Darren James Wilson

Physical and biological properties of a novel siloxane adhesive for soft tissue applications

The aim of this study was to investigate the adhesive properties of an in-house amino-propyltrimethoxysilane-methylenebisacrylamide (APTMS-MBA) siloxane system and compare them with a commercially available adhesive, n-butyl cyanoacrylate (nBCA). The ability of the material to perform as a soft tissue adhesive was established by measuring the physical (bond strength, curing time) and biological (cytotoxicity) properties of the adhesives on cartilage. Complementary physical techniques, X-ray photoelectron spectroscopy, Raman and infrared imaging, enabled the mode of action of the adhesive to the cartilage surface to be determined. Adhesion strength to cartilage was measured using a simple butt joint test after storage in phosphate-buffered saline solution at 37°C for periods up to 1 month. The adhesives were also characterised using two in vitro biological techniques. A live/dead stain assay enabled a measure of the viability of chondrocytes attached to the two adhesives to be made. A water-soluble tetrazolium assay was carried out using two different cell types, human dermal fibroblasts and ovine meniscal chondrocytes, in order to measure material cytotoxicity as a function of both supernatant concentration and time. IR imaging of the surface of cartilage treated with APTMS-MBA siloxane adhesive indicated that the adhesive penetrated the tissue surface marginally compared to nBCA which showed a greater depth of penetration. The curing time and adhesion strength values for APTMS-MBA siloxane and nBCA adhesives were measured to be 60 s/0.23 MPa and 38 min/0.62 MPa, respectively. These materials were found to be significantly stronger than either commercially available fibrin (0.02 MPa) or gelatin resorcinol formaldehyde (GRF) adhesives (0.1 MPa) (P < 0.01). Cell culture experiments revealed that APTMS-MBA siloxane adhesive induced 2% cell death compared to 95% for the nBCA adhesive, which extended to a depth of approximately 100–150 μm into the cartilage surface. The WST-1 assay demonstrated that APTMS-MBA siloxane was significantly less cytotoxic than nBCA adhesive as an undiluted conditioned supernatant (P < 0.001). These results suggest that the APTMS-MBA siloxane may be a useful adhesive for medical applications.

The permeability of silicone rubber to metal compounds: relevance to implanted devices.

Most implanted electrical devices use encapsulant as insulation. The encapsulant may remain functional for many years, bonded to the metallic surfaces, but eventually become partly detached allowing corrosion to occur. To understand whether the corrosion products will cause toxic effects, we need to know how quickly they will permeate through the encapsulant. In these experiments, silicone capsules (the encapsulant) containing metal compounds were left in jars of initially pure water for 6 months, and the concentration of the metal in the water was measured. The amount of metal depended on the type of compound; for the organometallic compounds tested, permeation was very rapid. However, for most of the other compounds, whether oxides or salts, the amount of metal was below the control level and therefore could have been the result of contamination. These compounds were tin sulfate and oxide (<10²), lead nitrate and oxide (<10²), copper sulfate (<10³), and nitrates of bismuth (<10¹), chrome (<10²), nickel (<10³) and zinc (<10²). The numbers in brackets are the maximum mass (ng) of permeated metal after 6 months. Three silver compounds were tested but without proper controls; however, the amount of permeated silver appeared to be low: silver oxide (1.3 × 10²), silver nitrate (6.3 × 10¹), and silver chloride (6 × 10⁰). The resolution of this method is limited by contamination that is detected by control capsules. The conclusion is that compounds that are likely corrosion products permeate through silicone encapsulant at a low rate and seem unlikely to cause toxic effects.

A single-channel telemetric intramedullary nail for in vivo measurement of fracture healing.

