Presurgical finite element analysis from routine computed tomography studies for craniofacial distraction: II. An engineering prediction model for gradual correction of asymmetric skull deformities

The Dynamics in Implantation for Patients with Clefts

Objective

To investigate the stresses and strains of an endosseous dental implant in patients with different types of cleft palate in a finite element model.

Materials and Methods

Seven three-dimensional (3D) maxillary models were designed on a personal computer according to computed tomography slice data obtained from seven dry skulls. Next, computer-aided modification was performed on each model to produce three other 3D models with different cleft patterns. Thus, four model types with different cleft patterns were designed and termed NORM (without cleft), ALVEOLAR (only alveolar cleft), PALATAL (only palatal cleft), and COMPLETE (complete cleft). An implant was embedded into the molar region of each model, and a 300-N vertical load and 50-N horizontal load were applied to simulate mastication. Under these conditions, the stresses occurring at the implant-bone interface were calculated by finite element analysis.

Results

Different stress patterns were observed between the models with a palatal cleft (PALATAL and COMPLETE) and those without palatal cleft (NORM and ALVEOLAR). Regarding vertical load application, greater stresses occurred in PALATAL and COMPLETE types than in NORM and ALVEOLAR types. On application of a horizontal load, though the stresses did not show quantitative difference, their vector patterns differed.

Conclusion

In patients with palatal clefts, characteristic stress patterns occur on the bone-implant interface during mastication. This should be taken into consideration when performing an implant treatment in patients with clefts.

EASEPort NPWT System to Enhance Skin Graft Survival--A Simple Assembly

Skin graft has been known to be prone to failure. This study was aimed to make a simplification of the negative pressure wound therapy (NPWT), which follows EASEPort (effective, affordable, safe, easily handled, and portable) concept to support the take of skin graft. The design of the EASEPort-NPWT was then made and technically verified. Thereafter, an animal experimental study comparing the EASEPort-NPWT to the classic tie-over technique on skin graft over exudative wound was conducted. The EASEPort-NPWT was verified to be able to yield and sustain the subatmospheric pressure needed. In the animal study, the treatment group showed better skin graft survival rate (97.55 ± 11.18% take) than the control group (54.88 ± 19.73%) on day-7. Histopathology examination showed good quality of the skin structures taken from the treatment group, which was better than the structures of the skin in the control group. In summary, this study has been able to fulfill its objective to create a device following EASEPort concept. Subsequently, the EASEPort-NPWT was able to enhance skin graft survival rate in exudative wound.

Biomechanical Analysis of the Effect of Intracranial Pressure on the Orbital Distances in Trigonocephaly

Objective

This biomechanical study aims to elucidate differences in how skulls with trigonocephaly, normal skulls, and postoperative trigonocephalic skulls respond to intracranial pressure and how this affects the orbital distances.

Materials and Methods

For 10 patients with trigonocephaly (8.2 ± 4.5 months), simulation models were produced based on the computed tomographic data of the skulls. These models were categorized as the Trigono group. For each model, a 15-mm Hg pressure was applied to the neurocranium to simulate the intracranial pressure. The interorbital distances expanded in response to the applied pressure. The amount of the change in the orbital distance was calculated using finite element analysis. The same processes were repeated for 10 models simulating normal skulls (the Control group) and postoperative trigonocephalic skulls (the Remodeled group). The changes in the orbital distance were compared among the three groups.

Results

The changes in the orbital distance were significantly smaller for the Trigono group than for the Control group. However, changes were significantly greater for the Remodeled group than for the Control group.

Conclusion

The expansion of interorbital distances in response to the cranial pressure is restricted in skulls with trigonocephaly. This restriction is eliminated by performing remodeling of the skull. These findings explain why spontaneous correction of hypotelorism occurs postoperatively in trigonocephaly.

