Sinus Floor Elevation Using Anorganic Bovine Bone Matrix (OsteoGraf/N) with and Without Autogenous Bone: A Clinical, Histologic, Radiographic, and Histomorphometric Analysis—Part 2 of an Ongoing Prospective Study

Stuart J. Froum, DOS*/Dennis P Tarnow DOS**/ Stephen S. Wallace, DD***"/Michael D. Rohrer, DOS****/ Sang-Choon Cho. DDS*****

One of the goals of the sinus elevation procedure is the creation of vital bone to effect the osseointegration of dental implants placed in the posterior maxilla. With this goal in mind, in 1993 the Department of Implant Dentistry at New York University College of Dentistry began a long-term clinical, histologic.  histomorphometric. And radiographic study of the sinus elevation procedure. The primary parameters included the effects of graft material selection, time allowed for graft maturation, and the effect of barrier membrane placement on the creation of vital bone in the sinus cavity The effect these and other parameters on implant survival rates were also to be evaluated. This paper reports the data collected on a subgroup of 113 sinus elevations that used anorganic bovine bone matrix (OsteoGraf/N) alone or in combination with autogenous bone and/or demineralized freeze-dried bone as a graft material. This is the second in a proposed series of papers that will result from this ongoing research project. The results of this study indicate that: OsteoGraf/N appears to be an effective graft material with a 98.2% implant survival rate to date: vital bone formation increased with time: vital bone formation increased  moderately when demineralized freeze-dried bone allograft was added, and increased substantially when intraoral autogenous bone was added or when an expanded polytetrafluoroethylene membrane was used: and the increased height achieved by the procedure was stable over a 3-year period. Because of the high overall implant survival rate, it was not possible to determine the relationship between vital bone formation or membrane usage and implant survival (Int J Periodont Rest Dent 1998:18:529-543.)

*Clinical Professor and Director of Clinical Research. Department of Implant Dentistry, and Clinical Professor Department of Surgical Services (Periodontics) New York University College of Dentistry, New York. New York
**Professor and Chairman. Deportment of Implant Dentistry. New York University College of Dentistry, New York. New York
***Associate Clinical Professor. Department of Implant Dentistry New York University College of Dentistry. New York. New York
'****Assistant Dean for Research. University of Oklahoma College of Dentistry. Oklahoma City. Oklahoma
*****Clinical Instructor, Department of Implant Dentistry New YorkUniversity College of Dentistry. New York. New York

Reprint requests: Dr Stephen S. Wallace, Central Connecticut Dental Implant Center. 140 Grandview Avenue, Suite 102, Waterbury, Connecticut 06708.

Lack of sufficient alveolar bone height has long been a common deterrent to the placement of root form dental implants in the posterior maxilla. This lack of height may be the result of alveolar bone loss following tooth loss, periodontal disease, pneumatization of the maxillary sinus, or a combination of the above. Grafting the floor of the maxillary sinus, a technique that was first presented by Tatum1 in 1977 and first published by Boyne and James2 in 1980, is a means of correcting
this deficiency. This technique was later modified by Tatum, 3 Misch, 4 Pierazzini,5 Smiler and Holmes, 6 Wood and Moore, 7 Kent and Block, 8 and Misch and Dietsh. 9 A recent paper by Smiler10 reviews the surgical technique and the various lateral window entry procedures. When used alone, this procedure can result in a dramatic increase in the height of bone available for implant placement. It may also be used in conjunction with buccal and crestal augmentation procedures11 to effect positive changes in implant angulation and improve interarch relationships.

