Anatomy, Appendicular Skeleton

John Ekblad; Bradley W. Anderson; Asa  C. Black; Bruno Bordoni; Asa  C. Black

Anatomy, Appendicular Skeleton

Free Review Questions

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
Metacarpals x5
Phalanx x14
Scaphoid
Lunate
Triquetrum
Pisiform
Trapezium
Trapezoid
Capitate
Hamate
Radius
Ulna
Humerus
Forearm
Wrist or Carpal Bones
Hand

Lower Limb

Ilium

Ischium

Pubis

Femur

Tibia

Fibula

Talas

Calcaneus

Cuboid

Medial, intermediate, and lateral cuneiform

Navicular

Metatarsals x5

Phalanx x14

Pelvic Girdle (Hip or Coxal Bone)
Thigh
Leg
Tarsal Bones
Foot

There are also various sesamoid bones not included in the list, such as the largest of the sesamoid bones, the patella, which protects the knee joint, and important attachment points for the ligaments that allow knee extension.[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 through 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 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 in 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 following 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, dividing into the anterior and posterior tibial arteries. 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 upper and lower limb lymphatics 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 most 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 several 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. This anomaly is rare in humans but often appears in animals; it seems to occur due to the 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 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, compared to complex, which involves bone fusion. Correcting syndactyl requires surgical intervention.[28][29]

Polydactyly is where a hand or foot has an extra digit. The condition 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, with preaxial on the medial side of the foot, postaxial on the lateral side of the foot, and central 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. In this condition, the thumb has three phalanges (proximal, intermediate, and distal) instead of the usual two phalanges (proximal and distal). This anomaly often makes 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. This process 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 fibula 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]

(Click Image to Enlarge)

Appendicular skeleton
Image courtesy S Bhimji MD

Details

References

[1]

Docherty B. Skeletal system: part four--the appendicular skeleton. Nursing times. 2007 Feb 20-26:103(8):26-7 [PubMed PMID: 17333873]

[2]

Panchal-Kildare S, Malone K. Skeletal anatomy of the hand. Hand clinics. 2013 Nov:29(4):459-71. doi: 10.1016/j.hcl.2013.08.001. Epub [PubMed PMID: 24209945]

[3]

Wobser AM, Adkins Z, Wobser RW. Anatomy, Abdomen and Pelvis: Bones (Ilium, Ischium, and Pubis). StatPearls. 2023 Jan:(): [PubMed PMID: 30137809]

[4]

Ficke J, Byerly DW. Anatomy, Bony Pelvis and Lower Limb: Foot. StatPearls. 2023 Jan:(): [PubMed PMID: 31536304]

[5]

Brockett CL, Chapman GJ. Biomechanics of the ankle. Orthopaedics and trauma. 2016 Jun:30(3):232-238 [PubMed PMID: 27594929]

[6]

Fox AJ, Wanivenhaus F, Rodeo SA. The basic science of the patella: structure, composition, and function. The journal of knee surgery. 2012 May:25(2):127-41 [PubMed PMID: 22928430]

[7]

Luo TD, Marino DV, Pilson H. Patella Fractures. StatPearls. 2023 Jan:(): [PubMed PMID: 30020702]

[8]

Dhawan R, Singh RA, Tins B, Hay SM. Sternoclavicular joint. Shoulder & elbow. 2018 Oct:10(4):296-305. doi: 10.1177/1758573218756880. Epub 2018 Apr 2 [PubMed PMID: 30214497]

[9]

Vleeming A, Schuenke MD, Masi AT, Carreiro JE, Danneels L, Willard FH. The sacroiliac joint: an overview of its anatomy, function and potential clinical implications. Journal of anatomy. 2012 Dec:221(6):537-67. doi: 10.1111/j.1469-7580.2012.01564.x. Epub 2012 Sep 19 [PubMed PMID: 22994881]

Level 3 (low-level) evidence

[10]

Frank RM, Ramirez J, Chalmers PN, McCormick FM, Romeo AA. Scapulothoracic anatomy and snapping scapula syndrome. Anatomy research international. 2013:2013():635628. doi: 10.1155/2013/635628. Epub 2013 Nov 28 [PubMed PMID: 24369502]

[11]

McKeon PO, Hertel J, Bramble D, Davis I. The foot core system: a new paradigm for understanding intrinsic foot muscle function. British journal of sports medicine. 2015 Mar:49(5):290. doi: 10.1136/bjsports-2013-092690. Epub 2014 Mar 21 [PubMed PMID: 24659509]

Level 3 (low-level) evidence

[12]

Al-Qattan MM, Kozin SH. Update on embryology of the upper limb. The Journal of hand surgery. 2013 Sep:38(9):1835-44. doi: 10.1016/j.jhsa.2013.03.018. Epub 2013 May 17 [PubMed PMID: 23684522]

