Aims. The risk of mechanical failure of modular revision hip stems is frequently mentioned in the literature, but little is currently known about the actual clinical failure rates of this type of prosthesis. The current retrospective long-term analysis examines the distal and modular failure patterns of the Prevision hip stem from 18 years of clinical use. A design improvement of the modular taper was introduced in 2008, and the data could also be used to compare the original and the current design of the modular connection. Methods. We performed an analysis of the Prevision modular hip stem using the manufacturer’s vigilance database and investigated different mechanical failure patterns of the hip stem from January 2004 to December 2022. Results. Two mechanical failure patterns were identified: fractures in the area of the distal fluted profile (distal stem fracture) and failure of the modular taper (modular fracture). A failure rate of 0.07% was observed for distal stem fracture, and modular fracture rates of 1.74% for the original and 0.013% for the current taper design. Conclusion. A low risk of mechanical failure for both fracture types was observed compared to other known complications in revision hip arthroplasty. In addition, the data show that a design change did significantly reduce the risk of a modular fracture. Cite this article: Bone Joint J 2024;106-B(2):151–157

Aims. Highly polished stems with force-closed design have shown satisfactory clinical results despite being related to relatively high early migration. It has been suggested that the minimal thickness of cement mantles surrounding the femoral stem should be 2 mm to 4 mm to avoid aseptic loosening. The line-to-line cementing technique of the femoral stem, designed to achieve stem press-fit, challenges this opinion. We compared the migration of a highly polished stem with force-closed design by standard and line-to-line cementing to investigate whether differences in early migration of the stems occur in a clinical study. Methods. In this single-blind, randomized controlled, clinical radiostereometric analysis (RSA) study, the migration pattern of the cemented Corail hip stem was compared between line-to-line and standard cementing in 48 arthroplasties. The primary outcome measure was femoral stem migration in terms of rotation and translation around and along with the X-, Y-, and Z- axes measured using model-based RSA at three, 12, and 24 months. A linear mixed-effects model was used for statistical analysis. Results. Results from mixed model analyses revealed a lower mean retroversion for line-to-line (0.72° (95% confidence interval (CI) 0.38° to 1.07°; p < 0.001), but no significant differences in subsidence between the techniques (-0.15 mm (95% CI -0.53 to 0.227; p = 0.429) at 24 months. Radiolucent lines measuring < 2 mm wide were found in three and five arthroplasties cemented by the standard and line-to-line method, respectively. Conclusion. The cemented Corail stem with a force-closed design seems to settle earlier and better with the line-to-line cementing method, although for subsidence the difference was not significant. However, the lower rate of migration into retroversion may reduce the wear and cement deformation, contributing to good long-term fixation and implant survival. Cite this article: Bone Joint J 2022;104-B(1):19–26

Introduction. Metal ion and particle release, particularly cobalt, has become an important subject in total hip arthroplasty, as it has shown to induce metal hypersensitivity, adverse local tissue reactions and systemic ion related diseases. The purpose of the following study was compare the ion release barrier function of a zirconium nitride (ZrN) multilayer coated hip stem for cemented use, designed for patients with metal ion hypersensitivity, against its uncoated version in a test configuration simulating the worst case scenario of a severely debonded hip stem. The ZrN multilayer coating is applied on a CoCrMo hip stem and consists of a thin adhesive chromium layer, five alternating intermediate layers out of chromium nitride (CrN) and chromium carbonitride (CrCN) and a final zirconium nitride (ZrN) shielding layer [1]. Methods. Hip stems with a ZrN multilayer coating (CoreHip AS, Aesculap AG, Germany) were tested in comparison with a cobalt-chrome uncoated version (CoreHip, Aesculap AG, Germany). In order to create a worst case scenario, the smallest stem size with the biggest offset in combination with an XL ceramic head (offset +7 mm) was used. The stems were embedded according to the ISO 7206-6 test in a bone cement sheet. Once the bone cement was bonded, the stem was pulled out and a PMMA grain was placed inside the femoral cavity in order to uprise the hip stem above its embedding line and simulate a debonded cemented hip stem with a severe toggling condition. The dynamic test was performed under bovine serum environment with an axial force of 3.875 kN [2] at 11.6 Hz for 15 million cycles. The test was interrupted after 1, 3, 5, 10 and 15 million cycles and the surfaces of the stems were analyzed through scanning electron microscopy (SEM) with energy dispersive X-Ray (EDX). Moreover, the test medium was analyzed for metal ion concentration (cobalt, chromium and molybdenum) using ICP-MS. Results. The SEM/EDX analysis demonstrated that the ZrN multilayer coating kept its integrity, as no trace of the substrate material (CoCrMo) could be detected. Furthermore, the taper of the ZrN group showed less fretting and corrosion than the taper of the CoCrMo stem (Fig.1). Moreover, the ion concentration analysis showed a reduction of up to two orders of magnitude in the release of cobalt, chromium and molybdenum in the ZrN coated stems in comparison with the uncoated version. Discussion. The results showed that, even in a worst case scenario of high micro-motion due to a severe stem debonding within the cement mantle, the hip stems with a ZrN multilayer coating substantially reduce the release of ions from the substrate material. For any figures or tables, please contact the authors directly

