Patent Publication Number: US-2011060339-A1

Title: Hip surgery assembly

Description:
PRIORITY CLAIM TO RELATED APPLICATIONS 
     This application claims priority to Dutch Application No. 1037265, filed Sep. 9, 2009, which application is incorporated herein by reference and made a part hereof in its entirety. 
     TECHNICAL FIELD 
     The present disclosure pertains to the field of surgery assistance apparatus, in particular to apparatus for assisting hip-replacement surgery. 
     BACKGROUND 
     Due to aging, disease or high load, patients may suffer from wear of joints (arthrosis). A common affliction is wear of the hip joint, in particular wear of the cartilage of the joint between the femoral head (head of the upper leg bone; convex part of the hip joint) and the interior of the acetabulum (bilateral cavity of the pelvis; concave part of the hip joint). Interaction and friction between the bare bone causes pain during movement. The pain and associated restriction in functionality further causes the patient to adapt its pattern of motions, in particular in walking. This again may lead to reactive pain in muscles and ligaments of the patient. Dependent on the amount of wear of the hip joint a total hip replacement operation may be performed. Goals of such operation are to enable pain-free movement and optimal regaining of active life prior to appearance of the symptoms. 
     In total hip replacement surgery both the femoral head and the acetabulum are replaced by prostheses. After resection of the worn femoral head a rod-like prosthesis is placed in the femoral shaft, usually made of titanium or cobalt-chrome which is provided with a ceramic or cobalt-chrome head, to replace the femoral head. Further, the interior side of the acetabulum is reamed and fitted with a acetabulum cup prosthesis, or cup for short, generally comprising polyethylene, titanium and/or ceramic material. The acetabulum cup replaces the original acetabulum. Both prostheses together form the new prosthetic joint. 
     Correct placement of both prostheses is crucial. Mal-positioning can lead to a number of post-operative complications such as impingement (contact of the components in non-desired locations as a consequence of motion) and limitation of the operative range of motion of the hip joint. Further, non-physiologic (directions of) forces and wear may accelerate wear and possibly detachment of (constituent parts of) the prosthetic joint. Such complications may therefore lead to hip-luxation. 
     Luxation is usually caused by a malpositioned acetabulum cup. In most cases the only solution is revision (restoration) surgery, in which the primary cup is replaced with a new cup. In order to avoid possible second malpositioning extra attention is required for proper positioning and placement of the new cup. 
     Aside from common risks associated with surgery—in particular of elder and/or delicate patients—an additional risk in cup revision surgery is that less bone is present for a good grip of the cup because usually significant amounts of bone material have been reamed away for fitting the primary cup. In almost all cases, this means longer duration of surgery, therefore additional risk of infections, and additional uncertainties for both the patient and surgeon. It is clear that positioning and placement of a cup should be done very carefully in both primary and revision surgery. For correctly positioning a cup it is well known that the angles of anteversion and inclination are determining factors. The angle of inclination is defined with respect to a frontal plane. The anteversion angle is defined with respect to a sagittal plane. 
     Despite a lack of consensus amongst orthopaedic surgeons on the correct value of these angles, a safe value for the angle of anteversion is generally considered to be between about 10 and 25 degrees with a preferred value of about 20 degrees. For the angle of inclination a range of about 35 to about 50 degrees is considered safe, with a preferred value of about 45 degrees. The preferred values allow a clinically acceptable margin of error. 
     Many orthopaedic surgeons are unaware that the colloquial anteversion angle can be divided in a true anteversion angle and a planar anteversion angle. The true anteversion angle is defined with respect to a pure sagittal plane and is independent of the position of the cup. The planar anteversion angle is defined in a plane perpendicular to the cup, the reference system thus is dependent on the position and orientation of the cup. Similar holds for true and planar inclination angles. See for more information e.g. L. Fabeck et al, “A method to measure acetabular cup anteversion after total hip replacement”, Acta Ortopaedica Belgica 65(4), 485-91 (1999). In other words, a variation in the angle of inclination affects the measured angle of planar anteversion. 
