Not Created Equal

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The skeletal structures of the knee joint are comprised of the femur, tibia and patellae. The stabilizing ligaments of the knee are the lateral collateral ligament (LCL), anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) and the medial collateral ligament (MCL). The meniscal cartilage of the knee is comprised of the medial meniscus and lateral meniscus.

Articular Cartilage Injuries

Articular cartilage covers the distal femur and the proximal tibia and is thickest in the patellofemoral compartment. The patella is a vital component of the native knee. One key function is allowing the extensor mechanism, a mechanical advantage via a fulcrum-like action to maximize its strength and function.

Articular cartilage may be injured traumatically or be part of a degenerative process known as chondromalacia or degenerative arthritis. Traumatic articular cartilage injuries have little if any ability to regenerate and undergo healing. Bone is rather coarse, even rough. Rubbing sandpaper on sandpaper would grind bones down to dust in brief time. The smooth articular cartilage covers the weightbearing surfaces of the bones.

Figure 1. Patellofemoral joint: Articular cartilage injury of the undersurface of the patella. All images courtesy Gordon Huie.
Figure 2. Axial view of the patella, demonstrating the patella-trochlear relationship.

Articular cartilage is a form of hyaline cartilage that contains no vessels and nerves and is predominantly comprised of water. It prevents the contact of bone at the synovial joint, contributes to joint stability, and absorbs shock to allow for the knee to function at high velocity.1 Seventy percent of articular cartilage is comprised of water, which prevents deformation of the cartilage when it is compacted. In contrast, only 2% is composed of chondrocytes, even though these substances play an influential role in the structure of articular cartilage.2

Chondrocytes are terminally differentiated cells that are highly specialized and multifunctional. They not only secrete angiogenesis inhibitors to maintain the avascular nature of the articular cartilage, but they also synthesize and maintain its extracellular matrix.2 These highly specialized and multifunctional cells predominantly produce type II collagen and aggrecan, a proteoglycan, the primary components of the extracellular matrix. Type II collagen molecules are organized in cross-linked fibrils that are able to trap the negative charge of aggrecan, the most abundant proteoglycan in the extracellular matrix.3 These fibrils aid in the maintenance of structural integrity of the extracellular matrix.

Another important factor in maintaining the extracellular matrix is the nourishment of the cells that produce its constituents. While glucose serves as the major source of energy in articular cartilage, synovial fluid lubricates and nourishes chondrocytes.4 Nutrients such as plasma and hyaluronan reach chondrocytes by means of diffusion during compression-relaxation cycles.5 This, in turn, facilitates the production of extracellular matrix components that contribute to the structural integrity of articular cartilage.

Mature articular cartilage is made up of four particular zones: superficial tangential zone, transitional zone, deep zone and the calcified cartilage zone. As each zone moves inward toward the subchondral bone, the concentration of proteoglycans produced by chondrocytes increases, while the concentration of water and collagen decreases. The superficial tangential zone is closest to the articular surface, has the highest concentration of water, and is made up of flattened chondrocytes.2,5

Relative to the three other zones, these chondrocytes produce a low concentration of proteoglycans and a high concentration of collagen. The transition zone is responsible for most of the weight in articular cartilage. It is large in diameter, rich in collagen, and is composed of spherical chondrocytes that produce a matrix.2

The deep zone has a comparatively higher concentration of proteoglycans, predominantly aggrecan. Chondrocytes in this zone are grouped in clusters and have a larger volume than the flattened cells of the superficial tangential zone.2 Here, collagen is grouped together and arranged in a radial fashion with small cell density.6 Collagen in the deep zone passes to the calcified zone and anchors articular cartilage to the subchondral bone. The calcified zone serves as a buffer between the deep zone, which is uncalcified, and the subchondral zone.7 Each of the four zones and their unique characteristics make articular cartilage a heterogeneous tissue.

Articular cartilage also covers the undersurface of the patellae. This allows the patellae to glide through the trochlear femoral groove, which aids in the extensor mechanism by being a fulcrum. Their thickness is about 3 millimeters. Articular cartilage injury is called chondromalacia. This is not limited to the patellofemoral joint; it can also affect the femorotibial joint. Chondromalacia is technically early arthritis of the knee. Once the disease is initiated, it cannot be cured and new cartilage cannot be regenerated (Figures 1 and 2).8

Meniscal Cartilage Tears

Meniscal cartilage is rubbery in consistency. One of its functions is to protect the knee during axial load, acting as shock absorbers. The meniscus is trapped when the knee is in the flexed position. A meniscal tear may occur with routine kneeling, coupled with a rotational motion. This applies a torque to the entrapped meniscus. The mechanism of injury is due to the static structures of the lateral collateral ligament and the medial collateral ligament not having the elasticity to permit the entrapped meniscus to gain freedom from a flexed position of the posterior femoral condyle articulating with the tibial plateau.


