Accuracy of Clinical and Radiological Classification of the Jawbone Anatomy for Implantation—A Survey of 374 Patients

The new era of modern implantology began in 1969 upon the publication of the first results of using titanium dental implants.1 Since that time, the shapes and surfaces of these implants have changed. Nevertheless, this method still ranks as the most popular and reliable. The mean implantation success rate is higher than 90%.2 The estimated cumulative implant survival rates 10 years after implantation were reported as 89.9% for the maxilla and 93.3% for the mandible.3–4 The results remained unchanged after 16.6 years. The success of implantation depends on a variety of factors, 1 of which is the selection of patients.5 After accepting the patient's state of health, the anatomical features of the jaw must be evaluated so that proper treatment may evolve.

Atrophy of the alveolar processes is expressed as reduction of height and width. This frequently occurs after the loss of teeth. New treatment methods, such as guided bone regeneration (GBR),6–10 sinus floor augmentation with bone or bone substitutes,11–16 and horizontal and vertical osteoplasty,17 have contributed to the reconstruction of atrophic jaws, thereby permitting a more liberal use of endosseous implants.

There is a wide choice of screw-type implant systems. Therefore, it is important to measure the alveolar process precisely so that the proper system may be chosen.18 There are many classifications suggested for assessment of the degree of atrophy of partially edentulous jaws.19–24 One of the most popular classifications of the quality and quantity of the jawbones was established in 1985.25 However, this classification, like many others, described changes only of jaw shapes in general and failed to indicate precise measurements of potential operative sites.26 In the system described in this paper, bone atrophy is evaluated according to analyses of jaw cross-sections. Jaw cross-sections consist of alveolar and basal bone. Usually, vertical and horizontal measurements of these are recorded when implant operations are planned. Vertical measurements involve the planned implant site and usually correspond to the basal bone. For example, the vertical distance from the crest of the ridge to the mandibular canal is important information for surgeons. Because the term cross-section does not correspond exactly with the implant position, the delineation of implant site is not strictly anatomical and therefore cannot be used in assigning clinical-anatomical classifications. Some authors have used terms such as the triangle of bone and bone profile at implant site.27–,28 These terms lack relevance, as do the others. It is much more accurate to use the term jaw dental segment (JDS). This is defined as a vertically cut jaw segment with tooth, alveolar bone, and all or part of the basal bone. The location of bone suitable for implantation is identical with the former location of a tooth in the jaw. The number of the JDS describing the position of a planned implant in the jaw can be shown. If the JDS is edentulous, the term edentulous jaw dental segment (eJDS) is used. In view of these considerations, the purpose of this study was to offer the most reliable clinical and radiological information of potential eJDS in order to optimize prognoses for implant surgery.

Three hundred seventy-four patients, 156 men and 218 women, were enrolled in the investigation. They were consecutive patients treated with dental implants in the Department of Maxillofacial Surgery, University of Kaunas. Their mean age was 46 years (SD = 127), ranging between 17 and 73 years (Table 1). Three hundred fifty patients (93.6%) were partially edentulous. Only 22 patients were fully edentulous; 10 had edentulous maxillae and 12 had edentulous mandibles. The implantations were performed only after the general health status of all patients had been deemed satisfactory. Contraindications for implantation were disorders of the immune system, uncontrolleddiabetes mellitus, significant osteoporosis, the presence of oncologic problems, the use of chemotherapy and radiotherapy, alcohol abuse, and heavy smoking.

To obtain clinical and radiological assessments of the eJDS, evaluation was begun at the widest point of each segment. Because the crest of the alveolar process was often thin, it was necessary to shave it and thus produce a plane surface for the planned implant installation. In such cases, the heights of eJDS would have been shortened by 1 to 3 mm; this change had to be considered when performing dental segment height evaluation.

