Alignment apparatus for use in hip arthroplasty

Abstract

Hip arthroplasty apparatus and methods are described to determine an orientation of an acetabular cup impactor, the acetabular cup impactor being moveable to a desired orientation relative to a patient's pelvic region for implantation of an acetabular cup. In one embodiment an electronic orientation sensor is transitionable between a first location on the patient's pelvic region and a second location on the acetabular cup impactor. At the first location, the orientation sensor is adapted to record a reference orientation of the patient's pelvic region. At the second location the orientation sensor is adapted to determine an orientation of the acetabular cup impactor relative to the reference orientation.

Claims

1. A method of determining an orientation of an acetabular cup impactor, comprising: recording a reference orientation of a patient's pelvic region using an electronic orientation sensor located at a first location on the patient's pelvic region, wherein in the first location the electronic orientation sensor is not coupled with a mechanical device while the mechanical device is engaged within an acetabulum of the patient, and monitoring an orientation of an acetabular cup impactor relative to the reference orientation using the electronic orientation sensor when the electronic orientation sensor is located at a second location on the acetabular cup impactor after being transitioned to the second location from the first location, the acetabular cup impactor being moveable to a desired orientation relative to the patient's pelvic region for implantation of an acetabular cup into the acetabulum.

2. The method of claim 1, further comprising engaging the orientation sensor and the pelvic region with a first mount to releasably fix the positions of the orientation sensor and the pelvic region relative to each other when the orientation sensor is located at the first location.

3. The method of claim 1, further comprising engaging the orientation sensor and the impactor with a second mount to releasably fix the positions of the orientation sensor and the impactor relative to each other when the orientation sensor is located at the second location.

4. The method of claim 1, wherein monitoring the orientation of the acetabular cup impactor relative to the reference orientation comprises monitoring the orientation in three-dimensional space.

5. The method of claim 4, wherein monitoring the orientation of the acetabular cup impactor relative to the reference orientation comprises monitoring an angle of anteversion and an angle inclination of the acetabular cup impactor.

6. The method of claim 1, further comprising providing the monitored orientation of the impactor or the recorded reference orientation to a clinician or other user.

7. The method of claim 6, wherein providing the monitored orientation of the impactor or the recorded reference orientation comprises displaying the orientation of the impactor or the recorded reference orientation on a display.

8. The method of claim 7, wherein the display is comprised in the electronic orientation sensor.

9. The method of claim 1, wherein the orientation sensor is comprised in a smartphone or a tablet computer.

10. The method of claim 1, wherein the monitored orientation or the reference orientation is determined using one or more of an accelerometer, a magnetic field sensor and a gyroscope comprised in the orientation sensor.

11. A non-transitory computer readable storage medium having stored thereon instruction which, when executed, cause a processor to perform a method comprising: recording a reference orientation of a patient's pelvic region using an electronic orientation sensor located at a first location on the patient's pelvic region, wherein in the first location the electronic orientation sensor is not coupled with a mechanical device while the mechanical device is engaged within an acetabulum of the patient, and monitoring an orientation of an acetabular cup impactor relative to the reference orientation using the electronic orientation sensor when the electronic orientation sensor is located at a second location on the acetabular cup impactor after being transitioned to the second location from the first location, the acetabular cup impactor being moveable to a desired orientation relative to the patient's pelvic region for implantation of an acetabular cup into the acetabulum.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) By way of example only, embodiments are now described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows apparatus according to an embodiment of the present disclosure with an electronic device at a first location;

(3) FIG. 2 shows the apparatus of FIG. 1 with the electronic device at a second location;

(4) FIG. 3 shows a schematic view of elements of the electronic device of FIG. 1;

(5) FIG. 4 shows a pelvic calibration display screen from the electronic device of FIG. 1;

(6) FIG. 5 shows an impactor orientation display screen from the electronic device of FIG. 1;

(7) FIG. 6 shows a display screen from an electronic device used in another embodiment of the present disclosure; and

(8) FIG. 7 shows apparatus according to another embodiment of the present disclosure.

