Introduction

The appendicular skeleton is one of two major bone groups in the body, the other being the axial skeleton. The appendicular skeleton is comprised of the upper and lower extremities, which include the shoulder girdle and pelvis. The shoulder girdle and pelvis provide connection points between the appendicular skeleton and the axial skeleton to where mechanical loads transfer. Of the 206 bones in the adult human body, a total of 126 bones form the appendicular skeleton. The bones that contribute to the appendicular skeleton include the bones of the hands, feet, upper extremity, lower extremity, shoulder girdle, and pelvic bones.[1] 

A single upper extremity includes 14 phalanges (proximal, intermediate, and distal), five metacarpals, eight carpal bones, two forearm bones (radius and ulna), the humerus, and the shoulder girdle (scapula and clavicle).[2] A single lower extremity contains 14 phalanges (proximal, intermediate, and distal), five metatarsals, seven tarsal bones, two leg bones (fibula, tibia), the femur, and the hip bone or coxal bone (ilium, ischium, and pubis).[3][4] These bones articulate with each other and are joined by a multitude of ligaments, cartilage, and tendons to form the appendicular skeleton. There are also bony prominences and protuberances that serve as muscle attachment sites on the surfaces of these bones. The appendicular skeleton is structured for a greater range of motion and locomotion generation when compared to the axial skeleton.[5]

Structure and Function

 There are 126 named bones of the appendicular skeleton (all bones exist in pairs) [1]:

• Upper Limb
• Shoulder girdle:
• Clavicle
• Scapula
• Arm
• Humerus
• Forearm
• Radius
• Ulna
• Wrist or carpal bones
• Scaphoid 
• Lunate
• Triquetrum
• Pisiform
• Trapezium
• Trapezoid
• Capitate
• Hamate
• Hand
• Metacarpals x5
• Phalanx x14
• Lower Limb
• Pelvic girdle (hip or coxal bone)
• Ilium
• Ischium
• Pubis
• Thigh
• Femur
• Leg
• Tibia
• Fibula
• Tarsal bones
• 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 protects the knee joint, and important attachment points for the ligaments that allow for the extension of the knee.[6][7]

There are two bilateral joints where the appendicular skeleton directly articulates with the axial skeleton. The first of these articulations is the sternoclavicular joint, where the sternum of the axial skeleton articulates with the clavicle of the appendicular skeleton. The sternoclavicular joint is a synovial joint.[8] The second point where the appendicular skeleton directly articulates with the axial skeleton is the sacroiliac joint, where the sacrum articulates with the ilium. The sacroiliac joint is both a synovial joint and a syndesmosis. The connection between the sacrum and the ilium is important to transfer the load of the axial skeleton to the lower limb of the appendicular skeleton.[9]

The thoracoscapular articulation is a second articulation between the upper limb of the appendicular skeleton and the axial skeleton. This articulation is not an actual joint and does not have a synovial membrane. The thoracoscapular articulation forms between the anterior surface of the scapula and the posterior ribs 2-7.[10]

The bones of the foot function to form a base where the skeleton contacts the ground while standing. During the gait cycle, the articulations between the bones of the foot combined with the fascia and ligaments allow for deformation of the arches, which create spring-like properties in the foot that are utilized during walking and running.[11]

Embryology

The appendicular skeleton first appears as limb buds near the end of the first month of embryogenesis. There are two upper limb buds and two lower limb buds. These form when the lateral plate mesoderm grows outwards. As these limb buds grow outwards, chondrification forms hyaline cartilage around the sixth week and continues cartilage growth the limb buds. This chondrification continues rapidly in a proximal to distal manner.[12] Around the tenth week, the ossification of the cartilage begins.[13] Ossification continues post-birth with secondary and ultimately complete ossification that is ongoing until around 20 years of age.[14]

Blood Supply and Lymphatics

The blood supply to the lower extremity of the appendicular skeleton originates from the common iliac arteries, which are the terminal branches of the descending aorta. The common iliac artery branches into the internal and external iliac arteries, supplying all the structures of the pelvis and the lower extremities.[15] The external iliac artery continues into the lower extremity to become the femoral artery as it passes under the inguinal ligament.[16] A major branch of the femoral artery is the deep femoral artery. The deep femoral artery supplies blood to the femur. The medial femoral circumflex artery and lateral femoral circumflex artery are early branches of the deep femoral artery that vascularize the hip joint.[16] The femoral artery continues posteriorly to the knee as the popliteal artery, then continues into the lower leg where it divides into the anterior tibial artery and the posterior tibial artery. The posterior tibial artery then bifurcates into the posterior tibial and fibular arteries, which distally contribute to the vasculature of the foot.[17][18][19]

