Optimal location of a single distal interlocking screw in intramedullary nailing of distal third femoral shaft fractures.
OBJECTIVE
This biomechanical study was done to determine the effect of the level of a single distal screw in a static intramedullary (IM) femoral nail on the stability of fixation of a fracture in the distal third of the femur.


METHODS
Fifteen composite fiberglass femora were osteotomized transversely in the distal third of the femur. A Grosse-Kempf nail was implanted into the femurs, which were divided into three groups of five specimens. Single screw distal nail locking was varied distal to the osteotomy site for each group at 2.5, 5.0, and 7.5 centimeters, respectively.


METHODS
All instrumented femurs were mounted on a servohydraulic testing machine and fitted with transducers to measure axial, rotational, and bending displacements. Specimens were cyclically loaded (one hertz) in simultaneous torsion (moment: +/- 10 newton-meters) and axial compression (amplitude: 2,000 newtons) for 500 cycles with a 250-pound abductor force.


METHODS
Data from linear and rotational transducers were sampled at 100 hertz for five cycles before cycling, every 100 cycles of loading, and immediately after cycling. Custom computer software was developed to convert transducer signals into static and dynamic measurements of axial motion (in millimeters), rotation (in degrees), and angulation (in degrees).


RESULTS
Osteotomy site dynamic rotation increased significantly in specimens locked at 7.5 centimeters when compared with the 2.5-centimeter group. There was minimal difference between the stability of the 5.0-centimeter and 7.5-centimeter groups. There was no significant change in position at the fracture site before or after cyclic loading with respect to axial shortening, rotation, or bending. Both dynamic axial and angular displacements were also unaffected by screw position.


CONCLUSIONS
The location of a single distal interlocking screw in static IM nail fixation of distal third femur fractures can significantly affect rotational stability but not axial or angular fixation.
George
C J
CJ
Joseph Barnhart Department of Orthopedic Surgery, Baylor College of Medicine, Houston, Texas 77030, USA.
Lindsey
R W
RW
Noble
P C
PC
Alexander
J W
JW
Kamaric
E
E
D001696:Biomechanical Phenomena; D001858:Bone Nails; D001863:Bone Screws; D003187:Compliance; D005264:Femoral Fractures / Q000601:surgery*; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation*; D008422:Materials Testing; D011474:Prosthesis Design; D012399:Rotation