Aims. Elevated blood cobalt levels secondary to metal-on-metal (MoM) hip arthroplasties are a suggested risk factor for developing cardiovascular complications including cardiomyopathy. Clinical studies assessing patients with MoM hips using left ventricular ejection fraction (LVEF) have found conflicting evidence of cobalt-induced cardiomyopathy. Global longitudinal strain (GLS) is an echocardiography measurement known to be more sensitive than LVEF when diagnosing early cardiomyopathies. The extent of cardiovascular injury, as measured by GLS, in patients with elevated blood cobalt levels has not previously been examined. Methods. A total of 16 patients with documented blood cobalt ion levels above 13 µg/l (13 ppb, 221 nmol/l) were identified from a regional arthroplasty database. They were matched with eight patients awaiting hip arthroplasty. All patients underwent echocardiography, including GLS, investigating potential signs of cardiomyopathy. Results. Patients with MoM hip arthroplasties had a mean blood cobalt level of 29 µg/l (495 nmol/l) compared to 0.01 µg/l (0.2 nmol/l) in the control group. GLS readings were available for seven of the MoM cohort, and were significantly lower when compared with controls (-15.5% vs -18% (MoM vs control); p = 0.025)). Pearson correlation demonstrated that GLS significantly correlated with blood cobalt level (r = 0.8521; p < 0.001). However, there were no differences or correlations for other echocardiography measurements, including LVEF (64.3% vs 63.7% (MoM vs control); p = 0.845). Conclusion. This study supports the hypothesis that patients with elevated blood cobalt levels above 13 µg/l in the presence of a MoM hip implant may have impaired cardiac function compared to a control group of patients awaiting hip arthroplasty. It is the first study to use the more sensitive parameter of GLS to assess for any cardiac contractile dysfunction in patients with a MoM hip implant and a normal LVEF. Larger studies should be performed to determine the potential of GLS as a predictor of cardiac complications in patients with MoM arthroplasties. Cite this article: Bone Joint J 2024;106-B(3 Supple A):51–58

Aims. Acetabular edge-loading was a cause of increased wear rates in metal-on-metal hip arthroplasties, ultimately contributing to their failure. Although such wear patterns have been regularly reported in retrieval analyses, this study aimed to determine their in vivo location and investigate their relationship with acetabular component positioning. Methods. 3D CT imaging was combined with a recently validated method of mapping bearing surface wear in retrieved hip implants. The asymmetrical stabilizing fins of Birmingham hip replacements (BHRs) allowed the co-registration of their acetabular wear maps and their computational models, segmented from CT scans. The in vivo location of edge-wear was measured within a standardized coordinate system, defined using the anterior pelvic plane. Results. Edge-wear was found predominantly along the superior acetabular edge in all cases, while its median location was 8° (interquartile range (IQR) -59° to 25°) within the anterosuperior quadrant. The deepest point of these scars had a median location of 16° (IQR -58° to 26°), which was statistically comparable to their centres (p = 0.496). Edge-wear was in closer proximity to the superior apex of the cups with greater angles of acetabular inclination, while a greater degree of anteversion influenced a more anteriorly centred scar. Conclusion. The anterosuperior location of edge-wear was comparable to the degradation patterns observed in acetabular cartilage, supporting previous findings that hip joint forces are directed anteriorly during a greater portion of walking gait. The further application of this novel method could improve the current definition of optimal and safe acetabular component positioning. Cite this article: Bone Joint Res 2021;10(10):639–649

Abstract. Objectives. Currently, total hip replacement surgery is an effective treatment for osteoarthritis, where the damaged hip joint is replaced with an artificial joint. Stress shielding is a mechanical phenomenon that refers to the reduction of bone density as a result of altered stresses acting on the host bone. Due to solid metallic nature and high stiffness of the current orthopaedic prostheses, surrounding bones undergo too much bone resorption secondary to stress shielding. With the use of 3D printing technology such as selective laser melting (SLM), it is now possible to produce porous graded microstructure hip stems to mimics the surrounding bone tissue properties. Method. In this study we have compared the physical and mechanical properties of two triply periodic minimal surface (TPMS) lattice structure namely gyroid and diamond TPMS. Based on initial investigations, it was decided to design, and 3D print the gyroid and diamond scaffolds having pore size of 800 and 1100 um respectively. Scaffold of each type of structure were manufactured and were tested mechanically in compression (n=8), tension (n=5) and bending (n=1). Results. Upon FEA validation of the scaffold in Abaqus, the desired scaffold for hip implant application was evaluated to have a young's modules of 12.15 GPa, yield strength of 242 MPa and porosity of 55%. Topology and lattice optimization were performed using nTopology to design an optimised graded porous hip implant based on stress shielding reduction. It was understood that the designed optimised hip implant can reduce the stress shielding effect by more than 65% when compared to the conventional generic implant. Conclusions. The designed hip implant presented in this work shows clinical promise in reducing bone loss while having enhanced osseointegration with the surrounding cortical bones. Hence, this will help reduce the risk of periprosthetic fracture and the probability of revision surgery

