Human error is usually evaluated using statistical descriptions during radiographic annotation. The technological advances popularized the “non-human” landmarking techniques, such as deep learning, in which the error is presented in a confidence format that is not comparable to that of the human method. The region-based landmark definition makes an arbitrary “ground truth” point impossible. The differences in patients’ anatomies, radiograph qualities, and scales make the horizontal comparison difficult. There is a demand to quantify the manual landmarking error in a probability format.

Taking the measurement of pelvic tilt (PT) as an example, this study recruited 115 sagittal pelvic radiographs for the measurement of two PTs. We proposed a method to unify the scale of images that allows horizontal comparisons of landmarks and calculated the maximum possible error using a density vector. Traditional descriptive statistics were also applied.

All measurements showed excellent reliabilities (intraclass correlation coefficients > 0.9). Eighty-four measurements (6.09%) were qualified as wrong landmarks that failed to label the correct locations. Directional bias (systematic error) was identified due to cognitive differences between observers. By removing wrong labels and rotated pelves, the analysis quantified the error density as a “good doctor” performance and found 6.77°-11.76° maximum PT disagreement with 95% data points.

The landmarks with excellent reliability still have a chance (at least 6.09% in our case) of making wrong landmark decisions. Identifying skeletal contours is at least 24.64% more accurate than estimating landmark locations. The landmark at a clear skeletal contour is more likely to generate systematic errors. Due to landmark ambiguity, a very careful surgeon measuring PT could make a maximum 11.76° random difference in 95% of cases, serving as a “good doctor benchmark” to qualify good landmarking techniques.

The spinopelvic alignment is often assessed via the Pelvic Incidence-Lumbar Lordosis (PI-LL) mismatch. Here we describe and validate a simplified method to evaluating the spinopelvic alignment through the L1-Pelvis angle (L1P). This method is set to reduce the operator error and make the on-film measurement more practicable.

126 standing lateral radiographs of patients presenting for Total Hip Arthroplasty were examined. Three operators were recruited to label 6 landmarks. One operator repeated the landmark selection for intra-operator analysis. We compare PI-LL mismatch obtained via the conventional method, and our simplified method where we estimate this mismatch using PI-LL = L1P - 90°. We also assess the method's reliability and repeatability.

We found no significant difference (p > 0.05) between the PI-LL mismatch from the conventional method (mean 0.22° ± 13.6) compared to L1P method (mean 0.0° ± 13.1). The overall average normalised root mean square error (NRMSE) for PI-LL mismatch across all operators is 7.53% (mean −3.3° ± 6.0) and 6.5% (mean −2.9° ± 4.9) for the conventional and L1P method, respectively. In relation to intra-operator repeatability, the correlation coefficients are 0.87 for PI, 0.94 for LL, and 0.96 for L1P. NRMSE between the two measurement sets are PI: 9.96%, LL: 5.97%, and L1P: 4.41%. A similar trend is observed in the absolute error between the two sets of measurements.

Results indicate an equivalence in PI-LL measurement between the methods. Reproducibility of the measurements and reliability between operators were improved. Using the L1P angle, the classification of the sagittal spinal deformity found in the literature translates to: normal L1P<100°, mild 100°<L1P<110°, and severe L1P>110°. Surgeons adopting our method should expect a small improvement in reliability and repeatability of their measurements, and a significant improvement of the assessment of the mismatch through the visualisation of the angle L1P.

Pelvic tilt (PT) is always described as the pelvic orientation along the transverse axis, yet four PT definitions were established based on different radiographic landmarks: anterior pelvic plane (PT_a), the centres of femoral heads and sacral plate (PT_m), pelvic outlet (PT_h), and sacral slope (SS). These landmarks quantify a similar concept, yet understanding of their relationships is lacking. Some studies referred to the words “pelvic tilt” for horizontal comparisons, but their PT definitions might differ. There is a demand for understanding their correlations and differences for education and research purposes.

This study recruited 105 sagittal pelvic radiographs (68 males and 37 females) from a single clinic awaiting their hip surgeries. Hip hardware and spine pathologies were examined for sub-group analysis. Two observers annotated four PTs in a gender-dependent manner and repeated it after six months. The linear regression model and intraclass correlation coefficient (ICC) were applied with a 95% significance interval.

The SS showed significant gender differences and the lowest correlations to the other parameters in the male group (−0.3< r <0.2). The correlations of SS in scoliosis (n = 7) and hip implant (female, n = 18) groups were statistically different, yet the sample sizes were too small. PT_m demonstrated very strong correlation to PT_h (r > 0.9) under the linear model PT_m = 0.951 × PT_h - 68.284.