Introduction and Objective. Some periprosthetic femoral fractures (PFFs) present history and radiographic aspect consistent with an atypical femoral fracture (AFF), fulfilling the criteria for AFF except that PFFs by themselves are excluded from the diagnosis of AFFs. The aim of this study was to evaluate in a single Institution series of PFFs if any of them could be considered a periprosthetic atypical femoral fracture (PAFF), and their prevalence. Materials and Methods. Surgical records were searched for PFFs around a primary hip stem from January 2013 to December 2019. Cases were classified according to Vancouver classification. Demographic and medical history were extracted. Fisher's exact test was used for statistical analysis. Results. One-hundred-fifteen PFFs were identified, 59 of them were type B1 and 16 were type C. Radiographs and medical records were available for all patients. Twenty-four patients (32%) have been treated with bisphosphonates (BPs) for longer than 4 years. Four patients presented a fracture with characteristics of PAFF. When enlarged to all PFFs of the series, no other PAFF was found: prevalence of PAFFs was 5.3% for type B1 and C cases and 3.5% for all surgically treated PFFs. Statistical significative difference between PAFFs and PFFs was found for prolonged BPs assumption and for the level of fracture clear of the stem. Conclusions. Fracture with characteristics of AFFs can also happen over a prosthetic stem, configuring themselves as PAFFs, and they are related to prolonged BPs use. As a correct diagnosis is mandatory for proper treatment, a revision of criteria for AFFs should be considered, accepting that PAFFs exist

A design modification to the DJO Linear hip stem was performed to facilitate use of the stem with the minimally invasive direct anterior approach. While the main design consideration was to reduce the overall stem length, it was also important to increase congruency of the implant and proximal cortical bone to ensure initial stability. An initial design attempt produced a geometry that was difficult to insert into the femur; therefore, reconstructed digital models of the femur (ADaMs by Materialise) were obtained and used to delineate the best fit implant cross section. The ADaMs models were constructed from 74 CT scans taken from northern Europeans undergoing investigations for cardio-vascular conditions. Using equivalency points, models representing the bone mean, ±1σ, and ±2σ were constructed. The ADaMs models are pictured in Figure 1. After importing the ADaMs models in the Solidworks CAD environment, the existing Linear stem was ideally positioned in the femur model and equally spaced planes parallel to the resection plane were defined as shown in Figure 2. At each plane, the shape of the cortical bone was determined and then used to define an implant cross section that was congruent to the bone, at least as large as the Linear hip stem, and symmetric about its midline. After using the base ADaMs models to drive the design's geometry, the final design fit was validated for very small patients using a hypothetical size −4σ extrapolation of the ADaMs models. The digital reconstructions improved the design process by providing accurate, tangible models of the actual femur geometry. From these models, the design team was able to visualize how implant geometry should be constructed to optimize congruency, symmetry, and favorable insertion characteristics. Additionally, the ADaMs models served to validate the design for a challenging condition and as a starting point for computer simulations that were able to predict the insertion difficulty encountered in the initial, pre ADaMs model design. The final redesign was launched in the US in 2014 as the TaperFill hip stem

Background: The purpose of this study was to assess the stability of short hip stem postoperatively using the radiographic parameters and a novel scoring system based on the unique fixation mechanism of the short hip stem system. Materials and Methods: Postoperative radiographic analysis was done for 31 total hip arthroplasties performed using Metha. ®. short hip stem prosthesis. The Metha. ®. short hip stem system (B. Braun Aesculap; Tuttlingen, Germany) design provides a unique fixation mechanism, with support at the medial calcar region, lateral neck, and dorso-lateral cortical contact distally. Additionally the modular design offers increased options and variations. We assessed the stability of stem by using a novel scoring system conceptually based on the unique fixation characteristics of the short stem design and included surgeon controlled variables like position and sitting of stem, dorso-lateral cortical contact and filling ratio of stem. The system also incorporated unmodifiable patient specific variables that ultimately render additional stability to the cementless stems in long run. Results: The positioning of stem was within normal range in all cases in terms of anteroposterior and lateral CCD angles and horizontal offset. The mean filling ratio of stem were 93.48%±4.38% (range 82–100%). 82.75% hips had acceptable sitting of the short stem at the osteotomy site at proximal end of femur. None of the hips showed any amount of subsidence, except one with a static first follow-up subsidence of 2mm. Using our scoring system all hips showed good to excellent results, with mean score of 44.29±3.83 (range 38–50, maximum score 50). Conclusion: Our scoring system acts as valuable tool for radiological assessment of Metha. ®. short stem postoperatively. Furthermore, in future longer follow-up studies are required to correlate the scoring system with the longevity and stability of the endoprosthesis

