[Femoral nail osteosynthesis. Mechanical factors influencing the femoral antetorsion].
BACKGROUND
Antegrade or retrograde intramedullary nailing is a common and well established procedure for the treatment of femoral shaft fractures. One drawback of this technique is the high incidence of clinically relevant malalignment. Despite intra-operative and radiological improvements this problem has not yet been solved efficiently. The aim of this study was the evaluation of the mechanical influence on the antetorsion angle of intramedullary nails during and after interlocking in femoral shaft fractures.


METHODS
A mechanical instrument was developed allowing a defined torque to be administered to the distal femur fragment. As an optical measurement system for the assessment of the antetorsion angle, a navigation system was applied. Initially the influence of the interlocking mechanism of the nail on the antetorsion deviation was investigated. The distal interlocking hole was fixed free handed or by using a navigation system. The multidirectional movement of the distal femur fragment was documented. Furthermore, the influence of the rotational stability on the antetorsion angle after mechanical stress of 4 NM was investigated by measuring the remaining rotational capacity of the distal femur fragment.


RESULTS
The average remaining rotational capacity of the distal femur fragment was 5.8 degrees after locking the nail by hand. The navigated locking resulted in a deviation of only 2 degrees , a significant difference compared to the free-hand procedure. The rotational stability under stress showed an average of 15.4 degrees deviation of the distal fragment. Even after complete interlocking of the intramedullary nail a 14.2 degrees rotational deviation was observed.


CONCLUSIONS
It could be shown that mechanical stability as well as the interlocking itself of femoral nails have a relevant impact on the antetorsional angle of the femur. Potential sources of error of the femoral antetorsion angle can be caused by the interlocking process as well as by forced rotation of the femur after interlocking. Clinical studies are needed to improve our findings, while the observed effects might have an essential influence on the incidence of femoral malalignment after osteosynthesis by intramedullary nailing of the femur.
Citak
M
M
Unfallchirurgische Klinik, Medizinische Hochschule, Carl-Neuberg-Strasse 1, 30625, Hannover, Deutschland. citak.musa@mh-hannover.de
Kendoff
D
D
Citak
M
M
Gardner
M J
MJ
Oszwald
M
M
Krettek
C
C
Hüfner
T
T
D001858:Bone Nails; D004548:Elasticity; D019544:Equipment Failure Analysis; D005264:Femoral Fractures / Q000503:physiopathology* / Q000601:surgery*; D005593:Fracture Fixation, Internal / Q000295:instrumentation* / Q000379:methods; D006801:Humans; D011474:Prosthesis Design; D013314:Stress, Mechanical