Letter to the Editor. Screw technique for lumbar spinal fusion

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  • 1 Saga University, Nabeshima, Saga, Japan
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TO THE EDITOR: We found the article by Matsukawa et al.1 quite interesting, with its conclusion that the cutoff value of the screw insertion depth for achieving bone fusion using the cortical bone trajectory technique is 40% in the vertebral body, which indicates that using longer screws and inserting them deeper into the vertebral body can achieve both anchor strengthening and load sharing within the vertebral body, leading to successful bone fusion (Matsukawa K, Yanai Y, Fujiyoshi K, et al. Depth of vertebral screw insertion using a cortical bone trajectory technique in lumbar spinal fusion: radiological significance of a long cortical bone trajectory. J Neurosurg Spine. 2021;35[5]:601-606). While we completely agree with this conclusion, we have some concerns regarding the radiographic evaluation.

First, in addition to bone fusion, it may have been good to evaluate vertebral endplate cysts, which can be a predictor of pseudarthrosis caused by delayed bone fusion due to micromotion or poor initial fixation.2 Vertebral endplate cysts are frequent, are simpler and easier to evaluate than bone fusion, and may have provided new insights to compensate for the drawback of the small number of non-bony fusion cases in their study.

Second, although we totally agree with the use of Hounsfield units for osteoporosis assessment, we would like to suggest a different region of interest (ROI). Since the authors evaluated loosening of the pedicle screw, the ROI of the pedicle screw trajectories should be evaluated. In contrast, for evaluating osteoporosis, placement of the trabecular ROI at the anterior-middle vertebral level is recommended over the cephalad or caudal3 to avoid distortion of attenuation measurements due to osteosclerosis around the vertebral endplates or posterior venous plexus.

Finally, as the authors also mentioned, it may have been better to investigate the timing of bone fusion. Initial biomechanical stability via posterior fixation until 6 months after posterior lumbar interbody fusion is essential for obtaining osseointegration at the intervertebral cages.4 Examining the period of bone fusion may provide new findings to confirm the strong initial fixation of this screw insertion technique.

We would like the authors to verify the efficacy of this technique with regard to vertebral endplate cysts, the area of the ROI, and the bony fusion period.

Disclosures

The authors report no conflict of interest.

References

  • 1

    Matsukawa K, Yanai Y, Fujiyoshi K, Kato T, Yato Y. Depth of vertebral screw insertion using a cortical bone trajectory technique in lumbar spinal fusion: radiological significance of a long cortical bone trajectory. J Neurosurg Spine. 2021;35(5):601606.

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  • 2

    Tanida S, Fujibayashi S, Otsuki B, et al. Vertebral endplate cyst as a predictor of nonunion after lumbar interbody fusion: comparison of titanium and polyetheretherketone cages. Spine (Phila Pa 1976).2016;41(20):E1216E1222.

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  • 3

    Lee BJ, Koo HW, Yoon SW, Sohn MJ. Usefulness of trabecular CT attenuation measurement of lumbar spine in predicting osteoporotic compression fracture: is the L4 trabecular region of interest most relevant? Spine. (Phila Pa 1976).2021;46(3):175183.

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  • 4

    Makino T, Takanaka S, Sakai Y, Yoshikawa H, Kaito T. Impact of mechanical stability on the progress of bone on growth on the frame surfaces of a titaniumcoated PEEK cage and a 3D porous titanium alloy cage: in vivo analysis using CT color mapping. Eur Spine J. 2021;30(5):13031313.

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  • 1 National Hospital Organization, Murayama Medical Center, Musashimurayama, Tokyo, Japan

Response

We would like to thank Dr. Morimoto et al. for their interest in our article and for their insightful and valuable comments. Here, we respond to their comments.

