The Knee Joint Has a Structure Made of Cartilage, Which Is Called the Meniscus Or Meniscal

INTRODUCTION

The knee joint has a structure made of cartilage, which is called the meniscus or meniscal cartilage. The menisci are the shock-absorbers of the knee - wedged horizontally inbetween the femur and the tibia. They fill in the incongruency between the rounded ends of the femur bone and the flattened ends of the tibia bone upon which the femur sits.

Menisci are squeezed between the rounded ends of the femur (the femoral condyles or rounded ends of the thigh bone) and the flat upper surface of the tibia (the tibial plateau or upper surface of the shinbone) - so they are difficult to see, and hard to explore.

A torn meniscus is a disruption of the fibrocartilage pads located between the femoral condyles and the tibial plateaus. The medial and lateral meniscus provides shock absorption and plays a role in joint lubrication.

Meniscal injuries are the most common surgically treated knee injury. Reported rates of meniscal injury are approximately 70 per one lakh (according to US Statistical Data). Men are affected more than women. Meniscal injuries can occur in all age groups. In older patients tears are predominantly degenerated and are commonly caused by activities of daily living, squatting or activities involving deep flexion. In younger patients upto 1/3rd of meniscal tears are sports related and are primarily caused by twisting or cutting movements, hyperflexion or trauma. In all sports with the exception of wrestling, tears of the medial meniscus occur more often than tears of the lateral meniscus.

Meniscal injuries often occur in knee pathology, although with different etiologies. Such injuries may occur (i) as part of a rotational trauma, (ii) due to bending, as a result of progression of a degenerative process, or (iii) as a spontaneous injury caused by fatigue.

The different etiologies converge into the same symptomatology, with similar clinical manifestations and treatments, although different therapeutic results are expected. When associated with the instability of the knee or with arthrosis at an advanced stage, meniscal injury is analyzed as a function of the major pathology.

The physiotherapy management of meniscal injuries involves shifting the focus of case towards increasing activity tolerance, prevention of recurrence apart from treating the pain alone.

DEFINITION

Injuries to the crescent-shaped cartilage pads between the two joints formed by the femur (the thigh bone) and the tibia (the shin bone). The meniscus acts as a smooth surface for the joint to move on.

The two menisci are easily injured by the force of rotating the knee while bearing weight. A partial or total tear of a meniscus may occur when a person quickly twists or rotates the upper leg while the foot stays still (for example, when dribbling a basketball around an opponent or turning to hit a tennis ball). If the tear is tiny, the meniscus stays connected to the front and back of the knee; if the tear is large, the meniscus may be left hanging by a thread of cartilage. The seriousness of a tear depends on its location and extent.

Types

The pattern of meniscus tear is important because it will determine the type of treatment receive (some tears will heal on their own, some can be treated surgically and some can't be fixed). Tears come in many shapes and sizes however there are 3 basic shapes for all meniscal tears: longitudinal, horizontal and radial. If these tears are not treated, they may become more damaged and develop a displaced tear (moving flap of meniscus). Complex tears are a combination of these basic shapes and include more than one pattern.

A Longitudinal meniscus tear (circumferential tear) extends along the length of meniscus and does not go all the way through. This tear divides meniscus into an inner and outer section; however the tear generally never touches the rim of the meniscus. It tends to be more medial than lateral, and results from repeated movements. It generally starts as a partial tear in the posterior horn, which can sometimes heal on its own. However if it doesn't heal properly it can lead to a displaced longitudinal tear, known as a displaced

Bucket Handle tear. This is a complete tear that goes all the way through and is located near the inner rim of medial meniscus; it is often associated with a radial tear. This tear accounts for 10% of all meniscus tears, and causes the knee to lock in flexion. It is seen most often in young athletes, and happens in conjunction with 50% of ACL injuries.

A Horizontal meniscus tear (cleavage tear) starts as a horizontal split deep in the meniscus. This tear divides the meniscus into a top and bottom section (like a sliced bun). It is often not visible, and moves from the posterior horn or mid section to the inside of the meniscus. This tear is rare and often starts after a minor injury from rotation in the knee or degeneration. It occurs frequently in the lateral meniscus; however it is noted in both menisci. A displaced.

Horizontal Flap tear can develop if the tear is overlooked or left alone. This type of tear is horizontal on the surface of the meniscus and creates a flap that flicks when the knee moves. It is a result of a strong force that tears the meniscus from the inner rim; it can easily become a complex tear. If this tear extends from the apex of the meniscus to the outer rim, one may develop a meniscal cyst (a mass that develops from a collection of synovial fluid along the outside rim of the meniscus).