     Known instruments for measuring on and/or placing an acetabular cup do not distinguish or even allow for the distinguishing between true or planar anteversion angles. 
     Thus reference systems for determining correct orientation and position of a cup may be inaccurately defined and/or differently defined between different medical practitioners. Predictability and reproducibility are therefore adversely affected. 
     As reproducible surgery results are only possible with respect to well defined reference systems, the position of the pelvis before and during surgery needs to be known. The position may be determined with respect to palpable portions of the pelvis (e.g. spina iliaca anterior superior dextra and -sinistra and os pubis). During surgery the position of the pelvis may be obscured by surgical drapes. Changes of the position with respect to reference points defined beforehand may therefore become inaccurate during surgery; this may go unnoticed to the surgeon. A reliable reference system and reliable determination of the actebular position and orientation with respect to such reference system are therefore desired. 
     An existing system described in U.S. Pat. No. 6,623,488 allows checking for movement of the patient in one direction only, and requires repositioning of the patient during operation to the initial position which is cumbersome, failure-prone and impossible for some patients and/or surgical procedures. 
     Systems according to US 2009/0105714 or US 2004/0210233 require sophisticated navigation instruments and computers which render the system complex and expensive. During surgery personnel may further be required to be conscious about blocking radiation and/or communication between (different parts of) navigation instruments, such that operation of the system interferes with (concentration on) the surgical procedure. 
     The hip endoprothesis implantation accessory described in US 2005/0107799 allows measuring an angle for implantation with respect to a reference object. However, no solution is provided for accurately and correctly orienting the accessory and/or the reference object with respect to a desired and/or predetermined orientation for the acetabular cup prosthesis. Furthermore, the described method requires initially reconstructing the femoral head, which most surgeons prefer to do after positioning the acetabulum cup since the femoral head may obscure parts of the surgical space and hinder subsequent procedures. 
     US 2008/0132903 discusses a goniometer for measuring artificial acetabular cup angles and a method for measuring thereof using the goniometer. The document distinguishes between operative anteversion and inclination angles on the one hand and radiographic anteversion and inclination angles on the other hand. The goniometer comprises two measurement units (defined in the document with  130  and  140  respectively) allowing establishing two angles in one operation. The document discloses that the first angle measurement unit ( 130 ) is configured to measure an inclination angle, the second angle measurement unit ( 140 ) is configured to measure an anteversion angle. However, upon closer inspection the second angle measurement unit is prone to introducing errors and proves only able to measure the planar angle of anteversion. Furthermore the goniometer is complex, unwieldy and clutters up the surgical space and hindering accuracy. 
     As a consequence there is a desire for apparatus for improving hip surgery addressing one or more of the aforementioned problems. 
     SUMMARY 
     In order to provide such apparatus an assembly according to claim  1  is provided. Preferably the goniometer is configured such that the first axis of rotation is arrangable at least parallel to the axis of extension, preferably also perpendicular to the axis of extension. For increased freedom of operation, the first axis of rotation is arrangable substantially continuously between being perpendicular and parallel to the axis of extension. The goniometer allows to measure an orientation of the axis of extension of the surgical tool in a reliable manner with respect to a desired plane, as will be set out in more detail below. The first axis of rotation may be arranged substantially in any direction to be normal to the desired plane of measuring, such that a substantially pure measurement may be performed and the risk of determining projections, rather than actual angles is reduced or even prevented. The axes of rotation are separated such that relatively well-defined rotations may be performed, opposite to for example a ball-joint with which it is extremely hard to perform a rotation in a single plane, and not accidentally also perform a rotation in another direction, i.e. about another axis of rotation or an effectively tilted axis of rotation. Such inexact determination is a source of errors, e.g. incorrect identification of planar and true anteversion angles as indicated above. In addition, quantifying angles of rotation about a ball joint is very difficult if not next to impossible in practice. 
     The assembly of claim  2  further provides improvements regarding at least one of simplicity and robustness, manufacturing cost and user friendliness. 
