Developing the System
When designing the Minuteman, Spinal Simplicity faced several challenges related to its size and strength. The goal was to develop a fusion device that could be percutaneously placed in the interspinous space. The device would then deploy folded arms that would affix to the spinous processes on either side of that placement. The smallest Minuteman implant is 8 mm in diameter, so mechanism space is limited. The challenge was to design a mechanism and deployable arms that were strong enough to provide rigid fixation of the implant. The resulting design consisted of two opposing wings that deploy from a linearly traveling plunger inside the body of the implant (see Figure 3). This plunger is held in a neutral position, not affecting the wings until the surgeon deploys the them (see Figure 4). In this second state, the plunger holds the wings open, while the implant is tightened into its final position (see Figure 5).
In order for the device to be minimally invasive and to perform as desired, it was determined that the device must have deployable wings (extension plates) as well as some sort of an anti-backout feature, locking the implant in place to prevent accidental dislocation. Eventually, these criteria drove the design of the plunger so that a single feature could deploy both extension plates at the same time. However, in order for this design to work, it was essential that the plunger lock in place in an initial (predeployment) position and in a final (postdeployment) position. In addition, it was critical that the mechanism be easy to assemble while providing outward pressure on the ID of the device.
The original design called for a linear compression spring for each extension plate; however, the springs took up too much space and the design was not feasible for manufacturing. Another design considered integration of a finger cutout that pressed outward within the plunger, but machining into a tiny plunger required expensive manufacturing and the force was not consistent or reproducible. Preliminary designs used an O-ring seated in a circumferential groove on the plunger that mated with a set of grooves in the implant. This design was rejected, however, because of concerns about wear, debris generation, and failure of the O-ring itself due to wear caused by material mismatch of the elastomer and the metallic properties. Ultimately it was determined that the design required a component that could easily and consistently reproduce the needed 3-4 lb force.
The Bal Spring, a tiny mechanical component designed and manufactured by California-based Bal Seal Engineering, was critical for providing this functionality. As a locking fastener, the spring offers game-changing technology for ISP devices. It can lock any two pieces together during surgery using minimal force, eliminating the need for other surgical tooling, and freeing up the surgeon's hands to conduct the surgery in a more efficient manner. For the Minuteman, the Bal Spring technology enabled the plunger to lock in place, while allowing precise control of the 3-4 lb force when deploying the extension plates to lock and unlock the plunger within the implant ID. The spring also prevented movement of the plunger within the implant ID. The Bal Spring met the small profile design (OD of 4 mm or less) requirements to fit within all implant sizes.
The Bal Spring is attached to the plunger, which is assembled into the ID of the Minuteman device. When the implant is assembled during manufacture (prior to sterile packaging), the plunger assembly is inserted into the ID of the implant until it engages a groove where the Bal Spring is housed. The spring holds the plunger in place to ensure that it does not move prior to implantation during surgical insertion of the device. Once the implant is in position, the surgeon uses the company's insertion instrument to advance the plunger forward. This inserter is designed to overcome the 3-4 lb force of the spring, allowing the plunger to advance within the ID of the device, thus deploying the extension plates. Once the extension plates are deployed, the surgeon can then start tightening the implant so that the spikes of the extension plates engage with the superior and inferior spinous process.
THE EVOLUTION OF SPINAL FIXATION TECHNOLOGIES
Pedicle Screws. The use of pedicle screws began in the 1960s, and these are still considered the gold standard for supplemental fixation. However, the procedure, which takes 1-3 hours, involves a large incision and cutting of the muscles and ligaments around the spine.
Spacers. Introduced in Europe in 2004, the Medtronic X-STOP inter-spinous process decompression system was the first spacer to market. It was designed to distract spinal segments in order to provide nerve impingement relief. Its long-term results are poor due to micro-motion, which causes bone erosion and thus loss of distraction and return of pain.
Interspinous Fusion. The Zimmer BioMet Aspen spinous process fixation system was the first ISP fusion device on the market. It was introduced in 2007, and more than 30,000 devices have been sold worldwide. The Aspen requires a posterior approach and often removal of the superspinous ligament at the implantation site.
Minimally Invasive Fusion. The Spinal Simplicity Minuteman G1 was the first MIS ISP fusion device. It received a CE mark in September 2011 and was indicated for stenosis and multilevel use in the international market. More than 500 have been implanted. The Minuteman G3 received FDA 510(k) clearance in January 2015, the hydroxyapatite-coated Minuteman G3 received FDA 510(k) clearance August 2015, and both received Canadian approval in October 2016.
The canted coil spring locks the device into place in two different cutout reliefs within the implant ID: the initial position (prior to plunger descent to deploy the extension plates) and then the second, final position to ensure that the plunger stays in place after deployment. It was essential that this be repeatable across all implants and predictably impart the same force from implant to implant. It was also important that the spring be composed of a medically implantable material that could withstand sterilization and meet cleanability requirements. In this case, titanium was selected because the implant itself is also made of titanium. This material provides superior strength and is highly resistant to corrosion and fatigue, all of which engender surgeon confidence.
Spinal Simplicity's MinutemanG3 is a prepackaged sterile fixation system that offers a less-invasive alternative to traditional pedicle screws and other spinous process plates that are implanted using open procedures. Compared with pedicle screws and other ISP devices, the Minuteman's lateral minimally invasive approach means patients experience less soft tissue disruption and less blood loss than during open spinal surgery as well as less postoperative pain. The minimally invasive system is especially promising for patients who are high-risk candidates for traditional spinal procedures that require sedation.
Developing a fusion device that could be percutaneously placed in the inter-spinous space was a challenge. The device needed to deploy folded arms that would affix to the spinous processes on either side of that placement. And it needed a single plunger that could deploy both extension plates at the same time and lock into two different positions. Integration of Bal Seal Engineering's Bal Spring has ensured that the Minuteman fixation system provides consistent, reliable, easy implementation. The Bal Spring technology provides the necessary force to lock the plunger in place and prevents movement of the plunger with the implant ID. The device's unique functionality, including its core threaded post and locking plunger, have revolutionized spinal fusion surgery by reducing risk, decreasing OR time, and minimizing blood loss, but most of all, providing for faster patient recovery.