Biomechanics

ERIC J. KROPF, MD

STRUAN H. COLEMAN, MD, PHD

ALEXANDER E. POOR, MD

Editor’s Note: Eric Kropf is a trusted colleague and Chairman of Orthopedic Surgery at Temple University. An excellent hip arthroscopist, Eric talks a lot about the importance of the different orientations of the hip and rest of the pelvis. Eric joins Struan and co-editor Alexander Poor in expanding upon the theme of pelvic geometry playing an important role in human performance and the surgery that we do. They present the basic concepts in a fun way. Keep in mind that very little basic research exists on this subject.

Come on, let’s twist again, like we did last summer.

Let’s twist again, like we did last year!

—Lyrics from the music of Chubby Checker (born Ernest Evans). Chubby may now be known also for propagating the roles of pelvic tilt and version. Popularizer of The Twist, The Limbo Rock, The Fly, and The Pony, Checker changed the way we dance to music. Before The Twist, grown-ups never danced to teenage music. Raised in the projects of Philadelphia, Checker tops Billboard’s list of the most popular singles ever and holds the record for placing 5 albums in the Top 12 at once.

CHUBBY’S NEW EYES

Think about Chubby and The Twist. How many twists, turns and contortions does your pelvis go through when you do The Twist? “Justin Timberlake, Britney Spears, all the rappers, they’re doing my dances and they’re making billions doing my dances. When they do that little thing they do with their hands? That’s The Fly and The Pony!”

Those were Chubby’s words. Chubby saw what’s going on; he saw the movements. Chubby lived all the different variations of pelvic tilt and version. No doubt, he also knows about all the other orientations of the bones and muscles of the pelvis and hips.

In the last chapter, Dr. Philippon traveled quickly through some clinical implications of the various anatomies of the hip. In the next chapter, Marc Safran digs deeper into the biomechanics. Let’s pause here for a moment, go up 30,000 feet, with Chubby, and place a few biomechanical definitions on what Chubby, Justin, Beyoncé, and Eminem are doing when they dance.

“THREE” ORIENTATIONS

No, we are not talking about sex again. We are talking about 3 different orientations that factor into movements that we create when we use our pelvis.

We will keep this simple. Think of the pelvis as having 3 important orientations. The pelvis, of course, has way more than 3 orientations. But just think about 3: tilt and version. Wait a minute, that’s just 2, isn’t it? We said 3. Before you designate us not-mathematical whiz kids, understand that when it comes to the hip, there are 2 types of version. That makes 3: tilt plus 2 types of version.

Now, think of the entire set of pelvic bones as one entity. Think of it as a planting pot holding a tree. Tilt refers to the orientation of that pot. The planting pot can tilt in any direction. The direction and degree of tilt will, obviously, affect the tree and whatever supports the pot (Figure 28A-1).

Version refers to each of the supporting posts of the pot. In the case of the human being, we have 2 supporting posts holding up that pot, or pelvis. Perhaps aliens have 3 or more. On Earth, we call these 2 supporting posts the lower extremities, and we call the connecting apparatus for each of those 2 posts the ball-in-socket hip joint. Therefore, now understand that we now have 3 types of orientation: one for the whole pelvis (tilt), and the second and third for each of the 2 hips jointly with their posts (versions). See, we have made this simple as promised. Thanky Panky, we only have to think about 2 types of orientation. What holds for one hip likely holds for the other.

Figure 28A-1.

Wait again. Two posts do not provide stability. Won’t it fall over? Doesn’t it take a triad of posts to stabilize the pelvis or a pot otherwise hanging in the air? Yes, that would be true if there were no movement, and all we did was stand still. Then we would usually simply fall over.

But we have movement here, and a lot of it. Our body has to stay dynamic to keep its balance. That is how we maintain our 2-leggedness and our superiority in the species hierarchy. Balance and stability occur via dynamic interplays of opposing forces. Tilt and version are big parts of normal body dynamism. To quote Coach Belichick, “It is what it is.” These orientations keep us doing what we are doing. Belichick probably said that, too.

The pelvis tree-pot can tip in any direction. It can tip in 3 ways, from (1) what is going on in the tree, (2) something within the pot itself, or (3) an issue involving either (or both) of its post connectors.

TILT TYPES

Anterior pelvic tilt occurs when the pelvis (or “pot”) tips forward. The anterior pubis drops, and the sacrum and rest of the posterior pelvis rise. Consequently, anterior soft tissue structures—such as the rectus abdominis, adductors, iliopsoas, rectus femoris, and sartorious—shorten, and posterior soft tissue structures—such as the erector spinae, hamstrings, and glutes—lengthen. Lengthening past normal tensile parameters causes weakness, and the whole process may lead to instability owing to inability of the shortened muscles to generate force enough to correct imbalances and restore neutrality.

