Artifical Vertebrae, by Matthew McNulty

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Motivation

The vertebral column is an essential structure in the human body. The column protects the spinal cord, a key component of the central nervous system. The protection comes in the form of shielding from impact, pinching, and other sources of trauma. Beyond protection, vertebrae provide sites of attachment for muscles, ligaments, and bones, specifically for ribs in the thoracic curve, all of which work in synchronization to allow mobility and structural stability for the body. [1]

There are many medical issues involving the vertebral column, including scoliosis, kyphosis, disc inflammation and slippage, and degenerative diseases such as arthritis.[1] Implications from such conditions, along with damage from fractures and tumors, can yield severely damaged vertebrae. Artificial vertebrae would provide such patients a normal, healthy life.[2]

Background

Vertebral column

Source A: General depiction of the spinal column regions..

Adult humans generally possess 33 vertebrae, 24 articulating and 9 fused, which compose the vertebral column. The column itself is defined in five regions[3]. From top to bottom:

  • cervical curve - 7 articulating vertebrae making up the neck
  • thoracic curve - 12 articulating vertebrae in the chest with ribs attached
  • lumbar curve - 5 articulating vertebrae supporting torso and head weight[4]
  • sacral curve - 5 vertebrae that fuse with age caged in the pelvis
  • coccyx - 4 vertebrae fused with age remnant of a vestigial tail[3]

Individual Vertebra

Individual vertebra differ significantly depending on the region of origin in the vertebral column. However, there are general structural characteristics of individual vertebra that enable full function.

Source B: General depiction of an individual vertebra with labeled structures..

The vertebral body is the largest structure of the vertebra. It serves as the main weight bearing area, taking about 80% of the load while standing. As so, the vertebral body increases in size each vertebra from the head down. The body also provides attachment for the cushioning discs between the vertebrae. The anterior body additionally gives protection to the spinal cord. [5]

The pedicles are two projections from the posterior vertebral body that provide side protection to the spinal cord. The pedicles help form the spinal canal as well as bridge the body to the rest of the vertebra. [5]

The lamina are flat plates that form the spinal canal and provide further support and protection for the spinal cord. [6]

The spinous process is a projection perpendicular to the lamina. Each spinous process is connected by ligaments to those above and below it in the vertebral column as well as being the site for important muscle connections necessary for movement and stability. These are the noticeable bumps you can feel running your hand down your spine. [6]

The transverse processes extend out towards the sides of the vertebra and provide additional attachment sites for ligaments and back muscles. These are normally absent in the cervical curve. If present, they often impair nerve roots and cause pain. [5]

The facet joints interlock with those above and below it to provide stability and at the same time mobility. Each vertebra possesses four facet joints, two superior and two inferior. The interlocking facet joints are surrounded by a synovial capsule to prevent grinding. [5]

The spinal canal is made up from various parts of the vertebra. The canal is a bony tunnel that surrounds the spinal cord and nerve roots for protection.

History

Source C: Robert Hibbs.
  • 1902 – experimented with steel rods and wire anchored to spine with autographs [7]
  • 1911 – Robert Hibbs performed first formal spinal fusion operation [7]
  • 1931 – Hibbs modifies the operation to achieve 20% success rate [7]
  • 1955 – Paul Harrington developed Harrington distraction instrument to straighten the spine and hold in place during spinal fusion operation. [7]
  • 1977 – George Bagby creates the “Bagby Basket”, a perforated stainless steel cylinder for stabilizing the cervical spine of horses. The operation with the cylinder yielded 20% death rate. [8]
  • 1984 – Cortrel and Dubousset develop segmental instrumentation system. It used two crosslinking rods for correction. [7]
  • 1996 – FDA approves redesigned titanium “Bagby Basket” for human use. [8]
  • 1997 – First expandable cage approved for use. [8]

Health Issues

Artificial vertebrae can be used to replace malfunctioning or severely damaged vertebrae. The main causes for replacement are fracturing, tumors, and degenerative diseases.

Fractures

Fracturing of the vertebral column is classified as either a burst, compression, or flexion-distraction fracture. The more common and less severe fracturing, compression fractures are due weakening of the vertebra, such as falling on the buttocks or ejection from an ejection seat of a jet fighter. As the vertebral body carries the majority of the load of the vertebra, compression fractures intuitively collapse the anterior vertebra (vertebral body) while mostly leaving the posterior portion alone. The thoracic curve is the most likely region for compression fracturing to occur as it supports a great deal of the axial load. [9]

Burst fracturing, accounting for 17% of spinal fractures, is a break from high-energy axial load causing the vertebra to shatter on impact. Neurological damage, occuring in 30% of cases, furthers mobility hindrance to the patient. As opposed to compression fracturing, burst fracturing damages the entire vertebral structure, not just the body. [10]

Flexion-distraction fracturing is caused from the spinal column being flexed in a way that causes one or more vertebrae to snap, in essence it is pulled apart. . This often occurs with force to the chest. As so, this type of fracturing is most common in a head-on car crash where the upper body is launched forward while the lower body is held in place by a lap seat belt. [12]

Tumors

Spinal tumors are rare but pose a great threat to the integrity of the spinal structure. These tumors often originate in other locations throughout the body and metastasize, most often lymphatically spread, to the spinal column.[6] This means that the tumors on the spinal column are often composed of abnormal cells of a different region of the body that had entered the blood stream from a primary tumor and reached the spinal column. [11]

Degenerative Diseases

Degenerative diseases can be cause in itself for need of artificial vertebrae but are often the root of a complication, such as with osteoporosis increasing the likelyhood of fracturing. There is a large variety of degenerative diseases affecting the spinal column and several are listed below with a brief description.

