Orthobiologics and Stem Cells in Orthopaedics – Principles, Applications and Evidence

Orthobiologics are biologically derived substances used to enhance or modulate the healing of bone, cartilage, tendon, ligament and other musculoskeletal tissues. They may be obtained from the patient, from donor tissue, or produced using biological and tissue-engineering techniques.

Common orthobiologic strategies include bone grafts, platelet-rich plasma (PRP), bone marrow aspirate concentrate (BMAC), growth factors, cell-based therapies, scaffolds and tissue-engineered constructs.

The term regenerative medicine is broader and refers to strategies intended to repair, replace or regenerate damaged cells, tissues or organs.

Orthobiologics aim to improve the biological environment for healing; they do not replace the fundamental principles of mechanical stability, alignment, vascularity and appropriate rehabilitation.

Musculoskeletal healing depends on both mechanical and biological factors. Even technically excellent fixation may fail if the biological environment is severely compromised.

Orthobiologics attempt to improve one or more components of healing by providing:

• Osteogenic cells.
• Osteoinductive signalling molecules.
• Osteoconductive scaffolds.
• Growth factors.
• Anti-inflammatory or immunomodulatory signals.
• Extracellular matrix support.
• Improved cellular recruitment and tissue repair.

Successful tissue regeneration is often conceptualised around three interacting elements:

The local mechanical and vascular environment represents an equally important component of successful healing.

• Bone grafts: autograft, allograft and synthetic substitutes.
• Blood-derived products: PRP and related platelet concentrates.
• Bone marrow-derived products: bone marrow aspirate and BMAC.
• Cell-based products: mesenchymal stromal/stem-cell-based therapies and other cellular preparations.
• Growth factors: BMPs and other signalling molecules.
• Scaffolds: natural and synthetic matrices.
• Cartilage cell therapies: autologous chondrocyte-based techniques.
• Emerging therapies: extracellular vesicles, exosomes, gene-based therapies and advanced tissue engineering.

Autologous bone graft remains one of the most important orthobiologic materials in fracture reconstruction because it can provide osteogenic, osteoinductive and osteoconductive properties.

Common Harvest Sites

• Iliac crest.
• Proximal tibia.
• Distal femur in selected procedures.
• Local bone obtained during surgery.
• Intramedullary reaming material using selected harvest systems.

Advantages

• No immunological incompatibility.
• Contains viable host cells.
• Contains native growth factors.
• Provides biological scaffold.

Disadvantages

• Donor-site pain.
• Additional surgical exposure.
• Haematoma.
• Infection.
• Limited graft quantity.
• Neurovascular injury in selected harvest sites.

Allograft bone is obtained from a donor and is commonly used when a larger quantity of graft is required or when donor-site morbidity is undesirable.

Depending on processing, allograft is primarily osteoconductive, with variable retention of osteoinductive properties and minimal or absent viable osteogenic cells.

Forms

• Cancellous chips.
• Cortical struts.
• Structural allografts.
• Demineralised bone matrix.

Demineralised bone matrix (DBM) is processed allograft from which much of the mineral component has been removed.

This exposes proteins within the extracellular matrix, including variable amounts of bone morphogenetic proteins.

DBM is primarily used as an osteoconductive and potentially osteoinductive adjunct rather than as a mechanically structural graft.

Biological activity can vary considerably between products, donors and processing techniques.

Synthetic bone graft substitutes provide predominantly an osteoconductive scaffold.

Common materials include:

• Hydroxyapatite.
• β-tricalcium phosphate.
• Calcium phosphate cements.
• Calcium sulphate.
• Composite ceramics.
• Bioactive glass.

Their resorption characteristics, mechanical properties and biological behaviour vary substantially.

Platelet-rich plasma (PRP) is an autologous blood-derived preparation containing platelets at a concentration above that of the patient's baseline whole blood, although exact definitions and preparation systems vary.

PRP is prepared by collecting peripheral blood followed by centrifugation or other separation techniques to concentrate the platelet-containing fraction.

Platelets contain alpha granules that release numerous biologically active proteins and growth factors.

PRP preparations differ considerably between commercial systems and protocols.

Important variables include:

• Platelet concentration.
• Leukocyte concentration.
• Red-cell contamination.
• Injection volume.
• Activation method.
• Number of injections.
• Interval between injections.
• Patient characteristics.

