Aims

Hydroxyapatite (HA)-coated collars have been shown to reduce aseptic loosening of massive endoprostheses following primary surgery. Limited information exists about their effectiveness in revision surgery. The aim of this study was to radiologically assess osteointegration to HA-coated collars of cemented massive endoprostheses following revision surgery.

Introduction

We report 10-year clinical outcomes of a prospective randomised controlled study on uni-compartmental knee arthroplasty using an active constraint robot.

Measuring the clinical impact of CAOS systems has generally been based around surrogate radiological measures with currently few long-term functional follow-up studies reported. We present 10 year clinical follow up results of robotic vs conventional surgery in UKA.

The purpose of the study was to identify risk factors that are associated with re-displacement of the hip after surgical reconstruction in cerebral palsy.

Retrospective review of children with cerebral palsy who had hip reconstruction with proximal femoral varus derotation osteotomy (VDRO) and Dega-type pelvic osteotomy, between 2005–2012, at a UK and European institution, was performed. Patient demographics, GMFCS, clinical and radiological outcome were assessed as well as the presence of pelvic obliquity and significant scoliosis (Cobb angle > 10 degrees). Redisplacement was defined as Reimer's Migration Index (MI) >30% at final follow-up. Logistic regression analysis was used to assess which factors were predictive of redisplacement and adjusted for clustered variables (α = 0.05).

Eighty hips were identified in 61 patients. The mean age at surgery was 8.8 years (± 3.3). Mean MI pre-op was 68% (± 23%) and post-op was 8% (± 12%). At a mean follow-up, of 3.2 years (± 2.0), 23 hips had a MI >30%. Of these; five were symptomatic, and one had required a salvage procedure. Metalwork removal was undertaken in 14 hips. Logistic regression demonstrated that the pre-operative MI and the percentage of acute correction were significant predictors of re-displacement. If the pre-operative MI was greater than 65 percent, the odds ratio (OR) for redisplacement was 5.99 (p = 0.04). If correction of the MI was less than 90% of the pre-operative MI, the OR for re-displacement was 4.6 (p = 0.03). Age at the time of surgery, GMFCS, pelvic obliquity and scoliosis were not predictive of re-displacement.

These results, firstly, highlight the importance of hip surveillance in children with cerebral palsy to allow timely intervention to ensure adequate radiological outcomes. Secondly, as in developmental hip dysplasia, full concentric reduction is essential to reduce the risk of re-displacement, with its associated clinical consequences.

To describe the prevalence and incidence of Methicillin-resistant Staphylococcus aureus (MRSA) colonisation during the patient journey for patients admitted to orthopaedic and trauma wards, we carried out a prospective audit at the University Hospital of North Staffordshire NHS Trust, England.

The Study Population comprised patients admitted to the trauma and elective orthopaedic wards, with an expected stay of 48 hours or more between March and May 2003.

Patients were swabbed for MRSA colonisation on ward admission, transfer to another ward and discharge from hospital. Elective patients undergoing major joint surgery were also swabbed at a pre-operative assessment clinic. Colonised patients were treated depending on individual risk assessment.

Five hundred and fifty-nine eligible patients were admitted to hospital. Of these, 323 (101 elective, 192 trauma and 30 non-orthopaedic) patients were included in the study, of whom 28 elective patients (28%), 43 trauma patients (22%), and seven non-orthopaedic patients (23%) were colonised with MRSA at any time during the audit period. Of the 80 patients identified as negative for MRSA colonisation at pre-assessment screening and included in the audit, ten (9.5%) were found to be colonised on admission.

There is a high prevalence of MRSA colonisation in patients admitted to the orthopaedic and trauma wards in our setting. A policy of pre-admission screening, though able to identify MRSA carriage does not guarantee that patients are not colonised in the period between screening and admission. Consideration should be given to screening all patients for MRSA who are admitted to an orthopaedic ward.

