Article Information

Dr. Harshadkumar A. Patel, MD¹, Francine Zeng, MD², Dr. Hardeep Singh, MD³, Dr. Scott S. Mallozzi, MD³, Mark Cote, P.T⁴, Dr. Isaac L. Moss, MD^5*

¹Assistant Professor, Department of Orthopedic surgery, Westchester Medical Center, New York Medical College, Valhalla, NY

²University of Connecticut, Farmington, CT

³Assistant professor, Department of Orthopedics surgery, Comprehensive Spine Center, University of Connecticut, Farmington, CT

⁴Biostatistician, Director of outcomes, research and quality, Department of orthopedic surgery, University of Connecticut, Farmington, CT

⁵Associate professor, Department of Orthopedics surgery, Comprehensive Spine Center, University of Connecticut, Farmington, CT

*Corresponding Author: Dr. Isaac L. Moss, MD, Chair and Associate professor, Department of Orthopedic Surgery, Comprehensive Spine Center, University of Connecticut, 263 Farmington Avenue, Farmington, CT 06030.

Received: 11 July 2023; Accepted: 17 July 2023; Published: 27 July 2023

Citation: Dr. Harshadkumar A. Patel, MD, Francine Zeng, MD, Dr. Hardeep Singh, MD, Dr. Scott S. Mallozzi, MD, Mark Cote, P.T, Dr. Isaac L. Moss, MD. Comparison of PEEK vs 3D printed titanium cage for ACDF- Is there any difference in subsidence? Journal of Spine Research and Surgery 5 (2023): 69-75

Abstract

Keywords

Cervical spine; Spondylosis; Anterior Cervical Discectomy and Fusion; Interbody Cages; PEEK; Titanium

Article Details

Introduction

Since its first introduction by Cloward[1] and Smith and Robinson[2], anterior cervical discectomy and fusion (ACDF) has become the gold standard of surgical treatment for patients with persistent symptomatic disc herniations, cervical myelopathy, or axial neck pain secondary to severe cervical spondylotic changes. After discectomy, a structural graft helps to achieve both patency of the neuroforamina and a solid fusion, as well as improvement of lordosis, if needed.

The traditional gold standard autologous iliac bone graft was associated with significant donor site morbidity, nonunion, and graft collapse[3,4] while allograft demonstrated slower incorporation and increased likelihood of collapse compared to autografts[5,6]. This has led to the development of interbody devices consisting of synthetic materials, with Titanium and Polyether-ether-ketone (PEEK) cages being most common[7]. Both Titanium and PEEK interbody cages have demonstrated favorable results when compared with allografts and autografts[8-10]. Titanium interbody devices offer immediate stability and osseointegration, with numerous studies revealing successful results after implantation[9,11,12]. However, these devices have been criticized for an increased risk of subsidence, likely secondary to the high modulus of elasticity of titanium relative to bone. Solid titanium cages have demonstrated subsidence rates of 13-45%, compared to 8-15% with PEEK interbodies[13]. Chou et al. have even concluded that Titanium cages have lower fusion rates and higher complication rates compared to both PEEK and autografts[14]. PEEK has a modulus of elasticity similar to cortical bone, which theoretically leads to improved load sharing and better stress distribution. So, PEEK cage likely leads to less subsidence and a possible reduction in associated complications [10,14]. However, PEEK surfaces have been found to be less hospitable for osteogenesis and bio-incorporation compared with titanium[15,16].

The new generation of 3D Printed Titanium cages (3DTC) are highly porous with reduced stiffness compared to the original solid titanium cages[17,18]. Theoretically, these implants could reduce the risk of subsidence while maintaining the ability to distract the disc space and enhance osseointegration. Although there are multiple studies comparing functional outcomes between PEEK and the conventional Titanium cage, there is a paucity of data comparing outcomes of the PEEK cage and 3DTC with anterior plate fixation. Apart from material properties, subsidence is affected by many aspects of surgical technique, including endplate preparation, area of interbody contact and over-distraction. In this retrospective study, we compared the radiological outcome of one/ two level PEEK cages vs 3DTC in ACDF, all performed by a single surgeon. A single surgeon case series was evaluated in order to control for factors related to surgical technique. We hypothesized that 3DTC cages will have comparable or perhaps even less subsidence compared to PEEK cages.

Material and Methods

A retrospective chart review was performed after approval from the hospital Institutional Review Board. The electronic medical record was queried for patients who underwent ACDF surgery based on the clinical procedural codes (CPT) codes (22551) between December 2014 and December 2019. Patients over the age of 18 years who underwent one/two level ACDF surgery by the single senior surgeon (IM) with the use of PEEK cage (Stryker AVS AS PEEK cage, Allendale, NJ) or 3D printed titanium cage (3DCT) (Stryker Tritanium C; Kalamazoo, MI) and an anterior plate were included. Patients with corpectomy, standalone cage ACDF, multilevel (≥3 levels) ACDF and combined anterior and posterior procedures were excluded.

