Assessment of lower limb alignment: supine fluoroscopy compared with a standing full-length radiograph.

BACKGROUND: While a full-length standing anteroposterior radiograph of the lower extremity provides the best radiographic method for assessing limb alignment, other methods must be used intraoperatively. We have employed intraoperative fluoroscopy with use of an electrocautery cord to assess limb alignment in the supine patient.
METHODS: We retrospectively compared the measurements of lower limb alignment that were obtained with use of supine intraoperative fluoroscopy with those that were obtained with use of a full-length standing anteroposterior radiograph of the lower extremity. A single examiner compared 102 sets of supine fluoroscopy images and full-length standing anteroposterior radiographs of the lower extremity to assess mechanical axis deviation and the joint line convergence angle. For the intraoperative fluoroscopic examination, an electrocautery cord was positioned overlying the center of the femoral head and the tibial plafond and an anteroposterior radiograph of the knee was made. The effect of age, gender, diagnosis, body mass index, pelvic height difference, joint line convergence angle, and the magnitude and direction of malalignment (varus or valgus) on the discrepancy in the observed mechanical axis deviation with use of the two methods was assessed.
RESULTS: The mean absolute difference between the two techniques was 13.4 mm for the measurement of mechanical axis deviation (p < 0.0001) and 2.8 degrees for the joint line convergence angle (p < 0.0001). The correlation coefficient (r) for the measurement of mechanical axis deviation with use of the two radiographic methods was 0.88. An increase in body mass index was associated with a greater magnitude of discrepancy in the measurement of mechanical axis deviation between the two techniques (p = 0.0014). Age, gender, pelvic height difference, and the direction of malalignment had no effect on the discrepancy in the measurement of mechanical axis deviation. Limbs with >2 cm of mechanical axis deviation and those with a joint line convergence angle of >3 degrees on the standing radiograph were significantly more likely to have >10 mm of discrepancy in the measurement of mechanical axis deviation with use of the two imaging techniques (p < 0.005).
CONCLUSIONS: Intraoperative fluoroscopy with use of the electrocautery cord method is a useful tool for assessing lower limb alignment in patients with a normal body mass index and </=2 cm of mechanical axis deviation and </=3 degrees of joint line convergence angle on the standing anteroposterior radiograph. However, the results obtained with fluoroscopy should be interpreted with caution in patients who are obese or who have substantial residual mechanical axis deviation or pathologic laxity of the knee joint.