THE INFLUENCE OF X-RAY ORIENTATION ON THE 1ST METATARSOCUNEIFORM JOINT ANGLE

The purpose of this study was to determine whether the first metatarso cuneiform (MC) joint angle is affected by the orientation of the x-ray beam, whether it can be reliably measured, and, therefore, whether it is a valid indication of tarsometatarsal fusion as an adjunct to hall ux valgus surgery. Each of seven cadaver feet were loaded to 350 newto ns in a Plexiglas apparatus that maintained the foot in a plantigrade position. The loading apparatus was positioned to simulate a 10-degree s, 20-degrees, and 30-degrees dorsal angulation of the x-ray beam. At each position, simulated weight-bearing radiographs were obtained for each foot. Three observers independently measured the first MC angle a nd the first intermetatarsal (IM) angle on all the radiographs. The da ta collected were analyzed by the components of variance. The inclinat ion of the first MC joint lessens significantly as the orientation of the x-ray beam changes from a 10-degrees to a 20-degrees tilt. The thr ee examiners' mean MC angles and mean IM angles from the three sets of radiographs were not statistically different from each other, implyin g that there were minimal examiner-to-examiner differences in measurin g these angles. There were no significant differences in measurements between examiners, suggesting that the first MC and first IM angles ca n be measured reliably. There was a significant difference in the firs t MC angle, with different orientations of the foot with respect to th e x-ray beam, suggesting that this angular measurement should not be u sed as an indication for tarsometatarsal arthrodesis.