Letter to Editor Regarding “Trabecular Bone Remodelling After Posterior Lumbar Interbody Fusion: Comparison of the Osseointegration in Three-Dimensional Porous Titanium Cages and Polyether-Ether-Ketone Cages” by Segi et al

Dear Editor,

I am writing to commend the authors of the recent article, “Trabecular Bone Remodelling after Posterior Lumbar Interbody Fusion: Comparison of the Osseointegration in Three-Dimensional Porous Titanium Cages and Polyether-Ether-Ketone Cages” by Segi et al, for their insightful research. ¹ The study offers a valuable comparison of 2 commonly used materials, three-dimensional porous Titanium (3DTi) and Polyether-Ether-Ketone (PEEK) in Posterior Lumbar Interbody Fusion (PLIF). The authors’ focus on Trabecular Bone Remodelling (TBR) as a novel imaging marker for assessing osseointegration is particularly compelling and adds a fresh perspective to the field of spinal fusion surgery.

The findings that 3DTi cages with early TBR (observed at 3 months postoperatively) demonstrated significant segmental stabilisation, while PEEK cages required TBR at 1 year to predict stabilisation, are fascinating. This suggests that the timing of TBR may vary depending on the cage material, which could have important implications for postoperative monitoring and patient care. Additionally, the observation that vertebral endplate cyst (VEC) formation was more common in 3DTi cages, while PEEK cages showed higher rates of subsidence, highlights the distinct biological and biomechanical responses elicited by these materials.

While the study is undoubtedly a significant contribution to the field, I would like to offer a few points for further consideration and discussion:

Sample Size and Statistical Power

The study included 101 patients, which, while reasonable, may limit the statistical power of certain analyses. For example, the comparison of TBR positivity rates between the 2 cage types at different time points did not reach statistical significance, possibly due to the relatively small sample size. Expanding the cohort in future studies could provide more robust conclusions, especially in subgroup analyses.

Discrepancy in the Number of Participants

The analysis identifies a minor discrepancy in participant numbers. Initially, 151 patients were selected, with 51 excluded (16 for specific medical reasons, 35 for lack of postoperative CT). This should leave 100 patients, but 101 were reported. Grouping (53 in 3DTi, 48 in PEEK) matches the 101 total, but the +1 discrepancy remains. Possible causes include data entry errors, exclusion overlaps, or an extra patient inclusion. While the discrepancy is small and unlikely to impact results, addressing it ensures accuracy and transparency in the study.

Evaluation of Osteoporosis

No mention about osteoporosis has been made to compare the bone quality which can impact the fusion rates and act as a confounding factor. It is a significant risk factor for mechanical complications of lumbar fusion. ²

Subjective Interpretation of Imaging Findings

The study relies heavily on the interpretation of CT imaging findings, such as TBR and VEC formation, which can be subjective. While the authors have provided clear definitions, inter-observer variability could still influence the results. Including an inter-observer reliability assessment in future research could strengthen the validity of these imaging findings. There is a lack of a proper scoring system like Burkus classification system which can be added to the study. ³

Long-Term Follow-Up

The study’s follow-up period of 2 years is sufficient for assessing early to mid-term outcomes, but extending this period could offer additional insights into the long-term durability of segmental stabilisation and the potential for late complications, such as delayed subsidence or pseudarthrosis. Long-term follow-up studies, have shown that complications like pseudarthrosis can manifest several years postoperatively, underscoring the need for extended observation periods.

Mechanism of VEC Formation in 3DTi Cages

The authors hypothesise that the higher elastic modulus of 3DTi cages may contribute to VEC formation. This hypothesis could be further explored through biomechanical studies or finite element analysis to better understand the stress distribution and its impact on vertebral endplates. Research by Wang et al (2023) has demonstrated that Finite Element Analysis (FEA) is critical for spinal IC studies. It helps in quantitatively and visually demonstrating the cage characteristics after implanting, lowering surgeons’ learning costs for new cage products, and probably assisting them in determining the best IC for patients. ⁴

While the study provides valuable imaging-based insights, correlating these findings with clinical outcomes, such as patient-reported pain scores, functional improvement, and revision surgery rates, would help bridge the gap between radiological findings and their clinical relevance. Studies have shown that as Patient reported outcomes help gain insight into individual patients’ experiences and integrate an appraisal of patients’ perspectives into clinical practice. ⁵

In conclusion, the study by Segi et al is a noteworthy contribution to the field of spinal fusion surgery, particularly in understanding the role of cage materials in osseointegration and segmental stabilisation. It highlights the importance of material selection in PLIF and lay the groundwork for future research. I commend the authors for their thorough investigation and look forward to seeing further studies that build on these findings.