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. Pre-operatively, 1215 patients received CT scans as part of their routine planning for THR. Within the 3D planning, each patient's native femoral neck anteversion, measured in relation to the posterior condyles of the knee, was determined. Patients were separated into eight groups based upon gender and age. Males and females were divided by those under 55 years of age, those aged 55 to 64, 65 to 74 and those 75 or older.Introduction
Methods
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. Pre-operatively, 3428 patients had their pelvic tilt (PT) and lumbar lordotic angle (LLA) measured in three positions; supine, standing and flexed-seated, as part of routine planning for THR. 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 excessive pelvic rotation, defined by rotation ≥13° in a detrimental direction, was determined.Introduction
Method
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. Pre-operatively, 90 total hip replacement patients had their pelvic tilt measured in 3 functional positions – standing, supine and flexed seated (posture at “seat-off” from a standard chair), Fig 1. 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, Fig 2.Introduction
Methodology
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. The position of the pelvis in the sagittal plane changes significantly between functional activities. The extent of change is specific to each patient. Spinal pathology can be an insidious “driver” of pelvic rotation, in some cases causing sagittal plane spinal imbalance or changes in orientation of previously well oriented acetabular components. Squeaking of ceramic on ceramic bearings appears to be multi factorial, usually involving some damage to the bearing but also usually occurring in the presence of anterior or posterior edge loading. Often these components will appear well oriented on standard views [Fig 1]. Runaway wear in hip resurfacing or large head metal-metal THR may be caused by poor component design or manufacture or component malorientation. Again we have seen multiple cases where no such malorientation can be seen on standard pelvic radiographs but functional studies demonstrate edge loading which is likely to be the cause of failure [Fig 2]. Clinical examples of all of these will be shown.Results and conclusions
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. The position of the pelvis in the sagittal plane changes significantly between functional activities. The extent of change is specific to each patient. Spinal pathology is a potent “driver” of pelvic sagittal rotation, usually unrecognised on standard radiographs. Pre-operative patient assessment can identify potential orientation problems and even suitability for hard on hard bearings. Optimal cup orientation is likely patient-specific and requires an evaluation of functional pelvic dynamics to pre-operatively determine the target angles. Post-operatively this technique can identify patient and implant factors likely to be causing edge loading leading to early failure in metal on metal bearings or squeaking in ceramic on ceramic bearings.Results and conclusions
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. Five cable systems were used in this study; Accord (Smith & Nephew), Cable Ready (Zimmer), Dall-Miles (Stryker), Osteo Clage (Acumed) and Control Cable (DePuy). Cables were looped over two 25 mm steel rods. Cable tension was applied to the maximum amount using the manufactures instrumentation. Devices were crimped by orthopaedic surgeon according to instructions. Crimping device/sleeve was secured in two different positions; 1. Long axis in-line with the load; 2. Long axis perpendicular to the load (Fig 1). Four constructs were tested for each cable system at each position. All constructs were tested following equilibration in phosphate buffered saline at 37 degrees Celsius using a servohydraulic testing machine (MTS 858 Bionix Testing Machine, MTS Systems) at a displacement rate of 10 mm per minute until failure. The failure load, stiffness and failure model (cable failure or slippage) was determined for all samples. Data was analysed using a two way analysis of variance (ANOVA) followed by a Games Howell post hoc test. One sample of each cable – crimping construct was embedded in PMMA and sectioned to examine the crimping mechanism.Introduction
Materials and Methods
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.
Hip Resurfacing in its current metal on metal hybrid fixation form has been performed in large numbers in Australia since 1999. Outcomes from the Australian Orthopaedic Association National Joint Replacement Registry are shown. While there is a wide range of outcomes these can be shown to depend on patient factors and implant factors. Use of one of the successful implants (for example the Birmingham Hip) in a young male patient with osteoarthritis by a suitably trained surgeon can lead to good results. In the AOA NJRR the 10 year cumulative percent revision rate for the Birmingham Hip in male patients under the age of 60 at the time of surgery is 3.3%
