Cartilage repair for the Knee Joint

Courtesy: Christopher McCrum, Assistant Professor, UT SouthWestern, Dallas, Texas, USA

Articular Cartilage Structure

Basic Components

• Water: Constitutes the majority of cartilage weight and is essential for load bearing.

• Chondrocytes: The only resident cells, responsible for maintaining the cartilage matrix.

• Extracellular Matrix:

  • Predominantly composed of Type Two collagen

  • Provides tensile strength and structural integrity

Zonal Organization

• Superficial Zone:

  • Flattened chondrocytes

  • High collagen content aligned parallel to the surface

• Intermediate Zone:

  • Round chondrocytes

  • Transitional collagen orientation

• Deep Zone:

  • Round chondrocytes arranged in columns

  • Highest proteoglycan concentration

• Calcified Cartilage Zone:

  • Separates articular cartilage from subchondral bone

  • Acts as a barrier isolating cartilage from the blood supply

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Molecular Biology of Articular Cartilage

Type Two Collagen

• Accounts for approximately ninety to ninety-five percent of total collagen content

• Represents about ten percent of cartilage wet weight

• Forms a highly cross-linked and interconnected fibrillar network

Proteoglycans

• Aggregating Proteoglycans are critical for compressive stiffness

Aggrecan

• Highly glycosylated core protein

• Contains glycosaminoglycan side chains

• Provides cartilage with its load-bearing properties

Hyaluronic Acid

• Long, unbranched polysaccharide

• Forms large proteoglycan aggregates with aggrecan

Decorin

• Binds to thicker collagen fibrils

• Fills gap regions within the matrix

• Expression increases during mechanical stress

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Biomechanics of Articular Cartilage

Response to Compression

• Proteoglycan aggregates carry strong negative charges

• This increases osmolarity and attracts water into the matrix

• Results in high internal tissue pressure and resistance to compression

Mechanical Loading and Nutrition

• Cartilage nutrition is regulated by changes in hydrostatic pressure

• During loading:

  • Compression expels interstitial fluid and metabolic waste

• During unloading:

  • Nutrient-rich fluid diffuses back into the matrix

• Fluctuating loads:

  • Stimulate matrix production

• Static loads:

  • Decrease aggrecan synthesis

  • No significant change in hyaluronic acid production

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Subchondral Bone

• The subchondral cortical endplate lies immediately beneath the calcified cartilage

• Injury to subchondral bone:

  • Does not regenerate to its original architecture

  • Has important implications for cartilage repair outcomes

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Natural History of Cartilage Lesions

Chondral Injury Mechanisms

• Single severe impact or repetitive blunt trauma

• Acute trauma leads to chondrocyte apoptosis

• Chronic repetitive loading results in:

  • Cartilage softening

  • Fissuring

  • Progressive tearing

Lamina Splendens

• Superficial layer of the superficial zone

• Contains a limited population of progenitor cells

Edge Loading

• Abnormal joint surface geometry causes increased load concentration at lesion margins

• Severity depends on lesion size and containment

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Classification of Cartilage Repair Strategies

Palliative Procedures

• Chondroplasty

Reparative Procedures (Marrow Stimulation)

• Microfracture

• Subchondral drilling

Restorative Procedures

• Microfracture augmentation techniques

• Autologous chondrocyte implantation

• Osteochondral autograft transfer

• Osteochondral allograft transplantation

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Marrow Stimulation Techniques (Microfracture)

Principle

• Allows bone marrow elements to enter the defect

• Stimulates formation of reparative fibrocartilage

Advantages

• Single-stage procedure

• Cost-effective

• Performed arthroscopically

Limitations

• Inferior results in larger lesions

• Does not adequately address subchondral bone defects

Prognostic Factors for Poor Outcome

• Inferior quality and quantity of repair tissue

• Smoking

• Longer duration of symptoms

• Obesity

Imaging Outcomes on Magnetic Resonance Imaging

• Complete defect fill: eighteen to ninety-five percent

• Poor fill: seventeen to fifty-seven percent

• Complete integration: four to eight percent

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Bone Overgrowth After Microfracture

• Development of intralesional osteophytes

• Represents overgrowth of the subchondral plate

• Associated with a ten-fold increase in failure rates

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Technical Considerations for Marrow Stimulation

• Complete removal of calcified cartilage layer is essential

• Subchondral drilling may be preferred over a conical awl in some cases

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Marrow Stimulation Augmentation With Cartilage Matrix Scaffolds

Allograft Cartilage Matrix Composition

• Type Two collagen

• Proteoglycans

• Endogenous growth factors

Potential Benefits

• Improved histologic quality of repair tissue

• Increased repair tissue volume

Early Clinical Outcomes

• Significant improvement in pain and functional scores for up to two years

• Approximately four percent failure rate due to delamination or mechanical symptoms

• Limited long-term evidence

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Cryopreserved Osteochondral Allograft Cartilage

• Perforated cartilage scaffold used with marrow stimulation

• Off-the-shelf product with approximately two-year shelf life

Clinical Outcomes

• Improved patient-reported outcomes at two years for isolated patellofemoral defects

