This aim of this study was to identify common factors in patients with the shortest length of hospital stay following total hip arthroplasty (THA). This would then allow a means of targeting suitable patients to reduce their length of stay.

This was a retrospective cohort study of all patients undergoing primary THA at our institution between September 2013 and August 2014. Demographic data were collected from the patient record. The cohort was divided into those discharged to home within two days of operation and the rest of the THA population. The demographics (age, gender, ASA grade, body mass index (BMI), primary diagnosis, socioeconomic status (Scottish Index of Multiple Deprivation, SIMD and SIMD health domain) were compared between groups. In addition for the early discharge group information on comorbidities, family support at home and independent transport were collected.

The study cohort was 1292 patients. 119 patients were discharged home on the first post-operative day. Those discharged earlier were on average younger (p<0.0001), more likely to be male (p<0.0001) and had a lower ASA grade (p<0.00001). Other demographics did not differ between groups. Patients who were discharged early also appeared to have few comorbidities (Diabetes 5.9%, Cardiac disease 7.6%, Respiratory disease 9%), high levels of family support at home (95%) and high levels of independent transport arrangements (97%).

Factors associated with those patients with the shortest lengths of stay were identified. Such factors could be used to target patients who are suitable for streamlined recovery programmes aimed at early discharge after THA and assist with service planning.

The knee joint displays a wide spectrum of laxity, from inherently tight to excessively lax even within the normal, uninjured population. The assessment of AP knee laxity in the clinical setting is performed by manual passive tests such as the Lachman test. Non-invasive assessment based on image free navigation has been clinically validated and used to quantify mechanical alignment and coronal knee laxity in early flexion. When used on cadavers the system demonstrated good AP laxity results with flexion up to 40°. This study aimed to validate the repeatability of the assessment of antero-posterior (AP) knee joint laxity using a non-invasive image free navigation system in normal, healthy subjects.

Twenty-five healthy volunteers were recruited and examined in a single centre. AP translation was measured using a non-invasive navigation system (PhysioPilot) consisting of an infrared camera, externally mounted optical trackers and computer software. Each of the volunteers had both legs examined by a single examiner twice (two registrations). The Lachman test was performed through flexion in increments of 15°. Coefficients of Repeatability (CR) and Interclass Correlation Coefficients (ICC) were used to validate AP translation. The acceptable limits of agreement for this project were set at 3mm for antero-posterior tibial translation.

The most reliable and repeatable AP translation assessments were at 30° and 45°, demonstrating good reliability (ICC 0.82, 0.82) and good repeatability (CR 2.5, 2.9). The AP translation assessment at 0°, 15°, 75° and 90° demonstrated moderate reliability (ICC ≤ 0.75), and poor repeatability (CR ≥3.0mm).

The non-invasive system was able to reliably and consistently measure AP knee translation between 30° and 45° flexion, the clinically relevant range for this assessment. This system could therefore be used to quantify abnormal knee laxity and improve the assessment of knee instability and ligamentous injuries in a clinic setting.

Knee osteoarthritis results in pain and functional limitations. In cases where the arthritis is limited to one compartment of the knee joint then a unicondylar knee arthroplasty (UKA) is successful, bone preserving option. UKA have been shown to result in superior clinical and functional outcomes compared to TKA patients. However, utilisation of this procedure has been limited due primarily to the high revision rates reported in joint registers. Robotic assisted devices have recently been introduced to the market for use in UKA. They have limited follow up periods but have reported good implant accuracy when compared to the pre-operative planned implant placement.

UKA was completed on 25 cadaver specimens (hip to toe) using an image-free approach with infrared optical navigation system with a hand held robotically assisted cutting tool. Therefore, no CT scan or MRI was required. The surface of the condylar was mapped intra operatively using a probe to record the 3 dimensional surface of the area of the knee joint to be resurfaced. Based on this data the size and orientation of the implant was planned. The user was able to rotate and translate the implant in all three planes. The system also displays the predicted gap balance graph through flexion as well as the predicted contact points on the femoral and tibial component through flexion. The required bone was removed using a bur. The depth of the cut was controlled by the robotically controlled freehand sculpting tool.

Four users (3 consultant orthopaedic surgeon and a post-doctoral research associate) who had been trained on the system prior to the cadaveric study carried out the procedures. The aim of this study was to quantify the differences between the ‘planned’ and ‘achieved’ cuts. A 3D image of the ‘actual’ implant position was overlaid on the ‘planned’ implant image. The errors between the ‘actual’ and the ‘planned’ implant placement were calculated in three planes and the three rotations. The maximum femoral RMS angular error was 2.34°. The maximum femoral RMS translational error across all directions was up to 1.61mm. The maximum tibial RMS angular error was 2.60°. The maximum tibial RMS translational error across all directions was up to 1.67mm.