Aims. The optimum clearance between the bearing surfaces of hip arthroplasties is unknown. Theoretically, to minimize wear, it is understood that clearances must be low enough to maintain optimal contact pressure and fluid film lubrication, while being large enough to allow lubricant recovery and reduce contact patch size. This study aimed to identify the relationship between diametrical clearance and volumetric wear, through the analysis of retrieved components. Methods. A total of 81 metal-on-metal Pinnacle hips paired with 12/14 stems were included in this study. Geometrical analysis was performed on each component, using coordinate and roundness measuring machines. The relationship between their as-manufactured diametrical clearance and volumetric wear was investigated. The Mann-Whitney U test and unpaired t-test were used, in addition to calculating the non-parametric Spearman's correlation coefficient, to statistically evaluate the acquired data. Results. The hips in this study were found to have had a median unworn diametrical clearance of 90.31 μm (interquartile range (IQR) 77.59 to 97.40); 32% (n = 26) were found to have been below the manufacturing tolerance. There was no correlation found between clearance and bearing (r. s. = -0.0004, p = 0.997) or taper (r. s. = 0.0048, p = 0.966) wear rates. The wear performance of hips manufactured within and below these specifications was not significantly different (bearing: p = 0.395; taper: p = 0.653). Pinnacles manufactured from 2007 onwards had a greater prevalence of bearing clearance below tolerance (p = 0.004). Conclusion. The diametrical clearance of Pinnacle hips did not influence their wear performance, even when below the manufacturing tolerance. The optimum clearance for minimizing hip implant wear remains unclear. Cite this article: Bone Joint Res 2020;9(8):515–523

Modern hip implants feature a modular design, whereby the individual components of the implant are assembled during the surgery. Increased reported failure rates associated with the utilization of modular junctions have raised many clinical concerns about the increased release of metal ions/debris leading to adverse local tissue reactions. Implant materials are subject to a myriad of mechanical motion and forces, and varying electrochemical conditions and pH changes from the surrounding environment. To date, no studies have attempted to model the collected data in order to predict the performance of the materials so that precautions can be taken before the problem reaches the critical stage. This study reports the effects of pH variation, displacement variation, and load variation on the mechanical and corrosion behavior of the hip implant modular junction system, tested with a custom-built fretting-corrosion apparatus. The main objective of this study is to combine the complete data set of the in-vitro experiments to create fretting-corrosion wear maps that can predict the dangerous domains of the hip implant modular system. For each test, the flat portions of two CoCrMo pins were loaded perpendicularly against a Ti6Al4V Rod (Ti alloy) in a Flat-on-flat configuration in a simulated synovial fluid in order to simulate the modular hip implant system. A schematic diagram of contact conditions is presented in Figure 1. A sinusoidal displacement was applied onto the rod, which articulated against the CoCrMo alloy pins, at a frequency of 1Hz. The experiential data from the fretting-corrosion tests has been used to create fretting-corrosion maps. The variables incorporated into the maps include: total mass loss, electrochemical destabilization, pH variation, load variation, displacement variation, and visual examination of the wear features of the contact zone. Total mass loss has been estimated via measurement of the simulator fluid by ICP-MS technique. Electrochemical destabilization was evaluated by a single parameter (V. Drop. ). The electrochemical destabilization of the tribosystem was evaluated by measuring the drop in potential, V. Drop. (V vs. SCE), resultant from the initiation of the fretting phase. The V. Drop. refers to the initial cathodic drop in potential in response to the initial onset of fretting motion. The data from the in vitro fretting-corrosion experiments has been combined to create four fretting-corrosion maps (Figures 2A–3D). Partial slip wear features and mechanical behavior was observed at 25µm displacement. 25–150µm displacement amplitudes showed gross slip behavior. Anything larger than 150µm displayed wear features that were indistinguishable from sliding wear. In general, total mass loss and V. Drop. increased with increasing displacement. Samples that were tested at pH 6.0 or higher showed signs of material transfer and higher V. Drop. Finally, there was a general decrease in V. Drop. with increased applied load and pH. In general, the wears maps were able to offer some predictive validity, however, there were some discrepancies between visual observations and the observed damage parameters. It is possible that other parameters could offer better correlation. Future studies will be conducted to measure other parameters. For figures/tables, please contact authors directly.