     The assembly of claim  3  further provides accuracy and user friendliness in that in at least one direction a substantially pure rotation is measurable, without one or more portions also performing a translation, which requires space for maneuvering and may introduce errors. In case all first, second and third axes intersect at or close to one point a cardan-like arrangement may be provided. Such an arrangement allows concurrent correct determination of two independent angles with respect to different, perpendicularly arranged reference directions such as reference planes. The assembly of claim  6  allows to provide the further object, preferably an elongated object such as a Kirschner wire or K-wire, with a reliably determined direction for further referencing purposes. 
     The assembly of claim  7  may facilitate surgery and may reduce errors in determination, since chances of forgetting and/or mixing up of values are reduced. The assembly of claims  8  and  9  improve accuracy and reliability of the surgery procedure. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       These and other aspects will hereafter be more fully explained with reference to the drawings showing embodiments by way of example. It should be noted that like elements are indicated with like reference numerals in the appended drawings, in which: 
         FIG. 1  schematically shows a human pelvis and the main anatomical planes; 
         FIG. 2  is a perspective view of an embodiment of an apparatus for hip joint surgery; 
         FIGS. 3A-3C  show different acetabulum bodies, here bodies for determining the opening plane of an acetabulum or a acetabular cup; 
         FIG. 4  shows a human pelvis and a gauge tool; 
         FIGS. 5A-6  indicate use of the assembly for determining different angles; 
         FIG. 7  indicate fixing an established orientation to the patient using an elongated reference object; 
       FIGS.  8  and  9 A- 9 D are schematic views of alternative embodiments of the assembly; 
         FIGS. 10-13  show different embodiments of gauge tools. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a human pelvis  1  in reclined position with the main anatomical planes, i.e. the frontal plane or coronal plane  2 , the transversal plane  3  and the sagittal plane  4 , all perpendicular to each other. The intersection between the frontal plane  2  and the transversal plane  3  defines a transversal axis  5 , the intersection between the frontal plane  2  and the sagittal plane  4  defines a longitudinal axis  6 , the intersection between the transversal plane  3  and the sagittal plane  4  defines a sagittal axis  7 . The pelvis comprises an acetabulum  8  left and right and anatomical features such as the spina iliaca anterior superior  9  left and right. In a complete hip joint the head of the femurs (not shown) reside rotatably in the acetabula. 
     True anteversion is rotation about the transversal axis  5 , or an axis or rotation parallel to the transversal axis  5  also known as a quasi-transversal axis  5 , and in the sagittal plane  4  or in a plane parallel to sagittal plane  4  known as a quasi-sagittal plane. True inclination is rotation about the sagittal axis  7 , or a quasi-sagittal axis, and in the frontal plane  2  or in a quasi-frontal plane. 
       FIG. 2  shows a hip joint surgery assembly  10  comprising a goniometer  12  and a surgical tool  14 . The goniometer  12  is connected to the surgical tool  14  with a joint assembly  16 . The surgical tool  14  comprises a shaft portion  18 , extending along an axis of extension L and an acetabulum body  20 , here an acetabulum trial  20  known per se. The surgical tool  14  is provided with an optional portion  22  for holding a further object in a predetermined position with respect to the shaft portion  18 . 
     The goniometer  12  comprises an indicator  24  and a contraindicator  26 , here in the form of a blade-like body  26  provided with markings  28  and a reference structure in the form of a straight edge  29 . The indicator  24  and contraindicator  26  are continuously rotatable with respect to each other over an angle α about a first axis of rotation A 1  (not shown in  FIG. 2 ) defined by a rotary joint  30  having a pivot  32 . The range of angles α available in the shown embodiment, α tot , is approximately 180 degrees (90 degrees both clockwise and anticlockwise). 
     The joint assembly  16  comprises a first rotary joint  34  and second rotary joint  36 . The first rotary joint  34  has a first portion  37  rigidly connected to—here integral with—the indicator  24 , a pivot  38  and a second portion  39 . The first joint  34  is continuously rotatable over an angle β about a second axis of rotation A 2  which is substantially perpendicular to the axis of extension L of the shaft portion  18 . The range of angles β available in the shown embodiment, β tot , is approximately 180 degrees (90 degrees both clockwise and counter clockwise). 