(9) FIG. 8 shows apparatus according to another embodiment of the present disclosure;

(10) FIG. 9 shows an image of a pelvic region captured by a camera of the apparatus of FIG. 8;

(11) FIG. 10 shows a schematic view of elements of an electronic device used in the apparatus of FIG. 8;

(12) FIG. 11 shows an outline of the area covered by the image of FIG. 9, with guidelines positioned at different locations in the area, the guidelines being indicative of positions in the area that corresponding to 10° intervals within the field of view of the camera;

(13) FIG. 12 shows a plurality of marker lines, each positioned with reference to one of the guidelines of FIG. 11, the marker lines being for guiding positioning of an acetabular cup impactor of the apparatus of FIG. 8;

(14) FIG. 13 shows the plurality of marker lines of FIG. 12 overlaid on the image of FIG. 9, with the acetabular cup impactor in a first position relative to the pelvic region;

(15) FIG. 14 shows the plurality of marker lines of FIG. 12 overlaid on the image of FIG. 9, with the acetabular cup impactor in a second position relative to the pelvic region;

(16) FIG. 15 shows apparatus according to another embodiment of the present disclosure;

(17) FIG. 16 shows an image of an acetabular cup impactor captured by a camera of the apparatus of FIG. 15;

(18) FIG. 17 shows a schematic view of elements of an electronic device used in the apparatus of FIG. 15;

(19) FIGS. 18a to 18d show calibration markers overlaid on images captured by the camera of the apparatus of FIG. 15;

(20) FIG. 19 shows an alignment marker overlaid on an image captured by the camera of the apparatus of FIG. 15;

(21) FIG. 20 shows apparatus according to another embodiment of the present disclosure;

(22) FIG. 21 shows an image of an acetabular cup impactor captured by a camera of the apparatus of FIG. 20;

(23) FIGS. 22a to 22d show calibration markers overlaid on images captured by the camera of the apparatus of FIG. 20;

(24) FIG. 23 shows an alignment marker overlaid on an image captured by the camera of the apparatus of FIG. 20.

DESCRIPTION OF EMBODIMENTS

(25) FIGS. 1 and 2 show apparatus according to an embodiment of the present disclosure. The apparatus includes an acetabular cup impactor 1, adapted to drive and implant an acetabular cup 11 into position at the acetabulum of a patient's pelvic bone 12, and an electronic device 2, the electronic device 2 being adapted to be located at a first location on the pelvic region (see FIG. 1) and subsequently located at a second location on the acetabular cup impactor 1 (see FIG. 2).

(26) With reference also to FIG. 3, the electronic device 2 acts at least in part as an orientation sensor through inclusion of a gyroscope 21, a magnetic field sensor 22 and an accelerometer 23 connected to a processor 24. In alternative embodiments, one or more of these sensors may be excluded. For example, the accelerometer 23 may be excluded or otherwise. The electronic device 2 further includes an input device connected to the processor 24 that is in the form of a touch screen display 25, which touch screen display 25 also provides an output device in conjunction with a speaker 26. A memory 27 is provided for data storage and retrieval. In this embodiment, the electronic device 2 is a Smartphone, e.g. an iPhone™, although a variety of different electronic devices may be used. Further, the sensors, processor, input and output devices need not be integrated into a single device. For example, in one embodiment, the display and speaker may be maintained at a location that is remote from the pelvic region and impactor, and may communicate with the processor 24 via wires or wirelessly.

(27) The acetabular cup impactor 1 includes a shaft 13 extending distally from the acetabular cup/pelvic region, and a handle 14 at the distal end of the shaft. In this embodiment, when at the second location as shown in FIG. 2, the electronic device 2 is releasably fixed to the distal end of the handle 14 such that planar face of the electronic device, which includes the display 25, is fixed at an orientation that is substantially orthogonal to the impactor shaft 13. A mount (not shown) is adapted to clamp the electronic device 2 to the handle 14. The electronic device 2 may be encased in a plastic covering. The plastic covering may hermetically seal the electronic device 2.

(28) The gyroscope 21, magnetic field sensor 22 and accelerometer 23 of the electronic device provide in combination with the processor 24 an orientation sensor that can track orientation of the electronic device 2, and hence the acetabular cup impactor 1 when mounted thereon. By sensing movement of the electronic device 2 within the surrounding gravitational and magnetic fields, and optionally also acceleration and deceleration of the device 2, changes in orientation about three orthogonal axes of a coordinate system can be monitored.

