Bovine or equine sintered xenograft

Bovine or equine derived bone is a xenograft. It is a carbonate-containing apatite with crystalline architecture and calcium–phosphate ratio similar to that of natural bone mineral in humans.⁵ With time, the sintered xenograft, mineralized bone graft material becomes integrated with bone. It may be slowly replaced by newly formed bone, but because the sintering process increases the crystallinity of the bone particles, it may not clinically resorb and will often be present years after placement.^6–9 When sintered xenograft material is used to graft an extraction site, 6 to 9 months may need to be allowed before placement of the implant, especially if the clinician plans to provisionalize the implant immediately. The relatively inert nature of this material delays revascularization and subsequent bone formation compared with more natural materials such as autogenous bone.

The preparation process for bovine and equine xenograft includes a process called sintering. This is removal of the organic material by heating the particulated bone under pressure. This increases the crystallinity of the graft material close to synthetic hydroxylapatite (HA). By using this process, the xenograft is very slow to resorb. Many clinicians view the extremely slow rate of resorption to reclassify this material as a nonresorbing material in the clinical sense.

Equine bone is sintered at a temperature approximating 900° C. The resulting material is predominantly b-type hydroxyapatite with a Ca to P ratio of 1.67. The Food and Drug Administration believes that anorganic bovine bone and equine sintered xenograft are chemically equivalent (information from Equimatrix manufacturer). There is no protein in these sintered xenografts.

There are several clinical uses for sintered xenograft. These include (but are not limited to) sinus augmentation, lateral ridge augmentation, limited vertical ridge augmentation, and onlay grafting sites where the xenograft is placed over allograft to maintain ridge contour.

Mineralized bone allograft

Human mineralized bone in particulate form can preserve most of an extraction site’s bone bulk and volume in preparation for the placement of implants. The advantages of an allograft are (1) the graft material is readily available without the need for a second surgical harvest site, and (2) the material is osteoconductive. Over time, the allograft resorbs and, it is hoped, is replaced with bone.

Human mineralized bone is available as particulate cortical or cancellous bone. The recommended particle size ranges from is 250 µm to 1.0 mm. The author’s preference is particle sizes from 350 to 800 µm. Particles smaller than 250 µm tend to flow with blood out of the site, and larger particles can be shed through the sites. Larger particles mixed with smaller sized particles are useful in larger graft defects. Allografts are prepared by bone banks. Sterile procedures are used to harvest the bone, which is washed with a series of delipidizing agents such as ethers and alcohol, lyophilized, and then sieved to the particle size necessary for a specific indication. The freeze-dried mineralized bone allograft usually is irradiated to sterilize it even though the entire process for harvesting to packaging is performed under strict sterile conditions. Comparative reports and clinical results involving different methods of processing mineralized bone are limited. The choice of allograft should be based on the ease of delivery, cost, consistency in the appearance of the graft material, and quality of the bone bank.

When placed in an extraction site, mineralized bone graft material is still present at 4 months.¹⁰ However, the bone forming around the mineralized bone particles usually is sufficiently mineralized to allow immediate provisionalization, with adequate primary stability after placement of the implant in the extraction site grafted with a mineralized allograft.

One goal of grafting of the extraction site is retention and preservation of the original ridge form and maintenance of the crestal bone after the implants have been restored. In one study in which no membrane was used at the time of extraction site grafting, the grafted sites felt “bone hard” at 4 months and appeared to be filled with bone.¹⁰ The average mesial crestal bone level was −0.66 ± 0.67 mm (range, 0 to −1.27 mm) at implant placement and 0.51 ± 0.41 mm (range, 0 to −1.91 mm) at final restoration. The average distal crestal bone level was −0.48 ± 0.68 mm (range, 0.64 to −1.91 mm) at implant placement and 0.48 ± 0.53 mm (range, 0 to 1.27 mm) at final restoration. A measurement of 1.27 mm from the top of the shoulder of the implants correlated to the level of the first thread of the implant.¹⁰ Bone heights were maintained with mineralized bone graft material.

The current technique for premolars, canines, incisors, and the maxillary palatal root sites advocates the additional use of a fast-resorbing material to retain the graft and promote epithelialization over the graft. The graft can be covered with a fast-resorbing hemostatic collagen material that resorbs in less than 7 days.¹⁰ In mandible molar sites and for coverage of the buccal root sites for maxillary molars, coverage with advancement of the gingiva is recommended with the periosteum used to retain the graft in the extraction socket.

Allograft is particularly useful material for sinus augmentation when combined with bone morphogenetic protein (BMP) and intraosseous defects when bone is desired within a relatively short period of time. When used in large defects, the patient may need more than 4 months for bone consolidation. Larger bone defects may require 6 months before implant or additional graft placement. When the labial or facial bone is not present or minimally present, allograft may resorb, resulting in a flat ridge rather than a ridge with convex form. In these situations, an onlay of sintered xenograft is recommended.