In conclusion, the results of this cadaver study reported low RMS errors in implant position placement compared to the plan. The results were comparable with those published from clinical studies investigating other robotic orthopaedic devices. Therefore, the freehand sculpting tool was shown to be a reliable tool for cutting bone in UKA and the system allows the surgeon to plan the placement of the implant intra operatively and then execute the plan successfully.

The Columbus® knee system was designed as a standard knee implant that allows high flexion without the need for additional bone resection. The aim of this retrospective study was to investigate the correlation between the maximum flexion achieved at five years and the slope of the tibial component. The hypothesis was that increased slope would give increased flexion.

The study design was a retrospective cohort study at a single centre. The inclusion criterion was having had a navigated cemented Columbus primary TKA implanted between March 2005 and December 2006 using the image free OrthoPilot® navigation system (Aesculap, Tuttlingen, Germany) in our institution. Follow-up had been carried out at review clinics by an independent arthroplasty team. Patient-related data had been recorded either in case notes, the departmental proprietary database or as radiographic images. In addition to demographics, five-year follow-up range of motion (ROM) was collected. All available radiographs on the national Picture Archiving and Communication System (Eastman Kodak Company, 10.1_SP1, 2006), whether taken at our institution or at the patient's local hospital, were analysed by a trainee orthopaedic surgeon (NCS) who was independent of the patients' care. Component position according to the Knee Society TKA scoring system was determined from the five-year review lateral x-ray. The tibial slope was calculated as 90° minus the angle of the tibial component so giving a posterior slope as a positive number and an anterior slope as a negative number. The correlation between maximum flexion angle and tibial slope was calculated. Further to this a subgroup of only CR prostheses and patients with BMI <35 were analysed for a relationship. The tibial slope of the group of patients having 90° or less of flexion (poor flexion) was compared to those having 110° or more (good flexion) using a t-test, as was the flexion of the those with BMI <30 to those with BMI > 35.

A total of 219 knees in 205 patients were identified. 123 had five-year radiograph and maximum flexion measurement available. Cohort demographics were mean age 68(8.6), mean BMI 32.0(5.9) and mean maximum flexion at five years of 101°(11°). The tibial slope angle showed variation around the mean of 2°(2.8°). There was no correlation between tibial slope and maximum flexion for either that whole cohort (r=-0.051, p=0.572, Figure 1b) or the subgroup of CR and BMI <35 patients (n=78, r = −0.089, p=0.438). The mean tibial slope of those patients having poor flexion was 2° (SD2.6°) and this was not significantly different to the mean for those with good flexion, 3° (SD3.1°) p=0.614. The mean flexion of those with BMI <30 was 100° (SD8.7°) and this was not significantly different to those with BMI >35, mean 101° (SD11.4°).

This study did not find any correlation between the tibial slope and maximum flexion angle in 123 TKAs at five year follow up. Further studies with a more accurate measurement of tibial slope should be carried out to confirm whether a relationship exists in the clinical setting.

Non-invasive assessment of lower limb mechanical alignment and assessment of knee laxity using navigation technology is now possible during knee flexion owing to recent software developments. We report a comparison of this new technology with a validated commercially available invasive navigation system.

We tested cadaveric lower limbs (n=12) with a commercial invasive navigation system against the non-invasive system. Mechanical femorotibial angle (MFTA) was measured with no stress, then with 15Nm of varus and valgus moment. MFTA was recorded at 10° intervals from full knee extension to 90° flexion. The investigator was blinded to all MFTA measurements. Repeatability coefficient was calculated to reflect each system's level of precision, and agreement between the systems; 3° was chosen as the upper limit of precision and agreement when measuring MFTA in the clinical setting based on current literature.

Precision of the invasive system was superior and acceptable in all conditions of stress throughout flexion (repeatability coefficient <2°). Precision of the non-invasive system was acceptable from extension until 60° flexion (repeatability coefficient <3°), beyond which precision was unacceptable. Agreement between invasive and non-invasive systems was within 1.7° from extension to 50° flexion when measuring MFTA with no varus / valgus applied. When applying varus / valgus stress agreement between the systems was acceptable from full extension to 20° & 30° knee flexion respectively (repeatability coefficient <3°). Beyond this the systems did not demonstrate sufficient agreement.

These results indicate that the non-invasive system can provide reliable quantitative data on MFTA and laxity in the range relevant to knee examination.

