Prevalence of Sinus Augmentation Associated With Maxillary Posterior Implants

Applications for the use of endosseous dental implants have expanded for partially and completely edentulous patients, including those with severely resorbed and atrophic residual ridges.¹  Clinicians considering implant reconstruction of the edentulous posterior maxilla have to account for several anatomic challenges, particularly the maxillary sinus.^1–5 

The maxillary sinus, the largest of the 4 paranasal sinuses, grows progressively as the skull matures; however, the presence of teeth prohibits the inferior growth of the sinus.^6,7  When teeth are lost, the sinus usually expands inferiorly at the expense of the surrounding bone.^6,7  Pneumatization or enlargement of the maxillary sinus limits the quantity of alveolar bone available for implant placement and may result in a lack of primary stability and difficulty in achieving osseointegration.^8–10  Augmentation of the maxillary sinus has become a routine treatment option for the atrophic posterior maxilla, allowing placement of implants using simultaneous or staged procedures.^1,3,4  Although the challenges are great, sinus augmentation procedures have been shown to have predictable implant survival rates greater than 90% over 3 to 5 years.^2–5 

When the pretreatment bone height is less than 10 mm, adequate stabilization of implants may not be possible with conventional implant surgical techniques.^6,11  By surgically increasing the alveolar height, a bony foundation can be established to permit the placement of implants for prosthetic reconstruction.^12,13  Although pretreatment bone height is frequently the primary predictor for determining the surgical technique that will be used to modify and reduce the sinus cavity,^14,15  the decision as to which bone augmentation technique is used can be affected by host factors, surgical experience, and clinical traditions.^14–16 

The most common technique for sinus floor elevation is the antrostomy, first described in 1980¹² with a crestal approach;⁹  later, access was achieved through the lateral wall of the maxilla.¹⁷  This can be done as a 1-stage technique, where the augmentation and implant placement occur simultaneously, or as a 2-stage technique, where the graft material is allowed to mature and implant placement is delayed.^3,4  Authors have noted that a minimum of 3 to 6 mm of residual crestal bone height is needed to consider the 1-stage technique,^17,18  otherwise, the 2-stage technique is recommended as primary stabilization cannot be ensured.¹⁵  Simultaneous augmentation and implant placement minimizes the cost and number of surgical procedures for the patient and allows for earlier loading.^8,17  On the other hand, a delayed approach allows for primary stabilization due to graft maturation, and it may be more predictable and result in more ideal implant positions.^13,19 

The osteotome technique is a less invasive alternative for sinus augmentation that also allows for simultaneous grafting and implant placement.²⁰  Concave tipped with a sharpened edge, tapered Summers²⁰  osteotomes are used to expand the osteotomy horizontally and vertically while cutting, compressing, and deforming the bone to facilitate the placement of implants.^20,21  This can be done with or without the addition of allograft material to achieve apical displacement of the sinus floor.^20,21  Although the osteotome technique has a narrower range of indications and can be uncomfortable for the patient,¹⁴  it conserves osseous tissue and allows implants to be placed in a greater variety of sites.²⁰ 

Regardless of the incidence of sinus augmentation, the posterior maxilla is known to have a thin bony cortex with poor medullary strength and low trabecular density.²²  This poor quality of bone has been associated with increased implant failure rates attributed to this insufficient cortex for implant stabilization.^16,23,24  The principles of osseointegration require that the presence of vital bone tissue with cells capable of connecting to the implant surface.²⁵  Rehabilitation of the oral cavity with implants has been shown to be successful and predictable in patients with normal bone volume and density where implants of a standard diameter and length can achieve initial stabilization.²⁶ 

Recent meta-analyses of the lateral approach and the transalveolar techniques concluded that sinus floor elevation is a predictable treatment method showing high survival rates and low complications.^3,4  A descriptive analysis by Lambert and colleagues⁵  stated that machined implants placed in augmented bone had statistically lower survival rates than those placed in native bone, and rough-surface implants had comparable survival rates when placed in augmented or native bone. Although Carr and colleagues²⁷  found that implants in augmented sites are 5 times more likely to fail than those placed in sites that did not require augmentation, several studies have found failure rates to be comparable.⁸  There also appears to be a correlation between the success of the bone graft and the success of the implant.¹⁹ 

The posterior maxilla presents a unique set of anatomic challenges for implant placement and survival because of the structural characteristics of the bone and pneumatization of the sinus. As dental implants have become a routine treatment option for the replacement of missing teeth, the need to assess the prevalence of sinus augmentation and identify factors associated with it is indicated. Even though several studies have examined survival rates of dental implants associated with sinus augmentation procedures, no previous research was found examining the prevalence of sinus augmentation and identifying variables associated with sinus augmentation in the posterior maxilla.

