Effect of compressive preload on the anterior lumbar interbody fusion cage construct stability

Abstract

Avinash G. Patwardhan, PhD, Gerard Carandang, MS, Hines, IL, USA; Alexander J. Ghanayem, MD, Maywood, IL, USA; Benjamin Cunningham, MD, Troy, MI, USA; Frank M. Phillips, MD, Chicago, IL, USA; Leonard I. Voronov, MD, Maywood, IL, USA; Robert Havey, BS, James Simonds, BS, Hines, IL, USA; Kevin Meade, PhD, Tae-Hong Lim, PhD, Chicago, IL, USA; Thomas M. Gavin, CO, Burr Ridge, IL, USA; Michael R. Zindrick, MD, Hinsdale, IL, USAIntroduction: Anterior lumbar interbody fusion (ALIF) using threaded cylindrical interbody fusion cages is used to treat segmental instability in symptomatic lumbar disc degeneration. The construct should minimize the unstable segment motion during activities of daily living to promote a solid fusion in proper alignment. The compressive load on the lumbar spine ranges from 200 N to 1,200 N during activities of daily living. Although previous cadaveric studies have applied physiologic levels of bending moments, the effect of in vivo compressive loads on fusion constructs has not been investigated. Without a realistic compressive preload, the loads on the implant in the ex vivo experiment differ significantly (both in magnitude and direction) for those in vivo [1]. The purpose of this study was to quantify the effect of compressive preload on the response of the ALIF cage construct and adjacent segments. A unique experimental model was used to subject human cadaveric specimens to loads of in vivo magnitudes [2].Materials and methods: Thirteen specimens (L1–sacrum; age, 52.15 years) were tested in flexion, extension and lateral bending with and without an L5–S1 ALIF cage construct. To simulate in vivo conditions, a follower preload was applied using cables [2]. The follower load cables maintained a compressive preload throughout the range of motion without compromising the mobility of the spine. The specimens were tested under an increasing compressive preload (0 to 1,200 N) with bending moments applied to L1 (0 to 8 Nm flexion, 0 to 6 Nm extension, 0 to 3 Nm lateral bending). Three-dimensional motions were measured using an optoelectronic motion monitoring system for at least two complete cycles of moment application. After intact specimen testing, two threaded, cylindrical interbody cages were inserted anteriorly at L5–S1 and the specimen was retested. Radiographic templating was used to select the cage size. Fusion cages of 13, 15 and 17 mm diameter were used in 2, 10 and 1 specimens, respectively. The load-displacement curves gave the range of motion (ROM) and stiffness at the instrumented and adjacent segments in flexion, extension and lateral bending. The ROM and stiffness after cage insertions were compared with the intact values using repeated-measures analysis of variance.Results: The ALIF cages decreased the L5–S1 ROM in the sagittal and frontal planes (p<.05). Their effectiveness improved with increasing compressive preload in flexion and extension. The cages decreased L5–S1 flexion ROM by 27% to 41% of intact for low preloads (0 to 400 N), and by 68% to 78% of intact under preloads of 800 to 1,200 N (p<.05). In extension, the ALIF cages permitted more than the intact segment with no compressive preload (p<.05), whereas the motion decreased by 48% at preloads of 800 to 1,200 N (p<.05). In lateral bending, the cages decreased the motion by 40% relative to intact at 400 N preload as compared with a 15% decease with no preload. At L4–L5 and L3–L4, there were few significant changes in ROM after insertion of the cages at L5–S1.Discussion: This study tested interbody cages under loads closely mimicking in vivo conditions. The response of the ALIF cage construct in flexion-extension under low compressive preloads is consistent with previous studies that tested the construct under zero or low compressive preloads [3,4]. The lack of segmental stabilization of the ALIF cage construct in extension with no preload has been observed in previous studies [3,4] and may be the result of a separation at the cage–end plate interface in extension. In contrast, the segmental stability of the ALIF cage construct in flexion and extension was significantly greater under high compressive preloads that correspond to standing and walking. This emphasizes the importance of preload for cage stability that may be only partially achieved by annular pretensioning. Relaxation of the annulus may be an important issue, because collagenous soft tissues relax to approximately 20% to 30% of the maximum load over time. Therefore, supplemental stabilization or postoperative immobilization may play an important role in protecting the ALIF cage construct during activities of daily living that induce minimal or low levels of compressive preloads on the lumbar spine.