Introduction. As new innovations are developed to improve the longevity of joint replacement components, preclinical testing is necessary in the early stages of research into areas such as osseointegration, metal-cartilage wear and periprosthetic joint infection (PJI). Large-animal studies that test load-bearing components are expensive, however, requiring that animals be housed in special facilities that are not available at all institutions. Comparably, small animal models, such as the rat, offer several advantages including lower cost. Load-bearing implants remain difficult to manufacture via traditional methods in the sizes required for small-animal testing. Recent advances in additive manufacturing (3D metal-printing) have allowed for the creation of miniature joint replacement components in a variety of medical-grade metal alloys. The objective of this work is to create and optimize an image-based 3D-printed rat hip implant system that will allow in vivo testing of functional implant properties in a rat model. Methods. A database of n=25 previously-acquired, 154μm micro-CT volumes (eXplore Locus Ultra, GE Medical) of male Sprague-Dawley rats (390–610g) were analyzed to obtain spatial and angular relationships between several anatomical features of the proximal rat femora. Mean measurements were used to guide the creation of a femoral implant template in computer-aided design software (Solidworks, Dassault Systemes). Several different variations were created, including collarless and collared designs, in a range of sizes to accommodate rats of various weights. Initial prototypes were 3D-printed 316L stainless steel with subsequent iterations printed in Ti6Al4V titanium and F75 cobalt-chrome. Implants were post-processed via sandblasting, hand-polishing, ultrasonic bath, and sterilization in an autoclave. Innate surface texturing was left on manufactured stems to promote osseointegration. Surgical implantation was performed in three live Sprague-Dawley rats (900g, 500g, 750g) with preservation of muscle attachments to the greater trochanter. Micro-CT imaging and X-ray fluoroscopy were performed post-operatively on each animal at 1 day, and 1, 3, 9 and 12 weeks to evaluate gait and component positioning. Results. Implantation of components was successful and each animal was observed to ambulate on its affected limb immediately following recovery from surgery. The 900g rat, given a collarless 316L stainless steel component, was kept for 11 months post-implantation before succumbing to old age. Micro-CT and fluoroscopic findings revealed no evidence of implant subsidence. The 500g animal, given a collarless 316L stainless steel implant, showed evidence of implant subsidence at 3 weeks, with full subsidence and hip dislocation at 12 weeks. The 750g rat, given a collared F75 cobalt-chrome implant, was observed ambulating on its affected limb, but experienced implant rotation and failure at 9 weeks. Conclusions. We report the first hip hemi-arthroplasty in a rat using a 3D-printed metal implant. This model aims to provide a low-cost platform for studying osseointegration, metal-cartilage interactions, and PJI using a functional, loaded implant. Efforts to further optimize the surgical approach will be made to reduce early implant loosening. A study with larger sample sizes is needed to determine if implants can be installed repeatedly, without complications, before the utility of this approach can be validated. Future work will include surface preparations on implant stems, with micro-CT to longitudinally track changes at the bone-metal interface, and gait analysis on a radiolucent treadmill to quantify post-operative kinematics

Aims

Hip arthroplasty does not always restore normal anatomy. This is due to inaccurate surgery or lack of stem sizes. We evaluated the aptitude of four total hip arthroplasty systems to restore an anatomical and medialized hip rotation centre.