Interaction of hydraulic and buckling mechanisms in blowout fractures

The etiology of blowout fractures is generally attributed to 2 mechanisms--increase in the pressure of the orbital contents (the hydraulic mechanism) and direct transmission of impacts on the orbital walls (the buckling mechanism). The present study aims to elucidate whether or not an interaction exists between these 2 mechanisms. We performed a simulation experiment using 10 Computer-Aided-Design skull models. We applied destructive energy to the orbits of the 10 models in 3 different ways. First, to simulate pure hydraulic mechanism, energy was applied solely on the internal walls of the orbit. Second, to simulate pure buckling mechanism, energy was applied solely on the inferior rim of the orbit. Third, to simulate the combined effect of the hydraulic and buckling mechanisms, energy was applied both on the internal wall of the orbit and inferior rim of the orbit. After applying the energy, we calculated the areas of the regions where fracture occurred in the models. Thereafter, we compared the areas among the 3 energy application patterns. When the hydraulic and buckling mechanisms work simultaneously, fracture occurs on wider areas of the orbital walls than when each of these mechanisms works separately. The hydraulic and buckling mechanisms interact, enhancing each other's effect. This information should be taken into consideration when we examine patients in whom blowout fracture is suspected.

Evaluation of craniofacial effects during rapid maxillary expansion through combined in vivo/in vitro and finite element studies

It is well documented in the literature that a contracted maxilla is commonly associated with nasal obstruction. Midpalatal splitting using the rapid maxillary expansion (RME) technique produces separation of the maxillary halves with consequent widening of the nasal cavity. Although clinicians agree about many of the indications for and outcomes of RME, some disagreements persist in relation to the biomechanical effects induced. The present research was based on the parametric analysis of a finite element model (FEM) of a dry human skull with the RME appliance cemented in place in order to evaluate these effects on the overall craniofacial complex with different suture ossification. The behaviour of the FEM was compared with the findings of a clinical study and to an in vitro experiment of the same dry skull. Comparisons refer to the opening pattern and associated displacements of four anatomical points located at the left and right maxilla (MI, UM, EM, CN). It was found that the maxillolacrymal, the frontomaxillary, the nasomaxillary, the transverse midpalatal sutures, and the suture between the maxilla and pterygoid process of the sphenoid bone did not influence the outcome of RME, while the zygomatico-maxillary suture influenced the response of the craniofacial complex to the expansion forces. Moreover, the sagittal suture at the level of the frontal part of the midpalatal suture plays an important role in the degree and manner of maxillary separation. Maximum displacements were observed in the area of maxilla below the hard palate, from the central incisors to second premolars, which dissipated at the frontal and parietal bone and nullified at the occipital bone.

Combined fixation with plates and transmalar Kirschner wires for zygomatic fractures

After repair of a fractured zygoma, the fixed zygoma occasionally becomes displaced. This phenomenon--generally called "relapse"--is a complication that can be prevented by fixing plural sites with plates. However, this impairs blood supply to the bone, which causes atrophy. To solve this dilemma, we developed a new concept for fixing the zygoma. Fractured zygomas are fixed by combining plate fixation at a single site with transmalar fixation with Kirschner wires. This secures stability of the fixed zygoma without impairing its blood supply. We evaluated the stability of fractured zygomas fixed by our method by doing dynamic experiments using simulation models. The stresses at the screw-bone interfaces were significantly reduced by the additional transmalar fixation with wire, indicating that zygomas fixed by our method are less likely to relapse than zygomas fixed with a single plate. We also reviewed the outcomes of patients treated by our method, which indicates its clinical usefulness.

Experimental evaluation of relapse-risks in operated zygoma fractures

OBJECTIVES

Prevention of relapse, or postoperative dislocation, of the fixed zygoma is necessary to achieve optimal results in the treatment of zygoma fractures. Assuming that the occurrence of intensified stresses on mastication at the screw-bone interface (SBI) constitutes the essential cause of the relapse, we evaluated the stresses for three different fixation methods-fixation at the frontal process (FP), inferior orbital rim (IOR), and zygomatico-maxillary buttress (ZMB).

METHODS

We used 10 computer-aided design (CAD) models simulating zygoma fractures in the experiment. For each CAD model, we fixed the fractured zygoma with four screws and one mini-plate at the FP, IOR, or ZMB. After applying a 5.5kg force simulating mastication, we calculated the intensity and distribution patterns of the stresses occurring at the SBIs of the fixation screws using the finite element method. Thereby, we evaluated dynamic stability of the fixed zygoma for each of the three fixation methods.

RESULTS

Greater stresses occur at the SBIs with IOR fixation than at those with FP and ZMB fixation. Although the stresses occurring at the SBIs on mastication demonstrated evenly distributed patterns with the FP and ZMB fixation, the stresses demonstrated concentration on one screw with the IOR fixation.

