Interspinous fusion devices are being evaluated as an alternative to bilateral and unilateral pedicle screw constructs in augmenting lumbar interbody fusion rates. The devices are designed to take advantage of the biomechanical loading processes of the posterior aspect of the vertebral column in order to immobilize the affected segment, thus stabilizing the spine. The interspinous devices are intended to create construct stability comparable to pedicle screws, while being less invasive, therefore reducing blood loss, risk of infection, and postoperative muscle pain [42–45]. In contrast to interspinous process devices (IPDs), which primarily work as stand-alone decompressive materials (i.e., X-STOP), interspinous fusion devices (IFDs) are designed for fixation and fusion. Early attempts at interspinous fusion failed, as the pilot implants had a small surface area with the spinous processes, meaning all of the force due to the axial load of the superior spine was applied on a small area. Contemporary devices include paired plates with teeth or U-shaped device with wings that attach to the spinous process [46].

The US Food and Drug Administration (FDA) have approved a significant number of IFDs (Table 25.1). Despite the fact that they are composed of different designs and materials, they share similar indications and implantation techniques with the aim of maintaining a constant degree of distraction between the spinous processes and stabilize the spine in a minimally invasive manner [46]. As per 510 (k) premarket notifications, the indications for the use of these devices are to achieve supplemental fusion in the following conditions: degenerative disk disease (defined as back pain of discogenic origin with degeneration of the disk confirmed by history and radiographic studies), spondylolisthesis, trauma (i.e., fracture or dislocation), and/or tumor (510 K doc). These devices can be used in elderly patients or those with bone quality too poor for pedicle screw instrumentation. The vast majority of devices are implanted via a midline incision followed by muscle dissection lateral to the supraspinous ligament. The paraspinal muscles are then stripped off the laminae, and the interspinous ligament is sacrificed. Before implantation, a microsurgical decompression is performed (per manufacturer instruction manual).

Currently, 15 IFDs have received clearance to market by the FDA. There are numerous designs to these implants as shown in Table 25.1, but each device claims the same advantages over the pedicle screw fixation including reduced risk of cerebrospinal fluid leakage and nerve damage, less muscle dissection and intraoperative estimated blood loss, shorter hospital stay and rehabilitation period, and reversibility of the surgical procedure that does not limit future surgical treatment options [47].

In contrast, while biomechanical studies indicate that IFDs may be similar to pedicle screw-rod constructs in limiting the range of flexion-extension, they may be less effective in reducing axial rotation and lateral bending [48]. Also, there is a potential for a negative impact on the interbody cage and bone graft due to focal kyphosis resulting from the interspinous device [48]. Due to the lack of long-term clinical studies and these uncertainties, further prospective clinical studies are needed to compare the functional outcomes between interspinous fusion devices and pedicle screw constructs.

The combination of fixation integrated into interbody fusion cages provides greater segmental rigidity and more physiologic loading through the segment, promoting optimal stability. Although integrated designs are used extensively in anteriorly placed cages both in lumbar and cervical, there are possible drawbacks associated with the introduction of these supplemental devices from a lateral approach. Screw angle, screw fixation, and plate designs are quite different than an anterior approach. However, additional surgical approaches may lead to prolonged operating time, larger skin incisions, soft tissue injuries, and higher infection rates. Certain authors reported screw or plate dislodgments, higher incidences of adjacent level degenerations, and heterotrophic ossifications [49] in multiple approach procedures. If sufficient stabilization can be achieved in a single approach without supplemental fixation from a secondary approach, then it may avoid such adverse effects resulting from additional posterior surgery and minimize the hospitalization time. Integrated fixation cages are a recently developed technology to reduce adverse effects from procedures involving interbody fusion cages.