Growth Factors in Bone Healing

BMP Family Overview

Bone morphogenetic proteins are members of the TGF-β superfamily and are the most potent osteoinductive growth factors known.

Key characteristics:

• Over 20 BMP family members identified
• BMP-2, BMP-4, BMP-6, BMP-7 have strong osteoinductive activity
• BMP-2 and BMP-7 are most clinically relevant
• Only growth factors that induce ectopic bone formation (true osteoinduction)
• Signal via Smad1/5/8 pathway to activate Runx2

Sources in bone healing:

• Produced by osteoblasts and osteoprogenitor cells
• Stored in bone matrix (released during resorption)
• Platelets contain small amounts
• Peak expression at week 2-3 post-fracture (days 14-21)

BMPs are essential for both fracture healing and skeletal development.

BMP Signaling Pathway (Smad Pathway)

BMP signal transduction follows the canonical Smad pathway:

Inhibitors of BMP signaling:

• Noggin: Binds BMPs extracellularly, prevents receptor binding
• Chordin: Similar to noggin, BMP antagonist, important in development
• Smad6 and Smad7: Inhibitory Smads (I-Smads) that block R-Smad phosphorylation
• Smurf1/2: E3 ubiquitin ligases that degrade Smads and receptors

The Smad pathway is the canonical BMP signaling mechanism for osteoblast differentiation and bone formation.

Clinical Use of Recombinant BMPs

FDA-approved indications:

Mechanism of action:

• BMP bound to collagen carrier placed at fracture/fusion site
• Local sustained release over weeks
• Induces mesenchymal stem cell differentiation to osteoblasts
• Stimulates bone formation without need for autograft
• Overcomes poor osteogenic potential of graft site

Advantages over bone graft:

• No donor site morbidity (iliac crest pain 10-20%, infection, hematoma)
• Unlimited supply (not limited by patient bone stock)
• Consistent osteoinductive potential (autograft quality variable by age, comorbidities)
• Avoids second surgical site (reduces operative time, patient preference)

Evidence base:

• BESTT trial (2002): rhBMP-2 superior to autograft for open tibia fractures, reduced infection
• Spine trials: Non-inferior to autograft for ALIF with less donor site pain
• Meta-analyses: Increased fusion rates (5-10% higher) but also increased complications
• Dose-dependent effects: Higher doses increase both efficacy and complications

rhBMP-2 is highly effective but requires careful patient selection, dose consideration, and informed consent about risks.

TGF-β Superfamily

TGF-β is the most abundant growth factor in bone matrix and a key regulator of bone remodeling and fracture healing.

Three isoforms in mammals:

• TGF-β1 (most abundant in bone, main regulator)
• TGF-β2 (development, wound healing)
• TGF-β3 (anti-scarring properties)

Sources in bone:

• Platelets (released during clotting in fracture hematoma, initiates healing)
• Bone matrix (released during osteoclastic resorption, coupling mechanism)
• Osteoblasts and inflammatory cells actively produce TGF-β
• Most abundant growth factor stored in bone matrix

Secretion and activation:

• Secreted as latent complex (inactive, bound to latency-associated peptide, LAP)
• Activation by proteases (plasmin, matrix metalloproteinases), acidic pH, or mechanical stress
• Active TGF-β binds type II TGF-β receptor (TβR-II)
• Activation is tightly regulated to control local effects

TGF-β has complex, context-dependent effects on bone cells at different stages.

Effects on Bone Cells (Biphasic)

Biphasic effects on osteoblasts:

This biphasic pattern is critical: TGF-β first expands the progenitor pool (more cells available), then must be downregulated to allow terminal differentiation. Persistent high TGF-β blocks bone formation despite abundant progenitors.

