As refreshing as this change may be to those of us who work with athletic injuries, the onset of disabling traumatic injuries is a source of heartache and real grief for the athlete. Resolving this grief inevitably triggers the question, “How and why did this happen to me?”. Although a seasoned athlete resolves this stage quickly and gets busy with their rehabilitation a.s.a.p., the question itself becomes important outside of the grieving process. Exploring this question objectively can prevent their injury from reoccurring and prevent it in the future athletes they will be coaching. This is the perfect forum to do just that
The laundry list of standard traumatic injuries suffered in these sports is quite long and which one is the most significant depends on which one we are suffering from at the moment. This piece will focus on one of the most dramatic and feared by field and court athletes, the ACL rupture and Unhappy Triad. The topic was chosen because a number of my favorite athletes have already gone down to anterior cruciate ligament (ACL) type injuries just preparing for their season ahead – two of them seniors this year. This article is dedicated to them, their spirit, and their resolve to continue to contribute to their teams and come off their injuries even stronger than before. Heal well - my thoughts are with you.
The Triad: ACL, MCL, Medial Menicus
Simply stating that too great a valgus force snaps the medial collateral ligament (MCL), which rips away the medial meniscus, and, being left the sole supporter, pops the ACL is a text book explanation of a trauma and does not develop an understanding of the structures at risk needed to build a cautious and proactive attitude. Taking a look at the forces involved and the intricate devices used to direct those forces is a more effective approach. Let’s start with a very basic look at how the power for our locomotion is generated and how the body transmits the energy from that power to create the forces that produce motion. We can then look at the unique way in which the knee controls that energy and consider how to prevent the mechanisms involved in that control from being damaged.
Power transmission
Humans are bipedal - a unique and efficient means of long distance locomotion, indeed one which current evolutionary theory believes came long before the rest of the unique changes we attribute to our survival. The power for this locomotion is initiated at the hips. The human pelvis is specially adapted for this and the rest of the body has developed around it. To increase efficiency and decrease power losses, a series of levers and power supplies are provided between the hips and the ground. This series allows raw power to be transformed into what we experience as fluid motion. At the hips, relatively large, broad muscles with broad attachment sites work with relatively few but thick ligamentous structures to transmit energy through the thighs, legs, and feet to the ground. Although power and control elements are found throughout this power chain, the closer we are to the beginning in this series (the hips), the more power production is emphasized and the further along the chain we move, the more control becomes a factor.
The hip joint is a single ball in socket joint, capable of circumduction, but lacks fine control. The last set of levers in this power transmission series are the ankle and feet joints, a complex conglomeration of joints which allow us to negotiate uneven surfaces. Much of the power produced in the hips that is transformed into motion is done so only with the aid of muscles in the legs that channels that energy by moving the feet and legs into the necessary position to take advantage of the power produced in the hips and thighs. This means that the feet and legs have to move in unison with, although somewhat differently from, the hip and thighs as we move through the gait cycle. The joint that negotiates this difference is the knee.
Power transmission is often performed most efficiently in nature in waves of energy. Our bodies are no different. Complex waves of electrical, chemical, mechanical and kinetic energy are exchanged to provide motion. This is worth taking a look at because the structures that exchange the force and ground reaction force that provide our movement are designed around this wave like action.
Our locomotion is though our gait. This involves the sum of movement in the frontal, transverse, and sagital plane (see www.becomehealthynow.com/article/anatom/704/ for a quick review of the planes). The hip rises (frontal), rotates (transverse), and move forward (sagital). The two wave like movements (up and down, side to side) then occur while we are going forward.
Graphing this we get a familiar shape, a spiral, or more precisely, a helix. Our axes here then are:
y = frontal ≈ a cosine function
x = transverse ≈ a sine function
z = sagital ≈ some constant (we’ll call it t)
Although an oversimplified model, it illustrates the point that we “spiral” through space and that the mechanical structures within our bodies are designed around this movement. This is quite obvious with one look at the anatomy illustrations in the Soarbody treatment room. The knee is no different. The complexity of its design allows it to negotiate the spiral transmission of energy from the larger moving femur to the smaller fixed foot and tibia. Check out the illustrations below – it’s a transverse view, superior aspect, of the tibial plateau. The spiral in the ACL is obvious, but you’ll notice the orientation of much of the rest of the supportive tissues also have a spiral configuration to them.