A Radial split meniscus tear (free-edge transverse tear) starts as a sharp split along the inner edge of the meniscus and eventually runs part way or all the way through the meniscus, dividing it into a front and back section (across the middle body instead of down the length). This tear generally occurs between the posterior horn and middle section and is seen frequently in the lateral meniscus. A small tear is difficult to notice, but when it grows and becomes a complete tear it will open up and look like a part is missing. This is called a Parrot's Beak tear (displaced radial tear with a curved inner portion). It generally occurs in the thicker portion of the lateral meniscus. As it gets larger, it will catch or lock more frequently, and prevent the meniscus from protecting the cartilage during weight bearing. This tear is a result of a traumatic event or forceful and repetitive stress activities; it is often associated with other injuries. Young athletes tend to suffer from combination tears called radial/parrot beak tears (the meniscus splits in 2 directions).

ANATOMY

Although the knee joint may look like a simple joint, it is one of the most complex. Moreover, the knee is more likely to be injured than is any other joint in the body. We tend to ignore our knees until something happens to them that causes pain. As the saying goes, however, "an ounce of prevention is worth a pound of cure."

The knee is essentially made up of four bones. The femur, which is the large bone in thigh, attaches by ligaments and a capsule to tibia. Just below and next to the tibia is the fibula, which runs parallel to the tibia. The patella, or what we call the knee cap, rides on the knee joint as the knee bends.

When the knee moves, it does not just bend and straighten, or, as it is medically termed, flex and extend. There is also a slight rotational component in this motion. This component was recognized only within the last 50 years, which may be part of the reason people have so many unknown injuries. The knee muscles which go across the knee joint are the quadriceps and the hamstrings. The quadriceps muscles are on the front of the knee, and the hamstrings are on the back of the knee. The ligaments are equally important in the knee joint because they hold the joint together.

The knee joint also has a structure made of cartilage, which is called the meniscus or meniscal cartilage. The meniscus is a C-shaped piece of tissue which fits into the joint between the tibia and the femur. It helps to protect the joint and allows the bones to slide freely on each other. There is also a bursa around the knee joint. A bursa is a little fluid sac that helps the muscles and tendons slide freely as the knee moves.

To function well, a person needs to have strong and flexible muscles. In addition, the meniscal cartilage, articular cartilage and ligaments must be smooth and strong. Problems occur when any of these parts of the knee joint are damaged or irritated.

The medial meniscus is semicircular and attached to the medial collateral ligament (medial collateral ligament) of the knee joint. It only moves 2-5 mm within the joint and is hence more prone to tears than the lateral meniscus which is more circular in shape and moves 9-11mm.

The lateral meniscus is often injured at the same time as the Anterior Cruciate Ligament (ACL), whereas the medial meniscus is itself more prone to tears in the chronically 'ACL deficient' kneeBucket Handle Meniscus Tear.

Blood supply

The blood supply to the menisci is limited to their peripheries. The medial and lateral geniculate arteries anastomose into a parameniscal capillary plexus supplying the synovial and capsular tissues of the knee joint. The vascular penetration through this capsular attachment is limited to 10-25% of the peripheral widths of the medial and lateral meniscal rims. In 1990, Renstrom and Johnson reported a 20% decrease in the vascular supply by age 40 years, which may be attributed to weight bearing over time.

The presence of a vascular supply to the menisci is an essential component in the potential for repair. The blood supply must be able to support the inflammatory response normally seen in wound healing. Arnoczky, in 1982, proposed a classification system that categorizes lesions in relation to the meniscal vascular supply.

An injury resulting in lesions within the blood-rich periphery is called a red-red tear. Both sides of the tear are in tissue with a functional blood supply, a situation that promotes healing.

A tear encompassing the peripheral rim and central portion is called a red-white tear. In this situation, one end of the lesion is in tissue with good blood supply, while the opposite end is in the avascular section.

A white-white tear is a lesion located exclusively in the avascular central portion; the prognosis for healing in such a tear is unfavorable.

BIOMECHANICS

The menisci provide several integral elements to knee function. These include load transmission, shock absorption, joint lubrication, and joint nutrition, distribution of load, amount of contact force and stability.

The menisci act as a structural transition zone between the femoral condyles and tibial plateau. As such, they increase the congruence between the condyles and the plateau. The menisci appear to transmit approximately 50% of the compressive load through a range of motion of 0 to 90 degrees. The contact area is increased, protecting articular cartilage from high concentrations of stress. The circumferential collagen fiber orientation within the meniscus is uniquely suited to this capacity. As load is applied, the menisci will tend to extrude from between the articular surfaces of the femur and tibia. In order to resist this tendency, circumferential tension is developed along the collagen fibers of the meniscus as hoop stresses. The circumferential continuity of the peripheral rim of the meniscus is integral to meniscal function.

