Introduction

The pelvis is not a static structure. It rotates in the sagittal plane depending upon the activity being performed. These dynamic changes in pelvic tilt have a substantial effect on the functional orientation of the acetabulum. The aim of this study was to quantify the changes in sagittal pelvic position between three functional postures.

Hip arthroplasty surgeons have various bearing choices to make on behalf of their patients. We make those choices based on our knowledge of pre-clinical wear testing data and the outcome of clinical and radiological follow-up studies. The initial use of conventional polyethylene revealed limitations in its use in younger patients. Modern highly crosslinked polyethylene is a vastly improved bearing surface that means less wear and its consequences. Despite this, registry data still suggests that loosening, lysis and dislocation are problematic causes of implant failure. The functional success of hip replacement surgery, the ageing population and younger patients requesting arthroplasty means we should predict ongoing issues consequent to wear related events even with the newer polyethylenes.

Ceramic-on-ceramic bearings surfaces have a long history of successful clinical use. The benefits of ceramic bearings are its superior wear characteristics, the minimal biological response to the ceramic wear products and the ability of ceramics to be offered in larger head sizes. Its limitations have been reports of fracture and squeaking.

Fourth generation ceramic articulations have reduced the fracture incidence. Squeaking has been reported to occur in 3% to 20% in different series but revision for squeaking is extremely, low suggesting it is not a significant clinical problem. Edge loading occurs in most hip articulations and is thought to be the primary mechanism in the squeaking event. Modern methodologies of “functional” implant orientation may reduce the incidence of squeaking.

While wear and its consequences remain significant issues in hip arthroplasty, the future will require a bearing with reduced wear and biologically inert wear products. This bearing exists already. “The future is now”.

Introduction

The primary purpose of Total Hip Arthroplasty (THA), aside from pain relief, is to restore hip biomechanics such that the patient experiences no discernible functional deficit, while also providing an environment conducive to implant longevity. Key factors in determining a successful THA include achieving the desired pre-operative femoral offset and leg length, as well as the restoration of range of motion (ROM). Minor leg length discrepancies (LLDs), less than a centimetre, are common after THA and usually well tolerated. However, in some patients, even these small discrepancies are a source of dissatisfaction. More significant discrepancies can be a risk factor for more serious concerns such as nerve injury, abnormal gait and chronic pain. The level of the femoral neck osteotomy is a critical step in reproducing a planned femoral stem position. Frequently the femoral osteotomy is too high and can lead to an increase in leg length and varus stem positioning. If the desired implant positions are identified from preoperative 3D templating, a planned femoral osteotomy can be used as a reference to recreate the correct leg length and offset. The aim of this study was assess the accuracy of a 3D printed patient-specific guide for delivering a pre-planned femoral neck osteotomy.

Introduction

Acetabular cup orientation has been shown to be a factor in edge-loading of a ceramic-on-ceramic THR bearing. Currently all recommended guidelines for cup orientation are defined from static measurements with the patient positioned supine. The objectives of this study are to investigate functional cup orientation and the incidence of edge-loading in ceramic hips using commercially available, dynamic musculoskeletal modelling software that simulates each patient performing activities associated with edge-loading.

Introduction

The pelvis is not a static structure. It rotates in the sagittal plane depending upon the activity being performed. These dynamic changes in pelvic tilt have a substantial effect on the functional orientation of the acetabulum. The aim of this study was to quantify the changes in sagittal pelvic position between three functional postures.

Sagittal stability of the knee is believed to be of significant importance following total knee arthroplasty. We examine four different knee designs at a minimum of twenty-four months postoperatively. Sagittal stability was measured at four degrees of flexion: 0°; 30°; 60°; and 90°, to examine the effect of design on mid-flexion stability.

The knee designs included were: the rotating platform LCS design (DePuy); the cruciate sparing Triathlon system (Stryker); SAIPH system (Matortho, UK); and the medial rotating knee design, MRK (Matortho, UK).

Following ethical approval, 64 cases were enrolled into the study, 22 male and 42 female. Inclusion criteria included: a minimum of 18 months from surgery; ability to flex beyond 90 degrees; and have no postoperative complications. 18 LCS, 18 MRK, 14 SAIPH and 14 Triathlon knee designs were analysed. Sagittal stability was measured using the KT1000 device. Active range of movement was measured using a hand held goniometer and recorded as was Oxford knee score, WOMAC knee score, SF12 and Kujala patellofemoral knee score.

Mean follow-up was 33.7 months postoperative, with a mean age of 72 years. Mean weight was 82.7kgs and height 164cms. There was no significant difference in preoperative demographics between the groups. Mean active post-operative range of motion of the knee was from 2–113° with no significant difference between groups.

Sagittal stability was similar in all four groups in full extension; however the MRK and SAIPH designs showed improved stability in the mid-range of flexion (30–90°). Patient satisfaction also showed a similar trend with MRK achieving better patient reported functional outcomes and satisfaction than that of the SAIPH, LCS and Triathlon systems.

All four knee designs demonstrated good post-operative range of movement with comparative improvement of patient scores to other reported studies. The MRK and SAIPH knee design showed an improved mid-flexion sagittal stability with better patient reported satisfaction and functional scores.

Malorientation of the acetabular cup in Total Hip replacement (THR) may contribute to premature failure of the joint through instability (impingement, subluxation or dislocation), runaway wear in metal-metal bearings when the edge of the contact patch encroaches on the edge of the bearing surface, squeaking of ceramic-ceramic bearings and excess wear of polyethylene bearing surfaces leading to osteolysis.

However as component malorientation often only occurs in functional positions it has been difficult to demonstrate and often is unremarkable on standard (usually supine) pelvic radiographs. The effects of spinal pathology as well as hip pathology can cause large rotations of the pelvis in the sagittal plane, again usually not recognized on standard pelvic views. While Posterior pelvic rotation with sitting increases the functional arc of the hip and is protective of a THR in regards to both edge loading and risk of dislocation, conversely Anterior rotation with sitting is potentially hazardous.

We developed a protocol using three functional positions – standing, supine and flexed seated (posture at “seat-off” from a standard chair). Lateral radiographs were used to define the pelvic tilt in the standing and flexed seated positions. Pelvic tilt was defined as the angle between a vertical reference line and the anterior pelvic plane. Supine pelvic tilt was measured from computed tomography.

Proprietary software (Optimized Ortho, Sydney) based on Rigid Body Dynamics then modelled the patients’ dynamics through their functional range producing a patient-specific simulation which also calculates the magnitude and direction of the dynamic force at the hip and traces the contact area between prosthetic head/liner onto a polar plot of the articulating surface. Given prosthesis specific information edge-loading can then be predicted based on the measured distance of the edge of the contact patch to the edge of the acetabular bearing.

Achieving optimal acetabular cup orientation in Total Hip Replacement (THR) remains one of the most difficult challenges in THR surgery (AAOR 2013) but very little has been added to useful understanding since Lewinnek published recommendations in 1978. This is largely due to difficulties of analysis in functional positions. The pelvis is not a static reference but rotates especially in the sagittal plane depending upon the activity being performed. These dynamic changes in pelvic rotation have a substantial effect on the functional orientation of the acetabulum, not appreciated on standard radiographs [Fig1].

Studies of groups of individuals have found the mean pelvic rotation in the sagittal plane is small but large individual variations commonly occur. Posterior rotation, with sitting, increases the functional arc of the hip and is protective of a THR in regards to both edge loading and risk of dislocation. Conversely Anterior rotation, with sitting, is potentially hazardous.

We developed a protocol using three functional positions – standing, supine and flexed seated (posture at “seat-off” from a standard chair). Lateral radiographs were used to define the pelvic tilt in the standing and flexed seated positions. Pelvic tilt was defined as the angle between a vertical reference line and the anterior pelvic plane (defined by the line joining both anterior superior iliac spines and the pubic symphysis). In the supine position pelvic tilt was defined as the angle between a horizontal reference line and the anterior pelvic plane. Supine pelvic tilt was measured from computed tomography.

Proprietary software (Optimized Ortho, Sydney) based on Rigid Body Dynamics then modelled the patients’ dynamics through their functional range producing a patient-specific simulation which also calculates the magnitude and direction of the dynamic force at the hip and traces the contact area between prosthetic head/liner onto a polar plot of the articulating surface, Fig 2. Given prosthesis specific information edge-loading can then be predicted based on the measured distance of the contact patch to the edge of the acetabular liner.

Delivery of desired orientation at surgery is facilitated by use of a solid 3D printed model of the acetabulum along with a patient specific guide which fits the model and the intra-operative acetabulum (with cartilage but not osteophytes removed) - an incorporated laser pointer then marks a reference point for the reamer and cup inserter to replicate the chosen orientation.

Introduction

The SAIPH™ (MatOrtho, UK) total knee replacement is a new fixed-bearing prosthesis design having attributes of a mobile bearing and the posterior stabilised categories for knee arthroplasties. The implant design goal is an articulation that provides definitive anteroposterior stability to beneficially control tibiofemoral translation, the ability for the tibia to axially rotate to accommodate various lifestyle activities, and to maintain a relatively posterior femoral position on the tibia to facilitate range of motion. This study aims to analyze knee kinematics of the SAIPH™ total knee arthroplasty (TKA) by videofluroscopy during four different weightbearing activities.

The need to demonstrate probity and fair market competition has increased scrutiny of the relationships between orthopaedic surgeons and the industry that supplies them with their tools and devices. Investigations and judgements from the US Department of Justice and the introduction of the AdvaMed and Eucomed codes have defined new boundaries for interactions between these groups. This article summarises the current interplay between orthopaedic surgeons and industry, and provides recommendations for the future.

Reasons for failure of hip resurfacing arthroplasty include femoral neck fracture, loosening, femoral head osteonecrosis, metal sensitivity or toxicity and component malpositioning.

Patient factors that influence the outcome include prior surgery, body mass index, age and gender, with female patients having two and a half times greater risk of revision by 5 years than males 1–4. In 2008, the Australian National Joint Replacement Registry (ANJRR) reported poorer results with small sizes, whereby component sizes 44mm or less have a five times greater risk of revision than those 55mm or greater 1. This finding is true for both males and females and after accounting for femoral head size, the effect of gender is eliminated.

We explore the relationship between component size and the factors that may influence the survivorship of this procedure, resulting in higher revision rates with smaller components.

These include femoral neck loading, edge loading, wear debris production and the effects of metal ions, cement penetration, component orientation, and femoral head vascularity. In particular the way the components are scaled from the large sizes down to the smaller sizes results in some marked changes in interactions between the implant and the patient.

Wall thickness of the acetabular and femoral component does not change between the large and small sizes in most devices. This results in a relative excessively thick component in the small sizes. This may cause more acetabular and femoral bone loss, increased risk of femoral neck notching and relative undersizing of the component where acetabular bone is a limiting factor. Stem thickness does not change throughout the size range in many of the devices leading to relatively more femoral bone loss and a greater stiffness mismatch between the femoral stem and the bone. Relatively stiffness between the femoral stem and the bone is up to six times greater in the small size compared to the large size in some designs.

The angle subtended by the articular surface (the articular arc) ranges from 170° down to as low as 144° in the small sizes of some devices. A smaller articular arc increases the risk of edge loading, especially if there is any acetabular component malpositioning. Acetabular inclination has been related to metal ion levels 5 and to the early development of pseudotumour6.

An acetabular component with a radiographic inclination of 45° will have an effective inclination anywhere from 50° to 64° depending on the type and size of the component. This corresponds to a centre-edge angle from 40° down to 26°. The effective anteversion is similarly influenced by design.

The result of a smaller articular arc is to reduce the size of the ‘safe window’ which is the target for orthopaedic surgeons.

Joint Registries are a valuable resource for defining the survivorship of prostheses and procedures undertaken for the treatment of joint disease. However, the use of this data as a basis for advocating specific implant designs is controversial because of the confounding effects of variations in patient selection, the training, skill and experience of surgeons, and the priorities of individual patients. Despite these challenges, the Australian Joint Registry has utilized its early survivorship data to identify specific designs that are expected to exhibit lower than average durability in the long term. The aim of this study was to assess the accuracy of this practice in identifying implants providing inferior long-term performance.

Over the period 2004–8, the Australian Registry identified 48 prosthetic components used in primary THA, HRA, TKA or UKA which exhibited a statistically significant increase in the early revision rate. For each of these components, we compared the rate of revisions per 100 “component-years” when it was first identified by the Registry, to its ultimate fiveyear cumulative survival in 2008. These survival parameters were also compared to average values based on procedure (eg.THR) and fixation method (i.e. cemented, cementless, hybrid).

Regression analysis was performed to determine the accuracy of initial relative revisions per 100 OCY as a predictive measure of eventual component revision rate.

