Patent Publication Number: US-2009240145-A1

Title: Pressing mechanism and ultrasound diagnostic apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pressing mechanism that applies a pressing force to a target bone in the diagnosis of mechanical properties, and also relates to an ultrasound diagnostic apparatus including the pressing mechanism. 
     2. Description of the Related Art 
     Easy quantitative measurement of mechanical properties is desired for diagnosing bone metabolic diseases such as osteoporosis, judging fracture risk, and quantitatively diagnosing bone union after treatment of bone fracture. The evaluation of mechanical properties of a bone (including strength and elasticity) greatly depends on X-ray photography, but quantitatively diagnosing bone strength by means of X-ray photography is very difficult. X-ray irradiation may also have adverse effects on the human body. 
     On the other hand, to simply evaluate mechanical properties (e.g., bone strength) of a bone, an ultrasound diagnostic apparatus discussed in Japanese Patent Laid-Open Publication No. 2004-298205 measures the shape of a target bone which deforms when a stress is applied, and diagnoses mechanical properties of the bone based on a measurement result. 
     A conventional diagnostic apparatus includes a dedicated pressing mechanism that can control a load value applied to a target bone and a position where the load is added, in the diagnosis of mechanical properties. For example, a conventional pressing mechanism includes a rigid pressing member which can be brought into contact with a surface of a human body which includes a bone to be pressed. The pressing mechanism is configured to control a load value applied to the bone by adjusting an advancing amount of the pressing member relative to the body. 
     A user can evaluate bone union of a fractured bone if mechanical properties of the bone are obtainable through the above-described diagnosis. In other words, the diagnosis of mechanical properties can be preferably executed for a fractured bone. In general, a plaster or an external fixation device can tightly hold a fractured bone. The external fixation device includes a plurality of screws and pins which are partly inserted into a human body to fix the bone in position. The external fixation device is fixed to the body in a stationary manner unless the fractured bone is completely cured. 
     In a state where an external fixation device is attached around a target body, a user cannot smoothly perform the ultrasonic mechanical property diagnosis on a fractured bone. Namely, if an external fixation device is attached around a target body, the above-described pressing mechanism of the diagnostic apparatus interferes with metal fittings of the external fixation device in the vicinity of the body. As a result, a user cannot apply an appropriate load to the fractured bone. Furthermore, regardless of the presence of an external fixation device, a conventional pressing mechanism is unable to apply an appropriate pressing force to a target bone if the type of diagnosis object is inappropriate, or if another method or device for curing a fractured bone is employed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a pressing mechanism capable of accurately pressing a predetermined pressing position, and an ultrasound diagnostic apparatus capable of appropriately performing diagnosis of mechanical properties. 
     According to an aspect of the present invention, a pressing mechanism applies a pressing force to a target bone when a diagnosis of mechanical properties is performed for the target bone. The pressing mechanism includes an annular belt detachably hung around a diagnosis object in which the target bone is present, a deadweight connected to a lower end of the annular belt and having a weight value corresponding to a desired pressing force, and a mobile base positioned under the deadweight and movable in an up-and-down direction. If the mobile base moves downward, the deadweight separates from the mobile base and is brought into a free state where a pressing force acts on the target bone. If the mobile base moves upward, the deadweight is supported by the mobile base and the target bone is released from the pressing force. 
     In a preferred embodiment, the annular belt is entrained around a plurality of rollers which are spaced from the diagnosis object. In this case, the rollers include a group of fixed rollers which are symmetrically disposed with respect to a pressing position in the right-and-left direction, and are in a fixed relationship relative to the annular belt, and a group of movable rollers which can slide along an inner surface of the annular belt and automatically move to a position where the gravity acting on the annular belt is balanced in the right-and-left direction. 
     In a preferred embodiment, the mobile base supports the deadweight via a plurality of balls that permits horizontal movement of the deadweight relative to the mobile base. In a preferred embodiment, an urging member is disposed between the mobile base and the deadweight to urge the deadweight in the upward direction. 