This study evaluated the feasibility of using published data from more than one register to define the performance of different hip implants. In order to obtain estimates of performance for specific types of hip system from different register, we analysed data from the annual reports of five national and one Italian regional register. We extracted the number of implants and rates of implant survival at different periods of follow-up. Our aim was to assess whether estimates of cumulative survival rate were comparable with data from registers from different countries, and our conclusion was that such a comparison could only be performed incompletely

Hip implant retrieval analysis is the most important source of insight into the performance of new materials and designs of hip arthroplasties. Even the most rigorous in vitro testing will not accurately simulate the behavior of implant materials and new designs of prosthetic arthroplasties. Retrieval analysis has revealed such factors as the effects of gamma-in-air sterilisation of polyethylene, fatigue failure mechanisms of polymethylmethacrylate bone cement, fretting corrosion of Morse taper junctions, third body wear effects of both hard-on-hard and hard-on-soft bearing couples, and the effects of impingement of components on the full spectrum of bearing surfaces, none of which was predicted by pre-implantation in vitro testing of these materials and combinations. The temporal sequence of the retrieval process is approximately six years from first implantation through retrieval analysis, laboratory investigation, and publication of results, and thus, in addition to rigorous clinical evaluation, represents the true development and insight cycle for new designs and materials

The philosophy of cemented total hip arthroplasty (THA) femoral components has become polarised. At one extreme are polished, collarless, tapered devices that are expected to subside; at the other extreme are roughened, non-tapered implants with a collar designed not to subside. Radiostereometric analysis (RSA) allows the accurate measurement of implant movement and has been extensively used for measurement of the in vivo migration of implants. The degree of migration as measured by RSA during the first years after surgery has been shown to correlate with the long-term performance of cemented femoral implants. The purpose of this study was to review the two-year RSA results of two different designs of primary cemented THA stems. Data from two previous prospective RSA trials with two-year follow-up were pooled. The first group included 36 patients who received a Spectron (Smith & Nephew, Memphis, USA) cemented stem. The second group included 13 patients who received an Exeter (Stryker, Mahwah, USA) cemented stem, and 15 patients who received a CPCS (Smith & Nephew, Memphis, USA) cemented stem. All patients underwent RSA examinations shortly post-operation, at 6 weeks, 3 months, 6 months, 1 years, and 2 years. Migration and rotation of the femoral stems was measured at each time point relative to the post-operative exam, and compared between the two groups. There was no difference in age at surgery (Spectron 78 ± 6 years, Exeter/CPCS 77 ± 5 years, p = 0.43), BMI (Spectron and Exeter/CPCS 28 ± 5 kg/m2, p = 0.92), or percentage of male patients (Spectron 23% male, Exeter/CPCS 21% male) between the implant groups. Subsidence was significantly greater (p < 0.0001) at all time points from three months to two years for the Exeter and CPCS stems (0.94 ± 0.39 mm at two years) compared to the Spectron stem (0.05 ± 0.16 mm at two years). There was no significant difference between the stem types for medial-lateral translation (p = 0.07) or anterior-posterior translation (p = 0.49), or for anterior-posterior tilt (p = 0.15), internal-external rotation (p = 0.89), or varus-valgus rotation (p = 0.05). Implant material, design, and surface finish are all factors in the long-term performance of cemented femoral hip implants. In this study, both femoral stem designs had a magnitude of sub-sidence that was within the limits of what is considered to be safe with respect to long-term performance. The continuous subsidence of the Exeter and CPCS stems is consistent with previous reports in the literature

There is continued debate as to whether cemented or cementless implants should be utilized in particular cases based upon chronological age. This debate has been rekindled in the UK and other countries by directives mandating certain forms of acetabular and femoral component fixation based exclusively on the chronological age of the patient. This editorial focuses on the literature-based arguments to support the use of cementless total hip arthroplasty (THA), while addressing potential concerns surrounding safety and cost-effectiveness.

Cite this article: Bone Joint Res. 2019;8(12):604–607.

