Retro-reflectors, markers and marker arrays

Abstract

Disclosed herein are retro-reflectors, markers and marker arrays for use with surgical tracking systems. Such a retro-reflector comprising a transparent optical window, a mask defining an electromagnetic radiation transmissible aperture into the transparent optical window, and a retro-reflective assembly comprising: a set of retro-reflective structures to reflect electromagnetic radiation received through the electromagnetic radiation transmissible aperture back through the electromagnetic radiation transmissible aperture; the set of retro-reflective structures having an extent that extends beyond a normal projection of the mask onto the set of retro-reflective structures.

Description

Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO RETRO-REFLECTORS, MARKERS AND MARKER ARRAYSTECHNICAL FIELD

[0001] The present application generally relates to retro-reflectors markers and markerarrays for optical tracking in, for example, Computer Assisted Surgery (CAS) systems(CASS) and methods. BACKGROUND

[0002] Optical tracking in CAS systems assist physicians to localize, navigate to andtreat the targeted tissues. Placed on or close to the patient or on the physician’sinstrument or instruments, markers are tracked by the CASS using electromagneticradiation (EMR). The EMR is either emitted by the tracking system and back reflected bythe marker, or simply emitted by the marker. The marker can be a rigid body that comprises optical references, optical structures, reflectors or light-emitting diodes (LEDs), called fiducials, which are generally packaged within a housing of the marker. In this context, reflectors are called “passive fiducials” whereas LEDs are called “active fiducials”. The passive fiducials are optical elements that are able to reflect electromagnetic radiation back to the source. Such passive fiducials are known as retro- reflectors. Markers containing such passive fiducials are known as retro-reflectormarkers, or simply markers. A marker array comprises multiple retro-reflectors.

[0003] The tracking performance of CAS systems depends on the quality of the reflectedEMR. Inhomogeneities or irregularities in the reflected EMR can give rise to tracking errors that result in inaccurate position and / or orientation data. Such inaccurate position and / or orientation data can result in anomalous control of a surgical tool. Such errors are also known as localization errors.

[0004] Accordingly, there is a desire to provide improved retro-reflectors, markers andmarker arrays for optical tracking in CAS systems.Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO BRIEF DESCRIPTION OF THE DRAWINGS

[0005] By way of illustration, specific non-limiting examples of a number of retro-reflectors, markers and marker arrays will now be described, with reference to theaccompanying drawings, in which:

[0006] FIG. 1 depicts a sectional view of an example retro-reflector;

[0007] FIG. 2 shows a sectional view of a further example retro-reflector;

[0008] FIG. 3 illustrates a sectional view of a yet further example retro-reflector;

[0009] FIG. 4 shows a sectional view of a still further example retro-reflector;

[0010] FIG. 5 depicts a sectional view of a refracted optical path;

[0011] FIG. 6 shows an example of a marker array comprising a set of retro-reflectors;and

[0012] FIG. 7 shows a view of an operating room comprising a Computer AssistedSurgery System (CASS) using such retro-reflectors.DETAILED DESCRIPTION

[0013] FIG. 1 shows a view 100 of a retro-reflector 102 for use in optical tracking by acomputer-assisted surgery system (CASS), the latter being described with reference toFIG. 7. The retro-reflector can be used alone, as a marker, or as part of a marker array.An example of such a marker array is described below with reference to FIG.6. Electromagnetic radiation is retro-reflected by the retro-reflector 102. In the case of a marker, that is, a single housed retro-reflector, the reflected electromagnetic radiation can be used to determine position. In the case of a marker array, the reflectedelectromagnetic radiation can be used to determine at least one, or both, of position andorientation for the marker array. Knowing the position and / or orientation of such amarker allows at least one, or both, of the position and orientation to be determined of anitem to which the marker is attached. The position of the retro-reflector 102 can bedetermined subject to a localization error, which is the degree or precision with which the position can be determined.Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO

[0014] The retro-reflector 102 comprises an optical window 104. The optical window104 is transparent to a predetermined wavelength or to a predetermined range of wavelengths of electromagnetic radiation (EMR).

