130S Proceedings of the NASS 23rd Annual Meeting / The Spine Journal 8 (2008) 1S–191S

significantly impaired physical and mental health as well as disability.

Multivariate regression analyses revealed that persistent ICBG complica-

tions were associated with significantly worse disability 3.5 years post sur-

gery, after adjusting for workers’ compensation status, surgical site pain,

and extremity pain (B5 1.7, p!0.02, R250.59). There was a trend asso-

ciation with physical health, after adjusting for age, workers’ compensa-

tion status, surgical site pain, and extremity pain (B5 �1.55, p!0.08,

R250.39). There was no association with mental health in the multivariate

model (B5 �1.61, p50.15, R250.30).

CONCLUSIONS: Chronic ICBG harvest site pain is reported by a signif-

icant percentage of patients undergoing this procedure over three years fol-

lowing surgery. In affected patients, persistent pain is associated with

a wide range of functional limitation and negatively impacts patient-

reported disability.

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

any applicable devices or drugs.

doi:10.1016/j.spinee.2008.06.302

P59. Spinal Hemiepiphysiodesis Affects Compressive Stresses in

Intervertebral Joint

Eric Wall, MD, David Glos, BA, Donita Bylski-Austrow, PhD,

Frank Sauser, Vivek Sharma, MD, Alvin Crawford, MD; Cincinnati

Children’s Hospital Medical Center, Cincinnati, OH, USA

BACKGROUND CONTEXT: Staple hemiepiphysiodesis methods have

been shown to alter spine growth in animal models by inducing physeal

structural gradients. The presumed mechanism is asymmetrically increased

compression of the joint, including interposed disc, bone, and physes.

PURPOSE: To determine normal physiologic dynamic disc compressive

stresses and whether staple implantation increased the mean baseline com-

pressive stress.

STUDY DESIGN/ SETTING: An in vivo animal model (approved by IA-

CUC) combined with in vitro biomechanical tests.

PATIENT SAMPLE: A series of 6 skeletally immature domestic pigs un-

derwent survival surgery and a post-operative evaluation time of 8 weeks.

For in vitro tests, 5 porcine motion segments were harvested from normal,

unoperated spines.

OUTCOME MEASURES: Compressive stresses measured bilaterally in

the disc annulus.

METHODS: For invivo tests, 4 custom sensors and a staple were implanted.

Custom stress sensors were placed in pairs at T7-8 and T5-6, and the staple

was placed across the right side of one disc space. Stresses were measured

during staple implantation, and then postoperatively under defined activities

and loading conditions. For in vitro tests, bilateral stresses were measured

during cyclic compression tests, before and after staple implantation, as well

as during staple insertion. Load-displacement curves were used to define

baseline and peak compressive stresses and tangent modulus.

RESULTS: Staple insertion resulted in an increase in baseline stress of 0.1

to 0.2 MPa in vivo (Fig. 1) and 0.10 MPa (6 0.03) in vitro. During normal

activities of the animal, compressive stresses were dynamic at both stapled

and control levels, including on the stapled side. After stapling, mean peak

dynamic compressive stresses were 77% of control values, while mean tan-

gent modulus was 81% of control.

CONCLUSIONS: Spinal hemiepiphysiodesis using a staple-like implant im-

mediately increased baseline static compressive stresses in the intervertebral

disc in a porcine model, both invivo and invitro. Treatment also decreased peak

dynamic compressive stresses, but not dramatically. Therefore, staples that

have been shown to slow growth and decrease physeal zone and cell size asym-

metrically increased the static baseline stress at time of implantation. The disc,

however, was not isolated from physiological compressive loads.

