Cartilage Restoration: The Complete Treatment Algorithm

Articular cartilage is often described as the "teflon" of the human body—a highly specialized, incredibly smooth, frictionless, and pain-free surface designed to withstand decades of complex biomechanical loading. However, this magnificent structure has a profound Achilles heel: it is avascular, aneural, and alymphatic.

Because it lacks a direct blood supply, native hyaline cartilage possesses virtually zero intrinsic healing capacity. Once damaged, the body cannot regenerate it. Instead, if the subchondral bone is breached, the defect heals with fibrous scar tissue (fibrocartilage), which consists primarily of Type I collagen. While fibrocartilage provides a temporary patch, it completely lacks the biomechanical durability, compressive strength, and shear resistance of native Type II collagen-rich hyaline cartilage.

For the young, active patient with a focal chondral defect, this represents a clinical crisis. Left untreated, these defects alter joint kinematics, increase focal contact pressures, and inevitably progress to early-onset osteoarthritis. For those in orthopaedic surgery training, mastering the algorithm for cartilage restoration is not just critical for fellowship exam preparation—it is essential for extending the athletic lifespan of your patients.

This comprehensive guide outlines the modern, evidence-based treatment algorithm for articular cartilage restoration, moving step-by-step from patient assessment to salvage procedures.

Phase 1: The Background Check (The "Hostile Knee")

The most common trap for junior surgeons—and a frequent pitfall in surgical education and board exams—is treating the focal defect while ignoring the joint's overall biomechanics. Before you even consider touching the cartilage, you must assess the Environment. A pristine, expensive cartilage graft placed into a hostile mechanical environment will predictably fail.

You must evaluate the "Unholy Triad" of joint mechanics:

1. Alignment (The Coronal Plane): Is the patient in varus or valgus alignment? A focal defect in the medial compartment of a varus knee is subjected to massive, concentrated overload. Correcting the mechanical axis (via High Tibial Osteotomy [HTO] or Distal Femoral Osteotomy [DFO]) to unload the affected compartment is often more critical to long-term success than the cartilage procedure itself. The goal is typically to shift the mechanical axis to the Fujisawa point (slightly lateral to the center of the tibial plateau) for medial defects.
2. Stability (The Sagittal and Coronal Planes): Is the ACL, PCL, or collateral ligament complex intact? Micro-instability generates abnormal shear forces that will easily strip or destroy any maturing cartilage graft. Ligamentous reconstruction must be performed either prior to or concurrently with cartilage restoration.
3. Meniscus (The Shock Absorber): The meniscus distributes up to 70% of the load in the lateral compartment and 50% in the medial compartment. If the patient is post-meniscectomy, the articular cartilage is bearing unphysiological stress. If the meniscus is functionally absent, you must strongly consider a concurrent Meniscal Allograft Transplantation (MAT).

Phase 2: The Defect Assessment and Classification

Once the background environment is optimized, you must precisely classify the defect. The International Cartilage Repair Society (ICRS) classification is the gold standard for describing chondral lesions.

Decision-making relies on four key defect characteristics:

• Size: This is the primary driver of the algorithm. Small (<2cm²), Medium (2-4cm²), or Large (>4cm²).
• Containment: Is the defect "shouldered" (surrounded by a healthy, vertical wall of native cartilage) or uncontained (open on one or more sides)? Contained defects hold grafts and clots much better.
• Location: Femoral Condyle (primarily compressive, weight-bearing forces) vs. Patellofemoral joint (high shear forces). Patellofemoral defects notoriously have worse outcomes and require careful assessment of patellar tracking (TT-TG distance, patellar height).
• Bone Involvement: Is this a pure chondral defect, or is the subchondral bone involved (an osteochondral defect)? Significant bone loss dictates that you must replace both bone and cartilage.

The Treatment Algorithm: Step-by-Step

1. Palliative: Debridement & Chondroplasty

• Indication: Low-demand patients, widespread diffuse osteoarthritic disease, or patients presenting purely with mechanical symptoms (catching, locking, giving way) from unstable cartilage flaps.
• Technique: Arthroscopic use of a shaver or curette to debride unstable, fibrillated flaps back to a stable, vertical rim of healthy cartilage. Avoid thermal ablation (radiofrequency wands) if possible, as peripheral heat necrosis can kill adjacent healthy chondrocytes and inadvertently widen the defect.
• Biology: Removes mechanical irritants.
• Pros: Quick recovery, arthroscopic, immediate relief of mechanical symptoms.
• Cons: Purely palliative. It does not restore native joint height or cartilage biology. Symptoms of dull, aching pain from bone-on-bone contact will persist or recur.

