Open the pores - Polydimethylsiloxane influences the porous structure of cancellous bone surrogates for biomechanical testing of osteosyntheses.

Synthetic materials used for valid and reliable implant testing and design should reflect the mechanical and morphometric properties of human bone. Such bone models are already available on the market, but they do not reflect the population variability of human bone, nor are they open-celled porous as human bone is. Biomechanical studies aimed at cementing the fracture or an implant cannot be conducted with them. The aim of this study was to investigate the influence of a cell stabilizer on polyurethane-based cancellous synthetic bone in terms of morphology, compressive mechanics, and opening of the cancellous bone structure for bone cement application. Mechanical properties of cylindrical specimens of the bone surrogates were determined by static compression tests to failure. Furthermore, a morphometric analysis was performed using microcomputed tomography. To prove the open-cell nature of the bone surrogates, an attempt was made to apply bone cement. Effects on the mechanical properties of the polyurethane-based bone surrogates were observed by the addition of polydimethylsiloxane. All mechanical parameters like Young's modulus, ultimate stress and yield stress increased statistically significantly with increasing amounts of cell stabilizer (all p > 0.001), except for yield stress. The analysis of morphometric parameters showed a decrease in trabecular thickness, spacing and connectivity density, which was accompanied by an increase in trabecular number and an increase in pore size. The open-cell nature was proven by the application and distribution of bone cement in specimens with stabilizer, which was visualized by X-ray. In conclusion, the results show that by adding a cell stabilizer, polyurethane-based cancellous bone substrates can be produced that have an open-cell structure similar to human bone. This makes these bone surrogates suitable for biomechanical testing of osteosyntheses and for osteosynthesis cementation issues.

Smart Artificial Soft Tissue - Application to a Hybrid Simulator for Training of Laryngeal Pacemaker Implantation.

Surgical simulators are safe and evolving educational tools for developing surgical skills. In particular, virtual and hybrid simulators are preferred due to their detailedness, customization and evaluation capabilities. To accelerate the revolution of a novel class of hybrid simulators, a Smart Artificial Soft Tissue is presented here, that determines the relative position of conductive surgical instruments in artificial soft tissue by inverse resistance mappings without the need for a fixed reference point. This is particularly beneficial for highly deformable structures when specific target regions need to be reached or avoided. The carbon-black-silicone composite used can be shaped almost arbitrarily and its elasticity can be tuned by modifying the silicone base material. Thus, objective positional feedback for haptically correct artificial soft tissue can be ensured. This is demonstrated by the development of a laryngeal phantom to simulate the implantation of laryngeal pacemaker electrodes. Apart from the position-detecting larynx phantom, the simulator uses a tablet computer for the virtual representation of the vocal folds' movements, in accordance with the electrical stimulation by the inserted electrodes. The possibility of displaying additional information about target regions and anatomy is intended to optimize the learning progress and illustrates the extensibility of hybrid surgical simulators.

Single or Double Plating for Acromial Type III Fractures: Biomechanical Comparison of Load to Failure and Fragment Motion.

BACKGROUND: Acromial Levy III fractures after inverse shoulder arthroplasty occur in up to 7% of patients. To date, it is not clear how these fractures should be treated as clinical outcomes remain unsatisfactory. The aim of this study was to evaluate the biomechanical performance of three different plating methods of type III acromion fractures. METHODS: Levy III fractures in synthetic scapulae were fixed with three different methods. Angular stable locking plates were placed on the spina scapula to bridge the fracture either dorsally, caudally, or on both aspects by double plating. In a biomechanical experiment, the pull of the deltoid muscle at 40° abduction of the arm was simulated by cyclic loading with increasing load levels until failure. Failure load, cycles to failure, and fragment motions were evaluated. RESULTS: The results showed that double plating (350 ± 63 N) withstood the highest loads until failure, followed by dorsal (292 ± 20 N) and caudal (217 ± 49 N) plating. Similarly, double plating showed significantly smaller fragment movement than the other two groups. CONCLUSIONS: Double plating appeared to provide the largest biomechanical stability in type III acromion fracture under arm abduction. Caudal plating in contract resulted in insufficient fracture stability and early failure and can thus not be recommended from a biomechanical point of view.

