Modern Total Hip Replacement (THR) is in general one of the most successful surgical treatments although the functional requirements of modern patients are more and more demanding. Challenges arise from an extended life-span, a higher activity level requiring more sophisticated artificial materials, and a larger required range-of-motion (ROM) caused by the younger patients’ eagerness to continue a sporty lifestyle. The design criteria for modern THR resulting from these patient demands also depend on the anatomical conditions as well as the socio-cultural circumstances of the patients.

Asian people require in general a higher ROM due to their habit to squat during daily activities which is not common in western societies. The outcome of a THR regarding the ROM is influenced by the size of the bearing couple, the design of the acetabular component, the head-to-neck ratio, and the implantation angles. In the case of a wrongly designed or a misaligned component, e. g. a verticalised socket, subluxations and impingement might occur leading to edge-loading between the ball head and the insert. This leads in all material couplings to problems: in hard-soft couplings (ceramic or metal ball head and polyethylene insert) to strongly increased polyethylene wear, in hard-hard bearings (metal-on-metal or ceramic-on-ceramic) to point loading followed by stripe wear and, in the case of a metal-on-metal coupling, a much higher metal ion level in the blood. Therefore, an appropriate choice of the prosthesis design together with the necessary surgeon’s diligence is necessary to avoid this kind of complication.

Other important design challenges come from possible anatomical differences between different ethnical groups. It has been shown that the “asian knee” has a different mean thickness in anterior-posterior as well as medio-lateral direction compared to caucasian. As another example, an extensive study of mexican people has shown a significantly different femur geometry concerning the height of the Trochanter major compared to the cross section of the femoral axis and the neck axis. For asian people it is widely accepted that the mean femoral size is smaller. The nonobservance of these geometrical factors in implant design may again lead to higher wear rates or subluxation and impingement followed by dislocation.

Wear and wear debris induced osteolysis is recognised as a major cause of long term failure in hip prostheses. Historically ultra high molecular weight polyethylene acetabular cups produced micron and submicron wear particles which accumulated in peri prosthetic tissues, and stimulated macrophages to generate wear debris induced osteolysis. Acceleration of wear and osteolysis was caused in historical materials by oxidative degradation of the polyethylene following gamma irradiation in air, and by third body damage and scratching of metallic femoral heads. Current conventional ultra high molecular weight polyethylene cups are irradiated in an inert atmosphere to reduce oxidative degradation and are articulated against ceramic femoral heads to reduce third body wear. More recently modified highly cross linked polyethylene has been developed, and while these materials produce a four to five fold reduction in wear volume the wear particles have been found to be more reactive, resulting in only a two fold reduction in functional osteolytic potential. The question remains as to whether this performance is adequate for high demand patients, particularly if larger diameter femoral heads are to be used. Recent interest in improved function, stability and reducing dislocations has generated interest in using larger diameter heads and hard on hard bearings. Alumina ceramic on ceramic bearings have shown a one hundred fold decrease in wear compared to highly cross linked polyethylene materials, and cell culture studies have shown the wear particles to be more bio-compatible and less osteolytic potential. Metal on metal bearings also produced very low wear rates compared to polyethylene. The wear particles are very small, 10 to 50 nanometers in size, some concern remains about the systematic release of metallic ions. These are lubrication sensitive bearings, and they unlike polyethylene wear decreases as the head size increases due to improved lubrication. Size 36 mm metal bearings are now commonplace for total joint replacements with even larger head sizes being used for surface replacement solutions. The demand for increased function and improved stability is leading to increased use of hard on hard bearings with larger diameter heads

