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J Korean Soc Spine Surg. 1998 May;5(1):1-8. Korean. Original Article.
You JW , Lim TH .
Abstract

STUDY DESIGN: This biomechanical study was designed to perform flexibility tests in multiple loading directions to compare the stabilizing effects of supplemental hook or pedicle screw fixation on short segment pedicle instrumentation system. OBJECTIVE: To compare biomechanical flexibilities of short segment pedicle instrumentation constructs added by hook or pedicle screw fixation in an unstable calf spine model. SUMMARY OF BACKGROUND DATA: Short segment pedicle instrumentation is using recently for the surgical treatment of the unstable burst fractures of the thoracolumbar spine, but a high incidence of early screw tai lure in short segment pedicle instrumentation has reported. MATERIALS AND METHODS: Ten fresh frozen calf spines (T10-L3) were loaded with pure uncosstrained moments in flexion, extension, axial rotation, and lateral bending directions. A maximum moment of 6.4 Nm was achieved in 5 steps using dead weights. After removal of L1 vertebral body, testing was performed on intact specimens first and then each specimen after laminar hook or pedicle screw insertion on the short segment pedicle instrumentation of ISOLA implant. Any kinds of graft material or transfixation device were not used to make the worst possible case of instability of an injured spine. Three different fixation methods were instrumented. These included: (1) one level aboye and one level below with pedicle screw, (2) 2 levels above with pedicle screw and hook and one level below with pedicle screw, and (3) 2 levels abode and one level below with pedicle screws. RESULTS: At the level of corpectomy, all fixation methods significantly reduced motions in flexion, extension, and lateral bending as compared to the intact motion (P<0.001). The differences between all constructs were not statistically significant. The addition of a hook or screw on the short segment pedicle screw construct was not significantly reduced the flexibi lite as compared to the short segment pedicle screw construct. Axial rotational motions in groups I, II, and III were similar with each other and with intact motion as well and there were no significant statistical difference. The addition of the hook or screw on the short segment pedicle construct showed more stability as compared to the pedicle screw construct, but statistical difference was not. The addition of hook or screw on the short segment pedicle screw construct showed similar stability in all motions with each other. At the level above corpectomyl all tested fixation methods did not improve the axial rotational stability beyond the intact case, but reduced flexion, extension, and lateral bending motions significantly (p<0.001). The addition of hook or screw on the short segment pedicle screw construct showed significant stability in all motions (p<0.001) as compared to the short segment pedicle screw construct and their stabilities were similar with each other. CONCLUSIONS: All fixation methods showed more stabilities in all motions than normal specimen. The addition of pedicle screw on the short segment pedicle screw constructs are more stable than the addition of hook, but there was no statistical difference. Adding one level of fixation cranial to the fracture using pedicle screws or hooks may be necessary to decrease the rate of clinical failure and to enhance the stability of the construct with short segment pedicle instrumentation

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