The use of modular systems adds versatility to the implant system, better restoration of hip biomechanics and lower inventory to the hospital. There have been reports of high metal ions, ARMD reactions and high implant failure rates due to potential problems from taper failures. These are more common in metal-on-metal hip replacements, but are being also reported in other bearings. Between 2001 and 2010, we performed 383 consecutive metal-on-metal (MoM) THRs through a posterior approach, using a BHR cup and Birmingham modular head with one of three different stems, all with 12/14 tapers. The earliest 104 hips employed a cemented MS30 stem (Zimmer GmbH, Winterthur, Switzerland). Subsequent 256 were Synergy and then 23 Anthology (both uncemented and both Smith and Nephew Orthopaedics, Memphis TN USA). There was no significant difference in the average age at surgery (65.4 years cemented vs 65.6 uncemented, p = 0.69), gender ratio (1.68 vs 1.89, p = 0.64), or bearing diameter (46.7 vs 46.8, p = 0.31). The earlier 203 Synergy stems were monoblock heads, while the remaining uncemented stems included a tapered sleeve in addition. There were 3 deep infections and 11 debris-related failures (overall revision rate 4.9%). The revision rate from aseptic failures (ALTR, effusion, osteolysis or component loosening) is 2.87%. Kaplan-Meier analysis of the entire cohort showed a 10-year implant survival of 96.8% with revision for any reason as the end-point. Cemented stems had a 100% survival at 10 years and 98.6% at 12 years. The uncemented stems had a 93.8% survival at 10 years. Within the uncemented group, the monoblocks had a 5 and 10-year survival of 99.0% and 96.4% respectively while the sleeved had 98.7% (5 years) and 96.3% (7 years) and 82.5% at 8 years. Retreival analysis showed clear evidence of taper failure. Our experience suggests taper failure leading to ALTRs and its sequelae. Others have reported ALTR type reactions in metal on polyethylene and ceramic on polyethylene bearing types as well in bearing diameters ranging from 28mm to 40mm. There is a need to improve taper design especially for use with large heads, and in high demand patients

Pseudotumour formation is being reported with increasing frequency in failing metal-on-metal hip resurfacings and replacements. This mode of failure complication has also been reported with metal-on-polyethylene bearing bearings when it is usually associated with evidence of surface corrosion and no apparent wear at the head–neck taper. We present a case with evidence of taper wear and damage secondary to corrosion in an uncemented total hip replacement with a metal on polyethylene articulation (TMZF (Titanium, Molybdenum, Zirconium and Ferrous) Accolade® stem, Trident® HA coated acetabular shell, Low Friction Ion Treatment (LFIT™) Cobalt-Chrome anatomic head (40 mm), X3® polyethylene liner). Case. A 69 year old woman had a THR in 2008. A year later she started to complain of lateral based hip pain. Clinical examination and initial imaging indicated trochanteric bursitis and heterotopic bone formation. The symptoms became worse over the next 3 years and the patient was listed for exploration and excision of heterotopic bone. Surgical findings. Extensive pseudotumour was encountered deep to bursa and adherent to capsule. Abductors, external rotators and vastus lateralis were spared. There was minimal calcar osteolysis and marginal erosion in superior acetabulum. There was no obvious wear on the articulating surfaces of the femoral head and polyethylene inlay of the socket nor was there any evidence of neck-to-rim impingement or edge loading. There was visible blackening of both the taper and trunnion after femoral head removal. Results. Peripheral blood samples taken at the time of surgery recorded Cobalt 107 nmol/L, Chromium 9 nmol/L. Wear Measurements. These were performed on the articulating surfaces and the head taper using the RedLux Artificial Hip Profiler. No discernable wear was noted at the articulating surfaces. Qualitative 3-D surface mapping demonstrated a trunnion imprint on head taper of 10.1 mm in length. At the distal end of the taper this was incomplete indicating possible taper/trunnion incongruity. The majority of the material loss was demonstrated at the proximal end on the polar opposite side to incomplete imprint (Figure 1). Corrosion analysis. The head was sectioned for more detailed surface analysis of areas of interest. This demonstrated a polished region just inside the taper (G), followed by a deep pit (F), a region of extensive pitting (E) and running along the taper length were longitudinal needle like pits (D) indicative of a path of fluid ingress (D-G ref Fig 2.). These findings were more suggestive of crevice corrosion than mechanical wear. Histology. There was evidence of aseptic lymphocytic vasculitis-associated lesion (ALVAL) scoring 8 out of 10 (as per Campbell et al). With the continuing increase in the use of larger head sizes the findings of pseudotumour formation and taper damage in a commonly used hip replacement raises considerable concern. Wear analysis has highlighted that although there is evidence of motion occurring at the taper / trunnion interface, corrosion appears to be the dominant mechanism of damage to the taper. In addition to the use of larger head sizes this case also raise concerns about the compatibility of certain metal alloy combinations

