The role of stiffness-matching in avoiding stress shielding-induced bone loss and stress concentration-induced skeletal reconstruction device failure

It is well documented that overly stiff skeletal replacement and fixation devices may fail and require revision surgery. Recent attempts to better support healing and sustain healed bone have looked at stiffness-matching of these devices to the desired role of limiting the stress on fractured or engrafted bone to compressive loads and, after the reconstructed bone has healed, to ensure that reconstructive medical devices (implants) interrupt the normal loading pattern as little as possible. The mechanical performance of these devices can be optimized by adjusting their location, integration/fastening, material(s), geometry (external and internal), and surface properties. This review highlights recent research that focuses on the optimal design of skeletal reconstruction devices to perform during and after healing as the mechanical regime changes. Previous studies have considered auxetic materials, homogeneous or gradient (i.e., adaptive) porosity, surface modification to enhance device/bone integration, and choosing the device's attachment location to ensure good osseointegration and resilient load transduction. By combining some or all of these factors, device designers work hard to avoid problems brought about by unsustainable stress shielding or stress concentrations as a means of creating sustainable stress-strain relationships that best repair and sustain a surgically reconstructed skeletal site. STATEMENT OF SIGNIFICANCE: Although standard-of-care skeletal reconstruction devices will usually allow normal healing and improved comfort for the patient during normal activities, there may be significant disadvantages during long-term use. Stress shielding and stress concentration are amongst the most common causes of failure of a metallic device. This review highlights recent developments in devices for skeletal reconstruction that match the stiffness, while not interrupting the normal loading pattern of a healthy bone, and help to combat stress shielding and stress concentration. This review summarises various approaches to achieve stiffness-matching: application of materials with modulus close to that of the bone; adaptation of geometry with pre-defined mechanical properties; and/or surface modification that ensures good integration and proper load transfer to the bone.

Efficacy of Allograft Versus Bioactive Glass-Ceramic Cage in Anterior Cervical Discectomy and Fusion: A Randomized Controlled Study

STUDY DESIGN

A randomized controlled trial.

OBJECTIVE

The aim of this study is to compare the efficacy of allografts and bioactive glass-ceramic (BG) cages for anterior cervical discectomy and fusion (ACDF) in treating cervical degenerative disc disease.

METHODS

We conducted a single-center, randomized controlled trial between August 2017 and August 2022. Participants were randomized into two groups, and consecutive patients requiring ACDF were randomly assigned to receive either the allograft cage or the BG cage. The surgical outcomes measured included pain levels, neck disability, surgical details, and radiological assessments.

RESULTS

Of the 45 assessed, 40 participants were included, with 18 in the allograft cage group and 22 in the BG cage group. By the 12-month follow-up, both groups exhibited significant improvements in pain levels and disability scores, with no notable intergroup differences. Over 85% of patients in both groups were satisfied with their outcomes. Radiological assessments revealed stability in the cervical spine with both cage types post intervention. Although both materials showed a trend toward increased subsidence over time, the difference between them was not statistically significant. Fusion rates were comparable between the groups at 12 months, with BG cage showing a slightly higher early fusion rate at 6 months. No significant differences were observed between the two groups in terms of complications.

CONCLUSIONS

Both allograft and BG cages are effective in ACDF surgeries for cervical degenerative disc disease, with both contributing to substantial postoperative improvements. Differences in disc height, interspinous motion, and subsidence were not significant in the last follow-up, indicating both materials' suitability for clinical use. Future research with a larger cohort and longer follow-up is needed to confirm these preliminary findings.

A novel hybrid design and modelling of a customised graded Ti-6Al-4V porous hip implant to reduce stress-shielding: An experimental and numerical analysis

