Pedicle Screw Fixation - Osteoporotic Bone

The pedicles are short, stem-like processes that extend posteriorly from the left and right sides of each vertebral body. The pedicles are one of the strongest parts of the vertebrae. In the anatomical illustration, pedicle screws are shown during and after implantation.

In one study, the height and width of lumbar pedicles in male and female subjects were measured. In these subjects, it was found pedicle height and width increased from L1 to L5.

For example, male pedicle height measured between 14-21 mm, width 8-15 mm and female height 14-19 mm and width 7-14 mm. A different study related the patient's height, not gender, as a predictor to the inner diameter of the cervical pedicle.

In most healthy patients, the pedicle is a good choice for screw implantation to provide fixation to instrumentation. Pre-operative radiographic evaluation is essential to help determine pedicle size in surgical planning.

Similar to other bones in the body, except synovial joints, pedicle bone is basically composed of an outer layer of cortical, or compact bone, and an inner core of cancellous, or trabecular bone. The cortical bone has an outer lining, the periosteum.

The outermost layer, the periosteum, consists of an outer fibrous and inner osteogenic layer. The fibrous periosteum is rich with capillaries and nerves and provides nourishment and sensation to bone. The osteogenic periosteum contains osteoprogenitor cells that produce new osteoblasts when prompted. Cortical bone is the dense external layer of bone that protects and supports the structure. Cancellous bone consists of a bone matrix, or layers - the lamellae, resembling latticework called trabeculae. Within the trabeculae is the medullary cavity, or red marrow for blood production and yellow marrow for adipose fat storage.

The illustration shows a sampling from the hip joint/head of the femur of healthy cancellous bone. Notice the compactness of the trabeculae.

Bone remodeling is an ongoing process and in healthy people, osteoclastic bone resorption and new bone formation by osteoblasts is in balance. This process is important to removing worn or injured bone, maintaining stress resistance, and preserving calcium reserves.

Illustrated is normal bone remodeling -- bone physiology, which involves continual bone remodeling with active bone resorption by osteoclasts, and new bone formation by osteoblasts. This process is in part regulated by circulating hormones such as calcitonin and parathyroid hormone, as well as estrogen, which are in turn affected by circulating calcium levels.

Osteoporosis, a progressive metabolic bone disease, affects 10 million Americans. It develops when the healthy balance of osteoclastic and osteoblastic activity is upset. In the figure, healthy bone matrix is compared to weakened bone matrix. The loss of bone density is obvious. Untreated, osteoporosis can devastate bone's trabeculae and significantly reduce bone mass density and increase fracture risk. Patients with osteoporosis, or low bone mineral density, are at risk for pedicle screw pullout.

Initially, osteoporosis primarily involves the inner layer of bone, the cancellous portion. The number and strength of trabeculae diminish, weakening the structural stability of the bone. In later stages, osteoporosis will weaken the cortical portion too.

Endocrine, metabolic, genetic, and nutritional disorders contribute to the development of osteoporosis. Related risks include long-term corticosteroid use, chemotherapy, radiation therapy, age and gender, genetics, and unhealthy lifestyle choices. Tobacco use has been postulated as a contributing factor in the development and severity of osteoporosis.

In addition, other processes, such as cancer, can alter the normal architecture of cancellous bone. By invading and replacing bone marrow, cancers can weaken individual bones in the same manner that osteoporosis does throughout the vertebral column.

Implanting pedicle screws into osteoporotic bone, or bone weakened by cancer, presents a greater risk for screw pullout. Screw channel preparation involves using an awl to create the path then drilling into the pedicle. Drilling into or placing screws into weak or osteoporotic bone can increase the chance of pedicle fracture. This can further weaken the bone and increase the risk of pullout. Osteoporosis is such a risk factor for spine surgery that many patients with severe pathology are deemed unsuitable surgical candidates because of the risk of device failure.

As demonstrated in this slide's radiographic series of implanted pedicle screws, placing instrumentation into the spine requires a solid interface between the screw and the bone it is implanted into. With progressive weakening of the cancellous bone, the amount of bone in contact with the implanted screw is reduced which reduces the resistance of the screw to pullout forces.

The aging spine is a growing problem in our population. Surgeons see patients with more advanced levels of pathology now than ever before. Many of these patients need pedicle screw stabilization. As patients live longer, their risk of osteoporosis increases. Consequently, osteoporotic bone is an increasingly common challenge to spine surgeons.

