Biomechanical analysis of a long-segment fusion in a lumbar spine ? A finite element model study

[+] Author and Article Information
Raghu N. Natarajan

Rush University Medical Center, Suite 204 F, Orthopedic Ambulatory Building, 1611 West Harrison, Chicago, IL 60612

Kei Watanabe

University Medical and Dental General Hospital, Niigata, Japan

Kazuhiro Hasegawa

Niigata Spine Surgery Center, Niigata, Japan

1Corresponding author.

ASME doi:10.1115/1.4039989 History: Received December 28, 2017; Revised March 27, 2018


Purpose. Examine the biomechanical effect of material properties, geometric variables and anchoring arrangements in a segmental pedicle screw with connecting rods spanning the entire lumbar spine using finite element models. This will help (1) to determine the variables with the greatest effect on spine kinematics and stresses in instrumentation, (2) to compare the multi-directional stability of the spinal instrumentation and (3) to determine less-rigid fixation systems that may reduce adjacent segment disc disease. Methods. A lumbar spine finite element model was used to analyze the biomechanical effects of different materials used for spinal rods (TNTZ or Ti or CoCr), varying diameters of the screws and rods (5 mm and 6 mm), and different fixation techniques (multi-level or intermittent). Results. The results based on the range of motion and stress distribution in the rods and screws revealed that differences in properties and variations in geometry of the screw-rod moderately affect the biomechanics of the spine. Further the spinal screw-rod system was least stable under the lateral bending mode. Stress analyses of the screws and rods revealed that the caudal section of the posterior spinal instrumentation was more susceptible to fatigue failure. Although CoCr screws and rods provided the greatest spinal stabilization, these constructs were susceptible to fatigue failure. Conclusion. The findings of the present study suggest that a posterior instrumentation system with a 5-mm screw-rod diameter made of Ti or TNTZ is advantageous over CoCr instrumentation system.

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