Abstract:
In a method for determining the angle of anteversion and/or the angle of inclination of an acetabulum in a pelvic bone, in order to achieve a simplification, a method is proposed, wherein prominent points on the pelvic bone are non-invasively determined, these prominent points are joined by an individual plane, and this individual plane is respectively rotated through predetermined, specific angular magnitudes for the angle of inclination and the angle of anteversion, respectively, and the angle between the rotated individual plane and a main plane of the pelvic bone is determined in order to determine the angle of inclination and the angle of anteversion.

Description:
[0001]    The present disclosure relates to the subject matter disclosed in German application number 10 2007 049 668.2 of Oct. 17, 2007, which is incorporated herein by reference in its entirety and for all purposes. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for determining the angle of anteversion and/or the angle of inclination of an acetabulum in a pelvic bone. 
         [0003]    When implanting an acetabulum, which generally has as an approximately hemispherical shape, the position of the acetabulum and its angular position relative to the pelvic bone must be precisely determined before the implantation. The position can be determined relatively easily by the center point of rotation of the hip joint being determined prior to the implantation, for example, by pivoting the femur relative to the pelvic bone. This center point then corresponds to the spherical center point of the hemispherical acetabulum. 
         [0004]    The determination of the angular position of the acetabulum proves more difficult. The acetabulum has a bottom rim which defines a plane delimiting the hemispherical acetabulum at the bottom end, and there extends perpendicularly to this plane a central axis of the hemispherical acetabulum, which passes through the center point of the acetabulum. This central axis of the acetabulum is generally arranged in space such that it includes different angles with the main planes of the pelvic bone. Main planes of the pelvic bone are to be understood as the transverse plane, i.e., a plane extending horizontally in an upright pelvis, the sagittal plane, i.e., a plane running from front to back and extending perpendicularly to the transverse plane in an upright pelvis, and the frontal plane which extends perpendicularly to the two aforementioned planes. 
         [0005]    To describe the position of the central axis relative to these main planes, it is standard practice to indicate the angle of anteversion and the angle of inclination. The angle of anteversion describes the angle which an anteversion plane containing the central axis and extending perpendicularly to the transverse plane includes with a frontal plane. The angle of inclination describes the angle which an inclination plane extending perpendicularly to the central axis of the acetabulum and extending perpendicularly to a frontal plane assumes relative to a sagittal plane. 
         [0006]    In practice, the angle of anteversion and the angle of inclination are determined by complicated preliminary examinations carried out on the patient, for example, by X-rays taken in various planes or by ultrasonic examinations, which involve a great deal of expenditure, and, in particular, in the case of X-ray examinations, expose the patient to a high dose of radiation. 
       SUMMARY OF THE INVENTION 
       [0007]    The object of the invention is to provide a method for determining these angular magnitudes, with which the angle of anteversion and/or the angle of inclination can be determined, where necessary, during the operation, in a simple way. 
         [0008]    This object is accomplished, in accordance with the invention, for determination of the angle of anteversion, by a method wherein the position of the spina iliaca anterior superior (anterior superior iliac spine) at one side of the pelvic bone is non-invasively determined, and the position of the spina iliaca posterior superior (posterior superior iliac spine) at the same side is non-invasively determined, a plane extending perpendicularly to a transverse plane of the pelvic bone is made to pass through these points, this plane is rotated about an axis of rotation extending perpendicularly to the transverse plane through a predetermined angular magnitude of between 35.6° and 40.4°, and the angle between the rotated plane and a frontal plane is determined as angle of anteversion. 
         [0009]    The predetermined angular magnitude is preferably 38°. 
         [0010]    The object is accomplished, in accordance with the invention, for determination of the angle of inclination by a method wherein the position of the spina iliaca anterior superior (anterior superior iliac spine) at one side of the pelvic bone is non-invasively determined, and, in addition, the position of the symphysis pubis (pubic symphysis) is non-invasively determined, a plane extending perpendicularly to a frontal plane of the pelvic bone is made to pass through these points, this plane is rotated about an axis of rotation extending perpendicularly to the frontal plane through a predetermined angular magnitude of between 14.1° and 17.9°, and the angle between the rotated plane and a sagittal plane is determined as angle of inclination. 
