Failure is not an option when it comes to medical equipment. From critical or sensitive devices like lasers, MRI scanners, knee joints and implantable pacemakers to instruments as straightforward as stethoscopes, when life or quality of life is at stake, medical equipment must be reliable.
In the medical field there's little room for the $15 billion per year spent by 25 of the largest U.S. manufacturers on warranty claims; and little room for processing those claims that eat up 2.5 to 4.5 percent of revenues for companies across all industries. The industry has higher aims than other fields and customer satisfaction and warranty problems associated with fastener failure must be prevented or quickly and economically handled.
Due to medicine's working environment inside and outside the human body, fasteners such as C-clips and specialized retaining rings can prove too cumbersome and costly. Because repetitive loads, shock and vibrational loosening must be decisively handled, traditional fasteners susceptible to self-loosening rotational movement, stripping and shearing are not always appropriate, either. Testing, in fact, has found that the first two threads of traditional fasteners can carry as much as 80 percent of the load, permitting stripping or shearing, while subsequent male threads “float” within the female threads.
Medical equipment manufacturers are successfully attacking these problems with a variety of new technologies. One of the most interesting solutions is also the simplest — an innovative self-locking fastener called Spiralock. By its unique design, Spiralock is capable of resisting loosening even under loads and vibrations strong enough to break the fastener.
Protecting Medical Equipment from Shock, Vibration and Warranty Issues
In a proactive design change to increase reliability in the field, the medical/aesthetic laser technology firm Lumenis recently sought to safeguard its precision critical laser alignments. These could potentially become misaligned during rough shipping or handling using standard thread forms and traditional locking techniques were not well-suited to the task.
“We wanted to increase the design's robustness to better withstand the shock and vibration of shipping, as well as carting from room to room in the field,” says Peter Hines, a Lumenis R&D engineer responsible for laser design.
Hines ruled out chemical adhesives because of their tendency to emit gaseous material which could collect on sensitive optics, degrading their performance. He was less than satisfied with traditional techniques, such as locking washers, which add weight and complexity with less than desired locking ability. Lumenis chose Spiralock self-locking fasteners.
What makes Spiralock unique is 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 Spiralock's wedge ramp. This not only eliminates sideways motion that causes vibrational loosening but also distributes the threaded joint's load throughout all engaged threads, a claim supported by a Massachusetts Institute of Technology research study. 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.
“We're using the Spiralock self-locking fastener to help alleviate our concerns regarding a system's critical precision alignment,” says Hines. “With its shock and vibration resistance, we're preventing screws from backing out and preserving critical alignment for enhanced reliability.”
Manufacturers of ultra-precision devices and instruments who face product reliability problems or whose components are used in larger OEM systems turn to Spiralock, whose self-locking fasteners are able to withstand micro-vibrations and restrict minor but noticeable torsional loads. The fasteners are ideal for securing Teflon-coated parts that are difficult to retain and yet, can be easily removed by hand.
By using the Spiralock self-locking thread form, manufacturers can achieve increased productivity, durability, more cost-effective and repeatable manufacturing processes, as well as the ability to retro-manufacture thousands of inventory parts that could end up as very costly scrap.
Production changeovers to Spiralock fasteners are typically quick and seamless, usually requiring just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock that have unreliable standard tapped holes.
Protecting Medical Implants from Extreme Loads and Load Cycles
Recently, Warsaw, IN-based Biomet Inc. developed an artificial knee implant which closely fits 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 makes possible the ability to build the knee in a modular fashion so each component's size can be selected by the surgeon based on the patient's specific anatomy. But, this innovative development is highly dependent on the threaded fastener that must guarantee repetitive loading without loosening.
Knee implant surgery is designed to be permanent and re-tightening the implant in a second or third operation is not an option. Studies have shown the human knee is subject to 1 million or more loading cycles per year. Since conventional fastening methods such as add-on locking components, adhesives, patches and inserts will be rejected by the body, they aren't acceptable methods for implant use.
The final choice came down to a snap-fit-type locking fastener and a Spiralock self-locking fastener. Extensive fatigue tests showed 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.
After several years of in-body application, not a single screw fastened with Spiralock fasteners has come loose.
Though many implants are designed for permanent function without revision some ultimately require revision due to 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 recent 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,” says 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 2,700 lb, at which point the screws broke but remained locked in the thread form.
Besides vibration resistance and trustworthy thread locking, the Spiralock thread also can provide 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,” adds 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.”
To decrease time in surgery, Spiralock makes threads as small as #0-80 (80 threads per inch), 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 pioneer innovative devices like their innovative internal limb-lengthening device, currently used to lengthen the femur or tibia in cases where bone has stopped growing or has been shattered.
Prior to the development of the internal limb-lengthening device, up to eight external pins had to penetrate from the skin to the femur or tibia in order to attach an external device, with a likely possibility of infection.
An unusual left-hand Spiralock thread now firmly joins the limb-lengthener's lead screw with a redesigned nut-screw, and 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 eighty millimeters while the patient remains mobile.
The Future of Medical Device Performance
As the reliability of medical equipment, devices and implants becomes increasingly important and tolerances become stricter, innovative technologies like Spiralock's self-locking fasteners are becoming more widespread in medical applications.
Because of their vibration-resistant, reliable locking features Spiralock fasteners hold various components together in artificial limbs, heart pumps and MRI machines and are also being considered for cardiovascular devices such as pacemakers and implantable defibrillators, as well as for dental and orthopedic surgical instruments and CT scanning applications.
Del Williams is a technical writer based in Torrance, CA.