The specific aim of this study was to retrospectively analyze a group of patients at the University of Minnesota School of Dentistry who had implants placed in the posterior maxilla and to calculate the prevalence of sinus augmentation and identify factors associated with it.

With institutional review board approval, dental records from a population of patients who had implants placed in the maxillary posterior region at the University of Minnesota School of Dentistry between January 2000 and December 2004 were used to create a database. This time period was selected to have the best opportunity for long-term survival rates. The criteria for study inclusion was that maxillary posterior implant placement was completed at the School of Dentistry during the time period specified. Exclusion criteria included patients with inadequate or unavailable dental records or cancer patients who had implants placed in a reconstructed maxilla, typically with bone from the iliac crest. The database was created using public domain statistical software for epidemiology (Epi Info, version 6, Centers for Disease Control and Prevention, Atlanta, Ga).

Information retrieved from the dental records was divided into continuous and categoric independent variables. The dependent variable was the incidence of a sinus augmentation procedure.

The 4 continuous variables were age of the patient at stage 1 implant surgery (S1), time between extraction and S1, time between extraction and sinus augmentation, and time between sinus augmentation and S1. The 11 categoric variables were subdivided into demographic, health status, anatomic, and implant variables.

The patient's gender was recorded as the demographic variable. Health status variables included classification according to the American Society of Anesthesiologists (ASA) system,²⁸  where patients are categorized as a normal healthy patient (ASA I), as having mild systemic disease (ASA II), or as having severe systemic disease (ASA III). Patients whose dental records identified them as a current tobacco user in the medical history form were recorded as smokers; otherwise, patients were considered nonsmokers. Patients were recorded as having osteoporosis if they indicated in the medical history documents that they had the condition.

Anatomic variables recorded included the residual crestal bone height (>12 mm, 8 to 12 mm, <8 mm) measured from a pretreatment panoramic radiograph and reduced to 80% of the original value to compensate for radiographic distortion.¹⁶  Residual crestal bone heights were grouped according to recommended or typical surgical techniques used to augment the sinus relative to the height of the residual bone. Heights >12 mm are most likely not involved in sinus augmentation procedures, those with 8 to 12 mm may have an osteotome procedure, and those with <8 mm are most likely to have a lateral antrostomy.^3,4  Other anatomic variables included the implant position (first premolar, second premolar, first molar, second molar), implant proximity (1 or 2 adjacent teeth, 1 tooth and 1 implant, 1 or 2 adjacent implants, no adjacent tooth or implant), and prosthesis type (single crown, fixed partial denture, complete denture). Implant variables included implant failure, diameter of the implant (<3.7 mm, 3.7 to 4.9 mm, >4.9 mm), and length of the implant (14 to 16 mm, 13 mm, 10 to 12 mm). Implant failure was defined as complete removal of the implant for any reason. Crestal bone loss typical of a failing implant was not included in the failed implant data.

Descriptive statistics were calculated for all study variables. Univariate analyses were used to assess the association of each of the independent variables on the prevalence of sinus augmentation. Because the independent variables were analyzed one at a time, the effect of each variable on the incidence of sinus augmentation was estimated using a simple logistic regression. A likelihood-ratio test was used to assess the statistical significance of the effect of each of the variables on sinus augmentation. The independent variables were considered statistically associated with sinus augmentation if the corresponding P value was less than the level of significance of .05. All analyses were conducted using statistical software (SAS, version 9.2, SAS Institute, Inc, Cary, NC).

The database included 581 implants, of which 66 were excluded because of inadequate or unavailable dental records and 13 were excluded because the implant was placed in a reconstructed maxilla of a cancer patient. The final database contained 502 maxillary posterior implants. The mean duration of follow-up for the 468 surviving implants was 35.7 months, with a range of 0 months to 8.3 years from S1 to the last recall appointment.

Table 1 summarizes the results of the descriptive statistics for the continuous variables as well as the odds ratios and P values for the incidence of sinus augmentation for the age of the patient at S1 and the time between extraction and S1. The time variables have different numbers of implants with available data as not all patients had the tooth extracted at the School of Dentistry and not all patients required a sinus augmentation procedure before implant placement. Of the 502 implants included in the study, the mean age of all patients was approximately 55 years with an age range of 18 to 84 years at the time of S1.