         [0011]    The predetermined angular magnitude is preferably 16°. 
         [0012]    Accordingly, in the described method it is sufficient to non-invasively determine a few prominent points of the pelvic bone, which can be done, for example, by palpating prominent points of the pelvic bone through the skin or by an ultrasonic examination, and from these points and data relating to the main planes of the pelvic bone and by using the predetermined angular magnitude, both the angle of anteversion and the angle of inclination can be calculated. Values for the angle of anteversion and the angle of inclination are obtained, which depend upon the respective geometry of the pelvic bone, i.e., which can be individually determined for each individual pelvic bone. This does not require complicated X-ray examinations. 
         [0013]    The object set forth hereinabove is also accomplished by an apparatus which operates in accordance with the methods described hereinabove and which is constructed in accordance with patent claims  9  and  11 , respectively. Further embodiments of these apparatuses are described in claims  10  and  12  to  16 . 
         [0014]    The following description of preferred embodiments of the invention serves in conjunction with the drawings for a more detailed explanation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a schematic representation of a patient lying on an operating table and a navigation system for detecting the position of navigated position sensors; 
           [0016]      FIG. 2  shows a schematic front view of a patient while determining a frontal plane of the pelvic bone; 
           [0017]      FIG. 3  shows a schematic view of a pelvic bone, a frontal plane, a transverse plane and an anteversion plane; 
           [0018]      FIG. 4  shows a view similar to  FIG. 3  with a frontal plane, a sagittal plane and an inclination plane; 
           [0019]      FIG. 5  shows a schematic plan view of a transverse plane of a pelvic bone with an individual plane passing through prominent points of the pelvic bone; and 
           [0020]      FIG. 6  shows a plan view of a frontal plane of the pelvic bone with an individual plane passing through prominent points of the pelvic bone. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    There is shown in  FIG. 1  a patient  1  lying on an operating table  2 . Operations may be carried out on the patient in this position. Located beside the operating table  2  is a navigation system  3  with a number of radiation emitters  4 ,  5 ,  6 , which at the same time are also configured as radiation receivers. The emitted radiation may be infrared radiation. This radiation is reflected by reflective surfaces  8 , which may be spheres arranged as markers  9  on various instruments  10 , for example, a palpation instrument  10  and an ultrasonic sensor  11 . 
         [0022]    The navigation system is therefore able to detect in a manner known per se the position of the instruments in space, i.e., their exact position and their orientation. 
         [0023]    A data processor  12  with a display device  13 , in the form of a monitor in the embodiment shown, is also associated with the navigation system  3 . 
         [0024]    To enable the position of a patient&#39;s pelvic bone  14  to be described, it is known to use three prominent points of the pelvic bone, which define a so-called pelvic inlet plane. This is a special frontal plane which is identified by special prominent points lying in this frontal plane. These prominent points are the following points:
   A) spina iliaca anterior superior left
       (left anterior superior iliac spine)   
       B) spina iliaca anterior superior right
       (right anterior superior iliac spine)   
       C) symphysis pubis
       (pubic symphysis).   
       
 
         [0031]    These three points may be palpated percutaneously, for example, by hand or with the aid of the navigated palpation instrument  10 . It is also possible to detect these points with the aid of the navigated ultrasonic sensor. In any case, it is in this way possible to locate the position of the three points A), B) and C) in space, and the navigation system can forward a set of data corresponding to the position of points A), B) and C) to the data processor  12 . 
         [0032]    The position of the pelvic bone in space is defined by the determination of these points. This plane serves as reference plane for all further determinations of location and orientation. 
         [0033]      FIG. 3  shows a pelvic bone  14  and, in addition, a frontal plane  15 . An acetabulum  16  is drawn schematically therein, more particularly, in such a way that a bottom terminating plane of the acetabulum  16  spanned by the bottom rim  17  extends perpendicularly to the frontal plane  15 . Therefore, a central axis  18  of the acetabulum, which extends perpendicularly to this bottom terminating plane of the acetabulum  16  and passes through the center point of the hemispherical acetabulum  16 , lies in the frontal plane  15  in the illustration in  FIG. 3 . 
