Why do we Need Menisci and How Can They Help the knee joint?

The knee joint is the biggest and the most important joint in the human body. This is because it does two main functions:

  1-Transmits the weight from the upper part to the lower part of the body.

  2-Moves in many different directions and in many different angles.

These two functions require the knee joint to be flexible and stable at the same time. The medial meniscus is mainly responsible for the stability of the knee because it is fixed, while the lateral meniscus is mainly responsible for the flexibility because it is not as fixed.

Why does the medial meniscus need to be more fixed?

The medial meniscus is closer to the center of the body than the lateral one. That means it receives higher pressure thanthe lateral meniscus because of the gravity. In the figure below, the line shows the direction through which the body weight transmit to the ground. The line is closer to the medial meniscus than the lateral meniscus, which means that the medial meniscus deals with much higher pressure. This is why the medial meniscus is more fixed.

Transition of the body weight 

What makes the medial meniscus more fixed than the lateral meniscus?

The medial meniscus is closely attached to two fixed structures called fibrous capsule and medial collateral ligament. These attachments contribute to the stability of the medial meniscus. on the other hand, the lateral meniscus is not closely attached to stable structures which makes it more flexible.

The attachments of the medial meniscus to the medial collateral ligament 

This great cooperation between the medial meniscus and the lateral meniscus results in a very fixed and flexible knee.

The Anterior Cruciate Ligament and The Medial Meniscus

As mentioned in one of my previous blogs, the medial meniscus is made up of three parts; anterior horn, posterior horn, and peripheral border. The most part that is subjected to be torn is the anterior horn.  Current anatomical research reported that there is a relationship between the medial meniscus anterior root tears and the anterior cruciate ligament position.
The anterior horn was recently found to be highly subjected to detachment from its normal point of insertion during anterior cruciate ligament reconstruction surgery-the surgery to relink the anterior cruciate ligament back to its position after tears-.

           

Why doctors can't do it accurately?
Doctors and researchers don’t really have a baseline data for the exact locations of the insertion points of the structures of the knee that they can use to detrain the insertion point. When doctors try to reconstruct the anterior cruciate ligament after tears they don’t usually reinsert it back to its exact insertion point. The exact point of insertion is also not always clear by just looking at the site of injury. Added to that, the two knees of one person don’t always have the exact insertion points, which makes using the other knee as a baseline for the insertion point almost useless.

Anterior Cruciate ligament 

What is the result if the reconstruction was not accurately done?
Scientists believe that, if the anterior cruciate ligament wasn’t inserted back to the exact point of insertion a bad consequence is more likely going to happen. The tension will increase on the anterior horn of the medial meniscus, which in turn will increase the chances of tears of the anterior horn of the medial meniscus.

The tension is equally distributed among knee structures and any change of the location of any structure will increase the tension on at least one structure and decrease the tension on at least one other structure, which in turn lead to injuries many problems to the knee.

What should happen?
Researchers should do more research to collect data about the exact points of insertions in all the knee structures. This will help doctors to be more precise when doing reconstruction surgeries to knee structures.

The Embryological Development of Menisci

The embryological meniscal development seems to have an effect over the shaped the of the complete meniscal discoid and the incomplete meniscal discoid. Many studies investigated the shaped the of the complete meniscal discoid and the incomplete meniscal discoid. In one of the studies they were found to be the main causative factors of pain in the knee joint. The variations of the shape of the discoid menisci are explained by embryological meniscal development.

Incomplete discoid 

                                                                Complete discoid 
Embryological meniscal development

The meniscal arises from the differentiation of mesenchymal tissue within the limb bud by the eighth week of fetal development. During O’Rahilly stage 22, the menisci arises from the eccentric portions of the articular inter zone but it is not clearly noticeable until the ninth week of development.

            The blood supply during embryologic development enters from the periphery and continues throughout the entire width of each meniscus. However, the central third will receive blood supply by the ninth week after birth, and by adulthood, the peripheral one third is the only part that will be vascular. At the ninth week of the development there will be no more microscopic structural changes and the meniscus will grow at the same level as all the other intra-articular structures. By the fourteenth week the menisci will have a normal adult relationship with the other structures of the knee.

Functions and Attachments of the anterior horn, posterior horn, and middle boarder of medial meniscus

The medial meniscus is an important primary stabilizer and weight-transmitter in the knee joint. It preforms an important function in limiting knee motion. The medial meniscus consists of three main parts. Each part has its own function and together the three parts make up the medial meniscus function. 

The three parts are:

1- anterior horn

2- posterior horn

3- peripheral border 

How do they function together?

The anterior horn of the medial meniscus carries most of the load during the first 30-A˚ of  the knee flexion, and after that the posterior horn carries most of the load. The anterior horn after 30 degrees of the knee flexion plays an important role in controlling the anterior femoral displacement. The peripheral border distributes the weight during the transition of force from the anterior horn to the posterior horn. 

A new study suggested that the attachments of the three parts of the of the medial meniscus are associated with the function and any abnormality in the attachment locations may decrease the quality of the function.

Anterior horn, posterior horn, and peripheral border attachments

 The anterior horn of the medial meniscus is attached to the anterior tibial intercondyler area anterior to the anterior cruciate ligament showing in fig. 1. and the posterior fibres of the anterior horn are continuous with the transvers ligament. The posterior horn is fixed with the posterior tibial intercondyler area, between the attachment of the lateral meniscus and the posterior cruciate ligament showing in fig. 2. The peripheral border is attached to the fibrous capsule and the deep surface of the medial collateral ligament. The tibial attachment is known as the coronary ligament. Together these attachments limit the movement of the medial meniscus and make it relatively more fixed and limited in movements than the lateral meniscus .

Fig. 1 The attachment of the anterior horn of the medial meniscus  indicated by the black arrow. Fig. 2 The attachment of the posterior horn of the medial meniscus indicated by the black arrow.