removal process as simple as possible. A Biomet elbow replacement device, for instance, joins a polyethylene bearing of an ulna component with two cobalt chrome articulating surfaces on the humeral component using the Spiralock thread.

"We had to find a way to lock the articulation together in a compact form without violating the elbow condyles," said Matt Brzozowski, a Biomet extremities product manager. "To replicate anatomy as closely as possible, we not only had to keep the threads secure, but also wanted to provide a simpler approach to the articulating components should additional surgery be necessary."

C-clips, specialized retaining rings, and other fasteners proved too cumbersome and costly for the design. For vibration control, loading, and shear strength, Spiralock threads were chosen in conjunction with titanium screws. The titanium screws survived a shear test to approximately 2700 lbs, at which point the screws broke but remained locked in the thread form.

Besides vibration resistance and trustworthy thread locking, the Spiralock thread also provided anatomic and surgical benefits. "The threads helped us match each patient specifically," says Brzozowski. "Because the bearing is consistent with respect to the screw, we're able to match any humeral sized component with any ulna sized component. For example, if a patient has a distal fracture or distal stress riser in the humerus and needs total elbow replacement, we can combine a small ulna component with a long humeral component to get past the fracture for more stability."

"Moreover, the way the threads work allows the surgeon to access them from the posterior side of the elbow," continues Brzozowski. "That way, if revision surgery is necessary, the humeral stem may remain in place and surgeons don't have to take the component out. For patients, that means less time in the operating room."

Longer limbs

To decrease time in surgery, Spiralock makes threads as small as #0-80, as well as double lead threads, which spin on twice as fast with every rotation. This wide range of wedge fastener solutions gives medical equipment manufacturers, such as Boston Endo-Surgical Technologies Inc., the ability to develop equipment like their internal limb-lengthening device, currently used to lengthen the femur or tibia in cases where bone has stopped growing or has been shattered.

With traditional external limb-lengthening devices, up to eight external pins had to be inserted through the skin into the femur or tibia in order to secure the apparatus, leaving the patient open to the risk of infection. A left-handed Spiralock thread was used to join the internal limb-lengthener's lead screw with a redesigned nutscrew--with appropriate torque applied, the screw joint will not loosen. When surgically placed in a patient's femur or tibia, the appropriate bone can now be lengthened up to 80 mm while the patient remains mobile.

Circle 135--S. S. White Technologies Inc., or connect directly at www.rsleads.com/509df-135

Circle 136--Spiralock Corp., or connect directly at www.rsleads.com/509df-136

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Miniaturizing Medical Devices

Changing global demographics, breakthroughs in medical research and diagnostics, and advances in healthcare has the medical products industry demanding smaller and more efficient devices. Additionally, the delivery of healthcare services has been moving away from large, centralized locations into remote facilities, smaller clinics, and at-home care.

As recently as five to ten years ago, oxygen concentrator and renal dialysis machines were so big and bulky that the idea of a portable machine seemed a futuristic dream. But smaller, more efficient components have made it possible for equipment to be ambulatory--saving time and resources in an industry where cost containment is a major issue while greatly increasing patient's well being. Such component scaling can be cited in the work carried out at SonoSite Inc., where engineers reduced a 300 lb dresser-sized ultrasound unit to a hand-carried device by reducing twenty circuit boards to one. Such changes could also produce a reduction in size of an oxygen concentrator for improved mobility, plus the patient can substantially reduce their expense by filling their gas bottle at home, an improvement in quality of life.

Small spaces and small equipment are critical in surgical scenarios where size reductions spell efficiency and often, better patient outcomes. Shrinking instrumentation is helping OEMs create products that better catheterize blood vessels during surgery, as well as heat up and cool down faster, creating a safer surgical environment.

Reducing the diameter of a cartridge heater for immunoassay test equipment had many challenges for the engineers at Watlow Electric Manufacturing Co., St. Louis, IL. Traditional cartridge heaters were limited to 1/4-in. dia. sizes, which resulted in larger thermal assemblies. The solution required development of new manufacturing processes that would allow use of smaller gage wire and more tightly compacted MgO dielectric material to produce the 1/8-in. FIREROD cartridge heater, which performs at a high watt density without compromising service life.

The delivery of fluids such as blood and saline to a patient requires warming the fluid to close to body temperature. Pre-heated fluids are easier on the patient and a possible aid to faster recovery. Older systems had bulky reservoirs remote from the IV and patient. By moving the heating mechanism closer to the patient, more precise temperature control and portability could be achieved. For this application, combining several components into a small reusable package provided the solution. Watlow engineers designed a Lexan heater to snap onto the delivery line. The heater assembly has an integral temperature and limit control that assures precise and safe fluid temperature.

Towards the goal of reducing size in thermal components, Watlow engineers have developed "layered heaters," a process where the heating element and dielectric insulation are deposited directly on the part that needs to be heated. The result is a single assembly that has eliminated size in total parts. Engineers can put heat in places they previously could not, the product has lower mass and is lower weight, is smaller, moisture resistant, and more energy efficient. Layered heaters today can be applied to a wide variety of substrates, including stainless steel, aluminum oxide, aluminum nitride, and quartz.

There are some limitations to how small medical devices can be and still be usable and effective. Surgeons must still be able to easily handle devices, and many products must be a certain size in order to generate the level of power needed for the application. However, the benefits of miniaturization will fuel innovation for years to come.

--RM

Circle 138--SonoSite Inc., or connect directly at www.rsleads.com/509df-138

Circle 139--Watlow Electric Mfg. Co., or connect directly at www.rsleads.com/509df-139