Biomechanical analysis of a novel posterior construct in a transforaminal lumbar interbody fusion model an in vitro study

Anil Sethi, Aditya M Muzumdar, Aditya Ingalhalikar, Rahul Vaidya
Spine Journal: Official Journal of the North American Spine Society 2011, 11 (9): 863-9

BACKGROUND CONTEXT: Spinal fusion is a commonly performed surgical procedure. It is used to treat a variety of spinal pathologies, including degenerative disease, trauma, spondylolisthesis, and deformities. A mechanically stable spine provides an ideal environment for the formation of a fusion mass. Instrumented spinal fusion allows early ambulation with minimal need for a postoperative external immobilizer. Several biomechanical and clinical studies have evaluated the stability offered by different posterior instrumentation techniques and the effects of reduced instrumentation.

PURPOSE: The aim of the study was to compare the biomechanics of a novel pedicle and translaminar facet screw (TLFS) construct. Also, in this study, comparisons were made with the more common pedicle screw/TLFS constructs for posterior fixation.

STUDY DESIGN: Human cadaveric lumbar spines were tested in an in vitro flexibility experiment to investigate the biomechanical stability provided by a novel pedicle and TLFS construct after transforaminal lumbar interbody fusion (TLIF).

METHODS: Seven fresh human lumbar spines (L2-L5) were tested by applying pure moments of ±8 Nm. After intact specimen testing, a left-sided TLIF with a radiolucent interbody spacer was performed at L3-L4. Each specimen was then tested for the following constructs: bilateral pedicle screws (BPS) and rods at L3-L4; unilateral pedicle screws (UPS) and rods at L3-L4; UPS and rods and TLFS at L3-L4 (UPS+TLFS); and unilateral single pedicle screw and TLFS and rod at L3-L4 (V construct). The L3-L4 range of motion (ROM) and stiffness for each construct were obtained by applying pure moments in flexion, extension, lateral bending, and axial rotation.

RESULTS: All instrumented constructs significantly reduced ROM in flexion-extension and lateral bending compared with the intact specimen. In axial rotation, only BPS constructs significantly reduced ROM compared with intact specimen. The V construct was able to achieve more reduction in ROM compared with UPS construct and was comparable to UPS+TLFS construct. Unilateral pedicle screws construct was the least stable in all loading modes and was significantly different than BPS construct in lateral bending.

CONCLUSIONS: The V construct exhibited enhanced stability compared with UPS construct in all loading modes. It provides bilateral fixation and preserves the anatomic integrity of the superior facet joint. The novel construct may offer advantages of less invasiveness, significant reduction in operation time, duration of hospitalization, and costs of implants, which would require further clinical evaluation.

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