57 Anterior Threaded Cage Revision Surgery

Migrated or malaligned cages can be evident by plain x-rays alone (Fig. 57–1). Computed tomography (CT) scan with reconstruction allows for better visualization of the placement of the cage as well as presence of fusion. Radiographic determination of fusion, however, is difficult and somewhat controversial with threaded cages. The most reliable indicator of fusion postoperatively is the presence of bridging bone anterior to the fusion cage. This finding is a late occurrence and may not always be present. Even when using CT scans with reconstructed images, it is difficult to assess visible bone within the hollow titanium cages. Data from our institution showed no difference in opacity of the bone within the hollow cages on Ferguson radiographs at periods immediately postoperative and at 3 months postoperative. The absence of the signs of pseudarthrosis rather than signs of fusion should be used as a criterion for fusion.

Signs of pseudoarthrosis include:

1. Visible motion on flexion and extension radiographs

2. Halo around implant

3. Sclerotic changes at end plate adjacent to the implant

4. Fractures of implant or vertebrae

5. Migration of implant

Indications for Surgery

1. Anterior prominence of cage: If migration of the cage occurs postoperatively, the cage should be revised because of risk of possible vascular impingement.

2. Lateral prominence with neurologic injury. Proper preoperative templating and complete surgical exposure to visualize the lateral extents of the annulus bilaterally should prevent this problem. Postoperative migration of the cages with neurologic injury requires revision of the cages.

3. Posterior migration is rare after anterior interbody fusion because of the intact posterior lip of the end plate and the intact posterior annulus and posterior longitudinal ligament. If the patient is symptomatic, the implant should be revised. If asymptomatic, the cage should be monitored closely. If there is any signs of progression of the migration, early posterior supplemental fixation is recommended.

4. Pseudarthrosis: If implant position is satisfactory, posterior supplemental fusion with instrumentation is recommended. Anterior revision surgery should be performed if there is an unacceptable cage position and the patient is symptomatic.

Contraindications

1. Calcified or aneurysmal anterior vessels

2. Prior vessel injury and repair during initial operation

3. Abundant scar expected because of unexpectedly high amount of adhesions noted at primary surgery

When these conditions exist, the risk of vessel tear is high. In addition, revision surgery is difficult due to less than adequate anterior exposure. Consideration should be given to a retroperitoneal approach through a flank incision to expose the lumbar spine laterally.

Advantages

Anterior approach to the revision directly addresses the area of the problem, but there is additional risk of vessel injury. It is recommended that a vascular surgeon perform the approach and remain in the operating room throughout the procedure.

Disadvantages

Posterior approach to revision of anterior lumbar interbody fusion is contraindicated. The extent of retraction required subjects the nerve roots to injury and arachnoiditis.

Procedure

Positioning

For a paramedian approach to the lumbar spine, the patient should be positioned supine on a radiolucent table with access for a fluoroscope. Proper positioning should be confirmed to ensure that the patient is not rotated, and that the axis of the body is parallel to the table.

Positioning Tips

1. Improved access into the disc space can be achieved by increasing lumbar lordosis using a roll towel under the lumbar spine.

2. Using two pillows under the knees flexes the hips to relax the psoas muscle and the anterior vessels for enhanced exposure of the lateral aspects of the spine.

Related posts:

Posterior Cervical Laminectomy and Fusion Odontoid Fixation Posterior Scoliosis Correction: King Type V/ Double Thoracic Curves Anterior Correction of Thoracic Scoliosis Using the Kaneda Anterior Scoliosis System (KASS) Posterior Scoliosis Correction: Pedicle Screws Total Lumbar Disc Replacement Using the SB III Charite Prosthesis

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