Robot-assisted invasive orthopaedic surgery

Pergamon

Mechatronics Vol. 6, No. 4, pp. 381-397, 1996 Copyright © 1996 Elsevier Science Ltd. All rights reserved.

Printed in Great Britain. 0957-4158/96 $15.00+0.00

PII:S0957-4158 (96) 00002-5

ROBOT-ASSISTED INVASIVE ORTHOPAEDIC SURGERY

K. BOUAZZA-MAROUF,* I. BROWBANK* and J. R. HEWIT +

*Department of Mechanical Engineering, Loughborough University of Technology, Loughborough, LEll 3TU, U.K. and +Department of Applied Physics and Electronic

and Mechanical Engineering, University of Dundee, Dundee DD1 4HN, U.K.

Abstract--Changes in orthopaedic practice have led to an increased reliance upon fluoroscopic image guidance during internal fracture fixation (osteosynthe- s/s) procedures. The resulting complexity of surgical techniques, and concerns over X-ray radiation exposure levels to orthopaedic surgeons, has prompted the introduction of new technologies into the operating room with the aim of improving the precision, repeatability and radiation safety of existing surgical procedures. The generic vision-guided robotic system for orthopaedic applications described in this paper is typical of this trend. In order to satisfy the stringent safety requirements of robotic-assisted surgery, the mechatronics design philosophy has been applied to the system. A purpose built manipulator has therefore been manufactured, which when interfaced to an existing C-arm unit allows a drill-bit guide to be automatically aligned with an intra-operatively planned drilling trajectory. Manual completion of the drilling stage can then be performed by the surgeon. The preliminary findings of an ongoing study into the potential for an invasive application of such a system, through the use of an actuated drilling unit incorporating a novel use of force feedback, are also presented. Initial results indicate that force feedback could play a major role in the safety protocol of such an invasive system. Copyright © 1996 Elsevier Science Ltd.

1. INTRODUCTION

Orthopaedic treatment of bone fractures initially involves a reduction stage, in order to restore the bone fragments to their normal anatomical position. Stabilisation of the reduced fracture site is then required to prevent misalignment occurring while the natural bone healing process takes place. In recent years, internal fixations (osteosyn- theses), medical factors permitting, have become the preferred fracture stabilisation method. These surgical fixation techniques, achieve excellent stabilisation through the use of internal mechanical devices [1], and as such allow early post-operative mobilisation of the patient. Recumbancy related problems associated with traditional treatments, such as prolonged traction or plaster-casting, are consequently avoided.

The current trend in osteosynthesis, as in other branches of medicine, is to minimise the trauma associated with gaining access to the surgical site. Accordingly,

381

382 K. BOUAZZA-MAROUF et al.

many of the "open" osteosynthesis techniques, which require large incisions and retraction to expose the fracture site, are being superseded by "closed" surgery counterparts in which the fracture site is not directly exposed. Without direct visualisation of the fracture site, X-ray imaging often becomes the only means of guidance during surgery. A direct result of these changes in clinical practice has, therefore, been a marked increase in the use of intra-operative fluoroscopic imaging.

The heavy reliance upon fluoroscopic guidance creates both technical difficulties and a potential health hazard for the orthopaedic surgeon. The use of two-dimensional X-ray images, to guide three-dimensional bone machining processes, creates a depth perception problem. As such, achieving optimum positioning of fracture fixation devices involves a certain amount of trial-and-error and may take several attempts. Both the duration and outcome of the procedure are, therefore, highly dependent upon the skill and experience of the surgeon. The increase in the number of X-ray images acquired during surgery has also led to concerns over the levels of radiation exposure received by orthopaedic surgeons. In particular, the direct exposure of the surgeon's hands to the collimated X-ray beam is a source of concern [2, 3]. Although a number of independent studies have quantified the actual radiation exposure received by orthopaedic surgeons [4-6], the accumulative effects of long term exposure to low-dose radiation are still not well understood. Consequently, no firm conclusions can be made as to how serious a health risk fluoroscopic guidance actually poses.