The PT_m and PT_h are interchangeable under a simple linear regression model, which enables study comparisons between them. In the male group, SS is more of a personalised spinal landmark independent of the pelvic anatomy. Female patients with hip implant may have more static spinopelvic relationships following a certain pattern, yet a deeper study using a larger dataset is required. The understanding of different PTs improves anatomical education.

Accurate measurement of pelvic tilt (PT) is critical in diagnosing hip and spine pathologies. Yet a sagittal pelvic radiograph with good quality is not always available. Studies explored the correlation between PT and sacro-femoral-pubic (SFP) angle from anteroposterior (AP) radiographs yet demonstrated conflicting conclusions about its feasibilities. This study aims to perform a cohort-controlled meta-analysis to examine the correlation between the SFP angle and PT and proposes an application range of the method.

This study searched PubMed, Embase, Cochrane, and Web of Science databases for studies that evaluated the correlation between SFP angle and PT. The Pearson's correlation coefficient r from studies were tabulated and compared. Pooled r for overall and gender/age (teenage or adult) controlled subgroup were reported using Fisher's Z transformation. Heterogeneity and publication bias were evaluated using Egger's regression test for the funnel plot asymmetry.

Eleven studies were recruited, with nine reported r (totalling 1,247 patients). The overall pooled r was 0.61 with high inter-study heterogeneity (I2 = 75.95%). Subgroup analysis showed that the adult group had a higher r than the teenage group (0.70 versus 0.56, p < 0.001). Although statistically insignificant (p = 0.062), the female group showed a higher r than the male group (0.72 versus 0.65).

The SFP method must be used with caution and should not be used in the male teenage group. The current studies did not demonstrate that the SFP method was superior to other AP landmarks correlating to PT. Identical heterogeneity was observed among studies, indicating that more ethnicity-segregated and gender-specific subgroup studies might be necessary. More data input analysing the errors will be useful.

Navigation in total hip arthroplasty has been shown to improve acetabular positioning and can decrease the incidence of mal-positioned acetabular components. The aim of this study was to assess two surgical guidance systems by comparing intra-operative measurements of acetabular component inclination and anteversion with a post-operative CT scan.

We prospectively collected intra-operative navigation data from 102 hips receiving conventional THA or hip resurfacing arthroplasty through either a direct anterior or posterior approach. Two guidance systems were used simultaneously: an inertial navigation system (INS) and optical navigation system (ONS). Acetabular component anteversion and inclination was measured on a post-operative CT.

The average age of the patients was 64 years (range: 24-92) and average BMI was 27 kg/m² (range 19-38). 52% had hip surgery through an anterior approach. 98% of the INS measurements and 88% of the ONS measurements were within 10° of the CT measurements. The mean (and standard deviation) of the absolute difference between the post-operative CT and the intra-operative measurements for inclination and anteversion were 3.0° (2.8) and 4.5° (3.2) respectively for the ONS, along with 2.1° (2.3) and 2.4° (2.1) respectively for the INS. There was significantly lower mean absolute difference to CT for the INS when compared to ONS in both anteversion (p<0.001) and inclination (p=0.02).

Both types of navigation produced reliable and reproducible acetabular cup positioning. It is important that patient-specific planning and navigation are used together to ensure that surgeons are targeting the optimal acetabular cup position. This assistance with cup positioning can provide benefits over free-hand techniques, especially in patients with an altered acetabular structure or extensive acetabular bone loss.

In conclusion, both ONS and INS allowed for adequate acetabular positioning as measured on postoperative CT, and thus provide reliable intraoperative feedback for optimal acetabular component placement.

Hip resurfacing arthroplasty (HRA) is a bone conserving alternative to total hip arthroplasty. We present the early 1 and 2-year clinical and radiographical follow-up of a novel ceramic-on-ceramic (CoC) HRA in a multi-centric Australian cohort.

Patient undergoing HRA between September 2018 and April 2021 were prospectively included. Patient-reported outcome measures (PROMS) in the form of the Forgotten Joint Score (FJS), HOOS Jr, WOMAC, Oxford Hip Score (OHS) and UCLA Activity Score were collected preoperatively and at 1- and 2-years post-operation. Serial radiographs were assessed for migration, component alignment, evidence of osteolysis/loosening and heterotopic ossification formation.