Surgical long shape tools, such as the arthroscopic hooked probe, are used during knee-arthroscopy procedures by surgeons to manipulate tissues and diagnose problems. These procedures allows surgeons to assess the physical properties of tissues (such as wear, tear, inflammation, stiffness, etc), which are impossible to evaluate using real-time video observation or MRI and CT mapping. This study focuses on the dynamic properties of the hooked probe and its ability to deliver tactile information, created at the tip of the hook as the tissue is being manipulated, to the handle where the surgeon is grasping the instrument.

From previous studies, it is known that when a probe comes into contact with hard tissues, such as bones, vibrations can occur that enhance the tactile feedback. To better understand the importance of the dynamic influence on the tactile feedback, initially a vibration analysis of the probe (Model 8399.95 by Richard Wolf UK Ltd) was performed; a stepped sine sweep was carried out to evaluate the dynamic behaviour of the probe, including its resonance response frequencies and the damping behaviour. Several vibration modes were identified in a range up to 2000Hz parallel and perpendicular to the probe. The measurement values were correlated to a finite element model of the probe and an error of less than 5% was found for all relevant resonance response frequencies, thereby validating the accuracy of the model.

Measurement and simulation results show that tapping on different materials excites different modes of the probe at different levels, leading to a tactile feedback that harder materials “shift” the probe resonances to higher levels. To verify this, a tapping experiment was performed and the resulting vibrations, while tapping on different materials, were recorded. The study shows that the dynamic behaviour of the probe are somewhat influenced by the fact that the probe is being held in hand leading to a slight reduction in its natural frequencies.

A study on an individual’s ability to discriminate between the stiffness of different materials while tapping on them using an arthroscopy hooked probe is currently underway. Ten subjects are being asked to sort five materials (silicon, latex, rubber, plastic, steel) from the softest to the hardest by simple tapping. During the test, each subject is exposed to two materials each time, iteratively until the sort is complete. The subjects are blindfolded and white noise is played through headphones, to blur the sounds of tapping. The resulting dynamic response of the probe is recorded, using an accelerometer, along with the impact forces on the material, measured by a force sensor. Results to date show that subjects can distinguish quite accurately between the soft materials (silicon and latex), but find it difficult to distinguish between stiffer materials (plastic and steel), but comprehensive statistics are not yet available.

Introduction: Lumbar disc disease comprises of a heavy portion of the workload in spinal as well as general orthopaedic clinic.

It is well accepted that nerve root tension signs such as straight leg raise (SLR) & Lasegue’s test are sensitive at diagnosing nerve root impingement secondary to lumbar disc degeneration. In isolation, however, they lack specificity & have a poor positive predictive value (PPV). This can lead to uncertainty in clinical diagnosis.

Our study proves that a structured approach to clinical examination with cumulative nerve root tension signs (RTS) significantly increases the tests’ specificity and PPV, therefore giving clinicians more confidence in their diagnosis.

Methods: Prospective review of 1303 patients seen in one Orthopaedic consultant’s spinal clinic from 2004 until 2008. Data was collected using a standardized proforma. Pattern of pain as well as RTS (SLR, Lasegue, bowstring and crossover) were recorded and cross-referenced with subsequent MRI findings. In our dataset a positive MRI result was one in which the demonstrated disc lesion and nerve impingement corresponded with patient symptoms. Patients included were all those presenting with lower back and/or neuropathic leg pain. Patients had to be excluded from series due to incomplete datasets & missing MRI scans.

Results: N = 858. Our results showed that as we progressed from 1 RTS up to 4 RTS there was a significant increase in the PPV : 1RTS PPV = 0.333 (CI 0.25 – 0.43), 2RTS PPV = 0.78 (CI 0.69 – 0.86), 3RTS PPV 0.87 (CI 0.81 – 0.91), 4RTS PPV 0.93 (CI 0.66 – 0.99). There was also significant increases in specificity compared with 1RTS: 1RTS 0.75 (CI 0.70 – 0.8), 2RTS 0.94 (CI 0.91 – 0.96), 3RTS 0.92 (CI 0.89 – 0.95), 4RTS 0.99 (CI 0.98 – 0.99).