Purpose. To determine whether there is a difference in the metal ion levels among three different metal-on-metal total hip systems: two monoblock large heads and one modular metal-on-metal total hip replacement system in patients who received these implants in our region. Method. A group of 56 patients were recruited that had either undergone total hip replacement (THR) with a Birmingham resurfacing socket, the Durom resurfacing socket, or a Pinnacle metal-on-metal bearing surface. All patients recruited were at least one year following their surgery in order that their ion levels had reached a steady state. We reviewed every patient clinically, radiographically as well as biochemically. Blood was obtained for cobalt (Co) and chromium (Cr) levels. Current radiographs were arranged to assess the stability and mechanics of the total hip systems. All patients signed an informed consent and completed three questionnaires, The Western Ontario and McMaster Universities (WOMAC) index, the Short Form 36 (SF36) and UCLA activity score. A Harris Hip score was completed in order to assess individual hip function. Statistical analysis was performed on the collected data to assess whether there were any other potential influence on the mean levels of Co and Cr. Results. The blood metal ion levels in the larger non modular acetabular sockets were significantly raised compared to the Pinnacle group. For Co, 1.95 parts per billion (ppb) and 2.70 ppb in the Durom and Birmingham groups respectively compared to only 0.52 ppb in the Pinnacle group (P< 0.001). Cr levels were the same in the two monoblock systems, 1.9 ppb compared to the Pinnacle sockets 1.2 ppb (P<0.001). In all groups however these levels were within an acceptable safe range. The mean head size used in the Birmingham group was 53.2mm (Range 44mm to 56mm), and in the Durom group, 47.1 (Range 42 to 54mm). The mean head size used in the modular group was 37.3mm (Range 36–44mm). There was no difference between the three groups in terms of functional outcome and patient demographics were similar in all three groups. There was no statistical difference between the groups in terms of anteversion and abduction angles. There was also a smaller spread in this group in terms of range of angles. There was also no relationship between these values and the metal ion levels. Conclusion. All three total hip systems demonstrated average metal ion whole blood levels in a safe range. Larger diameter metal on metal bearings had higher ion levels

Introduction. Although good mid-term results have been reported with some metal on metal hip replacements, reported complications due to metal on metal (MOM) related reactions are a cause for concern. We have assessed the early clinical outcome and results of routine metal artefact reduction MRI findings in a consecutive series of patients with a modern large head metal on metal hip replacement. Methods. 62 ASR XL Corail total hip replacements (THR) and 17 ASR resurfacings were performed at our hospital between 2005 and 2008. All patients were reviewed and assessed with an Oxford hip score (OHS), a plain radiograph and a MRI imaging was obtained on 76 (96%) hips. Implant position was assessed using Wrightington cup orientation software. Results. At a mean follow-up of 32 months, 8 ASR XL Corail THRs had been revised. 6 revisions were performed for MRI-confirmed MOM-related pathology. Histology confirmed a MOM reaction in all 6 cases. Of the 76 hips that were MRI scanned, 27 (36%) had typical features of a MOM reaction. These were classified as mild in 5 (7%), moderate in 18 (24%) and severe in 4 (5%). 78 patients completed an OHS and the mean score was 21. The mean OHS was 37 pre-operatively in those that had been revised, 19 in patients with abnormal MRI findings and 23 in those with a normal MRI. 10 patients with abnormal MRIs had a near perfect OHS (15 or less). Conclusion. The ASR XL Corail THR has an unacceptably high early failure rate. MRI detected metal debris related soft tissue related reactions were very common and often asymptomatic. We recommend that patients with this implant should be closely followed up and undergo routine metal artefact reduction MRI screening

Introduction:

Large diameter metal on metal total hip arthroplasty (MOM THA) have shorter lengths of implantation due to increased failure caused by wear either at the articulating surface as well as the taper-trunnion interface. Taper-trunnion wear may be worse in large diameter MOM THA due the increased torque at the taper-trunnion interface. However little has been done to understand how differences in taper-trunnion geometry and trunnion engagement effects wear. The purpose of this study was to (1) measure the differences in taper geometry and trunnion engagement on the head-taper of 11/13, 12/14, and Type 1 taper designs and (2) to determine if taper geometry affects fretting, corrosion, and wear at the taper interface.

In a cross sectional cohort study three different metal on metal total hip systems were assessed. Two monoblock acetabular designs; the Durom socket (Zimmer, Warsaw, In) and the Birmingham socket (Smith and Nephew, Memphis, TN), and one modular metal on metal total hip system (Pinnacle, Depuy Orthopaedics, Warsaw, In) in patients who have received these implants in the our region. 56 patients were recruited in total. All patients were assessed clinically, radiologically and biochemically. Whole blood Cobalt, Chromium and titanium levels were tested.

The median head size used in the Birmingham group was 52mm (Range 44mm to 56mm), and in the Durom group, 48mm (Range 42 to 54mm). The median head size used in the modular Pinnacle group was 40mm (Range 36–44mm).

The blood metal ions levels in the larger non modular acetabular sockets were significantly raised compared to the Pinnacle group. For Co 1.95 µg/l and 2.70 µg/l in the Durom and Birmingham groups respectively compared to only 0.52 µg/l in the Pinnacle group (P< 0.001). Mean Cr levels were the same for the two monoblock systems, 1.9 µg/l compared to the Pinnacle sockets 1.2 µg/l (P< 0.001).

Our study clearly demonstrates that there is a significant difference in metal ion levels in patients following a monobloc large head arthroplasty system compared to a smaller modular metal on metal hip arthroplasty. The smaller head size appears to produce less metal ions whilst at the same time a 36mm–44mm head size is large enough to increase hip stability and range of movement as well as decreasing the risk of impingement. In our practice we are no longer using this design and the safest strategy, when considering metal on metal bearings, is to use a modular, smaller head system such as the Pinnacle.

