Incidence and prevention of intervertebral cage overhang with minimally invasive lateral approach fusions

Study Design.: Radiographic review. Objective.: 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. Summary of Background Data.: 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. Methods.: 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. Results.: 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). Conclusion.: 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.