The width of the alveolar process was estimated intraorally with ridge-mapping callipers.29 This ridge-mapping method is simple to use and avoids exposure to radiation. The technique was modified by making measurements in a vertical plane in the eJDS at points 3, 4, 5, and 6 mm from the alveolar bone crest. This technique minimized discrepancies. The smallest measurement was accepted as the width of the eJDS (Figure 1).

The height of the alveolar process was estimated by the orthopantomogram and the stencil of the Osteofix dental implant system (Oulu, Finland), taking into consideration the average X-ray magnification of 20% (Cranex-3, Soredex, Finland). Spiral computerized tomography (CT) scans (Somatom Plus, Siemens, Erlangen, Germany) were used in cases of significant jaw atrophy, when particularly high-precision measurements of the jaws were necessary. The CT scans were derived following the standard exposure and patient positioning protocol of the Department of Radiology of the University Hospital of the Kaunas Medical School. Scanning was performed with a software program for soft and hard tissues of the maxillofacial region in slices of 2-mm thickness and in a 3-mm table step at 140 kV 130 mA, Kernel AH60, with the “head-legs” direction. The height measurements were taken in a vertical plane at the points of the eJDS from the tip of the alveolar bone to the maxillary sinus and nasal sinus in the upper jaw and mental foramen and mandibular canal in the lower jaw.

The minimal heights and widths of eJDS for proper implantation were estimated according to the principles of threaded implant insertion. Recommendations for successful results ideally require at least 1 mm of bone surrounding each implant.30 The diameters of Osteofix implants require that the minimal width of the alveolar process be 5.8 to 6.2 mm. Most implant systems require bone widths of 5 to 7 mm.8,25 We estimated that for proper implantation, the minimal width of an alveolar ridge should be 6 mm.

Several factors are considered when estimating the minimal height of an alveolar process. In areas where esthetic results were important (anterior parts of mandible and maxilla), the measurements were made with the implant head 2 mm lower than the necks of the adjacent teeth.31 If the operation was planned according to the orthopantomogram, implantation in the areas of the mandibular canal or the maxillary sinus mandated that the apices be at least 2 mm away from those vital structures. A minimum of 1 mm was demanded if the operation was planned with CT.32 When shorter implants were required, they were used in conjunction only with longer ones. When this was not feasible, sites of only the higher levels of bone density were selected.18,33 Essentially, the minimal height of the eJDS measured in orthopantomograms was at least 10 mm.

Patients whose dimensions satisfied these requirements were considered Type I. Patients with the less height (4–8 mm) were considered to be Type II. However, such heights were found to be sufficient to ensure primary stability of implants. Alveolar ridges with widths of 4 to 5 mm were deemed insufficient for proper implantation. Despite such deficiencies, it was expected that the wider parts of the implants would be covered by bone after insertion and that primary stability would be achieved. Patients whose eJDS ridge heights and widths were less than 4 mm were categorized as Type III. These measurements were considered to be insufficient for primary stability of implants.

Another important factor for proper implantation was the (anteroposterior) length of the eJDS. The minimal distance between 2 implants should be at least 3 mm34, and minimal distances between implants and natural roots should be at least 1.5 mm.35 Considering that the minimal recommended diameter of implants is 3 mm, all eJDS must have been at least 6 mm in length. This dimension was required in all cases irrespective of the degree of jaw atrophy.

The following clinical and radiological classification of alveolar bone is suggested in dental implantation, taking into consideration similar sizes of most screw-type implant systems and the traditional methods and principles of making measurements for their insertion.

The height of the eJDS is ≥10 mm and the width is ≥6 mm. Vertical defects in the anterior region must be ≤3 mm from the crest of alveolar bone to the necks of the adjacent teeth. In Type I areas, there is little or no atrophy of the jaws; therefore, sound principles of implant surgery may be practiced.

