the invention relates to a device and method for determining an aperture angle of a joint lying between two body structures , such as , for example , the aperture angle of a hip joint , a knee joint , an elbow joint or other joints . it is to be appreciated that the term “ aperture angle ” is intended to be understood , for example , in the case of the ball joint - like hip joint , both as the angle between a fixed upper leg axis and a plane or axis defined by the position of the hip , and as the spatial position or orientation of the angle . the intention is , for example , to ascertain how far the femur is inclined forwards , backwards or to the side , relative for example to the hip , i . e . the spatial angle which the joint forms is determined . the intention is also , for example , to ascertain how a joint or parts or structures thereof are rotated , for example , about a center axis . fig1 schematically shows an artificial hip joint 10 including a joint head 12 of a femur implant 14 , which is accommodated by a joint cavity 16 of the hip . reference elements or stars 20 , which include markers 22 are attached to the body structures connected to the joint head 12 and the joint cavity 16 , respectively , and / or to the joint head 12 and / or the joint cavity 16 themselves , in order to be able to determine the position of the joint head 12 and the joint cavity 16 . an camera 30 , such as an infra - red ( ir ) camera , records light reflected by the markers 22 of the reference stars 20 and forwards the signals to a computational unit 40 . the computational unit 40 can display the position and / or the spatial angle in the orthopaedically defined directions of abduction , adduction , flexion , extension , internal and external rotation and changes in leg length . it is also possible to calculate the angle between a previously defined axis 42 through the joint head 12 and a previously defined axis 44 through the joint cavity 16 . for calculating the angle , it is possible , for example , to fall back on data stored in a data base 50 , which includes , for example , information on the geometry of the joint head 12 , the joint cavity 16 , the fixing position of the reference stars 20 or other information . furthermore , reference values for a series of measurements performed can be stored in the data base 50 , where these values can be used for comparison with subsequent measurements . a robot 60 and a force measuring device 62 ( both in communication with the computational unit 40 ) can be employed for applying forces in defined directions to the joint as well as measuring respective applied forces . a coordinate system of the femur can , for example , be used as a reference system and can be defined by a plane formed by a neck axis 52 ( i . e ., the center axis of the neck of the femur ) and a shaft axis 54 ( i . e ., the center axis of the long bone of the femur ). this plane is oriented about the rotational point of the femur , parallel to the frontal pelvic plane . the healthy , opposite joint or leg to the joint to be implanted can be used as a comparative reference for , for example , the range of motion to be enabled or for the length of the leg . the spatial angle ascertained by the computational unit 40 can be outputted to a display unit , such as , for example , a screen , and there numerically outputted as a graphical representation of the joint and / or with additional information . with reference now to fig2 , a method for determining the aperture angle of a hip joint is provided . it is to be appreciated that while fig2 is described with reference to a hip joint , the methodology is applicable to other joints . in step 60 , a co - ordinate system of the hip can be defined by identifying a sagittal center plane and a front hip plane in a computer tomographic ( ct ) representation . in step 62 , a co - ordinate system of the femur is defined by identifying a rotational center point and a center axis in a computer tomographic representation . it is to be appreciated that steps 60 and 62 can be performed both simultaneously and sequentially , in any order , i . e ., step 62 can also be performed before step 60 . in step 64 , segmentation is performed , in order to be able to distinguish the individual bone structures , such as , for example , the hip , right femur and left femur , in the recordings of a body structure . in step 66 , a virtual movement of one or both femurs and the hip to a neutral or initial position is simulated in a simulation , wherein for example the coronal plane of the femur lies parallel to the front plane of the hip in an initial position , to define a zero position . in step 68 , markers , such as , for example , reference stars , can be attached to the femur and the hip and / or to the joint cavity and the joint head , on the side to be treated . in step 70 , the respective elements are then registered . in one embodiment , the registration can be performed using a navigation system as is described in co - owned u . s . pat . no . 6 , 351 , 659 , which is incorporated herein by reference in its entirety . in step 72 , the flexibility and / or the range of motion of a patient &# 39 ; s femur is verified . in step 74 , the maximum possible movements , for example , expressed by aperture angles of the joint in various directions , are ascertained , visualized and recorded . in this way , an abduction ( abducting or moving the joint backwards ), adduction ( guiding or moving the joint forwards ), flexion ( bending ) and / or extension ( stretching ) of the hip joint or of another of a patient &# 39 ; s joints can be performed . it is also possible to compare the movements possible with the flexibility of the patient &# 39 ; s still healthy joint . optionally , the flexibility or range of motion of a patient &# 39 ; s femur can be verified by comparing them with the flexibility or range of motion of a healthy joint lying symmetrical to said femur . the joint can be moved both manually and automatically , for example using a robot , in order to apply defined forces to the joint in defined directions . as described above , recordings of the natural or artificial joint and / or of body structures adjacent to the joint can be used to determine the aperture angle or spatial angle of a joint , wherein nuclear spin resonance ( mr ) methods , computer tomography ( ct ) methods , ultrasound methods or other suitable methods can be used . the recorded body structures can be sub - divided into individual elements , for example , using known segmentation or separating methods . in this way , the borders of adjacent structures can be obtained from the recorded data , in order to have data for calculating the aperture angle from positional data of the body structures or for calculating an optimum position for a joint to be implanted . in general , bone structures are detected in an image data set in a way that is substantially dependent on how the patient is positioned or lying at the time the image data are detected . there are rough instructions for radiology and for the patient with respect to a desired position for recording the image data set . however , the variable positions of the individual bone structures can be virtually moved to an initial position or neutral position to be defined , to provide a precise basis for comparison , which also enables comparison between a number of patients . after segmenting , three local co - ordinate systems can be defined , such as , for example , femur left , femur right and pelvis . these can then be oriented with respect to each other in accordance with a fixed specification and can thus be moved into a defined positional relationship . if one then wishes to visualize positional data , such as , for example , angles , length of leg or of a joint , then this neutral position can be used as a starting position with respect to which an aperture angle or a spatial position can be defined . in this way , initial conditions can be created , which may even be reproduced and compared with each other for various patients . fig3 a - 3d illustrate the procedure described above . an image data set , which can be recorded in an arbitrary position of the patient , is shown schematically in fig3 a . by segmenting , as shown schematically in fig3 b , the individual body structures , such as , for example , hip , femur left and femur right can be identified as individual elements . as shown in fig3 c , coordinate systems can be assigned to the individual , segmented structures . the coordinate systems can enable the structures to be virtually aligned in the neutral position shown in fig3 d . although the invention has been described using a hip joint by way of example , it is clear that the device and method in accordance with the invention can also be used to ascertain aperture angles for other joints , for example a knee joint or an elbow joint . although particular embodiments of the invention have been described in detail , it is understood that the invention is not limited correspondingly in scope , but includes all changes , modifications and equivalents coming within the spirit and terms of the claims appended hereto .
a device for determining the aperture angle of a joint includes a detection device for detecting positions of joint components and / or positions of structures connected to or to be connected to the joint and a computational unit for ascertaining the aperture angle of the joint based on the detected positions .