Introduction. Recent gains in knowledge reveal that the ideal acetabular cup position is in a narrower range than previously appreciated and that position is likely different based on femoral component anteversion. For that reason more accurate acetabular cup positioning techniques will be important for contemporary THA. It is well known that malalignment of the acetabular component in THA may result in dislocation, reduced range of motion or accelerated wear. Up to 8% of THA patients have cups malaligned in version by more than ±10° outside of the Lewinnek safe zone. This type of malalignment may result in dislocation of the femoral head and instability of the joint within the first year, requiring reoperation. Reported incidences of reoperation are 1-9% depending on surgical skills and technique. In addition, cup malalignment is becoming increasingly important as adoption of hard on hard bearings increases as the success of large head hard on hard bearings seems to be more sensitive to cup positioning. This study reports the accuracy of a haptic robotic system to ream the acetabulum and impact an acetabular cup compared to manual instrumentation. Methods. Six fresh frozen cadaveric acetabula were CT scanned and three-dimensional templating of the center of rotation, anteversion and inclination of the cup was determined pre-operatively. Half of the specimens were prepared with manual instrumentation while half were prepared with robotic guidance. Haptic and visual feedback were provided through robotics and an associated navigation system to guide reaming and impaction of the cup. The robot constrained the orientation and position of the instruments thus constraining the inclination, anteversion and center of rotation of the reamer, trial and the final cup. Post-operative CT's were used to determine the achieved cup placement and compared to the pre-operative plans. Results. In all cases, robotic guidance resulted in placement of the acetabular cup within ±3° of anteversion, ±3° of relative to the pre-op plan. The average absolute inclination error was 1.5±1.2° and the average absolute anteversion error 1.3±1.4°. Cup placement with robotic assistance was significantly more accurate and precise than with manual instrumentation. With manual instrumentation the errors were, on average, 4.2 times higher in inclination and 4.8 times higher in anteversion compared to robotic instrumentation. Conclusion. This haptic robotic system substantially improved the accuracy of acetabular reaming and placement of the final cup compared to traditional manual techniques. With greater knowledge of ideal acetabular cup position, highly accurate techniques may allow surgeons to decrease the risk of dislocation, promote durability and improve the ability to restore appropriate leg length and offset. Haptic robotics has proven to be safe and effective in both knee and hip surgery and provides the potential to redefine the “instrument set” used for orthopedic procedures

Design of hard-on-hard bearing couples has traditionally been characterized by the material of the bearing couple, clearance between the bearing surfaces, sphericity of the components, surface roughness, and the radii of the components. All of these factors play a role in the lambda ratio and fluid film thickness calculations. However, the fluid film for hard on hard bearings can be interrupted by issues like the presence of 3rd body particles, intermittent walking, jogging, and subluxation. Only recently have researcher begun to simulate some of these disruptions in the fluid film for hard on hard bearings. Recent laboratory testing has looked at the effects of utilizing different materials and methodologies to evaluate hard-on-hard bearings. Ceramic-on-metal is a unique combination of components that is currently available. Several authors have shown that this combination can reduce the amount of metal wear generated during the test by a factor of 4–100. However, an occasional anomaly has shown up in some of these tests where a wear couple in a steady state wear mode will have a several-fold increase in wear for a short duration. For bearing couples that have a metal component, ion analysis of the serum lubricant can be utilized to monitor the amount of wear. This technique can provide real-time data on the amount of wear seen in simulator testing without removing the specimens from the machine. Further, there are some designs of metal cups that cannot be removed from the simulator without causing damage to the component. Data from a ceramic-on-metal simulator test confirmed that the short-term anomaly in gravimetric wear correlated with an increase in metal ion levels. Distraction testing evaluates the change in wear due to the unintended subluxation of the hip. This may occur during a standard walking gait if the hip is loose, during impingement, or during deep-knee bends, squatting, or rising from a chair. Distraction testing has various effects on wear depending on the material of the bearing couple. UHMWPE is insensitive to this additional mode of simulator testing. Metal-on-metal and ceramic-on-ceramic can increase in wear by up to an order of magnitude. The utilization of Biolox-delta rather than Biolox-forte can reduce the amount of wear seen during distraction testing. Diamond-on-diamond is insensitive to this wear mode and showed immeasurable wear. Other issues during testing of hard-on-hard bearings are still being explored. It is well known that 3rd body particles will disrupt fluid films and can increase wear. But the results from adding particles is variable. Metal-on-metal tests can have one specimen with very little increase while another specimen has an order of magnitude increase. Deformation of the shell caused by insertion during surgery has been shown to occur. Currently, this deformation has not been able to be replicated in a simulator, therefore, its effects are unknown. The design and laboratory testing of hard-on-hard bearings has improved significantly over the past decade. Further research is still needed to evaluate designs that may potentially increase resistance to failure modes other than standard walking gait cycles

