Clinical implementation of finite element models in pelvic ring surgery for prediction of implant behavior: a case report

Fractures of the quadrilateral plate treated with a reconstruction plate and trans-plate quadrilateral screws: An experimental study on cadaveric specimens and finite element analysis

Quadrilateral plate fracture is one of the most complex and challenging pelvic lesions. Operative reduction and internal fixation are the gold standard management for displaced quadrilateral plate fractures. Traditional methods include various kinds of operative reduction and internal fixation through either anterior or posterior approaches using various combinations of plates and lag screws or acute total hip arthroplasty. Here we introduced a new fixation technique named reconstruction plate combined with trans-plate quadrilateral screws. We performed a cadaveric study to determine the biomechanical properties of this system comparing with conventional titanium plate combined with 1/3 tube titanium plate in a both-column acetabular fracture model in standing position. Besides, a finite element model of both-column acetabular fractures fixed by this system was developed and the mechanical properties of implants and acetabular fractures were analyzed. The biomechanical test showed the superiority of reconstruction plate combined with trans-plate quadrilateral screws over conventional titanium plate combined with 1/3 tube titanium plate in treating both-column quadrilateral plate fractures in standing position. Later finite element analysis confirmed the stabilities of the fractures under 1-legged stance. Thus, reconstruction plate combined with trans-plate quadrilateral screws provides an alternative method in treating quadrilateral plate fractures.

Evaluation of finite element modeling methods for predicting compression screw failure in a custom pelvic implant

Introduction: Three-dimensional (3D)-printed custom pelvic implants have become a clinically viable option for patients undergoing pelvic cancer surgery with resection of the hip joint. However, increased clinical utilization has also necessitated improved implant durability, especially with regard to the compression screws used to secure the implant to remaining pelvic bone. This study evaluated six different finite element (FE) screw modeling methods for predicting compression screw pullout and fatigue failure in a custom pelvic implant secured to bone using nine compression screws. Methods: Three modeling methods (tied constraints (TIE), bolt load with constant force (BL-CF), and bolt load with constant length (BL-CL)) generated screw axial forces using functionality built into Abaqus FE software; while the remaining three modeling methods (isotropic pseudo-thermal field (ISO), orthotropic pseudo-thermal field (ORT), and equal-and-opposite force field (FOR)) generated screw axial forces using iterative physics-based relationships that can be implemented in any FE software. The ability of all six modeling methods to match specified screw pretension forces and predict screw pullout and fatigue failure was evaluated using an FE model of a custom pelvic implant with total hip replacement. The applied hip contact forces in the FE model were estimated at two locations in a gait cycle. For each of the nine screws in the custom implant FE model, likelihood of screw pullout failure was predicted using maximum screw axial force, while likelihood of screw fatigue failure was predicted using maximum von Mises stress. Results: The three iterative physics-based modeling methods and the non-iterative Abaqus BL-CL method produced nearly identical predictions for likelihood of screw pullout and fatigue failure, while the other two built-in Abaqus modeling methods yielded vastly different predictions. However, the Abaqus BL-CL method required the least computation time, largely because an iterative process was not needed to induce specified screw pretension forces. Of the three iterative methods, FOR required the fewest iterations and thus the least computation time. Discussion: These findings suggest that the BL-CL screw modeling method is the best option when Abaqus is used for predicting screw pullout and fatigue failure in custom pelvis prostheses, while the iterative physics-based FOR method is the best option if FE software other than Abaqus is used.

Biomechanical evaluation of a healed acetabulum with internal fixators: finite element analysis

BACKGROUND

Treatment of complicated acetabular fracture with internal fixation usually has high risk of failure because of unbefitting fixation. However, evaluation of the biomechanical effect of internal fixation under physiological loading for fracture healing is still generally rarely performed. The purpose of this study is to analyze the biomechanical characteristics of a healed acetabulum with designed internal fixators under gait and to explore the biomechanical relationship between the healed bone and the internal fixator.

METHODS

A patient-specific finite element model of whole pelvis with designed internal fixators was constructed based on the tomographic digital images, in which the spring element was used to simulate the main ligaments of the pelvis. And the finite element analysis under both the combination loading of different phases and the individual loading of each phase during the gait cycle was carried out. The displacement, von Mises stress, and strain energy of both the healed bone and the fixation were calculated to evaluate the biomechanical characteristics of the healed pelvis.

RESULTS

Under the combination loading of gait, the maximum difference of displacement between the left hip bone with serious injury and the right hip bone with minor injury is 0.122 mm, and the maximum stress of the left and right hemi-pelvis is 115.5 MPa and 124.28 MPa, respectively. Moreover, the differences of average stress between the bone and internal fixators are in the range of 2.3-13.7 MPa. During the eight phases of gait, the stress distribution of the left and right hip bone is similar. Meanwhile, based on the acetabular three-column theory, the strain energy ratio of the central column is relatively large in stance phases, while the anterior column and posterior column of the acetabular three-column increase in swing phases.

CONCLUSIONS

The acetabular internal fixators designed by according to the anatomical feature of the acetabulum are integrated into the normal physiological stress conduction of the pelvis. The design and placement of the acetabular internal fixation conforming to the biomechanical characteristics of the bone is beneficial to the anatomical reduction and effective fixation of the fracture, especially for complex acetabular fracture.

