Biomechanical analysis of calcium phosphate cement in augmentation of anterior cervical screw

Yueping CHEN; Yong ZHU; Chao ZHANG

ABSTRACT

BACKGROUND:

Calcium phosphate cement (CPC) can strengthen the immediate and early fixation of screws by improving the quality of bone and the surface of screws.

OBJECTIVE:

To evaluate the biomechanical efficacy of CPC in augmentation of anterior cervical unicortical screw fixation.

DESIGN:

A study of controlled observation.

SETTING:

Department of Orthopaedics of First Xiangya Hospital of Xiangya Medical College of Central South University. MATERIALS Experiments were performed at the Room of Electromechanical Tensile Property of College of Material Science and Engineering of Central South University from September 2003 to January 2004. Anterior cervical unicortical screw (pure titanium) was produced by Zhangjiagang Xinda Medical Equipment Co., Ltd. Injectable calcium phosphate cement was produced by Shanghai Ruibang Biomaterial Co., Ltd. Axial pull-out sleeve was produced by Machine Manufacture Center of Central South University.

METHODS:

①Sixteen cervical vertebrae (C3-6) of four fresh young man cadavers with fine results in bone mineral density (BMD) test and sixteen cervical vertebrae (C3-6) of 4 fresh old man cadavers with poor results in BMD test were selected. Specimens were provided by Department of Anatomy of Xiangya Medical College of Central South University. Mortal remains of the deceased were donated according to the will of the dead. Informed consents were obtained from their family members. Twelve vertebrae were selected in each group for three tests. Six vertebrae were selected in pull-out test, and six vertebrae in cyclic bending test and shearing test. Screw holes, which were 8 mm apart from the midline and 5°inclined centrally, were made on both sides in the front part of the vertebra. Cortical bone was not penetrated. ②A screw was randomly inserted into one side hole, which was considered to be a control group. Pull-out test was performed on material testing machine at 5 mm/min. The screw hole was filled with CPC (0.10-0.15 mL) to repair the screw hole, and then the screw was implanted, which was considered to be a restoration group. The screw hole was injected with CPC, and then the screw was implanted, which was considered to be a augmentation group. 24 hours later, pull-out test was conducted again at 37 ℃. ③The screw was randomly implanted into one side hole (as a control group). The screw hole was injected with CPC, and then screw was implanted (as an augmentation group). Both groups received cyclic bending test and shearing test. MAIN OUTCOME

MEASURES:

①The maximal axial pull-out strength of screws, ②displacement of screws after cycle bending test, ③the maximal shear strength after cycle bending test.

RESULTS:

①The axial pull-out test In normal vertebrae, axial pull-out strength was (313±64)N, (376±88)N and (446±121)N in the control group, the restoration group and the augmentation group respectively. In osteoporotic vertebrae, axial pull-out strength was (106±47)N, (154±67) N and (191±80)N in the control group, the restoration group and the augmentation group respectively. The axial pull-out strength in the restoration group and augmentation group was significantly bigger than in the control group (P < 0.05). ②The cyclic bending test The displacement of the unicortical screw in the augmentation group was much smaller than in the control group (P < 0.05). ③ In normal vertebrae, the maximal shear strength was (301±79)N and (395±105)N in the control group and the augmentation group respectively. In the osteoporotic vertebrae, the maximal shear strength was (87±39)N and (149±63)N in the control group and the augmentation group respectively. The maximal shear strength was significantly bigger in the augmentation group than in the control group (P < 0.05).

CONCLUSION:

CPC can enhance the axial pull-out strength and shearing ability of the screw. The augmentation is more evident in osteoporosis patients.