Squeaking has become a more common problem following hard on hard bearings in total hip replacements. Although most squeaking is occasional and not concerning to either patient or health care practitioner, some reports of squeaking indicate high percentages (7% or higher) that can be constant and quite concerning. Much work has been done in this area, and although the exact mechanism is not yet understood, most of the data suggests a particular hip replacement system (metal alloy, taper design, cup design) significantly elevates to quantity and quality of the squeaking problem to concerning levels. Those specific details are described in depth along with future studies to improve our understanding in the nature of this acoustical phenomenon

Introduction: Polyethylene and metal has been the material of choice since the 1960’s. We are now seeing the third generation of cross-linked polyethylene along with work on alternative hard on hard bearings trying to reduce the generation of wear debris. Issues have been raised from squeaking to high trace elements and strength characteristics of current materials. Ideally, the surfaces for articulating bearing surfaces will be made from materials having high strength, high wear, and corrosion resistance, a high resistance to creep, and low frictional moments. This paper will review characteristics of a novel new approach for a bearing material. Methods: A review of past and current materials along with mechanical testing in creating a new approach to the development of a hydrophilic material replacing the polyethylene side of the bearing surface. Studies have demonstrated the advantages of the fullfluid film layer of lubrication in-terms of enhanced wear performance. An acetabular “buffer” bearing was developed that features a pliable bearing surface formulated, biocompatible polycarbonate urethane (PCU). A review of design objectives and testing will be highlighted in this paper. Results: Wear studies have demonstrated performance up to twelve times better compared to polyethylene. 34 components have been implanted reaching two years post-op. Two devices have been removed both for non-related implant issues. Retrieval analysis did not show any appreciable wear or damage to the bearing material. Conclusions: To date we are encouraged by the early basic and clinical science, however, only additional research and time will demonstrate the long-term viability of this material

Introduction: Increased wear is associated with aseptic loosening and late dislocations. Hard on hard bearings may reduce wear but still have topics of concern such as free metal ions in metal on metal bearings and the risk for fracture in ceramic articulations. Ceramic heads against conventional polyethylene is also used with the intention to reduce wear. But this effect has not been conclusively documented in the literature and is still discussed. 87 patients were operated consecutively by the same surgeon with the same surgical technique. All patients received a cemented all poly cup sterilized with irradiation in inert atmosphere and a cemented stem. Head size was 28 mm in all patients. 40 patients received cobalt-chrome heads and 47 patients aluminiumoxid heads. The patients were followed with RSA for 10 years and analysed for wear. Results: Mean (SEM) wear for the group with cobalt chrome heads was 0.93 mm (0.13) and for the group with aluminiumoxide was 0.43 mm (0.08) (p = 0.001). Discussion: We found significantly less wear with aluminumoxide heads compared to cobalt-chrome heads. The wear results in the cobalt-chrome group correlate well to wear values in the literature for conventional polyethylene. Although the polyethylene in this study is partly cross-linked (3Mrad) it is not clear whether these results can be extrapolated directly to the use of highly cross-linked PE. If longer follow-ups confirm the mechanical stability of highly cross-linked PE, ceramic heads might contribute additionally to the reduction of wear. In conclusion we found significantly reduced wear for aluminumoxide heads compared to cobalt chrome heads which could be beneficial for young and active patients

