Pedicle screw surface coatings improve fixation in nonfusion spinal constructs.
METHODS
Biomechanical and histologic analysis.


OBJECTIVE
To compare the strength of the bone-screw interface of standard uncoated pedicle screws with screws treated with hydroxyapatite (HA), titanium plasma spray (TPS), and a composite HA-TPS coating.


BACKGROUND
Transpedicular screw fixation has become the gold standard in the treatment of various thoracolumbar spinal conditions. Pedicle screw loosening, however, has been reported, especially in mechanically demanding constructs or in vertebrae with low bone mineral density.


METHODS
Six mature porcine were instrumented with 4 types of titanium monoaxial pedicle screws (uncoated, HA-only coated, TPS-only coated, and HA-TPS composite coated) in a systematically varied, single-blinded fashion. After a 3-month survival period, the spines were harvested en-bloc and "time zero" control screws were instrumented in adjacent vertebrae. Screw placement and bone mineral density were evaluated with a postharvest computed tomography, and the strength of the tissue-implant interface was evaluated with a torsional screw extraction analysis (60 screws) and a nondecalcified histologic analysis (16 screws).


RESULTS
At 3 months postoperative, peak torque increased for all 3 types of coated screws (increased fixation) and decreased significantly for the uncoated screws (P < 0.001). Although 3-month peak torque was not statistically different between the 3 screw coatings, 4 of 10 TPS-only coated screws had a peak torque that was nearly 0 (<0.1 N m) versus only 1 of 10 HA-only screws and 0 of 10 HA-TPS composite screws. Histologic analysis confirmed the biomechanical findings with improved osseointegration in the HA-only and HA-TPS composite screws.


CONCLUSIONS
Pedicle screw coatings that promote mechanical interlocking, TPS, or direct osteoblast bonding(HA) increased screw fixation in this nonfusion model. More non-HA coated screws, however, were thought to be "loose" with a nearly zero peak extraction torque and fibrous encapsulation. Increased osseointegration with HA may result in a decreased incidence of screw loosening and improved outcomes of transpedicular spinal instrumentation in nonfusion procedures.
Upasani
Vidyadhar V
VV
Department of Orthopedic Surgery, University of California San Diego, San Diego, CA, USA.
Farnsworth
Christine L
CL
Tomlinson
Tucker
T
Chambers
Reid C
RC
Tsutsui
Shunji
S
Slivka
Michael A
MA
Mahar
Andrew T
AT
Newton
Peter O
PO
D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D020099:Coated Materials, Biocompatible; D017886:Durapatite / Q000737:chemistry*; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D008159:Lumbar Vertebrae / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D008422:Materials Testing; D023421:Models, Animal; D016348:Osseointegration; D011474:Prosthesis Design; D013552:Swine; D013556:Swine, Miniature; D013904:Thoracic Vertebrae / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*; D014057:Tomography, X-Ray Computed; D019415:Torque
D020099:Coated Materials, Biocompatible; D017886:Durapatite; D014025:Titanium