[13]

Ortega N, Behonick DJ, Werb Z. Matrix remodeling during endochondral ossification. Trends in cell biology. 2004 Feb:14(2):86-93 [PubMed PMID: 15102440]

[14]

Verbruggen SW, Nowlan NC. Ontogeny of the Human Pelvis. Anatomical record (Hoboken, N.J. : 2007). 2017 Apr:300(4):643-652. doi: 10.1002/ar.23541. Epub [PubMed PMID: 28297183]

[15]

Yiming A, Baqué P, Rahili A, Mayer J, Braccini AL, Fontaine A, Leplatois A, Clavé A, Bourgeon A, de Peretti F. Anatomical study of the blood supply of the coxal bone: radiological and clinical application. Surgical and radiologic anatomy : SRA. 2002 May:24(2):81-6 [PubMed PMID: 12197024]

[16]

Zlotorowicz M, Czubak-Wrzosek M, Wrzosek P, Czubak J. The origin of the medial femoral circumflex artery, lateral femoral circumflex artery and obturator artery. Surgical and radiologic anatomy : SRA. 2018 May:40(5):515-520. doi: 10.1007/s00276-018-2012-6. Epub 2018 Apr 12 [PubMed PMID: 29651567]

[17]

Hirtler L, Lübbers A, Rath C. Vascular coverage of the anterior knee region - an anatomical study. Journal of anatomy. 2019 Aug:235(2):289-298. doi: 10.1111/joa.13004. Epub 2019 May 9 [PubMed PMID: 31070789]

[18]

Hyland S, Sinkler MA, Varacallo M. Anatomy, Bony Pelvis and Lower Limb: Popliteal Region. StatPearls. 2023 Jan:(): [PubMed PMID: 30422486]

[19]

Olewnik Ł, Łabętowicz P, Podgórski M, Polguj M, Ruzik K, Topol M. Variations in terminal branches of the popliteal artery: cadaveric study. Surgical and radiologic anatomy : SRA. 2019 Dec:41(12):1473-1482. doi: 10.1007/s00276-019-02262-3. Epub 2019 May 27 [PubMed PMID: 31134299]

[20]

Knudsen FW, Andersen M, Krag C. The arterial supply of the clavicle. Surgical and radiologic anatomy : SRA. 1989:11(3):211-4 [PubMed PMID: 2588097]

[21]

McCausland C, Sawyer E, Eovaldi BJ, Varacallo M. Anatomy, Shoulder and Upper Limb, Shoulder Muscles. StatPearls. 2023 Jan:(): [PubMed PMID: 30521257]

[22]

Haładaj R, Wysiadecki G, Dudkiewicz Z, Polguj M, Topol M. The High Origin of the Radial Artery (Brachioradial Artery): Its Anatomical Variations, Clinical Significance, and Contribution to the Blood Supply of the Hand. BioMed research international. 2018:2018():1520929. doi: 10.1155/2018/1520929. Epub 2018 Jun 11 [PubMed PMID: 29992133]

[23]

Ma CX, Pan WR, Liu ZA, Zeng FQ, Qiu ZQ, Liu MY. Deep lymphatic anatomy of the upper limb: An anatomical study and clinical implications. Annals of anatomy = Anatomischer Anzeiger : official organ of the Anatomische Gesellschaft. 2019 May:223():32-42. doi: 10.1016/j.aanat.2019.01.005. Epub 2019 Feb 1 [PubMed PMID: 30716466]

Level 2 (mid-level) evidence

[24]

Orebaugh SL, Williams BA. Brachial plexus anatomy: normal and variant. TheScientificWorldJournal. 2009 Apr 28:9():300-12. doi: 10.1100/tsw.2009.39. Epub 2009 Apr 28 [PubMed PMID: 19412559]

[25]

Di Benedetto P, Pinto G, Arcioni R, De Blasi RA, Sorrentino L, Rossifragola I, Baciarello M, Capotondi C. Anatomy and imaging of lumbar plexus. Minerva anestesiologica. 2005 Sep:71(9):549-54 [PubMed PMID: 16166916]

[26]

Dupont G, Unno F, Iwanaga J, Oskouian RJ, Tubbs RS. A Variant of the Sciatic Nerve and its Clinical Implications. Cureus. 2018 Jun 25:10(6):e2874. doi: 10.7759/cureus.2874. Epub 2018 Jun 25 [PubMed PMID: 30155377]

[27]

Montalvo N, Redrobán L, Espín VH. Incomplete duplication of a lower extremity (polymelia): a case report. Journal of medical case reports. 2014 Jun 12:8():184. doi: 10.1186/1752-1947-8-184. Epub 2014 Jun 12 [PubMed PMID: 24920152]

Level 3 (low-level) evidence

[28]

