The femur, knee and lower leg

The lower extremity, commonly named leg, is connected to the body at the pelvic girdle by the hip joint and is divided into two long segments separated by the knee joint: the thigh and the lower leg (calf and shin), the latter extending to the ankle and the foot. 

The lower extremity supports the body weight and permits ambulation. Its ability to bend and rotate facilitates many functions such as standing, walking, running, jumping, riding, driving and more.  Consequently, any pathology of the lower extremity severely affects our mobility, thus impairing most daily activities and occupations.

Femur

The femur or thighbone is the longest and strongest bone in the human skeleton. It extends from the hip to the knee joint. 

The femoral head is the distal (upper) end of the femur that inserts into the acetabulum of the hip joint. As it descends towards the knee, it is separated from the longest tubular bone, or shaft, by the femoral neck.

The greater trochanter is the protruding extremity of the upper femur that can be felt laterally at the hip. The great trochanter inserts the large gluteus muscle. Below and on the opposite side of the great trochanter is the lesser trochanter, which connects to the ileo-psoas muscle.

The femoral shaft contains the bone marrow. 

At the proximal (lower) end the femur forms two large structures on the outer side, the lateral epicondyle and the lateral condyle whereas at the inner side are the adductor tubercle, the medial epicondyle and the medial condyle.

The lateral and medial epicondyles articulate with the tibia and fibula to form the knee joint, including the trochlear groove, which holds the patella (kneecap).

The tibia and fibula

The lower leg consists of two bones: the tibia and fibula.

The tibia or shin bone is the largest bone located to the inner leg-side. It is the major weight bearing bone of the lower extremities, supporting 85% of the body weight.

The tibia extends from the knee, whereby its lower protuberances, the lateral and medial condyles, also named medial and lateral tibial plateau, articulate with the condyles of the distal femur to form the knee joint. The lateral condyle presents a round facet that accommodates the head of the fibula, whereas the medial condyle functions to insert the semimembranous muscle. Between the lateral and medial condyles is the intercondylar eminence. The tuberosity of the tibial head allows for the insertion of the patellae ligament. The tibial shaft has a triangular shape with the anterior prominent edge, the shin.

At the distal end the tibia becomes larger and assumes a curved profile with a lateral depression (fibular notch), which articulates with the fibula. The medial side of the tibia ends in the medial malleolus. The medial and lateral malleolus can be felt at both ankle sides.

The fibula runs parallel to the tibia to which is connected via the interosseus membrane. It is a thin bone with a head in the proximal end (outside the knee joint) and the triangular lateral malleolus to the distal end. It has four borders. Its main role is to connect the muscles of the lower leg rather than weight bearing mainly taken up by the tibia.

Bones and cartilage of the knee

The knee joint is formed by the distal end of the femur, the proximal end of the tibia and the patella, a mobile bone supported only by tendons and ligaments.

The articular surfaces of the knee bones (femur, tibial condyles and posterior side of the patella) are protected by cartilage. This is a rubbery, shiny and smooth tissue, which reduces the friction between bones during movement and absorbs the impact, thus preventing the degeneration of bone extremities.

Menisci

The knee is supported by two cartilaginous structures, the lateral and medial menisci, considered to be a form of ligaments. The menisci have a semilunar shape and are positioned laterally. They absorb the impact and stabilise the joint by distributing the energy of the round femur onto the flat tibia by forming a concave structure. They also protect the cartilage from grinding with use.

The knee joint capsule

As in all joints, the knee is also surrounded by a fibrous capsule enclosed by a synovial membrane, which produces a lubricating synovial fluid to facilitate the sliding of the knee bones: femur, tibia and patella.

The knee capsule is held together by ligaments that permit a wide range of movements. Its function is also to stabilise the knee when in standing and balancing positions. 

Knee bursas

The bursa is an additional important structure of the knee joint. It’s a fluid-filled sack acting as a cushion to absorb shocks and lubricate the joint.

Each knee has 11 bursas, the main ones being:

the supra-patellar bursa above and under the patella, which is connected to the cavity of the synovial joint

the pre-patellar bursa situated subcutaneously above the patella

the superficial and deep intra-patellar bursas located below the patella

Semimembranosus bursa on the knee posterior side.

Most often the bursas located over the patella are subject to inflammation or bursitis.

Ligaments of the knee

The knee is supported by four principal ligaments, which are elastic fibrous bands connecting bones to bones.

The medial and lateral collateral ligaments lie in the inner and lateral side of the knee and function to contain the sideways movements of the knee.

The anterior and posterior cruciate ligaments extend diagonally from the femur to the tibia and overlap forming a cross, giving the origin of the name from latin, crux. These ligaments limit the flexion/extension movements of the knee with the anterior cruciate also controlling the inward rotation. 

The lateral patellar retinaculum keeps the patella in line during flexion and extension of the knee.

The oblique popliteal ligament begins behind the tibial medial condyle and attaches to the posterior side of the joint capsule. It can rotate femur on the tibia to unlock the knee joint and allow flexion.

