Biomechanical comparison of expandable and locked intramedullary femoral nails.
OBJECTIVE
The expandable intramedullary (IM) nail does not require locking and fluoroscopy use is minimized. However, the lack of cross-locking screws may adversely affect the fixation's rotational stability. The purpose of our study was to compare the rotational stability afforded by an expandable nail with that of a standard locked nail.


METHODS
In a cadaver model of a diaphyseal femoral fracture (OTA type 32-A3), we compared first-generation expandable IM nails with standard locked IM nails in osteoporotic and nonosteoporotic femora (10 pairs each) and second-generation expandable nails with standard locked IM nails only in nonosteoporotic femora (10 pairs). To simulate torsional loads during walking, we applied an external rotation moment of -1 to 10 Nm at 1 Hz to each construct for 5000 cycles. Failure was defined as 15 degrees of rotation at the fracture site. We used McNemar's test to check for significant (P < 0.05) differences in failure between groups.


RESULTS
Of the first-generation expandable nails, 90% failed (9/10 in osteoporotic and 9/10 in nonosteoporotic femora) within the first 1000 cycles. Of the respective locked nails, significantly fewer failed in nonosteoporotic femora than in osteoporotic femora (0/10 and 3/10, respectively). Of the second-generation nails, 8/10 failed within 100 cycles of testing. Of the comparative locked nails, none failed at 5000 cycles.


CONCLUSIONS
We concluded that the expandable IM femoral nail, when tested in purely axial rotation, has poor rotational stability compared with the standard locked IM femoral nail.
Oliveira
Miguel L R
ML
International Center for Orthopaedic Advancement, Department of Orthopaedic Surgery, Johns Hopkins Bayview Medical Center, Baltimore, Maryland 21224-2780, USA.
Lemon
Michael A
MA
Mears
Simon C
SC
Dinah
A Feroz
AF
Waites
Matthew D
MD
Knight
Trevor A
TA
Belkoff
Stephen M
SM
D000368:Aged; D001858:Bone Nails; D002102:Cadaver; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D005264:Femoral Fractures / Q000503:physiopathology* / Q000601:surgery*; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation* / Q000379:methods; D006801:Humans; D008297:Male; D009038:Motion; D011474:Prosthesis Design; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D019415:Torque; D016896:Treatment Outcome