Bone Grafting Materials for Ridge Preservation

Bone grafting in humans was first reported more than 350 years ago by Dutch surgeon Job van Meekeren.^1,2 He corrected a traumatic defect in a soldier’s cranium using a xenograft, harvested from a dog’s skull. Since then, bone grafting techniques have become sophisticated, utilizing many types of bone material, barrier membranes, and growth factors to restore form and function to damaged bone.

This damage may occur through trauma, pathology, and even tooth loss. Historically when teeth were lost, they were replaced with various removable prosthetics or not at all. The damage that occurred to alveolar bone secondary to the extraction and healing process was expected, and little was done to restore the alveolar housing to its original form. However, with the advent of dental implants, it became apparent that ensuring proper form to the residual alveolar ridge would be critical to returning the patient to proper function through dental implant prosthetics.

What the Evidence Shows

Studies were first carried out in the 1960s to look at the processes involved in alveolar bone remodeling following tooth extraction.^3,4 Numerous authors over the years have reported data on the consequences of this alveolar remodeling. It was shown that bone loss due to remodeling could approach 56% on the buccal, 30% on the lingual, and the overall ridge width could be reduced by 50%.^5­­-7

Tan et al⁷ conducted a systematic review to determine both the hard and soft tissue changes that occurred over 12 months following tooth extraction. They found that up to 63% of horizontal bone loss and 22% of vertical bone loss were present; most of this was seen in the first 3 to 6 months post-extraction.

This pattern of remodeling lingualizes the crest of the ridge and presents challenges when considering implant therapy. Alveolar ridge preservation can reduce the effects of bone remodeling following tooth extraction, thus providing a more predictable and straight forward implant planning process.^8,9

Dental implants are a popular choice for tooth replacement both among patients and practitioners. Considering the impact of tooth extraction on the residual ridge over time, it’s important to closely evaluate the necessity and frequency of bone grafting when opting for dental implants. Cha et al¹⁰ reported in their study of 1,512 implants that more than half required bone grafting.

Clearly the need for effective alveolar ridge preservation following tooth extraction in a future implant patient is a necessity. And if this is true, then using the best bone grafting material for individual patient needs is paramount to success. While it is difficult to say if one technique is superior to another,⁸ educated decisions can be based on bone grafting material characteristics.

Discussion of Materials

Bone grafting materials can be broken down into two categories:

1. Human bone
2. Bone substitutes

These are classified further into four subcategories (see Table 1):

1. Autografts
2. Xenografts
3. Allografts
4. Alloplasts

Autografts

All bone grafting materials should posses some properties such as biocompatibility, safety, geometry, handling, surface characteristics, osteogenesis, osteoinductivity, and osteoconductivity (Table 2).¹¹ The ideal grafting material would offer all of these. Autografts are the only material considered close to ideal, while the other types possess some but not all of these properties. Understanding osteogenesis, osteoinduction, osteoconduction, and substitution rate and how they impact the success of alveolar ridge preservation is key.

An osteogenic graft contains mesenchymal osteoprogenitor cells (MSC), which are capable of laying down new bone once they differentiate into osteoblasts. Grafting material that is osteoinductive will recruit MSC and induce them to differentiate into mature bone-forming osteoblasts.

A graft that creates a scaffold that facilitates three-dimensional tissue and cell ingrowth is considered osteoconductive.¹¹ The rate of substitution of the graft material with host bone is a critical part of the planning process that is often not considered. Choosing a material that has a low substitution rate will maintain the volume of the alveolar housing.

Substitution rate is defined as the rate at which the material is resorbed and replaced by host bone. This substitution rate must match the rate at which the body produces bone to fill the void. If the graft material resorbs before the host bone fills the void, then alveolar volume loss will occur.

Using Autogenous Bone

When choosing a graft material for alveolar ridge preservation, using autogenous bone is appealing because it contains cells that can ultimately produce bone, provides an excellent scaffolding, and contains cytokines that will recruit and induce MSC.¹¹ However, its substitution rate is high and does not correspond to the rate of new bone formation. Because of this, it does not prevent loss of alveolar volume.

Wang et al¹² looked at autogenous bone’s ability to preserve the alveolar ridge following tooth extraction. They found that 25% of the coronal volume was lost. If the goal of alveolar ridge preservation is to preserve the residual ridge by preventing volume loss, then autogenous bone is not a desirable choice. Certainly, the additional cost and time required to harvest this bone are additional contraindications to the use of autogenous bone.

