Standard Radiographs Are Not Sufficient for Assessing Knee Mechanical Axis in Patients With Advanced Osteoarthritis

Area of Science:

• Orthopedic surgery
• Radiology
• Biomechanical analysis

Background:

• Standard radiographs and tibiofemoral angle (TFA) are commonly used to assess lower extremity alignment.
• The relationship between TFA and mechanical axis (MA) measured on hip-to-ankle (HTA) radiographs is not well understood.
• This study evaluates the accuracy of predicting MA using standard radiograph measurements.

Purpose of the Study:

• To assess the predictive accuracy of equations for estimating MA from standard radiographs.
• To develop gender-specific regression equations for MA prediction.
• To evaluate the clinical effectiveness of these equations using HTA radiographs as the gold standard.

Main Methods:

• Mechanical axis (MA) was measured on hip-to-ankle (HTA) radiographs.
• Tibiofemoral angle (TFA) and femoral angle were measured on standard radiographs.
• Data from 788 knee osteoarthritis patients were analyzed.

Main Results:

• TFA, femoral angle, and height were the strongest predictors of MA, explaining 83% (men) and 86% (women) of the variance.
• These factors predicted MA within ±3° in only 66% (men) and 69% (women) of cases.
• Previously reported equations showed similar predictive accuracy (61-63%).

Conclusions:

• Standard radiographs are inadequate for precise MA determination.
• HTA radiographs are recommended for surgical alignment decisions and post-total knee arthroplasty assessment.
• Previous surgical outcome studies relying on standard radiographs should be interpreted with caution.