Thigh Compartments: Anatomy

Thigh Compartments

• The thigh has three main compartments: anterior, medial, and posterior.
• The anterior compartment is at the front of the femur and is involved in hip flexion and knee extension, innervated by the femoral nerve.
• The medial compartment is medial to the femur and is involved in hip adduction, innervated by the obturator nerve.
• The posterior compartment is behind the femur and is involved in hip extension and knee flexion, innervated by the sciatic nerve.
• The main neurovascular bundle travels between the anterior and medial compartments, supplying vasculature and innervation to the lower limb.

Muscles of the Anterior Compartment

• The quadriceps femoris is the largest muscle in the anterior compartment and the principle extensor of the knee.
• It consists of four heads, which converge on the quadriceps tendon to attach to the patella and tibial tuberosity.
• The four heads are the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius.
• The rectus femoris originates from the anterior inferior iliac spine, descends through the thigh, and is enveloped by the other quadriceps muscles.
• It's the only quadriceps belly that crosses the hip and contributes to hip flexion and knee extension.
• The vastus lateralis originates from the gluteal tuberosity and lateral lip of the linea aspera, curving laterally to attach to the patella.
• The vastus medialis originates from the medial lip of the linea aspera, curving medially to attach to the patella.
• The vastus medialis obliquus (VMO) is the most inferior part of the vastus medialis, countering the lateral pull on the patella. Weak VMO is associated with poor patellar tracking and lateral dislocation.
• The vastus intermedius originates from the anterior surface of the femur and lies deep to the rectus femoris.
• The sartorius, also known as the "taylor" muscle, is the longest continuous muscle in the body.
• It originates from the anterior superior iliac spine (ASIS), curves medially along the vastus medialis, and crosses posterior to the knee joint.
• The sartorius inserts on the anteromedial aspect of the tibia at the pes anserinus and is the "cross-legged" muscle, flexing, abducting, and laterally rotating the hip, as well as flexing the knee.
• The iliopsoas is a combination of the psoas major, psoas minor (if present), and iliacus muscles, originating from the pelvis.
• It inserts via a common tendon to the lesser trochanter and is the principle flexor of the thigh.
• The pectineus is a "hybrid" muscle of the anterior and medial compartments, with dual innervation from the femoral and obturator nerves.
• It originates from the superior ramus of the pubis and inserts on the pectineal line of the femur, adducting, flexing, and medially rotating the hip.

Muscles of the Medial Compartment

• All muscles in the medial compartment are innervated by the obturator nerve and generate hip adduction.
• The obturator externus is the most superior, originating from the obturator membrane and inserting on the trochanteric fossa, laterally rotating and stabilizing the hip joint.
• The adductor longus originates from the pubic bone near the symphysis pubis and inserts on the middle third of the linea aspera.
• The adductor brevis originates from the pubic bone inferior to the adductor longus and attaches to the proximal portion of the linea aspera.
• The adductor magnus is the broadest adductor muscle, a "hybrid" with adductor and hamstring portions, originating from the inferior rami of the ischium/pubic bones.
• Its adductor portion inserts along the linea aspera, and the hamstring portion inserts on the adductor tubercle of the femur.
• The arch between portions at the femur attachment creates the adductor hiatus, allowing passage of vessels between anterior and posterior thigh compartments.
• The gracilis originates from the pubic bone inferior to the adductor brevis and runs posterior to the knee to insert with the sartorius on the pes anserinus, assisting with knee flexion and hip adduction.

The Femoral Triangle

• The femoral triangle is a triangular landmark of anatomical and clinical importance, with borders consisting of the inguinal canal (superior), adductor longus muscle (medial), and sartorius muscle (lateral).
• The apex is found inferiorly where the sartorius crosses the adductor longus, with floor formed by the pectineus and iliopsoas muscles
• It is covered by the facia lata and cruciform fascia (over saphenous opening).
• It serves as an important transit point for neurovascular structures.
• Vascular structures are embedded in the femoral sheath, a fascial tube about 3-4 cm long that blends with the superficial connective tissue of vessels distally, permitting gliding during hip flexion/extension.
• The most lateral structure in the triangle is the femoral nerve, found on the anterior surface of the psoas major and already divided into cutaneous and muscular branches.
• Its terminal branch, the saphenous nerve, travels through the abductor canal with femoral vessels.
• The femoral artery is the most lateral structure within the femoral sheath, found at the midway point of the subinguinal canal, and it's the primary artery supply of the lower limb.
• Injury to the proximal femoral artery can result in extensive blood loss; compress the artery by landmarking the mid-region of femoral triangle cutaneously and applying pressure
• The femoral artery projects down the adductor canal with the saphenous nerve, passing through the adductor hiatus to become the popliteal artery.
• The profunda femoris artery, or deep artery of the thigh, is the primary supply to the anterior compartment, with medial and lateral femoral circumflex branches supplying the femoral head and neck.