     The second rotary joint  36  has a first portion  39  rigidly connected to—here being identical to—the second portion  39  of the first rotary joint  34 , a pivot (not visible) and a second portion rigidly connected or connectable to the shaft  18 —here being identical with the shaft  18 —. The second joint  36  is continuously rotatable with an angle γ to a total angle γ tot  of a full 360 degrees about a third axis of rotation A 3  substantially parallel to and here coinciding with the axis of extension L of the shaft portion  18 . The range of angles γ available in the shown embodiment, γ tot , is a full 360 degrees but in other embodiments it may be restricted if desired. 
     A suitable origin for the determination of the angles (α, β, γ)=(0, 0, 0) is defined when all three axes A 1 , A 2 , A 3  are mutually perpendicular and when the indicator  24  and contraindicator  26  are in a default position, as shown in  FIG. 2 . 
     The first rotary joint  34  interconnects the goniometer  12  with the second rotary joint  36 , the second rotary joint  36  interconnects the first rotary joint  34  with (the shaft portion  18  of) the surgical tool  14 . The first and second axes of rotation A 1 , A 2  and the second and third axes of rotation A 2 , A 3  are pairwise substantially perpendicular. The rotary joints  30 ,  34 ,  36  each define a rotation in a plane substantially perpendicular to the respective axes of rotation A 1 , A 2 , A 3 . It should be noted that the first axis of rotation A 1  may be arranged substantially parallel to the third axis of rotation A 3  by rotation about the second joint for 90 degrees. Thus, (the indicator  24  of) the goniometer  12  may be manipulated in a solid angle Ω (not shown) defined by the (combination of) angles β and γ about an origin defined by the intersection of axes A 2  and A 3 . In case the axes A 2  and A 3  are merely crossing without intersection, a solid angle Ω′ is defined, of which the origin and extent are not clearly recognisable from the geometry of the assembly, complicating use of the device. Depending on the use of the assembly  10  and in particular the surgical tool  14  during surgery the acetabulum trial  20  may be exchanged for different objects. Examples of different acetabulum bodies are an acetabulum tripod  40  shown in  FIG. 3A , for determining the position and orientation of the acetabular rim and aperture and/or holding an acetabulum cup prosthesis, an acetabulum cup measurement tool or an acetabulum cupholder  41  shown in  FIG. 3B , for measuring an acetabular cup, e.g. a previously placed prosthesis, or the acetabulum trial/acetabulum measuring tool  20  shown in  FIG. 2  and in more detail in  FIG. 3C . Other objects may also be employed. The tool  14  and the goniometer  12  with joint assembly  16  may be detachable, such that the surgical tool  14  may be operated on with other tools, e.g. a hammer for impacting an acetabulum cup. 
     A method for performing hip surgery may comprise determination of the orientation an position of the anatomical planes of the patient, e.g. by palpating the pelvis. As explained above the correct determination of the planes during the surgical procedure is of the utmost importance. To that end, a gauge tool may be fixed to the patient, preferably fixed to the body part operated on, here the pelvis  1 . An improved gauge tool  42  fixed to the left spina iliaca ant. sup. of the pelvis  1  is shown in  FIG. 4 . 
     The gauge tool  42  comprises fixing means  44 , an indicator portion  46  and a joint  48 . The shown fixing means  44  may be applicable through the tissue and comprises a pin  44  provided with means for fixing the pin  44  to the bone, e.g. a screw thread, one or more sharp tips etc. A plurality of fixing points prevents rotation of the pin with respect to the bone. 
     The joint  48  is configured to arrange and fix the indicator portion  46  in a desired position relative to the fixing means  44 . A joint  48  with a one or two degrees of rotational freedom may suffice, but that requires accurate placement and/or manipulation of the fixing means  44 . A joint  48  with three degrees of rotational freedom is therefore preferred, e.g. the shown ball-joint  48  with fixation means. 
