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Technical Briefs

Cortical Screw Purchase in Synthetic and Human Femurs

[+] Author and Article Information
Rad Zdero1

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, Toronto, ON, M5B-1W8, Canadazderor@smh.toronto.on.ca

Khaled Elfallah

Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, M55-1A8, Canada

Michael Olsen

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, Toronto, ON, M5B-1W8, Canada; Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, M55-3G9, Canada

Emil H. Schemitsch

Martin Orthopaedic Biomechanics Laboratory, St. Michael’s Hospital, Toronto, ON, M5B-1W8, Canada; Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, ON, M55-1A8, Canada

1

Corresponding author.

J Biomech Eng 131(9), 094503 (Aug 06, 2009) (7 pages) doi:10.1115/1.3194755 History: Received December 04, 2007; Revised May 21, 2009; Published August 06, 2009

Biomechanical investigations of orthopedic fracture fixation constructs increasingly use analogs like the third and fourth generation composite femurs. However, no study has directly compared cortical screw purchase between these surrogates and human femurs, which was the present aim. Synthetic and human femurs had bicortical orthopedic screws (diameter=3.5mm and length=50mm) inserted in three locations along the anterior length. The screws were extracted to obtain pullout force, shear stress, and energy-to-pullout. The four study groups (n=6 femurs each) assessed were the fourth generation composite femur with both 16 mm and 20 mm diameter canals, the third generation composite femur with a 16 mm canal, and the human femur. For a given femur type, there was no statistical difference between the proximal, center, or distal screw sites for virtually all comparisons. The fourth generation composite femur with a 20 mm canal was closest to the human femur for the outcome measures considered. Synthetic femurs showed a range of average measures (2948.54–5286.30 N, 27.30–35.60 MPa, and 3.63–9.95 J) above that for human femurs (1645.92–3084.95 N, 17.86–24.64 MPa, and 1.82–3.27 J). Shear stress and energy-to-pullout were useful supplemental evaluators of screw purchase, since they account for material properties and screw motion. Although synthetic femurs approximated human femurs with respect to screw extraction behavior, ongoing research is required to definitively determine which type of synthetic femur most closely resembles normal, osteopenic, or osteoporotic human bone at the screw-bone interface.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

Grahic Jump Location
Figure 1

Screw insertion sites along the anterior side of femurs were located at the center of the full femur length, as well as 100 mm proximal and 100 mm distal to the center

Grahic Jump Location
Figure 2

Screw pullout test setup is comprised of (1) base plate, (2) side mounting plates equipped with horizontal support plate atop the femur to prevent excessive bending at the screw site, (3) optional thumb screws for positioning femurs, (4) U-channel screw extraction grip with tapered groove for multiple screw head sizes, and (5) universal joint swivel pin to minimize eccentric loads on load cell. Femurs had their distal condyles removed in order to be properly mounted. Diagram reproduced with permission (Ref. 22).

Grahic Jump Location
Figure 3

Cortical screw pullout force. The symbols indicate the presence of statistical differences (p<0.05) between screw sites for a given femur, femur types for a given screw site, and aggregate values for femur type (p<0.05). SSD means statistically significant difference. 4G16 femur (no SSD between any screws). 4G20 femur (no SSD between any screws). 3G16 femur (∗, proximal SSD compared with distal screw). Human femur (†, center SSD compared with distal screw). Proximal screw (€, human SSD compared with 4G16 and 3G16; ¥, 4G20 SSD compared with 3G16; £, 4G16 SSD compared with 4G20). Center screw (◆, 4G16 SSD compared with 4G20 and human). Distal screw (‡, human SSD compared with all other femurs). Aggregate screw (§, all femur comparisons showed SSD, except 4G20 compared with human femur). All other comparisons showed no SSD. Error bars indicate 1 standard deviation.

Grahic Jump Location
Figure 4

Cortical screw shear stress. The symbols indicate the presence of statistical differences (p<0.05) between screw sites for a given femur, femur types for a given screw site, and aggregate values for femur type (p<0.05). SSD means statistically significant difference. 4G16 femur (no SSD between any screws). 4G20 femur (no SSD between any screws). 3G16 femur (no SSD between any screws). Human femur (no SSD between any screws). Proximal screw (∗, human SSD compared with all other femurs). Center screw (†, 4G16 SSD compared with human). Distal screw (‡, human SSD compared with all other femurs; £, 4G16 SSD compared with 3G16). Aggregate screw (§, all femur comparisons showed SSD, except 4G20 compared with 3G16). All other comparisons showed no SSD. Error bars indicate 1 standard deviation.

Grahic Jump Location
Figure 5

Cortical screw energy-to-pullout. The symbols indicate the presence of statistical differences (p<0.05) between screw sites for a given femur, femur types for a given screw site, and aggregate values for femur type (p<0.05). SSD means statistically significant difference. 4G16 femur (no SSD between any screws). 4G20 femur (no SSD between any screws). 3G16 femur (no SSD between any screws). Human femur (no SSD between any screws). Proximal screw (no SSD between any femurs). Center screw (†, 4G16 SSD compared with human). Distal screw (‡, 4G16 SSD compared with human). Aggregate screw (§, human SSD compared with 4G16 and 3G16; ¥, 4G16 SSD compared with 4G20). All other comparisons showed no SSD. Error bars indicate 1 standard deviation.

Grahic Jump Location
Figure 6

Cortical material was sheared off at the screw-bone interface during screw pullout

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