Plastic Surgery Research Council

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Additive Drilling Significantly Improves Mechanically-Tested Bony Stabilization in Translational Spine Models
Fady G. Gendy, HSD1,2, Gregory Kurgansky, HSD3,2, Leyla Y. Cavdar, BA4, Christopher D. Lopez, BA5,2, Lukasz Witek, MSci, PhD2, Paulo G. Coelho, DDS, PhD5,2, Andrea Torroni, MD, PhD5.
1Baruch College, Manhattan, NY, USA, 2Department of Biomaterials and Biomimetics, New York University College of Dentistry, New York, NY, USA, 3Macaulay Honors College at Hunter College, Manhattan, NY, USA, 4New York Medical College, Valhalla, NY, USA, 5Hansjörg Wyss Department of Plastic Surgery, New York University Langone Medical Center, Manhattan, NY, USA.

Statement of Purpose:
Surgical fixation of implants into bone to treat skeletal pathology has positively influenced the well-being of patients and continues to be the basis of orthopaedic rehabilitation. Surgical fixation is dependent on osseointegration, the anchorage of bone around implant. Osseointegration evolves with primary and secondary stability between implant and bone: the initial mechanical interlocking between bone and implant and subsequent bone growth through the healing chambers of the implant, respectively. Osseointegration is dependent on multiple factors such as implant macrogeometry, host bone quality, and drilling techniques1. Implant geometries and bone quality have been well described, but different drilling techniques are not well explored. The traditional subtractive drilling techniques render the bony spicules excavated impractical, while additive techniques utilize them as nucleating surfaces for new bone. Therefore, we chose to investigate the effect of additive drilling on implant insertion.
Methods:
Utilizing a translational animal model, 64 implants were installed in the lumbar spine of 8 adult sheep (n=8/animal) bilaterally, with each pedicle screw measuring 4.5mm in diameter x 45mm length. The animals were separated into two time points, 6 and 12 weeks in-vivo. The left side of each lumbar vertebra underwent traditional subtractive drilling, while the right underwent additive drilling. The animals were sacrificed with anesthetic overdose, and the vertebrae were removed en bloc. Pullout strength was measured through mechanical testing using a universal testing machine. For histological analysis, non-decalcified histology was utilized. Biomechanical testing results were recorded and analyzed as mean values with the corresponding 95% confidence interval values (mean ± CI). Pull-out strength were compared using several factors of time in vivo (6- and 12-weeks) as well as surgical drilling method -subtractive and additive.
Results:
Mechanical pullout strength collapsed across all time points delineated no significant difference in outcomes between vertebrae. However, when comparing mechanical stability between additive and subtractive drilling at 6-weeks, there was significantly greater pullout strength for the additive group versus the subtractive group. The additive Group measured ~ 390 N, meanwhile the subtractive group only measured ~300 N. Furthermore, at the 12-week time point similar results were seen as the additive group had pullout strength of ~320 N and the subtractive group had ~230 N. All results were significant with p<0.05. Figure 1 demonstrates the initial histological evidence of increased bone growth in the additive group versus subtractive group.
Conclusion:
Mechanical pullout testing demonstrated that additive drilling provides better implant anchoring and stability compared with the subtractive group. The trend that pullout strength was greater at 6 weeks than that at 12-weeks can be explained by the further development of secondary stability at the 12-week time point.

Figure 1: Transverse histological sections of (left) subtractive and (right) additive drilling protocols.
1.Coelho PG, Jimbo R. Osseointegration of metallic devices: current trends based on implant hardware design. Archives of Biochemistry and Biophysics 2014;561:99-108.


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