Conventional computer navigation systems using bone fixation have been validated in measuring anteroposterior (AP) translation of the tibia. Recent developments in non-invasive skin-mounted systems may allow quantification of AP laxity in the out-patient setting.

We tested cadaveric lower limbs (n=12) with a commercial image free navigation system using passive trackers secured by bone screws. We then tested a non-invasive fabric-strap system. The lower limb was secured at 10° intervals from 0° to 60° knee flexion and 100N of force applied perpendicular to the tibial tuberosity using a secured dynamometer. Repeatability coefficient was calculated both to reflect precision within each system, and demonstrate agreement between the two systems at each flexion interval. An acceptable repeatability coefficient of ≤3mm was set based on diagnostic criteria for ACL insufficiency when using other mechanical devices to measure AP tibial translation.

Precision within the individual invasive and non-invasive systems measuring AP translation of the tibia was acceptable throughout the range of flexion tested (repeatability coefficient ≤1.6mm). Agreement between the two systems was acceptable when measuring AP laxity between full extension and 40° knee flexion (repeatability coefficient ≤2.1mm). Beyond 40° of flexion, agreement between the systems was unacceptable (repeatability coefficient >3mm).

These results indicate that from full knee extension to 40° flexion, non-invasive navigation-based quantification of AP tibial translation is as accurate as the standard invasive system, particularly in the clinically and functionally important range of 20° to 30° knee flexion. This could be useful in diagnosis and post-operative follow-up of ACL pathology.

Unicondylar knee arthroplasty (UKA) is a treatment for osteoarthritis when the disease only affects one compartment of the knee joint. The popularity in UKA grew in the 1980s but due to high revision rates the usage decreased. A high incidence of implant malalignment has been reported when using manual instrumentation. Recent developments include surgical robotics systems with navigation which have the potential to improve the accuracy and precision of UKA.

UKA was carried out using an imageless navigation system – the Navio Precision Freehand Sculpting system (Blue Belt Technologies, Pittsburgh, USA) with a medical Uni Knee Tornier implant (Tornier, Montbonnot Saint Martin, France) on nine fresh frozen cadaveric lower limbs (8 males, 1 females, mean age 71.7 (SD 13.3)). Two users (consultant orthopaedic surgeon and post doctoral research associate) who had been trained on the system prior to the cadaveric study carried out 4 and 5 implants respectively. The aim of this study was to quantify the differences between the planned and achieved cuts.

A 3D image of the ‘actual’ implant position was overlaid on the planned implant image. The errors between the ‘actual’ and the planned implant placement were calculated in three planes and the three rotations. The maximum femoral implant rotational error was 3.7° with a maximum RMS angular error of 2°. The maximum femoral implant translational error was 2.6mm and the RMS translational error across all directions was up to 1.1mm. The maximum tibial implant rotational error was 4.1° with a maximum RMS angular error was 2.6°. The maximum translational error was 2.7mm and the RMS translational error across all directions was up to 2.0mm.

The results were comparable to those reported by other robotic assistive devices on the market for UKA. This technology still needs clinical assessment to confirm these promising results.

INTRODUCTION

Leg length discrepancy following total hip arthroplasty (THA) can be functionally disabling for affected patients and can lead on to litigation issues. Assessment of limb length discrepancy during THA using traditional methods has been shown to produce inconsistent results. The aim of our study was to compare the accuracy of navigated vs. non navigated techniques in limb length restoration in THA.

Distal femur resection for correction of flexion contractures in total knee arthroplasty (TKA) can lead to joint line elevation, abnormal knee kinematics and patellofemoral problems. The aim of this retrospective study was to establish the contribution of soft tissue releases and bony cuts in the change in maximum knee extension in TKA.

Data were available for 209 navigated TKAs performed by a single surgeon using a medial approach. All patients had the same cemented implant, either CR or PS, which both required a minimum thickness of 10 mm for the tibial and 9mm for the femoral component. Intra-operatively pre- and post-implant extension angles and the size of bone resection were collected using a commercial navigation system. The thickness of polyethylene insert and the extent of soft tissue release performed (no release, moderate and extensive release) were collected from the patient record. A univariate linear regression model was used to predict change in maximum extension from pre- to post-implant.

