In an article recently published in NASSJ Weinstein et al. analyzed both clinical and radiographic outcomes of patients who underwent transforaminal lumbar interbody fusion (TLIF) with an expandable interbody device [1Weinstein MA Ayala GA Roura R Christmas KN Warren DH Simon P. Transforaminal lumbar interbody fusion with an expandable interbody device: Two-year clinical and radiographic outcomes.N Am Spine Soc J. 2023; 16 (Published 2023 Oct 13)100286https://doi.org/10.1016/j.xnsj.2023.100286Abstract Full Text Full Text PDF Scopus (0) Google Scholar]. The initial sample included 37 patients who underwent open TLIF with expandable cage by a single orthopedic spine surgeon, with 28 patients being included in final analysis. Clinical and radiographic outcomes were measured at baseline and post-operatively at 2-weeks, 6-weeks, 3-months, 6-months, 12-months, and 24-months. Patient reported outcomes collected included the Oswestry Disability Index (ODI) and the Visual Analogue Score (VAS). Extracted radiographic parameters included pelvic incidence, pelvic tilt, lumbar lordosis, segmental lordosis, and disc height at all instrumented levels. The study results showed significant improvement in both clinical measures, as well as segmental and lumbar lordosis which persisted at the final follow-up. The authors states that there were neither device-related complications nor verified instances of cage subsidence in their study population. As such, the authors concluded that the use of an expandable cage is both safe and effective for achieving increased case size and significant lordotic correction. Given the widespread use TLIF as one of the most common approaches for interbody fusion [2Mobbs RJ Phan K Malham G Seex K Rao PJ Lumbar interbody fusion: techniques, indications and comparison of interbody fusion options including PLIF, TLIF, MI-TLIF, OLIF/ATP, LLIF and ALIF.J Spine Surg. 2015; 1: 2-18https://doi.org/10.3978/j.issn.2414-469X.2015.10.05Crossref PubMed Google Scholar] and the emerging interest in expandable cages as a possible means of increasing the degree of restoration of segmental lordosis [3Jitpakdee K Sommer F Gouveia E Mykolajtchuk C Boadi B Berger J Hussain I Härtl R. Expandable cages that expand both height and lordosis provide improved immediate effect on sagittal alignment and short-term clinical outcomes following minimally invasive transforaminal lumbar interbody fusion (MIS TLIF).J Spine Surg. 2024 Mar 20; 10 (Epub 2024 Mar 15. PMID: 38567017; PMCID: PMC10982918): 55-67https://doi.org/10.21037/jss-23-106Crossref Google Scholar], this study is a commendable contribution to the literature. As the authors state, expandable interbody devices help to assuage the implicit biomechanical concerns posed by traditional cages [4Parisien A Wai EK ElSayed MSA Frei H. Subsidence of Spinal Fusion Cages: A Systematic Review.Int J Spine Surg. 2022; 16: 1103-1118https://doi.org/10.14444/8363Crossref PubMed Scopus (13) Google Scholar], allowing the device to be implanted in a less invasive manner and expanded to custom fit as per the surgeon's discretion [5Morrison R Rigal J Le Huec JC Schnake KJ Report of One Year Follow up of Patients after TLIF using a 3-Dimensional Expandable Cage for Lumbar Fusion with Lordotic Correction.Global Spine Journal. 2016; 6 (s-0036-1583147-s-0036-1583147)https://doi.org/10.1055/s-0036-1583147Crossref Google Scholar]. Though this work provides evidence of the positive clinical and radiological effects of expandable TLIF cages, upon a careful reading of this interesting study and the illustrative figures, it becomes apparent that the lower right panel of the original article's Figure 1 may show some degree of subsidence, contrary to the study's claim that there was no evidence of the phenomenon (Fig. 1). Outlined in red, the image appears to show subsidence of the interbody device into the posterior aspect of the inferior endplate at the L4 level. Although it is possible that this finding may be at least partially secondary to an imperfect lateral x-ray (note that the pre-operative top right image shows a dual shadow of the inferior endplate of L4 – Fig1 top right expanded image), there seems to be convincing evidence of at least some degree of subsidence of the posterior aspect of the cage upon the inferior endplate of L4, with a clear step-off (red marking) This finding not only puts into question the authors claim that there was no evidence of cage subsidence in this series but, most importantly, highlights the need of reliable imaging criteria for evaluation of subsidence and endplate violation in such type of studies. Ideally such type