Mechanical and morphometric characterization of custom-made trabecular bone surrogates.

Synthetic bones for biomechanical testing and surgeon training have become more important due to their numerous advantages compared to human bones. Several bone models are already available on the market, but most of them do not reflect the full range of versatile properties that characterize human bone like population-level influences, size, stiffness, bone-implant-interface or morphometry. Thus, the objectives of this study were to develop synthetic trabecular bone surrogates from polyurethane and varying additives and to determine their elastic and plastic mechanical compressive and additionally morphometric properties. Another aim was to investigate the influence of varying additives on aforementioned properties and finally compare the results with published data from human trabecular bone. Additives used were blowing agents to create a porous structure, mineral fillers to manipulate the basic polyurethane resin, and cell stabilizers to achieve an open porous composition. Mechanical properties were obtained from static compression tests until failure while morphometric analysis was carried out using microcomputed tomography. Thereby, the blowing agent showed the strongest influence on mechanical and morphometric properties with mean Young's moduli ranging from 627 ± 37 MPa (0% blowing agent) to 154 ± 15 MPa (0.25% blowing agent) while the variation of mineral filler content resulted in small standard deviations of approximately 10-20 MPa with a constant proportion of blowing agent. The achieved mechanical properties of the developed synthetic bones, such as the Young's modulus, ultimate stress and yield stress were in accordance with human trabecular bone, while yield strain for all groups was noticeably higher compared to human trabecular bone. Additionally, morphometric analysis showed results indicating similar morphometry of the custom-made synthetic bone and human cancellous bone. Although recreating bone structures in physiological conditions is not simple, the results of the current study show the possibility of developing synthetic bone materials with characteristics like human trabecular bone.

Three internal fixation methods for Danis-Weber-B distal fibular fractures: A biomechanical comparison in an osteoporotic fibula model.

A common agreement for the surgical treatment of osteoporotic ankle fractures has not been defined yet although locking plates are preferred for fractures with poor bone quality. This study aims to evaluate the mechanical stability of locked and conventional plates on osteoporotic Danis-Weber-B-fibula fracture models. Fractured custom-made osteoporotic fibulae were treated with neutralization plate plus lag screw, locking plate plus lag screw, or a standalone locking plate. Load until failure was applied mimicking single-leg stance. Stiffness, failureload, and interfragmentary movements were investigated. Stiffness, failureload and axial fragment movement showed no significant differences among groups. Shear movements and fragment rotation around the shaft of the neutralization plate were on average twice as high as those of the locking plates. Although no superiority was shown for overall mechanical performance, the locking plate groups exhibited higher shear and rotational stability than the neutralization plate.

Custom-made polyurethane-based synthetic bones mimic screw cut-through of intramedullary nails in human long bones.

Intramedullary nails are considered the gold standard for the treatment of tibial shaft fractures. Thereby, the screw-bone interface is considered the weakest link. For biomechanical evaluation of osteosyntheses synthetic bones are often used to overcome the disadvantages of human specimens. However, commercially available synthetic bones cannot adequately mimic the local mechanical properties of human bone. Thus, the aim of this study was to develop and evaluate novel cortical bone surrogate materials that mimic human tibial shafts in the screw-loosening mechanisms of intramedullary nails. Bone surrogates, based on two different polyurethanes, were developed and shaped as simple tubes with varying cortical thicknesses to simulate the diaphyseal cortex of human tibiae. Fresh frozen human tibiae and commercially available synthetic bones with similar cortical thickness were used as references. All specimens were treated with a nail dummy and bicortical locking screws to simulate treatment of a distal tibia shaft fracture. The nail-bone construct was loaded in a combined axial-torsional-sinusoidal loading protocol to simulate the physiological load during human gait. The loads to failure as well as the number of load cycles were evaluated. Furthermore, the cut-through length of the screws was analysed by additional micro computed -tomography images of the tested specimens. The failure load of custom made synthetic bone tubes with 6 mm cortical thickness (3242 ± 136 N) was in accordance with the failure load of human samples (3300 ± 307 N, p = 0.418) with a similar cortical thickness of 4.9 ± 1.4 mm. Commercially available synthetic bones with similar cortical thickness of 4.5 ± 0.7 mm were significantly stronger (4575 ± 795 N, p = 0.008). Oval-shaped migration patterns were "cut" into the cortices by the screws due to the cyclical loading. The cut-through length of the self-developed synthetic bones with 6 mm cortices (0.8 ± 0.6 mm, p = 0.516) matched the cut-through of the human tibiae (0.7 ± 0.6 mm). The cut-through of commercially available epoxy-based synthetic bones deviated from the human reference (0.2 ± 0.1 mm, p < 0.001). The results of this study indicate that the novel bone surrogates realistically mimic the failure and screw migration behaviour in human tibiae. Thus, they offer a new possibility to serve as substrate for biomechanical testing. The use of commercially available surrogates is discouraged for biomechanical testing as there is a risk of drawing incorrect conclusions.

