Defining Cage Subsidence in Anterior, Oblique, and Lateral Lumbar Spine Fusion Approaches: A Systematic Review of the Literature

Introduction: Cage subsidence is one of the most common complications of lumbar fusions, which leads to the collapse of disc height and the reappearance of the presenting symptomology. However, definitions of cage subsidence are inconsistent, leading to various subsidence calculation methodologies and thresholds. We aim to review previously published literature on cage subsidence and present the most common methods for calculating and defining subsidence in the anterior lumbar interbody fusion (ALIF), oblique lateral interbody fusion (OLIF), and lateral lumbar interbody fusion (LLIF) approaches.

Methods: A search was completed in PubMed and Embase with inclusion criteria focused on identifying any study that described the method, imaging modality, or subsidence threshold used to calculate the presence of cage subsidence.

Results: 69 articles were included in the final analysis; 18 (26.1%) discussed the ALIF approach, 22 (31.9%) the OLIF approach, and 31 (44.9%) the LLIF approach. Two studies reported multiple approaches. Most ALIF articles calculated disc height loss with a subsidence threshold of >2mm. Most OLIF articles calculated the total amount of cage migration into the vertebral bodies, with a threshold of >2mm. LLIF was the only approach in which most articles applied the same method for calculation, namely, a grading scale for classifying the loss of disc height over time.

Conclusion : We recommend future articles adhere to the most common methodologies presented here to ensure accuracy and generalizability in reporting cage subsidence.