     According to another aspect of the present invention, an ultrasound diagnostic apparatus is configured to transmit/receive ultrasonic waves to/from a target bone in a state where a pressing force is applied to the target bone, and to obtain deformation properties of the target bone subjected to the pressing force. The ultrasound diagnostic apparatus includes a pressing mechanism configured to press a predetermined position on a target bone, an ultrasonic probe configured to transmit and receive ultrasonic wave to and from the target bone, and a calculation unit configured to calculate a change in the target bone before and after the target bone is pressed by the pressing mechanism based on an echo signal obtained via the ultrasonic probe that performs transmission/reception of ultrasonic wave. The pressing mechanism includes an annular belt detachably hung around a diagnosis object in which the target bone is present, a deadweight connected to a lower end of the annular belt and having a weight value corresponding to a desired pressing force, and a mobile base positioned under the deadweight and movable in an up-and-down direction. If the mobile base moves downward, the deadweight separates from the mobile base and is brought into a free state where a pressing force acts on the target bone. If the mobile base moves upward, the deadweight is supported by the mobile base and the target bone is released from the pressing force. 
     According to the present invention, the gravity of a deadweight hung around a diagnosis object can be applied as a pressing force via an annular belt to a target bone. The annular belt is sufficiently compact in size and can be positioned closely to a pressing position. As a result, the main body of the pressing mechanism does not interfere with an external fixation device or another medical instrument. Thus, the pressing mechanism can appropriately apply a pressing force to a predetermined pressing position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a block diagram illustrating an ultrasound diagnostic apparatus according to an embodiment of the present invention; 
         FIG. 2  illustrates example tracking processing for obtaining echo signals from a bone surface; 
         FIG. 3  illustrates a perspective view of an example external fixation device; 
         FIG. 4  illustrates a perspective view of a pressing mechanism; 
         FIG. 5  illustrates a cross-sectional view of the pressing mechanism; 
         FIG. 6  illustrates a cross-sectional view taken along a line A-A of  FIG. 5 ; 
         FIG. 7A  illustrates a partly enlarged view of another pressing mechanism; and 
         FIG. 7B  illustrates a partly enlarged view of another pressing mechanism. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of the present invention is described below with reference to the drawings.  FIG. 1  is a block diagram illustrating an ultrasound diagnostic apparatus according to an embodiment of the present invention. An ultrasound diagnostic apparatus  10  is configured to diagnose mechanical properties of a bone (e.g., diagnosis object). For example, the diagnosis object is a shinbone  104  in a human leg  100 . 
     The ultrasound diagnostic apparatus  10  according to the present embodiment includes two probes  14  which are brought into contact with a body surface of a measurement target. The probe  14  is, for example, an ultrasonic probe capable of emitting ultrasonic beams toward the shinbone  104  (target bone) inside the body of the measurement target. 
     A transmission/reception unit  16  controls each probe  14  that performs electronic scanning with ultrasonic beams on a tomography surface. For example, if the probe  14  is a linear probe, the probe  14  successively emits a total of 120 ultrasonic beams for electronic scanning. The transmission/reception unit  16  obtains an echo signal from each ultrasonic beam. A transmission/reception control unit  18  controls the transmission/reception unit  16  according to a user&#39;s instruction input via an operation panel  20 . 
     The transmission/reception unit  16  outputs a plurality of echo signals (i.e., the signals having been obtained as described above) to a tomography image forming unit  22 . The tomography image forming unit  22  forms a tomography image (B mode image) of a target bone based on the received echo signals. The transmission/reception unit  16  also outputs the obtained echo signals to an echo tracking processing unit  24 . The echo tracking processing unit  24  performs echo tracking processing (i.e., extracts bone surface portions from the echo signals and performs tracking). For example, a technique discussed in Japanese Patent Laid-Open Publication No. 2001-309918 can be used for the echo tracking processing. The echo tracking processing uses a plurality of (e.g., three) tracking echo signals. The tracking echo signals can be selected from echo signals used for tomography image formation (e.g., 120 echo signals). Alternatively, three echo signals dedicated to the tracking can be used if available during interruption of the tomography image formation. 