CONCLUSIONS

The fixed zygoma is more likely to cause relapse with the IOR fixation than with the FP or ZMB fixation. Hence, in performing zygoma fixation at the IOR, care should be taken to minimize the likelihood of postoperative relapse that is caused by skewed distribution of the stresses on the fixation screws.

Dynamic analysis of the effects of upper lip pressure on the asymmetry of the facial skeleton in patients with unilateral complete cleft lip and palate

OBJECTIVE

Our purpose was to assess quantitatively the effect of increased upper-lip pressure on asymmetry of the facial bones in patients with unilateral complete lip-alveolar-palatal clefts.

METHODS

We collected computed tomographic images from 16 patients with unilateral complete lip-alveolar-palatal clefts and classified them into two groups based on absence/presence of alveolar bone grafting. We categorized eight patients (9.6 +/- 2.0 years old) who had not been treated with alveolar bone grafting as the ABG(-) group and the other eight patients (9.3 +/- 1.6 years old) who had received alveolar bone grafting as the ABG(+) group. After producing a computer-aided design model for each patient, we applied a uniform load on the anterior aspects of the maxilla, alveolus, and teeth of the model to simulate the upper-lip pressure. Then we calculated the degree of distortion each model presented using the finite element method. We compared the distortion pattern between the ABG(-) and ABG(+) groups.

RESULTS

In the ABG(-) patients, asymmetry of distortion between the cleft and noncleft sides was present in wide areas involving the orbit, nasal bone, piriform margin, and anterior wall of the maxillary sinus. In the ABG(+) patients, asymmetry of distortion was limited to rather small areas.

CONCLUSIONS

In unilateral complete lip-alveolar-palatal clefts patients, the upper-lip pressure works to dislocate the cleft-side segment to a more posterior position than the noncleft-side segment. This finding implies that the increased lip pressure exacerbates facial asymmetry of these patients. The exacerbating effect on facial asymmetry is alleviated by alveolar bone grafting.

Stress distribution on the thorax after the Nuss procedure for pectus excavatum results in different patterns between adult and child patients

OBJECTIVE

In the Nuss procedure, in which the deformed thorax is forcibly corrected by insertion of correction bars, considerable stresses occur on the patient's thorax. We performed the present study to elucidate how stress patterns on the thorax after this procedure differ between child and adult patients.

METHODS

Eighteen patients with pectus excavatum, constituting a child group (n = 10) and an adult group (n = 8), were included in the study. After a 3-dimensional computer-assisted design model was produced with computed tomographic data from each patient, simulation of the Nuss procedure was performed on the model. Then the stresses occurring on each thorax were calculated using the finite element method. The stresses were compared between the child and adult groups in terms of intensity on each rib and the distribution patterns over the whole thorax.

RESULTS

With all 12 ribs, significantly greater stress occurred in the adult group than stress in the child group. Although the stresses occurring on the thorax demonstrated concentrated patterns in the child group, widely distributed patterns were observed in the adult group.

CONCLUSIONS

The stresses that occur on the thorax after the Nuss procedure take different patterns between children and adults in terms of intensity and distribution. The differences should be taken into consideration in managing postoperative pain after the Nuss procedure.

Effectiveness of Additional Transmalar Kirschner Wire Fixation for a Zygoma Fracture

BACKGROUND

The purpose of this study was to verify the effectiveness of transmalar Kirschner wire fixation as additional fixation for the treatment of zygoma fractures.

METHODS

The authors compared two methods for zygoma fixation at the frontozygomatic suture from both theoretical and clinical viewpoints: miniplate fixation (plate fixation) and miniplate fixation with an additional transmalar Kirschner wire fixation (wire plus plate fixation). For the theoretical study, the authors produced zygoma fractures on 20 skull simulation models; these were generated on the basis of computed tomographic data of actual dry skulls. In their simulation surgery, they fixed the fractured zygoma with the above-mentioned two fixation methods, producing 20 plate fixation models and 20 wire plus plate fixation models. A 10-kgf load was then applied on the fractured zygoma in the anteroposterior and superoinferior directions. Finally, the stresses around the fixation screws and the deviation of the zygoma were calculated using finite element analysis. For the clinical study, the authors compared the postoperative zygoma alignment based on computed tomography of six patients treated with plate fixation and eight patients treated with wire plus plate fixation using a visual analogue scale.

