Introduction Functional spinal unit (FSU) defined as the cephalad vertebral body, caudad vertebral body, intervertebral disc, and the corresponding facet joints function is to provide physiologic motion and protect neural elements intradiscal pressure depends on position Spinal stability defined as the absence of abnormal strain or excessive motion in the FSU under physiologic loading maintained by login to view 4 more bullets Three Column Theory Denis three column model clinical relevance only moderately reliable in determining clinical degree of stability definitions anterior column login to view 2 more bullets middle column login to view 2 more bullets posterior column login to view 5 more bullets instability defined by login to view 4 more bullets Ligaments FSU is surrounded by 10 ligaments that function to: protect neural structures by restricting motion of the FSU absorb energy during high speed motion Contents all ligaments are composed of type I collagen except the ligamentum flavum (mostly elastin) are viscoelastic with nonlinear behavior Posterior Ligamentous Complex (PLC) Integrity of PLC is now considered one of the most critical predictors of spinal fracture stability one of three primary factors in the Thoracolumbar Injury Classification and Severity Score (TLICS). TLICS categorizes the PLC as: intact suspect/indeterminant ruptured Anatomy consists of: supraspinous ligament interspinous ligament ligamentum flavum facet capsule Evaluation determining PLC integrity can be challenging conditions where PLC is ruptured: bony chance fracture widening of interspinous distance progressive kyphosis with nonoperative treatment facet diastasis conditions with ambiguity: MRI shows signal intensity between spinous processes Treatment nonoperative according to TLICS, if PLC is intact (+0 points) in a compression (+1 point) burst fracture (+1 point) in a patient without neurologic deficits (+0 points), the patient should be treated nonoperatively login to view 1 more bullet operative according to TLICS, if PLC is ruptured (+3 points) in a compression (+1 point) burst fracture (+1 point) in a patient without neurologic deficits (+0 points), the patient should be treated with surgery login to view 1 more bullet Spinal Balance Sagittal balance due to the normal cervical lordosis, thoracic kyphosis, and lumbar lordosis cervical lordosis login to view 1 more bullet thoracic kyphosis login to view 2 more bullets lumbar lordosis login to view 3 more bullets normal alignment the vertical axis runs from the center of C2 → anterior border of T7 → middle of the T12/L1 disc → posterior to the L3 vertebral body → crosses the posterior superior corner of the sacrum on radiographs, this is estimated by a plumb line dropped from the center of C7 to the posterior-superior corner of S1 negative sagittal balance the axis is posterior to the sacrum and occurs in patients with lumbar hyperlordosis positive sagittal balance the axis is anterior to the sacrum and occurs in patients with hip flexion contracture or flat-back syndrome Motion The orientation of the facet (zygapophyseal) joints determines the degree and plane of motion at that level varies throughout the spine to meet physiologic function cervical spine (C3-7) planes login to view 2 more bullets function login to view 1 more bullet thoracic spine planes login to view 3 more bullets function login to view 2 more bullets lumbar spine plane login to view 2 more bullets function login to view 2 more bullets Instantaneous axis of rotation (IAR) axis about which the vertebra rotates at some instant in time normal FSU IAR is confined to a small area within the FSU abnormal FSU (e.g. degenerative disc) IAR shifts outside the physical space of the FSU can be dramatically enlarged Pedicle Anatomy Cervical C2 viable for pedicle screw placement C3-C6 pedicles are small, making pedicle screw instrumentation difficult login to view 1 more bullet C7 viable for pedicle screw placement Thoracic pedicle diameter the pedicle wall is twice as thick medially as it is laterally T4 has the narrowest pedicle diameter (on average) T7 can be irregular and have a narrow diameter on the concave side in AIS T12 usually has a larger pedicle diameter than L1 pedicle length pedicle length decreases from T1 to T4 and then increases moving distally in the thoracic spine login to view 3 more bullets pedicle angle transverse pedicle angle login to view 1 more bullet sagittal pedicle angle login to view 2 more bullets Lumbosacral landmarks midpoint of the transverse process is used to identify the midpoint of pedicle in the superior-inferior dimension lateral border of pars used to identify midpoint in the medial-lateral dimension pedicle angulation pedicles angulate more medially moving distally login to view 3 more bullets pedicle diameter L1 has the smallest diameter in the lumbar spine S1 has an average diameter of ~19 mm