The triggering of the arthrofibrosis cascade in the knee after injury or surgery sometimes takes clinicians by surprise.

First published by Dr Noyes in 2008, and reviewed August 2023 by Dr Sheila Strover (Clinical Editor)


As arthrofibrosis can be successfully alleviated if treated in its early stages, the clinical team must carefully watch for this complication relatively soon after the initial trauma or surgery, especially if there is an established risk such as in cruciate ligament surgery or arthroplasty.

How does the surgeon or therapist know that the process of arthrofibrosis has been triggered? How can they determine early that a patient's recovery is not progressing normally?


Normal Expectations after Surgery or Injury

We established in Part II the normal range of knee motion in regard to flexion and extension. In the early acute inflammatory phase following injury or surgery, the knee may be painful, swollen, warm, and unable to achieve full motion.

After approximately 2 to 3 weeks, patients should be able to regain normal flexion and extension or demonstrate a normal progression of motion, depending on the operative procedure which was performed. For instance, after high tibial osteotomy we expect patients to be able to achieve 0-110° by the end of the 2nd postoperative week; 0-130° by the end of the 4th week; and full motion by the end of the 6th week.

In patients in whom arthrofibrosis triggers, this 2-week 'turnaround' or normal progression of motion does not occur. In some cases, the knee may do well for the first few weeks following injury or surgery, but then motion becomes limited.

How can the clinician realize that there is an abnormal scarring reaction occurring in the knee?


High Index of Suspicion

The clinician should be alert to the possibility of this complication in disorders in which a higher than normal risk exists - we discussed these disorders in Part IV and briefly in Part I. In 'high-risk' disorders, the patient needs to be managed closely by the surgeon and rehabilitation unit so that healing can be assessed and serial measurements made of knee flexion and extension, the mobility of the patella and its related tendons,and the strength of the quadriceps muscles.


Subjective Alert Signals

  • Flexed knee gait - A patient who walks with a flexed knee gait should be assessed to determine if the quadriceps are inhibited or weak, or if there is a mechanical block to extension.
  • Worsening pain - Pain is a normal feature of injury and surgery, but usually settles in a predictable manner. When pain does not settle or becomes worse, arthrofibrosis must be excluded. Characteristically, pain may also be triggered by quadriceps contractions and tenderness to palpation around the patella and patellar tendon. NB It is important, when pain is severe, to keep in mind the disorder of complex regional pain syndrome which may need to be excluded.
  • Weak, wasted quadriceps - An inability to voluntarily perform a strong quadriceps contraction in the initial 1-2 weeks following knee trauma or surgery. No tension in the patellar tendon. Patella fails to move upwards when the quadriceps contract.
  • Warm joint - It is normal for the knee to be warm during the acute inflammatory stage after injury or surgey. This usually settles within 2 weeks. If the joint remains warm at this time, or any increase in warmth is suddenly experienced, the possibility of infection exists and the clinician should respond appropriately.
  • Continued tissue swelling - Tissue swelling in arthrofibrosis has a particular characteristic, making it relatively easy for the alert clinician to distinguish this from an effusion (fluid in the joint space) or haemarthrosis (blood in the joint space). The swelling associated with arthrofibrosis is a swelling in the soft tissues around the knee, giving a sensation of tissue bogginess or edema.


Objective alerts on serial measurement

  • Restricted patellar mobility - Restriction in patellar mobility when the examiner tries to move it up and down and side to side. This should be compared to the normal side.
  • Restricted active and passive ROM - The patient makes early gains in ROM, but then has no further improvement. The patient may lack more than 5 degrees of extension, and may have only 120 degrees (or less) of flexion.
  • Worsening ROM - The patient makes good early gains, but then loses some of the knee motion gained, despite aggressive exercise.
  • Downward migration of the patella - In an inflamed joint, there may be a transient patella infera due to fat pad and peri-patellar tissue contracture which is aggravated by quadriceps weakness. The patella may be visibly lower in position compared to the opposite normal knee. With progressing joint stiffness and limited ROM due to arthrofibrosis, the patellar tendon may permanently shorten, with the eventual onset of severe patellofemoral arthrosis (joint destruction).


Our research has shown (ref 1) that the progression of arthrofibrosis can trigger downward migration of the patella as early as a few weeks after ACL reconstruction, and crippling patella infera within 8 months to a year. An effective way to monitor this carefully is by the serial measurement of patellar height on a lateral X-ray of both knees.

Two ratios have become standard in the assessment of patellar position - these are the Insall-Salvati and Blackburne-Peel vertical height ratios.

insall-salvati ratio

Insall-Salvati ratio - length of the patellar tendon (lowest pole of the patella to the tibial tubercle on X-ray) compared to the height of the patella - usually 1.02 +/- 0.2. A ratio of less than 0.8 is considered to be patella infera.

blackburn-peel ratio

Blackburne-Peel ratio - lowest point on the patellar cartilage to the level of the tibial plateau compared to the length of the patellar articular surface - usually 0.54-1.06. A ratio of less than 0.54 is considered to be patella infera.

In 1991, (ref 2) two colleagues and I compared the two knees of 51 individuals and found that, although the ratio may vary slightly from person to person, it is remarkably consistent between the two knees of the same person. This allowed us to diagnose patella infera by comparing the ratio in the problem knee with that in the normal knee on serial lateral X-rays with the knee in 30-60° of flexion.


Key Message

It may be difficult to detect the onset of arthrofibrosis as early as 2 weeks postoperative or post-injury, as the features will be similar to a normal response but by 6 weeks postoperative, the process may already be quite advanced and the complication more difficult to successful resolve. Therefore, the physical therapist has the primary responsibility for picking up the warning signs and putting them into context (depending upon patient variability and established rehabilitation protocols). Unless the therapist knows what these warning indicators are, this complication may not be detected early enough to be resolved. Early referral by the alert therapist back to the surgeon to initiate serial x-ray measurements of patellar height will give the team an objective index to follow.


Caveat - Remember that a limitation of ROM may have causes other than arthrofibrosis

In the investigation of a patient's lack of progress, the careful clinician must exclude other mechanical causes for the joint stiffness -

These conditions may trigger or be present in combination with arthrofibrosis.


1. Noyes FR, Mangine RE, Barber S. Early knee motion after open and arthroscopic anterior cruciate ligament reconstruction. Am J Sport Med. 1987;15:149-60.

2. Noyes FR, Wojtys EM, Marshall MT. The early diagnosis and treatment of developmental patella infera syndrome. Clin Orthop. 1991;265:241-52.

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