J Korean Neurosurg Soc.  2015 Nov;58(5):412-418. 10.3340/jkns.2015.58.5.412.

Effect of Device Rigidity and Physiological Loading on Spinal Kinematics after Dynamic Stabilization : An In-Vitro Biomechanical Study

• Chun K
• Yang I
• Kim N
• Cho D

• KMID: 2191405
• DOI: http://doi.org/10.3340/jkns.2015.58.5.412

Abstract

OBJECTIVE
To investigate the effects of posterior implant rigidity on spinal kinematics at adjacent levels by utilizing a cadaveric spine model with simulated physiological loading.
METHODS
Five human lumbar spinal specimens (L3 to S1) were obtained and checked for abnormalities. The fresh specimens were stripped of muscle tissue, with care taken to preserve the spinal ligaments and facet joints. Pedicle screws were implanted in the L4 and L5 vertebrae of each specimen. Specimens were tested under 0 N and 400 N axial loading. Five different posterior rods of various elastic moduli (intact, rubber, low-density polyethylene, aluminum, and titanium) were tested. Segmental range of motion (ROM), center of rotation (COR) and intervertebral disc pressure were investigated.
RESULTS
As the rigidity of the posterior rods increased, both the segmental ROM and disc pressure at L4-5 decreased, while those values increased at adjacent levels. Implant stiffness saturation was evident, as the ROM and disc pressure were only marginally increased beyond an implant stiffness of aluminum. Since the disc pressures of adjacent levels were increased by the axial loading, it was shown that the rigidity of the implants influenced the load sharing between the implant and the spinal column. The segmental CORs at the adjacent disc levels translated anteriorly and inferiorly as rigidity of the device increased.
CONCLUSION
These biomechanical findings indicate that the rigidity of the dynamic stabilization implant and physiological loading play significant roles on spinal kinematics at adjacent disc levels, and will aid in further device development.

Keyword

Dynamic stabilization; Implant rigidity; Axial loading; Spinal kinematics; Cadaveric study; Robotic testing

MeSH Terms

Aluminum
Biomechanical Phenomena*
Cadaver
Humans
Intervertebral Disc
Ligaments
Polyethylene
Range of Motion, Articular
Rubber
Spine
Zygapophyseal Joint
Aluminum
Polyethylene
Rubber

Figure

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  Fig. 1 Testing set-up. Radiographic image of specimen with implantation of pedicle screws and placement of pressure transducers (A), testing specimen with active markers (B).

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  Fig. 2 Testing System. a : PcReflex motion analysis system, b : Robotic testing system, c : Data acquisition system for pressure transducer, d : Qualisys PcReflex camera system, e : Testing specimen.

• 

  Fig. 3 Five testing materials. A : Intact (without rod). B : Rubber. C : Low-density Polyethylene. D : Aluminum. E : Titanium.

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  Fig. 4 ROM for flexion (A), extension (B).

• 

  Fig. 5 Disc pressure for flexion (A), extension (B).

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  Fig. 6 Intersegmental COR of L3-4 and L5-S1. A : One specimen. B : All specimens.

• 

  Fig. 7 Transition of intersegmental COR at the adjacent levels.

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