Introduction & aims. Total hip replacement is an excellent treatment option for people with late stage degenerative hip disease. In addition to marked reduction in pain and improvement in sleep, most people regain range of motion, physical ability and quality of life. This study aimed at the functional outcomes of large diameter heads in THR patients. Method. This study is an analysis of a cohort of patients undergoing total hip replacement performed at our hospital from November 2011 to July 2013. A total of 70 hips, 40 males and 30 females, were operated upon with large diameter femoral heads. The mean age was 50.38 years (range 40–59 years). In our cohort, 32 patients had AVN of femur head, 19 had post traumatic secondary degeneration, 10 had RA, 6 had AS and 3 patients had OA of hip. The follow-up data included local complications, Harris Hip Score, medical complications, readmission, activity status and use of a walking aid. Results. Harris hip score at final follow up was, 33 cases had excellent, 26 cases had good result. None of the patient had fair or poor result. Two cases of superficial infection were observed. One case of dislocation was observed in post operative period which was reduced under GA and patient was given abduction brace. The mean follow up was 16.31 months ranging from 6 to 32 months. During the follow up, we had mortality of two patients. One died at 14 months due to myocardial infarction and the cause of other died due to CVA at 10 months follow up. Conclusions. Lower dislocation rate and better range of motion in majority of cases reinforces the advantage of large diameter head in THR in young and active patients. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Corrosion at the taper interface between the femoral head and the femoral stem is well described in metal-on-polyethylene (MoP) hips but previously was undetermined in large diameter head metal-on-metal (LHMoM) hips. The high failure rate of the articulating surface replacement (ASR) XL hip system has been partly attributed to susceptibility to corrosive damage at the taper interface. It was not known if other hip manufacturers are liable to taper corrosion. Therefore the aim of this study was to quantify the prevalence and severity of taper corrosion in LHMoM hips and compare corrosion across five different current generation manufacturers. Taper corrosion was analysed in a consecutive series of the five most common hip types at our retrieval centre: ASR XL, DePuy (n=49); Birmingham hip resurfacing, Smith & Nephew (n=33), Durom, Zimmer (n=31), M2a Magnum, Biomet (n=14) and Cormet, Stryker (n=10). A four-scale peer-reviewed qualitative corrosion scoring system was used to quantify corrosion (none, mild, moderate and severe). Evidence of corrosion was observed in 86% of components, with at least moderate corrosion observed in 61%. No difference in corrosion was observed between the ASR XL and the other manufacturers (p=0.202). There was still no difference seen when all manufacturers were compared individually (p=0.363). A positive correlation was observed between corrosion and femoral head diameter (r=0.224, p=0.021). However no relationship was observed with implantation time (r=0.163, p=0.118). Our study indicates that taper corrosion is common in LHMoM hips and affects all hip types equally. The clinical significance of this finding is that all hip types will be susceptible to the complications of corrosion, such as third body wear and osteolysis. Furthermore recent reports indicate that corrosive debris released from the taper interface may play a role in the formation of pseudotumours and adverse soft-tissue reactions. We found that larger femoral head sizes showed greater corrosion, which suggests that high torque increases fretting corrosion of the taper interface. Future work must determine the optimum femoral head size and investigate the chemical composition of the corrosive debris

In metal-on-metal (MoM) total hip arthroplasty, the taper interface is where the femoral head (female taper surface) attaches to the trunnion (male taper) of the femoral stem. Corrosion is well reported in metal-on-polyethylene hips but little is known about taper corrosion in MoM devices. The aim of this study was to quantify corrosion in modern-generation stemmed MoM hip systems and gain insight into the nature of the underlying corrosive attack.

Taper corrosion was quantified in 161 failed MoM components (head components n=128; femoral stem n=33) from nine hip types with the use of a qualitative subjective scoring system. An unanticipated finding on preliminary inspection of the hips was a region on the female taper surface that contained ridges that directly corresponded with the ridged microthread on the trunnion. The ridges were not present on unimplanted (control) female taper surfaces and therefore a novel four-scale subjective scoring system was devised to quantify the prevalence and severity of this ‘imprinting’ phenomenon.

Evidence of corrosion was observed in 81% (131/161) of components, with at least moderate corrosion observed in 58% (94/161). Corrosion was greater on the female taper surface than on the male taper (p=0.034) and the two scores were associated (r=0.784, p=0.001). Imprinting affected all manufacturers and was observed in 64% (82/128) of head components. The corrosion and imprinting scores were strongly correlated (r=0.694, p=0.001). Corrosion was largely confined to the area of the female taper interface where imprinting had occurred i.e. the region that had been in contact with the trunnion microthread. Scanning electron microscopy showed evidence of fretting corrosion and substantial mechanical wear within the ridged region on the female taper surface.

Our study indicates that MoM hips are susceptible to taper corrosion. We believe it occurs by a process of “mechanically-assisted crevice corrosion,” involving the following sequence of events: joint fluid enters the taper junction as a result of pumping of fluid along the machined microthread of the trunnion. This results in galvanic corrosion of the anodic surface (the cobalt-chromium femoral head or taper sleeve). The pattern of corrosion of the head taper is determined by the surface profile of the screw thread of the trunnion, thus leaving an imprinted appearance. Historically the ridged microthread was introduced to trunnions to minimise the risk of burst fracture of ceramic heads. However this study indicates that the ridges are detrimental in MoM hips by causing extensive mechanical wear. Thus the possibility that cobalt-chrome and ceramic femoral head components require different trunnion designs needs urgent investigation.