[0015] The retro-reflector 102 comprises a mask 106. The mask 106 is opaque to theEMR. The mask 106 defines an aperture 108 into the optical window 104 by which EMR can enter or exit the optical window 104. The aperture 108 is an example of a EMR transmissible aperture.

[0016] The retro-reflector 102 further comprises a retro-reflective assembly 110. Theretro-reflective assembly 110 can comprise at least one, or both, of: one, or more thanone, retro-reflective structure or a retro-reflective material. A collection of one, or morethan one, reflective structure is an example of a set of retro-reflective structures. The assembly 110 is an example of such a set. The set of retro-reflective structures cancomprise a single retro-reflective structure or a plurality of retro-reflective structures.The set of retro-reflective structures can be synonymously known as a retro-reflective plane. A retro-reflective structure or a retro-reflective materials is a structure or material that returns incident EMR back towards a source of the EMR, optionally along the sameoptical path. In the example depicted in FIG. 1, the retro-reflective assembly 110comprises a plurality of retro-reflective elements 112 such as, for example, retro-reflective beads. Each bead of the plurality of retro-reflective beads is arranged to reflectincident EMR back along the same incoming optical path. Although FIG. 1 depicts asectional view of the retro-reflector 102 that is rectangular, the retro-reflector 102 canhave a generally cylindrical shape, with a circular upper or outwardly directed face.

[0017] Incoming EMR such as ray 114 is incident upon the outwardly directed surface116 of the optical window 104. The ray 114 is refracted at the interface defined by the air, or environment, and the outwardly directed surface 116 of the optical window 104. The air has a respective refractive index, ^^. The optical window also has a respectiverefractive index, ^^, where ^^ > ^^. Accordingly, the ray 114 is refracted towards anormal 118 to the outwardly directed surface 116.Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO

[0018] The ray 114 propagates through the optical window 104 until the ray 114 isrefracted at a further interface between the optical window 104 and a cavity 120. The cavity 120 can comprise air, or some other material, that has a respective refractive index^^, where ^^ > ^^. In examples in which the cavity contains air, ^^ = ^^. Since ^^ >^^, the ray 114 is refracted away from a normal 122 to an inwardly directed surface 124 of the optical window 104.

[0019] The ray 114 traverses the cavity until encountering a retro-reflective structure 126of the set of retro-reflective structures 110. Examples can be realised in which the retro-reflective structure 126 comprises a spherical bead. The ray 114 traverses the retro-reflective structure 126 and is reflected back along the incident optical path by a rear surface 128 of the retro-reflective structure 126. Following such a reflection, the raytakes the same optical path as an exit route as the incoming optical path.

[0020] A normal projection 130 of an edge 132 of the mask 106, defining the aperture108, onto the set of retro-reflective structures divides the retro-reflective plane into two;namely, an inner extent 134 and an outer extent 136. In examples in which the retro-reflector 102 is cylindrical, the mask 106 will define a circular aperture such that theinner extent 134 is circular and the outer extent 136 is annular.

[0021] FIG. 1 also shows a further ray 138. The further ray 138 is incident on the opticalwindow 104 at the edge 132 of the mask 106. The further ray 138, in the same manner as ray 114, is reflected and refracted back along the incoming optical path and returns to the edge 132 where the ray 138 exits the optical window 104 at the edge 132. The ray 138 is retro-reflected by retro-reflective structures forming part of the outer extent 136 that lies normally beneath the opaque mask 106. The result is that the edge 132 of the opaque mask 106 is well-illuminated and, therefore, well-defined. A well-defined or well illuminated edge 132 provides a well-defined, EMR region governed by the aperture. The well-defined EMR is more uniform compared to previous markers due to EMR being retro-reflected by retro-reflective structures within the outer extent 136.

