Poster No. 1312 • 54th Annual Meeting of the Orthopaedic Research

Hydroxyapatite Surface Augmentation Improves Pedicle Screw Osseointegration for Posterior Dynamic Fixation Systems
Amy S. Lyons2, Kevin Foley3, Henry Bonin4, Eric Lange4, Anthony S. Turner1, Michael Beck4, Howard Seim1, Christian M. Puttlitz2
1Clinical Sciences, Colorado State University, Fort Collins, CO; 2Mechanical Engineering, Colorado State University, Fort Collins, CO; 3University
of Tennessee, Memphis, TN; 4Medtronic Spinal and Biologics, Memphis, TN
amylyons@engr.colostate.edu
Introduction: Prevention of pedicle screw loosening in dynamic stabilization
constructs, especially in patients with osteoporosis and/or poor bone quality, is a
significant challenge. Surface augmentation of orthopedic implants with osteoconductive coatings such as hydroxyapatite (HA) that provide a passive scaffold for
new bone formation has been proposed as a method of reducing the occurrence of
screw loosening over time. The purpose of this study was to compare the osseointegration potential of uncoated and HA-coated titanium pedicle screws when used
in conjunction with a dynamic fixation system in an ovine model.
Materials and Methods: Pedicle screws were inserted bilaterally into L4 and
L5 of the lumbar spine of 12 skeletally mature sheep (4 screws per animal, 48
screws total). Six animals received 4 standard anodized titanium pedicle screws
(non-HA) while the other six animals received 4 similar titanium pedicle screws
with a plasma-sprayed HA coating on the threaded portion of the screw (HA).
Each ipsilateral screw pair was connected with a dynamic fixation device. Plain
radiographs along with axial computed tomography (CT) scans of the lumbar
region were obtained at 3, 6, 9, and 12 months postoperatively. Animals were then
humanely euthanized and their spine harvested en bloc after 6 months (3 non-HA,
3 HA) and 12 months (3 non-HA, 3 HA) post-surgery.
Forty-two screws (n=12 six-month non-HA, n=10 six-month HA, n=10
twelve-month non-HA and n=10 twelve-month HA) were removed using a calibrated torque wrench with a digital readout to evaluate the peak extraction torque.
Following screw removal, specimens were analyzed for micro-fractures using the
12-month radiographs. The remaining in situ six screws (n=2 six-month HA, n=2
twelve-month non-HA and n=2 twelve-month HA) were scanned using microCT and processed for undecalcified histology. The circumferential contact area at
the bone-screw interface was calculated at 1.0 mm intervals along the screw length
using a virtual volume subtraction of the 3D reconstructed micro-CT scans.
Undecalcified histological sections were taken from the same specimens in the
sagittal plane along the long axis of the screw and histomorphometric measures
were used to quantify the longitudinal bone contact length at the bone-screw interface. Histopathology was also performed to evaluate the integration of the screw
with surrounding tissue. Screw orientation (cranial and caudal surface) was preserved throughout the histological processing (Figure 1). Extraction torque
between screw types at each time point was compared using a one way ANOVA on
ranks (alpha=0.05).
osseointegration may reduce the occurrence of screw loosening over time and
improve the overall success rates of pedicle-based, dynamic fixation procedures.
Figure 1: Top: micro-CT reconstruction (HA screw) showing circumferential bone-screw contact area (brown), non-bone contact area (green) and histological sagittal sectioning plane (red
line). Bottom: Sagittal histological section (bone stained pink) showing longitudinal screwbone contact, magnification 10X.
Results: Peak extraction torque was significantly higher at 6 (p<0.001)
and 12 months (p=0.002) for the HA screws compared to the non-HA
screws (Table 1). Radiolucent zones (defined as greater than 1mm in
width) were not found on any of the plain films or axial CT scans, and
micro-fractures were not observed. The HA coated screws had 170%
more bone contact circumferentially at 6 and 12 months and 450% more
bone contact longitudinally at 6 and 12 months compared to the non-HA
screw at 12 months (Table 1). Table 1: Extraction Torque and Percent Bone
Contact (mean ± stdev)
Histopathologically, the HA coated screws showed better osseointegration
with surrounding bone with no fibrous tissue encapsulation after 6 and 12 months.
Fibrous tissue encapsulation was frequently observed along the length of the screw
for the non-HA screw after 12 months (Figure 2).
Discussion: HA coated pedicle screws have significantly improved osseointegration compared to uncoated screws after 6 and 12 months of healing as evaluated by extraction torque. These data agreed with the micro-CT analysis, histomorphometry and histopathology results. Given the aging population and the increasingly frequent challenge of obtaining solid fixation in less dense bone, increased
Figure 2: A: 12 month non-HA specimen with fibrous tissue encapsulation of the screw, magnification 10X. B and C: Enlarged images of fibrous tissue encapsulation of the non-HA screw
showing poor osseointegration, magnification 40x and 20x respectively. D: 6-month HA specimen surrounded by new bone, 10X. E: Enlarged image showing very good HA incorporation,
100X. F: Image E enlarged to show HA coating with new bone apposition. No fibrous tissue
encapsulation was observed.
Acknowledgements: Institutional research support provided by Medtronic
Spinal and Biologics.
Poster No. 1312 • 54th Annual Meeting of the Orthopaedic Research Society