General Session: Biomechanics
Presented by: H.W.D. Hey - View Audio/Video Presentation (Members Only)
H.W.D. Hey(1), K.A. Tan(1), C.S.E. Neo(1), E.T.C. Lau(1), D.A.W. Choong(2), L.L. Lau(1), G.K.P. Liu(1), H.K. Wong(1)
(1) National University Health System, Orthopaedic Surgery, Singapore, Singapore
(2) National University of Singapore, Yong Loo Lin School of Medicine, Singapore, Singapore
Purpose of the Study: Adult spinal deformity correction sometimes involves long posterior pedicle screw constructs extending from the lumbosacral spine to thoracic vertebra. As fusion obliterates motion and places supraphysiological stress on adjacent spinal segments, it is crucial to ascertain the ideal upper instrumented vertebra (UIV) in order to minimize risk of proximal junctional failure (PJF). T10 is often chosen to allow bridging of the thoracolumbar junction into the immobile thoracic vertebrae on the basis that it is the lowest immobile thoracic vertebra strut by the rib cage. The purpose of this study is to characterize the range of motion (ROM) of each vertebral segment from T7 to S1 in order to determine if T10 is truly the lowest immobile thoracic vertebrae. Describe the methods used: This is a prospective, comparative study. 79 adults (mean age of 45.4 years) presenting with low back pain and/or lower limb radiculopathy, without previous spinal intervention, metastases, fractures, infection, or congenital deformities of the spine were recruited. Lumbar flexion-extension and neutral erect radiographs were obtained in randomized order using a slot scanner. Segmental ROM was measured from T7/T8 to L5/S1 and analyzed using t-tests for significant differences across two vertebral segments defined as >5° by the Cobb method. Age, gender, radiographical indices such as standard spinopelvic parameters, sagittal vertical axis (SVA), C7-T12 SVA, T1-slope, thoracic kyphosis (TK) and lumbar lordosis (LL) were studied via multivariate analysis to identify predictive factors for >5° change in ROM at the various segmental levels. There were no sources of funding and no conflicts of interests associated with this study. Summarize the
Findings: In the thoracolumbar spine, significant decreases in ROM when comparing to the adjacent caudad segment occurs up to T9/T10 with mean total ROM of 1.98 ± 1.47° (p< 0.001) seen in T9/T10, 2.19 ± 1.67° (p< 0.001) in T10/T11 and 3.92 ± 3.21°(p< 0.001) in T11/T12. The total ROM of T8/T9 (2.53 ± 1.79°) was not significantly different from that of T9/T10 (p=0.261). At the thoracolumbar junction, absence of scoliosis (OR 11.37, p=0.020), high pelvic incidence (OR 1.14, p=0.046), and low T1 slope (OR 1.45, p=0.030) were predictive of ROM >5°. Include statement of
Conclusions: Lumbar spine flexion-extension ROM decreases as it approaches the thoracolumbar junction. T10 is indeed the lowest immobile thoracic vertebra strut by the rib cage and the last significant decrease in ROM is observed at T9/T10, in relation to T10/T11. However, since this also implies that a UIV of T10 would mean there is only one level of fixation above the relatively mobile segment, while respecting other factors that influence UIV selection, we propose T9 as a more ideal UIV to fulfill the biomechanical concept of bridge fixation. However, this decision should still be taken on a case-by-case basis.