2. Reparative: Marrow Stimulation (Microfracture & Drilling)

• Indication: Small (<2cm²), acutely contained defects in low-to-moderate demand patients. Once the gold standard, its indications are shrinking rapidly in modern sports medicine.
• Technique: After creating vertical walls and removing the calcified cartilage layer (crucial step), an awl or fluted drill is used to penetrate the subchondral bone plate. This releases mesenchymal stem cells (MSCs) and growth factors from the bone marrow, allowing a "super clot" to fill the defect.
• Modern Update: Traditional microfracture uses an awl, which can compact the surrounding bone and cause subchondral cysts. Modern techniques favor drilling or nanofracture with a cooled K-wire to limit thermal necrosis and bone compaction.
• Biology: Produces Fibrocartilage (predominantly Type I Collagen).
• Pros: Arthroscopic, single-stage, inexpensive, technically straightforward.
• Cons: Fibrocartilage has inferior wear characteristics compared to hyaline cartilage. Clinical outcomes reliably deteriorate after 2 to 5 years. Not recommended as a first-line treatment for high-level pivoting athletes or patellofemoral defects.

3. Restorative: Osteochondral Autograft Transplantation (OATS / Mosaicplasty)

• Indication: Small-to-Medium (<2-3cm²) defects, especially in high-demand athletes requiring a fast return to sport. Ideal for osteochondral defects where subchondral bone is compromised.
• Technique: Cylindrical plugs of intact bone and overlying cartilage are harvested from non-weight-bearing zones of the knee (typically the superior lateral trochlear ridge or intercondylar notch). These are press-fit into precisely drilled recipient sockets in the defect.
• Biology: Transfers mature, native Hyaline Cartilage and living bone.
• Pros: True hyaline cartilage substitution. Bone-to-bone healing allows for immediate structural integrity and a faster return to sport compared to cell-based therapies.
• Cons: Donor site morbidity (anterior knee pain is common). Limited supply of expendable cartilage, meaning it cannot treat large defects. Multiple plugs ("mosaicplasty") leave a "cobblestone" surface with dead space between cylinders that heals with fibrocartilage. Plugs inserted proud (too high) will act as a wiper blade and destroy the opposing kissing cartilage.

4. Restorative: Cell-Based Therapies (MACI)

• Indication: Medium-Large (2-5cm²) defects. Ideal for pure chondral defects without significant bone loss. Excellent for the patellofemoral joint. Often used as a salvage procedure for failed microfracture.
• Technique: Matrix-Applied Characterized Autologous Cultured Chondrocytes (MACI) is a two-stage procedure.
  • Stage 1: A minor arthroscopic biopsy is taken from a non-weight-bearing area (e.g., intercondylar notch).
  • The Lab Phase: The chondrocytes are isolated, cultured, and expanded in a laboratory over 3-4 weeks, then seeded onto a highly purified porcine or bovine Type I/III collagen membrane.
  • Stage 2: Open arthrotomy or mini-arthrotomy. The defect is templated, and the cell-seeded membrane is cut to exact size and secured into the defect using fibrin glue (and occasionally fine sutures).
• Biology: Produces a durable, hyaline-like repair tissue (rich in Type II Collagen and aggrecan).
• Pros: Can treat massive areas without stripping the knee of healthy tissue (no major donor site morbidity). Highly customizable for complex topographies (like the patella). Excellent, proven long-term durability.
• Cons: Requires two separate surgeries. Highly expensive. The rehabilitation is notoriously slow, as the delicate cells require many months to mature and organize into load-bearing tissue.

5. Salvage: Osteochondral Allograft Transplantation (OCA)

• Indication: Massive defects (>4cm²), uncontained defects, Osteochondritis Dissecans (OCD) lesions with massive bony destruction, or salvage for failed prior cartilage surgeries.
• Technique: A fresh, size-matched cadaveric donor condyle or plateau is procured. A large cylindrical core (or sometimes a freehand shell) of the patient's defect is removed, and a perfectly matched plug from the donor is press-fit into the defect.
• Biology: Transfers living hyaline cartilage and mature, structurally sound trabecular bone.
  • The Cartilage: Chondrocytes are immunoprivileged (they lack MHC Class II antigens and are hidden in an avascular matrix), so they survive without systemic immunosuppression.
  • The Bone: The donor bone is dead but acts as a scaffold. It heals to the host via creeping substitution—the host bone slowly resorbs and replaces the donor bone over 1-2 years.
• Pros: A single-stage procedure that restores normal bony architecture and provides mature, functioning hyaline cartilage. Extremely high success rates (80-90% survivorship at 10 years).
• Cons: Severe logistical challenges. "Fresh" allografts must ideally be implanted within 14-28 days of procurement to maintain high chondrocyte viability, making scheduling a nightmare. The bone portion is slightly immunogenic, requiring thorough pulsatile lavage to remove donor marrow elements. High cost.