     The indicator portion  46  comprises two plane bodies  50 ,  52  perpendicular to each other. To provide reference information the bodies  50  may be arranged parallel to the frontal plane  2  and the body  52  may be arranged parallel to the sagittal planes  4 , to define quasi-anatomical planes. An intersection line  54  between bodies  50 ,  52  then is parallel to the anatomical longitudinal axis  6 . The planes  50 ,  52  have side faces  56 ,  58  at straight angles such that the perimeters of the planes  50 ,  52  may provide further reference structures. In such orientation side faces  56 ,  58  at the cranial and/or caudal side of the indicator portion  46  define a quasi-transversal plane, the side face  56  defines a quasi-sagittal axis and the side face  58  defines a quasi-transversal axis. Other (anatomical) reference planes and axes may also be employed. 
     Using the gauge tool  42  the reference system is fixed to the relevant portion of the patient, e.g. the pelvis, thus following any (inadvertent) movement or displacement of that portion and obviating relying on non-patient-fixed reference objects such as the surgery table or the operating room. 
       FIGS. 5A-5C  show determination of a desired angle of an acetabulum  8  of a pelvis  1  using the goniometer  12 . The surgical tool  14  is arranged in a desired position with respect to the acetabulum  8 , here by insertion of an acetabulum trial  20 . The shaft  18  is brought in approximately a desired position. The goniometer  12  is manipulated by appropriately rotating the first and second joints  34 ,  36  such that the indicator  24  and counterindicator  26  lie in or parallel to a desired plane and the axis of rotation A 1  of the goniometer  12  points in the desired direction. The angle α to be determined is found by appropriately rotating the counterindicator  26 . 
       FIG. 5A  indicates correctly determining the true inclination angle of the left hip: the goniometer  12  is oriented into a quasi-frontal plane and the axis A 1  parallel to the sagittal axis  7 . The correct orientation of the goniometer  12  may be checked with respect to the appropriate portions of the gauge tool  42 . The first and second joints  34 ,  36  allow the indicator  24  to point in any direction, but the additional constraint of the direction of the axis A 1  fixes the angles β, γ of the first and second joints  34 ,  36  to unique values. The counterindicator  26  is then rotated to bring the straight edge  29  parallel to the longitudinal axis  6  of the patient or rather the quasi-longitudinal axis  54  of the gauge tool  42 . The parallel directions are indicated in  FIG. 5A  with dashed lines. The angle α indicated by the goniometer  12  then corresponds to the angle of inclination. 
       FIG. 5B  indicates correctly determining the true angle of the left hip with respect to the sagittal plane: the goniometer  12  is oriented into a quasi-sagittal plane and the axis A 1  parallel to the transversal axis  5 . The correct orientation of the goniometer  12  may be checked with respect to the appropriate portions of the gauge tool  42 . The counterindicator  26  is then rotated to bring the straight edge  29  parallel to the sagittal axis  7  of the patient or rather the quasi-sagittal axis indicated by side  56  of the gauge tool  42 . The parallel directions are indicated in  FIG. 5B  with dashed lines. The angle α indicated by the goniometer  12  then corresponds to the true angle of the acetabulum in the sagittal plane. Using this true angle, the true inclination angle, and the anatomical planes&#39; mutual perpendicular orientation, the true anteversion angle may be determined in straightforward manner. 
     The true anteversion angle can also be measured directly, by orienting the goniometer  12  in a quasi-transversal plane with the axis A 1  parallel to the longitudinal axis  6  and bringing the straight edge  29  parallel to the sagittal axis  7 . 
       FIG. 5C  indicates correctly determining the planar anteversion angle of the left hip: the goniometer  12  is oriented such that the indicator  24  is aligned parallel with the axis A 3  of the shaft  18 , here overlapping the axis A 3 . Thus, the first rotary joint  34  is arranged with zero rotation and the axis A 1  is arranged in a (quasi-)frontal plane, perpendicular to the main axis of the acetabulum  8  and parallel to the opening plane of the acetabulum, but the axis A 1  generally will not be parallel to an anatomic axis. The counterindicator  26  is then rotated to bring the straight edge  29  parallel to the sagittal axis  7  of the patient or rather the quasi-sagittal axis indicated by side  56  of the gauge tool  42 . The parallel directions are indicated in  FIG. 5C  with dashed lines. The angle α indicated by the goniometer  12  then corresponds to the angle of planar anteversion. 
