Lumbosacral Fusion: Cages, Dowels and Pedicle Screws
History: Part 1
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. The earliest reports of anterior interbody arthrodesis were in association with the treatment of tuberculosis and lumbar spondylolisthesis,10, 49, 67 initially with transperitoneal approaches, 69 and later, retroperitoneal approaches.10, 49, 50 The first description of an anterior transperitoneal approach occurred in 1906 by Mueller,69 with Iwahara50 reporting the first lumbar arthrodesis performed through a retroperitoneal approach. In 1948 Lane and Moore61 in a classic description, were the first to report anterior lumbar interbody fusion (ALIF) for the treatment of lumbar degenerative disc disease. In 1950, Harmon42 described a retroperitoneal transabdominal approach for cases of acute intervertebral disc prolapse caused by disc degeneration.
Capener13 considered fusion of the lumbar spine by an anterior approach biomechanically ideal but technically impossible in 1932, however, over the ensuing decades surgical technical advances allowed anterior lumbar interbody fusion to become a common procedure. The anterior approach to the lumbar spine was increasingly utilized in the management of a variety of spinal pathologies, using a number of different grafting materials, including corticocancellous blocks,44, 45 corticocancellous dowels,41, 86 and femoral ring allografts.72 Hodgson44, 45 pioneered the anterior approach for spinal tuberculosis using corticocancellous blocks. Cylindrically shaped corticocancellous dowels were first used for an anterior lumbar fusion in 1963 by Harmon41 and 1965 by Sacks.86 Ralph Cloward17, 18, 19 pioneered the dowel technique beginning in 1953. While he utilized a posterior approach, his methods for disc removal, endplate preparation and grafting came to be used extensively. Later, Henry Crock22 adapted Cloward's dowel technique for use with an anterior approach to the lumbar spine using cylindrical allograft. O'Brien72 devised a hybrid interbody graft using a biological fusion cage (femoral cortical allograft ring) packed with autogenous cancellous bone graft. The concept of this hybrid is that the femoral allograft ring provides the acute stability of the construct, while the autogenous iliac crest graft provides for long-term stability.
Although the technical feat of exposing the anterior lumbar spine safely was reliable in the 1970s - 1980s, stand-alone anterior lumbar interbody fusion fell out of favor due to low fusion rates. Despite initial reports encompassing a heterogeneous group of patients and surgical techniques indicating fusion rates of 95% by Harmon,41 70% by Hoover,46 90% by Crock,22 and 96% by Fujimaki, 32 other reports demonstrated significantly poorer fusion rates. Calandruccio,12 Nisbet,71 Raney,79 and Flynn,28 respectively cited fusion rates of 19%, 40%, 45%, and 56%, but the 1972 study from the Mayo Clinic authored by Stauffer and Coventry90 drove the final nail in the coffin of stand-alone ALIF. They reported on 83 patients who had an anterior lumbar interbody arthrodesis without instrumentation between 1959 and 1967. They found an alarmingly low success rate with pseudarthrosis occurring in a discouraging 44%, and concluded that the only justification for this procedure was as salvage for failed posterolateral fusions. These outcomes resulted in a reassessment of ALIFs as a stand-alone procedure and a gradual decline in its popularity, particularly for the indication of lumbar degenerative disc disease and lumbar axial back pain.
The combination of anterior interbody fusion with a posterior fusion technique was developed with the aim of obtaining higher rates of fusion and improved outcome. Because of the significant drop in the fusion rate especially over multiple levels, combined anterior interbody fusion with posterior fusion and internal fixation became common. 58 The advantage of a very high fusion rate with these circumferential (360) procedures, however, must be balanced against the increased risk of morbidity related to the increased magnitude of the procedure.
Techniques to increase the fusion rate of anterior lumbar interbody fusion with an anterior-only approach to approximate the success of circumferential constructs began with anterior lumbar instrumentation, which was first reported by Humphries and Hawk in 1961. 47 They developed a slotted contoured plate to be placed over the anterior lumbar spine in an attempt to enhance arthrodesis, however, it was the threaded cylindrical cage that brought stand-alone anterior interbody fusion back as an option for the treatment of discogenic pain.
During the mid 1970s and early 1980s, Bagby and colleagues2 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. 23 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, 21, 23, 95 with up to 88% fusion success. 2 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. 74, 83 Similar to the method of Wiltberger,100 bilateral, parallel implants were designed for use in the lumbar spine. 21 This ultimately resulted in the current Bagby and Kuslich design (BAK, Spine-Tech, Minneapolis, MN), with the first human implantation occurring in 1992.60 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. Ray83 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 ALIF procedures. In 1985, Otero-Vich74 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 for anterior application 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)
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).
As a result of the improved clinical results associated with the use of many of these interbody fusion devices in stand-alone anterior procedures, avoiding many of the problems associated with instrumented posterolateral arthrodesis, their use has become more wide-spread; both in the United States and internationally. Approximately 80,000 lumbar interbody fusion cages have been implanted internationally over the past five years, with the United States accounting for 5000 implants per month. 64 In the U.S.A, it is estimated that in 1999 alone, sales of interbody fusion cages will have reached approximately 224 million dollars, 15% of which represents bone dowel products (Merrill Lynch personal communication August 2, 1999). The recent interest in performing lumbar interbody arthrodesis with use of cages is attributable to three factors according to McAfee. 64 First, the rate of failure associated with use of bone graft alone is high. Second, the rate of failure associated with use of posterior pedicle-screw instrumentation is high. And finally, the rate of success associated with use of stand-alone anterior fusion cages and autogenous bone graft is high. This obviates the need to perform a 360 (combined anterior and posterior) lumbar arthrodesis with use of posterior instrumentation. Interbody fusion cages have had a tremendous effect on anterior fusion. The rates of fusion after anterior interbody arthrodesis have improved from Stauffer and Coventry's90 56% to 93% with the use of the BAK titanium cage.60