of claim should be made based on CT-scan imaging but when x-rays are employed it is imperative clear that poorly aligned radiographs with double endplate shadow are a very suboptimal means of assessing cage subsidence. According to a systematic review of subsidence across different interbody fusion techniques, Parisien et al. found that subsidence occurs in 13% to 27% of patients. Further, the study results showed the TLIF subset to have a median occurrence of 21.4%, and a maximum occurrence of 51.2% [5Morrison R Rigal J Le Huec JC Schnake KJ Report of One Year Follow up of Patients after TLIF using a 3-Dimensional Expandable Cage for Lumbar Fusion with Lordotic Correction.Global Spine Journal. 2016; 6 (s-0036-1583147-s-0036-1583147)https://doi.org/10.1055/s-0036-1583147Crossref Google Scholar]. A number of studies have identified several factors associated with increased risk of subsidence, such as lower bone mineral density (BMD), sub-optimal device size, central device positioning, irregular endplate morphology, and a lower reading of lumbar vertebral body Hounsfield units [6Pisano AJ Fredericks DR Steelman T Riccio C Helgeson MD Wagner SC. Lumbar disc height and vertebral Hounsfield units: association with interbody cage subsidence.Neurosurg Focus. 2020; 49: E9https://doi.org/10.3171/2020.4.FOCUS20286Crossref PubMed Scopus (35) Google Scholar, 7Rickert M Fennema P Wehner D et al.Postoperative cage migration and subsidence following TLIF surgery is not associated with bony fusion.Sci Rep. 2023; 13 (Published 2023 Aug 3): 12597https://doi.org/10.1038/s41598-023-38801-7Crossref Scopus (2) Google Scholar, 8Zhou QS Chen X Xu L et al.Does Vertebral End Plate Morphology Affect Cage Subsidence After Transforaminal Lumbar Interbody Fusion?.World Neurosurg. 2019; 130: e694-e701https://doi.org/10.1016/j.wneu.2019.06.195Abstract Full Text PDF PubMed Scopus (24) Google Scholar]. Previous studies have also suggested that, despite the enthusiasm associated with expandable cages, such devices may be associated with increased risk of intra-operative subsidence [9Stickley C Philipp T Wang E Zhong J Balouch E O'Malley N Leon C Maglaras C Manning J Varlotta C Buckland AJ. Expandable cages increase the risk of intraoperative subsidence but do not improve perioperative outcomes in single level transforaminal lumbar interbody fusion.Spine J. 2021 Jan; 21 (Epub 2020 Sep 3. PMID: 32890783): 37-44https://doi.org/10.1016/j.spinee.2020.08.019Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar], a somewhat predictable event when considering the magnitude of the force exerted upon the vertebral bodies through the small surface area of the cage (or cages, if employed bilaterally). Ultimately cage subsidence may lead to loss of segmental lordosis, pseudarthrosis, cage migration and even hardware failure requiring revision surgery [6Pisano AJ Fredericks DR Steelman T Riccio C Helgeson MD Wagner SC. Lumbar disc height and vertebral Hounsfield units: association with interbody cage subsidence.Neurosurg Focus. 2020; 49: E9https://doi.org/10.3171/2020.4.FOCUS20286Crossref PubMed Scopus (35) Google Scholar]. In summary, although cage subsidence may sometimes be challenging to definitively assess because of inter-physician variability and measurement errors secondary to radiological artifact caused by the device material [9Stickley C Philipp T Wang E Zhong J Balouch E O'Malley N Leon C Maglaras C Manning J Varlotta C Buckland AJ. Expandable cages increase the risk of intraoperative subsidence but do not improve perioperative outcomes in single level transforaminal lumbar interbody fusion.Spine J. 2021 Jan; 21 (Epub 2020 Sep 3. PMID: 32890783): 37-44https://doi.org/10.1016/j.spinee.2020.08.019Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar], when a study purports to evaluate subsidence as a key radiographic outcome it is imperative that it employs meticulous and reproducible criteria for evaluation of such an important radiographic event. Overall we congratulate the authors for their interesting study on the expandable TLIF cages. However, we believe that the aforementioned concerns about the reliability of the radiographic technique employed for evaluation of cage subsidence in this study and the accuracy of the claim of "no subsidence" should be highlighted to NASSJ readers. [10Ha SK Park JY Kim SH Lim DJ Kim SD Lee SK. Radiologic Assessment of Subsidence in Stand-Alone Cervical Polyetheretherketone (PEEK) Cage.J Korean Neurosurg Soc. 2008; 44: 370-374https://doi.org/10.3340/jkns.2008.44.6.370Crossref PubMed Scopus (43) Google Scholar] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