     Similarly (but not shown), the planar angle of inclination may be found if the goniometer  12  is oriented such that the indicator  24  is aligned parallel with the axis A 3  of the shaft  18 , here overlapping the axis A 3 , arranging the first joint  34  with zero rotation and arranging the axis A 1  in a (quasi-) transversal plane, but it generally will not be parallel to an anatomic axis. The counterindicator  26  is then rotated to bring the straight edge  29  parallel to the longitudinal axis  6  of the patient or rather the quasi-longitudinal axis  54  of the gauge tool  42 . The angle α indicated by the goniometer  12  then corresponds to the angle of planar inclination. 
     It should be noted from the above that a goniometer with only two operative axes cannot provide the distinction between the values of true and planar anteversion and inclination. The second joint  36  allows manipulation of the goniometer  12  with respect to the shaft surgical tool  14  such that the surgical tool  14  need not be rotated with respect to the acetabulum  8  to correct the orientation of the goniometer  12  for proper determination of a desired angle. Such rotation could lead to damage of the acetabulum and/or an implanted acetabulum cup, and could lead to altering of the orientation of the shaft  18 , affecting determination of the combination of the required angles (inclination, anteversion). The substantially symmetric construction of the shown embodiment assists preventing asymmetric forces on the acetabulum and/or the assembly  10  and facilitates easy use. The assembly  10  may be held with one hand and the goniometer  12  may be manipulated with another hand. The goniometer may be made light-weight for further improving user friendliness. The desired angles may be measured for information or diagnostic purposes and a predetermined orientation for arranging a prosthesis may be established and/or checked. 
       FIG. 6  shows that a once established orientation may be fixed to the patient using a reference object. With the portion  22  such reference object  60 , preferably an elongated object such as a Kirschner wire  60  (or K-wire), a Steinmann pin etc. may be provided parallel to the direction of extension L of the shaft  18 . The assembly  10  may then be removed, if desired the gauge tool  42  as well ( FIG. 7 ), without significant loss of information and reliability. 
     Orientation of further objects during the surgery can then be compared with the reference object  60 . In alternative embodiments the reference object  60  can be provided with further features. Such further features may be for (additional) marking of reference directions such as rods, rings and/or planes, and/or for subsequent guiding a direction of a surgical tool. 
     Succinctly put: an initial reference system of anatomic planes X, is transferred to a patient-bound (or rather, to a relevant portion of the patient to account for partial motion) reference system X′ via the gauge tool  42 , and a reference direction Y is determined with respect to X′ via (the goniometer  12  of) the assembly  10 , which in turn is transferred to a patient-bound reference object Y′ (K-wire  60 ) for further use. Thus reducing or preventing displacement or reorientation of the patient (or the relevant portion of the patient) during a surgical procedure. 
     FIGS.  8  and  9 A- 9 D show second and third embodiments of an assembly  10 . In the second embodiment ( FIG. 8 ) the goniometer  12  the indicator  24  is formed as an arc in one plane perpendicular to the first axis A 1  and carrying markings  28 . The counterindicator  26  is formed as a pointer hand connected to the indicator. Operation of this embodiment is substantially identical to the first embodiment of  FIGS. 2 ,  5 A- 5 C, except that the counterindicator  26  should be aligned with the relevant axis, instead of the straight edge  29 . The second embodiment may further facilitate single-handed operation of the goniometer  12 . The shown second embodiment comprises a top portion  62  configured as an anvil such that the assembly  10  may be used as an impactor for applying an acetabulum cup. 
     In a variant of the first embodiment of  FIGS. 2 ,  5 A- 5 C, the indicator  24  may be configured to provide an anvil  62 . The portion  37  may for instance be a robust rod-like object provided with a first slot accommodating the counterindicator  26  and a second slot perpendicular to the first slot for acting as an indicator  24  to read a marking  28  on the counterindicator  26 . 
