Biomechanical efficacy of monoaxial or polyaxial pedicle screw and additional screw insertion at the level of fracture, in lumbar burst fracture: An experimental study

Abstract

Background: Use of a pedicle screw at the level of fracture, also known as an intermediate screw, has been shown to improve clinical results in managing lumbar fracture, but there is a paucity of biomechanical studies to support the claim. The aim of this study was to evaluate the effect of adding intermediate pedicle screws at the level of a fracture on the stiffness of a short-segment pedicle fixation using monoaxial or polyaxial screws and to compare the strength of monoaxial and polyaxial screws in the calf spine fracture model.

Materials and methods: Flexibility of 12 fresh-frozen calf lumbar spine specimens was evaluated in all planes. An unstable burst fracture model was created at the level of L3 by the pre-injury and dropped-mass technique. The specimens were randomly divided into monoaxial pedicle screw (MPS) and polyaxial pedicle screw (PPS) groups. Flexibility was retested without and with intermediate screws (MPSi and PPSi) placed at the level of fracture in addition to standard screws placed at L2 and L4.

Results: The addition of intermediate screws significantly increased the stability of the constructs, as measured by a decreased range of motion (ROM) in flexion, extension, and lateral bending in both MPS and PPS groups (P < 0.05). There was neither any significant difference in the ROM in the spines of the two groups before injury, nor a difference in the ROM between the MPSi and PPSi groups (P > 0.05), but there was a significant difference between MPS and PPS in flexion and extension in the short-segment fixation group (P < 0.05).

Conclusions: The addition of intermediate screws at the level of a burst fracture significantly increased the stability of short-segment pedicle screw fixation in both the MPS and PPS groups. However, in short-segment fixation group, monoaxial pedicle screw exhibited more stability in flexion and extension than the polyaxial pedicle screw.

Keywords: Monoaxial or polyaxial; burst fracture; intermediate screw at fracture level; lumbar spine; pedicular screw.

Figure 1

A line diagram of the specimens in the MPS and PPS groups showing. (a) The V-shaped corpectomy of the L3 vertebra; (b) the dropped-mass technique used after osteotomy to simulate an unstable burst fracture of the L3 vertebra; (c) short-segment pedicle screw construct involving transpedicular fixation of vertebrae one above and one below the fracture site; (d) use of an intermediate screw at the level of the fractured vertebra to test improvement in stability of the construct

Figure 2

X-ray film and photographs. (a) The film showing the burst fracture of vertebral body and the preservation of the anterior longitudinal ligament; (b) mechanical testing for lateral bending (intact specimen); (c) mechanical testing for axial rotation (intact specimen); (d) mechanical testing for flexion and extension (fracture specimen); (e) mechanical testing for flexion and extension (short-segment fixation specimen); (f) mechanical testing for flexion and extension (intermediate screw fixation specimen)

Figure 3

Flow chart showing the sequence of events for each specimen. BT = Biomechanical testing; MPS = Monoaxial pedicle screw; PPS = Polyaxial pedicle screw; MPSi = Monoaxial pedicle screws with intermediate pedicle screws fixation; PPSi = Polyaxial pedicle screws with intermediate pedicle screws fixation

Figure 4

Bar graphs showing the ROM compared with intact spine for (a) flexion, (b) extension, (c) lateral bending, and (d) axial rotation loading. MPS = Monoaxial pedicle screw; PPS = Polyaxial pedicle screw; MPSi = Monoaxial pedicle screws with intermediate pedicle screws fixation; PPSi = Polyaxial pedicle screws with intermediate pedicle screws fixation. *P, 0.05 for significant difference between groups