Preparations are often broadly described as:

• Leukocyte-rich PRP.
• Leukocyte-poor PRP.

This biological heterogeneity is one reason why results from different PRP studies cannot always be directly compared.

PRP has been investigated for numerous musculoskeletal disorders, including:

• Knee osteoarthritis.
• Patellar tendinopathy.
• Lateral epicondylitis.
• Achilles tendinopathy.
• Plantar fasciopathy.
• Rotator cuff pathology.
• Muscle injuries.
• Ligament injuries.
• Cartilage lesions.
• Adjunctive use during selected surgical procedures.

Evidence varies substantially according to the condition, preparation and outcome being evaluated. PRP should therefore not be considered uniformly effective across all orthopaedic indications.

Knee osteoarthritis is one of the most widely studied clinical applications of PRP.

Selected patients may experience improvement in pain and function following intra-articular PRP. However, results are influenced by disease severity, preparation technique and study methodology.

PRP should generally be viewed as a potential symptom-modifying biological treatment rather than a proven method of regenerating an osteoarthritic joint to normal cartilage.

Improvement in pain after an orthobiologic injection does not necessarily demonstrate structural cartilage regeneration.

Tendon healing is characterised by relatively limited vascularity, scar formation and altered extracellular matrix architecture.

PRP and cell-based therapies have therefore been investigated for chronic tendinopathies and tendon repair augmentation.

Outcomes vary considerably between anatomical regions and treatment protocols.

Appropriate rehabilitation and correction of mechanical overload remain essential even when biological therapies are used.

Bone marrow contains a heterogeneous population of:

• Haematopoietic cells.
• Platelets.
• Mononuclear cells.
• Progenitor cells.
• Mesenchymal stromal cells.
• Signalling molecules.

Bone marrow can be aspirated and used directly or processed to produce bone marrow aspirate concentrate (BMAC).

BMAC is produced by concentrating selected cellular and platelet components of aspirated bone marrow, usually by centrifugation.

Common harvest sites include the iliac crest and other selected cancellous bone sites depending on the procedure.

BMAC contains mesenchymal stromal cells but these represent only a small fraction of the total nucleated cellular population.

BMAC should not automatically be described as a purified or concentrated “stem-cell injection.”

• Fracture nonunion and delayed union as a biological adjunct.
• Bone defects.
• Cartilage restoration procedures.
• Osteochondral lesions.
• Early osteoarthritis in selected settings.
• Osteonecrosis of the femoral head in selected early-stage patients.
• Tendon or ligament augmentation in selected procedures.

Evidence remains indication-specific and should not be extrapolated from one condition to another.

A stem cell is characterised by the capacity for self-renewal and the potential to differentiate into specialised cell types.

Stem cells can be classified according to their developmental potential.

Cells historically called mesenchymal stem cells (MSCs) are increasingly referred to as mesenchymal stromal cells, particularly when true stem-cell properties have not been demonstrated for every cell within the preparation.

MSC populations have been isolated from tissues including:

• Bone marrow.
• Adipose tissue.
• Synovium.
• Periosteum.
• Umbilical and perinatal tissues in research or regulated products.

Bone marrow-derived MSCs remain among the most extensively studied populations in orthopaedics.

Laboratory definitions traditionally incorporate:

• Plastic adherence under standard culture conditions.
• A characteristic surface-marker profile.
• Ability to demonstrate osteogenic differentiation in vitro.
• Ability to demonstrate chondrogenic differentiation in vitro.
• Ability to demonstrate adipogenic differentiation in vitro.

These laboratory criteria should not be confused with proof that an injected cell preparation will regenerate a specific tissue clinically.

Early regenerative concepts focused heavily on the idea that transplanted cells would directly differentiate and permanently replace damaged tissue.

Current biological concepts also emphasise important paracrine and immunomodulatory effects.

MSC populations may release signalling molecules that influence:

• Inflammatory responses.
• Cell recruitment.
• Angiogenesis.
• Extracellular matrix metabolism.
• Local progenitor-cell activity.
• Tissue repair pathways.

Therefore, some potential therapeutic effects may arise from modification of the local biological environment rather than long-term engraftment of large numbers of transplanted cells.

