26

The biomechanics of these devices greatly surpass those of the interbody spacers commonly used (i.e., femoral ring composite grafts, tricortical wedges, and various impacted “containment devices” for cancellous bone), which have not offered significant stability in flexion/extension or torsion. In fact, several biomechanical studies have shown this recent class of construct to compare quite favorably with the segmental pedicular fixation/posterior lumbar interbody fusion (PLIF) constructs.1

Interbody spacers have traditionally been utilized as a component of an overall construct employing segmental posterior instrumentation. In contrast, when applied effectively, the threaded interbody cages are able to confer a degree of stability to the symptomatic motion segment so as to function as stand-alone constructs. The biomechanics of the interbody cage constructs are generally based on significant preoperative disk space narrowing. The act of reconstituting the anterior column height restores the annular tension, which is a key factor in the proper construct mechanics (Fig. 26–1). It is in such an application that the favorable comparison to pedicular fixation/PLIF constructs can be obtained.

Inappropriate size and/or placement of the threaded interbody devices may ineffectively create tension in the anulus fibrosus, resulting in a biomechanically compromised stand-alone construct. In these instances, the lack of annular tension results in the cage system functioning as an interbody spacer rather than as an interbody fixation device; supplemental stabilization techniques may be required. This failure to obtain adequate distraction of the anulus fibrosus has been among the most common reasons for failure of the procedure. With regard to the degree of disk space narrowing, the authors generally consider patients with 50% or more disk space narrowing to be appropriate candidates for a stand-alone interbody construct. In addition, radiographic findings of significant end-plate sclerosis and significant traction spurs correlate well with the ability to tension the anulus fibrosis intraoperatively. Increasing the tension of the anulus fibrosus in the interbody construct was found to be associated with higher biomechanical strength in biomechanical studies.

Laparoscopic Application of Threaded Interbody Fusion Devices

The laparoscopic application of interbody cage devices is simply a variance in the surgical approach to the spine. It is of paramount importance that the surgeon remains cognizant of the biomechanics of stand-alone interbody constructs regardless of the surgical approach selected for their placement. With adherence to these concepts, the authors have found laparoscopic techniques to offer a significant reduction in the operative morbidity associated with the anterior approach.

The authors have had the opportunity to participate at one of the initial clinical centers for the development of the laparoscopic application of the BAK, threaded femoral cortical bone dowel, and tapered LT Interbody Cage systems (Medtronic-Sofamor Danek, Memphis, TN) since early 1994. The first cases of the laparoscopic technique of interbody cage placement utilized the BAK cage.2 This initial experience demonstrated an important pattern of learning curves associated with this procedure: operative time, length of hospitalization, and complications. In comparing the open and laparoscopic insertion techniques of the BAK cage during the initial learning curve phase of the procedure, the authors were able to demonstrate a significant decrease in operative morbidity by the reduction in the length of hospitalization.

With further experience following this initial clinical trial, we were able to further decrease the length of hospitalization to an average of 23 hours for each of these cage systems, as well as the cylindrical Interfix cage. The open application of interbody cages has produced an average 3-day hospitalization over the same time period.

More recently, the authors participated at one of the initial pilot trial centers studying the use of recombinant bone morphogenic protein (rhBMP-2; InFuse, Medtronic-Sofamor Danek) with the LT Interbody Cage system (Medtronic-Sofamor Danek). We utilized the laparoscopic application technique for cage/BMP placement while the additional study sites employed a retroperitioneal open approach. The laparoscopic technique provided a remarkable 12-hour average hospitalization compared with the 3-day hospitalization for patients who underwent the open retroperitoneal technique.

On the one hand, the further reduction in the length of hospitalization (vs. 23 hours) realized with the use of rhBMP-2 (InFuse) is believed to represent the procedural morbidity of the iliac bone graft harvesting in the earlier clinical trials. On the other hand, the consistent 3-day hospitalization of the retroperitoneal open procedure with either rhBMP-2 (InFuse) or iliac crest bone graft represents the morbidity of the open exposure technique itself. With the possibility of a true same-day discharge, the approach to patient education must address this. Furthermore, the nursing unit staff will need to be educated on both this possibility as well as on the generally much more rapid mobilization of these patients compared with those who undergo an open anterior approach, or certainly the more common posterior instrumented spinal fusion procedures.

