Research: Design and Analysis of a Low-Stiffness Porous Hip Stem

A novel design of hip-stem with reduced strain-shielding

The use of uncemented stems in hip arthroplasty has been increasing, even in osteoporotic patients. The major concerns of uncemented hip-stems, however, are peri-prosthetic fracture, thigh pain, and proximal femoral stress-/strain-shielding. In this study, a novel design of uncemented hip-stem is proposed that will reduce such concerns, improve osseointegration, and benefit both osteoporotic and arthritic patients. The stem has a central titanium alloy core surrounded by a set of radial buttresses that are partly porous titanium, as is the stem tip. The aim of the study was to investigate the mechanical behaviour of the proposed partly-porous design, examining load transfer in the short-term, and comparing its strain-shielding behaviour with a solid metal implant. The long-term effect of implant-induced bone remodelling was also simulated. Computed tomography based three-dimensional finite element models of an intact proximal femur, and the same femur implanted with the proposed design, were developed. Peak hip contact and major muscle forces corresponding to level-walking and stair climbing were applied. The proposed partly-porous design had approximately 50% lower strain-shielding than the solid-metal counterpart. Results of bone remodelling simulation indicated that only 16% of the total bone volume is subjected to reduction of bone density. Strain concentrations were observed in the bone around the stem-tip for both solid and porous implants; however, it was less prominent for the porous design. Lower strain-shielding and reduced bone resorption are advantageous for long-term fixation, and the reduced strain concentration around the stem-tip indicates a lower risk of peri-prosthetic fracture.

Functionally graded stem optimizes the fixed and sliding surface coupling mechanism

Whether the optimization of fixed surface and sliding surface coupling mechanism is related to the hierarchical level of functionally graded porous stem is unknown. The functionally graded porous finite element stem models were constructed using tetrahedral microstructure with the porosities of 47-95%. The stress distribution for femoral bone gradually strengthened, the stress shielding was decreased along the increase of hierarchical levels of the stem after implantation. The coupling mechanism of fixed and sliding surfaces can be optimized by the functional gradient porous stem, the performance advantages become more prominent with the increase of hierarchical levels of the structure.

Additively manufactured controlled porous orthopedic joint replacement designs to reduce bone stress shielding: a systematic review

BACKGROUND

Total joint replacements are an established treatment for patients suffering from reduced mobility and pain due to severe joint damage. Aseptic loosening due to stress shielding is currently one of the main reasons for revision surgery. As this phenomenon is related to a mismatch in mechanical properties between implant and bone, stiffness reduction of implants has been of major interest in new implant designs. Facilitated by modern additive manufacturing technologies, the introduction of porosity into implant materials has been shown to enable significant stiffness reduction; however, whether these devices mitigate stress-shielding associated complications or device failure remains poorly understood.

METHODS

In this systematic review, a broad literature search was conducted in six databases (Scopus, Web of Science, Medline, Embase, Compendex, and Inspec) aiming to identify current design approaches to target stress shielding through controlled porous structures. The search keywords included 'lattice,' 'implant,' 'additive manufacturing,' and 'stress shielding.'

RESULTS

After the screening of 2530 articles, a total of 46 studies were included in this review. Studies focusing on hip, knee, and shoulder replacements were found. Three porous design strategies were identified, specifically uniform, graded, and optimized designs. The latter included personalized design approaches targeting stress shielding based on patient-specific data. All studies reported a reduction of stress shielding achieved by the presented design.

CONCLUSION

Not all studies used quantitative measures to describe the improvements, and the main stress shielding measures chosen varied between studies. However, due to the nature of the optimization approaches, optimized designs were found to be the most promising. Besides the stiffness reduction, other factors such as mechanical strength can be considered in the design on a patient-specific level. While it was found that controlled porous designs are overall promising to reduce stress shielding, further research and clinical evidence are needed to determine the most superior design approach for total joint replacement implants.

Femoral Stems With Porous Lattice Structures: A Review

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.

Additive manufacturing of Ti6Al4V alloy via electron beam melting for the development of implants for the biomedical industry

Additive Manufacturing (AM) or rapid prototyping technologies are presented as one of the best options to produce customized prostheses and implants with high-level requirements in terms of complex geometries, mechanical properties, and short production times. The AM method that has been more investigated to obtain metallic implants for medical and biomedical use is Electron Beam Melting (EBM), which is based on the powder bed fusion technique. One of the most common metals employed to manufacture medical implants is titanium. Although discovered in 1790, titanium and its alloys only started to be used as engineering materials for biomedical prostheses after the 1950s. In the biomedical field, these materials have been mainly employed to facilitate bone adhesion and fixation, as well as for joint replacement surgeries, thanks to their good chemical, mechanical, and biocompatibility properties. Therefore, this study aims to collect relevant and up-to-date information from an exhaustive literature review on EBM and its applications in the medical and biomedical fields. This AM method has become increasingly popular in the manufacturing sector due to its great versatility and geometry control.