When designing orthopedic and medical devices, engineers invariably encounter the same basic challenge: they need to connect two mating surfaces in a repeatable, predictable fashion.
While this may sound like a straight-forward task, there are hundreds of different ways to accomplish it. Among the traditional fastening methodologies are ball detents, snap rings, molded c-clamps, quick connects and o-rings, to name a few.
Most of these are relatively inexpensive, off-the-shelf solutions with varying life cycles, and for the most part, they work well for quick connect/disconnect applications. However, variability is common in these standard fastening solutions, and their inherently wide tolerances can lead to poor device performance and reliability issues. What’s more, they often fall short of satisfying the unique requirements (i.e., biocompatibility, cleanability and insertion/breakaway force accuracy) of the medical industry.
Designers know that target cycle life, ability to withstand autoclaving, and retention of original insertion and breakaway forces are all critically important in medical device applications. But these qualities aren’t always easy to come by in a fastener, and that makes finding the right fastening solution difficult – especially in light of the fact that product malfunction can result in loss of time and money, and even loss of life in some cases.
These factors have led a growing number of device manufacturers to choose another method for reliable, repeatable fastening: the canted coil spring. In many advanced medical applications, this spring is being used to perform a variety of functions and, most importantly, to meet and maintain strict insertion and breakaway force specifications.
Invented more than 50 years ago by engineer and entrepreneur Peter J. Balsells, the canted-coil spring consists of wire (in various materials, gauges and surface treatments) that has been precision coiled and resistance welded to a specific diameter. It’s a deceptively simple component with a lot of science behind it – and it just may be the ideal solution to your next medical device fastening challenge. Here’s why.
For starters, it’s versatile. In addition to fastening or “latching,” the canted-coil spring can be designed to permanently lock two pieces together, or to perform a holding function whereby it provides a specific amount of drag between two components. It can also perform EMI shielding and grounding functions, or serve as an electrical contact between two pieces.
Let’s look at some medical device and surgical instrumentation applications – both real-world and conceptual – in which all three of these spring functions can be employed.
The majority of medical grade canted-coil springs are used in latching applications which fasten two components together on a device in accordance with the force requirements specified by the designer. Surgical instrumentation design is trending more towards custom products for specific surgeons, and the spring makes this even easier for engineers because it enables them to tailor the instrument to the surgeon’s specific needs.
- Hex Driver Orthopedic Handpiece (latching spring) – This is an actual latching application in a surgical instrument. The spring is mounted onto the male hex driver, which latches into the female surgical screw. Insertion and breakaway forces in this specific application are tailored to the needs of the surgeon and the application. For example, insertion can be ~ 0.5 – 1 lbs. and breakaway can be ~ 2-3 lbs., which ensures that the hex driver does not prematurely release from the screw during surgery. The application of force by the surgeon releases the instrument, rather than a manual push-button release often used on these instruments. The latching spring application is also used regularly in surgical instrument applications where multiple heads need to be changed out on one tool. The spring is easy to use, resists compression set, generally has a long cycle life, and is easily cleanable. A wide offering of medical grade wire materials are available for medical applications such as the hex driver. 316SS, 316L both work very well for surgical handpieces that come in contact with the body, but are not implanted. Alternate materials approved for implantation in the body include Titanium, MP35N, 316LVM and Platinum Iridium. The canted-coil spring is passivated after it is coiled, cut and welded, and it’s cleanable via autoclave or alternative methods such as ETO or Gamma Ray Sterilization.
- Carpometacarpal Joint Design Concept (latching or locking spring) – To understand how the spring can be used in this type of application, a design concept for an implantable joint replacement device would employ the spring as a latching component. When used in this way, the spring provides flexibility and allows for smooth rotary movement similar to that which is normally found in a human carpometacarpal, or “CMC” joint. An implantable grade material such as Titanium or MP35N would likely be used in this type of application, although required spring force (a factor of tensile strength) would play a role in material choice.
