Optimization of a lumbar interspinous fixation device for the lumbar spine with degenerative disc disease

Abstract

Interspinous spacer devices used in interspinous fixation surgery remove soft tissues in the lumbar spine, such as ligaments and muscles and may cause degenerative diseases in adjacent segments its stiffness is higher than that of the lumbar spine. Therefore, this study aimed to structurally and kinematically optimize a lumbar interspinous fixation device (LIFD) using a full lumbar finite element model that allows for minimally invasive surgery, after which the normal behavior of the lumbar spine is not affected. The proposed healthy and degenerative lumbar spine models reflect the physiological characteristics of the lumbar spine in the human body. The optimum number of spring turns and spring wire diameter in the LIFD were selected as 3 mm and 2 turns, respectively-from a dynamic range of motion (ROM) perspective rather than a structural maximum stress perspective-by applying a 7.5 N∙m extension moment and 500 N follower load to the LIFD-inserted lumbar spine model. As the spring wire diameter in the LIFD increased, the maximum stress generated in the LIFD increased, and the ROM decreased. Further, as the number of spring turns decreased, both the maximum stress and ROM of the LIFD increased. When the optimized LIFD was inserted into a degenerative lumbar spine model with a degenerative disc, the facet joint force of the L3-L4 lumbar segment was reduced by 56%-98% in extension, lateral bending, and axial rotation. These results suggest that the optimized device can strengthen the stability of the lumbar spine that has undergone interspinous fixation surgery and reduce the risk of degenerative diseases at the adjacent lumbar segments.

Fig 1

Finite element models of (a) healthy lumbar spine, (b) degenerative lumbar spine with degenerative disc disease, and (c) lumbar spine inserted by the lumbar interspinous fixation device (LIFD).

Fig 2. Components of the lumbar interspinous fixation device (LIFD).

Fig 3

(a) Range of motion (ROM) of the healthy lumbar spine from entire L1 to L5 segments (i.e., healthy L1-L5 lumbar spine) under pure 7.5 N∙m moment (The red bar is the median value of previous in vitro experimental findings, and other bars represent previous and current findings of finite element analyses). (b) ROM of the healthy L1-L5 under pure moment of 0 to 7.5 N∙m (The red range bar represents previous findings of in vitro experiments under 7.5 N∙m moment). (c) Facet joint forces (FJFs) of the healthy L1-L5 under pure 7.5 N∙m moment (The red bar is the median value of previous in vitro experimental findings, and other bars represent previous and current findings of finite element analyses). (d) Nucleus pulposus pressure of the L4-L5 intervertebral disc under follower load of 0 to 1000 N (The red range bar represents previous findings of in vitro experiments under 300 N and 1000 N follower loads).

Fig 4

Range of motions (ROM) between two healthy lumbar spine segments was measured under a combination of (a) flexion moment (7.5 N∙m) and follower load (1175 N), (b) extension moment (7.5 N∙m) and follower load (500 N), (c) lateral bending moment (7.8 N∙m) and follower load (700 N), and (d) axial rotation moment (5.5 N∙m) and follower load (720 N) (The red bar is the median value of previous in vivo experimental findings, and other bars represent previous and current findings obtained from finite element analyses).

Fig 5

Nucleus pulposus pressure of the L4-L5 intervertebral disc measured under a (a) flexion moment (7.5 N∙m) and follower load (1175 N), (b) extension moment (7.5 N∙m) and follower load (500 N), (c) lateral bending moment (7.8 N∙m) and follower load (700 N), and (d) axial rotation moment (5.5 N∙m) and follower load (720 N) (The red bar is the median value of previous in vivo experimental findings, and other bars represent previous and current findings obtained from finite element analyses).

Fig 6

Facet joint force (FJF) of the two healthy lumbar spine joints under a combination of (a) flexion moment (7.5 N∙m) and follower load (1175 N), (b) extension moment (7.5 N∙m) and follower load (500 N), (c) lateral bending moment (7.8 N∙m) and follower load (700 N), and (d) axial rotation moment (5.5 N∙m) and follower load (720 N).

Fig 7. Range of motion (ROM) of the degenerative L3-L4 lumbar spine with degenerative disc disease under pure moment (10 N∙m) in flexion–extension, lateral bending, and axial rotation.

Fig 8

(a) Main effects plot of the means and (b) main effects plot of the S/N ratios (larger is better) for the maximum stress in the LIFD.

Fig 9

(a) Main effects plot of the means and (b) main effects plot of the S/N ratios (larger is better) for the Range of motion (ROMO of the L3-L4 lumbar spine segment.

Fig 10

Finite element analysis results of the healthy L3-L4 lumbar spine, the degenerative L3-L4 with degenerative disc disease, the L3-L4 lumbar spine inserted by the lumbar interspinous fixation device (LIFD) in flexion (1175-N follower load and 7.5- N∙m moment), extension (500 N follower load and 7.5 N∙m moment), lateral bending (700 N follower load and 7.8 N∙m moment), and axial rotation motion (720 N follower load and 5.5 N∙m moment): (a) range of motion (ROM) (b) facet joint force (FJF).