General comments

• Posterior to knee, diamond shaped, fat filled, passage of key structures to the lower limb

Boundaries, bones, ligaments, crossing structures, vasculature

• Superior boundary - apex of diamond formed by semimembranosus and semitendinosus diverging from biceps femoris
  • sciatic n. - enters from posterior thigh deep to hamstrings, divides into tibial (medial) and common peroneal (lateral) nn.
• Superior lateral boundary - biceps femoris
  • superior lateral genicular a.v. - leaves popliteal fossa femur at lateral femoral epicondyle superior to origin of gastrocnemius and deep to biceps femoris
• Superior medial boundary - semitendinosus and semimembranosus
  • superior medial genicular a.v. - leaves popliteal fossa at medial femoral epicondyle superior to adductor tubercle and deep to semimembranosus
• Inferior lateral boundary - lateral head of gastrocnemius
  • inferior lateral genicular a.v. - leaves popliteal fossa crossing posterior surface of popliteus fascia and arcuate ligament, deep to head of gastrocnemius
  • common peroneal n. - leaves popliteal posterior to lateral head gastrocnemius and lateral to neck of fibula
• Inferior medial boundary - medial head of gastrocnemius
  • inferior medial genicular a.v. - leaves popliteal fossa posterior surface of tibia near the plateau, deep to head of gastrocnemius
• Inferior boundary - apex of diamond formed by convergence of the two heads of gastrocnemius
  • posterior tibial a.v. - leaves popliteal fossa deep to the heads of origin of soleus
  • sural aa. - leaves popliteal fossa by entering the bellies of gastrocnemius
  • branches of tibial n. to the gastrocnemius
• Posterior boundary - popliteal fascia (deep fascia), transition from fascia lata to crural fascia
  • medial and lateral sural nn. - branches for tibial and common peroneal nn.
  • short saphenous v. - pierces popliteal fascia to enter the popliteal v.
• Anterior boundary - femur, tibial plateau, joint capsule, oblique popliteal lig., arcuate lig., popliteus m.
  • popliteal a.v. - enter through adductor hiatus (superior medial anterior in the fossa) as continuation of the femoral vessels; artery deep to vein.
  • middle genicular a. leaves popliteal fossa through the posterior aspect of the joint capsule

Innervation

• branches of the sciatic, tibial, and common peroneal provide sensory innervation to the region
• posterior femoral cutaneous nerve provides cutaneous innervation
• femoral and obturator nerves may, in part, supply the anterior reaches

Key Relations within the fossa

• superficial to deep: sciatic n and branches, femoral v., femoral a.
• arteries of genicular anastomosis applied to capsule and bone

Lymphatic drainage - popliteal lymph nodes

• superficial drainage of leg enters popliteal fossa along with the lesser saphenous v.
• deep drainage of leg enters popliteal fossa along with posterior tibial vessels
• lymph from popliteal fossa ascend along femoral vessels to deep inquinal nodes

General Comment

• Flexion of the digits, relative to extension, is more specific because of the independence of flexor tendons relative to extensor tendons

Primary Flexors

• Flexor Hallucis longus - inserts at base of distal phalanx of digit 1
  • Innervated by the tibial nerve
  • flexes the IP joint plus the MP and ankle joint
  • Origin from posterior compartment of leg - interosseous membrane and fibula
  • crosses ankle inferior to sustentaculum tali and within flexor sheath
  • receives guidance from the sesamoid bones within the lateral and medial heads of the flexor digitorum brevis
• Flexor hallucis brevis - inserts at base of proximal phalanx of digit 1
  • Innervated by the medial plantar nerve
  • flexes the MP joint
  • lateral and medial head each host a sesamoid bone near the head of the first metatarsal
  • sesamoid bones provide a "groove" to guide the tendon of flexor hallucis longus toward the distal phalanx
  • sesamoid bones provide site of attachment for the abductor hallucis and the adductor hallucis
• Flexor digitorum longus - inserts on the base of the distal phalanx of digits 2-4
  • Innervated by the tibial nerve
  • flexes the distal IP joint plus the proximal IP, MP, and ankle
  • lumbricals arise from the medial sides of the the tendons - extend IP and flex MP
  • origin from the posterior compartment of the leg - tibia and interosseous membrane
  • crosses ankle superior to sustentaculum tali and within the flexor sheath
• Flexor digitorum brevis - inserts at the base of the middle phalanx of digits 2-5
  • Innervated by the medial planter nerve
  • flexes the proximal IP joint plus the MP joint
  • origin from the calcaneus - intrinsic to the foot
  • tendons are approximated to planter surface of flexor digitorum longus
  • tendon splits and inserts onto the sides of the base of the middle phalanx
  • the tendon of flexor digitorum longus passes through the split tendon without tethering to the flexor digitorum brevis
  • independent action at the distal and proximal IP joint is preserved relative to the extensors (extensor hood)
• Flexor digiti minimi brevis - inserts at the base of the proximal phalanx of digit 5
  • Innervated by the lateral plantar nerve
  • flexes the the MP joint
  • intrinsic to the foot