Effects on other cell types:

• Mesenchymal stem cells: Chemotactic (recruits cells to healing site via gradient)
• Osteoclasts: Indirect effect via regulating osteoblast production of RANKL and OPG
• Chondrocytes: Promotes proliferation and cartilage matrix synthesis (soft callus)
• Fibroblasts: Stimulates collagen synthesis (can lead to fibrosis if excessive)

TGF-β coordinates the transition from resorption to formation in coupled bone remodeling.

TGF-β Signaling Pathway (Smad2/3)

Smad2/3 pathway (differs from BMP Smad1/5/8):

1. TGF-β binds type II receptor (TGFβR-II, constitutive serine/threonine kinase)
2. Type II recruits and phosphorylates type I receptor (TGFβR-I, also called ALK-5)
3. Activated type I receptor phosphorylates Smad2 and Smad3 (R-Smads specific to TGF-β)
4. Smad2/3 bind Smad4 (Co-Smad, common to both TGF-β and BMP pathways)
5. Complex translocates to nucleus and regulates gene transcription

Key difference from BMP pathway:

• TGF-β uses Smad2/3 (via ALK-5)
• BMPs use Smad1/5/8 (via ALK-2/3/6)
• Both pathways share Smad4 as common mediator
• This allows differential gene activation despite similar receptor mechanisms

Inhibitors:

• Smad7: Inhibitory Smad (I-Smad) specific for TGF-β pathway
• Decorin: Extracellular TGF-β antagonist, binds and sequesters TGF-β
• Follistatin: Binds activins (TGF-β family), prevents receptor binding

Understanding TGF-β signaling explains its role in coordinating bone remodeling and coupling.

VEGF in Bone Healing

VEGF is absolutely essential for bone healing because angiogenesis (blood vessel formation) is coupled to osteogenesis. No vessels means no bone formation.

Key roles in fracture healing:

• Vascular invasion of cartilage callus during endochondral ossification
• Coupling angiogenesis to osteogenesis (spatially and temporally)
• Osteoblast survival (anti-apoptotic effects, prevents cell death)
• Osteoclast recruitment (via indirect RANKL regulation)
• Delivery of osteoprogenitors via Type H vessels

Sources in fracture healing:

• Hypertrophic chondrocytes (highest production, signals vascular invasion of soft callus)
• Osteoblasts (maintain blood supply to forming bone)
• Macrophages and inflammatory cells (early and throughout healing)
• Platelets (released early in hematoma, first wave)

VEGF is the critical molecular link between angiogenesis and osteogenesis in bone healing.

VEGF Receptors and Signaling

VEGF family members:

• VEGF-A (most important for bone, multiple splice variants: VEGF121, VEGF165, VEGF189)
• VEGF-B (limited role in bone)
• VEGF-C, VEGF-D (primarily lymphangiogenesis)
• PlGF (placental growth factor, synergizes with VEGF-A)

Receptors:

• VEGFR-1 (Flt-1): Decoy receptor, low signaling activity, sequesters VEGF
• VEGFR-2 (KDR/Flk-1): Primary signaling receptor on endothelial cells, drives angiogenesis
• VEGFR-3: Primarily lymphatic endothelium

Signaling cascade (VEGFR-2):

1. VEGF-A binds VEGFR-2 on endothelial cells
2. Receptor dimerization and autophosphorylation (tyrosine kinase activation)
3. Activates multiple downstream pathways:
   • PI3K/Akt: Cell survival, migration
   • MAPK: Cell proliferation
   • PLCγ: Calcium signaling, permeability
4. Results in endothelial cell proliferation, migration, and tube formation (angiogenesis)

Regulation of VEGF expression:

• Hypoxia (low oxygen) is the major stimulus via HIF-1α (hypoxia-inducible factor)
• BMPs and TGF-β stimulate VEGF production in osteoblasts
• Inflammatory cytokines (IL-1, TNF-α) increase VEGF in macrophages
• Mechanical stress upregulates VEGF in osteocytes

VEGF signaling through VEGFR-2 is essential for coupling blood vessel growth to bone formation.

