Background
Posterior lumbar interbody fusion (PLIF) is the most common surgery for degenerative lumbar disease (DLD), such as lumbar disc herniation and lumbar spinal stenosis. However, PLIF achieves the clinical efficacy at the expense of mobility of the affected lumbar spine segments, increasing the load to adjacent segments and accelerating degenerative disc disease and facet joint degeneration, which further increases the risk of adjacent segment degeneration (ASD) [
1‐
4]. The incidence of ASD, following open or MIS lumbar instrumented fusions, ranged up to 30% [
5]. Some scholars [
1‐
3,
6‐
9] have found that the risk of ASD is related to various factors, including advanced age, increased body mass index (BMI), cross-sectional area of paraspinal muscles, pre-existing degeneration of adjacent discs, facet injury or tropism, the length of lumbar fixation, lower sacral slope, and post-operative sagittal alignment.
A non-fusion interspinous device, such as Coflex, is a known alternative technique to PLIF [
10]. It preserves a certain degree of vertebral activity, which reduces the incidence of ASD [
4,
10,
11]. Previously biomechanical and clinical literatures show that Coflex is a safe and effective treatment for DLD [
11,
12]. Topping-off surgery (PLIF combined with dynamic stabilization of interspinous space of Coflex in the superior segment) is designed to prevent ASD [
13,
14]. The protective effect of this approach was provided by Coflex insertion [
4,
11], and few studies are concerned with the clinical mechanism of the protective effect on the superior adjacent segment. Thus, the purpose of this retrospective study was to compare the clinical outcomes and radiographic parameters between Topping-off surgery and PLIF for DLD. In addition, we should investigate the working mechanism about the degeneration of superior adjacent segment from the radiographs.
Discussion
PLIF is the major treatment for DLD (e.g., spinal canal stenosis, lumbar disc herniation), though usually the expense of spinal mobility at the operated segment, which causes a compensatory increase in the mobility of adjacent segments and increases the risk of ASD [
1‐
3,
6‐
9]. Nancy [
5] reported that the incidence of ASD after PLIF is even as high as 30%, which is similar to that of ASD after PLIF in our study. The biomechanics study [
12,
17] has shown that due to the affected segment fixed, the mobility and stress of the adjacent segment increase as a result, leading to an apparent increase in the slipping and flexion and extension mobility than in the normal conditions. This finally leads to an increased risk of ASD. According to Alentado et al. [
3], 9% of the patients develop symptomatic ASD within 2 years after PLIF and require a second surgery. Therefore, preventing ASD is of high clinical significance.
Topping-off surgery, or PLIF combined with Coflex insertion in the superior segment, has been applied to DLD. Some researchers have shown that a similar effect is achieved for the segment with Coflex insertion as with PLIF [
4,
11‐
13,
18‐
20]. In our study, VAS and ODI scores were improved significantly at 3 years after surgery than before in both groups (
P < 0.05). That is to say, Topping-off and PLIF can both relieve the symptoms. The former had a significant reduction in surgical time and intraoperative blood loss than the latter (
P < 0.05). This is probably because there is no need to expose the intact facet joint and transverse process for Coflex insertion, but exposure of medial 1/2 of the facet joint was required. Moreover, no pedicle screw for internal fixation and bone graft in intervertebral space or between the transverse processes is needed, and that explains the reduction in surgical time and intraoperative blood loss. Patients with DLD are usually of an elder age. In the present study, the average age of patients in the Topping-off group was 53.5 years old and that of the PLIF group was 65.3 years old. These patients are more likely to have underlying diseases, such as hypertension and diabetes. Thus, less surgical time and intraoperative blood loss will contribute to safety and postoperative recovery. When Topping-off surgery was performed, severe intervertebral disk degeneration, instability, slippage, or isthmus fracture should be carefully watched for segments with Coflex insertion, so as to prevent failure of the implant.
Compared with fixed lumbar vertebrae after PLIF, the segment with Coflex insertion maintains a certain mobility. People have been working until 60 years old in our country, and it is very important for them to reserve lumbar activity. In this study, Topping-off surgery was chosen before 60 years old in order to remain maximize lumbar motion. In contrast, we chose PLIF when patients were older than 60 years old. Due to partial activity of the segment of Coflex insertion, there was a reduction in spinal stress to the superior adjacent segment, which is conducive to reducing the risk of ASD [
4,
11,
18‐
23]. Biomechanical studies [
10,
12,
17] have shown that the Coflex device exhibits excellent compression stiffness and tensile stiffness, thus providing good stability to the lumbar spine. In our study, Topping-off surgery involves partial resection of ligamentum flavum and articular process, enlargement of the nerve root canal, removal of the herniated disk or nucleus pulposus for the decompression of dural mater, and loosening of the nerve root. Special attention should be paid to protecting the lateral half of the articular process in the segment of Coflex insertion. The aim was to preserve the stable structure of the posterior spine as far as possible. With partial preservation of flexion and extension mobility, the spinal stress will not be excessively concentrated in the superior adjacent intervertebral space, which is conducive to protecting adjacent intervertebral space [
4,
12,
17].
In healthy subjects, all segments are involved in the overall flexion and extension mobility of the lumbar spine. With any segment immobilized, the other segments will have a compensatory increment of mobility [
1,
19]. In our study, we observed the changes in adjacent segment mobility by X-ray in the standing flexion and extension views. For the PLIF group, after complete restriction of mobility at L3–L5, the mobility of L2–L3 at 3 years after surgery was increased significantly than before (
P < 0.05). For the Topping-off group, with the complete restriction of mobility at L4–L5, Coflex insertion at L3–L4 had a certain reduction in mobility than before (
P < 0.05), but part of the mobility was still preserved. For the decline of mobility, this is because the inserted Coflex pushes aside the spinous process and stabilizes the posterior column, thus increasing the foraminal height [
14] and partially preventing posterior stretch of the spine. So Coflex insertion has a much less impact on spinal flexion [
4,
12]. Elastic immobilization ensures that Coflex has a certain mobility [
10] after its insertion. Because of the buffer provided by Coflex, the superior L2–L3 preserved similar mobility as before (
P > 0.05). Our study indicated that the intervertebral mobility (L2–L3) in the Topping-off group was much lower than that with PLIF (
P < 0.05), which was suggestive of the protective effect from the Coflex to the adjacent segment at L2–L3.
Surprisingly, we observed no significant difference between GASM in the Topping-off group and adjacent intervertebral mobility (L2–L3) in the PLIF group (
P > 0.05). The reason is probably that the original intervertebral mobility of the adjacent segment at L2–L3 if L3–5 was fusion was shared by both L3–L4 with Coflex insertion and the adjacent segment at L2–L3 if Topping off surgery was performed, thus reducing the risk of ASD at L2–L3. The intervertebral mobility (L2–L3) was above 10° in 2 cases from the PLIF group, which satisfied the criteria for ASD on X-ray. Three cases from the Topping-off group had GASM above 10°, but the mobility was shared by two segments, with each below 10°, which did not meet the criteria for ASD on X-ray. To more clearly detect intervertebral disc degeneration after surgery, lumbar MRI scan was performed for L2–L3 at 3 years after surgery. As expected, the incidence of ASD in the PLIF group, which was similar to the results reported in the literatures [
5,
7‐
9], was much higher than that of the Topping-off group (25.93% vs 4.44%,
P < 0.05). The reason is probably that the inserted Coflex partially undertook the mobility and stress of the proximal lumbar spine, which reduced the incidence rate of ASD.
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