       FIG. 2  illustrates example tracking processing for obtaining three echo signals  40  from a surface portion of the bone  104 . Three echo signals  40 , corresponding to three ultrasonic beams emitted toward the bone  104 , have a large value in amplitude (amplitude maximized portion  42 ) at a portion corresponding to the bone surface. The echo tracking processing unit  24  identifies the position on a bone surface based on the position of the amplitude maximized portion  42  (i.e., acquiring time of a corresponding waveform). Although the example tracking processing illustrated in  FIG. 2  uses three echo signals for echo tracking, the number of echo signals used for the measurement can be changed arbitrarily. In the echo tracking processing, a surface point being tracked for each echo signal  40  (i.e., for each ultrasonic beam) can be referred to as tracking point  106 . 
     An interpolation line generation unit  26  (refer to  FIG. 1 ) generates an interpolation line connecting three tracking points  106 . Namely, the interpolation line generation unit  26  generates an interpolated curve connecting respective tracking points  106  according to the spline interpolation or the least squares method interpolation. The generated interpolation line is a curve representing the shape of a target bone surface. The interpolation line, which can more accurately represent the surface shape of a target bone, can be obtained if the number of echo signals for the echo tracking processing is increased. The generated interpolation line can be temporarily stored in a memory  28 . A mechanical property calculation unit  30  calculates a mechanical property value (e.g., deflection amount) of a target bone with reference to the information (i.e., information relating to the shape of a bone surface) temporarily stored in the memory  28 . 
     A display image forming unit  32  forms an image to be displayed on a display unit  34 , as a result of diagnosis. The display image forming unit  32  receives a tomography image formed by the tomography image forming unit  22 , a mechanical property value (e.g., deflection amount) calculated by the mechanical property calculation unit  30 , and a load value applied to the target bone from the pressing mechanism  50 . The display image forming unit  32  forms a display image of a graph representing the received mechanical property value and/or the load value, and outputs the display image to the display unit  34 . For example, the display image forming unit  32  directly outputs the display image to the display unit  34 . Alternatively, the display image forming unit  32  can combine the display image with the B mode image. The display unit  34  displays the mechanical properties of a target bone to enable a user to evaluate the union or the strength of a fractured bone. 
     A pressing mechanism  50  is configured to apply a predetermined load to the shinbone  104  (i.e., a target bone to be diagnosed). As illustrated in  FIG. 1 , two probes  14  are disposed at both sides of a pressing position of the pressing mechanism  50 . Each probe  14  can measure a displacement of the shinbone  104  caused when the shinbone  104  is pressed by the pressing mechanism  50 . 
     The deflection amount, or another physical amount indicating mechanical properties, varies significantly depending on an applied load and a pressing position. For example, if the shinbone  104  (target bone) is in a fractured state, the strength of the bone  104  varies significantly depending on the distance from a fractured portion. Therefore, even if the applied load is set to the same value, the measured deflection varies significantly depending on the pressing position. If the deflection amount varies according to the pressing position, a user cannot accurately evaluate the strength of a target bone or the union of a fractured bone. Hence, the user determines a pressing position of the pressing mechanism for each measurement target considering the position of a fracture portion or the shape of a target bone. Then, to accurately perform the diagnosis of mechanical properties, the user is required to accurately press the determined pressing position. 
     A user can evaluate the union of a fractured bone based on a diagnosis result of mechanical properties. In this case, the fractured bone may be fixed by an external fixation device.  FIG. 3  illustrates a perspective view of an example external fixation device  110 . The external fixation device  110  includes a plurality of pins  114  which are inserted into a human body to fix the fractured bone in position and supported by metal fittings  112  As will be apparent from  FIG. 3 , when each pin  114  fixes the fractured bone at one end, the other end of the pin  114  and the metal fittings  112  holding the pin  114  are positioned outside the body. 
