Patella

Bone: Patella
Knee diagram.svg
Right knee
Latin patella
Gray's subject #60 255
MeSH Patella

The patella, also known as the knee cap or kneepan, is a thick, circular-triangular bone which articulates with the femur and covers and protects the anterior articular surface of the knee joint. It is the largest sesamoid bone in the human body.

Contents

Structure

Human left patella - anterior aspect

The patella is roughly triangular in shape with its base facing proximally (towards the torso) and its tip (apex patellae) facing distally (towards the feet). Its anterior and posterior surfaces are joined laterally (left/right) by a thinner margin and medially (towards centre) by a thicker margin. [1]

Anterior surface

The anterior surface can be divided into three parts:

  1. The upper third is coarse, flattened, and rough; it serves for the attachment of the tendon of the quadriceps and often has exostoses.
  2. The middle third has numerous vascular canaliculi.
  3. The lower third includes the distal apex which serves as the origin of the patellar ligament.

[1]

Posterior surface

Human left patella - posterior aspect

The posterior surface is divided into two parts.

The upper three-quarters articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape. Four main types of articular surface can be distinguished:

  1. Most commonly the medial articular surface is smaller than the lateral.
  2. Sometimes both articular surfaces are virtually equal in size.
  3. Occasionally, the medial surface is hypoplastic or
  4. the central ledge is only indicated.

In the adult the articular surface is about 12 cm2 (1.9 sq in) and covered by cartilage, which can reach a maximal thickness of 6 mm (0.24 in) in the centre at about 30 years of age.

The lower part of the posterior surface has vascular canaliculi filled and is filled by fatty tissue, the infrapatellar fat pad.

[1]

Variants

Emarginations (i.e. patella emarginata, a "missing piece") are common laterally on the proximal edge. Bipartite patellas are the result of an ossification of a second cartilaginous layer at the location of an emargination. Previously, bipartite patellas were explained as the failure of several ossification centres to fuse, but this idea has been rejected. Partite patellas occur almost exclusively in men. Tripartite and even multipartite patellas occur.

[1]

Muscles

It is attached to the tendon of the quadriceps femoris muscle, which contracts to extend/straighten the knee. The vastus intermedialis muscle is attached to the base of patella. The vastus lateralis and vastus medialis are attached to lateral and medial borders of patella respectively.

The patella is stabilized by the insertion of vastus medialis and the prominence of the anterior femoral condyles, which prevent lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise.

Function

The primary functional role of the patella is knee extension. The patella increases the leverage that the tendon can exert on the femur by increasing the angle at which it acts.

Ossification

In the patella an ossification centre develops between the ages 3-4 years. [1]

Evolutionary variation

The patella has convergently evolved in placental mammals and birds; marsupials have only rudimentary, non-ossified patellae.[2] In more primitive tetrapods, including living amphibians and reptiles, the muscle tendons from the upper leg are attached directly to the tibia, and the patella is not present.[3]

See also

Additional images
Lower extremity  
Right knee-joint. Anterior view.  
Sagittal section of right knee-joint.  
Capsule of right knee-joint (distended). Lateral aspect.  
Front and medial aspect of right thigh.  
Lateral aspect of right leg.  
Posterior surface of right patella  

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Platzer, Werner (2004). Color Atlas of Human Anatomy, Vol. 1: Locomotor System (5th ed.). Thieme. p. 194. ISBN 3-13-533305-1. 
  2. ↑ Herzmark MH (1938). "The Evolution of the Knee Joint". J Bone Joint Surg Am 20: 77–84. http://www.ejbjs.org/cgi/reprint/20/1/77.pdf. Retrieved 2007-11-17. 
  3. ↑ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. p. 205. ISBN 0-03-910284-X. 

External links

Bones of lower limbs (TA A02.5.04-18, GA 2.242-277)
Femur
upper extremity
head (fovea) · neck · greater trochanter (trochanteric fossa) · lesser trochanter · intertrochanteric line · intertrochanteric crest · quadrate tubercle
body
linea aspera · gluteal tuberosity / third trochanter · pectineal line
lower extremity
adductor tubercle · patellar surface · epicondyles (lateral, medial) · condyles (lateral, medial) · intercondylar fossa
Crus
upper extremity
Gerdy's tubercle · condyles (lateral, medial) · intercondylar eminence (lateral/medial intercondylar tubercle) · posterior/anterior intercondylar area
body
tuberosity · soleal line
lower extremity
medial malleolus · fibular notch
head · body · lateral malleolus
Other
patella (apex of patella)
Foot
Tarsus
calcaneus (sustentaculum tali, trochlear process) · talus (body, neck, head) · navicular · cuboid · cuneiform (medial, intermediate, lateral)
Metatarsus
1st metatarsal · 2nd · 3rd · 4th · 5th
Other
phalanges of the foot

: BON/CAR

anat(c/f/k/, u, t/p, l)//devp/cell

noco/cong/tumr, sysi/, injr

proc, drug()