Finite element analysis of screw fixation durability under multiple boundary and loading conditions for a custom pelvic implant

Despite showing promising functional outcomes for pelvic reconstruction after sarcoma resection, custom-made pelvic implants continue to exhibit high complication rates due to fixation failures. Patient-specific finite element models have been utilized by researchers to evaluate implant durability. However, the effect of assumed boundary and loading conditions on failure analysis results of fixation screws remains unknown. In this study, the postoperative stress distributions in the fixation screws of a state-of-the-art custom-made pelvic implant were simulated, and the risk of failure was estimated under various combinations of two bone-implant interaction models (tied vs. frictional contact) and four load cases from level-ground walking and stair activities. The study found that the average weighted peak von Mises stress could increase by 22-fold when the bone-implant interactions were modeled with a frictional contact model instead of a tied model, and the likelihood of fatigue and pullout failure for each screw could change dramatically when different combinations of boundary and loading conditions were used. The inclusion of additional boundary and loading conditions led to a more reliable analysis of fixation durability. These findings demonstrated the importance of simulating multiple boundary conditions and load cases for comprehensive implant design evaluation using finite element analysis.

Instability of the posterior pelvic ring: introduction of innovative implants

BACKGROUND

Increasing numbers of posterior pelvic ring fractures, especially in elderly patients, demonstrate the need for soft tissue protecting surgical techniques. Standard of care is iliosacral screw osteosynthesis. This type of osteosynthesis has its limitations especially in patients with reduced bone properties. Therefore, the development of new and straightforward surgical techniques and implant designs is favorable.

METHODS

Introducing this modular system for the posterior pelvic ring, known complications of iliosacral screw osteosynthesis, such as implant loosening and malpositioning may be reduced, due to innovative mechanical characteristics.

RESULTS

The shown cases demonstrate the potential benefits of the system with a wide range of treatment options due to its modularity.

CONCLUSION

The modular implant system presented here can significantly facilitate and improve the stabilization of posterior pelvic ring instabilities.

The Impact of an Open-Book Pelvic Ring Injury on Bone Strain: Validation of a Finite Element Model and Analysis Within the Gait Cycle

The threshold for surgical stabilization for an open-book pelvic fracture is not well defined. The purpose of this research was to validate the biomechanical behavior of a specimen-specific pelvic finite element (FE) model with an open-book fracture with the biomechanical behavior of a cadaveric pelvis in double leg stance configuration under physiologic loading, and to utilize the validated model to compare open book versus intact strain patterns during gait. A cadaveric pelvis was experimentally tested under compressive loading in double leg stance, intact, and with a simulated open-book fracture. An intact FE model of this specimen was reanalyzed with an equivalent simulated open-book fracture. Comparison of the FE generated and experimentally measured strains yielded an R2 value of 0.92 for the open-book fracture configuration. Strain patterns in the intact and fractured models were compared throughout the gait cycle. In double leg stance and heel-strike/heel-off models, tensile strains decreased, especially in the pubic ramus contralateral to the injury, and compressive strains increased in the sacroiliac region of the injured side. In the midstance/midswing gait configuration, higher tensile and compressive FE strains were observed on the midstance side of the fractured versus intact model and decreased along the superior and inferior pubic rami and ischium, with midswing side strains reduced almost to zero in the fractured model. Identified in silico patterns align with clinical understanding of open-book fracture pathology suggesting future potential of FE models to quantify instability and optimize fixation strategies.

Screw-in-screw fixation of fragility sacrum fractures provides high stability without loosening-biomechanical evaluation of a new concept

Surgical treatment of fragility sacrum fractures with percutaneous sacroiliac (SI) screw fixation is associated with high failure rates. Turn-out is detected in up to 20% of the patients. The aim of this study was to evaluate a new screw-in-screw implant prototype for fragility sacrum fracture fixation. Twenty-seven artificial hemipelvises were assigned to three groups (n = 9) for instrumentation of an SI screw, the new screw-in-screw implant prototype, ora transsacral screw. Before implantation, a vertical osteotomy was set in zone 1 after Denis. All specimens were biomechanically tested to failure in upright position. Validated setup and test protocol were used for complex axial and torsional loading applied through the S1 vertebral body to promote turn-out of the implants. Interfragmentary movements were captured via optical motion tracking. Screw motions were evaluated by means of triggered anteroposterior X-rays. Interfragmentary movements and implant motions were significantly higher for SI screw fixation compared to both transsacral and screw-in-screw fixations. In addition, transsacral screw and screw-in-screw fixations revealed similar construct stability. Moreover, screw-in-screw fixation successfully prevented turn-out of the implant that remained during testing at 0° rotation for all specimens. From biomechanical perspective, fragility sacrum fracture fixation with the new screw-in-screw implant prototype provides higher stability than an SI screw, being able to successfully prevent turn-out. Moreover, it combines the higher stability of transsacral screw fixation with the less risky operational procedure of SI screw fixation and can be considered as their alternative treatment option.

The three-dimensional bone mass distribution of the posterior pelvic ring and its key role in transsacral screw placement

To optimize the placement of iliosacral screws in osteoporotic bone it is essential to know where to find the best purchase. The aim of this study was to determine and visualize the distribution of bone mass in the posterior pelvic ring by using a color-coded thermal map, to differentiate the bone distribution patterns in normal pelvises and in pelvises with impaired bone density and to identify zones in S1 and S2 with particularly good bone quality, in both healthy and osteoporotic pelvises. A total of 324 pelvises were included. The bone density of the posterior pelvic ring, the fifth lumbar vertebral body (L5) and screw corridors S1 and S2 were visualized. Each individual pelvis was measured with a 3D automated program. Two groups were selected - patients with mean bone density in L5 of ≤100 HU (group 1, n = 52) and those with mean bone density >100 HU (group 2, n = 272). Color-coded thermal maps are presented of the bone density distribution in the pelvises. Bone density in L5 correlated significantly with S1 and S2; bone density was significantly higher in the S1 than in the S2 corridor (p < 0.001). Bone was denser in the posterior and upper parts of the S1 body. Bone density was significantly lower in group 2 than in group 1 (p < 0.001). The color-coded “thermal” maps of bone mass distribution can help surgeons to decide where sacroiliac screws are likely to find optimal purchase.

