Spine Stabilization With Cages

Spine Stabilization With
Titanium Cages

The introduction of titanium cages for the purpose of spinal "fusion" opened an important new era in spine stabilization. Not only have titanium cages, when appropriately utilized, created new, and often better, surgical alternatives for patients but they have also assisted in helping to break down the "fusion sound barrier." The need for better, and more productive, patient choices in spine surgery, has been clearly demonstrated by the present high number of spine surgery failures. The selective use of titanium cages as part of restorative spine surgery program is one such important option.

The modern titanium intervertebral cage was invented by orthopedist (and sometimes veterinarian), George Bagby. He pioneered the development of stainless steel cages which he initially used as a a treatment for "wobbler's syndrome" in horses; a condition where cervical spine degeneration produced segmental instability and spinal cord compression necessitating destruction of the horse.

The development of intervertebral cages by Bagby story is a fascinating true life adventure in its own right. His first horse cage was filled with the animal's own bone. The image above, and to the right, shows an actual specimen of a C6-7 intervertebral rigid stabilization with an advanced Bagby titanium cage of the type used to save the horse Seattle Slew in the year 2000. After Bagby first presented his work to a clinical audience of the North American Spine Society Meeting in Banff, Canada in 1984, a number of clinicians including Steven Kuslich, Gary Michelson, Charles Ray, and others developed different versions of Bagby's basic design for human application.

The Kuslich, Michelson and Ray cage designs are shown above. In addition to these a number of other metallic and non-metallic cage designs have been introduced into spine surgery in recent years.

Following the initial experimental work on animals the clinical investigational phase of titanium cage use in patients began in 1989. Based on these data the United States Food and Drug Administration approved these titanium cages for posterior interbody standalone use in 1996. Since that time they have also received approval for anterior interbody placement.

Of the many different cage designs the Ray cage was unique in having deep threads. These threads allowed this cage to be self-tapping and, as such, demonstrated the highest efficacy for standalone stabilization with posterior placement.

to achieve immediate mechanical stabilization of the intervertebral segment. Position of a cage(s) is shown below in relationship to the vertebral bodies. The success of the multi-site investigational studies across the United States led to their approval, for single or double standalone posterior implants, by the by the United States Food and Drug Administration (FDA) in 1996.

Following this FDA approval the use of these cages for anterior interbody arthrodesis was also then approved by the FDA. It is important to point out that while the anterior approach was easier for the surgeon, the same wasn't true for the patient.

Because anterior cage placement does not require direct contact with nerves and dura the majority of orthopedic spine surgeons chose this approach. The anterior approach, however, opened up a whole new area of potential complications such as injury to the great vessels and lymphatics as well as local nerves.

From an anatomic and biomechanical standpoint the most effective cage placement is by the posterior approach with a self-tapping cage. By maintaining an intact anterior annulus (red dot) and using appropriate techniques such a cage immediately creates a tight compartment and associated segmental stabilization.

The posterior approach is the most technically challenging but has the advantage of being able to decompress impaired nerves directly. When used to treat neurologic impairment (as opposed to "fusion" for low back pain." There is the advantage of having already created good exposure for placing a self-tapping cage.

Cage Placement Complications

Following the 1996 FDA approval of titanium cages two such interbody devices were routinely used for posterior placement. Subsequently they were also approved by the FDA for anterior placement as well.

A major advantage of posterior interbody cages has been the ease of implementing them when a patient requires the associated decompression of spinal nerves. It soon became evident that the use of two posterior cages was associated with significant potential liabilities such as:
■ Filling and overflowing the disc interspace.
■ A need to perform bilateral exposure.
■ Not infrequently producing nerve compression due to the cage edge extending into a foramen.
■ A tendency for two cages to compress the spinal level below and produce clinically significant lateral spinal stenosis if foraminal narrowing was present to start with.

Efficacy of Single Posterior Interbody Titanium Cage

In 1995 Tencer and associates published an article in the Journal Spine regarding the testing of the mechanical characteristics of a number of threaded cage implant options shown to the left. The results of this biomechanical study indicated that a single cage appeared to be as efficacious as two (Tencer et al., Biomechanical Properties of Threaded Inserts for Lumbar Interbody Spinal Fusion, Spine, Vol.20, No,22, 1995).

Posterior interbody titanium cages have now been in clinical use for over 15 years. Accumulated clinical and experimental data have shown that a short in length, single titanium cage, appropriately placed, is safer and more effective than two cages. This observation has been confirmed experimentally on bovine functional spinal units (Zhao JZ et al, Posterior Interbody Fusion Using Posterolateral Placement of a Single Cylindrical Threaded Cage, Spine, 25(4), 425-430, 2000) as well as investigationally in patients (Zhao JZ et al, One Versus Two BAK Fusion Cages in Posterior Lumbar Interbody Fusion to L4-5 Degenerative Spondylolisthesis, Spine, 27(4), 2753-2757, 2002) leading to the conclusion that:

The term "POLAr" (for "Posterior Oblique Lumbar ARthrodesis") was coined by the Zimmer Spine organization as a means of describing this single cage posterior interbody approach.

The Principle of Parsimony

William of Occam's dictum that "It is needless to do more when less will suffice" continues to be in evidence.

This image shows a L5-S1 stabilization procedure performed in 2003. Titanium cages were initially placed from an anterior approach. The patient was then turned over and a pedicle fusion was then also performed. A better, less expensive and less lengthy procedure could have been achieved for this patient by the POLAr posterior interbody technique with a self-tapping cage being used as a "standalone."

In this case single Ray cages were used at two vertebral levels. The pedicle screws and rods were actually extraneous given the fact that the Ray cages are effective as "standalones", are MRI "safe" and allow some segmental energy absorption, which rods and screws do not. In addition screws and rods are FDA approved only for "temporary" use (which is not the case for cages).

Shown on the left is a L5-S1 cage 9 years following surgery. A short cage was used and the patient's bone has not only grown through the cage but has also enveloped the cage. The Editor's clinical experience with over 700 posterior interbody cage procedures indicates the great clinical value of this form of stabilization. Particularly in maintaining a good result long-term.

EPILOGUE

There exist only a few stabilization surgeries available today allow both a speedy return to normal function and a good long term result than safely inserted posterior interbody titanium cage(s) for the purpose of arthrodesis. Unlike pedicle screws and rods which often have to be surgically removed and often prevent accurate future MRI imaging (because of metal artifact), the use of posterior interbody cages allows accurate follow-up MRI imaging and the ability to monitor and treatment future problems related to progressive degenerative spine disease.

The safe and effective use of interbody cages is akin to the relationship between a carpenter and a nail. Although the quality of the nail (in this case the cage, or other interbody device) is important; the training, skill, and quality of the "carpenter" have the most to do with the ultimate result. For spine surgeons familiar with the posterior surgical approach titanium cages represent an important alternative stabilization option to rigid pedicle screws and rods.

It is unfortunate that only a limited group of spine surgeons have had the skill set to safely and effectively implant cages by the posterior surgical method. This circumstance is exactly similar to Walter Dandy's "bloodless surgical approach" to the posterior fossa where knowing, and avoiding, the potential pitfalls allowed a high degree of surgical success. There is hope however for the interbody technique. Steinmetz et al pointed out in the February 2007 issue of the journal Operative Neurosurgery that: