Patella

Introduction

The patella (derived from latin means 'small plate') is a flat, triangular bone, situated on the front of the knee-joint. 

It is the largest sesamoid bone, developed in the tendon of the Quadriceps femoris, and resembles these bones

1. being developed in a tendon
2. center of ossification presenting a knotty or tuberculated outline;
3. being composed mainly of dense cancellous tissue.

Osteologic Features

It has an anterior and a posterior surface, three borders, and an apex.

SURFACES

• The subcutaneous anterior surface is convex in all directions, perforated by small apertures for the passage of nutrient vessels, and marked by numerous rough, longitudinal striae. This surface is covered, in the recent state, by an expansion from the tendon of the Quadriceps femoris, which is continuous below with the superficial fibers of the ligamentum patellae. It is separated from the integument by a bursa.
• The posterior surface is covered with articular cartilage upto 4 to 5 mm thick. This helps in dissipation of the force.
• presents above a smooth, oval, articular area, divided into two facets by a vertical ridge; the ridge corresponds to the groove on the patellar surface of the femur, and the facets to the medial and lateral parts of the same surface; the lateral facet is the broader and deeper. The medial facet shows significant anatomic variation. The 3rd odd facet exists along the extreme medial border of the medial facet.
  

BORDERS

• The base or superior border is thick,rough and sloped from behind, downward, and forward: it gives attachment to that portion of the Quadriceps femoris which is derived from the Rectus femoris and Vastus intermedius.
• The medial and lateral borders are thinner and converge below: they give attachment to those portions of the Quadriceps femoris which are derived from the Vasti lateralis and medialis. 

APEX

• The apex is pointed, and gives attachment to the ligamentum patellae
• In relaxed standing position, the apex lies just proximal to the knee joint line
  
  

Structure

The patella consists of a nearly uniform dense cancellous tissue, covered by a thin compact lamina. The cancelli immediately beneath the anterior surface are arranged parallel with it. In the rest of the bone they radiate from the articular surface toward the other parts of the bone. 

Ossification

The patella is ossified from a single center, which usually makes its appearance in the second or third year, but may be delayed until the sixth year. More rarely, the bone is developed by two centers, placed side by side. Ossification is completed about the age of puberty. 

Articulation

The patella articulates with the femur