Expert consensus on the application of intervertebral fusion devices for posterior lumbar spine surgery

source:

Chinese Journal of Spine and Spinal Cord Vol. 31, No. 4, 2021, pp. 379-384

Chinese Journal of Spine and Spinal Cord, 2021, Vol.31, No.4

Authors: Meng Hai, Yang Yong, Sun Tiansheng, Wu Ji, Li Zhongshi, Ruan Dike, Wang Bingqiang, Sun Changtai, Hong Yi, Li Chunde, Li Fang, Du Junjie, Liu Bo, Hai Yong, Li Li, Ding Lixiang, Liu Baoge, Lu Shibao, Tang Jiaguang, Zang Lei

Interbody fusion cage combined with pedicle nail rod system to implement lumbar posterior decompression fusion internal fixation is a common surgical method for the treatment of lumbar degenerative diseases such as lumbar spinal stenosis, lumbar spondylolisthesis, lumbar disc herniation with lumbar instability, including posterior lumbar interbody fusion (PLIF) and trans- trans- foraminal interbody fusion, TLIF), with good results. However, 0.9% to 4.7% of patients with posterior lumbar spine surgery have complications related to intervertebral fusion devices due to irregular application of intervertebral fusion devices, which makes it difficult to follow up treatment. In order to correctly and standardize the application of intervertebral fusion devices and reduce the occurrence of complications related to intervertebral fusion devices, the National Spine Salon organized by the editorial board of the Chinese Journal of Spine and Spinal Cord invited more than 20 spine surgery experts to form the following consensus for the reference of spine surgeons on the basis of evidence-based medicine and after multiple rounds of expert discussions.

01. Indications and contraindications for the application of intervertebral fusion devices in lumbar posterior surgery

In 1986, Bagby and Kuslich first designed an intervertebral fusion device for human use and applied it clinically, referred to as BAK (Bagby and Kuslich). Since then, the development of intervertebral fusion devices so far, regardless of their material/shape changes, have followed the "protrusion - compression" principle proposed by Baggy: after placing the intervertebral fusion device, the intervertebral space is propped open, so that the surrounding tissues such as fiber rings and anterior longitudinal ligaments are in a state of tension; at the same time, by the action of their own gravity and the contraction of the paravertebral muscles, the two antagonistic forces make the intervertebral fusion device reach a stable state and increase the spinal fusion rate.

1.1 Indications

Patients with lumbar degenerative disorders with indications for interbody fusion can be used as an indication for the use of intervertebral fusion devices in posterior interbody fusion techniques.

1.2 Contraindications

(1) Incomplete endplate due to various reasons; (2) spinal infectious diseases, spinal tumors; (3) patients with serious spinal canal adhesions, severe stenosis of the intervertebral space and severe osteoporosis should be used with caution; (4) patients with allergies to intervertebral fusion materials are banned.

02, the material of the intervertebral fusion device

There are four main types of materials for intervertebral fusion devices: (1) biological categories. Mainly refers to allotopic bone. Its advantage is that the source is sufficient, ensuring the amount of bone graft, and facilitating the rapid fusion of the intervertebral space. However, there is a risk of disease transmission, and it is difficult to maintain a complete structure to resist axial loads, which is not ideal for maintaining the height of the intervertebral space and physiological curvature. (2) Metals. Mainly including titanium alloys. Excellent biomechanical properties, good early stability, easy to 3D printing to prepare personalized intervertebral fusion devices. Its disadvantages are large elastic modulus, stress occlusion, large risk of sinking of the fusion device, occupying an effective bone growth space, X-ray examination is difficult to determine the bone fusion inside it, there are metal artifacts in CT and MRI examination, and the production of metal debris may cause long-term bone dissolution. (3) Polymer materials. There are carbon fiber, polyetheretherketone (PEEK) and so on. It is characterized by elastic modulus close to bone tissue, mechanical properties close to cortical bone, small stress occlusion, good light transmittance, and is conducive to imaging observation. However, its surface toothed protrusions are relatively shallow, the bite force is poor, and it is prone to displacement, which must be used in combination with strong internal fixation. (4) Degradable polymer materials. Such as polycarbonate, α-polyacids and the like. The advantages are good hydrophilicity and good biocompatibility; but the intrinsic fragility makes it easy to fragment during surgical operation, the bone replacement time is longer, and the high concentration of degradation products (acid and crystal components) can lead to serious tissue reactions, such as infection, bone dissolution, limiting its clinical application.

The ideal intervertebral fusion device material should have the following characteristics: (1) good biocompatibility; (2) stable chemical properties; (3) biomechanical properties similar to bone tissue; (4) promote bone fusion; (5) facilitate image observation; (6) be degradable.

03, the shape and type of intervertebral fusion device

There are a variety of design options for the profile of the blender. From the cross-sectional point of view: rectangular and kidney-shaped are currently two popular designs, rectangular fusion devices are usually used in pairs; the contours of the kidney-shaped and intervertebral discs are closer, which can better share the stress, usually used alone. Considering the sagittal surface, the fusion device is mainly wedge-shaped and olive-shaped. Wedge refers to the shape of the front high and low back, so that the physiological curvature of the lumbar spine can be effectively restored after the fusion device is placed, but this shape will also cause greater resistance when placed, and the insertion operation is relatively difficult, mainly for patients whose intervertebral discs have degenerated or become diseased; the olive-shaped contour is similar to the physiological curvature of the endplate surface, and the insertion operation is easier.

