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.

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.