Sagittal mechanical ratio: A novel technique to define sagittal alignment of the femur independent of distal anatomic landmarks

Abstract

BackgroundRadiographic measurements to study sagittal alignment in the setting of knee are frequently difficult to evaluate due the presence of a prosthesis or implant that obscures traditional radiographic landmarks. In this paper we present a novel method of determining sagittal femoral alignment in the presence of obscuring implants. Methods98 full-length femoral radiographs were reviewed and divided into two groups. In Group 1, the Distal Mechanical Point (DMP) was used to calculate the Distal Mechanical Ratio (DMR), defined as the ratio of the linear distance from the DMP to the anterior cortical axis divided by the distance from the anterior cortical axis to posterior condylar cortex. In group 2, the sagittal mechanical axis was measured using the true DMP (tDMP) and then separately measured using the DMR to find the calculated DMP (cDMP), and the angular variance between the calculated (cSMA) and true (tSMA) sagittal mechanical axis was calculated, as well as the linear distance between the tDMP and cDMP. Twenty additional patients with knee replacements were then selected and two observers used a cSMA to determine a femoral prosthesis flexion angle (FPFA), with intraobserver correlation calculated. ResultsThe mean DMR was found to be 0.24, with high intraobserver correlation and normal distribution. Validation of the model demonstrated angular variance between tSMA and cSMA less than 1 degree and linear distance between tDMP and cDMP less than 1 mm. Calculation of cCMA in the presence of total knee arthroplasty revealed very strong intraobserver correlation of 0.89. ConclusionThe Distal Mechanical Ratio reliably predicted the true Sagittal Mechanical Axis within 1 degree and true Distal Mechanical Point within 1 mm, indicating that it may be a valuable tool for evaluating sagittal femoral alignment in cases where anatomic landmarks may be absent or obscured.