BIOMECHANICAL ANALYSIS OF DIFFERENT SURGICAL STRATEGIES FOR THE TREATMENT OF ROTATIONALLY UNSTABLE PELVIC FRACTURE USING FINITE ELEMENT METHOD

A rotationally unstable pelvic fracture can lead to loss of function and limit moving ability. Immediate fracture fixation is needed for patients with the pelvic fractures. However, it may be difficult to evaluate different surgical strategies for the fracture treatments due to variations in patients’ anatomies and surgical techniques. Thus, the purpose of the present study was to analyze the biomechanical performances of the intact, injured, and treated pelvises based on different physiological movements of the spine using finite element method. Three-dimensional musculoskeletal finite element models of the spine-pelvis-femur complex were developed. The intact pelvis, the rotationally unstable pelvis, and six types of pelvic fixation techniques were analyzed. Additionally, seven types of physiological movements of the spine were also considered. The results showed that the posterior iliosacral screws combined with lower and anterior plate (PIS-LAP) had good fixation stability, lower plate stress, and lower pelvic stress. However, the PIS-LAP increased the stress of the posterior iliosacral screws. The right lateral bending, left lateral bending, and flexion significantly affect all the biomechanical performances compared to the other physiological movements of the spine. The present study can provide engineers and surgeons with the understanding of the biomechanics of various fixation techniques during different physiological movements for the treatment of rotationally unstable pelvic fractures.

Biomechanical Comparison of Three Internal Fixation Techniques for Stabilizing Posterior Pelvic Ring Disruption: A 3D Finite Element Analysis

OBJECTIVE

To compare the biomechanical stability and compatibility of two iliosacral screws (ISS), a tension band plate (TBP), and a minimally invasive adjustable plate (MIAP) for treating Tile C pelvic fractures.

METHODS

Three groups of finite element models of the intact pelvis, including the main ligament and the proximal one-third of both femurs, were developed to simulate vertical sacral fractures and treated with the three abovementioned internal fixation techniques. A 500 N vertical load, a 500 N vertical load plus a 10 Nm moment of forward sagittal direction, and 500 N vertical load plus a 10 Nm moment of right lateral direction were applied to the sacrum to simulate standing status, bending status, and flexion status, respectively. The maximum displacement value, the stress value, and the stress value of the fracture interface were compared among the three internal fixation techniques.

RESULTS

The results showed that all three internal fixation techniques effectively restored the biomechanical transmission of the injured pelvis. The stress on the implants in the TBP model was 167.47% and 53.41% higher than that in the ISS model and the MIAP model, respectively, and the stress shielding phenomenon of the TBP model was more obvious than in the other two models. Meanwhile, the stress between the fracture interfaces in the TBP fixation models was apparently higher than that in the other two models. However, the vertical displacement of the MIAP model was not significantly different from that in the ISS and TBP model; therefore, strong fixation could be obtained in all three models.

CONCLUSION

Based on our results, we believe that the stability of Tile C pelvic fracture fixed with MIAP was similar to that of fractures fixed with ISS and TBP, but the stress shielding phenomenon and safety of implants in the TBP models were inferior to those in the MIAP and ISS fixation models. Meanwhile, MIAP and ISS fixation were more helpful to the healing processing than was TBP fixation, especially at the fracture interface of the second and third vertebral body levels.

In Silico Pelvis and Sacroiliac Joint Motion: Refining a Model of the Human Osteoligamentous Pelvis for Assessing Physiological Load Deformation Using an Inverted Validation Approach

Introduction. Computational modeling of the human pelvis using the finite elements (FE) method has become increasingly important to understand the mechanisms of load distribution under both healthy and pathologically altered conditions and to develop and assess novel treatment strategies. The number of accurate and validated FE models is however small, and given models fail resembling the physiologic joint motion in particular of the sacroiliac joint. This study is aimed at using an inverted validation approach, using in vitro load deformation data to refine an existing FE model under the same mode of load application and to parametrically assess the influence of altered morphology and mechanical data on the kinematics of the model. Materials and Methods. An osteoligamentous FE model of the pelvis including the fifth lumbar vertebra was used, with highly accurate representations of ligament orientations. Material properties were altered parametrically for bone, cartilage, and ligaments, followed by changes in bone geometry (solid versus 3 and 2 mm shell) and material models (linear elastic, viscoelastic, and hyperelastic isotropic), and the effects of varying ligament fiber orientations were assessed. Results. Elastic modulus changes were more decisive in both linear elastic and viscoelastic bone, cartilage, and ligaments models, especially if shell geometries were used for the pelvic bones. Viscoelastic material properties gave more realistic results. Surprisingly little change was observed as a consequence of altering SIJ ligament orientations. Validation with in vitro experiments using cadavers showed close correlations for movements especially for 3 mm shell viscoelastic model. Discussion. This study has used an inverted validation approach to refine an existing FE model, to give realistic and accurate load deformation data of the osteoligamentous pelvis and showed which variation in the outcomes of the models are attributed to altered material properties and models. The given approach furthermore shows the value of accurate validation and of using the validation data to fine tune FE models.