Introduction

Current literature supports the use of total hip replacement (THR) for the treatment of displaced intra-capsular proximal femoral fractures (DIPFF). Case series of patients receiving this treatment show dislocation rates higher than that of patients who have THR to treat osteoarthritis. Large diameter THR have mechanical advantages in terms of dislocation and their role in PFF has yet to be assessed.

With the introduction of highly crosslinked polyethylene (HXLPE) in total hip arthroplasty (THA), orthopaedic surgeons have moved towards using larger femoral heads at the cost of thinner liners to decrease the risk of instability. Several short and mid-term studies have shown minimal liner wear with the use HXLPE liners, but the safety of using thinner HXPLE liners to maximize femoral head size remains uncertain and concerns that this may lead to premature failure exist. Our objective was to analyze the outcomes for primary THA done with HXLPE liners in patients who have a 36-mm head or larger and a cup of 52-mm or smaller, with a minimum of 10-year follow-up. Additionally, linear and volumetric wear rates of the HXLPE were evaluated in those with a minimum of seven-year follow-up. We hypothesized that there would be minimal wear and good clinical outcome. Between 2000 and 2010, we retrospectively identified 55 patients that underwent a primary THA performed in a high-volume single tertiary referral center using HXLPE liners with 36-mm or larger heads in cups with an outer diameter of or 52-mm or smaller. Patient characteristics, implant details including liner thickness, death, complications, and all cause revisions were recorded. Patients that had a minimum radiographic follow-up of seven years were assessed radiographically for linear and volumetric wear. Wear was calculated using ROMAN, a validated open-source software by two independent researchers on anteroposterior X-rays of the pelvis. A total of 55 patients were identified and included, with a mean age of 74.8 (range 38.67 - 95.9) years and a mean BMI of 28.98 (range 18.87 - 63-68). Fifty-one (94.4%) of patients were female. Twenty-six (47.7%) patients died during the follow-up period. Three patients were revised, none for liner wear, fracture or dissociation. Twenty-two patients had a radiographic follow-up of minimum seven years (mean 9.9 years, min-max 7.5 –13.7) and were included in the long-term radiographic analysis. Liner thickness was 5.5 mm at 45 degrees in all cases but one, who had a liner thickness of 4.7mm, and all patients had a cobalt-chrome head. Cup sizes were 52mm (n=15, 68%) and 50mm (n=7, 32%). Mean linear liner wear was 0.0470 mm/year (range 0 - 0.2628 mm) and mean volumetric wear was 127.69 mm3/year (range 0 - 721.23 mm3/year). Using HXLPE liners with 36-mm heads or bigger in 52-mm cups or smaller is safe, with low rates of linear and volumetric wear in the mid to long-term follow-up. Patients did not require revision surgery for liner complications, including liner fracture, dissociation, or wear. Our results suggest that the advantages of using larger heads should outweigh the potential risks of using thin HXLPE liners

High complication rates and poor outcomes have been widely reported in patients undergoing revision of large head metal-on-metal arthroplasty. A previous study from our centre showed high rates of dislocation, nerve injury, early cup loosening and pseudotumor recurrence. After noting these issues, we implemented the following changes in surgical protocol in all large head MOM revisions: One: Use of highly porous shells in all cases. Two: Use of largest femoral head possible. Three: Low threshold for use of dual mobility and constrained liners when abductors affected or absent posterior capsule. Four: Use of ceramic head with titanium sleeve in all cases. Five: Partial resection of pseudotumor adjacent to sciatic and femoral nerves. The purpose of the present study is to compare the new surgical protocol above to our previously reported early complications in this group of patients. We specifically looked at (1) complications including reoperations, (2) radiologic outcomes, and (3) functional outcomes. Complication rates after (Group 1), and before (Group 2) modified surgical protocol were compared using Chi-square test, assuming statistical significance p < 0 .05. Major complications occurred in 4 (8.3%) of 48 patients who had modified surgical technique, compared to 12 (38%) of 32 revisions prior to modification (p < 0 .05). Two hips of 48 (4.17%) endured dislocations in Group 1, compared to 9 of 32 (28%) in Group 2 (p < 0 .05). Four patients of 48 had repeat revision in Group 1: 2 for recurrence of pseudotumor, 1 for dislocation, and 1 for infection, compared to 6 patients who had 7 repeat revisions of 32 patients in Group 2: 3 for acetabular loosening, 3 for dislocation, and 1 for recurrence of pseudotumor (p=0.1). None of 48 revisions in Group 1 had acetabular loosening, compared to 4 of 32 in Group 2 (p=0.02). Two patients had nerve injury in Group 2, compared to none in Group 1 (p=0.16). The mean WOMAC pain score was 87.1 of 100 and the function score was 88.4 of 100 in Group 1, compared to a mean WOMAC pain score of 78 of 100 (p=0.6) and a function score of 83 of 100 in Group 2 (p=0.8). Modification of the surgical techniques described in the introduction has resulted in a significant decrease in complications in revision of large head MOM total hips. We continue to use this protocol and recommend it for these difficult cases

