NuNec: A PEEK-on-PEEK Cervical Arthroplasty System: In Clinical Perspective

Introduction

Currently anterior cervical discectomy and fusion (ACDF) is a widely accepted surgical treatment for patients with radiculopathy and myelopathy caused by degenerative disc disease (DDD). ACDF results in high patient satisfaction and high fusion rates and is a well accepted procedure. However, longer follow-up indicates that up to 25% of the patients develop recurrent radicular symptoms from adjacent level degeneration requiring surgical intervention within 10 years after the initial ACDF procedure. This adjacent level degeneration may be caused by the natural course of degenerative disc disease, but it is generally believed that fusion can significantly change the normal loading and mobility at adjacent levels which might attribute to the development of adjacent level disease.

More recently, cervical arthroplasty has gained more acceptance as potential alternative to ACDF. Disc arthroplasty aims to restore function and flexibility, reducing the chance to develop adjacent level disease. Short-term clinical results have shown that cervical arthroplasty has similar or better pain relief and maintains the motion of the index level , but long-term follow up has to show that cervical arthroplasty will reduce the incidence of adjacent level disease and accompanying need for surgical intervention.

NuNec™ Cervical Disc Arthroplasty System
The NuNec cervical arthroplasty system (Pioneer Surgical Technology BV) is a cervical arthroplasty device that is made of PEEK-OPTIMA and uses a unique CAM interference screw locking mechanism for endplate fixation (Figure 1).

CAM fixation provides for the ease of insertion and a low risk of vertebral body splitting in multi-level cases. The CAM fixation also has the advantage of easy reversibility either during surgery or later, if needed, over the keel design which is difficult to remove without the risk of damaging the vertebrae. Benefits of the PEEK-on-PEEK design include its biocompatibility, biostability, superb wear resistance and radiolucency. Most cervical artificial discs are manufactured in part from ferromagnetic materials, resulting in strong artifact on CT and MRI images (Figure 2). The outer surfaces of the NuNec device are coated with hydroxyapatite (HA) to enhance both the short- and long-term fixation to the vertebrae. A pilot clinical study was initiated to assess the safety and effectiveness of this device.

Preclinical Results
The NuNec has an HA coating that fulfills both applicable ISO and ASTM standards for coating of medical devices and it has been shown that the application of the HA coating does not affect the chemical and mechanical properties of the PEEK substrate.Figure 1. NuNec with unique CAM interference locking mechanism

Figure 2. MRI of NuNec (left) vs PCM (middle) and ProDisc (right), showing minimal MRI artifact.


The CAM design for fixation has been tested by bench-top pullout testing and has a fixation force significantly higher than most keel and flange-screw designs (Figure 3).

Figure 3. Fixation force of NuNec compared with other cervical arthroplasty devices and plate fixation
Figure 4. Flexion/ extension ROM restoration

The effects of NuNec on the unidirectional ROM (flexion-extension) were examined in human cadaver testing. Following discectomy the segmental ROM significantly increased. After implantation of the NuNec the normal kinematics of the motion segment were re-established. Treatment with ACDF using a cage and plate showed that the segmental ROM is significantly lower for ACDF when compared to NuNec (Figure 4).

Three months results from an in-vivo caprine model have shown excellent bony apposition to the HA coating (Figure 5).

Wear
For cervical disc arthroplasty devices long term wear resistance and durability are critical properties. Most of the cervical devices are made from traditional combinations of materials such as metal-on-metal and metal-on-polyethylene with known implications of either long term wear durability or metal ion release. To thoroughly assess the wear performance of NuNec, six PEEK-on-PEEK NuNec devices were tested following both ASTM and ISO load and motion profiles. The wear rate was respectively 0.26 ± 0.02mm3/ million cycles following ASTM method and 0.31 ± 0.02mm3/ million cycles following the ISO method. The wear rates are lower than other cervical arthroplasty devices that range from 0.38 to 3.00 mm3/ million cycles (Figure 6). These results indicate that PEEK might be a better choice as a self-mating articulating joint cervical arthroplasty.

Clinical Results
A prospective, longitudinal, multicenter clinical study was initiated in Europe and Asia to examine the safety and efficacy of the NuNec in patients presenting radiculopathy and/ or myelopathy caused by disc degeneration disease at C3 - C7 levels. Function and pain were investigated using self-administered patient questionnaires consisting of the NDI questionnaire to investigate function and VAS to investigate neck pain and arm pain. A typical anterior approach was used for discectomy and implantation.

Figure 5. Three months representative histological cross-section of NuNec. High magnification (right) showing PEEK (tan), HA coating (dark grey) and bone apposition (orange). Low magnification showing significant overall bone apposition (orange) and locking CAM (black)

This study reports our early clinical experience on our first 54 patients receiving the NuNec arthroplasty device at 94 levels starting in April 2008. There were 31 males and 23 females. The average patient age was 47.3 years ranging from 22 to 76 years old. The majority of the devices was implanted at C5-6 (44) followed by C6-7 (25), C4-5 (15) and C3-4 (10). 26 patients received the device at one level, 19 patients received the device at two levels, 6 patients received the device at three levels and 3 patients received the device at four levels.

Figure 6. Mean wear rate comparison
Figure 7. VAS and ODI scores

Average OR time was 110 minutes and no major intra-op or post-op complications occurred in this series.

The maximum follow-up was 12 months postoperatively. The NDI scores shows a functional improvement and the VAS scores show pain reduction for both the neck pain and the arm pain (Figure 7).

Figure 8. X-rays pre-operatively, 3 months after NuNec implantation and 6 months after NuNec implantation

Radiographic images showed no dislocation of the implant and maintenance of the normal range of motion (Figure 8).

Conclusion
Short-term results of previously reported studies indicate that cervical TDRs are safe and effective for the treatment of cervical radiculopathy and are equivalent or superior to ACDF. This study investigated the NuNec in a limited cohort and no long-term results are available yet, but the NDI and VAS scores indicate that the device is safe and effective, while demonstrating many of the design benefits. Longer term follow-up is planned and will be reported in the future.

References

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