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.

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.