Cartilage Repair, Grafting, and Tissue Engineering

Welcome to a comprehensive guide on Cartilage Repair, Grafting, and Tissue Engineering! This article will break down the essential aspects of cartilage, its repair mechanisms, surgical grafting techniques, and the exciting advancements in tissue engineering for reconstructive plastic surgery. Whether you're preparing for an exam or simply eager to understand this complex field, we'll cover everything from the basic types of cartilage to the latest innovations in creating engineered tissue.

Understanding Cartilage: Structure and Function

Cartilage is a vital type of connective tissue primarily made up of chondrocytes and their extracellular matrices (ECM). This ECM typically includes type II collagen fibers, proteoglycans, and elastic fibers. Its unique structure, with a low metabolic rate and avascular nature, means it's nourished by tissue fluid diffusion.

There are three main classifications of cartilage, each with distinct compositions and functions:

• Hyaline Cartilage: This is the most common type, found in costal, articular, tracheal, and nasal cartilage. Rich in glycosaminoglycans, it is characterized by its stiffness, allowing it to withstand sustained compressional loading. Most hyaline cartilage (except articular cartilage) is covered by a perichondrium.
• Fibrocartilage: Composed of thick collagen fiber bundles and unicellular islands of cartilage arranged in chains, fibrocartilage provides high tensile strength and support. It's found in areas subject to frequent stress, such as the meniscus, intervertebral discs, symphyseal joints, and bone/tendon/ligament junctions. Unique among cartilage types, it also contains type I collagen, moderate proteoglycan, and little glycosaminoglycans.
• Elastic Cartilage: Known for its extremely high elasticity due to abundant elastic fibers, it resembles hyaline cartilage but features an elastic fiber network alongside collagen. This provides great flexibility, enabling it to withstand repeated bending. It's primarily located in the outer ear, larynx, and epiglottis, and is also surrounded by a perichondrium.

The Role of Perichondrium in Cartilage Health and Repair

The perichondrium is a dense irregular connective tissue layer that covers most cartilage. It has two layers: an outer fibrous layer (collagen fibers and fibroblasts) and an inner chondrogenic layer. The inner layer contains partially undifferentiated mesenchymal stem cells and chondrogenic progenitor cells, which are crucial for cartilage repair and regeneration.

Cartilage Grafting: Surgical Repair Techniques

Cartilage grafting is a long-standing practice, with early forms reported by Sushruta Samhita in India. It's widely used in plastic surgery for nasal or auricular defects and other tissue reconstructions, either as a pure cartilage graft or a composite graft. Autologous cartilage grafts are preferred due to their immunological privilege and general lack of metaplastic changes.

Common donor sites for autologous cartilage include auricular (ear), nasal (septal), and rib cartilage.

Auricular Cartilage Grafts: Versatile Options

Auricular cartilage, being elastic, is an ideal and versatile graft due to its ease of fashioning and contouring. A significant portion of conchal cartilage can be removed without noticeable donor site deformity.

To avoid complications like a collapsed conchal bowl, specific techniques are employed:

• Using a postauricular incision to minimize visible scars.
• Harvesting the cymba concha and cavum concha separately, leaving at least 5 mm of the crus helicis and a 2-mm outer rim.
• Utilizing a tie-over bolster dressing to mold the conchal bowl.

Auricular cartilage is used for ear reconstruction, nasal and nipple reconstruction, and tarsal repair. For nasal reconstruction, it's often transferred as a composite chondrocutaneous graft.