Muscles of the upper leg

The motor function of the leg knee is mostly regulated by the muscles of the thigh.

The quadriceps is a group of four muscles located on the front of the thigh that contracts to straighten the knee joint by lifting the patella.

These muscles are:

The rectus femoris originating at the pelvis

The vastus medialis, vastus lateralis and vastus intermedialis originating at the femoral head.

The ligaments of these muscles extend over the knee and connect to the tibia to stabilise the joint.

The hamstrings consists of three muscles situated in the posterior side of the upper leg. They all originate at the ischium or sitting bone on the lower portion of the pelvic ring.

The semimembranosus attaches to the medial condyle of the tibia and is used for thigh extension, leg internal rotation, hip extension and knee flexion. It is a pelvis stabiliser.

The semitendinosus inserts below the medial tibial condyle of the tibia and its action is to flex the knee, extend the hip and internally rotate the leg.

The biceps femoris, is a muscle consisting of the long and short head, originating at the ischium and femoral bone, respectively. Both insert on the proximal lateral aspect of the fibula. This muscle functions to flex and rotate the knee (when bent), extend the hip joint and for leg external rotation.

Muscles of the anterior lower leg

The anterior part of the lower leg (shin) comprises the following muscles:

The tibialis anterior

The extensor digitorum longus

The extensor hallucis longus

The peroneus tertius 

These muscles dorsiflex (extend) the foot and toes upward. The controls turning the foot inward.

At the lateral side of the lower leg, are the muscles:

The peroneus longus

The peroneus brevis

These muscles are used to pull the foot outward (pointing the foot).

Muscle of the posterior lower leg

The posterior compartment of the lower leg, commonly named calf, consists of two large muscles:

The gastrocnemius

The soleus

The gastrocnemius muscle begins on the femur above the knee, whereas the soleus muscle inserts below the knee at the tibia. These muscles merge as they descend towards the ankle to form the Achilles tendon, which raps around the calcaneus or heel bone.

The calf muscles flex the ankle and lift the heel when raising the body on the ball of the feet.

In the deep posterior compartment of the lower leg are the following muscles:

The tibialis posterior

The flexor digitorum longus

The flexor hallucis longus

The tibialis muscle pulls the foot inward and the flexor muscles flex the toes and hallux, respectively. Together they also control the flexion of the foot plantar.

‍Tendons of the thigh and knee

Tendons are robust fibrous bands connecting muscles to bones. The main tendons of the lower extremity extend from the thigh into the lower leg, across the knee. The principal tendons are:

The quadriceps tendon is located above the knee and attaches the large quadriceps muscle to the patella

The patellar tendon is located below the patella in continuity with the quadriceps tendon. It connects the patella to the tibial tubercle to consolidate the insertion of the quadriceps to the shin

The ilio-tibial tendon runs laterally along the thigh from the tensor fasciae latae muscle to the knee

The hamstring tendons are situated on the posterior thigh inserting to the tibia and fibula along the medial and lateral side of the knee

The semimembranosus tendon extends from the muscle carrying the same name on the posterior thigh to insert onto the tibial medial condyle

The quadriceps and patellar tendons work in synergy with the quadriceps muscle during the extension of the leg.

The ilio-tibial tendon is a hip abductor whereas the hamstring tendons are knee flexors.

‍Tendons of the lower leg

The function of the muscles of the lower leg is coordinated by numerous tendons connecting the muscles to the bones. These tendons begin as an extension of the muscles and descend to the foot where they assist in the movement of the toes.

The peroneal or fibularis tendons (longus and brevis) run behind the fibula, along the lateral side of the calf to the foot. They are named as they corresponding muscles:

The peroneus brevis (short) tendon attaches to the 5th metatarsal bone of the foot (little toe)

The peroneus longus (long) tendon runs under the foot and inserts to the 1st metatarsal bone of the foot

The extensor longus digitorum and extensor hallucis longus tendons are the continuation of the muscles carrying the same names and extend to the foot and toes.

The extensor digitorum longus arises at the the tibial condyle, runs along the lateral side and inserts at the dorsal foot. It divides into four branches, each inserting in the small toes.

The extensor hallucis longus insert on the dorsal side of the big toe and extends it upward. It is clearly visible with foot/hallux flexion

The posterior tibial tendon arises from the calf muscle. It runs on the back side of the leg near the Achilles tendon and bends at the ankle and insert to the bones on the inner foot. It holds the arch and support the foot when walking.

The peroneal tertius tendon enters the dorsal surface of the 5th metatarsal bone.

‍Achilles tendon

‍The Achilles tendon is the largest and strongest tendon in the body. It connects the calf muscles to the back of the foot or calcaneus. It is also named the calcaneal tendon.

This tendon is formed conjointly by the gastrocnemius and soleus muscles of the calf and inserts into the calcaneus.

