gms | German Medical Science

Objectives: Progression of osteoarthritis over time is poorly understood. The aim of the current study was to find differences of cartilage degeneration per subregion of the knee in relation to different phenotypes of the lower extremity established by Hirschmann et al. in search of the toxic phenotype.

Methods: 466 arthritic knees (Kellgren and Lawrence grade 2-3) enrolled in the progressive cohort of the Osteoarthritis Initiative were categorized in nine different phenotypes of the lower extremity established by Hirschmann et al., based upon their distal mechanical angle of the femur (FMA) und proximal tibial angle of the tibia (TMA) in full-limb radiographs. Changes of knee cartilage in 16 different subregions of the knee joint were measured in MRI scans at baseline, 12- and 24-month visits. Statistical analysis was performed using correlation analysis and both classical Analysis of Variance and robust Analysis of Variance methods for possibly non-normal, heteroscedastic data in repeated measures.

Results and conclusion: 22 out of the 25 theoretically possible functional knee phenotypes were exhibited. Phenotypes of the first range (NEU_FMA/_TMA0°, VAL_FMA/_TMA3°, and VAR_FMA/_TMA3°) represented around 90% of the investigated cohort. Neutral alignment was the most common in both genders and age groups, even though varus tibial alignment (NEU_FMA0°, VAR_TMA3°) in males, and valgus tibial alignment (NEU_FMA0°,VAL_TMA3°) in females, were found to be the second most common phenotypes. Although a gradual decline in cartilage thickness and volume was observed, males demonstrated higher cartilage values in all visits. Valgus and varus knees demonstrated mainly lateral and medial compartment, respectively, cartilage degeneration. Nevertheless the biggest impact was seen of valgus knees on the internal medial tibial subregion (η²p = 0.930). Out of the nine most common phenotypes, VAL_FMA,VAL_TMA and VAL_FMA,VAR_TMA exhibited toxic effects on cartilage and progressive wear patterns within 12 months in subjects younger than 71 years. While 5.57% and 23.6% of the functional phenotypes represented anatomical and mechanical alignment targets, respectively, 78.75% of the patient cohort fit into safe zones targeting of restricted kinematic alignment.

Functional knee phenotypes demonstrated a wide distribution with specific effects on articular degeneration. The massive effects of VAL_FMA,VAL_TMA and VAL_FMA,VAR_TMA on articular wear, particularly on tibial subregions in subjects younger than 71 years of age, could be considered as toxic. While various total knee arthroplasty (TKA) alignment concepts are currently being debated, more than two-thirds of the investigated arthritic knees were found to represent a restricted alignment target within the safe zones for TKA. These findings suggest the reevaluation of knee kinematics and progression of cartilage wear and support a more individualized alignment approach when considering knee reconstructive surgery.