The mean bone resection was 19mm (15 to 28 mm) (Figure 1).79% of polyethylene inserts were 10mm thick (10 to 16 mm). 71% of knees had no soft tissue release. The mean increase in extension was 5° (11° decrease to 23° increase) (Figure 1). The analysis showed that bone cuts (p<0.001), soft tissue release (p=0.001) and insert thickness (p=0.010) were all significant terms in the model (r²_adj=0.170). This model predicted that carrying out a TKA with 19mm bone cuts, 10mm insert and no soft tissue release would give 4.2° increase in extension. It predicted that a moderate release would give a 2.8° increase in extension compared to no release, with an extensive release giving 3.9° increase over no release. For each mm increase in bone cuts the model predicted a 0.8° increase in extension and for each mm increase in insert size a decrease extension by 1.1°.

Preoperative FFC contracture is a frequent condition in TKA that the surgeon has to address either by resecting more bone or by extending soft tissue release to increase the extension gap to fit the knee implant. This analysis of 209 navigated knee arthroplasty showed that both options are suitable to increase the extension gap. The modelling results show that in general to increase maximum extension by the same as an extensive soft tissue release that bone cuts would have to be increased by 4–5mm. However this model only accounted for 17% of the variation in change in extension pre- to post-implant so is poor at predicting outcomes for specific patients. The large variation in actual FFC correction indicates that this relies on factors other than bone cuts and soft tissue releases as quantified in this study.

Introduction

Soft tissue balancing is an important aspect of total knee replacement surgery. Traditionally sequential medial soft tissue release is performed for balancing in varus deformity. Its effects on kinematics and dynamic Femoro-Tibial-Mechanical-Angle (FTMA) have been described in extension and 90° flexion in coronal plane. However most studies have missed what happens when the knee flexes from 0 to 90 degrees This study is one of the first to describe its effects on knee kinematics throughout flexion. The aim was to look at deviation of FTMA in coronal plane with traditional sequential medial release with and without measured stress applied in varus and valgus at each point of measurement through the range of flexion.

Traditionally sequential medial soft tissue release is performed for balancing in total knee arthroplasty for varus knees. Its effects on kinematics have been described in extension and 90° flexion in coronal plane. This is the first study to describe its effects on kinematics throughout flexion. 12 cadaveric knees were studied using a computer navigation system to assess kinematics. Femoro-Tibial-Mechanical-Angle(FTMA) was studied in extension, 0°, 5°, 30°,45°,60°,90° and maximum flexion. Sequential medial release was performed in 7 steps, described by Luring et al(Ref). At each step FTMA was measured without and with stressing. A 10 Newton Meter moment arm was applied for varus and valgus stress. Most of the initial release steps had little effect on FTMA without force applied, especially in the initial 60° of flexion. Application of varus force demonstrated very small changes. Application of valgus force demonstrated little change in initial arc of flexion until step 5 was reached (Table 1). Our study concludes the present sequence of medial release may not be correct and should be further investigated to modify the sequence for soft tissue balancing in TKR surgery.

Range of motion (ROM) is a well recognised outcome measure following total knee arthroplasty (TKA). Reduced knee flexion can lead to poor outcome after TKA and therefore identification at an early stage is important as it may provide a window for intervention with targeted physiotherapy, closer follow-up and in resistant cases possible manipulation or arthrolysis. ROM combines both flexion and extension and in contrast to flexion, fewer studies have recognised the importance of a lack of full extension or fixed flexion deformity (FFD) following TKA. A residual FFD can increase energy cost, decrease velocity during ambulation and result in pain with knee scores more likely to be diminished than if knee extension was normal. Recognition and early detection of FFD is therefore important. Methods of assessment include by visual estimation or goniometric measurement of knee flexion angle. While goniometers are inexpensive, easy to use and provide more accurate than visual estimates of angles, they have been shown to exhibit poor inter-observer reliability. Therefore they may not be sensitive enough to consistently identify FFD and therefore distinguish between grading systems based on absolute angular limits. The aim of this study was to investigate the accuracy of standard clinical ROM measurement techniques following TKA and determine their reliability for recognising FFD.

Ethical approval was obtained for this study. Thirty patients who were six weeks following TKA had their knee ROM measured. An infrared (IR) tracking system (±1°accuracy) that had been validated against an electro-goniometer was used to give a “true” measurement of the lower limb sagittal alignment with the knee fully extended and maximally flexed while the patient was supine. The patients were also assessed independently by experienced arthroplasty practitioners using a standardised goniometric measurement technique. For goniometric clinically-measured flexion (Clin_flex) and extension (Clin_ext) linear models were generated using IR-measured flexion and extension (IR_flex and IR_ext), BMI and gender as covariables. Data for extension were categorised in none, moderate and severe postoperative FFD as per Ritter et al. 2007 and agreement in classification between the two methods was assessed using the Kappa statistic.

