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The need to reduce ionising radiation absorption from the medical teams during orthopaedic surgeries has induced different firms to propose navigation systems allowing to visualize in Virtual Reality the instrument position with respect to the patient's body structure, on the base of radiographic or CAT initial images. These systems however, particularly suitable to the orthopaedic field, if on the one side supply the doctor a full vision of the operatory scene, eventually indicating on the screen the operations to be performed, on the other side do not supply any physical support to the doctor, that must in any event operate freely to reach the desired objectives. And this may be extremely difficult, since it is in any event necessary to position correctly the instruments, adjusting in the mean time the six degrees of freedom of an object in space. To overcome this problem operatory robots have been proposed that, on the basis of operatory preplanning, do substitute the doctor in operating the necessary resections, needed, for instance, to install a prosthesis. Now it is firm idea of the authors of this patent that the doctor should never be substituted by an equipment. One thing is to suggest, another is to perform a surgery on an human being. Only a doctor may have the experience and sensitivity to understand if a given operation, planned preoperatively on the base of images, is indeed to be performed. This originated the idea to develop a new equipment able to join the two characteristics of Navigator and Robot, changing from one form to another on doctor's request. However, while in the PCT application PCT/IT03/00322 of 27/05/2003, published by WIPO on December 4th 2003, then transformed into European Patent application EP03730474.8, reference was made only to a system able to compensate the weight acting on the suspended arms after the hinge, transmitting a variable torque that takes into account the actual position of the linkages, in order to allow the operator to move without feeling the weight of the kinematical chain supporting the operatory tools, proposing also to shift from different modes through the insertion/detachment of a system of frontal teeth, in the present application a few important novelties are inserted. First, should the self-balanced kinematic chain present some vertical axes hinges, or if the weight suspended to the structure following is small, then is no longer necessary to provide an active transmission of the torque through the hinge, thus is not necessary to use springs whose tension must continuously updated to connect the two arms of the joint, but is possible to leave it free to rotate, connecting it to the motor only when it must work in Robot mode. In particular the configuration chosen is similar to that presented in patent application EPl 109497, but differs strongly since that system was a purely passive one, while Navi-Robot is able to pass from passive to active mode and viceversa. Secondly, to avoid slacks always present in front dentures, and to allow blockage in any position, without having to use too high locking forces, specially designed drum brakes, producing blockage thanks to an enhancement of their self activation characteristics, have been used. In presence then of prismatic joints, for which it is also necessary to provide a transition to Robot mode, this may be obtained using a long screw parallel to the joint axis, whose rotational position is controlled by a motor, and by a female screw divided in two halves, that can be closed in robot mode taking care first of rotating the screw, whose angular position is always controlled, as it is the joint's vertical position, in order to close the female screw correctly with respect to the threads of the screw. Naturally a prismatic joint may be counterweighted so as to almost nullify the forces needed to move it vertically.
In order to further simplify the operation of the device, the portion of support of the surgical instrument, previously considered, and provided with further degrees of freedom that could be freed upon doctor's command, has been eliminated and substituted with simple cutting or drilling masks or simple probes, that must be sterilizable. As must be sterilizable also an intermediate element, positioned between the last link and the interchangeable masks, in order to allow substituting the end element without loosing sterility, while the Navi-Robot structure will have to be covered by sterile drapes.
As far as the joints having horizontal axis able to transmit balancing torques, a new hinge configuration is proposed, in which motor and torsion spring are hosted inside the proximal link, while the torques generated are transmitted to the distal link through a couple of bevel gears, in substitution to the endless screw and worm gear used previously. In this case two more brakes are added, one to block motor and reducer, the other to allow direct transmission between motor and distal link, bypassing the torsion spring and eliminating the elastic component. A third brake allows blocking the device configuration, once reached the correct position for the surgical act, in order to eliminate possibile errors caused by the force applied by the doctor.
The use of a unique system of reference is also introduced between patient and device, for every bone to be "treated". Such system may be similar to what described in Patent EP0783276, and consists essentially in a pair of pins to be inserted percutaneously in the bone to be treated. One of the pins may present on the base an enlargement or decrease in section, such to allow positioning in a unique way a special clamp, to be applied on the patient before detecting any X ray or CAT scanning, to be utilized in the pre-pralling surgical phase. The clamp presents also surfaces or reference pins allowing to connect it in a unique way to the Navi-Robot, as well as three spheres of known position with respect to the clamp itself, sufficiently detached in order to allow the unique identification of the position of the patient's bone structure with respect to the clamp, and hence the unique reference to the Navi-Robot.
More arms, presenting a Navi-Robot structure, or a simple Navigator structure (a measuring device) may be added to the first Navi-Robot arm of the device. They must be in any event lockable in any configuration with brakes, that must be applied also on any prismatic joint, for which a solution is also proposed.
Possible applications, in self balancing measuring mode, is the measure of the relative positions in space, in order to reconstruct in Virtual Reality and real time of the surgical