The majority of the published human data regarding this procedure has been in the form of clinical reports on implant survival over a given time period with a specific grafting material.8, 12-22Fewer papers23-26 have included significant histologic data or  histomorphometric analyses.6, 27-33 It is understood that implant success rates will be higher if the implants are placed in bone of favorable quality. The higher failure rates of implants placed in bone of poor quality34,35 and limited quantity36 attest to this fact. It is also known that implants placed in the maxillary sinus achieve osseointegration, at the light microscopic level, in a manner that is similar to that achieved in natural host bone.37-39

One of the goals of the sinus elevation procedure is the creation of vital bone to effect the osseointegration of implants placed in the posterior maxilla. With this goal in mind, in 1993 the Department of Implant Dentistry at New York University College of Dentistry began a prospective long-term histologic, histomorphometric, radiographic, and clinical study of the sinus elevation procedure. The primary factors chosen for consideration were the effects of graft material selection, time allowed for graft maturation, and the effect of barrier membrane placement on the creation of vital bone in the sinus cavity. Secondary to the above factors, the important variables of implant design, implant surface morphology, and long-term stability (repneumatization) of the grafted bone volume were to be evaluated, as was the correlation of these variables to the implant success rate

Method and materials

This study included patients from the clinical of the New York University Department of Implant Dentistry and from the private practices of the authors. All clinical participants were selected from a pool of patients who required sinus elevation procedures for the placement of posterior implants. All patients were informed about alternative treatment plans and selected the plan requiring maxillary sinus elevation. All patients with absolute contraindications for this procedure, such as uncontrolled diabetes, long-term steroid use, and blood disorders were excluded from the study. However, patients with relative contraindications, such as controlled diabetes or a smoking habit, were considered and included in this study. All patients were informed of the requirements for participation in the study (bone core harvesting) and signed an appropriate consent form. All patients had the option of withdrawing from the study at any time. The protocol for this study was approved by the New York University College of Dentistry Human Subject Committee.

As the enrollment of patients in this study is on an ongoing basis, decisions regarding graft materials and barrier membrane use were made on a randomized basis as each individual case presented. All graft materials used in this study had been previously used as graft materials in humans. All surgical procedures were performed under the direct supervision of one of the authors of the study. All bone cores were harvested by the authors. If, in the determination of the authors, a bone core could not be successfully harvested without risk to the patient, previously placed implants, or future implants sites, that patient was removed from the study (or from the histologic portion of the study).

Clinical information recorded in the database included patient age, medical history, habit history (e.g. bruxism, smoking), graft materials used, membrane use or nonuse, height of residual crestal bone, perforations, postoperative problems, time of core sample and time of loading.

Patient pool

As of February 18,1998, the overall patient pool for this ongoing study consisted of 236 sinus lifts on 169 patients with 136 histologic cores obtained for processing. The subgroup reported in this study consisted of 113 sinus lifts. In this subgroup, the grafting material consisted of OsteoGraf/N (CreaMed), an anorganic bovine bone matrix, used along or in combination with varying percentages of autogenous bone and/or other grafting materials. The most commonly used in combinations appear in Table 1.

Surgical procedure

Once the need for a sinus elevation procedure was determined, all patients were subjected to a protocol review. Diagnostic information varied by case need and included complete clinical and periodontal examination, panoramic radiographs, CT scans with radiographic templates, and SIM/Plant (Columbia Scientific) evaluation.

The vast majority of cases performed in the clinic and private practice environment were performed with local anesthesia that included a vasoconstrictor. The oral cavity was rinsed with chlorhexidine and the patients were draped for the procedure. The surgical technique most frequently used was the lateral window approach described by Tatum 3 and Misch  4 and later by Wood and Moore  7 (1986, 1987, and 1988 respectively).