Knee Mechanisms and the Triad
There is too much that occurs in gait to describe here, but to summarize, the foot, leg, and thigh oscillate from foot pronation, internal rotation of the tibia, and finally internal rotation of the thigh at heel strike to the opposite attitudes at toe off - sort of a coiling and uncoiling. To accomplish all this, there must be both flexion and rotation at the knee with the larger femur acting on the smaller tibia without slipping off the tibia or grinding down or fracturing the smaller tibial plateau. It’s a very neat trick that involves the triad (and more!) and some very exact timing.
Let’s start with how the big old femur can effectively transmit force to the smaller tibia without destroying the tibia. The meniscus shoulders much of this task. This unique structure is a semicircle of fibrocartilage secured to tibia tibia via the coronary ligament. It has a bowl shape, where its thicker vascular perimeter tapers down to where a very thin avascular part transitions to an opening that exposes the articular surface of the concave tibial condyle to the articular convex surface of the femur. Viewed from the center outward then, the slope of the meniscus increases rapidly as we move outward from the center to present a much thicker, vascular outer edge. This presents an increased surface contact area to the femur, distributing the force applied more evenly across the tibial condyle and thus decreasing wear on the articular surfaces (see figure below). The elastic quality of the fibrocartilage also allows it to distort somewhat, providing an extra cushion as axial and rotational force is applied to the knee. Notice I used the word “distort” above, rather than crush or squashed, when the pressure applied to the meniscus is controlled. The coronary ligament permits the meniscus to have some mobility and a number of other structures (which we will discuss below) are involved with keeping the meniscus in just the right place at the right time to keep from getting squashed and torn. See the illustration below.
Keeping the meniscus in the right place at the right time by itself is a nice trick not only because there is both flexion and rotation in the knee, but because flexion of the knee involves more than simple rolling. If the knee joint were simply a hinge joint that rolled, the bigger femur would roll right off the back of the smaller tibia. To deal with this incongruity, the condyles of the femur have adapted a shape that is not concentric, but rather a shape that creates a change in the radius from a changing axis of motion (an evolute for you geometry fans), allowing the condyles to rotate for only about the first 15-20 degrees after which it transitions into a sliding motion to complete the flexion. See illustration below.
Recalling that out knees are part of a conduit that transmits mechanical energy in a spiral fashion as it flexes and extends, it follows that there must be a rotational aspect to its flexion and extension. And so there is. To see this, while sitting (in shorts or buck naked is best), lift one thigh between your forearms and allow the knee to fall into passive flexion. Holding the thigh firmly between your forearms to prevent any influence from the hip rotators, extend the knee. You will see, through observation of the foot, that the tibia rotates medially as you approach full knee extension. This demonstrates rotation in the knee during extension, with the tibia medially rotating on the femur in this open chain configuration. With the foot planted in the closed chain model, it is the femur that rotates on the tibia. This aspect of the rotation in our cycle is what allows the femur to medially rotate and lock into place just in time to take the weight of the body. This is accomplished by a difference in size of the femoral condyles. The medial femoral condyle is actually bigger. This means that the lateral condyle will complete its roll and slide before the medial condyle. The medial continues it roll and slide, wheeling and locking the femur into a slightly medially rotated position when the foot is planted. See illustration below. (Note: because there is much focus on this “automatic locking mechanism”, I find people lose sight of the fact that it is only part of a continuum of rotation. Much of the rest of rotation through the knee into the leg is guided by the ligaments of the knee. This is worth your continued study.)
So how do the menisci keep up with all this rolling, sliding, and rotating without being crushed and torn? As it turns out there is a pretty precise dance that occurs between the muscles that flex and extend the knee and the positioning of the menisci. When we flex the knee, semimembranosus retracts the medial meniscus posteriorly in sync with the action of the medial femoral condyle. Popiteus performs the same trick with lateral meniscus. Then on extension, the meniscopatellar ligament pulls the menisci anteriorly through the action of the quads via the patella. It’s actually a whole lot more complex than this (again because of our spiral influence) with the end result of all the factors being more of rotation of both menisci in the same direction during knee flexion and both in the opposite direction during extension extension, but the idea is the same – it’s a complex dance!
