Hardness of Artificial Bone and Vulnerability of Reconstructed Skull—A Biomechanical Study

The skull can be damaged in trauma situations such as traffic accidents, causing defects of the skull.^1,2 Brain surgeons also often need to remove part of the skull temporarily to approach brain tumors.^3,4 Although the removed bone is placed back in its initial position, it can develop secondary infection and subsequent necrosis. When such complications occur, the infected bone must be removed, resulting in a skull defect.⁵ Plastic surgeons are requested to reconstruct such cranial defects. Various materials are used for skull reconstruction, including autologous bone,^6,7 titanium,^8-10 and hydroxyl apatite.^11-13 The physical qualities of these materials differ. Accordingly, skulls reconstructed with these materials are expected to exhibit different destruction patterns when impacted. What materials are favorable to minimize skull injury from secondary trauma? The present study elucidates this issue using biomechanical simulation.

Production of Simulation Models

Intact Skull Model

The morphological data of a skull was obtained by performing computer tomography scanning on a 23-year-old male with no congenital/acquired deformity of the skull. The data were transferred to a workstation (Dell Inspiron) and transformed into a 3-dimensional virtual skull model using software designed for this type of analysis (Scan IP, Simpleware Ltd). This model―simulating an intact skull and consisting of 158 000 elements―was defined as the Intact Skull Model.

Models of Reconstructed Skulls

Simulating injury of the skull, part of the skull was removed from the left hemi-frontal region of Intact Skull Model by using 3-dimensional graphic software (Geomagic Freeform, 3D SYSTEMS). Furthermore, the defect was reconstructed in simulation with an artificial bone piece with the same shape as the defect. The material of the bone piece was varied in 3 patterns as given below.

Hard material: A material with the same biomechanical strength as titanium (Young’s modulus 106000Mpa; specific gravity 4.5; yielding threshold 205MPa).

Moderately hard material: A material with the same bio mechanical strength as human bone (Young’s modulus 14000Mpa; specific gravity 2.0; yielding threshold 18.9MPa).

Soft material: A material with the same biomechanical strength
as hydroxyl apatite (Young’s modulus 100Mpa; specific
gravity 3.1; yielding threshold 1.6MPa).

Material properties (Young’s modulus, specific gravity, and yielding threshold) were obtained from the literature.^14,15 The models simulating the skulls with the hard material, moderately hard material, and soft material were defined as Hard Model, Moderate Model, and Soft Model, respectively.

Simulation of Impacts

Impacts to the 3 models (Hard Model, Moderate Model, and Soft Model) were simulated using the conditions of a lead ball with a diameter of 2.5 cm impacting the reconstructed skull at a velocity of 20 m/s. Each of the 3 models were impacted at 3 sites. The 3 sites were the frontal-zygomatic process, the glabella, and the parietal bone. Impact was applied to each of the 3 sites in 3 directions. The 3 directions were 30 degrees upward, horizontal (parallel to the Frankfort plane), and 30 degrees downward. Thus, impact was applied in 9 patterns to each of the 3 models. Subsequent fracture conditions were calculated using LS-DYNA (Livermore Software Technology Corporation) for a total of 27 conditions. LS-DYNA―commercially available software―is widely used to evaluate biomechanical behavior of various industrial products.

Evaluation of Fracture Conditions

The resulting fractures were compared among the 3 models for each of the 9 impacting conditions.

Essence of Results

Serious fracture is more likely to develop when Hard Model is impacted than when Moderate Model or Soft Model receive the equivalent impact. For instance, when the impact works upward (Figure 1) or horizontally (Figure 2) on the frontal process of the zygoma, serious fractures occur for Hard Model. However, fracture is localized for Moderate and Soft Model.

These findings indicate that skulls reconstructed with hard materials are more likely to develop serious fractures.

The results for each of the 9 impacting patterns are provided below.

Impact on the Frontal Process

Upward impact: A serious fracture extending to the parietal region occurs with Hard Model. With Moderate Model and Soft Model, fracture is localized to the impacted region (Figure 1).

Horizontal impact: A serious fracture involving the parietal region and contralateral side of the skull occurs with Hard Model. With Moderate Model and Soft Model, fracture is localized to the impacted region (Figure 2).

Downward impact: Major fracture doesn’t occur in any of the 3 models. The artificial bone dislocates with Hard Model (Figure 3).

Impact on the Glabella

Upward impact: In Hard Model, fracture occurs in wide areas, involving both the right frontal region and left temporal region. In Moderate Model and Soft Model, only minor fractures occur on the impacted site (Figure 4).

Horizontal impact: In Hard Model, fracture occurs in wide areas, involving the right frontal region, left frontal temporal region, left frontal region, and the left frontal process of the zygoma. In Moderate Model and Soft Model, only minor fractures occur at the site on which the impact works directly (Figure 5).

Downward impact: In Hard Model and Moderate Model, fracture develops in wide areas involving both sides of the skull. Only minor fracture occurs in Soft Model (Figure 6).

Impact on the Parietal Region

Upward impact: In Hard Model, fracture is serious, with almost the whole cranial vault destroyed. In Moderate Model and Soft Model, fracture is localized to the impacted region (Figure 7).

Horizontal impact: In Hard Model, almost the whole cranial vault is destroyed. In Moderate Model, although fracture involves wide areas of the skull as well, the degree of fracture is not as serious as in Hard Model. In Soft Model, the fracture is localized to the impacted region (Figure 8).

Downward impact: In Hard Model, almost the whole cranial vault—except the occipital region―is destroyed. In Moderate Model, fracture occurs in the right frontal region. In Soft Model, fracture is localized to the parietal region (Figure 9).