• High reoperation rates and moderate failure rates

• Evidence limited to small case series

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Allogenic Juvenile Minced Chondrocyte Implantation

Characteristics

• Uses fragments of juvenile articular cartilage

Short-Term Outcomes

• Significant improvement in patient-reported outcomes at two years

• Magnetic resonance imaging shows cartilage-like repair tissue

• Histology demonstrates predominance of Type Two collagen, with some Type One collagen present

Limitations

• Graft hypertrophy occurs in up to one-third of cases and may require surgical debridement

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Autologous Chondrocyte Implantation

Principle

• Cell-based therapy aiming to restore hyaline-like cartilage

Technique

• Articular cartilage harvested from a non-critical, non-weight-bearing area

• Chondrocytes isolated and expanded in a laboratory

• Cells implanted beneath a periosteal or collagen membrane

Advantages

• Produces more hyaline-like cartilage compared to marrow stimulation

• Suitable for larger defects

Limitations

• Requires intact full-thickness cartilage margins

• Two-stage procedure

• Prolonged protected weight bearing required

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Matrix-Associated Autologous Chondrocyte Implantation

Technique

• Cultured chondrocytes embedded in a scaffold

• Scaffold secured with fibrin adhesive

Advantages

• Can be performed arthroscopically

• Technically less demanding than traditional autologous chondrocyte implantation

• Effective for larger defects

Limitations

• Two-stage procedure

• Prolonged restriction of weight bearing

Repair Tissue Quality

• Hyaline-like cartilage reported in approximately fifty to seventy-five percent of grafts

• Increasing proportion reported in recent studies

• Cartilage-like tissue formation begins within approximately three weeks

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Clinical Outcomes of Matrix-Associated Autologous Chondrocyte Implantation

• Multiple cohort studies and randomized controlled trials available

• Significant improvement in patient-reported outcomes

• Low reoperation rates

• Small lesions show minimal difference compared to marrow stimulation in the short term

• Larger lesions demonstrate superior outcomes

• Treatment failure rate approximately ten percent

• Outcomes after failed marrow stimulation are inferior due to compromised subchondral bone

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Technical Pearls for Cell-Based Cartilage Repair

• Partial uncontained lesions are relative contraindications

• Graft containment can be achieved using sutures or anchors

• Ensure perpendicular defect walls

• Apply firm, uniform pressure until fibrin adhesive sets

• Complete removal of calcified cartilage layer is mandatory

• Prevent graft dislodgement during surgery and early postoperative period

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Osteochondral Repair Techniques

Osteochondral Autograft Transfer and Mosaicplasty

Principle

• Replacement of cartilage defect with autologous cartilage and bone plugs from non-weight-bearing areas

Advantages

• Viable chondrocytes

• Immediate range of motion possible

• Single-stage, arthroscopic, and cost-effective

Limitations

• Donor site morbidity

• Difficulty matching surface curvature

• Temporary reduction in fixation strength requiring weight-bearing restriction

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Osteochondral Autograft Harvest Sites

• Preferred site: Proximal to the medial sulcus terminalis

• Alternative site: Lateral aspect of the trochlea

• These areas experience the lowest contact pressures

Outcomes

• Faster bone integration compared to allografts

• Superior long-term results for small lesions, including higher hyaline cartilage content and improved patient-reported outcomes

• Donor site morbidity remains a limitation

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Osteochondral Allograft Transplantation

Principle

• Replacement of cartilage defect with mature hyaline cartilage and underlying bone containing viable chondrocytes

Graft Characteristics

• Fresh, refrigerated grafts preferred

Indications

• Large cartilage defects

• Significant bone loss

• Failed prior cartilage repair procedures

Advantages

• Ability to restore both cartilage and subchondral bone

• High rates of return to sport

Limitations

• Limited graft availability

• High cost

• Risk of infection and immune response

Long-Term Outcomes

• Survival rates:

  • Ninety-five percent at five years

  • Eighty-two percent at ten years

  • Seventy-four percent at fifteen years

  • Sixty-six percent at twenty years

• Return to sport at previous level in approximately seventy-six percent

• Reoperation rate approximately forty-six percent at twenty years

• Good subjective outcomes reported in seventy-five percent at twelve years

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Immunogenicity of Osteochondral Allografts

• Antibody-mediated immune response has been proposed

• Primarily Class One immune responses described

• Clinical significance remains unclear

• Pulsatile lavage of grafts used to remove donor marrow elements and reduce immunogenicity

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Effect of Graft Impaction

• Impaction may cause temporary chondrocyte death

• Extracellular matrix remains intact

• Chondrocyte viability typically normalizes by approximately eight days postoperatively

• Deeper graft insertion is associated with reduced chondrocyte viability

• Technical pearl: Use hydraulic principles by drilling recipient and donor sites with guide wires to reduce impaction force

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Osteochondral Allograft in Osteonecrosis

• Up to thirty-seven percent of patients on chronic corticosteroid therapy develop osteonecrosis

• Evidence remains limited

• Osteochondral allograft transplantation shows promising results:

  • Approximately ninety percent survivorship at five years

  • Eighty-two percent survivorship at ten years

  • Significant improvement in functional knee outcome scores