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Screws, Cages or Both?
History
Threaded titanium interbody fusion cages were developed as a stand–alone device to augment arthrodesis through an anterior or posterior lumbar interbody approach. Their introduction in 1996 caused resurgence in lumbar interbody fusion for degenerative disorders with over 33 thousand procedures performed in 1997, the first full year after release. This increased revenues by $111 million to the spine implant industry, propelling them as the fastest growing segment of the $9 billion worldwide orthopaedic market. This fast–paced growth continued with 58 thousand cage procedures performed in 1998 generating $182 million for the $950 million worldwide spine implant market. (Merrill Lynch: personal communication, August, 1999)
These threaded titanium cages trace their roots to veterinary medicine. During the mid 1970s and early 1980s, Bagby and colleagues' began treating "Wobbler Syndrome", a chronic cervical instability causing myelopathy in thoroughbred horses, by means of a smooth, stainless steel fenestrated cylinder (Bagby Basket) placed through an anterior approach. The standard Cloward technique had resulted in unacceptable morbidity due to the necessity of autogenous iliac bone graft harvest (2). Bagby eliminated the need for autograft harvest by packing his cage with cancellous bone chips obtained from the reaming of the cervical decompression. This novel device was designed with perforations in its walls to allow bone in–growth and enhance arthrodesis. They coined the term "distraction–compression stabilization", referring to their technique of distraction of the cervical interspace with this implant, achieving early stability while improving arthrodesis. Animal studies demonstrated excellent clinical results, particularly in comparison to previous techniques utilizing interbody allografts or xenografts (2,3,4), with up to 88% fusion success'. This stand–alone interbody fusion technique continued to evolve with material changes and the design of threaded cages to increase stability and decrease displacement rates.(5–6) Similar to the method of Wiltberger(7), bilateral, parallel implants were designed for use in the lumbar spine. This ultimately resulted in the current Bagby and Kuslich design (BAK, Spine–Tech, Minneapolis, MN), with the first human implantation occurring in 19928. This cylindrical titanium cage has threads to screw into the endplates, thereby stabilizing the device and allowing for increased fusion rate with a stand–alone anterior device. Ray (6) developed a similar titanium interbody fusion device (Ray TFC, Surgical Dynamics, Norwalk, CT) which was initially used in posterior lumbar interbody fusions (PLIF), but expanded to include anterior lumbar interbody fusion (ALIF) procedures. In 1985, Otero–Vich (5) reported using threaded bone dowels for anterior cervical arthrodesis, and femoral ring allograft bone has subsequently been fashioned into cylindrical threaded dowels for lumbar application.
Currently, there are a wide number of available interbody fusion devices of varying design and material, not all of which have gained Food and Drug Administration (FDA) approval in the setting of a stand–alone device.
These include:
1) Cylindrical threaded titanium interbody cages (BAK, Spine–Tech, Minneapolis, MN), (RTFC, Surgical Dynamics, Norwalk, CT), and (inter Fix, Sofamor Danek Group, Memphis, TN)
2) Cylindrical threaded cortical bone dowels (MD II, MD III, MD IV) (Sofamor Danek Group, Memphis, TN)
Figure 1: Inter Fix threaded interbody fusion device. (Sofamor Danek Group, Memphis, TN) 2) Cylindrical threaded cortical bone dowels (MD II, MD III, MD IV) (Sofamor Danek Group, Memphis, TN) Figure 2: MD 11 (left) and MD IV (right) threaded bone dowels. (Sofamor Danek Group, Memphis, TN)
3) Vertical interbody rings or boxes (Harms titanium–mesh cage, DePuy–Acromed, Cleveland, OH), (Brantigan carbon fiber cages, DePuy–Acromed, Cleveland, OH), and (Femoral Ring Allograft – FRA Spacer, Synthes, Paoli, PA).
Steffee (9) popularized posterior pedicle screw internal fixation in North America for augmentation of a posterolateral lumbar fusion. Zdeblick (10) demonstrated a high fusion rate with a stand–alone rigid posterior pedicle screw and rod device for degenerative lumbar disorders. Spinal fusion has become a widely used option in the treatment of degenerative conditions of the lumbar spine. Posterior, posterolateral, and interbody fusions, both anterior and posterior, have been used successfully alone or in combination. Although interbody fusion cages and pedicle screw devices have enjoyed some success as stand–alone devices, universal acceptance has not occurred for either strategy in the setting of degenerative lumbar disorders. This chapter will review the important issues and controversies regarding the appropriate use of stand–alone interbody cages (implanted through an ALIF or PLIF approach), stand–alone posterior screw constructs, and combined interbody cage and posterior screw techniques.
