Biomechanical Comparison of Fixation Stability among Various Pedicle Screw Geometries: Effects of Screw Outer/Inner Projection Shape and Thread Profile

The proper screw geometry and pilot-hole size remain controversial in current biomechanical studies. Variable results arise from differences in specimen anatomy and density, uncontrolled screw properties and mixed screw brands, in addition to the use of different tapping methods. The purpose of this study was to evaluate the effect of bone density and pilot-hole size on the biomechanical performance of various pedicle screw geometries. Six screw designs, involving three different outer/inner projections of screws (cylindrical/conical, conical/conical and cylindrical/cylindrical), together with two different thread profiles (square and V), were examined. The insertional torque and pullout strength of each screw were measured following insertion of the screw into test blocks, with densities of 20 and 30 pcf, predrilled with 2.7-mm/3.2-mm/3.7-mm pilot holes. The correlation between the bone volume embedded in the screw threads and the pullout strength was statistically analyzed. Our study demonstrates that V-shaped screw threads showed a higher pullout strength than S-shaped threads in materials of different densities and among different pilot-hole sizes. The configuration, consisting of an outer cylindrical shape, an inner conical shape and V-shaped screw threads, showed the highest insertional torque and pullout strength at a normal and higher-than-normal bone density. Even with increasing pilot-hole size, this configuration maintained superiority.

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The Effect of Pilot Hole Size on the Insertion Torque and Pullout Strength of Self-Tapping Cortical Bone Screws in Osteoporotic Bone

Journal of Trauma and Acute Care Surgery ◽

10.1097/ta.0b013e31802bf051 ◽

2008 ◽

Vol 64 (4) ◽

pp. 990-995 ◽

Cited By ~ 38

Author(s):

Suneel Battula ◽

Andrew J. Schoenfeld ◽

Vivek Sahai ◽

Gregory A. Vrabec ◽

Jason Tank ◽

...

Keyword(s):

Cortical Bone ◽

Bone Screws ◽

Osteoporotic Bone ◽

Insertion Torque ◽

Hole Size ◽

Pullout Strength ◽

Pilot Hole

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Effects of Pilot Hole Size and Bone Density on Miniscrew Implants' Stability

Clinical Implant Dentistry and Related Research ◽

10.1111/j.1708-8208.2010.00269.x ◽

2010 ◽

Vol 14 (3) ◽

pp. 454-460 ◽

Cited By ~ 15

Author(s):

Emily Hung ◽

Donald Oliver ◽

Ki Beom Kim ◽

Hee-Moon Kyung ◽

Peter H. Buschang

Keyword(s):

Bone Density ◽

Hole Size ◽

Pilot Hole ◽

Miniscrew Implants

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Designs and Techniques That Improve the Pullout Strength of Pedicle Screws in Osteoporotic Vertebrae: Current Status

BioMed Research International ◽

10.1155/2014/748393 ◽

2014 ◽

Vol 2014 ◽

pp. 1-15 ◽

Cited By ~ 55

Author(s):

Thomas M. Shea ◽

Jake Laun ◽

Sabrina A. Gonzalez-Blohm ◽

James J. Doulgeris ◽

William E. Lee ◽

...

Keyword(s):

Pedicle Screw ◽

Medical Condition ◽

Surgical Techniques ◽

Pedicle Screws ◽

The Body ◽

Current Status ◽

Screw Thread ◽

Pullout Strength ◽

Pilot Hole ◽

Advantages And Disadvantages

Osteoporosis is a medical condition affecting men and women of different age groups and populations. The compromised bone quality caused by this disease represents an important challenge when a surgical procedure (e.g., spinal fusion) is needed after failure of conservative treatments. Different pedicle screw designs and instrumentation techniques have been explored to enhance spinal device fixation in bone of compromised quality. These include alterations of screw thread design, optimization of pilot hole size for non-self-tapping screws, modification of the implant’s trajectory, and bone cement augmentation. While the true benefits and limitations of any procedure may not be realized until they are observed in a clinical setting, axial pullout tests, due in large part to their reproducibility and ease of execution, are commonly used to estimate the device’s effectiveness by quantifying the change in force required to remove the screw from the body. The objective of this investigation is to provide an overview of the different pedicle screw designs and the associated surgical techniques either currently utilized or proposed to improve pullout strength in osteoporotic patients. Mechanical comparisons as well as potential advantages and disadvantages of each consideration are provided herein.

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Effect of Pilot-Hole Size on the Pullout Strength of Flexor Digitorum Longus Transfer Fixed with a Bioabsorbable Screw

Foot & Ankle International ◽

10.3113/fai.2007.1078 ◽

2007 ◽

Vol 28 (10) ◽

pp. 1078-1081 ◽

Cited By ~ 8

Author(s):

Brian G. Donley ◽

Clinton Jambor ◽

Ahmet Erdermier ◽

James Sferra ◽

Peter Cavanagh

Keyword(s):

Insertion Site ◽

Hole Size ◽

Pullout Strength ◽

Flexor Digitorum Longus ◽

Interference Fit ◽

Pilot Hole ◽

Bioabsorbable Screw ◽

Significant Difference ◽

Eden Prairie ◽

Flexor Digitorum

Background: Fixation of tendon transfers with a bioabsorbable interference-fit screw has several advantages over other fixation methods: decreased dissection, operative time, and blood loss; preservation of tendon length; no interference with radiographic studies; no need for implant removal; and no barrier to revision surgery. Whether strength of fixation is affected by the size of the pilot hole has not been established. The purpose of this study was to determine the effect of pilot hole size on the pullout strength of a flexor digitorum longus (FDL) tendon secured into a bone analog using a 5.5-mm bioabsorbable screw. Methods: Thirty FDL tendons were harvested from 15 cadaver specimens and secured into predrilled 4 times 4 × 4 cm bone cubes with a 5.5-mm Arthrex bioabsorbable screw (Arthrex, Naples, FL). The use of bone analog foam cubes ensured consistent porosity at the insertion site, eliminating the variations associated with varying bone densities of cadaver specimens. Pilot hole sizes studied were 5.0 mm, 5.5 mm, and 6.0 mm. Pullout tests were done with an servohydraulic testing frame (MTS, Eden Prairie, MN). Results: There was no significant difference ( p = 0.4) between the pullout forces and stresses among the three pilot hole sizes. All specimens failed at the interface between the FDL and the bioabsorbable screw. In the 6.0-mm pilot hole group, there was a trend for increased pullout strength with increased tendon size. Conclusions: With a bioabsorbable 5.5-mm screw used for FDL transfer, a pilot hole the same size or a half millimeter larger or smaller than the screw had no statistically significant effect on the strength of the construct, even with tendons of different sizes.

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