Effects of Parallax and Distortion in Total Ankle Arthroplasty

Abstract

Background

Surgeons rely on intraoperative fluoroscopy to assist in placement of implant components during total ankle arthroplasty (TAA). Parallax alters the direction of an object when viewed from two different points, resulting in image distortion. The purpose of this study was to evaluate parallax/distortion in intraoperative fluoroscopic images during TAA.

Methods

A retrospective review of all TAAs performed by two surgeons (R.W.M. and B.S.) from August 2019 to April 2023 were reviewed. Intraoperative fluoroscopic anteroposterior (AP) ankle views were evaluated for any obvious parallax image distortion. Cases with obvious parallax distortion were included for angular evaluation of AP intraoperative fluoroscopic and first postoperative plain films. The tibia was marked at 2-centimeter intervals to create zones from the proximal stem of the implant. The anatomical axis of the tibia (AAT) was drawn at the mid-diaphysis. The anatomic lateral distal tibial angle (aLDTA) and anatomic axis deviation (AAD) were measured for each zone.

Results

A total of 22 TAAs were performed during the study period. Four cases were excluded due to inadequate imaging, leaving a total of 18 TAAs for review. We found 6 of 18 (33.3%) cases had obvious parallax distortion. We found the average aLDTA was 90.9° (84°-101°). At the most proximal tibial zone, the average aLDTA was 94° (91°-101°). We found the average AAD was 4.7 (0.5-17.2) mm. The AAD ranged from 0.5 to 17.2 mm lateral to 0.8 to 8.2 mm medial. Postoperative plain film radiographs displayed a normal aLDTA and an AAT centered within the ankle joint.

Conclusion

Parallax can distort the appearance of the tibia on fluoroscopic images. Deviation from the normal aLDTA and anatomical axis should be anticipated. Surgeons should be aware of the potential impact of parallax and ways to mitigate these effects.