Knee injuries are common injuries in most sports, commonly seen in football, soccer, basketball, and baseball. We have all heard of one of our favorite athletes sustaining a “knee injury,” however these injuries can range anywhere from meniscal tears and cartilage damage to tendon or ligament injuries. The knee joint is a large but simple joint. Unlike the shoulder and hip joint the knee joint basically functions as a hinge, creating both flexion and extension. The knee does have a rotational component as well. The function of the knee joint is heavily reliant on its strong muscles, tendons and ligaments. Tendons are strong bands of fibrous tissue that connect muscle to bones across joints in order to affect motion. Ligaments are strong fibrous bands that connect bone to bone across joints to help stability. (Figure 1) They prevent abnormal motion in any direction. By maintaining normal joint motion, these tendons and ligaments also protect the cartilage of the knee. This is perhaps one of their most important roles, as cartilage lines the surfaces of the bones within the joint, allowing for smooth, frictionless joint motion. When this joint cartilage is damaged, significant pain results. In fact, the definition of osteoarthritis (“arthritis”) is damage or loss of this joint surface cartilage.

The basic anatomy of the knee joint begins with three bones. The thigh bone (femur) sits on top of the shin bone (tibia), while the two femoral condyles (ends of the femur bone) mimic two balls resting in the grooves on top of the tibia. These grooves (called the tibial plateaus) are very shallow and there is almost no boney constraint to rotation, flexion or extension at the knee. The knee cap (patella) sits in front of the femur, also in its own groove (the trochlea), however even with normal flexion and extension it moves from side to side within this groove. Four major ligaments stabilize the knee. The ligaments on the medial (inside) and lateral (outside) sides of the knee – the medial and lateral collateral ligaments, respectively - insert on both the femur and tibia and prevent the knee joint from opening on the inside (MCL) and opening on the outside (LCL). The ligaments in the middle of the knee also connect the femur to the tibia, but in a cross-like configuration, hence named the anterior cruciate (ACL) and posterior cruciate ligaments (PCL). The ACL prevents rotation of the knee joint and keeps the tibia from sliding forward relative to the femur. The PCL is important in keeping the tibia from sliding behind the femur. There are also the posterior medial and posterior lateral corners of the knee (each made up of multiple structures) that aid in stability.

The kneecap, or patella, is encapsulated by the quadriceps tendon from above and the patellar tendon from below. The patella has the thickest cartilage in the body and is susceptible to significant shear forces during knee flexion and extension. The quadriceps and patellar tendons help keep the patella tracking appropriately, and in turn the patella maximizes efficiency of the quadriceps muscle. Without the patella, the quadriceps force needed for knee extension increases by about 30 percent.

Anterior cruciate ligament (ACL) injuries are becoming more common in athletic sporting events. There are about 150,000 ACL ruptures a year in the United States with around 100,000 surgical reconstructions a year. The most common mechanisms of injury for a torn ACL are rapid changes in direction, sudden deceleration, incorrect landing from a jump, twisting, catching a cleat, or a direct hit to the knee. Women generally have 2 to 8 times higher risk of ACL injury than men. Approximately 70 percent of ACL injuries are the result of a non-contact event. Most patients with an ACL tear can be diagnosed with a good history and physical exam. Generally at the time of injury the patient will feel a “pop,” pain, acute limitation in range of motion (usually a lack of full extension, or inability to straighten the knee) and significant swelling. The patient may or may not be able to bear weight. After the initial injury, the knee should be protected until examined by an orthopaedic surgeon. Most often ACL injuries can be diagnosed by the orthopaedic surgeon on the field or in the office. X-rays are important to evaluate any associated fractures or bony abnormalities. An MRI can confirm the diagnosis and also help evaluate concomitant injuries. After diagnosis is confirmed by MRI, the procedure of choice in active patients is an ACL reconstruction with a graft from the patient (autograft) or occasionally from a cadaver (allograft). Common sources for grafts are the patella tendon or hamstring tendons. Studies have shown that there are no differences in results based on the type of graft used. Prior to proceeding with ACL reconstruction, the patient needs to regain his or her full range of motion and have minimal knee swelling to prevent post-operative complications such as postoperative stiffness. After ACL reconstruction, the patient requires an extensive postoperative rehabilitation program. The patient is generally able to return to his or her sport in about 6-9 months. The return to sport or activity varies between athletes due to any concomitant injuries and progression in rehabilitation. Many studies have shown ACL reconstruction surgery to be very successful. Approximately 90 percent of athletes return to sports at the same level without any symptoms of instability.

