IThe Hips: Abduction
Graphics by Morgan Jeske ©2014

In our previous articles we have been looking at variations in our skeletons that affect how far we can move into certain yoga postures. We are all unique, and our yoga practice will reflect this uniqueness. Not everyone can do every pose: understanding where your limits are, and why they are, is essential for developing a safe and long lasting yoga practice. We have looked at the tibia and the femur. We will continue our investigation into human variations as we look now at the pelvis.

Your pelvis, just like your teeth and facial bones, is unique to you. This is a very complicated set of bones! The pelvis (which is Latin for basin or bowl) is made up of three bones that have fused together (the ilium, the pubis and the ischium) on either side (often referred to as the “hip bones”), and the sacrum, which is connected to the rest of the pelvis by strong ligaments.

Pelvis: Front and Back

The key landmark for our purposes is the acetabulum (which, also from the Latin, means vinegar cup, because that is what it sort of looks like. Today we would probably call it a sak cup.) The acetabulum is a hemispherical depression in the side of the hip, but it is not a perfect hemisphere. It can vary in width, depth and orientation. Your unique acetabular structure will determine how wide you can abduct your legs in a variety of yoga postures, such as the splits, and whether you can externally rotate the hips enough to safely achieve Lotus Pose. The head of the femur nestles snugly into the acetabulum and is held there by three strong ligaments and the joint capsule (these are not shown.) We will use the term “the hip” to refer to the joint where the femur and the acetabulum meet: note we are not defining the hip to be the top of the pelvis where most people rest their hands when standing like Superman, nor the bony prominence just below the belt line caused by the greater trochanter.

The key movements in the hips are extension and flexion (which move the legs back and forth), abduction and adduction (which move the legs side to side), and internal and external rotations (which rotate the femur in the horizontal plane.) See the article The Planes of Movement for detail descriptions of these three planes in which we can move our hips. Over the next few articles we will look at all the possible movements of the femur in the acetabulum in these three planes, and we will investigate the question, “What stops me?” The answer will be either tension from short tight muscles, fascia or joint capsules, or compression from points where the body is hitting another part of the body. In this article we will continue to investigate the movement called abduction. Abduction is a very common movement in standing postures, such as Triangle or Warrior 2. Any time we move the legs apart in the frontal plane or when we tilt the pelvis towards the thigh, we are abducting the hip socket. We have already looked at how variations in the shape of the neck of the femur can affect how deeply we can go into abduction. Time now to see how the shape of the pelvis also affects our range of movement.

In the drawings of the pelvis above, we showed an artist’s impression of the pelvis, however no one actually has a pelvis that looks like the drawings we find in the anatomy texts. Those drawings are averages taken from many cadavers and images. Here you see two images of a real pelvis: one is an x-ray image and the other is from computer tomography. These images are useful in helping to understand the relationship between the femur and the pelvis. In isolation, however, it is difficult to see the uniqueness of this pelvis, so let’s look at two other pelvises and see how they differ, and what these differences mean for the ultimate range of motion of the hips.

Notice these two pelvises below: do you see any differences? They are certainly not the same and there may be many differences that jump out at you. For now, lets focus on the acetabulums. Notice how the acetabulum of the pelvis on the left is orientated more toward the front of the body and it is pointing more downward. The one of the right is pointing out to the side and higher. These two orientations have technical terms: version and abduction. Version is the degree to which the acetabulum is oriented forward or backwards (called anteversion or retroversion respectively). Acetabular abduction describes the angle the acetabulum makes to a horizontal line running between the sitting bones.[1] (The pelvis imaged through computer tomography shows an acetabular abduction of 45.) The third key parameter of the acetabulum that affects our potential range of motion is the depth of the acetabulum, often measured in millimeters or cited as a percentage of the width of the acetabulum. The deeper the acetabulum, the less range of motion available. Not shown in these pictures of the pelvis bones is the rim of cartilage circling the acetabulum, known as the labrum (which is Latin for lip.) The labra (the plural for labrum) can effectively increase the depth of the acetabulums, making the joints more secure but also reducing range of motion.

Now, here is a pop quiz: assuming the femurs were identical for these two people (which they are not, but for the sake of simplicity we will make that assumption), which pelvis would allow for a greater ultimate abduction of the femur? Who will be able to spread their legs wider in Triangle, Goddess or the Splits postures? The answer: the pelvis on the right will ultimately be able to abduct much more than the pelvis on the left (once all the tensile resistance has been worked out.)

Some anatomy texts, like Thieme, state that the range of version changes as we age: from 7 at birth to 17 as an adult. (This is why children can more easily do Lotus pose than their parents.) The range of abduction also changes: from 51 at birth to 45 by 10 years of age to 40 as an adult.[2] (Again, young gymnasts can do the splits more easily than their older siblings.) By itself this creates the impression that everyone, when they become adults, will have version of 17 and acetabular abduction of 40. However, this is deceiving. The statistics just cited are averages: the range of human variation is quite dramatic, and you are more than likely not average, and neither are most of the students found in a yoga classroom. Below are some of the ranges observed in a variety of studies: notice the gender differences!