Spinal stenosis is a narrowing of the spinal canal by a build up of tissue. This can eventually compress nerves and the spinal cord.[13]

Osteoporosis reduces bone mineral density and deteriorates bone structure. This results in weaker bones more prone to fracturing. [ 14] 700,000 cases of compression fractures in the spine are due to osteoporosis alone in the United States each year. [9]

Artificial Vertebrae

Source D: CAD model of vertebral body replacement.

The most current form of artificial vertebrae is a vertebral body replacement cage. These vertebral body replacements, almost always made of titanium, can either have non-expandable or expandable cages. In the case of non-expandable cages, the cage must be produced with exact size and dimensions that the patient requires. Expandable cages offer advantages in that they can be inserted and expanded to the optimal size and orientation while in place that theoretically insures a tight fit and minimizes the size of the surgical opening.[15] A study in 2004 showed that in a comparison of expandable and non-expandable cages there were no significant differences in biomechanical properties. [16] However, there have yet to be extensive long term studies on the expandable cages.[15]

While titanium is the primary cage material, Polyether-etherketone (PEEK), a hard radiolucent plastic, is gaining popularity. Radio marker dots can be used for surgeons to help guide the surgery in cage placement. It has an elasticity modulus close to that of bone, 3.6Gpa. X-ray imaging can also by used for positioning during the surgery with this material. [22]

Vertebrae Replacement Surgery

The procedure begins with an incision on the left side of the upper abdomen and the spine is approached from the front for best access to the vertebral body. Fractured bone and the discs above and below the target vertebral body(ies) are removed carefully. A metal (titanium) cage filled with bone graft material is then inserted to replace the removed area. The bone graft is essential in growing bone around the cage for long term stability. Screws are then inserted with plates into the vertebral bodies adjacent to the replacement cage. A metal rod is attached on both plates for support and stabilty while the bone grows. The incision is then closed to complete the surgery. A back brace is often required in the 3-6 months of recovery time that it takes for the bone to fully grow.[21]

Issues

The biggest issue with vertebral body replacement is the potential for collapse. In such a case, the orientation and/or height of the replacement can be compromised. The failure of a single vertebra in turn may generate spinal instability which can lead to neurological damage as well as irregular spinal curvature and reduced mobility along with associated pains.

In June 2007, the company Synthes gained rank as one of three top companies in spinal devices worldwide. This was largely due to the distribution of the Ti Synex II Central Body Replacement. Two years later, several patient reports revealing the device's tendency to collapse brought the FDA to issue a recall. On top of the recall, Synthes was also indicted on charges of illegal testing of bone cement products. The FDA gave warnings that the bone cement was highly dangerous and studies showed it sometimes caused blood clots. [2]

Alternatives

Vertebral body replacement with artificial vertebrae is not the only technique addressing spinal column injuries. Prominently, there is spinal fusion and vertebroplasty/kyphoplasty that can meet the needs of many patients.

Spinal Fusion

Spinal fusion is a welding of painful vertebrae together. The source of the pain must be precisely located for this surgical procedure. The procedure fuses the vertebrae to prevent movement and thus prevent the symptoms of the issue from arising. The fusion reduces spinal flexibility, ranging from slightly to greatly depending on the extent of fusion. The spinal fusion itself requires a bone graft to stimulate bone growth. Autografts, bone grafts harvested from the patient, allograts, using cadaver bone, along with the use of several biomaterials comprise the main bone grafting options. Screws and metal rods are additionally used for initial fixation while the bone grows as in the artificial vertebrae procedure. The procedure costs in the range of $90,000.

Disadvantages of bone grafts include the chance of infection, nerve damage, blood clots, and most significantly, recurring symptoms. The fusion does not irradicate the issue but merely immobilizes the affected area and so the original symptoms in some patients have reemerged. [17]

Vertebroplasty & Kyphoplasty

In vertebroplasty, bone cement is injected into a fractured vertebra through a hole in the skin to stabilize the fracture. Kyphoplasty, a variation of vertebroplasty, injects bone cement into fractured vertebra after creating a void with a small balloon so as to correct the height and angle of kyphosis. The procedures are both minimally invasive. Due to the relative gentle procedure, this is often the choice for frail patients unable or willing to undergo a more drastic alternative. The procedure costs in the range of $3,000 - $16,000. [18]

While a viable alternative, vertebroplasty and kyphoplasty are not for the young and healthy due to a lack of long term information with the cement. In addition, two randomized and blinded studies found no benefit for patients with osteoporosis-related fractures.[19] Elderly patients with osteoporosis are also the main target for this treatment, wherein lies the potential issue.

Future Directions

Many spinal surgeons are agreeing that the future methodology in dealing with spinal injury will focus on improving mobility, specificity, and how invasive the procedure is. As the mortality rate and complications decrease, a device yielding unimpaired mobility is necessary. Long term case studies reveal potential issues and faults with using a standard artificial vertebral replacement. This so, special replacements will arise to target niche audiences and specific ailments. Regarding the surgical procedures, the less taxing the procedure, the larger the perspective benefit. Specifically, elder patients are currently deterred frequently from surgical procedures in fear of the steep recovery and proceed with nonsurgical methods.[20]

References

Images

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