Biomechanical models: key considerations in study design.

This manuscript summarizes presentations of a symposium on key considerations in design of biomechanical models at the 2019 Basic Science Focus Forum of the Orthopaedic Trauma Association. The first section outlines the most important characteristics of a high-quality biomechanical study. The second section considers choices associated with designing experiments using finite element modeling versus synthetic bones versus human specimens. The third section discusses appropriate selection of experimental protocols and finite element analyses. The fourth section considers the pros and cons of use of biomechanical research for implant design. Finally, the fifth section examines how results from biomechanical studies can be used when clinical evidence is lacking or contradictory. When taken together, these presentations emphasize the critical importance of biomechanical research and the need to carefully consider and optimize models when designing a biomechanical study.

The role of mechanical stimulation in the enhancement of bone healing.

The biomechanical environment plays a dominant role in the process of fracture repair. Mechanical signals control biological activities at the fracture site, regulate the formation and proliferation of different cell types, and are responsible for the formation of connective tissues and the consolidation of the fractured bone. The mechanobiology at the fracture site can be easily manipulated by the design and configuration of the fracture fixation construct and by the loading of the extremity (weight-bearing prescription). Depending on the choice of fracture fixation, the healing response can be directed towards direct healing or towards indirect healing through callus formation. This manuscript summarizes the evidence from experimental studies and clinical observations on the effect of mechanical manipulation on the healing response. Parameters like fracture gap size, interfragmentary movement, interfragmentary strain, and axial and shear deformation will be explored with respect to their respective effects on fracture repair. Also, the role of externally applied movement on the potential enhancement on the fracture repair process will be explored. Factors like fracture gap size, type and amplitude of the mechanical deformation as well as the loading history and its timing will be discussed.

Characterization of tissue properties in epidural needle insertion on human specimen and synthetic materials.

The force experienced while inserting an 18-gauge Tuohy needle into the epidural space or dura is one of only two feedback components perceived by an anaesthesiologist to deduce the needle tip position in a patient's spine. To the best of the authors knowledge, no x-ray validated measurements of these forces are currently available to the public. A needle insertion force recording during an automated insertion of an 18-gauge Tuohy needle into human vertebral segments of four female donors was conducted. During the measurements, x-ray images were recorded simultaneously. The force peaks due to the penetration of the ligamentum supraspinale and ligamentum flavum were measured and compared to the measurements of an artificial patient phantom for a hybrid patient simulator. Based on these force peaks and the slope of the ligamentum interspinale, a mathematical model was developed. The model parameters were used to compare human specimens and artificial patient phantom haptics. The force peaks for the ligamenta supraspinale and flavum were 7.55 ± 3.63 N and 15.18 ± 5.71 N, respectively. No significant differences were found between the patient phantom and the human specimens for the force peaks and four of six physical model parameters. The patient phantom mimics the same resistive force against the insertion of an 18-gauge Tuohy needle. However, there was a highly significant (p < 0.001, effsize = 0.949 and p < 0.001, effsize = 0.896) statistical difference observed in the insertion depth where the force peaks of the ligamenta supraspinale and flavum were detected between the measurements on the human specimens and the patient phantom. Within this work, biomechanical evidence was identified for the needle insertion force into human specimens. The comparison of the measured values of the human vertebral segments and the artificial patient phantom showed promising results.

