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ORIGINAL ARTICLE

MBARS: mini bone-attached robotic system for jointarthroplasty

A Wolf, B Jaramaz, B Lisien, A M DiGioia

A Wolf, B Jaramaz, B Lisien and A M DiGioiaICAOS, Institute for Computer Assisted Orthopaedic Surgery, The Western Pennsylvania Hospital, and theRobotics Institute at Carnegie Mellon University, Pittsburgh PA, USACorrespondence to: Alon Wolf, E-mail: [email protected]

AbstractA report on a new active, miniature bone-attached, robotic system including its design, high level and lowlevel control, is given together with a description of the system implementation and first experimental use.The system is capable of preparing the bone cavity for an implant during joint arthroplasty procedures.Without loss of generality, the report describes the implementation of the system for a Patellofemoral JointReplacement procedure. The system is image-free and all planning is performed intra-operatively in therobot coordinate system, eliminating the need for external tracking systems in the operating room.Experiments were conducted using the first robot prototype and the results supported the feasibility of theconcept. The methodology which is presented can be modified to other orthopaedic procedures and couldimprove the results in terms of accuracy and operational time. Moreover, it enables minimally invasiveprocedures and use of the next generation of more anatomically shaped implants.

Keywords: Medical robot, bone-attached robot, joint arthroscopy, minimally invasive surgery

Paper accepted: 1 December 2004

Published online: 15 January 2005. Available from: www.roboticpublications.com

DOI: 10.1581/mrcas.2005.010210

INTRODUCTIONAs the population ages, arthritis and joint diseasebecome more common and a major concern to theaging population (1) (Figure 1). The World HealthOrganization estimates that several hundred millionpeople already suffer from bone and joint diseases,with dramatic increases expected due to a doublingin the number of people over 50 years of age by2020 (2). This trend will result in an increasingnumber of total joint arthroplasty proceduresperformed by 2030 (3). Development of technolo-gies to support less and minimally invasive surgicalprocedures is one of the most important trendstoward confronting this increasing number ofprocedures and improving operational outcomes.

In this report we present the development of aminiature bone-attached robotic system for jointarthroplasty (Figure 2). The mini robot is rigidlyattached to the operated bone and acquires

anatomical-geometrical data in its own referencesystem in order to generate a surface model of theoperated anatomy. This data is then used for intra-operative planning of the operational procedure andimplant placement. Finally, once the intra-operativeplanning is completed, the information is down-loaded to the bone-attached robot which thenexecutes it very accurately and rapidly, just like acomputer numerical control (CNC) milling devicewould do in an industrial machine shop.

The hypothesis of this research is that aminiature bone-attached robot and its supportingtechnologies can be developed that accuratelycuts and shapes the bone to enable minimallyinvasive orthopaedic surgery procedures of the nextgeneration.

Following in our report we will overview thespecific aims of our research to support this system.These aims are:

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http://dx.doi.org/10.1581/mrcas.2005.010210

1. Development of the mechanical structure andcontrol of a miniature bone-attached robot thatcan perform minimally invasive joint arthro-scopy within limited space with minimalexposure and tissue distraction, with focus, asa case study, on patellofemoral arthroplasty.

2. Development of the surgical protocol for therobot-assisted joint arthroplasty procedure.

3. Development of methodologies to acquiresurface data from a patient and generate thesurgical plan for placement of the femoralimplant component.

4. Development of an algorithm to relate surfacefinish to characteristic operative times andto generate paths that guarantee completecoverage of cut volume.

BACKGROUND AND SIGNIFICANCEAlthough reconstructive orthopaedic proceduresrepresent a significant growing class of surgery theyhave lagged behind efforts in other areas of surgeryto make surgical procedures minimally invasive.The clinical benefits to patients are profound whenan ‘‘open’’ procedure can be made minimallyinvasive. By definition, performing any procedureless invasively results in less soft tissue disruption,with the effects of reduced pain, faster healing andrecovery, and fewer complications. Many areas ofsurgery have been revolutionized by the develop-ment of minimally invasive surgical (MIS) proce-dures developed after the introduction of fiber optictechnology (i.e. arthroscopy, endoscopy, laparo-scopy, etc.) Documented advantages of less invasiveprocedures include smaller incisions and fewerinjuries to major blood vessels and nerves (4); otherbenefits reported include reduced blood lossand decreased post-operative pain (5); with shorterhospital stays and faster return to normal activity forthe patient (6). Further, despite the higher capitalexpense of equipment needed to operate in aminimally invasive fashion, overall costs of mini-mally invasive procedures can actually be signifi-cantly lower (7, 8).

The application of minimally invasive approachesto orthopaedic surgical procedures is relatively new,though promising due to reports of superior patientoutcomes. Arthroscopy, the most widely usedminimally invasive technique, is used extensivelyin sports medicine for soft tissue injuries, such asACL repair in the knee and rotator cuff repair in theshoulder, and requires less soft tissue dissection andposes less risk to muscles (9). It has also recently beenapplied to minimally invasive selective osteotomy incombination with a miniature reciprocating saw,yielding similar beneficial results (10). In additionto joint surgery, arthroscopy has been successfully

Figure 1 Gallup Poll results.

Figure 2 Mini Bone-attached Robot.