FDA DEVICE/DRUG STATUS: HemiBridge Staple, Vertebral Locking

Screw: Investigational/Not approved.

doi:10.1016/j.spinee.2008.06.303

P60. Response Shift in Patients Following Lumbar Spine Surgery

Impact on Intepretation of Oswestry Disability Index

Carolyn Schwartz, ScD1, Rita Bode, PhD2, Helen Razmjou, MScPT,

PhD(C), MDT3, David Kim, MD1, Joel Finkelstein, MD3; 1New England

Baptist Hospital, Boston, MA, USA; 2Northwestern University, Chicago,

IL, USA; 3Sunnybrook Health Science Centre, Toronto, Ontario, Canada

BACKGROUND CONTEXT: Traditional outcome measures utilize pre-

and post-treatment self administered questionnaires measuring change

from baseline to follow-up. Response shift is a psychological phenomenon

that occurs with a change in health status and leads to (1) a change in in-

ternal standards of measurement (i.e. scale recalibration) (2) a change in

values (i.e. reprioritization) or (3) reconceptualization of QOL. In a pro-

spective longitudinal study, this may be a source contributing to occasional

discordance between patient and surgeon perceptions of outcome.

PURPOSE: To investigate whether there is evidence of response shift in

patients treated surgically for degenerative spinal conditions.

STUDY DESIGN/ SETTING: Prospective outcome study.

PATIENT SAMPLE: 215 patients with spinal conditions requiring sur-

gery (134 stenosis; 83 herniated disc; 24 degenerative disk disease; 10

spondylosis; 28 other); 43% female; mean age5 52.51 (15.30).

OUTCOME MEASURES: Oswestry Disability Index (ODI) scores.

METHODS: Differential Item Function (DIF) analysis is a statistical method

which controls for the overall disability score and analyzes how the same item

in a questionnaire is used or may have a different meaning to subgroups of peo-

ple over the trajectory of their recovery. DIF can be used to identify response

shift if an individual changes how s/he measures an item in a questionnaire.

DIF was examined by gender. Reconceptualization DIF examined whether

the groups conceptualize these items the same at each time point. Recalibration

DIF examined differences over time. Follow-up period for this study was 6

weeks which is an early point in the trajectory of recovery.

RESULTS: The pre-op ODI score was 33.04 (SD57.74;range 10–53) and

the post-op was 22.67(10.38, range 7–53). The DIF analysis suggested, how-

ever, that two of the ODI items demonstrated response shift. Both males and

females reported similar and significant improvement in their overall ODI

scores after surgery (p!.001). However, perceived improvement on individ-

ual items varied, with the greatest improvement in pain intensity and the least

improvement in pain when lifting. The lifting pain reported by females post-

surgery was similar to that reported by males pre-surgery, and pre-to-post

surgery improvement in lifting pain was non-significant for females. Relative

to their overall ODI scores, both males and females perceived their pain in-

tensity to be worse pre-surgery and their lifting pain to be worse post-surgery.

These findings reflect recalibration DIF such that surgical patients change

their internal standards on these items after experiencing surgery. Reconcep-

tualization DIF by gender revealed no differences pre-surgery but lifting pain

post-surgery was perceived to be worse by females compared to males at

a similar level of disability.

CONCLUSIONS: Recalibration response shifts were found for two ODI

items: pain intensity and lifting for all surgical patients. Reconceptualization

response shifts were found for female patients regarding the lifting item. Re-

sponse shift has the potential to obfuscate true impact of change in specific out-

comes. Our results indicate that response shift does occur in patients following

spine surgery. Future research is indicated to determine if our traditional out-

come measures as a whole are underestimating the true improvement in pa-

tients following surgery. Adjustment for response shift may give a more

accurate assessment of outcomes in patients undergoing spinal surgery.

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

any applicable devices or drugs.

doi:10.1016/j.spinee.2008.06.304

P61. Two and Five Year Lumbar Index Level Motion Following

Arthroplasty Impact of Heterotopic Ossification

Mohammad Majd, MD1, Richard Holt, MD1, Richard D. Guyer, MD2,

Robert Banco, MD3, Fred Geisler, MD, PhD4, Louis Jenis, MD5,