Unlike traditional implants primarily composed of bioinert materials, magnesium (Mg) -a degradable biomaterial - offers significant promise for next-generation bone healing implants, whether utilized as a primary structural component or a supporting material. The concept of incorporating cutting-edge techniques in Mg-containing implants creates an efficient ecosystem for bone healing.
While most research focuses on Mg's bioactive and osteoimmunological effect, this review highlights its mechanobiological role, summarizing the merits of Mg-containing implants in facilitating mechanotransduction and associated cellular events during bone healing.
The monitoring of the smart biosensors provides real-time information of Mg-containing implants behaviors and local microenvironment. After the intelligent computational analysis, rehabilitation protocols are optimized for better bone healing outcomes.
Bone, a connective tissue comprising trabecular and cortical regions, features a robust mineralized matrix deposition. Beyond providing exceptional mechanical strength functions for locomotion and support, bone requires exposure to mechanical stimuli including fluid shear stress, compression, and tensile strain, for its regeneration, homeostasis, and intrinsic development. Skeletal cells (osteoblasts, osteoclasts osteocytes, and progenitor cells) and adjacent tissue cells (e.g. endothelial cells, myocytes, fibroblasts) sense these stimuli.
Mechanical cues activate channels via membrane deformation or cytoskeletal transduction, triggering signaling cascades that converting physical stimuli into biological signals. This process, termed mechanotransduction modulates cellular behavior to adapt to the local mechanical environment.
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Fractures involving instability, displacement, or severe soft tissue injury, require transient or prolonged implantation of plates, screws, nails, or pins for anatomical alignment and stabilization. Traditional implants, typically composed of non-degradable materials like stainless steel, titanium (Ti), cobalt-chromium (Co-Cr) alloys, are designed for excellent biocompatibility, mechanical strength, fatigue resistance, corrosion resistance, and surface properties to integrate with native bone and facilitate mechanotransduction.
However, their non-degradability and bioinertness lead to significant limitations: stress shielding, inadequate osseointegration, and the frequent necessity for secondary removal surgery. Furthermore, their limited bioactivity poses challenges in pathological conditions such as osteoporotic fracture, osteonecrosis, bisphosphonates-related atypical femoral fracture (AFF), and diabetic settings.
In relation to the mechanical properties, implants are expected to match the surrounding bone tissue while maintain their mechanical support, which not only facilitates the mechanotransduction for new bone formation but also ensures the smooth transfer of physical loading. While the stiffness of implants (such as the traditional Ti and Co-Cr alloys) is far excessive, the loading stress is mainly concentrated on implants, depriving surrounding bone of appropriate mechanical stimulation.
This phenomenon is known as “stress shielding”, attributing to enhanced bone resorption and reduced new bone formation for poor osseointegration and compromise healing outcomes. Magnesium (Mg), as a biodegradable metal with excellent biocompatibility and a Young's modulus akin to the cortical bone, has emerged as a transformative material for implant engineering in recent years.
Its appeal lies in the ability to reprogram the local microenvironment to potentiate bone formation. Beyond obviating the need for surgical removal surgery, the degradation products of Mg-containing implants (Mg ions (Mg2+), hydroxide ions and hydrogen gas) exert direct biological effects that promote new bone formation and the coupled neovascularization.
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Consequently, engineering efforts for next-generation implants are increasingly focused on the degradable materials, with Mg as a leading candidate. While the biological effects of Mg-based implants are well-documented, this review explores a less-charted frontier: the crucial crosstalk between Mg-based implants and mechanical stimuli, offering a new perspective on their multifaceted benefits.
Fig. 1. Degradation products from Mg implants activate mechanotransduction signaling pathways to promote bone formation and neurovascularization.
Stress Shielding and Mechanotransduction
Traditional non-degradable materials, such as Ti and cobalt-chromium (Cd-Co) alloys, are favored for skeletal implants due to their excellent mechanical properties, biocompatibility, corrosion resistance and long-term stability. These bioinert materials provide reliable mechanical support and structural fixation to facilitate new bone formation.
However, they exhibit a significant drawback: high stiffness. The elastic moduli of the Ti alloys, stainless steel, and cobalt alloys (∼110 GPa, 200 GPa, and 230 GPa, respectively) are substantially higher than that of cortical bone. Cortical bone, with a porosity of <30 % and a density close to that of aluminum (Al) or Mg (approx. 1.85 g/cm3), is far less rigid. This mechanical mismatch leads to a phenomenon known as stress shielding.
The excessively stiff implant bears the majority of the mechanical load, creating a concentrated stress distribution within the implant itself. Consequently, the surrounding bone tissue is deprived of the physiological loading essential for mechanotransduction. This deficiency in mechanical stimuli hinders the normal healing process, resulting in impaired callus formation, reduced ossification, and bone non-union.