209 patients were identified of which 106 reached 2-year follow-up. Of these, 187 completed PROMS at 1 year and 90 at 2 years. There was significant improvement in HOOS (p< 0.001) and OHS (p< 0.001) between the pre-operative, 1-year and 2-years outcomes. Patients also reported improved pain (p<0.001), function (p<0.001) and reduced stiffness (p<0.001) as measured by the WOMAC score. Patients had improved activity scores on the UCLA Active Score (p<0.001) with 53% reporting return to impact activity at 2 years. FJS at 1 and 2-years were not significantly different (p=0.38). There was no migration, osteolysis or loosening of any of the implants. The mean acetabular cup inclination angle was 41.3° and the femoral component shaft angle was 137°. No fractures were reported over the 2-year follow-up with only 1 patient reporting a sciatic nerve palsy.

There was early return to impact activities in more than half our patients at 2 years with no early clinical or radiological complications related to the implant. Longer term follow-up with increased patient numbers are required to restore surgeon confidence in HRA and expand the use of this novel product.

In conclusion, CoC resurfacing at 2-years post-operation demonstrate promising results with satisfactory outcomes in all recorded PROMS.

Imageless computer navigation systems have the potential to improve acetabular cup position in total hip arthroplasty (THA), thereby reducing the risk of revision surgery. This study aimed to evaluate the accuracy of three alternate registration planes in the supine surgical position generated using imageless navigation for patients undergoing THA via the direct anterior approach (DAA).

Fifty-one participants who underwent a primary THA for osteoarthritis were assessed in the supine position using both optical and inertial sensor imageless navigation systems. Three registration planes were recorded: the anterior pelvic plane (APP) method, the anterior superior iliac spines (ASIS) functional method, and the Table Tilt (TT) functional method. Post-operative acetabular cup position was assessed using CT scans and converted to radiographic inclination and anteversion. Two repeated measures analysis of variance (ANOVA) and Bland-Altman plots were used to assess errors and agreement of the final cup position.

For inclination, the mean absolute error was lower using the TT functional method (2.4°±1.7°) than the ASIS functional method (2.8°±1.7°, ρ = .17), and the ASIS anatomic method (3.7°±2.1, ρ < .001). For anteversion, the mean absolute error was significantly lower for the TT functional method (2.4°±1.8°) than the ASIS functional method (3.9°±3.2°, ρ = .005), and the ASIS anatomic method (9.1°±6.2°, ρ < .001). All measurements were within ± 10° for the TT method, but not the ASIS functional or APP methods.

A functional registration plane is preferable to an anatomic reference plane to measure intra-operative acetabular cup inclination and anteversion accurately. Accuracy may be further improved by registering patient location using their position on the operating table rather than anatomic landmarks, particularly if a tighter target window of ± 5° is desired.

Imageless computer navigation systems in total hip arthroplasty (THA) improve acetabular cup position, thereby reducing the risk of revision surgery for all causes as well as dislocation. We aimed to evaluate the registration accuracy of 3 alternate registration planes.

A prospective, observational study was conducted with 45 THA in the supine position using two imageless navigation systems and 3 registration planes. Patient position was registered sequentially using an optical system (Stryker OrthoMap) and an inertial sensor-based system (Navbit Sprint) with 3 planes of reference: (Plane 1) an anatomical plane using the anterior superior iliac spines (ASISs) and the pubic symphysis; (Plane 2) a functional plane parallel to the line between the ASISs and the table plane; and, (Plane 3) a functional plane that was perpendicular to the gravity vector and aligned with the longitudinal axis of the patient. The 3 measurements of acetabular cup inclination and anteversion were compared with the measurements from postoperative computed tomography (CT) scans.

For inclination, the mean absolute error was significantly lower for Plane 3 (1.80°) than for Plane 2 (2.74°), p = .038 and was lower for both functional planes than for the anatomical plane (3.75°), p < .001.

For anteversion, the mean absolute error was significantly lower for Plane 3 (2.00°) than for Plane 2 (3.69°), p = .004 and was lower for both functional planes than for the anatomical plane (8.58°), p < .001.

Patient registration using functional planes more accurately measured the acetabular cup position than registration using anatomic planes.

Introduction

In total knee arthroplasty, the aim is to relieve pain and provide a stable, functional knee. Sagittal stability is crucial in enabling a patient to return to functional activities. Knee implants with a medial pivot (MP) design are thought to more accurately reproduce the mechanics of the native joint, and potentially confer greater antero-posterior stability through the range of flexion than some other implant designs.