Aim. In the last years, many short hip stem variants were developed, almost always sharing the principle of metaphyseal and proximal diaphyseal anchorage. In this study, we analyzed the midterm results after implanting short cementless hip stem of ESKA. Methods. A total of 380 total hip replacements using the ESKA-short cementless stems were performed in a series of 340 patients between November 2002 till May 2008. The clinical and radiological evaluation of the patients was done in September 2008. The mean follow up was 37 months (3–54 months). Results. The mean age of the patients was 65 years (35–85 years). The indications varied between osteoarthritis (n=342), avascular necrosis (n=23) and aseptic loosening of stem (n=15). Good clinical and radiological outcome without complications was observed in 361 cases. However, complications were recorded in 19 cases. Aseptic loosening was noted in 4 cases and was treated with stem exchange. Deep infection occurred in 5 cases, 3 of which had direct exchange and 2 had two-stage delayed reconstruction. Dislocation was noted in 2 cases, which were revisions after aseptic loosening. 8 patients suffered intraoperative femur fissures, 7 of which healed completely after prolonged partial weight bearing postoperatively (n=4 without osteosynthesis, n=3 needed adjunctive cerclage around the proximal femur). Only 1 spiral fracture was recorded and needed a periprosthetic NCB-plate. Conclusion. Our own experience with cementless short hip stems is, in accordance with other studies, also encouraging. Based on the metaphyseal and proximal diaphyseal anchorage, short stems established themselves as a surgical option in the hip replacement surgery. There was no increase in the postoperative complication rate compared with standard total hip arthoplasty. It is also an option for elderly population. However, long-term outcome is still to be investigated

Purpose of our study is to present the design rationale and the early clinical results for the Parva Stem, an innovative short hip stem. The Parva implant has been designed in order to address the bone sparing concept of a short stem able to achieve a good cervical and interthrocanteric primary fixation associated with a the reliable primary fixation the isthmus level. The stem has been designed to address the larger possible variety of anatomical variations too. The stem therefore features innovative design concepts including the latest generation Modular Neck System, coupled with a revolutionary manufacturing process and surface engineering technology. This manufacturing process (Powder manufacturing Technology) and Ingrowth Surface (Ti-Por) will be also briefly discussed in the presentation. Our early clinical results will be also presented (150 stem-one yr. maximum follow up will be presented) although they are not the main purpose of our study oriented more on the novel design and technological manufacturing advancement. The feed-back we had so far with this state of the art implant is extremely encouraging. Of course further data collection and longer follow up will be needed in order to confirm these early promising results

The stem and the rasp for cemented arthroplasty are typically designed to obtain a cement mantle 2–5 mm thick. However, sometimes a line-to-line cementation is preferred, where the femoral cavity is prepared with the same dimension as the actual stem. There are contrasting reports [1,2] about the suitability of this technique to withstand the long-term fatigue loads. While the theoretical geometry allows no space for the cement, a sort of cement mantle is formed as the cement penetrates in the spongy bone. The scopes of this study were: 1) developing a dedicated in vitro method to test line-to-line cementation; 2) assessing if a short, polished hip stem designed for a standard cementation can be safely cemented line-to-line. In order to perform long-term mechanical in vitro tests, composite bones must be used, as cadaveric bones cannot withstand millions of loading cycles [3]. For this study, the Sawbones Mod. 3406-4 were chosen: they feature an open-cell polyurethane core simulating low-density spongy bone. Post-implantation x-rays confirmed that a relevant cement-bone interdigitation was obtained. Four femurs were prepared with a CoreHip (Aesculap) with regular cement mantle (Regular). Another 4 femurs were rasped to the same rasp size, and implanted with line-to-line cementation with a larger stem (Line-to-line). The implanted femurs were subjected to an accelerated test derived from a validated protocol [3] which replicates the most demanding motor tasks of 24 years of patient activity. Implant elastic micromotions and permanent migrations were measured throughout the test. The implants were then sectioned and treated with dye penetrants to highlight the cement cracks. Elastic and permanent motions did not show any loosening trend, and never exceeded few micrometers. As expected, some damage was visible in the cement mantles after test completion, for both types of implantation (similar to retrieved cement mantles surrounding stable implants [3]. The cement damage was similar in all specimens. No sign of major disruption was visible, neither within the Regular nor in the Line-to-line specimens: in fact, the cracks were limited in length, did not seem to cross the entire mantle thickness, and did not result in any loose cement fragments. The cracks in the line-to-line implants showed the same position and distribution compared to those found in the regular implants, but were slightly longer in some specimens. This in vitro study confirmed the feasibility of simulating line-to-line cementation in vitro. Our results suggest that a stem designed for a regular cement mantle could induce slightly more damage when implanted line-to-line, but no significant trend toward loosening

