Middle-Column Gap Balancing and Middle-Column Mismatch in Spinal Reconstructive Surgery

Background Middle-column gap balancing (MCGB) is a reference measurement of the path of the posterior longitudinal ligament (PLL), which is reconstructed under tension and balanced by the combined height of the posterior one-third of the vertebral bodies and the posterior one-third of the disks, including any intervening load-sharing spacers. This measurement allows for a comparison of the ligamentous component of the middle column (PLL) with the load-sharing components (posterior one-third vertebral body + disk ). This difference gives rise to a “middle-column mismatch,” which provides a linear measurement of the redundancy of the ligaments and neural elements, which relates to the correct cage, spacer, or load-bearing height, which is optimized.

Methods For phase 1 measurement testing, 24 consecutive patients underwent reliable flexion, extension, and neutral lateral radiographic studies with a calibrated marker. The anterior, middle, and posterior columns were measured using a custom software program capable of measuring the length of curved lines specifically written for this purpose. For phase 2 measurement testing, 21 consecutive patients undergoing surgery with multilevel deformities for cervical, thoracic, and lumbar procedures had MCGB height pre- and postoperatively measured by 3 blinded observers. The preoperative and postoperative measurements were compared using a linear regression analysis and Pearson product-moment correlation.

Results In phase 1 measurement testing the flexion, extension, and neutral bending radiographs of spinal segments not containing deformities showed that the middle column had the most reliable measurements of spinal axial height both in the actual measurements of change from flexion to extension (mm) and in percentage of change. In phase 2 measurement testing, a Pearson product-moment correlation was run between each individual's pre- and postoperative middle-column measurements. There was a strong positive correlation between preoperative and postoperative measurements, which was statistically significant (r = 0.983, n = 21, P < .01).

Conclusions This consecutive series of 21 deformity patients demonstrated the utility of measuring the preoperative middle-column length in predicting the optimal height of the spacers and intervertebral disks, and posterior vertebral body height, simultaneously restoring sagittal and coronal plane alignment. Key points of this study include the following: (1) Spinal balance requires optimizing spinal height, which is a curved line in order to accommodate cervical lordosis, thoracic kyphosis, and lumbar lordosis. (2) Software programs can allow measurement of the preoperative curved circuitous course of the PLL and vertebral body misalignment; this curved length is predictive of the optimal postoperative middle-column height after spinal osteotomies and intervertebral spacer insertion. (3) All 3 dimensions are important to optimize in deformity correction: sagittal plane, coronal plane, and axial spinal height.