Active type uses computer systems in all or part of the preparation and operative procedure and the robot system is included in this type. In the robot system, the robot performs the surgery instead of the surgeon and the accuracy is very high. It was used limitedly in the early stage of computer assisted surgery, but it has been transformed into the modern robot assisted surgery since the development of the navigation system.

The robot can perform osteotomy and soft tissue release without any injury to the ligaments, nerves or vessels. Robot systems include the Robodoc and Caspar systems, and these systems perform concise, clear, accurate and effective three-dimensional operations (Fig. 10.1). Also, some of the surgical procedures such as intramedullary drilling can be avoided just like in the navigation surgery. It helps in cementless prosthesis due to an accurate osteotomy as it mills the surface without sawing.

Robot assisted surgery comprises of the pin type or the pinless type and preoperative CT is required for both types. In the pin type, a stab wound is made under local anesthesia before the operation and a self-tapping screw is fixed onto the pelvic bone, femur and tibia. These screws act as indicators of spatial orientation of the patient’s real bone for three dimensional verification. In the more recently developed pinless type, the process of inserting the pins before surgery is bypassed. Patient information based on the CT images including the position of the joint, size of the tibia and femur, and degree of deformity is stored in the computer, which enables to perform cyber surgery and to plan the operation. During the operation, the robot and the knee should be securely fixed to prevent micro-motion. The actual shape of the patient’s joint and bone is matched with the information already stored in the computer using a probe attached to the robotic system. This process of registration and verification is the most important step in robotic surgery. Once the registration is completed, the robot mills the bone finely and accurately with an error of less than 0.25 mm as planned during the preoperative cyber surgery, and then the pre-selected prosthesis is inserted to complete the operation (Fig. 10.2).

However, the robot system requires preoperative imaging and registration, it is bulky, it may hinder the normal operation process, and it is too time-consuming and expensive. A learning curve is also needed for using the robot system.

Saragaglia et al. reported that accuracy with less than 3° outliers can be achieved in 75 % of patients when the computer was not used, in 84 % of patients when the computer was used and in more than 95 % of patients when the robot system was used. Park and Lee reported the comparison between the robot system and the conventional method, which suggested that the robot system was clearly useful in terms of preoperative planning, accuracy and postoperative alignment, but the incidence of complications such as fracture or peroneal nerve palsy was higher in the early period of this technique.

Surgery is performed by both the surgeon and the robot; the robot guides the surgeon and controls the surgeon when osteotomy exceeds the normal range, or the surgeon can move the robot’s arms. The Acrobot can be classified into the active system or the semi-active system and it also requires preoperative imaging and registration during surgery.

Surgery is performed solely by the surgeon. The passive system includes the navigation system and the patient specific instrument (PSI) or the patient-specific templating system. The navigation system is used to guide the operation and it is more recommended by the US FDA.

In arthroplasty, navigation refers to registering the patient information in the computer in order to guide the performance of an accurate operation. Kinematic registration by joint motion detects the center of the joint and a probe is used for surface registration of the size and shape of the bone. This process simulates the real shape of the patient’s bone based on the registered information in order to secure accurate positioning of surgical instruments for osteotomy.

The patient-specific templating system, which is still in its toddler stage, uses preoperative CT or MRI. MRI has the theoretical advantages of detecting the cartilage and of being a radiation free imaging modality. However, MRI images have more segmentation difficulties than CT images, which may be the cause of malpositioning of the template. There are two methods of patient-specific templating system: one is the pin placement technique and the other is the cutting guide technique. In the pin placement technique, a preoperative CT or MRI is used to create the positioning guide in order to locate the position of the pin for osteotomy and the same conventional cutting jig is used (Fig. 10.3 ). In the cutting guide technique, the surgeon uses the custom made guides to perform the osteotomy without using the conventional cutting guide. There are two aligning method in PSI system: one is kinematic alignment and the other is mechanical alignment method. Mechanical alignment method needs detecting the center of hip, knee and ankle joint on CT or MRI to restore mechanical alignment. The MRI or CT of the joint is needed and release of collateral ligaments and retinacular ligament are not performed in kinematic alignment system. The concept of kinematic alignment, is quite different from that of conventional mechanical alignment. The purpose of kinematic alignment is to restore pre-arthritic state to improve clinical result. In case of mechanical alignment method, just similar to conventional instrument method, Nunley et al. experienced more varus outliers than the kinematic alignment method. During the operation, osteophytes should not be removed because they may be used as a reference for positioning the cutting guides. The femoral guides are usually used first to make more space for the tibial cutting guide. So, aligning the joint along the mechanical axis is just a reference. This is described in details under Sect. 4.​4.

The advantage of the patient-specific templating system is that the technique is relatively easier to use, less invasive, time saving and accurate. The drawback of this system is that a longer time is required to create the positioning guide, it does not usually provide the intraoperative measurements, lack of information for soft tissue balancing, more chances of poor positioning of the guides due to an MRI alignment error, and the surgeon should abandon the system in case the operation goes wrong. While the navigation system is universally used for all patients, the patient-specific templating system is based on the skeletal information of each patient.

Image-based navigation system is used for osteotomy based on the information acquired from CT scan or fluoroscopy. In this system, either the information from the preoperative CT scan is registered or fluoroscopy is used during the operation to acquire the information (Fig. 10.4).

During the registration, the information acquired from the CT scan or fluoroscopy is stored in the computer system, and the kinematic information of the joint motion and bony information are matched with the input data (kinematic registration) (Fig. 10.5).

Kinematic registration is followed by surface registration which is obtained from the probe during the operation, and these are used to determine the position and direction of the cutting guide in real time for an accurate osteotomy. Therefore, registration is the most important step and the software is also as important as the hardware. If the registration is incorrect, the operation becomes inaccurate. The causes of incorrect registration are the presence of borderline blurs and when segmentation of images occurs. The operation also tends to be inaccurate when the soft tissues impinge between the probe and the bone, when the probe points to a different site because the bone is not perfectly spherical, the cortical screw that fixes the rigid body moves and the computer hardware and software are inaccurate.

The method of using fluoroscopy is the automatic registration system through fluoroscopy during the operation. This method is not convenient to perform the TKA as it requires the use of fluoroscopy. It is more helpful to insert pins in fracture surgery than in arthroplasty.

Image-based system helps in the preoperative planning and postoperative evaluation, but it has been replaced by the image-free system because the image-based system is more time-consuming and it requires more effort since the acquisition of images before or during the operation is required.

Image-free system does not need preoperative CT imaging or intraoperative fluoroscopy and it incorporates the standard cutting blocks and cutting guides from a preexisting instrument. Currently, the image-free method for acquiring critical anatomic information has proved to be the most effective method for performing TKA. In this system, the standard skeletal structure is used as a basis to track and register the shape, position and axis of patient’s anatomy during the operation, and an osteotomy is guided by the navigation system. It identifies critical anatomic landmarks through both kinematic and surface registration.

For this method, a tracking camera and tracking marker (tracker) are required and this tracker is attached to the bone by a pin or a clamp (rigid body), which is fixed at more than three sites. Other trackers are attached to the surgical equipments and probe to guide the operation. Also, the accuracy can be verified before performing the osteotomy in order to correct malalignment. This system includes an optical system using the infrared light and an electromagnetic system using the electromagnets, and the method using the ultrasound is also being developed.