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applied to spinal procedures also resulting inminimal injury to soft tissue (11). Pioneering workin the field of less invasive joint replacement surgeryhas been reported by surgeons from the Hospital forSpecial Surgery in 1998 (12). They were the first toreport a significant reduction of surgical incision fortotal hip replacement surgery, reducing the incisionfrom an average of 25 cm to 6–8 cm. This wasachieved by modifying the surgical technique so thatthe bone preparation and implant insertion can bedone through a much smaller opening, while usingthe same surgical tools used in the traditionalapproach.

Realizing the potential of minimally invasiveprocedures, the implant industry is currently in theprocess of redesigning implants and, together withsurgeons, re-examining surgical procedures. It canbe expected that the next generation of implantswill be more suitable for the eventual developmentof less invasive procedures. Additionally, there is thepossibility of creating smaller implants with a moreanatomical shape, due to the ability to machinebone surface into a more complex shape that is notplanar or spherical. By utilizing a more anatomicalshape, bone lost to machining can be minimized,while simultaneously reducing the surgical expo-sure. Meanwhile however, there is an immediateopportunity to perform surgical procedures that arebetter tailored for localized joint arthritis, and thatcould either significantly postpone or completelyeliminate the need for total joint replacement. Thesepartial knee arthroplasties are also well suited forminimally invasive surgery, although they can bemore technically demanding than total joint repla-cements. Hence these efforts can only succeed if thesurgical and technological advances are developed inparallel and within a comprehensive collaborativeeffort.

Without loss of generality, we focus in this reporton the integration and implementation of our robotin improving the accuracy of the femoral-compo-nent preparation for patellofemoral arthroplasty.Although still not a very popular procedure, froman engineering point of view it simulates the futuregeneration of orthopaedic arthroplasty deviceswhere there will be a need to machine the bonesurface into a complex shape that is not planar orspherical. Therefore, the technology demonstratedin this research is not specific to the patellofemoralprocedure, but can be adapted to many other areas.The ability to do more precise, patient-specific

planning and surgical execution would create alarger demand for such applications among sur-geons, and would help lead to the developmentof additional minimally invasive procedures inorthopaedics.

Current state of the scienceMedical robotic devices can be categorizedinto three major groups: active roboticsystems, semi-active robotic systems, and passiverobotic systems. The first known active robotintroduced to the operating room was theRobodoc system (Integrated Surgical Systems,USA, www.robodoc.com). This robot is used tomill the medullary cavity of the femur for acementless femoral prosthesis in total hip replace-ment. Another application of the Robodoc systemis presented by Kazanzides et al. (13) where therobot actively mills the femur in order to optimallyfit an implant for knee surgery. A further approachfor active medical robotics is given in Brandtet al. (14), where a Stewart platform is used in hipreplacement surgery. Another active robotic systemis CASPAR, used for hip replacement (Figure 3).

The semi-active robotic system approach ispresented by Ho et al. (15–17). In Kienzle et al. (18)

the robot acts as an assistant during the operation byholding a tool in a steady position, accuratelyguiding a cutting tool, and preventing the tool frommoving out of the desired operative region. Thedefining feature of semi-active robotic systems isthat they do not perform any autonomous active acton the patient’s anatomy. Their main purpose is toguide or increase the surgeon’s control and accuracyof the operating tool. Such a system may, forexample: guide cutting jigs to the correct locationduring TKA (18), act to enforce constraintsby restricting a task within a pre-determinedenvelope (16, 19, 20), mimic a surgeon’s hand motions(telesurgery), or scale and filter hand motion duringsurgery. The main difference between the semi-active robotic group and the active one is that thecontrol is shared between the robotic tool and thesurgeon throughout the operation.

Acrobot (21) is an example of a semi-activerobotic system. Developed by Davies et al. (22–26),its core proprietary technology centers on thedevelopment of a new type of robotic control:Active Constraint RoboticsTM for orthopaedicsurgery. This concept facilitates a synergy betweenthe surgeon and the robot, actively assisting the

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surgeon and helping to prevent surgical error. Thisapproach, keeps the surgeon in the control loopthroughout the surgery. Moreover, the robot isguided by preoperative CT-based planning software.Other available robots which are mainly used fortool/camera/microscope support are EndoAssist,EndoWrist, AESOP, ZEUS, MKM and SurgiScope.

A third approach, passive robotic systems, ispresented in Grace et al. (27) and Jensen et al. (28),where a six degree-of-freedom (DoF) robot actssimply as a guided tool, fully controlled by thesurgeon who uses a multi-dimensional joystickinput device. McEwen et al. (29) use Arthrobot asan assistant in the operating room. The robot is apneumatically powered, electronically controlledpositioning device that intraoperatively holds thelimb during joint replacements of the knee and hip.

However, the most frequent examples of passiverobotic systems are surgical navigation systems (30).The basic concept of a surgical navigation system is

to ascertain the position, i.e. location andorientation, of the relevant components of thesystem and the patient’s anatomy in a globalcoordinate system, such that their relative positioncan be determined. One of the first systems wasHipNavTM (Figure 4), developed at CarnegieMellon University by DiGioia et al. (31). Some ofthe other navigation robotics systems which use thistype of technology are: VectorVision (BrainLab),SurgiGATETM (Medivision), NavitrackH (Orthosoft),StealthStationH (Medtronic), and Surgetic Systems.