Posterior pelvic tilt refers to the opposite situation. The pelvis tips backward; the flexors, adductors, and rectus abdominis lengthen; and the posterior muscles shorten. Instability may occur via a process complementary to anterior pelvic tilt (ie, weakened anterior muscles and/or ineffective posterior muscles). (See Figures 28A-2 and 28A-3.)

Figure 28A-2. Anterior and posterior pelvic tilt. (Reprinted with permission from Joel Kouyoumjian [admin@iseecrookedpeople.com] of Manchester-Bedford Myoskeletal [www.mbmyoskeletal.com] in Bedford, New Hampshire.)

Figure 28A-3. The bony arrangement of anterior and posterior pelvic tilt. Basically, think of tilt as the relationship between the entire bony pelvis and (1) the femur and rest of the bony lower extremity and (2) the spine. Note that both the lower extremity and spine must make adjustments.

Figure 28A-4. Schematic of lateral pelvic tilt. (Reprinted with permission from Dr. Jason Gray of My Rehab Connection [www.myrehabconnection.com].)

Lateral pelvic tilt, as you might surmise, occurs when the pelvis tips to either side (Figure 28A-4). This most commonly ensues from structural abnormalities such as limb length discrepancies or spinal scoliosis. Left vs right pelvic tilt refers to the side tipped down. In other words, with left tilt, the right iliac wing appears elevated, and with right tilt, the left iliac wing is higher.

When pelvic tilt matters, the designations permanent or variable hold importance with respect to designing treatment. Permanent refers to the inability to correct the actual tilt itself. This may be related either to inherent anatomical structure or the consequence of injury or an otherwise nonfunctional part (eg, after a stroke). Treatment then focuses on palliation via influencing the consequences of the permanent deficit. Variable refers to just that, the tilt is fixable either through physical therapy, more invasive intervention, alternative therapies, or a combination of treatments.

WHEN TILT MATTERS

Size and shape of the pelvis comes into this big time. Let’s start out with the horse, and then go to the monkey and human. Review Chapter 20 and some of the evolutionary/developmental/comparative anatomic factors.

Figure 28A-5. The horse’s pelvis. (A) The tail. (B) The long iliac bone. (C) The “tight” hip (ie, the acetabular socket completely enveloping the femoral ball).

Figure 28A-6. Chimp and human pelves have different structures. The most obvious difference is the chimp’s longer pelvis, probably important for locomotion.

Look at the horse’s pelvis (Figure 28A-5). Consider the anatomical factors that distinguish this 4-legged animal. There are, basically, 3 things: (1) the extension of the sacrum as a tail (okay, we don’t have that); (2) a long, vertical iliac bone; and (3) tight hips.

Now look at a gorilla or chimp and compare it to upright us (Figure 28A-6). First, look at the iliac bone. It is also much longer and more vertical in the gorilla. This anatomical feature distinguishes the 4-legged creatures from us upright hierarchal leaders. The height and size of the iliac bone helps fixate those creatures into 4-legged postures. The overall size and shape of the pelvis help determine acetabular depth and shape.

Think of the 4-legged posture as the extreme of anterior pelvic tilt (Figure 28A-7). We, as humans, normally vary in the size and shape of our pelvis. We can think of ourselves as all having some degree of “gorilla” in our posture. Could this be why the “set position” in most sports (see Chapter 7) involves just a little bit of anterior pelvic tilt?

Also see how much more limited anteriorly within the socket our hips appear to be. The monkey’s hips are more limited in their anterior range of motion. The horse and other 4-leggeders have grossly limited ranges of hip motion. In general, all 4-legged postures compellingly fixate both posture and hip motion; the exception indeed being the human baby.

In humans, alterations of tilt and version limit anterior hip range of motion.¹ In the extreme, we become more like 4-legged animals. Considering the species comparisons, doesn’t it seem best to think of this fixation as creating both an advantage and a disadvantage?

The advantage is that limitation of movement can also serve as a fulcrum, by its very nature of decreasing range of motion. If our muscles are fit, we can “fly” in a multitude of ways. But, of course, we can’t jump as high as many of the 4-leggeders. The ability to jump high seems obviously related to the vertical length of the pelvic bones, glutes, psoases, and other muscles. Think about it, think about all the parallels of comparative anatomy and how we might apply them in sports.

Figure 28A-7. Extremes of pelvic tilt shown by comparing (A) gorilla and (B) man. Think about where the third baseman’s baseball set position fits in.

How Variable Tilt Becomes Bad

The earlier discussion pertained mostly to the permanent type of pelvic tilt—something we are born with, either as a species or just individual variability. Alterations in resting pelvic tilt generally involve muscle weakness or contracture or joint or ligamentous instability or contracture. When one form of variable pelvic tilt stays for long, compensatory muscle activation patterns occur. The result may be chronic pain or even acute muscle injury. Sometimes, injuries occur in predictable patterns.

For example, with continuous anterior pelvic tilt, the spine must assume a relatively hyperextended position in order to maintain balance and strength. This results in paradoxical lengthening of the rectus abdominis and shortening or contracture of the erector spinae musculature. In this scenario, the spine may demonstrate increased lordosis or the athlete may compensate by bringing the hips into a flexed position. This further amplifies anterior muscular contracture of the hip flexors or even of the adductor compartment.

