Lumbar degenerative disc disease (DDD) is one of the most frequently encountered spinal disorders resulting in low back pain. Despite the fact that DDD starts in young patients and is often asymptomatic, it is believed that DDD is the major cause of low back pain, occurring at any stage of the degenerative process and varying from affecting the elasticity of the nucleus at the early degeneration to complete disc degeneration and severe loss of disc height, deformity, instability and compression of the neural elements at the final stage of DDD.
First step in the treatment of symptomatic DDD is nonoperative treatment. When no satisfactory results are obtained with nonoperative treatment surgical intervention might be considered as the next step. As currently no definite distinction can be made between both pain mechanisms the surgical intervention should address both potential causes of pain: removing of the nucleus and maintaining or restoring of natural anatomic structures and the stability of the index segment.
Currently the common surgical treatments are removal of the indexed disc followed by either fusion or disc arthroplasty. For decades fusion has been the gold standard for the surgical treatment of discogenic back pain. Although fusion can provide pain relief and restoration of spinal stability, there is a high rate of surgery related complications. In addition, 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, total disc arthroplasty (TDA) has gained more acceptance as a better surgical treatment for discogenic back pain. Disc arthroplasty aims at restoration of function and flexibility, reducing the chance for developing adjacent level disease. Most of the disc arthroplasty systems are based on removal of the entire disc; nucleus, annulus and endplates, and placement of an artificial disc with an inner ball/socket articulation.
Clinically TDA has shown reasonably good results, both in relieving pain and maintaining motion. However, the invasiveness of the TDA surgery, revision risk and accelerated facet degeneration may negatively impact the broad acceptance of TDA.
As there is a big gap in invasiveness and risk profile between non-surgical treatment and major surgeries such as fusion and TDA great efforts have been made to bridge this gap. One of those was the development of nucleus replacements. The designs and materials for nucleus replacements vary from different hydrogel or non-hydrogel elastomers, which are either preformed or formed in situ, to non-elastomeric materials, such as metal, PMMA and pyrolytic carbon. Most of these devices have been complicated by subsidence, extrusion, and reactive endplate changes. This could be explained by the fact that most of these devices are either too soft which has risk of extrusion or using rigid non-articulating constructs which do not allow for uniform load distribution. (see figure 2)
Based on the knowledge about the current technologies a new technology has been developed.
New technology: NUBAC™ Disc Arthroplasty System
In healthy discs the nucleus shares the compressive load with the annulus and distributes the load evenly over the endplate under all physiologic loading conditions. Restriction to the rotational motion is not based on the characteristics of the nucleus, but relies on the annulus and facet joints.
|Figure 1: The NUBAC Disc Arthroplasty System|
As in early to moderate DDD primarily the nucleus is affected leading to the mechanical and chemical origins of pain it seems to be a logic step only to replace the 'diseased' part of the disc and keep the healthy structures as much as possible. It is well known that removal of the nucleus can cause collapse of the disc height and lead to further instability of the index segment, which in turn can cause mechanical pain.
This newly developed device NUBAC which will be used for arthroplasty should maintain or restore the disc height and mechanical function of the disc (see figure 1).
The major benefits and pitfalls as described above have been taken into consideration in the design of the NUBAC Disc Arthroplasty System. NUBAC preserves the advantages of nucleus devices and is less invasive and less bridge burning, while capturing the articulating characteristics from TDA's, but being less bridge-burning than TDA's. The NUBAC has a unique two-piece design with an inner ball/socket articulation.
|Figure 2: Schematic drawings of stress distribution under compression and Bending.|
The NUBAC has an oval geometry to resemble the shape of the nucleus so it can conform to the cavity shape. Like the natural nucleus, the articulating characteristics of the device do not have constraint on physiological rotational motions, which are maintained by preserving the surrounding annulus and ligaments. In addition, the articulating properties allow more uniform stress distribution on the endplates independent from bending position (see figure 2 ). Lastly, it is expected that the risk of extrusion is minimised by the fact that the implant articulates in concert with the bending of the spinal segment under minimal shear forces.
A= Bulk polymer nucleus implant under compression; B= bulk polymer nucleus implant under bending; C= partial extrusion of bulk polymer nucleus implant under bending; D= NUBAC disc arthroplasty device under compression; E= NUBAC disc arthroplasty device under bending.
The NUBAC is made from PEEK-OPTIMA™ (Invibio, Greenville, NC, USA) with an elastic modulus close to that of the underlying bone. The device is radiolucent, while tantalum markers are used for visualization of the device on radiography. It is one of the top medical thermoplastics with a well-documented biocompatibility and biostability history, along with an excellent track record of being used as a permanent implant, including spinal implants.
NUBAC has a novel PEEK-on-PEEK articulating design for load bearing application. Therefore extensive wear tests have been conducted to assess the wear characteristics of this device. The NUBAC device was assessed for the wear properties by three different tests.
In the first test, six implants were subjected to dynamic axial load between 225-1024N coupled with a flexion/extension rotation in such a way that peak load coincides with the maximum rotation angle and the ROM in lumbar discs. The load magnitude was chosen based on the design goal to share the compressive load between device and annulus. The test was conducted up to 40 million cycles to address long wear durability while each 10 million cycles the devices were rotated for 90o to simulate flexion/extension and lateral bending profile. The wear rate was 0.28 ± 0.07 mg/million cycles over 40 million cycles.