The main problem with most available medicalrobots is that they are bulky and therefore occupy alarge space in the operating theatre. Moreover, theserobots are serial-type mechanisms with a large workvolume relative to the task in hand. A serialmechanism is composed of a single kinematic chainof links and joints connected in series from the baseto the end-effector. Therefore, they are somewhatcumbersome and heavy, and characteristically suffer

Figure 3 Examples of marketed robotic systems for surgery tasks: a) CASPAR; b) AESOP; c) ZEUS; d) Acrobot.




several known drawbacks including relatively lowstiffness and accuracy, and possess low nominalload/weight ratio. The fact that these robots areused for medical procedures, where accuracy andsafety are paramount, has motivated researchers tolook for manipulators with better kinematics anddynamic performance for specific surgical tasks.

Competing alternatives to conventional serialtypes of robot mechanisms are the parallel roboticstructures. A parallel mechanism utilizes two ormore independent serial kinematic chains to

connect the base to the end effector. A 6 DoFparallel robot, as shown in Figure 2, for example, iscomposed of two rigid platforms, one used as a baseplatform and the other as a moving end-effector.The two platforms are connected by ball-and-socketjoints to six independent links capable of changingtheir length. By controlling the length of each link,the mechanism can position and orient the movingend-effector relative to the base platform.Advantages of parallel robotic structures includelow weight, compact structure, high accuracy,high stiffness, restricted workspace, high frequencyresponse and low cost (32). Moreover, parallel robotsare significantly more robust to failure than serialdevices because in a serial device, one failure cancause the robot to dramatically move, whereas in aparallel structure, one failure will have little effect onthe overall position of the robot. This is importantin medical applications because surgeons want thedevice to maintain its last position in case of acatastrophic failure (33).

From a robotics perspective, the main drawbackof parallel mechanisms is their limited workspace.However, as pointed out by Khodabandehlooet al. (33), limited workspace is an advantage inmedical applications because the active in-situoperation volumes are limited to protect the patientand physician. Unfortunately, this advantage forcesthe robot to be deployed near the operation site inthe operating room, which is often unrealisticbecause the robot would interfere with thesurgeons. One of the solutions introduced to solvethis problem is to attach the entire robotic system tothe operating room’s ceiling so that the robot works‘‘upside down’’ (34). In this way, the robot does notinterfere during the standard operating procedure,and is activated and maneuvered to the operatingarea when required. However, this solution is notapplicable in all operating rooms and requires specialoperating room design.

In the mid 1990’s the Fraunhofer Institute forManufacturing Engineering and Automation (IPA),introduced the M-850 (35) which was a 6 DoFparallel robot which was used to reduce the tremoreffect of the surgeon’s hand and limit the effectiveworkspace of the robot to the operational scene.The next generation of the M-850 was theNonapod which has three ‘‘extra’’ legs that wereadded to the structure in order to increase thereliability of the robot while used for medicalapplications.

Figure 4 Screenshots of the HipNav preoperativeplanner; a) Implant position and orientation is plannedusing cross sectional views through the CT scan and the3D models of bones and implants. b) Range of motion(ROM) is simulated for any specified leg motion paths. Theeffect of any key variable on the simulation results isinteractively observed.




Making use of the advantage of a parallelrobotic structure, a novel semi-active medicalrobotic concept was introduced by Wolf andShoham et al. (36, 37) (Figure 5, Figure 6). In thiswork, the authors introduced a concept of aminiature bone-attached parallel robot. Takingadvantage of a parallel robot’s attributes such aslow weight, high accuracy, and compactness,they introduced the concept of a miniature,low mass, bone attached parallel robot speciallydesigned for spinal operations. The miniaturerobot was attached to the operative vertebraeby mechanical means, and guided the surgeon,

percutaneously, to selected anatomical sites inthe vertebra. The stated advantages of thisconcept over other navigation and roboticsystems (36, 37) are: 1) The operation outcomedoes not depend on the surgeon’s precision,because the system actively guides the surgeon indriving a k-wire to pre-selected sites during theoperation. 2) There is no need for an externalreferencing sensor because the robot is directlyattached to the operated vertebrae and moves withthe vertebrae as one rigid body. 3) The system isvery compact and does not consume operatingroom space.

Figure 5 A miniature medical robot attached to the vertebra; System concept; AP view; Lateral view.

Figure 6 Experimental results and the robot structure; a) Preplanning of insertion of K-wire to vertebral body of L1 (CTscan) b) Insertion of K-wire to pedicle of L1 as preplanned (CT scan) c) Miniature bone-attached parallel robotic system(Technion and Mazor).




Patellofemoral joint replacementsPatellofemoral arthritis remains one of the morechallenging problems in orthopaedic surgery. Itnormally presents as anterior knee pain, exacerbatedby stair climbing or arising from a chair. Mostpatients have little pain when walking on levelground. The condition is often associated with paindue to pressure on the patella during range ofmotion. The surgical options have included debri-dement, soft tissue releases, soft tissue or bonyrealignment, patellar osteotomy, patellar or com-bined patellofemoral prosthetic arthroplasty, andtotal knee replacement.

