Patellofemoral arthroplasty (PFA) is a delicate and challenging procedure. A PFA application has been developed for the Navio semi-active robotic platform (“NavioPFA”), to facilitate both planning and bone preparation. NavioPFA combines image-free navigation and planning with robotically assisted bone shaping, and is open to any implant design, provided that the feasibility and accuracy is confirmed in sawbones and cadaver tests. In this abstract we describe the accuracy tests of NavioPFA, with results for four different implant designs. The accuracy of prosthesis placement with Navio is evaluated by independent measurements that compare the final placement to the planned position.

NavioPFS™ is a hand-held robotic technology for bone shaping that employs computer control of a high-speed bone drill. There are two control modes – one based on control of exposure of the cutting bur and another based on the control of the speed of the cutting bur. The unicondylar knee replacement (UKR) application uses the image-free approach in which a mix of direct and kinematic referencing is used to define all parameters relevant for planning. After the bone cutting plan is generated, the user freely moves the NavioPFS handpiece over the bone surface, and carves out the parts of the bone targeted for removal. The real-time control loop controls the depth or speed of cut, thus resulting in the planned bone preparation. This experiment evaluates the accuracy of bone preparation and implant placement on cadaveric knees in a simulated clinical setting.

Three operators performed medial UKR on two cadaver specimens (4 knees) using a proprietary implant design that takes advantage of the NavioPFS approach. In order to measure the placement of components, each component included a set of 8 conical divots in predetermined locations. To establish a shared reference frame, a set of four fiducial screws is inserted in each bone. All bones were cut using a 5 mm spherical bur. Exposure Control was the primary mode of operation for both condylar cuts – although the users utilised Speed Control to perform some of the more posterior burring activities and to prepare the peg holes. Postoperatively, positions of conical divots on the femoral and tibial implants and on the respective four fiducial screws were measured using a Microscribe digitising arm in order to compare the final and the planned implant position.

All implants were placed within 1.5 mm of target position in any particular direction. Maximum translation error was 1.31 mm. Maximum rotational error was 1.90 degrees on a femoral and 3.26 degrees on a tibial component. RMS error over all components was 0.69mm/1.23 degrees.

This is the first report of the performance of the NavioPFS system under clinical conditions. Although preliminary, the results are overall in accordance with previous sawbones studies and with the reports from comparable semi-active robotic systems that use real time control loop to control the cutting performance.

The use of NavioPFS in UKR eliminates the need for conventional instrumentation and allows access to the bone through a reduced incision. By leveraging the surgeon's skill in manipulating soft tissues and actively optimising the tool's access to the bone, combined with the precision and reproducibility of the robotic control of bone cutting, we expect to make UKR surgery available to a wider patient population with isolated medial osteoarthritis that might otherwise receive a total knee replacement. In addition to accurate bone shaping with a handheld robotically controlled tool, NavioPFS system for UKR incorporates a CT-free planning system. This approach combines the practical advantages of not requiring pre-operative medical images, while still accurately gathering all key information, both geometric and kinematic, necessary for UKR planning.

Introduction

Precision Freehand Sculpting(PFS), is a hand-held semi-active robotic technology for bone shaping that works within the surgical navigation framework. PFS can alternate between two control modes – one based on control of exposure of the cutting bur (“Exposure Control”) and another based on the control of the speed of the cutting bur (“Speed Control”). In this study we evaluate the performance of PFS in preparing the femoral bone surface for unicondylar knee replacement (UKR).

Surgical navigation, coupled with preoperative plans, allows surgeons to plan and execute procedures to improve the likelihood of positive outcomes. In real life these navigation systems, which track both the patient and the surgical tools, are not absolutely accurate. Therefore, there is a need to know how much error there may be in the navigation system, so that the surgeon can assess the effects of possible errors in positioning.

The methodology for assessing the accuracy of a surgical navigation system is similar across surgical specialties. We developed a framework for assessing the accuracy of the HipNav system, a computer assisted surgical system used for planning and intra-operative surgical navigation for total hip arthroplasty. This framework can be adapted to other systems and surgical procedures. To assess navigational accuracy, we compared acquired values to a ground-truth model: rigid plastic Sawbones pelvii with mounted fiducials and acetabular implants, whose positions were measured with a coordinate measuring machine. We then identified the individual components of the system that can contribute to overall accuracy, and characterised their contributions to the accuracy of the system. We also measured the end-to-end accuracy of the HipNav system, from initial CT scan through to acetabular cup orientation. This value is of direct importance to the practicing surgeon, and indicates how far off the final measured orientation of the cup may be from its actual location. For the HipNav system, we found that the end-to-end square root of the mean square error was 0.82° in abduction and 0.76° in version.

The accuracy of a surgical navigation system is of vital importance to insure that a preoperative plan is executed properly. To measure the accuracy of a navigation system, accurate models that reflect the relevant anatomy are necessary, and allow true measurement of end-to-end and component accuracy. This example shows how the accuracy of HipNav was assessed, and that the final orientation of the acetabular implant was accurately guided.