The Fusion Rate of Cortical Bone Trajectory Screw Fixation and Pedicle Screw Fixations in L4‐5 Interbody Fusion: A Retrospective Cohort Study

Abstract

Objective

Although cortical bone trajectory (CBT) screw fixation has been used for several years, the number of studies on its fusion effects is limited. Furthermore, several studies report conflicting outcomes. We aimed to compare the fusion rates and clinical efficacy of CBT screw fixation and pedicle screw (PS) fixation for L4‐L5 interbody fusion.

Methods

This study was a retrospective cohort control study. Patients with lumbar degenerative disease who underwent L4‐L5 oblique lumbar interbody fusion (OLIF) or posterior decompression using CBT screws between February 2016 and February 2019 were included. Patients in whom PS was used were matched for age, sex, height, weight, and BMI. Record the operation time, blood loss. All enrolled patients underwent lumbar CT imaging at one‐year follow‐up to evaluate the fusion rate. At the two‐year follow‐up the visual analogue scale (VAS), Oswestry disability index (ODI), and Japanese Orthopaedic Association scores (JOA) were used to identify symptom improvement. Independent t‐test was used for the comparison, and score data were analyzed using the χ ² and exact probability tests.

Results

A total of 144 patients with were included. All patients were followed‐up postoperatively for 25–36 months (average 32.42 ± 10.55 months). Twenty‐eight patients underwent OLIF and CBT screw fixation, 36 underwent OLIF and PS fixation, 32 underwent posterior decompression and CBT screw fixation, and 48 underwent posterior decompression and PS fixation. The fusion rates following CBT screw and PS fixations in OLIF were 92.86% (26/28) and 91.67% (33/36), respectively (P = 1). The fusion rates following CBT screw and PS fixations in posterior decompression were 93.75% (30/32) and 93.75% (45/48), respectively (P > 0.05). Regardless of OLIF or posterior decompression, there were no significant differences in the VAS, ODI, and JOA scores between patients treated with CBT and PS (P > 0.05).

Conclusion

CBT screw fixation can achieve a satisfactory interbody fusion rate with a clinical efficacy similar to that of PS in patients with lumbar degenerative disease, regardless of whether OLIF or posterior decompression was performed.

Selected patients with lumbar degenerative disease who underwent L4‐5 oblique lumbar interbody fusion or posterior decompression using CBT screw fixation. Matched patients using pedicle screw with the same baseline data. The fusion rates of them were used in long‐term follow‐up.

Introduction

With the development of society and increase in the aging population, the number of patients with lumbar degenerative disease has been increasing annually. Decompression and fusion is the main treatment option for lumbar degenerative disease. ¹ The traditional pedicle screw (PS) fixation has been used in clinical practice for several years with excellent correction, spinal stability, and fusion results. PS fixation is the method of choice for surgical implantation in spine‐related diseases. ² However, it also has some shortcomings: screw placement often requires the dissection of more muscles; visual field is poor, especially in the lower lumbar spine or sacrum; tendency to invade the surrounding facet joints; and the nerve is prone to injury. ³ , ⁴ Moreover, PS loosens easily in older patients with osteoporosis because the screw insertion channel is encased in the cancellous bone. ⁵

In 2009, Santoni et al. advocated a new cortical bone trajectory (CBT) screw fixation method with specific characteristics, such as increased stability due to the greater contact surface with the bone cortex and less procedure‐related paravertebral muscle damage. ⁶ CBT screw fixation has significantly fewer complications than PS fixation and is safe and reliable. ⁷ Interbody fusion is an important index to evaluate after internal fixation. However, most of the current studies on CBT screw fixation focus on imaging, biomechanical and short‐term clinical efficacy. The focus of clinical research is mainly on the modification of the nail canal, use of a combination of different screws, or application in the correction of trauma and deformity. The number of studies on the fusion effects and long‐term clinical efficacy of CBT screw fixation is limited; furthermore, several studies report conflicting outcomes.