 The location of the primary incision was determined by the existing anatomy and by the presence and absence of natural dentition. In edentulous patients, the primary incision was most often a palatally displaced crestal incision. An anterior vertical releasing incision was often used to allow for the adequate reflection of a full-thickness mucoperiosteal flap and exposure of the surgical field. Flap reflection was modified to allow the placement and stabilization of barrier membranes when they were used. In the cases in which crestal defects approached the floor of the sinus, a crestal  window was used to elevate the sinus floor, and the initial incision was modified to distance it from the window. The size of the lateral window was determined by the number of implants to be placed by the desire to place long implant (13 to 15 mm) and by anatomic variations. A measurement was taken from the most coronal aspect of the usable (5-mm wide) crestal bone to a superior position at least 2 mm higher than the proposed implant position. This created 17 mm for implant placement and excluded the narrow unusable crest. The superior horizontal osteotomy was made at a location that would fall within the sinus cavity. Osteotomies were made to allow for sufficient graft positioning mesial and distal to the proposed implant sites. The window was then outlined with a round diamond or carbide bur. The corners were rounded to reduce the possibility of membrane perforation. The window was then infractured; either superior bone continuity was retained to create a hinge (hinge osteotomy), or a complete 360-degree osteotomy was performed (elevated osteotomy). The Schneiderian membrane was carefully reflected to the desired extent mesially, medially, and distally using curettes and elevators. Reflection was extended along the medial wall to the height of the superior bone incision to expose an additional bony wall for healing, and to ensure the creation of sufficient graft volume for implant placement. Small membrane perforations were covered with Gelfilm (Upjohn) or Collatape (Colla-Tec) to confine the particulate graft material. The graft material of choice was packed first anteriorly, then medially, and finally in a posterior direction; care was taken not to tear the membrane or overextend the graft posteriorly, The material was carefully packed until firm, but not so densely that vascular ingrowth would be impaired. If a membrane was used, it was trimmed to extend at least 2 to 3 mm beyond the window. Nonresorbable expanded polytetrafluoroethylene
(e-PTFE) membranes (GTAM, 3i) were stabilized with tacks or screws. Tension-free flap closure was accomplished with a combination of mattress and interrupted sutures using either # 4-0 silk or Gore-Tex sutures (3i)

Antibiotic coverage was accomplished with Augmentin (Smith, Kline, & Beecham) or amoxicillin 500 mg three times a day for 7 to 10 days, with coverage starting prior to surgery. Chlorhexidine rinses were used twice daily until soft tissue closure was achieved. Medrol (Upjohn) dose packs were often prescribed to control postoperative edema. Sutures were removed after 7 to 10 days.

The above surgical protocol was used for the majority of the patients in this study. The protocol was modified to include both immediate and staged implant placements. In the
staged procedure, implant placement was performed approximately 6 to 9 months after graft placement. In immediate placement procedures, second-stage surgery was planned for 6 to 9 months.

Harvesting of the bone core biopsies was accomplished at stage 2 surgery in the immediate placement cases. In staged procedures, harvesting was accomplished at implant placement, second-stage surgery, or at both times. At no time was there the entire vital bone that was present.

Nondecalcified technique

The majority (80%) of the specimens were processed using a nondecalcified technique. The cores were fixed in 10% neutral buffered formalin. At the Hard Tissue Research Laboratory, the specimens were dehydrated with a graded series of alcohols for approximately 14 days. Following dehydration, the specimens were infiltrated with a light-curing embedding resin (Technovit 7200 VLC). Following approximately 14 days of infiltration with constant shaking at normal atmospheric pressure, the specimens were embedded and polymerized by 450-nm light, with the temperature of the specimens never exceeding 40°C. The specimens were then prepared by the cutting/grinding method of Donath.40, 41 The specimens were cut to a thickness of 150 pm on an Exact cutting/grinding system (Exact Apparatebau). Following this, the slides were polished to a thickness of 35 to 45 urn using the Exact microgrinding system, and they were stained with Stevenel's blue and van Gieson's picric fuchsin. The specimens were analyzed using NIH Image, an image analysis software program developed by the National Institutes of Health, on a Power Macintosh 8500/132.

OG/N – OsteoGraf/N; DFDBA = demineralized freeze-dried bone allograft

Results
Histomorphometric analysis
All cores processed by one of the authors at the Hard Tissue Research Laboratory were analyzed for both percentage of total bone by volume and percentage of vital bone by volume. Table 2 demonstrates both the range of vital bone formation and the mean vital bone formation for the most common graft compositions in the database that included OsteoGraf/N.

Histomorphometric analysis of cores was performed at both the 6- to 9-month and 12- to 15-month intervals on nine patients. While the data was insufficient to compare graft maturation in each of the four graft categories, it was possible to compare the vital bone content at the two intervals for these nine cases. An increase in the mean value of vital bone was noted between the early cores and the later cores. The effect of the placement of a stabilized, nonresorbable e-PTFE membrane (GTAM) over the lateral window is demonstrated in Table 3.
Vital bone formation was enhanced when a membrane was placed, regardless of the graft material used.

