Anterior lumbar fusion with paired BAK standard andpaired BAK Proximity cages: subsidence incidence, subsidence factors,and clinical outcome

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BACKGROUND CONTEXT: Anterior lumbar interbody fusion (ALIF) procedures have a known incidence of subsidence. The individual risk of subsidence for specific lumbar levels in ALIF procedures has not been determined. PURPOSE: To evaluate the incidence of subsidence with two ALIF constructs. A paired Bagby and Kuslich (BAK) standard cage construct is compared with a paired BAK Proximity construct (Sulzer Spine-Tech, Minneapolis, MN). Study purpose is to evaluate lumbar intervertebral disc subsidence including the subsidence incidence for each disc level and with single- and two-level constructs. Also evaluated is the site of maximal subsidence within each end plate, risk with increased reaming depth, fusion incidence and clinical outcome. STUDY DESIGN: A consecutive series of 70 fused levels fused with paired standard BAK cages is compared with a subsequent series of 70 fused levels using paired Proximity BAK cages. PATIENT SAMPLE: The study population is derived from a consecutive series of ALIF procedures completed by a single surgeon. In 1998 the construct was changed from dual-standard to dual-Proximity cages. In the year 2000 there were 52 patients with a 2-year follow-up. These were matched to the previous 52 patients with dual-standard construct. OUTCOME MEASURES: Clinical outcome was determined using pre- and postoperative Oswestry questionnaires. Postoperative questionnaires were completed at the yearly follow-up. Radiographic outcome was determined by fusion status and evaluation of subsidence. Also evaluated was reaming depth and cage size for each fused level. METHODS: A total of 52 patients with ALIF procedures using paired BAK standard cages (the SS group) were studied with a group of 52 patients using paired BAK Proximity cages (the PP group). The study population was derived from an ongoing prospective study of consecutive ALIF fusion procedures completed by a single surgeon. Disc height measurements were used to determine subsidence. Reaming depth, fusion status and the site of maximal subsidence were all recorded. Clinical outcome was determined with a pre- and postoperative Oswestry functional questionnaire. RESULTS: Subsidence greater than 2 mm was noted in 5 of the 70 SS fused levels and in 9 of the 70 fused PP levels. Subsidence was always at the L4–L5 level in the SS subsided levels. Subsidence was in two-level fusions in all but one of the SS constructs. Subsidence was at the L4–L5 level in eight of the nine subsided PP levels. Subsidence was associated with increased reaming depth and the use of larger cage sizes. Subsidence was usually in the posterior and superior end plate. The clinical outcome was not affected by subsidence. Subsidence incidence was not associated with the age, sex or weight of the patient. CONCLUSIONS: There is a statistically significant increased incidence of subsidence at the L4–L5 level as opposed to other fused lumbar levels in ALIF fusions with BAK cage constructs. There is an increased incidence of subsidence with the PP constructs. Subsidence also is associated with increased reaming depth and with larger cage sizes. The lowest risk for subsidence was with single-level dual-standard cage constructs.