[0022] It can be appreciated that non-uniform, less uniform, or uneven, EMR wouldresult from a retro-reflector 102 comprising only the inner extent 134. Consider a stillSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO further, or third, ray 140 entering the optical window 104 at a position such that the third ray 140 is retro-reflected by an outer-most retro-reflective structure 141 of the inner extent 134. The absence of the retro-reflective structures of the outer extent 136 wouldresult in a region or volume 142 that is devoid of reflected EMR. Accordingly, but forthe retro-reflectors in the outer extent 136, the aperture would have a zone 144 that is not well illuminated by any retro-reflected EMR, which will present a smaller optical, lesswell-defined, image to a tracking system. Such a smaller optical image could result in aninaccurate estimate of the position of the retro-reflector 102, that is, could result in a localization error. The outer extent 136 comprising respective retro-reflective structures overcomes the foregoing limitation, which improves optical tracking and, in turn, reduces localization errors.

[0023] Still further, EMR incident upon a feature of the retro-reflector 102 other than aretro-reflective elements 112 will not be reflected back along the incoming or incidentoptical path. For example, EMR incident upon a side wall 146 will not be retro-reflectedand will not, therefore, contribute to the luminosity of the EMR output from the retro- reflector 102. Therefore, it can be appreciated that the angle of incidence of ray 138 establishes maximum angle of incidence for which retro-reflection can occur. Beyond such a maximum angle, which below is referred to as the maximum viewing angle, ^^,EMR will not be retro-reflected. A marker can be realised by encasing the above retro-reflector 102 in a respective housing 148. A marker array can be realised by housing, or grouping, a plurality of such retro-reflectors 102.

[0024] Various factors will influence the size of the outer extent 136, that is, the extent towhich the outer extent 136 extends beyond the projection of the normal 130. Thosefactors comprise at least one, or more than one, element of the set: {a prescribedmaximum angle of incidence for the EMR, the refractive index or indices of the material or materials used to construct the optical window, the thickness of the optical window,the depth of the cavity, the refractive index of the material within the cavity} all takenjointly and severally in any and all permutations. A mathematical relationship between above factors is described below with reference to FIG. 5.Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO

[0025] Referring to FIG. 2, there is shown a view 200 of a retro-reflector 202. Referencenumerals common to FIG. 1 and FIG. 2 refer to the same elements. The operatingprinciple of the retro-reflector 202 is substantially the same as that described above withreference to FIG. 1. The retro-reflector 202 is identical to the above-described retro-reflector 102, but for the addition of a set of anti-reflective (AR) coatings. The set of ARcoatings can comprise a single AR coating or a plurality of AR coatings. In the exampleshown, the retro-reflector 202 comprises two AR coatings 204 and 206. An upper ARcoating 204 is deposited on the outwardly directed surface 116 of the optical window 104while a lower AR coating 206 is deposited on the inwardly directed surface 124 of theoptical window 104. Examples can be realised in which only one or the other of the twoAR coatings 204 and 206 is provided. Examples can be realised that comprise only theupper AR coating 204. Examples can be realised that comprise only the lower AR coating 206.

[0026] Each AR coating is arranged to reduce reflections such as, for example, at leastone, or both, of: specular reflections and diffuse reflections. Reducing such reflectionsimproves the accuracy with which the position of the retro-reflector 202 can bedetermined by the tracking system. A marker can be realised by encasing the aboveretro-reflector 202 in a respective housing 248. A marker array can be realised by housing, or grouping, a plurality of such retro-reflectors 202.

[0027] Referring to FIG. 3, there is shown a view 300 of an example of a still furtheroptical retro-reflector 302. Reference numerals common to FIG. 1 and FIG.3 refer to thesame elements. The optical retro-reflector 302 is identical to the optical retro-reflector102 described above with reference to FIG. 1 but for the retro-reflective assembly 110.The retro-reflective assembly 110 of FIG. 3 comprises a set of structures other thanbeads. In the example depicted in FIG.3, the retro-reflective assembly 110 comprises a set of corner reflectors. The set of corner reflectors comprises a plurality of corner reflectors. An individual corner reflector 304 is identified by the dashed line rectangle.Each corner reflector reflects EMR in a parallel but opposite direction to that ofcorresponding incident EMR.Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO

[0028] The operating principle of the retro-reflector 302 is substantially the same as thatdescribed above with reference to FIG.1. A marker can be realised by encasing the above retro-reflector 302 in a respective housing 348. A marker array can be realised by housing, or grouping, a plurality of such retro-reflectors 302.

