A subject-specific method to measure dynamic spinal alignment in adult spinal deformity

BACKGROUND CONTEXTTwo-dimensional static radiography currently forms the golden standard in spinal alignment measurement in adult spinal deformity (ASD). However, these static measurements offer no information on dynamic spinal behavior. To fully understand the functionality and compensation strategies of ASD patients, tools to assess dynamic spinal alignment are needed.PURPOSETherefore, the aim of this study was to introduce, validate and assess the reliability of a new kinematic model to measure dynamic spinal parameters in ASD based on a polynomial function, taking into account the subject-specific anatomy.STUDY DESIGNValidation and reliability studyOUTCOME MEASURESRadiographic parameters, spinal kinematics and range of motion (ROM), Scoliosis Research Society Outcome Questionnaire (SRS-22), Core Outcome Measures Index (COMI).METHODSSpinal alignment of 23 ASD patients and 18 controls was measured using both x-rays and motion capture. Marker positions were corrected to the underlying anatomy and a polynomial function was fitted through these corrected marker positions. By comparing the polynomial method to x-ray measurements concurrent validity was assessed. Test-retest, inter- and intrarater reliability during standing and sit-to-stand (STS) were assessed on a subsample of eight ASD patients and eight controls.RESULTSThe results showed good to excellent correlations (r>0.75) between almost all x-ray and anatomy-corrected polynomial parameters. Anatomy correction consistently led to better correlations than no correction. Intraclass correlation coefficients for the polynomial method were good to excellent (>0.75) between sessions and between and within raters and comparable or even better than radiographic measurements. Also, during STS reliability was excellent. Fair to moderate correlations were found between spinal ROM during STS and quality of life, measured with SRS-22 and COMI.CONCLUSIONSThe results of this study indicate the polynomial method, with subject-specific anatomy correction, can measure spinal alignment in a valid and reliable way using motion capture in both healthy and deformed spines. This method makes it possible to extend evaluation in ASD from mainly static, by means of x-ray measurements, to dynamic and functional assessments.CLINICAL SIGNIFICANCEEventually, this newly obtained dynamic spinal alignment information might lead to new insights in clinical decision-making and new treatment strategies, based and oriented on dynamic parameters and functionality.