Young and active patients require bearing materials that can last up to 200 million walking steps, ten fold greater than conventional polyethylene bearings. Cross linked polyethylene provides reduced wear rate compared to conventional polyethylene, and further advantage is gained from using ceramic femoral heads. However in polyethylene bearings wear increases with the head diameter, and there is currently little opportunity to use head sizes greater then 36mm diameter. There is evidence of polyethylene fracture with steeply positioned cups. Ceramic on ceramic bearings provide substantially lower wear rates than polyethylene bearings. Steep cups, lateralised heads or neck impingement can lead to head contact on superior rim of the cup and stripe wear, but this still results in very low wear rates. Recently developed ceramic matrix composites Biolox Delta provide greater resistance to stripe wear. In a few patients stripe wear may lead to squeaking. Metal on metal bearings also provide substantially lower wear than polyethylene bearings. However there remains concern about elevated metal ion levels in a few patients and resultant risk of hypersensitivity reactions. In metal on metal bearings larger head sizes and reduced diametrical clearance can lead to reduced wear. Increased wear is associated with steep cups and lateralised heads resulting in rim wear. Ceramic on metal bearings have been introduced recently as the first differential hard on hard bearings. These bearings show substantial reduction in wear, corrosive wear mechanisms, metal ion levels in laboratory simulators and initial clinical studies have shown a reduction in metal ion levels in vivo compared to metal on metal bearings

Providing a long-lasting total hip arthroplasty for patients younger than 50 years remains one of the greatest challenges for modern arthroplasty surgery. Survival has been considered to be poor in young and active patients. We evaluated the benefit of total osteointegration of the prosthetic components in term of durable biological fixation. This study concerns a prospective series of 113 patients operated between 1986 and 1994. The femoral component (Corail, Landos-DePuy) and the acetabular shells (Atoll & Tropic, Landos-Depuy) were totaly coated with a 150μ thick layer of pure HA following a plasma-spray process. The mean age at the time of surgery was 40.3 (range 17 to 49.8). Two patients are now deceased, 11 patients (9.7%) are lost to FU. The mean FU for 100 patients still on file is 14 years. AVN represents 29 % of the cohort, primary arthritis 22% and dysplasia 17%. Functional results are excellent (mean PMA score 17.7 at the last control), as well as the subjective appreciation from the patients (94.9 % excellent or very good). 18 THA’s required components revisions: 6 without any implant removal (head or insert exchange); 1 stem (periprosthetic fracture) and 11 cups (6 well-fixed and 5 for loosening) were removed. Owing the high incidence of wear-related revision, actuarial survivorship, using re-operation for any reason as end-point, was 82% at 18 years ± 8.5, and considering aseptic loosening only, the survival probability of the stem is 99% ± 1.5 and 97 % ± 3.5. It is clear that HA-coatings have given lesser performance in the cups than in the stems. We advocate for HA and the eradication of wear debris using hard on hard bearings. This combination seems to be a reasonable solution in this subgroup of young patients

Introduction: The treatment of end stage hip osteonecrosis in patients with Sickle cell disease presents a unique set of challenges, with patients often needing arthroplasty in young adult life. Traditionally, this group of patients has a high incidence of complications and failure. We report the early results of THR in patients managed by the single hip surgeon working as part of the comprehensive Sickle Cell service. Methods: Data was collected prospectively on all sickle patients undergoing THR at our institution. 18 patients underwent surgery with a mean age of 37 (range 25–63). There were 16 primary and 2 revisions. All patients were optimised pre-operatively with an exchange transfusion to ensure the HB SS < 30%, and all possible sites of sepsis were treated aggressively. All patients received uncemented implants with hard on hard bearings and broad-spectrum prophylactic intravenous antibiotics for 48 hours. Results: 18 patients were followed up at a mean of 25 months. Despite technical challenges, all patients had a stable hip with good resolution of pain and radiographic evidence of bony ingrowth of all components. There were 3 minor intra-operative metaphyseal peri-prosthetic fractures, which all healed satisfactorily. There was a single early dislocation that has remained stable after closed reduction. There have been no superficial or deep infections. Discussion: This study shows that THR can be performed safely in patients with sickle cell disease within the context of a multi-disciplinary team approach. Operative technique involves the use of long drills under image intensifier to prepare the femur safely and use of a modular uncemented system to address the mismatch between the metaphysis and the diaphysis

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. Results and conclusions. 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

