Introduction
Lumbar spinal stenosis is defined as the narrowing of either the spinal canal or neural foramina.Underlying causes for the stenosis are hypertrophy of the ligament flavum and facet joints, osteophytes, spondylolisthesis and disc protrusion and commonly a combination of these causes is present (Figure 1)1,2.As spinal stenosis is a disease that is caused by the degeneration of the spine,patients aged 50 years and older are affected.Neurogenic intermittent claudication (NIC) is the most common and most described symptom associated with lumbar spinal stenosis3. Symptoms of NIC include back pain and radiating pain to the buttocks,thighs and/or legs and numbness buttocks,thighs and/or legs. The complaints become worse on prolonged standing,and are often exacerbated with walking,activity or lumbar extension.The symptoms are relieved with sitting,lying,or lumbar flexion explaining that patient with more severe symptoms may walk and stand in a characteristic bent position to relieve the pain. Biomechanically, relief of pain by lumbar flexion is explained by the increase of dimension of the spinal canal and neural foramina during flexion which reduces the compression of theneural structures of the spine and results in relief of the pain4.After diagnosis,initial treatment will focus on nonoperative treatment and may includeanalgesics, nonsteroidal anti-inflammatory medication, epidural steroid injections,oral steroids and physical therapy. For decades decompressive surgery alone or combined with fusion was the only surgical treatment option to patients who failed to respond to the nonoperative therapies. The reported success rate of decompressive surgery is quite variable,and the procedure is associated with a relatively high complication and reoperation rate5,6,7.Turner's meta-analysis of surgery for lumbar spinal stenosis reported good to excellent results ranging from 26 to 100%8. Decompressive surgery was seen as the gold standard for patients suffering from spinal stenosis. Recently, several studies indicate that patients who underwent surgery showed more improvement than patients who were treated nonsurgically, supporting a shared decision-making approach among physicians and patients when considering treatment options for lumbar spinal stenosis7,9. Based on these studies,the fact that the indicated patient population is an older population that may not be able to undergo general anaesthesia required for a decompressive surgery and that over the past several years, the current trend in surgery has been shifted to minimal soft-tissue dissection and to preserving as many structures of the spinal segment as possible, a new treatment alternative has been developed.
Interspinous decompression system
The new alternative treatment option is commonly known as interspinous decompression system as the device is implanted between the spinous processes.The working mechanism is based on the principle of reducing the pathologic extension at the symptomatic level, while allowing flexion, bending and axial rotation. Although interspinous decompression systems are a relatively new alternative, kinematic, biomechanical and clinical results with these systems, mainly the XStop (Medtronic) are published. Kinematic studies have confirmed that interspinous implants are able to decrease the range of extension, but does not affect flexion, bending or rotation10. Biomechanical studies concluded that interspinous decompression systems are able to reduce the load of the spinal structures, e.g. decrease of disc pressure in the annulus and decrease of the mean.
Interspinous decompression system
 |
| Figure 1a: Healthy spine unit |
 |
| Figure 1b: 1 Protrusion; 2 Hypertrophy ligament flavum; 3 Hypertrophy facet joints |
The new alternative treatment option is commonly known as interspinous decompression system as the device is implanted between the spinous processes. The working mechanism is based on the principle of reducing the pathologic extension at the symptomatic level, while allowing flexion, bending and axial rotation.
Although interspinous decompression systems are a relatively new alternative, kinematic, biomechanical and clinical results with these systems, mainly the X Stop (Medtronic) are published. Kinematic studies have confirmed that interspinous implants are able to decrease the range of extension, but does not affect flexion, bending or rotation10.
Biomechanical studies concluded that interspinous decompression systems are able to reduce the load of the spinal structures, e.g. decrease of disc pressure in the annulus and decrease of the mean facet pressure after implantation of an interspinous decompression system11,12. A cadaveric study, confirmed by a clinical study determined that after implantation of an interspinous decompression system that canal area, canal diameter, subarticular diameter, foraminal area and foraminal width all increased13,14.
Clinical studies demonstrate superior results for intervention with interspinous decompression systems, two years after surgery, 60% of the patients reported significant improvement of their symptoms, 57% reported significant improvement of their physical function and 73% were satisfied or very satisfied15.
The preclinical and Class I clinical data are mainly collected for the X Stop. As the design of other interspinous decompression systems widely vary from the X Stop, e.g., non-ligament sparing, preparation of cortical bone of spinous processes, surgical approach, limiting the non-symptomatic flexion, either very elastic (silicon) or hard (titanium) material, loading area, the preclinical and clinical data for these systems will also differ from the data obtained for the X Stop.
BacJac™ Interspinous Decompression System
 |
| Figure 2: BacJac Interspinous Decompression System |
The BacJac Interspinous Decompression System (Pioneer surgical Technology) is designed based on the X Stop, and has additional advantageous features (Figure 2). The BacJac is intended for patients with LSS and presenting symptoms of INC and for patients with degenerative spondylolisthesis no greater than grade 1 combined with intermittent neurogenic claudication. Other potential indication are Baastrup's syndrome/'Kissing spine', axial-load induced back pain, facet syndrome, degenerative and/or iatrogenic (post-discectomy) disc syndrome, adjunctive to micro-discectomy, and topping off to a lumbar fusion procedure, primary or secondary if the spinous process is intact.