Conversely, to maintain balance in the setting of posterior pelvic tilt, the spine will roll forward and “round,” resulting in contracture of the erector spinae. The lumbar spine then assumes a kyphotic position. With excessive lateral tilt, the hip abductors will need to increase their forces during single-leg stances. The glute medius and minimus exert maximal effort in order to elevate the pelvis in the single-leg stance so that the pelvis on the elevated side does not fall to the floor.

Standing and Sitting in the Office All Day Are Both Bad

Pelvic tilt issues occur for a variety of reasons. The most common clinical scenario is excessive anterior pelvic tilt. Most people remain unaware that their daily routine may be dangerous and result in the chronic muscle imbalances that create anterior pelvic tilt. Either standing or sitting throughout the day contribute to pelvic tilt issues. The best position for standing in neutral pelvic tilt involves mild knee flexion. Then the hip flexors and extensors become relatively balanced. However, this requires considerable muscular effort to maintain this position. Work creates fatigue.

Then it becomes more energy efficient to stand with the knees extended or hyperextended. In this position, the pelvis tips slightly anteriorly. The muscle groups in the anterior hip muscles contract and gradually tighten more and more. The muscles include the rectus femoris and iliopsoas. A repetitive cycle turns a mild muscular imbalance into a fixed contracture. A fixed anterior hip contracture then balances with increased lumbar lordosis. One ends up with a lot of stress across intervertebral discs and low back pain.

Our world has evolved into one in which people spend tremendous amounts of time sitting in front of a computer or television screen.2 Sitting turns out to be the worst way to spend our lives, from the standpoint of pelvic tilt problems. In our new cyberworld, jobs regularly require preposterously extended sitting. In the normal sitting posture, we usually lean forward for large amounts of time. Consequently, our trunks drift anteriorly. Of course, the position minimizes energy expenditure. And at the same time, it rocks the pelvis forward and forces the lumbar spine into lordosis, creating the condition for an anterior muscular contracture.

Let’s digress for a moment from the topic of sitting or standing all day and talk in general terms about muscular injury. Usually, muscular “pulls” occur when muscle fibers are stretched beyond their normal working length. All fitness instructors know that a lack of flexibility contributes to and increases the risk of injury. So, in layperson’s terms, the greater the tightness (resting contracture of a muscle unit), the less force (or excursion) it takes to stretch that muscle beyond acceptable length. In short, this is why pelvic tilt abnormalities result in muscle injury.

We often see patients reinjuring the same muscle groups over and over again. Plus, we see athletes with similar injury patterns year in and year out. Many of these recurrences likely come from tilt habits of daily living. Therefore, increasing our understanding of pelvic tilt shall likely yield new insights into correct rehabilitation protocols. One can guess that varying our postures during routine daily activity shall remain a component of the protocols. For now, we should study our job’s daily requirements and come up with basic biomechanical alterations to prevent injuries.

Affecting Impingement

In cam impingement, the misshapen, femoral ball does not fit perfectly into the spherical acetabular socket. This anatomical incompatibility creates shear that damages articular cartilage or labrum. In pincer impingement, the socket, rather than the ball, is the culprit. We should not underestimate the role of pelvic tilt as a cause or aggravator of either process. The exact position derives originally from the overall position of the pelvic bones. In extreme cases, clinical impingement may occur even without discernible ball-or-socket incongruities. For example, the anterior lip of the acetabulum may tip forward and down and abut the femur during hip flexion positions.

The previous observations emphasize the importance of recognizing pelvic tilt or other more regional anatomical imbalances for optimal treatment of clinical FAI. We see many patients with functional impingement and pain and no obviously significant cartilage or labral injury. We also see patients with miniscule or no apparent bony incongruity. Developing appropriate physical therapy regimens for these patients remains another matter in need of more thought and research. Some of these patients improve using only present-day knowledge about correcting pelvic tilt issues.

Aversions to Version

Version

Remember that we stated that we will discuss 3 orientations of the pelvis? That means we still have 2 left to cover. Think back to the planting pot holding a tree, and recall that version refers to the orientation of the 2 posts that hold up the pot. Version, like tilt, has a big impact on how well the ball and socket fit together. Think about version like the way a submarine periscope spins around. The femur even looks kind of like a periscope (Figure 28A-8). There are 2 types of version: femoral and acetabular.

Femoral Version

The orientation of the femur in the hip joint is femoral version. If the ball of the femur pushes toward the front of the hip joint, this is femoral anteversion. Folks with a lot of femoral anteversion walk around pigeon-toed. Subconsciously, they rotate their legs inward to keep their balls within their sockets. Femoral retroversion is just the opposite. The ball tucks deep into the back of the socket and makes the grinding of hip impingement more dramatic (Figure 28A-9).