Introduction. Squeaking is a potential problem of all hard on hard bearings yet it has been less frequently reported in metal-on-metal hips. We compared a cohort of 11 squeaking metal-on-metal hip resurfacings to individually matched controls, assessing cup inclination and anteversion between the groups to look for any differences. Methods. We retrospectively reviewed the patient records of 332 patients (387 hip resurfacings) who underwent hip resurfacing between December 1999 and Dec 2012. 11 hips in 11 patients were reported to squeak postoperatively. Each of these patients, except one, were matched by age, sex, BMI and implant to 3 controls. The final patient only had one control due to his high BMI. The latest post-operative radiographs of the squeaking group and controls were analysed using EBRA (Einzel-Bild-Roentgen-Analysis, University of Innsbruck, Austria) software to evaluate cup inclination and anteversion. Results. Post- operative audible squeaking occurred in 11 out of 387 hips (2.84%). The mean follow up of the squeaking group was 88.6 months (19–131 months). The mean time to squeak was 11.3 months (3–22 months). 8 (73%) patients were male, 10 (91%) patients had a Birmingham hip resurfacing and 9 (82%) patients had an operation on the left hip. The mean inclination angle of the cups in the squeaking group was 48.4° (43.9°–55.4°) compared to 50° (37.8° −63°) in the control group. The mean anteversion of the cups in the squeaking group was 17.1°(6.3°–25.7°) compared to 14.6° (4.3° −33.5°) in the control group. There was no statistically significant difference between the cases and their controls for cup inclination (p = 0.36) or cup anteversion (p = 0.31). The mean head size in the squeaking group was smaller at 49.3 mm (46 mm-54 mm), compared to 51.4 mm (48 mm-54 mm) in the control group (p = 0.026). The mean cup size in the squeaking group was also smaller at 56.5 mm (54 mm-62 mm), compared to 57.9 mm (48 mm-60 mm) in the control group (p = 0.007). Overall, 4 (40%) male patients in the squeaking group had a head size less than 50 mm, compared to 0 (0%) in the control group. 3 (27%) patients with squeaking resurfacings underwent revision surgery. 1 (9%) at 72 month for a pseudotumour, 1 (9%) at 114 months for persistant squeaking and 1 (9%) at 117 months for a subtrochanteric fracture after a fall. Conclusions. No difference was found between the radiographic inclination or anteversion of squeaking metal-on-metal hip resurfacing cups compared to a control group. Male patients with squeaking hips were noted to have smaller head and cup sizes than their controls

Traditional hip prostheses, which involve metal on poly-ethylene articulations, have shown good survivorship at ten years, but in the long term, wear debris induced osteolysis has been found to cause loosening and failure. Specifically, micron and submicron size polyethylene wear particles generated at the articulating surfaces enter the periprosthetic tissues, activate the macrophages causing adverse cellular reactions and bone resorption. Recent laboratory, retrieval and clinical studies have shown that oxidation of the traditional polyethylene irradiated in air, causes wear to increase by a factor of three following either storage on the shelf for five years or following implantation in vivo for 15 years. Furthermore, damage or scratching of metallic femoral heads has been shown to increase wear by a factor of two. In vitro cell culture studies with real polyethylene wear particles, have shown that the intensity of the adverse cellular reactions is critically dependent on the size of the polyethylene wear particle with the smallest particles 0.1 to 1 mm being the most active. A novel model has been developed to predict functional biological activity and osteolytic potential, by integrating wear rates, particle analysis and cell culture studies. Stabilised and crosslinked polyethylenes have been investigated and been found to reduce wear rates by a factor of three compared to oxidised and aged materials. A moderate level of crosslinking reduced wear from 50 to 35 mm3 per million cycles compared to non crosslinked materials. However, against scratched femoral heads, the wear rate of both stabilised and cross-linked polyethylene was elevated to levels where the functional biological activity remains a concern in the long term. Alternative bearing surfaces, metal on metal, and alumina ceramic on ceramic provide potential to substantially reduce wear. Metal on metal bearings have shown mean wear rates of 1.5 mm3/year in the hip joint simulator, with very small, 30 nm size particles. Alumina ceramic ceramic have also shown very low wear rates of approximately 1 mm3/year, even in the presence of microseparation and rim contact, with small 10 nm size wear particles and larger particles up to 1 mm in size caused by grain boundary fracture. The functional biological activity and osteolytic potential of the alumina ceramic couple is predicted to be at least ten times lower than crosslinked polyethylene. New ceramic materials (zirconia toughened alumina) have been shown to further reduce ceramic ceramic wear. Furthermore, novel differential hardness ceramic on metal bearings have shown even lower wear rates. The currently available hard on hard bearings and the recent further improvements of these bearing couples, indicate that osteolysis free lifetimes well beyond 20 years are now possible

