Interbody Fusion Cage

 

By studying the progress of the treatment of diseases such as worn out intervertebral disc including disc herniation, spinal canal stenosis, vertebral displacement and height reduction due to disc tissue corruption, we found that intervertebral cages are more effective in the Interbody spine fusion method. we will be
The advantage of different cages compared to placing dense bone between two vertebrae has led to the widespread use of this treatment system by surgeons
Considering the essential role of cages in treatment and considering that back pain is the second reason for patients to visit doctors in the United States and accounts for a large amount of money annually
Investigating the mechanical properties of different cage models will lead to knowing the design factors and trying to optimize them in order to design the cage with maximum advantage
Conducting clinical research to prevent and treat common back pain is one of the first priorities of medical and engineering research centers, and this is because a significant part of these pains are caused by damage to the vertebrae
Sometimes placing some things on the back causes pain in the intervertebral disc and articular appendage and sacral articular cartilage, and the fracture of some organs from this structure is dangerous, but there may also be a connection between them
Fractures of some forms of this structure can be produced by mechanical loading in the laboratory. Also, in some cases, by weakening genetics and age-related factors, we can gently remove an injury
Biomechanics can be used to determine spinal loads and their movements, as well as analysis and distribution of loads and mechanical injuries and progress and therapeutic intervention. For this reason, the science of biomechanics is considered as a tool to correctly understand the injury process and find prevention and treatment methods. In this topic, an attempt is made to address the principles of biomechanics of vertebrae. Therefore, the anatomy of the spine and the mechanical function of the intervertebral discs are mentioned first

Anatomy of the spine

The spine is the basic and important part of the body skeleton that extends from the head to the pelvis. In fact, the body, with the spine, can both move and be in a straight position when standing. It also supports an important structure called the spinal cord. The anatomy of the spine consists of 24 vertebrae, each of which is a bone
These 24 vertebrae of the spine are movable vertebrae. The vertebrae are connected to each other by muscles and ligaments, the combination of these ligaments and vertebrae is called the spine
When we look at the spine from the side, we see an S-shaped curve. This S-shaped shape helps to distribute the weight in the body and prevents the weight of the body from falling on one part and damaging an area. Also, the "S" curve to the column It helps the spine to withstand all kinds of stress. The reason why the spine resembles the letter S is that the cervical spine bends slightly inwards, the chest curves outwards and the curved back bends inwards. In this case, all parts need the power of other parts to function properly. So, the spine is made of four arc-shaped parts, which are

The upper arch that is placed under the head is the cervical arch and its convexity is forward, this arch consists of 7 vertebrae. These vertebrae are named C1-C7 from top to bottom and from one to seven

The central arch is called the thoracic part, which is convex towards the back and consists of 12 vertebrae. The thorax is formed around these 12 vertebrae. The thoracic vertebrae are named from top to bottom and from one to twelve as T1-T12

The lower arch is the lumbar part, which is convex forward and consists of 5 vertebrae. This area bears the weight of the trunk and back pain is felt more in this area. Lumbar vertebrae are named L1-L5 from top to bottom and from one to five

The sacrum part or sacrum is made from the joining of several small vertebrae. The sacrum or sacrum is 5 vertebrae attached together. This bone is a part of the pelvic ring, so the vertebral column is connected to the pelvis through the sacral vertebrae or sacrum
Vertebrae of the spine move relative to each other and this movement occurs in the place of the intervertebral disc and the joints between the intervertebral processes. In this way, the vertebral column can curve forward, backward and sideways

Anatomy of the intervertebral disc
The intervertebral disc is a cartilaginous structure located between the bodies of the vertebrae. Vertebral bodies are in the form of bony cubes. Discs absorb and neutralize the forces entering the spine. In fact, the disc works just like a shock absorber. In other words, the disc transfers the force of the body's weight as well as the forces that enter the spine evenly and symmetrically between the vertebral bodies in the spine. The structure of the disc is generally made of fiber and cartilage, which is called fibrocartilage

The intervertebral disc is one of the complex structures of the human body that connects two vertebral bodies. The primary function of intervertebral discs is to transfer compressive forces from one vertebral body to the next intervertebral body, by which we can see the shape change and small movement of the discs

Due to their structural nature, intervertebral discs have low hardness and therefore can play the role of mechanical absorbers in the spine. This is while there is a possibility of damage to this soft tissue due to compressive forces, shear forces, bending and twisting moments.

