Primary Stability of Kyphoplasty in Incomplete Vertebral Body Burst Fractures in Osteoporosis: A Biomechanical Investigation

Abstract

Background: The objective of our study was to biomechanically evaluate the use of kyphoplasty to stabilize post-traumatic segmental instability in incomplete burst fractures of the vertebrae. Methods: The study was performed on 14 osteoporotic spine postmortem samples (Th11-L3). First, acquisition of the native multisegmental kinematics in our robot-based spine tester with three-dimensional motion analysis was set as a baseline for each sample. Then, an incomplete burst fracture was generated in the vertebral body L1 with renewed kinematic testing. After subsequent kyphoplasty was performed on the fractured vertebral body, primary stability was examined again. Results: Initially, a significant increase in the range of motion after incomplete burst fracture generation in all three directions of motion (extension-flexion, lateral tilt, axial rotation) was detected as proof of post-traumatic instability. There were no significant changes to the native state in the adjacent segments. Radiologically, a significant loss of height in the fractured vertebral body was also shown. Traumatic instability was significantly reduced by kyphoplasty. However, native kinematics were not restored. Conclusions: Although post-traumatic segmental instability was significantly reduced by kyphoplasty in our in vitro model, native kinematics could not be reconstructed, and significant instability remained.

Keywords: biomechanics; incomplete burst fracture; kyphoplasty; lumbar spine; osteoporotic compression fracture; primary stability; spinal instability; spinal trauma; vertebral body.

Figure 1

Modified fracture creation by distance-controlled compression after osteotomy-like weakening of the upper endplate L1. Specimen before compression (left) and after fracture induction by axial compression (right).

Figure 2

Hydraulic material testing machine used to create standardized incomplete burst fractures and obtain radiographs. At the left, the mounted motion capture marker (rigid bodies) and radiographic reference (arrow) are shown. At the right is a magnified lateral view of a mounted sample.

Figure 3

Radiographs of the native sample (a), fractured specimen (b), balloon in position (c), inflated balloon (d), inserted cement (e), and after cement insertion using the anteroposterior technique (f).

Figure 4

Mounted specimen in the robot-based spine tester combined with active optical motion tracking to record each single segmental kinematic behavior in a multisegmental setup: overview (left) and magnified view with rigid bodies and follower-load applications (right).

Figure 5

Schematic presentation of height measurement via lateral radiographic projection: posterior (AB), anterior (CD), middle (EF), and posterior two-thirds (GH). I1 and I2 are perpendicular midline intersections for the construction of the points E and F [24].

Figure 6

Boxplot of lateral vertebral body height in millimeters. Anterior (cd), middle (ef), and posterior (ab) values are presented by condition: intact, blue; fractured, yellow; reconstructed by kyphoplasty (kypho), red.

Figure 7

Boxplot of kinematic median values of functional spinal unit Th12–L1 for axial rotation, extension–flexion, and lateral flexion. Intact values without (light blue) and with follower load (blue), fractured values with follower load (yellow), and values after kyphoplasty with follower load (red). Circle represents outliers and five-pointed asterisk represents extreme outliers.

Figure 8

Boxplot of kinematic median values of functional spinal unit L1–L2 for axial rotation, extension–flexion, and lateral flexion. Intact values without (light blue) and with follower load (blue), values for fractures with follower load (yellow), and values after kyphoplasty with follower load (red). Circle represents outliers and five-pointed asterisk represents extreme outliers.