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Background Total hip replacement surgery is a well-established intervention that restores joint function and provides excellent outcomes for patients. In some cases, bone resorption caused by stress shielding leads to implant loosening. Stress shielding occurs because implants are much stiffer than bone and transmit a significant proportion of the load, leaving the surrounding bone to carry less load compared to an intact femur. Methods To address stress shielding, we aimed to design patient-specific additively manufactured porous femoral stems with reduced stiffness. Diamond lattice structure test specimens of varying porosities were manufactured and tested to measure elastic moduli and yield strengths. These properties were used in subsequent implant optimisation based on finite element analysis. Four implant templates were created based on the region of the implant that was assigned porous material. These templates were referred to as fully porous (FP), proximally porous with a solid distal end (PP), solid distal shell (DS), and fully solid shell (SS). In addition, the elastic modulus within the porous region was assigned either a linear or a radial distribution, resulting in eight possible implant designs. Results Optimisation yielded six distinct solutions, which were evaluated based on the reduction in stress shielding and micromotion at the bone implant interface. While all implant designs reduced stress shielding compared to a solid implant, only the PP and SS stems were predicted to pass the standard fatigue test for femoral stems (ISO 7206-4). Implant-bone micromotion was conducive for bone implant integration for all designs, with the potential exception of the fully porous stem. Of the implants that were predicted to pass the fatigue test, the linear proximal porous stem resulted in the largest reduction in stress shielding (9.5% for walking, 8.1% for stair climbing). Conclusions Based on patient-specific computational models, the porous region of the stems influenced the reduction of stress shielding compared to fully solid stems. Considering implant fatigue failure and bone-implant micromotion, the PP and SS templates were found to be the most suitable design approaches to address femoral bone stress shielding after total hip replacement. Further investigation is required to fully comprehend the implications of porous femoral stems on long-term implant stability.

In revision THA surgeries, distally fixed, extensively porous-coated femoral stems are often used to achieve a solid initial diaphyseal fixation. In this study, we reported two cases of fracture of Solution Stem (DePuy, Warsaw, Indiana, USA) following revision THA in our institute, and aimed to identify some common risk factors for such a rare complication. A female patient suffered from juvenile rheumatoid arthritis (height: 147 cm, weight: 35.0 kg, and body mass index [BMI]: 16.2) received bilateral THA in 1974 at the age of 18. Extended trochanteric osteotomy (ETO) was performed to facilitate the removal of the prosthesis, and a Solution Stem (8-inch in length, 10.5 mm in diameter) was implanted [Figure 1]a. She had an uneventful recovery after the surgery. Since the distal stem had solid bone ingrowth, stress shielding was followed and subsequently aggravated the proximal bone loss. [5] A biomechanical analysis has shown that larger diameter femoral stems achieve greater torsional stability than smaller stems at a given diaphyseal contact length in revision hip arthroplasty, and a minimum diaphyseal contact length of 3 cm or 4 cm is recommended. [2] Both stems had diameters of 10.5 mm. Although the X-rays showed reasonably good contact between the distal stem and femur shaft, given the proximal bone loss, we assume a lager stem (12 mm) would offer better stem-bone contact in the proximal femur, even though much more bone would be sacrificed during canal reaming. Meneghini RM, Hallab NJ, Berger RA, Jacobs JJ, Paprosky WG, Rosenberg AG. Stem diameter...

Cementless femoral stems are prone to stress shielding of the femoral bone, which is caused by a mismatch in stiffness between the femoral stem and femur. This can cause bone resorption and resultant loosening of the implant. It is possible to reduce the stress shielding by using a femoral stem with porous structures and lower stiffness. A porous structure also provides a secondary function of allowing bone ingrowth, thus improving the long-term stability of the prosthesis. Furthermore, due to the advent of additive manufacturing (AM) technology, it is possible to fabricate femoral stems with internal porous lattices. Several review articles have discussed porous structures, mainly focusing on the geometric design, mechanical properties and influence on bone ingrowth. However, the safety and effectiveness of porous femoral stems depend not only on the characteristic of porous structure but also on the macro design of the femoral stem; for example, the distribution of the porous structure, the stem geometric shape, the material, and the manufacturing process. This review focuses on porous femoral stems, including the porous structure, macro geometric design of the stem, performance evaluation, research methods used for designing and evaluating the femoral stems, materials and manufacturing techniques. In addition, this review will evaluate whether porous femoral stems can reduce stress shielding and increase bone ingrowth, in addition to analyzing their shortcomings and related risks and providing ideas for potential design improvements.