The introduction of a generic vision-guided robotic system into the operating room is currently being investigated as a means of improving existing osteosynthesis procedures [7, 8]. In particular, two specific femoral fracture procedures have been identified as being ideal candidates for robot-assisted orthopaedic surgery [9]. Conventional techniques for locked intramedullary nailing of femoral shaft fractures are technically demanding and have been shown to expose orthopaedic surgeons to significant levels of X-ray radiation [10, 11]. Sliding hip screw fixation of proximal femoral fractures, arguably the most common orthopaedic procedure, is severely complicated by the depth perception problem. A purpose-built prototype manipulator, designed to work in partnership with an orthopaedic surgeon, has therefore been developed to perform the stages of surgical procedures which currently involve drilling of bone under X-ray guidance. In this way, a high precision "remote" surgery capability is provided to reduce both the scope for human error and the X-ray radiation hazard.

Mechatronics is concerned with the integrated design of complex systems involving mechanical engineering fused with electronics and computer control. The aim of the mechatronics approach to design is to achieve a high degree of synergy so that the resulting product has a performance well beyond that previously attainable. This demands the implementation of teamwork in the design process. The team will include specialists covering the various branches of technology involved in the product. These specialists, led by an individual with a broad base of design experience, are encouraged to communicate effectively. Examples of good mechatronics design can be found in modern SLR cameras, industrial robots and computer disc memories. Indeed, the Japanese dominance of world markets in these and similar products can be directly attributed to their espousal of mechatronics as the design philosophy [12].

Robot-assisted invasive orthopaedic surgery 383

The aim of the sections which follow is to give an indication of how the adoption of the mechatronics design philosophy has allowed solutions to be found for the unique requirements of invasive robot-assisted orthopaedic surgery. Following a brief discus- sion of the application of technology to current orthopaedic surgery practices, the primary design criteria of patient and theatre staff safety are discussed. Limitations imposed upon the design process by a need to ensure surgical sterility are also addressed. The preliminary findings of an ongoing study into the invasive aspects of the system, automated drilling of bone incorporating a novel use of force feedback, are then presented,

2. ROBOTIC-ASSISTED ORTHOPAEDIC SURGERY

Orthopaedics is primarily concerned with the correction of deformity, diseases of bone and joints, and injuries to the musculoskeletal system. The diversity of this branch of medicine requires surgeons-in-training to learn a wide range of technical skills and surgical procedures. Unfortunately, the inherently subjective nature of current surgical procedures creates a number of problems. Hand-held air-powered tools are used to manually perform bone machining procedures such as drilling, reaming, taping, sawing and screw insertion. This "wet carpentry" of bone requires the adoption of a trial-and-error approach, with qualitative X-ray imaging often providing the only means of surgical guidance. As a result, the necessary skills and experience can only be acquired over a long learning curve. Secondly, the precision to which surgical procedures can be planned, through the use of modern three-dimensional imaging modalities such as Computerised Tomography (CT) and Magnetic Resonance Imaging (MRI), is often not reproducible in the operating room using manual surgery techniques. New technologies are therefore beginning to be intro- duced into the operating room with the aim of improving the precision and repeatability of orthopaedic tasks.

The introduction of PC based technologies into the operating theatre is now generally referred to as "Computer-Assisted Surgery". Much of the early research in this area concentrated upon neurosurgical procedures. Intra-operative registration techniques, such as instrumented articulated pointing devices [13, 14], were developed in order to establish intra-operative correspondences between pre-operative MRI or CT images and actual anatomical features. In many orthopaedic applications, however, adherence to existing surgical practices precludes the use of the more expensive three-dimensional imaging modalities. Lower cost alternative intra-operative co- ordinate registration techniques are therefore under investigation at various research sites around the world. The use of the OPTOTRACK TM optical digitiser, in spine [15, 16] and knee ligament surgery [17], is a notable example of this trend. Sonic devices, which rely upon the time-of-flight measurement principle, have also been investigated for use in spine surgery [18]. Quantisation of the X-ray examination process, through the calibration of the mobile C-arm image intensifier unit which is widely used during orthopaedic surgery, is also an ongoing topic of research by the current authors (see section 3.4) and other research groups [19].