Five year survival data was available on 30 of the 48 implants identified by the registry. There was a strong correlation (R2=0.9614) between initial revisions per 100 component-years and the 5-yr survival of the identified designs. 29 of 30 designs (97%) exhibited lower than average survivorship at 5 years. Six designs (20%) had failure rates within 2% of average values, and 7 (23%) had a 5–year failure rate less than 50% above average values. Although, when identified by the Registry, 80% of identified components exceeded the average rate of revision by 100%, only 60% displayed more than twice the cumulative revision rate at 5 years post-op.

These results demonstrate that early data collected by Joint Registries can form the basis of accurate identification of designs which ultimately prove to be clinically unsuccessful. Predictions made by the Australian Registry concerning inferior designs have an accuracy of approximately 80%. Further work is recommended to enhance the valuable potential of Registry data in predicting the outcome of both implants and procedures.

Introduction: Although Hip Resurfacing Arthroplasty (HRA) has become a popular alternative to THR, the outcome of these procedures varies extensively between centres. This has been attributed to variations in patient selection, surgical experience, and patient volume. In this study we examine the effect of hospital volume on the outcome of hip resurfacing using a national database.

Methods: We examined data collected by the Australian Joint Registry between September 1999 and December 2006 relating to 8945 hip resurfacing procedures performed in 196 hospitals. Survivorship of the implanted components was calculated with revision as the end-point. The cumulative rate of revision at 4 years was compared between hospitals as a function of the number of cases performed during the study period (< 25, 25–49, 50–100, > 100 procedures). Using the log-rank test, differences in the risk of revision, corrected for age and sex of patients, were compared for low (< 25 cases) vs. higher volume centres (> 25 cases). We also estimated the number of cases/year of each centre and examined its apparent impact on revision rate.

Results: The majority (74%) of hospitals reporting performed less than 30 resurfacing procedures over the 7 year study period, with 64% of procedures performed at 16 “high volume” hospitals (> 100 cases), Overall, 249 of the 8945 resurfacing procedures (2.9%) were performed for revision of the original components. At 4 years, the cumulative revision rate dropped from 5.8% for hospitals performing less than 50 cases to 4.7% (50–99 cases) and 2.7% (> 100 cases) for larger volume centres. When adjusted for differences in patient age and sex, the risk of revision was 66% higher in hospitals performing < 25 cases. Based on the available data, the gap in revision rate between high and low volume centres is reduced by 50% once a surgeon’s operative volume exceeds 6 cases per year. On average, this corresponds to a learning curve of approximately 5 cases.

Conclusions: In this study, hospital volume is primarily a reflection of the operative experience of individual surgeons. Our results show that the outcome of hip resurfacing is strongly dependent on the experience of the surgeon and hospital performing the procedure. Even when adjusted for age and sex of the patients, the risk of revision increased by 66% when cases were performed at low volume centres. This supports the need for increased training of surgeons before undertaking hip resurfacing.

Introduction: Over the past decade metal-on-metal bearings in the form of hip resurfacings have been increasing in popularity and with it, an associated interest in the potential side effects of elevated serum metal ion levels.

Method: We prospectively measured the cobalt and chromium serum levels of 20 patients over a two-year period following Birmingham hip resurfacing. Cobalt was measured with inductively coupled plasma mass spectrometry and chromium using graphite furnace atomic absorption spectrometry.

Results: For serum cobalt there was an initial increase, to a peak at six months then a gradual decline. A similar pattern was observed for chromium, although the peak occurred slightly later at nine months.

Conclusion: One-off single measurements of metallic ion levels is of minimal clinical use, when the actual pattern of ion elevation and fall is not known. This study allows us to expect a peak at certain times following a hip resurfacing and a gradual decline thereafter.

Introduction and Aims: Hip resurfacing has undergone a resurgence of interest in the past five years, requiring surgeons to learn new principles and new operative techniques. For experienced surgeons, the learning curve is more transparent than in their earlier careers.

Method: We have reviewed the first 100 hip resurfacings performed by two experienced surgeons. Results of the first 20 and second 20 were compared for a difference, then the first 30 and second 30 and finally the first 50 and second 50. We evaluated accuracy of pre-operative planning to final sizing, pre-operative neck shaft angle and post-operative prosthesis angle, revision rates, complication rates, equipment problems, placement of the acetabular component.