The coated porous section of stem surface is initially filled with callus that undergoes osseointegration process, which develops a bond between stem and bone, lessens the micromotions and transfers stresses to the bone, proximally. This phenomenon attributes to primary and secondary stabilities of the stems that exhibit trade-off the stem stiffness. This study attempts to ascertain the influence of stem stiffness on peri-prosthetic bone formation and stress shielding when in silico models of solid CoCr alloy and Ti alloy stems, and porous Ti stems (53.8 GPa and 31.5 GPa Young’s moduli) were implanted. A tissue differentiation predictive mechanoregulation algorithm was employed to estimate the evolutionary bond between bone and stem interfaces with 0.5-mm- and 1-mm-thick calluses. The results revealed that the high stiffness stems yielded higher stress shielding and lower micromotions than that of low stiffness stems. Contrarily, bone formation around solid Ti alloy stem and porous Ti 53.8 GPa stem was augmented in 0.5-mm- and 1-mm-thick calluses, respectively. All designs of stems exhibited different rates of bone formation, diverse initial micromotions and stress shielding; however, long-term bone formation was coherent with different stress shielding. Therefore, contemplating the secondary stability of the stems, low stiffness stem (Ti 53.8 GPa) gave superior biomechanical performance than that of high stiffness stems.

In order to solve the loosening problem caused by stress shielding of femoral stem prostheses in clinical practice, an optimization design method of a personalized porous titanium alloy femoral stem is proposed. According to the stress characteristics of the femur, the porous unit cell structures (TO-C, TO-T, TO-B) under three different loads of compression, torsion, and bending were designed by topology optimization. The mechanical properties and permeability of different structures were studied. Combined with the porous structure optimization, a personalized radial gradient porous titanium alloy femoral stem was designed and manufactured by selective laser melting (SLM) technology. The results show that the TO-B structure has the best comprehensive performance among the three topologically optimized porous types, which is suitable for the porous filling structure of the femoral stem, and the SLM-formed porous femoral stem has good quality. The feasibility of the personalized design and manufacture of porous titanium alloy implants is verified, which can provide a theoretical basis for the optimal design of implants in different parts.

Stress shielding and aseptic loosening are complications of short stem total hip arthroplasty, which may lead to hardware failure. Stems with increased porosity toward the distal end were discovered to be effective in reducing stress shielding, however, there is a lack of research on optimized porous distribution in stem’s coating. This study aimed to optimize the distribution of the coefficient of friction of a metaphyseal femoral stem, aiming for reducing stress shielding in the proximal area. A finite element analysis model of an implanted, titanium alloy short-tapered wedge stem featuring a porous coating made of titanium was designed to simulate a static structural analysis of the femoral stem's behavior under axial loading in Analysis System Mechanical Software. For computational feasibility, 500 combinations of coefficients of friction were randomly sampled. Increased strains in proximal femur were found in 8.4% of the models, which had decreased coefficients of friction in middle medial areas of porous coating and increased in lateral proximal and lateral and medial distal areas. This study reported the importance of the interface between bone and middle medial and distal lateral areas of the porous coating in influencing the biomechanical behavior of the proximal femur, and potentially reducing stress shielding.

Multiscale techniques, namely homogenization, result in significant computational time savings in the analysis of complex structures such as lattice structures, as in many cases it is inefficient to model a periodic structure in full detail in its entire domain. The elastic and plastic properties of two TPMS-based cellular structures, the gyroid, and the primitive surface are studied in this work through numerical homogenization. The study enabled the development of material laws for the homogenized Young’s modulus and homogenized yield stress, which correlated well with experimental data from the literature. It is possible to use the developed material laws to run optimization analyses and develop optimized functionally graded structures for structural applications or reduced stress shielding in bio-applications. Thus, this work presents a study case of a functionally graded optimized femoral stem where it was shown that the porous femoral stem built with Ti-6Al-4V can minimize stress shielding while maintaining the necessary load-bearing capacity. It was shown that the stiffness of cementless femoral stem implant with a graded gyroid foam presents stiffness that is comparable to that of trabecular bone. Moreover, the maximum stress in the implant is lower than the maximum stress in trabecular bone.

