Thigh compartments

Three compartments, three nerves

Intermuscular septa from the fascia lata to the linea aspera divide the thigh into compartments, each with its own nerve and main action.

• All four parts of quadriceps, including vastus lateralis, are supplied by the femoral nerve. A femoral nerve lesion gives weak knee extension, a lost knee jerk and numb anterior thigh.
• Muscles with dual nerve supply: adductor magnus (obturator plus tibial part of sciatic), pectineus (femoral, sometimes obturator) and biceps femoris (two parts of the sciatic, one for each head). Adductor longus has a single supply (obturator).

Anterior compartment muscles

• Rectus femoris: the only part of quadriceps that crosses the hip. Straight head from the anterior inferior iliac spine, reflected head from above the acetabulum. It flexes the hip and extends the knee, so an injury (often avulsion of the AIIS in kicking adolescents) stops both actions together.
• Vasti: extend the knee only. Vastus medialis obliquus (lowest fibres) pulls the patella medially and resists lateral dislocation; it is the first part to waste after knee injury.
• Sartorius: the longest muscle, from the ASIS to the upper medial tibia. It flexes, abducts and laterally rotates the hip and flexes the knee (the cross-legged sitting position): the muscle that flexes both hip and knee.
• Iliopsoas: chief flexor of the hip, inserting on the lesser trochanter; also a lateral rotator. Its fascial sheath is continuous from the lumbar spine to the thigh, so a tuberculous psoas abscess tracks down inside it and points in the femoral triangle.

Medial compartment muscles

• The adductors arise from the pubis and ischium and insert along the linea aspera. Gracilis is the only one crossing the knee; it is often used as a muscle flap or tendon graft.
• Adductor magnus: the adductor part (from the ischiopubic ramus) is supplied by the obturator nerve; the hamstring part (from the ischial tuberosity, inserting on the adductor tubercle) by the tibial part of the sciatic nerve. The adductor hiatus between the two parts lets the femoral vessels reach the popliteal fossa.
• Obturator nerve and referred pain: the ovary lies in the ovarian fossa on the lateral pelvic wall directly over the obturator nerve. Ovarian disease, pelvic tumours or the nerve's injury at oophorectomy cause pain or numbness on the medial thigh. Obturator hernia gives pain referred along the medial thigh to the knee.

Posterior compartment: hamstrings

• True hamstrings arise from the ischial tuberosity, cross both hip and knee, and are supplied by the tibial part of the sciatic nerve: semitendinosus, semimembranosus, long head of biceps femoris and the hamstring part of adductor magnus. They extend the hip and flex the knee.
• The short head of biceps arises from the linea aspera, crosses only the knee and is supplied by the common peroneal part.
• Sprinting can avulse the hamstring origin from the ischial tuberosity.
• Pes anserinus: sartorius, gracilis and semitendinosus insert together on the upper medial tibia (one from each compartment, one from each nerve); its bursitis causes medial knee pain.

Remember One tendon each from the femoral, obturator and sciatic territories forms the pes anserinus.

Patella and the extensor mechanism

• The patella is the largest sesamoid bone, lying in the quadriceps tendon. It increases the lever arm of quadriceps and protects the tendon. The patellar ligament continues to the tibial tuberosity.
• Rupture of the quadriceps tendon (older people) or patellar ligament (younger people) prevents active straight leg raising.
• Q angle: the angle between a line from the ASIS to the centre of the patella and a line from the patella to the tibial tuberosity; normally about 14° in men and 17° in women. Because quadriceps pulls along the femur while the patellar ligament runs straight down, the resultant force on the patella is directed laterally. A wide pelvis, knock knees (genu valgum) or a laterally placed tibial tuberosity increase the Q angle, so young women with these features suffer recurrent lateral patellar dislocation. Resisting forces are the high lateral femoral condyle and vastus medialis obliquus. After a first dislocation, an X-ray looks for an osteochondral fragment.
• Osgood–Schlatter disease: a traction apophysitis of the tibial tuberosity in active children around 10–15 years, when repeated quadriceps pull through the patellar ligament irritates the growing apophysis. A painful, tender lump over the tuberosity worsens with running and settles with rest. Night pain or systemic symptoms need an X-ray to exclude a tumour.

Fascia lata and iliotibial tract

• The fascia lata is the tough deep fascia enclosing the thigh; the saphenous opening lies in it below the inguinal ligament.
• The membranous layer of superficial fascia (Scarpa's fascia, continuous with Colles fascia in the perineum) fuses with the fascia lata just below the inguinal ligament. Urine from a ruptured bulbar (spongy) urethra below the perineal membrane therefore spreads into the scrotum, penis and anterior abdominal wall but not into the thigh.
• The iliotibial tract is a thickening of fascia lata receiving tensor fasciae latae and much of gluteus maximus, inserting on the lateral tibial condyle (Gerdy's tubercle). It steadies the extended knee and hip; friction over the lateral femoral condyle causes iliotibial band syndrome in runners.

Blood supply and compartment pressure

• Profunda femoris and its four perforating arteries supply all three compartments; the obturator artery adds to the medial compartment.
• Thigh compartments are large and compliant, so compartment syndrome is less common than in the leg, but it follows crushes and femoral fractures.

Femoral shaft fractures

• High-energy injuries. The proximal fragment is flexed by iliopsoas, abducted by gluteus medius and minimus, and laterally rotated; the distal fragment is pulled upwards and medially by the hamstrings and adductors, causing shortening.
• A closed femoral shaft fracture can lose 1–1.5 litres of blood into the thigh.
• Fat embolism syndrome: confusion, hypoxia and petechiae (axillae, conjunctivae) 24–72 hours after a long-bone fracture.
• Myositis ossificans: a mass appearing weeks after a deep muscle bruise (often quadriceps), showing haemorrhage maturing into bone at the periphery (zonal pattern); not a sarcoma.

Exam trap Adductor magnus is supplied by both the obturator and the sciatic (tibial) nerves.

Exam trap The short head of biceps femoris is the only thigh muscle supplied by the common peroneal part of the sciatic nerve.

How FCPS asks about this

• Nerve to vastus lateralis or any quadriceps part: femoral nerve; weak knee extension and lost knee jerk.
• Muscle crossing hip and knee that flexes the hip and extends the knee: rectus femoris; hamstrings do the reverse; sartorius flexes both.
• Muscle with dual nerve supply: adductor magnus (obturator and tibial).
• Ovarian pain or post-oophorectomy numbness on the medial thigh: obturator nerve.
• Recurrent lateral patellar dislocation in a girl with wide hips and knock knees: increased Q angle; quadriceps pulls the patella laterally.
• Tender lump over the tibial tuberosity in an active 12-year-old: Osgood–Schlatter disease.
• Extravasated urine that does not enter the thigh: Colles and Scarpa fascia fuse with the fascia lata below the inguinal ligament.