[0029] Referring to FIG. 4, there is shown a view 400 of an example of a yet furtheroptical retro-reflector 402. Reference numerals common to FIG. 2 and FIG.4 refer to thesame elements. The optical retro-reflector 402 is identical to the optical retro-reflector202 described above with reference to FIG. 2 but for the retro-reflective assembly 110.The retro-reflective assembly 110 of FIG. 4 is identical to the retro-reflective assembly110 described above with reference to FIG.3. A marker can be realised by encasing the above retro-reflector 402 in a respective housing 448. A marker array can be realised by housing, or grouping, a plurality of such retro-reflectors 402.

[0030] FIG. 5 shows a view 500 of the relationship between the factors described abovethat influence the extent to the outer extent 136 for a given maximum viewing angle,which is the maximum angle at which incident EMR will be retro-reflected, as opposed to being incident on the side wall 146.

[0031] The maximum extent, ^, of the outer extent 136 for a given maximum viewingangle, is given by the following expression:

[0033] where

[0034] ^^ is the thickness of the optical window,

[0035] is the refractive index of the medium adjacent to the optical window, whichwill typically be air,

[0036] is the angle of incidence of the incident EMR 138, which, in the exampledepicted, is the maximum viewing angle,

[0037] ^^ is the refractive index of the optical window,

[0038] ^^ is the angle between the EMR ray 138 and a respective normal, that is, a firstangle of refraction,Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO

[0039] ^^ is the refractive index of the medium forming, or within, the cavity, whichwould typically be air, and

[0040] ^^ is the thickness of the cavity from the lower optical window interface to theretro-reflective assembly.

[0041] Examples can be realised in which ^^ > ^^, or in which ^^ >> ^^, such that theextent, ^, can be given by, or can be approximated by,

[0043] Given the above, EMR 138 entering the optical window at the maximum viewingangle, will be retro-reflected by the retro-reflective assembly 110.

[0044] The above equations are derived as follows:

[0050] ^^ = ^^^^^^^.

[0051] Substituting for ^^ gives

[0053] Similarly, from Snell’s Law

[0054] ^^^^^^^ = ^^^^^^^.

[0055] Therefore,^^^^^[^^] ^^ = ^^^^^, which gives

[0057] ^^=^^^^^^^.

[0058] Therefore,

[0059] ^ = ^^ + ^^=^^^^^^^ + ^^^^^^^.

[0060] Substituting for ^^ and ^^ givesSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO

[0062] FIG. 6 shows a view 600 of a marker array 602 comprising a set of retro-reflectors. The set of retro-reflectors can comprise a plurality of retro-reflectors. Theretro-reflectors in the set comprises any of the retro-reflectors described and / or claimed herein. In the example depicted in FIG.6, the marker array 602 comprises a set of three retro-reflectors 604 to 608. The three retro-reflectors are mounted in a housing 610 orbody of the marker array 602. In the example shown, the housing 610 comprises threelimbs 612 to 616 forming a triangle. The triangle can be a scalene triangle in order tofacilitate matching. The respective masks 618 to 622 and EMR transmissible apertures 624 to 628 of each retro-reflector 604 to 608 are shown.

[0063] An assembly of multiple retro-reflectors is known as a marker array.