Objective: Establishment of the new method to evaluate fill of the hip stem. Background: The fill of the hip stem is one of the important parameters to estimate the quality of planning or positioning of the cementless stem. It has been defined as a stem-canal width ratio on the A-P plain of X-ray images so far. However, it is quite a problem to get the correct AP images on basis so that positional difference may affect the measurement. According to our data, the fill was measured significantly different in 15, 30, 45, 60 degrees erroneous direction. First, we tried to figure out the fill of the hip stem three-dimensionally rather 2-dimensionally. Next, our new method was compared to conventional method. Material and Methods: Leg CAT scans were performed on 32 hips of 20 patients (2 male, 18 female). Images of the canal of femora were reconstructed using CAD software. We made 2-types of canal model with or without lesser trochanter. The geometries of our lateral flare stems with different sizes were compared to each canal geometry in the CAD software and proper size was decided. Then images were observed from an accurate vertical direction of the coronal plain of the stem. We measured the 2-D fill on this plane and the 3-D fill of every 5 mm slice from the 5mm above to the 100mm below the head of lesser trochanter line (reference line). We also examined the stems 1-size smaller or larger than the appropriate ones. Results: The mean age was 61.114 (range 24–82). The average of “3-D fill of Lateral flare stem was 51%/59% with/without lesser trochanter, and 2-D one was 74%/77%. The numerical and distributional results by these two methods to measure fill were alike but different. For example, in case without lesser trochanter, the 3-D fill showed the maximum value in the area just below the reference line. The maximum 2-D fill was recorded in 10mm caudal from the reference line. In general, this stem occupied much space in the distal area and around the lesser trochanter. Future Plan: Extension of this evaluation method into various kinds of stems

Introduction. Finite element (FE) models are commonly used to analyse the mechanical behaviour of the bone under different conditions. They provide detail information but they can be numerically expensive and this limits their use in cases where large or numerous simulations are required. On the other hand, 2D models show less computational cost but the precision of results depends on the approach used for the simplification. Three 2D approaches are commonly used: models without side-plate (WOSP)[1]; models with variable thickness side-plate and constant cortical thickness (SPCT)[2]; models with side-plate and variable cortical thickness (SPVT)[3]. The aim of this study is to determine which 2D approach reproduces best the FE results obtained with a 3D model involving hip stems. Methods. The 2D models were generated by the intersection of the 3D model with the stem symmetry plane. Three approaches were considered to assure 3D-2D correspondence: 1) consider variable thickness for the cortical elements so that their transversal area moment of inertia equals the cross-sectional area moment of inertia from the 3D model (model WOSP); 2) include an additional side-plate with variable thickness to match the area moment of inertia from the 3D model, and consider constant thickness for the cortical bone elements (model SPCT); 3) include the side-plate but consider variable thickness for the cortical bone elements, derived from the 3D model (model SPVT). In all cases, the cancellous bone and stem elements had variable thickness computed so that their transversal area moment of inertia was equal to the cross-sectional area moment of inertia measured in the 3D model. This was done at different levels (Fig.1), providing a thickness distribution for the 2D elements. FE analyses were carried out for the static loading condition simulating stair climbing[4]. All materials were defined as linear isotropic and homogeneous. The post-operative situation where bone ingrowth is achieved was considered, resulting in bonded contact between the bone and the implant. The comparison between the 2D and 3D models was done based on three physical quantities: the Von Mises stresses (σ. VM. ); the strain energy density (U) and the interfacial shear stress (t) along the stem-bone interface. Results. Fig.2 shows the σ. VM. , U and t distributions for the 3D model and in the three 2D models. In general, the values for the three physical quantities were under estimated in all 2D models although the differences were small. However, the maximum values of σ. VM. and U were larger in the 2D models than in the 3D model, whereas maximum t values were under estimated. Discussion. It is possible to use 2D simplifications based on the stem symmetry plane to perform FE analyses when large number of simulations is needed or when the computational cost needs to be limited. In this way, a side-plate with variable thickness should be considered to obtain close results to the 3D model, while cortical bone thickness can be kept constant and cancellous bone thickness is varied. For figures/tables, please contact authors directly.

