True Mechanical Alignment is Found Only on Full-Limb and not on Standard Anteroposterior Radiographs

Abstract

Purpose: The purpose of this study was to compare alignment measured on standard anteroposterior (AP) radiographs versus full-length weight-bearing radiographs.

Methods: Patients were prospectively enrolled from June 2019 to August 2019 from a single orthopedic surgeon's practice if they were ≥ 18 years of age, obtained both AP and full-length alignment radiographs and were capable of full weight-bearing with appropriate positioning. Patients were excluded if they were < 18 years of age, had previous knee arthroplasty, previous knee or hip osteotomy, were unable to bear full weight on both limbs, and if the patient's body habitus precluded appropriate visualization of necessary landmarks on the radiographs. Tibiofemoral angles were measured on AP radiographs using 2 techniques (AP angles 1 and 2). Linear regression and paired t tests were used to compare measurements. The minimal clinically important difference was defined as < 2°.

Results: There were 120 patients (62 males, 58 females) with an average age of 45 ± 17 years who were enrolled. There were positive correlations between average alignment on full-length weight-bearing and AP radiographs for AP angle 1 (r = 0.72) and AP angle 2 (r = 0.76) measurement techniques (P < .001). There was a significant difference in mean alignment between full-length weight-bearing and AP measurements (AP angle 1: 2.5° difference; AP angle 2: 4.4° difference; P < .001). Frequency distributions for the minimal clinically important difference between true mechanical alignment and AP views demonstrated that 46.7% of patients had ≥ 2° difference for AP angle 1, and 78.3% of patients had ≥ 2° difference for AP angle 2.

Conclusion: The average absolute difference in alignment measured between standard AP radiograph and full-length weight-bearing radiograph views was significant, with 46.7% to 78.3% of patients having a greater than 2° absolute difference between these 2 views. In cases where precise objective alignment measurement is necessary, full-length weight-bearing radiographs are recommended over standard AP radiographs for presurgical planning so as to reduce potential error in over- or underestimation of the true mechanical alignment.

Study design: Prospective case-comparison; Level of evidence, 1.

Fig 1

Limb alignment measured on standard AP compared to full-length weight-bearing radiographs. Assessment using the tibiofemoral angle 1 (AP angle 1) technique (left) demonstrates limb alignment of 184° (4° valgus); the AP angle 2 technique (middle) demonstrates limb alignment of 187° (7 ° valgus); the mechanical axis on full-length weight-bearing radiograph (right) demonstrates alignment of 172.8° (7.2° varus). Compared to the full-length weight-bearing view, the AP angle 1 an AP angle 2 techniques demonstrate a discrepancy of 11.2° and 14.2°, respectively.

Fig 2

Frequency distributions of differences between full-length weight-bearing (FLWB) and anteroposterior weightbearing (AP) knee radiographs (N = 120). (A) Absolute differences in mechanical alignment between FLWB and tibiofemoral angle measurement 1 (AP angle 1). (B) Absolute differences between FLWB compared to tibiofemoral angle measurement 2 (AP angle 2).