The concept of robotic-assisted orthopaedic surgery was initially demonstrated using modernised industrial robots. Safety issues, coupled with a need to gain clinical


acceptance, resulted in the majority of the procedures investigated having a non- invasive nature. The work of Matsen et al. [20], into the use of a robot to position knee arthroplasty jigs, is typical of these applications. Invasive procedures have, however, been successfully tackled in a few notable cases. The ROBODOC TM system developed by Taylor et al., to prepare femoral cavities during cementless hip replacement surgery, is now undergoing full clinical trials on human patients [21]. More recently, a noticeable trend towards the use of custom-built manipulators, in order to achieve full operating room compatibility at reduced cost levels, has emerged. The advantages of this "mechatronics in medicine" approach have already been seen in the field of urology, through the successful application of the 'SARP' (Surgical Assistance Robot for Prostatectomy) system [22]. The application of mechatronically designed systems to orthopaedic surgery procedures is a natural progression of this work.

3. MECHATRONICS APPLIED TO OSTEOSYNTHESIS

The mechatronics approach has been proposed as a possible solution to the safety and technical problems associated with robotic-assisted orthopaedic surgery. A vision-guided robotic system has therefore been developed to perform the drilling of bone during osteosynthesis procedures. This prototype system has the potential to augment the manual skills of surgeons, through improved precision and repeatability, as well as reducing radiation exposure to background levels. As such, reductions in both the duration of operations and fracture fixation failure rates could follow due to accurate right-first-time implant insertions. Given the large number of osteosynthesis procedures performed each year (during 1985 alone 46,000 proximal femoral fractures were reported in the U.K. [23]), even minor improvements would prove to be very significant in terms of medical resources.

Safety is the overriding design constraint in implementing a surgical robotic system. The patient, primary surgeon and theatre staff are all required to come into close proximity with the manipulator. This level of human/robot interaction is in complete contrast to industrial robot applications, where legislation aims to keep robots and humans apart through the use of fencing, interlocking and sensors. As these safety measures are impractical in the operating theatre, extensive modification of commer- cially available robots is required to meet the safety requirements of robotic-assisted surgery. Sterility of the robotic system must also be achievable using standard sterilisation methods. By adopting the mechatronics approach, these safety and sterility requirements have been addressed at the outset in the mechanical, interface and software design.

3.1. Prototype manipulator

The robotic-assisted orthopaedic surgery system is shown in Fig. 1. It incorporates a prototype manipulator, a PC based controller and a machine vision sub-system interfaced to a standard C-arm X-ray unit. Once intra-operative registration is established and the drilling trajectory defined by the surgeon, the machine vision sub-system outputs the trajectory data to the controller allowing automatic positioning


Ortho

Drilling

Fig. 1. Robotic-assisted orthopaedic surgery system.

of the drill holder unit. The machine vision aspects of this process are explained in section 3.4.

The prototype manipulator has five degrees-of-freedom (DOF). Four DOF, which constitute horizontal, vertical, pan and tilt motions, are used for the positioning of the drill-bit along the desired trajectory. The fifth DOF is used for the insertion of the drill-bit (for invasive surgery). All the DOF of the manipulator, with the exception of the pan joint, are driven by stepper motors through backlash-free baUscrews. The pan joint is driven by a stepper motor through a reduction gearbox fitted with an anti-backlash device.

The design of the drilling unit, the safety features and the sterility issues relevant to this system are discussed below.

3.1.1. Drilling unit. The drilling unit (Fig. 2) has been designed with the aim of improving upon the results which may be obtained by manual drilling with a hand-held guide [24]. This improvement is to be provided through quantification of the drilling process using force feedback. A strain gauge force sensor, which monitors the axial drilling force, has therefore been incorporated into the design of the drill-feed carriage. At present, a standard industrial air drill is being used to carry out laboratory trials of this drilling unit as discussed in section 4.

The drill-feed actuator consists of a drill-feed carriage driven through a ballscrew by a stepper motor. The drill holder unit is detachable, in order to maintain surgical sterility as described in section 3.3, and clamps onto the drill-feed carriage. In line with current surgical practices, a drill-bit guide is used to prevent the drill-bit from


Drill

] ' ( '~-~ ~ ~ l Detachable

II I//

Separate Manual Drill Bit Guide

Fig. 2. Drilling unit of robotic system (for both automatic and manual guide-wire insertion).

slipping on the bone surface. Drilling depth is measured using a linear potentiometer on the drill-feed actuator. Due to electromagnetic interference caused by operating theatre equipment, such as the C-arm unit, shielding of both the force sensor and potentiometer has been necessary in order to maintain signal integrity.