For the linear models for Clin_flex and Clin_ext neither BMI nor gender were significant variables. Therefore the final models were:

Clin_flex = 0.54 + 0.66∗IR_flex (r²_adj = 0.521)

Clin_ext = 0.23 + 0.50∗IR_ext (r²_adj = 0.247)

The model for Clin_flex showed that the IR and clinical measurements coincided at approximately 90° so that for every 10° increase in flexion above 90° clinical measurement only increased by 7° but for every 10° decrease in flexion below 90° clinical measurement only decreased by 7°. The model for Clin_ext showed that the IR and clinical measurements coincided at approximately 0° so that for every 10° increase in FFD angle, clinical measurement only increased by 5° but if the knee went into hyperextension this would be underestimated by the clinical measure. In identifying FFD there was moderate agreement between the two measurements (κ = 0.44). Clinically nine patients were assessed as having FFD but the IR measurements showed 18 patients having FFD, of which nine patients were not identified clinically.

When assessing knee ROM following joint arthroplasty manual goniometric measurements provided a poor estimate of the range when compared to the “true” angle as measured with a validated IR measurement tool. When the knee was held in maximum flexion there was a tendency to both underestimate and overestimate the true angle. However when the knee was held in extension there was a tendency to underestimate which we believe is important as it would underreport both the frequency and magnitude of FFD. In our study, 18 patients had a moderate FFD as identified by the IR system, only half of which were identified by goniometer measurement alone. Studies of comparisons of both visual and manual goniometry measurements of the knee in maximum flexion with lateral radiographs have shown most errors involved an underestimate of true flexion. It has been concluded that it was safer to underestimate knee flexion angle as it would result in higher pick up rate of cases being performing less well. In contrast however, underestimation while in extension is less desirable as it fails to pick-up FFD which may have benefited from intervention had they been identified. It is known that residual FFD can increase energy cost and decrease velocity during ambulation with pain and functional knee scores more likely to be reduced. Recognition and early detection is therefore important. With the use of more accurate systems to identify and measure FFD, such as the one used for this study may in turn allow more timely treatment and therefore hopefully improved outcomes.

Clinical laxity tests are frequently used for assessing knee ligament injuries and for soft tissue balancing in total knee arthroplasty (TKA). Current routine methods are highly subjective with respect to examination technique, magnitude of clinician-applied load and assessment of joint displacement. Alignment measurements generated by computer-assisted technology have led to the development of quantitative TKA soft tissue balancing algorithms. However to make the algorithms applicable in practice requires the standardisation of several parameters: knee flexion angle should be maintained to minimise the potential positional variation in ligament restraining properties; hand positioning of the examining clinician should correspond to a measured lever arm, defined as the perpendicular distance of the applied force from the rotational knee centre; accurate measurement of force applied is required to calculate the moment applied to the knee joint; resultant displacement of the knee should be quantified.

The primary aim of this study was to determine whether different clinicians could reliably assess coronal knee laxity with a standardised protocol that controlled these variables. Furthermore, a secondary question was to examine if the experience of the clinician makes a difference. We hypothesised that standardisation would result in a narrow range of laxity measurements obtained by different clinicians.

Six consultant orthopaedic surgeons, six orthopaedic trainees and six physiotherapists were instructed to assess the coronal laxity of the right knee of a healthy volunteer. Points were marked over the femoral epicondyles and the malleoli to indicate hand positioning and give a constant moment arm. The non-invasive adaptation of a commercially available image-free navigation system enabled real-time measurement of coronal and sagittal mechanical femorotibial (MFT) angles. This has been previously validated to an accuracy of ±1°. Collateral knee laxity was defined as the amount of angular displacement during a stress manoeuvre. Participants were instructed to maintain the knee joint in 2° of flexion whilst performing a varus-valgus stress test using what they perceived as an acceptable load. They were blinded to the coronal MFT angle measurements. A hand-held force application device (FAD) was then employed to allow the clinicians to apply a moment of 18Nm. This level was based on previous work to determine a suitable subject tolerance limit. They were instructed to repeat the test using the device in the palm of their right hand and to apply the force until the visual display and an auditory alarm indicated that the target had been reached. The FAD was then removed and participants were asked to repeat the clinical varus-valgus stress test, but to try and apply the same amount of force as they had been doing with the device.

Maximum MFT angular deviation was automatically recorded for each stress test and the maximum moment applied was recorded for each of the tests using the FAD. Means and standard deviations (SD) were used to compare different clinicians under the same conditions. Paired t-tests were used to measure the change in practice of groups of clinicians before, during and after use of the FAD for both varus and valgus stress tests.

