Total knee arthroplasty is a common procedure, and current navigation systems are gradually gaining acceptance for improving surgical accuracy and clinical outcomes. A new navigation system used within the surgical field, iAssist, has demonstrated reproducible accuracy in component alignment. All orientation information is captured by small electronic pods and transmitted via a local wireless network, which directs the surgical workflow automatically to the femoral and tibial resection instruments. This simple and accurate navigation system used completely in the surgical field, without optical trackers or preoperative imaging, seems to be the latest generation of smart instrumentation for total knee arthroplasty.
Key points
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A new novel navigation system used within the surgical field has demonstrated reproducible accuracy in component alignment.
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All orientation information is captured by small electronic pods and transmitted via a local wireless (Wi-Fi) network, which directs the surgical workflow automatically to the femoral and tibial resection instruments.
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The system demonstrates accuracy comparable to optical navigation.
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iAssist provides a novel navigation system for all users. For surgeons who prefer conventional instrumentation, the electronic pods provide intelligence and accuracy to the resection guide, whereas for surgeons who prefer computer navigation, this system provides the same accuracy with simplicity.
Introduction
Over the past years, the effect of alignment on implant survival has been well documented, with its influence on implant survival, component loosening, and clinical outcome scores. Numerous publications have documented the accuracy of computer-assisted navigation with knee arthroplasty, with some early postoperative benefits in recuperation, clinical function, and blood loss.
Total knee arthroplasty navigation systems have been around since 1995 and have taken various forms: image-based navigation, imageless navigation, fluoroscopy-based navigation, electromagnetic navigation, and optical navigation. Adoption of these systems in the overall number of procedures is still limited to approximately 5%, mostly because of the perceived complexity and the additional time required in the use of these systems.
Patient-specific instruments (PSI) appeared around 2006 and have reached a greater use rate than computer navigation. The surgical workflow with PSI is simpler than navigation, but requires preoperative imaging—either magnetic resonance imaging (MRI) or computed tomography (CT)—presurgical planning, and manufacturing of the resection guides. These personalized instruments are attractive, but have intraoperative obstacles that are not infrequently encountered, and therefore deviations from the preoperative plan may be necessary. In addition, once the bone isresected, there is no ability to validate the bone cuts.
More recently, navigation systems have been developed using inertial electronic components, which simplifies the tracking process, especially when compared with optical navigation systems. One of these new navigation systems is iAssist (Zimmer, Inc., Warsaw, IN, USA), which is an alignment system designed with the user interface built into disposable electronic pods that attach onto the femoral and tibial resection instruments. Within these pods are inertial electronic components or gyroscopes that exchange information using a secure local wireless (Wi-Fi) channel. After capture of the necessary data during the procedure, the alignment information is displayed to the surgeon directly on the pods within the surgical field. These pods are attached to either the femoral or tibial resection guides, which then guide the resection at the appropriate angles in both the coronal and sagittal planes. After bone resection, the accuracy of the alignment can be validated, confirming the position of both the femoral and tibial components.
The surgical workflow follows the classic method of femoral and tibia bone resection with each bone resected independently along the mechanical axis. The following section describes the surgical technique for the iAssist system.
Tibial Coordinate System Registration
As with other navigation systems, the coordinate system for each bone must be defined. For the iAssist system, an extramedullary tibial guide with an electronic digitizing pod secured to the tibia will identify the mechanical axis and guide the bone resection. The proximal position of the guide is positioned between the tibial spines in the center of the tibial joint surface. Two spikes secure the guide in this region. The distal portion of the alignment guide is mounted with a self-centering claw, which is positioned on the malleoli and fixes the instrument at the center of the ankle joint. The digitizer is then oriented in line with the medial third of the tibial tuberosity and fixed rigidly in place. Once secure, the alignment guide is positioned along the mechanical axis, as seen in Fig. 1 A . The angular relationship between the electronic pod of the digitizer and the bone reference is registered by the system through 3 movements of the limb: abduction, adduction, and neutral position. This function activates the inertial mechanism and creates the coordinate system required for navigation. Once the information has been registered, the digitizer is removed and the data are transferred wirelessly to the electronic pod on the tibial cutting guide, which has 2 knobs to adjust the varus/valgus alignment and the tibial slope. The electronic pod, viewed entirely within the operative field, accurately reports the data with incremental visual cues, including red lights when out of alignment and green lights when the cutting guide is within the desired alignment and slope (see Fig. 1 B). Once the position of the resection guide is satisfactory, the depth of resection is manually set and the cutting guide is secured to the proximal tibia, which is then cut in the usual fashion. After resection, the alignment and slope of the proximal tibial surface can be validated with an electronic pod. If any adjustment to the resection is needed, it can be performed at this time and the accuracy of the resection rechecked with the electronic validation tool.