The senior surgeon started using synthetic material cages for ACDF surgeries in December 2014. PEEK cages were used in consecutive patients from December 2014 to December 2017 and 3DTCs were used for all consecutive patients from December 2017 to December 2019. All patients meeting inclusion criteria were included in the study to limit selection bias. Demographic variables, including age, sex, body mass index (BMI), Charleston comorbidity index (CCI), smoking status and operative variables, including interbody type, size, and follow up data were collected. Radiographic evaluation was performed on immediate post-operative x-rays and then compared with post-operative x-rays at 6 months and at least 1 year. All patients underwent the standard Smith-Robinson surgical approach to the cervical spine, standard discectomy, preparation of endplates, cage placement with supplemental demineralized bone matrix (DBM) bone graft, and instrumentation using the same anterior cervical plating system. A total of 26 patients (44 levels) in the PEEK group and 31 patients (48 levels) in the 3DTC group were found to have ≥6 months of follow-up (Figure 1).

Figure 1: Patient selection flow chart

Figure 2: Subsidence measurement. (a and b): immediate post op and 1 year follow up neutral lateral x-ray of patient with one level ACDF with 3DTC; (c and d): immediate post op and 1 year follow up neutral lateral x-ray of patient with one level ACDF with PEEK; IVH(i): IVH on immediate post-operative x-ray; C2d(i): C2 inferior endplate depth on immediate post-operative x-ray; IVH(t): IVH at follow up; C2d(t): C2 inferior endplate depth at follow up)

Cage subsidence was measured as the change in the adjusted intervertebral height (IVH) between immediate post-operative and subsequent follow-up neutral position lateral radiographs[19]. IVH was measured from the midpoint of the superior endplate of the cephalad vertebrae to the midpoint of the inferior endplate of the caudal vertebrae. IVH was adjusted using the inferior endplate C2 vertebral body depth to account for magnification using the depicted formula (Figure 2).

[aIVH(t): adjusted IVH at follow up, IVH(t): IVH at follow up, C2d(t): C2 inferior endplate depth at follow up, C2d(i): C2 inferior endplate depth on immediate post-operative x-ray]

This technique was selected to avoid potential measurement errors from x-ray magnification. All the radiographic measurements were performed by single orthopedic spine surgeon. Severe subsidence was defined as the ≥ 3 mm of change in the aIVH at follow up, when compared to immediate postoperative radiographs[13,20].

Results

There was no significant difference noted in patients’ age, sex, BMI, smoking status and CCI between groups (Table 1). No statistical difference was noted between groups with regards to the various operative variables collected, including number of levels, cage size and cage heights (Table 1). The most commonly fused level was C5-C6 (Table 2). Overall, subsidence was 1.17 mm (95% CI: 0.82 – 1.51, p<0.001) at 6 months and 1.42 mm (95% CI: 1.07- 1.78, p<0.001) at 1-year follow up. The 3DTC group had significantly lower subsidence compared to the PEEK group at 6 months (0.84 mm vs 1.59 mm, p <0.05) and at 1 year (0.85 mm vs 1.92 mm, p<0.05) (Table 3, Figure 3 and 4). Overall, severe subsidence rates were 9.37% for 3DTC group versus 19.04% for the PEEK group at 1 year. There was no significant difference in the subsidence between cage sizes within both groups. Follow up rate at 6 months was 80.0% and at 1 year was 64.2%. Only 1 patient in the PEEK group had a superficial infection at 2 weeks which was managed with oral antibiotics. There was no significant difference in complications between groups (p<0.05) (Table 4).

* Analysis conducted using Fisher’s Exact test

† Analysis conducted using student’s t-test

‡ Cage size information was missing for 1 level in each group

¥ Cage height information was missing for 1 level in titanium group

Table 1: Basic demographic variable distribution between two groups

Table 2: Distribution of fused levels

*P-value is statistically significant

Table 3: Mean difference in subsidence and severe subsidence rate at 6 months and at 1 year

*Analysis conducted using chi² test

Table 4: Comparison of complications between the groups

Figure 3: Subsidence over time. ΔS : Mean difference in subsidence at particular time point , * P-value is significant

Figure 4: Subsidence is presented as a percentage of intervertebral height. PEEK had significantly higher subsidence compared to titanium at 6 month and 12 months follow-up. (P<0.05)

Discussion

The ideal interbody cage should provide immediate stability and promote osseointegration in order to achieve bony fusion and maintain indirect decompression through disc space distraction. Cage subsidence reduces foraminal height, possibly leading to malalignment, recurrence of radiculopathy and poor functional outcomes after ACDF surgery[20]. The rate of cage subsidence has been used as an outcome measure for success and compared between different types of interbody devices in the past. Prior studies have demonstrated that titanium induces greater osteoblast differentiation from stem cells compared to PEEK, leading to a more favorable bone-implant contact surface and superior osseointegration with surrounding bone[21]. This inherent characteristic leads to improved fusion rates compared to PEEK, but at the cost of increased subsidence from high modulus of elasticity[22].