  The intervertebral disc is composed of the core part located in the center and by the annulus

fibrosis and end cartilage plates are surrounded by upper and lower vertebrae

The core is a hydrostatic fluid and is a combination of water and Aggrecanproteoglycan gel in a combination of type two collagen and a network of elastin fibers. Another component of the disk is the annulus, a structure that forms 15 to 25 concentric layers around the nucleus. Each layer is a combination of type 1 collagen fibers, which are oriented at a positive and negative angle of 30 degrees to the horizon in successive layers. The intervertebral disc tolerates pressure due to the osmotic properties of Proteoglycans
Due to its gelatinous nature, the core of the disk is the place of compressive force distribution. This compressive force, which shows itself under hydrostatic pressure, causes the production of tensile stresses in the fibrous area

The ability of the disc to withstand lateral shear, anterior shear along with pressure and flexion has made this member the most important load-bearing component next to the facets in the spine
The nucleus acts like a gelatinous mass in the early years of life or in discs that have been destroyed a little. The compressive load reduces the height of the disc due to the reduction of the volume of the gelatinous mass. It also causes an increase in hydrostatic pressure and protrusion of the outer layers of the Annulus. During the day, the compressive force reduces the height of the disc because the disc loses its water and the collagen and viscoelastic fibers of the annulus undergo creep. Both effects are reversible with night rest and lying down, i.e. in a state where there is no load on the spine
Long-term loading on the spine causes swelling of the annulus and increased load on the facet joint. The destroyed disc changes the structure and function. Prolonged sitting will cause an axial compressive load that changes the viscoelastic properties of the disc and vertebra

Since the disc is a mechanical structure and is constantly moving, it undergoes wear and degenerative changes from the very beginning of childhood. These changes in the future cause back pain or diseases such as lumbar disc rupture, spinal stenosis and spinal curvature such as kyphosis and scoliosis. The actual cause of spinal diseases such as disc rupture is not known, but a mixture of genetic predispositions, old age, biomechanical changes, and environmental factors such as the type of job and lifestyle are known to be important in causing it. It is called multifactorial
 

Diseases treated with spinal cage

The most common problems treated with spinal fusion surgery and cage placement include pain and loss of function caused by

• Degenerative disc disease: a condition in which the disc begins to degenerate and lose function

• Spondylolisthesis: slippage of one vertebra on the other below it

• Spinal tumors: Tumor growth in the spinal tissues that may reach the spinal cord disrupts the stability of the spine

• Spinal canal stenosis: a complication that causes narrowing of the bony openings of the spinal nerves

• Herniated disc: A condition in which the internal contents of the discs leak out
Reversal back surgery (due to failed previous surgery) and correction of spinal deformities, such as scoliosis or kyphosis, may also require spinal fusion

 

Introduction of cages and how to place them between two nuts
1- The cage can be placed between two vertebrae with an incision in the abdomen through access from the front part, which is called this access
  Anterior Lumbar Interbody Fusion (ALIF)

2- The cage can be placed from the lower back, which is called Posterior Lumbar Interbody Fusion (PLIF)

3- Transforaminal Lumbar Interbody Fusion (TLIF) This cage is also placed from the lower back
One difference between TLIF and PLIF is the angle with which we access the disc, but both methods are performed through an incision made in the patient's back

4- Anterior Cervical Interbody Fusion This cage is placed between two vertebrae with a cut from the front of the neck

 

• The shape of the cage is a function of the method used for access. The cages used in the ALIF method are generally larger and more circular. PLIF cages are generally rectangular and right-angled. TLIF cages are usually curved or rectangular

 

• The geometry of the cages, when used with posterior stabilization, does not affect the structural stability of the cage, due to the type of posterior stabilizers, the tension shield phenomenon has occurred, so the load on the anterior column is significantly reduced
• The teeth on the surface of the cage are to lock the location of the cage and prevent it from moving
• In patients with osteoporosis, there should be sufficient contact surface between the cage and the plates of the two vertebrae to prevent subsidence
• In patients with degenerative disc, the plates of two vertebrae must be flat, in which case the flat cage is suitable to increase the contact surface and prevent subsidence

• The presence of a cage leads to an increase in the amount of compressive stress in the inner parts of the body of the vertebra compared to the normal and healthy state. Stress on bone tissue is a special feature of a good environment for successful fusion, which strongly affects bone growth and regeneration
• It should be considered that the body of the nut is almost oval and the cages have square ends and may be placed outside the disc. Care should be taken to place the cage around the disc because there is a possibility that the posterior side of the cage will enter the nerve cavities. and scratch the nerve nodes or the anterior-lateral part of its anterior edge