With the flexion of the calf muscles, the Achilles tendon will raise the calcaneus. This movement occurs when we stand on our toes, or during running, sprinting and jumping. Due to the poor blood flow, all tendons including the Achilles are prone to injury causing tears and severing.

Two bursas are located in the inner and outer aspect of the Achilles. They are the Retro-calcaneal and subcutaneous calcaneal bursa.

Arteries of the lower extremity

The popliteal artery originates from the femoral artery in the thigh and supplies the distal femur, knee joint and proximal tibia. Below the knee it branches first into the anterior tibial artery and subsequently into the posterior tibial artery and the fibular or peroneal artery.

The anterior tibial artery runs through the entire length of the leg into the foot and supplies the anterior part of the tibia and then forms the dorsalis pedis artery.

The posterior tibial artery descends through the deep posterior compartment of the lower limb and supplies the lateral leg. Downstream it forms the medial and lateral plantar artery of the foot. The plantar arch divides into the metatarsal and then plantar digital arteries to supply the toes.

The fibular artery runs between the tibia and fibula to supply the lateral side of the leg and dorsum of the foot.

Veins

Deoxygenated blood flows through the veins from the lower extremities and ascends to the heart.

The main veins of the lower leg are:

Lesser or small saphenous vein: originates at the foot and runs subcutaneously along the medial aspect of the calf to merge with the greater saphenous vein in the thigh and continue to the inguinal region.

Popiliteal vein: is located posteriorly in the subcutaneous region of the knee and continues upstream to merge with the femoral vein.

Anterior and posterior tibial veins and the fibular (or peroneal) vein run along the anterior, and posterior side of the tibia respectively.

Fibular vein runs at the lateral aspect of the calf along the fibula bone.

These latter three veins merge into the popliteal vein at the knee region.

The Calf Muscle Pump (CMP) and the venous foot pump

The CMP is referred to as the body’s secondary heart. When the calf muscles contract during movement (walking etc), they compress the veins of the CMP and help the blood to flow back to the heart. Similarly, the pressure on the calcaneus (rete calcaneus), the first part of the foot to hit the ground and the venous plexus of the foot plantar, contributes to compress the veins and pump the blood upstream. The good function of these pumps prevents the stagnation of venous blood and consequently the formation of deep vein thrombosis. A good prevention of vein thrombosis consists in avoiding a prolonged sedentary life and alternate sitting with frequent movements and wear compression stockings following surgery or during long flights.

Nerves of the lower extremity

The innervation of the lower leg begins upstream at the lombo-sacral region of the spine with a bundles of nerves that together form the lumbo-sacral plexus.

Sacral plexus

The nerves exiting the 4th and 5th lumbar vertebrae form the lumbo-sacral trunk merge with the sacral plexus, which consists of the sacral nerves from the 1st to the 4th sacral vertebrae. The sacral plexus descends along the posterior pelvic wall.

The most important nerve of the lower extremity is the sciatic nerve and its branches extending from the spine to the foot.

Sciatic nerve  

Sciatic nerve is the largest human nerve; it consists of nerves arising from the 4th lumbar to 3rd sacral vertebrae. It leaves the sciatic foramen to first innervate the gluteal muscle.

The sciatic nerve with all its components innervate several muscles of the leg.

It provides motor innervation to the muscles of the posterior aspect of the thigh, the adductor magnus and extends to all muscles of the lower leg and foot

It also provides sensory innervation to the skin of the leg anterolateral and postero-lateral aspect

Injury to the sciatic nerve produces a number of neurological deficits including motor and sensory loss of the posterior thigh, leg and foot.

Above the knee the sciatic nerve divides into two branches: the tibial nerve medially and the common peroneus nerve, laterally.

Tibial nerve

The tibial nerve runs in the posterior aspect of the leg along the tibial artery to reach the medial malleolus of the distal tibia at the ankle.

Below the medial malleolus, the tibial nerve bifurcates into the medial and lateral plantar nerve innervating the sole of the foot.

The tibial nerve affords motor and sensory innervation to the muscles of the posterior compartment of the leg and sensory function in the sole of the foot.

Injury to the tibial nerve causes:

loss of foot flexion (plantar flexion)

loss of toe flexion

reduced inversion of the foot

sensory loss throughout the areas above

Common fibular nerve

The common fibular nerve, or common peroneal nerve, originates from the tibial nerve and bends around the proximal fibula subcutaneously, which makes it prone to injury.

At the fibular head, the common peroneal nerve bifurcates into the superficial and deep fibular nerves.

The superficial fibular nerve is situated on the lateral aspect of the lower leg where it controls motor function during outwards movement (eversion) of the foot

It provides sensory function in the antero-lateral compartment of the leg and the dorsum of the foot.

The deep fibular nerve runs in the anterior aspect of the lower leg along the tibial artery. It confers motor function to the frontal muscles of the leg to control foot dorsiflexion and toe extension.

It regulates sensory innervation between hallux and second toe.

Injury to the deep fibular nerve causes the so called “foot-drop”, and deficits in the functions described above.