Post-operative Infection Treatment Score for the Spine (PITSS):

Validation of a Predictive Model to Define Need for Single versus Multiple Irrigation and Debridement

for Spinal Surgical Site Infection

Christian P. DiPaola, MD 1; Davor D. Saravanja 2, MD; Luca Boriani,MD 3; Michael Boyd, MD 4; Brian K. Kwon, MD, PhD 4; Scott Paquette 4, MD;

Marcel F. S. Dvorak, MD 4; Charles G. Fisher, MD, MHSc 4, John Street, MD, PhD 4

1University of Massachusetts Medical Center, Worcester, MA2 Sydney Children’s Hospital, Sydney, Australia

3Ospedale Maggiore , Bologna Italy4University of British Columbia, Vancouver, BC

Background

Rx=I&D

Closed Suction Irrigation for the Treatment ofPostoperative Wound Infections Following PosteriorSpinal Fusion and InstrumentationMichael T. Rohmiller, MD,* Behrooz A. Akbarnia, MD,†‡ Kian Raiszadeh, MD,§Kamshad Raiszadeh, MD,¶ and Sarah Canale, BS‡

SSI: Complex SSI Management V.A.C.SSI: Complex SSI Management V.A.C.

SSI: Complex ReconstructionSSI: Complex Reconstruction

Masri, B. A.; Duncan, C. P.; and Beauchamp, C. P.: Long-term elution of antibiotics from bone-cement: an in vivo study using the prosthesis of antibiotic-loaded acrylic

cement (PROSTALAC) system. J Arthroplasty, 13(3): 331-8, 1998

J Neurosurg Spine. 2006 Oct;5(4):287-93. Hollow cylindrical polymethylmethacrylate strut for spinal reconstruction after single-

level cervical corpectomy.Chen JF, Wu CTLee SC, Lee ST

PMMA struts have been used successfully for

reconstructing single level cervical vertebrectomy for

spondylodiscitis

SSI: Complex SSI ManagementSSI: Complex SSI Management

Purpose

• Build Predictive Model

• Single vs. Multiple I&D

Methods

• Select appropriate predictor candidates• Prediction model (internal validation)• External validation of the 1999-2005 model

was performed by applying it to the 2006-2008 population to produce predicted probabilities.

• Receiver Operating Characteristic (ROC) Curves were constructed and the Area Under the Curve (AUC) was calculated.

Hospital Factors• ICU stay Y/N• Duration of ICU stay• Duration in hospital prior to

operation

Spine Factors• Dx: oncology, deformity,

degenerative, trauma• Diagnosis Location• Neurology (ASIA score)

OR/Surgical Factors • # of levels• EBL• Approach • Presence of Instrumentation• Operative Duration• CSF leak/dural tear

Patient Factors• Medical co-morbidities • Age/Gender

Infection Factors• 1st Operative Deep Wound

Culture at I&D

• Distant site infxn: UTI, bacteremia, pneumonia

Hospital Factors

• ICU stay- Odds Ratio (OR)=1.75

• Duration of ICU stay- OR=1.33

• Days in hospital prior to operation- OR=1.15

– Odds Ratio >1 higher risk of multiple I&D

Spine Factors

• Spine Diagnosis: Oncology>trauma>deformity>degenerative

• Diagnosis Location- T/L >Cervical OR=2.0

• Neuro Deficit>Intact- OR=1.75

Odds Ratio >1 higher risk of multiple I&D

Surgical Factors

• CSF leak/dural tear-OR=2.63

• EBL & # of levels-OR=1.25-1.29

• Instrumentation-OR=1.23

• Revision surgery-OR=.66

• Operative Duration, Bone graft types, Sacrum/Ileum=no difference

•Approach-all 63 were posterior midlineOdds Ratio >1 higher risk of multiple I&D

Patient Factors

• Diabetes/elevated blood glucose- OR=1.39

• Age/Gender-No Difference

Odds Ratio >1 higher risk of multiple I&D

Infection Factors 1st Deep Wound Culture at I&D

• Polymicrobial w/ MRSA or MRSA alone > Gram+ OR=7.58

Odds Ratio >1 higher risk of multiple I&D

Infection Factors

Distant site infection

• Bacteremia & UTI > none- OR=50.74

• Bacteremia > none- OR=15.95

• UTI > none- OR=1.97

Odds Ratio >1 higher risk of multiple I&D

Purpose

137.408IPSIX17

135.85140CSF X16

134.45387GenderX15

132.45443Neurologic DeficitX14

130.55445LevelsX13

128.72450Previous SurgeryX12

126.79455Duration of ICU stayX11

124.89463Primary Spine DiagnosisX10

120.28464Index procedure durationX9

118.92486Hospital Duration PreopX8

117.77561Patient AgeX7

117.30M5+X6M6710Bone graftX6

119.32M4+X5M5721InstrumentationX5

124.38M3+X4M4725Distant site InfectionX4

125.97M2+X3M3739Medical ComorbiditiesX3

125.72M1+X2M2768

Spine Location

X2

122.81X1M1853

Wound CultureMicrobiology

X1

AICPredictors in the model

ModelFrequencyDescriptionRanked Predictor

Internal Validation of Predictor Selection using 1000 re-samples (predictability measure AUC=0.84 for model M6)

