Bones of the lower limb

The foot bone’s connected to the...leg bone. And the leg bone’s connected to the... thigh bone! Alright, as a quick recap… just kidding. The lower limbs actually have an incredibly detail-rich skeleton that can be divided into two functional components: the pelvic girdle, which connects the lower limb to the axial skeleton, and the bones of the free lower limb. 

The pelvic girdle is a bony ring, known as the pelvic ring, consisting of the right and left hip bones, and the sacrum which is common to both the pelvic girdle and axial skeleton. Each hip bone consists of the ilium, ischium, and pubic bone and has three articulations. Posteromedially, it articulates with the sacrum at the sacroiliac joint, anteromedially  it articulates with the other hip bone at the pubic symphysis. And finally, it articulates with the head of the femur to form the hip joint. 

Ok so, the ilium is the largest and most superior part of the hip bone and it can be divided into a body and a wing. The wing, or ala of the ilium,has a lateral and a medial surface, a crest, and two borders: anterior and posterior. Superiorly, there's the iliac crest, which begins at the anterior superior iliac spine and it extends posteriorly to the posterior superior iliac spine. The crest has an internal and external lip, and serves as an important attachment site for muscles and deep fascia. Next, the lateral surface of the ala has three rough arched lines called the posterior, anterior, and inferior gluteal lines, where the gluteal muscles attach, which are the muscles of your bottom. Next, the medial surface can be divided into two by the internal lip of the iliac crest. The anterior portion is concave, and it forms the iliac fossa - which is where the iliacus muscle attaches. 
The posteromedial portion is rough and it presents the auricular surface - which is shaped like an ear and articulates with an identical surface on the sacrum to form the sacroiliac joint.. Next, the anterior border presents the anterior superior iliac spine superiorly, where the inguinal ligament and the sartorius muscle attach; and underneath it there’s the anterior inferior iliac spine, where the straight head of the rectus femoris muscle and the iliofemoral ligament of the hip joint attach. Finally, the posterior border has a posterior superior iliac spine, which is where the oblique portion of the posterior sacroiliac ligaments and the multifidus muscle attach. Underneath it, there’s the posterior inferior iliac spine, below which is a deep notch, the greater sciatic notch. Ok, so the ala continues inferiorly with the body of the ilium, which joins the pubis and ischium to form the acetabulum. 

Next, let’s look at the ischium, which forms the postero-inferior part of the hip bone. The posterior border of the ischium forms the inferior margin of the greater sciatic notch.  At the end of the notch, there’s a triangular surface called the ischial spine, where the sacrospinous ligament and superior gemellus muscle originates. Below the spine, there’s the lesser sciatic notch, which separated from the greater sciatic notch by the sharp demarcation of the sacrospinous ligament. Going even further down, the body of the ischium has an ischial tuberosity, which is a thick, rough prominence that supports our weight while sitting and where the inferior gemellus muscle and the hamstrings attach. Unfortunately, these aren’t actual ham strings, but rather a pair of three posterior thigh muscles. Anteriorly, the ramus of the ischium joins the inferior ramus of the pubis to form a bar of bone called the ischiopubic ramus that represents the inferior border of the obturator foramen.

Ok, now, finally, there’s the pubis, which forms the anteromedial part of the hip bone. The body of the pubis has the symphyseal surface on its medial surface, which articulates with an identical surface on the body of the contralateral pubis via the pubic symphysis. The pubic symphysis is a cartilaginous joint that sits between and joins the pubic bones at their respective rami. Anterosuperiorly, the body presents a rounded thickening called the pubic crest, where the abdominal muscles attach. The crest extends laterally as the pubic tubercle, which is the distal attachment point of the inguinal ligament. Superior and laterally, the body of the pubis continues with its superior ramus, which forms part of the acetabulum. The lateral part of the superior ramus also has sharp raised edge, the pectineal line or pecten pubis, which joins the arcuate line of the ilium to form the iliopectineal line. Inferior and laterally, the pubis presents the inferior ramus, which joins the ramus of the ischium to form the ischiopubic ramus.

Now that we know a bit about the bones separately, let’s talk about what they form together. First, there’s the acetabulum, which is the large socket on the lateral face of the hip bone that articulates with the head of the femur to form the hip joint. Each of the three primary bones contributes to the acetabulum as follows: the ischium provides the posterior-inferior boundary, the ilium forms the upper boundary, and the rest, near the midline, is formed by the pubis. The margin of the acetabulum is incomplete inferiorly, where the acetabular notch is situated, which makes it look like a cup with a broken rim. There’s also a rough depression in the floor of the acetabulum - the acetabular fossa, right above the acetabular notch. These two structures, the acetabular notch and depression, are surrounded by the thick and smooth lunate surface of the acetabulum, the articular surface on which the head of the femur slides.  Moving on, there is a large oval opening of the hip bone called the obturator foramen, bounded by the pubis, ischium, and their rami. Normally, the obturator foramen is closed by the very strong obturator membrane, which has a tiny passageway for the obturator nerve and vessels called the obturator canal. The purpose of this foramen is to minimize bone weight, while its closure by the obturator membrane provides an extensive area for the fleshy attachment of internal and external obturator muscles.

Ok, so that was enough info to make your hip bones crack! Why don’t you pause for a minute, and see if you can label the most important features of the hip bones before we move on? 