Primary Extensors

• Extensor hallucis longus - inserts on the distal and proximal phalanges of digit 1
  • Innervated by the deep peroneal nerve
  • extends the IP joint plus the MP joint plus the ankle
  • origin from the anterior compartment of the leg - tibia and interosseous membrane
  • stabilized by the superior and inferior retinacula
  • tendon receives a lateral attachment at the level of the head of the first metatarsal from extensor hallucis brevis
  • The tendons of extensor hallucis longus and brevis are tethered to each other and can not act independently.
• Extensor hallucis brevis - inserts on the distal and proximal phalanges of digit 1 (via tendon of extensor hallucis longus)
  • Innervated by the deep peroneal nerve
  • extends the IP and MP joint
  • intrinsic to the dorsum of the foot
  • tendon attaches to lateral side of the tendon of extensor hallucis longus or to the proximal phalanx
  • origin is from the anterolateral aspect of the calcaneus
  • has been considered a named slip of extensor digitorum brevis
• Extensor digitorum longus - inserts on the bases of the middle and distal phalanges of digits 2-5 be way of the extensor hood
  • Innervated by the deep peroneal nerve
  • extends the distal and proximal IP joints plus the MP and the ankle joints
  • origin from the anterior compartment of the leg - interosseous membrane and the fibula
  • stabilized by the superior and inferior extensor retinacula
  • tendons receive lateral attachments from the extensor digitorum brevis on digits 2-4 at level of head of metatarsal
  • the combined tendons of the extensor digitorum longus and brevis contribute to the extensor hood
  • extensor hood has a center insertion on the middle phalanx and sends two lateral bands to insert on the sides of the base of the distal phalanx
  • extensor hood also receives fibrous contributions from the lumbricals and the interossei
  • The extensor digitorum longus and brevis are tethered to each other by the extensor hood and can not act independently
• Extensor digitorum brevis - inserts on the extensor hood and, thus, to the bases of the middle and distal phalanges of digits 2-4 (not 5)
  • Innervated by the deep peroneal nerve
  • extends the distal and proximal IP joints plus the MP
  • origin is from the anterolateral aspect of the calcaneus
  • tendons attach to the extensor at the lateral aspect of the extensor digitorum longus at the level of the head of the metatarsals

Abduction

• Dorsal interossei
  • Innervated by the deep branch of the lateral plantar nerve
  • Origin from the margins of the bodies of the metatarsal bones
  • Insertion onto the extensor hood proximal to the lumbricals
  • The extensor hood of the second toe receives two dorsal interossei and, thus, is the reference digit.
  • The first two dorsal interossei close the space between the first and second metatarsals and the second and third metatarsals
  • Bipennate
• Abductor hallucis
  • Innervated by the by the medial plantar nerve
  • Origin from the inferior medial margin of the calcaneus
  • Insertion on the medial base of the proximal phalanx of the great toe
• Abductor digiti minimi
  • Innervated by the deep branch of the lateral plantar nerve
  • Origin from the lateral posterior margin of the calcaneus
  • Insertion on the lateral base of the proximal phalanx of the fifth digit
• First lumbrical
  • Innervated by the superficial branch of the medial plantar nerve
  • Origin from the medial side of the first tendon (second digit) of the flexor digitorum longus
  • Insertion onto the medial dorsal margin of the extensor hood distal to the insertion of the first interosseus

