Knee Instability

Knee Instability

Matthew C. Bessette

Frank Winston Gwathmey Jr

Mark Miller


  • The knee is an encapsulated compound joint consisting of two condyloid joints at the medial and lateral articulations of the tibia and femur and one saddle joint between the patella and femur. Lacking intrinsic bony stability, most of its overall stability is derived from the major ligaments and smaller supporting structures in and around the knee. Disruption of one or more of these ligaments, commonly from injury in athletics or major trauma, results in instability. Injuries to the anterior cruciate ligament (ACL) and medial collateral ligament (MCL) make up the vast majority of ligamentous knee injuries.


Anterior Cruciate Ligament

  • The ACL is one of two major intra-articular ligaments of the knee. Its femoral origin is on the posteromedial aspect of the lateral femoral condyle. It inserts on the tibial plateau, just anterior to the area between the intercondylar eminences.

  • The ACL is compromised of two distinct bundles, each identified by their relative insertion on the tibia. The anteromedial bundle tightens in flexion, while the posterolateral bundle tightens in full extension. The overall tension on the ACL is greatest when the knee is in 30 degrees of flexion (59).

  • The blood supply to the ACL is from the middle geniculate artery, a branch of the popliteal artery. Innervation from branches of the tibial nerve aids proprioception.

  • The primary role of the ACL is to prevent excessive anterior tibial translation in relation to the femur, providing 90% of total anterior translational stability. The anteromedial bundle serves this function, while the posterolateral bundle adds protection against internal rotation (15). The ACL also stabilizes the knee against varus and valgus stress when the knee is in full extension. It contributes to the “screw home” mechanism, by which the femur is internally rotated over the tibia in full extension to “lock” the knee while standing (44).

Posterior Cruciate Ligament

  • The intra-articular posterior cruciate ligament (PCL) originates on the anterolateral aspect of the medial femoral condyle and inserts in a depression approximately 1-1.5 cm distal to the articular surface in a fovea between the posterior aspects of the medial and lateral tibial plateaus.

  • Like the ACL, the PCL is composed of two bundles, a larger anterolateral bundle that tightens in flexion and a posteromedial bundle that tightens in extension. Overall, the ligament bears the most tension in 90 degrees of knee flexion. These bundles are less distinct than those of the ACL and are described by some as a continuum of one ligament.

  • The PCL shares its neurovascular supply with the nearby ACL. Present in 90% of the population, the meniscofemoral ligaments of Humphry and Wrisberg originate on the posterior horn of the lateral meniscus and straddle the PCL anteriorly and posteriorly before inserting on the medial femoral condyle (27).

  • The primary role of the PCL is to resist posterior tibial translation, providing up to 100% of posterior stability at 90 degrees of flexion (3). It also protects against varus and valgus stress in full extension. With the ACL, it contributes to the “screw-home” mechanism (5).

Medial Collateral Ligament

  • The MCL is an extraarticular ligament composed of superficial and deep components. The superficial MCL is 10-12 cm long and originates just proximal and posterior to the medial epicondyle. It courses distally deep to the pes anserinus tendons and inserts on the medial tibial metaphysis approximately 5 cm distal to the joint line. The shorter deep MCL is deep and adherent to the superficial MCL. It is connected to the medial meniscus by its meniscofemoral and meniscotibial components.

  • Once thought to be a posterior portion of the superficial MCL, the posterior oblique ligament (POL) is a distinct, fanlike ligament spanning from a femoral origin posterior and superior to that of the superficial MCL distally to the joint capsule and distal semimembranosus tendon (33).

  • The superficial MCL is the primary restraint to valgus instability of the knee and aids in rotational stability. The POL acts synergistically with the MCL to stabilize the knee against
    internal tibial rotation and valgus deformity with the knee in extension (26).

  • The medial structures of the knee are described in three layers.

    • Sartorius and fascia

    • Superficial MCL, POL, semimembranosus

    • Deep MCL, capsule

Posterolateral Corner and Lateral Collateral Ligament

  • Multiple static and dynamic stabilizers form a complex, interrelated network to secure the lateral knee against varus deformity, external tibial rotation, and posterior tibial translation. Collectively, these are known as the posterolateral corner (PLC). The lateral collateral ligament (LCL), popliteus muscle, and popliteofibular ligaments are the most important stabilizing structures of the region (32).