The menisci follow the motion of the femoral condyle during knee flexion and extension. During extension, the femoral condyles exert a compressive force displacing the menisci antero posteriorly. As the knee moves into flexion, the condlyes roll back ward onto the tibial plateau. The menisci deform medial laterally, maintaining joint congruity and maximal contact area. As the knee flexes, the femur externally rotates on the tibia, and the medial meniscus is pulled forward.

AETIOLOGY

All the knee injuries are more common in women than men, men experience more meniscus injuries and tears (ratio 2.5:1 (Male : Female)) this is belief to be due to men’s participation in more aggressive sports and manual activities. The peak incidence of meniscal injuries for males is between 31 – 40 years whereas for females peak incidence is in between 11 - 20 years.

The two most common causes of meniscus injuries are acute trauma to the knee and degeneration of the knee joint.

Occupations such as mining or carpet laying (squat position), or participation in contact sports or repetitive stress activities (such as running and skiing) or prone to meniscus injuries.

Acute or traumatic meniscus damage:

It can result from forceful rotating of a straight or bent knee while foot is firmly planted and bearing weight, or from hyperflexion or hyper extension of knee. These injuries are experienced most frequently in activities such as Rugby, football, baseball, soccer, basketball when one twist or pivot on the knee, or slow down too quickly. The result will generally be a partial complete medical meniscus tear. This type of tear generally affects athletes or those under 40 years of age.

A medial meniscus tear will frequently occur along with other injuries such as MCL or ACL tear. The combined injuries are seen most often in contact sports, when an athlete gets hit on the outside of a bend knee.

A lateral meniscus tear will result more often from a knee i.e.., bent excessively and experiences full weight bearing, while the thigh bone is turning outward: seen in sports such as skiing. It can also be injured in collisions that involve deep knee bends.

Degeneration of the knee joint

It involves weakening of tissues with age, which results from small repetitive movements such as squatting or pivoting positions,. Or a minor meniscus injury that never healed properly. In the younger people meniscus is very flexible and pliable (like a new rubber tire) as they get older it becomes less flexible and more brittle, it also develops cracks in it (like those seen in an aged car tire).

Articular cartilage and meniscus detoriate as age advances, which can eventually lead to a degenerative tear without any major trauma. There will be a 20 percent decrease in blood supply to menisci by age 40 due to weight bearing over time; this inhibits body’s ability to heal itself. This wear and tear over the years may lead to an osteoarthritis condition. Approximately 60 percent of people over 65 years of age experience some form of degenerative meniscus tear.

A Discoid meniscus occurs when are born with a more flat, disc shaped meniscus rather than a crescent shaped, wedge meniscus. It is generally found in the lateral meniscus and in kids less than 11 years of age. The symptoms associated with a discoid meniscus can range from very mild to continuous clicking, snapping, buckling and locking of the knee joint, decreased range of motion, joint pain and tenderness, and atrophied quadriceps (muscles wasting away). The meniscus will often change to a C-shape with maturity and Kids/teens will grow out of their symptoms; however failure of normal development can be experienced.

PATHOPHYSIOLOGY

There are two different mechanisms for tearing a meniscus.

Meniscal tears are common and can be traumatic or degenerative. Traumatic tears occur classically during twisting forces on the knee in young active people, are often vertical longitudinal tears and can be associated with ligamentous injuries. Degenerative tears occur as part of progressive wear in the whole joint, most frequently in the over 40's. These tears are usually horizontal cleavage tears or flaps and have minimal healing capacity. Tears can be described as being complete or incomplete, stable or unstable and of various patterns.

Traumatic tears result from a sudden load being applied to the meniscal tissue which is severe enough to cause the meniscal cartilage to fail and let go. These usually occur from a twisting injury or a blow to the side of the knee that causes the meniscus to be levered against and compressed. A football clipping injury or a fall backwards onto the heel with rotation of the lower leg are common examples of this injury pattern. In a person under 30 years of age this typically requires a fairly violent injury although any age group can sustain a traumatic tear.

Degenerative meniscal tears are best thought of as a failure of the meniscus over time. There is a natural drying-out of the inner center of the meniscus that can begin in the late 20's and progresses with age. The meniscus becomes less elastic and compliant and as a result may fail with only minimal trauma (such as just getting down into a squat). Sometimes there are no memorable injuries or violent events which can be blamed as the cause of the tear. The association of these tears with aging makes degenerative tears in a teenager almost unheard of.