Background. Hip resurfacing arthroplasty (HRA) and total hip arthroplasty (THA) are treatments of end-stage hip disease. Gait analysis studies comparing HRA and THA have demonstrated that HRA results in a more normal gait than THA. The reasons may include the larger, more anatomic head diameter or the preservation of the neck of the femur with restoration of the anatomical position of the hip centre and normal proprioception. This study investigated (1) whether femoral head size diameter affects gait; (2) whether gait still differs between THA and HRA patients even with comparable head diameters. Methods. We retrospectively analysed the gait of 33 controls and 50 patients with a unilateral hip replacement, operated by the same surgeon. Follow-up ranged from 9–68 months. In 27 hips a small femoral head size was used (≤ 36mm); in 23 hips a large head size (>36mm). The small size group consisted of 11 long femoral stem THA and 16 short-stem THA and the large group of 5 long-stem THA, 8 short-stem THA and 10 HRA patients. There were 14 females/19 males in the control group; 22 females/5 males in the small size group; 13 females/10 males in the large size group. Results. (1) We found a significant difference in the step-length between the small head size group and the controls (p<0.01) at speeds ranging from 4.0 to 5.5 km/h but no difference between the larger head size and the controls. There was no significant difference in maximum speed, weight acceptance, push-off, mid-stance, impulse and cadence between the groups. (2) Analysis between THA and HRA in the large head size group revealed that there was a significant difference in the maximum speed (p=0.021) between the long-stem THA (6.338 km/h± 1.542) and HRA (7.756km/h± 0.7604) patients. At 5.5 km/h there was a significantly better weight acceptance (p=0.009) and mid-stance (p=0.041) of the HRA compared to short-stems. Impulse was significantly higher for HRA compared to long-stem THA (p<0.05) at all speeds ranging 4 to 5.5 km/h. (3) Males (7.1972 km/h ± .9700) had significantly higher maximum speeds compared to females (6.6524km/h± 1.019) (p=0.017) and lower gait impulse (p<0.01) at speeds ranging from 4 to 5.5km/h. (4) There was no significant difference in the Oxford Hip Score (OHS) and EQ-5D of patients in the small compared to the large head size group. Conclusions. Gait analysis demonstrated a significant difference in step length between THA patients with head size ≤ 36mm and normal controls. There was no difference in step length between normal controls and THA patients with larger head sizes. Compared to larger head size THA, the HRA still revealed higher maximum speeds and better weight acceptance. Males had significantly higher maximum speeds compared to females (controls and hip replacement patients). We could not demonstrate a correlation between better gait and Oxford scores or EQ-5D scores but these scores are known to have a ceiling effect. In a former study, better gait parameters such as longer step length and higher maximum speed have been associated with higher patient satisfaction

Increased femoral head size reduces the rate of dislocation after total hip arthroplasty (THA). With the introduction of highly crosslinked polyethylene (HXLPE) liners in THA there has been a trend towards using larger size femoral heads in relatively smaller cup sizes, theoretically increasing the risk of liner fracture, wear, or aseptic loosening. Short to medium follow-up studies have not demonstrated a negative effect of using thinner HXLPE liners. However, there is concern that these thinner liners may prematurely fail in the long-term, especially in those with thinner liners. The aim of this study was to evaluate the long-term survival and revision rates of HXLPE liners in primary THA, as well as the effect of liner thickness on these outcomes. We hypothesized that there would be no significant differences between the different liner thicknesses. We performed a retrospective database analysis from a single center of all primary total hip replacements using HXLPE liners from 2010 and earlier, including all femoral head sizes. All procedures were performed by fellowship trained arthroplasty surgeons. Patient characteristics, implant details including liner thickness, death, and revisions (all causes) were recorded. Patients were grouped for analysis for each millimeter of PE thickness (e.g. 4.0-4.9mm, 5.0-5.9mm). Kaplan-Meier survival estimates were estimated with all-cause and aseptic revisions as the endpoints. A total of 2354 patients (2584 hips) were included (mean age 64.3 years, min-max 19-96). Mean BMI was 29.0 and 47.6% was female. Mean follow-up was 13.2 years (range 11.0-18.8). Liner thickness varied from 4.9 to 12.7 mm. Seven patients had a liner thickness <5.0mm and 859 had a liner thickness of <6.0mm. Head sizes were 28mm (n=85, 3.3%), 32mm (n=1214, 47.0%), 36mm (n=1176, 45.5%), and 40mm (n=109, 4.2%), and 98.4% were metal heads. There were 101 revisions, and in 78 of these cases the liner was revised. Reason for revision was instability/dislocation (n=34), pseudotumor/aseptic lymphocyte-dominant vasculitis associated lesion (n=18), fracture (n=17), early loosening (n=11), infection (n=7), aseptic loosening (n=4), and other (n=10). When grouped by liner thickness, there were no significant differences between the groups when looking at all-cause revision (p=0.112) or aseptic revision (p=0.116). In our cohort, there were no significant differences in all-cause or aseptic revisions between any of the liner thickness groups at long-term follow-up. Our results indicate that using thinner HXPE liners to maximize femoral head size in THA does not lead to increased complications or liner failures at medium to long term follow-up. As such, orthopedic surgeons can consider the use of larger heads at the cost of liner thickness a safe practice to reduce the risk of dislocation after THA when using HXLPE liners

