Mechanical strength of biological canulated cancellous screw / 医用生物力学

ABSTRACT

Objective To develop a biological canulated screw and evaluate its mechanical properties, so as to provide theoretical basis for the bio therapy of fracture with enrichment technique of bone marrow stem cells, which could be diffused through the canula, particularly for promoting bone union of femoral neck fracture and preventing avascular necrosis of femoral head. Methods TC4 titanium alloyed canulated cancellous screw (7.3 mm in diameter) commonly used for internal fixation of femoral neck fracture was improved by designing an end sealing plug and side holes in a 900 mm-long canulated screw. The side holes were arranged along the axis of screw, and the first side hole was 20 mm away from the screw tip. The distance between each side hole was 10 mm, and the number of side holes ranged from 0 to 6. The screws without holes were tested as control, and the rest were divided into two groups, i.e., group A: force direction parallel to the side hole, group B: force direction perpendicular to the side hole. Three point bending test on the screw was conducted by using Instron material testing machine, so as to study the relationship between the mechanical strength of the screw and the number of side holes, and the loading direction. Results (1) When the force direction was parallel to the side hole, which was confined within 0, 1 or 2, the bending deformation of the screw reached 3 mm, and no differences were found in the maximum loads and elastic modulus of the screw. However, when the number of side holes was increased to 3 or more, a significant reduction in the maximum load and elastic modulus of the screw was found (P＜0.05). (2) When the force direction was perpendicular to the side hole, which was confined within 0,1 or 2, and the bending deformation of the screw reached 3 mm, no significant differences were found in the maximum loads of the screw. While no significant difference was found in the elastic modulus of the screw when the side hole was 0, 1, 2 and 3. With an increase in the number of side holes, the maximum loads (≧ 3 side holes) and elastic modulus (≧ 4 side holes) were significantly reduced (P＜0.05). (3) When the force direction was perpendicular to the side hole and the number of side holes was 3 or more, the maximum loads and elastic modulus of the screw were all significantly higher than the screw with the same number of side hole under force direction parallel to the side hole. Conclusions (1) For achieving better mechanical properties of the screw, the number of side holes in titanium alloyed canulated cancellous screw (7.3 mm in diameter) should be within 2; (2) If the number of side holes was equal or over 3, screws under force direction perpendicular to the side hole could provide better mechanical properties than screws under force direction parallel to the side hole. This study may provide some theoretical evidence and support for future clinical development and practice of the biological canulated screw.