Assessment of the Breakaway Torque at the Posterior Pelvic Ring in Human Cadavers.
OBJECTIVE
To enhance the diminished screw purchase in cancellous, osteoporotic bone following the fixation of posterior pelvic ring injuries by iliosacral screws an increased bone-implant contact area using modificated screws, techniques or bone cement may become necessary. The aim of the study was to identify sites within the pathway of iliosacral screws requiring modifications of the local bone or the design of instrumentations placed at this site.


METHODS
The breakaway torque was measured mechanically at the iliosacral joint ("ISJ"), the sacral lateral mass ("SLM") and the center of the S1 ("CS1"), at a superior and an inferior site under fluoroscopic control on five human cadaveric specimens (3 female; mean age 87 years, range: 76-99) using the DensiProbe™Spine device.


RESULTS
The measured median (range) breakaway torque was 0.63 Nm (0.31-2.52) at the "iliosacral joint", 0.14 Nm (0.05-1.22) at the "sacral lateral mass", 0.57 Nm (0.05-1.42) at the "S1 center." The "sacral lateral mass" breakaway torque was lower than compared to that at the "iliosacral joint" (p < .001) or "S1 center" (p < .001). The median (range) breakaway torque measured at all superior measurement points was 0.52 Nm (0.10-2.52), and 0.48 Nm (0.05-1.18) at all inferior sites. The observed difference was statistically significant (p < .05).


CONCLUSIONS
The lateral mass of the sacrum provides the lowest bone quality for implant anchorage. Iliosacral screws should be placed as superior as safely possible, should bridge the iliosacral joint and may allow for cement application at the lateral mass of the sacrum through perforations.
a Department of Orthopaedic and Trauma Surgery, Inselspital , University of Bern , Switzerland.
Johannes Dominik
JD
Bastian
b Institute of Anatomy , University of Bern , Switzerland.
Mathias
M
Bergmann
c AO Research Institute Davos , Switzerland.
Ronald
R
Schwyn
a Department of Orthopaedic and Trauma Surgery, Inselspital , University of Bern , Switzerland.
Marius Johann Baptist
MJ
Keel
a Department of Orthopaedic and Trauma Surgery, Inselspital , University of Bern , Switzerland.
Lorin Michael
LM
Benneker
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods; D006801:Humans; D008297:Male; D010024:Osteoporosis / Q000601:surgery; D010384:Pelvic Bones / Q000000981:diagnostic imaging / Q000293:injuries* / Q000601:surgery*; D011859:Radiography; D012447:Sacrum / Q000000981:diagnostic imaging / Q000601:surgery; D019415:Torque
DensiProbeTM; Posterior pelvic ring; breakaway torque; fragility fracture; osteoporosis; sacrum