Introduction

Component positioning is of great importance in total hip arthroplasty (THA) and navigation systems can help guide surgeons in the optimal placement of the implants. We report on a newly developed navigation system which employs an inertial measurement unit (IMU) to measure acetabular cup inclination and anteversion.

Background

Accurate implant positioning is of supreme importance in total knee replacement (TKR). The rotational profile of the femoral and tibial components can affect outcomes, and the aim is to achieve coronal conformity with parallelism between the medio-lateral axes of the femur and tibia.

Introduction

Kinematics post-TKA are complex; component alignment, component geometry and the patient specific musculoskeletal environment contribute towards the kinematic and kinetic outcomes of TKA. Tibial rotation in particular is largely uncontrolled during TKA and affects both tibiofemoral and patellofemoral kinematics. Given the complex nature of post- TKA kinematics, this study sought to characterize the contribution of tibial tray rotation to kinematic outcome variability across three separate knee geometries in a simulated framework.

We present a technique of single posterior longitudinal slot femorotomy. This technique allows the expansion of the metaphyseal-diaphyseal region of the proximal femur facilitating extraction of proximally coated uncemented femoral components while leaving the metaphysis and diaphysis intact. Since 1996 we have performed this technique in 18 revision total hip arthroplasties in 15 patients who had x-ray appearance of bony in-growth/on-growth and where found to have solidly ingrown stems at revision surgery. All were revised to a metaphyseal-fitting uncemented stem. At mean follow-up of 122.4 months, there were significant improvements in both pain and function. All revised stems achieved stable bony fixation. There were no complications due to this technique. No patient developed a limp or thigh pain post-operatively. There have been no re-revisions of the stem. With appropriate patient selection, this simple, reliable, and extensile technique is useful to assist in the extraction of uncemented proximally coated femoral components whether hydroxyapatite-coated or not.

Ceramic-on-ceramic bearings provide a solution to the osteolysis seen with traditional metal-on-polyethylene bearings. Sporadic reports of ceramic breakage and squeaking concern some surgeons and this bearing combination can show in vivo signs of edge loading wear which was not predicted from in vitro studies. Taper damage or debris in the taper between the ceramic and metal may lead to breakage of either a ceramic head or insert. Fastidious surgical technique may help to minimise the risk of ceramic breakage. Squeaking is usually a benign complication, most frequently occurring when the hip is fully flexed. Rarely, it can occur with each step of walking when it can be sufficiently troublesome to require revision surgery. The etiology of squeaking is multifactorial in origin. Taller, heavier and younger patients with higher activity levels are more prone to hips that squeak. Cup version and inclination are also relevant factors.

Osteolysis following metal-on-UHMW polyethylene Total Hip Arthroplasty (THA) is well reported. Earlier generation ceramic-on-ceramic bearings did produce some osteolysis, but in flawed implants. As third and now fourth generation ceramic THAs come into mid- and long-term service, the orthopaedic community has begun to see reports of high survival rates and very low incidence of osteolysis in these bearings. The technique used by radiologists for identifying the nature of lesions on Computed Tomography (CT) scan is the Hounsfield score which will identify the density of the tissue within the lucent area. Commonly the radiologist will have no access to previous imaging, especially pre-operative imaging if a long time has elapsed. With such a low incidence of osteolysis in this patient group, what, then, should a surgeon do on receiving a CT report on a ceramic-on-ceramic THA, which states there is osteolysis? This retrospective review aims to determine the accuracy of CT in identifying true osteolysis in a cohort of long-term third generation ceramic-on-ceramic uncemented hip arthroplasties in our department.

Pelvic CT scans were performed on the first 27 patients from a cohort of 301 patients undergoing 15-year review with third generation alumina-alumina cementless THAs. The average follow-up was 15 years (15–17). The CT scans were reviewed against pre-operative and post-operative radiographs and reviewed by a second musculoskeletal specialist radiologist.

Eleven of the CT scans were reported to show acetabular osteolysis, two reported osteolysis or a possible pre-existing cyst and one reported a definitive pre-existing cyst. After review of previous imaging including pre-operative radiographs, eleven of the thirteen patients initially reported to have osteolysis were found to have pre-existing cysts or geodes in the same size and position as the reported osteolysis, and a further patient had spot-welds with stress-shielding. One patient with evidence of true osteolysis awaits aspiration or biopsy to determine if he has evidence of ceramic wear or metallosis.

