How to evaluate bone tunnel widening after ACL reconstruction - a critical review.

Background: Comparing different imaging modalities and methods for assessment tunnel widening after ACL reconstruction and providing a detailed evidence-based literature overview.

Methods: PubMed was searched from 1970 to 2016 using the terms "ACL reconstruction" and "tunnel" and "imaging" or "CT" or "computerized tomography" or "MRI" or "magnetic resonance imaging" or "radiographs". 647 studies were found. 575 articles were excluded due to absence of specific radiological measurement methods of tunnel widening and 40 due to repetition of a previously published radiological measurement method. 32 articles were included reporting interand intraobserver reliabilities of tunnel measurement methods after ACL reconstruction.

Results: A variety of different algorithms and measurement methods using radiographs, magnetic resonance imaging, computed tomography or SPECT/CT evaluating tunnel position and bone tunnel enlargement have been described. Tunnel delination restricts an exact analysis using X-ray. Measurements using CT or MR were mostly obtained perpendicular to the tunnel axis or using specialized software for tunnel volume calculation in 3D.Based on the review the width of the femoral and tibial tunnels should be assessed perpendicular to the tunnel axis at different levels in relation to the joint. At least one measurement should be performed at the tunnel entrance, exit and midpoint of the tunnel.

Conclusion: CT should be considered the gold standard assessing tunnel widening in patients after ACL reconstruction. If specialized software is available calculating the tunnel volume, measurements should be preferably performed in 3D CT.

Level Of Evidence: II.