Type H Vessels and Osteogenesis

Type H vessels are a specialized subset of capillaries that couple angiogenesis to osteogenesis.

Characteristics of Type H vessels:

• High expression of CD31 (PECAM-1) and Endomucin (CD31-high, Emcn-high)
• Located at metaphysis and in fracture callus
• Strongly pro-osteogenic environment
• Deliver osteoprogenitors from circulation and marrow
• Support perivascular mesenchymal cell differentiation to osteoblasts

Type L vessels (comparison):

• Low expression of CD31 and Endomucin (CD31-low, Emcn-low)
• Located in diaphysis (cortical bone)
• Less osteogenic, primarily nutrient delivery
• Do not actively support bone formation

Mechanism of coupling:

1. VEGF and other factors induce Type H vessel sprouting
2. Type H vessels grow into callus along specific trajectories
3. Perivascular mesenchymal cells associate with vessels
4. Local BMP and Wnt signals drive osteoblast differentiation
5. Bone formation follows vessel network - osteoblasts line up along vessels and deposit osteoid

Type H vessels are the cellular and molecular mechanism linking blood vessel growth to bone formation.

Platelet-Derived Growth Factor (PDGF)

PDGF is the earliest growth factor at the fracture site, released immediately from platelet alpha granules when the fracture hematoma forms.

PDGF isoforms:

• PDGF-AA: Two A chains, binds PDGFR-α
• PDGF-BB: Two B chains, most potent, binds both receptors
• PDGF-AB: Heterodimer
• PDGF-CC, PDGF-DD: Newer isoforms, less studied

Functions in bone healing:

• Chemotaxis: Recruits inflammatory cells (neutrophils, macrophages) to fracture
• MSC recruitment: Attracts mesenchymal stem cells from periosteum, marrow, circulation
• Mitogenic: Stimulates proliferation of fibroblasts, smooth muscle cells, osteoblasts
• Angiogenesis: Indirect effect via inducing VEGF expression in stromal cells
• Matrix synthesis: Stimulates collagen and proteoglycan production

Temporal expression:

• Immediate release from platelets upon fracture (minutes to hours)
• Peak concentration at 24-48 hours in hematoma
• Sustained production by macrophages and fibroblasts during inflammation (days 3-7)
• Declines as inflammation resolves

Signaling pathways:

• Binds PDGF receptor alpha or beta (receptor tyrosine kinases)
• Activates Ras-MAPK pathway (cell proliferation)
• Activates PI3K-Akt pathway (cell survival, migration)
• Activates PLCγ pathway (calcium signaling, cytoskeletal reorganization for migration)

PDGF initiates the early inflammatory and cellular recruitment phases of fracture healing.

Fibroblast Growth Factors (FGFs)

FGF family comprises over 20 members involved in skeletal development, fracture healing, and homeostasis.

Key FGFs in bone:

• FGF-2 (basic FGF, bFGF): Most studied in bone healing
• FGF-18: Cartilage development and homeostasis
• FGF-23: Phosphate homeostasis (produced by osteocytes, regulates kidney phosphate handling)

FGF-2 (bFGF) effects:

• Mitogen for osteoprogenitors, chondrocytes, endothelial cells (cell proliferation)
• Stimulates alkaline phosphatase expression in osteoblasts
• Enhances angiogenesis (synergizes with VEGF for vessel formation)
• Modulates BMP signaling (can enhance or inhibit depending on concentration and context)
• Maintains mesenchymal cell pool (prevents premature terminal differentiation)

Expression in fracture healing:

• Produced by osteoblasts and chondrocytes during repair phases (weeks 2-8)
• Sustained low-level expression during remodeling and homeostasis
• Upregulated by mechanical loading

FGF-18 applications:

• Critical for growth plate chondrocyte proliferation and differentiation
• Maintains articular cartilage (chondroprotective effects)
• Sprifermin (recombinant FGF-18) in clinical trials for knee osteoarthritis

FGFR mutations and disease:

• FGFR3 activating mutations cause achondroplasia (most common skeletal dysplasia, short stature)
• Mechanism: Overactive FGFR3 inhibits chondrocyte proliferation in growth plate
• FGFR1/2 mutations cause craniosynostosis syndromes (premature suture fusion)

FGFs regulate mesenchymal proliferation and maintain progenitor pools during bone healing and development.

