D1: Orthopaedic Biomechanics I

MECHANICAL ANALYSIS OF PEDICLE SCREW FIXATION FOR ADOLESCENT IDIOPATHIC SCOLIOSIS

Michihiko Koseki1, Tsubasa Sato1, Hiroki Kanzawa2, Jun Takahashi1, Toshimasa Futatsugi1, Hiroyuki Kato1

1Shinshu University, Japan;
2Nomura Unison Co., Japan

Pedicle screw fixation is a surgical treatment used to treat patients with severe spine deformity such as adolescent idiopathic scoliosis. In this technique, pedicle screws are inserted to vertebrae and every screw are connected to a rod to correct the coronal and sagittal curves of the spine. Here, surgeons sometimes encounter bone fracture or screw back-out when the screws are connected to the rod, possibly because little is known on the precise mechanical conditions involved in the placement of screws. This study examined the placement and tightening order of screws to understand the mechanical states present in pedicle screw and rod systems.

In order to pursue the objective, a simple experimental spine model was constructed with seven resin vertebral bodies connected via a spring element to reduce error and increase measurement reproducibility. A triaxial strain gage was attached to each pedicle screw, and then the screws were fixed to vertebral bodies under the following three circumstances: Case 1) Screws were inserted into all consecutive vertebrae. Case 2) Screws were inserted into every second vertebra. Case 3) Screws were inserted into every third vertebra. Screws were tightened and maximum principle strain was computed from measured triaxial strains. Trials were performed five times for each case to ensure consistent results.

The examinations revealed that the mechanical stress on screws depended on the spacing and securing order during screw and rod fixation. In the case of skip pedicle screw fixation, the mechanical stress on screws was lower than that on screws inserted into every vertebra because the screws have a fewer number of constraints. In all cases, subsequently tightened screws displayed larger strains. Lower strains were generally observed in cases where vertebrae were skipped. These findings may help select the appropriate surgical approach when treating scoliosis patients with pedicle screw fixation.

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