The influence of zero-gravity on the volume of the augmented in the posterior maxilla bone and the survival of the dental implants placed in this bone

Introduction

Dental implants are effective medical devices to restore a person's ability to chew or their appearance nowadays. Dental implant survival, functional and esthetic reconstruction following implant treatment are highly dependent on sufficient volume and favorable architecture of the alveolar ridge. Forty percent of the American population loses all teeth in one or both jaws after 60 years old due to complicated decay [1]. It takes 40 days for a dental alveolus to heal [2, 3]. Unloaded alveoli lose bone immediately after heeling. It can be detected 3 to 4 months following tooth loss. Alveolar ridge can lose up to 1.5—2.0 mm high and 40—50% of width 6 to 12 months after tooth extraction [4]. Alveolar ridge deficiency worsens with time without functional load. Usage of removable dentures accelerates and deteriorates this process [5]. The adverse changes of the alveolar ridge bone can make it impossible to place dental implants requiring augmentation procedures to make it higher and broader [6]. Guided bone regeneration (GBR) is one of the procedures to augment the alveolar ridge bone. It employs the concept of promoting the migration of slow-growing osteogenic and angiogenic cells from the adjacent bone marrow into the defect while preventing fast-growing epithelial cells and fibroblasts from occupying that space [7, 8]. A bony defect is filled with bone grafts (autogenous or xenogenic bone or a mixture of both) to serve as an internal framework. The bone graft should be impregnated with blood to form a fibrin clot, which serves as a scaffold for the in-growth of progenitor cells. Platelets of the blood clot collapse, releasing the growth factors that induce osteogenic cells migration and blood vessels ingrowth. Four main principles are needed to be met for the GBR to be efficient: exclusion of epithelium and connective tissue, space maintenance, stability of the blood clot, and primary wound closure [9].

Tatum, Boyde, and James proposed maxillary sinus augmentation to solve vertical bone deficiency in the posterior maxilla [10, 11]. Maxillary sinusitis due to bone graft infection is one of the grave complications of the procedure. It can make a patient unfit for some occupations. Perforation of the maxillary sinus membrane can lead to this severe complication that requires long-lasting treatment. Nevertheless, a lateral window sinus augmentation is a highly effective and predictable procedure in case of adequate diagnostic, patient choice, and good surgical technique [12—14].

When placed into the bone reconstructed with maxillary sinus augmentation or guided bone regeneration, dental implants' success and survival rate can be as high as 97—100% [15]. Surgeons have no unique opinion on which bone graft material has the best effect on bone formation. It is believed that a surgeon's skills and defect characteristics influence the result of the operation to a greater extent than a type of bone graft material [16]. It is also unclear if different bone graft materials affect augmented bone stability.

Available literature has no data representing changes that might occur in the augmented bone under zero gravity conditions and how zero gravity might influence the success and survival rates of dental implants placed into the augmented bone.

This retrospective case series reports clinical and radiographic success and survival rates of dental implants placed after sinus augmentation in patients who had to periodically work in zero gravity conditions.

Materials and methods

Since 2013, unilateral maxillary sinus floor augmentation and guided bone regeneration procedures have been performed in three men patients (48 y.o. mean). All three patients had lost three teeth in the posterior maxilla and required missing teeth with dental implants. Before the treatment, each patient had spent 233 days in near-earth orbit on average. Two of them made spacewalks lasting an averagely of 12 hours and 11 minutes. In all cases, the mean period before the operation after the last space flight was one year and four months. They had deficient high and width of the alveolar ridge not sufficient for dental implants placement. CBCT showed the mean width of the alveolar ridge as follows: patient 1 — 4.7 mm, patient 2 — 3.0 mm, patient 3 — 4.4 mm. The mean high of the alveolar ridge was: patient 1 — 6.4mm, patient 2 — 7.7 mm, patient 3 — 5.0 mm

Maxillary sinus floor augmentation was performed through a lateral window. The incision was made into the gingiva at the top of the alveolar ridge. The incision went into the dentogingival sulcus three teeth medially and onto the maxillary tuberosity distally. In the exposed lateral boney wall was cut a 5x8 mm window to the sinus. The membrane was separated from the sinus floor and lifted 7—9 mm depending on the residual crestal bone height. The created space was filled with 1—2 mm particles of a xenograft Bio-Oss (Geistlich) mixed with auto venous blood only. Autogenous bone was not added to the xenograft. The amount of the blood was just enough to soak and cover the graft. The blood and graft mixture was prepared in advance to let the fibrin clot formation. Moreover, the graft particles glued together in the clot are more easily placed into the sinus than separate ones. We didn't use the collagen membrane to separate the graft from the sinus mucosa or the window from the gingiva.

Horizontal ridge augmentation was made with guided bone regeneration.