     If the external fixation device  110  is attached around a target body, a conventional pressing mechanism interferes with the external fixation device  110  and cannot appropriately press a pressing position determined based on the position of a fracture portion and the shape of a target bone. Hence, the pressing mechanism  50  according to the present embodiment is configured to overcome the above-described problems. 
       FIG. 4  illustrates a perspective view of the pressing mechanism  50  according to an embodiment.  FIG. 5  illustrates a cross-sectional view of the pressing mechanism  50 .  FIG. 6  is a cross-sectional view taken along a line A-A of  FIG. 5 . The pressing mechanism  50  is configured to apply a pressing force to the shinbone  104  (target bone) by hanging a deadweight  54  having a predetermined weight value on the leg  100  which includes the shinbone  104 . The deadweight  54  is connected to an annular belt  52  which is detachable from the leg  100 . The annular belt  52  is a belt-like member having separable/connectable end parts equipped with an appropriate fastener (e.g., hook). The annular belt  52  becomes an endless shape when the end parts of the belt-like member are connected to each other by the fastener. 
     More specifically, before attaching the annular belt  52  around the leg  100 , a user disengages the fastener of the annular belt  52 . In this state, the belt  52  is in an extended state with separated end parts. Next, the user ties the belt  52  around the leg  100  loosely and engages the fastener to connect the belt  52  into an endless shape. In this manner, the annular belt  52  can be temporarily extended into a belt with separated end parts and can be easily attached around the leg  100  even in a state where the leg  100  is supported by the external fixation device  110 . 
     A pressing board  60  is made of a rigid member and is fixedly bonded to an inner surface of the annular belt  52 . The pressing board  60  (pressing member) can apply a pressing force to the shinbone  104  when it is brought into contact with a surface of the leg  100 . Namely, a user adjusts the position of the annular belt  52  attached around the leg  100  so that the pressing board  60  is brought into contact with a predetermined pressing position. In this state, if the annular belt  52  is pulled downward by the deadweight  54 , the pressing board  60  can apply a pressing force to the leg  100  (namely, the shinbone  104  included in the leg  100 ). 
     The annular belt  52  is entrained around five rollers  62  disposed in a pentagonal shape so that an appropriate tension acts on the annular belt  52 . The five rollers  62  are appropriately spaced from an outer surface of the leg  100 . The annular belt  52 , when entrained around the rollers  62 , does not directly contact the leg  100 . In other words, the annular belt  52  can maintain good balance without causing any lack of uniformity due to frictional force. 
     Two rollers  62   a  and  62   b  are positioned at both sides of the pressing board  60  and are supported and connected by an upper metal holder  64  extending parallel to the pressing board  60 . The upper metal holder  64  and the pressing board  60  are physically connected with each other. Thus, the positional relationship between upper metal holder  64  (namely, the rollers  62   a  and  62   b  connected to the upper metal holder  64 ) and the pressing board  60  can be regulated. Two rollers  62   a  and  62   b  are symmetrical with respect to the pressing position in the right-and-left direction. Furthermore, two rollers  62   a  and  62   b  are in a fixed positional relationship relative to the annular belt  52 . In this respect, two rollers  62   a  and  62   b  function together as a “fixed roller group.” 
     On the other hand, three rollers  62   c ,  62   d , and  62   e  are supported and connected by a lower metal holder  66  configured into a Y shape. Two rollers  62   c  and  62   d  are symmetrical with respect to the roller  62   e  in the right-and-left direction. More specifically, three rollers  62   c ,  62   d , and  62   e  are disposed in a reversed isosceles triangular shape. Three rollers  62   c ,  62   d , and  62   e  can slide along an inner surface of the annular belt  52 . If any unbalance occurs in the gravity acting on the annular belt  52 , three rollers  62   c ,  62   d , and  62   e  automatically move to a position where the gravity acting on the annular belt  52  is balanced. As a result, the downward force acting on the annular belt  52  can be automatically adjusted to be symmetrical in the right-and-left direction. Namely, three rollers  62 ,  62   d , and  62   e  can slide along the inner surface of the annular belt  52  when the gravity is unbalanced in the right-and-left direction and adjust the gravity acting on the annular belt  52  to be symmetrical with respect to the pressing position. In this respect, three rollers  62 ,  62   d , and  62   e  function as a “movable roller group.” 