OG/N – OsteoGraf/N; DFDBA = demineralized freeze-dried bone allograft

Implant survival
By referencing the sinus lift study computer database, it was possible to calculate implant survival rates from the inception of the study to the time of the submission of this article for publication. The implant survival rates are presented as they relate to type of graft material used and 1o the presence or absence of a nonresorbable membrane over the lateral window (Table 4).

Radiographic analysis
Preoperative and postoperative panoramic radiographs were used to measure the increase in height that was achieved by the sinus elevation procedure. In a number of cases (13), the height was remeasured from panoramic radiographs taken over a 2- to 3-year interval postgrafting. The percent change in measured graft height over this interval was +1.4%. These measurements indicate a stable graft volume and show that repneumatization with this graft material is minimal.

Discussion
Graft material

The graft material reported on in this study, OsteoGraf/N, is a xenograft consisting of deproteinated, anorganic bovine bone. The processing of this material includes partial deproteinization using brine solution, followed by sequential calcining and sintering (in excess of 1.000°C) for sterilization and total deproteinization. The final product is a microporous, resorbable hydroxyapatite with chemistry and microtopography that are similar to human cortical/cancellous bone. The material comes in two particle sizes: N/300 and N/700. In the majority of grafts in this study, the particles were mixed in a 1:1 ratio. It was thought that the use of various particle sizes would prevent an overly dense compaction of the graft material and would thus be the most favorable environment for vascular ingrowth. Resorption time in vivo is estimated at 2 to 3 years via cellular and solution-mediated resorption. This material has been successfully used as an intraoral graft material for sinus24 and nonsinus grafting situations.42

Histologic and histomorphometric analysis

It should be noted that all cores were harvested from the lateral window, not from the crest as has been reported in most previous papers. This was done to eliminate the possibility of confusing previously existing crestal bone with bone resulting from the sinus graft. Additionally, taking cores from the superior aspect of the window provides a sample that is distant from the bony walls and presents the worst possible scenario for vital bone formation. The area adjacent to the crest, because it is narrow and close to multiple bony walls, would be expected to provide the most favorable environment for bone formation.

Histologic and histomorphometric analysis was performed on both decalcified and nondecalcified core biopsies in an attempt to determine which method would be most accurate for the determination of the amount of vital bone present. It became apparent that, while both the toluidine blue (decalcified) and the Stevenel's blue/van Gieson's picric fuchsin (nondecalcified) staining techniques were capable of distinguishing vital from nonvital bone, the nondecalcified specimens were easier to count. It was felt that both of these techniques would have an advantage over back-scattered electron imaging,43 which does not differentiate between vital and nonvital bone. In cases involving autogenous grafts this distinction is very important, as the donor graft loses its vitality and is replaced over time. The length of this replacement period is dependent on the cortical/cancellous nature of the donor bone, its endochondral or intramembranous origin, and the volume of the grafted area.

With decalcification of the specimens and hematoxylineosin staining, the distinction between vital and nonvital bone is made by the presence (vital) or absence (nonvital) of cells in the lacunae. A study by Rohrer et al44 has shown that dehydration, paraffin processing, and sectioning at 5 um causes a shrinkage of the osteocytes in the lacunae that can result in as many as 20% empty lacunae in normal vita! bone. Because of the possibility of empty lacunae in vital bone, combined with the fact that the pieces of vital and nonvital bone in these cores are so small, dependence on the presence or absence of cells in lacunae is a very inaccurate method for determining vitality in these specimens.

In addition to being a favorable staining technique for histomorphometric analysis, Stevenel's blue/van Gieson's picric fuchsin staining provides an insight into the biologic processes of graft turnover, bone formation, and mineralization. Figures 1 to 10 demonstrate these histologic events.