Study #1

Study #2

130.55

445LevelsX13

128.72

450Previous SurgeryX12

126.79

455Duration of ICU stayX11

124.89

463Primary Spine

DiagnosisX10

120.28

464Index procedure

durationX9

118.92

486Hospital Duration

PreopX8

117.77

561Patient AgeX7

117.30M5+X6M6710Bone graftX6

119.32M4+X5M5721InstrumentationX5

124.38M3+X4M4725Distant site InfectionX4

125.97M2+X3M3739Medical ComorbiditiesX3

125.72M1+X2M2768

Spine Location

X2

122.81X1M1853

Wound CultureMicrobiology

X1

AICPredictors in the model

ModelFrequencyDescriptionRanked Predictor

Internal Validation of Predictor Selection using 1000 re-samples (predictability measure AUC=0.84 for model M6)

Internal Validation of the Predictive Model

• AUC= .84

• 6 Key predictive factors

Predictors

• Microbiology• Spine Location• Medical Comorbidity• Distant Site Infection• Instrumentation• Bone graft

External validation

The predictability (measured by AUC – area under curve) of the final prediction model was checked with the validation dataset (2006-2008) and its re-samples. The curve referred to here is the ROC (Receiver Operating Characteristic). The closer AUC is to 1.0 the better the prediction power.

AUC=0.7095% CI = (0.51, 0.89)

Applying the Data

• By setting a probability cutoff of 0.24 the negative predictive value (NPV) for multiple I&D was 0.77

• A cutoff of 0.53 probability yielded a PPV for multiple I&D of 0.80.

MultipleI&D

Single I&D

.24 .53

NPV=.77 PPV=.85

0other graft

-0.4647autograft 0.0630

-1.7848none

Bone graft

0yes0.0450

-1.9550noInstrumentation

0Bacteremia+dist

0.0850Bacteremia+alone

-0.3082UTI/pneumonia0.0876

-1.7743none

Distant SiteInfection

0Others (ie. GI, renal etc.)

0.4218cardiovascular/pulmonary

2.0241diabetes0.1736

0.2039none

Comorbidities

0Lumbar/sacral (L1-S2)

-1.0658Thoracic or thoraco-lumbar (T1-L1) 0.1613

-1.5069Cervical (C0-7)

IP Location

0Polymicrobial with MRSA or MRSA alone

-1.9639Gram negative or polymicrobial wo MRSA 0.0389

-1.4416Gram positive

Microbiology

PvalueCoeffecientPredictor subgroupsPredictors

6Other (allograft, BMP, synthetic)

3Autograft

1None

Bone Graft

6Yes

2NoInstrumentation

Surgical Factors

6Bacteremia + PNA/UTI

5Bacteremia alone

3UTI/pneumonia

1none

Distant site infection

6Polymicrobial with MRSA or MRSA alone

4Gram negative or polymicrobial wo MRSA

2Gram positive

Microbiology

Infection Factors

4diabetes

1cardiovascular/pulmonary

0None/other

ComorbiditiesPatient Factors

4Lumbar/sacral

2Thoracolumbar

1Cervical

Spine LocationSpine Factors

Post-operative Infection Treatment Score for the Spine

Low risk: 7-14Indeterminate: 15-20

High risk 21-33

Applying the Data

MultipleI&D

Single I&D

14 20

NPV=.71 PPV=.98

Low risk: 7-14Indeterminate: 15-20

High risk 21-33

LowRisk

HighRisk

Unsure

6Other (allograft, BMP, synthetic)

3Autograft

1None

Bone Graft

6Yes

2NoInstrumentation

SurgicalFactors

6Bacteremia + PNA/UTI

5Bacteremia alone

3UTI/pneumonia

1None

Distant site infection

6Polymicrobial with MRSA or MRSA alone

4Gram negative or polymicrobial w/o MRSA

2Gram positive

Microbiology

Infection Factors

4Diabetes

1Cardiovascular/pulmonary

0None/other

ComorbiditiesPatientFactors

4Lumbar/sacral

2Thoracolumbar

1Cervical

Spine LocationSpineFactors PITSS Example Patient

28

MultipleI&D

Single I&D

14 20Low risk: 7-14

Indeterminate: 15-20High risk 21-33

Conclusions

• Context for each pt.• EBM treatment guidance• Supplemental options

Thank You