The radiographic appearance of the pattern of fusion in laparoscopic patients is of even greater experience. Whereas the open patient group underwent complete diskectomies, laparoscopic patients underwent “channel diskectomies”; that is, only the disk material specifically was removed. In all of these patients (i.e., open or laparoscopic) no bone graft material was used. Figure 26–2 demonstrates the postoperative CT images of a patient who underwent an L5–S1 laparoscopic anterior interbody fusion with rhBMP-2 (InFuse) and the LT Interbody Cage system. Ossification within the confines of the disk space is clearly visible, as are the areas where the intervertebral disk had not been excised. This is felt to represent a biological induction of the remaining nucleus pulposus to ossify.

Indications

The current indications for the use of threaded interbody cages, either metallic or allograft, include:

• One- or two-level degenerative disk disease at L2–S1 (particularly disk resorption)
  
• Discogenic pain/painful annular tear
  
• Revision of failed posterior fusions
  
• Grade I spondylolisthesis
  
• Segmental instability

As has been previously mentioned, the “stand-alone” use of the threaded interbody cages requires a significant preoperative narrowing of the symptomatic motion segment (Fig. 26–1). If the application is to a wide disk space, the use of supplementary posterior segmental instrumentation is appropriate.

In cases of spondylolytic spondylolisthesis, the lack of integrity of the pars intra-articularis seems to be associated with a greater laxity in the anulus fibrosus—this laxity, in fact, is one of the primary sources of the actual translatory deformity present. This fatigue, or laxity, of the anulus fibrosus can lead to a less definitive end point in disk space distraction with possible implications for fixation. This has led the authors to treat spondylolytic spondylolisthesis with an interbody cage device (as a stand-alone construct) only when there is marked disk space narrowing. The radiographic findings of end plate sclerosis and significant traction spur formation correlate clinically with the reasonable expectation of being able to place tension the anulus fibrosus and thereby produce a stable standalone construct. If distraction does not seem to produce an appropriate annular tension, however, the surgeon must recognize this and consider supplementary posterior fixation.

Last, in the instance of L5–S1 spondylolisthesis, the surgeon must ensure that the angle of the disk space does not pass into or below the pubic symphysis on the lateral radiograph. The anterior application of interbody cage devices requires a colinear approach to the disk space. Such an angulation would therefore prohibit appropriate cage placement.

Contraindications

Commensurate with the importance of significant preoperative narrowing of the involved disk space, a symptomatic motion segment with a well-preserved disk space would be contraindicated for a stand-alone interbody construct. The general contraindications include:

The patient is positioned supine on a radiolucent operating table. The authors prefer to elevate the patient’s pelvis and lumbar spine from the surface of the operating table with folded surgical blankets. This then permits the arms to be positioned along the patient’s side (tucked and padded) without obscuring the lateral fluoroscopic visualization of the symptomatic motion segment (Fig. 26–3). The spine should not be placed into a position of lordosis by way of bolsters or other positioning influences.

FIGURE 26–3 Patient positioning. Bucks traction boots help keep the patient from sliding up while in Trendelenburg’s position.

The ability to maneuver the C-arm into position and to visualize the pathologic motion segment clearly must also be assessed—the table upright will determine how high on the table the patient must be placed. Doublecheck the fluoroscopy with the requisite Trendelenburg positioning to be used during the procedure. In addition to visualizing the motion segment on the lateral image, the anteroposterior/Ferguson view must show a neutral motion segment rotation prior to proceeding.

The bladder is decompressed with a Foley catheter and the stomach with an orogastric tube, with both removed at the completion of the procedure. All routine positioning checks (i.e., padding of neurovascular structures/bony prominences) are also rechecked. It must be recalled that the laparoscopic procedure requires up to 30 degrees of Trendelenburg position throughout its duration, as opposed to the open anterior application of the interbody cages.

General Laparoscopic Technique

Exposure

For L4–L5 and L5–S1, the standard portal placement includes a periumbilical laparoscopic portal, 5 mm right and left of the lower abdominal quadrant dissection/retraction portals, and a suprapubic spinal working portal (Fig. 26–4A). The initial portals placed include the camera and right/left lower quadrant sites. Using these portals, the dissection is performed, and the symptomatic motion segment is exposed. The suprapubic spinal working portal is created only after the involved disk space is verified. This portal must present a colinear approach to the disk space, the line of approach, which will represent the orientation of the interbody cage within the disk space. In other words, if a nonlinear approach from the spinal working portal to the disk space exists, then this is the same orientation the implanted cage will take with the motion segment (Fig. 26–4B).