In the United Kingdom's National Joint Registry 2018 Annual Report, the combination of a POLARSTEM hip stem and R3 acetabular component has the lowest revision rate of any total hip arthroplasty (THA) construct combination at 7 years. Although revision rates remain a crucial measure of an implant combination's performance, there is increasingly more attention being given to patient-reported outcome measures (PROMs), which often reflect the endpoints that patients’ themselves consider of paramount importance in choosing to undergo THA. Therefore, the current analysis was undertaken to better understand the PROMs-based performance of this combination. Bespoke implant reports were requested for the POLARSTEM/R3 combination with OXINIUM™ heads and highly cross-linked polyethylene (XLPE) bearing. Reports used data from the National Health Service PROMs programme, which collected the Oxford Hip Score (OHS), EQ-5D and EQ-VAS. Health gain scores, calculated as differences between preoperative and 6-month post-operative scores, were adjusted to account for any differences in patient demographics between comparative groups. The mean OHS adjusted health gain score for the construct combination was 22.8 (95% confidence interval [CI]: 22.4 – 23.1; n = 1799 patients) compared to 21.2 (95% CI: 21.2 – 21.3; n = 111,055). For EQ-5D, the scores were 0.462 (95% CI: 0.451 – 0.473; n = 1685) for the construct and 0.434 (95% CI: 0.433 – 0.436; n = 102,448) for the class average. For EQ-VAS, the construct had adjusted scores of 14.2 (95% CI: 13.4 – 14.9; n = 1605) compared to the class average of 11.4 (95% CI: 11.3 – 11.5; n = 98,610). There were also more patients who rated their satisfaction as ‘excellent’ in the specific construct group. Comparisons were statistically significant in all cases (p < 0.001). In conclusion, in addition to excellent mid-term survivorship, the POLARSTEM/R3 construct combination has demonstrated superior PROMs that may improve patient outcomes

Purpose: More and more younger patients needs primary hip replacement. Specially for these patients the so called calcar prosthesis have been enveloped; short ste ms with fixation, bone incrow and loading only in the proximal parts of the femur. Using these type of prosthesis in cases of primary operation, later on in cases of first revision the so called standard prosthesis can be used. Materials and methods: We have experience with more than 500 calcar prosthesis type MAYO in the last 4 years. The indication for operation in these group of patients is different to the older patients group; the younger patients needs hip replacement because of rheumatic diseases, dysplasia or femoral head necrosis. The mean age of these patients is below 50 years. The implantations have been done by an modified anterolateral Watson Jones approach. Especially for the use of the MAYO stem we developed a minimal invasive operation technique to provide any trauma to the gluteal muscles. All the cases we have done are under clinical and radiological follow up. Results: Reporting all our cases according to the Harris-Hip-Score, we saw go od and excellent results; especially the good functional results could be reached in a short period of time after the operation. We have seen less complications by using the MAYO stem in comparison to the group of patients with our standard hip stems. 95% of the operations could be done without any incision to the gluteal muscles at the greater trochanter of the femur; the mean length of skin incision has been less than 8 cm. The x-ray follow up shows in none of our cases any osteolysis in the region of the calcar femoris. Conclusion: With the MAYO Hip System from our point of view good and excellent results can be reached; especially in cases of younger patients these type of short stem hip prosthesis should be used. The primary hip replacement therefore can be done with an minimum of bone lost at the calcar and with an maximum of atraumatic operation technique to the soft tissue around the hip joint

Introduction. Stress shielding is one of the major concerns of load bearing implants (e.g. hip prostheses). Stiff implants cause stress shielding, which is thought to contribute to bone resorption1. On the contrary, low-stiffness implants generate high interfacial stresses that have been related to pain and interfacial micro-movements². Different attempts have been made to reduce these problems by optimizing either the stem design3 or using functionally graded implants (FGI) where the stem's mechanical properties are optimized4. In this way, new additive manufacturing technologies allow fabricating porous materials with well-controlled mesostructure, which allows tailoring their mechanical properties. In this work, Finite Element (FE) simulations are used to develop an optimization methodology for the shape and material properties of a FGI hip stem. The resorbed bone mass fraction and the stem head displacement are used as objective functions. Methodology. The 2D-geometry of a femur model (Sawbones®) with an implanted Profemur-TL stem (Wright Medical Technology Inc.) was used for FE simulations. The stem geometry was parameterized using a set of 8 variables (Figure 1-a). To optimize the stem's material properties, a grid was generated with equally spaced points for a total of 96 points (Figure 1-b). Purely elastic materials were used for the stem and the bone. Two bone qualities were considered: good (Ecortical=20 GPa, Etrabecular=1.5 GPa) and medium (Ecortical=15 GPa, Etrabecular=1 GPa). Poisson ratio was fixed to v=0.3. Loading corresponded to stair climbing. Hip contact force along with abductors, vastus lateralis and vastus medialis muscles were considered5 for a bodyweight of 847 N. The resorbed bone mass fraction was evaluated from the differences in strain energy densities between the intact bone and the implanted bone2. The displacement of the load point on the femoral head was computed. The optimization problem was formulated as the minimization of the resorbed bone mass fraction and the head displacement. It was solved using a genetic algorithm. Results. For the Profemur-TL design, bone resorption was around 36% and 56% for good and medium bone qualities, respectively (Fig. 2). The corresponding head displacements were 11.75 mm and 21.19 mm. Optimized solutions showed bone resorption from 15% to 26% and from 44% to 65% for good and medium bone qualities, respectively. Corresponding head displacements ranged from 11.85 mm to 12.25 mm and from 16.9 mm to 22.6 mm. Conclusion. The obtained set of solutions constitutes an improvement of the implant performance for this functionally graded implant (FGI) compared to the original implant for both bone qualities. From these simulations, the final solution for the FGI could be chosen based on manufacturing restrictions or another performance indicator

