Posterior Compartment of Thigh

Introduction

• The posterior compartment of the thigh contains the muscles responsible for flexion at the knee and is therefore known as the flexor compartment of the thigh.
• It extends from the gluteal fold superiorly to the posterior aspect of the knee inferiorly.
• This compartment is bounded by the lateral intermuscular septum and the posterior intermuscular septum of the thigh.
• In the lower part of the thigh, the posterior intermuscular septum is poorly defined because the adductor magnus extends into both the medial and posterior compartments.

Contents of posterior compartment of thigh

• The posterior compartment contains the hamstring muscles, which include:
  • Semitendinosus
  • Semimembranosus
  • Biceps femoris
  • Hamstring (ischial) part of the adductor magnus
• The sciatic nerve is the principal nerve supplying this compartment.
• The arterial supply is maintained by vascular anastomoses located on the posterior aspect of the thigh, including:
  • Longitudinal anastomosis
  • Trochanteric anastomosis
  • Cruciate anastomosis

Cutaneous Innervation

• The skin over the posterior aspect of the thigh receives sensory innervation from the following nerves:
  1. Posterior cutaneous nerve of the thigh
  2. Cutaneous branches of the obturator nerve
  3. Medial branches of the anterior cutaneous nerve of the thigh
  4. Lateral cutaneous nerve of the thigh

Posterior Cutaneous Nerve of the Thigh

• The posterior cutaneous nerve of the thigh arises from the sacral plexus (S1, S2, S3).
• It enters the gluteal region through the greater sciatic foramen, inferior to the piriformis muscle.
• The nerve descends medial to the sciatic nerve beneath the gluteus maximus.
• It emerges at the lower border of the gluteus maximus and continues down the posterior thigh.
• Near the popliteal fossa, it pierces the deep fascia to become superficial.

Distribution

• Supplies the skin over the posterior aspect of the thigh and the upper part of the posterior leg.
• Gives gluteal branches to the skin of the inferior part of the gluteal region.
• Gives a perineal branch that supplies the posterior two-thirds of the scrotum in males or the posterior part of the labium majus in females.

Cutaneous Branches of the Obturator Nerve

• Supply the skin over the superomedial part of the posterior thigh.

Medial Branches of the Anterior Cutaneous Nerve of the Thigh

• Supply the skin over the lower medial part of the posterior thigh.

Lateral Cutaneous Nerve of the Thigh

• The lateral cutaneous nerve of the thigh is a branch of the lumbar plexus.
• It divides into anterior and posterior branches just below the inguinal ligament.
• Its posterior branch supplies the skin over the upper lateral part of the posterior thigh.

Muscles of Back of Thigh

• The posterior compartment of the thigh contains the following muscles:
  1. Semitendinosus
  2. Semimembranosus
  3. Biceps femoris
  4. Hamstring (ischial) part of the adductor magnus

• These muscles primarily produce extension at the hip joint and flexion at the knee joint.
• Clinically, the hamstring muscles are tested by asking the patient to flex the knee against resistance while the examiner palpates the contracted tendons or muscle bellies on the posterior aspect of the thigh.

<Hamstring muscles

• The muscles located in the posterior compartment of the thigh are collectively known as the hamstring muscles.
• The true hamstring muscles include:
  • Semitendinosus
  • Semimembranosus
  • Long head of the biceps femoris
  • Hamstring (ischial) part of the adductor magnus

• The short head of the biceps femoris is not considered a hamstring muscle because it arises from the femur rather than the ischial tuberosity.
• Only the ischial (hamstring) part of the adductor magnus belongs to the hamstring group; the adductor part is not a hamstring muscle.
• Historically, the term “hamstring” referred to disabling a person or animal by severing the hamstring tendons behind the knee.

Characteristic Features of Hamstring Muscles

True hamstring muscles share the following features:

• They originate from the ischial tuberosity.
• They insert onto the tibia or fibula.
• They are supplied by the tibial division of the sciatic nerve.
• They produce flexion of the knee joint.
• They produce extension of the hip joint.

Semitendinosus

• The semitendinosus is a long, superficial muscle situated on the posteromedial aspect of the thigh. It has a muscular upper part and a long tendinous lower part, which gives the muscle its name.

Origin

• Arises from the inferomedial impression on the upper part of the ischial tuberosity.
• Shares a common tendon of origin with the long head of the biceps femoris.