In-silico pelvis and sacroiliac joint motion-A review on published research using numerical analyses

BACKGROUND

Computational models of the human pelvis have become highly useful tools to assess mechanisms of injury, diagnostics and treatment options. The purpose of this systematic literature review was to summarize existing pelvic computer models, to assess their comparability and the measures taken for experimental validation.

METHODS

Research on virtual simulations of the posterior pelvis and sacroiliac joint available from the ISI Web of Knowledge, PubMed and Scopus databases available until January 2018 were included.

FINDINGS

From a total of 3938 articles, 33 studies matched the criteria. Thirteen studies reported on experimental biomechanics, of which seven were parametric. Thirteen studies focused on pelvic injury and surgery, three were clinical case reports. One study assessed the effects of lumbar surgery on the sacroiliac joint, three studies on diagnostics and the non-surgical treatment of the sacroiliac joint. The mode of load application, geometry, material laws and boundary conditions varied vastly between the studies. The majority excluded the lumbosacral transition as part of pelvic biomechanics, and used isotropic linear elastic material properties. Outcomes of the analyses were reported inconsistently with negative impact on their comparability, and validation was commonly conducted by literature with varying agreement of the loading conditions.

INTERPRETATION

Comparability and validation are two major issues of present computational biomechanics of the pelvis. These issues diminish the transferability of the in-silico findings into real-life scenarios. In-vitro cadaveric models remain the realistic standard to account for the present computational analyses which simplify the complex nature of musculoskeletal tissues of the pelvis.

3D statistical model of the pelvic ring - a CT-based statistical evaluation of anatomical variation

The pelvic ring is a highly complex construct with a central role for human stability and mobility. The observable interindividual differences in skeletal anatomy are caused by anatomical variation in the innominate bones as well as the sacrum, further to differences in the spatial arrangement of these bones to each other. The aim of this study was to generate a 3D statistical model of the entire pelvic ring in order to analyse the observed interindividual differences and anatomical variation. A series of 50 anonymized pelvic CT scans of uninjured Japanese adults [30 males, 20 females, average age of 74.9 years, standard deviation (SD) 16.9 years] were processed and analysed, resulting in a 3D statistical overall mean model and separate male and female mean models. Principal component analysis (PCA) of the overall statistical model predominantly showed size variation (20.39%) followed by shape variation (14.13%), and a variation of the spatial arrangement of the sacrum to the innominate bones in different anatomical peculiarities (11.39 and 8.85%). In addition, selected internal and external pelvic parameters were manually measured with the objective of further evaluating and quantifying the observed interindividual as well as the known sex-specific differences. A separate statistical model of the grey value distribution based on the given Hounsfield unit (HU) values was calculated for assessing bone mass distribution, thus an indication of bone quality utilizing grey values as a quantitative description of radiodensity was obtained. A consistent pattern of grey value distribution was shown, with the highest grey values observed between the sacro-iliac joint and the acetabulum along the pelvic brim. Low values were present in the sacral ala, in the area of the iliac fossa as well as in the pubic rami next to the symphysis. The present model allows a differentiated analysis of the observed interindividual variation of the pelvic ring and an evaluation of the grey value distribution therein. Besides providing a better understanding of anatomical variation, this model could be also used as a helpful tool for educational purposes, preoperative planning and implant design.

Effect of the screw type (S2-alar-iliac and iliac), screw length, and screw head angle on the risk of screw and adjacent bone failures after a spinopelvic fixation technique: A finite element analysis

PURPOSE

Spinopelvic fixations involving the S2-alar-iliac (S2AI) and iliac screws are commonly used in various spinal fusion surgeries. This study aimed to compare the biomechanical characteristics, specifically the risk of screw and adjacent bone failures of S2AI screw fixation with those of iliac screw fixation using a finite element analysis (FEA).

METHODS

A three-dimensional finite element (FE) model of a healthy spinopelvis was generated. The pedicle screws were placed on the L3-S1 with three different lengths of the S2AI and iliac screws (60 mm, 75 mm, and 90 mm). In particular, two types of the S2AI screw, 15°- and 30°-angled polyaxial screw, were adopted. Physiological loads, such as a combination of compression, torsion, and flexion/extension loads, were applied to the spinopelvic FE model, and the stress distribution as well as the maximum von Mises equivalent stress values were calculated.

RESULTS

For the iliac screw, the highest stress on the screw was observed with the 75-mm screw, rather than the 60-mm screw. The bones around the iliac screw indicated that the maximum equivalent stress decreased as the screw length increased. For the S2AI screw, the lowest stress was observed in the 90-mm screw length with a 30° head angle. The bones around the S2AI screw indicated that the lowest stress was observed in the 90-mm screw length and a 15° head angle.

CONCLUSIONS

It was found that the S2AI screw, rather than the iliac screw, reduced the risk of implant failure for the spinopelvic fixation technique, and the 90-mm screw length with a 15° head angle for the S2AI screw could be biomechanically advantageous.

Biomechanical evaluation of location and mode of failure in three screw fixations for a comminuted transforaminal sacral fracture model

Background

Pelvic ring–comminuted transforaminal sacral fracture injuries are rotationally and vertically unstable and have a high rate of failure.