Reports of osteolysis on CT should be interpreted with care in modern ceramic-on-ceramic THA to prevent unnecessary revision. Further imaging and investigations may be necessary to exclude other conditions such as geodes, or stress shielding which are frequently confused with osteolysis on CT scans.

INTRODUCTION

Medial ball and socket knee designs have a long history but are not yet widely used. The Saiph medial ball and socket knee passed preclinical testing before an introductory cohort of 20 patients were studied in detail for 2 years. Subsequently a multicenter study was undertaken by the developing surgeons.

The pelvis is known to undergo significant movement during Total Hip Replacement (THR). We developed a 4D-tracking device employing an inertial measurement unit (IMU) to track changes in pelvic orientation during THR.

The IMU was mounted on the iliac crest in 39 cases with tracking initiated at the commencement of surgery and digital logging of significant intra-operative milestones (i.e. acetabular impaction). The system was validated by videoing a select number of cases and the 4D model linked in real-time. Data were processed using a custom Java-based infrastructure to calculate roll (left/right) and tilt (flexion/extension).

19 patients underwent direct anterior approach (DAA) and 20 posterior approach (PA). Comparing DAA to PA, at acetabular impaction there was mean pelvic roll seen of 3.7°(range 0.5–10.1°) in the DAA group, and 5.6°(range 0.1–16.2°) in the PA group. Mean tilt in the DAA group was 3.7°(range: 0.2–7.1°) and in the PA group was 1.7°(range: 0.2–4.3°).

Mean BMI in the DAA group was 25.2(range: 18.4–34.2) and 29.1(range: 21.5–42.4). There was no direct correlation between BMI and the amount of roll or tilt recorded for individual patients.

The IMU tracking device provided a useful and real-time method of assessing pelvic orientation during THR via both the DAA and posterior approach. Specific variations in tilt and roll are consistent with previous literature. Significant variation in the pattern of pelvic movement was noted to be dependent on the approach and the position of the patient on the operating table.

Background

Since the development of modern total hip replacement (THR) more than 50 years ago, thousands of devices have been developed in attempt to improve patient outcomes and prolong implant survival. Modern THR devices are often broadly classified according to their method of fixation; cemented, uncemented or hybrid (typically an uncemented acetabular component with a cemented stem). Due to early failures of THR in young active patients, the concept of hip resurfacing was revisited in the 1990's and numerous prostheses were developed to serve this patient cohort, some with excellent clinical results. Experience with metal-on-metal (MoM) bearing related issues particularly involving the ASR (DePuy Synthes, Warsaw, Indiana) precipitated a fall in the use of hip resurfacing (HR) prostheses in Australia from a peak of 30.2% in 2004 to 4.3% in 2015. The effects of poorly performing prostheses and what is now recognised as suboptimal patient selection are reflected in the AOANJRR cumulative percent revision (CPR) data which demonstrates 13.2% revision at 15 years for all resurfacing hip replacements combined; with 11 different types of hip resurfacing prostheses recorded for patients less than 55 years of age and a primary diagnosis of OA. When this data is restricted to only those prostheses currently used in Australia (BHR; Smith and Nephew, Birmingham, UK & ADEPT; MatOrtho Ltd, Surrey, UK) there is a CPR of 9.5% at 15 years for all patients. Despite these CPR results, recognition is emerging of the important distinction between MoM THR and resurfacing.

Furthermore, in light of current consensus for patient selection and the surgical indications for resurfacing, a gender analysis demonstrates a CPR for females of 14.5% at 10 years compared to 3.7% for males. Similar difference for head size >50mm with 6% CPR at 10 years compared to 17.6% for head size <50mm (HR=2.15; 1.76, 2.63; p<0.001). Leading to renewed interest in resurfacing particularly in the young, active male. In addition to registry based CPR data, several studies have concluded that a true difference in mortality rates between HR and other forms of THR exists independent of age, sex or other confounding factors. We hypothesised that a difference in adjusted mortality rates between HR and other forms of THR may also be present in the Australian population.

We undertook an ad hoc data report request to the AOANJRR. The data set provided was deidentified for patient, surgeon and institution and included all HR and conventional THR procedures performed for the diagnosis of primary osteoarthritis recorded in the Registry since inception in 1999. We requested mortality and yearly cumulative percent survival (CPS) of patients for primary HR and THR with sub-group analysis by the mode of fixation.

