A two-cage reconstruction versus a single mega-cage reconstruction for lumbar interbody fusion: an experimental comparison

Background. Interbody cages are used as an adjunct to anterior lumbar interbody fusion, but exposure and insertion of two cages can be difficult. A biomechanical study was performed to compare the stability and exposed surface for fusion obtained with interbody reconstruction using two traditional cylindrical cages (18-mm diameter) vs. a single expanded mega-cage (24-mm diameter). A single-cage technique could result in safer exposure, shorter operating time, and less cost. Methods. Study design: nondestructive testing of L5-S1 motion segments with cages compared the two configurations, and direct measure of the size of the fusion bed was made. Patient sample: 16 human cadaveric lumbar motion segments. Outcome measures: significant differences in motion segment stiffness and cancellous surface areas were compared using a Wilcoxon rank sum test. Motion segments were biomechanically tested intact, and then tested again after insertion of two interbody cages (n=8) or a single mega-cage (n=8). Nondestructive biomechanical loading was performed consisting of: (1) compression (maximum load 900 N); (2) Flexion, extension, left and right lateral bending (maximum moment 18 Nm); and (3) left and right torsion (maximum moment 10 Nm). From the load-deformation curves obtained, stiffness values were calculated to compare the two-cage and the single mega-cage reconstructions. After testing, the specimens were disarticulated and the surface area of the endplate bed created in the cancellous bone (of both vertebrae) was measured to compare the potential vascular surface for osteogenesis with both constructs. Results. The averages of the normalized values of stiffness were significantly greater for the two-cage group as compared to the mega-cage group in flexion only (1.08 vs. 0.74, p<0.05). For extension, torsion and lateral bending there was no significant difference in stiffness. In compression the two-cage group was stiffer, although not significantly (0.92 vs. 0.68, p<0.07). The average cancellous bed surface area was slightly greater for the single-cage reconstruction (1,208 mm(2) vs. 1,155 mm(2)), although this difference was not significant. Conclusions. The stiffness with a single anterior mega-cage was significantly lower in flexion compared with two standard cages. However, in all other modes of testing the constructs were statistically equivalent, although neither construct was significantly stiffer than the intact specimen. Additionally, the single mega-cage provides an equivalent cancellous bed for fusion as compared to dual cages. While this study is not sufficient to recommend human application, these results and our previous experience with the successful in vivo use of a single cage in rhesus monkeys [4] suggest that the single expanded anterior cage may be an acceptable concept although subsidence risk needs further investigation. The potential advantages of a single mega-cage (safer for the foramen, safer for the vessels, more consistent decortication and possibly cheaper) further suggest that examination should be given to this method as an approach to anterior interbody reconstruction in selected patients.