Biomechanical Analysis of Osteotomy Type (OWO, CWO) and Rod Diameter for Treatment of Cervicothoracic Kyphosis

Proceedings of the NASS 25th Annual Meeting / The Spine Journal 10 (2010) 1S–149S (including bending films) and post-operative standing AP and lateral radiographs to evaluate the magnitude of the thoracic major curve and the lumbar secondary curve. Pre-operative and post-operative clinical rib hump measurements were performed. Univariate ANOVA was performed on the results to determine statistical significance. RESULTS: The pre-operative major thoracic Cobb angle measured on average 53.8 (40 to 75 ). The pre-operative secondary lumbar Cobb angle measured on average 43.9 (34 to 55 ). On bending films, the secondary curve corrected to 11.3 (0 to 35 ). The rib hump measured 15.0 (7 to 21 ). At latest follow-up the major thoracic Cobb angle measured on average 27.2 (20 to 41 ) (p!0.001) and the mean secondary lumbar curve was 27.3 (15 to 42 ) (p!0.001). This represented a correction factor of 37.8%. The rib hump measured 6.5 (2 to 15 ) at last follow-up (p!0.001). The figures demonstrated were comparable to published series when open surgery (both anterior and posterior approach) was performed. CONCLUSIONS: VATS is an effective method of correcting major thoracic curves with secondary lumbar curves resulting in a balanced spine. The behavior of the secondary lumbar curve is consistent with published series when open surgery, both anterior and posterior, is performed. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2010.07.305

P30. Biomechanical Analysis of Osteotomy Type (OWO, CWO) and Rod Diameter for Treatment of Cervicothoracic Kyphosis Justin K. Scheer, BS1, Jessica A. Tang1, Vedat Deviren, MD1, Jenni M. Buckley, PhD1, Murat Pekmezci, MD1, R. Trigg McClellan, MD1, Christopher P. Ames, MD2; 1UCSF/SFGH Biomechanical Testing Facility, San Francisco, CA, USA; 2University of California San Francisco, Department of Neurological Surgery, San Francisco, CA, USA BACKGROUND CONTEXT: Sagittal imbalance of the cervicothoracic spine often causes severe pain and loss of horizontal gaze. Traditionally, C7 opening wedge osteotomy (OWO) has been performed for patients with ankylosing spondylitis. For patients without ankylosing spondylitis, closing wedge osteotomy (CWO) may be considered for more controlled closure. Biomechanical characteristics of the two osteotomy alternatives have not yet been analyzed. PURPOSE: The goal of this study is to characterize the structural stability of the two types of cervicothoracic junction osteotomies and the independent effect of rod diameter. STUDY DESIGN/SETTING: Biomechanical pure moment testing on cadaveric cervicothoracic specimens with each receiving either an OWO or CWO and either a 3.5 mm or 4.5 mm diameter rod. PATIENT SAMPLE: 14 fresh-frozen human spines (M/F58/6; 60610y.o.; C3-T6). OUTCOME MEASURES: Maximum range-of-motion (ROM) in each anatomical bending direction. METHODS: Each specimen underwent sagittal alignment and posterior bilateral screw-rod fixation with lateral mass screws (4.0x16 mm) from C4-C6 and pedicle screws (4.0 x 34 mm) from T1-T3. Two different posterior spinal fusion rods were tested for each specimen: 3.5 mm Ti and 4.5 mm Ti. All instrumentation and insertion tools were standard (Ulrich Medical). Half of the specimens received OWO at C7 while the other half received CWO for a correction angle of approximately 30 . Non-destructive flexion/extension (FE), lateral bending (LB), and axial rotation (AR) tests were conducted to 4.5 Nm in each anatomical direction. 3D motion tracking was used to monitor primary range-of-motion (ROM) across the entire fixation construct (C4-T3) and localized to the osteotomy (C6T1). Results were expressed in terms of increase in bending rigidity (equivalent to decrease in ROM). RESULTS: Independent of osteotomy type, constructs instrumented with 4.5 mm rods exhibited a significant increase in rigidity compared to the 3.5 mm rods in all bending modes (p!0.01). Specifically, relative to

