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.