Definition of cage subsidence in transforaminal lumbar interbody fusion (TLIF) approach and posterior lumbar interbody fusion (PLIF) approach - A systematic review

Background: Although cage subsidence is one of the most common phenomenona associated with interbody fusions and was characterized more than 70 years ago, a standardized method for its measurement, detection, and reporting among different lumbar fusion procedures does not exist. Here, we review previously published literature on cage subsidence to present the most common methods for defining subsidence in the posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) techniques.

Methods: A search was completed in PubMed and Embase with inclusion criteria focused on identifying any study that provided descriptions of the method, imaging modality, and subsidence threshold used to define the presence of cage subsidence in study articles published between January 1, 2001 and December 31, 2022.

Results: A total of 54 studies were included in the final analysis. Among them, 32 (59.2 %) reported on TLIF, 20 (37.1 %) reported on PLIF, and 2 studies (3.7 %) reported on both approaches. For TLIF and PLIF procedures, the preferred method to determine subsidence was cage migration into the vertebral bodies rather than changes in disc height. In the TLIF group, the predominant criteria for defining subsidence were thresholds of ≥ 2 mm and > 2 mm utilized in each of 10 of the 34 studies (58.8 %). Similarly, in the PLIF group, the common criterion for defining subsidence was also ≥ 2 mm, observed in 5 of 22 studies (22.7 %), with > 2 mm observed in 4 studies (18.2 %). The methods for assessing cage migration or disc height change varied substantially among studies, with none of the measurements being consistently applied in more than 50 % of the studies.

Conclusions: Inconsistency persists in the methods used to determine, report, or measure the degree of subsidence. Surgeons and researchers should standardize these methods to ensure consistency and generalizability in reporting and studying subsidence.