End User Issues for Computer Assisted Surgical

End User Issues for Computer Assisted Surgical Systems A.M.M.A. Mohsen ** & *, K.P. Sherman **, T.J. Cain **, M.R.K. Karpinski **, F.R. Howell **. R. Phillips *, W.J. Viant *, J.G. Griffiths *, K.D.F. Dyer * ** Orthopaedic Department Hull Royal Infirmary, Hull. U.K. HU3 2JZ * Department Of Computer Science, University of Hull, Hull. U.K. HU6 7RX

Abstract Orthopaedic implants are manufactured to the highest degree of precision by some of the most precise machines known to man and inserted into patients by some of the most imprecise methods known. Computer assisted systems aim to overcome this dichotomy by improving the planning and implementation of orthopaedic surgery. This can be achieved by providing the surgeon with better information for planning and a more precise means of implementing the surgery. This surgical advancement will significantly change current orthopaedic practice if the appropriate surgical issues are considered during their development. Safety is obviously paramount and is being addressed, as is registration between the real (patient) and the virtual computer world. The more subtle, but never the less important, surgical issues have as yet not been fully identified or addressed satisfactorily. The following questions serve to highlight them. Is there an optimal system size, shape, reach, control and positioning in surgery? What are the salient environmental and functional requirements? Can there be intra-operative computer processing time? How important and what does timelessness, universality, communality and simplicity of the system mean? Should there be a relationship between training, surgical feedback and simplicity? What is partial or total sterilisation? Can capital outlay and running costs for the system be reduced or avoided by the hospitals? Are computer assisted orthopaedic surgical systems cost effective, necessary, desirable or indeed indicated in current cost containment in the NHS? The above questions are answered in this paper and points which are conducive to a positive response from the end user (surgeons, and hospital management) are discussed.

1. Introduction Orthopaedic implants are manufactured to the highest degree of precision by some of the most precise machines and inserted into patients by the most imprecise methods. Computer assisted systems aim to overcome the above dichotomy by improving the planning and implementation of orthopaedic surgery. This can be achieved by providing the surgeon with better information for planning and a more precise means of implementing the surgery [1].

Clearly, safety is paramount as is registration between the real and the virtual computer world [1]. The more subtle, but never the less, important surgical issues have as yet not been identified or addressed satisfactorily in the literature. This paper outlines these issues in relation to the CAOS (computer assisted orthopaedic surgical systems) projects under development by the authors. The CAOS projects include the development of a non invasive intelligent orthopaedic guide (effector arm). This guide utilises X-ray based two dimensional images to plan and implement a trajectory which allows placement of an object at a predetermined site in the human body, accurately, speedily and with minimum radiation. The guide is currently integrated with two orthopaedic procedures, distal locking of the intramedullary nail and internal hip fracture fixation. The issues raised and discussed in this paper are equally applicable to other computer assisted surgical systems.

2. System Presentation Successful introduction of any computer assisted system requires the close cooperation of surgeons, engineers, and theatre staff. In general the nearer a new system adheres to current surgical practices, the less the cost of introducing it. These costs may be economical or emotional resistance to yet proven medical technology by management and medical staff. Thus alterations in the theatre structure, design, change of surgical practice or display of unfamiliar views of anatomical regions is best avoided during the introduction and acceptance stage of a system. The flexibility afforded by the integration of various imaging modalities to produce hybrid images or three dimensional images will allow easier intraoperative surgical interpretation of complex anatomical relationships. When the technology proves itself, it becomes easier to alter current best surgical practice, this has occurred in the past, with total joint replacement [2] and arthroscopy.

Working

4.1 Size, Shape, Rigidity, Control and Reach

The freely available space in theatre which is not utilised during surgery is difficult to predict and is very variable but generally there is little unused space in the effective surgical field. This is especially so during intramedullary nailing or internal fixation of hip fractures.

The size is essentially the smallest and lightest design within engineering tolerances. The arm is required to withstand the forces applied to the drill guide by the surgeon during the actual drilling process. Any flex in the arms structure has been minimised, reducing an inherent cause of system inaccuracy. The reach depends on the distance of attachment from the surgical incision. Increased reach can be achieved by the inclusion of a sterilisable, adjustable bracket to the site of attachment for the effector arm.

