Accuracy of Tibia Deformity Correction with a Hexapod Multiplanar External Fixator with Measurements Taken from Radiographs, CT Scan, and Imaging Software: A Cadaveric Study

Abstract

Category: Other; Trauma Introduction/Purpose: The success of hexapod multiplanar external fixators for correction of tibial deformity has been well described. The ability to correct deformity is based on the accuracy of the reference and deformity data inputted into the prescription program. If the data is not accurate, multiple residual programs may be required for final correction. There are limited papers evaluating i accuracy of intraoperative measurements. We are unaware of any analysis on the use of CT scan to determine deformity and reference points. Newer software program allows surgeons to incorporate radiographs into the correction program to determine deformity and reference points. Our study objective was to compare deformity and reference point measurements taken from radiographs, CT, and imaging software to determine which method most accurately corrects tibia deformities. Methods: Hexapod external fixators were orthogonally applied on four full length tibia cadavers. Strut lengths were set to the same length. Each cadaver was assigned a different deformity: proximal quartile, distal quartile, midshaft, and segmental. An osteotomy was performed and the hexapod struts were loosened to create a multiplanar deformity. Radiographs and CT were taken orthogonal to the reference ring. Deformity and reference points were measured off each according to correction software data points. Pictures of the radiographs were imported into the software program (Stryker, Mahwah, NJ) and the correction reference points were determined with imaging software. All cadavers had a correction program for each method. Two surgeons performed the measurements twice to determine inter and intra-observer accuracy. Corrections were performed and post reduction radiographs were measured to determine length, angulation and translation. In addition, the total number of millimeters each strut was off from the pre-deformity position was recorded. Results: Line technique on software program was significantly better at measuring segmental deformity and the Image technique was statistically more accurate to correct proximal, midshaft and distal deformities (p<0.05). Overall, all methods were more accurate correcting Midshaft and Proximal deformities (44.8mm and 50.4mm, respectively) significantly better than Distal and Segmental deformity correction (88.9mm and 121.7mm, respectively (p<0.001). There was no difference in absolute measurements or intra-observer error between the four surgeons (p>0.05). Conclusion: Compared to the traditional method of measuring deformity and frame reference using radiographs, new imaging software appears to be superior and more reproducible in correcting tibia deformities. Surgeons should recognize the increased difficulty in correcting segmental and distal deformities regardless of the method used. Accuracy of determining deformity and frame reference points has the potential to decrease time patients are in a frame and reduces the potential for malunions. Further investigation is needed to determine sites of variation to further improve the process of correcting complex deformities of the tibia.