Stress shielding and aseptic loosening have been identified as adverse effects of short-stem total hip arthroplasty resulting in hardware failure. However, there is a gap in research regarding the impact of stress shielding in customized porous coatings. The purpose of this study was to optimize the distribution of the coefficients of friction in the porous coating of a metaphyseal femoral stem to minimize stress shielding. Static structural analysis of an implanted short, tapered-wedge stem with a titanium porous coating was performed with the use of Analysis System Mechanical Software under axial loading. To limit computational time, we randomly sampled only 500 of the possible combinations of coefficients of friction. Results indicate that the coefficient of friction in the distal lateral porous coating significantly affected the mid-distal medial femoral surface and lateral femoral surface. The resultant increased proximal strains resulted from an increased coefficient of friction in lateral porous coating and a reduction in the coefficient of friction in medial mid-distal coating. These findings suggest that a customized porous coating distribution may produce strain patterns that are biomechanically closer to intact bone, thereby reducing stress shielding in short femoral stems.

In this communication, a new methodological approach is proposed to develop a biomimetic metallic femoral stem. The design of this stem starts with the definition of an outer skin by reproducing the shape and overall dimensions of a Stryker® femoral stem to be implanted in an artificial femur model from Sawbones®. In-house algorithms are then used to generate two types of porous structures inside the outer skin: either a stochastic cubic-based porous structure or an ordered diamond-type porous structure. Next, a model of the femur-stem assembly is developed using the finite element method. The fully dense Stryker stem replica and two porous stems are fabricated using selective laser melting technology. Then, comparative mechanical testing is carried out using the ISO 7206-4 (2010) guidelines. These tests are conducted on an intact artificial femur (reference case) and on the identical femurs, but now implanted with the fully dense and porous stems. Using digital image correlation tools, the results of four series of tests are compared to assess which implant design leads to the lowest stress shielding in the implanted femur. Finally, the experimentally measured strain fields are compared to the numerical predictions to validate the numerical models.

Background Revision total hip arthroplasty (THA) with severe femoral bone defects remains a major challenge. The purpose of this study is to report the minimum 8-year clinical and radiographic results of revision THA with severe femoral bone defects treated with extensively porous-coated stems and cortical strut allografts. Methods We retrospectively identified 44 patients diagnosed with Paprosky type III and IV femoral bone defects between January 2006 and July 2011. The exclusion criteria were patients not eligible for surgery, revised with extensively porous-coated stems alone, lost to follow-up and deceased. A total of 31 patients treated with extensively porous-coated stems and cortical strut allografts were finally included in this study. The degree of femoral bone defects was categorized as Paprosky type IIIA in 19 patients, type IIIB in 9 patients and type IV in 3 patients. The mean duration of follow-up was 11.0 ± 1.5 (range, 8.1–13.5) years. Results The mean Harris Hip Score improved significantly from 43.4 ± 10.5 points to 85.2 ± 6.6 points ( P  < 0.001). Similarly, WOMAC and SF-12 scores also significantly improved. Twenty-eight stems achieved stable bone ingrowth, two stems showed stable fibrous ingrowth, and one stem was radiologically unstable. Complete union and bridging between cortical strut allografts and host bone was achieved in all 31 patients. The femoral width was augmented with cortical strut allografts after revision surgery (an increase of 10.5 ± 0.5 mm) and showed a slight decrease of 2.5 ± 4.8 mm after the 10-year follow-up. Using re-revision for any reason as an endpoint, the Kaplan-Meier cumulative survival rate of the stem was 96.2% (95% confidence interval, 75.7–99.5%) at 10 years. Conclusion Our data demonstrate that the use of extensively porous-coated stems combined with cortical strut allografts in revision THA with Paprosky type III and IV femoral bone defects can provide satisfactory clinical and radiographic outcomes with a minimum follow-up of 8 years.