Clinical outcomes for total knee arthroplasty (TKA) are sensitive to lower extremity alignment, implant positioning, and implant size. Accurate determination of femoral implant size is the focus of this paper. As existing methods (conventional instrumentation, preoperative images, navigation) can be limited by issues including inaccuracy, time required, exposure, and cost, this study assesses a novel method for determining femoral component size using navigation.

We used a commercially available navigation system (Exactech GPS, Blue Ortho, Grenoble, FR, with Total Knee V1.13 software). The system uses surface patches to collect small point clouds, and then computes points that match a given criteria (e.g. the most distal point). For femoral component sizing, the proposed method automatically defines a target area to be digitised on the anterior cortex.

To do this, the surgeon acquires anatomical landmarks (i.e., knee centre, distal condyles, etc.) for all femoral implant parameters but the size. The surgeon then moves the tip of the acquisition instrument near the anterior cortex, and the system computes the distance between the virtual posterior cut and the tip in real time. The theoretical implant size increases in real time as the instrument tip moves anteriorly and decreases as it moves posteriorly. The target area is displayed on the anterior cortex such that it covers all the bone in the medio-lateral direction, is centred on the most proximal part of the theoretical implant in the proximal-distal direction, and covers the current size plus or minus one size. As a result, the target area virtually moves in the proximal-distal direction as the surgeon moves the instrument tip closer to the anterior cortex surface. When the tip is in contact with the anterior surface, acquisition of the point cloud is performed. From a user point of view, the system does not move the target area relative to the bone on the display, but instead adjusts the relative position of the instrument tip, creating the impression that no matter the bone size, the target area does not move and the instrument tip is always guided to the right spot.

The method has been successfully implemented and used on more than 1,400 patients. A preliminary analysis on 189 surgical reports shows in 188 cases (99,5%) the proximal point of the selected implant is inside the target area (which means that the selected size is the one by default, plus or minus one).

We conclude the proposed method as implemented in the Exactech GPS has proven to be clinically effective. It can easily be extended to determination of other points when global criteria can be used to define an optimal area of digitisation determined from previously acquired data.

Major aspects on long-term outcome in Total Knee Arthroplasty are the correct alignment of the implant with the mechanical load axis, the rotational alignment of the components as well as good soft tissue balancing. To reduce the variability of implant alignment and at the same time minimise the invasiveness different computer assisted systems have been introduced.

To achieve accuracy as high as those of a robotic system but with a pure mechanically adjustable cutting block, the Exactech GPS system has been developed. The new concept comprises a seamlessly planning and navigation screen with an integrated optical tracking system for fast and accurate acquisition and verification of anatomical landmarks within the sterile field as well as a tiny cutting guide for accurate transfer of the planned bone resections.

Using a conventional screwdriver the cutting block could be accurately aligned with the planned resection by controlling the current position of the cutting block on the navigation screen. To save time, to maximise the ease of use and to minimize the surgeon's mental workload during adjustment, a smart screwdriver (SSD) has been developed being able to automatically adjust the screws.

The basic idea of the smart screwdriver is to have a system providing an automatic transfer of the planned data to the cutting guide similar to a robotic system, but with the actuators separated from the kinematic. The use of the SSD is as simple as follows: After planning of the intervention and rigid fixation of the cutting guide on the bone, the surgeon simply connects sequentially the screwdriver to all screws of the cutting guide.

To further maximise the ease of use and to avoid a mix-up of different screws, an identification means has been integrated into the positioning screws as well as into the smart screwdriver. For an automated identification of the screws different technologies have been analysed as position tracking, optical recognition or wired/wireless electronics.

A first prototype without screw identification has been used successfully on 4 cadaver knees. All guide positions could be adjusted automatically using the SSD. However, the absence of screw identification required that the surgeon follows indications given by the computer to turn screws sequentially.

A second prototype of the smart screwdriver has successfully been built up and is able to identify the different positioning screws in less than 1s with high reliability. The identification is realised as inductive coupling of different small resonance circuits that are integrated into the screw heads and the screwdrivers tip.

To adjust the cutting guide from neutral to the planned position, the screws have to be adjusted by 5 mm in average. The rotational speed of the current SSD implementation is 2 rounds per second, resulting in a mean time of about 3.5 s for each screw adjustment. The rotational accuracy of the screwdriver is ±5°. Taking into account a thread of the positioning screws of 0.7 mm, the theoretical translational error is about ±0.01 mm. Looking at the angular accuracy, the maximum distance of the screws of the current setup of the cutting block of 15 mm results in an angular error of less than ±0.05°.