1 Introduction

The knee joint is the largest and most complex joint in the human body and has the longest lever arms. The joint transmits muscle forces into motion of the human body. The large lever arms of the knee joint produce substantial loading moments. Axial load causes high mechanical stress in the knee joint. The mechanical load during walking on even ground amounts to 3.4 times body weight and as much as 4.3 times body weight when climbing stairs [1, 2].

2 Physiological axes of the leg

It is necessary to differentiate between anatomical and mechanical axes when considering the axes of the lower extremity.

The anatomical axes of the femur and tibia correspond to the diaphyseal midline of these long bones. The diaphyseal axis of the femur is not on a straight line with the anatomical axis of the tibia because of the orientation of the femoral neck. The anatomical axes of the femoral and tibial diaphyses form a laterally opened angle of 173-175° (aFTA) ( Fig 1-1a ).

The mechanical axis of the femur runs from the center of the femoral head to the center of the knee joint and forms an angle of 6° ± 1° with the anatomical axis of the femoral diaphysis (aMFA) (Fig 1-2 ). The mechanical and the anatomical axes of the tibia are almost identical. Both lines run parallel, whereby the anatomical tibial axis lies a few millimeters medial to the mechanical tibial axis. The mechanical axis of the leg (Mikulicz line) is the connecting line between the center of the femoral head and the center of the ankle joint (Fig 1-3 ). This line physiologically runs on average 4 (±2) mm medial to the center of the knee joint [3]. If the mechanical axis runs lateral or medial to this point, this indicates either a valgus or a varus deformity (see below). Due to the greater distance between the centers of the hip joints than between the centers of the knee joints and ankle joints, the weight-bearing axis of the leg runs slightly oblique from craniolateral to mediocaudal at an approximate angle of 3° to the perpendicular axis of the body (Fig 1-3 ).

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  The mechanical axis of the leg (Mikulicz line) runs from the center of the femoral head to the center of the ankle joint. Under physiological conditions this line runs on average 4 (±2) mm medial to the center of the knee joint.

  
  

  The anatomical and mechanical femoral axes form an angle of 6° (±1°) (aMFA).

Under physiological conditions the knee base line (tangent to the femoral condyles) and the tangent to the tibial plateau run almost parallel to each other (joint line convergence angle = JLCA, 0-1° medial convergence) (see Fig 1-1, Fig 1-6 ). Since the mechanical and anatomical axes of the tibia run parallel, the medial proximal angle between the tangent to the tibial plateau and the anatomical and mechanical axes is 87° ± 3° in both cases; the standard value for the lateral distal tibial angle at the line of the ankle joint is 89° ± 3° (see Fig 1-1a-b ).

The lateral distal angle between the mechanical femoral axis and the base line of the knee is also 87° ± 3°. Due to aMFA, the knee base line forms an angle of approximately 81° ± 2° with the anatomical axis of the femur (see Fig 1-1 a—b, Table 1-1 ).