Dual Mobility (DM) Total Hip Replacements (THRs), are becoming widely used but function in-vivo is not fully understood. The aim of this study was to compare the incidence of impingement of a modular dual mobility with that of a standard cup. A geometrical model of one subject's bony anatomy \[1\] was developed, a THR was implanted with the cup at a range of inclination and anteversion positions (Corail® stem, Pinnacle® cup (DePuy Synthes)). Two DM variants and one STD acetabular cup were modelled. Joint motions were taken from kinematic data of activities of daily living associated with dislocation \[2\] and walking. The occurrence of impingement was assessed for each component combination, orientation and activity. Implant-implant impingement can occur between the femoral neck and the metal or PE liner (DM or STD constructs respectively) or neck-PE mobile liner (DM only). The results comprise a colour coded matrix which sums the number of impingement events for each cup position and activity and for each implant variant. Neck-PE mobile liner impingement, occurred for both DM sizes, for all activities, and most cup placement positions indicating that the PE mobile liner is likely to move at the start of all activities including walking. For all constructs no placement positions avoided neck-metal (DM) or neck-PE liner (STD) impingementevents in all activities. The least number of events occurred at higher inclination and anteversion component positions. In addition to implant-implant impingement, some instances of bone-bone and implant-bone impingement were also observed. Consistent with DM philosophy, neck-PE mobile liner impingement and liner motion occurred for all activities including walking. Neck-liner impingement frequency was comparable between both DM sizes (metal liner) and a standard cup (PE liner).
Dual Mobility (DM) Total Hip Replacements (THRs) were introduced to reduce dislocation risk, which is the most common cause of early revision. The in-vivo mechanics of these implants is not well understood, despite their increased use in both elective and trauma settings. Therefore, the aim of this study was to comprehensively assess retrieved DM polyethylene liners for signs of damage using visual inspection and semi-quantitative geometric assessment techniques. Retrieved DM liners (n=20) were visually inspected for the presence of seven established modes of polyethylene damage. If embedded debris was identified on the external surface, its material composition was characterised using energy-dispersive x-ray analysis (EDX). Additionally, each liner was geometrically assessed for signs of wear/deformation using a validated methodology. Visual inspection of the liners revealed that scratching and pitting were the most common damage modes on either surface. Burnishing was observed on 50% and 15% of the internal and external surfaces, respectively. In addition, embedded debris was identified on 25% of the internal and 65% of the external surfaces. EDX analysis of the debris identified several materials including iron, titanium, cobalt-chrome, and tantalum. Geometric analysis demonstrated highly variable damage patterns across the liners. The results of this study provide insight into the in-vivo mechanics of DM bearings. For example, the results suggest that the internal bearing (i.e., between the head and liner) acts as the primary articulation site for DM-THRs as evidenced by a higher incidence of burnishing and larger, more concentrated regions of penetration across the liners’ internal surfaces. Furthermore, circumferential, and crescent-shaped damage patterns were identified on the articulating surfaces of the liners thus providing evidence that these components can rotate within the acetabular shell with varying degrees of mobility. The mechanics of DM bearings are complex and may be influenced by several factors (e.g., soft tissue fibrosis, patient activities) and thus further investigation is warranted. Finally, the results of this study suggest that DM liners may be susceptible to ex-vivo surface damage and thus caution is advised when handling and/or assessing these types of components.
Dual mobility (DM) total hip replacements (THRs) were introduced to reduce dislocation risk, which is the most common cause of early revision. Although DM THRs have shown good overall survivorship and low dislocation rates, the mechanisms which describe how these bearings function in-vivo are not fully understood. Therefore, the study aim was to comprehensively assess retrieved DM polyethylene liners for signs of damage using visual inspection and semi-quantitative geometric assessment methods. Retrieved DM liners (n=18) were visually inspected for the presence of surface damage, whereby the internal and external surfaces were independently assigned a score of one (present) or zero (not present) for seven damage modes. The severity of damage was not assessed. The material composition of embedded debris was characterised using energy-dispersive x-ray analysis (EDX). Additionally, each liner was geometrically assessed for signs of wear/deformation [1]. Scratching and pitting were the most common damage modes on either surface. Additionally, burnishing was observed on 50% of the internal surfaces and embedded debris was identified on 67% of the external surfaces. EDX analysis of the debris identified several materials including titanium, cobalt-chrome, iron, and tantalum. Geometric analysis demonstrated highly variable damage patterns across the liners. The incidence of burnishing was three times greater for the internal surfaces, suggesting that this acts as the primary articulation site. The external surfaces sustained more observable damage as evidenced by a higher incidence of embedded debris, abrasion, delamination, and deformation. In conjunction with the highly variable damage patterns observed, these results suggest that DM kinematics are complex and may be influenced by several factors (e.g., soft tissue fibrosis, patient activities) and thus further investigation is warranted.