3. The Orthopaedic Environment

The introduction of any computer assisted system into the operating theatre must conform to the current standards for electro-medical equipment [3].

4. The Effector Arm The limited space within the orthopaedic operating theatre makes a large heavy wheel based robot a poor design for the environment. It is not only difficult to accommodate in the surgical field, cumbersome to manoeuver, increases risk of sepsis by virtue of its size and shape [4] but it may also be perceived as alien, intimidating and out of control as was the case with industrial robots. Although such a setup has been utilised by Orthodoc [5], it is not without its shortfalls (set up time, prolonged operating time, training, supporting personnel etc.). Thus the optimal effector arm for assisting orthopaedic surgery should be seen “at work” and not when “at break”, i.e. minimal space occuppied in the working field and flat packed on the instruments tray or folded away when not in use. This implies a small, sterilisable, rapidly attachable/detachable effector arm with extreme ease of use. Safety is paramount when introducing new technologies to the operating theatre. To guarantee that patient safety is not compromised, only increased, it is essential that the surgeon is maintained within the decision making loop of the system. This important safety issue suggests the design of a safe mechanical device, capable of holding an instrument at a precise location, but never performing actual surgery. A passive rather than an active effector arm greatly reduces the inherent risk, removing the requirement for a safety critical control system. The communality between orthopaedic procedures utilising the basic orthopaedic principle [1], its timing during surgery, the perceived surgical requirements were all used to design an effector arm model in the CAOS project. This model was used by two surgeons (Mr. Mohsen and Mr. Sherman) in a simulated surgical trial to assess the arm’s surgical acceptability. This allowed identification of the necessary alteration in the design of the effector arm rapidly and economically. Thus the final design incorporated actual and perceived requirements which are summarised below under several headings: size, shape, rigidity, control reach, positioning during surgery, and surgical functionality.

The shape of the effector arm should be such that all corners are rounded off to allow for the sterile plastic / cloth sleeve to be easily slipped on over it without snagging or puncturing the sterilised cover, if this method of sterilisation is chosen. Maintenance of this requirement in a fully sterilisable effector arm avoids accidental damage to patient, staff and other drapes in use. For a large self supporting powered effector arm the proposed optimum form of a start-stop switch is a foot pedal, leaving a surgeon’s hands free and avoiding sterilisation problems. In fact two foot pedals are required, one allows the effector arm to move when pressed and freeze when the pressing force is stopped. The other pedal returns the arm to the starting position or the final position achieved when pressed. However with smaller passive self supporting effector arms as used in the CAOS projects [6], it is more appropriate to position a trigger in the arm’s end effector, which when pressed unlocks the effector arm which is then free to move. Release of the trigger locks (maintains) the effector arm in the last position through the use of a braking system incorporated in each arm joint. This is a similar arrangement to powered orthopaedic tools in current use e.g. drills, saws, etc. The arm’s end effector (the drill guide) is sterilisable in the autoclave.

4.2 Positioning during Surgery

of

the

Effector

Arm

Table 2 enumerates the different positions for mounting the effector arm in the surgical field and discussion of the two most viable options is given here. The effector arm can be mounted on either end of the Carm by a quick coupling device, for speed of attachment and detachment. This arrangement gives good surgical access and allows the image intensifier to be available for other procedures which do not require the effector arm. This mounting position will be perceived as safe (extension of the existing image intensifier rather than a total new system) allowing easy accommodation within the cluttered

theatre environment and maintaining a constant relationship between the image and effector arm at different viewing angles. The effector arm’s reach will always be less than the distance between the X-ray source and the image intensifier camera. The working volume of the effector arm will be similar to the viewed volume i.e. a maximum diameter of 30 cm. (visualised area of the anatomy by the image intensifier). 1. To the image intensifier on either end of the C-arm or to its base. 2. To the operating table (sides, groin post, base, either end of leg traction bars) with the help of a sterilisable quick release clamp and extensible bracket to give the arm the flexibility to reach the furthest surgical site with varying patient size. 3. To the theatre ceiling 4. On the floor 5. To the drape bars (horizontal pole or vertical poles)

