How structure relates to function of the meniscus of the knee

The meniscus is a fibrocartilaginous tissue that serves to transit and distribute force, stabilize the knee joint and aid in nutrition and joint lubrication, It also decreases contact stress and increases contact area and congruity of the knee joint.¹ Located between both the medial and lateral femoral condyles and tibial plateau, the meniscus is subjected to shearing stress, tensile hoop stress, compressive force and circumferentially directed force.² As a result of these multiple and complex functions, the meniscus has adopted a unique microscopic and macroscopic anatomy made up of cells, specialised extracellular matrix (ECM) and region specific innervation and vascularisation to become a complex tissue.³ 

Blood vessels and nerves from the surrounding joint capsule and synovial tissues innervate and provide nutrients for around 10–25% of the outside of the adult meniscus.⁴ Thus the meniscus can be categorised into three regions, the outer vascular/neural region (red/red zone), inner avascular/aneural region (white/white zone) and the conjoining middle region (red/white zone).⁵ 

Microscopic Anatomy

On a microscopic level these functions are facilitated by a heterogeneous ECM and cell population.⁶ Cells within the meniscus produce and maintain the ECM and the ECM determines the material proprieties of the tissue. The cells in the outer red/red zone are termed fibroblast like due to their oval, fusiform shape. These cells facilitate communication between other cells and the ECM. Additionally, the matrix surrounding these cells is largely made up of type I collagen fibres (80%). In contrast, cells within the inner white/white zone are termed fibrochondrocytes and appear circular and embedded in an ECM made of predominantly type II (60%) and I (40%) collagen fibres.^6,7 This discrepancy leads to a cartilaginous inner zone, similar to that of articular cartilage and a fibrous outer zone, similar to fibrocartilage. 

Collagen provides the tensile strength of the meniscus and due to the interspersed layers of radially and circumferentially orientated fibres, it is able to provide structural integrity.⁸ Under compressive load, radially oriented fibres resist shearing force and circumferential oriented fibres resist tensile hoop stress.⁹  As a result of these functions collagen can be considered a main structural component used to manage shearing stress, tensile hoop stress, compressive and circumferentially directed force. Additionally, within the outer red/red zone these interwoven layers create a honeycomb-like structure, known as tie fibres and it has been noted that blood vessels also follow the path of these tie fibres and may be mechanically protected by them.¹⁰ Mechanoreceptors within the meniscus also serve to convert tension and compression into an electrical nerve impulse, providing a proprioceptive function and aiding joint stability.¹¹ 

Proteoglycans are also present in the ECM and function in providing hydration and an increased capacity to resist compression.¹² Hydration is achieved though highly sulphated glycosaminoglycan side-chains which absorb and hold water, under compressive load this retention of water is able to provide resistance. Additionally synovial fluid secreted from the joint capsule is compressed into both the articular cartilage and meniscus allowing for the uptake of water and solutes found in synovial fluid.¹³ This highlights the relationship between compression, retraction and meniscus health. Proteoglycans are also more abundant within the inner white/white zone, possibly due to their high capacity to resist compression and the need for nutrient uptake within the avascular white/white zone.¹⁴ Therefore proteoglycans can be considered the structural component that enables the meniscus to function in the nutrition, lubrication, protection and both transmission and distribution of force.¹⁵ 

Macroscopic Anatomy

On a macroscopic level the meniscus is divided into either the medial and lateral menisci. The main stabilising ligaments include the medial collateral, transverse and meniscofemoral ligaments, as well as attachments at the anterior and posterior horns.¹⁶ The medial and lateral menisci increase the contact surface area of the tibial femoral joint thanks to their semilunar wedge shape which wraps the curved femoral condyles. As a result, meniscus is adept at distributing compression forces across a wider surface area, reducing stress and providing greater stability by limiting excessive motion. 

The meniscus’s wedged shape also converts compressive force into horizontal hoop stress and due to the collagen fibre orientation mentioned earlier, the meniscus is more adept at resisting circumferential tissue tension.¹⁷ Conversely, differences between the medial and lateral menisci, medial being more ‘C shaped’ and the lateral more circular shaped; is reflected in the distribution of stress. With deformation seen to be unchanged across the medial menisci, yet the lateral menisci is more flattened in the middle. This results in a greater fraction of load carried by the middle part of the lateral menisci and highlights a possible structural weakness.¹⁸ 

The lateral menisci also covers a greater surface area, possibly due to its function though knee flexion. As the knee reaches 90° of flexion the lateral menisci transmits 100% of lateral compartment load, whereas the medial menisci only transmits 50% of medial compartment load. This may seem a possible weakness. However, this allows the more ‘C’ shaped medial menisci’s to function as a secondary stabiliser against tibial anterior translations using its posterior horn attachment.¹⁹

Lastly the meniscus is said to function in shock absorption, yet a recent analysis of literature naming the meniscus as a shock absorber suggested the meniscus had very little capacity for energy absorption.²⁰ This due to its resilience under load rather than deformation.²⁰ However studies have shown a 20% decrease in shock absorption capacity following meniscectomy.²¹ This identifying controversy still amongst the literature. 

Injury & Function 

The predominance of circumferentially oriented fibres leaves the tissue highly susceptible to tears in parallel to the circumferentially oriented fibres, creating bucket handle and horizontal cleavage lesions.²² This can destabilise the tibial femoral joint though a decrease in contact area and an increase in contact stress. Furthermore this type of tear and dysfunction has been shown to accelerate osteoarthritis.²³ The serve outcome is due to region specific vascularisation which leaves the avascular zone of the meniscus incapable of healing.

References

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