Research

Purpose: To evaluate the utility of the C2 plumb line to evaluate sagittal balance when obscurity of the C7 landmarks prevent the accurate measurement of C7 plumb.
Methods: Sagittal balance was measured in a group of normal volunteers (n=34) and in deformity patients (n=42) who underwent long fusion from the sacrum to T10 or higher (74% idiopathic scoliosis, 24% degenerative scoliosis, and 2% other). In patients, standardized full-spine lateral radiographs were analyzed preoperatively, immediately postoperatively, and at an average 9 years follow-up. The anterior tip of the sacral promontory was marked to represent S1. A line bisecting the midpoint of the inferior endplate of both C2 and C7 was dropped to the sacrum. Distances from the promontory were measured in mm. Cervical alignment was measured with Cobb technique.
Results: In normals, the average C7 plumb line was 20 mm posterior to the sacral promontory and the average C2 plumb line was 5 mm anterior to the sacral promontory. The positional relationship between C2 and C7 was consistent with C2 always anterior to C7, by an average of 25 mm (correlation 0.946, p<0.01). In the deformity patients, the position of the C2 and C7 plumb lines were more variable but the relationship between plumb lines was consistent, C2 anterior to C7 by an average distance of 23 mm (correlation preop 0.944; postop 0.933; follow-up 0.949; p<0.01 for each). Cervical alignment and coexisting degeneration did not affect the relationship between C2 and C7.
Conclusion: The C2 plumb line can be used reliably to monitor sagittal balance when C7 is not clearly visualized. High correlation exists in deformity and post-operative patients.