Effects of Parallax and Distortion in Total Ankle Arthroplasty.

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.

Postoperative Maintenance of Sagittal Plane Positioning of the First Metatarsophalangeal Joint After Arthrodesis with an Isolated Dorsal Plate Construct: A Retrospective Review of 43 Feet.

BACKGROUND: First metatarsophalangeal joint arthrodesis with isolated dorsal plating without a lag screw and without a compressive mechanism incorporated into the plate is not well studied. Although surface area for bony fusion is increased, there is concern for lower fusion rates and progressive loss of sagittal plane positioning. We present fusion rates and progressive sagittal plane deviation with isolated dorsal plate fixation. METHODS: A retrospective review was performed of 41 patients (43 feet) who underwent first metatarsophalangeal joint arthrodesis with isolated dorsal plate fixation. Patients were excluded if another form of fixation was used, if there was a compressive feature to the dorsal plate, or if a lag screw was used. Preoperative, immediate postoperative, and final postoperative radiographs were reviewed to assess radiographic alignment and fusion about the first metatarsophalangeal joint. Specific attention was placed on hallux dorsiflexion in relation to the first metatarsal. Statistical significance was set at P ≤ .05 a priori. RESULTS: Patients were followed for an average of 55.7 weeks. Overall union rate was 97.62%. The average time to union was 42.55 days. Reoperation rate was 4.65%, with one patient requiring revisional arthrodesis with a lag screw construct. Hallux abduction and first-second intermetatarsal angle correction reached significance (P < .00001). Hallux dorsiflexion increased by 1.05° between initial postoperative and final postoperative radiographs (P = .542). CONCLUSIONS: Although fusion rates and progressive loss of sagittal plane position have been concerns for first metatarsophalangeal joint arthrodesis with an isolated dorsal plate construct, these results suggest this to be a stable construct without loss of positioning over time.

Parallax and Distortion in Fluoroscopy Units.

BACKGROUND: Parallax is an imaging phenomenon where an object appears to be at different positions when viewed from different angles. Distortion can occur secondary to internal fluoroscopic, or external environmental, factors. Fluoroscopy is a vital tool to assist surgeons intraoperatively. However, parallax and distortion can lead to inaccuracy, potentially leading to incorrect surgical decisions. The purpose of this study was to investigate the prevalence of parallax/distortion in large fluoroscopy units at a level-1 trauma center. METHODS: Two types of C-arm models were evaluated, including (1) round image intensifiers, and (2) flat plate detectors (FPD). A square plexiglass grid with embedded wire at ½-in intervals was created, with a round metal washer secured centrally. The grid was placed 16 in from the image intensifier. A metal ball bearing (BB) was secured to the center of the x-ray tube. Fluoroscopic images were obtained until the BB and washer were "center-center." A straight blade served as a fiducial marker to ensure there was no off-axis angulation. Standard anterior-posterior and lateral views were obtained. External factors were considered, tested, and limited. Images were printed and the patterns of parallax/distortion were identified. RESULTS: All 11/11 (100%) of fluoroscopy units had some degree of parallax and/or distortion. We noted 3 different patterns, including sigmoidal, converging, and diverging. The FPD units had less apparent distortion overall; however, two-thirds (66%) were off-axis in the x- and y-axes in relation to the fiducial marker. CONCLUSION: All fluoroscopy units had varying degrees and patterns of parallax/distortion. We noted less overall distortion in FPDs. However, some of these units may produce images that are off-axis. This research has important implications for improving the accuracy of intraoperative fluoroscopy. Musculoskeletal surgeons should understand the limitations of fluoroscopy and how to combat parallax distortion to improve surgical outcomes and reduce patient morbidity. LEVEL OF EVIDENCE: Level V.