  • The LCL originates just proximal and posterior to the lateral epicondyle and travels 7 cm distally to insert on the lateral portion of the fibular head. It is the primary restraint to varus instability of the knee and also aids in rotational stability in the extended knee.

  • From its origin on the posteromedial proximal tibia, the popliteus muscle arcs laterally and proximally. Its tendon becomes intra-articular, coursing through the popliteal hiatus before inserting on the femur deep, distal, and anterior to the LCL. It has interconnections to the fibula, tibia, and meniscus that form the popliteus complex, giving it both static and dynamic stabilizing properties.

  • The popliteofibular (arcuate) ligament is a “Y” shaped structure connecting the popliteus to the fibular head. The popliteus and popliteofibular ligament provide the primary restraint against external tibial rotation (34).

  • Other structures involved with the PLC include the fabellofibular ligament, iliotibial band, biceps femoris tendon, lateral head of the gastrocnemius, and the lateral joint capsule. The PLC works in concert with the PCL to prevent excessive posterior tibial translation, contributing more in extension (54).

  • The lateral structures of the knee can be considered in three layers.

    • Fascia, iliotibial band, and biceps femoris tendon

    • Patellar retinaculum and patellofemoral ligament

    • Popliteofibular ligament, popliteus, fabellofibular ligament, LCL, and capsule


Mechanism of Injury

  • It is estimated that more than 100,000 ACL injuries occur in the United States each year, many of them during athletic activity. Seventy percent of injuries to the ACL occur by noncontact mechanisms (11). Tension is created primarily by forces in the sagittal plane that cause large anterior tibial sheer forces. A combination of small knee flexion angles, large posterior ground reaction forces, and substantial quadriceps contractions all contribute to anterior sheer stress. This is augmented but not duplicated by rotational and valgus forces acting upon the knee. The posterior sheer forces on the tibia exerted by the hamstrings are protective from ACL injury.

  • Common mechanisms of injury include pivoting during acceleration or deceleration and forcefully landing on the heel with a small knee flexion angle (1). Contact-induced injury often involves valgus loading of the fixed and straightened knee (11).

  • While males and females sustain an approximately equal number of total ACL injuries, women who participate in athletics are three to eight times more likely than their male counterparts to sustain injury (23). Reasons for this discrepancy include smaller ligaments relative to body size, increased joint laxity, higher levels of estrogen, and smaller intercondylar notch dimensions. Females have also been shown to have different neuromuscular mechanics that predispose them to ACL injury. They tend to land from jumps with less knee flexion, have more valgus loading of the knee, and have greater activation of their quadriceps relative to their hamstrings.

  • Other risk factors for ACL injury include game situations in comparison to practice, an increased coefficient of friction of footwear, and prior ACL injury with or without reconstruction of the ipsilateral or contralateral knee (13).


  • A thorough history and physical exam is the most important aspect of the evaluation of a possible ACL injury. Patients often report pain and instability occurring after landing or cutting in a noncontact situation. An audible “pop” is common. Swelling within the joint occurs rapidly, and athletes are typically unable to return to play. It is not uncommon to see ACL, MCL, and lateral meniscal injury from the same event (5).

  • The Lachman test is the most important physical exam for ACL injuries, having a high sensitivity and specificity (85% and 94%, respectively). With the patient supine and the quadriceps relaxed, the knee is placed in 30 degrees of flexion. The patient’s thigh is stabilized, and the examiner grasps the patient’s proximal tibia and applies anterior force, noting the amount of anterior translation and the endpoint. Anterior translation of greater than 3 mm compared to the contralateral knee and a “soft” endpoint signifies ACL injury.

  • The pivot shift test is an important indicator of rotational instability. With the leg fully extended, the ankle is internally rotated while a valgus force is applied to the proximal tibia. In the ACL-deficient knee, this causes anterior subluxation of the lateral tibial plateau beneath the lateral
    femoral condyle. As the knee is slowly flexed past 30 degrees, the iliotibial band induces a sudden reduction of the subluxed lateral tibia, interpreted as a positive pivot shift. Although the test is not very sensitive (24%), it is extremely specific (98%). It is much more sensitive (74%) and comfortable for the patient when preformed under anesthesia (7).

