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Elimination of Mandibular Defects in Patients with Malignant Neoplasms Using the PoprusTiNi-alloys

The article presents the clinical observation of 16 patients with malignant neoplasms of the lower jaw. All the patients underwent resection of the mandible with the substitution for the flaws with endoprostheses of porous NiTi. In 10 cases a course of preoperative radiotherapy in a dose of 42-44 gr. was conducted. The results of surgical treatment showed high efficiency of the technology.


Introduction
In surgery of tumor lesions of the mandibular bone in connection with the topographic and anatomic features of the oral region, the lower face and neck area some difficulties occur associated with the removal of the extensive malignant neoplasms within the sound tissues due to the patient transfer from the category of a cancer patient in a group of patients with face defects and deformities, the functional disorders on the part of the masticatory apparatus, the temporomandibular joint/s (TMJ), etc. It follows that the radical excision of the pathological tissues often does not lead to full recovery of the patient. *Corresponding author: In individuals with malignant tumors in some cases the necessity of applying the combined or complex treatment method -a combination of surgical interference with radiation-and chemotherapy negatively influences on the rehabilitation measures effectiveness. Radiation-and chemotherapy have a negative impact on the reparative processes which greatly increase the risk of the reconstructive operations success, due to that the majority of experts don't carry out the primary bone grafting after the malignant tumors removal.
When choosing the method of operative treatment of tumor and osseous dysplasia it is necessary to consider the nature and type of the pathological process, the peculiarities of its pathologic behavior (the rate of growth, the tendency to recur), localization, prevalence, age, somatic state of the patient as well as one-step or subsequent possibility to restore the anatomical shape and functional features of the damaged bone and facial soft tissues.
Patients' rehabilitation with the lower jaw neoplasms should solve the following problems: 1. The impact on the affected structures (operative, radiation, chemotherapy depending on the lesion nature); 2. Elimination of bone and/or soft tissue defect; 3. Restoration of the functional characteristics of the temporomandibular joint/s and the masticatory apparatus by bone grafting, endoprosthesis replacement or dento-maxillary prosthetics.
In order to restore the anatomical and functional disorders in patients of this category the bone grafting is applied using various kinds of available graft materials [1,7], replacement of the mandibular bone and temporomandibular joint defects by endoprostheses made of stainless steel, titanium, chrome and cobalt alloy, polymers, ceramics, precious stones (sapphires) and others which do not exhibit the lag effect in terms of loading and unloading [2-4, 8, 14, 15]. However, the results of such operations can't meet the requirements of the patients and clinicians because of the transplant and implant materials resorption and rejection due to non-viability of allogenic and xenogenic tissues, the death of osteocytes in the autografts' thickness, the lack of biomechanical compatibility of the used implants with the body tissues. The use of vascularized grafts in combination with the microsurgical technique is traumatic [5,9,13]. Apart from that, the used bone grafts by form don't correspond to the replaceable defect and an artificially created

Experimental
In the scientific research Institute of Medical Materials and Implants with Shape Memory (Tomsk) the endoprosthesis was developed for the simultaneous substitution of the body, the angle and mandibular ramus including the head. This endoprosthesis is a design by configuration corresponding to the anatomical shape of the condylar process of mandible and the mandibular ramus to which the components are fixed which replace the angle and the body of the mandible with right and left variants. This design can be manufactured without a head and in the full version depending on the surgery task. To prevent the implant eruption through the oral cavity mucous membrane or facial soft tissues the body of the latter is covered with fibrous or woven thin-profile nickelid titanium by layer-bylayer winding in 2-3 or more layers (Fig. 1). The sizes and the endoprosthesis configuration are determined individually on the basis of Х-ray examinations (computed helical tomography -layer-by-layer and three-dimensional images images).  The peculiarity of the mandible resection in these patients is that after cutting off the muscles from the gross specimen the remaining parts are immersed inside with a thin-profiled knitted fabric structure based on nickelid titanium of "stocking" type and it is left in the surgical wound in this state. The specified measures (actions) during the implant replacement substitution stage allow to distinguish these muscles from the scar' modified tissues and carry out the subsequent fixation to the implant body to restore the function.
The made lower dentofacial prosthesis is fixed in the oral cavity of the patient, after that the surgical intervention is performed consisting of the scar modified tissues dissection in the area according to the previous operation, the soft tissues dissection up to the mucous or submucous layer to form the bed for the endoprosthesis, skeletization and the formation of the decorticating surface from the external edge of the mandible fragment, the fixation of the implant and the muscles according to the previous technology, covering the latter with soft tissues, the wound suturing. In patients with the preserved mandibular heads the endoprosthesis is fixed from the external side of the lower jaw fragments to the decorticating surfaces.
If required, to improve the ratio of the prosthetic bed tissues the subsequent dentofacial prosthetics in the zone of the reconstructed defect was performed individually, 1-3 months after the surgery.  Table 1.  Osteosynthesis with the devices made of nickelid titanium provides a stable fixation of the endoprosthesis to a mandible fragment and gives the opportunity for early masticatory load. Covering the endoprosthesis body with fibrous or woven thin-profile nickelid titanium by layer-by-layer winding creates the superelastic system, prevents the implant eruption through the surrounding soft tissues.

KnE Materials Science
Superelastic thin-profile mesh implants fixated on the stumps of the masticatory muscles and the muscles lowering the mandible enable the fixing strength of the latter to the fibrous layer of the prosthesis preventing dissociation, don't impede the muscle contractility and their return to the original position in the process of masticatory apparatus functioning.