For the GBR procedure, we used xenograft prepared in the manner described above. The surface of the alveolar ridge bone was stripped of any residual soft tissue and perforated with a 1.1 mm drill. In two cases, the bone substitute was packed into the bony defect and covered by a Bio-Gide (Geistlich) resorbable membrane; in another one by the non-resorbable membrane (Free-Oss). The latter was fixed to the bone with titanium screws 1.5x4.0 mm. We didn't pin the collagen-derived membranes. In one case, we used a bone block from the oblique line of the ramus of the mandible. We inserted the dental implants Xive S plus (Dentsply) simultaneously with augmentation procedures in one patient. The dental implants were allowed 195 days of integrating time before placing the teeth on them. In two other cases, the dental implants AstraTech (Dentsply) and Xive S plus (Dentsply)) were placed in a two-stage technique 184 days, on average, after the augmentation procedures.

The number of the implants matched the number of the lost teeth. We loaded the implants 152 days after their placement, on average. Two patients required recreation of the attached gingiva around the implants with transplantation of gingiva from the palate (Fig. 1).

Results

In all cases, the wounds healed primarily. The range of the gained bone volume was 4.3—9.2 mm in height and 4.0—6.5 mm in width (Table). Two patients had metal-ceramic crowns attached to the implants with screws; one patient had his all-zirconia crowns placed on the abutments with dental cement. After loading the implants, all the patients spent an average of 181 days in zero gravity conditions. Two of them had spacewalks lasting 17 hours and 14 minutes on average. Six months after returning from space, the patients were examined by a dentist with a cone-beam computed tomography (CBCT) scanning to check bone resorption around the implants, width, and height of the augmented bone. The implant restorations had no defects. CBCT showed no bone resorption at that time (Fig. 2). Later on, a dentist followed the patients with CBCT once a year. By the time of writing the article, the implants had been loaded for more than six years on average. The implants remained integrated with the bone. Seven years after loading the implants, the patient who didn't have gingivoplasty developed marginal bone resorption measuring 2.3 mm average (Fig. 3). Other patients have no problems. One restoration has a tiny chip that doesn't require replacement.

Discussion

Guided bone regeneration and maxillary sinus floor lift have been used to augment the maxilla's alveolar ridge for more than 25 years. The procedures are simple and highly predictable in different defects, making them popular among surgeons [17—19]. Having been reports on successful GBR without the membrane [20], it is one of the fundamental components of the procedure [21—23]. Utilizing the bone or bone substitutes without a membrane is associated with unpredictable results and inevitable bone loss in the long term [24, 25].

The xenograft Bio-Oss used in our operations has a remarkable property of becoming part of the ingrowing bone by gradually absorbing with osteoblasts [26—28]. There is no consensus on using xenograft per se. It is believed that its mixture with auto bone has more regenerative potential. The proportion of the xenograft and the auto bone depends on the regenerative property of the bone [19]. In our practice of reconstructing the bone defects of the same characteristics described in the article, we never mix the xenograft with the auto bone. In addition, the platelet-rich plasma did not provide the xenograft additional regenerative potential when compared with the auto venous blood. Strictly following the four main principles of guided bone regeneration guarantees predictable and stable results of the operation. The experience of alveolar ridge augmentation in patients with high professional health requirements described in the article can prove this statement.

Maxillary sinus floor lift is a kind of guided bone regeneration, the difference being the sinus walls serve as a barrier and framework and a source of the osteogenic cells.

The integrity of the maxillary sinus mucosa is a paramount requirement of the operation's success. The inner surface of the mucosa possesses properties compared to those of the periosteum, and its isolation is not consistent with the purposes of the operation. Some authors propose to enhance the mucosa with a collagen membrane before the placement of the bone material. We find this maneuver undesirable and resort to it only to cover minor rips of the mucosa. Meanwhile, there have been reports on the success of the maxillary sinus floor lift without the bone materials, using blood clots solely. These operations were performed with simultaneous placement of the dental implants [29—34]. These data prove the vital role of the intact sinus mucosa, flawless surgical technique, patient's adherence to the restrictions in the post-op period for a successful sinus lift procedure.

Conclusion

The bone created due to the guided bone regeneration and the maxillary sinus floor augmentation has the same properties as the native bone, maintaining volume even in extremal conditions such as zero gravity of Earth orbit. The xenograft Bio-Oss and the collagen membrane Bio-Gide (Geistlich) used in this operation are reliable and ensure predictable results when mixed with auto blood only. Survival of the dental implants placed in this newly created bone is 100% during a 7-year follow-up period despite a long-lasting stay in zero gravity. Our experience proved that maxillary sinus floor augmentation is predictable and safe for occupations with high health requirements, such as astronauts. The patients whose career necessitates infection-free maxillary sinus (pilots, divers, etc.) can be candidates for the procedure.