A novel PLIF PEEK interbody cage with an impactionless insertion technology: A case series with a mid-term follow up of three years

Abstract

BACKGROUND:

Spinal fusion surgery has become one of the most common spinal procedures during the recent years. Searching for an optimum structural stability of the vertebral interspace, surgical implants which can be inserted via a posterior lumbar interbody fusion (PLIF) approach have been enhanced recently.

OBJECTIVE:

Evaluation of safety and efficacy of a novel PLIF polyetheretherketone (PEEK) interbody cage (TWIST) with an impactionless insertion technique.

METHODS:

Surgical outcome in 15 patients treated with the new system were observed preoperatively, one and three years after surgery using the Oswestry Disability Index (ODI), a Visual Analogue Scale (VAS) for pain and radiological outcome analysis of fusion success.

RESULTS:

Fifteen patients (7 female, 8 male) were included in the test series. After three years ODI and VAS were significantly improved. The pain intensity was reduced by more than 75% after one year and after 3 years, pain intensity was about 60% below the initial situation. The Oswestry values (ODI) improved significantly in all patients after 3 years. The fusion evaluation showed a fusion success in 87% of the patients.

CONCLUSION:

The clinical and radiological results of this first series give a positive standing and important information on the efficacy and safety over 3 years. The follow-up checks with imaging techniques showed that the fusions were very successful and functional outcome as well as pain reduction were increased.

Keywords

Spine chronic low back pain posterior lumbar interbody fusion (PLIF)interbody cage

1. Introduction

Back pain as a result of degenerative changes in the spine is a worldwide problem. If symptomatic disc disease cannot be treated with conservative methods, fusion of intervertebral body may be a surgical alternative [1]. The objective of any fusion is to stabilize the spine and to achieve significant relief from pain restricting the ability of patients to function in their daily activities. Searching for an optimum structural stability of the interspace, surgical implants which are inserted into the disc space have been developed. Spondylodesis surgery is necessary to achieve adequate bony fusion. The fusion process is often compared to the process of fracture healing and will take three months or more [2, 3].

The TWIST PEEK PLIF cage (Twist Technologies S.à.r.l., Trélex, Switzerland) is a posterior lumbar interbody fusion cage made of polyetheretherketone which obtained the CE mark in 2010. It consists of a standard PLIF cage body with a cavity for bone-graft and a fixed flat rectangular tip positioned obliquely on the anterior face of the cage body. The tip serves as temporary distractor of the adjoining vertebrae to allow the smooth insertion of the cage. This is made possible by the height of the tip being greater than the height of the cage body. After the insertion of the tip, the cage is rotated laterally, causing it to distract the two vertebrae apart. A distance which exceeds the height of the body of the cage is formed. Thus, the cage can be inserted into the interbody space, gliding on its edges without the dents rubbing against the endplates, nor need of impactation. Only one instrument (inserter) will be necessary for all sizes. The use of this novel system has the advantage of a quick, non-traumatic and precise delivery because dents do not impair the progress of the cage between the vertebrae. Thanks to the distraction tip, a precise positioning of the cage is possible and repositioning can be performed without difficulties applying a new set of two rotations. The distraction pressure mainly affects the cortical rims at the time of the first rotation and pressure on cancellous bone is low while the cage glides between the two vertebrae. Nerve roots are better protected due to the absence of impactation.

As part of routine operations of posterior lumbar interbody fusion (PLIF) this novel intervertebral lumbar cage and its unique technique have been used in patients with appropriate indications for surgery. The patients were informed of the material and the type of surgical intervention.

2. Methods

All patients included in this test series were at least 18 years old and gave their informed consent prior to surgery. The average age at surgery was 64.5 years. Indications for surgery were osteochondrosis (n $=$ 8), spondylolisthesis (n $=$ 4) or both (n $=$ 3) with corresponding spinal stenosis and spinal claudicatio causing unbearable back and/or leg pain with partially starting paresis resistant to conservative treatment.

The Posterior Lumbar Interbody Fusion was conducted to the directive of the Department of Orthopedics and Traumatology, Bonn. During the operation, Fluoroscopic X-rays were taken to ensure that all devices were in the correct position.