Similar devastation to vertebral structures can be caused by metastatic spinal disease. The spine is the third most common area for malignant metastasis. Lung and breast cancers are primary sources for metastatic spinal disease. These, and other types of primary tumors, usually spread via arterial routes and may metastasize through vertebral bone marrow. Such tumors can cause vertebral instability and collapse.

Bone screws have been used in spinal fixation since the mid-1940s. King utilized a longer facet screw in spinal fixation. In 1959, Boucher reportedly was the first to implant pedicle screws and Roy-Camille was the first to connect pedicle screws to a posterior plate. Later bone screw pioneers include Harrington and Cotrel and Dubousset.

The history of pedicle screw fixation would be remiss without summarizing the past legal debacle.

In 1984, the U.S. Food and Drug Administration deemed pedicle screws risky business and classified them as Class III devices. It was illegal to market pedicle screws, although surgeons could use the screw for off-label conditions.

By 1993, the number of law suits filed by patients numbered into the thousands. Patients alleged pedicle screws caused them physical and emotional harm, including injury and loss of income.

During 1995 and 1996, surgeons' names, spine associations, and pedicle screw manufacturers were defendants named in various lawsuits. The litigious actions almost eliminated pedicle screw fixation altogether.

Throughout 1997, conspiracy claims were dismissed, defendants were allowed to use a Cohort Study in their defense, proposals were submitted to reclassify pedicle screws to Class II devices, and federal cases were returned to the district levels.

Finally, in 1998, the U.S. Food and Drug Administration reclassified pedicle screws as Class II devices. Manufacturers were no longer banned from providing surgeons information about off-label pedicle screw use in peer-reviewed journals, continued medical education and so forth, as long as the manufacturers supported off-label use along with full disclosure.

Pedicle screws are standard in posterior fixation procedures to treat spinal instability caused by deformity, degenerative disc disease, fracture, spinal stenosis, spondylolisthesis, or tumor. In the anatomical illustration, pedicle screws are shown during and after implantation. However, implanting pedicle screws in patients with osteoporosis presents surgeons with a challenge: How to increase screw purchase and reduce screw pullout.

During the past few decades, innovative surgeons have developed a number of strategies to stabilize the osteoporotic spine with instrumentation. Strategies include:

• Extend the fusion to more levels to decrease pedicle stress and load share
• Use pedicle hooks and laminar hooks to supplement pedicle screws
• Incorporate sacroiliac fixation in long constructs
• Use large diameter screws for purchase in cortical bone surrounding pedicle
• Use tapered screws for better bone compaction
• Obtain bicortical purchase by extending screws through the anterior vertebral body
• Place 2 pedicle screws in each pedicle where anatomically feasible
• Augment the bone and/or screw using polymethylmethacrylate (PMMA) placed down the pedicle shaft prior to pedicle screw insertion
• Use hydroxyapatite-coated pedicle screws
• Use balloon kyphoplasty to create a cavity for PMMA delivery
• Deliver PMMA using a fenestrated pedicle screw

An alternative is bone cement. Bone cement improves screw purchase and thereby reduces screw pullout. It also creates a large mass affixed to the screw shaft which is more resistant to pullout anatomically. Besides PMMA, calcium phosphate and carbonated apatite have been used with limited acceptance.

However, there are drawbacks. The alternative surgical options presented in the previous slides may not be ideal for the patient or surgeon for reasons such as:

Extending the fusion may mean longer surgical time, increased risk for screw malposition, as well as creating a longer moment-arm, which itself increases risk for pullout as anatomic junctions are crossed.

Likewise, more complex constructs increase operating time and can extend beyond the surgeon's capabilities. Bicortical purchase of screws increases the risk for soft tissue injury, especially great vessel injury anterior to the vertebral body. Upsizing or maximizing the screw size to pedicle size ratio also increases the risk of fracture.

Pedicle screw augmentation with cement placed down the pedicle shaft prior to screw insertion may cause sequestration of PMMA in the spinal canal or unintended tissues (e.g. lung) leading to complications such as neurologic or vascular injury.

Balloon kyphoplasty is an additional procedure that may increase surgical time. Although kyphoplasty creates a cavity, cement delivery and flow can be difficult to control.

Although pedicle screw fixation involves a steep learning curve, these screws offer surgeons greater versatility in spinal stabilization and reconstruction procedures. The pedicle is the strongest point for attachment and fixation using pedicle screws can stabilize both the posterior and anterior spine.