The 10 year survivorship of THR is generally over 95%. However, the incidence of revision is usually higher in year one. The most common reason being dislocation which at least in part is driven by inadequate range of motion (ROM) leading to impingement, subluxation and ultimately dislocation which is more frequently posterior. ROM is affected by patient activity, bone and component geometry, and component placement. To reduce the incidence of dislocation, supported by registry data, there has been an increase in the use of so-called ‘lipped’ liners. Whilst this increases joint stability, the theoretical ROM is reduced. The aim of this study was to investigate the effect of lip placement on impingement. A rigid body geometric model was incorporated into a CT scan hemi-pelvis and femur, with a clinically available THR virtually implanted. Kinematic activity data associated with dislocation was applied, comprising of five posterior and two anterior dislocation risk activities, resulting from anterior and posterior impingement respectively. Cup inclination and anteversion was varied (30°-70°, 0°-50° respectively) to simulate extremes of clinical outcomes. The apex position of a ‘lipped’ liner was rotated from the superior position, anteriorly and posteriorly in steps of 45°. Incidence and location of implant and bone impingement was recorded in 5346 cases generated. A liner with the lip placed superior increased the occurrence of implant-implant impingement compared with a neutral liner. Rotation of the lip from superior reduced this incidence. This effect was more marked with posterior rotation which after 90° reduced anterior impingement to levels similar to a neutral liner. Complete inversion of the lipped liner reduced impingement, but this and anterior rotation both negate its function – additional stability. This study comprises one bone geometry and component design and one set of activity profiles. Nevertheless, it indicates that appropriate lip placement can minimise the likelihood of impingement for a range of daily activities whilst still providing additional joint stability.
Dual mobility (DM) hip implants whereby the polyethylene liner is “free-floating” are being used increasingly clinically. The motion of the liner is not well understood and this may provide insight into failure mechanisms; however, there are no published methods on tracking liner motion while testing under clinically relevant conditions. The aim was to develop and evaluate a bespoke inertial tracking system for DM implants that could operate submerged in lubricant without line-of-sight and provide 3D orientation information. Trackers (n=5) adhered to DM liners were evaluated using a robotic arm and a six-degree of freedom anatomical hip simulator. Before each set of testing the onboard sensor suites were calibrated to account for steady-state and non-linearity errors. The trackers were subjected to ranges of motion from ±5° to ±25° and cycle frequencies from 0.35Hz to 1.25Hz and the outputs used to find the absolute error at the peak angle for each principle axis. In total each tracker was evaluated for ten unique motion profiles with each sequence lasting 60 cycles.Abstract
Objectives
Methods
Dual mobility (DM) total hip replacements (THRs) were introduced to reduce the risk of hip dislocation in at-risk patients. DM THRs have shown good overall survivorship and low rates of dislocation, however, the mechanisms which describe how these bearings function in-vivo are not fully understood. This is partly due to a lack of suitable characterisation methodologies which are appropriate for the novel geometry and function of DM polyethylene liners, whereby both surfaces are subject to articulation. This study aimed to develop a novel semi-quantitative geometric characterisation methodology to assess the wear/deformation of DM liners. Three-dimensional coordinate data of the internal and external surfaces of 14 in-vitro tested DM liners was collected using a Legex 322 coordinate measuring machine. Data was input into a custom Matlab script, whereby the unworn reference geometry was determined using a sphere fitting algorithm. The analysis method determined the geometric variance of each point from the reference surface and produced surface deviation heatmaps to visualise areas of wear/deformation. Repeatability of the method was also assessed.Abstract
OBJECTIVES
METHODS
