In Vivo Evaluation of a Novel Expanding Pedicle Screw: A Six-Month Study in a Sheep Model

96S Proceedings of the NASS 26th Annual Meeting / The Spine Journal 11 (2011) 1S–173S

vertebrae were divided into Black line or non-Black line. On lateral view at

6 months X-p, pseudarthrosis and kyphosis were evaluated. Dynamic mo-

tion or cleft sign in fractured vertebra was defined as pseudarthrosis. Ky-

phosis rate was obtained by subtracting the anterior vertebral height in the

injured vertebra from the average anterior vertebral height of the normal

vertebrae above and below the injured vertebra in the lateral images and

dividing the value by the average vertebral height of the normal vertebrae.

Kyphosis progression rate was obtained by subtracting the kyphosis rate on

initial examination from that at 6 months and dividing the value by the ini-

tial examination. All measurements were performed with the investigator

blinded concerning the patient’s name, clinical findings, and other imaging

data. Visual analogue scale for back pain (VAS) was evaluated at 6 months

examination. The chi-square test was used for pseudarthrosis rate, and the

Student’s t-test was used for kyphosis progression rate and VAS. p!.05

was taken as a statistically significant difference.

RESULTS: Fourteen of 63 vertebrae (22%) resulted in pseudarthrosis.

Powder sign (n516) included 0 (0%) pseudarthrosis, and non-Powder sign

(n547) included 14 (30%), and Powder sign included pseudarthrosis sig-

nificantly less than non-Powder sign (p5.01). Sensitivity of bone union by

Powder sign was 33%, and specificity of that was 100%. Black line (n524)

included 11 (46%) pseudarthrosis, non-Black line (n539) included 4

(10%), and Black line included pseudarthrosis significantly more than

non-Black line (p5.004). Sensitivity of pseudarthrosis by Black line was

71%, and specificity of that was 71%. Kyphosis progression rate of Powder

sign was 11% (621%) and that of non-Powder sign was 23% (625%), and

there was no significant difference (p5.08). Kyphosis progression rate of

Black line was 32% (622%) and that of non-Black line was 13%

(622%), and there was a significant difference (p5.003). The average

value of VAS of powder sign was 26 mm (618 mm) and that of non-

Powder sign was 28 mm (623 mm), and there was no significant differ-

ence. The average value of VAS of Black line was 35 mm (626 mm)

and that of non-Black line was 23 mm (617 mm), and there was a signif-

icant difference (p5.04).

CONCLUSIONS: On MRI STIR, Powder sign can predict bone union,

and Black line was the risk factor of pseudarthrosis, kyphosis progression

and back pain on osteoporotic vertebral fractures.

FDA DEVICE/DRUG STATUS: This abstract does not discuss or include

any applicable devices or drugs.

doi: 10.1016/j.spinee.2011.08.237

Friday, November 4, 20114:45–5:45 PM

Focused Paper Presentations: Biomechanics

183. In Vivo Evaluation of a Novel Expanding Pedicle Screw:

A Six-Month Study in a Sheep Model

Mark Levy, MD, ScD1, Ory Keynan, MD2, Orna Popper3, Elad Sapir3,

Yair Spanier3, Michal Ruchelsman3, Jaffar Hleihil3; 1Technion Israel

Institute of Technology, Haifa, Isreal; 2Kokhav Yair, Isreal; 3Expanding

Orthopedics, Caesarea, Isreal

BACKGROUND CONTEXT: Solid fixation of pedicle screws is the cor-

nerstone of successful spinal fusion. It’s particularly important in compro-

mised bone quality such as elderly population, various bone pathologies

and revision surgery. Early fixation of pedicle screws as well as long term

implant osteointegration, minimize the rate of failure optimizing the chan-

ces of fusion.

PURPOSE: The objective of the study was to evaluate in-vivo a novel ti-

tanium expanding pedicle screw, comprising a screw section in the pedicle

and a 4-wing expandable portion in the vertebral body. The study assessed

the mechanical performance and biological fixation with time, including

implantation and deployment, characterization of bone ingrowth on the

All referenced figures and tables will be available at the Annual Mee

exterior and interior surface of the expandable portion and the ability to

remove the screw after bone ingrowth.

