Cylops also called arthrofibrosis

Cyclops also known as arthrofibrosis is caused by scar tissue in the knee often after anterior cruciate ligament surgery. The symptoms are often described as a pain at the front of the knee that interferes with full range of motion. A cyclops occurs in an average of 5% of all people after anterior cruciate ligament reconstruction. Stretching the knee is particularly painful and sometimes there is an audible click described in English as "clunck". These symptoms usually occur between 8 and 32 weeks of rehabilitation. The exact mechanism of origin is not yet clear. Possibly, the scar tissue is a result of drilling the tunnels for the new anterior cruciate ligament, pinching of the graft between the tibia and femur, irritation due to fibers left over from the original anterior cruciate ligament taking up space in the joint. Either way, there is a mass of scar tissue taking up too much space in the knee and impeding function. The size of this scar tissue can vary between 3×5 mm to 8×12 mm

Further examination by means of MRI

As with other complications of the knee, an MRI can be definitive about the symptoms experienced. When there is a cyclops, a scarring is visible at the front of the knee joint. Specifically, at the attachment of the new anterior cruciate ligament to the lower leg (intercondylar notch at the insertion of the tibia). Studies show that cyclops in the knee increases after anterior cruciate ligament surgery. In 25% of people, cyclops in the knee is present after 6 months. This number increases to 33% 2 years after anterior cruciate ligament surgery. Cyclops or scar tissue is normal after anterior cruciate ligament surgery. It only becomes problematic when the mass is large enough to cause symptoms. 

inversion trauma

Figures and opportunities

Cyclops occurs approximately in 4% of anterior cruciate ligament surgeries. It is characterized by an inability to properly extend the knee. Usually we see this around 2-3 months after surgery. Before that, it is normal not to be able to stretch the knee properly due to the sensitivity of the knee after surgery. Often we see a dormant irritation of the knee joint as a result of cyclops. This is then particularly manifested in a restriction in extension, swelling in the knee, less good control of the quadriceps. When the symptoms do not subside, the surgeon may decide to surgically remove the cyclops. 

Possible symptoms in cyclops

  • Pain at the front of the knee
  • Problems with walking and speed (running)
  • An audible click while stretching the knee
  • Less mobility
  • A hard end feeling when stretching the knee
  • Impaired function of the quadriceps (upper leg muscle)
  • Swelling present in the knee (hydrops)

* this is after the acute phase. For the first 3 months, these symptoms are due to the surgery and not due to possible cyclops in the knee. 


Possible causes cyclops

Some causes and factors, which play into a cyclops:

  • Female gender due to less space in the knee joint
  • Increased volume of graft in relation to the notch 
  • Bony avulsion of anterior cruciate ligament (VKB) 
  • Bony avulsion of anterior cruciate ligament of femur 
  • Anterior placement of tibial tunnel 
  • Double-bundle VKB reconstruction because of larger volume of graft 
  • Bicruciate -maintaining arthroplasty because of VKB injury or sharp tibial bone island 
  • Hamstring Contracture


Around 1 in 10 people after anterior cruciate ligament surgery will experience cyclops in the knee. It is important to intervene on time. This is to prevent too much delay in the rehabilitation. Precious time can be lost when it is not spotted in time. Treatment of cyclops is only necessary for symptomatic lesions. This means that the pain symptoms and limitations are present with scar tissue in the knee large enough to cause problems.

In some cases, the treating physician may opt for an MRI or keyhole surgery to confirm the diagnosis. If necessary, the choice may be made to surgically remove the cyclops. Physiotherapy, in fact, has no effect on the cyclops. The physiotherapist's task is to recognize cyclops and inform the treating orthopedic surgeon. 

Surgical removal of a cyclops often gives good results. The pain decreases, mobility increases and the rehabilitation process can be resumed according to plan. 

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