(29) In use, as part of a calibration process, the electronic device 2 is mounted at the first location on the pelvic region of the body as shown in FIG. 1. In particular, in this embodiment in which the patient is in a supine position, it is mounted so that its bottom edge substantially lines up with a vector line extending between right and left anterior superior iliac spines (ASIS) of the pelvic bone 12. In alternative embodiments, the electronic device may be mounted so that its bottom edge is at a different angle to this vector line, such as a 45 degree angle. In FIG. 1 and subsequent Figures, for simplicity, the pelvic bone of the patient is represented independently of any other body parts or body tissue. In practice, other body parts and body tissue would, of course, be present.

(30) When the electronic device 2 is at the first location, the display 25 is adapted to display a pelvic calibration screen 3 as represented in FIG. 4. Three touch-screen buttons are provided on the screen 3. One of the buttons 31 enables input of the hip side of the patient, in particular so that a clinician or other use can indicate if the hip replacement is being carried out in relation to the left or right hip. Another of the buttons 33 enables input of the positioning of the patient, in particular so that the clinician or other user can indicate if the patient is in a supine or a lateral orientation. Finally, a zero button 32 is provided, which is to be pressed once the positioning of the patient and hip side have been inputted, and once the electronic device 2 is securely positioned at the first location (i.e. at the appropriate calibration position). When the zero button 32 is pressed, the electronic device 2 records its orientation, and hence the orientation of the pelvic region, and uses this as a reference orientation against which all subsequent changes in orientation of the electronic device 2 are compared.

(31) After calibration (‘zeroing’), the electronic device 2 is transitioned from the first location on the pelvic region to the second location on the impactor 1, in particular at the distal end of the handle 14 as shown in FIG. 2, where it displays an impactor orientation screen 4 as represented in FIG. 5. As it transitions from the calibration position, the electronic device 2 continually monitors changes in its orientation relative to the reference orientation such that, when mounted on the handle 14, it immediately knows its orientation, and hence the orientation of the impactor shaft 13, relative to the reference orientation. The electronic device 2 can therefore display on the screen 4 the orientation of the impactor shaft 13 relative to the reference orientation (in terms of angle of anteversion 41 and angle of inclination 42 in this embodiment) and it can monitor and update the orientation on the screen, as it moves with the impactor 1 thereafter. Thus, the clinician or other user can observe the angles of anteversion and inclination in ‘real-time’ on the display, allowing him/her to move the acetabular cup impactor 1 to a desired orientation. The desired orientation may be an angle of 20° anteversion and 45° inclination or otherwise. Once completed, or if recalibration of the reference orientation is desired, a button 43 can be pressed to restart the procedure.

(32) Example mathematics that may be employed in this or other embodiments is set forth below, where:

(33) RI=radiographic inclination pelvic reference frame

(34) RA=radiographic anteversion pelvic reference frame

(35) AI=anatomic inclination pelvic reference frame

(36) AA=anatomic anteversion pelvic reference frame

(37) ri=radiographic inclination gravity reference frame

(38) ra=radiographic anteversion gravity reference frame

(39) ai=anatomic inclination gravity reference frame

(40) aa=anatomic anteversion gravity reference frame

(41) y′−y=yaw

(42) r=roll

(43) P=pelvic roll

(44) Assuming no pelvic roll:

(45) Yaw gives radiographic inclination (RI)

(46) Roll gives radiographic anteversion (RA)

(47) To convert to anatomic anteversion (AA) and anatomic inclination (AI) per Murray (D. W. Murray: The definition and measurement of acetabular orientation. J Bone Joint Surg [Br] 1993; 75-B: 228-32):
Tan(AA)=Tan(RA)/Sin(RI)
Cos(AI)=Cos(RI)*Cos(RA)
Therefore:
Anatomic Anteversion=arctan(tan(r)/sin(y′−y))
Anatomic Inclination=arcos(cos(y′−y)*cos(r))
If there is pelvic roll ‘yaw’ is calculated about a vertical axis that has rolled and roll calculated against the same axis.
Supine position with pelvic roll to the right in a right hip:
AA−P=aa
AA=aa+P
AI=ai
ra=r
ri=y′−y
Cos(AI)=cos(ai)
=Cos(ri)*Cos(ra)
AI=arccos(cos(y′−y)*cos(r))
AA=arctan(tan(r)/sin(y′−y))+P

(48) And for a left hip:
AI=arccos(cos(y−y′)*cos(r))
AA=arctan(tan(r)/sin(y−y′))−P.

