First described by Santoni and colleagues in 2008, the cortical bone trajectory (CBT) for screw placement was presented as a less invasive method of performing spine surgery while using a familiar posterior, midline approach. Briefly, the CBT technique uses a midline incision with dissection of muscle to the lateral pars interarticularis. Screw placement is performed with a mediolateral, caudocranial trajectory, beginning at the lateral pars, caudad to the facet joint (Fig. 1).1 There are many theorized advantages to this approach. First, the medial start point requires less lateral exposure and muscle stripping than traditional pedicle screws (PS). Further, the trajectory away from the spinal canal may lead to fewer complications such as pedicle perforation, facet joint violation, and neurovascular injury (Fig. 2). In addition, traditional PS terminate in the cancellous bone of the vertebral body, whereas CBT screws terminate in denser cortical bone. Because of the difference in bone densities, biomechanical studies have reported greater pullout strength with CBT compared with PS. This increased screw purchase has been proposed to result in a lower frequency of screw loosening, pullout, and implant failure. These benefits may be particularly true for patients with osteoporosis, as cancellous bone of the vertebral body is more severely affected than cortical bone.
The predominant issue with using CBT for screw placement is the midline approach, which by nature involves stripping of the erector spinae and multifidus muscles. In order to reach the appropriate starting point for CBT screws, exposure of part of the facet joint and perhaps part of the transverse process is required. Although the extent of this lateral dissection may be less than what is necessary to place traditional PS, it is still a significant lateral dissection. Prior studies examining paravertebral muscle changes following lumbar fusion surgeries have reported significantly more multifidus atrophy following posterior lumbar interbody fusion (PLIF) when compared with minimally invasive transforaminal lumbar interbody fusion (MIS TLIF). This increased muscle damage has been correlated to inferior physical and mental outcomes, particularly in the early postoperative period.2
Although CBT screws can be combined with any approach for placement of interbody devices, many surgeons utilize PLIF because of the midline approach required for both. Although PLIF is a proven method for interbody placement, the technique has several drawbacks such as higher rates of dural and neurological injury, increased operative time, and lower arthrodesis rates.3 To reduce these difficulties, the dissection may be extended to perform a TLIF. However, this extension negates the benefits of the smaller exposure afforded by CBT, and is much more disruptive than the muscle sparing approach of a MIS TLIF. Although CBT could be performed after an anterior or lateral lumbar interbody fusion, this is logistically difficult. These methods require staging of the procedure to reposition the patient, and an additional dissection which significantly increases the operative time and complication profile—an avoidable risk when screws could be placed percutaneously quickly with minimal soft tissue dissection.
A primary purported advantage of CBT is that it reduces the complication profile when compared with traditional PS. However, the complication rate of traditional PS in the hands of an experienced surgeon is very small, and therefore it remains difficult to demonstrate a meaningful improvement. A meta-analysis comparing open versus percutaneous PS placement reported an overall screw malposition rate of 4.2% and 3.0%, respectively.4 Similarly, facet joint violation has been reported to be as low as 4.0%–6.8% when placing PS using navigation.5 Although limited in quantity, current literature with regard to CBT has reported a slightly higher complication rate. A study of 202 patients undergoing CBT using intraoperative fluoroscopy demonstrated facet joint violation in 11.8% of screws placed,5 while other small cohort studies have reported high early hardware failure and revision rates.6 To the authors’ knowledge, only one comparative study has been published to date; a prospective, randomized study of 79 patients demonstrated a higher rate of facet joint violation with traditional PS versus CBT. However, no difference in fusion rates, postoperative pain or disability were reported.7 Moreover, literature demonstrating that imperfect screw placement makes a significant clinical difference is minimal. Cortical breaches <2 mm rarely result in motor weakness, and have demonstrated similar strength to perfectly placed screws.8 Meta-analyses have reported neurological complications in as low as 2.3% of patients,9 and in <1.0% of PS placed using fluoroscopy.10
The authors’ preferred approach would be to perform a tubular MIS TLIF, which minimizes many of the issues with the midline CBT screws. A more direct approach to the disc space through the internervous plane and intervertebral foramen offers the ability to perform a direct decompression and/or a bilateral decompression without the need to perform a complete laminectomy. When compared with midline CBT, there is less muscle destruction, and the posterior attachments of the contralateral paraspinal muscles are preserved. In addition, the posterior tension band created by the interspinous and supraspinous ligament is spared. Lastly, the MIS approach results in less postoperative pain, reduced blood loss, and faster recovery and mobilization.
In conclusion, MIS TLIF offers the surgeon a muscle preserving approach with an acceptable complication rate, while allowing the surgeon to accomplish all necessary surgical goals; direct or bilateral decompression, as well as interbody fixation can be achieved without sacrificing graft size or requiring extended operative times. Although CBT may be beneficial in osteoporotic patients, this can be treated medically to improve their bone mineral density, minimizing the biomechanical benefit of CBT. To date, there is insufficient evidence to suggest that CBT offers a meaningful improved clinical risk profile over the proven MIS TLIF with PS technique, and does not overcome the dramatic adverse effects of a midline muscle stripping approach. On the basis of current literature, the only clear advantage of using a midline incision and CBT screws for interbody placement is this approach is easier to adopt for surgeons who do not want to overcome the learning curve associated with the more challenging MIS TLIF.
1. Santoni BG, Hynes RA, McGilvray KC, et al. Cortical bone trajectory for lumbar pedicle screws. Spine J. 2009;9:366–373.
2. Putzier M, Hartwig T, Hoff EK, et al. Minimally invasive TLIF leads to increased muscle sparing of the multifidus muscle but not the longissimus muscle compared with conventional PLIF-a prospective randomized clinical trial. Spine J. 2015;16:811–819.
3. Liu J, Deng H, Long X, et al. A comparative study of perioperative complications between transforaminal versus posterior lumbar interbody fusion in degenerative lumbar spondylolisthesis. Eur Spine J. 2015;25:1575–1580.
4. Phan K, Rao PJ, Mobbs RJ. Percutaneous versus open pedicle screw fixation for treatment of thoracolumbar fractures: Systematic review and meta-analysis of comparative studies. Clin Neurol Neurosurg. 2015;135:85–92.
5. Matsukawa K, Kato T, Yato Y, et al. Incidence and risk factors of adjacent cranial facet joint violation following pedicle screw insertion using cortical bone trajectory technique. Spine. 2016;41:E851–E856.
6. Patel SS, Cheng WK, Danisa OA. Early complications after instrumentation of the lumbar spine using cortical bone trajectory technique. J Clin Neurosci. 2016;24:63–67.
7. Lee GW, Son JH, Ahn MW, et al. The comparison of pedicle screw and cortical screw in posterior lumbar interbody fusion: a prospective randomized noninferiority trial. Spine J. 2015;15:1519–1526.
8. Stauff MP. Pedicle screw accuracy and the ramifications of imperfect screw placement. Spine J. 2013;13:1758–1759.
9. Verma R, Krishan S, Haendlmayer K, et al. Functional outcome of computer-assisted spinal pedicle screw placement: a systematic review and meta-analysis of 23 studies including 5,992 pedicle screws. Eur Spine J. 2010;19:370–375.
10. Tang J, Zhu Z, Sui T, et al. Position and complications of pedicle screw insertion with or without image-navigation techniques in the thoracolumbar spine: a meta-analysis of comparative studies. J Biomed Res. 2014;28:228–239.