This study retrospectively reviewed the clinical data of patients with lumbar degenerative disease who underwent interbody fusion at the L4‐L5 level in our hospital using either CBT screw or PS fixation and compared the fusion rates and surgical effects of both methods. The aim of our study was to determine: (i) whether the fusion rate and clinical efficacy of CBT and PS fixation after oblique lumbar interbody fusion (OLIF) are comparable; (ii) whether the fusion rate and clinical efficacy of CBT and PS fixation after posterior decompression are comparable; and (iii) provide a basis for future studies.

Through our study, we look forward to comparing the fusion following the two fixation methods using larger samples in prospective and multi‐center studies in the future. By changing the follow‐up time point or other factors, the postoperative recovery and intervertebral fusion of the fixation methods can be carefully compared.

Materials and Methods

Study Design and Participants

Patients with lumbar degenerative disease who underwent L4‐L5OLIF or posterior decompression and CBT screw fixation at Fujian Provincial Hospital between February 2016 and February 2019 were retrospectively analyzed. The inclusion criteria were as follows: (i) presence of severe symptoms of nerve compression, such as low back pain, leg pain, and intermittent claudication before surgery, with no improvement after conservative treatment for at least 3 months; (ii) preoperative images showing lumbar degenerative disease at the L4‐L5 level, including vertebral instability, intervertebral height loss, disc herniation, spinal stenosis, and grade I spondylolisthesis; and (iii) patients with a follow‐up time of >2 years and a complete data set. The exclusion criteria were as follows: (i) history of previous lumbar surgery or a medical condition; (i) incomplete patient data (including subsequent loss to follow‐up and refusal to follow‐up); and (iii) uncertain screw fixation method.

Patients who underwent PS fixation were matched for age, sex, height, weight, and BMI and the inclusion and exclusion criteria accepted by patients are the same as above.

This retrospective comparative study was approved by the Research Ethics Committee of Fujian Provincial Hospital (No: k2021‐06‐026).

Preoperative Preparations

All the enrolled patients underwent similar preoperative preparations. Complete routine preoperative examinations were performed and patients with operation‐related contraindications were eliminated. Preoperative anteroposterior radiography, CT, and MRI of the lumbar region were performed to evaluate the patients' lumbar spine and intervertebral discs. All patients were instructed to stop smoking at least 2 weeks before the surgical procedure. All patients underwent exercise and respiratory training required during confinement to the bed for 1 week. All patients and their relatives were briefed on the surgical risks and written informed consents were obtained. The final preoperative preparations included 8 h of fasting, 4 h of water deprivation, intravenous rehydration, and oral antihypertensive drug consumption the morning of the surgery.

Choice of Surgery and Identification of Fusion

After exclusion of patients with contraindications for surgery, OLIF is usually suitable for patients with sufficient vascular space and relatively less severe spinal stenosis. If facet hypertrophy and spinal stenosis are severe, complete laminar decompression is required, followed by posterior decompression. Different criteria have been used to assess fusion. The best non‐invasive process for identifying fusion is considered to be lumbar CT examination in combination with evaluation of the anterior and lateral views. Lumbar fusion is identified by the formation of a trabecular bone bridge at the target segment, with no obvious vertebral displacement on the anterior and lateral lumbar spine radiographs. ⁷ The possibility of bias caused by the availability of limited data cannot be ruled out. All lumbar spine operations in our department were performed under microscopic guidance, significantly reducing the surgical incision length to only 2.5–3 cm. Furthermore, the use of a microscope greatly improves the operator's visual field and the odds of effective CBT screw implantation and decompression. It also allows for a connection to an external display and recording for educational purposes.

Surgical Procedures

All patients were administered intravenous cefuroxime sodium (1.5 g) half an hour before the surgical procedure. After administration of general anesthesia the patients were intubated.