Aims: To develop and test in the surgical theatre an intra-operative device, which could help the surgeon assessing the primary stability of cementless hip stem. Methods: The device is basically a measurement instrument. It hosts two sterilization-proof sensors, giving separate signals of two magnitudes: a torque and an angle. The prototype has been designed to allow safe handling during surgical manoeuvres and easy insertion/ extraction in the stem under test. A series of leds gives the surgeon information concerning the load applied and the extent of implant stability achieved. A micro-motion threshold was set at 100 microns to discriminate between stable and unstable implants. The device was validated and calibrated in vitro on 5 cadaveric and 2 composite femurs (so as to cover a wide range of bone quality & size) hosting the same kind and size of stem, with increasing press-fit (Cristofolini, 2002). The prototype stability measurement was compared with that obtained with a previously validated in-vitro protocol (Monti et al.,1999), based on an additional high accuracy LVDT. A clinical trial is now in process. Results: The validation yielded satisfactory results in terms of noise and drift (for both sensors < 0,05% of the range).The final prototype was then calibrated, comparing the device readout with the reference data from the additional LVDT. Good linear correlation was found (R2B30,98). The overall accuracy of the prototype was estimated analysing the data of all the femurs. The final prototype can predict implant stability with an error of 10% on the displacement limit (100 micron). After the first test in the operating room, the surgeons confirmed that the device was easy to handle and to use, and that the parameters extracted were clinically significant. Micromotions were recorded in implants considered sufficiently stable by the surgeons (confirming the stability threshold of 100 micron). Conclusions: The prototype was successfully designed, validated and calibrated. The overall accuracy was deemed sufficient to discriminate between stable and unstable implants. The clinical trial highlighted the great advantage that this device can give to the surgeon in deciding if press-fit is sufficient

INTRODUCTION:. Good survival rates of cementless hip stems serve as motivation for further development, just like modular implant systems or short stems. New aims are worth striving for, e.g. soft tissue or bone sparing options with similar survival rates in case of short stems. Even minimal design modifications might result in complications, e.g. missing osseointegration, loosening of the implant or painful stem, as shown in the past. One of these developments is the Biomet – GTS™ stem [Fig. 1], a hybrid between conventional cementless straight stem and potentially sparing short stem. Aim of this biomechanical study was to analyze, if the biomechanical behavior of the stem is comparable to a clinically proofed design with respect to the stem fixation in the bone and to the mechanical behavior of the stem itself. That's why the primary stability of the GTS™ stem has been determined and subsequently was compared to the Zimmer – CLS® stem. MATERIAL & METHODS. Four GTS™ stems and four CLS® stems were implanted standardized in eight synthetic femurs. Micromotions of the stem and the bone were measured at different sites. A high precision measuring device was used to apply two different cyclic load situations: 1. Axial torque of +/−7 Nm around the longitudinal stem axis to determine the rotational implant stability. 2. Varus-valgus-torque of +/−3, 5 Nm to determine the bending behavior of the stem. Comparing the motions of the stem and femur at different sites allowed the calculation of relative micromotions at the bone-implant-interface. RESULTS:. Lowest relative micromotions were detected near the lesser trochanter within the proximal part of both stems. Maximum relative micromotions were measured near the proximal end of the stem for both designs, indicating a proximal fixation of both stems [Fig. 2]. Concerning varus-valgus-torque, a similar flexibility between proximal stem shoulder and distal tip of stem was shown for both stems. DISCUSSION & CONCLUSION:. The relative micromotions of both groups seem to indicate an adequate primary stability of the stems. Obviously, the shortened design might have no fundamental influence on the biomechanical rotational stability in the bone. Compared to the CLS® stem, the GTS™ seemed to act similar flexibel during varus-valgus-torque application. Both stems might follow the bending of the bone instead of ‘tilting’ within the femur. This study showed, that the CLS® stem and the GTS™ stem biomechanically behave similar. However, a clinical confirmation of these experimental results remains to be