     The deadweight  54  having a predetermined weight is attached to a proximal end of the lower metal holder  66 . When the deadweight  54  is pulled downward due to its gravity, a downward pressing force is applied to the annular belt  52  as well as to the pressing board  60  bonded together with the annular belt  52 . The downward pressing force acts on the shinbone  104  (i.e., target bone). The weight of the deadweight  54  is determined considering a load value to be applied to a target bone. If the purpose of diagnosis or the type of a target bone is changed, the deadweight  54  can be replaced with another deadweight having a different weight value. 
     A base  68 , movable in the up-and-down direction, is positioned under the deadweight  54 . If no pressing force is required for the shinbone  104 , the mobile base  68  can move upward to a higher position where the base  68  supports the bottom of the deadweight  54 . In the raised state where the mobile base  68  mounts and supports the deadweight  54 , no downward force acts on the annular belt  52  and the pressing board  60 . On the other hand, if a pressing force acting on the shinbone  104  is required, the mobile base  68  can move downward to a lower position where the deadweight  54  separates from the mobile base  68  and is kept in a free state. In this state, the deadweight  54  pulls the annular belt  52  with the pressing board  60  downward due to its gravity. The pulling force (i.e., pressing force) acts on the shinbone  104 . 
     A plurality of steel balls  70 , provided on an upper surface of the mobile base  68 , enables the deadweight  54  to easily move in the horizontal direction relative to the mobile base  68 . Namely, to accurately diagnose mechanical properties of the shinbone  104 , it is desirable to subject the shinbone  104  to only the force acting in the vertical direction. In other words, it is desirable to eliminate any deviation in the direction of the pressing force acting on the shinbone  104 . In a state where the deadweight  54  is mounted on the mobile base  68 , the deadweight  54  does not constantly exist directly below the pressing board  60  and therefore the deadweight  54  may be offset from the pressing board  60 . 
     If the mobile base  68  moves downward from the above-described state, the deadweight  54  is brought into a free state and the deadweight  54  automatically moves to a balanced position due to its gravity and stays directly below the pressing board  60 . In this movement, the pressing board  60  is subjected to not only a vertical force but also a horizontal force. As a result, a significant deviation is caused in the direction of the force acting on the shinbone  104 . To reduce such a deviation, the deadweight  54  is required to be constantly held at the position directly below the pressing board  60  in a state where the deadweight  54  is mounted on the mobile base  68 . Hence, the present embodiment provides a plurality of steel balls  70  on the upper surface of the mobile base  68  so that the deadweight  54  can easily change the horizontal position on the mobile base  68  in accordance with the movement of the pressing board  60  and the annular belt  52 . 
     The up-and-down movement of the mobile base  68  can be realized by a motor (i.e., a driving source not illustrated) and a transmission mechanism capable of transmitting driving force of the motor to the mobile base  68 . The transmission mechanism includes a screw  72  extending downward from the bottom surface of the mobile base  68 , a wheel  74  mating with the screw  72 , and a worm wheel  76  mating with the wheel  74 . The worm wheel  76  is connected to the output shaft of the motor and driven by the motor. The wheel  74  rotates when the worm wheel  76  rotates. The screw  72  rotates and shifts in the axial direction when the wheel  74  rotates. Thus, the mobile base  68  connected to the screw  72  can move in the up-and-down direction. The above-described arrangement of the driving mechanism is merely an example, and can be appropriately modified if necessary. 