• • Do careful preoperative templating on the adjacent normal-height disk using standard anteroposterior, lateral, and Ferguson spinal radiographs.
  
• – Template axial CT or MRI scans to assess the fit of LT cages within the motion segment end plates.
  
• – Use the lateral radiograph to determine the depthstop setting for the end-plate reamers.
  
• • Induce general anesthesia with the patient on the transportation gurney.
  
• – Place orogastric tube and Foley catheters.
  
• – Apply the TED hose.
  
• • Transfer patient to the prepared bed.
  
• – Fold surgical blankets to the width of the patient’s pelvis to allow the arms to be padded and tucked at the sides (Fig. 26–3).
  
• – Apply Bucks boots to prevent cranial migration of the patient while in the Trendelenburg position.
  
• – Ensure that all monitoring lines and IVs are out of the lateral fluoroscopic image view.
  
• – Do not extend the spine with bolsters or other padding.
  
• • Laparoscopic exposure:
  
• – The autonomic plexus rests in the retroperitoneal fat layer. Open the posterior retroperitoneum with laparoscopic scissors rather than with cautery.
  
• – Incise the posterior peritoneum to the right of the midline.
  
• – Ligate and divide middle sacral vessels for both L4–L5 and L5–S1 procedures.
  
• – Ligate and divide the iliolumbar vessel for L4–L5 procedures.
  
• – Expose the entire width of the disk space (Fig. 26–5).
  
• • Harvest the iliac cancellous bone graft if rhBMP-2 (InFuse) is not utilized.
  
• • Mark the midline of the disk space using anteroposterior fluoroscopy.
  
• – Check the skin midline before making the spinal working portal.
  
• – Keep the midline mark in view throughout the procedure.
  
• – The medial edge of the spinal working cannula should approximate this midline mark throughout the procedure.
  
• • Open the right and left diskal entry sites.
  
• – Use the starting guide appropriate for the templated implant size (Fig. 26–6).
  
• – Use the largest trephine that will fit in the disk space.
  
• • Clear the diskal entry sites using the pituitary rongeurs.
  
• – Perform this debridement with each step.
  
• – Utilize fluoroscopy to ensure the rongeur reaches the depth of the disk space.
  
• • Distract the disk space, beginning on the right diskal entry site.
  
• – Use the distractor size appropriate for the templated cage size.
  
• – Assess and confirm that this has resulted in firm annular tensioning. If not, determine if a larger implant should be used.
  
• • Seat the spinal working cannula over the distraction plug, and remove the distractor.
  
• • Ream the adjacent end plates at the right diskal entry site (parallel reaming of end plates).
  
• • Move to the left diskal entry site.
  
• – Repeat the sequence of disk space distraction and end-plate reaming on the left.
  
• – Save the lateral fluoroscopic image of the final reaming depth.
  
• – Debride the left diskal entry site with pituitary rongeurs.
  
• • Prepare the implant.
  
• – The implant holder secures the cage.
  
• – Pack cancellous bone firmly within the cage (if used).
  
• – If Infuse (rhBMP-2) is used, do not use suction over the implants from this point on.
  
• • Place the left implant.
  
• – Monitor the saved image of reaming depth to avoid stripping the implant.
  
• – Slightly recess the cage from the anterior vertebral margin, but maintain contact with the ring apophysis.
  
• – Orient the implant to the vertebral end plates.
  
• – Do not suction over the InFuse sponges.
  
• • Switch back to the 18 mm Ethicon laparoscopic cannula.
  
• • Center the spinal working cannula over the prepared right diskal entry site.
  
• – The end plates were previously reamed in a parallel manner.
  
• – Seat the spinal working cannula again.
  
• – Debride the disk space once again (debris can be pushed into the void of the right discal entry site with the process of preparing/placing the left cage).
  
• • Place the right implant.
  
• – Seat the right implant to an equal depth as the left.
  
• – Orient the implant to the end plates.
  
• – Do not suction over the InFuse sponges
  
• • Use the Adjuster instrument to fine-tune cage rotation.
  
• • Obtain final anteroposterior and lateral fluoroscopic images.
  
• • Close the posterior peritoneum (this helps avoid the risk of adhesions to the surgical site).
  