Discussion: This study shows that combining root tension signs as part of a structured assessment leads to a significant cumulative increase in the PPV and specificity of the diagnosis of nerve root impingement. Hence proving the importance of clinical examination. This method of sequential, cumulative RTS has not previously been documented in the literature.

Introduction: The treatment of femoral nonunions remains challenging despite modern surgical techniques and adjuncts to fracture healing. We present a series of 14 patients in whom a bifocal treatment technique has been used in order to achieve bony union and correct limb length.

Methods: Patients were identified from theatre records and their hospital notes and x-rays were retrospectively reviewed. All patients underwent bifocal treatment for femoral nonunions – debridement and internal fixation (single or double plating) of the nonunion and lengthening at the opposite end of the bone to correct limb length discrepancy. Initially the procedures were staged, with treatment of the non-union then subsequent lengthening. However, our technique has evolved to perform all procedures in a single stage. All lengthening procedures were done with a monolateral (Orthofix LRS) fixator.

Results: 11 patients had distal and 3 proximal femoral nonunions. 13 patients were male and 1 female. The non-union united with the index procedure in 13 patients, 1 is still undergoing treatment. Limb length discrepancy range 2–5 cm was fully corrected in all patients with no axial deviation of the regenerate. There were no pin site problems.

Discussion: Femoral nonunions are challenging due to multiple previous procedures, insecure grip on the smaller fragment and bone loss. Successful union can be achieved by ORIF with bone grafting, but this does not restore length. Treatment by the Ilizarov method alone is associated with significant morbidity, particularly knee stiffness. A bifocal strategy provides stable internal fixation of the non-union to allow bone healing, and any consequent loss of length is safely restored. We believe this to be a safe and effective technique to treat femoral nonunions.

A Prospective, randomised controlled trial demonstrates superior outcomes using an active constraint robot compared with conventional surgical technique in unicompartmental knee arthroplasty (UKA). Computer assistance should extinguish outliers in arthroplasty, with robotic systems being able to execute the preoperative plan with millimetre precision.

We used the Acrobot system to deliver tailor made surgery for each individual patient. A total of 27 patients (28 knees) awaiting unicompartmental knee arthroplasty were randomly assigned to have the operation performed either with the assistance of the Acrobot or conventionally. CT scans were obtained with coarse slices through hips and ankles and fine slices through the knee joint. Preoperative 3D plans were made and transferred to the Acrobot system in theatre, or printed out as a conventional surgical aid. Accurate co-registration was confirmed, prior to the surfaces of the femur and tibia being milled. The outcome parameters included measurements of the American Knee Society (AKS) score and Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index. These measurements were performed pre-operatively and at six, 18 weeks, and 18 months post-operatively. After 18 months two UKA out of the conventional trial (n =15) had been revised into a total knee replacement (TKA), whereas there were no revisions in the Acrobot trial group (n = 13).

Using an active constrained robot to assist the surgeon was significantly more accurate than the conventional surgical technique. This study has shown a direct correlation between accuracy and improvement in knee scores at 6, 18 weeks and 18 months after surgery. At 18 months there continues to be a significant improvement in the knee scores with again a marked correlation between radiological accuracy and clinical outcome with higher accuracy leading to better function based on the WOMAC and American Knee Society Score.

Whilst computer assistance enables more accurate arthroplasty to be performed, demonstrating this is difficult. The superior results of CAOS systems have not been widely appreciated because accurate determination of the position of the implants is impossible with conventional radiographs for they give very little information outside their plane of view.