The blood supply to the upper extremity of the appendicular skeleton comes from the subclavian artery. The subclavian artery is a branch of the brachiocephalic trunk on the right or a branch directly off the aortic arch on the left. The clavicle receives vascular supply from the suprascapular artery, thoracoacromial artery, and the internal thoracic artery.[20] The subclavian artery becomes the axillary artery after the lateral edge of the first rib. It then becomes the brachial artery after passing the inferior border of the teres minor muscle. The brachial artery bifurcates near the elbow into the radial and ulnar arteries, which distally contribute to the vasculature of the hands.[21][22]

The lymphatics of the upper and lower limb primarily follow the major blood vessels.[23]

Nerves

The upper extremity nerves originate from the brachial plexus. The brachial plexus is composed of roots, trunks, divisions, cords, and ultimately the five named branches. Spinal nerve roots C5 to T1 contribute to the brachial plexus. The terminal branches of the brachial plexus include the musculocutaneous, axillary, median, radial, and ulnar nerves. These named branches provide innervation to the upper limb.[24]

The lower extremity innervation originates from the lumbar plexus and the sacral plexus, which is formed by spinal nerve roots T12 to S3.[25] Part of the lumbosacral plexus forms the sciatic nerve, which contributes the majority of the innervation to the lower limb. The sciatic nerve divides into the tibial and fibular nerves, which continue distally to innervate the lower limb.[26]

Physiologic Variants

There are a number of physiologic variants of the appendicular skeleton. The text below briefly describes several of the anomalies.

Polymelia is a congenital duplication of a limb or appendage. It is rare in humans, but it often appears in animals. It seems to be due to incomplete separation of monozygotic twins. The gene responsible for polymelia may be related to the disorganization Ds gene seen in mice.[27]

Syndactyly, also known as webbed digits, is the partial or total conjoining of digits in the upper or lower limb; this is one of the most common limb malformations with a prevalence of 3 to 10 per 10,000 births. Syndactyly can present unilateral or bilateral. These are categorized as partial or complete, which is based on the extent of webbing or fusion. Syndactyly can then further sub-classify as simple or complex. Simple syndactyly only involves the fusion of soft tissue, in comparison to complex, which involves bone fusion. Treatment of syndactyl requires surgical intervention.[28][29] Polydactyly is where a hand or foot has an extra digit. It can manifest as anything from a small raised lump or partially formed digit to a fully formed and functioning extra digit. There are three classifications of polydactyly for the hand. The most common, postaxial polydactyly, is where the extra digit arises on the ulnar side of the hand. Preaxial polydactyly is where the extra digit arises on the radial side of the hand. The final classification, central polydactyly, is where the digit arises somewhere in between.[30] Polydactyly of the foot utilizes a similar classification system, although preaxial is on the medial side of the foot, postaxial is on the lateral side of the foot, and central is somewhere between the previous two.[31] These malformations are often surgically removed at a young age.[32]

Another digit malformation is triphalangeal thumbs, which is a rare congenital anomaly. This condition is where the thumb has three phalanges (proximal, intermediate, and distal) instead of the usual two phalanges (proximal and distal). This anomaly will often make the thumb appear “finger-like” due to its increased length. Like the other digit malformations, triphalangeal thumbs are repairable surgically.[33][34]

A supracondylar process is a bony projection on the anterior surface of the humerus. It points downward towards the medial epicondyle. Struthers ligament is a ligament that can accompany this bony malformation. The ligament attaches from the supracondylar process to the medial epicondyle. These variants are typically asymptomatic, although there are reports of cases where Struthers ligament entraps structures such as the median nerve.[35][36][37]

Clinical Significance

The appendicular skeleton is clinically relevant in many areas of medicine. External forces applied to the appendicular skeleton from traumas can lead to fractured bones. In the upper limb, the most common fracture is a distal radius and ulna fracture. The second most common fracture site is the phalanges and metacarpals of the hand.[38] Repetitive smaller forces acting on the appendicular skeleton can also lead to stress fractures. A study of lower extremity stress fractures in the United States military found that the tibia and fibular were the most common location of stress fractures.[39] 

Bones of the appendicular skeleton can also be a primary site for malignancy, such as multiple myeloma or osteosarcoma.[40][41] The joints of the appendicular skeleton are also susceptible to a wide variety of pathologies, including osteoarthritis, rheumatoid arthritis, and gout, to name a few. The bones of the appendicular skeleton often undergo imaging through various modalities, including X-ray, computed tomography scan, and magnetic resonance imaging. The imaging technique selected is dependent on the pathology that is being imaged.[42]