• – Surgeon’s preference as to the use of laparoscopic staples
  
• • Close 10 mm or larger fascial openings.
  
• • Skin closure and dressings
  
• • Remove the orogastric tube and Foley catheter.
  
• • Mobilization on the nursing unit:
  
• – Educate the floor nurses on early mobilization as opposed to the typical open fusion patient.
  
• – Oral postoperative analgesics are typically sufficient.
  
• – Discharge when ambulatory, voiding, and tolerating oral intake (thorough preoperative patient and family education is key).
  
• • Schedule of postoperative visits:
  
• – Plane anteroposterior, lateral, and Ferguson radiographs at each visit
  
• – Flexion/extension radiographs at each visit except the initial 2-week visit
  
• – CT scan (1.25 mm sections with sagittal and coronal reconstructions) at 3 months
  
• – Postoperative bracing at the surgeon’s discretion
  
• • Rehabilitation:
  
• – Encourage early ambulation.
  
• – Physical therapy beginning at 6 weeks for InFuse patients (12 weeks if autologous iliac bone graft is used)

A 62-year-old female underwent an L5–S1 laparoscopic interbody fusion with the LT interbody cage and InFuse (rhBMP-2). The preoperative diagnosis was a grade I L5–S1 spondylolytic spondylolisthesis and functionally limiting mechanical low back pain. Preoperative provocative diskography had confirmed the pain origin and also shown L4–L5 to be asymptomatic (Fig. 26–7).

The laparoscopic interbody fusion was performed uneventfully, and the patient was discharged to home that afternoon, independent in ambulation and requiring only limited oral analgesics. The preoperative mechanical low-back pain was relieved immediately, and her incisional discomfort resolved over the ensuing 2 weeks as expected. At 3 months a CT scan to assess fusion was performed (Fig. 26–8) and revealed a well-developed interbody fusion mass.

The formation of bone throughout the anatomical confines of the interspace was of particular interest, despite the fact that the disk space was debrided only where the interbody cages were placed. Furthermore, there was no evidence of heterotopic ossification. Repeat CT scans performed at 1 year revealed the further maturation of the interbody fusion without heterotopic extension (Fig. 26–9).

The patient recovered and rehabilitated very well clinically. She voiced complete resolution of the preoperative mechanical low-back pain and indicated she was functionally unlimited. In the authors’ experience, the same-day discharge is typical for patients treated with the laparoscopic approach to the spine and InFuse/LT interbody devices.

The particular advantages of interbody cages include the beneficial effects on bone healing associated with the anterior column location, the stabilization potential in appropriate motion segments, the ability to restore anterior column height and foraminal volume, the correction of segmental lordosis with the LT interbody device, and the avoidance of the fusion disease phenomenon. These advantages are not dependent on whether the approach of insertion is open or laparoscopic. The specific and demonstrated chief benefit of the laparoscopic approach to interbody cage insertion is the reduction in surgical morbidity. This is especially evident when the laparoscopic technique is combined with the use of bone morphogenic protein (rh-BMP2/In-Fuse). It is our belief that this significant reduction in patient morbidity will drive the further acceptance of the laparoscopic approach in a manner similar to the evolution of arthroscopic orthopedic surgery and laparoscopic general surgery.

The main disadvantage of the laparoscopic approach is the initial learning curve of the surgical team with the technique. In addition, the anterior approach to cage placement (open or laparoscopic) is limited, at this time, with regard to the inability to decompress the spinal canal directly.

It is recommended that interbody cages be considered as only one option in the stabilization of symptomatic motion segments. With care in patient selection, their ability to function in a stand-alone configuration has been successfully demonstrated. To evolve to the laparoscopic placement technique of interbody cages, the access surgeon and spine surgeon should begin as a team with the open approach, placing the particular instrumentation system they plan to use laparoscopically. Together, the access surgeon and spine surgeon should attend a hands-on laparoscopic course for the specific instrumentation system. In terms of initial case selection, they should begin with nondeformity L5–S1 cases and, most importantly, allow for time for the cases.

1. Zdeblick TA, Warden KE, Zou D, McAfee PC, Abitbol JJ. Anterior spine fixators: a biomechanical in vitro study. Spine. 1993;18:513–517.

2. Zucherman JF, Zdeblick TA, Bailey SA, Mahvi D, Hsu KY, Kohrs D. Instrumented laparoscopic spinal fusion: preliminary results. Spine. 1995;20:2029–2034.