Course and Direction of Fibres

• Begins as a flat aponeurotic tendon at its origin.
• The muscle fibres descend vertically and gradually form a long, cord-like tendon in the lower half of the thigh.
• In the distal thigh, the tendon passes posterior to the medial condyle of the femur before turning anteriorly toward its insertion.
• The long, rounded tendon is an important identification feature during dissection and clinical examination.

Insertion

• Inserts into the upper part of the medial surface of the tibia as part of the pes anserinus, lying posterior to the tendons of the sartorius and gracilis.

Nerve Supply

• Supplied by the tibial division of the sciatic nerve.

Actions

• Assists in extension of the hip joint.
• Produces flexion of the knee joint.
• Medially rotates the leg when the knee is partially flexed.

Semimembranosus

• The semimembranosus is a broad, flat muscle located on the medial side of the posterior thigh. It lies deep to the semitendinosus. The muscle derives its name from its broad membranous tendon at the upper part.

Origin

• Arises from the superolateral impression in the upper quadrangular area of the ischial tuberosity.

Course and Direction of Fibres

• Begins as a broad membranous tendon proximally.
• The tendon gradually becomes fleshy in the lower half of the thigh.
• The muscle descends deep to the semitendinosus throughout its course.
• The broad membranous upper tendon is an important identification feature during dissection.

Insertion

• Inserts into the groove on the posterior surface of the medial condyle of the tibia.

Nerve Supply

• Supplied by the tibial division of the sciatic nerve.

Actions

• Produces flexion of the knee joint.
• Medially rotates the leg when the knee is partially flexed.
• Assists in extension of the hip joint.

Oblique Popliteal Ligament

• The oblique popliteal ligament is a strong fibrous band that reinforces the posterior aspect of the knee joint capsule.
• It is formed predominantly by an expansion of the tendon of the semimembranosus muscle.
• The ligament extends superolaterally from the posterior part of the medial condyle of the tibia to the intercondylar region and lateral condyle of the femur.
• It forms part of the floor of the popliteal fossa.
• The ligament is pierced by the middle genicular artery and accompanying vein.

Biceps Femoris

• The biceps femoris is the lateral muscle of the posterior compartment of the thigh. It has two heads, namely the long head and the short head, which unite to form a common tendon.

Origin

Long Head

• Arises from the inferomedial impression of the upper quadrangular area of the ischial tuberosity.
• Also receives fibres from the sacrotuberous ligament.

Short Head

• Arises from the lateral lip of the linea aspera, between the insertions of the adductor magnus and vastus lateralis.
• Also originates from the upper two-thirds of the lateral supracondylar line of the femur.

Course and Direction of Fibres

• The long and short heads descend separately and unite in the lower third of the thigh at an acute angle.
• The combined muscle forms a broad fleshy belly that continues as a strong tendon.
• The tendon passes inferolaterally toward the head of the fibula.
• Near its insertion, the tendon lies in close relation to the fibular (lateral) collateral ligament, around which it is partially folded.

Insertion

• Inserts onto the lateral side of the head of the fibula, just anterior to its apex (styloid process).

Nerve Supply

The biceps femoris has a dual nerve supply:

• Long head: Tibial division of the sciatic nerve
• Short head: Common fibular (peroneal) division of the sciatic nerve

Actions

• Produces flexion of the knee joint.
• Laterally rotates the leg when the knee is flexed.
• Assists in extension of the hip joint through the long head.

Adductor Magnus

• The adductor magnus consists of two anatomically and functionally distinct parts:
  • Hamstring (ischial) part
  • Adductor part
• Only the hamstring (ischial) part belongs to the hamstring muscle group.

Hamstring (Ischial) Part

Origin

• Arises from the inferolateral part of the ischial tuberosity.

Note: The adductor part arises from the inferior ramus of the pubis and the ramus of the ischium (conjoint ramus).

Insertion

• Inserts into the adductor tubercle of the femur.

Note: The adductor part inserts along the gluteal tuberosity, linea aspera, and medial supracondylar line of the femur.

Nerve Supply

• Supplied by the tibial division of the sciatic nerve.

Note: The adductor part is supplied by the posterior division of the obturator nerve.

Actions

• Produces extension of the hip joint.
• Assists in flexion of the knee through its attachment to the tibial collateral ligament, which is considered the degenerated distal tendon of the hamstring part of the adductor magnus.