All three groups of clinicians initially produced measurements of valgus laxity with consistent mean values (1.5° for physiotherapists, 1.8° for consultants and 1.6° for trainees) and standard deviations (<1°). For varus, mean values were consistent (5.9° for physiotherapists, 5.0° for consultants and 5.4° for trainees) but standard deviations were larger (0.9° to 1.6°). When using the FAD, the standard deviations remained low for all groups for both varus and valgus laxity. Introducing the FAD overall produced a significantly greater angulation in valgus (2.4° compared to 1.6°, p<0.001) but not varus (p = 0.67) when compared to the initial examination. In attempting to reach the target moment of 18Nm, the mean ‘overshoot’ was 0.9Nm for both varus and valgus tests. Standard deviations for varus laxity were lower for all groups following use of the FAD. The consultants' performance remained consistent and valgus assessment remained consistent for all groups. The only statistically significant change in practice for a group before and after use of the FAD was for the trainees testing valgus, who may have been trained to push harder (p = 0.01). Standardising the applied moment indicated that usually a lower force is applied during valgus stress testing than varus. This was re-enforced by clinicians, one third of whom commented that they felt they had to push harder for valgus than varus, despite the FAD target being the same.

We have successfully standardised the manual technique of coronal knee laxity assessment by controlling the subjective variables. The results support the hypothesis of producing a narrow range of laxity measurements but with valgus laxity appearing more consistent than varus. The incorporation of a FAD into assessment of coronal knee laxity did not affect the clinicians' ability to produce reliable and repeatable measurements, despite removing the manual perception of laxity. The FAD also provided additional information about the actual moment applied. This information may have a role in improving the balancing techniques of TKA and the management of collateral ligament injuries with regard initial diagnosis and grading as well as rehabilitation.

Finally, the results suggest that following use of the FAD, more experienced clinicians returned to applying their usual manual force, while trainees appeared to use this augmented feedback to adapt their technique. Therefore this technique could be a way to harness the experience of senior clinicians and use it to enhance the perceptive skills of more junior trainees who do not have the benefit of this knowledge.

There is increasing interest in the use of image free computer assisted surgery (CAS) in total hip arthroplasty (THA). Many of these systems require the registration of the Anterior Pelvic Plane (APP) via the bony landmarks of the anterior superior iliac spines (ASIS) and pubic tubercles (PT) in order to accurately orient the acetabular cup in terms of anteversion and inclination. Given system accuracies are within 1mm and 1° and clinical validation studies have given accuracy by cup position. However, clinical outcomes contain not only system inaccuracies but also variations due to clinical practice. To understand the effects of variation in landmark acquisition on the identification of the acetabular cup orientation, independent bench testing is required. This requires a phantom model that can represent the range of pelvises, male and female, encountered during THA and introduce deliberate known errors to the acquisition to see the effect on anteversion and inclination angles. However, there is a paucity of information in the literature with regards to these specific pelvic dimensions (pelvic width and height). Therefore the aims of this work were to generate the normal expected range of sizes of the APP for both males and females and to use these to manufacture a phantom model that could be used to assess CT free navigation systems.

In the first part of the study 35 human cadavers and 100 pelvic computed tomography (CT) scans were examined.

All cadavers had no gross pelvic abnormalities or previous surgeries. Measurements were carried out with cadavers placed in a supine position. The first author made three sets of measurements using a millimeter ruler. Solid steel pins were used to identify the palpated ASISs and PTs. String was tied between the two ASIS pins and the pelvic width measured. The midpoint of the pubic tubercles was taken to be the midpoint of the pubic symphysis. Pelvic height was measured from the midpoint of the ASIS distance (marked on the string) to the midpoint of the PTs. One hundred pelvic CT scans with no bony abnormalities, previous surgery or metal prosthesis (due to artefacts) were obtained retrospectively from the hospital radiological online system (PACS, Kodak). Mimics software (Mimics12 Materialise, Leuven, Belgium) was used to automatically reconstruct three-dimensional (3D) models using the ‘Bone’ thresholding function. This eliminated any soft tissue from the 3D models. The most anterior ASIS and PT points were then identified on the 3D model surface and measurements of distances made. As the software did not allow identification of points not on the model surface it was not possible to directly obtain the midpoint of the ASIS distance. Therefore to obtain the pelvic height measurements the distance between each ASIS and the ipsilateral and contralateral PTs was also measured. The pelvic height was then calculated using trigonometric functions. The ratio of width to height was calculated (ratio > 1 indicating pelvis width greater than pelvis height). Student's t test was used analyse any differences between male and female pelvic measurements with a p<0.05 being statistically significant.