RESULTS

In the theoretical study, the wire plus plate fixation models demonstrated a significant decrease in both the stresses around the screws and the deviation of the fractured bone compared with the plate fixation models. In the clinical study, the visual analogue scale scores for the wire plus plate fixation group were significantly higher than those for the plate fixation group.

CONCLUSION

Because the additional transmalar Kirschner wire fixation can effectively increase the stability of the fractured zygoma that has been fixed with one miniplate, it should be recommended as an effective technique for the treatment of complicated zygoma fractures.

The Effect of Striking Angle on the Buckling Mechanism in Blowout Fracture

BACKGROUND

The buckling mechanism is widely accepted as a mechanism of blowout fractures, along with the hydraulic mechanism. Although many studies have been performed related to the buckling mechanism, none of them have taken the direction of the striking force into consideration. As the orbital floor is not parallel to the horizontal plane, a difference in the striking force direction might affect resultant fracture patterns. The present study aims to investigate whether fracture patterns in the orbital floor were influenced by the striking force direction in terms of the buckling mechanism.

METHODS

The authors produced three-dimensional models on a workstation simulating eight dry skulls and applied striking forces on the orbital rim of each model from three different angles (0, 15, and 30 degrees in the upward direction). Using finite element analysis, the authors calculated the width of the area where the resultant stresses exceed the bone's yielding criterion. The width was termed the "theoretical fracture width" because, theoretically, fracture was expected to occur in the area. Then, the authors compared the theoretical fracture width in groups with the three different striking force angles. Finally, the validity of the theoretical width was verified with an experiment on actual skull models.

RESULTS

The theoretical fracture width was the greatest when the striking force was directed at 30 degrees in the upward direction.

CONCLUSIONS

For the buckling mechanism, fracture would occur in a wider area of the orbital floor when striking force was directed upward than when the force was horizontally directed. This finding would be helpful in predicting fracture width in blowout fractures.

The Dynamic Role of Buttress Reconstruction after Maxillectomy

BACKGROUND

The purpose of this study was to investigate the dynamic effect of maxillary reconstruction after partial resection of the maxilla.

METHODS

On a personal computer, three-dimensional maxilla models were designed based on computed tomographic data obtained from 10 edentulous skull models. Simulation surgery was performed on each model, creating 10 pairs of half-removed maxilla models and corresponding models after reconstruction with a rib. The three different patterns seen in the 10 models were termed normal maxilla, half-removed maxilla, and reconstructed maxilla. After an implant was fixed on the molar region of each model, a 300-N vertical load and a 50-N horizontal load were applied. Using finite element analysis software, the deviation and stress on each model were calculated and compared between different model patterns.

RESULTS

Regarding deformity of the maxilla, when a vertical load was applied, no significant difference was observed among the three model patterns. However, a difference was observed in response to a horizontal load in that there was a tendency for deformation to occur, with that of half-removed maxilla being the greatest followed by reconstructed and normal maxilla. Regarding stresses around the implant, when the vertical and horizontal loads were applied, no significant difference was observed among the three model patterns in maximum stress around the implant.

CONCLUSIONS

A buttress reconstruction is effective in increasing the stability of the maxilla against a horizontal load. However, the maximum stress around the implant in the molar region is unaffected whether or not removal or reconstruction is performed.

Pre-surgical CT/FEA for craniofacial distraction: I. Methodology, development, and validation of the cranial finite element model

Recently, surgeons have begun to treat serious congenital craniofacial deformities including craniosynostoses with mechanical devices that gradually distract the skull. As a prospective means of treatment planning for such complex deformities, FE models derived from routine preoperative CT scans (CT/FEA) would provide ideal patient specific engineering analyses. The purpose of this study was to assess the dimensional and predictive accuracy of the CT/FEA process through the development of a 3D model of a dry human calvarium subjected to two-point distraction ex vivo. Comparative skull measurements revealed that CT/FEA construction error did not exceed 1% for transcranial dimensions, and the thickness error did not exceed 8.66% or 0.31 mm. CT/FEA strain predictions for the central region of the skull, between the distraction posts, were not statistically different from homologous gage values at P < 0.05. Peripherally, however, the strain fields were less well behaved and the FE predictions showed only general qualitative agreement with gage recordings.