2015 ISAKOS Biennial Congress ePoster #1449

A Comparison of Femoral Bone-Cut Angles Determined by Kinematic Alignment-Based Planning and Cylindrical Axis-Based Planning

Tomoki Sassa, MD, Fujisawa, Kanagawa JAPAN
Yasuo Niki, MD, PhD, Tokyo JAPAN
Kengo Harato, MD, PhD, Tokyo JAPAN
Takeo Nagura, MD, Tokyo JAPAN
Yoshiaki Toyama, MD, PhD, Shinjyuku JAPAN
Yasunori Suda, Prof, Yaita-Shi, Tochigi Prefecture JAPAN
Keio University, School of Medicine, Tokyo, JAPAN

FDA Status Cleared

Summary: Cylindrical axis-based planning is more confident for reproducing flexion-extension axis in Japanese subjects who have different cylindrical radius in medial and lateral femoral condyles.

Abstract:

Purpose

Flexion-extension axis (FEA) of the femur is increasingly focused to reproduce natural kinematics of the knee after TKA. Howell et al. advocated that femoral component be aligned so that the distal and posterior femoral resections were equal in thickness [1]. Such joint surface-reference technique enables kinematically aligned TKA through a restoration of natural FEA, but equal radius of medial and lateral femoral condyles is a prerequisite for this concept, and degree of cartilage loss should be accurately measured during TKA for end-stage osteoarthritis (OA). The purpose of the study was to assess the three-dimensional (3D) characteristics of cylindrical axis (CA) relative to surgical epicondylar axis (SEA), and to compare femoral bone-cut angles determined by CA-based planning with those determined by joint surface-based planning [2].

Materials And Methods

The present study enrolled 50 knees from 34 patients, who underwent primary TKA. CT data were reconstructed into 3D models using ATHENA® software. The varus/valgus orientation of the sagittal plane was determined, as the intersection of sagittal plane and coronal plane was aligned perpendicular to a tangential line connecting the most distal points of medial and lateral femoral condyles. The axial rotation of the sagittal plane was determined in the same way as described above. The radii of the femoral condyles were measured by circle-fitting technique, as the both condyles reportedly represented single radius of curvature from 10-160º [3]. The line connecting the cylindrical centers of the both condyles was defined as CA.
When the femoral components were assumed to be aligned to the CA, practical bone-cutting angles in three dimensions were determined. The bone-cut angles for coronal and axial alignment were compared between CA-based planning and joint surface-based planning aimed for kinematically aligned TKA using generic instruments [2].

Results

With regard to the angular relationship of the CA and SEA, difference between axes were 1.0±2.1º for coronal plane, 2.4±2.1º for axial plane, and 3.5±1.8º for 3D. Radii of the cylinders averaged 17.8 and 17.0mm for medial and lateral femoral condyle, respectively. Femoral bone-cut angles based on joint surface-based planning were 6.7º in coronal plane(relative to IM rod) and 0º in axial plane(relative to PCA), whereas those based on CA-based planning were 5.1º, 1.4º.
In practical planning of the femoral component position, CA-based planning indicated less valgus bone-cut of distal femur and larger external rotation of posterior condylar bone-cut than joint surface-based planning.

Discussion

Mechanically-aligned femoral component does not necessarily represent normal morphology of the femur, and the FEA of the femur is reportedly more approximated by the CA rather than SEA [4,5]. CA can be reproduced intraoperatively by preoperative 3D-planning. In the present Japanese subjects, positional difference between SEA and CA was small on 2D-plane, but a difference of 3.5º substantially existed in 3D space. Distal femoral bone-cut angle in coronal plane was smaller in CA-based planning than in joint surface-based planning, whereas that of posterior femoral condyle in axial plane was larger in CA-based planning than in joint surface-based planning which represented 0º. These differences were attributable to the difference in the radius of cylinder between medial and lateral femoral condyle. Howell et al. recommended joint surface-based bone cut using generic instruments during kinematically aligned TKA [2], due to an equivalent radius of the two femoral condyle cylinders in Caucasian subjects. However, we believe that CA-based planning is more confident for reproducing FEA in Japanese subjects who have different cylindrical radius in femoral condyles.