OLIF procedure

The oblique lateral and posterior approaches were combined for screw‐rod fixation. With the patient in the left lateral decubitus position, the intervertebral disc and fibrous ring were excised. The upper and lower endplates were prepared and a trial spacer was inserted. Subsequently, a cage filled with allogeneic bone graft was inserted under fluoroscopic guidance for fusion. Thereafter, the patient was placed in the prone position, and bilateral cortical screws were implanted using the posterior median approach after disinfection and redraping the surgical field. For pedicle screw fixation, small incisions were made before implantation, using the Westle approach. All the procedures were routinely monitored using intraoperative spinal cord electrophysiology.

Posterior decompression procedure

The posterior median approach, lumbar spinal fusion, and laminectomy were selected for this procedure. The patient was placed in a prone position with the abdomen suspended. The operative site was identified, confirmed, and delineated under C‐arm guidance before the incision was made. A posterior median skin incision was made over the identified diseased segment and each underlying layer was separated to reach both sides of the spinous process. Decompression was performed depending on the situation encountered. Unilateral lamina, lower articular process, and parts of the upper articular process were removed and decompressed depending on each patient's situation. Autologous bone graft was retained for cage insertion. The damaged and degenerated nucleus pulposus tissue was completely removed and the upper and lower vertebral cartilage endplates were scraped off. A corresponding equal‐sized cage filled with autologous bone graft was implanted. Thereafter, screws of appropriate size were inserted and connected with a rod. The physiological curve of the lumbar spine was adjusted and restored. Finally, a fluoroscopic image using the C‐arm was obtained to verify the position of the screws and rods.

Postoperative Care

All patients were started on a monitored physical exercise regimen on the second postoperative day while wearing a protective waist brace. An anteroposterior radiograph was obtained 1–3 days postoperatively to verify proper screw and cage positioning. The protective brace was worn for 3 months, and vigorous waist activities were prohibited.

Postoperative Evaluations and Follow‐ups

Surgical blood loss and time data were recorded. All patients underwent routine follow‐ups at 3 and 12 months, and a final follow‐up 2 years after the initial surgical procedure. The visual analogue scale (VAS) was used to record pre‐and post‐operative lower back and leg pain. The Oswestry dysfunction index (ODI) and Japan Orthopaedic Association score (JOA) were used to assess functional improvements before and after the surgical procedure.

The detection of continuous bridging trabeculae formation within the target segment on a CT, with no obvious vertebral displacements was observed in the anterior and lateral positions of the lumbar spine, indicating intervertebral fusion. ⁸ The patients were evaluated by doctors of the same team at the deputy director level or above.

Statistical Analysis

SPSS 22.0 (IBM Corp, Armonk, NY, USA) was used for the data analysis. Data with a normal distribution are expressed as x ± s. The independent sample t‐test was used for the comparisons, and the score data were analyzed using the χ ² and exact probability tests. Statistical significance was set at P < 0.05.

Results

Data were obtained from patients with lumbar degenerative disease who underwent interbody fusion at the L4‐L5 using CBT screw fixation or PS fixation between February 2016 and February 2019. According to the inclusion and exclusion criteria, patients were finally included as follows: 28 patients underwent OLIF with CBT screw fixation, 36 underwent OLIF with PS fixation, 32 underwent posterior decompression and CBT screw fixation, and 48 underwent posterior decompression and PS fixation (Fig. Fig. 1).

Fig. 1.

Follow the flow chart of the above criteria.

General Characteristics

A total of 144 patients were enrolled and followed up for 25–36 months, with an average follow‐up time of 32.42 ± 10.55 months. There was no significant difference between CBT screw fixation and PS fixation in the OLIF and posterior decompression procedures in basic information.(Tables 1 and 2).

TABLE 1.