[0064] Referring to FIG. 7, there is shown a view 700 of a CASS 702 according to anexample. In the example depicted, the CASS 702 is arranged to aid surgeons inperforming orthopedic surgical procedures such as, for example, an arthroplasty (e.g., atotal knee arthroplasty (TKA)) or a total hip arthroplasty (THA). An Effector Platform704 positions surgical tools relative to a patient 705 during surgery. The EffectorPlatform 704 can comprise a Robotic Arm 704A. For example, for a knee surgery, theEffector Platform 704 may include an End Effector 704B that holds surgical tools orinstruments during their use. Effector Platform 704 can include a Limb Positioner 704Cfor positioning the patient’s limbs during surgery. Resection Equipment (not shown inFIG. 7) performs bone or tissue resection using, for example, mechanical, ultrasonic, orlaser techniques. Effector Platform 704 can also include a cutting guide or jig 704D thatis used to guide saws or drills used to resect tissue during surgery. Such cutting guides704D can be formed integrally as part of the Effector Platform 704 or as part of theRobotic Arm 704A, or cutting guides can be separate structures that can be matinglyand / or removably attached to the Effector Platform 704 or Robotic Arm 704A.

[0065] The CASS 702 comprises an optical tracking system 706 uses one or moresensors to collect real-time position data that locates the patient’s anatomy and surgical instruments. Any suitable tracking system can be used for tracking surgical objects andSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO patient anatomy in the surgical theatre. For example, a combination of infrared (IR) andvisible light cameras can be used in an array. Such an optical Tracking System 706 canuse the EMR retro-reflected from any of the retro-reflectors described and / or claimedherein to determine real-time position data that locates at least one, or both, of thepatient’s anatomy and surgical instruments and / or the orientation(s) thereof.

[0066] Accordingly, the CASS 702 shown in FIG. 7 depicts a number of markers and / ormarker arrays comprising respective retro-reflectors. The markers can be as describedabove with reference to FIG.6. The retro-reflectors can be any of the retro-reflectorsdescribed and / or claimed herein. The marker arrays can be placed on objects or bodyparts to be tracked or for which respective positions and / or orientations are to bedetermined. Therefore, a set of retro-reflectors can be used to determine at least one, orboth, of: position and orientation of a respective item. As indicated above such a set ofretro-reflectors are formed as a marker array, as a marker or a plurality of markers. Therespective item can be such an object or body part. A set of retro-reflectors can comprise one retro-reflector or a number of retro-reflectors. Examples can be realised in which a set of retro-reflectors comprises at least three retro-reflectors. For example, a first markerarray 714 is situated on the robot arm 704A. Knowing the position of the first markerarray 714 can allow, for example, the position and orientation of the actuator 716 of therobot arm 704A to be determined. A second marker array 718 is placed on the handheldtool 704B to allow the position and orientation of the tool held by the End Effector 704Bto be determined and / or tracked in 3D space. A third marker array 720 can be situatedrelative to the jig 704D to allow the position and orientation of the jig 704D to bedetermined. A fourth marker array 722 can be placed on the Limb Positioner 704C toassist in determining the position of a respective distal actuator 724 for holding a limb. Afifth marker array 726 can be placed on eye-wear 728 of a surgeon 730. A sixth marker array 727 can be located on the operating table 704E.

[0067] Although the CASS 702 has been described with reference to a set of markerarrays comprising six marker arrays, examples are not limited thereto. Examples can berealised in which such a set of marker arrays comprises one or more than one markerSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO array to suit the needs of the operation to be performed. Still further, the deployment ofthe marker arrays can be realised other than in relation to the robot arm 704A, thehandheld tool 704B, the jig 704D and the limb positioner 704C.

[0068] Although the above has described a one to one relationship between an object orbody part and a respective marker array, examples can be realised in which an object orbody part can comprise a respective set of markers arrays. Such a set of marker arraysper object or body part can be used to improve or increase the accuracy with which at least one, or both, of: position and orientation can be determined.

