93SProceedings of the NASS 25th Annual Meeting / The Spine Journal 10 (2010) 1S–149S

2University of California, San Francisco, Neurosurgery, San Francisco,

CA, USA

BACKGROUND CONTEXT: Both expandable and fixed interbody cage

designs are available for reconstruction following single level lumbar

corpectomy. Anecdotal evidence from previous clinical and biomechan-

ical studies suggests that the expandable cage provides for a more consis-

tent and stable interface between the cage and the endplate; however,

there are several case reports of catastrophic adjacent-level vertebral

body fractures, presumably due to aggressive expansion of these cages

in patients with low bone density or stress concentration from cage end

cap to endplate contour mismatch. Previous studies have also noted poor

tactile feedback for some expandable cage designs in sensing the amount

of compressive load applied to the endplates. Little biomechanical evi-

dence exists to support these clinical assertions.

PURPOSE: To fully characterize the deployment mechanics for a typical

expandable interbody cage design. Variables of interest in this biomechan-

ical study are: imposed sagittal balance, expandable vs. fixed cage designs;

and intraoperative force-feedback in controlling the amount of cage

expansion.

STUDY DESIGN/SETTING: Cadaver-based biomechanical study.

PATIENT SAMPLE: Human thoracolumbar spines (N56; 81.5610.7 y.o.

5 Male; T12-L2 & L3-L5).

OUTCOME MEASURES: Endplate contact force and center of pressure

(COP). Manual torque needed for cage deployment (expandable only).

METHODS: Corpectomies were performed at L1 or L4, depending on

specimen, and real-time tactile pressure measurement film was posi-

tioned along the inferior vertebral endplate. Either an expandable or fixed

cage (VLIFT or VBOSS, respectively; Stryker Spine) was inserted into

the interbody space. Supplemental anterior hardware was added to fur-

ther secure the cage. For the fixed cages a best-fit endcap angle was used

in each instance. For the expandable cage, hypo-lordotic, hyper-lordotic,

and best-fit endcaps were applied sequentially, and pressure data was col-

lected for each deployment, along with real-time monitoring of the ap-

plied torque necessary to achieve full cage expansion.

RESULTS: For normal lordotic angles, the expandable cage had a signif-

icantly greater contact area than the fixed cage (p50.03). Contact area did

not differ with lordotic angle for the expandable cage design (pO0.05);

however, the COP location was significantly more anterior for the hyper-

lordotic endcap than other cases. There were no differences in total end-

plate contact force between expandable vs. fixed cage designs, nor

between different imposed lordotic angles for the expandable cage design.

However, total endplate compressive force trended higher with both hyper

and hypolordotic alignment versus normal. Cage expansion torque was

2.460.9 Nm, and there was no change in torque with lordotic angle

(pO0.05) despite mismatch with endplate.

CONCLUSIONS: This study’s results suggest that expandable cage de-

signs create greater contact area between the cage endcaps and the verte-

bral endplate, potentially providing a more stable base for bony union at

the endplate. However, the distribution of load along the endplate is sensi-

tive to endcap/endplate contour mismatch, which may be an explanation

for the clinically observed fractures in low bone density patients. Our

results also suggest that torque feedback on the ratcheting mechanism

for this particular expandable cage design may not be a reliable indicator

of cage fit. All of these findings warrant further investigation in a clinical

setting.

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

any applicable devices or drugs.

doi: 10.1016/j.spinee.2010.07.247

201. Complications in Cervical Arthroplasty Surgery - Single

Institution Experience in 100 Patients

Christianto B. Lumenta, MD, PhD, Julia F. Landscheidt, MD,

Andre Tomasino, MD, Matthias Krammer, MD, Werner Gerstner, MD;

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

Department of Neurosurgery, Bogenhausen Academic Teaching Hospital,

Technical University of Munich, Munich, Germany

BACKGROUND CONTEXT: Spinal arthroplasty is a result of modern

biomechanics, material sciences, and surgical technique. Although this

new technology may offer benefits over fusion, it also requires that the sur-

geons acquire new operative techniques, thus new complications may

occur.

PURPOSE: To determine the incidence of the surgical complications, we

analyzed our data obtained in a series of patients treated with the PCM�

(1) Disc prosthesis.

STUDY DESIGN/SETTING: Our prospectively recorded operative data,

complications as well as clinical and radiographic outcome data in 100 pa-

tients treated with PCM� Disc prosthesis in one to three levels were

analyzed.

PATIENT SAMPLE: One hundred sixteen discs were implanted in 100

patients.

OUTCOME MEASURES: Clinical, radiological findings including dy-

namic X-rays, NDI, VAS, and SF-36.

METHODS: Prospective analysis of 100 patients.

RESULTS: The perioperative complication rate was 3% (3/100). In one

patient with known status post radiotherapy due to neck lymphoma a hypo-

pharyngeal injury developed, requiring changing the site of the ventral ap-

proach. Permanent recurrent paresis occurred in 2 patients. Delayed

migration of the prosthesis was observed in six patients with two-level sur-

gery (A), and in four single- level cases (B) thus a rate of 8.6% (10/116).

