PATELLA

The patella is the largest sesamoid bone in the human body. It is a flat and rounded triangular bone which is distally tapered and proximally curved. It is commonly known as the kneecap. It is embedded in the tendon of the quadriceps femoris muscle, lying anterior to the distal femur, i.e. near to the femoral condyles.

The patella is located deep to the fascia lata and rectus femoris tendon & anterior to the knee joint. The patella can be felt by touching the knee.

FUNCTION OF PATELLA

·       The patella acts as a mechanical advantage for the quadriceps.

Ø Moment arm: Imagine a lever. The distance between the fulcrum (pivot point) and where the force is applied (effort arm) is the moment arm. A larger moment arm translates to greater force output.

Ø Patella as a fulcrum: The patella acts as a fulcrum for the quadriceps tendon, effectively increasing the moment arm of the quadriceps force acting on the tibia. This translates to more efficient force generation during knee extension.

Ø Increased torque for full extension: As the knee straightens, the patella's position relative to the femur and tibia changes. This strategically increases the moment arm, particularly during the final 15 degrees of extension. This allows the quadriceps to generate the additional torque (rotational force) needed to achieve full knee extension.

 

·       Multiple factors influence how the kneecap (patella) glides and interacts with the femur during knee movement:

Ø Active quadriceps contraction: The quadriceps muscle pulling on the patellar tendon directly influences patellar motion. As the quadriceps contract during extension, they pull the patella upwards (superior glide).

Ø Connective tissue extensibility: The flexibility of ligaments and tendons surrounding the patella can affect its gliding ability. Tightness in these tissues can restrict proper movement.

Ø Patellar and trochlear groove shape: The specific shape of the patella and the groove on the femur (trochlear groove) where it glides can influence tracking and stability. Abnormalities here can lead to malalignment.

 

·       Patellar movement during knee flexion and extension:

Ø Superior and inferior glide: During knee extension, the patella glides upwards (superiorly) on the femoral trochlear groove as the quadriceps contract. Conversely, as the knee flexes, the patella glides downwards (inferiorly).

Ø Medial and lateral glide: In normal movement, the patella should have minimal side-to-side (medial or lateral) gliding. However, with full knee extension, the patella may have a slight lateral position due to leg rotation.

 

·       Patellar contact area changes with knee flexion:

Ø Contact point shift: As the knee flexes, the area of the patella contacting the femur changes. The contact point moves downwards and backwards on the femoral condyles, and upwards on the patella itself.

Ø Initial contact: During early flexion, the patella's outer side (lateral facet) makes contact with the top of the outer femoral condyle.

Ø Equal distribution: By 30 degrees of flexion, the contact area spreads more evenly across both sides of the patella and femoral condyles.

Ø Expanding contact surface: With increasing flexion, the area of contact between the patella and femur increases, distributing pressure and reducing stress on the joint. This expands from around 2 cm² at 30° flexion to 6 cm² at 90° flexion.

Ø Deep flexion contact: In very deep flexion (around 90° and beyond), the patella glides over the notch between the femoral condyles, with contact only on the inner and outer edges. Finally, at full flexion, the only contact point is between a specific area of the patella (odd facet) and the outer surface of the inner femoral condyle.

 

STRUCTURE OF PATELLA

Patella is having anterior surface which is concave and posterior surface is convex. The anterior and posterior surfaces are joined by a thin margin and towards the centre by a thicker margin. It has 2 surfaces, 3 borders and an apex which is all palpable.

·       There are surfaces Anterior and posterior which is also articular:

     § Anterior Surface: The surface is subcutaneous, convex and is perforated by nutrient vessels. It is longitudinally ridged and is separated from the skin by the subcutaneous prepatellar bursa and is covered by the tendon of quadriceps femoris.

    §   Posterior Surface: It has a smooth proximal surface, oval articular area which is crossed by a smooth vertical ridge, which fits the intercondylar groove on the femoral patellar surface. The patellar articular area divides into 2 facets: medial and lateral.

§ Lateral Facet: It is larger than the medial surface. It articulates with the lateral condyle of the femur.

§  Medial Facet: It articulates with the medial condyle of the femur. It has an attachment of the patellar ligament and contains fatty pad known as infrapatellar fat pad.

There are 3 borders to the patella:

·       Medial and Lateral Borders: They are located medial and lateral to the patella. They are relatively thin, converge inferiorly and provides attachment to the muscles through the expansions of the lateral retinaculum.

·       Superior Border: It is also known as the base. It is thickest and slopes anteroinferiorly.

Apex

·       The apex is positioned proximally to the line of knee joint by 1 to 2 cm.

·       It is pointy, directing downwards and is non articular on the tip of bone.

MUSCLE ATTACHMENTS

·       Quadriceps femoris is attached to the superior surface of patella except near posterior margin. It extends distally onto the anterior surface.

·       Rectus femoris is attached anteroinferior to the vastus intermedius.

·       At the lateral upper 1/3^rd  and medial upper 2/3^rd  borders represent attachment to the vastus lateralis and medialis respectively.

·       At the apex of patella, there is attachment to the patellar ligament.

·       The non-articular area on the posterior surface provides attachment to the ligamentum patellae below.

BLOOD SUPPLY

The patella is having a huge vascular network which is separated into extraosseous and intraosseous:

·       Extraosseous anastomosis ring has anterior tibial recurrent arteries form a ring around patella and also the geniculate arteries

·       Intraosseous anastomosis has mid patellar vessels which enter the vascular foramina on the anterior surface of middle 1/3^rd of patella and the polar vessels enter between ligamentum patellae and articular surface of patella.

NERVOUS SUPPLY

The anterior cutaneous innervation of the knee is derived from L2 to L5 nerve roots. The genitofemoral, femoral, obturator and saphenous nerves supply the anteromedial part of the knee joint. The lateral femoral and lateral sural cutaneous nerves supply the anterolateral part of the knee joint.

OSSIFICATION

The patella develops from a continuous band of fibrous connective tissue in the mesenchymal interzone along the knee joint surface at the distal femoral margin. At about the 9^th week of gestation, chondrification of this fibrous band which begins and gives rise to the quadriceps tendon superiorly and the patellar ligament inferiorly. Patella becomes completely cartilaginous by 14^th week of the gestation. The medial and lateral patellar facets are equal in size at first but by the 23^rd week of gestation lateral facet becomes larger than the medial facet.

Primary ossification may be present by the age of 2 to 5 years. Periosteum forms early on the anterior patellar surface. All the ossifying centres fuse at the puberty.