Objective

Our study purpose was to use three-dimensional (3D) optical tracking to detect onset location of bone–implant interface failure and measure the distances and angles between screws and line of applied force for correlation to strength of pelvic fracture fixation techniques.

Methods

3D relative motion across sacral–rami fractures and screws relative to bone was measured with an optical tracking system. Synthetic pelves were used. Comminuted transforaminal sacral–rami fractures were modelled. Each pelvis was stabilised by either (1) two iliosacral screws in S1, (2) one transsacral screw in S1 and one iliosacral screw in S1 and (3) one trans-alar screw in S1 and one iliosacral screw in S1; groups 4–6 consisted of fixation groups with addition of anterior inferior iliac pelvic external fixator. Eighteen-instrumented pelvic models with right ilium fixed simulate single-leg stance. Load was applied to centre of S1 superior endplate. Five cycles of torque was initially applied, sequentially increased until permanent deformation occurred. Five cycles of axial load compression was next applied, sequentially increased until permanent deformation occurred, followed by axial loading to catastrophic failure. A Student t test was used to determine significance (p < 0.05).

Results

The model, protocol and 3D optical system have the ability to locate how sub-catastrophic failures initiate. Our results indicate failure of all screw-based constructs is due to localised bone failure (screw pull-in push-out at the ipsilateral ilium–screw interface, not in sacrum); thus, no difference was observed when not supplemented with external fixation.

Conclusion

Inclusion of external fixation improved resistance only to torsional loading.

Translational Potential of this Article

Patients with comminuted transforaminal sacral–ipsilateral rami fractures benefit from this fixation.

Development of finite element model for customized prostheses design for patient with pelvic bone tumor

The aim of this study was to design a hemi-pelvic prosthesis for a patient affected by pelvic sarcoma. To investigate the biomechanical functionality of the pelvis reconstructed with designed custom-made prosthesis, a patient-specific finite element model of whole pelvis with primary ligaments inclusive was constructed based on the computed tomography images of the patient. Then, a finite element analysis was performed to calculate and compare the stress distribution between the normal and implanted pelvis models when undergoing three different static conditions-both-leg standing, single-leg standing for the healthy and the affected one. No significant differences were observed in the stresses between the normal and reconstructed pelvis for both-leg standing, but 20%-40% larger stresses were predicted for the peak stress of the single-leg standing (affected side). Moreover, two- to threefold of peak stresses were predicted within the prostheses compared to that of the normal pelvis especially for single-leg standing case, however, still below the allowable fatigue limitation. The study on the load transmission functionality of prosthesis indicated that it is crucial to carry out finite element analysis for functional evaluation of the designed customized prostheses before three-dimensional printing manufacturing, allowing better understanding of the possible peak stresses within the bone as well as the implants for safety precaution. The finite element model can be equally applicable to other bone tumor model for biomechanical studying.

Three-dimensional reduction and finite element analysis improves the treatment of pelvic malunion reconstructive surgery: A case report

RATIONALE

Pelvic malunion is a rare complication and is technically challenging to correct owing to the complex three-dimensional (3D) geometry of the pelvic girdle. Hence, precise preoperative planning is required to ensure appropriate correction. Reconstructive surgery is generally a 2- or 3-stage procedure, with transiliac osteotomy serving as an alternative to address limb length discrepancy.

PATIENT CONCERNS

A 38-year-old female patient with a Mears type IV pelvic malunion with previous failed reconstructive surgery was admitted to our department due to progressive immobilization, increasing pain especially at the posterior pelvic arch and a leg length discrepancy. The leg discrepancy was approximately 4 cm and rotation of the right hip joint was associated with pain.

DIAGNOSIS

Radiography and computer tomography (CT) revealed a hypertrophic malunion at the site of the previous posterior osteotomy (Mears type IV) involving the anterior and middle column, according to the 3-column concept, as well as malunion of the left anterior arch (Mears type IV).

INTERVENTIONS

The surgery was planned virtually via 3D reconstruction, using the patient's CT, and subsequently performed via transiliac osteotomy and symphysiotomy. Finite element method (FEM) was used to plan the osteotomy and osteosynthesis as to include an estimation of the risk of implant failure.

OUTCOMES

There was not incidence of neurological injury or infection, and the remaining leg length discrepancy was ≤ 2 cm. The patient recovered independent, pain free, mobility. Virtual 3D planning provided a more precise measurement of correction parameters than radiographic-based measurements. FEM analysis identified the highest risk for implant failure at the symphyseal plate osteosynthesis and the parasymphyseal screws. No implant failure was observed.

LESSONS

Transiliac osteotomy, with additional osteotomy or symphysiotomy, was a suitable surgical procedure for the correction of pelvic malunion and provided adequate correction of leg length discrepancy. Virtual 3D planning enabled precise determination of correction parameters, with FEM analysis providing an appropriate method to predict areas of implant failure.