80 field, to substitute fluoroscopy, in robot mode, to be requested by the operator, positioning the equipment to allow, for instance, a precise bone resection by the doctor, that will be allowed to move the saw only in the plane indicated by the robot, positioning the cutting mask in the right place. Plane that the doctor will be able to modify, should he observe, during the surgery, that this modification is necessary. Differently from surgical robot, that

85 tend to substitute the doctor, this is an assistant, that participates upon request to stabilize the operations, never to substitute the doctor, that is and must remain the only responsible of the surgery.
But the applications of these instruments are not exhausted by the orthopaedic sector, nor by the medical, but can be applied every time is useful a measuring device having at least

90 three degrees of freedom, able to assume, upon command, a given precision position.
Description of the preferred embodiment.
The preferred embodiment of aNavi-Robot is made by at least one self-balanced kinematic chain having at least five degrees of freedom, and able both to block itself in any given position, and to assume a given position, always compatible with its work space.
95 Two the possible configurations of the kinematic chain, of which the first presents a first hinge monitored by an encoder, having vertical axis (1 of Figure 1), on which is mounted, through a second rigid link, a second hinge (2) having horizontal axis, and whose position is again monitored by an encoder, from which a third link departs, at the end of which a third hinge (3) bearing a measuring encoder, having axis horizontal and parallel to the

100 second, from which a fourth link departs, at the end of which a fourth hinge (4) equipped with its encoder and having axis parallel to the last two hinges. This fourth hinge is also the first hinge of the "wrist" of the measuring device, made by two more short links and relative hinges, of which the first (5) presents axis on a plane parallel or passing through the axis of the first encoder, followed by the last hinge (6), having axis, in extended

105 position, perpendicular to the last two being coaxial to the sixth link. Presence of a possible rotating counterweight placed on the first link, that could be mobile on an horizontal rail, in order to balance the weight of the arm as a function of the spatial configuration assumed. The idea of the system is in fact to balance the weight of the structure transmitting to the floor the system weight. Figure 1 shows the first scheme of constraints

110 described.
The second possible kinematic chain of a Navi-Robot derives from that of a SCARA robot, mounted on a first prismatic joint having vertical axis whose weight is balanced by a weight positioned on a second vertical prismatic joint. At the free extremity of the linkage the "wrist" is placed having three axes mutually perpendicular, the first vertical, followd by

115 two horizontal axes mutually perpendicular. Figure 2 shows the second scheme of constraints described.
Note also that, in order to precision position the surgeon's instrument with respect to the patient's body during the surgery, and thus also the correct Virtual Reality representation of the procedure, the system must be locked to the patient through a special reference clamp,

120 as well as to the surgical instrument, not being possible in any event to transfer its weight on the doctor nor, even more obviously, on the patient. In order to obtain this, at least two kinematic chains should be used, unless the total immobility of the patient could allow a self referencing system, utilizing hence a single kinematic chain. But this is unlikely. In particular the referencing clamp (Figure 3) must present an unique identification system