First, with regard to vertebral endplate cysts, as Dr. Morimoto and colleagues mentioned, the assessment of cyst formation is simpler than that of bone fusion, which may reduce interobserver bias. In our case series, a vertebral endplate cyst was observed in 30.2% of patients (29 of 96). Among the 29 patients, 22 (75.9%) attained bone fusion and 7 (24.1%) had nonfusion at the last follow-up, and a vertebral endplate cyst was not a good predictor of fusion failure in our study. Local micromotion on the interface between vertebral endplates and cages is reportedly thought to cause cyst formation; however, we speculate that bone fusion can be achieved with autologous bone grafted around the intervertebral cages even if the cyst sign is positive. Multivariate logistic regression analysis revealed that the only factor contributing to cyst formation was BMI (p = 0.02), and %depth was not a statistically significant factor.

Second, we completely agree with Dr. Morimoto and colleagues that Hounsfield units (HU) of screw trajectory is useful as a factor more directly related to screw fixation. Previous reports have shown that HU values of screw trajectory can be a reliable predictor of both primary and long-term screw fixation.1,2 Meanwhile, regional HU evaluation of screw trajectory was not possible in our study given the limitations of the imaging software. Accordingly, the vertebral bone properties were assessed by setting an oval-shaped ROI on the vertebral body following Schreiber’s method, excluding the osteosclerotic region and cortical rim.

Lastly, we received a valuable comment on the time to attain fusion. Bone fusion rates at 6 months postoperatively, 1 year postoperatively, and the last follow-up in patients with %depth > 39.2% and %depth ≤ 39.2% were, respectively, 68.8% and 12.5% (p < 0.001), 80.0% and 37.5% (p < 0.001), and 97.5% and 62.5% (p < 0.001). Statistically significant early bone fusion was observed in patients with %depth > 39.2%.

References

  • 1

    Matsukawa K, Abe Y, Yanai Y, Yato Y. Regional Hounsfield unit measurement of screw trajectory for predicting pedicle screw fixation using cortical bone trajectory: a retrospective cohort study. Acta Neurochir (Wien). 2018;160(2):405411.

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  • 2

    Sakai Y, Takenaka S, Matsuo Y, et al. Hounsfield unit of screw trajectory as a predictor of pedicle screw loosening after single level lumbar interbody fusion. J Orthop Sci. 2018;23(5):734738.

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    • PubMed
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Images from Zhou et al. (pp 274–282).

  • 1

    Matsukawa K, Yanai Y, Fujiyoshi K, Kato T, Yato Y. Depth of vertebral screw insertion using a cortical bone trajectory technique in lumbar spinal fusion: radiological significance of a long cortical bone trajectory. J Neurosurg Spine. 2021;35(5):601606.

    • Search Google Scholar
    • Export Citation
  • 2

    Tanida S, Fujibayashi S, Otsuki B, et al. Vertebral endplate cyst as a predictor of nonunion after lumbar interbody fusion: comparison of titanium and polyetheretherketone cages. Spine (Phila Pa 1976).2016;41(20):E1216E1222.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3

    Lee BJ, Koo HW, Yoon SW, Sohn MJ. Usefulness of trabecular CT attenuation measurement of lumbar spine in predicting osteoporotic compression fracture: is the L4 trabecular region of interest most relevant? Spine. (Phila Pa 1976).2021;46(3):175183.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 4

    Makino T, Takanaka S, Sakai Y, Yoshikawa H, Kaito T. Impact of mechanical stability on the progress of bone on growth on the frame surfaces of a titaniumcoated PEEK cage and a 3D porous titanium alloy cage: in vivo analysis using CT color mapping. Eur Spine J. 2021;30(5):13031313.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 1

    Matsukawa K, Abe Y, Yanai Y, Yato Y. Regional Hounsfield unit measurement of screw trajectory for predicting pedicle screw fixation using cortical bone trajectory: a retrospective cohort study. Acta Neurochir (Wien). 2018;160(2):405411.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2

    Sakai Y, Takenaka S, Matsuo Y, et al. Hounsfield unit of screw trajectory as a predictor of pedicle screw loosening after single level lumbar interbody fusion. J Orthop Sci. 2018;23(5):734738.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation

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