Note: The adductor part is primarily responsible for adduction of the thigh.

Sciatic Nerve

• The sciatic nerve is the largest and thickest nerve in the human body. It is the largest branch of the sacral plexus. The nerve begins in the pelvis, passes through the gluteal region and posterior compartment of the thigh, and terminates near the superior angle of the popliteal fossa by dividing into the tibial and common fibular (peroneal) nerves.
• It measures approximately 1.5-2 cm in width.

Note: The sciatic nerve divides in the popliteal fossa, not the cubital fossa.

Formation

• The sciatic nerve consists of two anatomically distinct components:
  • Tibial part
  • Common fibular (peroneal) part

Tibial Part

• Formed by the anterior divisions of the ventral rami of L4, L5, S1, S2, and S3 spinal nerves.

Common Fibular (Peroneal) Part

• Formed by the posterior divisions of the ventral rami of L4, L5, S1, and S2 spinal nerves.

Root Value

• L4, L5, S1, S2, S3

Extent

• The sciatic nerve extends through the following regions:
  • Pelvis
  • Gluteal region
  • Posterior compartment of the thigh

• It terminates at or just above the superior angle of the popliteal fossa, where it divides into its terminal branches.

Relations

In the Pelvis

• Lies on the anterior surface of the piriformis muscle, beneath its fascial covering.

In the Gluteal Region

• Enters the gluteal region through the greater sciatic foramen, inferior to the piriformis muscle.

Superficial Relations

• Gluteus maximus
• Long head of the biceps femoris

Deep Relations (Superior to Inferior)

• Body of the ischium
• Tendon of the obturator internus
• Superior gemellus
• Inferior gemellus
• Quadratus femoris
• Adductor magnus

Medial Relations

• Inferior gluteal nerve
• Inferior gluteal vessels

In the Posterior Thigh

Superficial Relation

• Long head of the biceps femoris, which crosses the nerve from medial to lateral

Deep Relation

• Adductor magnus

Medial Relations

• Semimembranosus
• Semitendinosus

Lateral Relation

• Biceps femoris

Branches

Articular Branch

• Supplies the hip joint.

Muscular Branches

From the Tibial Division

• Semitendinosus
• Semimembranosus
• Long head of the biceps femoris
• Hamstring (ischial) part of the adductor magnus

From the Common Fibular (Peroneal) Division

• Short head of the biceps femoris

Terminal Branches

• Tibial nerve
• Common fibular (peroneal) nerve

Arteries of Back of Thigh

• The posterior compartment of the thigh receives its arterial supply from the following vessels:
  • Profunda femoris (deep femoral) artery, which gives four perforating arteries.
  • Medial circumflex femoral artery, which gives ascending, transverse, and acetabular branches.
  • Lateral circumflex femoral artery, which gives ascending, transverse, and descending branches.

Arterial Anastomoses of the Posterior Thigh

Trochanteric Anastomosis

• The trochanteric anastomosis is situated in the trochanteric fossa, near the base of the femoral neck.
• It is formed by the following arteries:
  • Ascending branch of the medial circumflex femoral artery
  • Ascending branch of the lateral circumflex femoral artery
  • Descending branch of the superior gluteal artery
  • Descending branch of the inferior gluteal artery

H4: Cruciate Anastomosis

• The cruciate anastomosis is located on the upper posterior aspect of the femur at the level of the lesser trochanter.
• It is formed by the following branches:

Artery	Contributing Branch
Medial circumflex femoral artery	Transverse branch
Lateral circumflex femoral artery	Transverse branch
Inferior gluteal artery	Descending branch
First perforating artery	Ascending branch

Longitudinal Anastomosis

• The longitudinal anastomosis lies along the linea aspera, close to the insertion of the adductor magnus.
• It is formed by:
  • Ascending and descending branches of the first, second, third, and fourth perforating arteries
  • Ascending muscular branch of the popliteal artery

Functions of the Arterial Anastomoses

• These arterial communications provide an effective collateral circulation around the hip and thigh.
• They help maintain an uninterrupted blood supply to:
  • The hip joint
  • The gluteal muscles
  • The posterior compartment of the thigh
• They become particularly important when the femoral artery or one of its major branches is narrowed or occluded, ensuring continued perfusion of the lower limb.

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