Ok, great. Let’s now move on to the bones of the free lower limb, which include the femur, tibia, fibula, patella, and the bones of the feet. Now, the femur is the longest bone in the body and looks like the letter L turned upside down, making them oblique and directed inferomedially as the femur progresses distally. To place it in its anatomical position, the head of the femur, which is the part with a round prominence proximally, should be situated superiorly and medially. Its distal part, the bit with two round prominences, is situated inferiorly and bit more medial than the superior part because the femur has an oblique position in the body. And finally, on the posterior part of the bone there’s a sharp, longitudinal line - the linea aspera. The head of the femur articulates with the acetabulum to form the hip joint, while the distal femur articulates with the tibia and patella, known as the kneecap, to form the knee joint. The femur’s main purpose is to transmit body weight from the hip bone to the tibia when a person is standing, and, at the same time, to provide attachments for 23 muscles. 

Ok, so the proximal femur has a head, neck, and the greater and lesser trochanters. The round head of the femur makes up about two thirds of a sphere, and is covered in articular cartilage,  except for a medial place depression which creates a pit called the fovea capitis, which is where the ligament of the head of the femur attaches. The neck of the bone is trapezoidal, with its narrow end supporting the head and its broader base continuous with the shaft of the femur. Now, at the junction between the femoral neck and shaft, there are two large bumps called trochanters. The lesser trochanter is situated medial and inferior to the junction, and is where the iliopsoas attaches. Lateral and superior to the junction is the much larger greater trochanter, where the abductor and rotator muscles of the thigh attach. At the base of the greater trochanter, there’s a deep depression called the trochanteric fossa, which is where the tendon of the obturator externus muscle, obturator internus, and the superior and inferior gemellus all insert. Now, running from the greater to the lesser trochanter there’s also the intertrochanteric line on the anterior side of the proximal femur, which is where the neck of the femur and femoral shaft join. The iliofemoral ligament, considered the strongest  of the human body, attaches above this line, while the upper part of the Vastus medialis attaches to the lower part of the intertrochanteric line. Posterior to the trochanters, there’s a similar but smoother and more prominent ridge, the intertrochanteric crest, which, in turn, has a rounded elevation on its upper side called the quadrate tubercle, and the quadratus femoris muscle inserts on both these elements.

Most of the femoral shaft is smoothly rounded and it provides fleshy origin to extensors of the knee. The exception is a posterior, longitudinal, rough line, called the linea aspera - which is where the aponeuroses of the thigh adductor muscles attach. The linea aspera is situated around the middle third of the femoral shaft, and it presents a medial and lateral lip and an intermediate ridge. Superiorly, the lateral lip blends with a broad, rough patch of bone called the gluteal tuberosity, which is where part of the gluteus maximus attaches. The medial lip, on the other hand, continues superiorly as the spiral line, which extends toward the lesser trochanter and then passes to the anterior surface of the femur, where it is continuous with the intertrochanteric line. The intermediate ridge is called the pectineal line, and it extends from the central part of the linea aspera to the base of the lesser trochanter. Switching to its inferior part, here, the linea aspera divides into medial and lateral supracondylar lines, which lead to - you’ve guessed it, the medial and lateral femoral condyles.

Speaking of which, let’s look at the distal femur, which consists almost entirely of two round prominences called the medial and lateral femoral condyles. The femoral condyles articulate with the menisci, which are two plates of cartilage on top of the tibia, and with the tibial condyles to form the knee joint. The femoral shaft sits in about 9 degrees of valgus positioning from the vertical plane, which compensates for the oblique nature of the femur and allows the femoral condyles to be on the same horizontal level when articulating with the tibia.  The lateral surface of the lateral condyle has a central projection called the lateral epicondyle, whereas the medial surface of the medial condyle has a medial epicondyle. The condyles are separated posteriorly and inferiorly by the intercondylar fossa, but they merge anteriorly, forming a shallow longitudinal depression called the patellar surface, which articulates with the patella.  The epicondyles are where the medial and lateral collateral ligaments of the knee joint attach proximally. Superior to the medial epicondyle there’s another elevation, unsurprisingly called the adductor tubercle, where part of the adductor magnus muscle inserts. 

Ok, so that was enough info to make your fall on your knee! Why don’t you pause for a minute and see if you can label the most important features of the femur before we move on? 

Ready? Ok, so the patella or knee cap is a triangular bone located on top and in the middle of the femoral condyles. It has an anterior and a posterior surface, a superior base and an inferior apex, and two margins. The anterior surface is smooth and you can actually feel it on the anterior part of your knee . The posterior surface is covered with a thick layer of articular cartilage so it can articulate with the patellar surface of the femur. This surface is divided by a vertical ridge into a narrower medial and wider lateral articular surface. The ridge, along with the balanced pull of the vastus muscles, helps keep the patella centered in the intercondylar groove as it articulates with the femur during flexion and extension. The patella acts as the largest sesamoid bone in the body, and functions to provide leverage for the quadriceps during extension. 

Now, that was a breeze, wasn’t it? Let’s move on to the second largest bone of the body the tibia, which is located on the anteromedial side of the leg, nearly parallel to the fibula. This bone kind of resembles a scepter, so the thicker and flat proximal end is superior, and its thinner and rounder distal end is situated inferiorly. And because the body of the tibia is somewhat triangular, the tip of the triangle should be anterior, while its base should be posterior. The tibia articulates with the condyles of the femur proximally  and the talus distally , so it transmits the body’s weight to the foot and it provides stability to the ankle joint. The tibia can be divided into a proximal and a distal end and a shaft.

Ok, so the proximal tibia resembles half of a pyramid turned upside down. Superiorly, the base of the pyramid is rather flat, so it’s called the tibial plateau. This plateau has a medial and a lateral articular surfaces that articulate with the condyles of the femur. The articular surfaces are separated by an intercondylar eminence, formed by two raised ridges of bone -  the medial and lateral intercondylar tubercles, which are flanked by the anterior intercondylar area to the front, and posterior intercondylar area to the back.