Spinal Fusion

What is the spinal fusion?

Spinal fusion is a surgical procedure aiming to permanently immobilize a portion of the spine. It might be used alone or in combination with other surgical interventions (decompression, tumor removal, deformity compression etc.).

What is the structure of the human spine?

The spine is a mobile yet robust structure composed of vertebral bones and joints. It bears body weight and protects the spinal cord and nerves. There are 33-34 vertebral bones comprising the spine – 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 3-5 fused coccyx bones. The cervical, thoracic, and lumbar segments are mobile. There are three joints between each pair of vertebral bones: intervertebral disc joints and a two facet joints. Disc joints are the largest and are located in the front part of the spine between vertebral bodies. They carry the majority of the load and provide mobility to the spine. Facet joints are relatively small, paired accessory joints located in the back of the spine. They are formed by extensions of the vertebral bones called facet processes (or articular processes). The main function of the facet joints is to restrict excessive mobility of the disc joint and prevent slippage of the vertebral bones.

Disc and two facet joints connecting two neighbor vertebral bones comprise motion unit or articular triad. Besides joints there are several ligaments in spine holding vertebral bones together. These ligaments restrict excessive motion and protect spine from injury. There are many muscles surrounding the spine and most of them run parallel to the main its main axis. These muscles have two main functions – provide stability and motion.

Spine stability

Spine is a stable structure i.e. under normal circumstances its motion does not disrupt its structural integrity. This stability is provided by vertebral bones, joints, ligaments and muscles. Normally each motion segment of spine has physiological range of motion. Cervical and lumbar segments have high ranges of motion per segment. Thoracic spine is more restricted due to presence of the rib cage. But thoracic spine has more motion segments than cervical and lumbar. Therefore, the overall range of motion of thoracic spine is substantial. Sacrum and coccyx consist of fused vertebral bones and basically immobile.

The normal range of motion is very important for protection of neural structures running inside it i.e., spinal cord and nerves. Spinal cord runs in a vertical narrow tunnel called spinal canal. The spinal cord does not occupy the canal completely, there is some space around it for extra accommodation. The shape and dimensions of spinal canal change slightly with motion, yet extra room between spinal cord and the walls of canal prevent compression. Thus, spinal stability is very important for protection of vulnerable neural structures in it. Loss of this stability may compromise spinal canal and cause neurological damage.

What are the conditions leading to loss of spinal stability?

There are many conditions resulting in loss of spinal stability. Instability itself may be remarkable or subtle depending of causative factor and the ability of spine to compensate.

Trauma

Trauma is a frequent cause of spinal instability. Fractures of vertebral bones, lacerations and sprains in joints and ligaments result in loss of stability and misplacement of vertebral bones. As a result, the spinal canal may get compromised leading to spinal cord compression. Traumatic impacts act too fast for spine to adapt and compensate with catastrophic consequences. Trauma is the leading cause of spinal cord damage and disability worldwide.

Tumor

Another common cause of spinal instability is spinal tumors. Spine may be affected by various tumor types, yet the most frequent type is metastasis. Cancer spread to vertebral bones is very frequent. Usually these nests of cancer cells destroy bone (osteolysis) and occupy the empty space. Hence the growing tumor gradually weakens the spine and cause pathological fracture. In addition to loss of stability cancer may spread to spinal canal and compress the spinal cord and nerves directly. Usually, cancer-related instability is subtle due to slow growths speed and compensation. Very frequently cancer affects only one part of vertebral bone (usually vertebral body). The other parts remain intact and compensate for a period of time until their compensatory capacity is exceeded.

Infection

Spinal infection may affect vertebral bones (spondylitis) or disc joints (discitis or diskitis). Usually we encounter combination of thereof in form of spondylodiscitis. Bacteria are the main causes of spinal infections yet fungi and parasites occasionally may also spread to the spine. In most cases the spinal infections are caused by pyogenic (pus forming) bacteria like Staphylococcus or Streptococcus. Other bacteria causing spinal infections are Enterococcus, Escherichia coli, Proteus, Klebsiella, and Pseudomonas. Tuberculosis contributed to substantial portion of spinal infections in the past. Nowadays, the spinal tuberculosis is much less, yet not uncommon especially in developing countries. Cutibacterium Acnes (formerly known as Propionibacterium Acnes) causing skin acnes may also spread to spine and cause infection. Kingella kingae is a little known germ that may cause disc inflammation in young children. Viruses do not cause spinal infections.