A clinical study using BMAC cannot automatically be interpreted as evidence for culture-expanded stem-cell therapy, and vice versa.

Articular cartilage has limited intrinsic healing capacity because of its avascular nature and specialised extracellular matrix.

Biological approaches to cartilage pathology include:

• PRP.
• BMAC.
• Microfracture augmentation.
• Scaffold-based repair.
• Autologous chondrocyte implantation.
• Cell-seeded matrices.
• MSC-based therapies under investigation.

Focal cartilage defects and diffuse osteoarthritis represent biologically different problems and should not be considered interchangeable indications.

Autologous chondrocyte implantation is a cell-based cartilage restoration strategy in which chondrocytes are obtained from the patient, expanded and subsequently implanted into a cartilage defect.

Later-generation techniques commonly use a matrix or scaffold to deliver the cells.

Unlike nonspecific intra-articular “stem-cell injections,” autologous chondrocyte implantation is primarily intended for appropriately selected focal articular cartilage defects.

Nonunion frequently represents a combined mechanical and biological problem.

Before adding an orthobiologic, the surgeon should evaluate:

• Mechanical stability.
• Fracture alignment.
• Fracture gap.
• Infection.
• Vascularity.
• Bone loss.
• Soft-tissue condition.
• Smoking.
• Nutrition.
• Metabolic and endocrine abnormalities where relevant.

Biological strategies may include autologous bone graft, BMAC, graft substitutes and selected growth-factor-based interventions.

Biology cannot reliably compensate for gross mechanical instability.

The diamond concept highlights the multiple factors required for successful bone healing.

Important components include:

• Osteogenic cells.
• Osteoinductive mediators.
• Osteoconductive scaffold.
• Adequate mechanical stability.
• Adequate vascularity.
• Favourable host factors.

Orthobiologic treatment of nonunion is therefore best understood as one component of a comprehensive biological and mechanical reconstruction strategy.

Bone marrow-derived cellular preparations have been investigated as an adjunct to core decompression in early-stage osteonecrosis of the femoral head.

The proposed aim is to improve the biological environment within the necrotic femoral head by introducing progenitor cells and signalling factors.

Outcomes depend strongly on stage, lesion size, location, underlying cause and patient selection. Advanced femoral head collapse is primarily a structural problem and is unlikely to be reversed by an injection alone.

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor-β superfamily and have powerful osteoinductive activity.

Recombinant BMPs have been developed for selected clinical indications in bone healing and spinal fusion.

Their use must consider specific regulatory indications, dose, carrier, cost and potential complications.

Highly potent biological signalling molecules may produce effects beyond the intended treatment site.

Reported or potential concerns with selected BMP applications include:

• Ectopic or heterotopic bone formation.
• Local inflammatory response.
• Soft-tissue swelling.
• Osteolysis in selected situations.
• Procedure-specific complications.

Use should therefore be indication-specific rather than based simply on the concept that “more biology is better.”

A scaffold provides a three-dimensional environment in which cells can attach, migrate, proliferate and produce extracellular matrix.

Ideal scaffold characteristics include:

• Biocompatibility.
• Appropriate porosity.
• Suitable mechanical properties.
• Controlled degradation.
• Ability to support vascularisation.
• Ability to support cellular attachment and differentiation.

Scaffolds may be combined with cells and growth factors to create tissue-engineered constructs.

Cells communicate partly through extracellular vesicles containing proteins, lipids and nucleic acids.

Exosomes represent one category of small extracellular vesicles and have become an important area of regenerative-medicine research.

Proposed orthopaedic applications include modulation of inflammation and enhancement of cartilage, tendon and bone repair.

Clinical use remains an evolving area and claims of established regenerative effectiveness should be distinguished from experimental evidence.

Many orthobiologic therapies have attractive mechanisms demonstrated in laboratory or animal studies.

However, a treatment demonstrating cellular proliferation, increased growth-factor expression or improved histological appearance does not automatically mean that it improves pain, function, reoperation rates or long-term joint survival in patients.

Mechanistic evidence generates biological rationale; clinical trials determine clinical benefit.

One of the greatest challenges in orthobiologics is the absence of complete standardisation.

