Knee hyperextension is a condition caused when the knee straightens too far, beyond the normal maximum limit of 00 and often with the joint in varus malalignment. In the properly aligned knee, the load is borne on a line running down the center of the hip, knee and ankle, but in a varus deformity (bowleg form), the line is shifted outwards and back.
The most common injuries are posterolateral joint injury and avulsion of the bone attached to the anterior cruciate ligament, and they are especially common in young children due to their softer bones. Posterior cruciate ligament, however, is rare.
Knee hyperextension may occur because of:
- Trauma caused by impact to the front of the knee, which makes the joint move backward, putting high strain on the anterior cruciate ligament – such as when a football player faces a leg tackle.
- Pushing the femur or patella over the tibia, the lower leg bone, for example when one slides to a sudden stop using one leg, which stresses the major ligaments within the knee.
- Genu recurvatum: this is a deformity characterized by knee hyperextension over 5 degrees.
- Nerve damage, such as Brain injury (congenital, stroke-induced or traumatic), or poliomyelitis, may cause hyperextended gait because of muscle (quadriceps or calf muscle) atrophy, spastic plantar flexion of the ankle, or contractures of the heel tendons.
The cause of hyperextension predicts the symptoms, which may be minor pain and swelling when it occurs to a trivial degree, but may consist of sharp pain, strained or torn cruciate ligaments, avulsion of bone chips accompanying such tears, and bone bruising may occur on the anterior part of the knee joint. When the injury is at the posterolateral aspect, the ACL and the PCL typically tears. The rate of strain doesn’t seem to be the major factor as anterior cruciate ligaments tears vary at all rates of strain. Genu recurvatum may present with knee pain, abnormal gait, and a lack of proprioceptive perception, which makes it difficult to tell when the terminal extension of the knee is attained.
The hyperextended knee gait is marked by various degrees of abnormality. In some, the patient can walk only with the aid of a cane or a crutch. In others with minimal weakness, the knee becomes hyperextended only when the patient walks too much or indulges in other heavy work or exercise, leading to muscle fatigue and loss of joint support. The associated ligamentary and muscle weakness and joint integrity also contribute to the final abnormality of gait, as does the presence of arthritis in the knee joint.
Another symptom is knee instability, or a feeling of giving way at the knee, in part or full, during normal activities.
Pain may be felt over the knee or to the medial side, and is caused by compression of the soft tissues by the malaligned knee, as well as on the posterolateral aspect, due to stretching of the soft tissue. In acute hyperextension injury the knee may pop and acute swelling often occurs within a few hours of the injury. The pain can become too severe for normal support of weight during walking or running.
The knee joint is supported on the lateral and posterior aspects by the fibular collateral ligament, and the popliteus muscle with its tendon and ligament. This complex of structures keeps the knee compartment from widening laterally, prevents dislocation of the lateral surface of the tibial component of the knee to the back, keeps the tibia from rotating, and thus prevents both knee hyperextension and genu recurvatum. The bones may show varus alignment, in some cases. A triple varus knee is caused by three factors:
- Tibiofemoral alignment is disturbed.
- The lateral tibiofemoral compartment separation is increased because of weak supporting structures on the posterolateral aspect of the knee.
- The knee appears bowlegged in skeletal outline when fully extended.
In most cases both the posterolateral structures and the anterior cruciate ligament is damaged or at least weak, as following a knee injury or any other cause of muscle atrophy. Patellofemoral arthritis is another cause, but in this case the hyperextended knee gait is due to pain as well as muscle weakness.
The history will offer clues to the diagnosis, such as prior knee injury. A physical examination will help confirm and grade the injury, including joint mobility, visible injury, bruising or swelling, and locking of the joint. Imaging is needed in severe injury and especially if surgery is contemplated. MRI and X-ray imaging are typically performed.
Minor hyperextension of the knee may require only the R.I.C.E approach:
- Rest and avoidance of physical activities that strain the knee in any way for a few weeks.
- Ice application several times a day.
- Compression using a knee brace and crutches for support are helpful in protecting the ligaments of the knee against any further damage.
- Elevation to reduce edema.
Pain relief may be obtained using non-steroidal anti-inflammatory drugs.
Severe cases will require physical therapy, with graded exercises of the quadriceps and other hip and knee muscles. This should be done under supervision so that further injury does not occur, and full joint mobility is attained. Surgery is required to reattach torn ligaments and will again be followed by physical therapy.
Correction of the hyperextended knee gait is crucial if the deformity is to be corrected permanently, otherwise the excessive tensile force on the ligaments inside the joint and increased muscle force could increase the load on the joint capsules, especially the medial and lateral compartments. This can be harmful to joint integrity in varus malalignment.