Extensor mechanism alignment procedures alone,whether proximal or distal, are often ineffective inthe patient who has already developed advancedpatellofemoral arthritis. Patellar debridement pro-cedures, likewise, have a high failure rate whenperformed in the presence of extensive arthriticchanges. Replacement of both the patella and thetrochlear groove, has had mixed results in theliterature, with Smith et al. showing positive resultsfor only 64% of patients (38), while Kooijman etal. (39) showed 86% of knees had excellent or goodresults at a mean follow-up of 17 years. One groupstarted using the same implant as Smith et al., andstopped using the device because of poor resultscaused by the unforgiving requirements regardingdevice placement. This points to accuracy ofcomponent placement as an important factor inthe success of these procedures.

Patellofemoral arthroplasty is reserved for patientswith severe pain in the front or middle of the kneefrom cartilage degeneration. It is not used in the caseof maltracking of the patella. The advantage of thisparticular procedure over a total knee replacement isthat the surrounding cartilage, which may not besignificantly degenerated, the meniscal cartilage, andthe ligaments are left alone and not removed, as theywould be in a total knee replacement. However, ifmore widespread arthritis is identified throughoutthe knee joint, a total knee replacement may bemore appropriate. With this procedure, the patella iscleared of its degenerated cartilage and capped witha plastic prosthetic button. The mating femoralsurface is also cleared of degenerated cartilage and isthen capped with a metal prosthesis. Only thesurfaces of the patellofemoral joint are replaced, afterthe arthritic ends of the bone are shaved. This is aconservative procedure such that, if the patient laterdevelops osteoarthritis in the other compartments of

the knee joint, it can be easily revised to a total kneereplacement in the future.

Surgical planning is important for a successfuloutcome in patellofemoral joint replacement. Thecurrent tools to do the planning are not overlysophisticated. Typically, an X-ray of the femur iscombined with implant templates to determine theapproximate implant size. The correct size trochlearcomponent is determined intraoperatively using theintraoperative template/drill guides. The template isoriented in line with the trochlea and the excursionof the patella. The outline of the template is thenmarked on the cartilage and bone of the trochleausing a knife or marking pen, and all the cartilageand subchendral bone within the outline is resected,using high-speed burrs and small sharp osteotomesat the edges. This process of template and cut isrepeated until the trochlear template/drill guide isinlayed flush with the surrounding articular cartilagesurface. The trochlear trial component is thendriven into place using the impactor, and if jointfunction is satisfactory when the knee is flexed,utilizing the trial patellar component, the finalimplant components are inserted (Figure 7) (40).

During surgery, particular attention to tracking ofthe patella is required for a successful result. Failureto use the optimal size implant, failure to adequatelyseat the component adjacent to sufficient bonestock, and failure to ensure that the component isstable may result in dislocation, subsidence, fractureor loosening of the components.

As technology evolves, implants become moreanatomical and complex in shape. Additionally, it islikely that in the near future, a surgeon will be ableto replace a damaged surface with a tissueengineered composite graft made up of the patient’sown bone and cartilage grown in vitro. In order tosupport these new metal and biologic implants, it isnecessary to develop the technology that wouldallow the surgeon to accurately achieve thenecessary fit and fixation of the new implants tothe prepared bone surface to maximize positivepatient outcomes and implant longevity. Wehypothesize that this is an area where robotic systemperformance, in the realm of cutting accuracy,would be superior to that of a surgeon’s hand. Notonly this, but we also hypothesize that our newconcept of a mini bone-attached robot for jointarthroplasty would be able to perform the operationsuch that it would simplify the procedure, wouldnot require special modifications to the operating




room, would be user friendly, and most important;would improve the operative outcome andallow minimally invasive procedures of the nextgeneration.

MINIATURE ACTIVE ROBOTSMechanical designThe concept of using a miniature bone-attachedrobot was extended by our team for use in jointarthroplasty. The first prototype of the miniaturebone-attached robotic system (MBARS) has beenbuilt and tested on sawbones (Figure 10). The robotis composed of six linear actuators that areconnected in parallel between two rigid platforms:the lower reference platform and the upper plat-form, which is the moving end-effector of therobot. Each of the linear actuators is connected tothe lower platform by a universal joint (2 degrees offreedom), and to the upper platform by a sphericaljoint (3 degrees of freedom). This structure isknown as the classical Stewart-Gough six degrees-of-freedom robot. the robot is attached to the femurby three pins: one pin is placed into the medialepicondyle, one into the lateral epicondyle, and oneinto the metadiaphyseal region of the femur. A rigidconnection of the robot to the operated bone isobtained through these three pins. The robot isequipped with a milling device (we used aconventional off-the-shelf burr), which activelymills the bone according to the preoperative plan.

Ball screws were chosen for the linear actuatorsbecause of their high efficiency (compared to lead

screws) and effective speed reduction. The screwsare fixed in bearings mounted to the links, while thenuts drive clevises connected to the crosses of theU-joints. The screws are driven by brush-type,permanent-magnet, DC motors which can beoperated with simple, pulse-width-modulated(PWM) control. For compactness, the gearmotorand ball screw are placed side-by-side with a smalltoothed-belt drive connecting them. Each actuatoris mounted to the link through a steel flexure thataccommodates the slight lateral movement of thescrew as the joint angle changes. A novel feature of

Figure 7 Templating and component insertion during PFA; b) Template is used to select the imlpant size and outline thearea to be cut. a) Implant is inserted flush with the surrounding bone and cemented in place. (Depuy Surgical Technique).

