INTRODUCTION: Various studies have shown that intervertebral disc degeneration, lumbar stability, flexibility and low back pain are interrelated [1]. Hence, the relationship of lumbar spine flexibility to different grades of disc degeneration is of great interest in clinics. Finite element (FE) modeling has emerged in literature as an advantageous approach to study spinal biomechanics. Although complex exact FE modeling is reliable, its clinical application has been limited as it is time-consuming and constrained to the input geometry, typically based on one or few subjects. Therefore, the objective of this study was (1) to develop a validated parametric subject-specific FE model which can automatically update the geometry of the lumbar spine from different patients; and (2) to evaluate the influence of disc degeneration on the biomechanical response of lumbar spine. 

METHODS: The geometry of the lumbar spine (L1-S1) was automatically updated by inputting 125 independent parameters using a user-defined code based on defined geometrical constraints obtained from lateral and AP X-Ray images (Fig. 1A). Parametric lumbar FE model consisted of 5 vertebrae, sacral component, 5 IVDs, 7 ligaments and 5 pairs of facet joints and was developed using ABAQUS (SIMULIA, Providence, RI, USA). Five subject-specific FE models were developed based on images of 5 patients from data bank of the Chang Gung Memorial Hospital. The validity of the FE models was evaluated using pure moments of 1 to10 Nm in different directions, subsequent to a compressive preloading with a follower load to mimic the upper body weight. Further, parametric FE models of those lumbar spines were regenerated for L4-L5 disc degeneration in different grades (i.e., healthy, mild, moderate, and severe degeneration), in which the disc height reduction and the increasing of nucleus’ compressibility were considered (Fig. 1B) [2]. Biomechanical responses including motion patterns, intradiscal pressure (IDP) and experienced stress in different components were analyzed under the same loading and boundary conditions. The extracted ranges of motion (ROM) were compared using one-way ANOVA. The differences were considered significant at p<0.05.

RESULTS SECTION: The results of the intersegmental ROMs for healthy models were consistent with available experimental data in literature (Figure 1C) [3]. As well, the estimated IDP was in alignment with reported data in experiments [4]. The ROM was significantly decreased in moderate and severe degenerated disc level (i.e., L4-L5) in all directions. However, the ROMs of the adjacent levels were only altered in severe degeneration (Fig 1D). The IDP significantly decreased in severe degenerated level for flexion and extension but relevant experienced stress significantly increased in all directions (Table1). 

DISCUSSION: This work aimed to develop a parametric subject-specific FE modeling to provide an opportunity for assessing lumbar spine biomechanics in a wide range of individuals. Further, the feasibility of using this FE modeling in clinics was analyzed by simulating the effect of different degrees of disc degeneration on the biomechanical response of lumbar spine. We found that this novel validated subject-specific FE model provides a potential valuable tool for noninvasive time and cost effective analyses of lumbar spine biomechanical (i.e., kinematics and kinetics) changes associated with various diseases. The results show that increased severity of disc degeneration leads to decreased ROM for both degenerated and adjacent levels. Additionally, this study has demonstrated that higher degrees of degeneration reduce IDP but increase stress distribution. However, the variations are highly dependent on the degree of spine degeneration, and the direction of movement.

SIGNIFICANCE/CLINICAL RELEVANCE: This study provided a methodology for clinicians to use quantitative data towards subject-specific clinical evaluation and surgical planning. It shows that moderate and severe degeneration alter both intersegmental motions and load-sharing of the lumbar spine.

REFERENCES: [1] Mimura et al., (1994), Spine, 19 (12) p 1371-80.; [2] Schmidt et al, (2007), Clinical Biomechanics, 22 p 988-98.; [3] Panjabi et al., (1994), J Bone Joint Surg., 76 p 413-24.; [4] Wilke et al., (2001), J Biomechanics, 16 (S1) p S111-26.

ACKNOWLEDGEMENTS: The authors thank for the funding supported from the Chang Gung Memorial Hospital Research Program (CRRPG3E0131-2).

0820 - Different Degrees of Disc Degeneration Alter the Multidirectional Motions and Load-Sharing of the Lumbar Spine - Parametric Subject-Specific Finite Element Simulations