When engineers develop orthopedic surgical devices, they must often think beyond the design of the device, and find a way to make its installation easier, safer, and more accurate. Sometimes experimentation leads to an unusual solution: a tiny, canted coil spring.
For example, one orthopedic company was developing a femoral nail to address a variety of femoral fractures, ipsilateral neck/shaft fractures, fixation of femurs that have been surgically prepared (osteotomy) for correction of deformity, and more in both children and adults. Because the device could be used in children as young as seven or eight years old, it needed to be minimally invasive and minimize damage to growth plates.
As with any nailing system, one of the challenges for product engineers was not the nail itself but the precise positioning of the locking screws. The targeting screws must be precisely positioned. To do that, the surgeon's drill must be guided directly to the intended location.
Guiding and Positioning Drills in Surgery
Orthopedic surgeons rely on specially designed guide hardware to properly predrill holes prior to tapping and fixation of implants in surgeries requiring screws to fix bone and synthetic ligaments. Locking the nail is often the main complication in surgeries of this type. If the nail is not properly locked, it can move inside the bone — causing pain for the patient. In some cases, improper locking can cause the implant to fail prematurely.
Because of the smaller size of some patients and the increased possibility of complications from damage to growth plates, the accuracy of such a drill has tremendous implications on the procedure — even more so than in typical orthopedic surgeries. Understandably, there is generally a lower threshold for tolerating errors or pain when operating on children. In procedures involving femoral nails, the surgeon has to drill through soft tissue in order to fix the locking screws to the bone. For that reason, making drill and screw positioning more reliable is critical.
The Spring Solution
For the femoral nail system, for example, the engineers developed a targeting device to ensure proper positioning of the nail and the targeting screws. Once the nail is inserted into the femur at the appropriate depth, two guide tubes — an outer guide tube (4.5 mm) and an inner guide tube (3.5 mm) — are inserted into the targeting device and pushed down to the skin.
These tubes guide the surgeon's drill to the intended location and are essential to the proper positioning of the locking screws. Holding the tubes in place during this most critical step in the procedure are two Bal Spring® canted coil springs, manufactured by Foothill Ranch, CA-based Bal Seal Engineering, Inc.
It is important that the inner and outer guide tubes are advanced to the lateral cortex of the femur. Failure to do so could have an effect on proximal screw measurement and insertion. The Bal Spring canted coil springs ensure that the tubes do not retract once they are advanced to the lateral cortex of the femur, enabling the surgeon's drill to hit its intended target.
O-rings, snap rings, and ball detents are sometimes used in similar applications to guide and center drills in orthopedic procedures. While these components generally are an improvement over unassisted drill placement, their use can still result in imprecise positioning, requiring surgeons to use two hands to more accurately place their drill.
By contrast, the canted coil springs can be used for latching, locking, and holding applications. They are available in coiled diameters from 0.020 in. (0.51 mm), and in a variety of materials and surface finishes. 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.
Other methods that might secure the sleeves together are not viable because they involve more complex components. The inclusion of two Bal Springs provides a simple solution to the challenge of preventing the guide tubes from sliding out of the targeting guide under their own weight.
Since the introduction of the femoral nail, the Bal Spring has provided the reliability necessary for drill positioning and targeting in surgery. The consistent forces applied by these springs allow surgeons to easily target and predrill holes without having to use two hands.
Designers of femoral implants aren't alone in leveraging the benefits of the canted coil spring. In fact, the component has proven to be useful in the process of tibial trialing, as well. Knee replacement system engineers have incorporated the spring into specially designed shims, or “trials” that allow for accurate sizing of the joint during the implant procedure.
The trials, which are made from a medical-grade polymer (often a polyphenylsulfone such as Radel®), are temporarily placed into the orthopedic assembly, where they are securely latched in place by a tiny spring that provides the surgeon with feedback in the form a tactile click. Spring breakaway forces, which can be precisely determined and controlled by the device engineer, allow for easy removal of the trial once correct sizing is achieved.
Engineers at Spinal Simplicity, LLC incorporated the spring into their Minuteman G3 spinal fusion device (Medical Design Briefs, March 2017). In this application, the spring temporarily latches a linearly traveling plunger used by the surgeon to deploy folded arms, which affix to the spinous processes on either side of the placement.
The versatile canted coil spring component has even found its way into the instruments surgeons use to perform orthopedic procedures. For example, Waltham, MA-based Life Instruments Co. has designed the springs into its radiolucent cervical retractor, which is used by surgeons to spread both the skin and muscles surrounding the spine in order to access and repair the vertebrae. In the cervical retractor, two springs, positioned inside grooves in the retractor blades, serve as snap-connector components while providing circular referencing “footprints” that provide guidance for the surgeon under x-ray.