Conditions for Use and Implementation of Globally-Aligned Versus Local Baseplate Coordinate Systems When Computing...

Area of Science:

• Orthopedic biomechanics
• Medical imaging analysis
• Implant fixation stability

Background:

• Radiostereometric analysis (RSA) quantifies tibial baseplate migration to assess fixation stability.
• Current methods using a global baseplate coordinate system (BCS) are sensitive to imaging and surgical alignment deviations.
• Inconsistent migration direction measurements complicate patient and implant design comparisons.

Purpose of the Study:

• To summarize migration equations for global versus local BCS.
• To propose a method for defining a local BCS.
• To demonstrate migration differences between global and local BCS using a patient example.

Main Methods:

• Formulated migration equations for both globally-aligned and locally-defined BCS.
• Developed a novel method for establishing a local BCS based on baseplate features.
• Analyzed migration differences in a case study with rotational alignment deviations.

Main Results:

• Differences in migration between the two BCSs ranged up to ±0.5 mm translation and -0.4° to 0.7° rotation.
• Largest discrepancies occurred with internal-external rotation deviations; smallest with varus-valgus deviations.
• The global BCS incorrectly quantified migration as subsidence instead of liftoff in one instance.

Conclusions:

• A local BCS provides migration measurements independent of imaging and surgical alignment.
• Local BCS enhances consistency in migration direction across patients and baseplate designs.
• This approach improves the reliability of assessing fixation stability at the bone-implant interface.