     The mobile base  68  and the transmission mechanism are accommodated in a base body  56 . The base body  56  is a hollow box member. The base body  56  includes an intermediate support  56   b  protruding in the horizontal direction from the inner wall. The intermediate support  56   b  includes a guide hole  80  extending in the vertical direction. A guide shaft  78 , extending downward from the bottom surface of the mobile base  68 , is inserted into the guide hole  80 . The guide shaft  78  and the guide hole  80  cooperatively function as “guide unit” configured to regulate the moving direction of the mobile base  68 . 
     An internal space of the base body  56  communicates with an external space via an opening  56   a  formed on an upper surface of the base body  56 . The opening  56   a  has a sufficient size so that the deadweight  54  can be inserted via the opening  56   a  into the internal space of the base body  56 . The upper surface of the base body  56  is covered by a steel or other ferromagnetic member. A mount base  58 , including a magnet, is detachably mounted on the upper surface of the base body  56 . The mount base  58  supports the leg  100  in which the shinbone  104  is present. Each mount base  58  has a sufficient height so that the external fixation device  110 , when attached around the leg  100 , does not interfere with the upper surface of the base body  56 . However, the number of mount bases  58  and the shape of each mount base  58  can be appropriately changed considering the type of target bone or the shape of the external fixation device  110 . 
     A user can perform the diagnosis of mechanical properties of the shinbone  104 , using the above-described pressing mechanism  50  in the following manner. First, as a preparation for the diagnosis, a user determines a pressing position and a pressing force beforehand according to the purpose of the diagnosis and the type of bone (diagnosis object) and also considering the presence of any bone fracture. Then, the user selects the deadweight  54  having an appropriate weight value corresponding to the pressing force determined beforehand and hangs the selected deadweight  54  on the lower metal holder  66  of the pressing mechanism  50 . Furthermore, the user raises the mobile base  68  to a higher position where the bottom of the deadweight  54  can be supported by the mobile base  68 . 
     Then, the user puts the diagnosis object (the leg  100 ) on two mount bases  58  placed on the upper surface of the base body  56 , so that the pressing position determined beforehand can be roughly positioned above the opening  56   a.    
     Subsequently, the user ties the annular belt  52  around the leg  100 . More specifically, before attaching the annular belt  52  around the leg  100 , the user temporarily disengages the fastener of the annular belt  52 . Next, the user ties the belt  52  (in an extended state with separated end parts) around the leg  100  loosely and engages the fastener to connect the belt  52  into an endless shape. At this moment, the deadweight  54  is supported by the mobile base  68 . Therefore, no downward force acts on the annular belt  52  and the pressing board  60 , and no pressing force is applied to the shinbone  104 . 
     Then, the user performs a positional adjustment for the annular belt  52  so that the pressing board  60  can be accurately positioned at the pressing position determined beforehand. Furthermore, in addition to the positional adjustment for the annular belt  52 , the user can adjust the position of the deadweight  54  connected to the annular belt  52  so that the deadweight  54  can be positioned directly below the pressing board  60  in the vertical direction. The plurality of steel balls  70  provided on the upper surface of the mobile base  68  can reduce frictional force caused when the deadweight  54  moves. Therefore, the user can easily perform the positional adjustment for the deadweight  54 . In this case, instead of manually moving the deadweight  54 , a user can temporarily lower the mobile base  68  to a position where the deadweight  54  is kept in a free state and can automatically move to a correct position. 
     After completing the positional adjustment for the pressing board  60  and the deadweight  54 , the user attaches the probes  14  on an upper surface of the leg  100  at both sides of the pressing board  60  (i.e., pressing position). 
     Then, the user causes the ultrasound diagnostic apparatus  10  to start transmission/reception of ultrasonic waves using the probes  14 . The ultrasound diagnostic apparatus  10  obtains surface shape data of the shinbone  104  in a state free from any pressing force, based on echo signals obtained by transmission/reception of ultrasonic waves. 
     Subsequently, the user moves the mobile base  68  downward while the ultrasound diagnostic apparatus  10  continues transmission/reception of ultrasonic waves. If the deadweight  54  is brought into a free state, the downward force (i.e., the gravity of the deadweight  54 ) acts on the annular belt  52  as well as on the pressing board  60  bonded to the annular belt  52 . Thus, a downward pressing force acts on the shinbone  104 . 