Autogenous bone

Clinicians believe that the ideal bone replacement graft material has always been autogenous bone.11–14 Very few clinicians use a separate harvest site to obtain autogenous bone to graft extraction sites. The past use of bone harvested from the symphysis, ramus, or maxillary tuberosity is not a common, current procedure. Bone removed during alveoloplasty can be used as a graft. Bone can be scraped from adjacent sites, collected in a sieve after the bone has been shaved with a bur and collected with rongeur forceps from adjacent sites or the alveolar ridge.¹⁵

Past animal studies by Block and Kent¹⁶ showed that the addition of autogenous bone to HA particles results in more bone formation within the graft compared with HA used alone. This dog study can be extended to sintered xenograft, which because of the sintering process has similar chemistry to synthetic HA. Hallman et al.¹⁷ showed in sinus augmentation the advantage of adding autogenous bone to bovine xenograft. Hellem¹⁸ showed similar advantages to adding autogenous bone to bovine xenograft for ridge augmentation. Autogenous bone can add bone within a graft of xenograft particles, which is a perceived advantage when implants are planned to be placed within the graft.

The decision to harvest autogenous bone usually is made before the tooth is extracted. The harvesting of autogenous bone is reserved for patients who need autogenous bone for extended grafts or to promote bone formation when using xenografts. Incision designs should consider the need for subperiosteal tunneling or separate incisions to allow harvesting of bone. When multiple teeth are extracted, alveoloplasty can be performed and the particulated bone placed within the extraction sites. An alternative to using alveoloplasty bone is to use a subperiosteal tunnel and one of the available bone-scraping devices to collect bone from the external oblique ridge. Another alternative is to collect bone into a sieve placed in a suction line. Bone particles can be collected from implant preparation drills or by the use of a round bur in the chin or body or ramus region.⁵

Clinical studies and case reports indicate that comparable end results may be obtained with appropriately used nonautogenous grafting materials.^15,19–21

Gingival margin position

The level of the gingival margin over the implant’s final restoration is influenced by the gingival level before tooth extraction. If the gingival facial margin is ideal, the final implant restoration has an excellent prognosis, especially in a patient with thick gingiva. However, if the facial gingival margin is apically positioned and is not ideal because of crestal bone loss secondary to periodontal disease, attrition, or decay, or restorations invading the zone of biologic width, the final restorative facial gingival margin levels have a guarded prognosis unless interventional treatment is performed to correct or mask the gingival margin levels before tooth extraction (Figure 7-1).

If the gingival margin on the tooth to be extracted is apical to the ideal position for the planned esthetic restoration, the tooth may need to be extruded orthodontically; bone moved coronally using interpositional osteotomies; or in some situations, grafting an isolated horizontal concomitant deficiency can result with 1 to 2 mm of gingival coronal fill. Isolated labial bone defects can be grafted. If the tooth is extracted and the gingival margin is apical to the ideal level, the gingiva has a very high probability to be at a compromised location on the final restoration. Grafting bone defects at the time of tooth extraction does not usually correct problems with the gingival margin location.

With intact labial bone at the time of tooth removal, the gingival margin can be adjusted 1 to 2 mm coronally. These situations involve implant placement with grafting the gap between the implant and labial cortical bone with sintered xenograft at the time of tooth removal. The use of a space-filling vacuum form can be added to the treatment to create a negative pressure within the vacuum form, resulting in gingival hyperplasia and an additional 1⁄2 to 1 mm of soft tissue generation. Adjunctive procedures to mask this may require crown lengthening of the adjacent tooth to create symmetry.

If the patient has a thicker rather than thin gingival thickness and if there is labial bone present but the gingival margin is 1 to 1.5 mm apical to desired, the use of a combination of procedures at the time of tooth removal, including implant placement, xenograft grafting, and the use of concave healing abutments, can result in small movements of the gingiva coronally. This method is illustrated in the cases in this chapter.

Level of bone on adjacent tooth

Clinical evaluations by Tarnow et al.²² and Ryser et al.²³ confirm that the most important factor that predicts the presence of papilla between a tooth and an implant is the distance from the contact point of the final restoration to the level of bone on the adjacent tooth. The distance from the contact point to the level of bone on the implant itself is less discriminating. If the bone level on the adjacent tooth is at the cementoenamel junction (CEJ), the papilla will be adequate as long as the proportions of the final restoration are reasonable.

If the bone on the adjacent tooth is apical to the CEJ, then less desirable soft tissue migration may occur. The clinician must take into consideration bone crestal remodeling around an implant abutment interface. Implants with a straight interface without medializing the abutment implant interface will eventually have 1 to 3 mm of apical bone movement. In the interdental region this will add to the loss of papilla when the adjacent teeth have mesial or distal bone loss. The use of an implant that maintains bone at its interface, and the use of a concave abutment allows the subgingival concave shape to develop thick tissue around the concave shaped abutment. The final result will be at least maintenance of the preoperative gingival levels.