Biomechanics of Osteoporotic Fracture Fixation.

PURPOSE OF REVIEW: Fractures of osteoporotic bone in elderly individuals need special attention. This manuscript reviews the current strategies to provide sufficient fracture fixation stability with a particular focus on fractures that frequently occur in elderly individuals with osteoporosis and require full load-bearing capacity, i.e., pelvis, hip, ankle, and peri-implant fractures. RECENT FINDINGS: Elderly individuals benefit immensely from immediate mobilization after fracture and thus require stable fracture fixation that allows immediate post-operative weight-bearing. However, osteoporotic bone has decreased holding capacity for metallic implants and is thus associated with a considerable fracture fixation failure rate both short term and long term. Modern implant technologies with dedicated modifications provide sufficient mechanical stability to allow immediate weight-bearing for elderly individuals. Depending on fracture location and fracture severity, various options are available to reinforce or augment standard fracture fixation systems. Correct application of the basic principles of fracture fixation and the use of modern implant technologies enables mechanically stable fracture fixation that allows early weight-bearing and results in timely fracture healing even in patients with osteoporosis.

Development of open-cell polyurethane-based bone surrogates for biomechanical testing of pedicle screws.

Artificial bones made of polyurethane are frequently used as an alternative to human bone for biomechanical testing. However, the biomechanical characteristics of these materials are often not validated against those of human bones. Thus, synthetic bone surrogates reflecting procedure-specific biomechanical properties of human bones are necessary for reliable implant design and testing. The aim of this study was to evaluate novel custom made open- and close-cell bone surrogates through morphometry assessment and pedicle screw pullout tests as an alternative to human bone for biomechanical testing. Bone surrogates created from polyurethane resin, mineral fillers and varying amounts of blowing agent were customized to various densities. Pedicle screws were manually inserted and pullout tests with a feed rate of 1 mm/min were conducted until failure. Load and displacement curves were recorded and analyzed in terms of maximum pullout forces. The resulting pullout forces of open- (1437 ±â€¯665 N) and close-cell surrogates (951 ±â€¯578 N) showed comparable results to human bone (1417 ±â€¯812N) used as a reference. Furthermore, structural morphometric parameters were in accordance with human vertebral cancellous bone. In conclusion, the customized bone surrogates provide a new opportunity to design and test pedicle screws and further study the relationship between biomechanical properties and apparent density of artificial spongy bone.

Bone surrogates provide authentic onlay graft fixation torques.

Onlay graft bone augmentation is a standard practice to restore the loss of height of the alveolar ridge following loss of a tooth. Cranial grafts, lifted from the parietal bone, are sandwiched and used to bridge the bony defect in the jaw by means of small screws. During the elevation of the covering gum and subsequent screw placement, care has to be taken in order to preserve underlying nerves. Therefore, to avoid harm to the patient, a solid education of surgeons is essential, which requires training and experience. A simulator for cranial graft-lift training was already developed and shall be expanded to train the augmentation of the lifted implants. Therefore, in this study, synthetic bones for onlay block graft screw placement with realistic haptics for the screw application training were evaluated and compared with human specimens. Six different polyurethane based bone surrogate composites, enriched with varying amounts of calcium-based mineral fillers and blowing agents, were developed. The haptical properties of these synthetic bones were validated for screw placement and compared with human parietal bone specimens. For that, bones were pre-drilled, screws were automatically inserted using a customized testbench and the slope of the screw-insertion torques were analyzed. The slope of the screw insertion torques of the human reference bones was 56.5 ±â€¯14.0 * 10-3 Nm/deg, Surrogates with lower amounts of mineral fillers and blowing agents showed lower torques than the human bone. Synthetic bones, validated for drilling, milling and sawing in an earlier study, also achieved significantly lower torques, which were only the half of the human parietal bones. Two intermediate stages of the aforementioned material compositions, consisting of 75% mineral filler with 0.75% blowing agent and 100% mineral filler with 1.00% blowing agent revealed results comparable with human bone (57.4 ±â€¯10.2 *10-3 Nm/deg, p = 0.893 and 54.9 ±â€¯11.1 *10-3 Nm/deg, p = 0.795, respectively). In conclusion, our findings suggest that, two newly developed polyurethane-based materials mimicking the haptical properties of an onlay bone graft screw fixation, have been identified. Thus, these surrogates are capable of mimicking real bone tissue in our simulator for the education of novice surgeons.

