Arithmetic hip-knee-ankle angle (aHKA): An algorithm for estimating constitutional lower limb alignment in the...

Area of Science:

• Orthopedic surgery
• Biomechanical analysis
• Radiological assessment

Background:

• Determining pre-arthritic limb alignment is challenging once knee arthritis and deformity are present.
• Accurate restoration of constitutional alignment is crucial for successful total knee arthroplasty (TKA) outcomes.
• Existing methods may not reliably assess pre-arthritic alignment in deformed knees.

Purpose of the Study:

• To describe and validate the arithmetic hip-knee-ankle (aHKA) algorithm.
• To establish aHKA as a straightforward method for preoperative planning in TKA.
• To enable intraoperative restoration of constitutional limb alignment during TKA.

Main Methods:

• Comparative cross-sectional radiological study involving 500 normal knees and 500 arthritic knees undergoing TKA.
• The arithmetic hip-knee-ankle (aHKA) algorithm was defined as the lateral distal femoral angle (LDFA) subtracted from the medial proximal tibial angle (MPTA).
• Comparison of mechanical HKA (mHKA) and aHKA between normal and arthritic groups to assess alignment estimation and joint line convergence.

Main Results:

• Significant differences in mHKA were observed between normal and arthritic knees (p < 0.001).
• No significant difference was found between normal and arthritic groups when using the aHKA (p = 0.550).
• The medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) showed no significant difference between the groups.

Conclusions:

• The arithmetic hip-knee-ankle (aHKA) algorithm effectively estimates constitutional lower limb alignment post-arthritis onset.
• This validated algorithm is significant for surgeons aiming to restore constitutional alignment in TKA.
• The aHKA provides a reliable method for preoperative planning and intraoperative guidance in total knee arthroplasty.