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 rather than pedicles screws and rods but they have also assisted in helping to break down the “fusion hardware 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 has been an important medical story.
The modern titanium intervertebral cage was invented by orthopedist (and sometimes veterinarian), George Bagby who pioneered the initial development of stainless steel cages which he used as a treatment for “wobbler’s syndrome” in horses (cervical degenerative spondylolisthesis compressing the spinal cord) which was something, up until that time, routinely required destroying the horse. The Bagby story has been a fascinating true life adventure in its own right. His first horse cage was a homemade stainless steel cylinder with multiple drill holes which was then pounded into a dill opening in the horses’ neck. Not every horse survived this pounding.
Shown above is the more advanced and streamlined Bagby basket.Bagby then realized that the basket idea worked but a better, and safer, approach would be to add threads so that the cage could then be screwed into place, allowing a much higher level of horse horse surviviability.The image above is a post-mortem specimen of a C6-7 intervertebral cage fusion performed by Dr. Bagby with a threaded titanium cage of the type used to save the racehorse Seattle Slew in the year 2000. After Bagby first presented his veterinary work to a medical audience at the North American Spine Society Meeting in Banff, Canada in 1984, a number of spine surgeons in the audience including orthopedic surgeons Steven Kuslich and Gary Michelson, as well as neurosurgeon Charles Ray rushed to develop their own versions of Bagby’s brilliant design for human application.
Kuslich, Michelson Ray
Following the initial experimental work on animals titanium threaded fusion cages were first used on patients in 1989. Based on the subsequent published investigational data the United States Food and Drug Administration then approved titanium threaded fusion cages for posterior interbody standalone use in humans in 1996.
Of the many different cage designs the Ray cage was unique in having deep threads. These threads allowed this cage to be self-tapping much like a wood screw and, because of this, appropriate use was then associated with the highest degree of efficacy for immediate standalone stabilization of the spine utilizing anterior as well as posterior placement.
In addition to creating immediate spine stabilization, when properly employed, subsequent bone growth further fixated the device in place as shown above. It is important to point out that while the anterior approach was easier for the surgeon, the same wasn’t necessarily true for the patient. Because anterior cage placement did 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 has been the posterior approach with a self-tapping cage. By maintaining an intact anterior annulus (red dot) the cage is immediately capable of creating a tight compartment in association with segmental stabilization.
While the posterior approach has clearly been the most challenging from a technical standpoint it has also provided the opportunity for the surgeon being able to directly and adequately decompress impaired nerves. When used to treat neurologic impairment (as opposed to “fusion” for the treatment of low back pain alone”). There has been the advantage of having already created good surgical exposure for the placing of self-tapping cages.
Cage Placement Complications
Prior to the 1996 FDA approval of titanium cages single and dual interbody devices were studied but dual devices then became routinely used until consistently validate medical evidence demonstrated that there were significant advantages to the use of a single cage. The disadvantages of utilizing two posterior cages was determined to be associated with significant potential liabilities:
- 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 documented that a short, single titanium cage, appropriately placed, can be safer and more effective than two cage placements. 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 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 where 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 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 above 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.
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: