Holding power of variable pitch screws in osteoporotic, osteopenic and normal bone: are all screws created equal?
BACKGROUND
Biomechanical properties of four different commercially available small fragment cannulated screws (Twin fix (Stryker, Freiburg, Germany), Herbert, (Zimmer, Warsaw, USA), Omnitech (Unimedical, Torino, Italy), Barouk (Depuy, Warsaw, USA)), with variable pitch, used for fracture fixation were compared.


METHODS
Polyurethane foam blocks of three different densities with mechanical properties similar to osteoporotic, osteopenic and normal bones were used to conduct the tests. Each screw was tested for pushout and pullout holding power after a primary insertion and for pullout after a repeated insertion into the respective foam blocks.


RESULTS
The mean pullout and pushout strengths of all screws correlated to the foam density, and were significantly (p<0.001 and <0.001, respectively) better in foam with higher density. The mean pullout strength of each screw was consistently lower after reinsertion into the osteoporotic, osteopenic and normal bone densities by 4-30%, when compared to the index insertion (Fig. 4b). Yet, this difference was not found to be statistically significant (p=0.23). The Barouk screw performed significantly (p<0.0001) better than the other screws in all three different densities of foam for both for pushout and pullout after index insertion as well as for pullout tests after reinsertion.


CONCLUSIONS
The holding power of screws is directly correlated to bone density, thread design and number of threads engaging the bone. Reinsertion through the same hole could reduce the ultimate pullout strength. The surgeon should consider the advantages and disadvantages of each implant, depending on the clinical situation and choose accordingly.
Ramaswamy
Rajesh
R
Holland Orthopaedic and Arthritic Campus, Sunnybrook Medical Health Sciences Centre, University of Toronto, Ontario, Canada.
Evans
Samuel
S
Kosashvili
Yona
Y
D001696:Biomechanical Phenomena; D015519:Bone Density; D001851:Bone Diseases, Metabolic / Q000473:pathology; D001863:Bone Screws / Q000592:standards*; D001842:Bone and Bones / Q000473:pathology; D004867:Equipment Design; D004868:Equipment Failure; D019544:Equipment Failure Analysis / Q000379:methods; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006801:Humans; D008422:Materials Testing / Q000379:methods*; D011140:Polyurethanes; D016062:Porosity
D011140:Polyurethanes; C028279:polyurethane foam