Spondylodesis was conducted with two interbody spacer TWIST implants and a fixation system consisting of titanium screws and connection rods. Pedicle screws were first placed in the affected and the adjacent vertebrae. If decompression was necessary, it was conducted after ensuring the correct position of the screws. The PEEK cages were used to replace the intervertebral disc. These procedures were followed by the insertion of the flat tip of the cages between the vertebrae. The cage was then rotated approximately 85 ${}^{\circ}$ counter-clockwise to distract the intervertebral space before being pushed forward, gliding on two of its edges. After this, the cage was rotated clockwise about 20 ${}^{\circ}$ , and the dents anchored into the endplates. In all cases, autologous bone graft had been extracted via dorsal decompression and removal of the processus spinosus was used to perform bony fusion with the interbody cages. Figure 1 illustrates the insertion process again in detail. In case where there was not enough autologous material, the use of additional allogenic bone graft was necessary to fill up the cages. After discectomy and placing the interbody cages, bone graft was inserted ventral and between the cages and the two rods were inserted through the screw heads and locked to the screws to restore spinal stability in the end. Finally, the remaining bone graft was also placed dorsolateral to the cages. The patients were able to leave the hospital within 3–5 days after surgery.

Figure 1.

TWIST Cage – Insertion steps.

For the documentation of this test series, three follow ups were conducted. The first visit – prior to surgery – was intended for recording of history, clinical findings and results with imaging procedures. Clinical findings and evaluations of imaging procedures were repeated after one year and after three years. Pain intensity of patients’ back, legs and hips were documented using the Visual Analogue Scale (VAS 0–100 mm). The functionality of the patients was determined using the Oswestry Disability Index (ODI) and calculated as a percentage of the scores of the maximum possible values. Because this is a newly designed cage we chose 36 month of follow up to have a more accurate evaluation on the safety and efficacy.

Fusion results were most important after one year, but a second follow up after three years was done to reevaluate the fusion.

Radiological results were evaluated independently by an experienced orthopedic. Fusion results were classified by grades using the criteria of Kim et al. as shown in Table 1 [4]. Lumbar plain ap and lateral radiographs were taken before, one year and three years after operation. The TWIST Cage is radiolucent, making it easy to evaluate the presence of dense, mature bone fusion inside the cage.

Grade 1, fusion, was classified if there was no interspace between the cages and the vertebral body, the bony fusion area was more dense and more mature than originally achieved during surgery and complete mature bone trabeculae bridging was in fusion area. There was also no sign of traction spur.

Grade 2, ongoing remodeling, was classified, if there were all signs of Grade 1, but the bony bridging was incomplete.

Grade 3, uncertain fusion, bony bridging was incomplete, but there was no halo or interspace around the cages or screws

Grade 4, pseudarthrosis/non fusion, war classified when a halo or interspace was seen around the cages or screws.

Averages plus standard deviation (VAS) and median values plus first – 3rd quartiles (ODI) were calculated. For the calculations of the changes after one year and after three years, t-test (VAS) and the Wilcoxon matched-pairs-test (ODI) were applied. Based on these results, the respective effect sizes were estimated. VAS (parametric): Cohen’s effect size d $=$ Xsquare.Diff/Std.Dev.Diff. d $=$ 0.2 less effect, d $=$ 0.5 medium effect, d $=$ 0.8 large effect ODI (non-parametric): Effect size: r $=$ Z/sqrt (2 * n ’). Rosenthal r: 0.1 “low effect”, 0.3 “average effect”, 0.5 “large effect”.

Table 1

Classification of spinal fusion

Grade	Description
1	Fusion
2	Ongoing remodeling
3	Uncertain fusion
4	Pseudarthrosis

3. Results

A total of 15 patients (7 females, 8 males) were included in the test series. The average age of patients was 64.5 years (44–83 years). Before the operation, the average BMI of the patients was 31.6 $\pm$ 6 kg/cm ${}^{2}$ . Two-thirds of patients were obese. After 3 years, no significant change was found (mean: 32.1 5). Surgery was performed at one spine height (2 $\times$ L2/3, 2 $\times$ L3/4, 5 $\times$ L4/5, 6 $\times$ L5/S1) in 14 cases and at three heights in one case. Neurological deficits were registered in two patients prior to surgery. After surgery there were no more neurological deficits detected.

