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simply put- the knee
simply put- the knee
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By Fergus Connolly
Connolly Sports Performance Clinic
From a sporting perspective the knee is a critical joint. It plays a key role in absorbing and transferring powerful forces from the powerful hip muscles to the ground while maintaining balance and stability.
Unlike the hip or even the shoulder, the knee is isolated and unprotected by bony surfaces or armour such as the scapula or pelvis. Therefore, it is much more susceptible to injury. However, with proper training and strengthening the knee joint need not be the source of any injury or concern for athletes.
Nevertheless, as an exposed, unprotected joint, it is especially vulnerable to traumatic contact injury. Much of the stability of the joint is dependent upon the four main knee ligaments and the hamstring and quadricep muscle groups. If the knee is injured, the quadriceps muscle group can atrophy quite quickly. As a weight-bearing joint, the knee can be affected by osteoarthritis more than other joints. Unfortunately for athletes, there seems to be a growing and higher percentage of athletes suffering from OA after they retire, as OA tends to afflict joints subjected to trauma.
In this article I will discuss the knee, its structure, muscles ligaments and explain how it works. This will give us a clear understanding of how we can assess it and why we can rehabilitate the knee in the fastest and strongest way possible.
In future articles, I will describe some of the more common knee pathologies and the key secrets to rehabilitating them.
The knee is described as a “hinge-type joint,” but it also has a slight “twist” to help lock it into full extension. However, even this is a very simplistic view of the joint. It consists of a series of gliding and rotational movements that assist in the gross movement of flexion and extension.
First of all, let’s look at the bony elements that make up the knee. Overall, there are seven different types of tissue making up the knee joint - bones, ligaments, tendons, muscles, bursae, adipose tissue and articular cartilage.
The Bony Structures
The knee is made up of four main bony structures the patella, tibia, fibula and femur. The Femur is the largest of the three and transmits weight and force from the pelvis to the knee joint where it articulates with the Tibia and to a lesser extent the Fibula. The patella is a small bone located anteriorly to this joint.
Patella
The patella, or more commonly the ‘knee cap’, rides within the Rectus femoris tendon above the knee joint which in turn becomes the patellar tendon below the joint. It is the largest sesamoid bone in the body – meaning the largest bone encased in a tendon. It essentially floats in the tendon to the front of the knee. When observed from the front, it is similar in shape to an inverted triangle with the wider base superior to an inferior apex. The outward surface is convex and smooth with the inside divided into lateral and medial articular surfaces which meet creating a slight rise in the middle of the posterior of the patella. This is convenient as it acts as a slight tracking guide for the patella to run along between the condyles of the femur in the trochlear groove. The purpose of the patella is to protect the knee joint but more importantly to act as a pulley and increase the leverage pull of the quadriceps muscle group on the lower limb at their insertion into the tibia.
Femur
The Femur is the longest, heaviest bone in the body and has to withstand large forces. Its proximal end articulates with the hip at the acetabulum to form the ball-and-socket-shaped coxal joint. The lower distal end femur articulates with the tibia to form the tibio-femoral (knee) joint. The femoral shaft is surrounded by the thick muscles of the hamstrings and quadriceps. The tibiofemoral joint is a modified joint is a modified hinge joint, which means it can flex and extend and when flexed, can medially and laterally rotate the knee.
Tibia
The Tibia, or shinbone, bears 5/6th the weight of the body transferred from the femur. The Tibia is shorter in length than the femur with a large proximal end containing curved oval condyles to articulate with the medial and lateral condyles of the femur. However the distal end is much smaller to allow articulation with the fibula and talus bones in the ankle joint. The tibial plateau is the top of the tibia where there is a border surrounding the smoothened area of the condyles and a roughened intercondylar area where at its centre the intercondylar eminence provides attachment for the anterior and posterior cruciate ligaments and menisci. These intercondylar eminences match with the femur above it and the tibial eminences fit neatly into the intercondylar fossa of the femur. The tibial tuberosity is positioned proximally and anteriorly only separated from the skin by a bursa where the anterior condylar surfaces meet providing an attachment for the patellar ligament. Medial and inferiorly to this attachment is the pes anserine insertion for the attachment of the Sartorious, Gracilis and Semi-tendinosous, or ‘gooses foot’.
The Fibula
Lateral and parallel to the tibia separated by a interosseus membrane between, is the more slender fibula. Its role is while transmiting forces, provides an attachment for muscles extensor digitorum longus in front, peroneus longus antero-laterally and soleus behind, but takes no discernible part in the articulation at the knee joint. It’s included though as it is attcaheed ligamentosly at the proximal head to the femur above. Its proximal head articulates with the lateral tibial condyle at a height laterally the same level as the tibial tuberosity or attachement for the patella tendon.