     As shown in  FIG. 8  an assembly  10  with an anvil portion  62  may be provided with a sleeve portion  64  about the shaft  18  which may be fixed around the first and second joints  34 ,  36  for fixing these with respect to (the axis of extension L of) shaft  18 . The sleeve  64  may have a recess  66  for accommodating a portion of the indicator  24  and/or counterindicator  26 . 
     In a variant (not shown) of the second embodiment the goniometer  12  may be constructed symmetrical e.g. by providing a further arc opposite the shown indicator  24 , or by forming the indicator as an arc over substantially 360 degrees. 
       FIGS. 9A-9B  show a third embodiment of an assembly  10  comprising a cardan-like configuration integrating the joint of the goniometer and the joint assembly of the first joint and the second joint, such that the axes of rotation A 1 , A 2  and A 3  intersect perpendicularly at all times at an origin O. The assembly  10  comprises a mounting portion  68  with a rail portion  69  tracing a circle segment about the origin O along the circumference of the mounting portion  68 . 
     The joint of the goniometer  12  is formed by two joint portions  30 A,  30 B. The first joint  34  is likewise provided as two joint portions  34 A and  34 B. The second joint  36  is formed by the joint portions  30 A,  30 B;  34 A,  34 B being slidably arranged along the rail portion  69 . The second joint  36  allows rotation of the mounting joint portions  30 A,  30 B,  34 A,  34 D with respect to surgical tool  14 . The assembly  10  provides two goniometers  12 ,  12 ′ which are usable concurrently. A first, substantially semicircular, arc-shaped indicator  24  provided with markings  28  is attached to the mounting portion  68  with rotary joints  30 A,  30 B providing an operative axis of rotation A 1 . The first indicator  24  is rotatable about an axis of rotation A 2 . A second, substantially semicircular, arc-shaped indicator  24 ′ provided with markings  28  is attached to the mounting portion  68  with the rotary joints  34 A,  34 B providing an operative axis of rotation A 2 . The second indicator  24 ′ is rotatable about an axis of rotation A 1 . 
     A counterindicator  26  is movably attached to both the first and second indicators  24 ,  24 ′. The counterindicator  26  is provided with reference structures  70 ,  72  which here lie in the planes of the indicators  24 ,  24 ′ respectively. The counterindicator  26  is configured such that the indicators  24  and  24 ′ are maintained in a mutual perpendicular orientation at the position of the counterindicator  26 . The counterindicator  26  is movable with respect to each indicator  24 ,  24 ′; a displacement of the counterindicator  26  along the first indicator  24  corresponds to a rotation of the counterindicator  26  with respect to the origin O about the axis A 1  and causes a rotation of the second indicator  24 ′ about the associated axis A 2 , due to the rotary joint  34 A,  34 B. The position of the counterindicator  26  with respect to the second indicator  24 ′ may remain stationary during the displacement with respect to the first indicator  24 . Likewise, the counterindicator  26  may be rotated about the axis A 2  and displaced with respect to the second indicator  24 ′ and remain stationary with respect to the first indicator  24 . A displacement of the counterindicator  26  to with respect to both indicators  24 ,  24 ′ to determine a particular angle is enabled by the joint portions  30 A,  30 B;  34 A,  34 B, respectively, being movable with respect to the mounting portion  68 . Thus the axes A 1  and A 2  are rotatable about the axis A 3 , while keeping the axes mutually pairwise perpendicular to each other; axes A 1  and A 2  by means of the counterindicator  26 , axes A 1  and A 3  by the joint portions  30 A,  30 B and axes A 2  and A 3  by the joint portions  34 A,  34 B. 
     Thus, the indicator  24  is rotatable about A 3  with respect to the surgical tool  14  and the operative orientation of the axis A 1  of the goniometer  12  may be adjusted into any desired orientation and an angle α may be determined as described for the second embodiment. 
