The functional pelvic tilt when standing and sitting forward of 7402 cases on the OPS, Optimized Ortho, Australia Data Base were reviewed. All patients had undergone lateral radiographs when standing simulating extension of the hip, and sitting forward when the hip is near full flexion. Pelvic tilt was measured as the angle of the Anterior Pelvic Plane to the vertical Sagittal Plane, rotation anteriorly being given a positive value. Pelvises that had rotated more than 13 degrees anteriorly (+ve) when sitting forward or posteriorly (-ve) when standing were considered to place the hip at increased risk of dislocation or edge loading when flexed or extending respectively. This degree of rotation has the effect of changing the acetabular version by approximately10⁰. Most safe zones that have been described have given a range of anteversion of 20⁰ as safe. A change of 10⁰ would potentially place the acetabular orientation outside this range. Further, clinical studies have supported this concept. All lateral radiographs were reviewed to confirm that 281 had undergone instrumented spinal fusion at some level between T12 and S1. There was a large variability in the number and the levels arthrodesed. The range of pelvic mobility in the non-arthrodesed group in extension was −37⁰ to 31⁰ (mean −0.9⁰, Standard deviation 7.49) and in flexed position was −70⁰ to 49⁰ (mean −1.9⁰, Standard deviation 14.01). For the group with any fusion the range of pelvic tilt in extension was −31⁰ to 22⁰ (mean −4⁰, Standard deviation 8.21) and flexed −32⁰ to 46⁰ (mean 4.4⁰, Standard deviation 13.79). Of the 7121 cases without instrumented fusion, 15.5% were considered to be at risk when in flexion and 6.1% when extended. The risk for those with any fusion was approximately doubled in both flexion and extension. Further, those with extensive arthrodesis from T12 to S1 had a range of pelvic tilts similar to the non-fused group, although they had a significantly higher percentage of cases in the ‘at risk’ zones. The proportion of the cases in the ‘at risk’ zones decreased progressively as the arthrodesed levels moved from L5/S1 to the upper lumbar spine, and with decreasing number of levels fused.

Introduction

Appropriate femoral stem anteversion is an important factor in maintaining stability and maximizing the performance of the bearing after total hip replacement (THR). The anteversion of the native femoral neck has been shown to have a significant effect on the final anteversion of the stem, particularly with a uncemented femoral component. The aim of this study was to quantify the variation in native femoral neck anteversion in a population of patients requiring total hip replacement.

Introduction

The pelvis moves in the sagittal plane during functional activity. These movements can have a detrimental effect on functional cup orientation. The authors previously reported that 17% of total hip replacement (THR) patients have excessive pelvic rotation preoperatively. This increased pelvic rotation could be a risk factor for instability and edge-loading in both flexion and/or extension. The aim of this study was to investigate how gender, age and lumbar spine stiffness affects the number of patients at risk of excessive sagittal pelvic rotation.

The pelvis moves in the sagittal plane during functional activity. This can be detrimental to functional cup orientation. Increased pelvic mobility could be a risk factor for instability and edge-loading, in both flexion and/or extension. The aim of this study was to investigate how gender, age and lumbar spine stiffness, affects the number of patients at risk of excessive sagittal pelvic mobility.

Pre-operatively, 3428 patients had their pelvic tilt and lumbar lordotic angle (LLA) measured in three positions; supine, standing and flexed-seated. The pelvic rotation from supine-to-standing and from supine-to-seated was determined from the difference in pelvic tilt measurements between positions. Lumbar flexion was determined as the difference between LLA standing and LLA when flexed-seated. Patients were stratified into groups based upon age, gender, and lumbar flexion. The percentage of patients in each group with “at risk” pelvic rotation, defined by rotation ≥13° in a detrimental direction, was determined.

There was an increased incidence of “at risk” pelvic mobility with increasing age, and decreasing lumbar flexion. This was more pronounced in females. Notably, 31% of elderly females had “at risk” pelvic mobility. Furthermore, 38% of patients with lumbar flexion <20° had “at risk” pelvic mobility.

“At risk” pelvic mobility was more common in older patients and in patients with limited lumbar flexion. Additional stability, such as a dual mobility articulation, might be advisable in patient cohort. However, the majority of patients exhibiting “at risk” pelvic mobility were not older than 75, and did not have lumbar flexion <20°. This supports analysis of pelvic mobility on all patients undergoing THR.

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.

Introduction

There is increasing interest in the functional positions the pelvis assumes with activities of daily living and its effect on acetabular cup orientation. A number of systems are commercially available to assess these movements, and attempt to position the acetabular component of a total hip replacement in a patient specific safe zone.