Using the results from above an aluminium pelvic phantom model was designed and manufactured. It was machined from a billet of marine grade aluminium alloy using a vertical computer numerical controlled (CNC) milling machine. The top surface represented the APP and sides (which represented the acetabuli) were angled to give anteversion and inclination angles of 20° and 45° respectively. Co-ordinates for ASIS and PT points were given based on the 99% prediction intervals from the pelvic data and additional points were milled to give up to a 20 mm error mediolaterally and also in height. Each co-ordinate point was drilled with a 2.0mm diameter ball-nose cutter to a depth of 1.0mm, these holes designed to accommodate the ball-nosed pointer tip to ensure it remained at the same position in space at all orientations of the pointer. Further to this, known errors in height were introduced using accurately manufactured blocks with similar points milled on the surface to fit a ball-nosed pointer. These blocks could be secured to the top surface of the model using screws. A Perspex base unit with tracker attachments was made to hold the phantom and provide the reference frame. A further support that enables the phantom to also be used in the “lateral” position was manufactured.

For the assessment of pelvic size there were 66 females and 69 males, mean age 62.3 years (range from 20 to 99 years). The mean width was 238 mm (SD 20 mm) and mean height was 93 mm (SD 11 mm) with a mean ratio of 2.6 (SD 0.3). There were no statistically significant differences in mean between males and females (p>0.4 in all cases). From this data set the range of APP sizes required to cover 99% of population (width 186 to 290 mm and height 66 to 120 mm) and therefore the measurements for the model were generated. The manufactured model can be used to give the range of pelvis sizes from 170mm to 290mm in width and 60mm to 120mm in height and also to add up to 20 mm of error in palpation of each of the ASISs and PT.

This study generated APP sizes to cover 99% of the general population over a wide age range. It illustrated that a single pelvic model would fit both sexes. The model allows the determination of the effects of changes of the pelvic dimensions may have on the acetabular orientation measured on an image free CAS system including the assessment of point acquisition and deliberate errors. The model has been successfully used in preliminary testing and can be used to assess any CT free system.

Knee alignment is a fundamental measurement in the assessment, monitoring and surgical management of patients with osteoarthritis [OA]. In spite of extensive research into the consequences of malalignment, our understanding of static tibiofemoral alignment remains poor with discrepancies in the reported weight-bearing characteristics of the knee joint and there is a lack of data regarding the potential variation between supine and standing (functional) conditions. In total knee arthroplasty [TKA] the lower limb alignment is usually measured in a supine condition and decisions on prosthesis placement made on this. An improved understanding of the relationship between supine and weight-bearing conditions may lead to a reassessment of current surgical goals.

The purpose of this study was to explore the relationship between supine and standing lower limb alignment in asymptomatic, osteoarthritic and prosthetic knees. Our hypothesis was that the change in alignment of these three groups would be different.

A non-invasive infrared position capture system (accuracy ±1° in both coronal and sagittal plane) was used to assess the knee alignment for 30 asymptomatic controls and 31 patients with OA, both before and after TKA. Coronal and sagittal mechanical femorotibial (MFT) angles in extension (negative values indicating varus in the coronal plane and hyperextension in the sagittal plane) were measured with each subject supine and in bi-pedal stance. For the supine test, the lower limb was supported at the heel and the subject told to relax. For the standing position subjects were asked to assume their normal stance. The change in alignment between these two conditions was analysed using a paired t-test for both coronal and sagittal planes. To quantify the change in 3D, vector plots of ankle centre displacement relative to the knee centre from the supine to standing condition were produced.

Alignment in both planes changed significantly from supine to standing for all three groups. For the coronal plane the supine and standing measurements (in degrees, mean(SD)) were 0.1(2.5) and −1.1(3.7) in the asymptomatic group, −2.5(5.7) and −3.6(6) in the OA group and −0.7(1.4) and −2.5(2) in the TKA group. For the sagittal plane the numbers were −1.7(3.3) and −5.5(4.9); 7.7(7.1) and 1.8(7.7); 6.8(5.1) and 1.4((7.6) respectively. This change was most frequently towards relative varus and extension. Vector plots showed that the trend of relative varus and extension in stance was similar in overall magnitude and direction between the three groups.