Table 2 summarizes the results of the descriptive statistics for the categoric variables as well as the odds ratios and P values for the incidence of sinus augmentation. Of the 502 maxillary posterior implants placed, 272 (54.2%) were placed in association with a sinus augmentation procedure (210/272 or 77.2% with the lateral antrostomy). The majority of the patients were female (66.9%), were either healthy (ASA I; 36.5%) or had mild systemic disease (ASA II; 53.5%), were nonsmokers (88.5%), and did not report a history of osteoporosis (95.4%). Neither the demographic variable (P = .089) or the health status variables (ASA I, P = .510; ASA II, P = .689; and ASA III, P = .059) were statistically associated with the incidence of sinus grafting.

When the residual crestal bone height was >12 mm (48.9%), sinus augmentation was required for 47/240 implants (19.6%). Conversely, when the residual crestal bone height was <8 mm (25.2%), sinus augmentation was required for 118/124 implants (95.2%). The association between residual crest bone height and sinus augmentation was statistically significant (P < .001).

When considering the implants that survived (93.2%), sinus augmentation was performed for 244/468 implants (52.1%), whereas for those that failed (6.8%), sinus augmentation was carried out for 28/34 implants (82.4%). The association between implant failure and sinus augmentation was statistically significant (P < .01).

Table 3 summarizes the significant univariate relationships between the categoric independent variables and the incidence of sinus augmentation and reports confidence intervals. Implants placed in residual crestal bone height <8 mm were 80.76 times (P < .001) more likely to be associated with a sinus augmentation procedure than those placed in residual crestal bone heights >12 mm. Implants placed in the position of the first molar were 3.57 times (P < .001) more likely to be associated with a sinus augmentation procedure than those placed in the first premolar position. Implants restored with a fixed partial denture were 2.07 times (P < .001) more likely to be associated with a sinus augmentation procedure than an implant restored with a single crown. Implants that failed were 4.28 times (P < .01) more likely to be associated with sinus augmentation than were implants that survived. Additionally, the descriptive statistics on implant failure rates among the different sinus augmentation technique groups were performed; there was no significant difference (P = .117) in failure rate between the 3 sinus augmentation techniques (Table 4).

The overall prevalence of sinus augmentation within this implant database was 54.2%. The prevalence data needs to be interpreted cautiously, however, as some patients may have chosen not to have implant treatment because of the need for a sinus augmentation procedure. Another limitation of this study is that the management of residual crestal bone height may have been affected by the surgical experiences of the surgeons involved and the clinical traditions of the institute; however, the surgeons were not restricted to a particular surgical technique. Also, there were a large number of surgeons involved with this study, including oral surgery and periodontal faculty members and residents supervised by full-time and adjunct faculty members who have various educational and surgical experience backgrounds; this may balance out any biases of one surgeon.

With the apparent challenges of maxillary posterior implants, such as pneumatization of the sinus and low trabecular bone density, the survival rate of the implants in this database was 93.2% over a mean follow-up period of 35.7 months and a range from 0 months to 8.3 years.²  This survival rate compares well with those of other published studies.^3,4,19  Carr and colleagues²⁷  reported that implants were 5 times more likely to fail in augmented sites than nonaugmented sites. On the other hand, Olson and colleagues¹³  found that survival rates were higher for implants placed in grafted sinuses than for those is nongrafted sinuses. Even though this study reported an overall survival rate of 93.2%, the results also show that the failed implants had a 4.3 times higher chance (P < .01) of being associated with sinus augmentation. The posterior maxilla is known to have poor bone quality characterized by a thin cortex, poor medullary strength, and low trabecular density. This poor bone quality, compounded with inadequate bone quantity due to the resorption of the alveolar bone after tooth extraction and subsequent pneumatization of the maxillary sinus, affects the initial implant stability that can be expected and, ultimately, implant success.^6,16  Implant survival may be more related to the amount of residual bone supporting the implant than to the grafted bone.^1,5 

As far as different augmentation techniques are concerned, the 2-stage lateral antrostomy was the most frequently used technique among all the implants (30.7%), followed by the osteotome technique (12.4%) and the 1-stage lateral antrostomy (11.1%) (Table 4). Although the 2-stage lateral antrostomy had the highest failure rate (12.3%), there was no statistical difference between the 3 sinus augmentation techniques (P = .117). The lowest failure rate occurred when the implants were placed in sites that did not require sinus augmentation (2.6%). High success rates can be predictably achieved if certain preconditions are met, most importantly, primary implant stability. This initial implant stability is regarded as critical for the prognosis and future clinical performance of the implant.²⁹ 

