Commentary & Perspective

Commentary & Perspective on
"Lumbar Intervertebral Body Fusion Cages: Histological Evaluation of Clinically Failed Cages Retrieved from Humans"
by Daisuke Togawa, MD, PhD, et al.

Commentary & Perspective by
Dan M. Spengler, MD*,
Department of Orthopaedics and Rehabilitation, Vanderbilt University Medical Center, Nashville, Tennessee

The authors have reviewed an important area of clinical interest by studying tissue contents of lumbar cages that had been implanted for a variable time (mean, twenty-one months) in humans. The paper describes the type of cage and the type of graft material that was placed within the cage prior to implantation. The authors then analyze, by semiquantitative means, the tissue types and the debris that were retrieved from the cages. Seventy-eight cages were retrieved from forty-eight patients. The consistency and completeness of the data were unclear since the authors reported that the original diagnosis was not available for ten patients. In eleven additional patients, information on the type of graft material placed within the cages was unavailable. Since these data were not analyzed with use of statistics, the missing data sets do not influence a statistical analysis but do confuse the reader.

The cages were removed for a variety of reasons that included a failed fusion, malposition or migration, compression fractures at fusion sites, low-back pain, infection, nerve-root impingement, and progressive spondylosis. The migrated or malpositioned cages were not specifically identified. Because these cages would not be under a similar load environment, any osseous ingrowth would likely be minimal, although the presence of viable bone in these cages may in fact suggest that it was the graft within the cage—and not "ingrowth" from adjacent cell sources—that was responsible. Although this theory could not be defended by the results of this paper, additional studies could be designed to pursue the question.

Several graft types (within the cages) were evaluated, although more detailed clinical information would be required to understand the relationship between the graft type and the type of tissue retrieved from the cages. For example, the fate of graft contents in a cage implanted for six weeks in a seventy-year-old patient would be expected to differ from that of a graft in a cage implanted for eighteen months in a thirty-year-old patient. The authors did observe that large cortical bone fragments within a cage revealed only minimal formation of new bone. This likely affirms the concerns of many that regional autograft (e.g., spinous process) is less desirable than trabecular autograft from the iliac crest. The authors did provide information regarding fibrous tissue ingrowth and the relationship to total time of implantation (Fig. 7).

Cage types included eight carbon-fiber-reinforced polymer cages as well as seventy threaded metal cages. Because multiple, unknown factors could have influenced the presence of bone, cartilage, fibrous tissue, and vascular tissue within the cages, it is not possible to draw conclusions on the efficacy of any specific cage.

The authors are to be commended for undertaking a difficult task but a necessary beginning. I agree with them that little information has been provided in the literature to document the histology within lumbar intervertebral cages in humans. Because only "failures" can be analyzed in most situations, conclusions regarding the ideal graft type and cage design cannot be derived from this type of study design. I would hope that this paper will stimulate additional studies that advance our knowledge regarding the type of tissue that becomes incorporated into lumbar intervertebral cages. Such work may better identify the ideal cage design as well as the most efficient graft inclusion.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

Copyright © 2004 by the The Journal of Bone and Joint Surgery, Inc.