| Background:Lumbar degenerative disease is common in older peoples, and its incidence is high up to90%in those over the age of60years. Patients with poor effect of conservative therapy will need operative treatment. Posterior lumbar surgery is the most used approach of dealing with the lumbar degenerative disease, for that the intervertebral disc resected, the spinal canal decompressed and the intervertebral segment fused are currently the main operative procedures. However, reported in the literature, although the operation itself solves the nerve compression which usually cause pain and disability, the iatrogenic injury of paraspinal multifidus muscle can be the new source of back pain and disability in those postoperative patients, sometimes the postoperative symptoms are even more severe than the those preoperatively, which is called" fusion disease ", or" failed back syndrome" in literature. Therefore studying the mechanisms of iatrogenic multifidus muscle injury, and how to reduce this injury, have become an important topic in international spinal surgery area. Clinical reports have shown dissection, retraction and lumbar segmental fusion may be the main reasons of postoperative injury and atrophy in multifidus, but there is still lack of definite evidence to support these conclusions. Although animal studies have reported the retraction or fusion is the important reason of multifidus injury or atrophy, there is still some shortcomings:(1) lack of relative research about dissection;(2) lack of single factor-effect animal models;(3) potential mechanism of injury and atrophy has not been elucidated.Objective:1. Through animal lumbar spine anatomic study, to analyze the anatomic basis of multifidus muscle injury and atrophy in posterior lumbar surgeries, and to design the single factor-effect animal models of dissection, retraction and segmental fusion;2. To observe the morphological changes of the multifidus muscle after posterior lumbar spine surgery using MRI and histological methods at different postoperative time points through single factor-effect animal models; to analyze the process and related factors of multifidus injury; to analyze the effect of oxidative stress in the early stage of multifidus muscle injury, with measuring the contents of MDA, SOD and GSH.3. To observe the morphological changes of the multifidus muscle after posterior lumbar spine surgery using MRI and histological methods at different postoperative time points through single factor-effect animal models; to analyze the regeneration and remodeling process after injury and the related factors of atrophy; to investigate the denervation mechanism in multifidus atrophy through observing the motor endplate activity.Methods:1.8New Zealand rabbits (4male and4female) were selected. Preliminary analysis of bony structure of lumbar spine was taken by X-ray and CT scan. Distribution and structure of paraspinal muscles were preliminarily analyzed by MRI. Careful dissection of lumbar spine is taken according to the imaging results, to observe the distribution, course, and innervation of paraspinal muscles. As a result, the single factor-effect animal models were designed.2.36male rabbits were randomly divided into four groups:control group (group A), dissection group (group B),1-hour retraction group (group C) and2-hour re traction group (group D),9rabbits in each group. Muscle edema were evaluated by MRI T2weighted image and tissue wet/dry weight ratio at three different time points (3h,48h, and7d postoperatively). Histological edema, hemorrhage, necrosis and inflammatory cell infiltration were evaluated by HE staining, and fiber distribution was evaluated by NADH-TR staining at the same time. And oxidative stress was evaluated by MDA, SOD and GSH measurements.3.72male New Zealand white rabbits were randomly divided into six groups: control group (group A), dissection group (group B),1-hour retraction group (group C),2-hour retraction group (group D), anterior inter-body fusion group (group E) and posterolateral inter-transverse process fusion with internal fixation group (group F),12rabbits in each group. Fatty infiltration was assessed by MRI T1weighted coronal image at four different time points (3-,6-,12-, and24-week). Tissue composition was evaluated by HE and Masson staining (fibrous degeneration and fatty degeneration) at the same time. Fiber distribution was observed by ATPase staining, and neuromuscular junction was evaluated by acetylcholinesterase staining at the same time.Results:1. X-ray and CT scan showed the rabbit lumbar spine has7vertebrae with long and narrow shape, and the peak of iliac crest roughly located at the level of the bottom of L6vertebral body, near the position of the L6/7intervertebral disc. The mamillary process is long, and the transverse process grows to the ventral side. MRI showed paraspinal muscle distribution