PASS Principles for Predictable Bone Regeneration

Guided Bone Regeneration (GBR) is one of the most transformative advances in modern implant dentistry. By applying the concepts of Guided Tissue Regeneration (GTR) and tailoring them to hard tissue healing, GBR has enabled clinicians to regenerate bone in deficient areas, making implant placement possible in situations where it once was unfeasible.

This technique has proven effective in both partially and fully edentulous patients and is grounded in four essential biologic principles collectively refeinforred to as the PASS principles. These include:

• Primary wound closure
• Angiogenesis (blood supply)
• Space creation and maintenance
• Stability of both the wound and implant

Understanding these principles in depth not only enhances the predictability of bone regeneration outcomes but also helps clinicians troubleshoot complications and refine surgical protocols for long-term success.

Guided Bone Regeneration: The Foundation

GBR is a surgical technique that promotes the growth of new alveolar bone by using barrier membranes and bone graft materials to protect and guide the healing process. The primary objective is to selectively facilitate the proliferation of osteogenic cells—mainly osteoblasts—while excluding undesired soft tissue cells such as fibroblasts and epithelial cells that can interfere with proper bone healing.

In GBR, membranes act as protective barriers, enabling the formation of new bone in areas with deficient volume. The concept builds upon the work of Melcher, who established that regeneration is possible when cell types with regenerative capacity are given exclusive access to a wound. This was later validated in animal studies, where bony defects were successfully closed using membranes to exclude competing soft tissues.

With successful GBR, implants can now be placed in previously compromised sites with high survival rates—often above 95%. However, the success of GBR is not solely dependent on technique. It requires adherence to a solid biologic framework—the PASS principles—to ensure consistent and predictable results.

Primary Wound Closure: Creating a Stable Healing Environment

The first principle—Primary wound closure—is a critical surgical goal in GBR. Wound healing can occur through primary intention or secondary intention. Primary intention refers to healing where the wound edges are closely approximated, allowing for faster healing, minimal scarring, and reduced tissue remodeling. Secondary intention occurs when the wound edges cannot be brought together, resulting in prolonged healing, increased collagen deposition, and a higher likelihood of scarring.

Although achieving perfect primary closure can be challenging, it is essential for GBR procedures. When the surgical site is sealed, the internal environment remains undisturbed, minimizing contamination from oral bacteria and reducing mechanical disruption. This quiet environment is ideal for the wound to heal in a predictable manner.

Studies show that membrane exposure is detrimental to bone formation. For example, Machtei’s meta-analysis revealed that submerged membranes produced an average of 3.01 mm of new bone, whereas exposed membranes generated only 0.56 mm. Membrane exposure leads to the invasion of oral microflora, increased inflammation, accelerated resorption of graft materials, and a general decrease in regenerative outcomes.

Factors That Hinder Wound Closure

Several factors interfere with proper wound closure, including:

• Excessive tension on the flap
• Inadequate flap design
• Necrotic tissue or residual infection
• Foreign body contamination
• Poor blood supply at the wound margins

These factors increase the risk of membrane exposure and compromise bone regeneration. Therefore, careful surgical planning is critical.

Techniques for Achieving Tension-Free Closure

Numerous techniques have been developed to support tension-free closure:

• Lateral incision: Moves the incision line away from the grafted site.
• Buccal rotational flap: Provides additional soft tissue without compromising the blood supply.
• Palatal sliding flap and split palatal rotated flap: Allow coronal advancement of tissue from the palate.
• Palatal advanced flap: Enhances tissue volume in areas with limited keratinized gingiva.

Using collagen membranes also supports primary wound closure. These membranes encourage fibroblast migration, stabilize the clot, and prevent epithelial down-growth.

Fig. 1. Principles of successful GBR.

Angiogenesis: The Lifeline of Regeneration

The second PASS principle is angiogenesis, or the development of new blood vessels. Bone is a living tissue that requires oxygen, nutrients, and a continuous blood supply. Without vascularization, bone grafts will not integrate, and healing will be compromised.

Following GBR surgery, the healing sequence involves:

1. Formation of a blood clot within the grafted site
2. Infiltration of inflammatory cells like neutrophils and macrophages
3. Development of granulation tissue, which is rich in capillaries
4. Deposition of osteoid, the precursor to woven bone
5. Gradual remodeling into lamellar bone

This progression underscores the need for stable blood flow and cellular migration into the defect area. Importantly, osteogenic cells originate from three primary sources:

• Periosteum
• Endosteum
• Bone marrow

To enhance blood vessel infiltration and access to these progenitor cells, surgeons often employ decortication, or perforation of the cortical bone. This technique facilitates:

• Communication with the bone marrow
• Enhanced blood flow
• Mechanical interlocking with the regenerating bone
• Release of growth factors such as platelet-derived growth factor (PDGF) and bone morphogenetic proteins (BMPs)

Debating the Necessity of Decortication

While decortication has shown promising results in animal studies, evidence in humans remains inconclusive. Some studies suggest it significantly accelerates healing, while others report no noticeable differences. Nonetheless, the principle remains: a well-vascularized site is critical for successful GBR.

Space Creation and Maintenance: Making Room for Bone

The third principle of PASS—Space creation and maintenance—is fundamental to the success of GBR. Bone formation requires space for osteoblasts to populate and lay down new matrix. If this space collapses due to soft tissue pressure or lack of structural support, the regenerative process is halted or greatly reduced.

Role of Membranes in Space Maintenance

Membranes serve not only as barriers but also as tenting devices to preserve space for regeneration. Their effectiveness varies depending on their material properties:

• Resorbable collagen membranes: Biocompatible and do not require second surgery, but may collapse if not adequately supported.
• Nonresorbable reinforced membranes (e.g., titanium-reinforced PTFE): Offer excellent structural support but require surgical removal.
• Customized titanium mesh: Allows for precise shaping and control over the regenerative space, ideal for larger defects.

