Lower Limbs | Medical Science Optional Notes for UPSC PDF Download

Dermatomes of Lower Limb

A dermatome refers to a specific skin area primarily served by a single spinal nerve.

• The sensory supply to the skin of the lower limb originates from:
  (a) Predominantly from spinal cord segments L1 to L5 and S1 to S3; and
  (b) Partially from segments T12 and S4.
• Initially, each developing limb bud exhibits a cranial (preaxial) and a caudal (postaxial) border.
• During embryonic development, the preaxial border aligns with the great toe and tibia, while the postaxial border corresponds to the little toe and fibula.
• Subsequently, a 90° medial rotation occurs, shifting the great toe and tibia inward and the little toe and fibula outward.
• As a result, the original preaxial border becomes the tibial side, and the postaxial border becomes the fibular side of the lower limb.

The lower limb dermatomes follow a systematic numerical arrangement. 
Progressing along the preaxial border from top to bottom, dermatomes T12, L1 to L4 are observed. Segment L5 supplies the middle three toes, the adjacent region of the dorsum of the foot, and the lateral side of the leg. On the postaxial border, moving from bottom to top, dermatomes S1, S2, and S3 are present.
In instances of paraplegia, discerning the spinal lesion level is possible by considering dermatomes. The highest point of sensory impairment signifies the lesion level. Due to the overlapping nature of neighboring dermatomes, the extent of sensory loss resulting from spinal cord or nerve root damage is consistently smaller than the total area of the dermatome.

Venous drainage of lower limb

Perforator

'Perforating' veins establish connections between the long saphenous vein and the deep veins, especially in the vicinity of the ankle, distal calf, and knee. The arrangement of their valves is designed to impede the backward flow of blood from the deep veins to the superficial veins.

Sciatic Nerve

The sciatic nerve originates in the pelvis, formed by the ventral rami of the fourth lumbar to the third sacral spinal nerves (L4 to S3). It typically measures 2 cm in width at its origin, making it the thickest nerve in the body.

Course

Upon entering the lower limb through the greater sciatic foramen beneath the piriformis muscle, the sciatic nerve descends between the greater trochanter and ischial tuberosity. It traverses the posterior aspect of the thigh, intersected by the long head of the biceps femoris, and eventually bifurcates into the tibial and common fibular (peroneal) nerves proximal to the knee.

Supply

The sciatic nerve issues articular branches that provide innervation to the hip joint through its posterior capsule, with some branches occasionally originating directly from the sacral plexus. Additionally, the sciatic nerve supplies the knee joint.

The medial (tibial) component of the sciatic nerve innervates all hamstring muscles, including the ischial part of the adductor magnus, excluding the short head of the biceps femoris. The lateral (common fibular) component supplies the short head of the biceps femoris.

The sciatic nerve is responsible for supplying the knee flexors and all muscles below the knee. Consequently, a total paralysis of the sciatic nerve leads to a condition known as foot drop, causing significant challenges in walking.

Various factors can contribute to sciatic nerve injury, including hip posterior dislocation, piriformis syndrome, and iatrogenic causes such as misplaced therapeutic injections into the gluteus maximus or during total hip replacement. Iatrogenic incidents represent the most prevalent cause of severe sciatic nerve injuries.

Sciatica

Sciatica is not a disease but a symptom characterized by pain radiating down the back of the thigh and calf. Contributing factors include:

Arterial supply of Head and Neck of Femur

The primary source of blood supply to the head of the femur is through the lateral epiphyseal arteries, which originate from the retinacular branches of the medial circumflex femoral artery.
Following a fracture through the neck of the femur, there is a loss of both medullary and retinacular blood supply. In such cases, reliance shifts to the foveal vessels to ensure complete blood supply to the femoral head. In instances where this supply is inadequate, it can lead to a common complication in orthopedic practice known as avascular necrosis of the head of the femur.

Arches of Foot

Arches of Foot - Classification

Median Longitudinal Arch

Median Longitudinal Arch

Median Longitudinal Arch

Lateral Longitudinal Arch

Lateral Longitudinal Arch

Lateral Longitudinal Arch

Transverse Arches

Clinical Significance

• Pes planus 
• Pes cavus 
• Claw foot 
• Talipes equinus 
• Talipes calcaneus 
• Talipes varus 
• Talipes valgus

Lower limb -Repeats

• Arterial supply to head and neck of femur (1998) 
• Describe venous drainage of lower limb and its applied anatomy. (2011) 
• Describe the cutaneous nerve supply of dorsum of foot. (2016) 
• Define a dermatome. Illustrate by means of labeled diagram(s) dermatomes of lower limb. (2013) 
• Describe dermatomes of lower limb. (2016) 
• Describe in detail about the origin and distribution of the sciatic nerve. Add a note on its clinical importance. (2015) 
• Classify Arches of foot.  Write in details about medial longitudinal Arch. (2018)