The material composition varies. Pedicle screws may be titanium or stainless steel construction. Screw design differs as well. For example there are:

Polyaxial screws, which allow the head to rotate and lock onto the rod at any angle; this is important for ease of insertion. Monoaxial screws provide more rigidity and are important for deformity correction. A tulip head allows for less rod bending since the head can angle to engage the rod. Cannulated screws provide a central canal for placement over a K-wire during a percutaneous application. And, fenestrated screws are designed with multiple holes allowing for bony in-growth and/or influx of polymethylmethacrylate.

Properties of pedicle screws include profile, stiffness and yield load, and biomechanical aspects. For example, considering profile, tapered screws allow better bone compaction than straight screws.

Stiffness and yield load: Large diameter screws are stiffer and maximize resistance to pullout forces. In general, screw diameter is matched to the diameter of the pedicle in which the screw is to be implanted. And, longer screws share load better.

Biomechanical aspects: Cutting flutes on the first few millimeters of the shaft allows self-drilling/tapping. Graduated threads allow the screw to have better purchase and greater strength higher up as the screw enters the pedicle. And, coated screws covered with hydroxyapatite or porous titanium plasma spray help increase screw purchase.

Pedicle screws are commonly used in posterior fixation procedures to treat spinal instability caused by:

• Decompression of spinal stenosis
• Deformity, such as kyphosis or scoliosis
• Degenerative processes affecting discs or the facet joints
• Infection, although this is debatable as to whether screw implantation during active infection is indicated
• Fractures, such as vertebral compression fractures, traumatic facet, or posterior element fractures
• Pseudarthrosis revision
• Spondylolisthesis, such as congenital, degenerative, iatrogenic, traumatic, or Tumor

Contraindicated use of pedicle screws include: Severe osteopenia or osteoporosis, inadequately sized pedicles, pedicles compromised by fracture, metal allergy, or lack of anterior spinal column support may cause metal fatigue and subsequent screw fracture

A detailed patient history helps reveal the patient's risks for osteoporosis and vertebral fracture. Considerations include:

• The patient's sex. Women are at higher risk for osteoporosis than men
• The patient's age. Osteoporosis correlates with advancing age
• Does the patient smoke, or use other tobacco products
• Is the patient a postmenopausal female who is not taking estrogen
• Use corticosteroids, antiseizure or high-dose thyroid replacement medications
• Is the patient diabetic or have liver or kidney disease
• Has excessive collagen in the urine
• Has a history of a previous fracture
• Has a loss of height greater than 1-inch
• Is osteopeonic, or
• Has a tumor or cancer that can reduce bone strength in the affected vertebrae due to marrow replacement

Surgical planning may include a pre-operative bone mineral density test, such as DEXA or DXA, and/or lateral vertebral assessment under x-ray. Such testing is important, especially if poor bone health is suspected.

In most healthy patients, the pedicle is a good choice for screw implantation to provide fixation to instrumentation. Advance knowledge of the patient's general bone health, including the vertebrae and pedicles can help predict screw purchase and prevent fixation complications or failure. Pre-operative radiographic evaluation is essential to help determine pedicle size during surgical planning.

Intra-operative guidance can be of great assistance and may involve fluoroscopic guidance, such an AP or lateral screw trajectory views. The intra-operative image shown, demonstrates an intra-operative view of implanted pedicle screws. Other guidance includes intra-operative radiographs, Fluoro-Nav navigation, stereotactic navigation, O-arm Intra-operative CT, pressure and electrical probes to detect breach, intra-operative electrodiagnostics - such as free-running EMG, and screw stimulation testing.

Pre-operative radiographs and CT scans reveal the patient's anatomy and bone quality, as well as other imaging and intra-operative techniques outlined in previous slides. Pedicle screw technique considerations include:

• The surgical procedure to be performed, such as a TLIF and medial facetectomy, which typically opens the roof of the pedicle
• Local anatomy
• Size of the facets and size of the pedicles, and the
• Amount of exposure

Other considerations in pedicle screw implantation include landmarks, screw-hole preparation, and other techniques.

Landmarks include mamillary process and junction of the transverse process midpoint and the lateral facet.

Screw-hole preparation techniques include Leksell bite/high-speed drill to expose the proximal pedicle, awl/probe to develop the pedicle shaft, and drill.

Funnel, Wiltse or Roy-Camille approach, or Krag are other techniques.