Managing the donor site aesthetically is crucial. Techniques vary based on graft size and location:

• Small grafts (< 1 cm) from the helix root allow primary closure by running the dog ear anteriorly into the hairline.
• Wider grafts (1–1.5 cm) for alar rim defects can be closed by running the dog ear anteriorly and posteriorly into the triangular fossa, requiring a full-thickness wedge of cartilage removal from the triangular fossa to prevent buckling.
• For short vertical nasal defects, grafts from the anteroinferior helical rim allow primary closure by running the dog ear superiorly and inferiorly.
• Wider grafts (1–1.2 cm) from the helical base may be closed primarily by advancing the helical rim forward, potentially requiring a scaphomastoid suture to address overprojection.
• Grafts wider than 1.5 cm from the helical base require intricate closure, involving a V-shaped skin wedge and a “half-star” cartilage pattern excision at the apex of the V, along with a postauricular incision and scaphomastoid suture to prevent cupping.

Microvascularly transferred auricular composite grafts can overcome size limitations of free grafts, especially for large nasal defects.

Nasal Cartilage Grafts: Support and Reconstruction

Nasal cartilage, though limited in amount, is a key source, particularly septal cartilage. It's accessed via a hemi-transfixion incision. Crucially, an L-shaped septal strut must be preserved to provide nasal support and prevent collapse. The amount needed varies based on nasal septum strength and dimensions.

Applications include:

• Chondromucosal grafts for eyelid reconstruction.
• Upper lateral nasal cartilage grafts for eyelid repair, as described by Tessier.
• Pedicled nasal chondromucosal flaps (based on the dorsal nasal artery) for large upper eyelid defects, providing a thin, mobile eyelid with high viability.
• Dorsal augmentation.
• Tracheal repair.
• Extended septal grafts for nose tip projection.

Rib Cartilage Grafts: Strength and Volume

Costal (rib) cartilage is an excellent donor site for its substantial amount and mechanical strength, allowing it to be contoured into almost any shape and retain form. It's often used as a framework for total ear reconstruction.

Harvesting techniques, like those described by Tanzer, Thomson, and Brent, typically involve the synchondrosis of the sixth and seventh cartilages, as well as the eighth costal cartilage, often with perichondrium. Nagata's method may require harvesting up to four costal cartilages in stages.

Donor site morbidity can include pain, chest wall clicking, scarring, and chest wall deformity. Pneumothorax is a rare but serious concern, particularly with methods requiring more grafts.

To mitigate these issues, Kawanabe and Nagata developed a new method:

1. Harvesting costal cartilages en bloc while leaving the perichondrium completely intact at the donor site.
2. After framework fabrication, remaining costal cartilage is cut into small blocks and returned to fill the dead space in the perichondrial pocket.

This technique reduces infection and pneumothorax rates to less than 1% and prevents postoperative chest wall deformities. Follow-up studies show regeneration of hyaline and fibrocartilage within 6-12 months. Modifications to auricular frameworks can also reduce the amount of cartilage needed.

Rib cartilage is also used for:

• Significant saddle-nose deformity reconstruction.
• Treatment of maxillonasal dysplasia (Binder’s syndrome).
• Nipple reconstruction.
• Septorhinoplasty.
• Tracheal reconstruction.

Autologous Perichondrial Grafts: Regeneration Potential

Since Lester's 1959 report, the potential of transplanted perichondrium for neocartilage formation has been recognized due to its chondrogenic progenitor cells. However, clinical applications are limited to specific conditions, such as knee joint cartilage reconstruction, due to factors like harvest technique, recipient bed oxygenation, and progenitor cell presence.

Perichondrium can regenerate cartilage in proper microenvironments, forming neocartilage within weeks. Clinical applications have included:

• Joint repair, particularly for articular cartilage of the knee, with reports of articular cartilage regeneration after grafting rib perichondrium.
• Nasal reconstruction, either as a perichondrium graft (due to its thinness and malleability) or as a perichondrocutaneous graft (common in facial reconstruction, usually from the concha to prevent deformity).

Despite wide use, robust clinical evidence for significant cartilage regeneration from transplanted perichondrium alone is still developing.

Tissue Engineering for Cartilage Reconstruction and Repair

Tissue engineering is a groundbreaking biotechnology aiming to repair and regenerate human tissue using an engineering approach. Cartilage was one of the first tissues targeted, vividly demonstrating the potential for plastic surgery applications like auricular reconstruction, rhinoplasty, and facial contouring.

