Biomechanical evaluation of the fixation strength of lumbar pedicle screws using cortical bone trajectory: a finite element study

Keitaro Matsukawa Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and

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Yoshiyuki Yato Department of Orthopaedic Surgery, National Hospital Organization, Murayama Medical Center, Musashimurayama, Tokyo, Japan

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Hideaki Imabayashi Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and

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Naobumi Hosogane Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and

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Takashi Asazuma Department of Orthopaedic Surgery, National Hospital Organization, Murayama Medical Center, Musashimurayama, Tokyo, Japan

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Koichi Nemoto Department of Orthopaedic Surgery, National Defense Medical College, Tokorozawa, Saitama; and

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OBJECT

Cortical bone trajectory (CBT) maximizes thread contact with the cortical bone surface and provides increased fixation strength. Even though the superior stability of axial screw fixation has been demonstrated, little is known about the biomechanical stiffness against multidirectional loading or its characteristics within a unit construct. The purpose of the present study was to quantitatively evaluate the anchorage performance of CBT by the finite element (FE) method.

METHODS

Thirty FE models of L-4 vertebrae from human spines (mean age [± SD] 60.9 ± 18.7 years, 14 men and 16 women) were computationally created and pedicle screws were placed using the traditional trajectory (TT) and CBT. The TT screw was 6.5 mm in diameter and 40 mm in length, and the CBT screw was 5.5 mm in diameter and 35 mm in length. To make a valid comparison, the same shape of screw was inserted into the same pedicle in each subject. First, the fixation strength of a single pedicle screw was compared by axial pullout and multidirectional loading tests. Next, vertebral fixation strength within a construct was examined by simulating the motions of flexion, extension, lateral bending, and axial rotation.

RESULTS

CBT demonstrated a 26.4% greater mean pullout strength (POS; p = 0.003) than TT, and also showed a mean 27.8% stronger stiffness (p < 0.05) during cephalocaudal loading and 140.2% stronger stiffness (p < 0.001) during mediolateral loading. The CBT construct had superior resistance to flexion and extension loading and inferior resistance to lateral bending and axial rotation. The vertebral fixation strength of the construct was significantly correlated with bone mineral density of the femoral neck and the POS of a single screw.

CONCLUSIONS

CBT demonstrated superior fixation strength for each individual screw and sufficient stiffness in flexion and extension within a construct. The TT construct was superior to the CBT construct during lateral bending and axial rotation.

ABBREVIATIONS

BMD = bone mineral density; CBT = cortical bone trajectory; DEXA = dual-energy x-ray absorptiometry; FE = finite element; POS = pullout strength; TLIF = transforaminal lumbar interbody fusion; TT = traditional trajectory.
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