Malik S. Syndactyly: phenotypes, genetics and current classification. European journal of human genetics : EJHG. 2012 Aug:20(8):817-24. doi: 10.1038/ejhg.2012.14. Epub 2012 Feb 15 [PubMed PMID: 22333904]

[29]

Kvernmo HD, Haugstvedt JR. Treatment of congenital syndactyly of the fingers. Tidsskrift for den Norske laegeforening : tidsskrift for praktisk medicin, ny raekke. 2013 Aug 20:133(15):1591-5. doi: 10.4045/tidsskr.13.0147. Epub [PubMed PMID: 23970273]

[30]

Comer GC, Potter M, Ladd AL. Polydactyly of the Hand. The Journal of the American Academy of Orthopaedic Surgeons. 2018 Feb 1:26(3):75-82. doi: 10.5435/JAAOS-D-16-00139. Epub [PubMed PMID: 29309292]

[31]

Belthur MV, Linton JL, Barnes DA. The spectrum of preaxial polydactyly of the foot. Journal of pediatric orthopedics. 2011 Jun:31(4):435-47. doi: 10.1097/BPO.0b013e3182199a68. Epub [PubMed PMID: 21572282]

[32]

Kyriazis Z, Kollia P, Grivea I, Varitimidis SE, Constantoulakis P, Dailiana ZH. Thumb duplication: molecular analysis of different clinical types. European journal of orthopaedic surgery & traumatology : orthopedie traumatologie. 2019 Feb:29(2):421-426. doi: 10.1007/s00590-018-2343-3. Epub 2018 Nov 29 [PubMed PMID: 30498907]

[33]

Hovius SER, Potuijt JWP, van Nieuwenhoven CA. Triphalangeal thumb: clinical features and treatment. The Journal of hand surgery, European volume. 2019 Jan:44(1):69-79. doi: 10.1177/1753193418797922. Epub 2018 Sep 17 [PubMed PMID: 30223699]

[34]

Potuijt JWP, Galjaard RH, van der Spek PJ, van Nieuwenhoven CA, Ahituv N, Oberg KC, Hovius SER. A multidisciplinary review of triphalangeal thumb. The Journal of hand surgery, European volume. 2019 Jan:44(1):59-68. doi: 10.1177/1753193418803521. Epub 2018 Oct 14 [PubMed PMID: 30318985]

[35]

Shon HC, Park JK, Kim DS, Kang SW, Kim KJ, Hong SH. Supracondylar process syndrome: two cases of median nerve neuropathy due to compression by the ligament of Struthers. Journal of pain research. 2018:11():803-807. doi: 10.2147/JPR.S160861. Epub 2018 Apr 16 [PubMed PMID: 29713193]

Level 3 (low-level) evidence

[36]

Opanova MI, Atkinson RE. Supracondylar process syndrome: case report and literature review. The Journal of hand surgery. 2014 Jun:39(6):1130-5. doi: 10.1016/j.jhsa.2014.03.035. Epub [PubMed PMID: 24862112]

Level 3 (low-level) evidence

[37]

Gamble JG, Krygier JE. Fracture of the Supracondylar Process in a Child: A Case Report and Review of the Literature. JBJS case connector. 2019 Dec:9(4):e0396. doi: 10.2106/JBJS.CC.18.00396. Epub [PubMed PMID: 31633496]

Level 3 (low-level) evidence

[38]

Karl JW, Olson PR, Rosenwasser MP. The Epidemiology of Upper Extremity Fractures in the United States, 2009. Journal of orthopaedic trauma. 2015 Aug:29(8):e242-4. doi: 10.1097/BOT.0000000000000312. Epub [PubMed PMID: 25714441]

[39]

Waterman BR, Gun B, Bader JO, Orr JD, Belmont PJ Jr. Epidemiology of Lower Extremity Stress Fractures in the United States Military. Military medicine. 2016 Oct:181(10):1308-1313 [PubMed PMID: 27753569]

[40]

Eslick R, Talaulikar D. Multiple myeloma: from diagnosis to treatment. Australian family physician. 2013 Oct:42(10):684-8 [PubMed PMID: 24130968]

[41]

Rogozhin DV, Bulycheva IV, Konovalov DM, Talalaev AG, Roshchin VY, Ektova AP, Bogoroditsky YS, Strykov VA, Kazakova AN, Olshanskaya YV, Kachanov DY, Tereshchenko GV. [Classical osteosarcoma in children and adolescent]. Arkhiv patologii. 2015 Sep-Oct:77(5):68-74. doi: 10.17116/patol201577568-74. Epub [PubMed PMID: 27077157]

[42]

Wong AK. A comparison of peripheral imaging technologies for bone and muscle quantification: a technical review of image acquisition. Journal of musculoskeletal & neuronal interactions. 2016 Dec 14:16(4):265-282 [PubMed PMID: 27973379]