Impingement in total hip replacements (THRs), including bone-on-bone impingement, can lead to complications such as dislocation and loosening. The aim of this study was to investigate how the location of the anterior inferior iliac spine (AIIS) affected the range of motion before impingement. A cohort of 25 CT scans (50 hips) were assessed and nine hips were selected with a range of AIIS locations relative to the hip joint centre. The selected CT Scans were converted to solid models (ScanIP) and THR components (DePuy Synthes) were virtually implanted (Solidworks). Flexion angles of 100⁰, 110⁰, and 120⁰ were applied to the femur, each followed by internal rotation to the point of impingement. The lateral, superior and anterior extent of the AIIS from the Centre of Rotation (CoR) of the hip was measured and its effect on the range of motion was recorded.Abstract
Objectives
Methods
Many factors contribute to the occurrence of edge-loading conditions in hip replacement; soft tissue tension, surgical position, patient biomechanical variations and type of activities, hip design, etc. The aim of this study was to determine the effect of different levels of rotational and translational surgical positioning of hip replacement bearings on the occurrence and severity of edge-loading and the resultant wear rates. The Leeds II Hip-Joint Simulator and 36mm diameter alumina matrix composite ceramic bearings (BIOLOX delta, DePuy Synthes, UK) were used in this study. Different levels of mismatch between the reconstructed rotational centres of the head and the cup were considered (2, 3 and 4mm) in the medial-lateral axis. Two cup inclination angles were investigated; an equivalent to 45 and 65 degrees in-vivo, thus six conditions (n=6 for each condition) were studied in total with three million cycles completed for each condition. The wear of the ceramic-on-ceramic bearings were determined using a microbalance (Mettler Toledo, XP205, UK) and the dynamic microseparation displacement was measured using a Liner Variable Differential Transformer.Background
Method
Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres. The Leeds II hip joint simulator with a standard walking cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta, DePuy Synthes Joint Reconstruction, Leeds, UK.) were used. The study was in two parts. Part one: a biomechanical study where the dynamic separation, the maximum load during edge loading, and the duration of edge loading alongside the magnitude of forces under edge loading (severity of edge loading) were assessed. Part two; a wear study where the wear rates of the bearing surfaces were assessed under a series of input conditions. These input testing conditions included inclining the acetabular cups at 45° and 65° cup inclination angle (in-vivo equivalent), with 2, 3, and 4mm medial-lateral component mismatch between the centres of the head and the cup. This equated to six conditions being assessed, each with three repeats for the biomechanical test, and six repeats completed for the wear study. The severity of edge loading was assessed as described in Equation 1. Severity of Edge Loading = ∫tt0 F(x) dx + ∫tt0 F(y) dy … Equation 1, where F(x) is the axial load, F(y) is the medial-lateral load and t-t0 is the duration of edge loading. The wear of the ceramic bearings were determined using gravimetric analysis (XP205, Mettler Toledo, UK).Introduction and Aims
Methods
There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator. The Leeds II hip joint simulator with a standard gait cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. The study was in two stages; [1] a biomechanical study where the magnitude of dynamic separation, the duration of edge loading and the magnitude of force under edge loading (severity) were assessed under variations in component positioning and SPLs. [2] A wear study to assess edge loading with selected input conditions. For the biomechanical study, a combination of four mismatches, three cup inclination angles, and eight SPLs (Table 1) were investigated. For the wear study, three SPL conditions were selected with one cup angle and one mismatch (Table 1). Three million cycles were completed under each condition. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed (significance taken at p<0.05). Table 1: Study matrixIntroduction and Aims
Methods