We report on the use of low dose (approximately a quarter of a conventional pelvic scan), low cost CT to robustly measure and demonstrate the efficacy of computer assisted hip resurfacing. In this study we demonstrate 3 methods of using 3D CT to measure the difference between the planned and achieved positions in both conventional and navigated hip resurfacing.

The initial part of this study was performed by imaging a standard radiological, tissue equivalent phantom pelvis. The 3D surface models extracted from the CT scan were co-registered with a further scan of the same phantom. Subsequently both the femoral and acetabular components were scanned encased in a large block of ice to simulate the equivalent Hounsfield value of human tissue. The CT images of the metal components were then co-registered with their digital images provided by the implant manufactures. The accuracy of the co-registration algorithm developed here was shown to be within 0.5mm.

This technique was subsequently used to evaluate the accuracy of component placement in our patients who were all pre-operatively CT scanned. Their surgery was digitally planned by first defining the anterior pelvic plane (APP), which is then used as the frame of reference to accurately position and size the wire frame models of the implant. This plan greatly aids the surgeon in both groups and in the computer assisted arm the Acrobot Wayfinder uses this pre-operative plan to guide the surgeon.

Following surgery all patients, in both groups were further CT scanned to evaluate the achieved accuracy. This post-operative CT scan is co-registered to the pre-operative CT based plan. The difference between the planned and achieved implant positions is accurately computed in all three planes, giving 3 angular and 3 translational numerical values for each component.

Further analysis of the CT generated results is used to measure the implant intersection volume between the pre-operatively planned and achieved positions. This gives a single numerical value of placement error for each component. These 3D CT datasets have also been used to quantify the volume of bone resected in both groups of patients comparing the simulated resection of the planned position of the implant to that measured on the post-operative CT.

This study uses 3D CT as a surrogate outcome measure to demonstrate the efficacy of CAOS systems.

Last year at CAOS UK we reported on the development of the Acrobot® Navigation System for accurate computer-assisted hip resurfacing surgery. This paper describes the findings of using the system in the clinical setting and includes the improvements that have been made to expedite the procedure. The aim of our system is to allow accurate planning of the surgery and precise placement of the prosthesis in accordance with the plan, with a zero intra-operative time penalty in comparison to the standard non-navigated technique.

The system uses a pre-operative CT-based plan to allow the surgeon to have full 3D knowledge of the patient’s anatomy and complete control over the sizes and positions of the components prior to surgery.

At present the navigation system is undergoing final clinical evaluation prior to a clinical study designed to demonstrate the accuracy of outcome compared with the conventional technique. Whilst full results are not yet available, this paper describes the techniques that are being used to evaluate accuracy by comparing pre-operative CT-based plans with post-operative CT scans, and gives initial results.

This approach provides a true measure of procedure outcome by measuring what was achieved against what was planned in 3D. The measure includes all the sources of error present within the procedure protocol, therefore these results represent the first time that the outcome of a navigated orthopaedic procedure has been measured accurately.

We used computer tomography (CT) to measure the outcome of knee-arthroplasty in our prospective double-blind randomised controlled study of our active constraint robotic system ACROBOT.

All patients in our trial had pre-operative CT scan and proprietary software used to plan the size, position and orientation of the implants. Post operatively a further CT scan was performed and measurement studies performed using 3 different methods of manipulating the CT dicom data.

Method 1, a quick and simple method of implant assessment that measures the varus-valgus orientation of the implants relative to the axes of the long bones

Two landmarks each are used to define the individual mechanical axis for both the femur and tibia, for consistency these landmarks are the very ones used in the planning stage on the pre-operative CT.

Landmarks are then placed on the implants in order to measure their tilt relative to the mechanical axes. An appropriate Hounsfield threshold (2800) was used to image the metal components. The angle between the individual mechanical axis and the prosthetic component was calculated.

Method 2, detailed and accurate comparisons between the planned and achieved component positions in 3D are made. Co-registration of the precisely planned CT based models with surface models from the post-op scan gives real measurements of implant position enabling the measurement of the accuracy of component in an all six degrees of freedom giving both translation and rotation errors in all three planes.