The basic principle involves three major components:

1. Seed Cells: These cells produce, deposit, and form the tissue matrix.
2. Scaffold: A 3D structure that provides space for cells to reside, proliferate, and produce matrix.
3. Tissue Formation Environment: A proper environment where cells grow, produce ECM, and remodel the tissue as the scaffold gradually degrades, leading to mature engineered tissue.

Significant progress has been made in scaffold biomaterials, cell sources (including stem cells), chondrogenic induction, and in vitro cartilage reconstruction.

Engineering Auricular Cartilage: Crafting a New Ear

A notable achievement is the generation of human ear-shaped cartilage. One pioneering procedure involved:

• Harvesting chondrocytes (e.g., from calf cartilage) and expanding them in vitro.
• Creating an ear-shaped scaffold from polyglycolic acid (PGA) nonwoven fibers coated with polylactic acid (PLA), using a plaster cast mold.
• Seeding chondrocytes onto the scaffold and culturing in vitro for a week.
• Implanting the cell–scaffold construct into a nude mouse subcutaneously, supported by an external stent.

After 12 weeks, an ear cartilage structure formed, almost identical in 3D shape to a human ear. Histology confirmed well-formed cartilage. This demonstrated the potential for engineered auricular reconstruction.

Further advancements include using computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies:

• Scanning a patient's normal ear with computed tomography (CT) to get geometric data.
• Using CAD to generate positive and negative image data of the ear.
• Inputting data into a CAM system to print a 3D mold.
• Inserting PGA fibers into the mold, coating with PLA, to create an ear-shaped scaffold.

This method achieves over 97% similarity to the original ear shape. Human chondrocytes seeded on these scaffolds can generate elastic ear cartilage in vitro with good elasticity and mature histological structures within 12 weeks. The morphological similarity can reach 82.6% to the positive ear mold. Future efforts focus on the in vivo fate of this in vitro-engineered cartilage.

Achieving the flexibility and elasticity of native ear cartilage is also a goal. One method involves sandwiching chondrocyte–fibrin constructs between two layers of lyophilized swine perichondrium. These trilayer constructs, implanted in athymic mice, resulted in engineered cartilage with mechanical properties similar to native swine ear, tolerating torsion and bending without fracture.

Clinical trials have also shown promising results. Chondrocytes from microtia patients, cultured and expanded, can form a gelatinous chondroid mass. This mass, injected into a subcutaneous pocket (e.g., in the lower abdomen), can generate an elastic cartilage block. This block can then be sculpted into an auricular framework and implanted, supporting the reconstructed auricle for years without significant absorption.

Engineered Cartilage for Rhinoplasty and Facial Contouring

Tissue-engineered cartilage also holds promise for rhinoplasty and facial contouring. Injectable cartilage is a significant form for these applications.

• Early research explored slowly polymerizing gels (e.g., calcium alginate, Pluronic gel) for injectable cartilage.
• Human chondrocytes have been shown to form injectable cartilage using various scaffolds.

Clinical trials have been reported for injectable cultured autologous chondrocytes for nasal augmentation. Cartilage harvested from the auricular concha, cultured in vitro into a gel-type mass, is transplanted into a subperiosteal skin pocket on the nasal dorsum. This has shown to maintain shape for years without resorption, correcting saddle-nose deformities and enhancing facial contours. Similar techniques are used for chin augmentation and correcting temporal/forehead depressions.

In vivo-engineered cartilage can also be used for nasal reconstruction. Chondrocytes from the nasal septum, seeded on hyaluronan meshes, cultured in vitro, and then implanted into nude mice, showed well-formed cartilage with good mechanical properties within two weeks in vivo. These could provide autologous grafts for septum or alar reconstruction, or nasal augmentation.

Engineered Cartilage for Joint Repair and Reconstruction

Engineered cartilage can serve as a composite graft for articular cartilage repair in temporomandibular joints (TMJ) and digital joints.