The Knee Muscles
The muscles acting on the knee have a nerve supply from about L3 and L4 and act on the knee mainly through facilitating knee extension. Though in the case of the Rectus Femoris it acts as a hip flexor also.
In terms of muscles, we have four powerful muscles making up the Quadriceps group - Rectus femoris, Vastus lateralis, Vastus intermedialis, Vastus medialis. Each of these attach to the patella which demonstrates the importance of the patella when we consider all knee extension force is directed through it.
The Rectus Femoris is the most anterior muscle in the quadriceps group and the only one of the group that crosses the hip joint, meaning it is also a hip extensor.
While it arises from the ilium or pelvis it meets the other quadriceps muscles at the patella and eventually inserts at the tibial tuberosity.
Vastus Lateralis is the most lateral of the muscle group originating at the proximal end of the femur and like the others meets the patella at the quadriceps tendon.
Vastus Medialis, Vastus Medialis Obliqus or VMO originates at the inner groin area and linea aspera of the femur and attaches to the medial aspect of the rectus femoris tendon. It is the teardrop shaped muscle at the medial side of your inner keen.
The deepest of the group is Vastas Intermedialis which has its origin a little lower than the others, starting about 1/3rd of the way from the top of the femur, but like the others, meets the quadriceps tendon and patella.
The Auxiliary muscles
Popliteus
While not one of the main knee joint muscles, the popliteus muscles is often ignored in knee pain, but has a key role in almost every knee movement. The Popliteus sits deep in the popliteal fossa which is a diamond-shaped depression behind the knee. It is bordered by the Biceps femoris, Semimembranosus, semitendinosus and from below by the heads of the gastrocnemius muscles. Innervated by the tibial nerve it runs from the lateral condyle of the femur to the proximal tibia and is the key muscles used in unlocking the knee joint as it medially rotates the tibia on the femur. It is also a weak knee flexor assisting the hamstrings and gastrocnemius and often is the culprit in limiting knee extension.
The Auxiliary Tissues
Bursa
Many bursae are located around the knee joint in order to reduce friction between the tissues. While not often discussed the bursa of a joint are critical in maintaining joint health. The knee bursa is a sac or cavity found near areas in a joint that are subjected to friction and particularly separating muscle or tendon from bony prominences. In the knee, there are 12 sacs of synovia lined with synovial membrane. Synovial membrane lines almost all the structures of the knee joint. It encloses the cruciate ligaments and covers all non articular structures within the knee capsule.
There are four bursa on the lateral side including one between the lateral collateral ligament and biceps femoris tendon. There are fours further bursae separating the tendons of sartorius, gracilis and semitendinosus from the upper medial aspect of the tibia. The rest of the bursae lie to the back of the knee
The Prepatellar bursa
Sitting on the anterior part of the patella this large bursa is designed to permit the skin to glide freely over the patella especially when the knee is flexed and can be easily inflamed by excessive kneeling leading to a condition known as “Housemaid’s knee.
The Infrapatellar bursa
Also inflamed through excessive kneeling this smaller bursa is in two parts superficially between the distal part of the tibial tuberosity and skin and deeper between the tibia and patellar ligament.
The Suprapatellar bursa
This large suprapatellar bursa resting between the femur and quadriceps femoris tendon is an extension of the joint capsule and permits a smooth riding of the tendon over the femur.
The Pes Anserine bursa
This bursa sits laterally and superiorly to the attachments of the the tendons of sartorius, gracilis and semitendinosus protecing it from excessive friction.
Cartilage
Articular cartilage or menisci is the shock absorber connective tissue of the knee joint. Attached to the tibia it allows a smooth articulation between bones and in the knee this is particularly important since the femur and tibia are not congruent themselves. The knee contains two semi-lunar shaped menisci located on the tibial plateau which hold the femoral condyles in place. The peripheral areas of the menisci are vascularized with the the inner regions avascular. The menisci and bursa are critical in maintaining the close smooth fit of the knee joint and maintaining knee stability. The meniscus consists of fibrocartilage with thin collagen bundles at their articular parts and synovium covering the outer larger bundles. The medial meniscus is does not move as freely on the tibia as the lateral meniscus and can become caught between the surfaces of the tibia and femur
The articular capsule is thick in some parts and thin in others and tendons and their expansions assist in strengthening the joint capsule in some places. For example the patellar ligament replaces the capsule anteriorly along with the tendon expansions from vastus lateralis and medialis.
