135. Neutral cervical sagittal vertical axis (SVA) varies with T1 tilt

BACKGROUND CONTEXT With an aging population and increasing incidence of cervical spine surgery, more attention is being focused on cervical alignment and deformity. Iatrogenic deformity can result from under- or overcorrection of cervical sagittal alignment during fusion surgery. We sought to explore the relationship between T1 tilt and cervical SVA. In our cadaveric study, the sagittal vertical alignment (SVA) was altered for a given T1 tilt until the least extrinsic force was required to maintain alignment. This correlates with the in vivo state requiring the least muscle contraction to maintain alignment. PURPOSE To determine if an increase or decrease in T1 tilt from baseline will require a compensatory increase or decrease in SVA in order to maintain neutral posture. STUDY DESIGN/SETTING Cadaver biomechanical study. PATIENT SAMPLE Six cadaveric cervical spines (occiput-T1). OUTCOME MEASURES C0-T1 SVA (mm). METHODS Six cadaveric cervical spines (occiput-T1) were mounted with a 5 kg mass simulating head weight. Forces and moments were recorded with a load cell. Motion was recorded with optoelectric sensors placed at each level, and a thin cut CT was used to create a 3-dimensional reconstruction. The head was constrained to maintain horizontal gaze. T1 tilt and SVA were adjusted by rotating or translating the test bed (respectively) to find the condition requiring the least extrinsic force to maintain. T1 tilt was then decreased, and SVA was adjusted to minimize extrinsic force and measured. T1 tilt was then increased and SVA adjusted and measured. RESULTS T1 tilt was decreased an average of 6.49 ± 1.39° (mean ± SD) from baseline, requiring a decrease in C0-T1 SVA of 28.14 ± 17.41 mm to achieve neutral posture. T1 tilt was increased 35.12 ± 37.08° from baseline, leading to an increase in SVA of 8.80 ± 2.72 mm. CONCLUSIONS As T1 tilt was increased from baseline, an increase in C0-T1 SVA was required to maintain neutral posture. As T1 tilt was decreased, a decrease in SVA was required. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.