Characteristics of the patients undergoing OLIF

Groups	Age (years)	Follow‐up time (month)	Height (cm)	Weight (kg)	BMI (kg/m2)	M‐F ratio (n)
CBT screw fixation (n = 28)	61.21 ± 11.21	31.14 ± 5.90	161.30 ± 4.52	59.81 ± 6.39	22.98 ± 2.22	12:16
PS fixation (n = 36)	60.28 ± 9.72	31.61 ± 9.73	160.39 ± 5.54	59.13 ± 5.84	23.00 ± 2.08	14:22
t value	0.36	−0.65	0.71	0.45	−0.03
P value	0.72	0.51	0.48	0.66	0.98	0.75

Abbreviations: CBT, cortical bone trajectory; F, female: M, male; OLIF, oblique lumbar interbody fusion; PS, pedicle screw.

TABLE 2.

Characteristics of the patients undergoing posterior decompression

Groups	Age (years)	Follow‐up time (month)	Height (cm)	Weight (kg)	BMI (kg/m2)	M‐F ratio (n)
CBT screw fixation (n = 32)	64.66 ± 12.34	33.78 ± 2.43	164.01 ± 6.03	66.34 ± 8.48	24.69 ± 3.11	12:20
PS fixation (n = 48)	63.08 ± 9.36	32.85 ± 3.12	165/71 ± 6.86	67.56 ± 8.05	24.57 ± 2.10	20:28
t value	0.65	1.21	−1.15	−0.65	0.21
P value	0.52	0.23	0.26	0.52	0.83	0.71

Abbreviation: CBT, cortical bone trajectory; F, female; M, male; PS, pedicle screw.

Blood Loss and Operation Time

The blood loss and operation time of each group were as follows. No statistically significant differences (P > 0.05) were observed between CBT and PS in blood loss and operation time. (P > 0.05) (Tables 3 and 4).

TABLE 3.

Blood loss and operation time who underwent OLIF(Mean ± SD)

Groups	Blood loss (ml)	Operation time (min)
CBT screw fixation (n = 28)	108.93 ± 22.99	175.36 ± 30.98
PS fixation (n = 36)	111.94 ± 30.88	175.28 ± 50.28
t value	−0.43	0.28
P value	0.08	0.17

Abbreviations: CBT, cortical bone trajectory; PS, pedicle screw;

TABLE 4.

Blood loss and operation time who underwent posterior decompression (Mean ± SD)

Groups	Blood loss (ml)	Operation time (min)
CBT screw fixation (n = 32)	247.42 ± 39.83	139.68 ± 28.69
PS fixation (n = 48)	249.17 ± 39.78	136.88 ± 28.97
t value	−0.19	0.42
P value	0.16	0.95

Abbreviations: CBT, cortical bone trajectory; PS, pedicle screw.

Fusion Rate

Postoperative scans indicated satisfactory screw and fusion cage placement in all the patients. No screw loosening, cage sinking, or screw or internal fixation fractures were detected up to the last follow‐up (>2 years after the operation) in any of the patients. Postoperative lumbar CT images at 1‐year follow‐up revealed fusion rates of 92.86% (26/28) and 91.67% (33/36) for CBT screw fixation and PS fixation, respectively, in patients who underwent OLIF. There was no significant difference between the two fixation methods. The fusion rates of CBT screw fixation and PS fixations were 93.75% (30/32) and 93.75% (45/48), respectively (P > 0.05), in patients who underwent posterior decompression. The remaining 10 patients in whom non‐fusion was detected had no obvious clinical symptoms. All fusion rates were determined on CT by the same doctor.

Clinical Efficacy Analysis

None of the 144 patients exhibited complications, such as nerve injury or incision site infection. All patients experienced significant relief of lower back and leg pain after the surgical procedure and the physical function improved. The postoperative VAS, ODI, and JOA scores were significantly better than the preoperative scores (P < 0.05). No statistically significant differences (P > 0.05) were observed between the postoperative VAS, ODI, and JOA scores at various time points (P > 0.05) (Tables 5 and 6).

TABLE 5.

