Types of femoroacetabular impingement (FAI). (a) Normal shaped hip joint, (b) cam type has aspheric feature at the head-neck junction, (c) pincer type has over-covered femoral head because of protrusion at acetabulum, and (d) mixed type has bump at the head-neck junction and protrusion at acetabulum.
Bone-mounted navigation system for FAI. (a) User interface show the in the three-dimensional model of femur and the position of the surgical tool. (b) Bone mounted measurement device is linked to the femur. It is used to obtain a number of point points at bone surface for registration. It measures the position of the surgical tool.
Components of the bone-mounted measurement device. (a) Serial link structure that consists of five joints with encoder and one passive joint without encoder, (b) bone screw and T-shaped adaptor to link the measurement device and femur, (c) probe to measure points at bone surface and surgical tool position, (d) medical pneumatic milling tool to shave bone, and (e) installed position of the bone-mounted measurement device.
Degrees of freedom of the surgical instruments at the port. The surgical tool has four degrees of freedom. Three degrees of freedom is the tip position (X, Y, Z) and one degree of freedom is rolling motion along the surgical tool's axis.
Structure of the bone-mounted measurement device. It has serial link structure with six degrees of freedom. It has five revolute joints with an encoder (from R1 to R5) and one revolute joint without an encoder (R6). The rotational motion along the R6 joint does not change the position of surgical tool that is attached to the last link.
Workspace at the femoral head-neck junction. The digitizer's workspace is fan shape because of the incision points. L is the distance between the base of the digitizer to the incision port along superior-inferior direction. D is the distance between the base of the digitizer to the incision port along medial-lateral direction. R is the radius of the workspace.
Design of the link length. The dimension of two main link lengths (L1, L2) was decided at the full stretched posture of the digitizer. L3 is the surgical tool length. θ is the angle of the cone-shaped workspace.
Kinematic modeling of bone-mounted measurement device for calibration. The Denavi-Hartenberg parameters was used.
Flowchart of the ICP algorithm. The two main procedures of ICP are obtaining the closest points on the model to the digitized points and calculating the relationship between the corresponding points.
Anatomical points for the estimation of the initial transformation. (a) Three anatomical points at patient's femur. The medial condyle and lateral condyle points can be digitized percutaneously and the head center can be calculated by the sphere fitting of the digitized points at the femoral head area. (b) Three anatomical points are extracted from the three-dimensional model of the femur.
Flowchart of the modified ICP algorithm. Perturbations are added to the parameters of the initial transformation as calculated from anatomical points. The iterative calculation of the ICP is then applied to each starting transformation. Transformation with the minimal mean square error is then done.
Validation of the reconstructed 3D model of the femur. The diameter of markers on 3D model and inserted marker were compared. The diameter of markers on 3D model was calculated using CAD program. The diameter of manufactured marker was measured with digital caliper.
Changing the TRE due to the perturbation of each parameter of the transformation matrix. Transformation parameters are three translational terms (δx,δy,δz) and three rotational terms (δα,δβ,δγ). The translational perturbation along y axis and z axis has a large effect on the target registration error.
Target registration error vs. number of points and perturbation range. Under the same perturbation range condition, the magnitude and range of the TRE were decreased when the number of points was increased. Under the same number of points condition, the TRE was affected by the range of perturbation. From the results, 20 points with 5 mm perturbation show optimal condition for registration.
Position accuracy and precision in navigation systems.
Comparison of target registration errors in different navigation systems.
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