In the current study, we measured the changes of kinematics in spinous processes in patients before and after DLIF at different body positions in axial rotation. We found that, in general, altered kinematics mainly took place in rotation. In the supine position, significant differences were mainly detected in proximal-distal and left-right translation before and after operation. In the standing position, L4-5 exhibited increased extension and left rotation compared to those from before the operation. In the flexion/extension posture, alterations in 6 DOF of the inter-spinous processes (ISP) showed different trends in different DDD patients. Generally, these changes imply that disc degeneration and fusion intervention distinctly correlate to alterations in ISP motions at involved levels [
13].
Only several studies report on the 3D kinematics of the LSP, especially regarding pathological and intervention conditions, such as artificial disc replacement or lumbar discectomy followed by fusion [
6,
8,
14]. Ihm et al. investigated the lumbar ISP distance and demonstrated a declining trend regarding the ISP distance along with increased age [
15]. Sobottke et al. measured the anatomical features of inter-spinous space and spinous processes. They reported that the anterior position is a optimum choice for a stand-alone inter-spinous spacer [
8]. However, the 2D classification methods led to increased result variability due to the limited and inexact identification of the identical anatomic landmarks [
16,
17]. The 2D-3D registration method is a more accurate approach compared to 2D classification methods [
18]. Xia et al. investigated the 3D motion characteristics of ISP distances in healthy subjects and found that changes in ISP distances are positively correlated to vertebral levels and body postures [
1]. Yao et al. reported that, in supine, standing, and extension positions, ISP were physically smaller in patients with DDD than healthy subjects. [
11]. The repeatability is 0.3 mm for translation and 0.6° for rotation. However, we can speculate that ISP distances cannot exactly define the tracking of ISP because ISPD is an indirect measurement method with which to explore ISP kinematics. In the study by Xia et al. [
1], ISPD is defined as the shortest distance between the lowest tip of the inferior spinous process and the highest tip of the superior spinous process. To some extent, it represents the partial change of adjacent joints between spinous processes as only vertical translation was described in this method. Movement in other directions was not evaluated.
No researchers have performed kinematic analysis of spinous processes in patients who have DDD during in vivo weight-bearing movements. Quantitative knowledge about adjacent spinous processes is very important to understanding spinal pathology. It is also very critical to improve the current surgical treatment approaches for spinal diseases. In recent years, it has become very popular to measure the kinematics (including 6DOF and ROM) in adjacent joints, such as knee joints and lumbar inter-vertebrae, using fluoroscopic technology [
10,
19].
Rotation
In our study, we found that ROM in flexion/extension rotation was smaller in post-operation patients at all investigated levels when compared to pre-operation, which is likely to increase the risk of adjacent segmental disorders. Decreased ROM happened in trunk standing, flexion, and extension positions. One of the major disadvantages of anterior lumbar interbody infusion (ALIF) is that it demonstrated abnormal kinematics of the spine, and therefore, may lead to degeneration at adjacent segments. These abnormal alterations potentially require additional fusion. Other potential complications include ileus, vascular injury, and retrograde ejaculation. Thus, in recent years, a lateral trans-psoas approach (DLIF) has been utilized to prevent from the limitations at the involved levels and allows patients to quickly return to routine activities [
20]. In addition, this method can improve spinal instabilities as well as deformities and avoids to perform a posterior approach. Here, we detect alterations in kinematics after DLIF. Except for sagittal rotation, we also found that ROM in left/right bending and transverse rotation decreased. We speculate that this is due to the enhanced tissue strain, facet joint pressures, and increased intradiscal at the levels that are adjacent to the fusion. This motion data concurs with our ISP kinematic alterations between preoperative and postoperative groups.
Translation
Statistical differences were mainly detected in the trunk flexion position. In this position, LSPs at all levels exhibited statistical differences in left/right translation. We found increased left translation in all three ISP. Besides, significant differences were found in left/right rotation at all three levels. Yao et al. reported that, in non-weight-bearing supine, standing, and extension positions, ISP were physically smaller in patients with DDD than healthy subjects [
11]. In the current study, similar trends were exhibited in all groups. Cinotti et al. reported that, compared healthy intervertebral discs, degenerated intervertebral discs result in the reduced height of posterior structures [
20]. DLIF has the advantage of protecting the facet joint, the posterior longitudinal ligament, and the anterior longitudinal ligament, which enables it to improve spinal alignment and stabilization. Therefore, it is likely to replace conventional options of lumbar interbody fusion, like ALIF, as an alternative surgical approach [
21]. DLIF is able to provide strong mechanical stability via a large interbody constructure and the sparing of ligamentous structures [
22]. In our data, these different patterns of kinematic alteration imply that after DLIF, different strategies were shown in different vertebral spinous processes from dysfunctional to re-stabilization conditions.
The results of the current study have some clinical significance regarding the interpretation of LSP in the pathogenesis of low back pain that is not clear. Our study indicates that, for patients with DDD, the ROM of the ISP is reduced during in vivo functional movements. Lately, ISPD has been used as an alternative surgical approach to conventional decompressive surgery for the treatment of DDD and favorable clinical outcomes have been shown [
23,
24]. However, some complications with the use of inter-spinous implants have been reported, such as implant movement [
23‐
25]. Kim et al. argued that after inter-spinous process spacer (IPS) surgery, degenerative lumbar spondylolisthesis is strongly correlated with the prevalence of spinous process fracture [
23]. In the current study, we found that the ROM of ISP change increased for two postures (maximum flexion and extension) after operation. This implies that, after the introduction of inter-spinous implants in DDD patients, hypermobility should be restricted to sustain the position of a loosely-fitted device in case the ISP dislocates. The kinematic characteristics of LSPs provide some biomechanical information that can help with decisions about the use of compressible or rigid devices.