Table 2. Mounting Positions for Effector Arm Unfortunately, the C-arm of many image intensifiers flex under its own weight and the addition of the effector arm will compound this problem. Overcoming the flexion requires strengthening of the C-arm or attachment of a support to the base which compromises surgical access and alters the image intensifier. The second position for the effector arm is vertically in an upward direction (towards the ceiling) from the outside of the traction bar proximal to the hinge. This is the best compromise option in term of access to the patient by the surgeon, effector arm and image intensifier and has the least flexion distortion problem. The other possible positions suffer from one or more of the following shortfalls: flex at the site of attachment, inadequate image intensifier access in at least one view, or difficulty in satisfactory positioning of the patient on the operating table.

4.3 Drill Guide Arm Surgical Functional Requirements Orthopaedic procedures, particularly in the case of minimal access spinal surgery, require accuracy better than 0.5 mm. The drill guide arm has to repeatably achieve this figure if any improvement over the current surgical practice is to be achieved. The drill guide should be constructed from a material which is autoclavable (see sterilisation) and radiolucent (transparent to X-ray) The radiolucent property allows an X-ray image to be taken to confirm the correctness of the systems implemented trajectory.

An alternative or a secondary confirmation of the drill guide trajectory can be obtained by the virtual tip concept [7]. A virtual tip implies that the system shows the tip of the guide to be further into the tissue than it actually is. This concept allows the proposed trajectory to be assessed prior to its implementation, thus the surgeon can confirm that the systems intentions and his intentions match. This concept is important in spinal surgery but less so in hip fractures and is not required for distal locking of the intramedullary nail. The attachment of the drill guide arm to the effector arm should be a snap on method of male/female configuration in order to allow for its sterilisation by autoclaving independently of the effector arm and subsequent attachment to the effector arm intraoperatively if the arm is not fully sterilisable. The size of the drill guide arm should allow comfortable handling with the flange resting onto the index finger and thumb of the clenched fist. The effector arm is not operation specific but the drill guide is. Hence for our purposes there will be two drill guides, one for the distal locking of the intramedullary nail and the other for hip fracture fixation. The drill guide tube for the dynamic hip screw should extend by four centimeters from the drill guide handle, to allow accommodation of soft tissue between the skin and the bone. This tube should be of appropriate diameter and have teeth at its end to provide some degree of stability on the bone during drilling.

5. Mapping of the real to the virtual computer world A computer assisted orthopaedic system functions in two worlds; a real (patient) and a virtual (computer) world. These two worlds need to be linked or related to each other by fiducials (markers, registration systems or patient tracking systems). Fiducials are reference features locatable on both the actual and the virtual patient accurately and are used by the system to relate the real world to the virtual computer world [5,8,9,10]. The number of fiducials required to establish the coordinate frame of the anatomic object depends on the type of fiducials being used. The currently available fiducials are either artificial, anatomical or a combination of both and can be plane, spatial or point fiducials which are attached to a frame which is fixed to the patient, or frameless i.e. free markers fixed directly onto the anatomy and appear on the scanned images. The frameless system either registers fiducials on deformable (e.g. skin or soft tissue), or rigid (e.g. bone) anatomical structures, or a combination of the above or the fiducials may be anatomical features of the structures themselves.

Frameless strategies that track markers on deformable structures suffer from errors introduced by the distortion of these structures. Whilst framed or inserted markers (pins) are uncomfortable and expose the patient to additional risk associated with their insertion and may still move. Anatomical fiducials are non-invasive and are not liable to move in relation to the anatomy of the object making them the optimal fiducials. Anatomical fiducials have been used in experimental settings and can utilise lasers, Fluoroscopic, ultrasound or optical data as their imaging methods. The ideal registration system should be accurate, inexpensive, not require extra surgical procedures, safe, automatically register three dimensional data sets, and track changes in the data set’s position over time, without requiring the attachment of any devices to the patient. This ideal system is not currently available and will have to depend on anatomical fiducials. In distal locking of the intramedullary nail and hip fractures the fiducial marker system cannot be based on their positions being identified by a CT or MRI as these investigations are not used for the management of fractured shaft or neck of the femur. However insertion of fiducial wire markers in the femur during surgery is possible or a raytec or steridrape with radiopaque lines may be attached at the site of surgery and the lines utilised as fiducials. Both these methods are in current surgical use and the fiducials can be viewed by the image intensifier. The fiducials suggested here are not as good as the anatomical fiducials but are surgically acceptable for hip fracture fixation and distal locking of the intramedullary nail.