Presence or absence of root prominence

For a patient with a high smile line, the gingival morphology apical to the gingival margin usually has a convex form, known as the root prominence. When a tooth is extracted and the site is not grafted, some degree of labial bone loss occurs, resulting in a flat ridge form rather than the convex root prominence. Grafting the extraction site may help preserve the prominence of the root, which enhances the esthetics of an implant restoration in the esthetic zone. If sufficient bone is present for implant placement but the ridge form is still flat, the ridge form is augmented with a clinically nonresorbable material, such as a sintered xenograft. Patients with thick gingiva have less need of augmentation of the root prominence because the thickness of the gingiva aids the final esthetic appearance. Root prominence augmentation is important primarily in patients with high smile lines and the thin gingiva phenotype. In addition, the use of concave abutments promotes thick tissue generation at the crest, resulting in a more esthetic root prominence appearance.

If there is loss of labial bone, an allograft is typically placed into the extraction site. Allograft will resorb and remodel, resulting in a flat contour. In this situation, the use of a sintered xenograft placed over the allograft at the time of extraction can result in an excellent long-term ridge contour.

Proportions of tooth to replace with regard to adjacent teeth

In the preoperative evaluation, if the new tooth restoration is planned to be longer or shorter than the tooth to be extracted, the implant’s vertical position may need to take into consideration the position of the planned gingival margin, which may be more apical than the current tooth’s position. If the tooth proportions indicate that a more coronal positioning is required, appropriate grafting may be necessary to achieve the desired result. If the implant is placed too superficially and the esthetic restoration requires lengthening of the tooth without moving the incisive edge, the resultant problem is caused by improper vertical positioning of the implant. It is critical to place the implant with the final crown form illustrated by a guide stent, as determined from preoperative planning using ideal crown proportions.

Local deficiency of bone in the implant site

If bone loss has occurred anywhere around the tooth to be extracted, including apical bone, labial bone concavities, and labial or palatal cortical bone, grafting of the site is indicated. If previous surgical procedures have been performed on the tooth to be extracted, such as an apicoectomy or periodontal bone reduction, or if the tooth has a history of previous avulsion and replacement, the clinician must assume that the bone around the tooth has a local deficiency.

Cone-beam scanning is essential to determine the amount of bone available without a surgical surprise. Apical procedures may result in concavities, which have a direct effect on implant positioning and stability. If an apical bone concavity or labial bone loss is expected, grafting can be performed at the time of the extraction to augment the site before the implant is placed (Figures 7-2 and 7-3). Careful evaluation of these sites is critical so that the necessary surgical procedures can be performed and the desired result achieved. Imaging may be necessary with a tooth in the planned position to assess bone.

Labial defects

A labial defect is a common finding when a tooth is extracted. Labial defects can be classified according to the amount of bone loss.

With less than 3 mm of labial bone loss, the gingival margin may be adequate or may be apical depending on the bone loss and the presence of thin gingiva, which tends to recede with the bone. If the gingival margin pre-extraction is apical, the final esthetic result will benefit from orthodontic extrusion; otherwise, the patient and restorative dentist must realize that the final result may have a gingival margin slightly more apical than ideal. In these situations, depending on the lip line and the adjacent teeth, adjunctive masking procedures can be used. The adjacent tooth can be lengthened to match the implant crown. Pink material can be placed on the labial surface of the restoration. No treatment is considered if the final result is satisfactory to the patient. Because the implant is placed 3 mm apical to the gingival margin of the planned restoration, a small amount of labial bone loss may not be an esthetic disaster, especially in patients with a relatively normal starting gingival margin position. It is important to choose an implant that does not routinely have crestal bone loss secondary to a flush and not media-lized abutment implant interface. Implants with conical internal connections anecdotally have less crestal bone loss than flush type connections even in the presence of platform medicalization.

With more than 3 mm but less than 6 mm of labial bone loss, the implant usually can be placed in the appropriate position, but a bone defect may exist, with exposure of the implant threads. The clinician has two choices with labial bone loss after extraction of the tooth: (1) the site can be grafted with mineralized allograft bone with an overlay of sintered xenograft, with the patient returning 16 weeks later for implant placement (see Figure 7-3), or (2) the implant can be placed and the labial defect grafted with or without placement of a membrane to maintain the graft’s position. As the labial bone loss increases, the chance to obtain an ideal, perfect long-lasting gingival margin position diminishes. With moderate labial bone loss (3–6 mm), immediate provisionalization of the implant should be performed only if the radiofrequency index is greater than 60.²⁴ In general, the more bone loss and less healthy gingiva, the less predictable is the resolution of the gingival compromise if an aggressive implant placement and graft scenario is pursued.

With greater than 6 mm of labial bone loss, implant placement is not as predictable because less labial bone is present, and the chance of micromotion affecting implant integration is higher. In this scenario, the site is grafted, and after the graft has consolidated, an implant is placed (Figures 7-4 to 7-6).

Surgical technique

When the treatment plan calls for extraction and grafting without immediate implant placement, an Essix type of a vacuum formed clear thermoformed plastic material temporary prosthesis should be made to provide the patient with immediate temporization with a removable tooth supported device that can be made with no pressure on the surgical site. The crown within the Essix gently approximates to the papilla to provide support without putting pressure on the crestal aspect of the ridge. Tissue-borne temporaries are not recommended. Sixteen weeks after extraction and grafting, the implant can be placed and immediately provisionalized if indicated (Table 7-1).