ACL Injury: Causes and Symptoms

The anterior cruciate ligament (ACL) is responsible for joining the upper leg bone to the lower leg bone. It also helps keep the knee stable.

A tear in one of the knee ligaments is called an ACL injury.

An ACL injury can range from mild (small tears) to severe (complete tears or when part of the bone as well as the ligament is separated from the rest of the bone).


ACL injuries occur when the knee is twisted, bent side to side, or hyperextended (straightened beyond its normal limits).

Common scenarios that can result to ACL injuries include:

  • Drastic direction changes or cutting around obstacles while one foot is planted on the ground. (This is prevalent among sports that put a great demand on the ACL like gymnastics, football, soccer, basketball, and skiing).
  • Landing after a jump while the leg is slightly bent or straight.
  • Jumping from moderate or extreme heights.
  • Falling from a ladder.
  • Missing a step when walking down the stairs.


Telltale indicators of ACL injuries include:

  • Hearing (or feeling) a “pop” in the knee when the injury occurs.
  • Experiencing pain on the back and outside of the knee.
  • Noticeable knee instability (knee gives out, buckles, or feels wobbly).
  • Swelling (especially the first few hours after the injury occurred). Swelling may indicate a bleeding in the joint. Sudden swelling is also a typical sign that the injury may be serious.
  • Movement of the knee becomes limited secondary to pain or swelling.

Risk Factors

The following are some of the factors that can increase one’s risk of ACL injuries:

  • Accidental movements that can twist the knee.
  • Engaging in sports that entail drastic directions changes.
  • Loss of muscle tone (secondary to inactivity or age).
  • Imbalance in the strength of the leg muscle (i.e. quadriceps are stronger than the hamstrings).
  • Previous injuries of the ACL.

Treatment Options

Treatment goals for ACL injuries include:

  • Restore normal (or at least close to normal) knee stability.
  • Minimize loss of knee function.
  • Lessen pain.
  • Restore normal knee function.
  • Prevent further injury or damage to other structures of the knee.
  • Ward off osteoarthritis.

ACL injury treatment will depend on the following:

  • Severity of the rupture or tear (grade I, II, or III)
  • Stability of the knee
  • Age
  • Overall health
  • Willingness and commitment to complete a rigorous (and long) rehabilitation


Medications are given to help:

  • Alleviate pain brought about by a recent ACL injury or a long-term (chronic) ACL deficiency.
  • Ease the pain while patient is rehabilitating.

Over-the-counter pain medications like ibuprofen, naproxen, acetaminophen, and other nonsteroidal anti-inflammatory drugs (NSAIDs) are often prescribed.


Surgical treatment goals include:

  • Restoration of level of function prior to the ACL injury.
  • Restoration of normal (or at least close to normal) knee stability.
  • Lessen loss of knee function.
  • Preventing further degeneration or injury of other knee structures.

Surgery for ACL injuries often entails replacement of the problematic ACL with another tissue. This replacement tissue is called a graft. In most cases, an autograft (tendon tissue from another part of the body) is used.

Avulsion (ACL torn from the lower or upper leg bone) will also require surgical repair.

Most ACL surgeries nowadays are carried out by inserting surgical tools through small incisions. This procedure is called arthroscopic surgery. However, open surgery (making large knee incisions) might be required in other cases.

Majority of individuals who have had ACL surgery report favorable results (i.e. restored knee stability and function, less pain, and ability to do normal routine, among others).

Athletes and other individuals who participate in sports can expect to return to playing sports a few months after the surgery. However, time frame of full recovery would also depend on the intensity of the rehabilitation program.