The process of alignment was achieved by surface-to-surface registration. An implementation of the iterative closest point algorithm was used to register matching surfaces on the objects to be registered. A polygon mesh of the implant, provided by the manufacturer, defined the surface shape of each size of implant. This was used both to define the planned position and to register to the post-operative scan. Method 3, in this study we quantified post-operative error in knee arthroplasty using one value for each component whilst retaining 3D perspective.

The position of the prosthetic components in the post-op scan is calculated and individual transformation matrix computed which is matched to the transformation matrices for the planned components.

The pre-operative CT based component positions were co-registered to the post-operative CT scan and values for the intersection (volumetric) between the digitised images (both planned and achieved) were calculated. Both the co-registered femoral and tibial component’s intersection was quantified with software packages supporting Boolean volume analysis

Method 1, the sum of the two, independently measured, angles allows an estimate of the post-operative alignment of the load bearing axes in the two bones.

Method 2, 3D CT allows precise measurements of the achieved position for each component in all three planes. Six values, three angular and three translational, define the achieved component position relative to the planned position.

Method 3, the greater the percentage intersection between the planned and achieved images, the greater the accuracy of the surgery. Owing to the shape of the components (large articular surface) large intersections demonstrate more accurate reconstruction of the joint line.

In the recent past the lack of a sufficiently accurate tool to plan and measure the accuracy of component placement has resulted in an inability to detect and study radiological and functional outliers and hence the hypnotised relationship between prosthetic joint placement and outcome has been difficult to prove.

CT offers us the ability to accurately describe the actual position and deviation from plan of component placement in knee arthroplasty. Whilst X-ray has the intrinsic problems of perspective distortion magnification errors and orientation uncertainties CT can be used to define ‘true’ planes for two dimensional (2D) measurements and permits the comparison in three dimensions (3D) between the planned and achieved component positions.

Hip resurfacing has advantages over hip replacement for younger, more active patients. However, it requires that surgeons learn new techniques for correctly cutting bone and positioning the components. Pre-operative planning systems exist for conventional hip replacement. Planning software for hip resurfacing is described, with the resulting plans available as a visual aid during surgery, or transferred to the Acrobot^® Navigation system for intra-operative guidance.

CT data is acquired from the top of the pelvis to immediately above the acetabulae in 4 mm slices, and from there down to just below the lesser trochanter in one mm slices. This keeps radiation doses low while providing high image quality in the important regions for planning. This is segmented semi-automatically, and bone surface models are generated.

Frames of reference are generated for the pelvis and femur, and the acetabular and femoral head positions are computed relative to these.

Prosthesis components are initially positioned and sized to match the computed anatomy. They can then be adjusted as required by the surgeon. While adjusting their positions, he is able to visualize their fit onto the bone to ensure good placement without problems such as femoral neck notching.

Twenty one hip resurfacings have been planned including two navigated cases. In addition, visualization of hip geometry for osteotomy and impingement debridement has been performed on 14 cases, giving the surgeon a good understanding of hip geometry prior to surgery. Initial evidence indicates surgeons find the planner useful, particularly when the anatomy is not straightforward.

Accurately planning the intervention and precisely measuring outcome in computer assisted orthopaedic surgery (CAOS) is essential for it permits robust analysis of the efficacy of these systems.

We demonstrate the use of low dose computer tomography (CT) radiation for both the planning and outcome measurement of robotic and conventionally performed knee arthroplasty.

Studies were initially performed on a human phantom pelvis and lower limb. The mAs (milliampere seconds) were varied from 120 to 75 at the pelvis and from 100 to 45 for both the knee and ankle whilst keeping the kV (kilovolt) between 120 and 140. Image quality was evaluated at the different doses.