Figure 1            Newly formed vital bone (red) bridges particles of OsteoGraf/N at 6 months (Original magnification x 50 Stevenel’s blue/ van Gieson’s picric fushcin stain)
Figure 2            Newly formed vital bone connects particles of OsteoGraf/N at 8 months in decalcified section (Original magnification x 125 hematoxylin-eosin stain)
Figure 3            Newly mineralized vital bone (red) with osteoid (green) between particles of OsteoGraf/N at 6 months (Original magnification x 50 Stevenel’s blue/van Gieson’s picric fushcin stain)
Figure 4            View similar to Fig 3 shown under polarized light. The linear orientation of the collagen matrix of the remineralizing bone is evident in the osteoid (green) and in the immature bone (orange). Osteoblasts line the osteoid layer (arrows) (Original magnification x 50 Stevenel’s blue/ van Gieson’s picric fushcin strain)
Figure 5            Osteoblasts (arrow) line the surface at newly formed bone that is attached to the surface of the OsteoGraf/N (Original magnification x 100. Stevenel’s blue/ van Gieson’s picric fushcin stain)
Figure 6            Polarized view shows bone formation within particles of OsteoGraf/N (Original magnification x 25 Stevene;’s blue/ van Gieson’s picric fushcin stain)

Vital bone formation for each category was reported as both a range and as a mean. While the number of cases evaluated by histomorphometric analysis was large for a sinus elevation study, these statistics will be updated as more cases are processed in this ongoing study.

The wide range of vital bone production for each category of graft material was reduced when factoring in the time of graft harvesting. Cores were generally harvested at stage 2 surgery (6 to 9 months) for cases with immediate implant placement, and at stage 1 surgery (6 to 9 months) and/or stage 2 surgery (12 to 15 months) for cases with delayed implant placement. This allowed our clinical team to harvest core biopsies without performing additional surgical procedures.

The increase in vital bone production with time was substantial. In nine sinuses, there was 24% mean vital bone content after 6 to 9 months (range 9 to 34), and a 33% mean vital bone content after 12 to 15 months (range 10 to 65). The increase in both vital bone volume and graft maturity with time is shown in Figs 11and 12. This increase was likely a result of many factors. It is axiomatic in wound healing that angiogenesis precedes osteogenesis. The vascular supply, and therefore the source of cellular activity, for wound healing in the maxillary sinus comes from the surrounding bony walls. For sinus graft healing, this model was postulated by Boyne and James2 and reported in a primate study by Misch and Dietsh.45 The proliferation of blood vessels is a function of time. The formation of woven bone and its transformation to lamellar bone would be expected to begin in the vicinity of the bony wails and advance toward the central area of the graft. Histologic and histomorphometric analysis of four sinuses grafted with 100% Interpore-200 (Interpore Dental) showed woven and lamellar (composite) bone penetrating the cores to a distance of 2 to 3 mm in 6 months and 5 to 6 mm in 12 months.27 In regard to vascular proliferation rates, the variables of patient age and vascularity may be significant. Another extremely important variable is the volume of the grafted area, as the vascularization of a large graft occurs over a longer time period.

The slower production of vital bone in the cases in which OsteoGraf/N was the sole grafting material was to be expected, especially in large grafts where the distance to the bony walls was greater. This is a result of the fact that a xenograft is designed to replace the inorganic components required for vital bone production. The organic, or cellular, components must be derived from the host blood supply, from other graft materials such as autogenous bone, or from added growth factors. In rhesus monkey studies using autogenous bone mixed in a 3:1 ratio with the porous hydroxyapatite alloplast Interpore-200,46 and in studies in which lnterpore-200 was used alone/7 new bone formation averaged 47.1% and 27.7%, respectively (biopsies were taken at 15 months loaded and at 8 months unloaded). One would expect the volume of bone produced in the rhesus monkey to be greater than that produced in a human in a given period of time because of the anatomically smaller sinus in the monkey and the more rapid bone apposition rate.