STUDY DESIGN/SETTING: A GLP-compliant animal study using stan-

dard and expanding pedicle screws for lumbar spine fusion.

PATIENT SAMPLE: Eleven skeletally-mature Suffolk sheep, 80-120 kg

at implantation day.

OUTCOME MEASURES: Clinical and radiographic data, histology of

specimens, deployment and un-deployment, removal of the expanding ped-

icle screw.

METHODS: Lumbar spines of 11 sheep were implanted with a total of

44 expanding pedicle screws Ø5.8 mm and 22 standard pedicle screws

Ø5.5 mm (control), using standard spinal fusion techniques and instrumen-

tation. In each spine 6 screws were inserted, one per vertebra, through

a standard posterior trans-pedicle approach, in 3 one-level unilateral fusion

pairs: one pair of controls and 2 pairs of expanding pedicle screws. Each

pair was connected using Ø5 mm rods. No discs were removed and no

bone graft was added. Pre and post-operative x-ray films were obtained.

The animals were kept up to 180-day follow-up period with no activity

restriction. Intermediate time points were at 35, 77 and 90 days post-

operation. At the end of each time point, the animals were sacrificed

and the spines were harvested for removal of the screws or histological

evaluation.

RESULTS: Successful deployment of the expanding pedicle screws was

achieved in all implanted screws using an active mechanism. The histologi-

cal analysis demonstrated osteointegration of the expanding pedicle screw in

exterior and interior surface of the expandable portion from the 35-day time

point and onward. The average amount of bone ingrowth inside the expand-

able area was elevated with each consecutive time point showing maturation

of the bone matrix from woven bone to lamellar osteonal bone over time.

Removal of the expanding pedicle screwswas achieved for all screws despite

the bone ingrowth, due to an active undeployment mechanism allowing col-

lapse of the expandable area and removal through the pedicle.

CONCLUSIONS: Implantation and deployment of a novel expanding

pedicle screw was demonstrated in the lumbar spine of mature sheep.

The increased surface area of the metal-bone interface allowed immediate

fixation. The increased osteointegration with time, as well as bone in-

growth inside the expandable area, provided a progressively robust fixation

of the screw within the vertebral body. The immediate mechanical fixation

followed by the long-term biological fixation was designed to enhance

construct stability and subsequent fusion. The ability to undeploy and re-

move the screws demonstrated the mechanical properties of this novel

screw and its instrumentation.

FDA DEVICE/DRUG STATUS: Expanding Pedicle Screw: Not approved

for this indication.

doi: 10.1016/j.spinee.2011.08.239

184. Biomechanical Aspects of Pedicle Screw Loosening in the

Lumbar Spine

Joel Boerckel, PhD1, Liz Peters2, Lalissie Merga3, Tamkeenat Syed4,

Mona Arabshahi3, Rick Chappuis, PA-C3, James Chappuis, MD, FACS5;1Georgia Tech, Atlanta, GA, USA; 2Ball Ground, GA, USA; 3Atlanta, GA,

USA; 4Crystal Lake, IL, USA; 5Orthopaedic & Spine Surgery of Atlanta,

Atlanta, GA, USA

BACKGROUND CONTEXT: Previously at NASS, we reported that

removal of pedicle screws following fusion resulted in significant reduc-

tion in pain and medication use and increased activity and quality of life.

As posterior pedicle screw systems increase in strength and rigidity, more

loads are present at the bone screw interface and loosening can occur.

PURPOSE: The purpose of this study was to quantify pedicle screw loos-

ening using insertion and removal torque and evoked EMG measurement

and identify factors which may predict loosening.

STUDY DESIGN/SETTING: Patients received either pedicle screw in-

sertion or removal at Emory University Hospital, Midtown, Atlanta, GA.

ting and will be included with the post-meeting online content.