Biomechanical study of different fixation techniques for the treatment of sacroiliac joint injuries using finite element analyses and biomechanical tests

The pelvis is one of the most stressed areas of the human musculoskeletal system due to the transfer of truncal loads to the lower extremities. Sacroiliac joint injury may lead to abnormal joint mechanics and an unstable pelvis. Various fixation techniques have been evaluated and discussed. However, it may be difficult to investigate each technique due to variations in bone quality, bone anatomy, fracture pattern, and fixation location. Additionally, the finite element method is one useful technology that avoids these variations. Unfortunately, most previous studies neglected the effects of the lumbar spine and femurs when they investigated the biomechanics of pelvises. Thus, the aim of this study was to investigate the biomechanical performance of intact, injured, and treated pelvises using numerical and experimental approaches. Three-dimensional finite element models of the spine-pelvis-femur complex with and without muscles and ligaments were developed. The intact pelvis, the pelvis with sacroiliac joint injury, and three types of pelvic fixation techniques were analyzed. Concurrently, biomechanical tests were conducted to validate the numerical outcomes using artificial pelvises. Posterior iliosacral screw fixation showed relatively better fixation stability and lower risks of implant failure and pelvic breakage than sacral bar fixation and a locking compression plate fixation. The present study can help surgeons and engineers understand the biomechanics of intact, injured, and treated pelvises. Both the simulation technique and the experimental setup can be applied to investigate different pelvic injuries.

Experimental investigation of the structural behavior of equine urethra

BACKGROUND AND OBJECTIVE

An integrated experimental and computational investigation was developed aiming to provide a methodology for characterizing the structural response of the urethral duct. The investigation provides information that are suitable for the actual comprehension of lower urinary tract mechanical functionality and the optimal design of prosthetic devices.

METHODS

Experimental activity entailed the execution of inflation tests performed on segments of horse penile urethras from both proximal and distal regions. Inflation tests were developed imposing different volumes. Each test was performed according to a two-step procedure. The tubular segment was inflated almost instantaneously during the first step, while volume was held constant for about 300s to allow the development of relaxation processes during the second step. Tests performed on the same specimen were interspersed by 600s of rest to allow the recovery of the specimen mechanical condition. Results from experimental activities were statistically analyzed and processed by means of a specific mechanical model. Such computational model was developed with the purpose of interpreting the general pressure-volume-time response of biologic tubular structures. The model includes parameters that interpret the elastic and viscous behavior of hollow structures, directly correlated with the results from the experimental activities.

RESULTS

Post-processing of experimental data provided information about the non-linear elastic and time-dependent behavior of the urethral duct. In detail, statistically representative pressure-volume and pressure relaxation curves were identified, and summarized by structural parameters. Considering elastic properties, initial stiffness ranged between 0.677 ± 0.026kPa and 0.262 ± 0.006kPa moving from proximal to distal region of penile urethra. Viscous parameters showed typical values of soft biological tissues, as τ₁=0.153±0.018s, τ₂=17.458 ± 1.644s and τ₁=0.201 ± 0.085, τ₂= 8.514 ± 1.379s for proximal and distal regions respectively.

DISCUSSION

A general procedure for the mechanical characterization of the urethral duct has been provided. The proposed methodology allows identifying mechanical parameters that properly express the mechanical behavior of the biological tube. The approach is especially suitable for evaluating the influence of degenerative phenomena on the lower urinary tract mechanical functionality. The information are mandatory for the optimal design of potential surgical procedures and devices.

Development of a Patient-Specific Finite Element Model for Predicting Implant Failure in Pelvic Ring Fracture Fixation

Introduction. The main purpose of this study is to develop an efficient technique for generating FE models of pelvic ring fractures that is capable of predicting possible failure regions of osteosynthesis with acceptable accuracy. Methods. Patient-specific FE models of two patients with osteoporotic pelvic fractures were generated. A validated FE model of an uninjured pelvis from our previous study was used as a master model. Then, fracture morphologies and implant positions defined by a trauma surgeon in the preoperative CT were manually introduced as 3D splines to the master model. Four loading cases were used as boundary conditions. Regions of high stresses in the models were compared with actual locations of implant breakages and loosening identified from follow-up X-rays. Results. Model predictions and the actual clinical outcomes matched well. For Patient A, zones of increased tension and maximum stress coincided well with the actual locations of implant loosening. For Patient B, the model predicted accurately the loosening of the implant in the anterior region. Conclusion. Since a significant reduction in time and labour was achieved in our mesh generation technique, it can be considered as a viable option to be implemented as a part of the clinical routine to aid presurgical planning and postsurgical management of pelvic ring fracture patients.

Finite Element Analysis of Sacroiliac Joint Fixation under Compression Loads

BACKGROUND

Sacroiliac joint (SIJ) is a known chronic pain-generator. The last resort of treatment is the arthrodesis. Different implants allow fixation of the joint, but to date there is no tool to analyze their influence on the SIJ biomechanics under physiological loads. The objective was to develop a computational model to biomechanically analyze different parameters of the stable SIJ fixation instrumentation.

METHODS

A comprehensive finite element model (FEM) of the pelvis was built with detailed SIJ representation. Bone and sacroiliac joint ligament material properties were calibrated against experimentally acquired load-displacement data of the SIJ. Model evaluation was performed with experimental load-displacement measurements of instrumented cadaveric SIJ. Then six fixation scenarios with one or two implants on one side with two different trajectories (proximal, distal) were simulated and assessed with the FEM under vertical compression loads.

RESULTS

The simulated S1 endplate displacement reduction achieved with the fixation devices was within 3% of the experimentally measured data. Under compression loads, the uninstrumented sacrum exhibited mainly a rotation motion (nutation) of 1.38° and 2.80° respectively at 600 N and 1000 N, with a combined relative translation (0.3 mm). The instrumentation with one screw reduced the local displacement within the SIJ by up to 62.5% for the proximal trajectory vs. 15.6% for the distal trajectory. Adding a second implant had no significant additional effect.