(49) In another embodiment of the present disclosure, the apparatus described above with reference to FIGS. 1 to 4 is adapted for use in tracking changes in orientation of the pelvic region during surgery. An electronic device is mounted to the pelvis, e.g. as represented in FIG. 1. However, after carrying out a calibration process as described with reference to FIG. 4, the electronic device 2 is maintained in position on the pelvic region and is used to track motion of the pelvic region in at least two rotational axes (pitch (tilt) and roll) or preferably three rotational axes (pitch, roll and yaw). The device 2 is adapted to display a pelvis tracking screen 5 as represented in FIG. 6, which presents the current orientation of the pelvis substantially in ‘real-time’ during the surgical procedure. The electronic device 2 is adapted to record the pelvic movement in the memory 27 throughout the surgical procedure. In one embodiment, predetermined limits on the degree of motion of the pelvis are inputted by the clinician into the electronic device 2, and an audible signal using the speaker 26 or other type of alarm is provided as a warning when these limits are exceeded.

(50) In yet another embodiment, the approach described with respect to the two preceding embodiments is combined through the provision of two electronic devices 2a, 2b. Referring to FIG. 7, a first one of the electronic devices 2a is used as described above to record a reference orientation of the pelvic region prior to transitioning to the second location where it determines the orientation of the impactor 1 relative to the reference orientation. Further, a second one of the electronic devices 2b is used as described above to record a reference orientation of the pelvic region and is then maintained on the pelvic region to track changes in orientation of the pelvic region during surgery. The second electronic device 2b is adapted to wirelessly communicate with first electronic device 2a to provide information about changes in the orientation of the pelvic region, allowing correction of the reference orientation recorded by the first electronic device 2a to be made substantially in ‘real-time’.

(51) FIG. 8 shows apparatus according to an embodiment of the present disclosure. The apparatus includes an acetabular cup impactor 10, adapted to drive and implant an acetabular cup 110 into position at the acetabulum of a patient's pelvic bone 120, and an electronic device 20, the electronic device 20 being mounted on the impactor 10. With reference also to FIG. 10, the electronic device 20 includes an image capture device in the form of a video camera 210, a digital display 220, a tilt sensor 230, a processor 240, a touch keypad 250 and a memory 260 for data storage and retrieval. In this embodiment, the electronic device 20 is a Smartphone, e.g. an iPhone™, although a variety of different electronic devices may be used. The camera 210, display 220, tilt sensor 230 and processor 240 need not be integrated into a single device 20, nor mounted on the impactor 10. For example, in one embodiment, the display and/or processor may be located remotely from the impactor 10.

(52) The electronic device 20 is releasably fixed to the shaft 130 of the impactor 10 via a mount 30 such that the camera of the electronic device faces the pelvic bone 120 and, more generally, the pelvic region of the patient. The mount 30 is adapted to clamp to the shaft 130 of the impactor 10 through provision of a sleeve portion 310 that at least partially extends around the impactor shaft 130. The mount 30 is also adapted to clamp to the electronic device 20 through provision of one or more arms 320 that project from the sleeve portion 310 and abut opposing sides or edges of the electronic device 20. The electronic device 20 may be encased in a plastic covering. The plastic covering may hermetically seal the electronic device 20.

(53) The camera 210 of the electronic device 20 is adapted to sequentially capture a plurality of images of the pelvic region of the patient (i.e. video the pelvic region of the patient), and the images are presented, substantially in ‘real time’, on the display 220. The pelvis 120 includes a first marker 140 thereon, more particularly a vector line 140 extending between right and left anterior superior iliac spines (ASIS) 121 that is imagined or drawn on bone and/or tissue between ASIS 121. With reference to FIG. 9, which shows an example image (frame) 270 as presented on the display 220, the ASIS vector line 140 is represented in the image 270. In FIG. 8 and subsequent Figures, for simplicity, the pelvic bone 120 of the patient is represented independently of any other body parts or body tissue. In practice, other body parts and body tissue would, of course, be present.