Objective: The objective of this study was to develop a single-channel telemetric intramedullary nail that measures anterior-posterior bending strains and determine whether these forces decrease sigmoidally when normalized to the ground reaction force during fracture healing. Methods: A transverse midshaft femoral osteotomy (1 mm) was stabilized using a customized TriGen intramedullary nail incorporating a strain gauge in the anterior-posterior plane. Fourteen skeletally mature sheep (2-3 years old) were treated in two pilot studies (n = 3/pilot) and a pivotal study (n = 8). Three animals were excluded as a result of welfare issues. Static strain measurements were acquired at approximately 130 Hz during leg stance. In vivo gait analysis was carried out weekly to assess ground reaction forces and biweekly x-rays to assess stability and fracture healing. Animals were euthanized 12 weeks postoperatively. Callus formation was assessed by microcomputed tomography and histomorphometry. The degree of load share between bone and the nail was determined postmortem by three-point bending. Results: A significant preload was generated during implantation, most notably during placement of the four interlocking screws and by the action of attached soft tissues. Eight animals showed evidence of bone healing by x-ray, microcomputed tomography, and histology. However, a reduction in implant load was only observed with two of the eight. The degree of load sharing observed in vivo in these animals (50%-75%) compared favorably with the in vitro observations (approximately 50%). In the nonhealing ambulating animals, nail forces did not change over time. Three-point bend tests carried out on “healed” femurs suggested that load sharing between the bone and nail could be detected more easily in the absence of soft tissues. Conclusion: No clear correlation between implant strain and fracture healing was observed using the single-channel system when subjected to one external loading regime (leg stance phase). However, ex vivo biomechanical testing demonstrated that load share changes could be detected when loads were directly applied to the bone in the absence of muscle and ligament forces. These data emphasize the need to fully characterize the complex biomechanical environment of the limb to determine the load changes resulting from fracture healing.

Strain response of an instrumented intramedullary nail to three-point bending

Objectives: An experimental biomechanical evaluation of an instrumented intramedullary nail (TriGen® META Nail, Smith&Nephew®) was undertaken. The objectives were two-fold. The first was to identify the most sensitive strain gauge positions and orientations on the nail, and the second was to demonstrate that the nail was capable of detecting changes in stiffness of the nail–bone composite. The function of the instrumented nail is to quantify fracture healing objectively and directly, and so to predict delayed repair or non-union 2 months before current methods. Methods: Eight flat pockets were machined onto the surface of the nail and three strain gauges attached in each pocket. The instrumented nail was inserted into fourth generation biomechanical grade Sawbones® tibiae with three different fracture configurations as well as into a non-fractured bone. The nail–bone composite was loaded in three-point bending at five positions to determine the strain changes in each of the eight strain gauge pockets located along the length of the nail. To simulate callus in the simplest way and to increase the stiffness of the nail–bone composite, loops of duct tape in multiples of four were applied over the fracture locus. A three-point loading jig was used to obtain the change in strain with increasing stiffness. Relative displacement of the bone ends was quantified using radiostereometric analysis. Results: There was no single position of greatest strain sensitivity for all fracture types. The greatest change in strain occurred when the strain gauge pocket and fracture line were closest. Applying the loading moment directly over the strain gauge pocket also maximised its sensitivity. The duct tape callus simulation showed that the instrumented nail was able to detect a change in stiffness of less than 4.1 Nm/°. Conclusions: It has been shown that the instrumented nail can detect physiologically relevant changes in stiffness, and so to provide a useful function as an objective monitor of fracture repair.

Structural Health Monitoring of Long Bone Fractures Using Instrumented Intramedullary Nails

Clinical assessment of fracture healing is usually subjective, relying upon the detection of movement (‘feel’) by the surgeon, the patient’s response in terms of pain and confidence, and radiographic evidence of callus and primary bone union. A more quantitative, objective method of measuring the strength of a healing callus would be useful in assessing many aspects, such as the effectiveness of different forms of treatment, the pattern and rate of healing, and the stage at which the patient can return to full weight-bearing activity. The results presented in this paper demonstrate the complexity of monitoring fracture healing in leg stance phase using an instrumented intramedullary (IM) nail equipped with a single sensor. The bone healers exhibited both sigmoidal and linear load responses during fracture healing. Ambulating non-healers demonstrated high nail forces which did not change significantly over time whereas lame non-healers demonstrated a decreasing nail load due to reduced GRF or loosening of fixation.Copyright