Is the Center of Gravity Located at the Bottom of the C7 Plumb Line?**

• b, e - Medtronic

Introduction: Defining the optimal state of spinal balance is difficult. In hip and knee reconstruction, it is clear that well-aligned joints function well, while mal-aligned joints are susceptible to early degenerative changes. It appears that sagittal alignment may be related to the risk of symptomatic disk disease, spinal stenosis, progression of deformity, or failed surgical intervention. What constitutes normal and pathologic alignment in the spine has not been established in a manner similar to that for the hip or knee. The body’s center of gravity with respect to the spinal column is an important determinant of stability and balance. A plumb line from C7/T1 is said to correlate with the patient’s center of gravity. To test this hypothesis, 120 normal volunteers were x-rayed in a stabilized standing position using a digital force plate. We have also attempted to determine if a geometric relationship between the location of the top of the spine and the pelvis in the sagittal plane exists in normal subjects.

Materials and Methods: 120 young adult volunteers without symptoms of spinal or orthopedic disease were enrolled. The patients were asked to stand on a custom built, rotating, digital force plate while holding two aluminium supports. The force plate recorded the projection of the gravity line at the time of each radiographic exposure. The computer assisted analysis of digital x-rays used in this study has been previously described. The radiographic landmarks of the spine and pelvis and the digital projections of the C7 plumb line and the gravity line were established. The horizontal differences between the gravity line, the C7 plumb line, and the posterior edge of the sacral plate and the centre of the bicoxofemoral axis were measured. Correlations between the geometric parameters of pelvic and lumbar alignment were determined using the Pearson correlation coefficient. Statistical difference was determined by the Student’s T test, p <.05.

Results: The gravity line is located at an average of + 21.03mm from the posterior edge of S1. In contrast, the C7 plumb line is located at an average of - 27.4mm from the same point. The position of the plumb line does not correlate with the position of the center of gravity (R = 0.357, p <.002), demonstrating that they are not co-existant.

Constant geometric relations between the position of the top of the spine and the pelvis exist. Irrespective of the variation of sacral slope from 25° to 64°, T1 is positioned over the center of the sacrum with an angle of 95° +/ - 2° (R = 0.93). This demonstrates that healthy subjects auto-regulate their spinal alignment to position C7- T1 over the sacral endplate as determined by the formula: 99° - 0.1SS.

Conclusions: This study has two significant findings: (i) the C7 plumb line and the gravity line in a stabilized standing position are not coexistant; (ii) the relationship between the center of T1 and the sacral endplate is a constant marker of spinal balance in individuals without symptoms of back pain or radiographic evidence of deformity.

**The FDA has not cleared a drug and/or medical device the use described in this presentation (i.e., the drug or medical device is being discussed in an “off-label use).

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