in fig1 a video - graphic arthroscopy system 10 is shown including an arthroscope 12 and a housing 16 for the position sensing apparatus which is attached to a patient &# 39 ; s joint 14 ( shown in phantom ). the arthroscope includes conventional saline injection means ( not shown ) to distend the joint and also a conventional source of illumination ( not shown ) to illuminate the joint . light from the observation region is transmitted via optical fiber 24 to a video - image processor 18 , which generates a video image of the biological structures under observation in the patient &# 39 ; s joint . at the same time , a position sensing apparatus in housing 16 generates electrical signals via cable 26 , which are transmitted to a position graphics processor 20 to generate a display of the arthroscope &# 39 ; s position relative to a fixed reference point ( typically the point of entry ). in the illustration of fig1 a graphic image is generated by the graphics processor 20 which is superimposed upon the video image using half silvered mirror 22 to provide a display for the arthroscopist . in fig2 the position sensing apparatus 30 of the present invention is shown in more detail ( with the housing removed for illustration ), including a rotation sensor 32 which is mounted upon the cannula 46 of arthroscope 12 . within the rotation sensor 32 , a rotor 34 includes a keyway notch 48 which mates with a keyway or spline 36 carried upon sleeve 38 . the sleeve 38 is attached to sheath 40 of the arthroscope 12 by set screw 42 . a depth sensor 44 is also employed to measure the depth of penetration of the arthroscope 12 into the patient &# 39 ; s joint . the depth sensor 44 can be attached to the cannula 46 ( or alternatively to the sleeve 38 ). in the illustrated embodiment , position sensing apparatus 30 utilizes depth sensor 44 and nylon line 60 , coupled to sleeve 38 and wound around a spool in depth sensor 44 , to provide information representative of the depth of penetration of arthroscope 12 into the patient &# 39 ; s joint . depth sensor 44 translates the degree to which nylon line 60 is wound or unwound from depth sensor 44 into an analog voltage which is supplied to a processor . as sleeve 38 is inserted or retracted , nylon line 60 is unwound or wound , respectively . as a result , a magnet , which is coupled to the spool in depth sensor 44 , rotates correspondingly . a hall effect sensor measures the motion of the magnet and generates a signal which is conditioned by conventional electronics . ( alternatively , the depth sensor can employ a rack and pinion - type drive mechanism , whereby a pinion gear drives the sensor and mates with a rack mounted upon sleeve 38 .) semicircular sloted yokes 52 , 56 define an x - axis and a y - axis for measuring the tilt of the arthroscope during usage . an x - axis sensor 58 is joined to the center of rotation of the x - axis yoke 56 , and a similar y - axis sensor 54 is joined to the center of rotation of y - axis yoke 52 . the cannula 46 is disposed between the yokes 52 , 56 so that the sensors 54 , 58 , can provide a direct relationship between sensor rotation and the angular movement ( tilting ) or the arthroscope 12 . the two - member system effectively decouples the motion in the x and y directions . an angular range of motion in each direction of approximately ˜ 40 degrees is typically sufficient for arthroscopic operations . various commercially available sensors can be employed to measure the four degrees of motion . for example , the rotary sensor 32 has been implemented employing a rotary variable differential transformer , such as the model ral sensor manufactured by transducers and systems , inc . ( northbranford , conn .). the other sensors ( for measuring tilt and penetration ) can also employ rotary sensors or simpler devices for example , commercially available hall effect sensors such as those manufactured by texas instruments ( dallas , tex .) can be employed . the analog voltages generated by the sensors are transmitted , via cable 26 , to a processor which includes an analog to digital converter to translate the analog votage to a digital representation thereof . as detailed above , the sensor information is utilized to generate positional information of the probe of arthroscope 12 with respect to the patient &# 39 ; s joint , and to generate computer graphics simulating objects not generally present in the patient &# 39 ; s joint . the processor , which can be a general purpose computer or other processing device , in conjunction with the information from the sensors can provide a computer - generated model of the joint in question . this computer graphic model can augment the video display of the arthroscope . a variety of commercially available computer graphics engines can be used , such as the rennaissance graphics engine manufactured by hewlett - packard ( andover , mass .) for use on the hp - 9000 computer , a general purpose work station built around a motorola 68020 processor . the rennaissance engine is capable of generating high resolution images for both 2 and 3 - dimensional displays . the graphics operations of this system can be implemented using a variety of commercially available graphics programs , such as the hewlett - packard starbase graphics package . the main loop of such programs continually poles the position of the arthroscope and then performs various tasks . in one demonstration of the system , a wire frame cross - section data base of the major bones of the knee was entered into the processor memory . the program was then configured to generate a line representing the probe and display it over the cross section . the probe orientation with respect to the wire frame illustration of the knee could then be displayed from the sensor signals by using a set of transformational matrices . in one embodiment , three orthogonal views can be presented to the viewer . ( for further details on computer graphics , see newman and sproul , principles of interactive computer graphics ( mcgraw - hill , 1979 ) and rogers , procedural elements for computer graphics ( mcgraw - hill , 1985 ) ( both of which are incorporated herein by reference ). more realistic knee illustrations can , of course , be generated with appropriate graphics programs using more detailed data bases of the biological structures . in a further embodiment of the invention , such graphic displays can be merged with the video images from the arthroscope . if the video signals generated by graphics engine are incompatable with the video signals generated by the viewing probe &# 39 ; s camera , either a standards conversion box which reduces the high resolution graphics picture to a ntsc resolution interlaced video image can be employed . alternatively , it is possible to convert the ntsc resolution arthroscope image up to the same resolution as the graphics image , and electronically merge them . in yet another alternatively , the two images can be optically merged using a half - silvered mirror . use of such a mirror enables one to merge the two images , both inexpensively , and with no loss of resolution . the images are merged together by placing a mirror , which is partially reflective and transmisive , at a 45 ° angle in between two monitors which are perpendicular to each other . the viewer sits in front of one monitor and sees the image from the other monitor superimposed upon the first monitor &# 39 ; s image . various alternatives and auxiliary devices can also be incorporated into the present invention . for example , the graphics processor can include additional user input interfaces , such as a graphics tablet , mouse , knob box , button box or keyboard . these input devices can provide the user with a means of &# 34 ; pointing &# 34 ; to objects within the scene . it will be understood that changes may be made in the above construction and the forgoing sequences of operations without departing from the spirit or scope of the invention . it is accordingly intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative rather than in a limiting sense .
a position sensing system which allows the arthroscopist to readily determine the location of an arthroscope &# 39 ; s tip in relation to the point of entry has been developed . the position sensing apparatus can be employed in conjunction with a graphics module to display the location of the arthroscope in real time and provide perspective views of the instrument &# 39 ; s location .