Computer Platform

The most basic component of a computer navigation system is the computer on which the system relies for coordination of inputs from the surgical field, mathematical interpretation of the datasets, and display of the resultant information on a monitor. Currently utilized systems require hardware capable of a robust, real-time calculation. Commonly, this results in pairing of powerful microprocessors and software platforms based on Windows or Linux systems. These base operating systems are considered more responsive and stable for the use of mission critical applications. The measurement system is designed in such a fashion that the three-dimensional position of objects or targets in the operative field can be determined with low rates of error—much as a global positioning satellite system would function. Computer platforms may be considered closed, or proprietary, if the navigation provides support limited to a specific implant system or surgical technique. The other possibility is an open system that is more general and allows, for example, a software protocol to support the implantation of total knee implants from different manufacturers. The advantage of a proprietary system is that more elaborate representations are usually supported such that virtual implant sizing to a virtual reconstruction of the patient’s anatomy can be performed.

A typical capital system will have a rolling cart with computer, keyboard, mouse, LCD monitor, foot pedal activator, and optical tracking camera. The optical camera may be placed on a boom or a separate tower to allow placement in its appropriate position during the operative procedure. The optical camera system typically will have two charged-coupled device (CCD) receivers that will pick up laser impulses from an active tracker or reflected beam from passive balls attached to a passive tracker. Portable options have been developed that allow similar hardware and applications but with smaller desktop computers and microcameras or tracking devices that may be quickly assembled from a suitcase. This allows manufacturers’ representatives to conveniently bring in a full navigation system for limited one-time use as a service that may be purchased by the hospital. This option is a great opportunity for the surgeon who is new to computer navigation and who may not be committed to asking his hospital to make the several hundred thousand dollar (U.S.) purchase of a capital system. It is an excellent option for low-volume surgeons and hospitals that may not have resources to afford the large investment for more permanent systems, in that it provides similar technology to perform computer-aided surgery.

Tracking Technologies

An important element of any computer navigation system is the mechanism or technology chosen to track the target or object. The basic elements are trackers that may be attached to the patient’s bones or surgical instruments. These trackers are then used in an environment that consists of a camera, electromagnetic coil, or ultrasonic probe that will pick up laser or electromagnetic pulses that originate from the trackers. Recently, video monitoring has been added as a “real-time” tracking option, but this method has not reached significant clinical application.

Optical tracking systems require two or three CCD cameras to pick up laser impulses from the trackers that are recognized by a minimum of three and possibly four or five active emitters or passive reflective balls. The computer calculates the three-dimensional position of the trackers based on recognition of the spatial footprint of the tracker emitters. The footprint of each tracker is unique and allows differentiation of bones, instruments, and implants. Conventional optical cameras function by being placed 6 to 8 feet from the object trackers, and must have an unobstructed “line of sight” to the trackers. This requires that operating personnel must be aware of this relationship, but if positioning is optimal, staff readily adapt to the requirement. Clinical validation studies of optical tracking systems have demonstrated very high reliability and accuracy with a typical translational error of 0.25 mm.^23,35,52 This absolute measurement error increases trigonometrically with increasing distance from the camera.⁴³

Electromagnetic (EM) tracking relies on small trackers that create an electromagnetic impulse that is recognized by an electromagnetic coil placed 20 to 30 inches away.²⁹ These trackers may be placed inside the wound but require small wires that go directly to the computer system for activation. The magnetic coil then measures interference created by the tracker as it moves within the electromagnetic field. The disadvantage of current electromagnetic tracking systems is distortion of the field created by ferrous metals that are inherently magnetic, but any metal such as brass and copper and even nonmetals such as Kevlar may interfere. Current computer algorithms have been calibrated to shut down the system if distortion is recognized. However, the EM approach still remains vulnerable to many other potentially distorting fields that are found in the typical operating room. Clinical validation studies have identified this problem, and although the electromagnetic system seems to perform with the precision of current optical systems at the 0.5 mm level, the occasional outlier may be off by several degrees, which makes this method less reliable.^1,11,13,24

Referencing Methods

For the surgeon, referencing of target objects is the most significant problem and requires a thorough knowledge of both the technology and the desired anatomic points to be matched on the virtual computer model. The process basically is to define points in space with a tracker that can be triangulated by the tracking system. For surgical instruments, a referencing tool allows the surgeon to capture the “marked” instrument such as a pointer probe. The precision of the instrument will be within 250 to 500 µ of error.²³ Imageless referencing is possible if the targeted objects are directly visible, and is most applicable in TKA. Numerous studies have demonstrated the efficacy of imageless referencing as compared with conventional instrumentation for TKA, but these results depend on the expertise of the surgeon to choose the correct reference points.¹⁹ Computer algorithms are written with the assumption that the ideal point will be selected. For example, referencing of one universal total knee protocol prescribes that the femoral center is chosen as a point that is under the roof of the intercondylar notch and lies on both the transepicondylar line and the anteroposterior axis of Whiteside. Deviation from this “ideal” point adds error.

Kinematic referencing in TKA as pioneered by Lavallee has been a novel innovation for determining the center of the hip and ankle joints, markedly simplifying this procedure.³⁸ Because the hip joint is not directly visible, a method was needed by which to accurately reference the hip center. This was accomplished by tracking the femur with the optical camera as the femur was rotated in a circular motion. The movement of the tracker described the base of a cone, which when projected to its zenith closely approximates the center of the hip joint. The computer algorithm calculates the root mean square error or the standard deviation, which must fall within a limited range for the computer to accept the hip center reference point.

A secondary method of referencing is bone morphing, which involves selecting hundreds of surface match points by “painting” the bone with the pointer probe.^46,47 This method does not require the segmented three-dimensional model typical of computed tomography (CT) but uses a virtual model that is then constructed by the computer algorithm. The virtual image created allows enhanced capabilities such as prosthetic sizing, “live” bone resection, and kinematic assessment. However, this additional technology usually adds time and complexity to the operative procedure and may limit the surgeon’s choice of prosthetic implants. One current morph technology is to limit the referenced areas to small patches. For example, surgeons find difficulty referencing the femoral posterior condyle in finding the most posterior or dorsal position. This process can be simplified and made much more precise by morphing a small patch, allowing the computer to find the optimum position.

Ultrasound image capture is a newer method of referencing that is evolving as a potential technique whereby multimodal referencing is performed.^8,26 Depending on frequency and the acoustical properties of the object, point localization is accurate to submillimeter levels on the order of 0.25 to 0.75 mm with ultrasound, whether it is 2-dimensional, 2.5-dimensional, or 3-dimensional in the modality of image capture. Segmentation is possible when this image may be matched with a preoperative CT image or even an intraoperative “bone morphed” image. However, the clinical applications remain limited for a variety of reasons. Definition of baseline anatomic points is difficult with ultrasound, which creates an error on the order of 2 to 5 mm, which is unacceptable for clinical practice. However, the promise of ultrasound is that it can be done through the tissues intraoperatively without the need for skin incision or radiation exposure.