However, these functional positions are assessed pre operatively when the patient still has the arthritis which may affect the range of movement of the hips, and thus affect the functional position of the pelvis. Obviously the planned acetabular position must take into account any changes in the functional movement of the pelvis as a result of the surgery.

Ishida et al showed that a pelvis with more than a 10° anterior tilt when standing can be expected to correct towards neutral by 12 months post-surgery. However many of Ishida's cases were dysplastic. Hip arthritis in the Caucasian population is far less likely due to dysplasia and this may affect these pelvic tilt changes post-operatively.

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.

Dislocation after Total Hip Replacement (THR) remains the second most common reason for revision in the Australian Orthopaedic Association National Joint Replacement Registry (AOA NJRR) and is the most common reason out to 3 years post operatively. There are many causes and associations of dislocation, including patient behaviour and (often unrecognized) spinal pathology leading to adverse component orientation. Femoral ball head size along with the head:neck ratio and the head:cup ratio are all important. Data from the AOA NJRR demonstrates a lower revision rate for dislocation with larger head sizes in all bearing surface combinations.

Data from the AOA NJRR confirms that the revision rate for replaced hips using non cross-linked polyethylene cups increases along with the head size, but this is not seen with cross-linked polyethylene cups.

THR using cross-linked polyethylene has a lower revision rate than THR using non cross-linked polyethylene, this difference is evident after only 3 months and the difference increases with time. The 12 year Cumulative Revision Rate (CRR) is 5.3% compared to 10.1%.

This lower rate of revision is due to a reduced revision rate for both dislocation and loosening/lysis. The revision rate for dislocation at 1 year was 0.4% for THR with cross-linked polyethylene and 0.7% with non cross-linked polyethylene.

Head sizes of 32mm and greater were used in 56.5% of THR with cross-linked polyethylene but only 12.7% of those with non cross-linked polyethylene. There was no difference in the revision rate for dislocation when head sizes of 32mm and less were compared, the difference was due to the higher proportion of larger head sizes used with cross-linked polyethylene.

However there are reasons why the benefits of larger femoral ball heads may not increase with increasing head size, this is chiefly because of altered cup subtended angles (and femoral head offset) geometries incorporated into cup and liner designs, especially the ceramic on ceramic bearings.

Larger head sizes may also increase the risk of taper disease, especially with smaller tapers and softer metal alloys. Exchangeable neck prostheses, introduced to allow surgeons more control over orientation and offset have a higher revision rate in the AOA NJRR and this increased revision rate is due to prosthetic dislocation as well as loosening/lysis.

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

Mechanical stabilization following periprosthetic fractures is challenging. A variety of cable and crimping devices with different design configurations are available for clinical use. This study evaluated the mechanical performance of 5 different cable systems in vitro. The effect of crimping device position on the static failure properties were examined using a idealized testing set up.

This study looks at Australian Orthopaedic Association National Joint Replacement Registry figures to try to see if any questions can be answered about the role of the trunion in some THR failures.

The Registry shows that large head (≥50 mm) hip resurfacings are doing well in appropriate patients, but the same size resurfacing cups with stemmed THR are doing poorly, while the smaller sizes in metal/metal stemmed THR continue to perform well.

The Registry also shows all stemmed THR with exchangeable (modular) necks have twice the revision rate of non exchangeable necks, and that these revisions are for dislocation as well as “loosening/lysis”.

One possible reason for the failure of large head metal – metal THRs is the trunion, which has been designed for use with small (22 mm–32 mm) femoral heads and is now being used with large (up to 56 mm) femoral heads.

We postulated that if the trunion were failing it might be seen more commonly with the smaller tapers such as the Stryker V40 taper, and that this might be seen with large (36 mm–44 mm) metal heads used on these tapers even in metal-poly hips.

Results from the Registry are shown.

Thirty nine Birmingham Resurfacing Hip Replacements were inserted between March 1999 and December 2000. The very early results are very satisfactory in a high demand group of patients.

Because of the relative ease of revision the implants have a role as a “pre” total hip replacement. The also have an important role in cases such as osteopetrosis and previous femoral osteotomy were an intra medullary stem is difficult. The series includes two cases of arthrodesis take down.

Complications have included one dislocation in a patient with cerebral palsy and one intra operative fractured neck of femur.

However, the early studies suggest that the large metal on metal bearing does produce serum chrome and cobalt levels which may be elevated in comparison with Metasul bearings at one year. The clinical significance of this is unknown.