The anatomy of the atlantoaxial joint is complex and variable. A careful under- standing of the C1 and C2 anatomy is critical to the success of any type of screw fixation. Several anatomic studies have described the surgically relevant anatomy of the atlas.
The dimensions of the lateral mass of C1 allow screw fixation in nearly all cases.
Christensen and colleagues34 performed a study of 120 human specimens at the Cleveland Museum of Natural History. The anteroposterior length of the lateral mass averaged 16.93 mm (minimum, 13.13 mm) below the posterior ring of C1. The width of the lateral mass between the vertebral artery foramen and the spinal canal averaged 8.68 mm (minimum, 5.25 mm). The cephalocaudal height from the top of the pedicle analog to the inferior facet averaged 8.99 mm (minimum, 4.73 mm).
Others have confirmed these findings.35,41 Christensen and associates also studied the dimensions of the pedicle analog to see how often it is feasible to insert a screw through the posterior inferior portion of the arch of C1 to avoid dissecting out the lateral mass as described by Resnick and associates.35 They found that the bone beneath the groove of the vertebral artery in the arch of C1 is less than 4-mm thick in 20% of cases.34 This would potentially cause a 3.5-mm screw to cut out superiorly, placing the vertebral artery at risk, unless the surgeon intentionally causes the screw to cut out inferiorly36 (Figures 12-15 and 12-16).
Tan and co-workers38 published results of a study of 50 dried atlas specimens that identified basic anatomic dimensions, including the posterior arch of the atlas. The thinnest part of the groove as measured by coronal CT scan was 4.72 ± 0.65 mm on the left and 4.72 ± 0.68 mm on the right. Only 8% of the cases in their series had a thickness of less than 4 mm, whereas Lee and co-workers37 found that the bone in that location was more than 4 mm in only 46% of cases. The preference is to place the screw into that location when the preoperative contrast-enhanced CT scan shows that it can be done safely.
FIGURE 12-15 Posterior view of C1 at the confluence of the posterior ring and the lateral mass. The posterior arch can be seen superimposed over the caudal aspect of the vertebral artery as it passes over the groove. The black-and-white dot denotes the starting point for a screw on the posterior aspect of the lateral mass.
(From Christensen DM, Eastlack RK, Lynch JJ, et al: C1 anatomy and dimensions rela- tive to lateral mass screw placement, Spine 32:844–848, 2007, Figure 1. By permis- sion of Mayo Foundation for Medical Edu- cation and Research. All rights reserved.)
Level I Studies
Multiple morphometric studies demonstrate the wide anatomic variability of the atlan- toaxial complex.22,23,25,28,34,41 All of these investigations highlight the need to carefully plan the procedure before placing instrumentation in the upper cervical spine.
COMMENTARY
C1-C2 fusions are technically challenging operations, but they are also among the most gratifying procedures spine surgeons do. Nowhere in the spine is the anatomy more exquisite or more important to the success of the operation than in the upper cervical spine. The region is rife with anatomic variations that alter the technique for the informed and lead to complications for the unaware. Preoperative planning is the key to success.
Surgeons should be comfortable with multiple techniques since each one has pros and cons. The primary advantage of using the Harms method or transarticular screw fixation over simple wiring techniques is the added stability conferred by the screws; the enhanced stability allows a comparable or higher fusion rate without halo immobilization. The main advantages of the transarticular screw technique over the Harms technique are lower cost and simplicity. The major advantages of the Harms technique over the transarticular screw technique are broader application (it can be used when transarticular screw fixation is contraindicated by variations in the anatomy) and the ability to place screws before the spine is reduced.
The surgeon should watch an experienced colleague and practice in the cadaver laboratory before attempting these techniques in the operating room. A surgeon who is not yet comfortable with placing C1 and C2 screws should consider referring the patient to a colleague with more experience or using a simple wiring technique, with the following caveats: The use of simple wiring techniques without the added stability of postoperative halo immobilization results in an unacceptably high pseud- arthrosis rate. Unfortunately, halo use is associated with a high complication rate in the elderly and should be avoided in that patient group. Nonoperative care may be the best option in the elderly if the spine is stable. Remember that the benefits of an advanced technique outweigh the risks only when the operation is accomplished by a surgeon who is adequately prepared.
REFERENCES
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FIGURE 12-16 Lateral view of C1 dem- onstrating the ideal orientation of screw placement within the sagittal plane (10 to 15 degrees caudad to cephalad). Note the starting position below the posterior ring confluence with the lateral mass. (From Christensen DM, Eastlack RK, Lynch JJ, et al: C1 anatomy and dimensions relative to lateral mass screw placement, Spine 32:844–848, 2007, Figure 2. By permis- sion of Mayo Foundation for Medical Edu- cation and Research. All rights reserved.)
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