A novel cementless tapered wedge femoral hip implant has been designed at a reduced length and with a geometry optimized to better fit a wide array of bone types (Accolade II, Stryker, Mahwah, USA). In this study, finite element analysis (FEA) is used to compare the initial stability of the new proposed hip stem to predicate tapered wedge stem designs. A fit analysis was also conducted. The novel stem was compared to a predicate standard tapered stem and a shortened version of that same predicate stem. Methods. The novel shortened tapered wedge stem geometry was designed based on a morphological study of 556 CT scans. We then selected 10 discrete femoral geometries of interest from the CT database, including champagne fluted and stove pipe femurs. The novel and the predicate stems were virtually implanted in the bones in ABAQUS CAE. A total of thirty FEA models were meshed with 4 nodes linear tetrahedral elements. Bone/implant interface properties was simulated with contact surface and a friction coefficient of 0.35. Initial stability of each stem/bone assembly was calculated using stair-climbing loading conditions. The overall initial stability of the HA coated surface was evaluated by comparing the mean rotational, vertical, gap-opening and total micromotion at the proximal bone/implant interface of the novel and predicate stem designs. To characterize the fit of the stem designs we analyzed the ratio of a distal (60 mm below lesser trochanter) and a proximal (10 mm above lesser trochanter) cross section. A constant implantation height of 20 mm above the lesser trochanter was used. The fit of the stems was classified as Type 1 (proximal and distal engagement), Type 2 (proximal engagement only) and Type 3 (distal engagement only). Results. The mean % micromotion of the HA coated surface greater than 50 mm was lowest at 40.2% (SD 11.5%) for the novel tapered wedge stem compared to the clinically successful predicate stem design (Accolade TMAZ, Stryker, Mahwah, USA) at 44.9% (SD 13.2%) and its shortened version at 48.5% (SD 9.0%) as shown in Figure 1. Improved initial stability of the new stem was also confirmed for rotational, vertical and gap-opening micromotion. However, there was no statistically significant difference. The novel tapered stem design showed a well balanced proximal to distal ratio throughout the complete size range. The novel tapered stem design showed a reduced percentage of distal engagements (2.8%) compared to the predicate standard stem (17.2%). In the 40 to 60 year old male group the distal engagement for the standard stem increases (28.2%), whereas the distal engagements for the novel stem remains unchanged (1.3%). Discussion. It appears that through optimization of the novel tapered wedge geometry, a reduced length of a tapered wedge stem can be accomplished without jeopardizing initial stability. This data also shows that simply shortening an existing tapered wedge design may reduce the initial stability, albeit not statistically significant in this model. Optimising the shape of the stem has also significantly reduced the incidence of distal only type fixation in a computer model

Introduction. Hip stem taper wear and corrosion is a multifactorial process involving mechanical, chemical and biological damage modes. For the most cases it seems likely that the mechanically driven fretting wear is accompanied by other damage modes like pitting corrosion, galvanic corrosion or metal transfer. Recent retrieval studies have reported that the taper surface topography may affect taper damage resulting from fretting and corrosion [1]. Therefore, the current study aimed to examine effects of different taper topography parameters and material combinations on taper mechanics and results regarding wear and corrosion have been investigated. Materials and Methods. Combined experimental and numerical studies were conducted using titanium, cobalt-chromium and stainless steel generic tapers (Figure1). Uniaxial tensile tests were performed to determine the mechanical properties of the materials examined. For the taper studies macro-geometry of ceramic ball heads (BIOLOX. ®. delta) and tapers were characterized using a coordinate measuring machine, and assembly experiments according to ISO7206-10 were conducted up to 4kN. Before and after loading, taper subsidence was quantified by assembly height measurements. Taper micro-geometry, taper surface deformation, and contact area were determined by profilometry. Initial numerical studies determined coefficients of friction for the three material combinations. Macro- and micro-geometries of the tapers were modelled, and taper subsidence and assembly load served as boundary conditions. Further studies used simplified models to examine effects of varying profile depths and angular gaps on surface deformation, taper subsidence, contact area, engagement length and pull-off force. Results. Largest coefficient of friction and pull-off forces were calculated for steel (µ=0.32), cobalt-chromium revealed the lowest with µ=0.18. Titanium showed largest deformations and taper subsidence throughout all calculations (Figure2, Figure3). Taper subsidence, engagement length and deformations increased with increasing profile depth while contact area decreased. Pull-off forces were almost constant for different profile depths while they increased for increasing angular gaps. Taper subsidence and deformations also increased with increasing angular gap while engagement length decreased and contact area almost remained constant. Discussion. In order to decrease wear and corrosion micromotions should be minimized. Therefore, smaller angular gaps and smaller profile depths seems to be beneficial since deformation and taper subsidence are reduced. Literature data confirmed the results for different angular gaps showing that a larger angular gap is associated with larger amounts of micromotion and wear [2, 3]. Additionally, larger angular gaps and larger profile depths result in larger plastic deformation facilitating subsurface crack initiation and propagation. A large angular gap may also facilitate particle release [4]. Larger pull-off forces can indicate larger resistance against micromotion. Therefore, steel may tend to later develop fretting-corrosion in situ. However, among the metals examined steel also showed the largest equivalent plastic strain. This study is limited to pairings involving ceramic heads. These can help mitigating fretting corrosion resulting from micromotion between ball head and cobalt-chromium or titanium alloy tapers [5]. However, future studies will include other ball head materials. In conclusion, this study showed that taper surface topography affects taper mechanics and is important in terms of wear and corrosion