In order to gain initial clinical acceptance of the system, a manual drill-bit guide has been incorporated into the drilling unit as shown in Fig. 2. This allows the manipulator to be used as a non-invasive tool positioning device, aligning the manual drill-bit guide with a calculated drilling trajectory. Manual completion of the drilling task can then be undertaken by the surgeon, using the guide to achieve optimum drilling results.

3.2. Safety features

The safety of the patient and the theatre staff must be ensured at all times. The surgeon must, therefore, be in complete control of the robotic system which in turn should incorporate all the necessary safety measures. This is attained by providing the surgeon with a dead man's handle, and incorporating electromechanical safety devices and a safety protocol in the control software. During alignment of the drill-bit guide with a computed drilling trajectory, software imposed motion limits confine the manipulator to a "safe" working envelope. In the unlikely event of these software limits being exceeded, limit switches are fitted to all the joints to prevent overdrive. The maximum safety condition of the system must also be established before drilling can be started. Hence, fail-safe locking mechanisms (brakes) fitted to all the drives,


except the drill-bit feed drive, are activated prior to initiating the drilling procedure. In addition, the drilling depth is continuously measured and checked against a predefined value obtained in the intra-operative surgical plan. Another safety enhancement is the measurement of the axial drilling force and the monitoring of the profile of this drilling force by a "safety supervisor" algorithm, as discussed in section 4. Confirmation of the final drilling depth is obtained fluoroscopicaUy.

3.3. Sterility issues

Osteosynthesis procedures involve the insertion, often via large incisions, of foreign bodies (metallic fixation devices) into human anatomy. The risk of infection associated with these procedures is consequently significant and can be as high as 40% when extensive soft tissue damage is present [25]. The prevention of contamination in the operating room is therefore an overriding design criterion for the robotic system. Compatibility with existing sterilisation techniques, and the need to maintain sterile areas in the operating room, has been ensured through the adoption of a modular manipulator design.

Sterilisation of the manipulator is not practicable due to size limitations imposed by the chamber dimensions of clinical sterilisation units. Sterilised draping will, therefore, be used to isolate the manipulator from the sterile areas of the operating theatre. A similar method of draping is currently employed during most orthopaedic surgery procedures to isolate the image intensifier and X-ray source housings of the C-arm unit. The sterilised drill holder unit is then clamped onto the manipulator outside the isolation drape, as shown in Fig. 3, thus maintaining surgical sterility.

The drill holder unit, which may come into contact with the patient, is to be steam sterilised in an autoclave and has been designed accordingly. Effective steam sterilisation requires items to be exposed to saturated steam in the range 121-134 °C for up to 15 minutes. These conditions preclude the use of many common engineering materials, and in particular most polymers. Special high temperature seals made out of virgin polytetrafluoroethylene (PTFE) have therefore been used. The drill-bit guide has also been manufactured from stainless steel to ensure adequate corrosion resistance and biocompatibility [1].

In addition to the manipulator and drill holder unit, other items which are handled by the surgeon and operating room staff have to be either sterilised or isolated. These items may include a PC mouse, teaching pendant or even a keyboard. Electrical equipment cannot obviously be subjected to steam sterilisation conditions. Low temperature sterilisation by gaseous ethylene oxide (EO) may therefore be employed. This process is slower and more expensive to operate than steam sterilisation, and may also leave toxic residues behind. A long aeration time is therefore required to reduce residual EO to a safe level. When practical, isolation of these items using sterilised plastic covers may be a more cost effective alternative to EO.

3.4. Machine vision interface

X-ray imaging is used during most osteosynthesis procedures to verify that satisfactory reduction and fixation device insertion have been achieved. Each fluoroscopic examination stage involves the acquisition of two approximately orthogonal,

388 K. BOUAZZA-MAROUF et al .

/; i i " t i ?

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Fig, 3. Manipulator isolation using sterilised drape.

posterioanterior (PA) and lateral, views of the fracture site using a mobile C-arm unit. The design of these units incorporates an X-ray source and image intensifier/ video camera system mounted on opposite ends of a C-shaped beam. This beam may be manually positioned and oriented to obtain X-ray images of the required portion of the patient's body from a number of viewpoints. Images acquired in this way can be instantly displayed on a monitor for examination by the surgeon.