Bone availability is a primary factor when evaluating patients as a candidate for implant therapy.^6,11,13  Block and Kent¹¹  stated that when the thickness of bone between the alveolar crest and floor of the sinus is <10 mm, sinus augmentation is necessary to support long implants and the subsequent prosthesis. Misch⁶  proposed 4 subantral treatment options for the posterior maxilla that varied depending on the available bone height. Most authors base the selection of the sinus augmentation technique on the vertical dimension of the residual bone, from a 2-step lateral antrostomy in situations with a severely resorbed maxilla to the less invasive osteotome technique when there is only mild resorption.^{9,12,15,17,20,21}  The results of this study are in agreement with those studies as the prevalence of sinus augmentation increased by a factor of 12.71 when the residual crestal bone height was between 8 and 12 mm and by a factor of 80.76 when the residual crestal bone height was <8 mm (P < .001). In those sites in this study that had residual crestal bone height >12 mm and a sinus augmentation procedure, it is likely the procedure was actually indicated for an adjacent edentulous site.

Resorption of the alveolar bone and enlargement of the maxillary sinus subsequent to tooth extraction combined with a possible loss of alveolar bone due to periodontal disease before tooth extraction leaves a clinical situation that complicates the placement of implants.^{6,11,12,16,30}  Even though time between extraction and S1 was not significantly associated with sinus augmentation in current study, it was borderline (P = .096). For those who had a sinus augmentation procedure, the mean time between the extraction and S1 was 26.4 months (mode = 7–12 months), while for those who did not require a sinus augmentation procedure, the mean time decreased to 10.4 months (mode = 1–6 months) (Table 1). The mean time between extraction and sinus augmentation was 23.6 months, and the mean time between sinus augmentation and S1 was 3.3 months, considering that 43% (115/268) were placed simultaneously.

Resumption of maxillary sinus pneumatization after the age of 20 or the eruption of the third molars can occur after posterior tooth extraction.⁷  The proximity of the inferior wall of the maxillary sinus to the dentition is closest in the second molar region, which frequently causes a loss of the thin bone between the alveolar socket and the sinus during extraction, resulting in expansion of the sinus.⁷  This relates to the results of this study showing that sinus augmentation was more prevalent in molar sites than premolar sites (P < .001).

Because of reduced bone resistance to sinus expansion, pneumatization has also been shown to be higher when adjacent posterior teeth are extracted.⁷  In posterior single-tooth edentulous sites, roots of the adjacent teeth appear to prevent or decrease sinus pneumatization as negligible expansion has been found.⁷  Those findings also agree with the results from this study, where implants sites adjacent to another implant or to an edentulous site had an increased prevalence of sinus augmentation compared with those adjacent to natural teeth (P < .001). The type of prosthesis, such as a complete denture compared with a single crown, suggests that adjacent teeth are missing and those prostheses had an increased requirement for sinus augmentation (P < .001).

Implant diameters higher than 4.9 mm had a 2.8 times greater chance to be associated with sinus augmentation than regular (3.7 to 4.9 mm) or narrow diameter (<3.7 mm) implants (P < .01). Relatively wide buccolingual ridge dimensions of the posterior maxilla might have encouraged surgeons to place wider implants to improve the biomechanics of the implants placed in augmented bone. Those implants that were 13 mm in length had a 1.4 times higher chance to be associated with sinus augmentation than implants that were 14 to 16 mm in length, although this was not significant (P = .22). It is quite interesting to find out that a 13-mm implant was the most popular implant length used in augmented bone (73%). Regardless of the technique used to augment the sinus, the resultant vertical bone available at S1 might have been close to 13 mm.

Osteoporosis had a borderline significance (P = .059), showing a 2.5 times higher chance to be associated with sinus augmentation. The small sample size (23/502) of the osteoporosis group raises questions about the significance of the association, however.

Within the limitations of this retrospective study, the results suggest that more than half (54.2%) of the maxillary posterior implants in this database were associated with a sinus augmentation procedure. The prevalence of sinus augmentation increased with decreased residual crestal bone height, more posterior implant locations, and complete or partial edentulism. Sinus augmentation was significantly associated with implant failure and wide implants.

Abbreviation

ASA

American Society of Anesthesiologists

S1

stage 1 implant surgery