Adduction

• Plantar interossei
  • Innervated by the deep branch of the lateral plantar nerve
  • Origin from the medial margin of the metatarsal bone of the digit that acts
  • Insertion onto the medial dorsal margin of the lateral three extensor hoods proximal to the insertion of the lumbricals
  • Unipennate
• Lateral three lumbricals
  • Innervated by the lateral plantar nerve
  • Origin from the medial side of the tendons of the flexor digitorum longus
  • Insertion onto the medial dorsal margin of the extensor hood distal to the insertion of the interossei
• Adductor hallucis
  • Innervated by the deep branch of the lateral plantar nerve
  • Transverse head origin from the heads of the metatarsals and the deep transverse metatarsal ligament
  • Oblique head origin from the base of the metatarsal bones and the adjacent tarsal bones
  • Insertion onto the lateral posterior base of the proximal phalanx of the great toe

Flex the metatarsophalangeal joints while holding interphalangeal joints in extension

• Lumbricals (4)
  • Tendons of insertion cross the plantar side of the horizontal axis of the metatarsophalangeal joint
    • Flex the metatarsophalangeal joint
  • Insertion onto the medial margin of the extensor hood distal to the insertions of the interossei
    • Extend the proximal and distal interphalangeal joints
  • Holding the IP joints in extension while flexing the MP joint contributes to the execution of a "graceful" grasp
• Interossei (7)
  • Tendons of insertion cross the plantar side of the horizontal axis of the metatarsophalangeal joint
    • Flex the metatarsophalangeal joint
  • Insertion onto the medial margin of the extensor hood proximal to the insertions of the lumbricals
    • Extend the proximal and distal interphalangeal joints
  • Holding the IP joints in extension while flexing the MP joint contributes to the execution of a "graceful" grasp

Additional comments

• It is the relative independence of the flexor tendons and the action of the lumbricals that permit greater specificity of flexion.
• Mesotendons (vincula) drived from tendon sheaths provide vascular and nervous supply to the distal flexor tendons
• The deep and superficial transverse metatarsal ligaments stabilize the heads of the metatarsals and contribute to fine movement.
• Collateral ligaments at the MP and IP joints stablize flexion and extension.

Orientation, and Function

• Located to the right and anterior within the pericardial sac
• Site for the pooling of caval blood that subsequently enters the right ventricle
• Atrial contraction assists the "sucking" of blood into the right ventricle

Structure

• Quadrangular chamber receiving deoxygenated blood from the superior and inferior vena cava
• The right auricle is a diverticulum at the superior anterior margin
• Sulcus terminalis marks the internal crista terminalis and the sinuatrial node
  • Landmark for location of sinuatrial (SA) node
• Interatrial septum
  • Fossa ovalis - remnant of fetal foramen ovale, a circulatory shunt between the atria
  • Coronary Sinus - landmark for location of Atrioventricular (AV) node
• Openings
  • Atrioventricular ostium
    • Tricuspid valve into the right ventricle
  • Superior vena cava
    • landmark for location of sinuatrial (SA) node
  • Inferior vena cava
    • Ostia of coronary sinus is to the left of IVC
  • Coronary sinus
    • Receives venous blood from the great, middle, and small cardiac veins
  • Anterior cardiac veins
    • Drain directly into right atrium

Muscles

• Crista terminalis - muscle
  • Separates the smooth posterior wall (sinus venarum) from the muscular anterior wall (pectinate mm)
  • Marks the surface projection of the sulcus terminalis
  • Superior extent marks location of sinoatrial node near the superior vena cava
• Pectinate muscle

Innervation

• Sinoatrial node
  • The sinoatrial node is a crescent shaped heart region about 8mm long and located at the superior aspect of the sulcus terminalis within the wall of the right atrium. It is referred to as the pacemaker of the heart owing to its electrical characteristics and crucial role in timing of the cardiac cycle.
  • The SA node connects to the AV node by internodal tracts.

Vasculature

• Right coronary artery
• Marginal branch of right coronary artery
• Sinuatrial artery - nodal branch to SA node
• Anterior cardiac vv
• Small cardiac vv

Valves

• Tricuspid valve (AV Ostium)
  • Supported by the fibrocartilaginous skeleton of the heart
  • Three cusps are held by chordae tendinea that prevent eversion during right ventricular contraction
  • Passively open to the flow of atrial blood into the right ventricle
• Valve of the inferior vena cava
• Valve of the coronary sinus

Relationship to Pericardium

• Reflections of visceral pericardium (epicardium) define the venous mesocardium of the superior and inferior vena cava
  • The visceral pericardium moves against the parietal serous pericardium with the benefit of a layer of serous fluid to reduce friction
• The sinoatrial node is located immediately deep to the epicardium and has access to exogenous autonomic nervous supply at this location