Using the Anatomical Axis as an Alternative to the Mechanical Axis to Assess Knee Alignment

Area of Science:

• Orthopedic surgery
• Radiology
• Biomedical engineering

Background:

• Knee osteoarthritis (OA) management and total knee arthroplasty (TKA) require precise preoperative planning using repetitive imaging.
• Full-length standing anteroposterior (AP) radiographs are the gold standard for measuring the mechanical axis, crucial for limb alignment assessment.
• Limited knee radiographs offer a less costly and lower-radiation alternative, but their correlation with mechanical axis measurement needs validation.

Purpose of the Study:

• To investigate the correlation between the anatomical axis and the mechanical axis of the knee.
• To determine if anatomical axis measurements from limited knee radiographs can reliably estimate the mechanical axis for TKA planning.
• To assess the influence of imaging technique and patient factors on this correlation.

Main Methods:

• A cross-sectional prospective study analyzing 209 total knee arthroplasty radiographs.
• Comparison of mechanical axis measurements from full-length standing AP radiographs with anatomical axis measurements from fixed-flexed posteroanterior (Rosenberg view) radiographs.
• Utilizing linear regression to determine the correlation and optimal offset angles.

Main Results:

• A significant correlation was found between the anatomical axis and the mechanical axis, regardless of whether measured from standing AP or fixed-flexed PA radiographs.
• Linear regression analysis, incorporating an offset angle, demonstrated a closer correlation between the two measurements.
• Body mass index and Kellgren-Lawrence grade did not significantly impact the correlation.

Conclusions:

• The anatomical axis, measured from limited knee radiography, can serve as a plausible estimate of the mechanical axis.
• A neutral angle of offset is recommended for this estimation, with the optimal offset angle varying by gender and imaging technique.
• This finding supports the potential use of limited knee radiography for preoperative planning in TKA, reducing radiation exposure and cost.