Laxity and function in knee replacements. A comparative study of three prosthetic designs

Area of Science:

• Orthopedic Surgery
• Biomedical Engineering
• Biomechanics

Background:

• Knee replacement surgery aims to restore function, but post-operative laxity can be a limiting factor.
• Assessing anteroposterior (AP) laxity is critical for understanding clinical outcomes in total knee arthroplasty.
• Different knee implant designs may have varying responses to AP laxity.

Purpose of the Study:

• To investigate the relationship between AP laxity and clinical function after knee replacement.
• To determine if laxity thresholds influence range of motion and flexion deformities.
• To evaluate the impact of AP laxity on clinical outcomes across different knee implant designs.

Main Methods:

• Computerized measurement of AP laxity in three types of knee replacements: Insall-Burstein (posterior stabilized), Kinemax Condylar (posterior cruciate retaining), and Oxford bicompartmental (anterior and posterior cruciate retaining).
• Correlation of measured laxity with passive range of motion, flexion deformity, and American Knee Society Clinical Rating system scores.
• Analysis of laxity's influence on clinical function specific to implant design.

Main Results:

• Laxities less than 5 mm were associated with reduced passive motion and increased risk of significant flexion deformity (>4 degrees).
• The Oxford bicompartmental knee replacement showed particular sensitivity to AP laxity in terms of clinical function.
• A minimum AP laxity exceeding 5 mm appears necessary for optimal prosthetic knee function.

Conclusions:

• Anteroposterior laxity greater than 5 mm is desirable for unimpaired joint function after knee replacement.
• Ligamentous tension during surgery is a key factor influencing post-operative laxity and function.
• Clinical rating systems for knee replacements should incorporate appropriate weighting for AP laxity.