Effect of Bone Mineral Density and Endplate Thickness on the Compressive Strength in the Cervical Spine / 대한척추외과학회지

STUDY DESIGN: This is a biomechanical study to evaluate the subsidence at the cervical endplate from seven fresh cadavers. The study performed after evaluating a bone mineral density(BMD) and a thickness of the endplate. OBJECTIVES: To evaluate the effect of BMD and endplate thickness on the biomechanical strength in an anterior cervical interbody fusion model. MATERIAL AND METHODS: A total of 7 cervical spines(C3-C7) were obtained from fresh cadavers and dissected through the intervertebral disc to obtain the isolated vertebrae. BMD of each vertebral body was measured using a dual-energy x-ray absorptiometry(DEXA) and thickness of endplates was measured by CT images. Each vertebral body was cut into halves through the horizontal plane and prepared specimens were assigned one of the following 3 groups so that group mean BMDs became similar. In group 1, the endplates were preserved intact. The endplates in group 2 were burred into approximately 1/2 of the intact thickness. In group 3, the endplates were totally removed. Each specimen underwent the destructive compression test by using an Instron material test system(MTS). RESULTS: There was significant linea relationship between BMD and load to failure. The load to failure of group 1 was significantly greater than group 3. CONCLUSION: Preoperative consideration of BMD would be important for patient selection and the choice of a surgical technique. And it may be important to preserve the endplate as much as possible to reduced the incidence of subsidence when performing the cervical interbody fusion.

Biomechanical Motion Characteristics of Lumbar Motion Segments : Effects of Radial Tear of the Annulus Fibrosus / 대한척추외과학회지

STUDY DESIGN: Lumbar disc degeneration and segmental instability of the lumbar spine are causes of low back pain. Disc degeneration causes specific changes of the intervertebral disc, and could affect anatomic variations of end plate and vetebral body. However, the exact relationship between degenerative changes of the intervertebral disc and segmental motion characteristics is not known. It is known that radial tears of the annulus fibrosus initiate or accompany degenerative process of nucleus pulposus and the motion segment. It is hypothesis of this study that the existence of radial tear in the annulus fibrosis affects 3 dimension motion characteristics of motion segment. For the purpose, the degree of intervertebral disc degeneration is newly classified by existence of radial tear. Then, the resulting biomechanical motions are investigated. OBJECTIVES: To investigate effects of disc degeneration by the classification on kinematic motions of the motion segment from human lumbar spine and to suggest a quantified method to determine spinal instability in vivo. MATERIALS AND METHODS: A total of 60 spinal motion segments from human lumbar spine was used for this study. To measure 3 dimensioal motion of the motion segments, Vicon system(Oxford, England) with 3 cameras reflective markers and VAX station was used. 6 kinds of pure moments(flexion, extension, right and left axial rotation, and right and left lateral bending) were applied to the motion segments using dead weight for each loading step. At the end of test(maximum loading), motion segments were frozen for anatomical study. For making clear the degree of the degeneration of the disc, a new classification based on MRI results was used: Grade 1 is a normal young disc without tear; Grade 2 is a normal aging disc without radial tear; Grade 3 is a degenerative disc with radial tear; and Grade 4 is a severely degenerative disc with radial tear and other degeneration such as showing decreased disc height. RESULTS: The upper lumbar specimens with radial tears has increased flexion motions as compared to the normal group. Also, the right and left axial rotation in radial tear group increased as compared to the normal group. However, there were no statistical differences in other motions. For the lower lumbar specimens, there were no significant differences in measured motions in all directions between the normal and radial tear groups CONCLUSIONS: These results suggests that the segmental motions are affected by radial tear in the intervertebral disc. Thus, the radial tear in the annulus fibrosus of lumbar intervertebral disc could cause the instability of lumbar spine. Further research is required to determine the relationship between other structural changes and biomechanical characteristics, and future studies should include in vivo investigations to correlate these findings to patients'symptoms.

Biomechanical Evaluation of Supplemental Hook or Screw Fixation in Short Segment Spinal Instrumentation / 대한척추외과학회지

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