Abstract
Correct alignment and ligament tension are widely accepted conditional features of successful TKA. The technical route of achieving this goal remains a matter of debate. Two philosophies prevail: measured resection as a geometry based system, and tensioned gaps, based upon the dependent relation between tibia and femur. Both techniques claim the best results and are often presented as radically different. From a conceptual standpoint, however, the dependent technique is not purely ligament based as it starts with a cut of the proximal tibia, which is geometrically ruled by sagittal and coronal alignment targets. As such, geometric alignment is the starting point of both techniques.
The use of ligament tensioners as the main basis for obtaining stability and alignment can be a treacherous route to follow. In the native knee that is not affected by arthritis, the functional characteristics of the ligaments are determined by their insertions and the articular geometry. Once the arthritis sets in, the articular surface deforms and leg alignment deviates. It is difficult to restore these parameters, making use of ligament tension as a guide, for the following reasons.
Ligament tension is hard to measure reproducibly intra-operatively. The stress-strain curves of ligaments are different and the magnitude of the optimal distraction force is unknown.
In flexion, the dislocation or eversion of the patella will affect the obtained result and might induce significant bias.
Ligament tensioners evaluate available space in flexion and in 90° of flexion. No information about the mid- and deep flexion range can be obtained. Articular geometry of the implant will affect this.
The ligament tensioner does not control relative position AP position of femur and tibia. Especially in the presence of the PCL, distraction of the femur from the tibia at 90° of flexion will push the tibia forwards as the PCL straightens out and finds a more vertical position.
The medial side of the knee is the more stable and isometric side, where the lateral side is more dynamically controlled by muscle loads. Applying an equal distraction force to the medial and lateral compartment in an anesthetised patient can overestimate the available space in the lateral compartment and induce excessive external rotation.
Traumatic or chronic ligament injury caused by the arthritic process can skew the obtained results
Measure resection is obviously not void of potential errors. Especially in case of dysplasia or prior trauma, bone geometry and landmarks can be deformed. In contrast to ligament assessment however, pre-operative imaging under the form of x-rays or CT allows for an objective and reproducible evaluation of the amount of deformity and subsequent surgical correction.
