The Ray Threaded Fusion Cage for Posterior Lumbar Interbody Fusion

To the Editor: In the January 1998 issue of Neurosurgery, Onesti and Ashkenazi (4) described the use of interbody fusion cages for posterior lumbar fusion. I herein address some of the comments made by the article's reviewers. Sonntag (7), among other comments, voices concern that independent biomechanical models, animal models, and cadaver studies are not available. Studies are available for review. A comparison of insertion torques and pushout loads of interbody cages in bovine and human cadaver spines was presented by Rapoff et al. (5). Brodke et al. (2) reviewed spine stiffness in a calf spine model comparing cages, pedicle screw fixation, and bony posterior lumbar interbody fusion to the intact and destabilized spine. Both authors concluded that cages significantly increase segmental stiffness. Similar results were reported by Tencer et al. (8). These and other studies detailing the biomechanical properties of the cage and anatomic studies of the canal and the foraminal area after cage insertion are available in the medical literature. Rich and McCormick (6), in their review, mention the potential for neurological morbidity and instability with the cage procedure as well as the potential for long-term complications. Three-year follow-up data for 143 eligible patients treated with lumbar fusion cages (BAK; Spine Tech, Minneapolis, MN) revealed that neurological complications occurred in only 2.7% (9). The fusion rate was 98.3% despite that most surgeons destabilize or remove the facet joints during posterior cage insertion. Although concerns regarding late complications are appropriate, my experience (using lumbar interbody fusion cages for ∼5 yr in one community) is that late complications, neurological or otherwise, are not occurring. As part of a long-term study, 4-year follow-up data on the BAK patient group are now being tabulated. Personal communication indicates that no unexpected late complications are being encountered. Benzel (1) describes posterior lumbar interbody fusion and the cage technique as having "essentially unproved efficacy." The Food and Drug Administration evaluated interbody fusion cages commencing in 1993 with two separate clinical studies. The BAK device study enrolled 947 patients, with 94.6% of eligible patients available for data review at 24 months. In that study, the average pain score had decreased from marked to mild by 24 months. Of those patients in the study deemed work-eligible, only 28.4% were working preoperatively. Data from the 2-year follow-up evaluation revealed that 83% of nonwork compensation and 70% of work compensation patients were on the job. The Food and Drug Administration reviewed the clinical data from the Ray cage and the BAK cage regarding safety and effectiveness and released fusion cages for the treatment of low back pain caused by painful disc disorders in 1996. As I reported in 1997 (3), in a consecutive series of 54 patients, 73% with an average preoperative disability period of 22 months returned to work and 89% reported satisfactory outcomes. Sixty-eight percent rated their outcomes as good or excellent. The authors and reviewers both identify the most important issue of interbody fusion cages: patient selection. Those who treat discogenic pain are advised to exhaust conservative therapies and obtain extensive preoperative evaluations. Surgical decisions based on motion x-rays or magnetic resonance images with "dark discs" will lead to disappointment. Of equal importance is the need for neurosurgeons to familiarize themselves with the principles of fusion and discogenic pain. At this time, the majority of physicians learning the techniques and writing about interbody fusion cages are orthopedists. If neurosurgeons want to influence the direction of this technology, greater interest from academic centers and neurosurgeons in general will be required. Robert J.