In all robotic-assisted orthopaedic surgery applications, it is essential that the position of the patient (fracture site) with respect to the robot is accurately known at


all times. As there is often no direct access to, or visualisation of, the fracture site, intra-operative co-ordinate referencing can only be achieved indirectly using X-ray images. Co-ordinate referencing is therefore a machine vision task, facilitated by interfacing the C-arm unit to a PC equipped with a frame grabber. Unfortunately, C-arm units are designed solely for qualitative diagnostic examination and as such have a number of inherent characteristics which are highly undesirable for metrology applications. The positioning degrees of freedom are neither instrumented nor rigidly lockable, resulting in an unconstrained imaging geometry of low repeatability; in addition, the displayed image is noticeably distorted. A means of introducing artificial calibration markers into the standard clinical X-ray views is therefore required in order to compensate for these shortcomings.

3.4.1. Intra-operative registration. The intra-operative registration scenario is illus- trated in Fig. 4. The ultimate aim of this calibration process is to accurately establish the three-dimensional co-ordinates of anatomical target points with respect to an arbitrarily defined manipulator co-ordinate system (MCS). In practice, the reliance upon intra-operative fluoroscopy means that the position of a target point in the MCS has to be obtained from the informational content of the PA and lateral images. The use of a flame grabber, to digitise these X-ray images, allows the two-dimensional image co-ordinates of the target point to be obtained with respect to an image display co-ordinate system (IDCS). If the target point is visible in both views, triangulation based techniques can therefore be used to reconstruct its position, provided that a relationship can be established between the manipulator (MCS) and image display (IDCS) co-ordinate systems.

The relationship between the two frames of reference, MCS and IDCS, can be obtained in a number of ways. One option is to rigidly insert radiopaque pins into the bone during a pre-operative surgical procedure. The manipulator can then be "taught" the positions of these pins intra-operatively. Another possible method, is to instrument the C-arm unit and establish a physical link with the manipulator. However, these options are not compatible with the overall project aims which include minimal deviation from existing surgical practices and no modification of existing equipment. An alternative technique, which uses radiolucent calibration frames containing radiopaque reference markers, has therefore been adopted.

An anatomically compatible perspex calibration flame is positioned around the body part of interest. By attaching the calibration frame to the manipulator, the transformation between the calibration frame co-ordinate system (CFCS) and the MCS is established by direct kinematics. Once manipulator calibration is performed (off-line), the position of the calibration frame is calculated from a combination of calibrated dimensions and intra-operative potentiometer (or encoder) readings. In this way, the positions of the radiopaque calibration markers are determined in terms of the MCS. By acquiring fluoroscopic images of the calibration frame together with the fracture site, the co-ordinates of any visible calibration marker with respect to both the IDCS and MCS can be obtained, thus allowing the relationship between the manipulator and image display to be established.

The X-ray projection process is almost linear and can be modelled as a cone of beams diverging from the point source to the image intensifier input screen. An undistorted pattern of X-ray radiation, attenuated by the calibration markers and

390 K. BOUAZZA-MAROUF et al .

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/ _! Cross Section / Target Point --~ _-- o f Thigh

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Fig. 4. Intra-operative registration between the patient and manipulator.

anatomical features, is consequently incident at the input screen of the image intensifier. Unfortunately, the image intensifier/video camera chain, used to convert the incident X-ray radiation into a visible image, exhibits highly non-linear optical properties. As such, the output image which is digitised and displayed by the frame


grabber is noticeably distorted. Pre-operative internal calibration is therefore required to compensate for this image distortion. This is achieved by placing a radiolucent calibration object, containing radiopaque reference markers, as near as possible to the input screen of the image intensifier unit. An image dewarping mapping is then derived, which relates the measured (distorted output image) IDCS calibration marker co-ordinates to their known (undistorted) co-ordinates in the image intensifier co-ordinate system (IICS). By applying this mapping and a grey-scale interpolation function over the entire distorted image, an undistorted image can be displayed allowing quantitative interaction with the surgeon. This "true" image represents a reconstruction of the image which would have been formed by the X-ray projection process had radiographic film been used as the means of X-ray transduction rather than the non-linear image intensifier.