Additive manufacturing (i.e. 3D printing) with titanium allows the creation of open cellular lattices with different densities to modulate implant stiffness, with the goal of more closely approximating the properties of spinal bone[23]. The highly porous architecture also mimics cancellous bone, leading to a reduction of stress shielding at the bone-implant interface. Along with this added benefit, 3D printing also allows the feature of “in growth” topologies and pores that specifically encourage osseointegration and increases bone bonding[18]. Therefore, 3D printing technology can increase, or at least maintains, the fusion ability characteristic of the standard titanium, with a potential added advantage of subsidence reduction[17]. Our study found that 3DTC has 1.07 mm (95% CI 0.39 – 1.76) less subsidence compared to PEEK cages at 1 year follow up (p<0.05). Brecevich et al. previously demonstrated 95.37% fusion rates in their 108 ACDF levels with 3DTC, without incidence of severe subsidence (≥3 mm) or cage migration. However, they did not present actual subsidence numbers regarding each fusion level[17]. Despite our study revealing a minor, but statistically significant reduction in subsidence with 3DTC, there is no available literature to suggest the clinical significance of this minor subsidence.

In general, ACDF surgery leads to significant improvement in postoperative outcomes for all primary diagnoses over long term follow up, but several studies have shown lower patient satisfaction and outcome scores in patients with severe subsidence (≥ 3mm)[24]. Cabraja et al. evaluated subsidence in 86 single level ACDF patients and noted that solid titanium group had 20.5% subsidence while PEEK group had 14.3% subsidence[25]. Overall, severe subsidence rate was higher in PEEK (19.04%) compared to 3DTC (9.37%) in our study at ≥1 year follow up, but the difference was not statistically significant. Post-hoc analysis showed that a new study would need a sample of 204 in each group to be powered at 80% with an alpha level of 0.05. In other words, our study revealed that severe subsidence rates in 3DTC is likely comparable to PEEK, which was a drawback to the traditional solid titanium cage.

Literature has been regionally biased due to the fact that European and Asian studies predominantly report results of standalone cages without any screws while, while studies performed in the United States have predominantly reported outcomes of cage/screw combinations (standalone with screw(s) or cage with anterior plate and screws). Noordhoek et al. systematically compiled seventy-one studies and reported subsidence for different cage types and cage-screw combinations (CSC) [20]. They found that cage-screw combination has significantly lower subsidence rates, followed by PEEK, titanium and PMMA cages (15.1%, 23.5%, 24.9% and 30.2%, respectively; p<0.001) [20]. Our overall subsidence rate (14.86%) was similar to the CSC subsidence rate. Anterior plates for vertebral body fixation lead to stress shielding and likely reduces the degree of subsidence. Therefore, anterior plating or standalone cage with screws, which is a nearly universal ACDF technique in the United States provides inherent protection against subsidence, irrespective of the cage material.

The strength of this study includes the involvement of a single surgeon, which provides consistency in surgical technique- especially endplate preparation, the consistent use of same design PEEK cages and 3DTC, and a relatively large number of levels in both groups. The limitations of this study include its nature of being a retrospective cohort study, which has its inherent flaws and biases. There is also a lack of associated patient reported outcomes or satisfaction score to further elucidate the clinical effect of the subsidence difference. Even though, study has reported 1 year follow-up, it would be ideal to have at least 2 years follow-up. As this study only focus on subsidence and most subsidence occur during initial bone-resorption stage of bony healing, there is less likelihood that further longer follow up would change the result. Nearly 35.8% levels did not follow up for 1 year and later visit, which could have led to attrition bias in the result. It is crucial to carefully interpret the final result. Even though, overall study power is small, there were a total of 92 levels available to evaluate the radiological outcome, which is reasonable number considering the single surgeon cohort.

Conclusion

3DCT has a small, but significant reduction in subsidence compared to PEEK cages. Overall, the severe subsidence rate is relatively comparable between 3DTC and PEEK. A large power randomized control trial would be helpful to further evaluate clinically significant severe subsidence and long-term surgical outcomes of newer 3DTC interbody devices.

Funding:

No funding was received for the study.

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