(54) The processor 240 of the electronic device 10 is adapted to receive orientation data related to the impactor 10 (and the acetabular cup 110). In this embodiment, the patient is located in a supine position, and the orientation data received by the processor 240 includes a desired inclination angle for the impactor and measured anteversion angles for the impactor. The desired inclination angle, which is 45° in this example, is input into the electronic device 20 using the touchscreen keypad 250. The anteversion angle is continually measured using the tilt sensor of the electronic device 20.

(55) Based on the received orientation data, and with reference to FIGS. 13 and 14, the processor 240 is adapted to overlay one or more second markers, more particularly alignment lines 271a-e, in images 270a, 270b displayed by the display device 220 such that, when the ASIS vector line 140, as seen in the images, is substantially aligned with one or more of the alignment lines 271a-271e, the acetabular cup impactor 10 will be oriented at the desired angle of inclination.

(56) In order to provide this guidance for the inclination angle, the processor 240 is adapted to determine the appropriate orientation for the plurality of alignment lines 271a-e, when overlaid at respective positions in the images 270. The appropriate orientation of the alignment lines 271a-e, when overlaid in the images, is partially dependent on the position in the images at which they are to be overlaid, due to the angular range of the field of view of the camera. This means that the orientations of items as seen within images, such as the ASIS vector line 140, are dependent not only on their actual orientation relative to the impactor 10, but on where in the field of view of the camera those items are positioned.

(57) In this embodiment, the processor 240 is adapted to overlay five alignment lines 271a-e in the images 270a, 270b in accordance with equally spaced angular distances along the vertical axis of the field of view of the camera 210. In this embodiment, the camera 210 has a field of view of about 50° to 60° and the alignment lines are located, and their orientation determined, with respect to angular distances in the vertical axis of −20°, −10°, 0°, +10° and +20°, from the central horizontal axis of the camera's field of view. These angular distances are represented by guidelines 272a-e in FIG. 11, where FIG. 11 shows an outline 273 of the area covered by the image 270 of FIG. 9.

(58) Using Equation 1, the processor 240 is adapted to determine for each angular distance (d) from the central horizontal line within the field of view of the camera, and for a measured anteversion angle (x) and a desired inclination angle (y), the angle (g) at which to orient alignment lines 271a-e that are to be overlaid in the images presented on the display.
tan g=tan(y).Math.sin(x+d)  Equation 1

(59) Example orientations for the alignment lines 271a-e as determined using Equation 1 for each of the angular distances (d) are represented in FIG. 12, each alignment line 271a-e being overlaid next to a respective guideline 272a-272e. The orientations angles (g) can continually change as a result of the measured anteversion angle (x) changing as indicated above, and thus the alignment lines 271a-e can be seen to rotate within the screen as the impactor 1 is moved.

(60) FIG. 13 shows a first image 270a as seen on the display by the surgeon, when the alignment lines 271a-271e have been overlaid by the processor 240. In the corner of the image 270a, the measured anteversion angle 274 is presented and continually updated as the impactor 10 moves.

(61) The desired angle of inclination of the impactor 10 is achieved when the ASIS vector line 140 is substantially aligned with the nearest alignment line or lines 271a-e. In FIG. 13, the vector line 140 can be seen in image 270a positioned nearest the top two alignment lines 271a, 271b. The vector line 140 is substantially misaligned with these alignment lines 271a, 271b. This indicates that the impactor 10 is not at the desired angle of inclination. Furthermore, the anteversion angle 274 as presented on the display is at 23°, rather than a desired angle of 20°.

(62) However, through movement of the impactor 10, and observation of the display 220, the surgeon can move the impactor 10 to a position as represented in the image 270b of FIG. 14. In this image 270b, the vector line 140 is substantially aligned (i.e. substantially parallel) with the nearest alignment lines 271a, 271b and the anteversion angle 274 as presented on the display is at the desired angle of 20°. At this point, the desired orientation of the impactor 10, and thus the acetabular cup 110 connected to the impactor 10, is achieved.