6. Patient Surgery

Stabilisation

during

Immobilisation during imaging (data acquisition) and subsequently during implementation of the procedure in the operating theatre is an important issue in orthopaedic systems. Intraoperative immobilisation provides a fixed relationship between a patient’s limb and the computer assisted instrument which is important for achieving the desired accuracies [8]. Patient immobilisation has been undertaken by a vacuum pack (a bag that hardens and molds to the contour when the air is removed) and fixation clamps [5,8,9]. In distal locking of the intramedullary nail and hip fracture fixation the patient is on the traction table i.e. is immobilised. Although movement is possible, by applying some force to the table or traction setup, this does not normally happen. However any movement can be monitored and taken into account by sensors attached distally to the operating fields.

The distal attachment of the sensors makes use of the fact that the further the fiducials are from the surgical site, the less rigorous the immobilisation needs to be [8] and avoids sterilisation of the sensors.

7. Processing Time Non surgical time, in computer assisted orthopaedic systems is defined as the time available for computational needs i.e. image capture, processing, trajectory planning and implementation. Surgical procedures can be classified into three levels depending upon the availability of non surgical time. • Level I surgery: Real time computation only. • Level II Surgery: Six to forty eight hours preoperatively followed by five to ten minutes intraoperatively then real time computation. • Level III surgery: A few weeks preoperative then as in Level II intraoperatively Level I surgery requires the capturing, processing of the image and planning of the trajectory to occur in real time. Here the data needed by the system to calculate and implement the trajectory is only available intraoperatively e.g. distal locking of the intramedullary nail. Level II surgery is demonstrated in hip fracture fixation where two incidental processing times are available. The first occurs between the patients hospital admission and arrival in the operating room (up to forty eight hours). The non-surgical time available here can be utilised in digitising the plain radiographs and extracting anatomical data and planning of the trajectory. Acquisition of intra-theatre data by fluoroscopy is followed by the second non surgical time of five to ten minutes. This hiatus occurs after hip fracture reduction and prior to start of definitive surgery i.e. time used by the surgeon to scrub up, don the surgical gown, gloves, etc. and the patient being draped. During surgery more up to date information, in real time is available by fluoroscopy and real time solutions are then required. Finally, an example of level III surgery is seen in spinal surgery, here the precise and detailed anatomical and pathological data is available days or weeks prior to the surgery. This data can be utilised to formulate the necessary trajectory preoperatively. Intraoperatively fluoroscopy provides real time, but less detailed, data on the spine. Here again a surgical hiatus, similar to that of hip fractures, can be engineered to occur.

8. Timelessness Orthopaedic surgical practice is continuously evolving, with new procedures being introduced and old procedures undergoing rapid evolution, as our knowledge increases.

Thus an element of timelessness needs to be in-built into the system otherwise it is at risk of becoming obsolete very rapidly. This necessitates an open ended, modular system which aids a number of orthopaedic procedures and allows alteration or modification in the software or hardware easily.

factor in acceptance and take up of any procedure, instrument or technology. It is possible to create a system which is accurate, fast, sterilisable and contained within the image intensifier but which prices itself out of the market if it is not governed by economic reality.

Modularity and open endedness implies that the core technology of the system is reasonably constant but new sub-systems are added to allow new procedures or modification of old procedures to be undertaken. Modularity also allows the skills acquired in learning one procedure to be utilised in other procedures.

The affordability of the system has to depend on its widespread application to a variety of orthopaedic procedures. The greater the number of procedures assisted in, the more universal the system, the less the cost per procedure.

9. Universality

The following are the main economic issues of computer assisted orthopaedic systems: capital investment, maintenance, utilisation, running cost, technology spread, cost savings and cost effectiveness.