Seven patients (A:3, B:4) needed revision surgery with changing the

PCM� to cage fusion in 4 patients and with using the flange in 3 patients.

An insufficient preparation of the intervertebral space with poor press fit

was found in 9 of these 10 patients. The intraoperative imaging has dem-

onstrated unsatisfied implant position, which was not corrected at that

time. In another patient after fusion surgery, a flange was not used in the

old PCM standard prosthesis without teeth. Two patients with known disc

herniation and stenosis at the adjacent level needed second surgery 6 and

12 months after the first surgery for this level using PCM�. Heterotopic

ossification and spontaneous fusion occurred in seven prostheses (7/

11656%). The clinical outcome of these patients, however, was good. Mo-

tion was preserved in the remaining 99 prostheses. The segmental motion

after surgery shows an angle between 1.4� in flexion and 11.1’’in exten-

sion. The mean motion of the operated segments was 7.2�. We do not es-

tablish any neurological deterioration nor postoperative infection or

hemorrhage. The survey up to 24 months after surgery shows a significant

improvement of NDI-Score and of the VAS. The analysis of the SF - 36

demonstrates a significant improvement of all points postoperatively.

CONCLUSIONS: The PCM� prosthesis is effective in maintaining spinal

motion, Major perioperative complications were infrequent. Strong pa-

tient’s selection and meticulous surgical technique, however, are required

in order to obtain good results.

FDA DEVICE/DRUG STATUS: PCM: Investigational/Not approved.

doi: 10.1016/j.spinee.2010.07.248

202. An Analysis of Agreement between MMG vs. EMG Systems for

Identification of Nerve Location During Spinal Procedures

Edward Anderson, MD1, Christopher Wybo, MS2, Stephen Bartol, MD3;1Wilford Hall Medical Center, Lackland AFB, TX, USA; 2Innovative

Surgical Solutions, LLC, Royal Oak, MI, USA; 3Henry Ford Hospital,

Detroit, MI, USA

BACKGROUND CONTEXT: Neural injury is a major risk of surgical

procedures. Traditional surgical exposures emphasize landmark recogni-

tion in order to avoid neural injury. As newer MIS techniques develop,

neural protection has become an ever-increasing challenge. The utilization

of intraoperative neurophysiologic monitoring is becoming a common tool

to aid surgeons with neural identification and avoidance. Standard

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

94S Proceedings of the NASS 25th Annual Meeting / The Spine Journal 10 (2010) 1S–149S

techniques for nerve localization and avoidance employ electromyography

(EMG). EMG functions by measuring the electrical activity generated by

muscle cells during contraction. This electrical activity is measured and re-

corded by either surface electrodes or percutaneous needle electrodes.

EMG signals are prone to interference, and the analysis of EMG signals

is a complex process that is poorly understood by most surgeons, usually

requiring specialty training for interpretation. An alternative to EMG, Me-

chanomyography (MMG), functions by detecting and measuring the mo-

tion of contracting muscle using accelerometry. Interpretation of an

MMG signal does not require a specialty trained technician or neurologist,

it is simply ‘‘STOP - the muscle is contracting. a nerve must be nearby’’

or ‘‘GO - no muscle activity detected’’.

PURPOSE: To compare the effectiveness of a novel MMG based nerve

avoidance system to that of a commercially available EMG system during

minimally invasive spine surgery.

STUDY DESIGN/SETTING: A comparative animal study to assess

statistical agreement.

METHODS: Eight adult male castrated sheep weighing 60–80 kg were

obtained. Following general anesthesia induction, the sheep were sheered

devoid of wool and EMG needle electrodes were placed into the biceps

femoris, vastus lateralis, semitendinosus, and gastrocnemius muscles.

MMG sensors were placed adjacent the EMG electrodes onto the surface

of the skin. A direct lateral dissection was used to enter the retroperitoneal

space, exposing the psoas muscle. A customized nerve stimulation grid

was positioned about the psoas muscle and fixed into place. The grid con-

tained an 8x8 matrix of holes for a stimulator probe to be inserted. An elec-

trical current stimulating probe was then inserted through the first hole on

the grid and advanced through the psoas down to the intervertebral disc.

An electrical current ramp was then applied starting at 1 mA, and ending

at 15 mAs of intensity with a pulse duration of 100us and a frequency of

5 Hz. Binary outputs from the EMG and MMG sensor systems were re-

corded. This process was repeated for all 64 hole locations on each of

the 8 lumbar nerve roots per animal, resulting in a total of 4,096 trials.

An analysis was performed to assess the agreement between the two

systems.

RESULTS: The MMG system had an overall agreement of 91% relative to

the EMG system, with a 98% positive agreement for confirming the pres-

ence of a nerve. The MMG system exhibited greater sensitivity and de-

tected the presence of a nerve up to 4 seconds earlier than the EMG

system.