Graft Materials as Structural Supports

Bone graft materials play a crucial role in maintaining space beneath the membrane. They may also contribute biologically via:

• Osteoconduction (serving as a scaffold for new bone)
• Osteoinduction (stimulating progenitor cells)
• Osteogenesis (providing live cells, in the case of autografts)

However, their primary role in GBR is structural—keeping the membrane tented and stable.

Studies comparing membranes alone versus membranes plus grafts show superior outcomes in the latter group. For example, when absorbable membranes were used without grafts in non-space-making defects, the results were poor due to membrane collapse.

Preventing Membrane Collapse

To prevent collapse, clinicians may use:

• Bone graft materials under the membrane
• Tenting screws or pins
• Internal frameworks built into the membrane
• Coronally advanced flaps to reduce pressure

Fig. 2. Case No. 2. Augmentation of horizontal ridge defect in conjunction with implant placement. A, Preoperative view showing inadequate ridge width and height. B, Presurgical radiograph illustrated potential apical lesion. C, Initial incisions depict 2 divergent vertical releasing incisions. D, Surgical view showing ridge defects with granulomatous tissues. E, Area was de´ brided to the bare bone. F, Initial implant drill following the surgical guide to indicate ideal buccolingual location. G, Implant placement with horizontal ridge deficiency. H, Intra-bone marrow penetration using half-round bur. I, Sandwich bone augmentation. First layer of bone graft aimed at promoting better bone to implant contact (human mineralized cancellous bone allograft, Puros; Zimmer Dental Inc.). J, Sandwich bone augmentation. Second layer of bone graft aimed at creating/maintaining space (human mineralized bone cortical allograft, Puros) was used for barrier support and space creation, both horizontally and vertically. K, Sandwich bone augmentation. Outer layer used for barrier support and space creation, both horizontally and vertically (collagen membrane, BioMend Extend). L, Suture with 4-0 and 5-0 Vicryl suture (primary coverage with passive flap tension). M, Four-week healing indicated uneventful healing. N, Reentry at 6 months showing new bone formation.

Stability: The Final Piece of the Puzzle

The fourth and final principle is Stability—both of the implant and the regenerative site. Initial stability ensures that the blood clot formed after surgery remains undisturbed, allowing for the proper progression through the healing stages.

Importance of Clot Stability

A stable blood clot is rich in growth factors, signaling molecules, and inflammatory mediators. It initiates the formation of granulation tissue, which transitions into woven and then lamellar bone.

Membranes, especially those with high integrity, help immobilize the clot and seal the area, thereby reducing the risk of micromotion and tissue ingrowth.

Implant Stability and Long-Term Success

For implants, primary stability (mechanical engagement with bone) is crucial. Lack of stability leads to:

• Micromotion
• Fibrous tissue formation
• Failed osseointegration

Methods to assess implant stability include:

• Tactile assessment during surgery
• Torque testing
• Resonance Frequency Analysis (RFA), a non-invasive method that measures vibration response to determine fixture stability

Lower RFA values correlate with higher risk of implant failure. Studies have shown that RFA can effectively monitor stability throughout the healing process.

Surgical Technique: Practical Application of the PASS Principles

A clinical case illustrates how the PASS principles come together in real-life surgery:

1. Assessment and Planning (Fig. 2A–B)
   Radiographs and clinical examination reveal a horizontal ridge deficiency.
2. Incision and Flap Design (Fig. 2C–D)
   Vertical releasing incisions are placed to allow access and soft tissue mobility.
3. Flap Reflection and Periosteal Releasing (Fig. 2E)
   Full-thickness flaps are reflected, and periosteal scoring allows tension-free closure.
4. Debridement and Implant Site Preparation (Fig. 2F–G)
   Granulation tissue is removed, and implants are placed using a surgical guide.
5. Cortical Perforation (Fig. 2H)
   Decortication is performed to encourage angiogenesis and cell migration.
6. Bone Grafting (Fig. 2I–J)
   A layered grafting technique is used (“sandwich augmentation”) to improve contact and support.
7. Membrane Placement (Fig. 2K)
   A collagen membrane is adapted over the graft and defect area.
8. Closure and Suturing (Fig. 2L)
   The flap is repositioned and closed with resorbable sutures under no tension.
9. Postoperative Follow-Up (Fig. 2M–N)
   Healing is monitored at 4 weeks and evaluated at 6 months for bone fill.

Postoperative Care and Monitoring

Postoperative care is essential for GBR success. The protocol typically includes:

• Antibiotics (e.g., Amoxicillin 2 g/day for 10 days)
• Warm saltwater rinses for 2–3 weeks
• Chlorhexidine rinses (0.12%) afterward
• Avoiding mechanical trauma to the site
• Suture removal at 10–14 days
• Bi-weekly follow-up for the first 2 months
• Final evaluation at 4–6 months before implant loading

Conclusion: The Power of PASS

The PASS principles provide a biologically sound and clinically proven framework for predictable bone regeneration:

• Primary Closure minimizes disruption and contamination.
• Angiogenesis ensures a robust blood supply and cellular support.
• Space Maintenance provides room for osteogenesis.
• Stability locks the clot and implant in place for successful integration.

By respecting these principles and applying meticulous surgical technique, clinicians can regenerate bone in even the most challenging sites. GBR is no longer a risky gamble—it is a reliable solution when the biologic groundwork is respected.

The original article published by: Hom-Lay Wang, DDS, MSD, and Lakshmi Boyapati, BDS†