(63) As indicated, in this embodiment, the patient is in a supine position. However, the approach described above can be carried out, mutatis mutandis, with a patient in the lateral recumbent position. In this variation, the tilt sensor will provide the angle of inclination of the impactor, and the alignment lines will be used instead to arrive at the desired angle of anteversion. More particularly, when the ASIS vector line, as seen in the images, is substantially aligned with one or more of the alignment lines, the acetabular cup impactor will be oriented at the desired angle of anteversion.

(64) Equation 2 can be utilised in place of Equation 1. In particular using Equation 2, the processor is adapted to determine for each angular distance (d) from a central horizontal line within the field of view of the camera, and for a measured inclination angle (y) and a desired anteversion angle (x), the angle (g) at which to orient alignment lines that are to be overlaid in the images presented on the display.
tan g=tan(x).Math.sin(y+d)  Equation 2

(65) FIG. 15 shows apparatus according to another embodiment of the present disclosure. The apparatus includes an acetabular cup impactor 10, adapted to drive and implant an acetabular cup 110 into position at the acetabulum of a patient's pelvic bone 120, and an electronic device 200, the electronic device 200 being mounted to the pelvic region, e.g. on the pelvic bone 120. With reference also to FIG. 17, the electronic device 200 includes an image capture device in the form of a video camera 201, a digital display 202, a processor 203, a touch keypad 204 and a memory 205 for data storage and retrieval. A tilt sensor may also be included. In this embodiment, the electronic device 200 is a tablet, e.g. an iPad™ although a variety of different electronic devices may be used. The camera 201, display 202, and processor 203 need not be integrated into a single device 200, nor all mounted on the pelvic region. For example, in one embodiment, the display and/or processor may be located remotely from the pelvic region.

(66) The electronic device 200 is releasably fixed to the pelvic bone 120 or pelvic region via a mount (not shown) such that the camera 201 of the electronic device 200 faces the impactor 10. The electronic device 200 may be encased in a plastic covering. The plastic covering may hermetically seal the electronic device 200.

(67) The camera 201 of the electronic device 200 is adapted to sequentially capture a plurality of images of the impactor 10 and the images are presented substantially in ‘real time’ on the display 202.

(68) A navigation element 40 in the form of two circular disks 410, 420, connected together by a spacer 430, is releasably mounted to the distal end of the impactor 10. The two disks 410, 420 are concentric and the centres of the disks 410, 420 are aligned with the longitudinal axis of the impactor 10. The disk 410 closest to the impactor 10 has a smaller diameter than the disk 420 furthest from the impactor 10. The edges 401, 402 of the disks define circles that provide two first markers. With reference to FIG. 16, which shows an example image (frame) 206 as presented on the display 202, the two first markers 401, 402 are visible in the image 206.

(69) The processor 203 of the electronic device 200 is adapted to receive orientation data related to the impactor 10 (and the acetabular cup 110). In this embodiment, the patient is located in a supine position, and the orientation data received by the processor includes a desired inclination angle and a desired anteversion angle for the impactor. The desired inclination and anteversion angles, which are 45° and 20°, respectively, in this example, are input into the electronic device 200 using the touchscreen keypad 204.

(70) In this embodiment, a calibration procedure is performed to determine the pivot point of the impactor 10 relative to the camera 201 and the positions of the first markers along the longitudinal axis of the impactor 10. With reference to FIG. 18a, during the calibration procedure the processor 203 is adapted to overlay a third marker 208 in a first position in images 206a displayed by the display device 202. The impactor 10 is then moved by a surgeon, generally in a direction as indicated by arrow 209, such that one of the disks, in particular the larger disk 402 in this embodiment, is aligned with the third marker 208. Once aligned, the user is required to touch the screen, or ‘click’ a cursor on the screen, at the position in the image at which the other of the disks, in particular the smaller disk 401 in this embodiment, is located. This process is repeated for a number of different positions (e.g. second to fourth positions) of the third marker 209, as represented in images 206b-206d of FIGS. 18b to 18d. This enables a determination to be made of the exact and relative positions of the two first markers 401, 402 in the images 206a-206d, and through application of trigonometric functions, calibration data including the pivot position of the impactor relative to the camera, and the positions of the first markers along the longitudinal axis of the impactor, can also be determined.