[0069] The registration process that registers the CASS 702 to the relevant anatomy ofthe patient 705 can also involve using of anatomical landmarks, such as landmarks on abone or cartilage. For example, the CASS 702 can include a 3D model of the relevantbone or joint and the surgeon 730 can intraoperatively collect data regarding the location of bony landmarks on the patient’s actual bone using a probe (not shown) that isconnected to the CASS. Alternatively, the CASS 702 can construct a 3D model of thebone or joint without pre-operative image data by using location data of bony landmarks and the bone surface that are collected by the surgeon using a CASS probe or other means.

[0070] A Tissue Navigation System (not shown in FIG. 7) provides the surgeon with oneor more intraoperative, real-time visualizations of the patient’s bone, cartilage, muscle,nervous, and / or vascular tissues surrounding the surgical area.

[0071] The CASS 702 can comprise a Display 708 to provide graphical user interfaces(GUIs) that display images collected by the Tissue Navigation System as well otherinformation relevant to the surgery to the surgeon 730 or other operating theatre staff.For example, the Display 708 can overlay image information derived from variousmodalities (e.g., CT, MRI, X-ray, fluorescent, ultrasound, etc.) collected pre-operatively or intra-operatively to give the surgeon various views of the patient’s anatomy as well asreal-time conditions. A Surgical Computer 710 provides control instructions to variouscomponents of the CASS 702, collects data from those components, and provides generalprocessing for various data needed during surgery. In the example depicted in FIG. 7, theSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WOsurgeon 730 is shown as wearing the protective eye-wear 728 or an augmented realityheadset, which can also comprise or bear a respective marker 726.

[0072] Although the retro-reflective entities described with reference to FIG. 7 abovewere marker arrays, examples are not limited thereto. Examples can be realised in which one or more, or all, of the marker arrays are markers instead.

[0073] Any of the retro-reflectors, markers or marker arrays described herein mayadditionally comprise a wipeable outer surface. Such examples can be realised in which the wipeable outer surface comprises a hydrophobic surface or an oleophobic surface.

[0074] Further examples can be realised according to the following clauses.

[0075] Clause 1: A retro-reflector; comprising

[0076] a transparent optical window,

[0077] a mask defining an electromagnetic radiation transmissible aperture into thetransparent optical window, and

[0078] a retro-reflective assembly comprising: a set of retro-reflective structures toreflect electromagnetic radiation received through the electromagnetic radiation transmissible aperture back through the electromagnetic radiation transmissible aperture; the set of retro-reflective structures having an extent that extends beyond a normal projection of the mask onto the set of retro-reflective structures.

[0079] Clause 2: The retro-reflector of clause 1, in which the extent of the set of retro-reflective structures comprises a portion of the set of retro-reflectors outside of the normal projection; the portion being arranged to reflect the electromagnetic radiation entering the optical window at an edge of the mask back through the optical window at that edge of the mask.

[0080] Clause 3: The retro-reflector of any preceding clause, in which the extent of theset of retro-reflective structures comprises a portion of the set of retro-reflectors outside of the normal projection; the portion being arranged to define an optical path from and to at least an edge of the aperture of the mask.

[0081] Clause 4: The retro-reflector of any preceding clause, in which theelectromagnetic radiation transmissible aperture has an associated prescribed maximumSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO viewing angle; the extent being defined by at least the associated prescribed maximum viewing angle.

[0082] Clause 5: The retro-reflector of any preceding clause, in which the extent isdefined by at least the refractive indices of electromagnetic radiation transmissible media associated with an optical path between an edge of the mask and a portion of the set of retro-reflective structures outside of the normal projection.

[0083] Clause 6: The retro-reflector of any preceding clause, in which theelectromagnetic radiation reflected from the set of retro-reflective structures within the extent outside of the normal projection is arranged to illuminate the mask defining the electromagnetic radiation transmissible window.

[0084] Clause 7: The retro-reflector of any preceding clause, further comprising an anti-reflective coating.

[0085] Clause 8: The retro-reflector of clause 7, in which the anti-reflective coating isdeposited on at least the optical window.