Femoral head diameters in THA have been increasing due to good long-term outcomes of 1. st. generation HXLP cups. Furthermore, some 2. nd. generation HXLP cups allow 36mm or larger heads. However, larger femoral head diameters increase the frictional torque and may lead to early cup migration and loosening. And there is a concern that larger head diameters and reduced liner thickness may increase polyethylene wear. In this study, we compared early acetabular component migration and wear rates between a group of larger heads using a 2. nd. generation HXLP and a group smaller heads using a 1. st. generation HXLP. The larger head group comprising 30 hips underwent THA between February 2010 and March 2011 with the use of a sequentially cross-linked polyethylene liner (X3). 30 patients were included in this study (30 women). Their mean age was 59.3years; mean weight was 53.6kg. Trident HA-coated cementless cups were used and the sizes ranged from 46mm to 56mm (mean 50.5mm). The head diameters were 36mm in 23hips, 40mm in 5 hips, and 44mm in 2hips. All X3 liners were 5.9mm or less in thickness. A control group was selected from a previous case series that had undergo THA between July 2007 and January 2008 using a 1st generation HXLP liner (Crossfire) by matching age and sex. Therefore 30 patients were included in this study (30 women) too. Their mean age was 60.0 years; mean weight was 55.5kg. The same Trident cups were used and the sizes ranged from 46mm to 56mm (mean 49.5mm). The head diameters were 26mm in 19hips and 32mm in 11hips. The liner thicknesses were 7.8mm or more. All hips had standardized anteroposterior pelvic digital radiographs performed postoperatively and cup migration was measured on digital radiographs at the immediate postoperative period and two year using EBRA-CUP software. We analyzed horizontal and vertical cup migration distance and the difference in cup anteversion and inclination angle at two years. Additionally, total head penetrarion and polyethylene liner volumetric wear rates were measured using a computer-assited method with PolyWear software. The larger head group revealed an average of 0.48mm of horizontal migration, 0.75mm of vertical migration, 0.19degree of inclination change, and 1.26 degrees of anteversion change. The control group showed an average of 0.63mm of horizontal migration, 0.36mm of vertical migration, 0.07 degree of inclination change, and 0.88 degree of anteversion change. Based on the EBRA-CUP measurements, there were no cases of significant early loosening which was indicated by more than 1mm of migration, more than 2.5 degree of inclination change, or more than 3.3 degree of anteversion change. The liner penetration rates were 0.388±0.192mm/yr in the large head group and 0.362±0.178mm/yr in the control group. The difference was not significant (p=0.64.) The volumetric wear rates were 42.8±27.9mm⁁3/yr in the large head group and 42.0±33.0mm⁁3/yr in the control group. Again, the difference was not significant (p=0.94). No significant early cup migration or increased wear rate were detected in THA with the sequentially cross-linked polyethylene liner and 36mm or large heads at two years

The number one reason to consider large heads in total hip arthroplasty (THA) is for increased stability. Large diameter femoral heads substantially increase stability by virtue of increased range of motion and increased jump distance, which is the amount of displacement required to sublux the head out of the socket. Prevention is the best means for reducing dislocation, with requisites for stability being appropriate component position, restoration of leg length, and restoration of offset. In a review from our center studying the frequency of dislocation with small diameter femoral heads (≤32 mm) in 1262 patients (1518 hips) who underwent primary THA performed via a direct lateral approach, we observed a dislocation rate of 0.8% (12 of 1518). In a subsequent study of 1748 patients (2020 hips) who underwent primary THA at our center with large diameter heads (mean 43 mm, range 36–60 mm), we observed a substantially lower 0.04% frequency of dislocation (one of 2010) at a mean followup of 2.6 years. Our findings have been echoed in studies from several other centers. Howie et al. reported a prospective controlled trial of 644 low risk patients undergoing primary or revision THA randomised to receive either a 36 mm or 28 mm metal head articulated on highly crosslinked polyethylene. They observed significantly lower frequency of frequency of dislocation with 36 mm heads both overall (1.3%, 4 of 299 versus 5.4%, 17 of 216 with 28 mm heads, p=0.012) and in primary use (0.8%, 2 of 258 versus 4.4%, 12 of 275 with 28 mm heads, p=0.024), and a similar trend in their smaller groups of revision patients (5%, 2 of 41, versus 12%, 5 of 41 with 28 mm heads, p=0.273). Lachiewicz and Soileau reported on early and late dislocation with 36- and 40 mm heads in 112 patients (122 hips) at presumed high risk for dislocation who underwent primary THA. Risk factors were age >75 for 80 hips, proximal femur fracture for 18, history of contralateral dislocation for 2, history of alcohol abuse in 2, large acetabulum (>60 mm) in 6, and other reasons in 14. Early dislocation (<1 year) occurred in 4% (5 of 122), all with 36 mm heads. Late dislocation (>5 years) did not occur in any of the 74 patients with followup beyond 5 years. Stroh et al. compared 225 patients (248 hips) treated with THA using small diameter heads (<36 mm) to 501 patients (559 hips) treated with THA using large diameter heads (≥36 mm). There were no dislocations with large diameter heads compared with 1.8% (10 of 559) with small diameter heads. Allen et al. studied whether or not large femoral heads improve functional outcome after primary THA via the posterior approach in 726 patients. There were 399 done with small heads (<36 mm), 254 with medium heads (36 mm), and 73 with large heads (>36 mm), analyzed pre-operatively, at 6 months, and at 12 months. The authors could not find a correlation between increasing head size and improved function at one year, but observed that dislocation was reduced with large diameter heads. Optimization of hip biomechanics via proper surgical technique, component position, and restoration of leg length and offset are mandatory in total hip arthroplasty. Large heads enhance stability by increasing range of motion prior to impingement and enhancing jump stability