Table 2 shows the VAS of five different parts of the body before and after the operation. The comparison shows that the pain intensity was reduced by more than 75% in all regions after one year. After 3 years, pain intensity of about 60% below those of the initial situation was recorded ( $p<$ 0.00001).

Table 2
Visual analogue scale (VAS: 0–100 mm) before and after surgery given as mean $\pm$ SD

Localization	VAS (mm) preoperatively	VAS (mm) after 1 year	VAS (mm) after 3 years
Back	82.2 $\pm$ 18	19.4 $\pm$ 16	35.0 $\pm$ 24
Left leg	73.5 $\pm$ 29	14.6 $\pm$ 16	21.4 $\pm$ 25
Left hip	60.1 $\pm$ 38	13.3 $\pm$ 16	24.7 $\pm$ 29
Right leg	74.7 $\pm$ 30	16.3 $\pm$ 15	24.7 $\pm$ 20
Right hip	65.1 $\pm$ 35	11.6 $\pm$ 15	23.5 $\pm$ 21

The calculations of the differences between pre-surgery registration and checks after 1 year yielded effect sizes between d $=$ 1.4 (left hip) and d $=$ 3.2 (back), corresponding to a very strong potency. Even after 3 years, the controls still showed effect sizes in the range between d $=$ 1.1 (left hip) and d $=$ 1.9 (back). All patients stated improvements of pain in the back.

The median of the Oswestry score was 76% (Qu1: 66 – Qu.3: 88%) before surgery. After 1 year, the median was reduced to 17% (6%–42%). This corresponds to a difference (Hodges-Lehmann estimator) of 52 percentage points and an effect size of r $=$ 0.6 ( $p<$ 0.0001). Three years following the surgical procedure, a median of 28% was calculated, representing a decrease of 43.5 percentage points compared to the base-line; an effect size of r $=$ 0.6 was calculated. The evaluation showed that the Oswestry values had improved in all patients ( $p<$ 0.001). The medians of the variables are shown graphically in Fig. 2.

The patient with the nonunion had severe back pain (VAS: 65 mm while showing low Oswestry values of only 28%. Revision surgery was necessary after a period of conservative treatments was not successful. A severe Pseudarthosis of both screws in the upper vertebrae was confirmed intraoperativly. The patient with uncertain fusion mainly suffered from hip pain (VAS: 60 mm; ODI 22%) and there was also an affection of the left sacroiliac joint detected. This patient received intense conservative treatment and reduced his nicotine consumption.

Table 3

Results of fusion examination

Fusion examination	After one year	After three years
Fusion (1)	47% (n $=$ 7)	87% (n $=$ 13)
Ongoing remodeling (2)	53% (n $=$ 8)
Uncertain fusion (3)		6.5% (n $=$ 1)
Pseudarthrosis (4)		6.5% (n $=$ 1)

Figure 2.

Comparison of Oswestry scores (Medians; Min – Max, Quartiles. 1 and 3) prior to surgery versus three years later. Topic categories: 1: Pain intensity, 2: Personal care, 3: Lifting, 4: Walking, 5: Sitting, 6: Standing, 7: Sleeping, 8: Sex Life, 9: Social Life, 10: Travelling.

Fusion results are shown in Table 3. Radiographic examples of early and late stages of fusion as well as a reduced spondylolisthesis are shown in Figs 3–5.

Figure 3.

Patient plain radiograph one year after surgery. Fusion was classified as grade 2 (ongoing remodeling).

Figure 4.

Patient plain radiograph three years after surgery. Fusion was classified as grade 1 (complete fusion).

Figure 5.

Preoperative plain radiograph with severe spondylolisthesis and complete fusion after three years; classified as Grade 1 (complete fusion).

At baseline, all patients reported that they were taking analgesics (NSAIDs continuously: n $=$ 11; NSAIDs plus opioid WHO II: n $=$ 2; NSAID plus opioid WHO III: n $=$ 2). After three years, four patients no longer required analgesics, seven patients took NSAIDs when necessary and one patient permanently. Two patients took additional opioids WHO II and one patient opioids WHO III.

Of the 15 cases only one was working before surgery; six patients were unable to exercise their work due to their illness, and eight patients were pensioners. Three years after surgery, six patients were able to work again, and nine were retired.

The following adverse events were recorded: Perioperative: wound infection: n $=$ 2. Within three years after surgery: nonunion: n $=$ 1, uncertain fusion: n $=$ 1.