Characterization of polyurethane-based synthetic vertebrae for spinal cement augmentation training.

Vertebral augmentation techniques are used to stabilize impacted vertebrae. To minimize intraoperative risks, a solid education of surgeons is desirable. Thus, to improve education of surgeons as well as patient safety, the development of a high-fidelity simulator for the surgical training of cement augmentation techniques was initiated. The integrated synthetic vertebrae should be able to provide realistic haptics during all procedural steps. Synthetic vertebrae were developed, tested and validated with reference to human vertebrae. As a further reference, commercially available vertebrae surrogates for orthopedic testing were investigated. To validate the new synthetic vertebrae, characteristic mechanical parameters for tool insertion, balloon dilation pressure and volume were analyzed. Fluoroscopy images were taken to evaluate the bone cement distribution. Based on the measurement results, one type of synthetic vertebrae was able to reflect the characteristic parameters in comparison to human vertebrae. The different tool insertion forces (19.7 ± 4.1, 13.1 ± 0.9 N, 1.5 ± 0.2 N) of the human reference were reflected by one bone surrogate (11.9 ± 9.8, 24.3 ± 3.9 N, 2.4 ± 1.0 N, respectively). The balloon dilation pressure (13.0 ± 2.4 bar), volume (2.3 ± 1.5 ml) of the synthetic vertebrae were in good accordance with the human reference (10.7 ± 3.4 bar, 3.1 ± 1.1 ml). Cement application forces were also in good accordance whereas the cement distribution couldn't be reproduced accurately. Synthetic vertebrae were developed that delivered authentic haptics during transpedicular instrument insertion, balloon tamp dilation and bone cement application. The validated vertebra model will be used within a hybrid simulator for minimally invasive spine surgery to educate and train surgeons.

Characterization of an artificial skull cap for cranio-maxillofacial surgery training.

Cranial grafts are favored to reconstruct skeletal defects because of their reduced resorption and their histocompatibility. Training possibilities for novice surgeons include the "learning by doing" on the patient, specimens or simulators. Although the acceptance of simulators is growing, the major drawback is the lack of validated bone models. The aim of this study was to create and validate a realistic skull cap model and to show superiority compared to a commercially available skull model. Characteristic forces during machinery procedures were recorded and thickness parameters from the bony layers were obtained. The thickness values of the bone layers of the developed parietal bone were comparable to the human ones. Differences between drilling and sawing forces of human and artificial bones were not detected using statistical analysis. In contrast the parameters of the commercially available skull model were significantly different. However, as a result, a model-based simulator for tabula externa graft lift training, consisting of a brain, skull bone cap and covering soft tissues was created. This simulator enables the training of all procedural steps of a "split thickness graft lift". In conclusion, an artificial skull cap suitable for parietal graft lift training was manufactured and validated against human parietal bones.

Procedure-Specific Validation of Artificial Vertebrae.