Morphology

The meniscus is located between the femoral condyles and tibial plateaus and is composed of the lateral and medial meniscus. It is a fibrocartilagenous structure that is anteriorly and posteriorly attached to subchondral bone by insertional fibers.6 The lateral meniscus is circular in shape, and the medial meniscus is C-shaped and larger in diameter. Both menisci are attached to at the anterior horn by the intermeniscal ligament.6

Figure 3. Illustration of the “C” shaped menisci. Thicker outer periphery comes into a tapered edge.
Figure 4. Arthroscopic picture of a normal meniscus. Note the tapered inner third.
Figure 5. Illustration of a radial tear of the meniscus. All illustrations by Gordon Huie.
Figure 6. Arthroscopic picture of a radial tear of the menisci. Image courtesy the authors.

The meniscus is predominantly composed of circumferentially oriented collagen fibers and radial and perforating collagen fibers. The orientation of tightly woven collagen fibers plays a role in the differentiation of meniscal tears. The meniscus is thicker in the outer periphery and comes in to a tapered edge. This is thought to contribute to medial lateral stability of the knee (Figures 3 and 4).

Radial Meniscal Tears: Radial tears can occur simultaneously with other tears. The injury begins at the inner part of the meniscus and moves to the periphery. Radial meniscal tears are vertically oriented and can be incomplete or complete, depending on the extent of injury. Isolated radial tears can be small and propagate to a more complete tear (Figures 5 and 6).10

Bucket Handle Meniscal Tears: Bucket-handle tears are unstable and complete. A bucket handle meniscal tear may start as acute longitudinal tear. Tears can occur at the posterior segment of the medial or lateral meniscus. A complete tear may start on the inferior surface and extend through the body to the superior surface (spans from the tibial to femoral surfaces).11 Smaller tears may be asymptomatic. In larger tears (larger than 1 cm), symptoms such as extension limitations and locking of the knee may occur.6 Locking of the knee occurs when the offending piece gets dislodged from its normal anatomical site and interferes with the femoral-tibial surfaces. This will result in an inability to full extend the knee (Figures 7 and 8).

Figure 7. Illustration of an exaggerated Bucket handle tear of the meniscus.
Figure 8. MRI coronal view of a displaced medial meniscus tear (offending piece is next to the plateau spine).

Flap Meniscal Tears: Flap tears are offending pieces that protrude into the joint space. Some larger tears are called parrot beak tears due to the shape of the tear. They can be painful and cause a grinding sensation. The torn meniscus can catch in the joint space of the articulating femur against the tibial plateau (Figures 9 and 10).6

Cleavage Meniscal Tears: Cleavage tears are common in older patients when tissue becomes less healthy. A parallel tear occurs and forms a flap between the superior and inferior part of the meniscus (Figures 11 and 12).11

Degenerative Meniscal Tears: Degenerative tears result from pre-existing damage to the meniscus. They are complex and cannot be repaired due to poor tissue quality. However, these tears usually stay in place and do not cause mechanical symptoms (Figure 13).10,11

Intrasubstance Meniscal Tears: These tears are not visible to the naked eye during an arthroscopic procedure. They involve a lesion within the body that does not violate the superior or inferior surfaces of the menisci. These tears are discovered when the arthroscopic probe is inserted. Upon palpation of the menisci, a “dip” can be felt.

Figure 9. Illustration of a flap tear of the meniscus.
Figure 10. Arthroscopic picture of a flap tear flipped over. Note the metal probe inserted within the offending flap.
Figure 11. Illustration of a split cleavage tear of the menisci.
Figure 12. Arthroscopic picture of a cleavage tear of the menisci, demonstrating a superior and inferior flap.
Figure 13. Illustration of a degenerative tear of the meniscus.
Figure 14. MRI of medial meniscus tear.

Diagnosis

The history and physical will provide valuable clues to which structures are injured. The patient is usually able to relate his or her ambulatory status; this provides clues to skeletal integrity as a differential diagnosis of fractures.

Patients with articular cartilage injuries may be able to relate the mechanism of injury and activity at the time of an acute injury. In the chronic painful knee, this may be more challenging. Not everyone’s knees are the same. For example, different patients may track their patella (while flexing and extending) at different angles to their trochlear groove. Subluxating or dislocating patellas would tend to give a more focal premature wearing away of the articular cartilage.

Axial loading can injure both articular cartilage and meniscal cartilage. The patient may be able to recount doing certain types of repetitive motion as part of exercise (running, treadmill, bicycle). This specific activity can wear down the articular cartilage over time.11

Not all meniscal cartilage injuries are due to sports or violent impact. Simply sitting in a squatted position and twisting the knee (as when washing the floor or picking up an item on the floor) can apply a shear force/torque to an entrapped meniscus, thus tearing the soft rubbery cartilage. Depending on the size and location of a meniscal tear, the patient may complain of a “locking” sensation.