Study Background

Plastic surgeons are often requested by neurosurgeons to reconstruct skulls damaged in injury or after the decompression of the cranial pressure following intracranial hemorrhage. In performing reconstruction of the skull, plastic surgeons have made various efforts to achieve functionally and aesthetically optimal outcomes. In particular, it is a current trend to produce artificial bones that accurately fit into the defect by making use of 3-dimensional printers.^16-18 Despite these efforts in meticulously pursuing morphological accuracy, little attention seems to be paid to the biomechanical strength of reconstructed skulls. Patients who have undergone skull reconstruction often have weakened perceptual and motor functions because of the initial etiology that caused the skull defect. Therefore, compared with intact persons, they are more likely to fall down while walking or be involved in traffic accidents. In performing skull reconstruction, plastic surgeons should keep this in mind and make effort to minimize the patients’ damage by taking preventive measures. To achieve this purpose, plastic surgeons should care about the vulnerability of the skull. The authors hypothesized that reconstructed skulls can present different biomechanical vulnerability depending on the reconstruction material and conducted the present study to evaluate this hypothesis.

Methodology

In most previous biomechanical studies regarding the mechanisms of skull fractures, actual skulls were used as the study material. For instance, Waterhouse¹⁹ and Fujino²⁰ elucidated the mechanisms of orbital floor fractures by actually impacting human skulls. In terms of methodology, the usage of actual skulls is persuasive and easy to understand. However, current ethical standards discourage destruction of actual skulls, even in academic studies. Therefore, we have employed biomechanical simulation instead—finite element analysis—as the study’s method. Employment of finite element analysis allows the authors to evaluate fracture patterns without destroying actual skulls. The finite element method is an established technique in medical research used in biomechanical studies regarding the skull,²¹ the thorax,²² and the skin.²³

In the present study’s design, impact was applied to the models, varying the patterns of impact sites and the direction of impact. The impact patterns were varied because in actual trauma situations, the skull can be impacted in random patterns.

Analyses were performed using models of a bare skull. In actual situations, fracture can be less serious when impacts are applied to actual heads because the muscle, fat tissue, and skin covering the skull functions serve as a shock absorber. Hence, analyses with models containing these soft tissues can provide more accurate results. This is a limitation of the present study, and improvement is expected in future studies.

Hypothetical Explanation of Findings

The main finding of the present study is that serious fractures are more likely to develop in Hard Model than in the other 2 models. From a clinical standpoint, this finding means that serious fracture is more likely to develop when a skull reconstructed with a hard material is impacted.

The mechanism of this finding is explained as follows: assume a wall with a defect; a circular piece that fits the defect is placed to restore the wall.

First, imagine the circular piece is made of glass. When the wall is impacted, the area of the wall neighboring the impacted site and the glass part will break (Figure 10 above). The area of the wall on the opposite side of the impact is unlikely to break because the glass breaks instead. Herein, the glass part functions like the bumper of a car, absorbing some impact.

Next, imagine the defect is filled with an iron piece. When this wall is impacted, the area of the wall over the iron part can break, as well as the area close to the impacted site (Figure 10 below). This phenomenon occurs because the iron piece transmits the destructive energy.

In essence, the replaced part functions as a shock absorber when it is made of a vulnerable material. Contrarily, the replaced part functions as a shock transmitter when it is made of a hard material. The present study’s main finding—that serious fractures tend to occur in Hard Model—can be explained by this hypothesis.

Clinical Meaning of Results

The results demonstrate that when impacted, greater damage can occur in Hard Model than in Moderate Model and Soft Model. This finding leads to the clinical proposition that in secondary injury after reconstruction, skulls fixed with hard artificial bones are more likely to develop serious fractures than skulls fixed with moderately hard or soft materials. As materials for artificial bones, hydroxyl apatite,^11-13 meshed titanium,^8,9 and solid titanium10,^24,25 are widely used. The findings of the present study suggest that the usage of solid titanium should be avoided because serious fracture can develop if the patient is involved in a subsequent trauma.

Prospects of Future Study

The present study elucidates the vulnerability of reconstructed skulls using biomechanical techniques. Though the study provides significant information to the archives of plastic surgery, some might argue that it is merely theoretical and does not necessarily reflect real trauma conditions. Hence, it is preferable that the vulnerability of reconstructed skulls be verified by clinical studies. This can be achieved by collecting the profiles of statistically significant numbers of patients who have undergone skull reconstruction and conducting follow-up. The authors plan to conduct this study in the future. Because limited sample size makes it difficult to carry out the study at only a single institute, multiple institutes should be involved. It is the authors' hope that interested readers will join this clinical study.

Using biomechanical simulation, the present study elucidated the relationship between the vulnerability of reconstructed skulls and the hardness of the artificial bone. It was found that serious fractures are likely to occur when a skull reconstructed with hard artificial bone is impacted. Therefore, skull reconstruction with a hard material—such as rigid titanium—should be avoided to prevent serious skull damage in a possible secondary trauma.

Affiliations: ¹Department of Plastic and Reconstructive Surgery, Faculty of Medicine/Graduate School of Medicine, Kagawa University, Kagawa, Japan; ²Department of Plastic and Reconstructive Surgery, International University of Health and Welfare, Sanno Hospital, Tokyo, Japan; ³Department of Plastic and Reconstructive Surgery, Faculty of Medicine, Kindai University, Osaka, Japan

Correspondence: Tomohisa Nagasao; nagasao.tomohisa@kagawa-u.ac.jp

Funding: Part of this study was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) 18K12034.

Ethics: This work complies with the Helsinki Declaration related to research carried out on human subjects. This study’s design was reviewed and approved by the institutional review board of Kagawa University (Approval #2019-138).

Disclosures: The authors have no relevant financial or nonfinancial interests to disclose.