Once the IICS undistorted co-ordinates of the calibration markers are known, an intra-operative external calibration technique can be applied to establish the position of the C-arm with respect to the manipulator (IICS to MCS transformation). Calibration methods originally developed for both camera [26] and X-ray film based applications [27] are currently being used to perform this task.

3.4.2. Quantitative fluoroscopic imaging. The provision of an intra-operative quantitative fluoroscopic imaging capability, through the use of internal and external calibration of an unmodernised C-arm unit, offers significant benefits to orthopaedic surgery. The need for surgeons to "think" in three-dimensional space can be reduced using human-computer interfaces to build up an intra-operative surgical plan from sets of target points. These target points are indicated by the surgeon, using a mouse or similar pointing device, and are displayed as overlays on top of the "true" X-ray images. The end points of a drilling trajectory are initially indicated in two-dimensions on the PA view. Transformations derived during calibration then allow the lines which correspond to these target points to be displayed as overlays on the lateral view. In this way, the surgeon's indication of the drilling trajectory on the lateral view is quantitatively guided by the information from the PA view, minimising the depth perception problem.

After confirmation by the surgeon, the three-dimensional drilling trajectory is downloaded via an RS-232C serial link to the manipulator control system. The manipulator is then automatically positioned to align either the automatic drilling unit or manual drill-bit guide with this drilling trajectory. An accurate execution of the surgical plan can then be performed. The potential for right-first-time fixation device insertion is thus provided by the vision-guided robotic-assisted surgery system.

4. DRILLING TESTS

Bone is an inhomogeneous organic material of variable density, which can occur as either a compact (cortical) or spongy (cancellous) form. A typical human bone consists of an outer shell of cortical bone around a central mass of cancellous bone. In long bones such as the femur some of the cancellous bone may be replaced by a marrow cavity (medullary cavity). The density of cancellous bone varies considerably but can be considered to be about 1.3 g/era 3, whereas cortical bone has a density in

392 K. BOUAZZA-MAROUF et al. the region of 1.8 g/cm 3 [28]. The strength of cancellous bone is very variable; it is usually less than one tenth of that of cortical bone [1]. It has been proposed that the resulting variations in bone properties across a cross-section of the femur can be utilised, through the provision of drilling force feedback allowing comparison with theoretical drilling force profiles, as part of the safety protocol for automated drilling procedures. In particular it is expected that sharp changes in axial drilling force will be detected at a cortical/cancellous bone interface.

In order to empirically investigate the effect of bone density/strength upon axial drilling force at a cortical/cancellous bone interface, the drilling unit has been instrumented with a force sensor as described in section 3.1.1. A preliminary set of qualitative, with respect to drilling depth, tests were then performed on fresh porcine femurs to investigate the drilling force characteristics at both the shaft and head/neck (proximal) sections of the femur. Pig bones were used as they are a close representation to human bones in terms of mechanical properties and composition.

Figure 5 is representative of the drilling force profiles obtained at the shaft section (diaphysis) of a pig femur using a surgical pilot drill-bit (or guide-wire) of diameter

~ . ~ Medullary Cavity

~ Threaded Trocar

Pilot Drill B[[* ( A : ~ * / I _ _ ~ B ?_L_~2~_~ C TiP.cortlcalBone

Force (NI ......................... i ......................................................... ~' ................ I

i::;::iiiii.i. ....... ii 17N

Drilling Time

Fig. 5. Drilling force profile at the shaft section of a pig femur (feed late = 2 mm/s, drilling speed = 3300 rev/min).


2.5 mm. The drilling force can be seen to increase steadily as the guide-wire enters the cortical bone (region A) before reaching a peak just prior to the penetration of the medullary cavity (region B). Subsequently, there is a rapid drop in force as the guide-wire enters the medullary cavity. The drilling force then remains more or less constant until the far cortical bone (region C) is entered. Again, the force increases steadily and drops rapidly upon the tip of the guide-wire breaking through the bone. As predicted, a sharp change in drilling force is observed at both cortical bone/medullary cavity interfaces.