Figure 8 ‘‘Snubber’’.




this design is the overload mechanism or ‘‘snubber’’(Figure 8). It is designed to absorb the kineticenergy of the links and motors when the mechanical

stops are reached, and to accommodate imposedloads on the snake without damage to the actuatorsor structure. Belleville spring washers, (four series

Figure 9 a) CAD sketches of MBARS b, c) CAD sketch of the Ball screw mechanism d) Actual ball screw mechanism.




sets of three parallel-stacked washers) are mountedin the ‘‘snubber housing’’ such that the ball screwcan move axially by 1 mm if the preload value isexceeded. Belleville washers are hardened steelwashers with a slight conical shape. They can becompressed flat to produce an axial spring force.

They can be combined in parallel or series, bystacking the cone sections parallel or opposing, toadjust the force and deflection characteristics. Thethrust load of the screw is taken by a custom-made,four-point-contact bearing integrated into the‘‘snubber’’ housing.

Figure 10 a) MBARS mounted on a Sawbones model b) General view, close up on the milling tool c) view on the baseplatform and one of the pins.




The primary means of motion delivery to ourhyper-redundant robot joints is through a ball screwmechanism (Figure 9). The ball screws are 6 mmdiameter with 1 mm lead, rated at 400N. Themotors used are Maxon RE-10 (10 mm diameter)gearmotors with 16:1 planetary gear reducers and16-count encoders (64 counts per revolution withquadrature encoding). The motors develop about 21mNm of continuous torque with permissibleintermittent torque of 42 mNm; this translates toabout 120N of force at the ball screw (well belowthe rated load), with a 2:1 belt drive and assumingtransmission efficiencies. The snubber mechanismsare preloaded to about 600N to protect the ballscrews and bearings from overload; no displacementoccurs until this load value is reached, so the normalstiffness of the structure is not compromised. Testsof the joints indicate that the robot can deliver about450N to its moving platform; this is based on thecalculated axial force.

Operational and robot workspaceWorkspace evaluation is an essential stage in robotdesign for a specific task, as one of the primaryfunctions of a manipulator is to reach a set of desiredpoints in space with its end in prescribed orienta-tion. When dealing with parallel manipulators thisissue is of great important since, as most authorspoint out, the major drawback of parallel manip-ulators is their limited workspace. Since the requiredworkspace dramatically affects the robot dimensionand structure we first define the operational work-space for knee arthroplasty. The following data(Figure 11) is a summary of the work presented bySeedham which investigated the anatomical size ofthe knee for implant design (41).

As can be seen from Figure 11 and Table 1, thelargest dimension of the knee is about 80 mm (a) by70 mm (b), which means that any robot for kneesurgery should be capable of reaching any point inthis workspace. We optimized the dimensions andstructure of MBARS to cover this workspace,however this is difficult to display since it is a 6dimensional entity. The workspace of parallelmanipulators can be described as a six-dimensionvector, with three components as a location vectorand three as an orientation. Hence it can’t bedecoupled into two 3D workspaces characterizingthe possible translation and orientation motion.Therefore the workspace is completely embeddedin R36So(3) (three of translation and three oforientation) and there is no human readable way torepresent it. However, some projections of the fullworkspace can be drawn (usually the translation) ina fixed orientation and altitude of the mobileplatform (42). Such projections of the MBARSworkspace are shown in Figures 12a-c.

Low level controlThe mechanical design of the robotic system is notcomplete without integrating the low-level controlcircuitry. For the low-level control of the robot weuse a PIC controller. Hard wiring to all 6 actuatorsand encoders would require (666) 36 conductors toconnect to the mini-robot. This solution wasdeemed too cumbersome; hence we decided to

Figure 11 Anatomical measurements of the knee(Seedham (40)).

Table 1 Mean anatomical measurements of the knee (Seedham (40))

Mean dimension [mm] A b c D e f g

X-ray Based 77.2 67.2 66.4 28.8 20.2 27.9

Cadaveric Based 82.4 73 72.5 30.4 21.9 29.9 81.1




utilize technology based on an I2C control bus(by Koninklijke Philips Electronics N.V., theNetherlands), and requires a cable with just 6 wiresto control the robot. In our current design, there isone custom circuit board which contains sixindependent microcontrollers (one per link). Theboard is located inside the base platform of therobot (Figure 13). Each microcontroller includes aMicrochip PIC16F876 microcontroller, an LSI/CSILS7166 24 bit (quadrature) counter, an AllegroA3953 H-bridge amplifier IC, a linear voltageregulator for logic power supply, and a currentreader for force feedback and force control. Thefirmware running on the PIC microcontrollerincludes support for the I2C protocol, both sendand receive mode. Moreover, the code also per-forms a PID control loop for the motor connectedto it. For this use, the PIC reads the joint angle(quadrature encoder counter), and drives the H-bridge amplifier with a PWM signal according tothe data received on the I2C bus. It also has theability to communicate with the host computer overthe bus and provides, upon request, the current jointangle. The I2C bus is running on a 120 kbits/swhich are used for a 50 Hz data update rate onthe bus. The PID control loop on board the PICmicrocontroller runs at a 1 kHz rate. This frequencyis limited by the current microchip that is beingused and can be increased using a different model.Both the software and the control parameters can beupdated over the I2C Bus.