Illustration inspired by Calais-Germain, 195
Just to stay on point here: Why all this sliding and rolling and rotating within the knee? Because it’s working within a spiral. The rolling, sliding, and rotating within the knee will demand some degree of lateral, medial, anterior, and posterior translation and, with that, structures that set limits on these movements will also be needed. Although the muscles that cross the knee play an important role in stabilizing the knee, the tough passive structures that limit rotation, anterior/posterior translation and lateral/medial flexion of the knee are the ACL, PCL, MCL, LCL. In limiting the range of these movements, they define the boundaries at this juncture of a conduit through which the energy can be transmitted to and from the hip. If disrespect these limits (and perhaps rupture one of these boundaries) we can expect a dramatic power loss. Take a look again at the two illustrations below. Can you see intuitively now where there are limits in magnitude to the helix and that passive elements are designed to both aid and set limits on that motion? You should also be able to recognize adopting postures (active or static) that push these passive structures to extremes also compromises, and possibly jeopardizes, the flow of energy between the hips and ground.
So here then are the primary responsibilities of main structures that facilitate the passage of and set limits on the energy that passes through the knee.
Anterior Cruciate Ligament
The main role of the ACL it to prevent the femur from slipping backwards off the tibia. It’s more robust counterpart, the posterior cruciate ligament (PCL) prevents the femur from slipping forward off the tibia. The way the fibers of the ACL are oriented, it’s taut in both extremes of flexion and extension, and offers the least support and the most slack at about 30 degrees of knee flexion. When the tibia is rotated inward (medial rotation) the ACL wraps around (crosses) the PLC to resist medial rotation. The cruciate ligaments pass between the condyles of the femur and are therefore outside of the synovial joint capsule.
The Medial Collateral Ligament
The collateral ligaments run along the inside and outside of the knee to prevent the knee from buckling inward or outward. The medial collateral ligament is taut and provides the most support when the knee is extended and the least support, when it’s lax, during full knee flexion.
The Medial Meniscus
The menisci are asymmetrical cartilaginous discs (sort of like horseshoes in shape, similar but not exactly the same shape) that lie on the tibial condyles within the synovial joint capsules and play an important role in weight distribution as the knee goes through its range of motion. They are thinner toward the middle and thicker toward the outside. The medial meniscus communicates with the medial collateral ligament through the joint capsule.
Onset of injury
These three passive elements (and more!), along with the muscles that surround the knee, support the knee to keep it stable. Stressing any of the ligaments past their tolerance can result in damaging the structure. The ACL in particular is at risk when the knee is at or near full extension or at 90 degrees of flexion and a force is applied from the outside of the knee toward the inside. This force can be applied by foreign body (getting tackled – “clipping”), or can be the result of the athletes stance and own force, which is often the case. Specifically, if an athlete has their tibia externally rotated, with their foot planted, and their femur has started to internally rotate as they apply force to the knee, they create their own valgus force at the knee. See the figure below. This can happen, for example, when a football player attempts to “cut” sharply, a soccer player attempts to reach out too far with their foot, or a volleyball player lands in that same position - that is, attempting deceleration in the position described above. If the force is great enough in this position, the MCL and the medial meniscus can also be ruptured, resulting in the “unhappy triad” of simultaneous injuries as the ACL ruptures as well.
When the force is great enough to rupture all three elements of the triad, the order of collapse and rupture is usually: MCL, meniscus, ACL. The force snaps the MCL, which rips away the meniscus with it, and with no support left the ACL then pops as well.
What about the active elements, the muscles? Can’t they stop the knee from collapsing inward? Unfortunately, no, not once the chain of events that results in the unhappy triad begins. Although conditioning and training can prevent the non-contact ACL/unhappy triad injuries from starting (providing more resilient connective tissue, increased awareness in the athlete, and quicker response times) once a “stance of no return” is adopted and the athlete’s weight is applied, the muscles will not be able to support the knee in time to stop the injury. The average time for the the reflex signal from the ligament to spine and back and to the muscle is 87 msecs and then to fully engage the muscles in the co-activation response needed to make an attempt to protect the ligaments is another 115 msecs. This is way to slow a response considering an MCL rupture can occur 73 msecs after pressure is applied and the spindles may not start their reflex communication until 128 msecs have passed. (Saidoff, p. 695) When the ACL ruptures by itself, the athlete often hears or feels a pop and experiences an involuntary collapse of the knee. Swelling occurs within 2 to 24 hours, but usually won’t occur at onset.
