Experimental evaluation of susceptibility to fractures of the orbital floor in patients with unilateral complete cleft palate.

We set out this study to verify a hypothesis that orbital floor fractures tend to affect wider areas in patients with unilateral complete cleft palate. Using a striking machine, the inferior orbital rims of eight normal skulls (intact skull group) and eight skulls with parts of the maxillas removed to simulate alveolar and palatal clefts (cleft skull group) were impacted. The fractured areas were compared. Models designed using a computer were produced to simulate the skulls of 12 normal people and 12 patients with left unilateral complete cleft palate, and were classified as the intact model group and the cleft model group, respectively. Computer simulation of applying external forces to the inferior orbital rim of each model was performed. Areas where stresses exceeded the bone-yielding threshold were compared. Actual fractured areas were significantly larger in the cleft skull group than in the intact skull group. Theoretical fracture areas were also significantly greater in the cleft model group than in the intact model group. We conclude that orbital floor fractures develop in wider areas in patients with unilateral complete cleft palates, because of the instability of the maxilla on the cleft side.

Double-bar application decreases postoperative pain after the Nuss procedure.

OBJECTIVE: This biomechanical study aims to elucidate whether additional bar application increases postoperative pain after the Nuss procedure for pectus excavatum. METHODS: Clinical evaluation: The intensity of postoperative pain was compared between patients for whom a single-bar was used (single-bar group: n = 14) and those for whom double bars (double-bar group: n = 10) were used to correct the thoracic deformity. The evaluation was performed by referring to the frequency with which local anesthetics were self-injected in a patient-controlled anesthetic system and how many days were needed for the patients to resume ambulation. Theoretical evaluation: An original simulation system for the Nuss procedure was developed by producing 3-dimensional finite element analysis models from computed tomographic data of patients with pectus excavatum. With this system, single-bar and double-bar placement was simulated separately for the thorax models of the double-bar group. The stresses occurring on the thoraces were then compared between the two situations. RESULTS: Clinical evaluation: Self-injection of local anesthetic was more frequent for the single-bar group than for the double-bar group; single-bar patients restarted ambulation later than the double-bar group. Theoretical evaluation: Stresses on the thoraces were smaller when double bars were applied than when a single bar was applied. CONCLUSIONS: Performing double-bar placement decreases postoperative pain. Therefore, surgeons should not hesitate to perform double-bar correction in patients in whom the deformity extends to multiple intercostal spaces, requiring correction of the thorax shape at multiple sites.

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.

Age-related change of postoperative pain location after Nuss procedure for pectus excavatum.

OBJECTIVE: The present study aims to evaluate age-related change of postoperative pain after the Nuss procedure by referring to clinical cases, and to elucidate the biomechanical aetiology of the change by using the finite element method. METHODS: Twelve paediatric patients (paediatric group: 9.4+/-2.3 years old) and 13 adult patients (adult group: 26.3+/-5.5 years old) who received the Nuss procedure for pectus excavatum were included in the study. On the second postoperative day, the patients were asked to indicate regions on the thorax where they felt the greatest pain. The locations of these regions were compared between the two groups. In addition, stress-distribution patterns were examined using finite element models produced by simulating the thoraces of the patients. The stress-distribution patterns were compared between the two groups. RESULTS: The patients of the paediatric group and adult group tend to have pain on the anterior and posterior regions of the thorax, respectively. The finite element study revealed that paediatric thoraces and adult thoraces develop intensified stresses in the anterior region and the posterior region, respectively. CONCLUSION: Postoperative pain tends to occur in the anterior part of the thorax for paediatric patients and in the posterior part of the thorax for adult patients, reflecting the stress distributions of these two distinct patient populations.

A comparison of stresses in implantation for grafted and plate-and-screw mandible reconstruction.