The volumes scanned were defined on the scout film; they included the whole femoral head (0.5cm above and below the head), 20cm at the knee (10cm on either side of the joint line) and 5cm at the ankle (the distal tibia and the talus). Effective dose (mSv) was calculated using two commercially available software packages. This protocol was subsequently used to image patients in our prospective double-blind randomised controlled study of our active constraint robotic system ACRO-BOT.

With the reduction in the mA and scanned volume the effective dose was reduced to 0.761 mSv in females and 0.497 mSv in males whilst maintaining a sufficient image resolution for our purposes. We found that a mAs of 80 for the hip joint, 100 for the knee and 45 for the ankle was sufficient for imaging in both pre-op planning and pos-operative assessment in knee arthroplasty. This contributed on an average effective dose to the hip of 0.61 mSv, the knee 0.120 mSv and to the ankle 0.0046 mSv.

The results of our study show that we have considerably reduced the effective dose (0.8 mSv) to one third of the Perth Protocol (2.5mSv) by reducing the areas of the body scanned and adjusting the mA for the various parts of the body whist maintaining the x, y and z axis throughout the scan. The areas between the knee, hip and ankle that were not exposed to radiation are not strictly necessary for the planning of knee arthroplasty, but it is essential that the leg does not move during the scanning process. In order to prevent this leg was placed in a radiolucent splint. For post op three dimensional (3D) assessments only the knee component of the protocol is necessary.

This paper presents initial results of the Acrobot® Navigation System for Minimally Invasive (MI) Hip Resurfacing (HR) which addresses the problems of conventional HR. The system allows true MI HR – mini-mising the incision and tissue retraction required, and conservation of bone in contrast to other MI total hip procedures.

Pre-operative CT-based software allows the surgeon to plan the operation accurately. Use of CT gives the greatest accuracy, and is the only method which can give an accurate assessment of procedure outcome (planned versus achieved implant position). Intra-operatively, the bones are registered by touching points using a probe connected to a digitising arm. Next a series of tools is connected so that bone preparation and implant insertion is performed using on-screen guidance.

The accuracy of the registration probe is within 0.6mm, inside the acceptable margin for optical tracker systems. We have validated this acceptability using registration simulations leading to a protocol which restricts registration errors to within 1.5mm and three degree. These error margins are within those in the literature for acetabular component placement using optical tracker based systems (five degree inclination, six degree anteversion). No comparable data could be found regarding the accuracy of femoral component placement during computer-assisted HR.

The system is currently undergoing clinical tests at one alpha site, with three further beta sites planned for early 2006. The methods described by Henckel et al (CAOS International Proceedings 1994, pp. 281–282) are being used to evaluate the performance of the system, comparing pre-operative to post-operative CTs to obtain a true, accurate measure of performance.

The primary objective of this study was to evaluate the performance of the Acrobot^® Sculptor system in achieving a surgical plan for implantation of unicompartmental knee prostheses, compared with conventional surgery. The Acrobot^® Sculptor is a novel hands-on medical device, consisting of a high speed cutter mounted on a robotic device which the surgeon holds and directs.

A prospective, randomised, double-blind (patient and evaluator), controlled versus conventional surgery study was undertaken and has been fully reported in Journal of Bone and Joint Surgery (British), 88-B.

All (13 out of 13) of the Acrobot^® cases were implanted with tibio-femoral alignment in the coronal plane within ±2° of the planned position, while only 40% (six out of 15) of the conventionally performed cases achieved this level of accuracy.

There was also a significant enhancement in the extent of post-operative improvement, as measured by American Knee Society (AKS) Scores at six weeks, in the cases implanted with the Acrobot^®. The difference between type of surgery is statistically significant (p=0.004, Mann-Whitney U test). Operating time (skin to skin) is higher in Acrobot treated subjects, but the difference between the two types of surgery fails to reach significance.