Variations in component position can lead to dynamic separation and edge loading conditions. In vitro methods have been developed to simulate edge loading conditions and replicate stripe wear, increased wear rate, and bimodal wear debris size distribution, as observed clinically [1, 2]. The aim of this study was to determine the effects of translational and rotational positioning on the occurrence of dynamic separation and severity of edge loading, and then investigate the wear rates under the most severe separation and edge loading conditions on an electromechanical hip joint simulator. A hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter ceramic-on-ceramic (BIOLOX®delta, PINNACLE®, DePuy Synthes, UK) hip replacements. Three axes of rotation conditions (ISO 14242-1 [3]) was applied to the femoral head. This study was in two parts. I) A biomechanical test was carried out at 45° (n=3) and 65° (n=3) cup inclination angles with 1, 2, 3 and 4 (mm) medial-lateral translational mismatch between the centres of the head and cup. The amount of dynamic separation displacement between the head and cup was measured using a position sensor. The severity of edge loading was determined from the area under the axial force and medial-lateral force outputs during the time of separation [4]. II) A wear test was carried out at 45° (n=6) and 65° (n=6) cup inclination angles for three million cycles with translational mismatch of 4mm between the head and cup. The lubricant used was diluted new-born calf serum (25% v/v). Volumetric wear measurements were undertaken at one million cycle intervals and mean wear rates were calculated with 95% confidence limits. Statistical analysis was carried out using ANOVA and a t-test with significance levels taken at p<0.05.Introduction
Materials and Methods
In order to improve the longevity and design of an implant, a wide range of pre-clinical testing conditions should be considered including variations in surgical delivery, and patients' anatomy and biomechanics. The aim of this research study was to determine the effect of the acetabular cup inclination angle with different levels of joint centre mismatch on the magnitude of dynamic microseparation, occurrence and severity of edge loading and the resultant wear rates in a hip joint simulator. The six-station Leeds Mark II Anatomical Physiological Hip Joint Simulator and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. A standard gait cycle, with a twin-peak loading (2.5kN peak load and approximately 70N swing phase load), extension/flexion 15°/+30° and internal/external ±10° rotations, was applied. Translational mismatch in the medial-lateral axis between the centres of rotation of the head and the cup were considered. In this study, mismatches of 2, 3 and 4 (mm) were applied. Two acetabular cup inclination angles were investigated; equivalent to 45° and 65° in-vivo. These resulted in a total of six conditions [Figure 1] with n=6 for each condition. Three million cycles were completed under each condition. The lubricant used was 25% (v/v) new-born calf serum supplemented with 0.03% (w/v) sodium azide to retard bacterial growth. The wear of the ceramic bearings were determined using a microbalance (XP205, Mettler Toledo, UK) and a coordinate measuring machine (Legex 322, Mitutoyo, UK). The stripe wear was analysed using RedLux software. The dynamic microseparation displacement was measured using a linear variable differential transformer. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed with significance taken at p<0.05. Results Increasing the medial-lateral joint centre mismatch from 2 to 3 to 4mm resulted in an increased dynamic microseparation [Figure 2]. A similar trend was observed for the wear. A higher level of medial-lateral mismatch increased the wear rate under both 45° and 65° cup inclination angle conditions [Figure 3]. The mean wear rates obtained under 65° were significantly higher compared to those obtained under the 45° cup inclination angle conditions for a given medial-lateral mismatch in the joint centre (p=0.02 for 2mm mismatch, p=0.02 for 3 mm mismatch, and p<0.01 for 4mm mismatch).Introduction and Aims
Methods
Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions. Ceramic-on-ceramic bearings where the ceramic liner was inserted into a titanium alloy cup (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes, UK) were tested on the six-station Leeds II hip simulator. The first test was run with the cups inclined at an angle equivalent, clinically, to 45° (n=6) and the second test was run with the cups inclined at an angle equivalent, clinically, to 65° (n=6). A standard gait cycle was run. A fixed surgical translational mal-positioning of 4mm between the centres of rotations of the head and the cup in the medial/lateral axis was applied on all stations. Both tests ran for three million cycles each. The lubricant used was 25% new-born calf serum. Wear was assessed gravimetrically using a microbalance (XP205, Mettler Toledo, UK) and geometrically using a coordinate measuring machine (CMM, Legex 322, Mitutoyo, UK). Statistical analysis was done using one way ANOVA with significance taken at p<0.05.Introduction
Materials and Methods