[0086] Clause 9: The retro-reflector of clause 8, in which the anti-reflective coating isdeposited on at least one, or both, of: an outwardly directed surface of the optical window or an inwardly directed surface of the optical window.

[0087] Clause 10: The retro-reflector of any preceding clause, comprising a wipeableouter surface.

[0088] Clause 11: The retro-reflector of clause 10, in which the wipeable outer surfacecomprises a hydrophobic surface or an oleophobic surface.

[0089] Clause 12: The retro-reflector of any preceding clause, in which the opticalwindow is transmissible to electromagnetic radiation of a predetermined wavelength.

[0090] Clause 13: The retro-reflector of clause 12, in which the predeterminedwavelength is associated with a wavelength of electromagnetic radiation associated with an optical tracking system.

[0091] Clause 14: A computer assisted surgery system comprising an optical trackingsystem and at least one, or both of: a marker or marker array; the at least one, or both of: the marker or marker array comprising: an optical window, an opaque mask defining anSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO aperture into the optical window for electromagnetic radiation (EMR); a retro-reflector arranged to reflect the EMR back through the aperture; the reflective material having an outer perimeter or extent extending beyond a normal projection of the aperture to define a return optical path for the EMR from the retro-reflector to an edge of the aperture of the opaque mask.

[0092] Clause 15: The computer assisted surgery system of clause 14, in which the retro-reflector comprises one or more than one of: a reflective substrate or at least one retro-reflective 3D structure.

[0093] Clause 16: The computer assisted surgery system of either of clauses 14 and 15,in which at least one, or more than one, of the following taken jointly and severally in any and all permutations: outer perimeter, a maximum dimension of the aperture, a thickness of the optical window, the refractive index of the optical window, a thickness of a void between an inwardly directed surface of the optical window or a reflector facing surface of the optical window and the retro-reflector, the refractive index of the void, defines a maximum prescribed viewing angle of the tracking device, or defines a maximum angle of incidence for the EMR relative to a normal of the optical window.

[0094] The computer assisted surgery system of any of clauses 14 to 16, comprising ananti-reflective coating.

[0095] Clause 18: The computer assisted surgery system of clause 17, in which the anti-reflective coating is deposited on at least one, or both, of: an outwardly directed surface of the optical window or an inwardly directed surface of the optical window.

[0096] Clause 19: The computer assisted surgery system of any of clauses 14 to 17, inwhich the outer perimeter or extent extends beyond a normal projection of the aperture byan amount, ^, given by:

[0097] is a maximum viewing angle, ^^ is an angle of refraction at an interfaceof the optical window that is related toby ^^^^^^^ = ^^^^^^^, where ^^ is therefractive index of a medium to carrying incident EMR to the interface, ^^is the refractive index of the optical window, ^^is the refractive index of the medium formingSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO the cavity, ^^is the depth of the optical window, and ^^is the distance between an inwardly directed surface of the optical window and the retro-reflector.

[0098] Clause 20: A marker comprising a set of retro-reflectors of any of clauses 1 to 13.

[0099] Clause 21: A marker array comprising a plurality of retro-reflectors of any ofclauses 1 to 13.

Claims

Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO CLAIMS What is claimed is:

1. A retro-reflector; comprisinga. a transparent optical window,b. a mask defining an electromagnetic radiation transmissible aperture intothe transparent optical window, and c. a retro-reflective assembly comprising:i. a set of retro-reflective structures to reflect electromagneticradiation received through the electromagnetic radiation transmissible aperture back through the electromagnetic radiation transmissible aperture; the set of retro-reflective structures having an extent that extends beyond a normal projection of the mask ontothe set of retro-reflective structures.

2. The retro-reflector of claim 1, in which the extent of the set of retro-reflectivestructures comprises a portion of the set of retro-reflectors outside of the normal projection; the portion being arranged to reflect the electromagnetic radiation entering the optical window at an edge of the mask back through the optical window at that edge of the mask.