One of the more dangerous stances. Here, weight is applied to one side, near full extension, with tibia laterally rotated, femur is medially rotating. This can result while trying to pivot with foot planted (rather than on the ball of the foot), landing from a jump in this position, etc.
Probability of Injury
There are several factors that increase the probability of incurring a triad injury. Currently, the percentage of noncontact ACL lesions seems to be higher in female than male athletes. A greater Q-angle (see figure below) and a general tendency to have increased laxity of supportive connective tissues (ligaments and joint capsules) are some of the intrinsic factors that are thought to account for much the disparity between the percentage of women verses men that incur ACL and triad injuries. Training and experience can influence many of the extrinsic factors, making the best of increased strength, flexibility, resilience and elasticity of connective tissue, coordination, spacial awareness. The particular sport too, of course, increases the likelihood of these types of injuries.
The Q-angle is the angle formed between lines drawn between the ASIS and the center of the patella and from the center of the patella along the midline of the tibia. The greater the Q-angle, the greater the valgus pressure to the knee. Women in general have an average greater Q-angle because in general they have broader hips.
Prevention of Injury
Training and conditioning are used to decrease the likelihood of the triad injury. Drills that increase agility and improve alignment and sports specific training that focus on proper, safe execution of specific skills along with conditioning that builds the resilience of the connective tissue that support the knee are effective in decreasing the probability of injury.
Summary
The body moves through space by executing a combination of rotational movements that transmit energy through a spiral pattern. The body is designed to facilitate the transmission of this energy, as demonstrated by its spiral architecture. The knee facilitates this transmission at a juncture that negotiates the generation of power in the hips and thighs with the legs and feet that perform the complex adjustments necessary to transfer energy to and from ground potential. The structures within the knee permit a certain amount of flexibility to facilitate this negotiation and thereby set limits for this spiral transmission. If these limits are exceeded, the conduit will lose it’s integrity. Power may be lost immediately and entirely if the breach is severe enough and the conduit will need to be repaired to restore energy flow and power transmission. This is the case with the Unhappy Triad. To prevent such a breach, efforts should be made to strengthen the structures that support the knee and to train the athlete to adopt safe stances and avoid putting the knee into stances that risk violating the limits the connective tissues of the knee.
References Sited:
1)Saidoff, David C. , McDonough, Andrew L. “Critical Pathways in Therapeutic Intervention, Extremities and Spine” Ed. White, Kellie, St Louis, MO, Mosby, Inc., 2002
2) Calais-Germain, Blandine “Anatomy of Movement”
Seattle, WA Eastland Press, Inc 1993
Other texts used for research:
Houglum, Peggy. “Therapeutic Exercise for Musculoskeletal Injuries, 2nd Edition.” Primary and Secondary Healing. Ed. David H. Perrin. Champaign, IL, Human Kinetics, 2005.
Clemente, Carmine D. “Anatomy, A regional Atlas of the Human Body, 5th Edition”
Baltimore, MD, Lippincott Williams & Wilkins, 2007
Hyde, Thomas E., Gengenbach, Marianne S. “Conservative Management of Sports Injuries, 2nd Edition” Sudbury, MA Jones and Bartlett Publishers, 2007
Hammer, Warren I. “Functional Soft Tissue Examination and Treatment by Manual Methods, 3rd Edition” Sudbury, MA Jones and Bartlett Publishers, 2007
Moore, Keith L., Dalley, Arthur F. “Clinically Oriented Anatomy, 4th Edition”
Ed. Kelly, Paul J. Baltimore, MD, Lippincott Williams & Wilkins, 1999
Smith, Laura K., Weiss, Elizabeth L., Lehmkuhl, L. Don “Brunnstrom’s Clinical Kinesiology, 5th Edition” Philadelphia, PA F.A. Davis Company, 1996
Levangie, Pamela K., Norkin, Cynthia C. “Joint Structure and Function, A comprehensive Analysis, 3rd Edition” Philadelphia, PA F.A. Davis Company, 2001