The Acrobot^® System was found to significantly improve both accuracy and short term outcome in this investigation. By permitting the creation of bone surfaces that can be machined by means other than an oscillating saw, the Acrobot^® System paves the way for novel implant designs to be developed, facilitating bone conserving arthroplasty in the knee, hip and spine with a new generation of even less invasive but more reliable procedures.

We performed a prospective, randomised controlled trial of unicompartmental knee arthroplasty comparing the performance of the Acrobot system with conventional surgery. A total of 27 patients (28 knees) awaiting unicompartmental knee arthroplasty were randomly allocated to have the operation performed conventionally or with the assistance of the Acrobot. The primary outcome measurement was the angle of tibiofemoral alignment in the coronal plane, measured by CT. Other secondary parameters were evaluated and are reported.

All of the Acrobot group had tibiofemoral alignment in the coronal plane within 2° of the planned position, while only 40% of the conventional group achieved this level of accuracy. While the operations took longer, no adverse effects were noted, and there was a trend towards improvement in performance with increasing accuracy based on the Western Ontario and McMaster Universities Osteoarthritis Index and American Knee Society scores at six weeks and three months. The Acrobot device allows the surgeon to reproduce a pre-operative plan more reliably than is possible using conventional techniques which may have clinical advantages.

Objective- To evaluate three femoral cement pressurization techniques in a laboratory setting.

Design- Observational study using a plastic femur (sawbone) model.

Materials and Methods- 12 femoral bone models were cemented and pressurized using three different cement pressurization techniques (standard technique, pressuriser in situ technique, and thumb pressurisation technique). Four sets of observations were taken for each technique. Intramedullary pressure readings were obtained using proximal and distal pressure monitoring transducers. The peak pressure and the time for which the pressure was above a particular cut off level (5 KPa and 100 KPa) were compared.

Results- There were significant variations between the peak pressure and the duration for which the pressure was above 100 KPa. The pressuriser in situ technique yielded significantly (p< 0.001) higher peak pressure both proximally (397.5 ± 40.2 KPa) and distally (597.3 ± 102.4). The standard technique produced the optimum pressure of 100 KPa for significantly (p< 0.001) longer duration proximally and distally (66.8 ± 29.5 and 45.2 ± 15.5 seconds respectively) compared to the other two techniques (less than 5 and 17 seconds for thumb pressurisation technique and pressuriser in situ technique respectively, both proximally and distally). There were no significant statistical differences between the three methods for the time for which the cement pressure was higher than 5 KPa.

Conclusion- Although the pressuriser in situ technique produced highest peak pressure, the standard technique produced optimum pressure for longer duration. The standard technique seems to be adequate to achieve optimum pressurization during femoral cementing without increased risk of embolisation.

The Acrobot®, an active constraint “hands-on” robotic system, gives navigation cues to the surgeon, and also assists him in the surgery, using active software constraints if he tries to depart from the preoperative plan. It has just entered clinical trials. We report the first 5 cases.

The Acrobot® system for precision total knee arthroplasty comprises the following components:

1. A CT-based planning system

a gross positioning device with separate brakes and encoders, locked off for safety during the procedure,

4. The Acrobot’s software which:

imports the preoperative plan,

Each patient’s knee scores were monitored and postoperative CT scan was compared with the preoperative plan.

Seven robot assisted arthroplasties have been performed. No significant complications have been encountered. The Knee and Womac Scores show that the procedure is safe and comparable to conventional surgery in the early postoperative period. The envelope of error on postoperative CT scans has been within the accuracy of the method of measurement, at < 1 mm and < 10 without the outliers which haunt every clinical series.

The Acrobot® system for total knee arthroplasty has completed its preliminary trial satisfactorily. It provides a handson operation but with robotic levels of accuracy. It is suitable for conventional open surgery, but its real place will be in the arena of minimally invasive unicondylar knee arthroplasty, hip arthroplasty and resurfacing, and in the spine, where active constraint will prevent potentially dangerous surgical errors.