According to whether the internal hollow design of the intervertebral fusion device is divided into three types: closed intervertebral fusion device, open intervertebral fusion device and porous intervertebral fusion device. Closed intervertebral fusion devices are generally solid structures with or without surface coatings in metal materials; open intervertebral fusion devices, that is, common rectangular structures, hollow design, which can be placed in granular bone; porous intervertebral fusion devices are often made of tantalum metal, and their micropore rate is as high as 80%, and the structure similar to bone trabecles can make the fusion rate of bone higher.

The ideal intervertebral fusion device design should meet the following conditions: (1) Immediate stability. For example, the tooth process on the surface of the intervertebral fusion device increases the bite force, or the expansion propping device increases the stability after implantation. (2) Long-term stability. The large graft cavity capacity inside the intervertebral fusion device and the large contact area with the adjacent vertebral body facilitate the final fusion. (3) Close to the normal physiological curvature of the lumbar spine, in line with the normal anatomy of the human body's intervertebral space. (4) Can adjust the size, easy installation and non-destructive placement path.

04. The height of the intervertebral fusion device

The high choice of intervertebral fusion device is crucial for the successful completion of intervertebral implantation. If the height of the intervertebral fusion device is selected, it may lead to difficulty in insertion, excessive stretching of the intervertebral space, and increase the probability of nerve root traction; at the same time, the stress of adjacent segments is increased, increasing the incidence of degeneration; and the overload of the intervertebral fusion device is too high, which also leads to an increase in sedimentation rate. Insufficient intervertebral fusion device height does not restore intervertebral height and lumbar precociousness, and may also lead to intervertebral fusion device displacement and fusion failure.

Recommended protocol: (1) Preoperative planning. The height selection of the intervertebral fusion device is related to the patient's sex, height, surgical segment, and intervertebral height, especially the height of the intervertebral fusion device is estimated based on the normal intervertebral height adjacent to the surgical segment. (2) Intraoperative mold trial. After the intervertebral space treatment is completed, the fusion device mold trial operation is carried out from small to large, if the test mold does not match the preoperative estimated height, the mold test shall prevail. (3) Intraoperative perspective. After the completion of the pressurization operation, the C-arm X-ray machine is used for intraoperative fluoroscopy, and it is generally believed that the height of the posterior edge of the lateral position like the upper intervertebral fusion device should be slightly greater than the height of the posterior margin of the intervertebral fusion, which is conducive to increasing stability and reducing the incidence of loose exit of the intervertebral fusion device.

05, the number and length of intervertebral fusion devices

Bilateral use of 2 intervertebral fusion devices during PLIF surgery is the standard procedure. Two intervertebral fusion devices are usually placed with relatively short length fusion devices, and the placement trajectory tends to be parallel, avoiding the collision of the head end and affecting the stability.

In order to reduce the damage to the bony structure of the posterior lumbar column and reduce the interference with nerve tissue, more and more scholars have advocated the unilateral use of a fusion device in recent years. Finite element analysis and clinical trials have shown that there is no statistical difference in the stability and final fusion rate of the use of double fusion devices and single fusion devices, among which the use of single intervertebral fusion devices can reduce the operation time and bleeding volume, reduce medical costs, and reduce the pressure of adjacent segments, but in terms of maintaining the height of the intervertebral discs, some literature reports that the use of double intervertebral fusion devices is more advantageous.

The use of a single intervertebral fusion device should be in line with its design philosophy and is usually applied to TLIF surgery. More literature reports that the use of a single intervertebral fusion device does not increase the risk of its sinking and can effectively maintain the height of the disc. However, the premise is that the length of a single intervertebral fusion device is sufficient to make both ends reach the edge area of the end plate and reduce the risk of sinking of the fusion device; secondly, optimize the placement trajectory of the fusion device, and the entry point is as close as possible to the outer disc away from the midpoint of the intervertebral disc, so that the fusion device as a whole is more easily located in the intervertebral midpart, so as to achieve balanced compression force on both sides and improve stability.

06. The position of the intervertebral fusion device

The placement of intervertebral fusion devices should follow its design philosophy. According to the clinical characteristics of different patients, their specific positions are adjusted accordingly. If it is necessary to restore lumbar anterior convexity, the position of the intervertebral fusion device is placed in front of the midpoint of the intervertebral center; if the focus is on restoring the height of the intervertebral space and the intervertebral foramen, the intervertebral fusion device should be properly placed back; if there is scoliosis deformity, the intervertebral fusion device should be placed on the left or right side as needed.