In Type IIA, the height of the eJDS is ≥10 mm and the width is 4 to 5 mm (narrow eJDS). In Type IIB, the height of the eJDS is 4 to 9 mm and the width is ≥6 mm (shallow eJDS). In Type IIC, the height of the eJDS is 4 to 9 mm and the width is 4 to 5 mm (shallow and narrow eJDS). In Type IID, the height of the eJDS is ≥10 mm and the width is ≥6 mm; the vertical cosmetic defect in an anterior region is more than 3 mm from the crest of alveolar bone to the necks of the adjacent teeth (Figure 2).

Immediate implantation in such cases is possible only with (1) horizontal alveolar augmentation with GBR, (2) vertical alveolar augmentation with GBR or sinus floor augmentation, (3) vertical alveolar augmentation with GBR or sinus floor and horizontal alveolar augmentation with GBR, or (4) horizontal alveolar augmentation with GBR.

The height of the eJDS is <4 mm and the width is <4 mm (eJDS is both too shallow and too narrow for dental implantation). Implantation can be performed as a second stage only after augmentation or reconstruction of the potential host site has been completed (Figure 3).

Implantation was performed with Osteofix implants, which were of 8, 10, 12, and 14 mm in length and 3.8 and 4.2 mm in diameter. The implantation was performed according to a standard protocol.36 The patients were given 2 g of VK-penicillin 1 hour before the surgery and 2 g twice a day for 7 days postoperatively. Chlorhexidine gluconate 0.2% (Adams Healthcare) oral rinses were prescribed twice a day for 2 weeks. The sutures were removed after 10 days.

The accuracy of the planning classifications was compared with the clinical and radiological eJDS assessments. Insufficient accuracy was noted for the following parameters: (1) implant threads were not completely covered after implantations, (2) bony walls of the implant host sites fractured during the drilling procedures or implant insertions, and (3) insufficient primary implant stability.

Statistical analyses were performed with the SPSS/PC+ version 10.0.1 program (SPSS, Inc, Chicago, Ill). Standard deviations of the means were calculated. The accuracy of the clinical and radiological classifications was calculated as the number of properly installed implants divided by the total number of implants, and the resulting number was multiplied by 100. This yielded the results in percentages.

The height and width assessments of eJDS measured clinically and radiologically in different segments of maxillae and mandibles are presented in Table 1. When analysing the results of the measurements, we found that proper implantation was possible at 476 implant sites (60.1% of the total number) for 232 patients (96 men and 136 women) in the Type I group (Table 2). Patients within the age range of 17 to 45 years (mean 40.9 years, SD 11.8) dominated this group (50% of the total). The distribution of the defects of the arch according to defect localization is shown in Figure 4.

The Type II group consisted of 100 patients, and eJDS with insufficient measurements for proper implantation were found in 222 implant sites (28% of the total number) (Table 2). The mean age of the individuals in this group was 45.8 (SD 13.4) years.

In 52 patients of the Type II group for whom GBR was applied, we found vertical atrophy in 48 segments, horizontal atrophy in 53 segments, and both vertical and horizontal atrophy in 19 segments (Table 3). If sufficient primary stability of the implant was achieved, the remaining bone defects around the implant were filled by using GBR10 with deproteinized bovine bone mineral (Bio-Oss, Geistlich AG, Wolhusen, Switzerland) and covered with a collagen membrane (Bio-Gide, Geistlich AG, Wolhusen, Switzerland). The membrane was extended onto the intact bony walls around the defect and fixed in place with resorbable pins (Resor Pin, Geistlich AG, Wolhusen, Switzerland).

In the anterior region, vertical defects of more than 3 mm from the alveolar bone margin to the necks of the adjacent teeth had been noted for 11 patients (12 implants). To obtain better cosmetic results, these implants were inserted with their shoulders placed 2 mm below the necks of the adjacent teeth. Guided bone regeneration was used for the filling of accompanying bone defects.