  • Although not as sensitive or specific as the Lachman test, the anterior drawer test is useful in the evaluation of chronic ACL injury. With a supine patient’s knee in 90 degrees of flexion and foot resting on the table, anterior force is applied to the tibia. The difference in anterior translation between the two knees is evaluated (7). All tests have been shown to be highly examiner dependent and are most effectively preformed by trained orthopedists (23). Mechanical devices such as the KT-1000 or KT-2000 arthrometers (MEDmetric, San Diego, CA) can evaluate the anterior translation of the tibia in a more standardized fashion but are typically used for research purposes only (44).

  • Although radiographs typically do not diagnose ligamentous injury, they are helpful to evaluate for other injuries. Avulsion of the tibial spine and Segond fractures (lateral tibial capsular avulsion fractures) can be seen with ACL injuries, the latter of which is pathognomonic. A magnetic resonance imaging (MRI) is usually not necessary to make the diagnosis of an ACL rupture, but it is useful to visualize the ligament and to evaluate for other pathology (5).

Natural History

  • Injury to the ACL rarely occurs in isolation. Characteristic bone bruise on the central lateral femoral condyle and posterior lateral tibial plateau can be found in nearly all patients on MRI (24), although the clinical significance is unknown at this point (20). Approximately half will have meniscal tears, commonly located in the posterior horn of the lateral meniscus in acute injury (6). Chronic instability stresses the posterior horn of the medial meniscus, resulting in tears in this region. When meniscal repair is performed concomitantly with ACL reconstruction, there is a greater chance of healing of the meniscus in comparison to delayed treatment. Meniscal repairs in the chronically unstable knee typically have poor outcomes (6).

  • There is a 50% incidence of clinical and radiographic evidence of osteoarthritis in patients between 10 and 20 years after ACL injury regardless of their treatment. This increased risk of osteoarthritis is especially evident in those who sustain injury to the menisci (39).


  • Indications for surgical reconstruction include athletes, patients with associated repairable meniscal injury, patients with complete tears to any of the three other major knee ligaments, and patients experiencing instability that interferes with activities of daily living (22).

  • Conservative (nonoperative) treatment focuses on regaining normal knee range of motion and strengthening the secondary stabilizers of the knee, in particular the hamstrings. Rotation and lateral movement should be avoided for 6-12 weeks, and competitive situations should be avoided for at least 3 months (20). Bracing may be used for patient comfort but is not necessary. It is important to counsel the patient about episodes of instability and behavioral modifications to protect the joint (30). These patients are typically self-reported to be satisfied with their condition, but suffer from instability issues and a reduction in activity levels (47).

  • If surgical treatment is elected, it should be performed after acute hemarthrosis and accompanying synovitis have resolved and knee range of motion has returned to normal, usually 3-4 weeks after injury. Early reconstruction may result in arthrofibrosis, whereas late repair may be associated with additional injury to the joint (30). Restoring range of motion, weight bearing, and closed-chain exercises are key aspects of both preoperative and postoperative management.

  • Primary repair does not result in adequate healing due to the intra-articular environment, and thus, reconstruction is necessary. This involves drilling tunnels through the tibia and femur and placing a graft through the tunnels to serve as scaffolding for new ligament growth. Surgeons commonly use the central third of the patient’s patellar tendon with bone plugs from the patella and tibial tubercle (bone-tendon-bone [BTB]), the patient’s own semitendinosus and gracilis tendons (hamstring), or cadaveric (allograft) grafts.

  • BTB autografts incorporate into the bone tunnels more rapidly and have been shown by some to have lower failure rates than hamstring grafts, making them ideal for more active, younger patients. They are associated with increased radiographic osteoarthritis changes as well as anterior knee pain (29).

    Only gold members can continue reading. Log In or Register to continue

May 22, 2016 | Posted by in SPORT MEDICINE | Comments Off on Knee Instability
Premium Wordpress Themes by UFO Themes