Studies may differ in:

• Patient age.
• Disease severity.
• Biologic preparation method.
• Cell concentration.
• Platelet concentration.
• Leukocyte concentration.
• Injection volume.
• Number of injections.
• Concomitant procedures.
• Rehabilitation protocols.
• Outcome measures.
• Follow-up duration.

Two studies both labelled “PRP” or “stem-cell therapy” may therefore be evaluating substantially different biological products.

Autologous orthobiologics are themselves influenced by the biology of the individual patient.

Factors that may influence cell or platelet characteristics include:

• Age.
• General health.
• Smoking.
• Metabolic disease.
• Medications.
• Local tissue environment.
• Severity and chronicity of disease.

Therefore, an autologous biologic is not necessarily identical between two patients even when the same commercial preparation system is used.

Risk depends heavily on the specific product and the degree of tissue manipulation.

Potential complications include:

• Pain following injection.
• Local inflammatory reaction.
• Bleeding or haematoma.
• Infection.
• Donor-site morbidity following marrow or graft harvest.
• Failure to improve symptoms.
• Unexpected tissue response.
• Risks associated with unregulated or extensively manipulated cellular products.

Risks associated with simple autologous PRP should not be assumed to be identical to those associated with cultured cells, allogeneic cell products or experimental regenerative therapies.

Regulation of orthobiologic products depends on the country, source of tissue, extent of processing, intended use and whether cells are substantially manipulated or expanded.

Clinicians should distinguish between:

• Established orthobiologic procedures.
• Approved or specifically regulated cellular products.
• Experimental interventions performed within clinical trials.
• Commercially promoted interventions lacking adequate evidence or regulatory approval.

Regulatory status should be verified within the jurisdiction where treatment is being performed.

“Stem-cell therapy” is frequently used as a broad marketing term despite major differences between products.

The clinician should determine:

• What tissue is being harvested?
• Is it autologous or allogeneic?
• What cells are actually present?
• Are the cells isolated or merely contained within a mixed preparation?
• Have they been culture expanded?
• What is the cell dose?
• What evidence supports that exact product for that exact indication?
• What is its regulatory status?

The presence of a small number of progenitor cells within a preparation does not justify claiming that the treatment will regenerate cartilage, tendon or bone.

This is one of the most important misconceptions surrounding orthobiologic treatment.

Improvement in pain or function following a cellular or platelet-based injection does not prove restoration of normal hyaline cartilage.

Advanced osteoarthritis involves not only cartilage loss but also changes in:

• Subchondral bone.
• Synovium.
• Meniscus.
• Ligaments.
• Joint alignment.
• Muscle function.

A single biological injection should therefore not be presented as a proven method of reconstructing a severely degenerated joint.

• Variation in cell source.
• Variation in cell isolation and processing.
• Variation in cell number and viability.
• Age-related changes in autologous cell populations.
• Uncertain optimal dosage.
• Uncertain delivery method.
• Limited survival of transplanted cells in some environments.
• Difficulty controlling differentiation.
• Cost.
• Manufacturing complexity.
• Regulatory requirements.
• Insufficient long-term clinical evidence for many proposed indications.

The safety profile of a cellular intervention depends on cell source, processing, culture conditions, genetic stability, route of administration and patient factors.

Theoretical or documented concerns with inadequately regulated cell-based interventions can include:

• Infection.
• Immune reactions with selected allogeneic products.
• Contamination during processing.
• Unintended differentiation.
• Ectopic tissue formation.
• Abnormal cellular proliferation.
• Unpredictable biological activity.

These concerns become increasingly important as cellular manipulation becomes more extensive.

Orthobiologic therapy should be evaluated using the same evidence-based principles applied to drugs, implants and surgical procedures.

Important questions include:

1. What exact biological product was used?
2. Was its composition adequately characterised?
3. What pathology was being treated?
4. Was there an appropriate control group?
5. Were patients randomised?
6. Were clinically meaningful outcomes measured?
7. Was follow-up sufficiently long?
8. Was structural improvement actually demonstrated?
9. Were complications systematically recorded?
10. Can the results be reproduced with another preparation system?

• Safe.
• Effective for a defined indication.
• Biologically reproducible.
• Standardised.
• Easy to obtain or manufacture.
• Low morbidity.
• Cost-effective.
• Stable during storage and administration.
• Capable of delivering the appropriate cells or signals to the target tissue.
• Supported by high-quality clinical evidence.

