The quadrate line is an anatomical feature of the upper portion of the femur, the long bone of the thigh that is the largest bone in the human body. A vertical elevation that is only a few centimeters in length, it is found on the inner rear side of the upper femur, just below the part of the bone known as the femoral neck. Also known as the linea quadrata, the quadrate line is significant as the attachment site for the quadratus femoris muscle of the posterior hip. It also is where a small number of fibers from the top end of the adductor magnus muscle of the inner thigh connect to the femur.
At the top of the femur, the portions of bone known as the head and neck angle inward from the shaft of the femur to insert into the hip socket. The topmost portion of the shaft, which is situated to the outside of the neck and head, forms an irregular surface known as the greater trochanter. On the rear aspect of the greater trochanter is where most of the deep rear hip muscles attach: the piriformis, obturator internus, gemelli, and gluteus minimus muscles among them.
Dividing the neck of the femur from the shaft at an angle perpendicular to the direction of the neck is the intertrochanteric ridge, the raised border of the shaft. This ridge circles the femur as it descends, so it spirals downward from the greater trochanter while angling diagonally, wrapping the shaft below the femoral neck. Forming a second, vertical ridge that begins at about the midpoint of the intertrochanteric ridge on the medial posterior femur is the quadrate line. This line, which is only barely perceptible, directs downward roughly five centimeters from the intertrochanteric ridge on the rear inside aspect of the bone. It ends where it meets the top of the lesser trochanter, a smaller rounded prominence where the neck meets the shaft on this side of the femur.
Along most of the length of the quadrate line, fibers from the quadratus femoris muscle attach to the bone. The quadratus femoris arises from a thickened surface on the posterior aspect of the ischium bone of the lower pelvis known as the ischial tuberosity. Shaped like a flat, wide band, it then crosses horizontally from the ischium outward to the intertrochanteric region of the femur, passing behind the hip joint on its course. Some of its fibers attach near the upper end of the intertrochanteric ridge on the intertrochanteric crest, as the rest follow the quadrate line downward.
The lowest portion of the quadrate line gives rise to the topmost fibers of the adductor magnus muscle. Covering most of the length of the inner thigh, the adductor magnus stretches from the bottom of the femur just above the knee joint all the way to the ischial tuberosity and the pubis bone in the lower pelvis. Near the top, it gives off a few fibers that insert along the bottom of the quadrate line below those of the quadratus femoris muscle.