OBJECTIVE: During oral rehabilitation by dental implantation for mandibles reconstructed with plate and screws, intensified stresses can develop at the implant-bone interfaces and fixation screw-bone interfaces that might cause complications at these interfaces. The present study aims to evaluate the stresses occurring at these sites using the finite element method. STUDY DESIGN: Ten computer-aided design models simulating 10 mandibles were produced and were termed normal models. Simulation surgery was performed on these normal models, where parts of the models were removed and replaced by fibula or reconstruction plates plus screws. Depending on the replaced part (body [B] or body and symphysis [BS]) and the reconstruction materials (fibula [Fib] or reconstruction plate [Plate]), the modified models were termed B-Fib, B-Plate, BS-Fib, and BS-Plate models, respectively. For each model, an implant was embedded in the first molar region. Stresses occurring at the implant-bone interfaces and fixation screw-bone interfaces on mastication were calculated using the finite element method and compared among the model groups. RESULTS: The stresses at the implant-bone interfaces showed no statistically significant differences among the 5 model groups. With the B-Plate and BS-Plate models, stresses at the fixation screw-bone interfaces were nearly twice as intense as those at the implant-bone interfaces. CONCLUSION: If it is allowed that complication risks correlate with stresses, fixation screws are more subject to failure than implants in performing implantation for mandibles reconstructed with a plate and fixation screws. Therefore, the fixation screws deserve special attention in performing oral rehabilitation for such patients.

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.

Biomechanical evaluation of surgical correction of prominent ear.

BACKGROUND: Mustardé sutures and conchal setback are widely used for surgical correction of prominent ear, and numerous cartilage-manipulation techniques accompanying these two methods are also available. However, it is unknown how each technique works biomechanically. The effects of otoplasty were evaluated by finite element analysis. METHODS: Data of eight prominent ears were obtained with a noncontact three-dimensional digitizer, and three-dimensional auricular cartilage models were produced. These models were modified to cartilage-manipulation models: scaphal-incision, abrasion, conchal-incision, and conchal-excision models. The loads corresponding to Mustardé sutures were then applied to the no-manipulation, scaphal-incision, and abrasion models. Loads corresponding to conchal setback were applied to the no-manipulation, conchal-incision, and conchal-excision models. Stresses and deformed shapes were evaluated using finite element method. RESULTS: When Mustardé sutures and conchal setback were compared in the no-manipulation models, maximal stresses occurred around loads in Mustardé sutures and at the root of the helix in conchal setback. The entire ear laid down in conchal setback, whereas in Mustardé sutures, strong bending occurred at the upper third. In Mustardé sutures, maximal stresses decreased to 82 percent with scaphal incision and 95 percent with abrasion. However, the setback effect was also reduced in these two models. In conchal setback, high stresses were markedly decreased with conchal incision or excision. There was no significant difference between incision and excision. CONCLUSIONS: For successful otoplasty, precise understanding of biomechanical reactions is essential. This study provides improved insight into otoplasty for many surgeons.

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.

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.

Adaquate fixation of plates for stability during mandibular reconstruction.

PURPOSE: To determine the most appropriate plate fixation for dispersing the stress around screws in mandibular reconstruction. MATERIAL AND METHODS: Forty-eight three-dimensional reconstructed mandibular models with Central (C) or right Lateral (L) defects were created and divided into three groups, fixed with: (1) two screws on each side, (2) three screws on the left side with the third (middle one) located distally, or (3) three screws on the left side with the third (middle one) located proximally. A 300N vertical load was applied to the left molar region. The maximum stress was calculated using a finite element method and statistically evaluated. RESULTS: Stress was concentrated around the screw at the distal end of the mandibular halves on the loaded side ('crucial screw'). For the C defect, stress concentrated around the 'crucial screw' was greater when there were only two screws for fixation (20.55MPa) than when there were three screws (16.17MPa; p=0.022). No significant difference was found between the two groups with three screws for fixation. For the L defect on the other hand, stress on the 'crucial screw' was relatively greater when there were three screws for fixation (18.9MPa) than when there were only two screws for (12.83MPa; p=0.051). No significant difference was found between the two groups with three screws for fixation. As for the reconstruction plate, the stresses on the plates were not different among the various screw groups for fixation nor among the C and L defects. CONCLUSION: This paradoxical result is explained by plate bowing. Thus in large defects screw positions should take into consideration the potential for plate bowing.

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.