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3.5 mm rods, 4.5 mm constructs showed 31612% greater rigidity in FE, 37639% in LB, and 31611% in AR. Localized to the osteotomy site, there was a 43623% increase in FE rigidity, 45636% in LB, and 41617% in AR. Independent of rod diameter, CWO was significantly stiffer than OWO in FE bending only (p!0.05). Relative to OWO, CWO demonstrated 42% greater rigidity in FE for the entire fusion construct and 56% increase localized to the osteotomy site. CONCLUSIONS: Results indicate that CWOs are more mechanically stable than OWOs in flexion-extension and, as anticipated, increasing rod diameter from 3.5 mm to 4.5 mm results in a non-trivial increase in rigidity in all modes of bending. OWOs and CWOs demonstrated similar sensitivity to changes in rod diameter, meaning that the surgeon can expect a similar increase in construct rigidity in switching from 3.5 mm to 4.5 mm rod independent of osteotomy type. The increased stiffness observed in specimens receiving closing wedge PSOs has an anatomical basis. Compared to OWOs, CWOs leave the anterior longitudinal ligament (ALL) intact, along with both discs above and below the osteotomy site. OWOs, in contrast, result in disruption of the ALL and leave a significant anterior gap. Since CWOs are performed with a wedge through the vertebral body, closure leaves no anterior gap providing greater axial loading stability. This greater bone on bone contact in CWOs is likely a significant reason for the anterior stiffness and may provide greater fusion rates in the non ankylosing spondylitis patient population. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. doi: 10.1016/j.spinee.2010.07.306

P31. Importance of Sagittal Spinal Alignment for the Prevention of Vertebral Fracture Cascade Tetsuya Kobayashi, MD, PhD, Kiyoshi Aono, MD, Issei Senoo, MD, Yuji Atsuta, MD, PhD, Takeo Matsuno, MD, PhD; Asahikawa Medical College, Asahikawa, Japan BACKGROUND CONTEXT: Physiological spinal curvature plays an important role in absorbing axial stress onto spinal column, and its modification has been shown to increase the incidence of vertebral compression fracture (VCF). The fact that VCF itself increases the risk of subsequent fracture propels physicians to predict the risk of first VCF, and evaluation of sagittal spinal alignment might be helpful in this scope. PURPOSE: To investigate the relationship between sagittal spinal curvature and the incidence of VCF in subjects without baseline fractures in a prospective cohort. STUDY DESIGN/SETTING: This study was a component of the ASAP study, an ongoing prospective cohort study since 1983. PATIENT SAMPLE: 228 community-based female volunteers aged over 40 years, recruited from population register. OUTCOME MEASURES: Radiographic measurements of sagittal spinal parameters and evaluation of VCF. METHODS: Among 228 female volunteers, final total of 198 subjects without VCF at baseline upright entire spine radiograph were included and followed for more than 8 years. Serial entire spine radiographs were taken and used to identify radiographic VCF using both quantitative and semi-quantitative method. Radiological measurements included thoracic kyphosis (T4-12) and lumbar lordosis (L1-5) measured by Cobb method. Grade of osteoporosis was also evaluated using modification in vertebral trabecular pattern. Sagittal spinal alignment was classified using mean value and standard deviation (SD). RESULTS: During mean 12.0-year observation, 52 VCF occurred in 30 subjects. In univariate analyses, age, grade of osteoporosis, 1 SD decrease in T4-12 and L1-5 angles were identified as risk factors. In multivariate analyses, 1 SD decrease in L1-5 (RR 3.106, 95% CI 1.193–8.084, p50.0202) was an independent predictor of VCF, as well as the grade of osteoporosis.

All referenced figures and tables will be available at the Annual Meeting and will be included with the post-meeting online content.