High level control and path planning for completecoverage of operational surfaceOne of the most important steps in any jointarthroplasty procedure is bone shaping. In this phaseof the operation, the surgeon sculpts the bone to fitan implant that would replace the diseased/damagedbony surface. Our algorithm that results in the pathfor the root to follow in order to move the cuttingtool and shape the bone uses cellular decompositionand sweep lines to generate a set of meaningful waypoints for the bone burring robot to visit, and thennavigate between these way points using potentialfunctions. The algorithm allows equal removal ofwaste bone with maximum control over the controlparameters such as depth, and roughness of the finalsurface.

Without loss of generality we choose to imple-ment our method in patellofemoral joint arthro-plasty (PFA) (Figure 7).

Figure 12 MBARS effective workspace for fixedplatform orientations.




The surgery requires the centerline of the implantto be aligned with patella tracking to ensure properalignment. Via software, a user (surgeon) marks thepatellar tracking line on the bone surface. Theimplant model is moved such that its centerline iscoplanar with the patellofemoral centerline(Figure 14a). A translation to put the distal tip ofthe implant surface in contact with the knee isperformed, and an exhaustive search for differentorientations of the implant is performed(Figure 14b): the algorithm selects the rotation,about the lateral-medial axis of the trochlearcomponent, which minimizes the distance betweenpoints on it and their corresponding closest pointson the femoral surface (Figure 14c).

Once the implant position on the surface isdetermined, we then calculate a trajectory for theMBARS cutter to achieve uniform coverage. To doso, we treat the bone burr tip as a point in 3D space.Once the trajectory for the particle is calculated, weuse inverse kinematics to find the configuration ofthe robot. Because MBARS is a parallel manip-ulator, inverse kinematics becomes a trivial problemof calculating link lengths.

For the task of coverage, we use cell decomposi-tion: a technique to achieve complete coverage ofconnected free spaces. By using cell decomposition,we break a space into a connected set of cells whichare easily covered. Therefore, covering the entirespace becomes an easily solved problem of visitingevery cell. In two-dimensional spaces, boustrophe-don cell decomposition (one particular kind of celldecomposition) is carried out by moving a scan lineacross a space, and marking the points where thescan line changes continuity. These points markwhere the border of a cell occurs (43). We illustratethe cellular decomposition of an example space as ascan line is moved from left to right (Figure 15a).The cell decomposition of the implant from a scanline moving in the distal-proximal direction yields 3cells (Figure 15b).

Once the cells are determined, one needs togenerate a coverage path within each cell. We havechosen to use a simple back and forth pattern toachieve coverage (Figure 15c), but we also regulatethe distance between passes to ensure uniformity.We ensure a uniform surface of the bone aftermilling, by maintaining equal sweeps of the burrsuch that scallop height, or the height of ridges on thesurface, remains constant (44).

Once nodes along the surface for MBARS to visitare chosen, we calculate ‘‘local navigation’’ usingpotential functions. Potential functions have alreadybeen widely used in mobile robot navigation. Insummary, a potential function is a differentiablereal valued function whose gradient defines a vectorfield. Navigation of a robot is done by treating thetip of the milling device as a particle movingthrough the vector field. By making a goal havean attractive potential and obstacles have arepulsive potential, virtual forces generated bythe potential field direct a robot from the startto goal position.

The result is a trajectory that navigates from pointto point at a fixed distance above the surface. Wechose the fixed distance to be significantly smallerthan the width of the burr head to keep the amountof bone being cut at a single pass small. A step in theiterative process is illustrated in Figure 16a. Finallyby varying constants, smoother trajectories for therobot to follow can be formed (Figure 16b). Moredetailed description of the algorithm can be found inAbraham et al. (44).

This algorithm can be applied on any surfacemodel whether it is image-based or anatomy based,

Figure 13 Electronics board embedded in the baseplatform.




i.e. palpated, since it is not source dependant.Currently we applied this algorithm on an image-based surface model. However, we are concurrentlyimplementing an image-free version where we usethe robot to scan the cartilage surface and create athree-dimensional model that is the basis of thesurgical plan. Again, the concept of attaching a smallyet very rigid robot to the bone is for the two tobecome a one rigid body with no relative motionsbetween them during operation. Since the robot isattached to the operated bone, the intraoperativeplanning can be performed in the robot’s end-effector coordinate frame. The system would utilize

the force feedback capabilities embedded in therobot in order to assist the surgeon to navigate apoint probe over the bony surface, by using a hapticdevice (such as a PHANTOM, www.sensable.-com). Once a contact is detected between the pointprobe and the bone, surface points are beingrecorded in the robot’s end-effector coordinatesystem (Figure 17).