CONCLUSION

A comprehensive finite element model was developed to assess the biomechanics of SIJ fixation. SIJ devices enable to reduce the motion, mainly rotational, between the sacrum and ilium. Positioning the implant farther from the SIJ instantaneous rotation center was an important factor to reduce the intra-articular displacement.

CLINICAL RELEVANCE

Knowledge provided by this biomechanical study enables improvement of SIJ fixation through optimal implant trajectory.

Sacral Bone Mass Distribution Assessed by Averaged Three-Dimensional CT Models: Implications for Pathogenesis and Treatment of Fragility Fractures of the Sacrum

BACKGROUND

Fragility fractures of the sacrum are increasing in prevalence due to osteoporosis and epidemiological changes and are challenging in their treatment. They exhibit specific fracture patterns with unilateral or bilateral fractures lateral to the sacral foramina, and sometimes an additional transverse fracture leads to spinopelvic dissociation. The goal of this study was to assess sacral bone mass distribution and corresponding changes with decreased general bone mass.

METHODS

Clinical computed tomography (CT) scans of intact pelves in ninety-one individuals (mean age and standard deviation, 61.5 ± 11.3 years) were used to generate three-dimensional (3D) models of the sacrum averaging bone mass in Hounsfield units (HU). Individuals with decreased general bone mass were identified by measuring bone mass in L5 (group 1 with <100 HU; in contrast to group 2 with ≥100 HU).

RESULTS

In group 1, a large zone of negative Hounsfield units was located in the paraforaminal lateral region from S1 to S3. Along the trans-sacral corridors, a Hounsfield unit peak was observed laterally, corresponding to cortical bone of the auricular surface. The lowest Hounsfield unit values were found in the paraforaminal lateral region in the sacral ala. An intermediate level of bone mass was observed in the area of the vertebral bodies, which also demonstrated the largest difference between groups 1 and 2. Overall, the Hounsfield units were lower at S2 than S1.

CONCLUSIONS

The models of averaged bone mass in the sacrum revealed a distinct 3D distribution pattern.

CLINICAL RELEVANCE

The negative values in the paraforaminal lateral region may explain the specific fracture patterns in fragility fractures of the sacrum involving the lateral areas of the sacrum. Transverse fractures located between S1 and S2 leading to spinopelvic dissociation may occur because of decreased bone mass in S2. The largest difference between the studied groups was found in the vertebral bodies and might support the use of transsacral or cement-augmented implants.

An Atraumatic Symphysiolysis with a Unilateral Injured Sacroiliac Joint in a Patient with Cushing's Disease: A Loss of Pelvic Stability Related to Ligamentous Insufficiency?

Glucocorticoids are well known for altering bone structure and elevating fracture risk. Nevertheless, there are very few reports on pelvic ring fractures, compared to other bones, especially with a predominantly ligamentous insufficiency, resulting in a rotationally unstable pelvic girdle. We report a 39-year-old premenopausal woman suffering from an atraumatic symphysiolysis and disruption of the left sacroiliac joint. She presented with external rotational pelvic instability and immobilization. Prior to the injury, she received high-dose glucocorticoids for a tentative diagnosis of rheumatoid arthritis over two months. This diagnosis was not confirmed. Other causes leading to the unstable pelvic girdle were excluded by several laboratory and radiological examinations. Elevated basal cortisol and adrenocorticotropic hormone levels were measured and subsequent corticotropin-releasing hormone stimulation, dexamethasone suppression test, and petrosal sinus sampling verified the diagnosis of adrenocorticotropic hormone-dependent Cushing's disease. The combination of adrenocorticotropic hormone-dependent Cushing's disease and the additional application of exogenous glucocorticoids is the most probable cause of a rare atraumatic rotational pelvic instability in a premenopausal patient. To the authors' knowledge, this case presents the first description of a rotationally unstable pelvic ring fracture involving a predominantly ligamentous insufficiency in the context of combined exogenous and endogenous glucocorticoid elevation.

A double-barrelled fibula graft restoring pelvic stability after late posterior ring instability related to a surgical treated osteitis pubis: a case report

BACKGROUND

Osteitis pubis or symphysitis pubis is a rare occurring non-infectious inflammation of the symphysis, the adjacent pubic bones and surrounding tissue. The therapy might be conservative or surgical by a resection of the symphysis and involved parts of the pubic bone. Nevertheless, this resection might lead to an anterior instability impairing the posterior arch and the sacroiliac joints in the aftermath.

CASE PRESENTATION

Here, we report about a 50-year-old women suffering from osteitis pubis treated by wedge resection of the symphysis and parts of the pubic bone. To maintain stability and for local antibiotic treatment a cement spacer was implemented. By clinical inconspicuous findings and the patient's desire, no further surgery was performed. However, 2 years after surgery the spacer dislocated and the patient complained about pain in the posterior arch due to an impaired mobility. Reconstruction surgery was planned including the bridging of the accrued space with a vascularized double-barrelled fibula graft, plate osteosynthesis and rectus abdominis flap coverage. The performed surgery led to pain relief and increased mobility.

CONCLUSION

The present case highlights the possible complication of surgical treated osteitis pubis leading to anterior arch instability affecting the posterior arch and thus impairing pelvic ring stability and patient mobility. Furthermore, we describe an opportunity to treat this complication or other etiologies contributing to anterior pelvic ring stability with large bone defects using a vascularized double-barrelled fibula graft to restore pelvic stability.

Fragility fractures of the sacrum: how to identify and when to treat surgically?