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The mechanobiology underlying this failure involves an upregulation of bone resorption. Stress shielding has been shown to nearly double osteoclast numbers and RANKL expression while reducing OPG production. This imbalance exacerbates cortical bone osteoporosis and lowers bone mineral density.
To prevent stress concentration on implants, the mechanical properties must closely match those of the adjacent bone to avoid a load-bearing mismatch. Mg-based metals are considered innovative implant materials due to their high tensile strength (200 MPa) and an excellent elastic modulus (∼45 GPa) that is remarkably similar to cortical bone (15-30 Gpa). This compatibility is demonstrated by finite element analyses and clinical observations.
For instance, Luo et al. reported that Mg screws transfer approximately 50 % more stress to the surrounding bone tissue than Ti screws. Similarly, the stress shielding ratio for bone fixed by Mg plate and screws was calculated to be 1.52, lower than titanium (1.98) or stainless steel (3.82). In an ankle fracture model, Mg screw fixation resulted in a total deformation of 0.1275 mm and the maximum von Mises stress of 68.351 MPa.
Crucially, the maximum stress on the Mg screws and themselves was 59.5 MPa, below both the stress on Ti screws and the failure threshold for the implant. The beneficial effect of eliminating stress shielding also extends to Mg-coated alloys. In a proximal femur fracture model, a biodegradable Mg alloy bionic Gamma nail (Young's modulus ∼45 GPa) reduced the maximum stress (18.7 MPa versus 32 Mpa), the minimum stress (−12.6 MPa versus −23.5 Mpa), and the von Mises stress (14.0 MPa versus 31.3 MPa) compared to the traditional implants.
Furthermore, commercial alloys like MgYREZr, Mg-Zn-Y-Al, and LAE442 exhibit a similarly Young's modulus, underscoring their broad potential to mitigate stress shielding and promote bone formation. Therefore, whether used as a supportive core or an additive coating, Mg efficiently attenuates stress shielding to create an optimal mechanotransduction for bone healing.
Fig. 2. Comparison of traditional implants and Mg-containing implants.
Osseointegration and Mg-Containing Implants
In addition to stress shielding, another challenge presented by traditional implants is the unsatisfying osseointegration owing to their bioinert nature. “Osseointegration”, a term first described by Branemark, describes the structural and functional connection between an implant and native bone, a concept that extends beyond simple bone growth. Its defining criteria include implant stability under functional loading without micromotion, and new bone or marrow deposition directly on the implant surface without the intervention of fibrotic tissue or connective tissue.
Failure to achieve osseointegration - a well-documented cause of peri-prosthetic aseptic loosening in arthroplasty - increases the risk of fracture, post-surgical pain and infection. This failure induces significant micromotion at the bone-implant interface under loading. The excessive micromotion at the first week post-surgery is a primary inhibitor of bone ingrowth, while persistent micromotion (typically 50-150 μm) can provoke chronic inflammation and bone resorption, leading to osteolysis and early symptomatic loosening.
Establishing stable bone-implant contact hinges on osteoinduction and osteoconduction, processes heavily influenced by implant composition. Traditional bioinert implants achieves only a physical connection, lacking biochemical binding to the surrounding bone tissue. In contrast, Mg-containing implants, actively promote osseointegration through multiple mechanisms: they accelerate osteogenic differentiation, mineralization and neurovascularization; inhibit bone resorption, and provide immunomodulation. Evidence for enhanced osseointegration with Mg is substantial.
During the corrosion of Mg-Ca-Zn alloys, a biomimetic mineralized layer formed on the degrading outer surface, fostering integration. In physiological solutions, Mg pins developed a calcium phosphate-rich corrosion layer. The pro-osseointegration effects of Mg are also mediated by other biological interactions.
For instance, increased innervation featuring local accumulation of neuronal CGRP can be detected after Mg alloys implantation, which activates the Protein Kinase A (PKA)/FAK signaling pathway in bone marrow mesenchymal stem cells (BMSCs) to drive osteogenesis. Furthermore, Mg alloys exhibit potent immunomodulatory effects by promoting M2 macrophage polarization and the secretion of anti-inflammatory cytokines.
This modulates the local immune environment, curbing excessive inflammation and orchestrating a regenerative microenvironment conductive to the subperiosteal bone formation and neovascularization, thereby significantly strengthening the bone-implant interface. Furthermore, the features of the implant surface (e.g., roughness, wettability and hydrophilicity) and the bioactivity surface modifications are critical for directing cell migration, adhesion, and differentiation.