Rehabilitation: The 4 Phases of Cartilage Maturation

Cartilage rehabilitation is a delicate dance between protecting a fragile graft and providing the mechanical stimulus required for cell differentiation and matrix synthesis. In surgical education, understanding rehab is just as important as the surgical steps.

• Phase 1 (0-6 weeks): Protection and Proliferation. The goal is to avoid shear and heavy compression. Weight-bearing is strictly limited (non-weight bearing or toe-touch). Use of a Continuous Passive Motion (CPM) machine is often prescribed for 6-8 hours a day. Why? Because cyclic, low-load motion physically pumps synovial fluid nutrients into the avascular graft, stimulating chondrocyte survival and alignment.
• Phase 2 (6-12 weeks): Transition. Progressive normalization of gait. Initiation of controlled, closed-kinetic-chain exercises (e.g., leg press) to safely load the joint without sheer.
• Phase 3 (3-6 months): Strengthening. Progression to unilateral loading. Building quadriceps and gluteal mass to absorb shock and protect the joint.
• Phase 4 (6-12+ months): Return to Sport (RTS). Impact activities (running, jumping) are slowly introduced. Return to pivoting sports is delayed until 9-12 months for OATS, and often 12-18 months for MACI or large OCAs. RTS requires symmetrical strength testing, passing functional hop tests, and ideally, MRI evidence of graft integration and bony healing.

Evidence Corner: What the Literature Says

A strong grasp of landmark literature is essential for any orthopaedic surgery trainee. Here is what the major trials tell us:

• Microfracture vs. OATS: The landmark trial by Gudas et al. demonstrated that for young, athletic patients with femoral condyle defects, OATS is vastly superior to microfracture. At long-term follow-up, the microfracture group showed a severe drop-off in clinical scores and a much lower rate of return to pre-injury sports levels compared to the OATS group.
• Microfracture vs. MACI: The prospective, randomized SUMMIT trial firmly established that MACI is clinically and statistically superior to microfracture for defects greater than 2cm², particularly in durability and pain reduction at 2 and 5 years out.
• Osteochondral Allograft (OCA): Long-term data from Gross et al. and Bugbee et al. confirm that fresh OCA is the ultimate salvage procedure, demonstrating remarkable survivorship exceeding 80% at 10 years, and >70% at 15 years, even in complex, multi-operated knees.

Conclusion and Summary Algorithm

There is no single "magic bullet" in cartilage restoration. The successful surgeon acts as a biological architect, matching the procedure to the defect size, location, patient demand, and concomitant pathology.

The Exam Day Summary:

• Small (<2cm²), Low Demand: Microfracture / Drilling.
• Small to Medium (1-3cm²), High Demand/Athlete: OATS (Autograft).
• Medium to Large (2-5cm²), Pure Chondral: MACI.
• Large (>4cm²) or Massive Bone Loss: Fresh Osteochondral Allograft (OCA).

The Ultimate Clinical Trap: Do not attempt to microfracture a lesion > 4cm²—it will invariably fail and burn a bridge. Conversely, do not perform MACI if there is profound subchondral bone loss or deep cystic changes; the delicate collagen membrane requires a stable, flat, healthy bony bed to survive. If the bone is gone, you must use bone (OATS or OCA).

References

1. Saris, D., et al. (2014). "Matrix-Applied Characterized Autologous Cultured Chondrocytes Versus Microfracture: Two-Year Follow-up of a Prospective Randomized Trial." Am J Sports Med.
2. Gudas, R., et al. (2005). "A prospective randomized clinical study of mosaic osteochondral autologous transplantation versus microfracture for the treatment of osteochondral defects in the knee joint in young athletes." Arthroscopy.
3. Gross, A. E., et al. (2008). "Fresh osteochondral allografts for posttraumatic defects in the knee: a survivorship analysis." JBJS.
4. Brittberg, M., et al. (1994). "Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation." N Engl J Med.
5. Nho, S. J., et al. (2008). "Osteochondral allograft transplantation in the knee." Am J Sports Med.