Anatomy, Appendicular Skeleton

Article Author:
Bradley Anderson
Article Editor:
Bruno Bordoni
Updated:
12/16/2018 1:42:22 PM
PubMed Link:
Anatomy, Appendicular Skeleton

Introduction

The appendicular skeleton is made up of the upper and lower extremities that attach to the axial skeleton at the shoulder and pelvis. Appendicular is an adjective for "appendage." In biology, appendage means a projecting part of a vertebrate that has an important function and purpose. Out of the 206 bones in the human body, a total of 126 bones form the appendicular skeleton, including the bones of the hands, feet, upper extremity, lower extremity, and pelvic bones. The upper extremity comprises the 27 hand bones, eight wrist bones, two forearm bones (radius and ulna), the humerus of the upper arm, and the shoulder girdle (scapula and clavicle.[1][2] The lower extremity comprises the 26 bones of the feet, seven tarsal bones, the leg bones (fibula, tibia, and femur), and the pelvic girdle or pelvis (Ilium, ischium, and the pubis).[3] These bones, joined by a multitude of joints, ligaments, and cartilage, form the appendicular skeleton. This allows for greater motion and movement when compared to the axial skeleton. 

The appendicular skeleton bones are vital for movement, locomotion, and overall function of the human body. They provide support and attachment locations for the major muscles groups that impart motion and stability of the physical structure of the body. The appendicular skeleton also provides support and attachment points for ligaments and the structures that for the joints. With these traits and skeletal form, the human body can perform movements and complex activities that most mammals are unable to perform. 

Structure and Function

 The 126 named bones of the Appendicular Skeleton (all bones are paired):

  • Shoulder Girdle:
    • Clavicle
    • Scapula
  • Arm or upper extremity
    • Humerus
    • Radius
    • Ulna
  • Wrist or Carpal Bones
    •  Scaphoid 
    • Lunate
    • Triquetrum
    • Pisiform
    • Trapezium
    • Trapezoid
    • Capitate
    • Hamate
  • Hand
    • Metacarpals x5
    • Phalanx x14
  • Pelvic Girdle 
    • Ilium
    • Ischium
    • Pubis
  • Leg or lower extremity
    • Femur
    • Tibia
    • Fibula
  • Tarsal bones (midfoot and the hindfoot)
    • Talas
    • Calcaneus 
    • Cuboid
    • Medial, intermediate, and lateral cuneiform
    • Navicular
  • Foot
    • Metatarsals x5
    • Phalanx x14

There are also various sesamoid bones not included in the list such as the largest of the sesamoid bones the patella that provides protection for the knee joint and attachments for the ligaments that allow for the extension of the knee. 

Embryology

The appendicular skeleton first appears as limb buds around the end of the first month of embryogenesis. This occurs through condensation of the mesenchymal tissue with begins the formation of the bones through cartilage. Chondrification forms hyaline cartilage around the 6th week within the pelvis and continues into the limb buds. This chondrification continues rapidly and around the 10th-week ossification of the cartilage begins. Fusion of the girdle and pelvis occurs around the end of the 10th week. Ossification continues post-birth with secondary and ultimately complete ossification that continues into adulthood.[3]

Blood Supply and Lymphatics

 The pelvis blood supply comes from the common iliac artery, the terminal branches of the descending aorta. The common iliac branches into the internal and external iliac arteries that supply all the structures of the pelvis and the lower extremities. The internal iliac artery branches into the Obturator artery, Umbilical artery, Inferior vesical artery, Vaginal artery (female), Uterine artery (female), Middle rectal artery, Internal pudendal artery, Inferior gluteal artery. The external artery continues into the lower extremity as discussed below.[4][5]

 The blood supply to the upper extremities comes primarily from the subclavian artery which is a branch of the brachiocephalic trunk. It then continues as the axillary artery and then ultimately the brachial artery near the elbow in most humans. The brachial artery bifurcates into the radial and ulnar artery which eventually feeds the hands.[6][7][8]

 The blood supply to the lower extremities comes primarily from the femoral artery. The femoral artery is a continuation of the external iliac artery. It continues posteriorly to the knee as the popliteal artery, then into the lower leg where it divides into the anterior tibial artery and the tibiofibular trunk. The tibioperoneal trunk bifurcates into the posterior tibial and fibular arteries which ultimately feed the foot.[9][10]

The lymphatics primarily follow the major blood vessels and pass through major lymphatic plexuses near major joints.

Nerves

The upper appendage or extremity nerves are provided primarily through the brachial plexus. The brachial plexus breaks into roots, trunks, divisions, cords, and ultimately the names branches. The names branches are musculocutaneous, axillary, median, radial, and ulnar. These named branches provide the nerve supply to the arm and hand.[7]

The lower appendage or extremity and pelvic nerves are provided primarily through the lumbar plexus, sacral plexus, and femoral nerve. These include nerve branches from T12 to S4.[4][11][12]

Physiologic Variants

There are many physiologic variants of the appendicular skeleton. The text below describes several disease processes in brief. 

Polymelia is a very rare congenital duplication of a limb or appendage. It is rarely seen in humans but commonly described in animals. It is believed to be due to incomplete separation of monozygotic twins. Limb development occurs through a wide arrange of genes. One gene thought to be related to polymelia is the heterogeneous mutation of disorganization Ds gene.[13]

Syndactyly, often know as webbed digits, is the partial or total conjoining of digits. These can be categorized as simple, with conjoining through the skin or soft tissue, incomplete, with only partial conjoining of the digit, or complex, with the joining of bones complete with the entire conjoining of the digit. This anomaly often requires through surgical intervention.[14][15]

Polydactyly is the term for an extra digit, either finger or toe, on the hand or foot, respectively. It can include anything from a small raised lump or soft tissue, partially formed digit, to fully formed and usable extra digit. The most common form of treatment is surgical removal.[16][17]

A supracondylar process of the humerus is a bony projection on the anterior surface of the humerus. It is pointed downwards towards the medial epicondyle. Struthers ligament is a variant that attaches this process to the medial epicondyle. These variants are typically asymptomatic, but at times the Struthers ligament can lead to medial nerve entrapment.[18]

Clinical Significance

The appendicular skeleton is vital in many aspects of medicine. The bones of the appendicular skeleton are commonly fractured leading to multiple hospital visits and treatment cost. The bones of the appendicular skeleton routinely undergo imaging through various modalities including x-ray, CT scan, and MRI. The joints of the appendicular skeleton have a multitude of disease processes that lead to a variety of issues. 



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      Image courtesy S Bhimji MD

References

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