Quantitative measurement of lower limb mechanical alignment and coronal knee laxity in early flexion

Background: Non-invasive quantification of lower limb alignment using navigation technology is now possible throughout knee flexion owing to software developments. We report the precision and accuracy of a non-invasive system measuring mechanical alignment of the lower limb including coronal stress testing of the knee. Methods: Twelve cadaveric limbs were tested with a commercial invasive navigation system against the non-invasive system. Coronal mechanical femorotibial (MFT) alignment was measured with no stress, then 15 Nm varus and valgus applied moments. Measurements were recorded at 10 degrees intervals from extension to 90 degrees flexion. At each flexion interval, coefficient of repeatability (CR) tested precision within each system, and limits of agreement (LOA) tested agreement between the two systems. Limits for CR & LOA were set at 3 degrees based on requirements for surgical planning and evaluation. Results: Precision was acceptable throughout flexion in all conditions of stress using the invasive system (CR <= 1.9 degrees). Precision was acceptable using the non-invasive system from extension to 50 degrees flexion (CR <= 2.4 degrees), beyond which precision was unacceptable (>3.4 degrees). With no coronal stress applied, agreement remained acceptable from extension to 40 degrees (LOA <= 2.4 degrees), and when 15 Nm varus or valgus stress was applied agreement was acceptable from extension to 30 degrees (LOA <= 2.9 degrees). Higher angles of knee flexion had a negative impact on precision and accuracy. Conclusion & clinical relevance: The non-invasive system provides reliable quantitative data in-vitro on coronal MFT alignment and laxity in the range relevant to assessment of collateral ligament injury, pre-operative planning of arthroplasty and flexion instability following arthroplasty. In-vivo validation should be performed. (C) 2014 Elsevier B.V. All rights reserved.