Femoral Coordinate System Registration
Similar to most optical navigation systems, the mechanical axis is determined by the anatomically located center of the distal femur and the kinematic determination of the femoral hip center. A line connecting those 2 landmarks then defines the mechanical axis of the femur.
With the iAssist system, a bone spike is impacted into the distal femoral sulcus approximately 10 mm anterior to the posterior cruciate ligament, in the anatomic location of the distal femoral mechanical axis. A sleeve with an electronic pod attached is then inserted over the spike and fixed to the trochlear groove. The femoral hip center is then registered through multiple stop-and-go movements by the surgeon. The system provides an auditory cue when the bone is stopped, signaling the surgeon to proceed to the next movement. Multiple acquisitions are taken and recorded by the electronic pod, and the mechanical axis of the femur is acquired. The distal femoral resection guide with a separate electronic pod is then coupled to the femoral spike. The alignment information from the first pod is transferred wirelessly to the second pod, which guides the resection of the distal femur, including the mechanical axis and flexion. The electronic pod, viewed entirely within the operative field, accurately reports the data with incremental visual cues, including red lights when out of alignment and green lights when the cutting guide is within the desired alignment and flexion ( Fig. 2 ). Once the position of the resection guide is satisfactory, the depth of resection is set and the cutting guide is secured to the distal femur. The femoral spike is then removed and the distal femur is resected in the usual fashion. After resection, the alignment and flexion of the distal femur surface can be validated with an electronic pod. If any adjustments to the resection are needed, these can be performed at this time and the accuracy of the resection rechecked with the electronic validation tool.
Preclinical Validation Study
Preclinical validation was performed with a comparative study on cadaveric specimens using an optical navigation system and iAssist simultaneously. The intent was to compare the values for both the femoral and tibial resections. In addition, the limb alignment was assessed with fluoroscopic images of the whole limb. The mechanical axes were compared on a bone-to-bone basis.
For all optical measurements, a Navitrack (Zimmer, Inc.) system running TKR 3.2 was used. As required with the optical navigation protocol, bone trackers were installed on both the femur and the tibia, and bone registrations performed. The iAssist system was then used as described earlier concurrently with the optical system. The iAssist cut guide was adjusted to position the resection guides in a neutral mechanical axis for both the femur and the tibia. The preselected tibial slope was adjusted to 7° and the femoral flexion angle was preselected to 3°. Before any bone cuts were made, the optical navigation system was used to register the orientation of the cut guide and any discrepancy was noted. After bone cuts were made, iAssist was used to measure and validate the resected bone surfaces, and the optical navigation system was used to measure the same final bone surface; any discrepancies with the iAssist system were noted.
After the validated bone resection, each cadaver knee had a posterior stabilized total knee arthroplasty implanted. Each knee then underwent fluoroscopic measurements with an OEC 9600 C-Arm (General Electric, Fairfield, CT, USA). Multiple fluoroscopic images were taken from the ankle to the head of the femur. Specific software was used to stitch the images together to obtain the equivalent of a long-leg radiograph ( Fig. 3 ). Mechanical axes were computed for the femur and then for the tibia. Discrepancies were recorded and compared with the angles obtained with the optical navigation system.