The frictional torque of ceramic-on-ceramic bearings tended to increase with increasing the bearings size (32, 48, 56mm). However, the frictional torque was significantly lower than that measured on metal-on-metal bearings under well positioned and well lubricated conditions. Larger head size in total hip replacement theoretically provides increased range of motion and enhanced stability. However, there are potential clinical concerns regarding increased frictional torques with large diameter metal-on-metal bearings causing loosening of the acetabular cups and corrosion at the taper. The aim of this study was to determine the frictional torques of large diameter BIOLOX® delta ceramic-on-ceramic bearings.Summary Statement
Introduction
Ceramic-on-metal hip replacements (COM, where the head is a Biolox Delta ceramic and liner is Co Cr alloy), have demonstrated reduced wear under standard conditions in vitro compared to metal-on-metal (MOM) [1]. Early clinical results are also encouraging [2]. Recently concerns have been raised regarding the poor clinical performance of MOM hip resurfacings [3], particularly when cups are steeply inclined. Laboratory hip simulator testing has been used to replicate edge loading, also demonstrating elevated wear [4]. Therefore, a range of conditions to replicate sub-optimal use clinically to better predict in vivo performance should be used. The aim of this study was to compare the wear rates of MOM and COM under adverse edge loading conditions in an in vitro hip simulator test. Ceramic-on-metal (n=3) and metal-on-metal (n=3) 36mm hip prostheses (supplied by DePuy International Ltd, UK) were tested in the Leeds Physiological Anatomical Hip Joint Simulator. Liners were mounted to provide a clinical angle of 45o, and stems positioned anatomically. A simplified gait cycle and microseparation was applied as previously described [5] for two million cycles in 25% new born calf serum. Gravimetric analysis was completed every million cycles and wear volumes calculated.INTRODUCTION
METHODS
Concerns regarding UHMWPE wear particle induced osteolysis in total hip replacement (THR, [1]) have led to alternative materials to be sought. Carbon-fibre reinforced poly-ether-ether-ketone (CFR-PEEK) has shown reduced wear in hip and knee configurations compared with conventional polyethylene [2-4]. The aim of this study was to investigate the wear performance of a ceramic-on-CFR PEEK THR through a simulator study. Five 36mm diameter Biolox Delta heads were paired with extruded CFR-PEEK cups and tested in a hip wear simulator (Simulator Solutions, UK) for 10 million cycles (Mc). Tests were performed in a Prosim hip simulator, which applied a twin peak loading cycle, with a peak load of 3kN. Flexion-extension of −15 to +30 degrees was applied to the head and internal-external rotation of +/− 10 degrees was applied to the cup, components were mounted anatomically. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (w/v) sodium azide. Wear was assessed gravimetrically at several intervals adjusted for moisture uptake using loaded and unloaded soak controls.Introduction
Methodology
Hip wear simulation is a widely used technique for the pre-clinical evaluation of new bearing designs. However, wear rates seen in vitro can often be significantly different to those seen clinically. This can be attributed to the difference between the optimal conditions in a simulator and wide ranging conditions in real patients. This study aimed to develop more clinically relevant simulator tests, looking specifically at the effects of cup inclination angle (in vivo) and stop-dwell-start (SDS) protocols on a clinically available product. Five tests using a Paul type walking cycle (ISO 14242) were carried out on two ProSim hip simulators:
28mm MoM, standard walking, cup inclination 45°, (n = 5) 36mm MoM, standard walking, cup 45°, (n = 4) 36mm MOM, SDS: 10 walking cycles and pause of 5s with stance load of 1250N cup 45°, (n = 5) 36mm MOM, SDS: 10 walking cycles and pause of 30s with stance load of 1250N, (n = 5) cup 45° 36mm MOM, standard walking, cup 55°(n = 5), and 65°(n = 5). All samples had matched clearances, measured using a CMM (Prismo Navigator, Zeiss, Germany). Wear was measured gravimetrically (Sartorius ME235S: 0.01mg).Introduction
Method
A metal ion study was undertaken in patients who had received an articular surface replacement. The design of these components is optimised in line with lubrication theory and produces low levels of wear in hip joint simulators. Patients were recruited in four centres. Whole blood samples were analysed for metal ion levels using high resolution ICP-MS (inductively coupled plasma mass spectrophotometry). A total of 75 patients was enrolled into the study and 65 and 47 patients were assessed after 12 and 24 months implantation respectively. Results are included irrespective of clinical outcome.Introduction
