3D Meniscus Atlas

Teaching notes

The menisci are fibrocartilaginous wedges covering one-half to two-thirds of the tibial articular surface, transmitting 40% to 60% of load during standing. Their circumferential collagen fibers convert axial compression into hoop tension directed to the tibial roots; any disruption of that ring has immediate consequences for contact stress.

Anatomy and Biomechanics

Each meniscus has a triangular cross-section: the inferior surface is flat while the superior surface is concave, conforming to the femoral condyle. The thick peripheral border attaches via the coronary ligament; the inner border is free. The medial meniscus is C-shaped, firmly attached to the deep MCL throughout its periphery, and less mobile. The lateral meniscus is nearly circular, more mobile, and lacks capsular attachment at the popliteal hiatus. Collagen fibers run predominantly circumferentially; loading perpendicular to this orientation reduces tensile strength to less than 10% of the circumferential value.

• Lateral meniscus bears approximately 70% of lateral compartment load; medial bears roughly half of medial compartment load.
• Perimeniscal vessels supply only 10% to 30% of medial and 10% to 25% of lateral meniscal width.
• Attachment sequence anterior to posterior: medial anterior horn, ACL, lateral anterior horn, lateral posterior horn, medial posterior horn, PCL.

Tear Patterns

Longitudinal tears run parallel to the margin, preserving hoop stress. A complete longitudinal tear displacing the inner fragment into the intercondylar notch is a bucket-handle tear, presenting as true locking. A ramp lesion is a longitudinal tear within 3 mm of the meniscocapsular junction of the medial posterior horn, associated with ACL injury and missed on MRI in more than half of cases.

Radial tears extend perpendicular to the circumferential fibers. When one reaches the capsular margin, hoop stress is abolished — equivalent to total meniscectomy in load transmission. Root tears are radial tears at or within 1 cm of the bony attachment; extrusion greater than 3 mm on MRI confirms hoop failure. Horizontal tears divide the meniscus into superior and inferior leaflets and are more common in older patients.

• Stable longitudinal tear: inner fragment cannot be displaced more than 3 mm from the peripheral rim with a probe.
• Bucket-handle inner fragment spans at least two-thirds of the meniscal circumference.
• Medial meniscus is affected by longitudinal tears about 3 times more often than the lateral.

Healing Potential and Repair Indications

Healing depends on vascular supply: the red-red zone heals reliably; the white-white zone does not. The ideal repair candidate is an unstable vertical longitudinal tear longer than 1 cm in the vascular zone in an active patient with a stable or simultaneously reconstructed knee; combined with ACL reconstruction, approximately 90% heal. Tears longer than 4 cm fail to heal in more than half of cases. Root tears warrant transtibial pull-out repair in active patients without advanced arthritis (Outerbridge 3 or 4); partial meniscectomy does not halt arthritic progression after a root tear.

Repair Techniques

Inside-out repair uses a cannula from the contralateral portal; vertical mattress sutures are placed every 3 to 4 mm on the medial side and 4 to 5 mm on the lateral side and tied over a posteromedial or posterolateral accessory incision. The knee is held at 15 to 20 degrees of flexion medially and near 90 degrees laterally to protect the saphenous and common peroneal nerves. Outside-in repair with an 18-gauge needle and suture shuttle suits the anterior horn where the inside-out angle is poor. All-inside devices use two capsular anchors and a pre-tied sliding knot through standard portals; the needle stop should be set to 14 or 16 mm. Bucket-handle repair success rates: 83% inside-out versus 81% all-inside.

Meniscectomy and Consequences

Partial meniscectomy removes only unstable tissue while preserving a balanced peripheral rim. Resecting even one-third of the meniscus can raise contact forces by up to 350%. Medial total meniscectomy reduces contact area by 50% to 70% and raises contact stress by 100%; lateral total meniscectomy raises contact stress by 200% to 300%. Subtotal meniscectomy removes part of the peripheral rim in the posterior horn, opening the circumferential ring. Fairbank changes follow total meniscectomy: joint space narrowing, femoral condyle flattening, and osteophytes.

• After medial partial meniscectomy, 88% to 95% good to excellent results at medium-term follow-up.
• Meniscal allograft is considered for symptomatic patients under 40 to 50 after total or subtotal meniscectomy with normal alignment and Outerbridge I or II cartilage.