Simulation and Visualization Research Group
Computer-assisted post-operative audit

Introduction

The occurrence of clinical trials involving computer assisted surgery (CAS) systems is increasing.  A direct consequence is a requirement for an objective method to quantitatively assess CAS performance in implant placement when compared to conventional surgery. The conventional approach to performance assessment involves an element of “eyeballing” with the generation of very little quantitative data.

The objective of this new approach is to provide a computer-assisted objective approach to post-operative evaluation of implant position for both compression / dynamic (DHS) and cannulated hip screw fixation procedures. The new approach is:

• Simple
• Easy to use
• Reliable
• Inexpensive
• Reproducible
• Validated

Computer-assisted approach

The assessment of implant placement is based on software analysis of two near orthogonal x-ray images (either plain film of fluoroscopic) of the proximal femur containing the implant.  Significant anatomical features along with the implant(s) are extracted from each image.  The scale of the image is determined from known implant properties, and thus invariant to imaging modality position.  Anatomical fiducials consistently available across varying quality images and differing anatomical specimens are developed.  The position of the implant(s) is calculated relative to these fiducials, and recorded as both an objective score and precise measurement.

Validation with CMM and CT

The approach has been validated using two independent measurement tools. DHS guide wires were inserted into ten plastic femurs.

• The position of the guide wires within the femurs were precisely measured with a co-ordinate measurement machine (CMM).
• Axial slice CT scans of the femurs were used to determine the position of  the guide wires. A specific protocol enabled metal artefact free images to be obtained

CMM measurement

CT scan of head

CT scan of neck

Measurement with software

Fluoroscopic image intensifier x-ray images provided the input to the Computer-Assisted Post-Operative Audit software.  Corresponding cross sections through the femoral head and neck were compared

Lateral image

AP image

Femur head results

Enlarged cross section through head, showing the comparison between CT and S/W errors.

Cross section through head, with overlaid measurements for CT (blue), CMM (red) and S/W (green).

Femur neck results

Enlarged cross section through neck, showing the comparison between CT and S/W errors.

Cross section through neck, with overlaid measurements for CT (blue), CMM (red) and S/W (green).

Discussion

The Computer-Assisted Post-Operative Audit proved reliability in determining the position of the implant within the head and neck of the femur. The software also proved considerably more accurate than CT-scan measurements in determining the position of the implant within the femur head.  The positional accuracy for implant position within the neck region was comparable to the CT scan approach.  This is to be expected as the neck region is a more complex three dimensional structure and inherently more difficult to reconstruct in two dimensions.

Future developments include a grading system to correlate the outcome and accuracy, along with a quantifiably assessment of the quality of fracture reduction. The main benefit of this approach is quantitative objective audits of implant position, without the need for 3D image reconstruction