Squeaking in ceramic on ceramic bearing total hip arthroplasty is well documented but its aetiology is poorly understood. In this study we have undertaken an acoustic analysis of the squeaking sound recorded from 31 ceramic on ceramic bearing hips. The frequencies of these sounds were compared with in vitro acoustic analysis of the component parts of the total hip implant. Analysis of the sounds produced by squeaking hip replacements and comparison of the frequencies of these sounds with the natural frequency of the component parts of the hip replacements indicates that the squeaking sound is due to a friction driven forced vibration resulting in resonance of one or both of the metal components of the implant. Finite element analysis of edge loading of the prostheses shows that there is a stiffness incompatibility between the acetabular shell and the liner. The shell tends to deform, uncoupling the shell-liner taper system. As a result the liner tends to tilt out of the acetabular shell and slide against the acetabular shell adjacent to the applied load. The amount of sliding varied from 4–40μm. In vitro acoustic and finite element analysis of the component parts of a total hip replacement compared with in vivo acoustic analysis of squeaking hips indicate that either the acetabular shell or the femoral stem can act as an “oscillator’ in a forced vibration system and thus emit a squeak. Introduction: Squeaking has long been recognized as a complication in hip arthroplasty. It was first reported in the Judet acrylic hemiarthroplasty. 1. It was the squeak of a Judet prosthesis that led John Charnley to investigate friction and lubrication of normal and artificial joints which ultimately led to the concept of low friction arthroplasty. Ceramic on ceramic bearings were pioneered by Boutin in France during the 1970’s, but experienced unacceptably high fracture rates. Charnley demonstrated in vitro squeaking when he tested one of Boutin’s ceramic-on-ceramic bearings in his pendulum friction comparator. 2. Squeaking has also been reported in other hard on hard bearings, and can also occur after polyethylene bearing surface failure resulting in articulation between metal on metal or ceramic on metal surfaces. 3–6. Recently, squeaking has been increasingly reported in modern ceramic-on-ceramic bearings in hip arthroplasty. However, although well-documented, the aetiology of squeaking in ceramic on ceramic bearings is still poorly understood. The incidence ranges from under 1% to 10%. 7–10. It has been reported in mismatched ceramic couples,11and after ceramic liner fracture. 12,13. An increased risk of squeaking has been demonstrated with acetabular component malposition, as well as in younger, heavier and taller patients. 9. However, it may also occur in properly matched ceramic bearings with ideal acetabular component position and in the absence of neck to rim impingement. 7–9. In rare cases, the squeak is not tolerated by the patient and has prompted a revision. Under ideal conditions hard-on-hard bearings are assumed to be operating under conditions of fluid film lubrication with very low friction. 14,15. However, if fluid film lubrication breaks down leading to dry sliding contact there will be a dramatic increase in friction. If this increased friction provides more energy to the system than it can dissipate, instabilities may develop in the form of friction induced vibrations and sound radiation. 16. Friction induced vibrations are a special case of forced vibration, where the frequency of the resulting vibration is determined by the natural frequency of the component parts. Running a moistened finger around the rim of a wine glass is an example of this. [Appendix]. The hypothesis of this study is that the squeaking sound that occurs in ceramic on ceramic hip replacement is the result of a forced vibration. This forced vibration can be broken down into a driving force and a resultant dynamic response. 17. The driving force is a frictional driving force and occurs when there is a loss of fluid film lubrication resulting in a high friction force. 14,15,18. The dynamic response is a vibration of a part of the device (the oscillator) at a frequency that is influenced by the natural frequency of the part. 16. By analyzing the frequencies of the sound produced by squeaking hip replacements and comparing them to the natural frequency of the component parts of a hip replacement this study aims to determine which part produces the sound. Materials and methods: In vitro determination of the natural frequencies of implant components Modal analysis has suggested that resonance of the ceramic components would occur only at frequencies above the human audible range and that resonance of the metal parts would occur at frequencies within the human audible range. Furthermore, that resonance of the combined ceramic insert and titanium shell would not be within the human audible range. To test this hypothesis we performed a simple acoustic analysis. The natural frequency of hip replacement components was determined experimentally using an impulse-excitation method (Grindo-sonic). Components were placed on a soft foam mat in a quiet environment and struck with a wooden mallet. The sound emitted from the component was