Study of single-level lumbar degenerative diseases treated by unilateral wiltse access with unilateral nail rod fixation assisted by a new automatic retraction device

The main muscles around the lumbar spine are divided into two groups: the anterior group includes the psoas major and psoas square, and the posterior group consists of the multifidus and sacrospinous muscles. In the postero-lateral region of the lumbar spine, three separate muscle gaps are formed between the multifidus, longissimus, iliopsoas muscle and lumbar square muscle. The longissimus has relatively small muscle fibers and forms a clear muscle gap with the medial multifidus muscle, which can be filled with a small amount of fatty tissue and can be easily separated bluntly without vascular or nerve penetration [11]. The Wiltse approach has been widely used in lumbar spine surgery. The current wiltse muscle gap approach to TLIF surgery is more damaging to the posterior spinal musculature and is not conducive to the preservation of low back muscle function [12]. The new retraction device in our study is designed with an L-shaped support post, which is fixed to the pedicle screw at the caudal end of the segment with no excessive pulling effect on the longest lateral muscle. The support end is designed as an angle-adjustable detent in the axial plane to "push" the multifidus muscle towards the midline. The retractor device is secured once a suitable position has been obtained. The area to be decompressed is clearly exposed. Direct visualization of the affected synovial joint, the vertebral plate and the base of the spinous process minimizes damage to the posterior spinal musculature during surgery, making it fully feasible for a single operator, especially in lower lumbar surgery. The multifidus muscle is tightly attached to both sides of the spinous process of the lumbar vertebrae, and the muscle bundle gradually thickens from the top downwards [13]. At the L1-L3 level, the gap is close to the midline and the small joints and transverse processes of the lumbar spine can be easily revealed, whereas at the L4-S1 level, the Wiltse gap deviates from the midline and the angle of inclination of the gap becomes greater [14]. It can be seen that the Wiltse gap can safely reveal different structures at different levels of the lumbar spine. The new automatic retractor we have designed makes intra-vertebral manipulation easy and convenient.

The advantages of unilateral pedicle screw fixation over bilateral pedicle screw fixation are: soft tissue damage is reduced by stripping only the paravertebral muscles on the affected side, preserving the structural integrity of the contralateral muscles and the posterior median ligament, reducing postoperative scarring of the paravertebral muscles and localized denervated atrophy, minimizing the occurrence of intractable postoperative low back pain [15]. For unilateral pedicle screw fixation, the spinal bony structures are reduced and the healthy side of the vertebral plate and articular processes are preserved, while maintaining the structural integrity of the spinal canal. The intraoperative bleeding, operating time and costs are significantly less than for bilateral pedicle screw fixation [16]. In our study, although we have cut off the inferior articular eminence, we retained the superior articular eminence as well, and we confirmed that the unilateral fixation group had significantly less operative time, intraoperative bleeding and postoperative drainage than the bilateral fixation group. H Xue et al. also found that a modified unilateral TLIF with only a unilateral screw had the advantages of less trauma, less bleeding and fewer complications, with a satisfactory clinical outcome rate better than conventional TLIF with bilateral pedicle screw fixation [17].

A three-dimensional finite element model was used to build the relevant surgical model, and biomechanical analysis revealed that single segment unilateral pedicle screw fixation without interbody fusion could not control the lateral flexion and rotation load well, and the pedicle screws were subjected to higher stresses; once a single fusion device was attached, the stability of the fused segment could be reestablished, and the stresses on the screws were significantly reduced [18]. After unilateral internal fixation with an intervertebral fusion, the stress on the pedicle screws was significantly reduced, although the peak stress was still higher than that of bilateral pedicle screw fixation, but the difference was minimal. This suggests that the intervertebral fusion device can significantly reduce the stress on the screw, thus reducing the incidence of screw fracture. Unilateral and bilateral internal fixation have similar resistance to flexion, extension, and compression, and unilateral pedicle screw fixation with a single fusion placement may provide sufficient stability as an option for internal fixation in the treatment of degenerative lumbar spine disease. Previous mechanical experiments using animal specimen modeling revealed that the mean stress intensity and axial stiffness of the lumbar spine after fusion with ipsilateral, contralateral, and bilateral pedicle screw fixation combined with fusion interbody fusion were significantly higher than those of simulated injury specimens and normal specimens, and that fusion implantation with additional unilateral and bilateral pedicle screw system internal fixation could achieve stabilization of the lumbar spine. The biological stability of bilateral specimens in the directions of forward flexion, left bending, and left and right rotation did not differ between the unilateral model and the bilateral model [19]. A large number of in vitro tests have confirmed to some extent the feasibility and stability of unilateral pedicle screw fixation combined with fusion of the intervertebral fusion. Unilateral decompression is difficult to perform intra-vertebral interspace double fusion placement. The common peek bullet type interbody fusion is not accurate for maintaining stability of the anterior and middle spinal columns, especially when the patient is in a twisted and lateral position, due to the limited angular tilt during transmural placement and the small cross-section and bone contact area of the fusion [20]. Moreover, the unilateral nail bar combined with the bullet-type fusion cage suffers from stress imbalance, does not provide sufficient stability due to its asymmetric fixation, and is less resistant to rotation with unilateral internal fixation [21]. To overcome this problem, we adopted a kidney-like fusion cage with more uniform stress distribution. The kidney-like fusion is larger and has a larger contact area with the bony endplate, and the extra-large bone graft window allows for greater bone filling. When the kidney-like fusion is placed into the intervertebral space, the forces on the left and right sides of the anterior mid column become equalized. The implantation of a renal fusion after simultaneous gap sparing and the restoration of tension in the anterior and posterior longitudinal ligaments both reduce the impact on the healthy synovial joint during torsion and lateral bending, and the preservation of synovial mobility on the healthy side reduces the occurrence of accelerated degeneration in the adjacent segment. Overall, the use of unilateral screw fixation with a kidney-like fusion device can maintain three-column stability, no less than bilateral fixation. It also preserves the partial mobility of the operated segment and reduces the occurrence of adjacent segment disease.

