How to avoid unintended valgus alignment in distal femoral derotational osteotomy for treatment of femoral torsional malalignment - a concept study

Abstract

Background: Defining the optimal cutting plane for derotational osteotomy at the distal femur for correction of torsion in cases of patellofemoral instability is still challenging. This preliminary study investigates changes of frontal alignment by a simplified trigonometrical model and demonstrates a surgical guidance technique with the use of femur cadavers. The hypothesis was that regardless of midshaft bowing, a cutting plane perpendicular to the virtual anatomic shaft axis avoids unintended valgus malalignment due to derotation.

Methods: A novel mathematical model, called the Pillar-Crane-Model, was developed to forecast changes on frontal alignment of the femur when a perpendicular cutting plane to the virtual anatomical shaft was chosen. As proof of concept, eight different torsion angles were assessed on two human cadaver femora (left and right). A single cut distal femoral osteotomy perpendicular to the virtual anatomical shaft was performed. Frontal plane alignment (mLDFA, aLDFA, AMA) was radiographically analyzed before and after rotation by 0°, 10°, 20°, and 30°. Measurements were compared to the model.

Results: The trigonometrical equation from the Pillar-Crane-Model provides mathematical proof that slight changes into varus occur, seen by an increase in AMA and mLDFA, when the cutting plane is perpendicular to the virtual anatomical shaft axis. A table with standardized values is provided. Exemplarily, the specimens showed a mean increase of AMA from 4.8° to 6.3° and mLDFA from 85.2° to 86.7 after derotation by 30°. Throughout the derotation procedure, aLDFA remained at 80.4° ± 0.4°SD.

Conclusions: With the use of this model for surgical guidance and anatomic reference, unintended valgus changes on frontal malalignment can be avoided. When the cutting plane is considered to be perpendicular to the virtual anatomical shaft from a frontal and lateral view, a slight increase of mLDFA results when a derotational osteotomy of the distal femur is performed.

Keywords: Distal femoral derotational osteotomy; Mechanical anatomical axis; Patellofemoral instability; Torsion correction; Valgus-varus alignment.

Fig. 1

Pillar-Crane-Model frontal view; a virtual anatomical axis regardless of femoral bowing; b introducing “AMA at cutting”, which is needed for exact calculations; c when derotation of the proximal limb is performed, increase of AMA at cutting, and increase of mLDFA will occur

Fig. 2

Overview of cadaveric femur shaft from frontal and lateral view for definition of the proximal middle point of the shaft and distal middle point of the shaft at the proposed cutting plane, a lateral view, virtual anatomical axis (black) and perpendicular cutting plane (red), despite bowing (dotted line); b frontal view, virtual anatomical axis (black), perpendicular cutting plane (red)

Fig. 3

Equation of change of AMA at cutting point

Fig. 4

frontal view x-ray; four different torsion angles on one specimen. a torsion = 37°, AMA = 4.9°, mLDFA = 84.6°; b torsion = 27°, AMA = 5.4°, mLDFA = 85.7°; c torsion = 17°, AMA = 5.8°, mLDFA = 86.1°; d torsion = 7°, AMA = 6.4, mLDFA = 86.4°