OBJECTIVE: The development of a novel hybrid patient simulator was initiated to provide a safe training possibility for novice surgeons. Integrated artificial vertebrae should be able to realistically mimic the haptics of transpedicular vertebroplasty instrument insertion and pedicle screw placement. Therefore, new open-celled material compositions were developed, tested, and validated with reference to elderly human vertebrae. METHODS: Vertebroplasty tool insertion force and pedicle screw torque measurements were performed. To validate the new bone surrogates for transpedicular tool insertion, a novel parametric model of the procedure was developed identifying three characteristic insertion parameters (weighting factors, cutting, and clamping forces). Furthermore, the slope of the insertion torque was used to validate the new materials against the human vertebrae for pedicle screw placement. RESULTS: A relative error less than 6% confirmed the suitability of the parametric model for validation. The weighting factors () and the clamping forces ( ) of the human reference were met by the bone surrogate with 1.25% of blowing agent ( and , respectively). However, no material was able to reflect the instrument cutting forces. The slope obtained during pedicle screw placement in human vertebrae was  Nm/m. The material composition with 1% blowing agent achieved similar results ( N m/m). CONCLUSION: Two suitable materials that deliver realistic haptics during both instrument insertions were validated. The parametric model suitably modeled the transpedicular instrument insertion. SIGNIFICANCE: These newly developed models provide a realistic haptic feedback during transpe-dicular instrument insertions with the potential of cement application during surgical skill training.

Validation of a simulator for cranial graft lift training: Face, content, and construct validity.

PURPOSE: Surgical skills can be improved through practical exercise. The use of specimens, human as well as animal, or live animals for surgical training is limited due to ethical concerns. Drawbacks of simulators are costs, fidelity and creditibility. Thus, simulators must be evaluated objectively to determine their validity before they can be used as teaching modalities. The aim of this study was to verify the face content and construct validity of a novel model-based simulator for lifting tabula externa transplants from the parietal skull. MATERIALS AND METHODS: Participants were invited to perform a tabula externa graft lift during a training session on the simulator. Task performance was analyzed with a standardized assessment tool evaluating realism and appropriateness. Specialist ratings were used to evaluate the performance of the participants. This was an exploratory study using a questionnaire, at Kepler University Hospital, Linz, Austria, a university hospital. According to their expertise in craniomaxillofacial surgery, 17 participants were subdivided into 3 groups: 8 novices, 7 experts and 2 raters. RESULTS: The face validity (realism) obtained an average score of 4.2 of a maximum of 5 points. Likewise, the content validity (appropriateness as a teaching modality) obtained an average score of 4.8 of maximum 5 points. No differences were found between experts and novices concerning the recorded surgery completion times (p = 0.418) or the sizes of the lifted grafts (p = 0.110). During the evaluation of task performance, the expert surgeons (46.9 ± 3.7) were graded significantly better than the novices (36.4 ± 8.5), which proved the construct validity of the simulator (p = 0.001). CONCLUSION: All investigated validities were confirmed and approved the simulator as a valid training tool for parietal graft lift.

Transpedicular Approach on a Novel Spine Simulator: A Validation Study.

OBJECTIVE: The popularity of simulation in the medical field has increased dramatically over the last decades. However, the majority of studies focused on laparoscopic or other endoscopic procedures. In this study, participants performed an image-guided surgery task on a novel spine simulator. Face, content, construct, and concurrent validity were examined. DESIGN: A surgical access through both pedicles (transpedicular) into the vertebral body of artificial L3 vertebrae was performed. Questionnaires, a simulation-based performance score, and a specialist rating were used to evaluate the various forms of validity. SETTING: Klinikum Wels-Grieskirchen, Wels, Austria; tertiary hospital PARTICIPANTS: According to their expertise in image-guided surgery and pedicle tool insertions, 43 participants were subdivided into 3 groups: 22 novices, 12 intermediates, and 9 experts. RESULTS: Of the novice group, the vast majorities were impressed with the attractiveness and the general appearance of the simulator. The majority of intermediates (92%) and experts (89%) would recommend the simulator to others. According to a simulation-based performance score, experts performed significantly better than novices (p = 0.001, d = 1.52) and intermediates (p = 0.01, d = 1.26). The association between the simulation-based performance score and the specialist rating was strong (R = 0.86, p < 0.01). CONCLUSIONS: The novel spine simulator provides an applicable tool for the training of image-guided surgery skills in a realistic design. Its simulation-based assessment score classifies different levels of expertise accurately.