The number one reason to consider large heads in total hip arthroplasty (THA) is for increased stability. Large diameter femoral heads substantially increase stability by virtue of increased range of motion and increased jump distance, which is the amount of displacement required to sublux the head out of the socket. Prevention is the best means for reducing dislocation, with requisites for stability being appropriate component position, restoration of leg length, and restoration of offset. In a review from our center studying the frequency of dislocation with small diameter femoral heads (≤32 mm) in 1262 patients (1518 hips) who underwent primary THA performed via a direct lateral approach, we observed a dislocation rate of 0.8% (12 of 1518). In a subsequent study of 1748 patients (2020 hips) who underwent primary THA at our center with large diameter heads (mean 43 mm, range 36–60 mm), we observed a substantially lower 0.04% frequency of dislocation (one of 2010) at a mean followup of 2.6 years. Our findings have been echoed in studies from several other centers. Howie et al. reported a prospective controlled trial of 644 low risk patients undergoing primary or revision THA randomised to receive either a 36 mm or 28 mm metal head articulated on highly crosslinked polyethylene. They observed significantly lower frequency of frequency of dislocation with 36 mm heads both overall (1.3%, 4 of 299 versus 5.4%, 17 of 216 with 28 mm heads, p=0.012) and in primary use (0.8%, 2 of 258 versus 4.4%, 12 of 275 with 28 mm heads, p=0.024), and a similar trend in their smaller groups of revision patients (5%, 2 of 41 versus 12%, 5 of 41 with 28 mm heads, p=0.273). Lachiewicz and Soileau reported on early and late dislocation with 36- and 40 mm heads in 112 patients (122 hips) at presumed high risk for dislocation who underwent primary THA. Risk factors were age >75 for 80 hips, proximal femur fracture for 18, history of contralateral dislocation for 2, history of alcohol abuse in 2, large acetabulum (>60 mm) in 6, and other reasons in 14. Early dislocation (<1 year) occurred in 4% (5 of 122), all with 36 mm heads. Late dislocation (>5 years) did not occur in any of the 74 patients with follow up beyond 5 years. Stroh et al. compared 225 patients (248 hips) treated with THA using small diameter heads (<36 mm) to 501 patients (559 hips) treated with THA using large diameter heads (≥36 mm). There were no dislocations with large diameter heads compared with 1.8% (10 of 559) with small diameter heads. Allen et al. studied whether or not large femoral heads improve functional outcome after primary THA via the posterior approach in 726 patients. There were 399 done with small heads (<36 mm), 254 with medium heads (36 mm), and 73 with large heads (>36 mm), analyzed preoperatively, at 6 months, and at 12 months. The authors could not find a correlation between increasing head size and improved function at one year, but observed that dislocation was reduced with large diameter heads. Optimization of hip biomechanics via proper surgical technique, component position, and restoration of leg length and offset are mandatory in total hip arthroplasty. Large heads enhance stability by increasing range of motion prior to impingement and enhancing jump stability

Abductor deficiency after THA can result from proximal femoral bone loss, trochanteric avulsion, muscle destruction associated with infection, pseudotumor, ALTR to metal debris, or other causes. Constrained acetabular components are indicated to control instability after THA with deficient abductors. However, the added implant constraint also results in greater stresses at the modular liner-locking mechanism of the constrained component and bone-implant fixation interface, which can contribute to mechanical failure of the constrained implant or mechanical loosening. Use of large heads has been effective in reducing the rate of dislocation after primary THA. However, relatively large (36mm) heads were not found to be effective in controlling dislocation in patients with abductor deficiency. Dual mobility implants which can provide considerably larger head diameters than 36mm may offer an advantage in improving stability in patients with abductor deficiency. However the utility of these devices in controlling instability after THA with deficient abductors has not been established. Whiteside has described a transfer of the tensor muscle and anterior gluteus maximus to the greater trochanter for treatment of absent abductors after THA. Transposition of the tensor muscle requires raising an anterior soft tissue flap to the lever of the interval between the tensor muscle and sartorius, which is the same interval used in an anterior approach to the hip. The muscle is transected distally and transposed posteriorly to attach to the proximal femur. This can result in soft tissue redundancy between the posterior tensor muscle and anterior gluteus maximus. This interval is separated and the anterior gluteus maximus also attached to the proximal femur. The transposed tensor muscle provides muscle coverage over the greater trochanter, which may be beneficial in controlling lateral hip pain. In our practice, 11 patients were treated with Whiteside's tensor muscle transfer. Six patients had absent abductors, one had an avulsed greater trochanter, and four intact but weak abductors. One patient had a muscle transposition alone, one had an ORIF of the greater trochanter and muscle transposition, two had a muscle transposition and head/liner exchange, three had a muscle transposition and cup revision, two had a femoral revision and liner exchange with muscle transposition, and two had a muscle transposition with both component revision. None of the patients had constrained components. The mean pre-operative abductor strength was 2.2 (0/5 in four patients 3/5 in four patients, and 4/5 in three patients). Pre-operative lateral hip pain was none or mild in two patients, moderate in three, and severe in six patients. Mean post-operative abductor strength was 3.2 (2/5 in four patients, 3/5 in three, 4/5 in two, 5/5 in two patients). Post-operative lateral hip pain was none in five and mild in six patients. One patient sustained a dislocation four weeks after surgery which was treated with open reduction. All of the other hips have remained stable. Treatment of patients with hip instability and abductor deficiency has generally required use of a constrained acetabular component. In our experience, transfer of the tensor muscle and anterior gluteus maximus to the greater trochanter can improve abductor strength by one grade and also reduce lateral hip pain. The combination of a large head and tensor muscle transposition may be a viable alternative to use of a fully constrained component in patients with deficient abductors after THA. However, the need for implant constraint should also be individualised and based on factors such as the viability of the transposed muscle, patient compliance with post-operative activity restrictions, femoral head/neck ratio, and cup position