Forty-eight patients (91 implants) with 4 to 8 mm of bone height from the alveolar crest to the maxillary sinus floor were assigned to a separate group of Type II patients. Immediate implant insertion accompanied by maxillary sinus floor augmentation (with autogenous bone graft taken from the chin) was performed for 8 patients (16 implants). Xenogenic bone (Bio-Oss, Osteohealth, Shirley, NY) had been used for 40 patients (86 implants).11,12 Autogenous bone was used for patients with low eJDS (4–5 mm). However, after the insertion of the implants into the sinus floors, sufficient primary stability was noted. Implantations in the region of premolars and molars of both jaws were the most common sites for the Type II patients (Figure 5).

The Type III group consisted of 42 patients, 12 men and 30 women (94 implants, 11.9% of the total number), with a mean age of 46.6 years (SD = 12.9). Immediate implantation, including bone grafting, was not available for these patients (Table 2). Primary stability of these implants could not be assured because of shallow, narrow alveolar (less than 4 mm). Therefore, grafting of these defects followed by later implantation was the universal approach for Type III patients (Figure 6). The site most frequently used in maxillae was the anterior region. Immediate implantation was not possible because of the severe deficiencies. Augmentation with later implantation was performed in these cases to preserve the thin, shallow alveolar processes.

When comparing the preoperative clinical and radiological eJDS measurements with the intraoperative observations for the Type I patients, we found that implant thread coverage was incomplete in 14 cases. In 6 cases, lateral surfaces of the implants were exposed in the posterior maxillae eJDS. We noted significant buccal concavities in all those 6 cases. After analysis of possible causes of this complication, we concluded that, despite cases that showed minimal thicknesses of 6.0 to 6.7 mm with implant diameters of 3.8 mm, poorly made osteotomies were responsible for exposure and the need for unanticipated GBR surgery.

Nonetheless, the accuracy of matching preoperative findings with those observed intraoperativally for the patients of Type I was 95.8%.

Comparing the pre- and intraoperative assessments for the Type II group was less satisfactory. Primary implant stability failed in 7 of the cases. The main reason for this was fracture of the walls of the shallow and narrow alveolar processes. It was estimated that the bony walls of host sites were fractured during the drilling procedures in 3 cases and during implant insertion in 4 cases. Insufficient primary implant stability was noted in 2 cases when an implant was inserted into an eJDS in the maxillary sinus region. In all these failures, the widths of eJDS were from 4 to 5 mm and the heights were from 5 to 6 mm.

Despite the higher percentage of failures and complications, the accuracy of the clinical and radiological classifications among the Type II patients was established intraoperatively at 95.9%. The accuracy of clinical and radiological classification for the patients of Type III was not assessed.

Most of the known classifications used to characterize the degree of jaw atrophy are based on schematic terms of the jaw: jaw cross-section,25 "the triangle of bone,"28 and bone profile at implant site. However, these terms are not anatomical. As compared with these previously published delineations, eJDS are most readily identified references assigning anatomical areas to implant sites because they consist of the alveolar bone and a part of the basal bone. These anatomic sites can be established when teeth are present or soon after tooth extraction when immediate implant insertion has been performed. The exception is in the region of the lower incisors, where the tooth roots are very narrow. To do so here is to couple 2 incisor sites.37 The mesiodistal diameters of the mandibular central and lateral incisor roots at the cementoenam-el junction are 3.5 and 4.0 mm, respectively.