Stress shielding secondary to bone resorption is one of the main causes of aseptic loosening, which limits the lifespan of hip prostheses and exacerbates revision surgery rates. In order to minimise post-hip replacement stress variations, this investigation proposes a low-stiffness, porous Ti6Al4V hip prosthesis, developed through selective laser melting (SLM). The stress shielding effect and potential bone resorption properties of the porous hip implant were investigated through both in vitro quasi-physiological experimental assays, together with finite element analysis. A solid hip implant was incorporated in this investigation for contrast, as a control group. The stiffness and fatigue properties of both the solid and the porous hip implants were measured through compression tests. The safety factor of the porous hip stem under both static and dynamic loading patterns was obtained through simulation. The porous hip implant was inserted into Sawbone/PMMA cement and was loaded to 2,300 N (compression). The proposed porous hip implant demonstrated a more natural stress distribution, with reduced stress shielding (by 70%) and loss in bone mass (by 60%), when compared to a fully solid hip implant. Solid and porous hip stems had a stiffness of 2.76 kN/mm and 2.15 kN/mm respectively. Considering all daily activities, the porous hip stem had a factor of safety greater than 2. At the 2,300 N load, maximum von Mises stresses on the hip stem were observed as 112 MPa on the medial neck and 290 MPa on the distal restriction point, whereby such values remained below the endurance limit of 3D printed Ti6Al4V (375 MPa). Overall, through the strut thickness optimisation process for a Ti6Al4V porous hip stem, stress shielding and bone resorption can be reduced, therefore proposing a potential replacement for the generic solid implant.

EVALUATING THE EFFECTS OF EXPERIMENTAL SETTINGS DURING ISO 7206-4 ENDURANCE AND PERFORMACE TESTS: A FINITE ELEMENT ANALYSIS

ISO 7206-4:2010 is used to determine the endurance properties and performance of stemmed femoral components of hip prosthesis. Experimental set-ups are allowed for tolerances with respect to the desired settings. The effects of the acceptable accuracies are not known nowadays. For this reason, this study aims at evaluating how and how much the precision of the experimental settings can affect the outputs under ISO 7206-4:2010 boundaries. Thus, a finite element model was first defined and verified against experimental test. Then, a sensitivity analysis involving accepted variations for potting level, angle in the frontal plane, angle in the lateral plane and material in use for the cement block, was performed. The results of sensitivity analysis show that both stress and deformation outputs were affected up to 100[Formula: see text]MPa and 0.5[Formula: see text]mm, respectively, for varus configurations. The material properties of the cement block had a main effect on the displacement of the head. The collected information through the performed sensitivity analysis on the verified model against experimental test has a double benefit. It supports the understanding of the potential effects during the experimental set-ups and it is also helpful in case of determining ranges for verification of developed numerical models.

Influence of the Surface Roughness of PEEK GRF30 and Ti6Al4V SLM on the Viability of Primary Human Osteoblasts Determined by the MTT Test

The aim of the study was to clearly determine whether selected modern medical materials and three dimensional printing allow for satisfactory viability of human osteoblasts, which is important from the point of view of the subsequent osseointegration process. Moreover, as implants are produced with various topography, the influence of surface roughness on viability of bone cells was evaluated. To conduct the research, primary human osteoblasts (PromoCell) were used. Cells were seeded on samples of glass-reinforced polyetheretherketone (30% of the filling), Ti6Al4V manufactured with the use of selective laser melting technology and forged Ti6Al4V with appropriately prepared variable surface roughness. To assess the viability of the tested cells the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide was used. Results showed that all evaluated materials do not exhibit cytotoxic properties. Moreover, on their basis it can be concluded that there is a certain surface topography (designated i.a. as roughness equal to approx. Ra = 0.30 μm), which ensures the highest possible viability of human osteoblasts. On the basis of the received data, it can also be concluded that modern glass-reinforced polyetheretherketone or Ti6Al4V produced by rapid prototyping method allow to manufacture implants that should be effectively used in clinical conditions.

Development and in vitro validation of a simplified numerical model for the design of a biomimetic femoral stem

BACKGROUND

Dense and stiff metallic femoral stems implanted into femurs for total hip arthroplasties produce a stress shielding effect since they modify the original load sharing path in the bony structure. Consequently, in the long term, the strain adaptive nature of bones leads to bone resorption, implant loosening, and the need for arthroplasty revision. The design of new cementless femoral stems integrating open porous structures can reduce the global stiffness of the stems, allowing them a better match with that of bones and provide their firm fixation via bone ingrowth, and, thus reduce the risk of implantation failure.

METHODS

This paper aims to develop and validate a simplified numerical model of stress shielding, which calculates the levels of bone resorption or formation by comparing strain distributions on the surface of the intact and the implanted femurs subjected to a simulated biological loading. Two femoral stems produced by laser powder-bed fusion using Ti-6Al-4V alloy are employed: the first is fully dense, while the second features a diamond cubic lattice structure in its core. The validation consists of a comparison of the numerically calculated force-displacement diagrams, and displacement and strain fields with their experimental equivalents obtained using the digital image correlation technique.

