Lag-Screw Osteosynthesis in Thoracolumbar Pincer Fractures

Abstract

Study design: Biomechanical.

Objective: This study evaluates the biomechanical properties of lag-screws used in vertebral pincer fractures at the thoracolumbar junction.

Methods: Pincer fractures were created in 18 bisegmental human specimens. The specimens were assigned to three groups depending on their treatment perspective, either bolted, with the thread positioned in the cortical or cancellous bone, or control. The specimens were mounted in a servo-hydraulic testing machine and loaded with a 500 N follower load. They were consecutively tested in 3 different conditions: intact, fractured, and bolted/control. For each condition 10 cycles in extension/flexion, torsion, and lateral bending were applied. After each tested condition, a computed tomography (CT) scan was performed. Finally, an extension/flexion fatigue loading was applied to all specimens.

Results: Biomechanical results revealed a nonsignificant increase in stiffness in extension/flexion of the fractured specimens compared with the intact ones. For lateral bending and torsion, the stiffness was significantly lower. Compared with the fractured specimens, no changes in stiffness due to bolting were discovered. CT scans showed an increasing fracture gap during axial loading both in extension/flexion, torsion, and lateral bending in the control specimens. In bolted specimens, the anterior fragment was approximated, and the fracture gap nullified. This refers to both the cortical and the cancellous thread positions.

Conclusion: The results of this study concerning the effect of lag-screws on pincer fractures appear promising. Though there was little effect on stiffness, CT scans reveal a bony contact in the bolted specimens, which is a requirement for bony healing.

Keywords: biomechanics; human specimen; lag-screw; pincer fracture; spinal fractures; spinal fusion; spine; weightbearing.

Figure 1.

Scheme of the experimental setup. In the frontal view (a), the sideward guides for the cable of the follower load are visible. The lateral view (b) exhibits the position of the angular actuator for extension/flexion and lateral bending of the specimen.

Figure 2.

Dissected and embedded specimen fixed to a monoaxial servo hydraulic testing machine.

Figure 3.

Wedged (left side) V-shaped metal section to induce pincer fractures. The arrow indicates the tip, which was inserted between the endplate and the intervertebral disc.

Figure 4.

Lateral radiograph of a specimen (Th11 to L1) with an artificially created pincer fracture in the Th12 vertebral body, with parts of the intervertebral disc pushed into the fracture gap.

Figure 5.

Computed tomography scans. (a) Lateral view of a lag-screw in situ, thread positioned in the specimen’s cortical bone. (b) Lateral view of a lag-screw in situ, thread positioned in the specimen’s cancellous bone. Specimens with a lag-screw position as shown in (a) are referred to as bolted COT, specimens with a lag-screw position as shown in (b) are referred to as bolted CAT.

Figure 6.

Computed tomography scan of a fractured vertebral body (lateral [left] and coronal [right] view). Red arrows indicate: I: length of cranial endplate, II: length of vertebral midline, III: length of caudal endplate. Green arrow indicates posterior vertebral height; blue arrow indicates anterior vertebral height.

Figure 7.

Computed tomography scan: lateral view of a control specimen after fatigue loading, arrows indicating a broad fracture gap with bone debris and intervertebral disc tissue impressed into the fracture gap.