CONCLUSIONS:MMG is an effective method for locating nerves during

procedures that do not allow direct visualization on nerves. MMG exhibits

greater sensitivity over EMG and overall demonstrated a faster response to

electrical stimulus compared to EMG. MMG represents an easy to use al-

ternative means of nerve monitoring that should be considered when motor

nerve roots are at risk.

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

any applicable devices or drugs.

doi: 10.1016/j.spinee.2010.07.249

203. Does the NHSN Adequately Stratify Infection Rates for Spine

Surgery?

Steven Takemoto, PhD, A. Kakis, DPM, Lawrence Block, MPH, MPA,

Amy Nichols, RN, Sigurd Berven, MD; UCSF, San Francisco, CA, USA

BACKGROUND CONTEXT: The CDC-sponsored National Health Sur-

veillance Network (NHSN) stratifies surgical spine procedures into three

categories: laminectomy, primary and re-fusion; when reporting rates of

surgical site infection (SSI).

PURPOSE: Hypothesis: the number of segments fused has a significant

impact on infection rates and should be incorporated into risk adjustment

models.

STUDY DESIGN/SETTING: A retrospective review of administrative

claims at a single hospital were linked to hospital epidemiology and

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

infection control (HEIC) data for nine attending surgeons from the Ortho

and Neurospine services.

PATIENT SAMPLE: The study included 2,478 cases over a two-year pe-

riod (FY08–09).

OUTCOME MEASURES: HEIC validated surgical site infection.

METHODS: Procedures, stratified using diagnosis related group (DRG)

codes included: laminectomy/discectomy (748 cases), cervical fusion

(489), spine fusion (578), fusion with curvature, malignancy or 9+ seg-

ments (CMS, 185), combined anterior/posterior fusion (407).

RESULTS: The overall SSI rate for all procedures was 3%. Rates were

similar when distributed by patient ethnicity (White 72% of cases,

Hispanic 8%, Asian 6%, Black 4%), payor (commercial 52%, Medicare

35%, Medicaid 10%) and age. Patient factors associated with increased

SSI rates were risk index category 2 (RIC 2, 7% SSI rate) and DRG-

associated MCC (major comorbidity or complication, 7%). Surgical

factors associated with increased rates included spine CMS (8%), O8

levels (8%),O5 hour operation time (6%), more than two previous pro-

cedures (5%). Multivariate logistic regression indicates O8 segments

(Odds ratio 2.5, 95% confidence interval 1.1–5.4), 4–8 segments (1.7,

1.0–2.9), op time O5 hours (1.6, 0.9–3.0), and RIC 2 (2.1, 1.2–3.7)

independently increased the risk of SSI. Interestingly, cases with 1 pre-

vious operation had a similar SSI rate (2.5%) to the index operation,

indicating staging did not increase SSI rates. The burden of SSI is illus-

trated by the finding that the SSI rate was 9% for patients hospitalized

longer than 2 weeks.

CONCLUSIONS: These results indicate fusions with more than 8 seg-

ments independently increased the risk of surgical site infection. Models

that stratify the risk of infection after spine surgery should consider these

complex cases separately from shorter fusions.

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

any applicable devices or drugs.

doi: 10.1016/j.spinee.2010.07.250

204. Increased Incidence of Cage Migration and Non-union in

Instrumented Transforaminal Lumbar Interbody Fusion with

Bioabsorbable Cages

Arien Smith, MD1, Marc Arginteanu, MD2, Alfred Steinberger, MD2,

Martin Camins, MD2; 1Spine Institute of New York, New York, NY, USA;2Mount Sinai Medical Center, New York, NY, USA

BACKGROUND CONTEXT: Recent advances in the field of spinal im-

plants have led to the development of the bioabsorbable interbody cage.

Although much has been written about the advantageous characteristics,

little has been reported regarding their complications.

PURPOSE: To compare the fusion rates and complications in patients un-

dergoing TLIF using carbon fiber and bioabsorbable PLDLA implants.

STUDY DESIGN/SETTING: A prospective cohort study was conducted

to compare fusion rates and complications in patients undergoing transfor-

aminal lumbar interbody fusion (TLIF) with carbon fiber cages versus bio-

absorbable cages made from 70/30 poly(L-lactide-co-D,L-lactide)

(PLDLA).

PATIENT SAMPLE: 81 patients undergoing single- or multi-level TLIF

for degenerative disc disease, scoliosis, or spondylolisthesis.

OUTCOME MEASURES: Fusion and complication rates.

METHODS: Between January 2005 and May 2006 eighty-one patients

with various degenerative and/or structural pathologies affecting the lum-

bar spine underwent single- to multi-level TLIF with posterior segmental

pedicle screw fixation using implants made of carbon fiber (37 patients)

or 70/30 PLDLA (44 patients). Clinical and radiologic follow-up was per-

formed at 6 weeks, 3 months, 6 months, and one year, and is ongoing. The

incidence of nonunion, screw breakage, and cage migration were com-

pared between the two groups.

RESULTS: There was no significant difference in demographic data be-

tween the two groups, average number of lumbar levels operated, or

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