Introduction. Currently, knee and hip implants are evaluated experimentally using mechanical simulators or clinically using long-term follow-up. Unfortunately, it is not practical to mechanically evaluate all patient and surgical variables and predict the viability of implant success and/or performance. More recently, a validated mathematical model has been developed that can theoretically simulate new implant designs under in vivo conditions to predict joint forces kinematics and performance. Therefore, the objective of this study was to use a validated forward solution model (FSM) to evaluate new and existing implant designs, predicting mechanics of the hip and knee joints. Methods. The model simulates the four quadriceps muscles, the complete hamstring muscle group, all three gluteus muscles, iliopsoas group, tensor fasciae latae, and an adductor muscle group. Other soft tissues include the patellar ligament, MCL, LCL, PCL, ACL, multiple ligaments connecting the patella to the femur, and the primary hip capsular ligaments (ischiofemoral, iliofemoral, and pubofemoral). The model was previously validated using telemetric implants and fluoroscopic results and is now being used to analyze multiple implant geometries. Virtual implantation allows for various surgical alignments to determine the effect of surgical errors. Furthermore, the model can simulate resecting, weakening, or tightening of soft tissues based on surgical errors or technique modifications. Results. The model revealed PCL weakening leads to paradoxical anterior slide of both femoral condyles. This paradoxical slide reduces maximum flexion and increases knee forces as seen in TKA fluoroscopic studies. Cam/post kinematics in posterior-stabilized designs were also analyzed, revealing cam/post forces increasing linearly with flexion. While cam/post engagement should ideally occur superiorly on the post and move inferiorly throughout knee flexion, fluoroscopy documented implants contacting inferiorly and rolling superiorly with flexion. Thus, a theoretical new implant was simulated to overcome this problem such that TKA design would experience the desired motion, yielding inferior contact in later flexion when forces approach 1.0 × BW. At the hip, the model predicts maximum compressive hip forces of 1.5–2.5 xBW throughout stance phase of gait. The model determines how this force is distributed on the femoral head and acetabular cup throughout the entire activity, allowing wear patterns on implant components to be predicted. During stance phase, the model predicts posterior-to-anterior sliding of the femoral head, with larger magnitudes of motion occurring on the supero-lateral aspect of the cup. The model can predict femoral neck impingement on the acetabular cup and shows that excessive anteversion of the cup leads to the femoral component levering away from the acetabular cup, yielding up to 2.0 mm of hip separation. Conclusions. This study demonstrates the ability of an in-vivo data based forward solution model to evaluate the impact of variation upon implant forces, motion and performance. This will improve understanding of observations such as polyethylene wear, pain associated with excessive soft-tissue forces, subluxation and dislocation, among others. Ultimately, the model could become a theoretical simulator that could evaluate implants much quicker for longer time durations, be less costly and provide comparative analyses when compared to present day experimental simulators

Introduction. The use modular total hip arthroplasty is associated with potentially serious local and systemic complications. Each modular interface introduces a source for wear particle generation. Research suggests the etiology of wear particle generation and subsequent corrosion begins with mechanical fretting and disruption of the protective oxide layer leading to release of metal ions at the taper interface. The purpose of this study was to conduct three dimensional (3D) surface scans of the mating surfaces of the neck-stem taper to identify features that may contribute to the fretting and surface corrosion. Methods. Eighteen modular hip implant components (9 stems and 9 necks) received 3D surface scans to examine the neck-stem taper junction. The study analyzed the neck-stem taper in an as assembled condition so relative surface positions and surface features could be studied. The 9 stems and 9 necks were scanned using an optical scanner. The implant image volume was resolved to a point spacing of 0.5 mm. Measurements were made to determine the normal distance between the surfaces of the neck taper as seated in the stem slot. These measurements were used to produce a color map of the contact proximity between the neck and stem surfaces (Figure 1). Circumferential surface points from the neck and stem at corresponding taper axis heights were used to create surface contour plots to identify surface shape variation and contact. The angle measurements and neck seated depth were analyzed by regression. Results. The typical features observed in these contact maps were: 1) a distinct vertically running line of contact at one end of the transition from the flat surface section to the radius surface section, and present on opposite surfaces in the same location; 2) distinct vertically running line of contact in the radius surface section just past the centerline on the side further away from the transition contact, and also present on the opposite radius section in the same location; 3) a concavity or area of no contact along the flat surface exists between the neck and stem components; and 4) one of the neck flat surfaces was closer to its mating surface on the stem. The plot colors show contact proximity ranging from 0 – 0.025 mm, 0.025 – 0.050 mm, and 0.050 – 0.075 mm. The average neck seated depth in the stem was 14.181 mm, ranging from 13.796 mm to 14.422 mm. Regression analysis showed that the seated depth of the neck was dependent on the taper angles in the flat section of the neck (R. 2. = 0.5000, p = 0.0332). Conclusions. Three dimensional scans and analysis suggest that the shape of the neck and stem tapers deviate from ideal design dimensions, which results in a contact pattern and component fit with gaps between the mating surfaces and rotated alignment. The probable cause of the dimensional deviation is due to machine tool deflection during manufacture. The combination of the contact and fit is expected to contribute to relative motions between the neck and stem, exacerbating the MACC

