Introduction Functional spinal unit the cephalad and caudad vertebral body as well as the 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 when, under physiologic loading, there is neither abnormal strain or excessive motion in the FSU maintained by FSU muscular tension abdominal and thoracic pressure rib cage support Three Column Theory Denis three column system clinical relevance only moderately reliable in determining clinical degree of stability definitions anterior column anterior longitudinal ligament (ALL) anterior 2/3 of vertebral body and annulus middle column posterior longitudinal ligament (PLL) posterior 1/3 of vertebral body and annulus posterior column pedicles facets ligamentum flavum spinous process posterior ligament complex (PLC) instability defined by injury to middle column as evidenced by widening of interpedicular distance on AP radiograph loss of height of posterior cortex of vertebral body disruption of posterior ligament complex combined with anterior and middle column involvement Ligaments FSU is surrounded by 10 ligaments with the functions: protecting neural structures by restricting motion of the FSU absorb energy during high speed motions Contents all ligaments are composed of type I collagen except ligamentum flavum (mostly elastin) are viscoelastic, with nonlinear behavior Posterior Ligamentous Complex Integerity of PLC now considered to be one of the most critical predictor of spinal fracture stability one of three primary factors in TLICS scoring system. TLICS measures as intact suspect/indeterminant ruptured Anatomy consists of supraspinous ligament interspinous ligament ligamentum flavum facet capsule Evaluation determining the integrity of the PLC can be challenging conditions where PLC is ruptured bony chance fracture widening of interspinous distance progressive kyphosis with nonoperative treatment facet diastasis conditions where ambiguity MRI shows signal intensity between spinous process Treatment nonoperative according to TLICS, if PLC is intact (+0 points) in a compression (+1 point) burst fx (+1 point) in a patient without neurologic deficits (+0 points) than the patient should be treated nonoperatively total score = 2 points (score < 4 points = nonoperative) operative according to TLICS, if PLC is ruptured (+3 points) in a compression (+1 point) burst fx (+1 point) in a patient without neurologic deficits (+0 points) than the patient should be treated with surgery total score = 5 points (score > 4 points = operative) Spinal Balance Sagittal balance is due to the normal cervical lordosis, thoracic kyphosis and lumbar lordosis. cervical lordosis normal range 20-40° thoracic kyphosis average 35° normal range 20-50° lumbar lordosis average 60° normal range 20-80° as much as 75% of lumbar lordosis occurs between L4 and S1 with 47% occurring at L5/S1 normal alignment the vertical axis runs from the center of C2 to the anterior border of T7 to the middle of the T12/L1 disc, posterior to the L3 vertebral body, and crosses the posterior superior corner of the sacrum. on radiograph 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 facets (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 0° coronal 45° sagittal (angled superio-medially) function allows flexion-extension, lateral flexion, rotation thoracic spine planes 20° coronal 55° sagittal (facets in coronal plane) 6 degrees of freedom function allows some rotation, minimal flexion-extension (also limited by ribs) prevents downward flexion on heart and lungs lumbar spine plane 50° coronal 90° sagittal (facets in sagittal plane) function allows flexion-extension, minimal rotation helps increase abdominal pressure Instantaneous axis of rotation (IAR) axis about which the vertebra rotates at some instant in time normal FSU is confined to a small area within the FSU abnormal FSU (e.g. degenerate disc) shifts outside the physical space of the FSU is enlarged dramatically Pedicle Anatomy Cervical C2 viable for pedicle screw placement C3-C6 pedicle small making pedicle screw instrumentation difficult lateral mass scews placed at C3-C6 as alternative C7 viable for pedicle screw placement Thoracic pedicle diameter the pedicle wall is twice as thick medially as 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 larger pedicle diameter than L1 pedicle length pedicle length decreases from T1 to T4 and then increases again as you move distal in the thoracic spine T1: 20mm T4: 14mm (shortest pedicle) T10: 20 mm pedicle angle transverse pedicle angle varies from 10deg (mid thoracic spine) to 30deg (L5) sagittal pedicle angle 15-17deg cephalad for majority of thoracic spine neutral (0deg) for lumbar spine except L5 (caudal) Lumbar-Sacral landmarks midpoint of the transverse process used to identify midpoint of pedicle in superior-inferior dimension lateral border of pars used to identify midpoint in medial-lateral dimension pedicle angulation pedicles angulate more medial as you move distal L1: 12 degrees L5: 30 degrees S1: 39 degrees pedicle diameter L1 has smallest diameter in lumbar spine S1 has average diameter of ~19mm