There are a variety of methods, instruments and implants utilised by the orthopaedic surgeon to achieve similar end results. Computer based instruments that assist the orthopaedic surgeon will thus ideally be largely independent of the various implants, methodologies and manufacturers. This independence is achieved by relying on the communality principle of orthopaedic surgical procedures i.e. the common denominator in each procedure is the key to the universality of the system being created. For example the common denominators in distal locking of the intramedullary nail is the presence of a distal hole for locking and for hip fractures is the positioning of the fixation in the head and neck of the femur. Obsolescence of the system is an important element in determining the cost. If the system is dependable on a principle and not an implant or methodology (which is bound to change rendering the system obsolete) the cost should be less. A non universal system, i.e. one which is company or implant specific, will limit the wide spread use of the system unless the company controls the majority of the implant market. However a non universal system can be advantageous, as most orthopaedic companies provide their equipment on long term free loan to hospitals. The hospital in turn utilises that company’s implant. This arrangement allows the orthopaedic company to recoup the cost of the instrumentation from the implant profits and at the same time saves the hospital capital expenditure on instrumentation which will be obsolete in a few years. Thus a non universal product allows computer assisted systems to be introduced at no direct cost to the hospitals

10. Financial A principal objective is to keep the costs of the system as low as possible. The use of a 486 based PC, for CAOS, reflects this. In the current climate of market economy in the NHS (National Health Service) cost is an important

Cost effectiveness analysis is the comparison of the positive and negative economic consequences of using alternative technologies [11]. It is difficult to determine the cost effectiveness of the new technology when clinical efficacy has not been established. This is a “catch 22” situation, as the financial issues need to be considered during system development in terms of what is economically viable. Practically it is not possible to assess the financial implications until the perceived advantages of the system prove to be founded as the system is dealing with a patient i.e. in essence a biological entity which possesses an inherent ability to adjust for surgical inaccuracies and self healing. A technology that increases both cost and benefit may or may not be considered cost effective, depending on its cost effectiveness ratio, defined as the marginal cost divided by the marginal benefit [12]. In conclusion computer assisted orthopaedic systems may be costly to start with but will probably follow past trends of technologies i.e. plummeting prices. The cost effectiveness can only be determined after clinical accuracy trials and outcome trials

11. Commonality The orthopaedic procedures in the CAOS projects involve the use of an image intensifier as a vision system and require the accurate placement of an object at a predetermined site within specific anatomical constraints. The implementation of the trajectory in all the projects takes place through a common effector arm. Maintenance of commonality throughout the system for such tasks as image processing, trajectory planning and implementation has implications in terms of simplicity of use, training and understanding of the principles involved in these new surgical instruments by the orthopaedic surgeons.

12. Simplicity To the established surgical generation, computer assisted instruments are intimidating objects which are not easy to come to terms with. This would not be the case with the “Nintendo Sega generation” of surgeons, who are currently going through the education system. They will be more adapt at using computer assisted instruments by virtue of the skills they have acquired during activities of daily living. Hence the importance of simplicity is for the benefit of the current orthopaedic surgeon who is the gatekeeper to future surgical practice Computer assisted instruments are complex engineering tools which are to be used in constructing intelligent surgical systems which aim to simplify technically demanding orthopaedic procedures. Simplification is achieved by hiding the complex engineering solutions behind a surgeon friendly interface. This implies a black box approach i.e. simple in its use like the post office; where a stamp (image) an envelope (operation selection) and an address (trajectory planning) allows a letter to be delivered (trajectory implementation) by the postman (the surgeon) without “Joe Public” (the staff) being aware of the complex procedures it undergoes. To meet the simplicity requirements the system must be engineered to allow it to function autonomously with the surgeon’s role being limited to control (skill) and verification (judgment) of the placement of the drill guide arrived at by the system. The simplicity - complexity of the system can be divided into utilisation, training and layout issues.