Spinal infections may erode disc joint and adjacent vertebral bones and cause instability. Sometimes bones may weaken so much that they break up under body’s weight. In other cases, infection may spread directly into the spinal canal and cause epidural abscess.

Disc degeneration

Disc degeneration is a very common condition leading to herniation and/or spinal stenosis. The degeneration is a destabilizing factor for motion segment. The disc joint consists of two parts – nucleus pulposus (or simply nucleus) and annulus fibrosus (annulus). Nucleus is damaged in lumbar disc degeneration and looses its main function – ability to withstand body weight. However this process is slow and other parts of affected spinal segment (ligaments and facet joints) try to compensate by becoming bigger and stronger. This process leads to spinal stenosis. It is not surprising that common surgical treatment like laminectomy, facetectomy, discectomy fail to provide benefit because they do not restore the stability. Conversely they may aggravate the condition by weakening the damaged segment even more.

Surgery

Surgical intervention itself can be source of instability. Usually accessing spinal cord and/or nerves requires removal of parts of vertebral bones and ligaments. Although, spine may initially tolerate and later restore some degree of instability there is threshold it can bear. This is especially true when surgery includes several vertebral segments. For example, post-laminectomy syndrome is a well-known clinical scenario of iatrogenic instability.

It should be noted that in many cases surgeons are unaware of underlying instability. Degenerative disc disease and its derivate conditions like disc herniations and stenosis are frequently treated with decompression surgery alone. Usually this procedure involves removal of the part of spinal bones, ligaments and joints to accomplish the goal. Vertebral lamina, facet joints, ligamentum flavum, posterior longitudinal ligament and disc joints are frequently removed/disrupted by surgical procedure. Combined with pre-existing degeneration-induced instability, surgery further aggravates segmental instability and worsens patient condition.

Radiation therapy (RT)

Radiation is one of the most important tools to suppress cancer growth. Several types of spinal tumors are frequently treated with RT. Some tumors are very sensitive to radiation (multiple myeloma) others are resistant to it (chordoma). Yet in the vast majority of cases RT is valuable option to stop cancer cells from multiplication. Yet RT affects not only the tumor cells but also normal cells. As a result, the weakened segment may develop later instability. Usually, RT induces instability develops years later after treatment. For most cancer patients with short life expectancy this is not a problem. But in long term survivors may develop RT related spinal instability.

In many spinal oncological cases the radiation is added to surgery. For example, surgeon may use laminectomy to access the spinal tumor and remove it. Then the radiation therapy is given to further suppress the tumor. The combined effect of tumor, surgery and radiation may compromise spine’s integrity and may induce instability. Usually the effect of multiple factors (tumor, RT, surgery etc) is much more than the sum of individual factors.

Congenital conditions

In some instances spine fails to develop properly during embryological period. For example, lumbar spondylosis is frequent condition characterized by improper formation of lumbar vertebrae. As a result the back portion of the vertebra is disconnected from the front making facet joints ineffective. Usually, though this conditions do not become symptomatic until adulthood. Increased mechanical stress on disc joint due to lack of protection by facet joints leads to accelerated degeneration. Combined with congenital instability this leads to slippage of vertebra – the condition is knows as spondylolisthesis. Another example of congenital problem potentially leading to instability is os odontoideum i.e., erroneous formation of 2nd cervical vertebra. Specifically the odontoid process fails to fuse with rest of vertebra and may result in atlantoaxial instability.

What is spinal fusion procedure?

In many cases spinal instability develops due to irreversible loss of bone and/or ligaments. Therefore, the only practical way to restore the stability is to fix the damaged sector by merging vertebral bones. Spinal fusion aims to permanently fuse one or more vertebral bones together and restore stability. The mobility of the segment is compromised in this process as a trade-off for stability.

What is the difference between spinal instrumentation and spinal fusion?

Spinal fusion is a commonly performed procedure for treating various spine conditions. Unfortunately, it is a misleading term since fusion is not achieved during surgery. Rather spinal instrumentation and bone grafting are implemented. These terms are used interchangeably and cause confusion even among medical practitioners.