As a locking component, the spring has the potential to offer game-changing technology in the orthopedic surgical implant world. It can lock any two pieces together during surgery using minimal force, eliminating the need for hammers and other surgical tooling, and freeing up the surgeon’s hands to conduct the surgery in a more efficient manner. This efficiency can translate to shorter surgical procedures, and it can also reduce the patient’s exposure to multiple tools, which are sources of potential infection. Depending on design requirements, a spring may be engineered to require as much as 900 to 1,000 shearing lbs. to release the two pieces. The medical grade locking spring can be used in an implantable joint, such as a hip or elbow. The stem can easily lock into the head of the implant, allowing a degree of freedom, or the spring can lock the stem into a v-groove, allowing no axial play. MP35N or 316LVM are the materials of choice for this type of application.
Holding canted-coil springs are most often designed into surgical instrumentation. Drill guides, sleeves or any other sliding components that require a specific amount of sliding force can incorporate a spring. The drag force can be specified during the design process, and it requires consideration of the housing dimensions, tolerances and surface finishes. In some cases, the spring can also be customized to fit existing hardware, depending on force requirements. Typically, passivated 316SS canted-coil springs are used in surgical instrumentation. Their holding function can also be combined with latching in any given application. The spring can drag across a surface until it reaches a latching point. The drag force as well as the insertion and removal force can be specified for the spring.
Electrical Contact/EMI Shielding & Grounding
For years, medical device OEMs have been feeling the pressure to reduce package sizes while improving performance and reliability. It’s no coincidence that many of these device makers have embraced the spring as an efficient means to connect and conduct electricity. It can be used to provide electrical contact to sensors and other devices, and it performs extremely well in grounding or shielding applications for long periods inside or outside the body. In these types of applications, the spring can be used alone or in a metal housing made from MP35N or Platinum Iridium.
It’s Down to the Wire
The wire is the heart of the canted-coil spring, and it’s what determines how the component will perform in an application. Various wire grades, materials and diameters are used to address a broad range of design criteria, such as galvanic compatibility (dissimilar metal corrosion), material hardness compatibility (to prevent the spring from scratching softer surfaces during travel), MRI compatibility, tensile strength in response to force requirements, cycle life requirements and media compatibility.
Standard material offerings for mechanical or electro-mechanical canted-coil springs are as follows:
- Platinum Iridium
- Beryllium Copper
- Platings – gold, silver, nickel or tin
Springs used in medical applications require a special gas welding process that eliminates any black carbon deposits. Special chemical sonication cleanings, protective handling gloves and workstation cleaning protocols are all required when processing these springs for medical applications.
As a fastener, the canted-coil spring can be mounted into a housing or onto a piston, depending on which is more effective for the application. For example, if the piston is disposable, the spring can be placed in the housing to reduce cost and leverage the maximum cycle life.
Spring performance life can vary, and it’s largely dependent upon force ratios. Smaller medical device applications typically have a maximum 1:4 force ratio (1 lb of insertion, 4 lbs of breakaway force). The general rule is the higher the force ratio, the fewer the number of cycles. Larger industrial applications can range as high as 10:1 force ratio, with limited cycle life. The insertion and breakaway force is always tailored to the specific application. This is achieved by manipulating the groove and counter groove dimensions, as well as the lead-in chamfer angles, wire thicknesses and the cant of the spring coils. The groove can be designed with minimal axial play when the two pieces latch together, or a very specific amount of play if an application calls for it.
Using a spring can also prove more cost-effective in certain applications, because spring grooves are usually simple “o-ring grooves” that are easy to machine. This is much less complicated than, for example, machining threaded components. The spring is also resistant to vibration or rotational movements unlike a threaded two-piece assembly. In addition, the spring’s unique force vs. deflection curve gives it the ability to take up significant amounts of slack and still perform effectively, eliminating the need for precise dimensional tolerances and further reducing machining costs. The diameter and cross-sectional tolerances of the spring are also much tighter as compared to an o-ring, resulting in repeatable and quantitative entry and exit forces. Canted-coil springs are even occasionally used to take up dimensional slack in a design that needs to be free of any play.
A Solution to Consider
The canted-coil spring won’t be the answer to every medical device design challenge. In fact, there may be many instances in which the variability of more traditional fastening methods will be within acceptable limits for the application at hand.
But for engineers who want to advance technology and gain an added level of customization and performance from their design, the simple spring offers a proven, versatile alternative.