Three-dimensional analysis of interbody cage-apophyseal ring contact to predict endplate subsidence following transforaminal interbody fusion

BACKGROUND: There is a higher risk of subsidence following transforaminal lumbar interbody fusion (TLIF) relative to other approaches. Decreased subsidence risk is associated with anterior cage placement, speculated to be because of increased apophyseal ring contact. However, this hypothesis is largely based on in vitro evidence and, to date, has not been investigated in a clinical cohort. METHODS: Pre-operative and post-operative computed tomography (CT) images from 42 TLIF patients were used to segment the endplate and cages. The apophyseal ring boundary was manually landmarked for each endplate. The pre-operative endplates were rigidly registered with the post-operative cage position, and an iterative closest point approach was used to calculate the contact area between the cage, apophyseal ring, and endplate. Subsidence was categorised based on severity (No Subsidence: <2 mm; Moderate Subsidence: 2-4 mm; Severe Subsidence: ≥4 mm) from post-operative CT. FINDINGS: Apophyseal ring contact was significantly lower in both Moderate and Severe Subsidence, relative to No Subsidence (Moderate: -19.6 ± 7.0%; Severe: -21.5 ± 6.5%; P < 0.05), and negatively correlated with subsidence depth (P < 0.05). Injury risk analysis demonstrated that a 50% subsidence risk was associated with 45.7% (38.4-53.6%) apophyseal ring contact. Suprajacent endplate apophyseal ring contact, but not subjacent, was significantly predictive of subsidence at the respective endplate (P < 0.05). INTERPRETATION: The risk of subsidence in TLIF patients can be mitigated by ensuring that at least half of the interbody cage surface area is in contact with the peripheral endplate rim, particularly at the endplate superior to the cage.