The present invention relates to the field of spinal surgery, in particular to the field of interbody implants (cages), intended for implantation between two adjacent vertebrae. Interbody spinal fusion requires the use of a cage capable of supporting approximately 500 kilograms without deformation in order to carry out the distribution of axial stresses on the operated segment. Cages can be surgically installed from the anterior or posterior approaches. The main advantage is that interbody cages are capable of restoring and maintaining the vertical distance between the vertebrae and sustaining the permissible norms of spinal curvature.

However, patients often return, and the pain syndrome persists, which according to some authors leads to repeated surgical interventions in 6-32% of cases. The most common cause of repeated operations is spinal segment instability due to the absence of fusion between bone and implant surfaces, as well as the migration of implanted cages.

In addition, the different moduli of elasticity between the cage the vertebra where it is implanted leads to biomechanical conflict. In some cases, the titanium implant can damage the nearby vertebrae with which it comes into contact.

These different moduli of elasticity can cause risk factors for the development of adjacent segment syndrome, a complication of spinal fusion, when adjacent spinal segments begin to become unstable.

Risk factors for adjacent segment syndrome include: the use of several cages in multi-level spinal fusion, post-operative incorrect sagittal balance, degenerative processes in the adjacent segment and facet joints that develop before the operation and the patient’s age.

Currently, implants are being developed with the intention of making the biomechanical properties of spinal fusion implants compatible with the biomechanical properties of the body's own tissues. There are attempts to introduce other porous metals (tantalum and titanium nickelide) as new materials for cages, but so far these attempts have been unsuccessful, and synthetic materials have been found to have biocompatibility limitations. Cages of porous titanium nickelide are discussed in prior art (see the article: «Application of an implant from porous titanium nickelide in the anterior cervical corpectomy surgery with reference to the compression fracture of the fifth cervical vertebra. Case study» Journal of Surgery of Kazakhstan N. 4, 2011 , p. 75-76). Porous titanium nickelide has elasticity similar to those of bone tissue in that it has a block structure with perforating porosity and capillarity that permits liquids to penetrate the entire structure of the implant. Due to this porosity, which is similar to vertebra bone porosity, the post-operative intergrowth of regenerative fibrous and bone tissue in the implant results in the implant's reliable fixation.

The multiphase alloy titanium nickelide is not recommended for use in the production of porous titanium cages due to its risk of releasing toxic nickel into body tissue. Another analogue of the invention is the spinal fusion cage produced by

Medbiotech Ltd. (http://medbiotech.bv/product/keidai)· There are two types of flat cages, type 1 being paralellepiped-shaped and type 2 banana-shaped. Both cages have a rigid titanium frame, anti-migration teeth on the top and bottom edges and a center cavity to be filled with bone graft materials. The rigid titanium frame takes all axial pressure while the cavity filled with bone graft materials is intended to stimulate osseointegration. The thickness of the rigid frame wall is 2-3 mm, and the height of the anti-migration teeth is up to 1 mm.

The disadvantage of the design lies in the fact that any weight falling on the working surface is distributed only on the surrounding frame’s upper and lower parts. This is because the bone graft materials have no dimensional stability and do not support the axial load. At the same time, the titanium alloy’s modulus of elasticity is 50-70 times higher than that of the overlying and underlying vertebrae's bone tissue, which leads to cage protrusion into the vertebral body, surrounding bone tissue lysis and cage migration. Another technical solution as a prototype is the utility model RU 160822, published on 04/10/2016, which eliminates the need of using bone graft materials. The patented implant consists of a rigid titanium frame in the shape of a parallelepiped which is pre-filled with pressed titanium mesh. This pressed titanium mesh has osteoconductive properties, holds the load of up to 500 kg and is not subject to lysis. When connective, bone and cartilage tissues begin to be regenerated through the implant, a complex is formed between implant and tissues that has an elasticity modulus close to that of bone tissue which might remove complications caused by differing moduli. Titanium has the highest bio-inertness of all metals, so the human body has no immunological reaction after implantation.