Future orthobiologic treatment may increasingly become personalised rather than using one biological preparation for every patient.

Treatment selection could potentially consider:

• Patient age.
• Disease stage.
• Bone quality.
• Inflammatory phenotype.
• Mechanical alignment.
• Activity level.
• Cellular characteristics.
• Biomarkers.
• Individual response to previous treatment.

Future orthobiologic research is moving toward more precisely defined and reproducible therapies.

Areas under investigation include:

• Optimised PRP formulations.
• Better characterisation of BMAC.
• Cell selection and targeted cell populations.
• Advanced biomaterial scaffolds.
• 3D-bioprinted constructs.
• Extracellular vesicles and exosomes.
• Gene-enhanced tissue repair.
• Controlled growth-factor delivery.
• Cell-free regenerative therapies.
• Personalised orthobiologic protocols.

1. Define the pathology accurately. Do not treat “knee pain” or “tendon pain” without establishing the underlying diagnosis.
2. Correct the mechanical problem. Alignment, instability, fracture gap and mechanical overload may determine outcome more than the biologic used.
3. Identify the biological deficit. Consider vascularity, bone loss, tissue quality, host factors and chronicity.
4. Choose an evidence-supported product. Evidence for one orthobiologic cannot automatically be extrapolated to another.
5. Know exactly what is being administered. “PRP,” “BMAC” and “stem cells” are not synonymous.
6. Explain realistic expectations. Symptomatic improvement should not be described as proven tissue regeneration unless structural evidence demonstrates it.
7. Discuss alternative treatments. Orthobiologics should be integrated into a complete treatment strategy rather than marketed as a universal cure.

• Orthobiologics are biologically derived substances used to enhance musculoskeletal tissue healing.
• Autologous bone graft provides osteogenic, osteoinductive and osteoconductive properties.
• Allograft is predominantly osteoconductive after processing.
• Osteogenesis refers to new bone formation by viable bone-forming cells.
• Osteoinduction stimulates progenitor cells toward osteogenic differentiation.
• Osteoconduction provides a scaffold for new bone growth.
• PRP is a platelet-concentrated autologous blood product containing multiple signalling molecules.
• PRP preparations vary in platelet, leukocyte and red-cell composition.
• BMAC is a heterogeneous bone-marrow concentrate and is not equivalent to a purified stem-cell product.
• Mesenchymal stromal cells may exert important paracrine and immunomodulatory effects in addition to differentiation potential.
• Bone marrow and adipose tissue are important sources of mesenchymal stromal cells.
• BMPs are potent osteoinductive signalling proteins.
• Cartilage regeneration, symptom improvement and structural repair are different outcomes.
• Improvement after PRP or cellular therapy does not automatically mean regeneration of normal hyaline cartilage.
• Orthobiologics cannot compensate for gross mechanical instability in fracture nonunion.
• The biological treatment of nonunion should be integrated with mechanical stability, vascularity and host optimisation.
• Cell source, preparation technique, cell dose and indication must be specified when interpreting “stem-cell” studies.
• Evidence supporting one orthobiologic cannot automatically be extrapolated to another.
• Extensive processing or culture expansion introduces additional regulatory and safety considerations.

1. Understand the biological problem: determine whether healing is limited by cells, signalling, scaffold, vascularity or host factors.
2. Do not forget mechanics: stable fixation, alignment and appropriate loading remain fundamental.
3. Define the orthobiologic: PRP, BMAC, bone graft and cultured cell therapies are biologically different products.
4. Match treatment to pathology: evidence from knee osteoarthritis cannot automatically be extrapolated to tendon injury, nonunion or cartilage defects.
5. Separate symptom modification from regeneration: improvement in pain does not prove restoration of normal tissue.
6. Interpret “stem-cell therapy” carefully: identify the source, processing method, cell population, dose and regulatory status.
7. Demand clinical evidence: laboratory promise should be confirmed by appropriately designed clinical studies.
8. Use orthobiologics as part of comprehensive care: rehabilitation, mechanical correction, patient optimisation and established surgical principles remain essential.

The future of orthobiologics lies not in a universal “stem-cell cure,” but in precisely defined, evidence-based biological treatments matched to the correct patient, tissue and mechanical environment.