PEEK Biomaterials in Trauma, Orthopedic, and Spinal Implants

The advent of 'biological internal fixation' is an important development in the surgical management of fractures. Locked nailing has demonstrated that flexible fixation without precise reduction results in reliable healing. While external fixators are mainly used today to provide temporary fixation in fractures after severe injury, the internal fixator offers flexible fixation, maintaining the advantages of the external fixator but allowing long-term treatment. The internal fixator resembles a plate but functions differently. It is based on pure splinting rather than compression. The resulting flexible stabilisation induces the formation of callus. With the use of locked threaded bolts, the application of the internal fixator foregoes the need of adaptation of the shape of the splint to that of the bone during surgery. Thus, it is possible to apply the internal fixator as a minimally invasive percutaneous osteosynthesis (MIPO). Minimal surgical trauma and flexible fixation allow prompt healing when the blood supply to bone is maintained or can be restored early. The scientific basis of the fixation and function of these new implants has been reviewed. The biomechanical aspects principally address the degree of instability which may be tolerated by fracture healing under different biological conditions. Fractures may heal spontaneously in spite of gross instability while minimal, even non-visible, instability may be deleterious for rigidly fixed small fracture gaps. The theory of strain offers an explanation for the maximum instability which will be tolerated and the minimal degree required for induction of callus formation. The biological aspects of damage to the blood supply, necrosis and temporary porosity explain the importance of avoiding extensive contact of the implant with bone. The phenomenon of bone loss and stress protection has a biological rather than a mechanical explanation. The same mechanism of necrosis-induced internal remodelling may explain the basic process of direct healing.

Evolution of the internal fixation of long bone fractures: the scientific basis of biological internal fixation: choosing a new balance between stability and biology

Adhesion to wet and dynamic surfaces, including biological tissues, is important in many fields but has proven to be extremely challenging. Existing adhesives are cytotoxic, adhere weakly to tissues, or cannot be used in wet environments. We report a bioinspired design for adhesives consisting of two layers: an adhesive surface and a dissipative matrix. The former adheres to the substrate by electrostatic interactions, covalent bonds, and physical interpenetration. The latter amplifies energy dissipation through hysteresis. The two layers synergistically lead to higher adhesion energies on wet surfaces as compared with those of existing adhesives. Adhesion occurs within minutes, independent of blood exposure and compatible with in vivo dynamic movements. This family of adhesives may be useful in many areas of application, including tissue adhesives, wound dressings, and tissue repair.

Tough adhesives for diverse wet surfaces

Objective:To review the biomechanical principles that guide fracture fixation with plates and screws; specifically to compare and contrast the function and roles of conventional unlocked plates to locked plates in fracture fixation. We review basic plate and screw function, discuss the design ration

Biomechanics of locked plates and screws.

In many animals, the bonding of tendon and cartilage to bone is extremely tough (for example, interfacial toughness ∼800 J m(-2); refs ,), yet such tough interfaces have not been achieved between synthetic hydrogels and non-porous surfaces of engineered solids. Here, we report a strategy to design tough transparent and conductive bonding of synthetic hydrogels containing 90% water to non-porous surfaces of diverse solids, including glass, silicon, ceramics, titanium and aluminium. The design strategy is to anchor the long-chain polymer networks of tough hydrogels covalently to non-porous solid surfaces, which can be achieved by the silanation of such surfaces. Compared with physical interactions, the chemical anchorage results in a higher intrinsic work of adhesion and in significant energy dissipation of bulk hydrogel during detachment, which lead to interfacial toughness values over 1,000 J m(-2). We also demonstrate applications of robust hydrogel-solid hybrids, including hydrogel superglues, mechanically protective hydrogel coatings, hydrogel joints for robotic structures and robust hydrogel-metal conductors.