Consequently, strategic modification of the Mg-containing alloys can optimize both their degradation kinesis and bone-implant integration. For example, micro-arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), can be applied to create a micro/nano-porous coating on the Mg alloys. This layer enhances structural integrity, improves corrosion resistance, and promotes cell adhesion, proliferation, osteogenic differentiation, and angiogenesis.
Studies by Rendenbach and Berger, demonstrated that PEO-treated WE43 Mg screws and plates exhibit superior osteoconductivity, increased osseointegration, and a more controlled degradation rate over the first six months compared to their unmodified counterparts. Nanospiked modification of Mg or the surface coating of tantalum/Poly(ether imide) exhibits superior osseointegration as well as controlled degradability and corrosion resistance, showing promising orthopedic implant applications.
A firm initial fixation is a prerequisite for successful osseointegration and eventually functional restoration. While the degradable nature of Mg eliminates the need for surgical removal surgery, its degradation must be carefully controlled. In the critical early post-operative phase, characterized by an inflammatory environment and low pH that can accelerate corrosion, the implant must retain sufficient mechanical strength to facilitate callus formation and maturation.
Subsequently, as hard callus maturation and remodeling commence (around 1.5-2 months), the implant's corrosion should progress in tandem with new bone formation, creating complementary trajectories where Mg erosion gradually makes space for regenerating new bone without inducing a shielding effect. Importantly, Mg-containing implants maintain their osteointegrative efficacy even in challenging pathological conditions like osteoporosis and diabetes.
In osteoporotic rats, Mg-loaded Ti screws stimulated robust new bone formation on their surfaces within the first week post-implantation, a process associated with BMP6 upregulation driven by the release of Mg2+. Similarly, MgO-coated implants have been shown to counteract diabetes-induced endothelial dysfunction and promoted osseointegration during alveolar bone healing.
Wireless Charging Compatibility with Magnesium Implants
Wireless charging technology has advanced significantly, with standards like Qi and MagSafe becoming increasingly prevalent. However, compatibility issues can arise, especially with devices that have internal magnets or metallic components.
Apple introduced MagSafe with the iPhone 12 series, which includes an array of magnets around the receiver coil. This design ensures precise alignment and efficient charging. However, some Qi chargers may leak magnetic fields onto these magnets, causing heat generation or triggering foreign object detection (FOD), which can interrupt charging.
To address these issues, manufacturers must thoroughly analyze MagSafe compatibility and develop solutions that minimize magnetic field leakage and ensure efficient charging.
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MagSafe Compatibility: Key Considerations
- Magnet Array: Apple uses 18 precisely arranged circular magnets around the charging coil to ensure perfect alignment and secure attachment.
- Qi Wireless Charging: MagSafe chargers use Qi technology, adding magnets for alignment. Without Qi support, charging won't work.
- Magnetic Shielding: Proper shielding prevents interference with sensitive electronics and improves charging efficiency.
- Power Transfer: MagSafe delivers up to 15W charging, requiring perfect coil alignment, which magnets facilitate.
For Android devices, MagSafe compatibility can be achieved through MagSafe-compatible cases or accessories with built-in magnet rings that match Apple's specifications. It is crucial to ensure that these accessories provide sufficient magnetic strength and proper shielding to avoid interference and ensure efficient charging.
How to Check MagSafe Compatibility
To check if an iPhone is MagSafe compatible, look for models starting from the iPhone 12 and later. These models have the magnetic array inside, enabling improved wireless charging speed and secure accessory attachment. Newer iPhones also receive firmware updates supporting MagSafe’s different accessory standards.
MagSafe vs. Normal Wireless Chargers
Regular wireless chargers often misalign, charge slowly, or get interrupted if the phone shifts even slightly. MagSafe's magnetic system prevents misalignment, ensuring better charging reliability.
To make an iPhone MagSafe compatible, users can attach a magnetic ring or use a MagSafe-compatible case. These accessories align the phone with the charger.
Future Developments
The EU wants common charging standards, which might expand MagSafe-like systems. More Android phones could adopt magnets, but universal standards don't exist yet. For now, MagSafe remains Apple-focused.
Table: MagSafe Compatibility Across iPhone Models
| iPhone Model | MagSafe Compatibility |
|---|---|
| iPhone 11 and Earlier | Requires MagSafe-compatible case or adapter |
| iPhone 12 Series (Mini, Standard, Pro, Pro Max) | Native MagSafe support |
| iPhone 13 Series | Native MagSafe support |
| iPhone 14 Series | Native MagSafe support |
| iPhone 15 Series | Native MagSafe support |
| iPhone 16 Series | Native MagSafe support |
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