Advantages of most of the interspinous decompression systems are that these are minimally invasive, preserving ligaments, tissue-sparing and keeping alternative surgical options open. The BacJac has additional advantages like implantation via a true unilateral approach with fascial incison on only one side, self-deploying, only axial force is needed for distraction, large contact area and no CT/MRI artifact (Figure 3).
 |
| Figure 3: Advantages and disadvantages of Interspinous Decompression Systems |
The BacJac is designed as a self-deploying device. In the undeployed configuration the arm that will be positioned between the spinous process has an height of maximal 8 mm (Figure 4). Previous research showed that for other interspinous implants with an height of 10 and 12 mm the insertion load was significantly less than the spinous process failure load, although it is expected that the insertion load will increase with larger (higher) implants and may subsequently result in trapdoor fractures. As the initial height of the BacJac is maximal 8 mm, it is expected that the insertion load for the BacJac is even lower, so decreasing the risk for trapdoor fractures during insertion16.
 |
 |
| Figure 4a: Initial insertion height of BacJac | Figure 4b: Final height of BacJac |
The final height of the BacJac is obtained by deployment via instrumentation. As the BacJac is already on its final position between the spinous processes, only axial force is applied on the spinous process during the increase of the height of the BacJac. This will also decrease the risk for damage of the bony structures of the spinous processes. For most other systems the implant is pushed laterally between the processes, so combining lateral insertion force with increasing height, this may results in fractures of the spinous processes or other damage.
The design of the BacJac include some features to minimize the risk for subsidence. Subsidence into the spinous process may be the result of point loading or the stiffness of the implant material.
Point loading may occur for implants with a, for example, cylindrical configuration. This configuration results in a small contact area between implant and spinous process, resulting in a higher average contact stress. If the contact area stress exceeds the strength of the bone of the spinous process subsidence will occur. To decrease the risk for subsidence the contact area should therefore be maximized. The BacJac design has included a larger contact area by two characteristics. Firstly, the contact area of a flat, not spherical, surface resulting in a increase of the contact area. Secondly, as anatomical dimensions of man differ, the length (distance anterior-posterior) of the spinous process can vary. Therefore, the BacJac is available in two footprints with respectively AP depth of 12 and 15 mm, to maximize the contact area to personal anatomical dimensions.
The BacJac is made from polyetheretherketone (PEEK) (PEEK Optima, Invibio, Greenville, NC, USA). Next to excellent wear characteristics, PEEK has an elasticity modulus comparable to the bone. Implants of stiffer materials may be subject to subsidence as the implant respond different on pressure by anatomical positions. As the BacJac is made of a material with a comparable stiffness of bone and a design that allows this modulus to take effect, it is expected that less subsidence will occur. Accompanying advantage of PEEK is that it does not cause any CT/ MRI artifacts.
Surgical Technique
As indicated, the BacJac will be inserted unilaterally with fascial incision on only one side. The procedure is a simple, 3-step, procedure that shortens the OP-time. Patients may be operated under local anaesthesia with light intravenous sedation or under general anaesthesia. The supraspinous ligamentous complex is preserved, which serves to stabilize the implant, and is also important to prevent postoperative kyphosis.
The 3-step technique consists of the following steps: dilation, sizing and implanting (Figure 5). The intraspinous ligament is pierced and dilated, but retained. Following the dilation, sizing and implantation are conducted.
 |
 |
 |
| Figure 5a: Dilation | Figure 5b: Sizing | Figure 5c: Insertion of BacJac |
The Ti-pivot of the BacJac can be used as marker for the positioning of the BacJac during and after surgery (Figure 6).
 |
 |
 |
 |
| Figure 6a: Pre-operative AP X-ray | Figure 6b: Post-operative AP X-ray | Figure 6c: Pre-operative lateral X-ray | Figure 6d: Post-operative lateral X-ray |
PEEK-Optima
The BacJac Interspinous Decompression System is, like the other Pioneer Peek-on-Peek (P3) products Nubac and NuNec, made of PEEK Optima. PEEK is a thermoplastic with an elastic modulus close to that of bone and is radiolucent. Previously biocompatibility and biodurability testing showed no significant material changes after aging and no cytotoxic and histopathologic responses or other inflammatory responses17.
Axial fatigue and static testing were conducted with the BacJac. The test were conducting following ASTM WK 7479, standard test method for the static, dynamic and wear assessment of lumbar extradiscal spinal motion preserving implants. The axial static test indicated an axial static offset yield load of 4.8kN. During the fatigue testing, BacJac samples successfully pass the fatigue test with run-out load 2100 N. Previously, a static mean failure load of 1250 ± 627 N was suggested for spinous process failure18. As both the static and fatigue failure testing loads are significantly greater the failure load of spinous process, these results suggest that BacJac can withstand loads significantly above the expected in vivo loads of the lumbar spinous process.
The reported wear rate for NuBac Disc Arthroplasty device, member of the P3 family for PEEK motion-preserving devices showed excellent wear characteristics. Coupled-motion testing was conducted with the NuBac in accordance with ISO/DIS 18192-1, one without frequency shifting and one with frequency shifting to mimic the cross-shear effect. A dynamic compressive load of 225 to 1024 N was utilized. The wear rate compares very favorably to the wear rates reported for ProDisc-L and Charité using the same standard (Figure 7).
 |
| Figure 7: Wear characteristics lumbar disc arthroplasty devices |
Since the BacJac has a non-articulating design, the amount of wear generated is considered to be insignificant under expected in vivo loading.
Conclusion
The BacJac is a new minimally invasive, stand-alone interspinous decompression device which can be used as an alternative to conservative and decompressive treatments. Its design is based and improved on current insights on the working mechanism, benefits and disadvantages of known interspinous decompression systems. As the BacJac design incorporates the unique material characteristics of PEEK with the unilateral approach respecting a maximum of anatomical structures it is expected that this device will be a less invasive treatment option for patients with spinal stenosis caused by DDD.
In summary, the BacJac Interspinous Decompression System is:
- based on a clinically proven concept
- a safe and effective treatment for NIC
- improved for unilateral approach
- self-deploying
- tissue sparing
- ligament preserving
- minimal risk of subsidence (large contact area & near-physiologic modulus)
