Biomechanical analysis of atlas fractures: a study on 40 human atlas specimens.

STUDY

DESIGN

Forty isolated specimens of the first cervical vertebra were tested by the application of pure axial force to failure. To exclude ligamentous side effects, transverse ligaments were dissected in all specimens.

OBJECTIVE

To investigate the biomechanical characteristics of the human atlas and to describe the influence of different speeds of force impact on the fracture types.

SUMMARY OF BACKGROUND DATA: Atlas fractures have been reproduced in some studies in the literature. However, the characteristics of isolated atlas fractures under pure axial loading at different speeds has not been reported so far.

METHODS

After dissection of soft tissue and generation of a peripheral quantitative computed tomography scan, the atlas preparations were tested to failure by displacement-controlled axial force application at constant speeds of either 0.5 mm/s (Group 1) or 300 mm/s (Group 2). The fracture types were classified according to Gehweiler.

RESULTS

At slow loading speed (Group 1), 2 Type-I (anterior arch), 3 Type-II (posterior arch), 2 Type-III (anterior and posterior arch), and 13 Type-IV (lateral mass) fractures occurred out of 20 specimens. At high loading speed (Group 2), Type-III fractures (burst fractures of 2 to 4 parts) occurred in all 20 tested specimens.

CONCLUSION

The presented results strongly suggest that the Type of atlas fracture depends on the speed of axial force impact. The present study demonstrates that Type-III fractures (2- to 4-part burst fractures) result from fast force impact whereas slow force impact is responsible for Type-IV atlas fractures of the lateral mass.