The kneecap: Structure and function

On the human body, there are about 49 sesame bones of different sizes. Sesamoid bones are small bones located below tendons, commonly found in the joints of the hands and feet. It acts as a pulley for the tendon to slide up, increasing the efficiency of muscle contraction. One of the best known and also the largest sesamoid bones in the body is the patella.

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• 1. Position of the kneecap
• 2. Anatomical structure of the kneecap in normal people
• 3. Function of the patella in the knee complex
• 4. What is the common pathology of the patella?

1. Position of the kneecap

The kneecap is a slightly rounded triangle, located in front of the lower head of the femur as a protective cap for the knee joint.

This sesamoid bone is located close to the skin, so it is easy to be injured in traffic accidents, labor or life accidents.

We can feel and move it most clearly in the extended knee position.

The position of the patella when viewed externally.

2. Anatomical structure of the kneecap in normal people

At birth, this bone has a cartilage structure, then begins to ossify into bone at 3-4 years of age.

Mature bone is covered with dense bone on the outside and spongy bone inside. The anatomical structure of the patella includes: 2 faces, 2 edges, 1 apex below and 1 base above.

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Two bones:

• Front: slightly convex, rough, with many notches for the quadriceps tendon to attach. If the kneecap is lost, the quadriceps lose their firm support, resulting in a weakened knee extension.
• Posterior surface, also known as articular surface: the articular surface occupies 4/5 of the posterior surface area and articulates with the patellar surface of the femur. In an adult, the joint surface is about 12 cm2 and is covered with cartilage. Due to the great pressure in the knee flexion, the articular cartilage is the thickest of the articular cartilages in the human body. This layer of cartilage can be up to 6 mm thick in the center by about 30 years of age. A ridge divides this joint face into two parts, the outer surface and the inner face. The outer surface is wider and deeper than the inner surface. The inner surface also includes a small area, called the odd face.

Bone edge

• There are 2 banks (inner and outer shore). Place of attachment of the quadriceps tendon components and the medial and lateral patellar reticular fibers, respectively.

Background

• The base for the quadriceps tendon to attach to.

Top

• Below, there is an attachment of the patellar ligament.

In rare cases there may be variations of the patella. Such as: double kneecap, one-piece defect bone.

Double patella occurs mostly in men and is divided into 3 types. Differential diagnosis is required for some fractures.

3. Function of the patella in the knee complex

As mentioned above, the kneecap is the largest sesamoid bone in the body. Therefore, its basic role is the same as that of other sesamoid bones.

The primary function of this large sesamoid bone is as a pulley for the quadriceps tendon. It increases the arm length during quadriceps contraction. As a result, the torque generated by the quadriceps muscle increases by about 33-55%. Therefore, this bone facilitates the knee extension process to occur more effectively.

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In the past, the kneecap was seen as a frictionless pulley.

In which, the force of the patellar tendon is equal to the force of the quadriceps tendon. However, recent studies show that this bone acts as a balance. It adjusts the length, direction, and force of the patellar and quadriceps tendons each to different clavicle positions from different knee flexions. When flexing the knee, the kneecap moves downward. Thus, the femoral contact position of the patella moves from distal to proximal (top to bottom). Changing the contact area results in a change in the clavicle arm and facilitates quadriceps contraction.

The patella is located between the quadriceps tendon and the condyle of the femur, so it also acts as a cushion to protect the quadriceps tendon from friction. In addition, this bone also helps to reduce the force of the quadriceps on the thigh bone, by distributing the force evenly to the bone below.

In summary, the patella plays an important role in the function of the knee joint. Both protect and stabilize the knee joint; It also plays a role in supporting the action of the quadriceps muscle.

Structure of the knee joint.

4. What is the common pathology of the patella?

Some common conditions in the kneecap include:

4.1. Dislocated kneecap

Dislocations are common and often dislocated to the side. Diagnosis is based on physical examination and radiographs to exclude fracture. Treatments are dislocation and immobilization.

Possible related injuries

• Fracture of bone cartilage or lateral condyle of the femur.

Complications may include

• Osteoarthritis.
• In patients with patellar abnormalities, recurrent dislocation or subdislocation of the patella may occur.

Diagnose

• Clinical assessment.
• X-ray to rule out fracture.

In dislocations, unless spontaneously straightened, the clinical diagnosis is based on history. If the joint is still dislocated, the patella will be palpable. The patient holds the knee slightly flexed and cannot straighten the leg. If the joint is dislocated, it will present with knee effusion and often pain around the patella.

X-ray of straight and inclined knee joints to diagnose cartilage rupture with accompanying joint dislocation.

Treatments

• Adjustment.
• Motionless.

Immediately dislocate the kneecap joint. Most patients do not need sedation or pain relief. Initially, the dislocation is hip flexion. The straightener then gently pushes the patella inward while slowly extending the knee. When the patella is out of the way, you will hear a clear click and reduce distortion around the pillow. After correcting the dislocation, immobilize the joint immediately with a knee brace or an angled knee brace with the knee flexed 20 degrees.

4.2. Patella fracture

Causes and mechanisms of bone fractures

• Mechanism of direct injury

Commonly caused by a fall hitting the knee on the ground or hitting a hard object while the knee is in a flexed position or by hitting the kneecap directly.

• Indirect injury mechanism is less common

Can be seen in athletes due to sudden flexion of the lower leg when the quadriceps muscle is in flexion. The movement puts pressure on the patella, causing a transverse fracture.

Diagnose

• Diagnosis is confirmed by clinical examination. The clinical symptoms alone are enough to confirm the diagnosis of patellar fracture. X-ray film of the knee joint to clearly determine the fracture location, degree of displacement and damage in adjacent bones….

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Treatment of kneecap fractures

• First aid:

In case of kneecap fracture, first aid should be followed by the following steps:

Use pain relievers.

+ Temporarily fixed with wooden splints.

+ Then transfer the patient to the orthopedic specialist.

• Conservative treatment with cast:

+ Indications: cases of patellar fracture with displacement less than 3mm and difference in the posterior facet of the kneecap less than 1mm or fracture of the kneecap.

• Surgical treatment is used in cases of:

+ Cases of open fracture of the kneecap.

+ Displaced patellar fractures greater than allowable conservative treatment.

+ Patellar prosthesis.

In addition, there are a number of other diseases including: Osteoarthritis of the kneecap, the patella is in a higher or lower position than usual, and the bone spurs on the kneecap.

To better understand the diseases and treatments, you need to learn detailed information about each disease.

Understanding the structure and function of the patella is important for the correct diagnosis of its pathology. It is important not to miss this bony lesion in the examination of the knee joint. This omission can greatly affect a patient's long-term walking function.

Doctor Luong Sy Bac