recorded on a personal computer with an external microphone with a frequency response which ranges from 50Hz to 18,000Hz (Beyerdynamic MCE87, Heilbronn, Ger-many). The computer has an integrated sound card with a frequency response from 20Hz to 24kHz (SoundMAX integrated digital audio chip, Analogue Devices Inc, Norwood, M.A.) and we used a codec with a frequency response from 20Hz to 20kHz (Audio Codec ’97, Intel, Santa Clara, CA). Sound files were captured as 16 bit mono files at a sample rate of 48000Hz using acoustic analysis software (Adobe Audition 1.5, Adobe Systems Incorporated, San Jose, California, USA). We performed fast Fourier transform (FFT) of the sound using FFT size 1024 with a Blackmann-Harris window to detect the frequency components of the emitted sound. (Fast Fourier transform is an accepted and efficient algorithm which enables construction of a frequency spectrum of digitized sound). We tested the following components: modular ceramic/titanium acetabular components, which included testing the titanium shell and the respective ceramic inserts both assembled according to the manufacturer’s instructions and unassembled; titanium femoral stems and ceramic femoral heads both assembled and unassembled. A range of sizes of each component was tested according to availability from our retrieval collection. In vivo acoustic analysis: Sound recordings were collected from 31 patients. Nineteen recordings were made at our institution: 16 of these were video and audio recordings and 3 were audio only recordings. Video recording was with a digital video camera recorder (Sony DCR-DVD101E Sony Electronics, San Diego, CA, USA) with the same external microphone used in the in vitro analysis. For 3 patients who could not reproduce the sound in the office we lent them a digital sound recorder for them to take home and record the sound when it occurred (Sony ICD-MX20, Sony Electronics, San Diego, CA, USA). This device has a In vivo acoustic frequency range from 60Hz to 13,500Hz. The remainder of the recordings were video and audio recordings made by surgeons at three other institutions on digital video camera recorders. Sound files were captured and analyzed by the same method used in the in vitro analysis. Each recording was previewed in the spectral view mode which allows easy visual identification of the squeak in the sound recording. In addition all sound recordings were played, listening for the squeak. Once a squeak was identified a fast Fourier transform (FFT) was performed. We used FFT size 1024 with a Blackmann-Harris window which allowed us to easily pick out the major frequency components. All prominent frequency components were recorded at the beginning of the squeak and at several time points during the squeak if there was any change. A range was recorded for the fundamental frequency component. We were able to determine the frequency range of the recording device used by observing the frequency range of the background noise on the recording. We found that if a squeak was audible on the recording we had no difficulty determining its frequency regardless of the quality of the device used to make the recording or the amount of background noise. The mean age of the patients was 54 years (23 to 79 years), mean height was 171cm (152 to 186cm) and mean weight was 79kg (52 to 111kg). There were 17 female and 14 male patients. There were nineteen ABGII stem and ABGII cup combinations, 10 accolade stem and trident cup, 1 Exeter stem and trident cup and 1 Osteonics Securfit stem with an Osteonics cup. Ethics committee approval was obtained for this project from our institution and from the referring institutions and informed consent was gained from the patients. Finite element analysis of edge loading: Edge-loading wear which may provide a mechanism for failure of fluid film lubrication and may therefore play a role in squeaking. To evaluate edge loading further we conducted finite-element analysis (FEA). 9. Computed tomography (CT) scans of an intact pelvis were obtained from visual human data set (VHD, NLM, Bethesda, Maryland). Slices were taken at 1mm thick with no inter-slice distance through the entire pelvis. The CT files were then read into a contour extraction program and saved into an IGES file format which was imported into PATRAN (MSC Software, Los Angeles, CA) to develop the pelvic geometry. The pelvis was meshed with 10 noded modified tetrahedral elements. The model was reconstructed with a 54mm titanium alloy generic acetabular shell and a 28mm alumina ceramic liner. The acetabular shell and ceramic liner were meshed using 8 noded hexahedral elements. The shell-liner modular taper junction incorporated an 18° angle. The implant contact conditions (Lagrangian multiplier) allowed the liner and shell to slide with a friction coefficient of 0.9. Tied contact conditions were applied between the generic acetabular shell and the bone representing bone ongrowth. Bone material properties were extracted from the CT files by taking the Hounsfield value and the coordinates and mapping to the element in the model allowing us to calculate the Young’s modulus for each element . 19. Material properties for the shell and liner were based on published values. 20. for titanium alloy and alumina ceramic