The increasing prevalence of fragility fractures of the sacrum (FFS) occurring predominantly in osteoporotic individuals poses a diagnostic and therapeutic challenge. The clinical presentation varies from longstanding low back pain without the patient remembering a traumatic event to immobilized patients after suffering a low-energy trauma. FFS are often combined with a fracture of the anterior pelvic ring; hence they are classified as a part of fragility fractures of the pelvis (FFP). If not displaced, the patients are treated with weight bearing as tolerated and analgesics; however, we advocate to treat displaced fractures surgically according to the fracture personality and the patient’s comorbidities. Surgical options include minimal invasive sacro-iliac screws, trans-sacral bar osteosynthesis, open reduction and internal fixation, or spinopelvic stabilization. In the light of the high complication rate associated with immobilized patients, an operative approach often is indicated to accelerate the patient’s mobility.

The extent of ligament injury and its influence on pelvic stability following type II anteroposterior compression pelvic injuries--A computer study to gain insight into open book trauma

Surgical stabilization of the pelvis following type II anteroposterior compression pelvic injuries (APCII) is based on the assumption that the anterior sacroiliac, sacrospinous, and sacrotuberous ligaments disrupt simultaneously. Recent data on the ligaments contradict this concept. We aimed at determining the mechanisms of ligament failure in APCII computationally. In an individual osteoligamentous computer model of the pelvis, ligament load, and strain were observed for the two-leg stance, APCII with 100-mm symphyseal widening and for two-leg stance with APCII-related ligament failure, and validated with body donors. The anterior sacroiliac and sacrotuberous ligaments had the greatest load with 80% and 17% of the total load, respectively. APCII causes partial failure of the anterior sacroiliac ligament and the pelvis to become horizontally instable. The other ligaments remained intact. The sacrospinous ligament was negligibly loaded but stabilized the pelvis vertically. The interosseous sacroiliac and sacrotuberous ligaments are likely responsible for reducing the symphysis and might serve as an indicator of vertical stability. The sacrospinous ligament appears to be of minor significance in APCII but plays an important role in vertical stabilization. Further research is necessary to determine the influence of alterations in ligament and bone material properties.

Mechanical testing of a device for subcutaneous internal anterior pelvic ring fixation versus external pelvic ring fixation

Background

Although useful in the emergency treatment of pelvic ring injuries, external fixation is associated with pin tract infections, the patient’s limited mobility and a restricted surgical accessibility to the lower abdomen. In this study, the mechanical stability of a subcutaneous internal anterior fixation (SIAF) system is investigated.

Methods

A standard external fixation and a SIAF system were tested on pairs of Polyoxymethylene testing cylinders using a universal testing machine. Each specimen was subjected to a total of 2000 consecutive cyclic loadings at 1 Hz with sinusoidal lateral compression/distraction (+/−50 N) and torque (+/− 0.5 Nm) loading alternating every 200 cycles. Translational and rotational stiffness were determined at 100, 300, 500, 700 and 900 cycles.

Results

There was no significant difference in translational stiffness between the SIAF and the standard external fixation when compared at 500 (p = .089), 700 (p = .081), and 900 (p = .266) cycles. Rotational stiffness observed for the SIAF was about 50 percent higher than the standard external fixation at 300 (p = .005), 500 (p = .020), and 900 (p = .005) cycles. No loosening or failure of the rod-pin/rod-screw interfaces was seen.

Conclusions

In comparison with the standard external fixation system, the tested device for subcutaneous internal anterior fixation (SIAF) in vitro has similar translational and superior rotational stiffness.

Statistical shape and appearance models of bones

When applied to bones, statistical shape models (SSM) and statistical appearance models (SAM) respectively describe the mean shape and mean density distribution of bones within a certain population as well as the main modes of variations of shape and density distribution from their mean values. The availability of this quantitative information regarding the detailed anatomy of bones provides new opportunities for diagnosis, evaluation, and treatment of skeletal diseases. The potential of SSM and SAM has been recently recognized within the bone research community. For example, these models have been applied for studying the effects of bone shape on the etiology of osteoarthritis, improving the accuracy of clinical osteoporotic fracture prediction techniques, design of orthopedic implants, and surgery planning. This paper reviews the main concepts, methods, and applications of SSM and SAM as applied to bone.

Characterization of the anisotropic mechanical behaviour of colonic tissues: experimental activity and constitutive formulation

The aim was to investigate the biomechanical behaviour of colonic tissues by a coupled experimental and numerical approach. The wall of the colon is composed of different tissue layers. Within each layer, different fibre families are distributed according to specific spatial orientations, which lead to a strongly anisotropic configuration. Accounting for the complex histology of the tissues, mechanical tests must be planned and designed to evaluate the behaviour of the colonic wall in different directions. Uni-axial tensile tests were performed on tissue specimens from 15 fresh pig colons, accounting for six different loading directions (five specimens for each loading direction). The next step of the investigation was to define an appropriate constitutive framework and develop a procedure for identification of the constitutive parameters. A specific hyperelastic formulation was developed that accounted for the multilayered conformation of the colonic wall and the fibre-reinforced configuration of the tissues. The parameters were identified by inverse analyses of the mechanical tests. The comparison of model results with experimental data, together with the evaluation of satisfaction of material thermomechanics principles, confirmed the reliability of the analysis developed. This work forms the basis for more comprehensive activities that aim to provide computational tools for the interpretation of surgical procedures that involve the gastrointestinal tract, considering the specific biomedical devices adopted.