Both menisci are held together anteriorly where they are not strongly attached to the tibia by the transverse ligament. The main function of the menisci is to deepen the articulating surfaces of the tibia receiving the femur so that it can facilitate the rolling and sliding of the femur while cushioning compressive forces also.
Ligaments of the Knee
The 14 ligaments of the knee joint connect bone to bone, helping to provide stability and integrity to the joint. There area eight interior and six external ligaments.
The Patellar Ligament
Starting out a tendon from the quadriceps femoris muscle it goes from the apex of the patella attaching to the tibial tuberosity. A strong, flat ligament It is separated by a large infrapatellar fat pad from the synovial membrane and from the tibia by a bursa.
The Cruciate Ligaments
There are two very powerful ligaments in the knee - an Anterior (ACL) and Posterior Cruciate ligament (PCL) whose key role is preventing dislocation of the tibia on the femur, anteriorly and posteriorly. Both these short intracapsular ligamanets lie deep inside the knee originating on the tibia and twisting over on themselves as the insert into the inner sides of the femoral condyles. ACL injury is a very common injury, the stronger PCL is injured much less frequenty.
Both of these ligaments remain taut throughout extension and flexion of the knee, and as both ligaments are small and positioned at the centre of the knee neither ligament can prevent these actions.
Understanding the Knee
Collateral ligaments
The Collateral ligaments are as the name suggests on two ligaments on both sides of the knee. The Medial Collateral Ligament is a broad flat band that on the side of th knee insertion at the medial epicondyle of the femur distal to the adductor tubercle and resists valgus forces and abduction of the knee. Injuries to it though generally include damage to the medial meniscus since they are linked. The Medial Lateral ligament is also strengthened by the tendons of the sartorious, gracilis and semitendinousus muscles.
The stouter Lateral Collateral Ligament originates at the lateral femoral epicondyle and inserts on the lateral fibular head. Like the Medial Ligamanet the lateral ligamanet is supported by the tendon of biceps femoris. Though injured far less than the medial collateral ligament it also has less consequences if so injured since it is not attached to the lateral meniscus.
Coronary ligaments
Coronary ligaments join the menisci to the edges of the of the head tibial plateau If these ligament tear it can cause the meniscus to become detached.
Popliteal ligament
The oblique popliteal ligament extends from the intercondylar fossa of the femur and lateral femoral condyle, to the head of the tibia. It is an expansion of the tendon of semimembranosus, and runs up to the lateral femoral condyle, strengthening the posterior surface of the joint. It fuses with the fibrous capsule and becomes obvious if the tendon of semimembranosus is pulled on. The arcuate popliteal ligament strengthens the lower lateral part of the posterior surface of the joint. It is a y-shaped mass of capsular fibres extending from the lateral condyle of the femur to the styloid process of the head of the fibula.
Tendons
The tendons connect each of the muscles to bony attachments the biggest being the patellar tendon to the tibial tuberosity.
The Knee Joints
The knee joint is the largest joint in the human body but it actually consists of three joints in one. While the main knee joint is the tibiofemoral joint which has a lateral and medial aspect the knee complex consists of a patellofemoral joint and a tibiofibular joint.
Patellofemoral joint
While the patella-femur joint is not a conventional joint there is a movement of the patella along the femur where the artciluating surfaces are the lateral and medial posterior surfaces of the patella articulating with the lateral and meidal condyles of the femur as the knee travels through its full range for flexion. The interior articlaur surfaces of the patella are covered by a protective articular cartilage which is the largest single layering of cartilage in the body up to 4-5mm thick (1/8th of an inch) thick. There is an extensive synovial membrane attached to the joint in a pouch known as the suprapatellar bursa. These are necessary, as even relatively straightforward things like walking up and down stairs can put over 600 pounds of pressure down on top of the patella – imagine what squatting does to it! This is an unusual design of joint but is necessary to allow such a degree of movement yet transmit such force. This movement is vital for ease of movement, but any slight disturbance of this mechanism can cause the articulating surfaces to come into contact with each other, causing many issues and wearing of cartilage.
By relaxing the quadriceps group and removing the tension on the quadriceps tendon we can move or glide the tendon and patella laterally and medially across the knee joint. As the patella is situated in the middle of the quadriceps tendon it is vital that they remain tracked down the middle of the femur to prevent dislocation and as each of the individual quadriceps muscles act separately on the patella we need to ensure they are all firing and pulling separately and equally to maintain its straightness.