In the second and third tests, coupled-motion in accordance with ISO/DIS 18192-1 was used, one without frequency shifting and one with frequency shifting to mimic the cross-shear effect. Like the previous described test a dynamic compressive load of 225 to 1024 N was utilised. These compares very favorably to the wear rates reported for ProDisc-L and Charité using the same standard (See figure 3).
As expulsion is one of the most commonly seen complications for nucleus replacements tests were conducted to determine the risk for expulsion. Six adjacent pairs of human cadaver lumbar functional spine units (FSU's) were tested. Disc level L3-4 remained intact and served as control while L4-5 was implanted with a NUBAC. Each specimen was tested for 100,000 cycles of unilateral bending opposite to the annular window. The average bending level was 4.4o at the beginning and 9.2o at the end. Implant expulsion did not occur for any of the samples.
These results demonstrate a low risk for implant expulsion that could be due to the inner articulating design which allows the two plates to have full contact with the endplates in any position and helps to remain the implant in the center of the disc.
|Figure 4: Operative Level Kinematics|
The effects of the NUBAC on the multi-directional ROM and neutral zone and the effect on disc height, ROM and stiffness were examined in two separate studies. Following annulotomy/nucleotomy the segmental ROM and neutral zone significantly increased while the disc height showed a significant decrease. After implantation of the NUBAC the kinematics were re-established to the intact condition indicating that the in vitro biomechanical function of the spine with the NUBAC is similar to the intact spine. (See figure 4)
(a) Range of motion. * Indicates statistical difference at p<0.05. No other differences were observed. Error bars indicate one standard deviation. (b) Neutral zone. *Indicates statistical difference at p<0.05. No other differences were observed. Error bars indicate one standard deviation.
A prospective, longitudinal, multicenter clinical study was initiated to examine safety and effectivity of the NUBAC in patients with symptomatic single-level or double-level DDD in Europe and Asia. The first patient was included in December 2004 and at submission of this manuscript over 100 NUBAC devices were implanted. No major intra-operative and post-operative vascular and neurological complications have been reported.
Patient evaluation occurred pre-operative and at respectively 6 weeks, 3, 6, 12 and 24 months post-operatively with self-assessment Oswestry Disability Index (ODI) and Visual Analog Scale (VAS) for pain.
|Figure 5: Mean ODI scores and improvement of ODI scores compared with pre-operative condition.|
|Figure 6: Mean VAS scores and improvement of VAS scores compared with pre-operative condition.|
The ODI score improved from 50.4 preoperatively to 31.1, 26.6, 24.5, 23.0 and 10.3 respectively at 6 weeks, 3, 6, 12, and 24 months post-operatively.(see figure 5) In addition the VAS decreased from 76.6 pre-operatively to 30.6, 31.1, 27.0, 26.5 and 23.5 respectively at 6 weeks, 3, 6, 12, and 24 months post-operatively. (see figure 6)
The main indication for the NUBAC device is discogenic back pain secondary to DDD, similar to that for interbody fusion and TDA devices. In literature a significant decrease in VAS and ODI scores are reported for both ProDisc and Charité TDA's. , The preliminary results obtained with the NUBAC shows a similar trend for these outcomes. (see figure 5 & 6)
For the groups “ProDisc-L” and “NUBAC pre ODI > 40” patients with an pre-operative ODI score greater than 40 were included. For the group “Charité” patients with a pre-operative ODI score greater than 30 were included.
Although these results show similar trends it should be stated that the results obtained for the TDA's are based on randomized controlled studies with a completed follow-up period of 2 year, while the results obtained with the NUBAC are based on a longitudinal study.
While both NUBAC and TDA devices are intended for DDD, the NUBAC device differs from TDA devices in the requirement for a certain minimal disc height. This is based on the clinical objective to restore/maintain disc height and natural load sharing between nucleus and annulus. If one uses disc height as indicator for the stage of DDD, significant disc height loss represents a late stage of DDD, indicating that the NUBAC device is more adequate for patients at an early to moderate stage of DDD.
Preliminary radiographic analyses show that the center of rotation (COR) was restored to the normal location for a healthy disc after implantation of the NUBAC device. (see figure 7) This might be explained by the fact that most of the disc tissues, like annulus and ligaments, are preserved and the stability of the index segment is restored after the implantation of the NUBAC device.
Next to the above described study, NUBAC received conditional approval from the FDA to proceed with a feasibility study in July 2006, and the first patient was enrolled in July 2006.
NUBAC disc arthroplasty is a less invasive technology indicated for patients with discogenic back pain caused by early to moderate DDD, while fusion and TDA should be indicated more for patients at the late stage of DDD due to their invasiveness and more bridge-burning nature.
Other than the less invasive and less bridge-burning characteristics of the NUBAC device the NUBAC has the advantage of being able to be implanted with different approaches and has been implanted successfully via posterior, lateral (ALPA) and retroperitoneal antero-lateral approach.
The initial encouraging clinical results and the absence of any major intra-operative and post-operative vascular or neurological complications suggests that the NUBAC could be a viable, first line surgical treatment option for patients with low back pain caused by DDD.
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