The final judgment of the patients regarding the entire treatment were: Very satisfied: n $=$ 6; satisfied: n $=$ 5, relatively satisfied: n $=$ 2; relatively unsatisfied: n $=$ 1; unhappy: n $=$ 0.1. This corresponds to a success rate of 73%. Twelve patients would be willing to undergo such an operation again.

4. Discussion

Spondylodesis operation has become one of the most common orthopedic operations. In the United States, lumbar fusion is performed in more than 400 000 patients per year, and its frequency is increasing [5]. Fusion of the lumbar spine is a treatment intervention of last resort for patients with severe chronic back pain due to osteochondrosis and spondylolystesis resistant to conservative treatment. Therefore good indications for spondylodesis treatment are patients with osteochondrosis and/or spondylolisthesis also causing neurological deficits and claudicatio spinalis.

An adequate fusion is essential in order to reduce spinal instabilities, restore spinal deformities and finally achieve an improvement of the patient’s quality of life. The bone formation process is similar to the process of fracture healing. Success of spondylodesis surgeries is highly variable and reported non-fusion rates vary from 0% to 27% after at least two years of follow up [6, 14]. In our study population one patient ( $=$ 6.7%) suffered from recurring back pain after three years and non union was detected, which is quiet a low incidence. Patients with non-fusion have the possibility to be treated conservatively or operatively. Our patient needed revision surgery, which confirmed the radiological results of screw loosening, but neither the screws nor the cages were broken.

In a retrospective study, reoperations were necessary in 13.2% of all patients. Also, disc herniation after lumbar fusion is reported [7]. Delayed fusion after a follow up period of 24 month was reported in 6% by Okuda et al. [6]. Our study showed out one patient with uncertain fusion (6.7%) and one with pseudarthrosis (6.7%) after 3 years. Regarding these facts, continuous optimization of the surgical methods, instruments and cages used for fusion proves necessary. The recently developed methods with minimally invasive technique yield a minimum of blood loss, shortened hospital stay, and decreased post-operative back pain [3].

The clinical and radiological results of this first series with the newly designed cage for PLIF give some important information on the efficacy and safety over 3 years. The fact that the tip is higher than the height of the cage body proves to be a particular advantage, so that “overdistraction” of the vertebral segment can be gained.

The use of TWIST has also the advantage of a quick, non-traumatic and precise delivery because dents do not impair the progress of the cage between the vertebrae.

The distraction pressure mainly affects the cortical rims at the time of the first rotation and pressure on cancellous bone is low while the cage glides between the two vertebrae. Nerve roots are better protected due to the absence of impactation.

Other studies report about a high risk of adjacent segment pathology (ASP) after spondylodesis operation [8, 9]. The progression of adjacent segment degeneration can be considered as a part of normal aging process but this phenomenon seems to be at least partly influenced by the stress of lumbar arthrodesis. The immediate next segment or the two adjacent segments appear to be mainly influenced [8]. Aota et al. found a significantly higher incidence for patients 55 years of age and older to develop adjacent segment instability after spondylodesis [10]. The average age of patients in this study was 64.5 years but we could not detect any ASP after 36 month of follow up.

Patients experienced a rapid post-operative reduction of pain and malfunctions. These clinical effects reached the optimum after 1 year. The slight deterioration in the course of the following years can be attributed to other pathological e.g. smoking changes in the bones or the soft tissue structures facet degeneration or sacroiliac joint affection. It has to be mentioned, that the patient with pseudarthrosis as also the patient with uncertain fusion are smokers. Other studies reported, that smoking has a negative influence on bony fusion and smokers exhibit a worse or prolonged fracture healing as compared to non-smokers [11, 12, 13].

The follow-up checks with imaging techniques, however, showed that the fusions were very successful.

The results of the present study have to be interpreted carefully because the patient group is small. Further studies with a larger population could offer greater insight and a more accurate comparison of further factors that may also have an impact on clinical outcomes.

5. Conclusions

The clinical and radiological results of this first case series with a new PLIF PEEK cage indicate an excellent efficacy and safety after three years. The follow-up checks with imaging techniques showed that bony fusions were successful and functional outcome as well as pain reduction were increased and could be maintained.

Conflict of interest

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

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