Femoral Artery Branches

• The lateral femoral circumflex gives off transverse and descending branches.
• The transverse branch supplies blood to the head of the femur.
• The descending branch supplies the lateral part of the anterior compartment and the knee.
• Perforating branches pierce adductor muscles to access the posterior compartment, ensuring blood flow while sitting.
• The femoral vein lies medial to the femoral artery in the femoral triangle.
• It projects with the femoral artery to the adductor hiatus, where it becomes the popliteal vein, draining perforating branches and the great saphenous vein.
• The femoral canal is an empty space in the femoral sheath medial to the femoral vein, allowing artery expansion during systole.
• Deep lymphatic vessels/nodes are in the most medial region of the femoral sheath, draining lymph from deep structures in the lower limb, passing it to deep nodes within the pelvis.
• They may occasionally drain lymph from superficial lymphatics.

Adductor (Subsartorial) Canal and Obturator Canal

• The adductor canal is a muscular compartment distal to the apex of the femoral triangle, with borders including the vastus medialis (anterolateral), adductors longus and magnus (posterior), and sartorius muscle (covering).
• It allows passage of femoral vessels to the adductor hiatus, the saphenous nerve to the leg, and a motor branch to the VMO, with vessels covered by subsartorial fascia.
• The obturator canal is formed by the incomplete covering of the obturator foramen by the obturator membrane, typically found at the medial margins of the membrane.
• It permits access of the obturator artery and nerve to the medial compartment of the thigh.

Posterior Thigh Overview

• The posterior thigh region lies posterior to the femur, medial, and lateral intermuscular septum of the thigh.
• It primarily consists of hip extensors and knee flexors.
• It receives vascular supply from perforating arteries from the profunda femoris.
• It is innervated by branches of the sciatic nerve, which passes in the intermediate aspect of the compartment.

Posterior Thigh Muscles

• Three of the four muscle bellies are part of the hamstring muscle group, originating off the ischial tuberosity and supplied by the tibial nerve portion of the sciatic nerve.
• The semitendinosus is a hamstring muscle with a slim, tendinous distal portion commonly used in ACL autograft reconstructions, passing medial and posterior to the knee to insert on the pes anserinus.
• It contracts to generate hip extension and knee flexion.
• The semimembranosus is a broad muscle deep to the semitendinosus, passing medial and posterior to the knee to insert inferior to the medial condyle, contracting to generate hip extension and knee flexion.
• The biceps femoris is a convergent muscle with long and short heads.
• The long head is part of the hamstring muscle group, running from the ischial tuberosity to the lateral surface of the head of the fibula, contracting to generate hip extension and knee flexion and the most important extensor of the hip.
• The short head originates off the distal half of the femoral shaft, lateral to the linea aspera, and inserts with the long head on the head of the fibula, contributing only to knee flexion because it is not innervated by the common fibular nerve.

The Knee Joint Overview

• The knee joint is an articulation between the thigh and leg, a synovial hinge-type joint between the femur and tibia allowing subtle gliding and rotational movements,the fibula does not contribute to knee motion.
• It has an unstable architecture, as the larger superior  bone is supported by a smaller inferior bone with a smaller base of support, but the extensive array of ligamentous/cartilagenous attachments provides joint stability.
• The Q-angle measures the obliqueness of the femoral shaft.
• The femoral shaft projects medially and inferiorly in anatomical position, redistributing weight to the midline.
• The knee joint lies directly inferior to the hip joint, which is important for postural stability.
• The Q-angle is the angle between a vertical line through the knee and a line connecting the ASIS to the patella, typically greater in females due to a wider pelvis.
• Genu varum ("bowlegged") is when femoral shaft is closer to vertical, with a low Q-angle and increased weight-bearing load on medial condyles, making them susceptible to osteoarthritis.
• Genu valgum ("knock-knee") is when the femoral shaft lies more oblique, with a Q-angle > 170, a weight-bearing line lies lateral to the knee increasing load on lateral condyles, quadriceps femoris tend to pull the patella laterally, causing patellofemoral syndrome.

