Medical Device Link.

FEATURE

Fastening woes

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When designing orthopaedic and medical devices, engineers invariably encounter the same basic challenge: how to connect two mating surfaces in a repeatable, predictable fashion. Although this may sound like a straightforward task, there are many different ways to accomplish it. Traditional fastening methodologies include ball detents, snap rings, moulded c-clamps, quick connects and o-rings, to name just a few. Most of these are relatively inexpensive, off-the-shelf solutions with varying life cycles and they usually 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. In addition, they often fall short of satisfying the unique requirements of the medical industry: biocompatibility, cleanability and insertion/breakaway force accuracy.

Achieving target cycle life, the ability to withstand autoclaving and retention of original insertion and breakaway forces are essential requirements for medical device applications. However, these qualities are not always available in a fastener and that makes finding the right fastening solution difficult, especially when 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 medical device manufacturers to choose another method for reliable, repeatable fastening: the canted-coil spring. In many advanced medical applications, this is being used to perform a variety of functions and, most importantly, to meet and maintain strict insertion and breakaway force specifications.

The canted-coil spring

Invented more than 50 years ago by engineer and entre-preneur Peter J. Balsells, the canted-coil spring consists of wire (in various materials, sizes and surface treatments) that has been precision coiled and resistance welded to a specific diameter. It is a deceptively simple component with a lot of science behind it.

The canted-coil spring has gained in use recently mainly because of its versatility. In addition to fastening or “latching,” it can be designed to permanently lock two pieces together, or to perform a holding function whereby it provides a specific amount of friction between two components. It can also perform electromagnetic interference shielding and grounding functions or serve as an electrical contact between two pieces. The following are applications, both real-world and conceptual, in which these spring functions can be employed.

Latching

The majority of medical grade canted-coil springs are used in latching applications in which two components are fastened together on a device in accordance with the force requirements specified by the designer. Surgical instrumentation design is moving more towards custom products for specific surgeons and the spring makes this easier for engineers, because it enables them to tailor the instrument to the surgeon’s specific needs.

Hex driver orthopaedic 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. (~227–453 g) and breakaway can be ~2–3 lbs. (~0.9–1.4 kg), 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, which is often used on these instruments. The latching spring application is also used regularly in surgical instrument applications where multiple heads need to be changed on one tool. The spring is easy to use and clean, resists compression set and generally has a long cycle life. A wide offering of medical grade wire materials is available for applications such as the hex driver. Materials 316SS and 316L work well for surgical handpieces that come into 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 can be autoclaved or sterilised with ethylene oxide or gamma ray.

Figure 1. (click to enlarge) A human carpometacarpal joint design concept employs a canted-coil spring to ensure smooth rotary motion and reduce the possibility of dislocation.

Carpometacarpal (CMC) joint design concept (latching or locking spring). To understand how the spring can be used in this type of application, consider this design concept for an implantable joint replacement device that employs the spring as a latching component. When used in this way, the spring provides flexibility and allows for smooth rotary movement similar to that typically found in a human CMC joint (Figure 1). An implantable grade material such as titanium or MP35N is likely to be used in this application, although required spring force (a factor of tensile strength) would play a role in the type of material selected.

Locking

Figure 2. (click to enlarge) In this model of an insulin pump, the spring is used to hold the insulin cartridge in place during the dispensing process.

Having a locking component means that the spring offers advantages for orthopaedic implants. The spring can lock any two pieces together during surgery using minimal force, which eliminates the need for hammers and other surgical tooling and frees up the surgeon’s hands to conduct the surgery in a more efficient manner. This efficiency can translate into shorter surgical procedures and 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 1000 shearing lbs. (408–454 kg) 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 to allow a degree of freedom, or the spring can lock the stem into a v-groove to eliminate axial play. MP35N or 316LVM are the materials of choice for this type of application.

Holding

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 customised 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 slide across a surface until it reaches a latching point. The running force as well as the insertion and removal force can be specified for the spring.

Electrical contact/EMI shielding and grounding

Figure 3. (click to enlarge) Ultrasound shield. In ultrasound transducer applications like this one, the spring can be used to shield sensitive electronics against electromagnetic interference.

For years, medical device manufacturers have been under pressure to reduce package size while improving performance and reliability. It is no coincidence that many 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 (Figure 3).

Importance of the wire

The wire is the heart of the canted-coil spring: it determines how the component will perform in an application. Various wire sizes, 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), magnetic resonance imaging 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 302SS, 316SS, MP35N, 316L, 316LVM, platinum iridium, titanium, beryllium copper, and 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 maximise cycle life.

Performance life

Spring performance life can vary and is largely dependent on 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 that 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 specific amount of play, if an application calls for it.

Figure 4. (click to enlarge) The canted-coil spring has the ability to retain a relatively uniform force over a wide deflection range.

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 versus deflection curve gives it the ability to take up significant amounts of slack and still perform effectively, which eliminates the need for precise dimensional tolerances and further reduces machining costs (Figure 4). The diameter and cross-sectional tolerances of the spring are also much tighter compared with an o-ring, which results 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 will not 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 acceptable for the application. But for engineers who want to advance technology and gain an added level of customisation and performance from their design, the simple spring offers a proven, versatile alternative.

Alicia Hackley is a Medical Device Market Manager for Bal Seal Engineering Inc., 19650 Pauling, Foothill Ranch, California 92610-2610, USA, tel. +1 949 460 2100, e-mail: sales@balseal.com, www.balseal.com