Surgical protocol for the robot-assisted jointarthroplasty procedure.Without loss of generality we present here thesurgical flow for a PFA procedure. The surgical flow

Figure 14 Automatic Implant fitting to trochlear groove and bony surface.




in the robot-assisted procedure is modified slightlyfrom the conventional manner. Mainly, roboticmilling of the cavity for the femoral implant replacesthe freehand milling in the current procedure.Figure 18 shows the basic surgical flow for therobot-assisted technique, and is described in greaterdetail below*.

The operation is performed under tourniquetcontrol. After inflation of the tourniquet, a linearmidline incision is created similar in nature to theincision utilized for a total knee arthroplasty (TKA).It begins distally just medial to the tibial tubercle. Itextends in a linear fashion proximally to a pointroughly two fingerbreadths above the superior pole

of the patella with the knee in roughly 45˚ offlexion. This incision is significantly shorter than theincision required for traditional total knee arthro-plasty. It is similar to the incision currently utilizedfor minimally invasive TKAs. Sharp dissection iscontinued down to the level of the extensormechanism. Subcutaneous flaps are raised for easeof retraction. A medial parapatellar arthrotomy iscreated.

A traditional approach is to continue thearthrotomy to the proximal portion of the patellaand continue proximally into the quadricepstendon. Other alternatives include the midvastusor subvastus approach, which could be utilizedbased on surgeon preference and experience. Theseapproaches are advantageous in that the quadricepsitself is not violated and postoperative rehabilitation

Figure 15 a) Cellular decomposition: resulting cells, marked by circular icons, shown in step d b) Cellulardecomposition of implant with scan line Shown at critical step c) Coverage of the leftmost cell from step b d) The circularcutting tool tip leaves ridges known as scallops. Scallop height is dependent on the amount of overlap of the cuts made bythe tool.

* Thanks to Dr. Yram Groff from ICAOS at The WesternPennsylvania Hospital, Pittsburgh PA, USA




is more rapid. A significant advantage of the robot-assisted surgery is that the required exposure and softtissue dissection are minimized, and the patient’spostoperative recovery should proceed morequickly, while the implant can be placed in aprecise and more controlled way.

Before subluxing the patella, the surgeon per-forms a flexion-extension test to identify the patellartracking line. The surgeon then marks that line onthe femur, or marks the line that best represents the

path that the patella should track if the patella is nottracking ideally. The patella and extensor mechan-ism are then subluxed laterally, exposing the distalfemur and, in particular, the trochlear groove. Bladeretractors are placed medially and laterally topreserve the exposure. Alternatively, a self-retainingretractor is placed to ensure constant exposure.At this point, the robot’s sub-base is affixed to thedistal femur. This is accomplished by placing threeSteinman pins in a triangular configuration to create

Figure 16 Typical trajectory as formed by virtual forces. a) First position demonstrates the surface as a repulsive force,second as an attractive force; Simulated paths across a 4th order polynomial. b) Curves further away from the surfacecorrespond to smaller values of the control parameters.

Figure 17 a) b) Scanning of the femoral surface to construct the surface model.




a stable base. The pins are placed through a circularsub-base of the robot, so that the base of the robot iscentered at the cutting zone and roughly parallel toit. One Steinman pin is placed into the medialepicondyle, roughly 45˚ anterior to the coronalplane. This anatomic location is easily accessiblethrough the acquired incision and requires noadditional incisions on the patient’s skin. Thesecond pin is placed into the lateral epicondyle,also at an orientation of 45˚ anterior to the coronalplane. This anatomic landmark is often times easilyaccessible through the surgical incision. Othertimes, due to the patient’s anatomy, and difficultyof exposure, the epicondyle is not as readilyapparent. In these circumstances, a separate stabwound is made through the skin on thelateral aspect of the knee just over the lateralepicondyle. The incision required would be only

the diameter of the Steinman pin itself, andpercutaneous placement of this pin would be easilyaccomplished.

The third pin is placed on the anterior aspect ofthe femur proximal to the trochlear groove itself.The pin is oriented roughly 45˚cephalad to the axialplane. This anatomical location is easily accessedthrough the standard approach. However, if thisarea was not easily exposed due to anatomicvariation, then this pin could also be placed throughanother simple percutaneous stab wound.

The robotic mechanism is then placed on thesub-base, establishing the rigid fixation between therobot and the distal femur. The robot mechanism isplaced inside a sterile donut-shaped bag, andadditional sterile tool interfaces are being clampedto the top of the robot. The tool is capable ofholding, alternately, the digitizing pointer and the

Figure 18 Surgical flow for robot-assisted PFA.

Figure 19: a) MBARS attached to the femur bone; b) Model of PFA implant.




cutting tool that is in contact with the bone andcartilage surface. The pointer and the cutter aredocked in a unique and repeatable way, so that theycan be alternated as the procedure requires and suchthat calibration is not necessary.

At this point in time, the surface scanning,surgical planning and robotic machining of thetrochlear groove are performed. These steps aredescribed in detail in the following sections. Thefinal result is that the bone cavity is machined toperfectly match the undersurface of the trochlearcomponent of the prosthesis. Once the trochleargroove is finished, the femoral trial componentwould be applied to ensure an accurate fit. After thistest, the robotic arm is removed.

