Unstable Knee – Causes and Management

The knee joint is the largest joint of the body. The knee helps in day to day activities such as walking, sitting, climbing stairs, running, etc. The unstable knee is the feeling of the knee giving away. Also known as buckling of the knee, it is usually associated with pain and swelling.


The knee is a large weight-bearing hinge joint formed by the thigh bone, kneecap and shin bone. The various structures forming the joint are:

  • Articular cartilage is a glistening white smooth tissue covering the ends of bones. It is toughened yet flexible enough to cushion the gliding of the bones reducing friction and acting as a shock absorber.
  • The menisci are pads of tissue between the joint dampening the impact. Both the meniscus and cartilage have a very limited blood supply, therefore, limiting any healing if subjected to damage.
  • Ligaments are tough tissues connecting one bone to another. Ligaments prevent slipping one bone over the other, maintaining stability.
  • Synovium is a thin tissue lines the inside of the joint secreting a watery thin, sticky, clear lubrication called synovial fluid. Just like machine oil, it helps in the smooth movement of the joint.
  • The lower end of the thigh bone and upper part of the shin bone along with the kneecap form the joint. The muscles attached to them help to bend or straighten the knee joint. Proper alignment between both is necessary to perform the movements.


The ligaments of the knee along with the meniscus provide stability to the knee. Tears or rupture of the ligaments and menisci are the most common cause of knee buckling.

Anterior Cruciate Ligament (ACL)
The ACL runs from the front and middle of the tibia to the outer and backward side of the femur. The ligament provides stability mainly in the front and back of the knee. The ligament also stabilizes the rotatory movement of the knee.

Posterior Cruciate Ligament (PCL)
The PCL is present behind the knee joint. Along with ACL, it provides stability in the front and back of the knee. Similar to ACL, the PCL also provides rotational stability.

Medial & Lateral Collateral Ligaments
The medial collateral ligament provides stability on the inner side of the knee. Similarly, the lateral collateral ligament provides stability on the outer side of the knee.

Causes of unstable knee

Twisting injuries or sudden forces on the knee in some positions may lead to tears or complete rupture of the ligaments.

The ACL is commonly injured during the sudden twisting of the knee during non-contact sports. The ACL tear is usually associated with a medial collateral ligament and medial meniscus injury. The PCL injury is associated with a violent force acting in front of the knee. The PCL injury may occur isolated or associated with other injuries.

The medial collateral ligament is injured by a force acting on the outer side of the knee. The knee opens up on the inner side with the rupture of the medial collateral ligament. Meniscus injuries may occur with small twisting/turning forces acting while playing field sports or running.

Anterior Instability

Instability in the front may be due to injury of a number of structures such as:

  • ACL
  • Lateral capsular (partial or complete)
  • Medial capsular (partial or complete)

Posterior Instability

Instability behind the knee may be due to injury to :

  • PCL
  • Arcuate complex (partial or complete)
  • Posterior oblique ligament (partial or complete)

Medial Instability

Instability on the inner side of the knee is due to injury of :

  • Medial collateral ligament
  • Medial capsular ligament
  • ACL
  • Posterior oblique ligament
  • Sometimes PCL

Lateral Instability

Instability on the outer side of the knee is due to injury of :

  • Lateral collateral ligament
  • Lateral capsular
  • ACL
  • Biceps tendon
  • Arcuate complex (partial or complete)
  • ITB (Iliotibial band)


The instability is often described as a feeling of the knee giving away or buckling. The instability is more evident in activities that require turning/twisting of the knee such as getting out of a vehicle. The buckling is mostly associated with knee pain and swelling of the knee.


The patient’s knee is thoroughly examined by the orthopedic surgeon. The physical examination involves a detailed history of the events leading to the injury. Special tests are done to look for instability in the various range of motion.

Radiological tests are usually done after clinical examination. An X-ray gives information about any fractures or arthritis of the knee joint. An MRI is able to detect any minor tears/rupture of the ligaments or a torn meniscus. The gold standard test for the diagnosis of knee instability is an arthroscopic examination.


The treatment options depend upon the severity of the injury and the structure damaged.

Most of the medial collateral ligament tears and isolated PCL injuries can be managed conservatively. Conservative management involves knee braces, rest, icing, compression and elevation, and anti-inflammatory drugs. Physical therapy is aimed at strengthening the muscles around the knee.

Surgical treatments for knee instability is mostly carried out with an arthroscope. A tiny instrument about the size of a pencil is inserted in the shoulder joint. The instrument has magnifying lenses and a camera.

The video feed is displayed on a large monitor. The operating surgeon utilizes the live video feed to guide minuscule instruments to repair the ligament injury.

Postop, the patient undergoes rehabilitation in the form of physical therapy and home exercise program to regain maximum function and strength.

My name is Dr. Suhirad Khokhar, and am an orthopaedic surgeon. I completed my MBBS (Bachelor of Medicine & Bachelor of Surgery) at Govt. Medical College, Patiala, India.

I specialize in musculoskeletal disorders and their management, and have personally approved of and written this content.

My profile page has all of my educational information, work experience, and all the pages on this site that I've contributed to.