Cervical Spinous Process and Its Attached Muscles Maintain Lower Disk Lordosis: A Retrospective Study of 155 Patients Who Underwent Muscle-Preserving Double Laminectomies

STUDY DESIGN: A retrospective study conducted at a single academic institution. PURPOSE: This study compared the postoperative alignment of consecutive double laminectomies according to their decompression levels and investigated the influence of the extension unit of the spinous process and its attached muscles on postoperative alignment. OVERVIEW OF LITERATURE: Many reports have investigated bony and soft tissue factors as the causes of postoperative cervical alignment disorders. To-this-date, no other article has clarified the importance of the attached muscles between the spinous processes of C3 and C6 to maintain local cervical alignment. METHODS: In total, 155 consecutive patients who underwent muscle-preserving consecutive double laminectomies for cervical spondylotic myelopathy from 2005 to 2013 were included in this study. The imaging parameters included the C2–C7 angle, range of motion, C2–C7 sagittal vertical axis (SVA), C7 slope, C2–C5 angle, C5–C7 angle, local disk angle caudal to the decompression level, and the disk height between C2/C3 and C7/Th1. RESULTS: The caudal disk angle of the decompression level decreased after consecutive double laminectomies, thus suggesting that the extension unit maintained the local lordosis at the lower disk of the decompression level. Postoperatively, in the C3–4 decompression cases, the C2–C7 angle decreased by 7.3°, and the C2–C7 SVA increased by 8.6 mm, thus indicating the appearance of an alignment disorder. Multivariate logistic regression analysis showed that cephalad laminectomy was a risk factor for C2–C7 angle decreases >10°. However, the postoperative recovery rate of Japanese Orthopedic Association scores after consecutive double laminectomies was reasonable, and the overall cervical alignment was well maintained in all decompression levels except C3–C4. CONCLUSIONS: The cervical extension unit maintained lordosis at the disk caudal to it. The extension unit was found to contribute more to the maintenance of lordosis of the entire cervical spine at the cephalad side.