Biomechanical analysis of thoracolumbar interbody constructs. How important is the endplate?

A biomechanical study of human cadaveric thoracic vertebral bodies was conducted using several anterior fusion options subjected to axial loads. This study emphasized the contribution of the endplate to resistance of graft subsidence. To determine the importance of the vertebral endplate in resisting subsidence of various constructs into the vertebral body; the relative efficacy of potential alternative graft constructs such as iliac crest, ribs, humerus, and titanium mesh cage; and the importance of bone mineral content, vertebral level, and cross-sectional graft area on construct subsidence. As the fixation length of anterior and posterior spinal constructs is reduced, load sharing of the anterior column has become more important to reduce failure of the shorter devices. Several alternative graft constructs and surgical techniques have been used for reconstruction of the anterior column. There exist little comparative data as to whether any of these constructs are superior and whether the vertebral endplate contributes significantly to the integrity of the construct. Sixty-three isolated human cadaveric vertebral bodies from T3 to T12 were used to test seven different constructs in direct axial load onto prepared endplates with an electrohydraulic testing device. These constructs were: 1) titanium mesh cage (17 x 22 mm) on intact endplate, 2) C-shaped humerus on intact endplate, 3) tricorticated iliac graft in "tee configuration" on intact endplate, 4) tricorticated iliac graft in cancellous trough, 5) triple rib strut graft, 6) single rib on endplate, and 7) single rib on cancellous body. Dual X-ray absorptiometry assessment of bone mineral content was performed. A uniaxial load was applied with force and displacement data collected to determine maximal load to "failure" of the vertebral body. Preservation of vertebral endplate did not significantly increase the resistance to graft subsidence. The titanium cage construct provided the greatest resistance to axial load. Preservation of the vertebral endplate may not offer a significant biomechanical advantage in reconstructing the anterior column. Several alternative constructs are mechanically equivalent.