The Tibiofemoral joint
Surprisingly this hinge joint is not a close fitting joint and there is quite a gap between the articluar surfaces of the tibia and femur and most of the strength comes from the close fitting muscles, tendons and ligaments of the knee. The articulating surfaces of the Tibia and Femur is divided into lateral and medial condyles. The medial oval articulating surface area of the femoral and tibial condyles is slightly concave but small and the lateral tibial and femoral articular surface is circular but even smaller. Since these surfaces are not congruent the attached menisci increase the depth and concavity of the surfaces and stability of the joint.
The Proximal Tibiofibular Joint
While this is not exactly a knee joint rather a distal fibrous tibiofibular joint.
It involves the articulation of the lateral tibial condyle with the fibular head. The fibular facet on the lateral condyle that articulates is usually circular and almost flat, while facing distally and posterolaterally. Both surfaces are covered with hyaline cartilage and anterior and posterior ligaments stabilize the articulation. These ligaments are not part of the fibrous capsule that attaches to the margins of the facets on the tibia and fibula. Anteriorly and posteriorly the capsule is thicker. The synovial membrane in about 10% of knee joints extends to the proximal tibiofibular joint according to Gray (Gray, p711).
Movements of the knee joint
As a hinge joint flexion and extension are the two main movements but also with slight lateral rotation. Passive knee flexion is about 160 degrees. Active knee flexion varies, dependent primarily on the hamstring contraction, and has a range of 140 degrees with hip flexed and 120 degrees with hip extended. Knee rotation can only be performed with the knee flexed and has with a range 30 degrees of internal rotation and 40 degrees external rotation. Passive internal rotation has a range of 30-35 degrees and external rotation 45-50 degrees.
The “Screw Home Principle” of the Knee
While often described as a hinge joint, the knee joint actually has a rotating action that finally helps pull it into proper full knee extension. The rotation helps the laterala nd medial aspects of the tibiofemoral joint line into place. The nature of the rotation depends on the orientation and placement of the femur and the tibia.
The flexion and extension of the knee joint will incorporate additional movements of the femur or tibia in the last 300 of movement if the tibia bearing weight, or free to move. Unweigthed the femur internally (medially) rotates and glides backwards over the tibia into its place. However, with the foot fixed the movements are reversed as the knee flexes the femur laterally rotates and moves forward over the tibia.
These auxiliary movements are necessary as the two tibio-femoral articulating condyle surfaces are not of equal area and the lateral articular surfaces fall meet first, 300 short of full extension and the last 300 of knee extension must pull the medial condyle into place. The two key muscles in these actions are the popliteus and vastus medialis muscles. The popliteus muscle laterally rotates the femur at the beginning of the move from extension to flexion. It must also be noted that at full extension, the interior tissues such as the anterior cruciate, tibial, fibular collateral ligaments are all taut and rigid.
Optimal Athlete Knee Rehabilitation
One of the areas most misunderstood is the whole area of knee rehabilitation. Knee rehab from many injuries can be hastened by understanding the exact nature of the forces operating on the knee during rehab exercises, particularly the much-maligned squat.
One of the single biggest factors in successful rehab is the level of strengthening before the athlete goes for the operation. By developing strength pre-op the athlete can slow the total atrophy of the knee and greatly speed the strengthening and rehab process.
Role of the Hip and Ankle in ACL Rehab
The Hip ad ankle joint are vital cogs in the ACL rehab wheel also. Poor flexibility particularly dorsiflexion in the ankle can increase the imposed loads on the knee during activity. Also, while a weak knee flexor the Gastrocnemius is still an important muscle to be trained to assist in knee flexion, but it’s ROM must be maintained while strengthening. Underdeveloped or atrophied strength in the hip post knee injury can also prevent the body adjusted to imposed loads and forcing the knee to absorb the weight.
Weak Hamstrings leading to ACL
One of the key areas of ACL rehab is to increase hamstring strength. The ACL is stressed when an anterior shear force is applied to the tibio-femoral joint. The Hamstrings can act as a braking force preventing this anterior draw.
To Leg Press or Squat?
One of the key debates in knee rehabilitation has been the debate between using the Leg Press of the Squat to develop and strengthen the Lower limb and muscles surrounding the knee, particularly the Quadriceps and Hamstring group. There have been studies attempting to quantify knee forces and muscle activity while performing squat and leg press exercises using variations technique.
As a rule the squat demands far more muscle activity than the Leg Press and so is a better choice for muscular strength development. Though it is not just as simple as that. Proprioception, Tibiofemoral (TF) compressive force, Patellofemoral (PF) compressive force, neuromuscular demand and total muscular stabilising requirement are all valid considerations in exercise choice.
Shear Forces on the ACL and PCL
Another vital factor in exercise choice are the actual forces the knee is exposed to during the squat and leg pres. The Squat has been shown to create a greater tensile force on the PCL so those rehabbing PCL injuries should be mindful of this especially during early stages of rehab.