There were 12,910 hip resurfacings (79% male) compared to 234,484 conventional THR (46.8% male) over the study period. When adjusted for age and gender over the 15 years of available data, there was a statistically significant difference in cumulative percent survival (CPS) between conventional THR and hip resurfacing (HR 1.66 (1.52, 1.82; p<0.001)) and between cemented THR and hip resurfacing (HR 1.96 (1.78, 2.43; p<0.001)); between uncemented THR and hip resurfacing (HR 1.58 (1.45, 1.73; p<0.001)); and between hybrid THR and hip resurfacing (HR 1.82 (1.66, 1.99; p<0.001)). When adjusted for age, gender and ASA over the 3 years data available, there was no statistically significant difference in CPS between hip resurfacing and any individual fixation type of THR.

Ceramic-on-ceramic bearings provide a solution to the osteolysis seen with traditional metal-on-polyethylene bearings. Sporadic reports of ceramic breakage and squeaking concern some surgeons and this bearing combination can show in vivo signs of edge loading wear which was not predicted from in vitro studies. Taper damage or debris in the taper between the ceramic and metal may lead to breakage of either a ceramic head or insert. Fastidious surgical technique may help to minimise the risk of ceramic breakage. Squeaking is usually a benign complication, most frequently occurring when the hip is fully flexed. Rarely, it can occur with each step of walking when it can be sufficiently troublesome to require revision surgery. The etiology of squeaking is multifactorial origin. Taller, heavier and younger patients with higher activity levels are more prone to hips that squeak. Cup version and inclination are also relevant factors.

Fifty-five ceramic bearings revised at our center were collected over 12 years. Median time to revision was 2.7 years. Forty-six (84%) cases had edge loading wear. The median femoral head wear volume overall was 0.2mm³/yr, for anterosuperior edge loading was 2.0mm³/yr, and the median volumetric wear rate for posterior edge loading was 0.15mm³/yr (p=0.005).

Osteolysis following metal-on-polyethylene total hip arthroplasty (THA) is well reported. Earlier generation ceramic-on-ceramic bearings did produce some osteolysis, but in flawed implants. As 3rd and now 4th generation ceramic THAs come into mid- and long-term service, the orthopaedic community has begun to see reports of high survival rates and very low incidence of osteolysis in these bearings. The technique used by radiologists for identifying the nature of lesions on Computed Tomography (CT) scan is the Hounsfield score which will identify the density of the tissue within the lucent area. Commonly the radiologist will have no access to previous imaging, especially pre-operative imaging if a long time has elapsed. With such a low incidence of osteolysis in this patient group, what, then, should a surgeon do on receiving a CT report on a ceramic-on-ceramic THA, which states there is osteolysis? This retrospective review aims to determine the accuracy of CT in identifying true osteolysis in a cohort of long-term 3rd generation ceramic-on-ceramic uncemented hip arthroplasties in our department.

Introduction

The continued improvement of ceramic materials for total hip arthroplasty led to the development of Zirconia and Zirconia toughened Alumina materials such as BIOLOX® delta. Zirconia exists in a tetragonal phase in new ceramic heads and can transform to a monoclinic phase in response to loading giving the material improved fracture toughness. It is known that surface transformation occurs in this material under hydrothermal conditions (i.e. in vivo condition), and ISO standards recommend parts are artificially aged prior to testing to include any effect of surface transformation on new designs. Accelerated aging procedures have been used to predict the amount of phase transformation that will occur in vivo, but validation of these models requires the study of retrieved hip joints. Here 26 BIOLOX® delta retrievals are analysed to determine the degree of phase transformation that occurs in vivo. The levels of phase transformation measured are compared with those predicted based on accelerated aging tests.

Introduction

Dislocation is one of the leading causes of revision after primary total hip arthroplasty (THA). Polyethylene wear is one of the risk factors for late dislocations (>2 years). It can induce an inflammatory response resulting in distension and thinning of the pseudocapsule, predisposing the hip to dislocation. Alternatively, eccentric seating of the femoral head in a worn out socket may result in an asymmetric excursion arc predisposing the hip to impingement, levering out and dislocation. Highly cross linked polyethylene has a significantly lower wear rate as compared to conventional polyethylene. Incidence of late dislocations has been shown to be significantly greater with conventional polyethylene bearings as compared to ceramic bearings. However, there is no literature comparing the risk of dislocation between ceramic- on- ceramic (CoC) bearings with metal/ceramic- on- cross linked polyethylene (M/CoP) bearings and this was the aim our study