Measurement and prediction of outcome. Application in fusion surgery for chronic low backpain. The Swedish lumbar spine study

Abstract

Background: Outcome of surgery for chronic low back pain should be documented ina standardised and reproducible fashion to establish evidence of its benefit. Measurementof outcome requires validated instruments, which have the ability to detect functionalchanges perceived as beneficial by the patient. The outcome instruments shouldbe practical in clinical decision-making and dependable in research conditions. Predictivefactors need to be isolated, and diagnostic instruments should be evaluated, toimprove the selection of surgical candidates.Aims: To validate a new instrument for measurement of physical function. To evaluatea simplification of over all outcome measurement. To establish the clinical importanceof outcome instrument score changes. To search for predictors of outcome. To evaluatethe predictive value of the pain-drawing.Study population: The main study population comprised 289 patients in the SwedishLumbar Spine Study, a multi-centre randomised controlled trial of surgery for chroniclow back pain. An age- and sex- matched control group of 287 randomly allocatedSwedish citizens, and four separate cohorts of patients treated surgically for variousspinal conditions were also employed.Procedures: The new instrument, The General Function Score, was tested for validityin a practical performance test, for reliability in a test-retest setting, and for responsivenessand feasibility, using the four separate cohorts. Simplification of outcome measurementwas tested by comparing retrospective patient global assessment with a set ofprospective multi-item outcome instruments, specifically evaluating responsiveness andbias. The minimal clinically important score changes of the General Function Score,the Oswestry Disability Index of physical disability, the Zung Depression Scale and theVisual Analogue Scale of pain, were estimated with patient global assessment as externalcriterion. The score changes were compared with the measurement errors of eachinstrument. Predictors of clinical outcome were evaluated by comparing base line characteristicsof patients with functional and occupational outcome. The pain-drawing wastested for predictive properties using four different interpretations of its features.Results and conclusions: The General Function Score demonstrated acceptable validity,reliability and feasibility. Patient global assessment appeared to be a valid outcomemeasure, which could replace multi-item outcome instruments in randomised controlledtrials of smaller sample size. The minimal clinically important score changes ofthe General Function Score, the Oswestry Disability Index and the Zung DepressionScale were less than the limits of the measurement errors and may, thus, not be detectedby the outcome instruments. The Visual Analogue Scale of pain was responsiveenough to confidently detect such a score change. Improved clinical outcome aftersurgical treatment was associated with non-neurotic personality traits and radiographicsigns of severe disc degeneration. Work resumption was associated with low age and ashort period of sick leave. No interpretation of the pain-drawing could predict theclinical outcome. A widespread pain-drawing was associated with a depressed state ofmood and high pain intensity, but not with personality traits.