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Structural Mechanics for Jaw Bone Reconstruction Surgery

Introduction

Currently there are a variety of methods available for the treatment of oral cancer; however, surgery is the most well established mode of treatment for the majority of oral cancers [1]. Mandibulectomy is a surgery that includes removing all or part of the mandible (the jaw bone). If an x-ray of a jaw bone finds that the tumor has grown into the bone then segmental mandibulectomy will need to be performed to remove a large part of the jaw [2]. If an x-ray is done on the tumor and there’s no evidence that the cancer has grown into the jaw bone then it may only be required to perform a marginal mandibulectomy. A marginal mandibulectomy includes the removal of only part or a piece of the jaw bone [2]. Using Scan&Solve Pro, I performed two studies regarding the mechanical properties of bone in the mandible related to these surgeries. The first study analyzes the mechanics of the mandible while biting when different material properties are used for the bone. The second study focuses on the mechanics of the mandible while biting when a segment of the bone is healing due to a marginal mandibulectomy. The mandible model I will be using was obtained from GrabCAD and consists of 117912 polygons. Scan&Solve is ideal for the studies because it supports assemblies and also doesn’t require us to convert the mandible model to a finite element mesh, which can be hard for such a complex geometry.

Structural Role of Cortical and Cancellous Bone Tissue Study

A key aspect of the jaw bone is that it is comprised of both cortical and cancellous bone tissue. Cortical and cancellous bone have completely different mechanical properties. The elastic modulus of cortical bone is 15,000 MPa and only 1,500 for cancellous bone [3]. Cortical bone is much stiffer and forms the outer cortex of bones. Cancellous bone is the inner tissue of the bone that is porous and contains red bone marrow, making it weaker and more flexible than cortical.

   

          

             

 

Figure 1: Model of the mandible in Rhinoceros                                                 Figure 2: Cancellous bone component of the model.        

In this study, I used Scan&Solve Pro to compare the mechanical properties of the cortical and cancellous bone in the mandible during mastication (chewing) by running tests on a mandible with just the cortical tissue, with both cortical and cancellous tissue, and with both tissues being cortical. This allows us to see the role that cortical and cancellous tissue plays in the mechanical properties of the mandible. The forces I applied to represent a unilateral molar bite were taken from another study [4] and can be seen in Figure 4 below. Fb is the bite force =100 N. Fjw and Fjb are the joint forces at the balancing and working sides, Fjw=60 N and Fjb=90 N. Fmw and Fmb are the muscle forces both =125 N. I put the restraints on the temporomandibular joints (see Figure 3 below). I created custom materials based upon the mechanical properties of the cortical and cancellous bones described above. Figure 2 above shows the cancellous component of the mandible. In the three tests performed the forces and restraints remained the same. For the Only Cortical Layer test, the only component used was the outer cortical layer. For the Cortical and Cancellous Layers test, both the cortical and cancellous layers were used as components with their corresponding mechanical properties. Lastly, for the Both Layers as Cortical test, the cortical and cancellous components were used, but both used cortical mechanical properties. Results from the 3 tests are summarized in Table 1.

 

 

Figure 3: Restraints at the temporomandibular joints.                

                                                                                                                                             Figure 4: Forces during mastication.           

Table 1: Scan&Solve results for Structural Role of Cortical and Cancellous Bone Tissue Study.

Marginal Mandibulectomy Study

Spontaneous bone regeneration can heal the bone cavity caused by a marginal mandibulectomy without the use of any filling material [5]. In this study I used Scan&Solve Pro in order to see the mechanical properties of healing cortical bone in the jaw. Figure 5 below shows the portion of the cortical jaw that was removed and where spontaneous bone generation is healing the cavity. It was found that bone can completely heal within 19 weeks [6]. I wanted to see how the mechanical properties of the mandible changed during chewing when the segment was 25% healed, 50% healed, 75% healed, and 100% healed. I used the same boundary conditions on Scan&Solve as the previous study except I changed the elastic modulus of the healing bone segment. The results can be seen in Table 2 below.

 

Figure 5: Healing bone segment used in Scan&Solve.                                                                                                                                                                                                                                                Figure 6: Total displacement of the mandible as the bone

                                                                                                                                                     segment healed.                                 

Table 2: Scan&Solve results for Marginal Mandibulectomy Study.


The results showed that as the bone healed, it was able to take on more of the stress from the bite. This allowed the stress to distribute more over the mandible and reduce the total displacement as seen in Figure 6 above. After the bone segment had healed 25% it was already causing minimal effect to the mandible during mastication. By 50% healing the mandible was very close to having the same displacement and von mises stress as if the bone segment was fully healed. This shows that a patient who undergoes marginal mandibulectomy can almost fully regain chewing capabilities when the bone cavity is 50% healed.

Conclusion

The Scan&Solve Pro allowed us to make important findings about the mechanical properties of the mandible during chewing. Knowing the role that cortical and cancellous bone plays in the mechanics of the mandible will be very important to consider for an implant needed after the removal of a large part of the jaw due to cancer (segmental mandibulectomy). Additionally, when a smaller cavity is removed from the jaw (marginal mandibulectomy) and spontaneous bone regeneration is occurring, the results from the mechanical tests with the healing bone segment help us to learn the point in healing that it is safe to return to chewing.



References

[1] Gou, L., Yang, W., Qiao, X., Ye, L., Yan, K., Li, L. and Li, C. (2018). Marginal or segmental mandibulectomy: treatment modality selection for oral cancer: a systematic review and meta-analysis. International Journal of Oral and Maxillofacial Surgery, 47(1), pp.1-10.

[2] Cancer.org. (2018). Surgery for Oral Cavity and Oropharyngeal Cancer. [online]

[3] Nagasao, T., Kobayashi, M., Tsuchiya, Y., Kaneko, T. and Nakajima, T. (2002). Finite element analysis of the stresses around endosseous implants in various reconstructed mandibular models. Journal of Cranio-Maxillofacial Surgery, 30(3), pp.170-177.

[4] van Eijden, T. (2000). Biomechanics of the Mandible. Critical Reviews in Oral Biology & Medicine, 11(1), pp.123-136.

[5] Di Dio M, Scarapecchia D, Porcelli D, Arcuri C. Spontaneous bone regeneration after removal of cysts: one-year follow-up of 336 consecutive cases.

[6] Claes, L., Grass, R., Schmickal, T., Kisse, B., Eggers, C., Gerngro, H., Mutschler, W., Arand, M., Wintermeyer, T. and Wentzensen, A. (2002). Monitoring and healing analysis of 100 tibial shaft fractures. Langenbeck's Archives of Surgery, 387(3-4), pp.146-152.

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