The medial collateral ligament (MCL) is the most commonly injured ligament of the knee. It is usually injured from a direct blow to the outside side of the knee as seen in football. Because the ligament is spanning the gap between the femur and tibia on the inside (medial side) of the knee, it becomes stretched with this mechanism of injury. It can also be injured in sports such as skiing when the knee bends inward or twists. Most MCL tears are diagnosed on physical exam by testing the stability of the knee at 30 degrees of flexion. These injuries rarely need an X-ray or MRI for diagnosis. MCL injuries are often called sprains and classified by the amount of ligament fibers that are torn. Grade 1 injuries have only a few fibers torn, while grade 2 have a significant number of fibers torn and the ligament becomes loose, but it’s not a complete tear. Grade 3 injuries are complete tears of the ligament. Treatment most often is non-surgical. Grade 1 injuries usually don’t need bracing, but grade 2 and grade 3 injuries are typically treated with bracing and rehabilitation. Athletes in high-risk sports can wear a prophylactic (preventative) brace to protect the MCL from the forces that cause MCL injury. Wearing a preventative brace, especially for football interior linemen, appears to decrease the incidence of MCL tears.

Posterior cruciate ligament (PCL) injuries are not as common as ACL injuries, and can often be missed or misdiagnosed as a knee “sprain.” PCL injuries can occur with forced hyperextension (the knee bends backwards) or hyperflexion of the knee (the knee bends more than it normally does), or with a direct blow to the knee while it is flexed, which pushes the tibia posteriorly (a “dashboard injury in motor vehicle accidents”). Initial symptoms can be similar to those seen in ACL injuries – with acute pain and swelling, and potential difficulty putting weight on that leg. The most sensitive exam to test for a PCL injury is the posterior drawer test, which is positive if the tibia can be translated posteriorly with the knee in flexion. As with ACL injuries, X-rays can identify boney injuries or abnormalities, and an MRI can confirm the PCL injury. Most isolated PCL injuries are treated non-operatively, with particular focus on strengthening the knee extensors. PCL reconstruction is performed in patients with an unstable knee or in those with a combined PCL/PLC injury.

The lateral collateral ligament (LCL) is rarely an isolated injury. The LCL, spanning the outside (lateral side) of the knee, is generally injured along with another ligament such as the posterior cruciate ligament (PCL). These injuries are graded the same way as MCL injuries. However, the mechanism for these injuries is usually high energy, such as motor vehicle accidents or a direct blow. Also, the patient can sustain nerve damage and in severe injuries there can be arterial (blood vessel) damage as well. It is important to realize that multiple ligament knee injuries can be limb threatening and should be evaluated by an orthopaedic surgeon emergently. The most common nerve injured is the peroneal nerve. Peroneal nerve damage can have a devastating effect on the patient, such as a drop foot, where the patient can’t lift the foot. Diagnosis is made with a careful clinical examination including a complete neurovascular exam. An MRI is helpful for surgical planning and can diagnose any concomitant injuries. Isolated partial LCL injuries may be treated non-operatively in a brace. However, as a majority of these injuries are combined injuries with another ligament such as the PCL, surgery is usually recommended. Acute tears are treated with surgical repair of the ligaments and chronic tears are treated with reconstruction utilizing a ligament graft.

Ruptures of the tendons about the patella can occur, more often in the quadriceps tendon than the patellar tendon. When either the quadriceps or patellar tendon ruptures, the extensor mechanism of the knee will be disrupted, and the patient will be unable to extend/straighten his or her knee. Often a defect or divot can be felt just below the skin at the site of the tendon rupture. The best initial treatment is to immobilize the knee in full extension. With quadriceps and patellar tendon ruptures, primary repair with heavy suture can reapproximate the tendon and restore the knee’s extensor mechanism.

The ligaments and tendons of the knee are integral components of the knee joint. They keep the knee highly functioning, powerful and pain-free.

Quick Reference:

  • Tendon: structure that connects bone to muscle
  • Ligament: structure that connects bone to bone
  • Medial side of the knee: inside of the knee
  • Lateral side of the knee: outside of the knee
  • Flexion: bending the knee
  • Extension: straightening the knee
  • ACL: anterior cruciate ligament
  • PCL: posterior cruciate ligament
  • PLC: posterolateral corner
  • MCL: medial collateral ligament
  • LCL: lateral collateral ligament
  • Valgus: knock-kneed deformity at the knee
  • Varus: bow-legged deformity at the knee


Courtesy of Johns Hopkins Sports Medicine

Andrea Spiker MD
Orthopaedic Resident- Post Graduate Year 4
Johns Hopkins Department of Orthopaedics

Bashir A. Zikria MD, MSc
Assistant Professor
Johns Hopkins Department of Orthopaedics
Team Physician Baltimore Orioles and Johns Hopkins Athletics

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