Study Design: Retrospective analysis of factors related to kidney-type interbody spacer subsidence (SS) in transforaminal interbody fusion (TLIF). Objective: To determine the risk factors for SS in TLIF using kidney-type spacers. Summary of Background Data: SS into the vertebral body, a major complication of TLIF, has not been studied extensively. Methods: Between July 2004 and May 2006, 54 consecutive patients with lumbar dysplastic changes or degenerative disc diseases underwent TLIF using 82 kidney-type spacers with iliac bone grafts. All were followed-up for more than 2 years (mean, 2 y, 11 mo). SS was defined as disc height loss > 2 mm. Risk factors analyzed for SS included spacer location in the intervertebral space (IVS, anterior, center, and posterior), sex, age, body mass index, and disease (spondylolisthesis, degenerative disc disease). Clinical outcomes were assessed using the Japanese Orthopedic Associated Score for back pain, as were adjacent segment disorder and nonunion. Results: Of the 82 spacers, 66 were located in the center of the IVS and 16 anteriorly. There were 18 SS. Spacer position at the center of the IVS (P < 0.001) and older age (P < 0.001) were significantly associated with SS, with the first 2 factors having a synergistic effect. Adjacent level disorder (n = 5) and nonunion (n = 3) were observed only in patients with spacers positioned at the center of the IVS. Conclusions: Kidney-type spacers should be located in the anterior portion of the IVS to prevent subsidence of the intervertebral body, especially in patients with elderly age.
