Abstract
The zygomaticus implant was developed for patients with severe bone resorption of the posterior maxilla. These may eliminate or minimize the need for bone grafting. Although the zygomaticus implant has shown a remarkable success rate in this difficult-to-treat patient population, the method requires an advanced surgical technique and carries an increased risk of complications. There have been few anatomical studies on the zygomatic bone in relation to the insertion of zygomaticus implants. The height and thickness of the zygomatic bone for the insertion were measured in this study. The thickness at the 90° angle point, where the upper margin of the zygomatic arch and the temporal margin of the frontal process of the zygomatic bone intersect and where the apex of the implant penetrates, was found to be 1.8 ± 0.4 mm; this gradually increased inferiorly and anteriorly. Thus, the penetration point of the apex of the zygomaticus implant should be located more inferoanterior to the 90° angle point, as the thickness in this region is thinner than the diameter of the implant. Based on the results of this study, a newer and safer insertion method for the zygomaticus implant using a drill guide is proposed.
Implants of certain lengths often cannot be installed in the molar region of the maxilla due to severe alveolar bone resorption and the presence of the maxillary sinus. Although onlay or inlay bone grafts are normally used in these cases, there is usually a low rate of success for the implants placed in the bone graft area. Lekholm et al. reported the success rates of implants placed in autogenous bone grafts performed using five separate techniques: local onlay graft, full onlay graft, maxillary inlay graft (sinus floor augmentation), combined onlay/inlay graft, and Le Fort I osteotomies. A total of 781 Brånemark implants were inserted, and the majority of patients were treated with simultaneous bone grafting. The overall implant survival rate was approximately 80% after 3 years. Onlays, inlays, and Le Fort I osteotomies showed almost identical success rates (76–84%), whereas the onlay/inlay technique resulted in a less favourable outcome (60%). In addition, these bone grafting techniques often require the harvesting of iliac bone, which results in severe mental and physical strain for the patient.
The zygomaticus implant (Brånemark System; Nobel Biocare, Göteborg, Sweden) was developed for patients with severe bone resorption of the posterior maxilla. These can eliminate or minimize the need for bone grafting and can support an implant-anchored fixed prosthesis. The overall success rate for the zygomaticus implant was reported to be 97.9% in a systematic review that included a total of 1541 implants placed over a 3-year time span. Regarding the management of similar patient populations, this success rate surpasses all previously published success rates for implants in grafted bone in the severely resorbed maxilla.
When compared with bone grafts, zygomaticus implants have the advantage of a reduced total treatment time, as bone grafts require an extended healing time, especially with the two-stage procedure, in which the bone grafting is followed by the placement of the implants.
Although the success rates of zygomaticus implants are high, treatment requires an advanced surgical technique and carries the risk of severe complications, such as perforation of the orbital floor or infratemporal fossa. Therefore, it is important for dentists to have a detailed understanding of the anatomy of the zygoma before performing surgery for the insertion of zygomaticus implants. Although there have been anatomical studies reporting the linear and angular measurements for the insertion of zygomaticus implants, there is a paucity of information regarding the thickness of the zygoma.
The height and thickness of the zygomatic bone were measured in this study, and a new method for zygomaticus implant insertion is proposed that is safer than existing methods. This method of zygomaticus implant insertion consists of the pilot hole technique using a drill guide; this can be performed easily without the use of a computer-aided surgical navigation system.
Materials and methods
Cadavers
Eleven Japanese cadavers used for practical anatomy training at the University of Tokushima were included in this study. The body heights of the cadavers ranged from 148 to 179 cm (159.4 ± 9.2 cm), and they ranged in age from 62 to 88 years (79.9 ± 8.3 years). According to their medical histories, they had not had any diseases involving the zygoma, maxilla, or maxillary sinus, and they did not show any facial asymmetry. The sinus membranes were all shown to be healthy when the maxillary sinus was exposed. The maxilla and zygoma were taken out of each specimen and the surrounding soft tissue was removed from the bone. Seven of the 26 specimens were excluded as they had maxillary molars. Nineteen specimens without molar teeth were evaluated with the measurement system outlined below.