The physical examination of a meniscal injury involves assessing range of motion, identifying specific areas of pain (medial joint line, lateral joint line, anterior knee), palpation for “clicks” (as in chondromalacia), and the McMurray test for detection of medial or lateral meniscus tear. There are many causes for knee effusion. Knee effusion in itself is not specific enough to provide clues to cartilage injuries only.

Diagnostic Modalities

Radiographs with weightbearing AP, lateral, sunrise (patella) and a tunnel (notch) views will allow the provider to rule out obvious fractures and loose bodies (Figure 14). Magnetic resonance imaging (MRI) is an excellent follow-up when x-rays are not conclusive. MRI lacks ionizing radiation, is noninvasive, and provides superb soft tissue contrast.

Treatment

Treatment of chondromalacia consists of an escalating approach involving conservative management, invasive and eventually operative procedures. In grade I or grade II (Outerbridge classification) chondromalacia, physical therapy should not be discounted and is effective as an initial treatment option. Making the knee muscles (hamstring and quadriceps) strong will minimize the pain associated with early arthritis, besides improving the range of motion. Applying cold therapy to induce numbness will allow the patient to increase his or her exercises. The need for weight reduction should be emphasized as well (if it is an issue).

If the patient can tolerate nonsteroidal anti-inflammatory drugs, prescribe them to reduce inflammation and permit less painful exercise.

Injection of cortisone has its benefits, but it also has limitations. Cortisone is a powerful anti-inflammatory drug. It works more as a “shotgun” than a “smart bomb.” It not only attacks the diseased tissue, it also attacks the healthy adjacent tissue. Excessive cortisone use can make tissues such as tendons prone to spontaneous ruptures. In our practice, we reserve the use of cortisone to three injections per year.

Viscosupplementation injections are commonly used in early onset arthritis. They are particularly useful in patients who are taking blood thinners and or have gastrointestinal symptoms. Between 60% and 70% of patients receive pain relief for periods averaging 6 months. Viscosupplementation may improve the patient’s knee by thickening the existing joint fluid and increasing viscosity. Its effects are temporary. Depending on which medication is selected, the relief may span 2 to 8 months. Repeat treatment regimens are available should the initial response be favorable.

Surgical arthroscopy is warranted when all the previous conservative modalities have failed.

For chondromalacia and arthritic knees, arthroscopic interventions can remove loose bodies or debris and smooth jagged articular surfaces. Surgical arthroscopy choices are limited for isolated articular cartilage injuries. Pain relief is dependent on multiple factors, such as where the injury is located (weightbearing surface?), and how thin or badly degraded the remaining articular cartilage is.

This is in contrast to surgical arthroscopy of meniscal tears. Meniscal cartilage injuries often require excision and debridement of the offending piece. If, however, the nondisplaced tear is 1 cm or less and is located in the outer one-third of the meniscus, repair of the cartilage is possible. It is more successful in the younger athletic population. Research has determined that vascular supply is located primarily in the outer one-third of the menisci.13 Repairing a meniscal tear in an area where no blood supply exists will most likely result in a poor outcome.

Understanding Needed

In today’s information era, misinterpretation or reading incorrect information about an injury can bring about a dismal result because patient expectations do not match the realities of that injury. Often, the patient compares his or her specific injury to that of friends or family members undergoing the same surgical procedure (arthroscopy) for “cartilage injury of the knee.” Healthcare providers must work to ensure that patients are fully equipped with accurate information after sustaining a meniscal injury.

Carissa Jeannette is chief research associate in the Emergency Trauma Department at Hackensack University Medical Center in Hackensack, N.J. Gordon Huie is research coordinator in the Emergency Trauma Department, Douglas Finefrock is vice chairman of the Emergency Trauma Department, and Joseph Feldman is chairman of the Emergency Trauma Department. Michael Kelly is chairman of the Department of Orthopaedic Surgery, within the Emergency Trauma Department at Hackensack University Medical Center.

References

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8. Gahunia HK, Pritzker KPH. Effects of exercise on articular cartilage. Orthop Clin N Am. 2012;43:187-199.

9. Miller III RH, Azar ST. Knee Injuries. In: Canale ST, Beaty JH. Campbell’s Operative Orthopedics. 12th ed. Philadelphia, PA: Mosby; 2013.

10. Phillips BB, Milhalko MJ. Arthroscopy of the Lower Extremity. In: Canale ST, Beaty JH. Campbell’s Operative Orthopedics. 12th ed. Philadelphia, PA: Mosby; 2013.

11. Lento P, Akuthota V. Meniscal injuries. In: Canale ST, Beaty JH. Campbell’s Operative Orthopedics. 12th ed. Philadelphia, PA: Mosby; 2013.

12. Huie G, et al. Hazards of high-tech exercise and anterior knee pain. Physician Assistant. 1997;21(3):101-104.

13. Arnoczky SP, Warren RF. The microvasculature of the meniscus and its response to injury. Am J Sports Med. 1983;11(3):131-41.

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