Does stabilization of the degenerative lumbar spine itself produce multifidus atrophy?
Introduction
Instability of the spine may have several causes, including degenerative disease, trauma, or cancer. For the treatment of instability of the spine, spinal stabilization through several types of instruments is the major surgical option. However, subsequent mechanical change and physiological alteration can cause a secondary problem in the spinal column. Furthermore, several clinical studies have shown that iatrogenic injuries to the paraspinal muscles during posterior lumbar surgery cause a reduction in their cross-sectional area over time post-surgery [1], [2], [3]. Dissection and retraction of the paraspinal musculature could lead to the denervation and atrophy of paraspinal muscles. However, minimally invasive transforaminal lumbar interbody fusion and anterior lumbar interbody fusion also produce back musculature atrophy [2], [4]. This finding suggests that factors inducing back musculature atrophy include not only direct invasion of the back muscle, but also postoperative changes in muscle activity depending on stabilization. Although the reason for this change in muscle activity is unknown, this alteration in muscle activity could be regarded as adaptation to changes in spinal stability.
The mechanoreceptors embedded in the disc play a key role in the perception of joint status and adjustment of the muscle tone of the vertebral column [5]. This enhances spinal function and protects the spinal joints against additional injury and future degenerative processes. Based on this concept, we have already proposed computational models in which the intervertebral disc has a transducer function and the paraspinal muscles are activated according to a sensor-driven control mechanism to maintain the stability of the spine [6], [7]. The muscle forces calculated in these studies provided a fairly even resisting force to the flexion moment generated by the trunk weight, and the activity level in some superficial muscles matched well with previously reported EMG data. According to this concept, the eliminated or reduced motion at the stabilized segment for the treatment of instability of the degenerated spine will generate a different mechanical signal than those in intact segments, and the paraspinal muscles will change their activity to minimize the alteration in joint status. This could explain the postsurgical multifidus atrophy over time by some mechanism other than the direct invasion of the back muscle. In this study, the paraspinal muscle activity change after stabilization of a degenerative disc was analyzed in erect standing and flexed postures.
Section snippets
Materials and methods
A previously developed musculoskeletal model was used for this analysis. The model consisted of a validated finite element (FE) lumbar spinal column model, simplified trunk and the pelvis model, and model of 98 muscle fascicles [7]. In the current model, the lumbar spinal column FE model was slightly modified in the direction of the fiber forming the interspinous ligament. The motion characteristics of the modified model were almost identical to those of the previous model, and other verified
Results
The simulation shows that stabilization produced changes in the pattern of the paraspinal muscles, while the trunk position was maintained as in the intact case. Compared with the intact case, fusion generated a 12% reduction in the total multifidus muscle force in an erect standing posture, and its reduction was 6.6% after PBDS application. However, difference in the reduction of multifidus muscle force between the stabilization types decreased in 20° flexed posture. Reductions of 10.5% and
Discussion
Most previous studies dealing with paraspinal muscle changes after stabilization at one or more levels of the spine have focused only on postoperative multifidus atrophy. In those analyses, it was pointed out that muscular damage associated with the open dorsal approach to the lumbar spine produced a reduction in the cross-sectional area of the multifidus muscle. This was directly related to the muscle retraction time during surgery, the large area of muscle stripping and mechanical and heat
Conclusions
The present study explored, for the first time, the effect of stabilization on changes in the pattern of paraspinal muscle activity using a computational model in which paraspinal muscles were activated through a sensor-driven control mechanism. It is not yet possible to validate directly these results in the human spine in vivo; however, the results of this computational study could allow us to explain the degeneration of the multifidus muscle after stabilization. Compared with the intact
Conflict of interest
The authors declare that there is no conflict of interest.
Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1A2A2A01008329).
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