[Development and biomechanical study of Ni-Ti shape memory alloys scaphoid arc nail].

OBJECTIVE

To develop a new internal fixator Ni-Ti shape memory alloy scaphoid arc nail (NT-SAN) for fracture of the scaphoid waist and to test the biomechanical characteristics of NT-SAN so as to provide the biomechanical proofs for clinical application.

METHODS

According to the acquired measurements and anatomic features of the scaphoid in Chinese population, a special internal fixator--NT-SAN was designed. The biomechanical intensity experiment: The 32 specimens of fractures of the waist of scaphoid were divided randomly into 4 groups (n=8). Reduction and internal fixation were carried out in each specimen, with Kirschner wires (Group A), with Kirschner wires stapling (Group B), with screw (Group C) and with NT-SAN (Group D). Their fixed strength was tested by a hydrolicpressure biomechanical system AG-1. The biomechanical fatigue experiment: The models of 24 waist scaphoid fracture of adult upper limbs specimens were made and randomly divided into 3 groups (n=8). Fracture was fixed with Kirschner wire stapling (Group E), with screw(Group F) and with NT-SAN (Group G). Wrist joint was vertically pressured load of 98 N, palmar flexion and dorsal extension motion was simulated; the range of movement was from palmar flexion 5 degrees to dorsal extension 30 degrees and the frequency was 2,000 cycles. Displace data of fragment were measured at every 500 cycles. Finally, the biomechanical features of NT-SAN were evaluated according to the tested data. NT-SAN was used to treated 1 patient with fracture of the waist of scaphoid, who was typed as IIb according to Herbert classification.

RESULTS

The biomechanical tests showed that the traction forces were (15.18+/-3.55), (36.04+/-4.30), (64.88+/-11.62), (65.84+/-12.22) N and (20.28+/-12.09), (75.95+/-47.64), (120.91+/-26.68), (130.21+/-31.55) N when the displacements of the fracture lines distracted in 1 mm/min were 1 mm and 2 mm; showing significant differences between Group D and groups A, B (P < 0.05), and showing no significant difference between Group D and Group C (P < 0.05). The biomechanical fatigue experiment showed that there were significant differences between Group G and groups E, F (P < 0.05) according to the measured results of the displacements of the fracture lines. When wrist joint were circularly moved; showing significant differences between Group G and Group E (P < 0.05) after 500 circular movements, and showing significant differences between Group G and Group F (P < 0.05) after 1,500 circular movements according to the measured results of the "stage-shape" displacements in the fracture position. Incision healed by first intention and the X-ray films showed good NT-SAN fixation 3 months after follow-up.

CONCLUSION

The design of NT-SAN is in accordance with the anatomic features of the scaphoid. The fixed strength can meet the demand of the biomechanics and the range of fatigue strength can meet the demand of bony union.