As the population ages, the incidence of lumbar spinal stenosis and the number of available surgical options for its treatment continue to increase. To date, posterolateral spinal fusion via pedicle screw and rod fixation has been regarded as the mainstay surgical treatment option for stabilization of a degenerative spinal segment. The intended benefits of this treatment include reduced back pain and prevention of continued degeneration of the segment through stabilization and development of bony fusion. However, pedicle screw and rod constructs carry significant morbidity, such as cerebrospinal fluid leaks, neural injury from misplacement of pedicle screws, adjacent segment degeneration and disease, and instrumentation-related complications. In addition, this procedure often injures the unfused rostral facet joints and its musculotendinous attachments. Furthermore, pedicle screw and rod fusion constructs require greater operative times, which may be difficult for elderly patients to tolerate.
In many patients, pedicle screw and rod fixation may be an overtreatment. In the degenerative spine in which there is mechanical back pain and the presence of neurologic symptoms with or without grade I spondylolisthesis, the unfavorable adverse outcomes and increased morbidity of instrumented fusion may not be worth the risks. In contrast, decompression without fusion may fail to address all of the patients’ symptoms and may fail to slow the progression of degenerative changes. The interlaminar lumbar instrumented fusion (ILIF) technique was developed to overcome the potential shortcomings of these current treatment standards by avoiding pedicle screw fixation while creating stabilization in a minimally disruptive surgical technique.
The ILIF procedure is indicated in cases where decompression and spinal stabilization is required. The most common indication is grade I degenerative spondylolisthesis and spinal stenosis. The authors typically choose this procedure in a patient with advanced age where a more aggressive pedicle screw construct is less desirable.
There are a number of limitations of ILIF. The patient’s site of spinal compression must be located at the interspinous space. If the compression is significant in the craniocaudal direction and the spinous process and significant portions of the laminae will have to be resected to perform an adequate decompression, the ILIF procedure will not be possible. The procedure is not possible in revision cases in which the spinous processes have previously been resected. Care must be taken in patients with severe osteoporosis. In these patients it may be easy to fracture the spinous process during plate placement. This procedure is not ideal for those patients with high-grade instability. Lastly, there is limited bony surface area for bony fusion during this procedure. The facets may be packed with auto or allogenic bone; many of the implants also permit the placement of fusion material within the device.
The patient is placed on the operative table in the prone position. It is possible to perform this procedure in the lateral decubitus position, but the authors do not prefer this. The authors prefer to use a radiolucent Jackson table, but any table is appropriate as long as it can accommodate imaging modalities ( Fig. 14.1 ).
Following preparing and draping of the lumbar region, the incision is planned. The incision is typically slightly caudal to a standard incision planned for a laminectomy; specifically, it only is required to expose the interspinous space, which is typically slightly caudal to the disk space on lateral imaging. An approximately 2.5- to 4-cm incision is made, the subcutaneous fat and lumbodorsal fascia are then divided and retracted. After dividing the fascia, electrocautery is used to perform a subperiosteal dissection so that the bilateral erector spinae muscles can be mobilized off of the cranial and caudal spinous processes to the spinolaminar junction ( Fig. 14.2 ).
Many surgeons believe that the ILIF procedure is best performed with distraction laminoplasty. The distraction laminoplasty technique, as first described by O’Leary and McCance, allows for robust distraction and greater access to the spinal canal, while minimizing the risk of spinous process fracture during device implantation. Furthermore, this technique has the advantage of minimizing bony resection, including maintenance of the bilateral facet joints. At this point, a Kerrison rongeur is used to excise the supraspinous and interspinous ligaments at the index level ( Fig. 14.3 ). If required based on bony overgrowth, the Kerrison rongeur is used to remove the caudal one-third of the superior spinous process and laminae, as well as the cranial edge of the inferior spinous process. This step is not required in all patients and if not needed should be avoided because the spinous process may be fractured and the procedure aborted for a standard laminectomy and pedicle fixation and fusion. The facet joint capsules should be spared as much as possible. If significant facet hypertrophy is present, the spinolaminar junction between the cranial and caudal vertebrae is then thinned down with a high-speed burr in order to create a surface to accept an interlaminar distractor.
Surgeon preference and implant choice will determine how spinous process distraction is next performed. This step is required to provide adequate visualization of the intralaminar space, permit laminoplasty, and decompress the neural elements. The authors prefer to place Caspar posts in the top of the spinous processed to apply distraction with a post distractor as one would use during an anterior cervical diskectomy and fusion. Pilot holes are drilled into the cranial and caudal spinous processes and Caspar pins are inserted into the drill holes. These posts should be placed in the cranial portion of the spinous processes, which is the thickest portion of the spinous process. Distraction of the segment is then performed, enabling enhanced access to the interlaminar space without additional excision of the spinous processes ( Fig. 14.4 ). It is important to insert the Caspar pins parallel to each other so that iatrogenic spinous fracture is avoided. Also, overly aggressive distraction should be avoided to minimize damage to the spinous process. As mentioned above, other tools for distraction are available. Bilateral hemilaminoties and medial facetectomies may next be performed to gain access to the compressed neural elements ( Fig. 14.5 ). Next, curettes are used to mobilize the ligamentum flavum insertion from the laminae. Kerrison rongeurs are then used to remove soft tissue from the medial aspect of the facet joints just lateral to the ligamentum flavum. Again, care should be taken to leave the capsular tissues undisturbed. The ligamentum flavum insertion on the superior lamina is then released with angled curettes, and the ligamentum flavum is removed partially or completely at the stenotic segment ( Fig. 14.6 ).