44S Proceedings of the NASS 24th Annual Meeting / The Spine Journal 9 (2009) 1S-205S

PURPOSE: Hence, we conducted an international, multi-center, prospec-

tive cohort study to assess the outcomes of operative intervention in this

important cause of spinal cord dysfunction.

STUDY DESIGN/SETTING: Prospective international, multi-center

cohort study.

PATIENT SAMPLE: Patients with radiologic and clinical evidence of

cervical spondylotic myelopathy.

OUTCOME MEASURES: Modified Japanese Orthopedic Association

Scale (mJOA), Neck Disability Index (NDI) and SF-36.

METHODS: Patients underwent anterior or posterior decompressive/

reconstructive surgery with the surgical techniques chosen by the treating

team.

RESULTS: There were 40% females with no significant differences in the

proportion of females across the regions. Patients in Asia/Pacific region

were significantly younger than in other three regions (41.5 and 56.5 years

of age, respectively, P ! .01). 32% of subjects were smokers with no

differences noted among the regions. There was a significant difference

among the regions in the number of spinal levels operated. In Europe

and Asia/Pacific patients had fewer number of levels treated operatively

(3.2 levels on average) while North and South American subjects had

a higher number of levels treated surgically (4.2 on average; P ! .01).

The anterior approach was used in 70% of the cases in Europe, 75% in

Asia Pacific, 56% in North America and 18% in South America

(P !.01). The duration of symptoms at the time of surgery was 20 months

in Europe, 18 months Asia Pacific, 32 months North America, and 50

months in South America (P!.01). There were major differences in the

average length of hospital stay: Europe 12.7 days; Asia/Pacific 10.7 days;

North America 3.2 days; South America 6.4 days (P ! .01). The average

Nurick score was highest in Asia/Pacific region (4.7), followed by Europe

and North America (4.1 each) and lowest in South America (3.5) (P! .01).

The average NDI was 42, average mJOA was 13, average SF36 PCS was

34 and average SF36 MCS was 38. There were no differences among the

regions in these parameters.

CONCLUSIONS: This study presents novel data which highlight ma-

jor variations in the clinical presentation and treatment protocols used

to treat cervical spondylotic myelopathy. While some of these varia-

tions reflect differences in health care systems, other variations reflect

an opportunity to develop more uniform evidence-based treatment

protocols.

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

any applicable devices or drugs.

doi: 10.1016/j.spinee.2009.08.104

Thursday, November 12, 200911:00 AM–12:00 PM

Concurrent Session 2: Biomechanics

84. Cobalt Chromium Rods: How Do They Stack Up?

Thomas Nunn1, Eric Varley, DO2, Rattalerk Arunakul, MD3,

Arnel Aguinaldo, MA1, Peter Newton, MD1; 1Rady Children’s Hospital

and Health Center - San Diego, San Diego, CA, USA; 2University of

California, San Diego, San Diego, CA, USA; 3University of California,

Irvine, Orange, CA, USA

BACKGROUND CONTEXT: Spinal rods have traditionally been avail-

able in stainless steel and titanium. While stainless steel offers the advan-

tages of a high stiffness and a high yield point it comes at the cost of

producing a large amount of MRI artifact. Titanium on the other hand min-

imizes MRI artifact but comes at the expense of decreased rod stiffness.

Several implant manufacturers have developed cobalt chromium (CoCr)

in an effort to reduce MRI artifact while maintaining material properties

similar to steel. An understanding of the biomechanical properties of CoCr

rods in context of more commonly used rod materials is essential for sur-

geons to select the optimum rod type.

PURPOSE: To compare CoCr rods to steel and titanium rods in 4 point

bending mechanical testing with the goal of understanding how these rods

differ with regards to bending stiffness, strength (as expressed by yield

point), and memory (deformation at yield point).

STUDY DESIGN/SETTING: Prospective/Biomechanical Research

Laboratory.

PATIENT SAMPLE: N/A.

OUTCOME MEASURES: Young’s Modulus, Stiffness, Yield Point, Ul-

timate Load, Memory.

METHODS: Five spine rod materials: Standard Strength Steel (SS), High

Strength Steel (HSS), Ultra Strength Steel (USS), Titanium Alloy (Ti), and

Cobalt Chromium (CoCr) were tested in a four-point rod bending system

fixed to an MTS test frame (MTS, Eden Prairie, MN). For each material,

nine samples measuring 200 mm in length and 5.5 mm in diameter were

bent to varying angles of deformation (3 to 60 degrees). Load (N) and dis-

placement (mm) of the sample’s midpoint were measured directly at

100 Hz utilizing the MTS machine and a three camera motion capture sys-

tem. Load-deformation curves were extracted and the Young’s modulus,

stiffness, yield load, ultimate load and memory values were calculated.

The yield point was defined as the load resulting in permanent (plastic)

rod deformation while memory referred to the angle of deformation at

the yield point.

RESULTS: The mechanical properties for each rod are listed in Table 1.

The bending stiffness was highest for CoCr (24% greater than SS, 47%

greater than Ti) and the Young’s Modulus was greatest for CoCr (20%

greater than SS, 55% greater than Ti). The HSS and CoCr rods had similar

Young’s Moduli (260 GPa and 282 GPa), with the lowest values for the SS

and Ti rods (226 GPa and 155 GPa). CoCr rods displayed a memory lower

than that of Ti and USS rods and similar to SS rods.

CONCLUSIONS: The elastic and plastic properties of rods used to per-

form spinal instrumentation vary substantially. Aggressive correction of

large curves may result in rod deformations beyond the elastic limit.

Choosing the best rod for the individual case and correction strategy re-

quires an appreciation of these differences. CoCr rods offer both a high

stiffness and a low yield point option best suited for secure boney fixation

and in-situ bending.

FDA DEVICE/DRUG STATUS: Cobalt Chromium Spinal Rod: Approved

for this indication; Titanium Alloy, Standard Strength Steel, High Strength

Steel, Ultra High Strength Steel Spinal Rods: Approved for this indication.

doi: 10.1016/j.spinee.2009.08.106

85. Biomechanical Determination of Distal Level for Fusions Across

the Cervicothoracic Junction

Ivan Cheng, MD1, Alexander Iezza, MD1, Eric Sundberg, MD1,

Derek Lindsey, MS2, K. Daniel Riew, MD3; 1Stanford University, Stanford,

CA, USA; 2Bone and Joint Center of Excellence, VA Palo Alto HCS, Palo

Alto, CA, USA; 3Washington University in St. Louis, St. Louis, MO, USA

BACKGROUND CONTEXT: When a cervical fusion needs to be ex-

tended down to the thoracic spine for adjacent level disease, many sur-

geons prefer to fuse down to T2 or lower in order to diminish the

possibility of further adjacent disease. The optimal distal level, however,

remains to be established.

PURPOSE: We undertook this study to determine the effect of ending

long cervicothoracic fusions at different caudal levels on adjacent-level in-

tradiscal pressure. We hypothesized that stopping a fusion at T1 would be

biomechanically less sound than stopping at a more caudal level.

STUDY DESIGN/SETTING: This was a biomechanical study.

PATIENT SAMPLE: Four cadaveric cervical spine specimens with an in-

tact rib cage and sternum were carefully dissected to preserve costoverte-

bral joints, costosternal joints, facet joints, intercostal muscles, and all

ligaments.