Figure 6 shows a typical drilling force profile at the proximal section of a porcine femur using the same guide-wire. A rise in the drilling force (up to peak P1) is initially observed as the guide-wire enters first a thin layer of cortical bone and then an area of cancellous bone (region A). A sharp drop in the drilling force then occurs as the guide-wire penetrates a low density area of bone (region B). In this section of

Medium Density Low Density High Density Cancellous Bone Cancellous Bone Cancellous Bone '\

Conical Bone

i ........... Y ........... i ......... ~ ........... i ........... T. ........... i "F~V; Force(N) ~ i i ~ i i ~ . . . ~ . . . . . . ~

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........... ........... ...................... . . . . . . . . . . . . i I o I

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Fig. 6. Drilling force profile at the proximal section of a pig femur (feed rate = 2 mm/s, drilling speed = 3300 rev/min).


the femur, an almost constant drilling force is observed. As the guide-wire reaches the femoral head (region C), the force increases steadily (up to peak P2) and drops rapidly upon the tip of the guide-wire breaking through the femoral head.

An interesting feature of this particular test case, is an unexpected drop in force at point T1, which indicates the guide-wire has passed through an anomalous lower density area of bone. Once again, the force profile is as expected with the highest magnitude drilling forces being measured in the area of the femoral head.

While factors such as compliance within the present drilling system have obviously contributed to the specific form of the force profiles obtained, necessitating a more in-depth quantitative investigation of the overall drilling process, the preliminary tests have demonstrated that the expected rapid change in drilling force at cortical/ cancellous bone interfaces is quantifiable. As such, force feedback is now being incorporated into the safety protocol of the robotic system and applied to internal fixation procedures.

During the intramedullary nailing of femoral shaft fractures, it is often necessary to interlock the nail (a hollow stainless steel tube inserted into the medullary cavity) with the bone in order to achieve the desired level of fracture stabilisation [9]. Part of this interlocking procedure involves the "blind" insertion of distal locking screws through one cortex of the femur, a pair of holes in the nail, and finally the far cortex. During this process, it is not uncommon for the drill-bit to come into contact with, and damage, the nail. By setting a force limit in the safety protocol, the drilling can be stopped immediately in the event of such a contact, which is indicated by a sudden rise in drilling force.

The internal fixation of proximal femoral fractures using one of the sliding hip screw devices, requires the insertion of first a guide-wire and then a lag screw into the femoral head [9]. Several recent studies have shown that there is a reduced rate of fixation failure if the lag screw is in the lower half of the femoral head [29, 30]. However, there still remains a debate within orthopaedic surgery as to whether the traditional central lag screw position, with respect to the femoral neck, produces better results than this lower half placement. It is hoped that a quantitative investigation of the drilling force characteristics of the guide-wire insertion stage of this procedure will provide data which contribute to this argument.

Quantitative sets of drilling tests, employing different drill speeds, feed rates and drill-bit designs, are planned to determine drilling force profiles at different positions and sections of the femur. A custom built, low compliance test rig is currently being manufactured to facilitate these tests. It is hoped that the permanent bone strength data obtained during these tests, can form the basis of future clinical investigations. The instrumentation of a standard surgical drill is also in progress, with the aim of investigating the drilling forces associated with manual orthopaedic surgery procedures. This will allow comparisons to be made with force profiles obtained during automated invasive procedures.

5. CONCLUSIONS

The potential benefits arising from the application of a mechatronic system to orthopaedic surgery procedures have been investigated. It has been shown that the


development of suitable calibration techniques allows quantification of the standard biplanar fluoroscopic examination. This permits drilling trajectories based upon quantitative information, rather than subjective judgements, to be planned via human/computer interfaces. A fail-safe alignment, with these drilling trajectories, of a guide for a drill-bit (or guide-wire), can be achieved using a purpose built manipulator which has been described. The guided manual drilling, non-invasive alignment scenario represents a clinically acceptable means of introducing robotic technology into the operating theatre. However, scope for human error still remains, along with a reliance upon fluoroscopic imaging to assess drilling depth. As confidence is gained in the performance of the system, it will be possible to implement an invasive drilling procedure. Such an automated procedure necessitates additional safety precautions; preliminary results have indicated that force feedback can be utilised to enhance the safety protocol during invasive procedures.

Acknowledgements--This research project has been supported by the Wishbone Trust of the British Orthopaedic Association. The authors are grateful for the financial support provided. This work is carried out in collaboration with Mr R. G. Shedden, Consultant Orthopaedic Surgeon, Mayday University Hospital, Surrey, England. The authors also wish to thank Mr F. R. Ong for his contributions to the experimental tests and desgin of the drilling unit.

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