3D-ANALYSIS OF LOWER LIMB ANATOMY: NORMAL VALUES FOR JOINT ANGLES AND ALIGNMENT

AIMS AND OBJECTIVES: The analysis of lower limb anatomy is mandatory in deformity surgery and total knee replacement. Standard values for joint angles and alignment are only available for 2D images using long standing and lateral radiographs. In contrast, 3D bone models realistically represent the anatomy and are independent of position and projection. Using a self-developed, standardised and validated protocol for 3D analysis of lower limb geometry, standard values for specific joint angles and axes can be obtained in a young cohort for the first time. MATERIALS AND METHODS: After power and sample size analysis, 60 thin-slice CTs of the entire leg were analysed (30 patients, 13 f, 17 m; age = 33.8 [18-50 years]). After segmentation all 24 relevant bony landmarks were set using our standardised analysis method. Results of joint angles and mechanical axis deviation are calculated automatically. Mean values are stated with standard deviations to describe a possible reference range. RESULTS: The mean neck-shaft angle (NSA) was 133±10° and the mean medial proximal femoral angle (MPFA) 85±9°. Around the knee joint, the mean mechanical lateral distal femoral angle (mLDFA) was 87±4° and the medial proximal tibial angle (MPTA) 88±5°. Medial and lateral slope averaged 12±7° and 9±8°. The deviation of the frontal mechanical leg axis from the knee joint center (MAD frontal) was 7±15mm and the mechanical femoro-tibial angle (HKA) 175±6°. Women showed a smaller mLDFA than men (86.4° vs. 87.8°, p<0.05) and a larger joint line convergence angle (JLCA 2.5° vs. 1.4°, p<0.01) representing a more valgiform anatomy. CONCLUSION: The standardised 3D analysis enabled reference ranges for the relevant joint angles and axes to be defined on the 3D bone model for the first time. The results redefine reference ranges for 3D measurements similar to existing 2D measurements on long standing radiographs. The systematic analysis of the complex 3D anatomy results in a multitude of new questions and possibilities in preoperative planning.