Insulin-Like Growth Factors (IGFs) and Other Factors

IGF-1 and IGF-2:

IGF-1 is the primary systemic anabolic factor for bone and cartilage.

IGF-1 functions in bone:

• Stimulates osteoblast proliferation and differentiation
• Increases type I collagen synthesis (matrix production)
• Enhances mineralization of osteoid
• Anti-apoptotic (prolongs osteoblast lifespan, prevents premature death)
• Mediates many skeletal effects of growth hormone (GH)

Sources of IGF-1:

• Endocrine: Liver produces IGF-1 in response to GH, circulates bound to IGFBPs
• Autocrine/paracrine: Osteoblasts produce IGF-1 locally, stored in bone matrix
• Released from bone matrix during osteoclastic resorption

Signaling pathway:

• IGF-1 receptor (IGF-1R, receptor tyrosine kinase, homologous to insulin receptor)
• Activates PI3K-Akt pathway (cell survival, protein synthesis, glucose uptake)
• Activates MAPK pathway (cell proliferation)

IGF Binding Proteins (IGFBPs):

• IGFBP-1 through IGFBP-6 modulate IGF bioavailability
• Sequester IGF in circulation and bone matrix
• Proteolysis of IGFBPs releases active IGF at target sites
• IGFBP-3 is most abundant in circulation

Other growth factors:

IGFs and other factors work synergistically with primary growth factors to regulate all phases of bone healing.

BMP-2 and BMP-7 Clinical Use

BMPs are the only growth factors with robust FDA approval and widespread clinical use in orthopaedics.

rhBMP-2 (InFuse, Medtronic):

• FDA-approved indications:
  1. ALIF (anterior lumbar interbody fusion) single level L4-S1
  2. Open tibial shaft fractures (Gustilo type IIIA, IIIB, IIIC)
  3. Oral maxillofacial reconstructive surgery
• Mechanism: Osteoinduction (induces bone formation in non-skeletal sites, true ectopic bone)
• Delivery: Absorbable collagen sponge (ACS) carrier for sustained release
• Typical dose: 12 mg total for ALIF (1.5 mg/mL concentration on sponge)
• Efficacy: 94-100% fusion rate in ALIF vs 85-90% with autograft
• Advantages: Avoids donor site morbidity (iliac crest pain 10-20%, infection risk)

Evidence:

• BESTT trial (2002): Level I RCT, rhBMP-2 superior to autograft for open tibia fractures
• Reduced infection risk, faster healing, fewer secondary procedures
• Spine trials: Multiple RCTs showing non-inferiority to autograft for fusion
• Industry-funded trials showed higher fusion rates with BMP

Off-label use (controversial, common in practice):

• Posterolateral lumbar fusion
• Cervical spine fusion (FDA warning - airway complications)
• Revision spine surgery
• Long bone nonunions
• Pelvis and acetabular fractures

rhBMP-7 (OP-1, Olympus/Stryker):

• Humanitarian Device Exemption (HDE) for recalcitrant long bone nonunions
• Less availability (withdrawn from many markets)
• Lower osteoinductive potency than BMP-2
• Potentially fewer inflammatory complications
• Requires IRB approval for use in United States

BMP products are highly effective but require careful risk-benefit assessment.

rhBMP-2 Complications and Controversies

Serious complications (rates vary by indication and dose):

Cervical spine complications (FDA Public Health Notification 2008):

• Life-threatening prevertebral soft tissue swelling
• Airway compromise requiring intubation
• Dysphagia, hoarseness, wound complications
• FDA warning against off-label use in anterior cervical spine

Dose-response relationship:

• Higher doses increase fusion rates but also complications
• Some spine surgeons use doses above FDA-approved levels
• "More is not better" - optimal dosing still debated

Understanding complications allows appropriate patient selection and risk mitigation strategies.