The standard for the insertion depth of the intervertebral fusion device is that the front end of the fusion does not exceed the anterior edge of the vertebral body, and the posterior end is located at least 3 mm in front of the vertebral posterior edge of the vertebral body. On the one hand, the intervertebral fusion device can avoid stimulating the dural sac and nerve roots; on the other hand, because the endplate of the vertebral body is slightly curved, the middle height of the intervertebral space is greater than the height of the posterior edge of the intervertebral space, and after the pressure of the upper and lower end plates, the height of the posterior edge of the intervertebral space is less than the height of the fusion device, which can effectively prevent the fusion device from moving backwards. Try to avoid shorter lengths of the fuser located in the central area of the intervertebral, where the trabecular bone is weak, the compressive resistance is low, and it is easy to sink; the intervertebral fuser should be placed close to the edge area of the end plate to increase stability.

07. Preparation of bone grafting bed

Posterior decompression exposes the intervertebral disc, partial resection of the intervertebral disc, the amount of resection of the intervertebral disc should first meet the smooth implantation operation of the intervertebral fusion device, and secondly, there is enough space to fill the bone graft to achieve an effective fusion contact area. The amount of bone grafting in the space of intervertebral fusion devices is limited, and pure cancellous bone should be selected as much as possible, and its peripheral bone graft is more important. The use of bone graft funnels or homemade tubular channels in bone graft operations can not only push bone graft particles into the intervertebral layer efficiently and accurately, but also avoid the spilling of bone graft particles into the spinal canal and cause nerve interference.

After partial resection of the intervertebral disc, protect the dural sac and nerve roots, scrape the upper and lower cartilage endplates of the intervertebral space with a spatula, scrape the bone end plates into a rough surface or punctate bleeding state, and maintain the longitudinal support function of the bone end plates. Avoiding the deep penetration of the spatula into the anterior fibrous ring and anterior longitudinal ligament injuring the large blood vessels in front of it, preventing rupture in front of the fibrous ring is also an important factor in the anterior prolapse of the intervertebral fusion device, and avoiding damage to the dural sac and nerve roots when the spatula enters and exits the intervertebral space.

08. Fusion device space and intervertebral bone grafting

Intervertebral fusion device can maintain intervertebral height and stability, in order to obtain the best spinal fusion rate, the internal or/and surrounding environment of the intervertebral fusion device needs to be further supplemented by other osteogenesis, bone conductivity, bone induction graft materials, including autologous skin, cancellous bone, bone marrow, allogeneic bone and bone morphogenesis protein (BMP), etc., where autologous bone is considered the "gold standard" of the transplant material, and care should be taken to remove the soft tissue around it when preparing the autologous bone graft material.

Adequate bone graft volume and fusion contact area are the guarantee of effective integration of the spine. It is generally believed that the amount of intervertebral bone graft should be no less than 5ml, and the contact area between bone graft and end plate should be greater than 30% of the end plate area, otherwise the spinal fusion rate will be significantly reduced. The amount of bone grafting in the space of intervertebral fusion devices is limited, and pure cancellous bone should be selected as much as possible, and its peripheral bone graft is more important. The use of bone graft funnels or homemade tubular channels in bone graft operations can not only push bone graft particles into the intervertebral layer efficiently and accurately, but also avoid the spilling of bone graft particles into the spinal canal and cause nerve interference. Early autologous cancellous bone often comes from the iliac region, and existing studies have confirmed that autologous fragments of decompression removal are consistent with iliac bone grafting. The combination of bone graft materials and bone growth factors is increasingly widely used in the clinic, the most common is bone morphogenesis protein-2 (BMP-2), which can induce undifferentiated mesenchymal cells to differentiate into osteoblasts and chondrocytes, and then induce the formation of new bone. Clinical studies have confirmed that autologous bone combined with BMP-2 can increase spinal fusion rates.

09. Fixed in the back road

Intervertebral fusion devices are used in posterior lumbar decompression fusion surgery, usually in conjunction with a strong internal fixation system. After the intervertebral fusion device is placed, it often needs to go through the pressurized operation of the internal fixation system, so that the fusion device and the graft bone can better contact the vertebral body, the stability is higher, and it is conducive to spinal fusion. The compression operation needs to be moderate in strength, such as insufficient strength, the intervertebral fusion device is more likely to loosen; while the pressure strength is too large, it is easy to damage the bone end plate, and the sinking and instability of the intervertebral fusion device occur.

The intervertebral fusion device combined with the lumbar posterior semi-rigid/elastic internal fixation system can also obtain satisfactory clinical efficacy, which not only ensures the stability of the fusion segment, but also makes the load forward and stress distribution more extensive and uniform, which is conducive to reshaping the stress conduction of the fusion segment of the spine, reducing the gravitational attraction of the bone-screw interface, increasing the load of the bone graft area, and promoting bone graft fusion.

10. Summary

In summary, posterior lumbar fusion surgery using intervertebral fusion devices, as a mature technique for the treatment of degenerative diseases of the lumbar spine, can provide excellent segment stability and create a good fusion environment for implanted bone. However, how to reduce the complications caused by the use of intervertebral fusion devices is worthy of further exploration and research; at the same time, it is also expected that the new intervertebral fusion devices and internal fixation systems will meet the needs of different patients and achieve individualized treatment purposes.