Landmarks and measurements
From the lateral view, the point of intersection (the so-called 90° angle point) between the upper margin of the zygomatic arch and the temporal margin of the frontal process of the zygomatic bone was designated as point A ( Fig. 1 ). A line perpendicular to the upper margin of the zygomatic arch was drawn from point A. The intersection point between the perpendicular line and the lower margin of the zygomatic arch was designated as point B. Segment A–B was divided into four parts, which were designated, starting from the superior-most aspect, as points A1, A2, and A3. The middle point on the line extending from the upper margin of the frontal process of the zygomatic bone, between the orbital margin and the temporal margin of the frontal process of the zygomatic bone, was designated as C1. A line perpendicular to the upper margin of the zygomatic arch was drawn from point C1, and the intersection point between the perpendicular line and the lower margin of the zygomatic arch was designated as point C5. Segment C1–C5 was divided into four parts, which were designated, starting from the superior-most aspect, as points C2, C3, and C4. The thickness at each point (A, A1, A2, A3, C1, C2, C3, and C4) was measured five times using callipers (Mitutoyo Corporation, Kanagawa, Japan). The starting point for the insertion of a zygomaticus implant was in the second premolar region of the maxillary alveolar process, in a slightly palatal position. Because the specimens had no molar teeth, the position of the second premolar was estimated from the infraorbital foramen, according to the method of Uchida et al. The starting point for the insertion of the zygomaticus implant was shifted 3 mm towards the palatal side from the second premolar. As a reference for the length of the possible zygomaticus implant, the distances from the starting point to each point on the outer surface of the zygomatic bone were also measured.
Statistical analysis
The Student’s t -test was applied to identify measurement differences between males and females using StatView 5.0 software program (Abacus Concepts Inc., Berkeley, CA, USA). For the correlation analysis, Pearson’s correlation coefficient was calculated using the same program; the correlation was tested by applying Fisher’s test. The critical P -value was set at 0.05.
Results
The height and thickness of the zygomatic bone at each point are summarized in Table 1 .
| Group | Number of cadavers | Age, years | Number of specimens | Height of zygomatic bone, mm | Thickness of zygomatic bone, mm | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A–B | C1–C5 | A | A1 | A2 | A3 | C1 | C2 | C3 | C4 | ||||
| Male | 5 | 77.0 ± 9.6 | 10 | 19.7 ± 3.4 | 23.1 ± 4.1 | 2.0 ± 0.3 | 4.1 ± 0.4 | 4.9 ± 0.7 | 4.5 ± 1.0 | 5.9 ± 1.6 | 6.4 ± 1.8 | 7.2 ± 1.7 | 6.4 ± 1.5 |
| Female | 6 | 82.3 ± 7.0 | 9 | 18.2 ± 2.5 | 20.5 ± 2.8 | 1.6 ± 0.5 | 2.9 ± 0.5 *** | 3.5 ± 0.6 *** | 3.4 ± 0.8 ** | 3.8 ± 0.7 ** | 4.6 ± 0.8 * | 4.9 ± 0.7 ** | 4.9 ± 0.9 * |
| Total | 11 | 79.9 ± 8.3 | 19 | 19.0 ± 3.0 | 21.9 ± 3.7 | 1.8 ± 0.4 a | 3.5 ± 0.8 a | 4.2 ± 0.9 | 3.9 ± 1.0 b | 4.9 ± 1.6 | 5.6 ± 1.7 | 6.1 ± 1.7 a | 5.7 ± 1.4 a |
The thickness at point A (corresponding to the 90° angle point) was a mean 1.8 ± 0.4 mm. The thickness increased gradually from point A to A2 and from point C1 to C3. The thicknesses at points A, A1, A2, and A3 were thinner than the corresponding anterior points C1, C2, C3, and C4, respectively. In other words, the more inferoanterior portion was thicker than point A. Although no significant difference in the height of the zygomatic bone was observed between males and females, the zygomatic bone in females was significantly thinner than in the males at all points, except point A.
Table 2 summarizes the distances from the starting point to each point on the outer surface of the zygomatic bone. For all specimens, the mean distance ranged from 42.4 to 55.5 mm. There were no significant differences between males and females in relation to the distances from the starting points to their respective corresponding points.
| Group | Distance from the starting point to each point, mm | |||||||
|---|---|---|---|---|---|---|---|---|
| A | A1 | A2 | A3 | C1 | C2 | C3 | C4 | |
| Male | 55.3 ± 3.2 | 53.2 ± 2.4 | 50.5 ± 2.3 | 47.4 ± 2.6 | 52.4 ± 3.1 | 49.4 ± 3.0 | 45.9 ± 2.8 | 42.0 ± 3.4 |
| Female | 55.9 ± 2.1 | 52.8 ± 2.7 | 50.1 ± 2.6 | 47.7 ± 3.1 | 52.7 ± 2.3 | 49.0 ± 2.9 | 46.1 ± 2.8 | 42.9 ± 3.1 |
| Total | 55.5 ± 2.7 | 53.0 ± 2.4 | 50.3 ± 2.4 | 47.5 ± 2.8 | 52.5 ± 2.8 | 49.2 ± 2.9 | 46.0 ± 2.8 | 42.4 ± 3.2 |
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