Introduction: Clinical experience has shown that addressing variations in bone morphology is important in the development of successful hip implant designs. Numerous studies of femoral bone morphology have been published utilizing various techniques. This study has developed a method which consistently measures large quantities of 3-dimensional digital femura geometry segmented from computed tomography (CT) scans and can accurately make anatomical measurements from these images. Methods: CT images of left femora on five hundred fifty six left femura (57% male, 43% female), consisting of 69% Caucasian, 16% Asian and 14% unknown were analyzed. The average age was 66 years, ranging from 40 to 93 years. Segmentation of the outer cortical, inner cortical, and marrow boundaries were consistently performed over all CT scans. The positions identified on the reference bone are transformed to the equivalent position on the clinical bone images, from which the dimensional data is extracted and stored. The mediolateral width (MLW), medial offset (MO) and lateral offset (LO) were measured in 10mm increments, ranging from 20mm above the lesser trochanter (LT) to 130mm below the lesser trochanter. The canal flare index was defined as a ratio of the mediolateral width at a section 20mm above the lesser trochanter to the mediolateral width at the isthmus level. Results: The mean mediolateral width at 20mm above the lesser trochanter was 47.0 ± 4.5 (35.1–61.8; n=556). Noble reported 45.4 ± 5.3 (31.0–60.0; n=200), Husmann reported in a neck oriented study 46.3 ± 6.9 (27.6–63.6; n=310) and Laine reported 47.1 ± 4.9 (n=50). The mean medial offset at a section 20mm above the lesser trochanter was 25.1 ± 2.9 (16.7–33.4). In the study by Husmann, a mean of 25.0 ± 5.2 (9.4–45.5) was reported. The mean canal flare index was 4.49 ±.8. Noble reported a mean canal flare index of 3.80 ±.074, Husmann 3.81 ±.83 and Laine 4.3 ±.93. Discussion: In general, the study showed minor differences to published data of proximal bone morphology. However, this more powerful study has shown that there is a higher mean canal flare index than determined by Noble and a similar mean canal flare index as determined by Laine. As reported by Laine, the canal flare index varies significantly with the placement of measurements in the canal. In this study the measurements were performed in a plane oriented by the femoral neck as a hip stem would be placed. The CFI over the isthmus width showed a greater correlation than previously shown by Noble. The novel software tool allows for anatomical measurements that can be applied to an unlimited population size enabling further applications and studies

Alumina–alumina bearings are among the most resistant to wear in total hip replacement. Examination of their surfaces is one way of comparing damage caused by wear of hip joints simulated in vitro to that seen in explanted bearings. The aim of this study was to determine whether second-generation ceramic bearings exhibited a better pattern of wear than those reported in the literature for first-generation bearings. We considered both macro- and microscopic findings.

We found that long-term alumina wear in association with a loose acetabular component could be categorised into three groups. Of 20 specimens, four had ‘low wear’, eight ‘crescent wear’ and eight ‘severe wear’, which was characterised by a change in the physical shape of the bearing and a loss of volume. This suggests that the wear in alumina–alumina bearings in association with a loose acetabular component may be variable in pattern, and may explain, in part, why the wear of a ceramic head in vivo may be greater than that seen after in vitro testing.

Background

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.

We undertook a review of the literature relating to the two basic stem designs in use in cemented hip replacement, namely loaded tapers or force-closed femoral stems, and the composite beam or shape-closed designs. The associated stem fixation theory as understood from in vitro studies and finite element modelling were examined with reference to the survivorship results for each of the concepts of fixation.

It is clear that both design principles are capable of producing successful long-term results, providing that their specific requirements of stem metallurgy, shape and surface finish, preparation of the bone and handling of the cement are observed.

Background

There has been a trend in the evolution of total hip arthroplasty towards increased modularity, with this increase in modularity come some potentially harmful consequences. Modularity at the neck shaft junction has been linked to corrosion, adverse reaction to metal debris and pseudotumor formation.

The aim of this retrieval study is to assess whether the surface integrity of the polyethylene (PE) liner is affected by metal wear debris in a single implant design series of THA revised for trunnionosis.

