INTRA-OPERATIVE ASSESSMENT OF THE SOFT TISSUE ENVELOPE IS INTEGRAL TO THE PLANNING OF UKA COMPONENTS

Introduction Recent advances in 3D printing enable the use of custom patient-specific instruments to place drill guides and cutting slots for knee replacement surgery. However, such techniques limit the ability to intra-operatively adjust an implant plan based on soft-tissue tension and/or joint pathology observed in the operating room, e.g. cruciate ligament integrity. It is hypothesized that given the opportunity, a skilled surgeon will make intra-operative adjustments based on intra-operative information not captured by the hard tissue anatomy reconstructed from a pre-operative CT scan or standing x-ray. For example, tibiofemoral implant gaps measured intra-operatively are an indication of soft-tissue tension in the patient9s knee, and may influence a surgeon to adjust implant position, orientation or size. This study investigates the frequency and magnitude of intra-operative adjustments from a single orthopedic surgeon during 38 unicondylar knee arthroplasty (UKA) cases. Methods For each patient, a pre-operative plan was created based on the bony anatomy reconstructed from the pre-operative CT. This plan is analogous to a plan created with patient-specific cutting blocks or customized implants. With robotic technology that utilizes pre-operative imaging, intra-operative navigation and robotic execution, this “anatomic” plan can be fine-tuned and adjusted based on the soft tissue envelop measured intra-operatively. The relative positions of the femur and the tibia are measured intra-operatively under a valgus load (for medial UKA, varus load for lateral UKA) for each patient from extension to deep knee flexion and used to compute the predicted space between the implants (gaps) throughout flexion. The planned position, orientation and size of the components can then be adjusted to achieve an optimal dynamic ligament balance prior to any bony cuts. This is the plan that is then executed under robotic guidance. Intra-operative adjustments are defined as any size, position or orientation changes occurring intra-operatively to the pre-operative anatomic plan. Results The surgeon adjusted the pre-operative implant plan in 86.8% of cases, leading to combined RMS changes of 2.0 mm and 2.1 degrees to the femoral implant, and 0.9 mm and 1.4 degrees to the tibial implant. The RMS femoral implant translations and rotations were 1.0, 1.5, 0.9 mm and 1.0, 1.0, 1.7 degrees in the medial, anterior, and superior directions, respectively. The RMS tibial implant translations and rotations were 0.2, 0.4, 0.8 mm and 1.3, 0.4, 0.6 degrees in the medial, anterior, and superior directions, respectively. Implant sizes were adjusted in 36.8% of cases, with all changes occuring to the femoral implant, and 13 out of those 14 cases showing a reduction in the femoral implant size. Conclusions These data support the hypothesis that surgical planning of UKA components based on accurate 3D dimensional reconstructions of anatomy alone is not adequate to create optimal implant gap spacing throughout flexion. Measurement and knowledge of the patient9s soft tissue envelope allows for signficiant changes to the implant plan prior to any bony cuts.