• TMJ Reconstruction: Studies have shown that PLA-coated PGA scaffolds molded into a mandible condyle shape, seeded with osteoblasts and chondrocytes, and implanted in vivo, can form condyle-shaped bone tissue with articular cartilage on the surface, suggesting a potential for TMJ reconstruction. Engineered cartilage is also proposed for TMJ disc reconstruction.
• Digital Joint Reconstruction: Preliminary studies for small phalanges and whole joints use PLA or PGA scaffolds wrapped with periosteum for bone engineering, and seeded with chondrocytes and tenocytes for articular cartilage and tendon tissue. These efforts have successfully formed composite tissues resembling human phalanges and joints.

Future Directions in Cartilage Tissue Engineering

Despite promising preliminary clinical trials, significant research is ongoing to translate cartilage engineering to widespread clinical application. Key future directions include:

• Stem Cell-Based Cartilage Engineering: Given the limited availability of chondrocytes from auricular, nasal, or rib cartilage, adult mesenchymal stem cells (from bone marrow or adipose tissue) are a feasible alternative. Their multidifferentiation potential and strong proliferation capabilities make them ideal. Optimization of chondrogenic induction (growth factors, chondrogenic matrices, scaffold topographical structure) is crucial for enhancing differentiation and improving stem cell-engineered cartilage.
• Enhanced Mechanical Strength of In Vitro-Engineered Cartilage: While in vitro engineering avoids two-stage operations and reduces risks, its mechanical strength is often weaker than in vivo cartilage. Mimicking in vivo microenvironmental factors, such as dynamic mechanical loading, can significantly increase Young's modulus and the production of key matrix proteins and collagens, improving mechanical properties for immediate transplantation.
• Design and Precise Control of 3D Structure: For auricular and nasal reconstruction, precise control over the 3D structure is paramount. CAD/CAM systems are essential. Fabricating scaffolds that maintain the exact 3D structure during cartilage formation, possibly with slow-degrading inner core stents, is important for generating flexible, anatomically accurate cartilage.

Frequently Asked Questions (FAQ) about Cartilage Repair and Tissue Engineering

What are the main types of cartilage and where are they found?

There are three main types: Hyaline cartilage (most common, found in ribs, joints, trachea, nose), Fibrocartilage (strongest, found in meniscus, intervertebral discs), and Elastic cartilage (flexible, found in outer ear, larynx, epiglottis). Each type has unique structural components contributing to its specific function.

Why is autologous cartilage grafting preferred in plastic surgery?

Autologous (self-donated) cartilage grafting is preferred because cartilage is considered "immunologically privileged," meaning it's less likely to be rejected by the body. This ensures better graft survival and reduces complications, making it the most applicable cartilage graft option.

What is tissue engineering and how does it apply to cartilage?

Tissue engineering is a biotechnology that aims to repair or regenerate human tissues and organs by combining cells, scaffolds, and suitable biochemical/physicochemical factors. For cartilage, it involves growing patient-specific cells on a biocompatible scaffold in a controlled environment to create new cartilage tissue that can be implanted to repair defects, such as reconstructing an ear or nose.

What are the potential complications of harvesting rib cartilage for grafts?

Common complications include pain, clicking of the chest wall, scarring, and chest wall deformity. A more serious, though rare, complication is pneumothorax (collapsed lung) during the surgical procedure. Newer techniques aim to minimize these risks by preserving the perichondrium and returning excess cartilage blocks to the donor site.

What are mesenchymal stem cells and their role in future cartilage repair?

Mesenchymal stem cells (MSCs) are adult stem cells that can differentiate into various cell types, including chondrocytes (cartilage cells). They are a promising alternative cell source for cartilage engineering due to their strong proliferation capability and ability to form cartilage. Future research focuses on optimizing their differentiation into functional cartilage tissue to overcome limitations of traditional chondrocyte sources.