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MR-Based Measurement of Spinal Cord Motion During Flexion of the Spine: Implications for Intradural Spinal Cord Stimulator Systems 2013

Category General Spine Charles Smittkamp
Stephanus Viljoen
Brian Dalm
Saul Wilson
Chandan Reddy
Matthew Howard
George Gillies
Purpose We are developing the Human Spinal Cord Modulation System (HSCMS) as a new means of delivering electrical stimuli directly to the pial surface of the spinal cord for treatment of intractable pain via neuromodulation of targeted dorsal column pathways.  To optimize device performance and avoid risk of injury to the spinal cord, the HSCMS implant must remain in gentle yet direct contact with the spinal cord even as it moves within the spinal canal. Functional anatomic knowledge of the spinal cord movement within the spinal canal is therefore needed to inform design requirements for the HSCMS.  We have conducted an experiment to assess this movement at the anticipated lower thoracic location of the HSCMS. Materials & Methods The lower thoracic cord in healthy volunteers was imaged in both supine neutral and supine maximal spinal flexion positions.  A bariatric MR was utilized as its larger bore size could accommodate volunteers while in the flexed position.  CISS pulse sequences were selected for their high spatial frequency, allowing for the most accurate measurements.  The distance between the T10 dorsal nerve root entry zone (DREZ) and T11 DREZ on the cord surface was measured in flexed and neutral positions and used as an assessment of spinal cord compression/expansion along the cranial-caudal axis.  Next, the distance from the T10 DREZ to the inferior aspect of the T10 pedicle was measured.  The difference in this measurement between flexed and neutral position is a measurement of cranial-caudal movement of the cord within the canal.  Finally, a similar measurement was made on the conus tip to assess cranial-caudal movement at the distal-most cord. Results The inter-DREZ distance across all patients between the neutral and flexion positions ranged from -2.0 mm to + 6.7 mm, with a mean and standard deviation of 3.5 ± 2.6 mm. The measured change in the pedicle-to-DREZ distance across all patients between the neutral and flexion positions ranged from 1.9 mm to 18.0 mm, with a mean and standard deviation of 8.5 ± 6.0 mm.   The mean and standard deviation for the rostral-caudal conus movement was found to be 6.4 ± 4.1 mm within an overall range of 1.1 to 11.4 mm. Conclusion Although mean values for cranial-caudal movement and compression/expansion are calculated and reported as intended, much more interesting results are seen when evaluating the ranges of measurements obtained.  An unexpected result was immense inter-subject variability in how the spinal cord accommodates maximal flexion.  Some subjects have nearly two centimeters of cranial cord movement with very little stretching while others have almost no cord movement while stretching over 25% along the measured segment.  Because the spinal cord’s structure and anchoring elements vary somewhat over its length, some difference in degree of motion and stretch is expected.  Our measurement data further suggests that the inter-subject variance in these quantities can be quite large even over a localized segment of the cord.  Our findings thus highlight the need for the HSCMS design to accommodate large patient-to-patient variations in spinal cord dynamic movement properties. 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