         [0034]    Such an arrangement of the acetabulum would correspond to an angle of anteversion of O° which is, however, not realistic. 
         [0035]    In reality, the central axis  18  of the acetabulum  16  lies in an anteversion plane  19  which, like the frontal plane  15 , extends perpendicularly to a transverse plane  20  and is rotated in relation to the frontal plane  15  about an axis of rotation extending perpendicularly to the transverse plane  20 . The angle α included between the anteversion plane  19  and the frontal plane  15  is referred to as angle of anteversion. 
         [0036]    In a similar way, a pelvic bone  14  is shown in  FIG. 4  together with a frontal plane  15  and a sagittal plane  22  and, in addition, with an inclination plane  21 , which coincides with the bottom terminating plane spanned by the bottom rim  17  of the acetabulum  16 . The inclination plane  21  corresponds to the actual position in which the acetabulum  16  is incorporated into the pelvic bone  14 , as does the anteversion plane  19  correspond to the actual position in which it is incorporated therein. 
         [0037]    The angle β between the inclination plane  21  and the sagittal plane  22  is referred to as angle of inclination. 
         [0038]    To determine the angle of anteversion, a further prominent point of the pelvic bone  14 , as will be apparent from the schematic representation in  FIG. 5 , is additionally determined non-invasively in the manner described hereinabove, namely point D) spina iliaca posterior superior (posterior superior iliac spine). An individual plane  23 , which extends perpendicularly to a transverse plane, is made to pass through this additionally determined point D) and through the spina iliaca anterior superior (anterior superior iliac spine) on the same side of the pelvic bone  14 . This plane includes an individual angle γ with the frontal plane  15 . 
         [0039]    In a further step, the individual plane  23  is rotated about an axis of rotation extending perpendicularly to the transverse plane through a predetermined angle δ, which for determination of the angle of anteversion ranges from between 35.6° and 40.4° and is preferably 38°. After the rotation, the rotated plane includes an angle with the frontal plane  15 , and this angle is the angle of anteversion α. Accordingly, after the rotation, the rotated plane forms the anteversion plane  19  or coincides with it. 
         [0040]    In a similar way, the angle of inclination is determined in the manner shown schematically in  FIG. 6 . An individual plane  33 , which extends perpendicularly to a frontal plane  15  and includes with it an individual angle γ′, is made to pass through points B) and C), i.e., through the spina iliaca anterior superior (anterior superior iliac spine) on one side of the pelvic bone and through the symphysis pubis (pubic symphysis). This individual plane  33  is then rotated about an axis of rotation extending perpendicularly to the frontal plane through a predetermined angle δ′, which for determination of the angle of inclination ranges between 14.1° and 17.9° and is preferably 16°. After the rotation, the individual plane  33  coincides with the plane of inclination  21 , which includes an angle of inclination β with a sagittal plane  22 . 
         [0041]    Accordingly, merely by determining the position of the prominent points of the spina iliaca anterior superior (anterior superior iliac spine) and the spina iliaca posterior superior (posterior superior iliac spine) and of the symphysis pubis (pubic symphysis), it is possible to determine values for the angle of anteversion and the angle of inclination, which are individually dependent upon the shape of the pelvic bone  14 , without complicated X-ray examinations or the like having to be carried out. 
         [0042]    The method steps described hereinabove can be readily performed by the data processor  12  on the basis of the position data made available by the navigation system  3 , so that the values of the angle of inclination and the angle of anteversion can be displayed to the operating surgeon immediately after determination of the position of the described prominent points. 
         [0043]    Surprisingly, it has been found that the predetermined magnitudes of the angles through which the planes connecting the prominent points, i.e., the individual planes, must be rotated, in order to transfer them to the inclination plane and the anteversion plane, respectively, may be utilized independently of the individual dimensions of the pelvic bones in different patients, and that results are obtained for the values of the angle of inclination and the angle of anteversion that readily correspond to those achieved by much more complicated X-ray examinations.