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Chapter Summary
• Context: Pedicle screw fixation represents the gold standard for three-column spinal instrumentation across the thoracic, lumbar, and lumbosacral spine. By traversing all three mechanical columns of the vertebra through the dense cortical cylinder of the pedicle, pedicle screws provide unmatched multiplanar rigidity, enabling powerful deformity correction, rigid stabilization for trauma and tumors, and superior fusion rates. The free-hand technique, perfected by Kim and Lenke, allows safe, accurate, and reproducible pedicle screw insertion relying strictly on visible anatomical bony landmarks and tactile feedback without continuous fluoroscopic exposure. However, pedicle morphometry varies widely along the spinal column: thoracic pedicles are narrow and closely border the spinal cord medially, pleura anteriorly, and aorta anterolaterally, while lumbar and sacral pedicles are wider but lie near exiting nerve roots and major vascular bifurcations. Mastering the free-hand technique requires meticulous anatomical landmark recognition, tactile probe palpation, and structured intraoperative verification.
• Chapter Objective: Detail the anatomical landmarks, morphometric differences, tactile feedback principles, and step-by-step execution of the free-hand pedicle screw insertion technique from T1 to S1. The reader will master entry point identification, gearshift probing, five-wall palpation, screw placement, and intraoperative safety testing to prevent neurological, vascular, and visceral complications.
• Anatomical landmarks and morphometry from T1 to S1Thoracic pedicles: Entry points transition along the thoracic spine. In the upper thoracic spine (T1-T3), the entry point is at the junction of the superior border of the transverse process and the lateral margin of the superior facet. In the mid-thoracic spine (T4-T8, narrowest pedicles), the entry point moves superiorly, near the superior edge of the transverse process and medial to the facet joint. In the lower thoracic spine (T9-T12), the entry point returns toward the midpoint of the transverse process. Lumbar pedicles: The entry point is located at the intersection of the vertical line along the lateral margin of the superior articular facet and the horizontal line bisecting the transverse process (the 'mammillary process'). S1 pedicle: Entry point is at the inferolateral base of the S1 superior articular process, directed 15°–25° medially and straight toward the sacral promontory.
• The 5-step free-hand surgical technique (Kim-Lenke)Step 1: Cortical burr decortication of the entry point. Step 2: Gearshift probing: A curved pedicle probe is gently advanced through the pedicle into the vertebral body with a blunt, tactile push (pointed lateral initially to avoid medial wall breach, then rotated medial after entering the vertebral body). Step 3: 5-wall ball-tip palpation: A flexible ball-tip feeler confirms an intact bony floor and all four walls (medial, lateral, superior, inferior). Step 4: Undertapping with a line-to-line or undersized tap. Step 5: Final ball-tip re-palpation and screw insertion.
• Tactile feedback and managing pedicle breachesThe surgeon relies on tactile resistance: passing through cancellous bone feels smooth and resilient; encountering a cortical breach feels like a sudden drop in resistance or a hard, gritty bone wall. A medial breach threatens the spinal cord or cauda equina; an inferior breach threatens the exiting nerve root; an anterior/lateral thoracic breach risks pleural or aortic laceration.
• Neurophysiological testing and safety confirmationTriggered EMG (t-EMG) stimulation of pedicle screws provides real-time confirmation of medial wall integrity. In the lumbar spine, stimulation thresholds >11–15 mA indicate an intact medial wall, while thresholds <8 mA raise suspicion of a medial cortical breach or direct root contact.
• Complications and preventionNeurological deficits (radiculopathy, cord contusion), vascular laceration (aorta, vena cava, segmental vessels), dural tears, screw loosening, and pedicle wall fracture. Preoperative CT measurement of pedicle diameter and strict adherence to the 5-step protocol ensure complication rates below 1-2%.
• Clinical Application: In clinical practice, the free-hand technique allows rapid, low-radiation multi-segmental pedicle screw placement in scoliosis, trauma, and degenerative fusion. Preoperative CT must be reviewed to identify dysplastic, sclerotic, or narrow pedicles (<4 mm diameter). During thoracic probe insertion, the probe should never be forced with a mallet; advancing must be done exclusively by controlled hand pressure and wrist rotation. If the probe meets unexpected hard resistance, it must be removed and the trajectory redirected. After probing, systematic 5-wall palpation with a ball-tip feeler is mandatory before tapping. If a medial breach is palpated in the thoracic spine, the track should be abandoned or redirected laterally to prevent spinal cord injury. Intraoperative triggered EMG and final AP/lateral fluoroscopy confirm accurate 3D placement.
