RELIABILITY OF CT BASED CLASSIFICATION OF INCORPORATION OF FEMORAL ALLOGRAFT IN ANTERIOR COLUMN RECONSTRUCTION OF THE THORACOLUMBAR SPINE

Abstract

Introduction: Diaphyseal femoral allograft is well suited to anterior column reconstruction of the thoracolumbar spine due to its inherent structural properties and bio-compatibility. The Bridwell system of interbody fusion assessment¹ is based on plain x-rays and therefore lacks sensitivity. A new classification system of bony union is proposed using high-speed spiral CT imaging.

Methods: Twenty-six patients who underwent anterior thoracolumbar reconstruction for burst fracture using femoral allograft were followed for a minimum of 2 years. Each subject underwent high speed spiral CT scanning through the reconstructed region of the thoracolumbar spine and a classification system of graft to endplate union and central cancellous autograft incorporation was established.

The classification system reflects gradually increasing biological stability of the construct. Grade I (complete fusion) implies cortical union of the allograft and central trabecular continuity. Grade II (partial fusion) implies cortical union of the structural allograft with partial trabecular incorporation. Grade III (unipolar pseudarthrosis) denotes superior or inferior cortical non-union of the central allograft with partial trabecular discontinuity centrally and Grade IV (bipolar pseudarthrosis) suggests both superior and inferior cortical non-union with a complete lack of central trabecular continuity. Intra- and inter-observer error studies were carried out involving spinal surgeons, radiologists and trainees to examine reliability of the classification

Results: In this series 84% of cases demonstrated Grade I or Grade II characteristics. 1 case (4%) was identified as Grade IV. The classification showed good reliability with a kappa score of over 0.7

Discussion: Plain radiographs have always proved unsatisfactory for the accurate assessment of incorporation of grafts in the thoracolumbar spine. The use of CT imaging in the assessment of graft union has allowed a more accurate assessment of union. The classifi cation system presented allows a reproducible and relevant categorisation of allograft incorporation.