• Spinal instrumentation or stabilization is a surgical procedure that includes placing hardware and bone grafts in the spine in order to achieve spinal fusion in the future.
• Spinal fusion is the end-result of healing which generates new bone binding vertebrae. Fusion does not develop instantly after the surgery; rather, it requires some time for bone-forming cells to multiply and make new bone. It is very similar (but not identical) to fracture healing. Arthrodesis is another term for fusion.

Therefore, spinal fusion is no more than desired effect of spinal instrumentation surgery. Whether this effect materialize or not depends on the number of factors. Surgical technique, material type, hardware quality, type and amount of bone graft material, age of the patient, his general condition, health of skeletal system and even smoking status are all variables that affect the outcome.

How does the spinal fusion develop?

Following surgery successful bone fusion needs some time to materialize. The eventual goal is to unite two or more adjacent vertebral bones. There are several important player contributing to the surgical success:

Surgical technique

Surgical technique is probably the most important factor contributing to successful outcome. There are numerous surgical methods and hardware types used for spinal stabilization. Unfortunately, not all of them are equally effective. Years of practical experience have proven that some are much better than others. The combination of a cage inserted between vertebral bodies with supplemental screws is the most robust stabilization technique. The cage acts like a pillar bearing the weight and screws hold the segment still. Clinically this combination yields the best outcomes. The cage can be inserted into the emptied disc space (interbody cage) or after entire vertebral body has been removed (corpectomy cage). Generally, the bigger the cage the better the outcome. Specifically the footprint area of the cage is of great importance. Big cages with wide footprint areas are best but may be technically challenging.

Among various types of screws used in spine surgery pedicle screws provide the best stabilization. They are inserted from the back through pedicle screws into the vertebral body. Then they are connected via metallic rods and fasteners. In cervical region the operation is usually performed from anterior and pedicle screws cannot be placed. Alternatively, anterior plate screw system is a plausible alternative to pedicle screws.

In order to achieve the best clinical outcomes both cages and screws should be implemented. In many cases though surgeons may not use cages but screws only. In cervical spine, frequently cages are placed without supplemental screws and plates. Unfortunately, these procedures rarely end up with fusion. Lack of fusion is called non-union or pseudoarthrosis is not an uncommon problem following spinal stabilization procedure. Pseudoarthrosis may be the source of significant disability for patients.

There are many other spinal fixation devices used for stabilization. Facet screws, dowels, interspinous stabilizers, and many other devices are available. However, in terms of stabilization robustness pedicle screw and cage combination is far more superior.

Hardware

One of the most important aspects of the procedure is the robustness of the hardware. Fusion does not develop if bones are mobile. It is similar to casting in order to achieve bone healing following a fracture. However, casting the spine is of limited value for achieving fusion. Doctors usually have to place metallic hardware in order to immobilize the spine robustly.

Bone graft

Another important factor is the quality of the bone graft material. Unlike fractures, spine fusion is usually performed under conditions when bones have a particular distance between each other. Therefore, in order to fill the gap, the surgeon has to place bone material into that space. The ideal bone graft should have three properties for successful fusion:

• Osteoconduction is a process by which graft material provides a scaffold for newly developing bone. Bone-forming cells (osteoblasts) migrate into the graft, make a new home, and start forming new bone.
• Osteoinduction is a process of stimulating osteoblasts to migrate, divide, and make new bone. This process is complex and involves multiple proteins that promote the proliferation and division of the cells.
• Osteogenesis is the process in which osteoblasts located in the graft material proliferate and create new bone tissue.

There are many bone graft options available for surgeons.

• Autograft– a graft obtained from the patient themselves. It is considered the best graft material since it has osteoconductive, osteoinductive, and osteogenetic properties.
• Allograft– a graft material obtained from another person. It does have osteoinductive and weak osteoconductive properties. But there is no osteogenesic properties.
• Xenograft– a graft material obtained from animals. It has a limited value in spinal surgery.
• Artificial bone substitutes – artificially created grafts that contain calcium crystals. They have only osteoconductive properties. Sometimes, they are augmented with growth promoters to have additional osteoinductive properties. However, this feature is usually weak.

Patient’s health

3. The patient’s condition is very important for developing fusion. Advanced age, osteoporosis, poor bone quality, and bad general condition are factors impeding the fusion process. Smoking is also very detrimental to fusion, and therefore, patients undergoing fusion surgery should seriously consider smoking cessation.

What are the conditions that require treatment with spinal fusion?