The limited resection of bony structures in the posterior column and the small amount of autologous bone obtained by the interosseous approach TLIF procedure do not meet the need for intervertebral fusion bone grafting [22]. Currently, alternative grafts for clinical use include allograft bone, synthetic bone, and bone morphogenetic protein (BMP). Allogeneic bone is mainly derived from cadaveric tissue, including freeze-dried bone, fresh frozen bone, cancellous bone strip bone, and decalcified bone matrix (DBM), which are inexpensive and easily available [23]. BMP is a specific bone growth factor, which can form new bone in ectopic or normal location through chondrogenic bone and intramembranous bone, and has strong bone repair ability [24]. However, BMP can only act locally and is easily diffused and degraded in vivo, and easily loses its activity. In our study, the rate of intervertebral fusion was high in two group. Since the study group using both allogeneic bone and BMP to replace autologous bone in the intervertebral space, fusion rate appear to be higher, which indicating that good fusion results can be obtained with kidney-like fusion cage.

Pfirrmann et al. developed a system for classifying disc degeneration based on MR signal intensity, disc structure, distinction between the nucleus pulposus and nucleus pulposus, and disc height, which with 5 grades and has been accepted and used in clinical applications [25]. However, the Pfirrmann Grading System was previously found to be ambiguous in classifying intervertebral discs, so the modified Pfirrmann grading system, which has 8 grades, is more commonly used in clinical practice to better distinguish the degree of disc degeneration [26]. Our study found no difference in Modified Pfirrmann Grading of the upper intervertebral disc between the study and control groups, either preoperatively or postoperatively. The height of the intervertebral space of the upper intervertebral disc can be used to reflect the degeneration of the lumbar spine [27]. We found no significant difference in the intervertebral space height of the upper intervertebral disc between the two groups, either preoperatively or postoperatively. It is possible that both types of surgery are effective in treating herniated discs, with no harmful impact on the nearby segments.

There are 3 isozymes of creatine kinase, mainly found in skeletal muscle, brain and cardiac muscle. Since plasma levels of skeletal muscle isoenzymes account for more than 96% of the total, creatine kinase values can be used to reflect skeletal muscle damage in surgery that does not affect the brain or myocardium. Creatine kinase is an enzyme that catalyzes the reaction of creatine and adenosine triphosphate (ATP) with phosphocreatine and adenosine diphosphate (ADP). Creatine kinase concentrations have been used to study skeletal muscle damage caused by lumbar spine surgery [28]. The elevation of creatine kinase isoenzymes in skeletal muscle is mainly related to the process of muscle damage and is generally highest on the first postoperative day, returning to normal by 1 week after surgery [29]. In our study, creatine kinase levels increased significantly at 24 h after surgery and decreased significantly at 48 h. Interestingly, Creatine kinase levels were significantly lower in the study group than in the ctrl group at 24 h postoperatively, suggesting that the unilateral wiltse muscle gap access TLIF procedure combined with the treatment procedure was less damaging to the muscle.

Some deficiencies exist in our study, first, the sample size of patients was single-centered and insufficient in number. Second, in addition to the difference between unilateral and bilateral fixation treatment, our study differs from the ctrl group used a kidney-like fusion cage in the unilateral treatment group, and it is not well distinguishable whether unilateral fixation or kidney-like fusion cage or their combined effect has a greater impact on the outcome, and more subsequent studies are needed to confirm this.

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