As a squat is performed there are substantial posterior and anterior shear forces acting on the tibiofemoral joint which draw the tibia forwards or backwards across the femur above. The posterior shear force drawing the tibia back puts strain on the PCL whereas anterior shear force draws the tibia forward directing the strain on to the ACL. While there is a posterior shear force on the PCL caused by the quadriceps activity it is not enough to cause undue stress to the PCL and even at that the PCL stress does not begin until 60 degrees or greater. Again the squat is a safe activity for ACL rehab as the forces acting on it are quite small. The ACL is stressed during angles les than 60 degrees when the Quadriceps force is anterior in nature.
Compressive Forces while Squatting
The Squat has also been shown to place far greater compressive stress on the Patello-Femoral joint especially in lower angles and can aggravate athletes who are predisposed to patella disorders. In fact anterior knee pain on squatting is a sign of patella misalignment or perhaps hypertonicity in the quadriceps group.
For those suffering from Patellofemoral injuries the variations in compression force on the Patellafemoral joint vary greatly with two factors – Knee Flexion and Stance width. Knee flexion increases the compressive force aggravating the friction in the Patellofemoral joint greatly after an angle of 45 degrees. The narrower the stance - the less force on the joint.
Does Foot Stance Affect Rehabilitation?
The wider the stance in the Leg Press the greater the hamstring recruitment and this is even in the case of the squat where in some cases in early rehab very deep squats involve such high compression forces that they may be unsuitable for patients suffering with acute knee injuries. Narrow foot placement in the squatting will produce greater gastrocnemius activity as it crosses the knee than a wider stance squat which uses the hamstring group more eliminating the demand on the gastrocnemius.
Squatting for ACL Rehab
Rehabbing the ACL we focus first on increasing the Range of motion along with developing the overall strength of each muscles surrounding the knee joint, both anterior and posteriorly. The aim is to increase the ROM and strength gradually, but steadily challenging all aspects of the knee function and this can be done using many variables. Of course the key advantage of the squat is that it as a closed-chain exercise the ankle, knee and hip joints must all be co-ordinated helping develop the functional application of the muscles in a linked movement. Not only does it train the particular movements, it also trains them to operate together.
One final point - remember that the Squat is not an exercise. Rather, it is a name for a collection of variations of a basic lift. It can be performed in many forms to suit and challenge the body. It can be performed using barbells, dumbbells, held on the shoulders, behind the back above the head, changing variations in knee flexion angle, foot position or foot angle. And that’s not even considering loads or set/rep/tempo variations. Generally normal Tempo rules apply, but it must be noted that the tempo of squatting also affects shear forces as the greater the sped the greater the shear force. Using slower tempos during early rehab will protect the ACL more.
One of the most effective methods of knee developing strength is using timed concentric and eccentric phases as the knee recovers. While we use eccentric training for many athletes recovering from tendon injuries we have found substantial improvements in athletes using timed eccentric phases helping develop smoother muscular control. The main focus here should be on smooth muscular movements with proper neuromuscular control. They should be prescribed carefully and judiciously monitoring the Nervous system loads.
Proper Squat Form
Performing squats with proper form is essential any time, but certainly post ACL surgery. Thing s that should be looked for excessive forward movement of the knees that increase anterior shear forces putting more stress on the ACL. As a client is squatting observe their form and look for any forward lean. While slight lean is not dangerous it points to the client increasing the hamstring demand and therefore increasing the posterior pull on the tibia to support the ACL. One other myth to be dispels is that post ACL surgery athletes cannot lift heavy weights. This is not the case at all. Research studies have shown that while increased weight does have greater force on the Tibio-femoral joint and the articulating surfaces of the tibia and femur, it doesn’t affect or increase the stress on the ACL or PCL once correct form and therefore muscle recruitment is observed. Again widening the stance may increase compressive force but it has no affect nor does it increase stress on the ACL or PCL.
The squat is an excellent movement lift for muscular development, recruiting the muscles of the hamstrings, quadriceps, adductors and Gastrocnemius. The Gastrocnemius is the least involved muscle grouping during the squat assisting in knee flexion in the ascent phase. The hamstring and adductor groups are recruited most during the ascent and in the lowest position, coming out of the lift. Since it is a maximum lift the degree of recruitment and activity is dependent on the load, the heavier the load the more muscle fibres recruited, but the risk increases as the athletes reaches muscular fatigue. Quadriceps activity is highest at the higher lock-out stage of the lift, where the vastus medialis and vastis lateralis are recruited most putting increased stress on the Patello-femoral joint.
