Abstract
We report the use of CT with 3D reconstruction to determine the accuracy of knee arthroplasty.
Method Pre- and post-operative CT scans have been performed in a cohort of 20 total and unicompartmental knee replacements in fine detail of hips, knees and ankles but with minimal dosage elsewhere.
Three different methods have been used to measure the position of the implant.
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‘True’ anterior posterior views are reconstructed from the post-operative CT data and tibiofemoral angles computed.
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A wire frame model of the implants is registered to the post-op scan. This defines the orientation of the implant relative to bony reference points.
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The position of the prosthetic components in the post op scan is calculated. The post-op scan is registered with the pre-op scan.
Results Alignment can be determined within 0.5° using method 1. With the addition of method, 2 rotational malalignment can be computed. With method 3 a transformation matrix is provided, showing the position in space of the prosthesis relative to the pre-op plan, with accuracy of under 0.5 mm. Poor function is explained.
Conclusion We have used 3-D reconstructions from CT scans and digital measurements to compute the precise position of the implant in the bone. Showing where the implant lies in 3D space explains accurately why poor results have occurred. CT based planning has been used to ensure that the bone cuts are planned correctly. Postoperative CT scans confirm that if the plan is achieved, function will be good.
Correspondence should be addressed to Roger Smith, Honorary Secretary, BASK c/o Royal College of Surgeons, 35 – 43 Lincoln’s Inn Fields, London WC2A 3PN