Figure 7            New bone formation at 6 months within a granular particle of OsteoGraf/N Note osteocytes within lacunae (arrows) in the immature bone (orange) above the mature bone (red). (Original magnification x 50; Stevenel's blue/van Gieson 's picric fuchsin stain.)
Figure 8            Higher magnification of top of Fig 7 shows bone formation within OsteoGraf/N particle. Note concentric layers of immature bone (orange) and osteoid (green). Osteoblasts trapped in the newly mineralizing bone are now osteocytes (open arrow). Osteoblasts can be seen on the bone surface (closed arrow). (Original magnification x 700 Stevenel's blue/van Gieson's picric fuchsin stain.)
Figure 9            New bone formation within OsteoGraf/N particle at 6 months. Note concentric layers of mature bone (red), immature bone (orange), and osteoid (green). (Original magnification x 50 Stevenel's blue/van Gieson's picric fuchsin stain.)
Figure 10           OsteoGraf/N particle at 12 months. Note multinucleated cells around the periphery (arrows) in decalcified section. (Original magnification x 200; hematoxylin-eosin stain)
Figure 11           Core composed of OsteoGraf/N 300 (40%). OsteoGraf/N 700 (40%) and intraoral autogenous bone (20%) at 6 months in decalcified section (Original magnification x 31.5 hematoxylin-eosin stain)
Figure 12           Core from same patient as Fig 11 at 12 months. Note increase in bone volume in decalcified section (Original magnification x 31.5 hematoxylin-eosin)

The percentage of intraoral autogenous bone in the grafting mixture may also be of  significance to the outcome of this procedure. It has been shown, in an earlier paper by the authors 24 that autogenous bone placed in the sinus appeared to lack vitality at 4 months. This loss of vitality may be associated with the release of bone morphogenetic proteins (BMP) and other growth factors that enhance bone formation by an effect on cell attraction and/or cell differentiation. A greater concentration of autogenous bone in the graft mixture may enhance this osteoinductive effect. In this study, the addition of 20% autogenous bone to the xenograft mix significantly increased the amount of vital bone in the core samples. The minimum amount of autogenous bone that is necessary for this increase to occur could not be determined in this study.

In this study, the use of demineralized freeze-dried bone as a grafting material resulted in a small increase in the production of vital bone in the grafted sinuses. This occurred 6 to 9 months postgrafting. In light of the long turnover time of demineralized freeze-dried bone allograft (DFDBA) (over 4 years for the graft material to be replaced by vital bone) reported by Simion et al, 48 and of the variable amount of BMP found in different samples of commercial DFDBA as reported by Schwartz et al,49 further research into the healing response to DFDBA is warranted. An evaluation of DFDBA as a grafting material is now in preparation.

Bone formation may be increased through modification of the surgical protocol in ways that enhance biologic activity. Elevating the Schneiderian membrane exposes the bony walls that take part in wound healing; by elevating this membrane from the medial wall of the sinus to the height of the lateral window as proposed by Misch,50 an additional surface area of bone is made available for wound healing. Elevating the Schneiderian membrane from the bony wall also acts as a stimulus to elicit a healing response.

Figure 13           Reentry and core harvest at 6 months with no membrane use.Note particulate appearance of graft material.
Figure 14           Reentry and core harvest at 6 months with e-PTFE membrane use (membrane removed to show graft surface). Note contiguity of graft material described by Murray as the "caging effect.”
Figure 15           Core shown in Fig 14 composed of OsteoGraf/N (40%). DFDBA (40%), and intraoral autogenous bone (20%). Note lamellar bone formation beneath the e-PTFE membrane at top in decalcified section. (Original magnification x31.5;  hematoxylin-eosin stain.)
Figure 16           Higher magnification of specimen in Fig 15. (Original magnification x 125.)

Hurzeler and coworkers46, 51 and Quinones et al47 have shown, in sinus lift studies in rhesus monkeys, that the implant bone interface is greater after delayed implant placement than after simultaneous placement. The placement of an implant in the healing sinus graft may act as a second wounding process (the grafting procedure being the first wound), which again initiates a healing response.

As the study progressed, a significant positive effect was recorded with the placement of a stabilized, nonresorbable e-PTFE membrane (GTAM) over the lateral window. This result is in accordance with the positive effects of membrane placement reported by Jensen and Greer.52 This finding favored the use of barrier membranes in cases with limited crestal bone. When a membrane was not used over the lateral window, it was not uncommon to observe loose particles of the graft, sometimes with soft tissue invagination, upon reentry at 6 to 9 months (Fig 13). In most cores obtained when a membrane was in place, the graft material appeared to be contiguous and corticalized (Fig 14). This reaction beneath a membrane, called the "caging effect," was reported by Murray et al.53 In core samples where the membrane can be retained intact, bone formation was apparent directly beneath the membrane. This effect, an increase in the volume of bone, appeared throughout the entire length of the core. Figures 15 and 16 show a core that was taken from the superior, central area of a lateral window that was approximately 15 x 20 mm in size. The results of membrane placement with a variety of graft materials will be discussed in a separate paper that is currently in preparation.

