This case illustrates the use of patient specific instrumentation to perform a total knee replacement. The Visionaire technology is from Smith & Nephew, being introduced in 2010. It utilises a single weight bearing AP alignment film of the patient’s leg and a focused MRI of their knee to provide the data for cutting jig and implant design. The information is relayed to technicians who are based in the United States of America who design the knee replacement for you within the parameters you have set for them.
The surgeon sets the sizing constraints for example to upsize unless medial-lateral overhang, the rotational axis, the varus/valgus alignment, the tibial slope and tibial rotation. These usually correspond to the the typical component features used in a standard knee replacement, but can be varied.
Once the implant has been designed the plan is e-mailed to the surgeon and it is then checked and accepted. Printed 3-D printed nylon cutting blocks are manufactured for both the tibial and femoral sides of the knee replacement. The blocks are applied to the patient intra-operatively, cuts made and then the final preparation of the bone is performed using standard instrumentation and the total knee replacement is then implanted.
The use of Visionaire technology is associated with more reproducible implant placement, decreased operative times, fewer equipment trays in theatre, a reduced tourniquet time and reduced blood loss. Technically the Smith and Nephew Visionaire instrumentation is particularly useful in cases where there is difficulty passing standard instrumentation such as the intramedullary femoral rod in cases of femoral mal-union or canal occlusion.
OrthOracle readers will also find of interest the following associated instructional techniques:
This overview is brought to you by Orthoracle - the online e-learning Orthopeadic Surgery Atlas
- preparation and exposure for the proximal tibial cut:
- hyperflexion of the knee to allows the vessels to fall more posteriorly;
- curved knee retractors are positioned anterior to mid-coronal plane, so that patella tendon and MCL will not be injured;
- inadvertent transection of the lateral patellar tendon may occur w/ inadequate retraction during the proximal tibial cut;
- homan retractor may be placed behind posterior tibia and levered off of distal femur, which allows tibia to be translated anteriorly;
- extramedullary guides:
- depth of the tibial cut (avoid lowering of the joint line)
- posterior slope:
- rotation of tibial component:
- mechanisms can be used to avoid tendency to internal rotation;
- look down on tibial plateau & cutting jig w/ knee in flexion;
- face of jig should be parallel to plane of the femoral condyles;
- plane of resection of the mechanical axis:
- after fixing tibial cutting jig in place, gentily flex and extend knee while watching that axis of cutting jig is parallel to axis of rotation of knee;
- current techniques involve making tibial cut parallel to the tibial anatomic axis (and cutting distal femur at 5-7 deg to its anatomic axis);
- this reduces the probability of inadvertent excessively varus cut and thus the likihood of varus alignment of the limb;
- as its easier to cut right angles than 3 deg off right angle & varus error in tibia is most common error w/ significant implicantions,
most techniques call for resection of upper tibia at 90 deg to its long axis;
- valgus errors are well tolerated while varus errors are not;
- original PCA technique involved making the proximal tibia in 3 deg of varus in relation to its longitudinal axis;
- this means that distal femoral cut must be approx 9 deg of valgus in relation to anatomic axis of femur to have overall mech axis of 0 deg;
- peroneal nerve:
- at the level of tibial bone resection, the common peroneal nerve lies superficial to the lateral head of the gastrocnemius;
- mean distance from the bony posterolateral corner of the tibia to the nerve was 1.49 cm, with no distance less than 0.9 cm.