Attention would then be turned to the patella.Traditionally, the patella is everted to create thepatellar cut. This would not be required. It couldsimply be turned 90 .̊ Typically, the degenerativesurface of the patella is removed using either anoscillating saw or a reaming device. A minimum of12 mm of patellar thickness is retained. A trialprosthetic patella is then placed onto the cut surfaceof the patella. Typically, the patellar component ismedialized on the cut surface of the patella as muchas possible to optimize patellar tracking. After thejoint is tested with the trial components in place, thetrial components are removed. The cut surface ofthe bones, having been properly prepared, is driedcarefully and bone cement is typically used for theimplant fixation. It is applied to the cut surface ofthe bone as well as to the back surface of theimplants and these are held under pressure until thecement cures completely. Once the components arein place, a test of range of motion is undertaken.Patellar tracking is observed. If necessary, a lateralrelease is performed. In certain case of grossmaltracking, a tibial tubercle osteotomy would beperformed as well.

The robotic procedure has several benefits overthe conventional one in case of a PFA. In thesimplest sense, it eliminates the human errorassociated with the sculpting of the trochlear grooveto accept the prosthesis. Ideally, this creates a perfectfit between the sculpted surface of the bone andthe undersurface of the trochlear component.Additionally, the femoral component is recessedinto the bone and cartilage at the appropriate depthto avoid impingement between the patellar compo-nent and the edge of the femoral component. Thisprecise fit is anticipated to increase the longevity of

these components. Furthermore, the robotic bonepreparation decreases the required exposure for theoperation. The exposure currently required isequivalent to that required for minimally invasivetotal knee arthroplasties. But with the assistance ofthe robot, it is conceivable that the requireddissection would be minimized further. This hassignificant potential benefits to patients in terms ofthe time required to convalesce from the operationand long term recovery of function, particularlyfunction of the quadriceps.

Finally, the robot is capable of accomplishingthese goals in a more expedient fashion as comparedto surgeons, due to the accuracy of milling the bone.This decreases the required total surgical time and,in particular, tourniquet time. This is also antici-pated to have a positive impact on patient recoveryand on overall operating room efficiency.

FIRST EXPERIMENTSWe have conducted several experiments on saw-bones with MBARS in order to validate the systemconcept. During the experiment, the robot wasattached to the femur by using three pins asdescribed in the previous section (Figure 19), inorder to create the cavity for the PFA implant. Foran implant we have used a ‘‘home made’’ implantwhich was designed with CAD software and wasprinted out in an FDM machine so that the surfacemodel of the implant was available. Then the robotwas attached to the femur and a registration processwas carried out. Once registration is completed therobot burrs the bone to create a cavity for theimplant by manipulating a conventional StrykerTPS tool over the bony surface. First, the robotburred the outer contour of the cavity (Figure 20a)followed by removal of the rest of the waste bone(Figure 20b) following a trajectory generated by themotion planner described in the previous sections.The entire process of preparing the cavity tookabout two minutes. The result of this process canbe observed in Figure 20c (45). We are currentlyconducting experiments to evaluate the robotaccuracy.

CONCLUSIONMedical robotics has the potential to revolutionizethe conduct of surgical procedures on bonyanatomy. It can assist surgeons by preparing bonesmuch more accurately than mechanical guidesor freehand cutting. It can help improve patient




outcomes by decreasing surgical errors. In theorthopaedic community however, medical robotshave not been very successful due to a variety of

issues ranging from robot size, to surgical time, andsoft-tissue difficulties. This report proposes toovercome these difficulties by utilizing a miniaturerobotic milling device that attaches directly to thebone.

Realizing the potential of minimally invasiveprocedures, the implant industry is currently in theprocess of redesigning implants and, together withsurgeons, re-examining surgical procedures. It canbe expected that the next generation of implantswill be smaller and more suitable for eventualdevelopment of less invasive procedures. Oneexample of such procedures in knee arthroplasty ispatellofemoral resurfacing of the knee. Without lossof generality we examined the capability of therobot in improving the accuracy of the femoral-component preparation for patellofemoral arthro-scopy. From the engineering point of view thisprocedure simulates the future generation oforthopaedic arthroplasty where there will be a needto machine bone surface into a more complex shapethat is not planar or spherical, but complex surfaces.Therefore, the technology demonstrated in thisreport, though not specific to the patellofemoralprocedure, is adaptable to many other areas.

The reported work presents the development ofa miniature robot that is rigidly affixed directly tothe bone. The robot itself scans the shape of thefemur directly, removing any need for preoperativeimaging or intraoperative registration and trackingof the bone. With the additional input of thedirection of patellar tracking, it automaticallyoptimizes the planned position of the implant toensure that it is properly aligned and congruent withthe surrounding healthy cartilage. Congruency is arequirement for this procedure to make certain thatthe patellar component does not impinge on theedge of the femoral component and lead to earlyfailure of the implant. The robot then mills out thecavity to within 1 mm of the planned location,guaranteeing complete coverage of the area with adefined surface uniformity. The process presented inthis work theoretically reduces the operational time,increases accuracy and allows MIS arthroplastyprocedures of the next generation.

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Figure 20 MBARS a) Milling the outer contour of theimplant; b) Milling the inner surface insuring completecoverage; c) Implant placed after milling is completed.




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