Comparison of the CBT fixation and PS fixation groups patients who underwent OLIF (Mean ± SD)

Groups	VAS			ODI (%)			JOA
	Preop	Postop (3 months)	Final	Preop	Postop (3 months)	Final	Preop	Postop (3 months)	Final
CBT screw fixation (n = 28)	6.29 ± 1.46	2.39 ± 1.01	0.72 ± 0.60	50.04 ± 5.68	10.54 ± 2.17	8.32 ± 2.16	11.57 ± 2.20	25.32 ± 1.77	25.29 ± 1.80
PS fixation (n = 36)	6.22 ± 0.99	2.69 ± 0.79	0.81 ± 0.40	51.22 ± 3.41	11.22 ± 1.57	8.50 ± 1.21	10.67 ± 2.21	25.67 ± 1.45	25.78 ± 1.48
t value	0.21	−1.3	−0.73	−1.03	−1.47	−0.42	1.63	−0.86	−1.2
P value	0.84	0.2	0.47	0.3	0.15	0.68	0.11	0.4	0.24

Abbreviations: CBT, cortical bone trajectory; JOA, Japanese Orthopedic Association score; ODI, Oswestry disability index; OLIF, oblique lumbar interbody fusion; Postop, postoperative; Preop, preoperative; PS, pedicle screw; VAS, visual analogue scale.

TABLE 6.

Comparison of the CBT fixation and PS fixation groups in patients who underwent posterior decompression (Mean ± SD)

Groups	VAS			ODI (%)			JOA
	Preop	Postop (3 months)	Final	Preop	Postop (3 months)	Final	Preop	Postop (3 months)	Final
CBT screw fixation (n = 32)	6.50 ± 0.96	2.97 ± 0.65	1.41 ± 0.98	53.31 ± 4.93	12.19 ± 2.63	10.25 ± 1.67	10.5 ± 2.40	24.13 ± 2.88	24.63 ± 2.08
PS fixation (n = 48)	6.52 ± 1.27	2.73 ± 0.82	1.15 ± 0.62	52.19 ± 5.94	11.83 ± 2.83	9.50 ± 2.31	11.17 ± 2.39	24.88 ± 1.78	25.38 ± 1.95
t value	−0.08	1.31	1.46	0.89	0.56	1.58	−1.39	−1.44	−1.64
P value	0.94	0.17	0.15	0.38	0.58	0.12	0.17	0.15	0.11

Abbreviations: CBT, cortical bone trajectory; JOA, Japanese Orthopedic Association score; ODI, Oswestry disability index; Postop, postoperative; Preop, preoperative; PS, pedicle screw; VAS, visual analogue scale.

Case Presentation

An 80‐year‐old male patient underwent successful lumbar fusion with CBT screws using the posterior median approach for L4‐L5 lumbar spinal stenosis (Fig. 2). Preoperative VAS, ODI, and JOA scores were 6, 56, and 9, respectively. The VAS, ODI, and JOA scores at the three‐month postoperative follow‐up were 2, 11, and 25, respectively. At the final follow‐up, which was performed 4 years after the initial operation, the VAS, ODI, and JOA scores were 1, 9, and 25, respectively. Preoperative and postoperative CT images are shown in Fig. 2.

Fig. 2.

(A, B): Patient's preoperative MRI scan images showing stenosis of the canal at the L4‐L5 level. (C, D): Radiographs obtained at 6 months follow‐up indicating satisfactory internal fixation. (E): CT images obtained at one‐year follow‐up showing satisfactory interbody fusion at L4‐L5. (F): Healed incision of approximately 2 cm length at one‐year follow‐up.

Discussion

We compared the fusion rates following CBT screw fixation and PS fixation in OLIF and posterior decompression. We found that CBT screw fixation, whether applied during posterior decompression or OLIF, achieved satisfactory fusion rate, and VAS, ODI, and JOA scored similar to that of PS implantation.