Novel synthetic vertebrae provide realistic haptics for pedicle screw placement.

During vertebral surgery, misplaced pedicle screws can harm vital neural and vascular structures. Haptic distinction between cortical and cancellous bone structures is therefore essential for correct screw placement. This tactile experience during pedicle screw placement can be obtained by training on human or animal specimens even if expensive or ethically questionable. In this study, novel synthetic vertebrae were evaluated within a hybrid simulator to provide realistic haptics for the training of spine surgeries. Synthetic vertebrae were custommade of calcium powder-based composites imitating both, cancellous and cortical bone. The mechanical properties of synthetic surrogates were validated for pedicle screw placement and cement augmentation and were compared with those obtained from human vertebrae and insertion torques were analyzed. In human vertebrae pedicle screw torque measurements resulted in mean torque slopes of 82±33Nm/m. Calcium carbonate-based materials achieved lower torques than the human bone whereas calcium phosphate-based bone surrogates showed comparable results. A further differentiation of the calcium phosphate-based vertebrae revealed, that synthetic vertebrae with lower amounts of blowing agent, achieved suitable torques (83 ± 28Nm/m) in comparison to the human reference (p = 0.39). Cement application and subsequent fluoroscopy images confirmed, that the cancellous core of the synthetic vertebrae enabled cement augmentation. In conclusion, our findings suggest, that the artificial bone samples mimic the properties of human bone during pedicle screw placement and cement augmentation and are therefore suitable as synthetic vertebrae in a hybrid surgical simulator.

Characterization of synthetic foam structures used to manufacture artificial vertebral trabecular bone.

Artificial materials reflecting the mechanical properties of human bone are essential for valid and reliable implant testing and design. They also are of great benefit for realistic simulation of surgical procedures. The objective of this study was therefore to characterize two groups of self-developed synthetic foam structures by static compressive testing and by microcomputed tomography. Two mineral fillers and varying amounts of a blowing agent were used to create different expansion behavior of the synthetic open-cell foams. The resulting compressive and morphometric properties thus differed within and also slightly between both groups. Apart from the structural anisotropy, the compressive and morphometric properties of the synthetic foam materials were shown to mirror the respective characteristics of human vertebral trabecular bone in good approximation. In conclusion, the artificial materials created can be used to manufacture valid synthetic bones for surgical training. Further, they provide novel possibilities for studying the relationship between trabecular bone microstructure and biomechanical properties.

Novel bone surrogates for cranial surgery training.

Parietal graft lifts are trained on human or animal specimens or are directly performed on patients without extensive training. In order to prevent harm to the patient resulting from fast rotating machinery tools, the surgeon needs to apply appropriate forces. Realistic haptics are essential to identify the varying parietal bone layers and to avoid a penetration of the brain. This however, requires experience and training. Therefore, in this study, bone surrogate materials were evaluated with the aim to provide an anatomically correct artificial skull cap with realistic haptic feedback for graft lift training procedures. Polyurethane composites made of calcium carbonate and calcium phosphate were developed and were used to create customized bone surrogates, imitating both cancellous and cortical bone. Mechanical properties of these surrogates were validated for drilling, milling and sawing by comparison with human parietal bones. For that, surgical tool tips were automatically inserted into artificial and human bones in a customized test bench and the maximum axial insertion forces were analyzed. Axial tool insertion measurements in human parietal bones resulted in mean maximum forces of 1.8±0.5N for drilling, 1.7±0.3N for milling and 0.9±0.1N for sawing. Calcium carbonate-based materials achieved higher forces than the human bone for drilling and milling, and lower forces for sawing. The calcium phosphate-based bone surrogates showed comparable axial insertions forces for all investigated tools and were identified as a suitable surrogate for drilling (p=0.87 and 0.41), milling (p=0.92 and 0.63) and sawing (p=0.11 and 0.76) of the cortical layer and the cancellous bone, respectively. In conclusion, our findings suggest, that a suitable material composition for artificial parietal bones has been identified, mimicking the properties of human bone during surgical machinery procedures. Thus, these materials are suitable for surgical training and education in simulator training.