 

Males

 

 

Females

 

 

Overall Mean

 

Study 1 28 – 42 28 – 42
37~38
Study 2 36 – 43 37 – 47
40~41
Study 3 48 – 66 51 – 67
56~57
Study 4 29 – 57 35 – 45
39~40
Ranges of Acetabular Abduction [3]
 

Males

 

 

Females

 

 

Overall Mean

 

11 – 21 12 – 24
17
13 – 24 14 – 28
20
16 – 28 18 – 28
23
8.5 – 32 14 – 33
22
14 – 24 18 – 29
21
Ranges of Acetabular Version [4]

As you can see there is a vast difference in variations between people. There is no one angle that is the “right” amount of version or abduction. You may fall within these ranges, but there is no guarantee that everyone else will – there are always outliers beyond these ranges.

What is the difference in range of abduction of the leg for someone with an acetabular abduction angle of 28(the lowest found in the studies above) versus someone with 67 (the highest listed)? Here are two simulations: in both cases we are assuming that the femur has the same neck-shaft angle of 134(which is the North American average), but you can imagine how the femur neck-shaft angle will also have a bearing on range of motion (which we saw in the previous article discussing the femur.) What we are showing are the femur and the outline of the part of the pelvis surrounding the acetabulum.

Compression will occur either at the point where the neck of the femur hits the rim of the acetabulum or where the greater trochanter pinches flesh between it and the side of the pelvis. In the case of the pelvis with an acetabular abduction angle of 67, this happens after 70 of abduction (remember, if the FSNA is higher, the amount of abduction available also increases.) In the case with acetabular abduction angle of 28, the amount of abduction of the leg is considerably less because the acetabular abduction angle is also much smaller.


Abduction with Acetabular Abduction Angle of 67
Abduction with Acetabular Abduction Angle of 28

We find that, all other things being equal, such as acetabular depth, acetabular version and the neck-shaft angle of the femur, there is an extra range of 35 of abduction of the leg for the hip that has 67 of acetabular abduction versus the one with only 28.

If we have an acetabulum that is deeper than those shown above, this too will affect the amount of movement possible. In some people, the lips of the acetabulum have grown outwards more: this over-coverage condition is known as a pincer – the edges of the acetabulum “pinch” the neck of the femur more securely, but with consequences for the range of motion. The blue coloring of the lip of the acetabulum in the graphic below shows this. Indeed, it is the growth of the overhang of the acetabulum top and bottom that not only dictates the depth of the socket, but also the angle of abduction! If the top lip is recessed and shallow, while the bottom lip is extended, the angle of abduction is likely to be higher than if the top lip was prominent and the bottom lip recessed. In some cases, however, the whole rim of the acetabulum can be extended, causing the pincer, which deepens the socket.


Abduction with Acetabular Abduction Angle of 67 but 20% deeper Acetabulum

The relationship between acetabular depth and pincer has been noticed in studies.[5] In the example shown here, the acetabular angle and the femur neck-shaft angles are the same as we have seen earlier at 67 and 134 respectively, but the depth of the acetabulum is 20% more, thanks to the pincer over-coverage. (One study found that acetabular depth varied by about 40%, so 20% variation is not unusual.[6]) This results in the ability of the leg to abduct to be reduced from 70 to 45.

We have been only looking at abduction, but similar restrictions to adducting the legs could also be deduced. An acetabulum that has a lower acetabular abduction angle will promote greater adduction of the leg than one that has a higher angle.

The purpose of this article is to illustrate the point that human variations will and do affect how far you can move your limbs in any direction. In our next article, we will continue to look at the femur and pelvis and see what stops us in the other movements of the femur in the acetabulum: the movements of external and internal rotation, and flexion and extension.

Footnotes:

  1. — Just to make things more confusing, different researchers and anatomists will use different names for these: for example, Thieme calls the acetabular abduction angle the transverse angle of theacetabular inlet plane. It can also be referred to as Sharp’s Angle.
  2. — See Thieme Atlas of Anatomy page 379
  3. — Study #1: From Acetabular Dysplasia – The Acetabular Angle May 2, 1961 by Ian Sharp
    Study #2: From Differences in Acetabular Morphology related to side and sex in a Chinese Population by Zeng et al in Journal of Anatomy in March, 2012. In this study, we have only cited the range as being one standard deviation from the mean.
    Study #3: From Variations in Acetabular anatomy with reference to total hip replacement by P.E. Murtha et al in Journal of Bone Joint Surgery, March 2008
    Study #4: From Multilevel Measurement of Acetabular Version Using 3-D CT-generated Models by Aimee C. Perreira et al published Sept 25, 2010 ClinOrthopRelat Res (2011) 469:552-561 DOI 10.1007/s11999-010-1567-2
  4. — This is a summary of a number of different studies found in Multilevel Measurement of Acetabular Version Using 3-D CT-generated Models by Aimee C. Perreira et al published Sept 25, 2010 ClinOrthopRelat Res (2011) 469:552-561 DOI 10.1007/s11999-010-1567-2. Included are only those finding that provided ranges.
  5. — See Cams and Pincer Impingement Are Distinct, Not Mixed: The Acetabular Pathomorphology of Femoroacetabular Impingement by Justin Cobb et al. published in the Clinical Orthopaedics and Related Research August 2010.
  6. — See Acetabular morphometry for determining hip dysplasia in the Singaporean population by M Umer et al., published in Journal of Orthopaedic Surgery, 2006. They observed that the mean depth to width ratio of the acetabulum is .32, however it varied within one standard deviation by .06, which is 19%. This means that ~68% of the people were within that range of variation, but almost 1/3rd of people vary by more than that!