(71) Based on the calibration data and the received orientation data (i.e. the desired inclination and anteversion angles), the processor 203 is adapted to determine where in the displayed images a second marker 211 should be located to guide the impactor so that it has the desired inclination and anteversion angles. In this embodiment, with reference to FIG. 19, the processor 203 is adapted to overlay the second marker 211 in the images 206e displayed by the display device 202 such that, when the larger disk 402, as seen in the images, is substantially aligned with the second marker 211, the acetabular cup impactor 10 will be oriented at the desired orientation.

(72) In a variation of this embodiment, the processor is adapted to use feature detection to determine the positions and shapes of the first markers 401, 402 within the images 206. The feature detection may be used in place of a user being required to touch or ‘click’ on the position of one of the first markers 401, in order to identify the position of that marker. Alternatively, feature detection may be used to remove the need for the calibration procedure entirely.

(73) In more detail, to the extent that the centre of the camera 201 is misaligned with the longitudinal axis of the impactor 10, the first markers 401, 402 will appear as ellipses in the images 206. The shape (e.g. minor to major axis ratio) and relative positioning of the ellipses is dependent on the angle at which the impactor 10 is located. Following from this, feature detection can be used to determine the inclination and anteversion angles for the impactor 10, and these angles can be presented by the processor 203 substantially in ‘real time’ on the images 206, e.g., within boxes 2011a, 2011b in the image 206 as shown in FIG. 16. This enables a surgeon to move the impactor 10 to the desired orientation based on observation of changes to the displayed angles. Alternatively or additionally, based on the feature detection and user input of the desired inclination and anteversion angles, a second marker can be overlaid on the images to guide movement of the impactor 10 to the desired orientation.

(74) With reference to FIG. 20, in an alternative embodiment, apparatus is provided that is substantially identical to the apparatus shown in FIG. 15, but which employs a different type of navigation element, in particular a navigation element in the form of a sphere 400 that is releasably mounted at the distal end of the impactor 10. The sphere 400 provides a first marker. With reference to FIG. 21, which shows an example image (frame) 212 as presented on the display, the first marker 400 is visible in the image 212.

(75) Again, in this embodiment, a calibration procedure is performed to determine the pivot point of the impactor relative to the camera 201, and the positions of the first marker 400 along the longitudinal axis of the impactor 10. With reference to FIG. 22a, during the calibration procedure the processor 203 is adapted to overlay a third marker 213 in a first position in images 212a displayed by the display device. The impactor 10 is then moved by the surgeon, generally as indicated by arrow 214, such that the first marker 400 is aligned with the third marker 213. Once aligned, the user is required to touch the screen, or ‘click’, at one of a plurality of guidelines 215a-215e that are overlaid on the screen, which guideline 215a-215e has the closest angular relationship to the angle of extension of the shaft 130 as seen within the image 212a. This process is repeated for a number of different positions (e.g. second to fourth positions) of the third marker 213, as represented in images 212b-212d of FIGS. 22b to 22d. This enables a determination to be made of the positioning of the first marker 400 and the angle of extension of the shaft 130 of the impactor 10 within the images, and through application of trigonometric functions, calibration data including the pivot position of the impactor relative to the camera, and the positions of the first marker along the longitudinal axis of the impactor, can also be determined.

(76) Based on the calibration data and the received orientation data (i.e. the desired inclination and anteversion angles), the processor 203 is adapted to determine where in images a second marker 216 should be located to guide the impactor 10 so that it has the desired inclination and anteversion angles. In this embodiment, with reference to FIG. 23, the processor 203 is adapted to overlay the second marker 216 in the images 212e displayed by the display device 22 such that, when the sphere 400, as seen in the images, is substantially aligned with the second marker 216, the acetabular cup impactor 10 will be oriented at the desired orientation.

(77) While the use of navigation elements, feature detection, and calibration steps, etc., is described in conjunction with FIGS. 15 to 23, where the image capture device is mounted to the pelvic region, substantially the same navigation elements, feature detection, and calibration steps, etc., may be employed, mutatis mutandis, when the image capture device is mounted on the impactor 10, e.g. as shown in FIG. 8. In this variation, navigation elements similar to those described in FIGS. 15 to 23 may be mounted on the pelvic region, for example.

(78) It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.