Despite these advantages, this cage has two disadvantages. Firstly, the 2 mm or more width of the cage edge, too wide to allow proper insertion between vertebrae, doesn’t permit the cage’s teeth to grasp the vertebrae securely, which can result in the cage’s migration. Secondly, the cage’s 2mm teeth cannot be lengthened to improve fixation since doing so would result in a wider space between cage and vertebrae surfaces, which risks migration as well as a delayed regeneration process. Therefore, the insecure contact between implant and bone surfaces precludes use of the porous titanium material advantages.

The qualities of this invention permit the best fusion because the two types of fixation improve on those used in earlier models: Firstly, since this cage has a wall thickness of the rigid frame and its edge is 0.3-1 .6 mm, located along the longitudinal axis of the porous platform, primary fixation due to longer, sharper teeth allows the inserted cage to grasp bone tissue more securely when the cage is inserted; since the cage’s rigid titanium frame edge is narrower than the previous model's more space is available for pressed titanium wire that makes the use of screws possible to secure the cage. Secondly, the secondary fixation of this invention is better than that of previous models because of its use of porous titanium instead of bone graft materials, which provides better osseointegration due to porosity and bioinertness. Moreover, the cage design makes the connection between cage and bone surfaces tighter, which facilitates regeneration from bone tissue rather than connective tissue which promotes better spinal fusion around and through the implant.

In a first aspect of the invention, a cage is provided in the form of a three- dimensional pattern comprising two parts: a rigid frame with top and bottom edges and a porous platform, which is located inside the rigid frame and is made of extruded titanium wire with a wall thickness of the rigid frame and its edge is 0.3- 1.6 mm, located along the longitudinal axis of the porous platform.

In preferred embodiments, the wall thickness of the rigid frame and its edge is 0.8-1.1 mm.

In some embodiments, the cage has teeth on the top and bottom edges of the rigid frame. In preferred embodiments, the teeth have a front cutting edge. In further preferred embodiments, the teeth have a height of more than 1.5 mm.

In further embodiments, the rigid frame has apertures on the side walls. In certain embodiments, the rigid frame has special elements for fixation such as screws and/or plates. In preferred embodiments, the rigid frame is affixed with screws that are guided by a 0-50 degree angle relative to the porous platform’s nearest contact surface.

This invention is illustrated by drawings: Fig.1 shows a general view of a lumbar cage and Fig. 2 indicates a cervical cage. According to the illustrations, the cage (Fig.1) comprises two major parts: a rigid titanium frame and porous titanium material pressed into the frame. The rigid frame (1) might be made of titanium alloy, titanium nickelide, ceramics or PEEK as the most biocompatible materials, but for the best osseointegration the preferable option is titanium alloy. The cage has teeth on the top and bottom edges of the rigid frame (3). Besides the fact that the rigid titanium frame’s teeth and screws allow the cage to have perfect primary fixation, the frame holds the cage’s different shapes, which depend on the spinal area in which it is implanted. The porous titanium material is also retained within the frame regardless of the shape it takes as the frame prevents the porous titanium material from going beyond the frame boards. The porous titanium material (2) is formed as the porous platform to provide very tight contact between implant and vertebrae surfaces for excellent osseointegration. The posterior part of the rigid frame has special features (6) such as holes, grooves, etc. for connection between the implant and special surgical installation tools. The porous titanium material is made of pressed titanium wire (the mark of the titanium alloy is GRADE 1 , GRADE 4, etc.), the diameter of which is 0.05-1 mm. For the best intergrowth between the cage and the spinal segment the porosity of the titanium porous platform ⁰ is 40-70% and pore size is 50-400 micrometers. The porous titanium material creates the contact surfaces between the implant and vertebrae and takes the entire vertical load. The rigid frame has anterior and posterior parts. The posterior part has a special feature (6) for fixation between the cage and a surgical tool. The other side of cage is anterior (4), which might be tapered or rounded to facilitate cage insertion into the interbody space. In the side view, the cage is considered to have symmetrical, equal parts relative to the longitudinal axis dividing the top and the bottom edge of the frame. The side part has apertures (5) as additional means for tissue intergrowth into the implant.

