To evaluate the incidence and degree of cage overhang in minimally invasive spinal (MIS) fusions, when using either the direct lateral interbody fusion (DLIF) or extreme lateral interbody fusion (XLIF) techniques.
Among the difficulties surgeons face during a MIS lateral interbody fusion is to assess the proper placement of the cage without the use of direct visualization. Determining the proper length of the cage using AP view fluoroscopy can be misleading. As the axial profile of the vertebral body is oval, inserting the cage anterior or posterior to the maximal width point requires adjustment of the cage's length.
The incidence and degree of cage overhang were measured using magnetic resonance imaging (MRI) and computed tomography (CT) studies from patients that underwent a MIS lateral interbody fusion. To determine the adjustment needed when the cage is inserted at various sagittal sites, the coronal spans of normal vertebral endplates were measured.
Forty-five percent of the cages were placed in the central portion, 34% were located in the anterior 1/3, and 7% were located in the posterior 1/3 of the disc space. Of the anterior positioned cages, 45% were found to be overhanging outside of the boundaries of the intervertebral disc space. The average measured lateral protrusion was 7.8 ± 3.6 mm, and anterior protrusion was 9.8 ± 3.3 mm. The vertebral body width measured 41.7 ± 6 mm at the anterior 1/3, 50 ± 4 mm at the mid, and 49 ± 1 mm at the posterior 1/3. Compared with the midvertebral width, the vertebral body width at the anterior 1/3 was decreased by 16.5% ± 0.9% (P < 0.05).
The risk of placing an excessively long cage, when the insertion site is located in the anterior 1/3 of the disc, is relatively high, when performing MIS lateral approach interbody fusions. When using an anterior entry point for the insertion of the cage, choosing a 15% shorter cage length compared with that measured on the AP should prevent anterolateral protrusion of the cage.
Using AP view fluoroscopy to determine the proper cage length during the XLIF/DLIF procedure can be misleading because of the oval profile of the vertebral body. This study evaluates the incidence of cage protrusion beyond the intervertebral space and determines cage length adjustments in relation to the sagittal insertion point.
From the *Department of Orthopaedic Surgery, University of California, San Diego, CA; †Spine Surgery Unit, Tel-Aviv Sourasky Medical Center, Tel-Aviv, Israel; ‡Department of Radiology, University of California; and §Center for Minimally Invasive Spine Surgery at Alvarado Hospital, San Diego, CA.
Acknowledgment date: April 12, 2009. First revision date: July 11, 2009. Second revision date: August 24, 2009. Acceptance date: August 26, 2009.
The manuscript submitted does not contain information about medical device(s)/drug(s).
Funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Supported by NCRR of the NIH grant MO1-RR00827 for the UCSD General Clinical Research Center and American Physician Fellowship for Medicine in Israel (to G.J.R.).
Address correspondence and reprint requests to Choll W. Kim, MD, PhD, Spine Institute of San Diego, Center for Minimally Invasive Spine Surgery at Alvarado Hospital, 6719 Alvarado Road Suite 308, San Diego, CA 92120; E-mail email@example.com