Articular Surfaces and Joint Capsule

• Articulations occur between the lateral/medial condyles of the femur/tibia, shaped incongruently.
• Femoral condyles are condyloid, while tibial condyles are planar.
• Best surface contact/stability occurs in the extended position.
• Articulation between the anterior intercondylar surface of the femur and the patella also occurs.
• The fibrous joint capsule attaches to the femur and tibia peripheral to articular surfaces, relatively thin and continuous with the patella and patellar tendon/ligament anteriorly.
• It's incomplete posteriorly over the lateral epicondyle, allowing passage of the popliteus tendon.
• The synovial membrane lines all non-articulating surfaces internal to the capsule, reflecting into the intercondylar region to line cruciate ligaments and infrapatellar fat pad.
• There's an incomplete division into left/right femorotibial cavities, and extends superiorly as the suprapatellar bursa.

Intrinsic and Interarticular Ligaments

• The patellar ligament is a continuation of the patellar tendon beyond the patella.
• The tibial (medial) collateral ligament is a strong, flat band on medial aspect of the joint capsule that resists valgus forces to the knee, also works with fibular collateral ligament to resist lateral rotation.
• Deep fibers insert on the medial meniscus within the capsule.
• The fibular (lateral) collateral ligament is a strong cord on lateral surface of the joint that attaches to the fibular head and resists varus forces to the knee, also works with tibial collateral ligament to resist lateral rotation.
• The anterolateral ligament (ALL) originates with the LCL and runs obliquely to insert on the tibia.
• The oblique popliteal ligament is a recurrent extension of the semimembranosus tendon that provides posterior reinforcement.
• The arcuate popliteal ligament forms an arch over the popliteal ligament and also provides posterior reinforcement.
• Cruciate ligaments are located within intracondylar region of joint,internal to joint capsule, external to synovial cavity
• There are two ligaments which corkscrew around each other in clockwise fashion as they run superiorly, winding tight during medial rotation limited to 100 versus 600 lateral rotation in flexed knee position and maintain proximity between femur and tibia

Cruciate Ligaments

• The anterior cruciate ligament (ACL) is named for anterior attachment to intercondylar region of the tibia.
• ACL projects posteriorly, lateral to PCL to insert just medial to lateral condyle of femur and prevents anterior glide of tibia on femur
• The posterior cruciate ligament (PCL) is named for posterior attachment to intercondylar region of the tibia.
• PCL projects anteriorly, medial to ACL to insert just lateral to medial condyle of femur and prevent posterior glide of tibia on femur

Menisci

• Located in the knee joint
• Fibrocartilagenous discs anchored to the tibia through coronary ligaments.
• Crescent wedge shape fits contours of femoral condyles
• Distensibility/elasticity serve as shock absorption during foot strike with walking/running.
• Medial meniscus - C-shape
• Broadest in posterior region
• Lateral margin anchored to MCL
• Lateral meniscus is smaller and more mobile, anchored to popliteus tendon, PCL (through posterior meniscofemoral ligament), and circular in appearance.
• Menisci- greater surface are contact between articulating surfaces which distrubutes pressure across a greater surface area

Joint Stability

• In flexion, collaterals and the posterior joint capsule become loose, due to glide of femoral condyles on tibia decreasing distance between attachment points.
• Cruciate ligaments maintain some level of taughtness due to oblique course of ligaments.
• In extension, all ligaments become taught, with lateral rotation of the tibia on the femur due to greater rolling of the smaller medial femoral condyle, generating further tension in collateral ligaments with a greater pull of tibia towards femur due to slight oblique angles of fibers.
• The tautness allows "locking" of knee, minimizing energy expenditure from muscular stabilization.
• Tension in ligaments and rigidity of knee joint in extension means greater susceptibility to ligament sprains.

Knee Injuries

• Collateral ligament sprains occur because they are taught in the extended position and resist lateral rotation of tibia.
• Medial collateral ligament (MCL) is susceptible to tearing during pivoting or from valgus stressor (blow to lateral aspect of knee).
• Lateral collateral ligament (LCL) is not as commonly injured (excessive varus stressors are uncommon).
• Cruciate ligaments sprains occur because they are taught throughout joint angles and resist medial rotation of the tibia on the femur.
• ACL is susceptible to hyperextension injury and may tear along with MCL with valgus stressor.
• PCL is infrequently torn when falling on flexed knee (pushes tibia backward).
• Menisci injuries occur as a result of accepting axial load from femur and anchored to joint capsule through numerous ligamentous attachments.
• Medial meniscus is commonly injured from shearing forces of femur in ACL deficient knee. A "bucket handle" tear describes the appearance of a torn central flap hanging loose and may also tear with MCL tears due to close connection.
• Lateral Meniscus is commonly torn from compression forces of femur during valgus stressor and is less commonly torn from rotational movements and less anchored, more mobile.
• The "unhappy triad" describes tearing of the MCL, ACL, and meniscus with excessive valgus stressor.
• Originally medial meniscus but studies show lateral meniscus commonly torn due to connection with MCL.