Introduction: Computed tomography (CT) with Hounsfield unit (HU) is being used with increasing frequency for determining bone density. Established correlations between HU and bone density have been shown in the literature. Some investigators have used measurements obtained from clinical computed tomography for assessing fracture risk, implant stability, and spinal fusion success without additional examinations. The aim of this retrospective study was to determine the bone density changes of the stabilized and adjacent segment vertebral bodies by comparing HU values before and after lumbar posterior stabilization.

Methods: Sixteen patients who had same diagnosis of lumbar spondylosis and stenosis were evaluated in this study. Same surgical procedures were performed to all of the patients with L2-3-4-5 transpedicular screw fixation, fusion and L3-4 total laminectomy. Bone mineral density measurements were obtained with clinical computed tomography. Measurements were obtained from stabilized (L2, 3, 4, 5) and adjacent segment (L1, S1) vertebral bodies. Densities of vertebral bodies were evaluated with HU before the surgeries and one year after the surgeries. The preoperative HU value of each vertebra was compared with postoperative HU value of the same vertebrae by using statistical analysis (Figure 1, 2).

Results: The HU values consistently decreased after the operations. There were significant differences between the preoperative HU values and the postoperative HU values of the all evaluated vertebral bodies (Table 1, 2).

Conclusions: Lumbar posterior stabilization caused decreases in the bone density of the stabilized and the adjacent segment vertebral bodies. Posterior stabilization and fusion may protect stabilized vertebral segment. Therefore, the vertebra in the adjacent segments seems to be at greater danger. This condition may be one of the major predisposing factor for adjacent segment disease.

Patient Care: In this manner, we can avoid from unending adjacent segment disease.

Learning Objectives: The mineral bone densities of all vertebral bodies in the stabilized and adjacent segments consistently decreased after the operations. Decrease of mineral bone densities in adjacent segments may be major cause of adjacent segment disease. Strengthening vertebral bodies in the adjacent segments can reduce the risk of adjacent segment disease.

References: 1. Johnell O, Kanis JA. An estimate of the worldwide prevalence and disability associated with osteoporotic fractures. Osteoporos Int. 2006;17:1726-33. 2. Marshall D, Johnell O, Wedel H. Meta-analysis of how well measures of bone mineral density predict occurrence of osteoporotic fractures. BMJ. 1996;312: 1254-9. 3. Silva MJ, Keaveny TM, Hayes WC. Computed tomography-based finite element analysis predicts failure loads and fracture patterns for vertebral sections. J Orthop Res. 1998;16:300-8. 4. Prevention and management of osteoporosis. World Health Organ Tech Rep Ser. 2003;921:1–164. 5. Lewiecki EM, Keaveny TM, Kopperdahl DL, Genant HK, Engelke K, Fuerst T, Kivitz A, Davies RY, Fitzpatrick LA. Once-monthly oral ibandronate improves biomechanical determinants of bone strength in women with postmenopausal osteoporosis. J Clin Endocrinol Metab. 2009;94:171-80. 6. Lund T, Oxland TR, Jost B, Cripton P, Grassmann S, Etter C, Nolte LP. Interbody cage stabilisation in the lumbar spine: biomechanical evaluation of cage design, posterior instrumentation and bone density. J Bone Joint Surg Br. 1998;80:351-9. 7. Hasegawa K, Abe M, Washio T, Hara T. An experimental study on the interface strength between titanium mesh cage and vertebra in reference to vertebral bone mineral density. Spine (Phila Pa 1976). 2001;26:957-63. 8. Lim TH, Kwon H, Jeon CH, Kim JG, Sokolowski M, Natarajan R, An HS, Andersson GB. Effect of endplate conditions and bone mineral density on the compressive strength of the graft-endplate interface in anterior cervical spine fusion. Spine (Phila Pa 1976). 2001;26:951-6. 9. Turkyilmaz I, Sennerby L, McGlumphy EA, T¨oz¨um TF. Biomechanical aspects of primary implant stability: a human cadaver study. Clin Implant Dent Relat Res. 2009;11:113-9. 10. Eswaran SK, Gupta A, Adams MF, Keaveny TM. Cortical and trabecular load sharing in the human vertebral body. J Bone Miner Res. 2006;21:307-14. 11.Schreiber JJ, Anderson PA, Rosas HG, Buchholz AL, Au AG : Houn¬sfield units for assessing bone mineral density and strength : a tool for osteoporosis management. J Bone Joint Surg Am 2011; 93 : 1057-1063. 12. Tay WL, Chui CK, Ong SH, Ng AC : Osteoporosis screening using areal bone mineral density estimation from diagnostic CT images. 2012; Acad Ra¬diol 19 : 1273-1282. 13.Watanabe K , Lenke LG , Bridwell KH , et al. Proximal junctional vertebral fracture in adults after spinal deformity surgery using pedicle screw constructs: analysis of morphological features . Spine 2010 ; 35 : 138 – 45 . 14. Kim YJ , Bridwell KH , Lenke LG , et al. Is the T9, T11, or L1 the more reliable proximal level after adult lumbar or lumbosacral instrumented fusion to L5 or S1? Spine 2007 ; 32 : 2653 – 61 . 15. Shuffl ebarger H , Suk SI , Mardjetko S . Debate: determining the upper instrumented vertebra in the management of adult degenerative scoliosis: stopping at T10 versus L1 . Spine 2006 ; 31 : S185 – 94 . 16. McLain RF . The biomechanics of long versus short fixation for thoracolumbar spine fractures . Spine 2006 ; 31 : S70 – 9 . 17. Kim YJ , Bridwell KH , Lenke LG , et al. An analysis of sagittal spinal alignment following long adult lumbar instrumentation and fusion to L5 or S1: can we predict ideal lumbar lordosis? Spine 2006 ; 31 : 2343 – 52 . 18.Phillips FM , Reuben J , Wetzel FT . Intervertebral disc degeneration adjacent to a lumbar fusion. An experimental rabbit model . J Bone Joint Surg Br 2002 ; 84 : 289 – 94 . 19. Rahm MD , Hall BB . Adjacent-segment degeneration after lumbar fusion with instrumentation: a retrospective study . J Spinal Disord 1996 ; 9 : 392 – 400 . 20. Schlegel JD , Smith JA , Schleusener RL . Lumbar motion segment pathology adjacent to thoracolumbar, lumbar, and lumbosacral fusions . Spine 1996 ; 21 : 970 – 81 . 21. Lee CK . Accelerated degeneration of the segment adjacent to a lumbar fusion . Spine 1988 ; 13 : 375 – 7 . 22. Dekutoski MB , Schendel MJ , Ogilvie JW , et al. Comparison of in vivo and in vitro adjacent segment motion after lumbar fusion . Spine 1994 ; 19 : 1745 – 51 .