Methods
The search for the ideal bearing surface in Total Hip Replacements continues. The current ‘best’ materials are felt to be combinations of metal, ceramics and cross-linked polyethylene. Laboratory studies suggest that ceramic-on-metal articulations may provide distinct advantages. This study aims to identify the best bearing surface combination with the lowest adverse side effect profile. Between February 2004 and September 2007, 164 hips were replaced in 142 patients. 39% were male and 69% were female. The average age at surgery was 53 years (17-72 years). Follow-up assessment included radiographs, the Harris Hip Score and whole blood samples for metal ion levels. Complications to date included 3 hips which needed femoral revision because of surgery related factors, and 3 cases of sepsis of which 1 settled and 2 needed revision. One hip needed revision of head and liner to a larger bearing size for recurrent dislocations, and is no longer being followed up for blood metal ions. Post-operative whole blood metal ion levels were compared to pre-operative levels to determine the increase or decrease in metal ion levels. There were no changes in those patients with ceramic-on-ceramic and ceramic-on-polyethylene articulations. Moderately raised whole blood metal ion levels were noted at 3 months in the ceramic-on-metal group, while the metal-on-metal group show the greatest increase. This study agrees with laboratory bearing surface wear studies demonstrating lower wear rates in the ceramic-on-metal group compared to the metal-on-metal group. With concerns related to high blood metal ion levels in metal-on-metal articulations, ceramic-on-metal bearing surfaces may well become a bearing surface of choice in the future, but progress needs to be monitored in the longer term.
The long-term performance of surface replacement devices remains unknown. One area of concern is the viability of the bone in the proximal portion of the femur. Previous studies, using a single time point, have shown reduced loss in bone stock compared to total hip replacement and minimal difference with the contra-lateral, unaffected hip. The aim of this study is use DEXA scanning to report the changes in bone mineral density (BMD) which take place at different time points up to 5 years post-op in the same patients following implantation. The effect of component placement will also be considered Thirty patients were recruited (24 male, 6 female; 16 left hip 14 right, mean age at surgery 53 years, range 28–65). DEXA measurements were taken post- operatively and at 3, 12, 24 and 60 months. During the study 2 patients were revised and 8 were lost to follow-up. Thus the number of patients available at each follow-up were 28, 29, 23 and 20 respectively. The regions of interest were R1 (neck region), R2 (Gruen zones 2,6) and R3 (Gruen zones 3,5). BMD in zones R1, R2 and R3 post-operatively were significantly different, 0.955, 1.114 and 1.457g/cm2 respectively (p<
0.0001). In the R1, BMD reduced at 3m to 95.0% (p=0.005) and then recovered to higher than the post-op level 102.2% (p=0.241) by 12m, and further increases to 103.5% (p=0.019) at 24m and 103.9% (p=0.057) at 60m. In zone R2, BMD reduced at 3m (97.4%, p=0.02) but recovered to post-op levels after 12m and is maintained thereafter. In zone R3 there were no significant differences from post-op. In zone R1 at 3m, 20/28 cases (71%) had a BMD that was less than the immediate post-operative value. At 12m only 12/29 cases (41%) had reduced BMD, the balance (59%) undergoing an increase. The comparable values at 60m follow-up were 43% and 57%. There was a trend for patients with higher post-op BMD to undergo a greater reduction at 3m whilst showing a greater level of recovery after 60m. However patients with higher post-op BMD had the highest 3m and 60m values. There was a trend for older patients to have a lower post-op BMD although this was not translated into greater reductions in BMD. There was no obvious correlation between femoral component angle and BMD. However there was a trend for components with a higher cup angle to undergo a greater reduction in BMD at both 3m and 60m. The current cohort was dominated by male patients and therefore comparison by gender was not possible. Changes in BMD were confined to the neck region (R1) and Gruen zones 2, 6 (R2). The finding that BMD reduces in R1 at 3m but by 12m has recovered to postop levels in R2 and in R1 has exceeded post-op levels, strongly suggests that whatever inter-operative trauma takes places is quickly repaired and changes beyond 12m are minimal out to 60m.