     At the same time, the counterindicator  26  may be used to measure a second angle α′ with respect to the second indicator  24 ′. Since in general a goniometer can be used with either the indicator indicating a reference direction and the counterindicator indicating the direction to be determined, but also the other way around, in which case the angle α may carry a different sign or be offset by ±90 degrees. Since further the true anteversion angle and the true inclination angle are determined with respect to mutually perpendicular planes and axes, and the operative axes A 1 , A 2  of both goniometers  12 ,  12 ′ can be arranged in perpendicular planes without introducing an offset leading to a projection, both true angles may be determined concurrently. This is indicated in  FIG. 9B  for relatively large angles α, β.  FIGS. 9C-9D  show a variant of the embodiment of  FIGS. 9A-9B  in the similar positions as  FIGS. 9A-9B . In  FIGS. 9C-9D  each goniometer  12 ,  12 ′ is rotatably connected to the surgical tool with an individual second joint  36 ,  36 ′, respectively, attached to the joint portions  30 A,  30 B;  34 A,  34 B, respectively, with rod-like structures  39 ,  39 ′. The rod-like structures  39 ,  39 ′ are configured such that the joint portions  30 A,  30 B;  34 A,  34 B are arranged in one plane, so as to provide a cardan-like arrangement with all axes A 1 , A 2 , A 3  continuously intersecting at the origin O. Thus, operation of the assembly  10  is as described before. The embodiment of  FIGS. 9C-9D  may reduce weight of the assembly  10  with respect to the embodiment of  FIGS. 9A-9B  and the individual second joints may provide reduced friction and smoother operation than the embodiment of  FIGS. 9A-9B . 
       FIGS. 10-13  show various embodiments of a gauge tool.  FIG. 10  shows a simple embodiment of a gauge tool  42 , comprising an anvil portion  62  for insertion into bone by hammering. Reference axes are indicated with rod-like structures  74 ,  76  and  78  attached to the fixing portion  44 . With such embodiment significant care is required during fixation to ensure correct positioning. 
     The gauge tool  42  of  FIG. 11  is a combination of the embodiments of  FIGS. 4 and 10 . Depending on the robustness of the ball joint  48  this embodiment may be hammered into the bone or fixed via other means. This embodiment allows adjustment of the orientation of the reference structures  74 - 78 . 
     The gauge tool of  FIG. 12  combines an indicator portion as in the embodiment of  FIG. 4  with the simplicity of the gauge tool of  FIG. 10 . The plane  52  may comprise an anvil portion. 
     The gauge tool of  FIG. 13  comprises a joint  48  in the form of a joint assembly  80  in turn comprising a cardan joint  82  and a rotary joint  84  allowing swivelling about an axis of extension of the fixing portion  44 . This allows to arrange the indicator portion  46  in substantially any orientation, in which orientation the indicator portion  46  may be fixed by any suitable means, e.g. a wing nut or a quick-release skewer (not shown). 
     The disclosure is not restricted to the above described embodiments which can be varied in a number of ways within the scope of the claims. For instance, the goniometer may be formed separable from the surgical tool, e.g. as an accessory to an existing acetabulum cup impactor. 
     The goniometer, the surgical tool and the gauge tool may be marketed and sold separately and/or as kit of parts. 
     The gauge tool may be modular such that different indicator portions may be mounted to the joint of the gauge tool if so desired, damaged indicator portions and/or fixing portions be exchanged and/or facilitating sterilisation of the tool. The fixing means may comprise a screw thread, one or more sharp tips etc. A plurality of fixing points prevents rotation of the pin with respect to the bone. The pivot  32  of the goniometer  12  of the first and second embodiments may be provided with a reference structure such as a protrusion or a rod-like structure for clearer indication of the direction of its axis of rotation A 1 . 
     An assembly may comprise two separate goniometers, e.g. provided on different branch portions of a Y-shaped shaft portion  18 . Such assembly may be used for educational purposes and/or facilitate concurrent measuring of plural angles. 
     Elements and aspects discussed for or in relation with a particular embodiment may be suitably combined with elements and aspects of other embodiments, unless explicitly stated otherwise.