P2. To cross or not to cross: a biomechanical investigation of crossing cervicothoracic junction in posterior cervical laminectomy and fusion

Abstract

BACKGROUND CONTEXT Lower stopping point for the construct in posterior cervical laminectomy and fusion (PCLF) that involves the cervico-thoracic junction is highly debated. Constructs with C7 as lowest instrumented vertebra (LIV) have been implicated in higher incidence of adjacent level disease and junctional kyphosis, but the literature is not conclusive about these findings and many studies claim in difference in adjacent level disease or PJK. PURPOSE To investigate the biomechanical implications of having C7 as the lowest instrumented vertebra versus crossing the cervico-thoracic junction. STUDY DESIGN/SETTING Biomechanical cadaveric study PATIENT SAMPLE N/A OUTCOME MEASURES N/A METHODS Human cadaveric specimens of full subaxial cervical spine and torso with intact ribcage were used for the study. Six degrees of freedom pure-moment simulator were used to test the specimens in Flexion-Extension (FE), Lateral Bending (LB) and Axial Rotation (AR). Torque-controlled protocol (2.5 Nm, 1.5o/s) was utilized for testing of the intact specimens, while displacement control (based on the angle values obtained from the intact testing) was used for testing the constructs. After the intact testing, C3–7 laminectomy was performed in a standard fashion, and three different fusion constructs were tested in the same specimen: C3–C7, C3–T1, C3–T2. Optic tracking system was used to record range of motion (ROM) and intradiscal pressure (IDP) monitors with custom LabView based software was used to study the intradiscal pressures at the levels adjacent to the fusion constructs. Univariate analysis along with multivariate model was used to analyze the differences between the three constructs. RESULTS A total of nine cadaveric specimens were included in the study. There was partial IDP data missing on one of the specimens and the data was unreliable (statistical outlier on the other specimen) those specimens were excluded from the IDP analysis. ROM data were available for all nine specimens. There was a statistically significant difference in an absolute increase in the adjacent segment range of motion in both torque-controlled and displacement-controlled protocol on FE between constructs stopping at C7 and the ones stopping at T1 or T2. There was also a statically significant difference in absolute increase in the IDP using displacement-control protocol at the adjacent level in FE between constructs stopping at C7 and the ones stopping at T1 or T2. There was no statistically significant difference between changes in the ROM or IDP at the adjacent level in LB or AR. There was also no statistically significant difference in absolute change in ROM or IDP at the adjacent level between constructs stopping at T1 and T2, although a trend was towards lower increase in ROM and IDP with C3–T2 construct. CONCLUSIONS This is the first definitive biomechanical study demonstrating a significant difference in the increase in both ROM and IDP at the adjacent level for constructs terminating at C7 versus the ones crossing the cervico-thoracic junction. These findings suggest higher stress on the adjacent levels with likely implications for the adjacent level disease and PJK for multilevel constructs that involve C7 and do not cross the cervico-thoracic junction. FDA DEVICE/DRUG STATUS QUARTEX (Globus Medical) (Approved for this indication)