3. The retro-reflector of claim 1, in which the extent of the set of retro-reflectivestructures comprises a portion of the set of retro-reflectors outside of the normal projection; the portion being arranged to define an optical path from and to at least an edge of the aperture of the mask.

4. The retro-reflector of claim 1, in which the electromagnetic radiationtransmissible aperture has an associated prescribed maximum viewing angle; the extent being defined by at least the associated prescribed maximum viewing angle.

5. The retro-reflector of claim 1, in which the extent is defined by at least therefractive indices of electromagnetic radiation transmissible media associatedSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO with an optical path between an edge of the mask and a portion of the set of retro- reflective structures outside of the normal projection.

6. The retro-reflector of claim 1, in which the electromagnetic radiation reflectedfrom the set of retro-reflective structures within the extent outside of the normal projection is arranged to illuminate the mask defining the electromagnetic radiation transmissible window.

7. The retro-reflector of claim 1, further comprising an anti-reflective coating.

8. The retro-reflector of claim 7, in which the anti-reflective coating is deposited onat least the optical window.

9. The retro-reflector of claim 8, in which the anti-reflective coating is deposited onat least one, or both, of: an outwardly directed surface of the optical window or an inwardly directed surface of the optical window.

10. The retro-reflector of preceding claim 1, comprising a wipeable outer surface.

11. The retro-reflector of claim 10, in which the wipeable outer surface comprises ahydrophobic surface or an oleophobic surface.

12. The retro-reflector of claim 1, in which the optical window is transmissible toelectromagnetic radiation of a predetermined wavelength.

13. The retro-reflector of claim 12, in which the predetermined wavelength isassociated with a wavelength of electromagnetic radiation associated with an optical tracking system.

14. A computer assisted surgery system comprising an optical tracking system and atleast one, or both of: a marker or marker array; the at least one, or both of: the marker or marker array comprising: a. an optical window,b. an opaque mask defining an aperture into the optical window forelectromagnetic radiation (EMR); c. a retro-reflector arranged to reflect the EMR back through the aperture;the reflective material having an outer perimeter or extent extending beyond a normal projection of the aperture to define a return optical pathSmith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO for the EMR from the retro-reflector to an edge of the aperture of the opaque mask.

15. The computer assisted surgery system of claim 14, in which the retro-reflectorcomprises one or more than one of: a. a retro-reflective substrate, orb. at least one retro-reflective 3D structure.

16. The computer assisted surgery system of claim 14, in which the at least one, ormore than one, of the following taken jointly and severally in any and all permutations: a. outer perimeterb. a maximum dimension of the aperture,c. a thickness of the optical window,d. the refractive index of the optical window,e. a thickness of a void between an inwardly directed surface of the opticalwindow or a reflector facing surface of the optical window and the reflector, f. the refractive index of the void,defines a maximum prescribed viewing angle of the tracking device, or defines amaximum angle of incidence for the EMR relative to a normal of the optical window.

17. The computer assisted surgery system of claim 16, comprising an anti-reflectivecoating.

18. The computer assisted surgery system of claim 17, in which the anti-reflectivecoating is deposited on at least one, or both, of: an outwardly directed surface of the optical window or an inwardly directed surface of the optical window.

19. The computer assisted surgery system of claim 14, in which the outer perimeter orextent extends beyond a normal projection of the aperture by an amount, ^, givenby:Smith & Nephew Docket No.: PT-6040-WO-PCTAttorney Docket No.: P364766WO b. wherei. ^^ is a maximum viewing angle of the tracking device,ii. ^^ is an angle of refraction at an interface of the optical windowthat is relatedtherefractive index of a medium to carrying incident EMR to the interface, ^^is the refractive index of the optical window, ^^is the refractive index of the medium forming the cavity, iii. ^^ is the depth of the optical window, andiv. ^^ is the distance between an inwardly directed surface of theoptical window and the retro-reflector.

20. A marker comprising a set of retro-reflectors as claimed in claim 1.

21. A marker array comprising a plurality of retro-reflectors as claimed in claim 1.

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