Xenografts

The most utilized xenograft is anorganic bovine bone mineral (ABBM).¹³ Bovine bone has been studied extensively in bone grafting and is considered an excellent scaffold with a low substitution rate.¹³ Due to its low substitution rate, bovine bone is an excellent space maintainer. ABBM is widely used in procedures, such as guided bone regeneration and sinus augmentation, for this property.

Wang and his group¹² found that unfilled sockets had three times the amount of horizontal bone loss compared to those filled with a xenograft, but it served only as a scaffold and did not stimulate new bone formation. A meta-analysis that looked at biomaterials for alveolar ridge preservation found that xenografts performed better than other materials in preserving al­ve­olar ridge dimension.¹³

Perhaps the best choice for volume maintenance is a xeno­graft. However, xenografts cannot stimulate the formation of new bone. Their resorption can be so slow, so it may remain years after its placement. A longer healing time would be required to assure that adequate amounts of host bone had filled the socket prior to implant placement.

Allografts

Allografts provide properties found in both autogenous bone and xenografts. Several types of allografts with varying attributes are available (Table 3). An understanding of these properties enables the clinician to best choose which type of allograft will perform best in each circumstance. This article focuses on the properties that enhance alveolar volume, maintain space through a favorable substitution rate, and induce bone formation through osteoinduction and osteoconduction.

Mineralized/demineralized cortical cancellous material provides these characteristics more so than the other allografts. Mineralized cortical bone provides excellent space maintenance and volume enhancement. A study has shown that 5 months following alveolar ridge preservation, the volume change in the alveolar housing was similar when using ABBM or mineralized cortical allograft.¹⁴

Wood et al¹⁵ compared demineralized bone to mineralized bone in alveolar ridge preservation and found that demineralized corticol bone provides excellent osteoinduction and osteoconduction, leading to the most vital bone formation over the shortest time compared to mineralized cortical bone.

These studies show that mineralized allograft is excellent at space maintenance and demineralized corticol bone is effective at stimulating vital bone growth. But they cannot be used individually as they lack other necessary qualities.

Another study compared two groups. In the first group, 100% mineralized freeze-dried corticol allograft was placed in fresh first molar sockets. In the second group, 70% mineralized freeze-dried allograft and 30% demineralized allograft were placed in fresh first molar sockets. The conclusion was that the combination graft created more vital bone while achieving similar alveolar dimensional stability as the 100% mineralized graft.¹⁶ A combination graft — such as this — will offer the desired properties of an ideal graft, except for osteogenesis.

Alloplasts

Some patients will not accept an allograft or a xenograft for various reasons. As mentioned, autogenous grafts are not ideal for alveolar ridge preservation and the additional surgery to harvest the bone is not practical. An alternative for these patients is an alloplast.

While historically this has not been the most ideal choice because alloplasts were not osteoinductive, today an osteoinductive alloplast is available.¹¹ This alloplast is osteoinductive, osteoconductive, and has a favorable substitution rate.

Alloplasts in general have been shown to preserve alveolar volume and create a more favorable residual ridge compared to no alveolar ridge preservation.¹⁷ While more research is needed, this relatively new osteoinductive alloplast seems to be a viable option for alveolar ridge preservation for patients who refuse an allograft or xenograft.

Conclusions

The loss of alveolar volume and architecture following tooth extraction is well established. Alveolar ridge preservation is a viable procedure for the prevention of this deleterious change to the alveolar ridge.

Selecting the most appropriate graft material will lead to the best results. The horizontal bone loss will be minimized, and a higher percentage of vital bone will be present in the healed socket. This will facilitate future implant placement in a restoratively driven location, leading to better biomechanics and a more predictable long-term treatment outcome.

A combination graft of 70% mineralized and 30% demineralized allograft will provide the most favorable results with all things being equal. It will maintain space and enhance ridge volume through a favorable substitution rate. This allograft will facilitate the formation of the most vital bone in the shortest time due to its high osteoinductive and osteoconductive potential and it meets the other requirements of biocompatibility, safety, and handling. For most alveolar ridge preservation procedures, this allograft may be the best choice.

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References

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From Decisions in Dentistry. April/May 2024; 10(3):28-31