RESULTS AND CONCLUSIONS

The numerical models showed reasonable agreement between the force-displacement diagrams. Also, satisfactory results for the correlation analyses of the total displacement and equivalent strain fields were obtained. The stress shielding effect of the implant was assessed by comparing the equivalent strain fields of the implanted and intact femurs. The results obtained predicted less bone resorption in the femur implanted with the porous stem than with its dense counterpart.

Influence of Femoral Implant Alignment in Uncemented Total Hip Replacement Arthroplasty: Varus Insertion and Stress Shielding

BACKGROUND

The influence of varus insertion of femoral implants in uncemented total hip replacement arthroplasty (THR) remains unclear. Thus, in this study, we retrospectively assessed the clinical impact of uncemented THR with femoral implants that were inserted in varus on the basis of radiological findings.

MATERIALS AND METHODS

The study participants included 89 patients who underwent uncemented THR for 106 joints and were followed-up for >3 years. From clinical records, we retrieved Japanese Orthopaedic Association (JOA) pain scores and the range of motion (ROM) of flexion and abduction both preoperatively and at the final follow-up. The presence of varus insertion of the femoral implant and stress shielding were also retrospectively reviewed from X-rays. We defined varus insertion of the femoral implant as the axis of the femoral implant that was inclined to the femoral shaft by 2° or more. Stress shielding was judged in accordance with Engh's classification system.

RESULTS

Of the 106 joints, varus insertion was observed in 40 (37.3%) (the varus group) but not in 66 (62.3%) (the non-varus group). The JOA pain score significantly improved in both groups; however, there were no significant differences between the groups. Although ROM improved in both groups, there were no significant differences between the groups. The appearance rate of stress shielding of ≥third degree in the varus group was significantly greater than that in the non-varus group.

CONCLUSION

These results revealed that varus insertion of femoral implants had no influence on short- to mid-term clinical outcomes because the pain score and ROM significantly improved in both the varus and non-varus groups. However, high rates of severe stress shielding appeared with varus insertion of femoral implants, suggesting an influence on long-term clinical outcomes.

In vitro implant-bone interface pressure measurements for a cementless femoral implant. A preliminary study

PURPOSE

Implants endurance as well as a good clinical tolerance depends on the recovery of a physiological stress distribution within bone after implantation. The purpose of the present work was to develop an alternative technique using Force Sensing Resistors (FSR) to gather in vitro pressure values at the implant-bone interface for a cementless implant.

METHOD

Eight cementless femoral stems were instrumented with six calibrated FSR bonded on each facet and then implanted in eight cadaver femurs. Compression tests were performed until failure and FSR pressure values were recorded.

RESULTS

The average failure load was 4241 N. The maximum contact pressure measured with the FSR averaged 1.965 MPa.

CONCLUSION

FSR reached many of the requirements for an ideal implant-bone interfacial sensor. This experimentation provided in vitro quantitative data on contact pressure at the implant-bone interface, which could help understanding stress shielding phenomenon and developing relevant numerical model.

Bone bonding ability of a chemically and thermally treated low elastic modulus Ti alloy: gum metal

The gum metal with composition Ti-36Nb-2Ta-3Zr-0.3O, is free from cytotoxic elements and exhibits a low elastic modulus as well as high mechanical strength. We have previously demonstrated that this gum metal, once subjected to a series of surface treatments--immersion in 1 M NaOH (alkali treatment) and then 100 mM CaCl2, before heating at 700 °C (sample: ACaH-GM), with an optional final hot water immersion (sample: ACaHW-GM)--has apatite-forming ability in simulated body fluid. To confirm the in vivo bioactivity of these treated alloys, failure loads between implants and bone at 4, 8, 16, and 26 weeks after implantation in rabbits' tibiae were measured for untreated gum metal (UT-GM), ACaH-GM and ACaHW-GM, as well as pure titanium plates after alkali and heat treatment (AH-Ti). The ACaH-GM and UT-GM plates showed almost no bonding, whereas ACaHW-GM and AH-Ti plates showed successful bonding by 4 weeks, and their failure loads subsequently increased with time. The histological findings showed a large amount of new bone in contact with the surface of ACaHW-GM and AH-Ti plates, suggesting that the ACaHW treatment could impart bone-bonding bioactivity to a gum metal in vivo. Thus, with this improved bioactive treatment, these advantageous gum metals become useful candidates for orthopedic and dental devices.