12.1 Training It is important that utilisation of the system by the surgeon requires virtually no prior knowledge of computers or training in their use. Surgical training is a long, stressful and dedicated process. During surgery the surgeon must be in control of the surgical environment including his instruments. No surgical instrumentation should be employed unless the surgeon knows the proper indications for its use, its potential capabilities, and its limitations. Thus a training program is envisaged for the system. As with any new instrument training of the personnel is the key to success. Training time needs to be kept to the minimum, this is achieved by the simplicity of the user system. The learning of one particular process should impart to learners a core skill which they then use during other procedures of which the system is capable. Training will be undertaken in two sessions. The first session is a scientific one i.e. indication, advantages, disadvantages and a brief engineering overview, etc. This will be followed by the practical session which aims to

familiarise the surgeon and his supporting team with the system and practical procedures utilising synthetic bones with simulated X-ray images, possibily, within a virtual environment training set up. Orthopaedic innovations follow a predictable pattern of events. This is summarised as an introduction period followed by a honeymoon period (enthusiastic utilisation which may be inappropriate) subsequent to this disappointment, disillusion and crisis stage occur i.e. the relationship between the surgeon and the innovation sours. If the crisis stage is overcome a steady well defined relationship is established. Limiting the distribution initially and having formalised training structures may avoid or curtail the above cycle of events.

12.2 Layout The layout of a computer assisted orthopaedic system is an important surgical functionality issue. The layout of the controls will be largely dictated by the feedback from the surgeon and radiographer but with multiple personnel using the system, permanent fixation of the controls is advisable. Currently the CAOS project’s system architecture comprises of a 486 based PC and its monitor placed next to the image intensifier’s viewing unit. A more optimal approach would be the integration of the PC functionality based in the image intensifier viewing unit, and the overlaying of information of to a shared display. The control panel of the system would be placed on the Carm control panel. Cables connecting the system to the control panel would be integrated into the image intensifier cable. The radiographer would control the image acquisition and manipulation via the control panel situated on the C-arm unit, using a set of menus. Concentration of controls within one organised system, for the radiographer, enhances surgical sterility and operative safety and efficiency. In conclusion the learning of new methods of performing surgery, use of a keyboard to enter information, taking of measurements intraoperatively are issues that need to be avoided in a computer assisted surgical system. The radiographer’s input into the system should be graphically based using a mouse or touch sensitive screen and not a keyboard.

13. Surgical Feedback In initial CAOS systems it is expected that instructions for movement of apparatus be displayed to the surgeon and radiographer. Such feedback from the system, in the form of images or instructions, must be simple and comprehensible to the staff with no possible misunderstanding.

One of the difficulties of on screen surgery (e.g. arthroscopy, operating microscopes and image intensifiers) is the fact that surgical movement in the operating field can be seen on the screen as occurring in the opposite direction or mirroring the actual movements. This phenomenon is disorientating hence graphic or other instructions on screen must make allowances for this. In the case of the CAOS projects, it is proposed that the system displays positioning instructions for the image intensifier to the surgeon, in the manner outlined below, thus avoiding mirroring effects. A three dimensional diagram of the image intensifier or effector arm appears in the corner of the screen giving the axis which is being adjusted, The position adjustment occurs in three dimensional planes, X, Y, Z, thus each planar movement is displayed on the screen on its own with the direction of movement being signposted. The screen is split into three parts. One displays a three dimensional view of the image intensifier with the section to be adjusted in a different colour. The second display indicates the direction in which the adjustment should occur. The final part shows the current position and the required position with real time update as the adjustment is implemented. When the required position is achieved the next axis is adjusted. It is possible to use this principle to undertake all axis adjustment on one split screen.

14. Safety Issues Automation increases surgical accuracy, flexibility and shortens operating time but introduces background complexity into orthopaedic procedures along with increased risk of equipment malfunction and decreased safety in the surgical procedure. Safety issues can be divided into • theatre environment safety, • hardware - software safety and • surgical safety. Theatre safety issues have been discussed in the working environment section, thus no further discussion is undertaken here. The critical safety aspects of the individual components of the system (sensors, processing of information and effector arm) and the systems overall safety features must be a priority. The discussion of engineering safety issues are beyond the scope of this paper. Surgical safety issues include confirmation of the trajectory prior to its implementation (this is discussed in section 4.3), the system should not interfere with the surgeon's manoeuvers during performance of the surgery, the potential of the system to initiate surgical complications should be minimal, the failure mode of the system should

be in the safe position thus allowing the surgeon to simply remove the system from the surgical field and continue with the surgery without its assistance.