Uncemented hip implants commonly have porous coated surfaces that enhance the mechanical interlock with bone, encourage bone ingrowth and promote the formation of a stable interface between prosthesis and bone. However, the presence of tissue, either fibrous or with parts of osseous tissue, at the interface between the implant and the bone has been commonly observed after a few years in vivo. The exact mechanisms that govern the type of tissues formed at the interface are not fully understood and several theories have been proposed. This study aims to employ finite element analysis (FEA) to simulate tissue formation and differentiation around the AML (DePuy, Warsaw, USA) femoral implant by employing a tissue differentiation algorithm based on a mechanoregulatory hypothesis of fracture healing. FE models of the femur were generated using computer tomography (CT) scans. The AML prosthesis was then implanted into the bone and a granulation tissue layer of 0.75mm was created around the implant. The mechanoregulatory hypothesis of Carter et al (J.Orthop, 1988) originally developed to explain fracture healing was used with selected modifications, most notably the addition of a quantitative module to the otherwise qualitative algorithm. The tendency of ossification in the original hypothesis was modified to simulate tissue differentiation to bone, cartilage or fibrous tissue. Normal walking and stair climbing loads were used for a specified number of cycles reflecting typical patient activity post surgery. The transformation of granulation tissue to one of the three simulated tissue types was evident as the iterations progressed. The majority of the tissue type formed initially was cartilage and bone (~40% each), and occupied the mid to distal regions of the implant respectively. After tissue stabilisation, the prominent tissue type was bone (65%), occupying most of the mid-distal regions with a significant decline in cartilage tissue formed. This has been shown in clinical retrieval studies with the same implant, where maximum bone ingrowth is in the mid-distal regions of the implant, directly corresponding to the region where there is minimal micromotion. This would be the case with a diaphyseal fixation, which most AML prostheses employ for stability. Fibrous tissue formation was limited to the proximal-medial regions (~10%), with the remainder of the proximal regions filled with cartilage tissue. In addition, predicted bone formation was along the lines of the more stable cartilage tissue as opposed to directly replacing fibrous tissue. The formation of bone would require repeated periods of minimal micromotion and stress at the interface tissue; this was facilitated by the presence of cartilage tissue around the mid regions of the implant. The micromotion and interface stresses in the proximal regions of the implant were too high to encourage bone ingrowth, resulting in the presence of tissue that remained fibrous throughout the process. The FE model, employing a very simple tissue differentiation hypothesis and algorithm was able to predict the formation of different tissues at the interface. Initial bone formation was rapid, occupying the distal regions of the implant, and then gradually occupying a larger portion of the mid-regions around the implant. The proximal regions were largely occupied by a combination of fibrous and cartilage tissue. Overall, the presence of bone and cartilage tissue accounted for nearly 85% of the tissue formed which would suggest a very stable interface as predicted by the Carter’s hypothesis

The lastest biomechanical studies have shown that the load in the proximal femur is transferred not only medially but also laterally. Following these new philosophies, an original ultra-short stem with extensive proximal load transfer was developed. The main features of this implant are: an almost complete absence of the diaphyseal portion of the stem; a well defined lateral flare with load transfer also on the lateral column of the femur; and a high femoral neck cut which allows the preservation of most of the anterior, posterior and medial wall of the femoral neck (giving a complete proximal circumferencial bone in-growth). The implant, which we began to use in 1995 as a custom made prosthesis based on pre-operative CT data (140 cases), and later as a standard prosthesis (Proxima Hip 347 cases) was, in the first years, recommended only for young and active patients before being extended with very large indications also to elderly ones. Purpose of this paper is to present clinical and radiological results of 487 implants with an 7 yrs average follow-up. (14 yrs to 3 months). Harris Hip Score (HHS) formed the basis of the clinical assessment. Serial post-operative AP and lateral radiographs were taken for all patients.

Excellent results were reported: patients were followed-up for up to 14 years in all the series there were no revisions for aseptic loosening and only one case was revised for a deep infection. The mean HHS increased from 44.8 pre-operatively to 98.6 post-operatively at the latest follow-up. Tight pain was recorded only in one case. Other complications included 3 dislocations and 2 superficial wound infections. Radiographically good periprosthetic bone remodelling with increase of the bone stock around the implant. No radiolucent lines, subsidence or loosening have been observed.

In conclusion after a 14 year experience we can assert that neck preservation combined with a proximal lateral flare support guarantees a more natural loading of the femur and large indications. The absence of the stem makes this implant ideal not only for conventional surgical approach but also for MIS.