Introduction. Sir John Charnley introduced his concept of low friction arthroplasty— though this did not necessarily mean low wear, as the initial experience with metal on teflon proved. Although other bearing surfaces had been tried in the past, the success of the Charnley THR meant that metal-on-polyethylene became the standard bearing couple for many years. However, concerns regarding the occurrence of peri-prosthetic lysis secondary to wear particles lead to consideration of other bearing surfaces and even to the avoidance of cement (although this has proven to be erroneous). Bearing combinations include polymers, ceramic and metallic materials and are generally categorised as hard/soft or hard/hard. In general, all newer bearing surface combinations have reduced wear but present with their own strengths and weaknesses, some of which are becoming more apparent with time. Bearing surfaces must have the following characteristics: low wear rate, low friction, Biocompatibility and corrosion resistance in synovial fluid. Hard/soft. Femoral head components are generally made of cobalt, chromium alloy, either cast or forged. Both alumina and zirconia ceramics have been used as femoral head materials and the hardness is thought to reduce the incidence of surface damage to the femoral head. The hard femoral heads have been traditionally matched with conventional ultra high molecular weight polyethylene. (UHMWPE) which has been produced by either ram extrusion or compression moulding. Over the past 10 years, most implant companies have moved to highly cross-linked UHMWP which in both laboratory and human RCTs have shown appreciably less wear. Hard/hard bearings – Metal-on-metal (M-O-M). The first generation of metal bearings were based on stainless steel couples but the metal on metal design by. McKee-Farrar was made from CoCrMo alloy with large head diameters. The second generation M-O-M bearing were introduced by Weber using wrought. CoCrMo alloy with low surface roughness and wear rates about 100 to 200 times less than traditional metal/UHMWPE. The re-introduction of resurfacing hip arthroplasty has been made possible by the improvement in metal technology. Concerns however exist with the long term biologic effects of metal ions, the reported incidence of sensitivity reactions to metal and the more demanding techniques required for implantation. Ceramic on Ceramic (C-O-C). Alumina ceramic bearing surfaces are extremely hard, have high wear resistance and reported low concentration of wear particles in peri-prosthetic tissues. Unlike M-O-M there is no ion release. While the reported fracture rate for ceramic couplings is extremely low their proper implantation is important to minimise impingement. There is an incidence of squeaking not seen in other bearing couples and because of the hardness of the bearing, long term concerns with stress shielding of bone remain. Clinical outcomes. Data will be presented from the Australian Orthopaedic. Association National Joint Replacement Registry on clinical outcomes of bearing surfaces. Overall metal on UHMWPE has the least revision of any bearing surface couple used with conventional hip replacement. Future trends. Further research into hard/soft bearings will look at ways to reduce UHMWPE wear without compromise of clinical results based on over 40 years use. Hard-on-hard bearings may focus on combining the best features of both. M-O-M and C-O-C couplings without fracture risk or metal iron release. When deciding which bearing surface is suitable for your patients it must be emphasised that wear reduction is only one of several considerations when taking into account the most appropriate implant