Implant survival

The value of histomorphometric analysis as an evidence-based predicator of clinical implant survival could not be assessed in this study because of the low failure rate for all groups. Only 3 of 82 implants placed in sinus grafts without membranes and 1 of 133 implants placed in sinus grafts with membranes have failed in our study population to date, with an overall survival rate of 98.2%. While an implant survival rate of 98.2% appears high, it is in line with similar studies of implant survival. 8, 13-22 This result is also in line with the reported success rates for implants placed in augmented bone in nonsinus
(ridge) sites.54-56

The time of loading of these cases ranged from 0 (stage 2 surgery) to 48 months. The minimal vital bone value necessary for survival may have been exceeded in this group of patients within the 6- to 9-month protocol. Larger numbers of implants, placed at various postgraft healing times, may shed more light on this question. To date, previous studies on implant survival have not attempted to show a correlation between vital bone volume and implant survival. However, data from our larger patient pool will be analyzed in a future paper to determine if such a correlation exists.  Similarly, a determination of differences in survival based on either implant surface characteristics or amount of crestal bone present at the time of sinus lift surgery could not be performed in this study because of the low failure rate. Implant survival for this group of patients was also not affected by the presence or absence of an e-PTFE membrane over the lateral window, This appears to make a difference with other graft materials, however, as our database for all graft materials showed a substantial increase in success rate when a membrane was used. This data will be published in a separate paper.

Radiographic analysis
The volume of bone produced with the graft material used in this study appeared to be stable over time. Repneumatization of the sinus following the sinus elevation procedure has been reported with other grafting materials such as bone from the iliac crest and DFDBA, Panoramic radiographs were used in our study to evaluate changes in graft height during the maturation, implantation, and maintenance phases of therapy (2 to 3 years after sinus grafting). The range of percentage change in the initial graft height for the 13 cases in which OsteoGraf/N was used was -5.9% to +12.5%. with a mean change of +1.4%. Measurements of two DFDBA grafts showed a loss in height of -15.5% and -16,5%, respectively.

Conclusion
This paper reports the clinical, histologic, radiographic, and histomorphometric results following the use of the xenograft OsteoGraf/N as a grafting material for the sinus elevation procedure. The database for this study included 113 sinus elevation procedures with 215 implants placed and followed to date. OsteoGrat/N was used alone and in combination with other grafting materials. Histologic and histomorphometric analysis was predominantly performed at intervals of 6 to 9 months and 12 to 15 months; the overall time range was from 4 to 20 months. Procedures were performed with and without nonresorbable membranes over the lateral window.

Under the conditions of this study, the results showed that:

1. OsteoGraf/N appears to be an effective graft material for the sinus elevation procedure.
2. Vital bone formation was time dependent.
3. Vital bone formation increased substantially when intraoral autogenous bone was added to the graft mixture.
4. Vital bone formation showed a moderate increase when DFDBA was added to the graft mixture.
5. Vital bone formation increased substantially when a nonresorbabie e-PTFE membrane was placed over the lateral window as compared to cases in which no membrane was used.
6. The increase in bone height achieved with OsteoGraf/N was maintained over a 2- to 3-year period without repneumatization.
7. The relationship between vital bone formation and implant survival could not be determined in this study because of the high rate of implant survival in all groups
(98,2%).
8. The relationship of implant survival rate to use and nonuse of a nonresorbable e-PTFE membrane could not be determined in this study because of the high rate of implant survival.

Further studies are under way to determine if the above trends are universal with other graft materials and other barrier membranes.

Acknowledgment
The authors wish to acknowledge the help of Gloria Turner of the Diagnostic Pathology Laboratory at the New York University College of Dentistry, and of
Research Assistant Hail Prasad. BS, MDT and Senior Histology Technician Tom Sprowl of the Hard Tissue Research Laboratory at the University of Oklahoma
College of Dentistry,

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