Introduction. Wear plays a key role in the clinical outcome of total hip replacements (THR). In addition, increased frictional moment can stress the implant interfaces which may lead to high torsional loadings in the intermodular taper junction (fretting) and cup loosening and to the development of noise (squeaking). Against the background of larger head diameters (increased range of motion and decreased risk of dislocation), the friction induced by the joint articulation is of particular interest. As of now, the investigation of friction with the use of relevant joint kinematics and loadings are limited to numerical studies. Experimental approaches use simplified models which do not take into consideration complex activities. Thus, with the aim of this study is the identification of articular frictional moments that consider critical in vivo loading conditions and kinematics as well as the clinical cup inclination, head size and clearance of ceramic-on-ceramic hip bearings. Materials and Methods. A standard hip simulator (Minibionix 852 with 4 DOF Hip setup, MTS, Eden Prairie, USA) was modified in order to allow for high-precision friction measurements during head-insert articulation in all 6 DOF (MC2.5D-500, AMTI, Boston, USA). Disturbing systemic effects have been minimized by using quasi frictionless aerostatic lateral force compensation (Eitzenberger, Wessobrunn, Germany) and cross talk compensation. Beside the standard protocoll for in vitro wear assessment (ISO 14242-1), more complex profiles from in vivo patient data (Heidelberg Motion Lab and Orthoload database) have been used: normal walking with different walking speeds and patient's weights, stairs up/down and start-stop conditions. All-ceramic bearings (Biolox delta, Ceramtec, Plochingen, Germany) have been orientated in clinically relevant cup inclinations (30, 45, 60 and 75 deg). For each head diameter (28, 36 and 48 mm) n=8 specimens have been devided in two groups: small and large clearance according to the manufacturer's specification. All tests were run at 37°C in diluted bovine serum (20 g/l protein content). Results and Discussion. For all continuously running activities (normal walking, stairs up/down and ISO standard), increased resulting frictional moments have been measured with larger head diameters and smaller clearances in a range of less than 6 Nm. This data corresponds well to the results of a well-lubricated ceramic-on-ceramic bearing from numerical studies. In addition, the initial breakaway torque after a short resting period (start-stop initiation) was increased, where the highest maximal moments have been measured with increasing resting durations and larger head diameters (large clearance: up to 11 Nm, small clearance: up to 20 Nm). Interestingly enough, not in all cases a negative effect on the resulting moment was seen with increasing cup inclination, even though no subluxation was induced. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Head sizes used in total hip arthroplasty (THA) has increased drastically from the original 22mm used by Charnley. This is due to two factors: the use of hard-on-hard materials for the bearing articulation and the increasing problem of dislocation. The tribological aspect. Hard-on-hard materials enable mixed or fluid film lubrication due to their good wettability. The development of a fluid film layer is encouraged by smaller surface pressures (larger area) and higher velocity at the articulating interface (larger radius), suggesting that larger diameters exhibit better lubrication and such less wear. This was effectivly proven in pre-clinical simulator studies and used as argument to increase the diameters of metal-on-metall and ceramic-on-ceramic bearings. Clinically the tribological advantage of larger diameters has not yet been shown. For hard-on-soft bearings the situation is different. Due to the bad wettability of Polyethylene (PE), the abrasive wear regime is dominant. This means that the longer wear path of a larger diameter will inevitably carry a larger amount of wear debris. Despite this relation, the heads used in combination with PE were also increased up to 40mm diameter, justified by the overall greatly reduced wear amount of the new generation(s) of cross-linked PE and favourable simulator results. First in-vivo studies have shown that larger heads carry larger amounts of wear particles. Whether this increase is relevant with respect to osteolysis is still unclear and will have to be shown in longer term studies. The biomechanical aspect. Larger heads require a larger “jumping” distance until they dislocate. Consequently the use of larger heads reduces dislocation rates, which was shown in multiple clinical studies. However, the reduction in dislocation rate achieved by increasing diameters varies greatly. Some centres achieve dislocation rates below 1% with 28mm heads, other centres require 36mm heads to achieve the same result. No study shows any further advantage with head diameters larger than 36mm. Despite their obvious biomechanical advantage with regard to stability, larger heads also have large disadvantages. Larger heads carry inevitably larger friction moments, requiring better anchoring of the components. In unfavourable conditions (start-up, break-down of lubrication film), friction moments of hard-on-hard bearings can get very high and reach or even exceed the losening torque of the head on the taper. Depending on the head impaction foce during assembly, the loosening torques amount to 8 to 17Nm. Movement at the head-taper connection possibly causes wear and increased corrosion at this interface. Larger head diameters also require thinner shells and/or liners, leading to problems with liner chipping or incomplete seating. Large head diameters have also lead to the use of sub-hemispherical cups with reduced covering surface, increasing the risk of fluid film break down due to edge loading if not well positioned. Finally, larger heads might give the surgeon a wrong feeling of security regarding a sub-optimal positioned cup. The question regarding “the optimal” head diameter is open for discussion and needs to consider the bearing material used. Head size should be limited to a reasonable compromise, which based on the information currently available, could be 36mm. Join the “36 and under” club