Study design:

Retrospective Cross-sectional review of a large database.

Objective:

Little is known regarding extension K-lines for treatment of cervical myelopathy. Therefore, this study seeks to examine differences between K-lines drawn in neutral and extension.

Summary of background data:

The modified K-line is a radiological tool used in surgical planning of the cervical spine. As posterior cervical decompression and fusion often results in patients being fused in a more lordotic position than the preoperative neutral radiograph, a K-line measured in the extension position may offer better utility for these patients.

Methods:

97 patients were selected with T2-weighted, upright cervical MRIs taken in neutral and extension. For each patient, the K-line was drawn at the mid-sagittal position for both neutral and extension. The distance from the most posterior portion of each disc (between C2-C7) to the K-line was measured in neutral and extension and the difference was calculated. Paired t-test was used to assess significant differences.

Results:

Across all levels between C2-C7 there was an increase in the distance between the dorsal aspect of the disc and K-line when comparing neutral and extension radiographs. The average change in difference (extension minus neutral) at each cervical spinal level was 0.9 mm (C2-C3), 2.5 mm (C3-C4), 2.6 mm (C4-C5), 2.0 mm (C5-C6), and 0.9 mm (C6-C7). A paired t-test showed that the K-line increase from neutral to extension was statistically significant across all disc levels (P<0.001).

Conclusion:

When positioned in extension, patients experience a significant increase in distance from the dorsal aspect of a disc to the K-line compared to when positioned in neutral, especially between C3-C6. This is clinically relevant for surgeons considering a posterior cervical decompression and fusion in patients with a negative modified K line on preoperative MRI imaging, as these patients may have enough cervical cord drift back when fused in an extended position, maximizing likelihood of improving postoperative CSM functional outcomes.