The femur or thigh bone is the longest bone in the body, extending from the hip and descending down, curving slightly toward the midline of the body until it reaches the knee. It is also the heaviest and strongest bone in the body, yet around 250,000 people experience a broken femur in the U.S. each year. The femur supports the weight of the body during many simple activities including standing, walking, running, and jumping. Not only does the femur account for one-fourth of a person’s height, but it can also resist forces of 1800 to 2500 pounds.

Anatomy of the Femur

The femur consists of three parts: the proximal, the shaft, and the distal. On the top or proximal end, the femur is part of the ball-and-socket joint, the acetabulum of the hip. At its lower or distal end, the femur forms part of the knee joint. Like other bones in the body, the femur is made up of three layers -- the outside skin or periosteum, a hard compact bone, and the bone marrow, which contains gelatin-like material. These three layers encase both the bloodstream and the nerve signals that move between the layers.

xray femur bone yodiyim / Getty Images

The Proximal End of the Femur

The bulbous head of the femur is essential because it connects with the hip joint. The femoral neck joins the head with the femoral shaft; this is the weakest part of the femur due to its much smaller diameter compared to the rest of the bone. The femoral neck consists primarily of spongy rather than hardened bone. At the base of the neck, the lesser trochanter faces toward the midline of the body. The greater trochanter, a large four-sided part of the bone on the lateral side of the femur, faces away from the midline of the body. The intertrochanteric line runs between the greater trochanter and the lesser trochanter, connecting them on the anterior or front side of the femur. On the posterior or back side of the femur, the intertrochanteric crest connects the greater and lesser trochanters.

doctor femur hip joint Jan-Otto / Getty Images

The Shaft of the Femur

The femoral shaft is the long, straight section of the femur. It descends in a slightly medial direction and ensures that the knees are closer to the body’s center of gravity when the femur connects with them, which improves stability. Rough ridges of bone, the linea aspera, split at both the top and bottom. At the top of the shaft on the proximal boundary is the pectineal line. The gluteal tuberosity is on the top lateral side and attaches to the gluteus maximus muscle. At the lower end of the shaft, the linea aspera widens and forms the popliteal fossa floor, the lateral supracondylar line, and the medial supracondylar line. On the distal or lower side of the linea aspera, the medial supracondylar line comes to an end at the adductor tubercle. The adductor magnus attaches to the adductor tubercle.

femur bones shaft Drbouz / Getty Images

The Distal End of the Femur

The primary function of the distal end of the femur is to connect with the tibia and the patella to form the knee joint. Pieces of cartilage called the menisci fill the joint space where the two bones, the femur and the tibia, meet. The medial and lateral condyles are rounded areas on the lower end of the femur. The lateral condyle’s surface protrudes more than the medial’s does. This protrusion prevents the natural lateral movement of the patella which could cause a dislocation of the kneecap. The medial and lateral epicondyles are the bony elevations that interact with the muscles and connective tissues. The intercondylar fossa is a notch on the femur’s posterior that separates the condyles. It connects the intracapsular knee ligaments.

knee femur joint Jan-Otto / Getty Images

Muscle Origin Points

Twenty-three individual muscles originate from or attach to the femur. Three of the four quadriceps in the thigh begin here: the vastus lateralis, vastus medialis, and vastus intermedius on the front of the thigh. These muscles allow the knees to straighten from a bent position. Other important muscles that originate from the femur include the popliteus muscle that unlocks the knee and the gastrocnemius which allows the knee and ankle to flex.

thigh muscles femur pictorico / Getty Images

Muscle Insertion Points

Insertion is the point of attachment in a muscle where movement transpires. The five adductor muscles of the thigh help stabilize the hip bone and stretch from the pelvis to the femur. They include the adductor magnus, adductor longus, adductor brevis, and obturator externus. These adductor muscles are on the inside of the thigh. The adductor longus’ main function is to provide movement toward the median axis of the body and laterally rotate the thigh. The three hamstrings that run up and down the back of the thigh -- the semimembranosus, semitendinosus, and biceps femoris -- allow the knee to bend.

hamstring muscles insertion Jan-Otto / Getty Images

Femoral Fractures

Femur fractures are common despite the great deal of force it takes to break this large bone. These injuries come with risks such as bleeding inside the thigh, which can result in significant blood loss, and blood clots. Serious fractures may even result in long-term disability. These fractures can occur for a variety of reasons, and the chance of a broken femur increase with age. The Journal of American Academic Orthopedic Surgery reports that by the year 2050, the number of femur fractures will double. An individual can experience a femoral break at the femoral head, shaft, or condyles. Fractures of the femoral neck are the most common, and osteoporosis is a primary risk factor.

femur fracture xray praisaeng / Getty Images

Types of Femoral Shaft Fractures

Doctors use a classification system to identify femur fractures based on their location and pattern. They also consider torn skin or muscle.

  • Transverse fractures are straight horizontal lines that extend across the femoral shaft.
  • Oblique fractures have an angled break across the femoral shaft.
  • Spiral fractures of a twisting force to the thigh result in a fracture line that encircles the femoral shaft.
  • Open or compound fractures force a bone fragment to protrude through the skin. Or, in some cases, a wound extends down to a broken bone. Both may result in damage to muscles, ligaments, and tendons and are highly susceptible to infections.
  • Comminuted fractures occur when the bone breaks into three or more pieces.
femur fracture hip Sutthaburawonk / Getty Images

Causes of Femoral Fractures

High-impact trauma from a significant perpendicular force commonly cause transverse fractures, while a rotational force accounts for spiral or oblique fractures. Car collisions, ATV and motorcycle crashes, industrial accidents, falls, and gunshot wounds are the cause of most femoral fractures, though health conditions such as osteoporosis and cancer make a break more likely during an incident with less force. The severity of the break determines the healing time. Femoral fractures tend to require surgery and a longer healing time than other fractures, but they usually repair within six months.

doctors xray femure fracture avid_creative / Getty Images

Stress Fractures

Over time, running, jumping, dancing, and other activities requiring excessive weight-bearing may cause femoral stress fractures. This type of fracture is an incomplete crack in the thigh bone, the result of repetitive activities that force the bone to work beyond what it can withstand. In many cases, stress fractures start with a bony stress reaction that worsens following an increase in training or a change in footwear, technique, or training surface.

pain injury femoral stress filadendron / Getty Images

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