The shape that the cage finally takes depends on the operated spinal segment and the surgical approach. For example, the cervical cage has a flattened cylindrical shape. The cage for lumbar fusion might be bullet- or bean-shaped. The cage for ALIF (Anterior Lumbar Interbody Fusion) as a rule is heart-shaped.

The cage’s height might be the same along its entire length or it might decrease from its anterior to posterior parts. In that case, the cage has a lordotic angle, 1- 8° , which adjusts naturally to the shape of the cervical or lumbar lordosis. The cage has additional fixation features such as screws and plates, and provides three ways to use screws. First of all, for screw fixation a plate with holes might be used. The plates are necessary in cases when the screws need to be affixed to top and bottom vertebral bodies. In that case, the distance from any hole in the plate to the contact surface of the porous platform is 6-25 mm. The second option for screwing is provided by the special 3-5 mm flanges which protrude above and below the rigid frame’s surface. The third option consists of the possibility of guiding the screws through the rigid frame and thickness of the porous titanium material. In that case the screw holes are located in the rigid frame (Fig.2). All three options set the angle between the direction of the screw’s main axis and the contact surface of the implant in the range of 0-50°. Such different screw fixation options are necessary to adjust the cage to the operated spinal segment by taking into account the height of the vertebrae.

One innovation of this invention in comparison to the cage’s model described in patent RU 160822 is the changing of proportions between the cage’s rigid and porous parts that result in a range of advantages. The technical solution is to reduce the cage edge’s thickness located along the longitudinal axis of the porous platform up to 0.3-1 .6 mm which permits the load to be shifted from the rigid frame to the porous part thereby expanding the porous platform for the best contact between cage and vertebrae surfaces, which by minimizing the risk of the implant falling through the vertebrae is crucial for osseointegration. The fact of the load’s shift from the frame (1) to the porous platform (2) makes it possible to lengthen the teeth (3) to more than 1.5 mm and to sharpen them by creating a special anterior cutting edge on the tooth. This edge makes the insertion of the cage in the spinal interbody space easier than in the previous version of the cage. It is important to exercise the most expedient means to implant the insertions in order to reduce surgery time and minimize damage to the anatomical structures around the insertion space. Moreover, all of these construction features provide the cage’s most outstanding anti-migration properties.

Case 1. Patient L. 52-year-old female. Intervertebral disc herniation, level L4-L5. A discectomy through the PLIF (Posterior Lumbar Interbody Fusion) was performed. The measured intervertebral space was found to be reduced to 6 mm instead of the patient’s normal 10 mm. The intervertebral distance was restored using the cage (length 25 mm, width 10 mm, height 10 mm, rigid frame edge and wall thickness 0.5 mm, teeth length 2 mm, the teeth being located on the top and bottom edges of the rigid titanium frame). The cage was impacted into the intervertebral space by the special impactor tool which was connected with the implant through the cage’s aperture. The insertion of the cage was performed by rapping on the impactor’s handle. The cage’s anterior cutting edge made the slits on the vertebrae cortical bone surfaces. By this expedient the cage was inserted into the intervertebral space relatively easy. As a result, the height of the intervertebral space was reconstructed. 1 year later a CT scan showed the proper position of the cage with no dislocation.

Case 2. Patient P. 56-year-old male. Intervertebral disc herniation, level C5- C6. An ACDF (Anterior Cervical Discectomy and Fusion) was performed. The height of the intervertebral space was found to be 5mm. The cervical cage for the ACDF was used (Fig.2) (length 14 mm, width 14 mm height 5 mm, cage wall thickness 0.6 mm). The cage was affixed with the screws (diameter 3 mm, length 15 mm). After surgery the patient was observed for 3 years. During this time the position of the cage according to CT-scan date was stable and signs of osseointegration were clear. The fusion was assessed as well-formed.