is similar to the human's, and the boundary of multifidus muscle can be clearly identified. The sacrospinalis group consists mainly of the multifidus medially, longissimus intermedially and iliocostalis laterally, separated by the fascial compartments. The multifidus muscle can be separated into superficial and deep muscle fibers, and the superficial fibers arise from the spinous process, interspinal ligament, and attach to the medial cephalic side of mammillary process, posteromedial side of iliac crest and sacrum, and course over2-3intervertebral segments. The sacrospinalis is innervated with the superior one-level lumbar dorsal rami who projects from spinal nerve innervates and runs dorsocaudally through the intertransverse space, where the rami divided into3branches termed as medial, intermediate and lateral. The medial branch passes dorsally and caudally through the bottom of transverse process and the basement of mamillary process, then turns medially and caudally across the vertebral lamina and lies deep to multifidus.2. MRI T2signal intensity ratio, tissue weighting and histopathological assessment all showed:the edema of group B and D gradually increase after surgery, except for group C, in which the edema reached the maximum at3hours postoperatively, then reduced gradually; and correlations between these three methods are significant (P<0.001). Severe bleedings were observed in group C and D at3hours postoperatively. Different degrees of necrosis, fiber disordered and infiltration of inflammatory cells were observed in group B, C and D, and all are the most serious in group D (P<0.05). Compared to group A, the oxidation index of MDA were significantly higher in group B, C and D throughout the period of observation (P<0.05), and the MDA index of group D was significantly higher than this of group B and C3hours and48hours postoperatively (P<0.05), but there were no significant differences between group B, C and D7days postoperatively. As well, compared to group A, the antioxidant indexes of SOD and GSH decreased much in group B, C and D throughout the period of observation (P<0.05); and the indexes of group D were significantly lower than those of group B and C at3hours and48hours postoperatively (P<0.05), but there were no significant differences between group B, C and D7days postoperatively.3. There were no obvious fibrous and fatty infiltration and degeneration of neuromuscular junction in group A, E and F postoperatively, except that the type â… fibers increased and became hypertrophy significantly in the group F (P<0.05). There were different degrees of fibrous degeneration3and6weeks postoperatively and fatty infiltration at12and24weeks postoperatively in group B, C and D. The histological changes in group B showed an aggravating trend, and the most area of muscle fiber or fibrous tissue was replaced by fat tissue24weeks postoperatively; and AchE staining showed obvious motor endplate degeneration throughout the whole observation period, and most of the endplates were unable to identify at24weeks postoperatively, that indicated denervation of multifidus in group B. The muscle fibers regenerated and remodeled, the fibrosis reduced and the degenerative motor endplates recovered from3weeks to24weeks postoperatively in group C and D, only with a little fatty infiltration24weeks postoperatively. However, throughout the whole period of observation, the fibrosis, fatty infiltration and motor endplate degeneration in group C were significantly lower than those of group D (P<0.05).Conclusion:1. Paraspinal muscles distribution and structure provided anatomic basis for designing the single factor-effect animal model. The anatomic positioning and innervation of multifidus showed the potential reasons of multifidus injury and made it easy to get the specimen accurately.2. Dissection and retraction are the main factors of multifidus injury in posterior lumbar surgery. The injury degree is related with the continuing retraction time in retraction injury, and intermittent relaxation can obviously reduce this injury. Inflammatory cell infiltration and oxidative stress may be the main causes of secondary injury after surgery.3. Dissection is the most important factor for multifidus atrophy after posterior lumbar surgery. Intra-operative nerve injury and denervation after surgery are probably the main reasons of atrophy caused by dissection. Regeneration and remodeling can be observed in retraction injury after posterior lumbar surgery, and the longer continuing retraction time, the more poor ability of regeneration. Two spinal segmental fusions can not cause atrophy of the multifidus muscle after24-week observation postoperatively. Anterior inter-body fusion model is better for studying the pure fusion influence on multifidus muscle after posterior lumbar surgery. |
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