There are several reasons for which surgeon may be willing to perform a fusion procedure on a patient:

• Instability. It is a condition when the spine’s integrity is compromised, leading to abnormal motion. Instability is not a diagnosis itself; rather, it’s a condition seen with other disorders (trauma, tumor, etc.). Instability causes significant pain since the spine’s load-carrying capacity is compromised. Additionally, it may cause abnormal displacement of vertebral bones with spinal cord compression and lead to paralysis.
• Tumor. In some cases, tumors growing in the spine may cause bone destruction and cause instability. Sometimes, in order to reach and eliminate the tumor, a surgeon has to remove a part of the spine. This procedure may further compromise the spine’s stability, and stabilization should be performed.
• Degeneration. Disc degeneration may cause loosening of the supportive system and cause instability. This effect is frequently seen in lumbar and cervical disc herniations. In this case scenario, the surgeon has to restore stability during surgery. In other cases, stabilization is performed to achieve permanent fusion and avoid recurrence of symptoms in the future.
• Infection. Infections of the spine may destroy bone and ligaments and cause instability.
• Spondylolisthesis, or simply spinal slippage, is a condition when one vertebral body is slipped either to the front (anterolisthesis) or to the back (retrolisthesis) regarding the vertebra below. There are many reasons for spondylolisthesis, such as trauma, degeneration, and congenital disorders.
• Trauma. Trauma is one of the most common reasons for fusion. Vertebral bone fractures and soft tissue disruption may cause instability and compromise the spinal cord. Therefore, stabilization must be performed in these cases to restore normal structures and function of the spine.
• The deformity is a condition when a part of the spine has lost its normal appearance. Scoliosis and kyphosis are the two main deformity types. If surgery is performed, an affected segment is corrected, and hardware is placed to hold that part until the fusion develops.

What are the types of spinal fusion?

There are several types of spinal stabilization and fusion used for various reasons. Generally, the spine is divided into anterior and posterior in regard to the spinal canal. Spinal fusion may be anterior, posterior, or combined. Sometimes, spinal fusion is performed from the side, and the procedure is called lateral. However, strictly speaking, even though the procedure is performed from the side, stabilization is done anterior to the spinal canal, and therefore, it is considered a variant of anterior surgery.

Spinal fusion is also classified according to the segment of the spine involved. Cervical, thoracic, lumbar, cervicothoracic, thoracolumbar, and craniocervical fusion types are examples of these fusion types.

There are several common types of spinal fusion:

• Anterior cervical discectomy and fusion (ACDF). The surgery is performed for cervical disc herniation from the front of the neck. Following the removal of the disc herniation, the surgeon performs stabilization by using cages, plates, and screws. Usually, fusion develops within several weeks following the surgery.

In the lumbar spine, there are several interbody fusion options:

• Posterior Lateral Interbody Fusion (PLIF) is performed from the back. Following discectomy, stabilization is performed by inserting a straight cage and placing bone pedicle screws
• Transforaminal Lateral Interbody Fusion (TLIF) is performed from the back slightly lateral than PLIF by inserting a bigger, curved cage and bone pedicle screws
• eXtreme Lateral Interbody Fusion (XLIF)is performed from the side through the psoas muscle, and a big cage is placed. It may require the placement of additional bone pedicle screws.
• Anterior Lateral Interbody Fusion (ALIF)is performed from the front, and a big cage is placed. It may require the placement of additional bone pedicle screws.

What is the failed fusion syndrome?

In some cases, fusion fails to develop after the stabilization procedure. This condition is called nonunion or pseudofusion. It is one of the most common reasons for failed back syndrome. In these cases, adjacent bones become connected by soft tissue instead of solid bone. There are several reasons for nonunion. These factors are patient or technique-related. Patient-related factors are advanced age, bad general condition, smoking, and poor bone quality. Technique-related factors are lack of robust stabilization and/or lack of adequate grafting.

There are two possible scenarios for failed fusion: hardware loosening or breakage. Screws, plates, and cages become loose in case of failed fusion and move. Loose screws usually pull out from the original location. Loose cages erode and dig into nearby bones – a condition called subsidence. In some cases, hardware may not tolerate constant bending forces and break down.

Failed fusion is one of the most common reasons for spinal revision. However, this type of revision surgery is very complex and, therefore, should be performed by a highly experienced surgeon.