Knee alignment can change from supine to standing for asymptomatic and osteoarthritic knees, most frequently towards relative varus and hyperextension. The similarities between each group did not support our hypothesis. The consistent kinematic pattern for different knee types suggests that soft tissue restraints rather than underlying joint deformity may be more influential in dynamic control of alignment from lying to standing. In spite of some evidence suggesting a difference between supine and standing knee alignment a mechanical femorotibial (MFT) angle of 0° is a common intra-operative target as well as the desired post-operative weight-bearing alignment. These results indicated that arthroplasties positioned in varus intra-operatively could potentially become ‘outliers’ (>3° varus) when measured weight-bearing. Mild flexion contractures may correct when standing, reducing the need for intra-operative posterior release. These potential changes should be considered when positioning TKA components on supine limbs as post-operative functional alignment may be different.

Computer assisted surgery is becoming more frequently used in the medical world. Navigation of surgical instruments and implants plays an important role in this surgery. OrthoPilot™ Hip Suite (BBraun Aesculap) is one such system used for hip navigation in orthopaedic surgery. However the accuracy of this system remains to be determined independently of the manufacturer. The manufacturer supplies a technical specification for the accuracy of the system (± 2 mm and ± 2°) and previous research has been undertaken to compare its clinical accuracy against conventional hip replacements by x-ray. This clinical validation is important but contains many sources of error or deviation from an ideal outcome in terms of the surgeons' use of the system, inaccurate palpation of landmarks, variation in actual cup position from that given by the navigation system and measurement of the final cup position. It is therefore not possible to validate the claims of the manufacturer from this data. There is no literature evaluating the technical accuracy of the software i.e. the accuracy of the system given known inputs. This study had two main aims 1) validating the accuracy of the OrthoPilot data while navigating the surgical instruments and 2) validating the accuracy of navigation algorithm inside the OrthoPilot system which determines cup implant placement. The OrthoPilot validation was performed and compared against the gold standard of a VICON movement analysis system.

The system used was OrthoPilot™ with a Spectra camera from Northern Digital Inc. (Ontario, Canada). Software investigated was the Hip Suite THA cup only navigation software Version 3.1. The validation was performed and compared against the VICON Nexus version 1.4.116 with Bodybuilder software version 3.55. An aluminium pelvis phantom was used for measurement allowing accurate and repeatable inputs. The OrthoPilot system has three types of instruments sets; passive, active and hybrid. This study was carried out with the passive instruments set. Data were captured simultaneously from both the OrthoPilot and VICON systems for the supine position of the phantom. Distances between the anatomical land marks on the phantom were compared to test the data capturing accuracy of the OrthoPilot system. Anatomical land marks of right anterior superior iliac supine (RASIS), left anterior superior iliac supine (LASIS) and Pubic Symphasis (PS) were palpated to define the Anterior Pelvic Plane (APP). Distances between the anatomical landmarks of RASIS to LASIS, RASIS to PS and LASIS to PS were considered for comparison. Width and height of the pelvis was varied to examine different APPs. The width and height used were 170 mm and 53 mm, 230 mm and 88 mm, and 290 mm and 123 mm respectively. One hundred APP data sets were captured at each instance.

The accuracy of the hip navigation algorithm was tested by applying similar algorithm to calculate the native anteversion and inclination angles of the acetabulum using the VICON system. Data were captured simultaneously from both OrthoPilot and VICON systems. Radiographic anteversion and inclination angles were obtained with phantom model, which had 14° of anteversion angle and 45° of inclination angle. APP of 230 mm in width and 88 mm in height was used to obtain anterior pelvic plane data. Position vectors for each anatomical land mark from the OrthoPilot system were extracted from relevant transformation matrices, while position vectors from the VICON system were extracted from static trial modelling.

The distance data from both systems were compared with calibrated distance data from the phantom model. Mean values of the distances between anatomical landmarks were found to be similar for both OrthoPilot and VICON systems. In addition, these distances were comparable with the pelvic phantom model data, within 1 mm for all measured distances for the VICON and 2 mm for the OrthoPilot. Furthermore, the standard deviations were less than 1% of the measured value. Comparison was also made for the anteversion and inclination angles of the acetabulum of the pelvic model with OrthoPilot and VICON data. Both systems produced similar results for the mean angle values, within 0.5° of the known angles for the VICON and 1° for the OrthoPilot and with standard deviations of the measured values of less than 1%.

All the data were captured simultaneously from both OrthoPilot and VICON systems under the same laboratory conditions. According to the above results it is clear that the distance readings obtained from the OrthoPilot are comparable to the results obtained from the gold standard VICON system and the calibrated distance readings of the phantom. In addition, acetabular angle results obtained from OrthoPilot are almost equivalent to results obtained from VICON and the calibrated phantom angles. Finally it is can be concluded that, both the data palpation with OrthoPilot system and acetabular angle calculation algorithm of the OrthoPilot system are accurate enough for the real world clinical tasks they are expected to perform.