Is a single anterolateral screw-plate fixation sufficient for the treatment of spinal fractures in the thoracolumbar junction? A biomechanical in vitro investigation

Abstract

Controversy exists about the indications, advantages and disadvantages of various surgical techniques used for anterior interbody fusion of spinal fractures in the thoracolumbar junction. The purpose of this study was to evaluate the stabilizing effect of an anterolateral and thoracoscopically implantable screw-plate system. Six human bisegmental spinal units (T12-L2) were used for the biomechanical in vitro testing procedure. Each specimen was tested in three different scenarios: (1) intact spinal segments vs (2) monosegmental (T12/L1) anterolateral fixation (macsTL, Aesculap, Germany) with an interbody bone strut graft from the iliac crest after both partial corpectomy (L1) and discectomy (T12/L1) vs (3) bisegmental anterolateral instrumentation after extended partial corpectomy (L1), and bisegmental discectomy (T12/L1 and L1/L2). Specimens were loaded with an alternating, nondestructive maximum bending moment of +/-7.5 Nm in six directions: flexion/extension, right and left lateral bending, and right and left axial rotation. Motion analysis was performed by a contact-less three-dimensional optical measuring system. Segmental stiffness of the three different scenarios was evaluated by the relative alteration of the intervertebral angles in the three main anatomical planes. With each stabilization technique, the specimens were more rigid, compared with the intact spine, for flexion/extension (sagittal plane) as well as in left and right lateral bending (frontal plane). In these planes the bisegmental instrumentation compared to the monosegmental case had an even larger stiffening effect on the specimens. In contrast to these findings, axial rotation showed a modest increase of motion after bisegmental instrumentation. To conclude, the immobilization of monosegmental fractures in the thoracolumbar junction can be secured by means of bone grafting and the implant used in this study for all three anatomical planes. After bisegmental anterolateral stabilization a sufficient reduction of the movements was registered for flexion/extension and lateral bending. However, the observed slight increase of the range of motion in the transversal plane may lead to loosening of the implant before union. Therefore, the use of an additional dorsal fixation device should be considered.

Fig. 1

Spine testing setup in neutral position (left) and maximal deflection (right)

Fig. 2

macsTL implant system (Aesculap, Germany) in the “twin-screw” configuration with bisegmental framing plate (left); principle of the polyaxial clamping, with an overview of the various system components (right)

Fig. 3

Test step 2: left—a specimen with partial corpectomy (L1) and discectomy (T12/L1); right—b monosegmental anterolateral implant (macsTL, Aesculap) with c inserted autogenous bone graft

Fig. 4

Test step 3: left—a specimen with extended partial corpectomy of L1; right—b bisegmental anterolateral implant (macsTL, Aesculap) with c inserted autogenous bone graft

Fig. 5

Box-plot-diagram for flexion/extension showing the grouped bisegmental and monosegmental results: step 1 (intact), step 2 (macsTL-monosegmental) and step 3 (macsTL-bisegmental)

Fig. 6

Load/deformation curves (hysteresis) for specimen No. 3; bisegmental view in left and right lateral bending, showing the three tested steps: step 1 (intact), step 2 (macsTL-monosegmental), step 3 (macsTL-bisegmental)

Fig. 7

Box-plot diagram for lateral bending showing the grouped bisegmental and monosegmental results: step 1 (intact), step 2 (macsTL-monosegmental) and step 3 (macsTL-bisegmental)

Fig. 8

Box-plot diagram for axial rotation showing the grouped bisegmental and monosegmental results: step 1 (intact), step 2 (macsTL-monosegmental) and step 3 (macsTL-bisegmental)