When a person suffers from degenerative disc disease, spine trauma, or cancer of the spine, the result is decreased stability that can produce pain — often severe — during normal activities. Surgeons will first attempt conservative measures such as cortisone injections and physical therapy to strengthen the muscles of the lower back, but if these conservative treatments are unsuccessful, the only alternative is to address the pain through surgery. This procedure, known as a fusion and fixation, prevents movement in the unstable portion of the spine, thus reducing the pain.
A standard fixation device consists of plates and rods that are attached to the vertebrae with wire, pedicle screws, and hooks. Bone graft material is often used to promote fusion for long-term stability and relief from pain. Unfortunately, these fixation devices often require the use of bulky instrumentation, which surgeons may find awkward and difficult to use.
Interspinous process (ISP) fusion has been shown to be a safe and effective alternative to traditional methods of posterior spinal fixation (i.e., pedicle screws). A high degree of biomechanical stability and reliable fusion rates are achieved with preservation of adjacent level structures when using ISP fusion implants. Because ISP fusion devices do not violate adjacent facet joints, they can potentially decrease the likelihood of adjacent segment degeneration, changes in the motion segments above and below the surgery site.
A new type of ISP fusion device offers a less-invasive alternative to traditional pedicle screws and other spinous process plates that are placed in open procedures. Spinal Simplicity's Minuteman™ G3 innovative prepackaged sterile fixation system has made the procedure simpler, faster, and accessible to a greater patient population (see Figure 1). The spinous process fusion plate received FDA 510(k) clearance in January 2015 and the first procedure in the United States was performed the following April. A similar device, known as the Minuteman™ G1, has been available in Europe since 2011. To date, over 500 procedures have been performed in Europe with the Minuteman™ G1 device. In 2016, the company introduced a hydroxyapatite-coated version of its fixation device. The FDA-cleared coating, also known as a bone mineral, occurs naturally in bones and teeth, and promotes osto-integration.
The Minuteman device consists of a core threaded post with graft window, expanding plates that open on the far side of the spinous processes, a fixed plate for the near side of the spinous processes, and a locking hex nut to secure the device in place. An inserter is used to surgically implant the device by expanding the wings and to tighten it to the inferior and superior spinous process using a simple instrument design. The components of the system, particularly the Bal Spring® canted coil spring, are critical to the device's unique design. This article examines the inner workings of the Minuteman and how the innovative design of the device has advanced the treatment of complex spinal disorders.
Minimally Invasive Procedure
Much like traditional spinal fixation devices, the Minuteman device is intended for fixation to the spinous processes to provide supplemental fixation for degenerative disc disease, spondylolisthesis, spine trauma, or spinal tumors. It can also be used as an adjunct to lumbar interbody fusions [e.g., anterior (ALIF), direct lateral (DLIF)], thus reducing the approach-related trauma to normal healthy tissue. The Minuteman reduces approach-related morbidity, decreases OR time, minimizes blood loss, and provides for quicker patient recovery and return to normal activities.
Unlike other ISP devices, the surgical technique is easily reproducible. The Minuteman device can be implanted under minimal fluoroscopy using a direct lateral or posterior unilateral approach. Direct lateral surgeons can now insert a supplemental posterior fixation device with the patient remaining in the lateral decubitus position (lying on one side) in 10-15 minutes. The same access port used for the anterior interbody is used during the direct lateral approach to gain access to the inter-spinous space. Also unlike all other devices, the Minuteman does not require dissection, stripping, and retracting of the paraspinal muscles, or removal of the superspinous ligament. Following the dilation of the surgical access site (using an incision that is smaller than the diameter of a golf ball), the surgeon uses Spinal Simplicity's patented sizing instrument to lightly decorticate the fusion site in a controlled manner and to determine the appropriate implant size. To accommodate various patient anatomy, the fixation device is provided in five different sizes. Regardless of the implant size, all use the same size plunger, which helps ensure a consistent experience for the surgeon from one surgery to the next.
The lateral minimally invasive approach to the spine is posterior to the level of the facet joints, in what is also considered a safer zone. This reduces the potential of damaging the neural and vascular structures, avoiding the risks associated with misplaced pedicle screws. 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 procedure occasionally has been performed under local anesthesia (with IV sedation in an outpatient setting), providing a treatment option for patients who should avoid general anesthesia due to their age or because of comorbidities such as coronary artery disease and aortic stenosis. These patients, who are high-risk candidates for traditional spinal procedures, were offered relief from pain through a minimally invasive approach that doesn't require “going under.”
The Nuts and Bolts
The device's fixed plate, which mates with the locking hex nut, consists of two sets of spike grips that attach to the spinous processes to provide stabilization of the spinal segments (see Figure 2). Each spike grip set contains three spikes. The extension plate has two spike grips that attach to the spinous processes, the bony projections that extend from the back of each vertebrae. Each spike grip on the extension plate also contains three spikes. The extension plate is stowed during insertion and deployed for final fixation.
The Minuteman's core post sits on the flat surface of the device, not the threads. The core threaded post is unique to the Minuteman device. It allows for the device to distract the spinous processes in a controlled manner to help prevent spinous process fractures. The core threaded post also provides indirect decompression of the posterior column of the spine, which leads to widening of the foraminal diameter of the vertebral openings that house the spinal nerve roots. This reduces the soft tissue compression by stretching the ligamentum flavum, posterior annulus, and facet capsule. The threaded post also has the largest graft window of all ISP devices available to the market. This window size is on par with TLIF inter-body cages, and promotes the ingrowth of bone by means of the graft material that is delivered to the spinous process in this window. The Minuteman G3 has hydroxyapatite coating on both the core and the extension plates to promote the ingrowth of bone to the implant.
Auto-lock technology provides secure, reversible, zero-step locking fixation of plate halves, regardless of plate positon on the spinous processes. The auto-lock allows the surgeon to tighten the Minuteman in variations — depending on the anatomy and quality of bone — without backing out or loosening the device. The vibration-resistant auto-lock is reversible and can be repositioned during the surgery to provide optimal positioning.
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.