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Posterior fixation using an interspinous device for the treatment of degenerative spine disease
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It is generally accepted that spinal fusion, in conjunction with decompression, produces better clinical outcomes in patients with degenerative spondylolisthesis, and that the use of instrumented fixation achieves greater fusion rates beyond that of onlay alone.
Historically, pedicle screws or facet screws have been the choice for vertebral fixation. Each of these, however, presents certain risks to the patient, since screw placement is in close proximity to nerve roots and vasculature. Furthermore, C-arm fluoroscopy is required to ensure proper screw placement, exposing the patient and surgical team to high levels of radiation.
Depending on the patient’s condition, anatomy, age, activity level and concomitant procedures performed, implantation of a spinous process fixation system such as the Aspen Spinous Process System (Figure 1; Lanx) has shown construct stability comparable to pedicle screws, but with fewer risks, as reported by Karaholios and colleagues in a 2010 issue of the Journal of Neurosurgery: Spine.
Differing from interspinous process decompression systems, this device is designed for fixation and fusion. The posterior fixation technique described here is applicable to multiple fusion procedures, including: posterior fusion; anterior lumbar interbody fusion (ALIF); transforaminal lumbar interbody fusion (TLIF)/posterior lumbar interbody fusion (PLIF); and lateral interbody fusion.
Patient positioning
We position the patient prone on a Jackson frame (Mizuho OSI). When performing a procedure that requires optimal decompression, this frame provides the most accurate representation of the patient’s standing anatomy. Wilson and Andrews frames (Mizuho OSI) flex the spine forward, increasing the interspinous process space, which may result in placement of an oversized implant. This would have a number of potentially deleterious effects. First: the spinous process may become stressed leading to the possibility of fracture. Second: an oversized implant would lead to distraction of the facet joints, thereby decreasing the rotationally stability of the construct. Third: placing a supra-physiologic flexion moment on the operative level could lead to problems at the adjacent motion segments. The device is an adjunct to fusion; therefore, surgical decompression is recommended prior to implantation (Figure 2).
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Images: Hardenbrook MA |
Surgical approach
An incision approximately 5 cm in length is made using a standard subperiosteal, midline approach (Figure 3). Muscle injury should be minimized during the procedure, since the dynamic stabilization that intact muscles provide promotes recovery and lessens the possibility of adjacent segment degeneration.
The paraspinal musculature and other soft tissues are retracted to expose the spinous processes and lamina to the medial border of the facet joints. On a posterior-anterior fluoroscopic image, the spinous process is aligned with the pedicles of the inferior vertebral body. This is deceiving and can lead to incorrect device placement. For this reason, we use the lamina rather than the spinous process to mark the operative level. Typically, we place a marker (Woodson dental probe [Surgimax Instruments] or Penfield #4 [Life Instruments]) underneath the lamina of the upper level. This marker will point directly to the degenerative level (Figure 4).
We have not found it necessary to dissect out to the posterior lateral gutters. Our fusion rates have been good with facet and posterior-only fusion. Exposure to the posterior lateral gutters can be considered, but this does greater injury to the multifidus muscles, slowing recovery and potentially causing future adjacent segment degeneration if the adjacent facet capsule is violated.
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Supraspinous ligament removal
Depending on preference, the supraspinous ligament may be removed to permit more accurate sizing of the device. With the ligament in place, an abrupt halt occurs during distraction when it is tensioned. Removing the ligament offers a better feel and more accurate visualization of the motion involved and, therefore, a more physiologic fit. It should be noted that super-physiologic distraction can be applied inadvertently when there is no ligament in place, so surgeons should be mindful of this risk.
Additionally, the device plate is often easier to drop directly between the processes when the supraspinous ligament is excised, rather than slipping it in from the side. With the ligament intact, directly dropping the plate is not an option and approaching from the side requires more space, mandating a larger exposure and additional dissection.
Device sizing, decompression, distraction
If the desired distraction falls between two device size options, we choose the smaller to maintain a certain degree of construct stability. The fixation device has a central barrel designed for load sharing and may have some effect on both the central canal and the foramen. However, it does not replace standard decompression and is designed solely as a fixation device for achieving successful fusion. Keep in mind, there is a point beyond which indirect decompression destabilizes the construct without providing additional benefits.
Depending on indications, we perform a formal hemi-laminotomy or bilateral hemi-laminotomies.
To ensure adequate decompression, the ligamentum flavum (which in many cases is the primary cause of compression) is excised as well as any bone contributing to nerve compression. We size the construct for optimal stability (Figure 5).
We use the spreader to determine the appropriate implant size by ratcheting it up to dilate the space and then sizing down, and we take a lateral image to assess the amount of distraction and the accuracy of device sizing.
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Placing the fusion device
A rasp is inserted into the interspinous space and moved cephalad and caudad to prepare the fusion site.
With the spreader, the spinous processes are distracted and the post plate is dropped (or slid) into position between the two spinous processes. Device orientation, from left to right or right to left, will be dictated largely by the anatomy of the spinous processes.
The device is placed as far anterior as possible as the spinous process is most robust at the base. Align the conical tips of the compressors into the lateral divots in each plate. Placement should be reconfirmed fluoroscopically.
The plates are then clamped against the spinous processes. The fixation spikes on the plates securely engage both spinous processes with gentle compression. The lock plate angulates ±10° coronally for varying anatomy. Maintain compression on the plates and ensure the post protrudes at least 3 mm beyond the lateral face of the lock plate before tightening the set screw to 30 inches-per-pound, verified by an audible click. Remove the compressors and inspect the plates manually and visually to confirm fixation. Check device placement fluoroscopically.
The device is not intended for stand-alone use. The barrel of the post plate is designed to accept bone graft material to accomplish fusion. Resecting the supraspinous ligament allows for burring of the tip of the spinous processes and placement of bone graft material there as well. Fusion between the spinous process is secondary to the intended posterior fusion and/or facet fusion.
While we have never experienced an intraoperative fracture using this procedure, we always have a backup plan in place before we begin. Traditional pedicle screws are available in the event that alternate instrumentation is needed.
Conclusion
Using the system for interspinous process fixation presents fewer risks than placing pedicle or facet screws and accomplishes fusion rates comparable to posterolateral stabilization techniques (Figure 6), according to Lanx documentation.
The procedure is considerably less invasive than pedicle screw fixation, thereby reducing the trauma of surgery and allowing for more rapid postoperative recovery. Additionally, because of the superficial nature of the spinous process anatomy, placement of the spinous process fixation can be performed under direct visualization, minimizing the need for fluoroscopic imaging and greatly reducing the exposure of the surgical team and patient to intraoperative radiation.
References:
- Karaholios DG, Kaibara T, Porter RW, et al. Biomechanics of a lumbar interspinous anchor with anterior lumbar interbody fusion. J Neurosurg Spine. 2010;12:372-380.
- Data on file at Lanx: Histopathological preparation and evaluation of sheep lumbar arthrodesis. Charles River Laboratories Preclinical Services, Montreal, September 10, 2009.
- Mitchell A. Hardenbrook, MD, can be reached at Uniformed Services University of Health Sciences, The Boston Spine Group, 299 Washington St., Newton, MA 02458; e-mail: drhardenbrook@yahoo.com.
