Radiographic assessment of interbody lumbar interbody fusion: the cage-o-gram

Abstract

William J. Beutler, MD, Lori Dunn, DO, Walter C. Peppelman, Jr., DO, Steven B. Wolf, MD, Harrisburg, PA, USAIntroduction: The determination of lumbar interbody bone fusion status is often difficult secondary to interbody metal constructs, posterior fixation limiting range of motion or anatomic factors. A method of radiographic evaluation of lumbar interbody fusion using discogram technique is described.Methods: Thirty patients were referred for evaluation of lumbar interbody fusion and adjacent levels by discogram technique. Referrals specified evaluation of interbody fusion status with inability to determine fusion status on radiographic studies. Adjacent levels were routinely evaluated during the study. Presenting symptom was predominantly low back pain (n = 30) with some lower extremity symptoms (n = 14). Pseudarthrosis was dramatically documented when dye flowed around and through interbody cages or surrounded allograft arthrodesis. Solid arthrodesis was documented with ability to inject dye or with dye flowing around solid construct. Pain response with injection at arthrodesis level and adjacent levels was documented. Prereferral studies were requested and independently reviewed for confirmation of inability to determine pseudarthrosis with established studies. Appearance of fusion prereferral was determined by lack of halo formation and less than five degrees of motion on flexion-extension lateral radiographs.Results: Thirty patients (11 male, 19 female) underwent evaluation of 45 levels of previous interbody fusion and 43 adjacent levels from May 1998 through August 2000. An average of three levels per patient were injected. Average time from the fusion to the discogram study was 17 months. Twenty-three levels involved interbody cages, four levels with carbon fiber cages and 18 levels with allograft construct. Eleven subjects had posterior instrumentation in place in addition to the anterior construct. Dye flowed through interbody cages in eight levels, surrounded carbon fiber cages in one level and surrounded allograft constructs in nine levels. Solid constructs with inability to inject dye or dye flowing around solid constructs was documented in 27 levels. A concordant pain response on injection was noted in 18 operated levels and 3 nonoperated levels. One level with a solid fusion had a concordant pain response. In all but one of the levels with documented pseudarthrosis by discogram, a concordant pain response was reported (p <.001, Fisher's exact test). Independent review of prereferral fusion status noted only two referrals, both allograft interbody constructs, with evidence of pseudarthrosis on the prereferral studies.Discussion: A method for evaluation of lumbar interbody fusion is presented. The technique documents pseudarthrosis in radiographic challenging situations, including combined anterior and posterior instrumentation. The technique appeared to be most useful for evaluation of fusion status with interbody metal constructs, with or without posterior instrumentation. Dramatic documentation of fusion failure with dye flowing through and around constructs is demonstrated in 18 levels. There is a strong statistical association of concordant pain response on injection at levels of incomplete fusion.