15. Sterilisation Options Sterilisation is a process whereby all living organisms, including spores are destroyed [4]. Disinfection is a process which destroys only the vegetative forms of the organisms but which leaves intact any spores that may be present [4]. This distinction is important as the skin of patients is disinfected while surgical instruments are sterilised. There are two methods of disinfection, namely: physical and chemical. The physical ones are the most important as they are the only ones that ensure sterilisation [4]. The physical methods are dry heat, (160°C for 1 hour), moist heat (not used surgically), dry steaming under pressure (autoclaving 135 KPa, 121°C for 15 minutes or 220 KPa 134°C for 3 minutes), light (ultraviolet for surface disinfection only), ionising radiation (Gamma and Beta outside the realm of hospital sterilisation) [4]. Chemical disinfectants e.g. glutaraldehyde or alcoholic chlorhexidine (0.5%) are the ones in current use. The soaking time varies from 20 minutes to 24 hours [4]. Gaseous disinfectants (Ethylene oxide, Formaldehyde gas) are both problematic in their use [4] and typically not available in the standard orthopaedic operating theatre, hence no further discussion on them is undertaken. Hybrid methods include, dry steaming under pressure at 80°C in Formalin for two hours. In a computer assisted orthopaedic system the effector arm will come in contact with the surgical wound thus it must be sterilised. There are three possible approaches to the sterility of the effector arm. Total sterilisation by one of the above methods, partial sterilisation and draping or draping alone.

15.1 Total Sterilisation The effector arm is constructed of materials which are sterilisable by one of the above methods. Practically, autoclaving is the only viable option. Total sterilisation may also be achieved by sealing the effector arm in a container which prevents the components from being damaged by the sterilisation method selected. Such a set up may comprise a shell or a suit of armour for the effector arm which can be autoclaved and the arm then placed in it and the shell locked, or a well insulated air tight shell or suit of armour may contain the effector arm while it is in the autoclave. Sterilisation by ionising radiation is not a viable option in a hospital. However, if the effector arm were to ever become cheap enough that several units are in the hospital

at the same time then, theoretically these units can arrive in the theatre sterilised by radiation and then be sent back to be re-sterilised or even become disposable components.

15.2 Partial Sterilisation and Draping This is currently used for sterilising some arthroscopic cameras. The effector arm of the computer assisted surgical systems can be covered by a sterile cape (plastic, paper or cloth) prior to its introduction into the surgical field. This cape can be in the form of multiple folds which will unfold over the effector arm. This method is not easy surgically as it tends to be cumbersome and fiddly and maintenance of sterility is something of a challenge to the surgeon’s ingenuity during the operation. This shortfall is known from our current experience of using drapes. Alternatively a sterile open cover is placed on the sterile table surface and the effector arm placed on it. Then the cover is fastened to itself with Velcro, zip, or studs. this is technically easier than passing a cape over the effector arm. In this solution the drill guide which comes into contact with the patient, the attachment clamp and the attachment bracket are sterilised in the autoclave.

15.3 Total Draping This is similar to the above but no parts of the effector arm, the drill guide, attachment clamp or bracket is sterilisable. This is not a viable option surgically as it is very difficult technically to place the effector arm in the sterile draping and maintain its sterility during the surgery.

15.4 Introducing the Effector Arm into the Surgical Field The first step is to attach the sterile drill guide to the effector arm by means of its male/female snap on attachment (female end in the arm, male end on the guide). The effector arm plus the drill guide are then attached to the sterile bracket and attachment clamp. Thus the effector arm of the system is now sterile and ready for use.

16. Acknowledgments The CAOS group projects are supported by the following funding bodies: • British Orthopaedic Association • Henry Smith Charity • Sir Samuel Scott of Yews Trusts • Hull Royal Infirmary • University of Hull • Radiology Dept. HRI - Loan Of Image Intensifier

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