Introduction. Periprosthetic femur fractures are a serious complication after hip replacement surgery. In an aging population these fractures are becoming more and more common. Open reduction and plate osteosynthesis is one of the available treatment options. Objective. To investigate hip stem stability and cement mantle integrity under cyclic loading conditions after plate fixation with screws perforating the cement in the proximal fragment. Methods. Polished tapered hip stems were implanted in 16 biomechanical testing femora with Palacos cement (3rd generation technique) according to the manufacturer's recommendations. 8 testing bones were osteotomised distal to the stem representing the fracture group (Vancouver Type C). The osteotomy was fixed with a polyaxial locking plate, the other 8 specimens served as a control group. The specimens were tested in a biaxial material testing machine under axial compression (including adduction and torsion moments) for 100.000 cycles at physiological loads. Stem subsidence was measured in 3 planes with a stereoscopic image correlation system during the tests. Subsequently the sliced and crack dyed specimens were investigated microscopically for cement cracks. Results. In the control group no specimen failed during testing. There were no statistically significant differences in stem subsidence along the longitudinal axis (control group mean ± SD −15.4 ± 12.2 μm, fracture group −14.1 ± 13.1 μm). In the fracture group two specimens fractured through the most proximal screw hole after 74.000 and 80.000 cycles. Overall 15 out of 36 screws in the proximal fragment had direct stem contact. No cement cracks were detected in the sliced specimens in both groups. Conclusion. Drilling the cement mantle and placing screws in the cement did not increase stem subsidence under cyclic loading. No cracks or cement mantle failure were observed. Large screw diameters proximally weaken the lateral cortex resulting in tension failure of the bone. Plate fixation of a periprosthetic femoral fracture with a stable, cemented prosthesis does not lead to early cement mantle failure

Accurate detection of migration of hip arthroplasty stems without the burden of bone markers and stereo-radiographic equipment is of interest. This would facilitate the study of stem migration in an experimental setting, but more importantly, it would allow assessing stem loosening in patients with a painful hip outside a study protocol. We developed and validated a marker-free automated CT-based spatial analysis method (CTSA) to quantify stem-bone migration in successive CT scan acquisitions. First, we segmented the bone and stem within both three-dimensional images, then we pairwise registered those elements (Fig. 1). By comparing the rigid transformations of stem and bone, we calculated the migration of the stem with reference to the bone and transferred the three translation and three rotation parameters to an anatomic coordinate system. Based on the rigid transformation, we also calculated the point of the stem that presented the maximal migration (PMM). Accuracy was assessed in a stem-bone model (Fig. 2) by imposing 39 predefined stem rotations and translations, and by comparing those with values calculated with the CTSA tool. In all cases, differences were below 0.20 mm for translations and 0.19° for rotations (95% tolerance interval (95% TI) below 0.22 mm and 0.20°, largest standard deviation of the signed error (SDSE) 0.081 mm and 0.057°). Precision was defined as stem migration calculated in eight clinical relevant zero-migration scenarios. In all cases, precision was below 0.05 mm and 0.08° (95% TI below 0.06 mm and 0.08°, largest SDSE 0.012 mm and 0.020°). The largest displacement of the PMM on the stem was 0.169mm. The precision estimated in five patients was very dependent on the CT scan resolution and was below 0.48 mm and 0.37° (95% TI below 0.59 mm and 0.61°, largest SDSE 0.202 mm and 0.279°, largest displacement of the PMM 0.972 mm). In optimized conditions, the precision in patients improved largely and was below 0.040 mm and 0.111° (largest SDSE 0.202 mm and 0.279°, largest displacement of the PMM 0.156 mm). Our marker-free automated CT-based spatial analysis can detect hip stem migration with an accuracy and precision comparable to that of radiostereometric analysis (RSA), but without the burden of bone markers and the cost of stereo-radiographic equipment. As such, we believe our tool could make accurate measurement of stem migration available to departments without access to RSA and boost this type of research. Moreover, as CTSA does not rely on bone makers, it is applicable to all-comers with a painful hip arthroplasty. Indeed, in those patients with a reference CT scan after hip replacement, a new CT scan could demonstrate stem migration. If no initial CT scan is available, a reference scan could be taken during a first visit and repeated later. Additionally, a “stress test” of the hip could be performed. During such test, comparing CT images acquired during forced maximal intern and external rotation could demonstrate stem loosening