A comparative biomechanical investigation of anterior lumbar interbody cages: Central and bilateral approaches

Background: Some biomechanical studies have been performed to evaluate the stabilization provided by interbody cages, but there are virtually no compa rative data for the different designs. Furthermore, most investigators have used animal models, which may have led to different results due to morphol ogical variation in the end plates and articular facets. The objectives of the current study were to evaluate whether two different anterior cage desi gns (BAK and SynCage) performed differently with respect to immediate stabi lization of the spine, whether the cages stabilized the spine significantly compared with its intact condition, and whether the addition of supplement ary translaminar screw fixation further stabilized the spine. Stabilization was defined as a reduction in motion after insertion of an implant. Methods: Twelve lumbar functional spinal units from human cadavera were tes ted under pure moments of flexion, extension, bilateral axial rotation, and bilateral lateral bending to a maximum of ten newtonmeters. The relative i ntervertebral motions were measured, with use of an optoelectronic camera s ystem, under three test conditions: with the spine intact, after insertion of anterior interbody cages, and after insertion of anterior interbody cage s supplemented with translaminar screw fixation, Six specimens were tested for each type of cage: a bilateral, porous, threaded cylinder (BAK) and a c entral, porous, contoured implant with end-plate fit (SynCage), Results: The cages performed in a similar manner in all directions of loadi ng, with no significant differences between the two designs. The cages sign ificantly stabilized the spine compared with its intact condition in flexio n, axial rotation, and lateral bending (the median value for motion was 40, 48, and 29 percent of the value for the intact condition, respectively; p = 0.002 for all three directions). Compared with the cages alone, translami nar screw fixation provided no additional stabilizing effect in these direc tions but it significantly increased the stability of the spine in extensio n (the median value for motion was 34 percent of the value with the cages a lone; p = 0.013), Conclusions: There were no differences in the stabilization provided by the two different cage designs. Use of the cages alone stabilized the spine in all directions except extension, and use of supplementary translaminar scr ew fixation provided additional stabilization only in extension. Clinical R elevance: This study demonstrated that interbody cages do not stabilize the lumbar spine in extension, and this observation was not altered by the use of substantially different designs. If the lack of stabilization in extens ion is a clinical problem, possible solutions include the avoidance of exte nsion postoperatively or the use of supplementary fixation.