     If a pressing force is applied to the shinbone  104 , the shinbone  104  causes a deformation in shape (e.g., deflection). The ultrasound diagnostic apparatus  10  obtains surface shape data of the shinbone  104  in a state where the shinbone  104  is subjected to the pressing force, based on echo signals obtained by transmission/reception of ultrasonic waves. After completing the measurement of surface shape data, the user moves the mobile base  68  upward to release the shinbone  104  from the gravity of the deadweight  54 . Meanwhile, the ultrasound diagnostic apparatus  10  calculates a mechanical property value (e.g., deflection amount) of the shinbone  104  based on the surface shape data obtained before and after application of a pressing force. If the mechanical property value is obtained, the user completes the diagnosis of mechanical properties. 
     As will be apparent from the foregoing description and  FIGS. 4 through 6 , in the pressing mechanism  50  according to the present embodiment, the components located in the vicinity of a pressing position are limited to the annular belt  52 , the pressing board  60 , and the roller  62 , which are compact in size. Therefore, even if the pins  114  and the metal fittings  112  (i.e., constituent members of the external fixation device  110 ) are present in the vicinity of the pressing position, the pressing mechanism  50  can accurately press a designated pressing position without causing any interference with the constituent members of the external fixation device  110 . As a result, the present embodiment enables a user to appropriately perform the diagnosis of mechanical properties even in a state where the external fixation device  110  is attached around a target body. 
     The arrangement of the above-described pressing mechanism is merely an example, and can be appropriately modified if an appropriate pressing force can be applied to a target bone when a deadweight having a predetermined weight value is hung. For example, a pressing mechanism according to another embodiment may not include the rollers  62 , the pressing board  60 , and the steel balls  70 . 
     In the above-described embodiment, the mobile base  68  directly supports the deadweight  54 . However, if an appropriate urging member is provided between the deadweight  54  and the mobile base  68 , the urging member can gradually apply a force to the deadweight  54  in the upper direction. The pressing force can be slowly applied to the shinbone  104  via the urging member. The deflection amount of a bone or other mechanical property value is variable depending on how a pressing force is applied. Namely, a mechanical property value obtained when the pressing force is slowly applied is different from a mechanical property value obtained when the pressing force is quickly applied. 
     Furthermore, according to the purpose of mechanical diagnosis, it may be necessary to obtain a mechanical property value under a static load. In such a case, it is preferable to provide an urging member between the deadweight  54  and the mobile base  68  so that the pressing force can be slowly applied to the deadweight  54 . 
       FIGS. 7A and 7B  illustrate a shock absorbing mechanism including a coil spring  84  and a damper  86  which are provided in a recessed portion  82  formed on the bottom of the deadweight  54 . If a user slowly moves the mobile base  68  downward and the mobile base  68  starts separating from the deadweight  54 , the shock absorbing mechanism can continue supporting the deadweight  54 . The weight of deadweight  54  supported by the shock absorbing mechanism gradually decreases with increasing distance between the mobile base  68  and the deadweight  54 . Finally, the entire weight of the deadweight  54  acts on the pressing board  60 . Namely, if an appropriate shock absorbing mechanism is provided between the deadweight  54  and the mobile base  68 , the load amount acting on the shinbone  104  can be gradually increased. 
     Furthermore, the above-described embodiment uses transmission/reception of ultrasonic waves to detect a change in the shape of a target bone subjected to a pressing force. However, according to another embodiment, indirectly measuring a change in the shape of a target bone is feasible. For example, a laser displacement gauge can be used to measure a bending amount of a body part in which a target bone is present (e.g., a leg if the target bone is a shinbone) and diagnose mechanical properties of the target bone based on a measurement result. In this case, although the tissues of a human body have some effect on a measurement result, a user can easily diagnose mechanical properties of a target bone. 
     Although the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that modifications and variations may be made without departing from the spirit or scope of the appended claims.