The requirements for the height of the alveolar and basal bone of the jaw as well as the optimal length of the screw-type implant necessary for successful implantation have been described in the past decade.26,–29 Implants of 8 mm or longer were chosen in our study.18,33 Requirements for the necessary width of the host bone and the minimal diameter of the implant were reported as well.18,21,25 Some systems have very small (3-mm diameter) or very large (5-mm diameter) implants. Indications for use of the smaller implants are restricted because of poor biomechanics. Implants of large diameter are rarely used because alveolar ridges can rarely accommodate them.38,39 The literature indicates that the minimal distance between implants or between an implant and an adjacent tooth should be at least 3 mm and 1.5 mm, respectively. When all the anatomic requirements for the implant site are known, it is possible to establish the real anatomic eJDS status and characterize it with certain mea-surements for proper implantation. If both, or only 1, of the measurements of eJDS are insufficient, it becomes necessary to determine the minimal values of eJDS width and height in order to ensure the primary stability of the implant. If the implant is stable, the missing amount of bone can be restored by various methods of jaw augmentation. When mea-surements of eJDS cannot ensure the primary stability of the implant, the same methods of augmentation are used initially with the intention to perform implantation at a later date. To be sure in the efficacy of our methods, we examined 374 patients and installed 792 screw-shaped Osteofix implants. The well-known techniques of orthopantomograms, spiral CTs, and clinical examination with special ridge-mapping callipers for measurement of alveolar process widths were used to establish alveolar bone anatomy. All these methods are required to establish dimensions. Unanticipated supplementary surgical procedures such as GBR augmentation may be required, but they do inconvenience the patient.

According to the results of measurements, proper implantations were made at 476 (60.1%) implant sites for 232 (72.4%) Type I patients (96 men and 136 women). The results agreed with a past study that estimated that proper implantation was possible for 80% of the patients.15 Implantation was most common in the regions of premolars and molars (Figure 4). The Type I patients were younger than those in the other groups.

The cervical threads were not completely covered in 14 cases in Type I patients. In 6 other cases, the lateral surfaces were dehiscent in the regions of the buccal concavities. It is also possible that bone width had been overestimated by improper use of the ridge-mapping callipers. Finally, we concluded that in all borderline cases that involved thread exposure, the small differences between ridge width and implant diameter (0.7 mm) had been responsible for cervical exposure. The best results for implant placement were achieved with the following rule: eJDS width = dental implant diameter + 3 mm.

The alveolar processes were of insufficient height or width for implantation in the patients of the Type II group. When such deficiencies were present, augmentation of the jaws with immediate implantation was performed. GBR was used to cover the implants and bone defects. The therapy used deproteinized bovine bone mineral, bioresorbable membrane, and resorbable pins.

Sinus floor augmentation with autologous bone or bone restitution with Bio-Oss were used in cases when the height of the alveolar process was insufficient in the posterior maxilla. We noted that primary stability of the implants was achieved by sinus grafting with Bio-Oss in cases where the alveolar process were 4 to 5 mm in height. This finding met the opinion of some authors who have noted that the minimal height of the alveolar process for primary stability of an implant should be at least 4 mm.40 In 2 cases, insufficient primary stability was reported when immediate implantation with sinus floor augmentation had been performed. In both cases, the alveolar process was 4 mm in height. In such cases, primary stability was realized when autogenous bone derived from the chin was used.

The number of men (48) and women (52) was almost equal in the Type II group. The mean age was 45.8 years (SD = 13.4) years, which was higher than in the Type I group. Alveolar bone defects in the premolar and molar areas were noted, and implantation was performed at these sites with augmentation as they had been in the Type I population (Figure 5).

The patients of the Type III group had significant atrophy of the jaw, and primary stability of the implants could not be achieved. The number of women (30) in this group was larger than the number of men (12). The more frequently noted atrophy in the female jaws may be caused by physiological and endocrine factors.5,41 The patients in this group were older than in the Type I and Type II groups; the mean age was 46.6 years (SD = 12.9). Guided bone regeneration and other jaw augmentation and reconstruction methods were performed before implant placements for all the patients in the Type III group.

Clinical and radiological investigation of the jaws and the benefits accrued from the surgery may be considered as clinical advantages of establishing the classifications cited in this article. We demonstrated that the selection of patients by using the system in this study led to successful implantation. Treatment strategy and collaboration among specialists would be facilitated by the uniform criteria of measurements as described herein.

The main anatomic characteristics that determine implant operative decisions are height, width, and length of eJDS in prospective site regions. In this study, 3 anatomic types of alveolar bone were shown to define some of the criteria governing implantation. Classifications for management were derived for Types I, II, and III as interpreted by the anatomic characteristics classified as eJDS.