[Adjusted mechanical alignment: operative technique-Tips and tricks]

Mechanical alignment (MA) is a standardized procedure that aims to achieve a neutrally aligned leg axis. An alignment of the prosthesis closer to the patient's anatomy can be an approach for better clinical outcomes. The surgical technique of adjusted mechanical alignment (aMA) presented here is a modified extension-gap-first technique that takes into account the natural ligamentous tension of the knee joint so that ligamentous releases can be avoided as far as possible.

The aMA technique can be used for primary and secondary varus gonarthrosis of up to 20° of varus.

The aim of the operation is to achieve a balanced ligament tension through a femoral osseous correction rather than ligament releases. TEA and the sulcus line are marked to control the ligament-based femoral rotation. The osteophytes are removed to ensure a reliable ligament tension. A quantitative ligament tensioner is stretched with great care, and gap width as well as medial and lateral ligament tension are read off. In order to correct an extension gap asymmetry, instead of the typical medial soft tissue release, the asymmetry is compensated by a special femoral cutting block. Now, the flexion gap is assessed, whereby the transverse femoral rotation follows the soft tissue tension. The tensioner adjusts a rectangular flexion gap with balanced ligament tension. After a final balancing of the gaps, the femoral preparation is completed and the trial components are inserted. Here, the rotation of the tibial component is set by repeated flexion-extension cycles.

The technique presented combines a measured-resection technique with individual ligament tension. The maximum deviation of the femoral alignment in the coronal plane from the neutral alignment is 2.5°. In order to avoid problems, it is recommended, as with the described technique, to achieve a component alignment based on the patient anatomy by adjusting the femoral component. The measured-resection technique carries the risk of flexion instability. With the gap-balancing technique symmetrical ligament tension can be achieved, assuming precise proximal tibial cuts. When aligning the femoral component rotation, flexion gap stability and patella tracking should be considered. Long-term studies of high case numbers are necessary to evaluate the good short-term results of the presented surgical technique.