125 both in the image plane (for instance three spheres of known position) and with respect to Navi-Robot.
Passing to the description of the typical joints allowing the Navi-Robot behavior, let us start with the one having horizontal axis, thus allowing torque transmission. It is presented in Figure 4, where the proximal (7) and distal (8) link, whose relative position is monitored

130 by an encoder (9), while motor (10), reducer (11) and possible dedicated encoder transmit motion through a couple of bevel gears, of which the driven is fixed to the distal link (8), while the driving il coupled to the torsion spring (14), fixed on the other extreme to the motor. Three brakes are present, one blocking the relative link position (12), the second blocking the motor (15), not to be forced to keep it on all the time, and the third causing

135 direct power transmission from the motor, bypassing the torque spring.
In the case of hinges having vertical axis, with links aligned, presented in Figure 5, note the presence of only two brakes, one (12), placed under the upper encoder, to block the relative link position (7 and 8), the other (13), placed in diametrically opposed position, connecting the reducer frame to the proximal link (7), while the output shaft of the reducer in always

140 , fixed to the distal link (8). Blocking this brake, transition to Robot mode is obtained.
Passing to the case of vertical axis hinges, whose distal link axis is inclined by 90°, (Figure 6), note that, differently from the previous case, the motor-reducer group is also rotated by 90°, while mechanical transmission to the following link is ensured by bevel gears.
Figure 7 shows a drum brake characterized by tiny shoes (16), activated by solenoids (17),

145 used to allow blockage, while the following Figure 8 shows a different type. Both brakes produce blockage, thanks to the fact that the shoe hinge is positioned very internally, which increments the moment produced by the friction forces, till the condition in which the breaking equilibrium force becomes negative, causing self-blockage, even with relatively low friction coefficients (0,35 in this case). Also notice that blockage occurs in both

150 directions of rotation, being in the first case caused by one shoe only, the left when rotating clockwise, while in the second case, being the hinge a cam, both shoes oppose rotation. Their activation them may be assigned to solenoids, or, more effectively, to electric motors (18) coupled with reducers and endless screw acting on cams (19) for final actuation.
Figure 9 shows the mechanism for blocking and transmitting motion of the vertical joint,

155 actuated by solenoids or electric motors as in the previous case, in which note two halves female screws (20), the step motor placed above the screw, and the actuation system.
Figure 10 shows instead the linear vertical brake, in which two L shaped shoes (16), computed according to the Reye assumption on wear, to block the joint. Also in this case the locking action is obtained simply bringing in contact the shoes with the lateral joint

160 surfaces.
Figure 11 shows a possible connecting element to the Navi-Robot (21), a drilling mask (22), two cutting masks, for minimally invasive (23 a) and traditional (23 b) surgery, and finally a measuring probe (24), all sterilizable and automatically recognizable by Navi- Robot. Finally Figure 12 shows a possible configuration of Navi-Robot for Orthopaedy,

165 where (25) indicates the active / passive arm, while (26) indicate the passive ones.
It is worth noting, as already underlined in the previous patent applications, that the object of the patent is an actively or passively self balancing device, allowing real time measurements of the end effectors position with respect to a given reference, able also to become a robot, and to lock itself in any configuration, starting from both Navigator or

170 Robot mode, without needing to pass through the other mode.
Only with such an instrument the doctor may freely move the instrument, characterized by the self balancing system, in proximity of the cutting zone, as indicated by the Augmented Reality representation, and require the transition in Robot mode, that, locking the joints, will assume the correct position for cutting or drilling through the suitable mask, and

175 operate in total safety, being also able to order the system to modify the cutting plane by a given amount, if this would seem necessary to him, knowing that the system will take that into account in all following operations, including the prosthesis component's choice. The same instrument can also be used with a single 6 DOF kinematic chain, for dimensional controls for the industry, eventually eliminating the locking brakes. Self balancing characteristics and possibility to pass in Robot mode would remain, peculiarities that distinguish this device from any other present on the market.