Platelet-Rich Plasma (PRP) and Bone Marrow Concentrate (BMC)

Platelet-Rich Plasma (PRP):

Rationale: Concentrate platelets (and growth factors) from autologous blood to deliver supraphysiologic doses of PDGF, TGF-β, VEGF, FGF-2, IGF-1 to healing site.

Preparation methods:

• Centrifuge whole blood to separate platelet-rich layer
• Activate with thrombin, calcium chloride, or freeze-thaw to release granule contents
• Concentration factor: 2-8x platelet baseline (typical target 1 million/μL)

Growth factor content (varies widely by preparation method):

• PDGF-BB: 2-10x baseline concentration
• TGF-β1: 2-5x baseline
• VEGF: 2-10x baseline
• IGF-1, FGF-2: Present in variable amounts

Clinical evidence:

Limitations of PRP:

• Variable preparation methods (no standardization across studies or clinics)
• Platelet-poor plasma may work similarly (physiologic doses may be sufficient)
• Activated platelets release pro-inflammatory mediators, not just growth factors
• Placebo effect substantial (injection procedure, belief in treatment)
• Cost not covered by insurance, patient pays out-of-pocket

Bone Marrow Concentrate (BMC):

Rationale: Concentrate nucleated cells from bone marrow aspirate to deliver mesenchymal stem cells (MSCs) and endogenous growth factors.

Preparation:

• Aspirate bone marrow (typically posterior iliac crest)
• Centrifuge to concentrate nucleated cells (remove RBCs and plasma)
• Inject concentrated cell fraction at treatment site

Contents of BMC:

• MSCs: 0.001-0.01% of nucleated cells (very low concentration)
• Hematopoietic progenitor cells (majority of nucleated cells)
• Growth factors: Lower than PRP (fewer platelets in marrow aspirate)
• Cytokines and chemokines

Clinical evidence:

• Fracture nonunion: Case series show benefit, no high-quality RCTs vs autograft
• Osteoarthritis: Minimal evidence, short-term pain reduction at best, no structural benefit
• Cartilage defects: Limited benefit, inferior to established procedures (ACI, OATS)

Limitations:

• MSC concentration very low (most cells are hematopoietic, not mesenchymal)
• MSC potency declines dramatically with age (limited efficacy in elderly)
• Variable aspiration technique and cell yield between operators
• No FDA-approved indications (used off-label based on surgeon judgment)
• No standardization of preparation or dosing

PRP and BMC are autologous, relatively low-risk adjuncts but lack robust evidence of efficacy for most applications.

Advanced Assessment Modalities

Clinical Decision-Making for BMP Use

Technical Considerations by Application

Complication Management and Prevention

Differential of impaired bone healing (by disrupted biological axis)

When a fracture or fusion fails to heal, reason through which corner of the diamond and which growth-factor axis is disrupted. This is the high-yield differential examiners probe from a "growth factors" stem.

Specific Considerations

Evidence Summary and Meta-Analysis

Global regulatory status of growth-factor products

Approved indications are deliberately narrow and consistent across major regulators; almost all heavy clinical use of rhBMP-2 worldwide is off-label.

Major guideline / advisory positions, side by side

Registry and surveillance evidence

Unlike arthroplasty, growth-factor use is not systematically captured by joint registries (NJR England/Wales, AJRR USA, AOANJRR Australia, SHAR Sweden, Norwegian, NZJR). Safety knowledge therefore comes from regulatory pharmacovigilance and large administrative datasets rather than implant registries.

Global practice variation