In osteoporotic bone, pedicle screw-hole preparation may be easier as there is little cancellous bone. However, upsizing the screw to adequately capture cortical bone is a challenge given the risk of pedicle fracture using an over-sized implant.

Osteoporosis, or osteopenia, and related screw pull-out are obvious challenges in pedicle screw implantation. Others include:

Screw breakage: This is reduced by using stainless steel or titanium implants and usually is a long-term complication in the setting of fusion failure.

Cortical breach: Redirected screws still take the path of least resistance; therefore, redirection is more difficult if pedicle fracture occurs. Further, medial or inferior breach typically causes nerve root irritation and/or spinal cord/nerve root injury and anterior breach can cause intestinal or vascular injury.

Severe spondylosis can obscure landmarks or impede screw head rotation.

Thick musculature can impede the lateral exposure needed for proper screw trajectory.

Inadequate pedicle size, and

Preservation of the superior facet capsule and reduction of iatrogenic trauma to the neighboring level.

Innovative surgeons have developed a number of strategies to stabilize the osteoporotic spine with instrumentation. Such as:

Inserting a few cc's of Polymethylmethacrylate (PMMA) into the pilot-hole and then implant the screw. Currently, this is an off-label use to strengthen the bone-screw interface.

Use of polymethylmethacrylate combined with a biodegradable Calcium-Phosphate-Cement (CaP). This too is an off-label use to strengthen the bone-screw interface.

Employ balloon kyphoplasty to create a cavity for polymethylmethacrylate delivery, or the latest technique using fenestrated pedicle screws.

There are specific advantages and disadvantages to cement augmentation.

Advantages: Bone cement improves screw purchase and thereby reduces screw pullout. It also creates a large mass affixed to the screw shaft which is more resistant to pullout anatomically. Besides PMMA, calcium phosphate and carbonated apatite have been used with limited acceptance.

Disadvantages: Pedicle screw augmentation with cement may cause sequestration of PMMA in the spinal canal or unintended tissues, such as the spinal cord or lung, leading to complications such as neurologic or vascular injury. Further, refluxing cement may block the pedicle screw or interfere with rod insertion.

This is more commonly associated with the initial methods of placing cement into the pedicle shaft prior to screw insertion, as the surgeon has little control where the cement migrates with screw insertion.

Neurologic complications related to pedicle screws include: A medial and inferior breach in the pedicle can affect an exiting nerve root, severe medial breach can affect a traversing nerve root, and slippage of the probe, tap, or screw/screw driver on insertion can cause cauda equina injury

Vascular complications related to pedicle screws include the following:

In the thoracic and lumbar spine, lateral trajectory can injure the aorta, vena cava, or common iliac artery/vein. (Occlusion, dissection, penetration)

If the screw is too long, these same complications can occur.

Screw pullout and breakage are problems discussed in the literature. Pain is commonly reported, however it is not well-defined.

Screw pullout is reported as 1.7% according to The Lumbar Spine: Official Publication of the International Society for the Study of the Lumbar Spine.

That same publication reports screw breakage at 1%. Other publications report:

0.5% to 25% - The Adult and Pediatric Spine: Principles, Practice and Surgery.

2.2% - The Journal of Bone and Joint Surgery, and

12.4% - in an issue of the European Spine Journal

In order to overcome the problem of cement migration, as seen in earlier methods, balloon kyphoplasty has been used to create a pocket in the vertebral body for cement placement after pedicle preparation - but prior to screw placement. This has reduced the incidence of cement migration into the spinal canal, but paradoxically weakens the screw-bone interface initially by further destroying the remaining trabecular architecture.

Fenestrated pedicle screws represent the latest solution to the surgeon's challenge of implanting pedicle screws into osteoporotic bone.

Fenestrated pedicle screws are typically cannulated, meaning they have a central channel running down the center of the shaft. This is the channel through which bone cement is injected. The cement then disperses into the surrounding bone through multiple fenestrations along the length of the shaft. These fenestrations are placed only in the distal aspect of the screw so none are adjacent to the spinal canal. Bone cement then interdigitates into the trabecular bone like a sponge absorbing water. Only the part of the screw that is buried in bone is fenestrated to avoid egress of cement towards neural tissue.

This solves the problem of cement migration, as cement is placed through the implanted screw and only through the distal fenestrations, reducing the likelihood of cement migration into the spinal canal, or back up the pedicle shaft. Further, as cement is introduced AFTER screw placement -- the only method in which this is the case, the native screw-bone interface is not compromised by the process, only strengthened.