PS fixation is the most commonly used method in spine surgery for spinal canal stenosis. ⁹ However, PS implantation has high prerequisites, especially related to incision and exposure; excessive dissection of the muscular tissues is often required to expose bony markers that may lead to damage of the surrounding muscles. ¹⁰ CBT screw fixation was first reported by Santoni et al. in 2009. ⁶ A biomechanical study of cadavers revealed that CBT screw fixation could achieve a better pull‐out force than PS for vertebral stabilization.

Fusion Rate

The fusion rates in our study ranged from 91% to 94%, which was marginally higher than those reported in previous studies (88%–91%). ¹¹ , ¹² , ¹³ This might be because only single‐segment cases were included and the sample size was small. Some studies have reported single‐segment CBT fusion rates of >95%. ¹⁴ , ¹⁵ In our study, patients often wore protective waist braces for 3 months after surgery to limit the waist movements, which is conducive to vertebral fusion. Five of the patients in this study in whom fusion was not achieved did not strictly follow orders to restrict lumbar movement. The association between non wearing of the waist brace and non‐fusion remains to be determined.

Clinical Efficacy

There was no significant difference in the blood loss, operation time, AS, ODI, or JOA scores between the CBT screw fixation and PS fixation groups, regardless of whether posterior decompression or OLIF was performed. Due to its medial position, CBT screw fixation could reduce the probability of peripheral muscle and joint injuries, reduce procedural bleeding and ensure higher screw‐rod stability. ¹⁶ , ¹⁷ However, there was no significant reduction in CBT group blood loss in this study, which may be because we were not skilled enough to place nails in one step. No significant complications were observed in any patient during long‐term follow‐up. However, previous studies have identified some CBT screw‐related complications, such as screw fall‐off, pedicle fractures, and pseudo‐joint formation. ¹⁸

Comparison with Previous Studies

Numerous studies have shown that CBT screw fixation can achieve an effect equal to that with PS implantation, with the advantage of being less traumatic. ¹⁹ In our study, we focused on the fusion rate and obtained a satisfactory result, which might be related to the habit of placing longer screws. The load‐bearing capacity of 45 mm screws is 28% higher than that of 35 mm screws. ²⁰ Additionally, significantly greater fusion rates with lower screw loosening probabilities were seen with screw placements to depths of >39.2% of the bone width. ²¹ , ²² However, this may have been due to the small sample size of the study.

All existing studies have concluded that CBT screw fixation had no obvious shortcomings, and may even be better than PS fixation in terms of safety, stability, and curative effects. Its disadvantages include the shorter application time, higher surgical skill requirement of the surgeon, and longer learning curve. ²³ , ²⁴ Currently, available studies have primarily focused on analyzing CBT screw and PS clinical effects by evaluating their impact on the VAS, ODI, and JOA scores. To date, no significant differences in short‐term clinical effects (within 1 year after the initial operation) were found between the two methods. ²⁵ , ²⁶

Strengths and Limitations

Most of the existing literature on this topic include biomechanics, imaging, and short‐term clinical studies. Studies on the fusion rate and long‐term clinical efficacy of CBT are limited, and most have focused on demonstrating its viability.

Our study had the following limitations: first, this was a single‐center, retrospective cohort study with a relatively small CBT screw fixation sample size. A multi‐center prospective randomized study with a larger sample size has been planned. Second, we did not collect the CT data within 1 year after surgery, and the exact fusion time could not be observed.

Conclusion

In this single‐center retrospective cohort study, we compared the fusion rate and VAS, ODI, and JOA scores following CBT screw fixation and PS fixation in OLIF and posterior decompression. We found that CBT screw fixation achieved an interbody fusion rate and improvement in postoperative pain and function similar to that of PS in patients with lumbar degenerative disease.

Author Contributions

Lin Yuhe: formal analysis; data curation; writing—original draft; visualization. Xu Jie: methodology; writing—review and editing. Zheng Wu: formal analysis; writing—review and editing.