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Tough bonding of hydrogels to diverse non-porous surfaces

Stability in plate fixation of fractures relates to the motionless fastening between plate and bone. When the plate is affixed to the bone, high shear force may appear between plate and bone, particularly near the end screws, which can lead to motion under weight-bearing. Two mechanisms might be involved in the prevention of motion: the bending stiffness of the screws or the friction between the screw and the plate. Experiments performed in vivo and in vitro show that with conventional plates, motion is prevented by friction and depends upon the axial force of the screw, pressing the plate onto the bone. The torque applied to the screws is crucial. Motion appears under smooth plates under relatively low physiological loads. With newly developed internal fixators, the motion is prevented by the structural stiffness of the plate-screw system.

Force transfer between the plate and the bone : relative importance of the bending stiffness of the screws and the friction between plate and bone

https://onlinelibrary.wiley.com/doi/pdf/10.1016/j.orthres.2003.11.007

The influence of cyclic compression and distraction on the healing of experimental tibial fractures.

Loads acting in an intramedullary nail during fracture healing in the human femur.

Conventional fracture treatment implants, i.e. bone plates and intramedullary nails, but also a great variety of special devices, rely on a mixed mode of load transfer between the bone segments and the implant. The bending moments and transverse forces are usually balanced by reactive forces created by compression in the areas of contact, while the torsional and the axial forces are most frequently transferred by bone screws, directly, or by screw-dependent friction between the implant and the bone. In all cases, contact between the implant and the bone is required and is achieved either by careful adaptation of the implant to the bone or by chance fit. Vascular damage in these areas of direct contact has been shown to account for most of the implant-related damage to the bone. The Point Contact Fixator (PC-Fix: an internal plate and screw fixation system, the function of which is based on similar mechanical principles to the external fixator) is a general purpose internal fixation system in which the mixed mode of load transfer has been eliminated in favour of screw-only transfer, made possible by locking of the screw head into the “plate” of the PC-Fix. The possibility to contour the implant and to use the screws without any special preparatory steps gives the PC-Fix the versatility of conventional plating systems, while practically eliminating their major drawback of direct contact to the bone. Locking of the screw head in the PC-Fix permits transfer of the moments between the screw and the implant. This has allowed a further reduction of damage to the bone by the use of monocortical screws. A theoretical comparison of the mechanisms of load transfer between the Point Contact Fixator and the Dynamic Compression Plate has been demonstrated by in vitro experimental data. The bone-implant construct is generally stronger in the case of the internal fixator provided the bone treated can withstand implant yielding loads. The mechanical conditions at the fracture site for sub-yield levels of loading are dominated by the technique of application and are expected to be comparable for the two implants. It appears that most of the advantages of the PC-Fix observed to date in in vivo experimental fracture treatment can be explained by the reduction of implant-related damage to the bone without any compromise on the mechanical function of fracture stabilization.

The biomechanics of the PC-Fix internal fixator

A generally accepted idea has been that plate fixation of fractures may result in the structural adaptation of bone (bone loss) to reduced stress (stress protection) with the subsequent danger of refracture after implant removal. This was the negative aspect of stress protection. For this reason, it was proposed that plates made from more deformable materials be used (titanium, polymers or carbon fibres). A theoretical analysis using composite beam theory, with different loading conditions (axial load and bending), demonstrates that stress protection, i.e. early temporary porosis, is a myth. Mechanics of materials shows that when an over-large plate is fixed to small bones (as in small animals, e.g. rabbits), the reduction of bone strain is exaggerated; in contrast, using plates of varying flexibility (steel, titanium or carbon fibre) on large bones leads to strain reduction with an astonishingly similar amplitude.

S.M. Perren

Papers

Force transfer between the plate and the bone : relative importance of the bending stiffness of the screws and the friction between plate and bone

The influence of cyclic compression and distraction on the healing of experimental tibial fractures.

Loads acting in an intramedullary nail during fracture healing in the human femur.

The biomechanics of the PC-Fix internal fixator

Stress protection due to plates: myth or reality? A parametric analysis made using the composite beam theory.