Results: On comparing the first 50 procedures performed with the second 50 performed, there was a significant difference (p< 0.001) in positioning of the femoral prosthesis, notching of the femoral neck, seating of the femoral and acetabular component position and seating. When comparing the first 20 and second 20 procedures no significant difference was noted.

Conclusion: We note there is a definite learning curve associated with the hip resurfacing procedure and it was longer than we estimated.

Introduction: Hip resurfacing has undergone a resurgence of interest in the past five years. With it, the awareness of femoral neck fractures associated with this prosthesis has grown. We report the first national review of femoral neck fractures over a four-year period.

Method: All surgeons using the Birmingham hip resurfacing in Australia were contacted and asked to supply details of any femoral neck fractures they had experienced. Routine co-morbidities and demographics were collated.

Results: Fifty fractures have occurred in the time period. The fracture rate was twice as high in women than men and this was statistically significant (p< 0.001). Mean time to fracture was 15 weeks and was slightly longer in women than men. Technical difficulties included notching of the femoral neck, varus placement of the femoral prosthesis, poor exposure due to obesity and poor impaction of the femoral component.

Conclusions: The national fracture rate associated with the Birmingham hip resurfacing is 1.46%.

Introduction: Over the past decade metal on metal bearings in the form of hip resurfacings have been increasing in popularity and with it an associated interest in the potential side effets of elevated serum metal ion levels.

Methods: We prospectively measured the cobalt and chromium serum levels of 20 patients over a 2 year period following Birmingham hip resurfacing. Cobalt was measured with inductively coupled plasma mass spectrometry and chromium using graphite furnace atomic absorption spectrometry.

Results: For serum cobalt there was an initial increase to a peak at 6 months then a gradual decline. A similar pattern was observed for chromium, although the peak occurred slightly later at 9 months.

Conclusions: One off single measurements of metallic ion levels are of minimal clinical use, when the actual pattern of ion elevation and fall is not known. This study allows us to expect a peak at certain times following a hip resurfacing and a gradual decline thereafter.

220 consecutive hip resurfacing procedures were reviewed at a minimum of two years follow up to assess the incidence of heterotopic ossification and its effect on function and clinical outcome. We also reviewed the pre-operative diagnosis, gender and previous surgery. The overall percentage of heterotopic ossification was 58.63%. The incidence of Brooker 1 was 37.27%, Brooker 2 was 13.18% and Brooker 3 was 8.18%.

Male osteoarthritics had the highest incidence of heterotopic bone formation. Three males underwent excision of heterotopic bone, two for pain and stiffness and one for decreased range of movement.

Both antero-posterior and lateral radiographs were reviewed for evidence of heterotopic bone formation. 12.7% had no evidence of heterotopic bone formation on one view but clearly had on the second view.

Overall we found no evidence that heterotopic bone formation affected the clinical or functional outcome of the hip resurfacing at a mean of 3 years follow up.

We describe our early experience with the implantation of the first consecutive 231 primary Birmingham Hip resurfacings. At a mean follow up of 33 months, survivorship was 99.14 %, with revision in one patient for a loose acetabular component and one unrelated death.

Mean Harris Hip score improved from a mean of 62.54 ( Range : 8–92) to 97.74. (Range: 61 – 100) Mean flexion improved from a mean of 91.52°, ( Range : 25° –140°) to a mean 110.41°. ( Range : 80° – 145°)

1 patient presented at 6 weeks post resurfacing with pain and no history of trauma. An undisplaced fracture of the superior femoral neck was seen, which healed with a period of non-weight bearing.

96.94% of patients rated their prosthesis as good / excellent, the remainder rated it good/fair.

Introduction: Hip resurfacing has undergone a resurgence of interest in the past 5 years. This has required the acquistion of new surgical principles and techniques. Allowing us to study the learning curve associated with acquiring these skills.

Methods: The first 50 hip resurfacings and the second hip resurfacings performed by two consultant surgeons were compared for femoral fit, pre-operative femoral neck shaft angle, post-operative prosthesis angle, acetabular cup abduction angle, seating of the acetabular component, post-operative complications and revision rates.

Results: There was a statistically significant improvement in the alignment of acetabular components, seating of the acetbaular component and fewer problems associated with the acetabular introducer in the second cohort. There was more consistent placement of the femoral prosthesis in the neck,less notching and less variation in position in the second cohort.

Conclusions: There is a definite leaning curve associated with hip resurfacing and it was longer than expected.