Diagnosing Arthrofibrosis – Differential Diagnosis and Clinical Evaluation

This blog summarises part of the information presented in Sebastiano Nutarelli’s IAA’s webinar “What is Arthrofibrosis and How to Diagnose It”. Sebastiano is an IAA Director, physiotherapist and clinical researcher at the European Knee Arthrofibrosis Centre.  The webinar transcript has been edited for clarity and brevity by Kayley Usher, and contains some additional comments. Watch the full video, including example MRIs and descriptions, at the end of this blog.

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Diagnosing arthrofibrosis is a largely process of differential diagnosis, which means we need to understand when we're dealing with arthrofibrosis or with something else that causes similar symptoms. 

So, we might confuse arthrofibrosis with a loose prosthesis, a bucket handle tear of the meniscus, a cyclops lesion after an anterior cruciate ligament reconstruction, mispositioning of an ACL graft, or complex regional pain syndrome. And then sometimes you also need to evaluate everything in case of fractures.

Bucket Handle Tears in the Meniscus. So, bucket handle tears are traumatic in nature. Something happens, it's a sudden movement usually associated with deep flexion, and the meniscus tears. You can get a tear that is shaped like the handle of a bucket, and the meniscus flap flips out into the joint, so usually you have a mechanical block. This is completely different from arthrofibrosis. You don't have a persistent inflammatory situation. You have a clear history of a trauma and then you have a mechanical issue. So, as a clinician it’s really important not to confuse the two.

Cyclops Lesions in the Reconstructed ACL. In this case, the patient has undergone an ACL reconstruction, otherwise there cannot be a cyclops lesion. Cyclops lesions are the formation of a fibrotic mass on the reconstructed ACL, which are visible on an MRI. You have a sort of ball, which is a rounded formation of scar tissue. We can have some fibrosis elsewhere, but a big lump of fibrosis on the ACL is limiting knee extension. Usually, it's painful and the lump can be quite large, it can even bulge out into the Hoffa’s fat pad. Cyclops lesions can be detectable as soon as a month after an ACL reconstruction. If you're dealing with a knee that is progressing as it should after an ACL reconstruction, but the knee has a painful lack of terminal extension, you should think about contacting the orthopaedic surgeon because it might be necessary to take another MRI to check for a Cyclops lesion. And if this is found to be the case, the treatment is surgical. The right approach here (for a Cyclops lesion) is immediate surgery, the mass is easily removed and because the fibrosis is usually isolated, these patients come back the following day with complete extension. This mass of scar tissue, the Cyclops lesion, sort of resembles a big head of garlic.

Poorly Positioned ACL Graft. Next, we have to consider the possibility of a technical error in terms of the surgical reconstruction of the ACL. Sometimes, the drilling of the tunnel is way too anterior (towards the front), which means in the end it leads to situations like this. When the tunnel in the tibia is positioned too anteriorly, then mechanically the ACL can limit extension. The ACL is the braking system, and in this case, it gets loaded too early. It's a technical error. We need to recognize it and understand that this is not arthrofibrosis. This needs to be surgically revised, we need to repeat the procedure, otherwise this patient won’t recover extension.

Partial Rupture of the ACL. And then we also have to consider the possibility of a partial rupture of the ACL. So, let's say the ACL is the native one – not reconstructed. A native ACL is built with two bundles, the anteromedial and the posterolateral. Sometimes, you have a fascicle or a bundle of the ACL that gets ruptured. This bundle flaps down into the joint and mechanically blocks the joint. The intact part of the ACL is still attached. An MRI helps to diagnose this, but sometimes you need to perform an arthroscopy (surgery) to see that there is a mechanical problem. The surgeon trims out the flap and usually this works just fine. Yes, you're left with a smaller ACL, but that's still functional most of the time. It's not the arthrofibrosis.

Complex Regional Pain Syndrome (CRPS). There are the Budapest criteria to diagnose complex regional pain syndrome. So, most of the time with arthrofibrosis we are not dealing with complex regional pain syndrome. CRPS is not just a stiff knee with a constant inflammatory process, a warm knee. We have other domains that are impacted with CRPS, the sensory domain. So, there is a mechanical allodynia, which means these patients perceive gentle touch to be painful touch. There’s an altered representation of pain, but this can also happen with central sensitization.

A lot of arthrofibrosis patients do feel gentle touch as painful and that's called mechanical allodynia. A good example is a lot of patients can no longer wear long trousers after the onset of arthrofibrosis for months, sometimes for years, because their nerves are over-sensitized. But that does not mean they have CRPS. In CRPS you also have some discoloration, which means the knee is either discoloured or dark coloured. There is often a temperature change. There is oedema (swelling). Things are just different. And aside from just having a stiff knee, the knee tends to be a bit sweaty. You might lose all the hair. So, all these things are linked to the vegetative nervous system are disturbed and they're clinically telling you that there's something else apart from arthrofibrosis. 

But I must tell you, we see a lot of knee complex knees, but rarely CRPS. Quite often patients get diagnosed with an umbrella diagnosis of CRPS and they're told you don't have arthrofibrosis, you have CRPS. Often, no, they don't. So, we need to be careful with that.

Infection. If the patient has had surgery, we want to make sure that there is no infection. A low-grade infection might not be clinically evident, the knee might not be swollen, and the patient doesn’t have fever or a positive blood test. But there might be a low-grade infection in the knee which is feeding the constant skyrocketing level of the inflammation. And this causes the fibrotic process to continue forever. This means we need a blood test, but we also need to tap the knee and check the synovial fluid because sometimes we need to get into the knee to see if there's bacteria.

In the blood test we check certain indicators such as C-reactive protein and the Erythrocyte Sedimentation Rate, white blood cell count, etcetera. But in general, if the patient has had a total knee replacement, it’s really important (to test the synovial fluid) because what we might be dealing with is an infection, and this is a much more serious situation when you have a prosthesis in the knee. When there is an infection, the surgeon can perform an arthroscopic wash-out surgery, which is washing out the infection for 10 minutes or 20 minutes, whatever it takes. You literally wash it out with Liters of saline, and this gets rid of the inflammation along with oral antibiotics. The problem is when you have a total knee replacement this often isn’t successful. You often must remove the implant (perform a revision TKR), otherwise the interface between the metal and the bone isn’t washed out.

Implant Loosening after a TKR. When a total knee replacement evolves into arthrofibrosis we need to rule out implant loosening. When this occurs, the implant does not fuse with the bone properly and there is micro-movement of the prosthesis happening. The patient doesn’t perceive any movement, but there is a constant trigger which is feeding inflammation and fibrosis.

So, when you have a total knee replacement that is still warm after 10 months or longer, we do a bone scan or a scintigraphy. It's the same thing, they’re synonymous. A bone scan can show either infection or implant loosening. If the scan is positive and you suspect implant loosening, it's because you believe there is no infection. There has been no fever, the blood tests are all fine and when you tap the knee (perform needle aspiration of synovial fluid) there are no bacteria. So, you have nothing to indicate that there is an infection happening, even a low grade one. Hence you think maybe there is an implant loosening.

So, I want to have this patient checked with a triphasic (imaging at three time points) scan. It's a kind of scan (with radioactively tagged tracer FDG [2-deoxy-2-[18F]fluoro-D-glucose]) in which they radioactively mark cells with a very low dose. This exam (FDG-PET/CT) is done at the hospital in nuclear medicine. Once injected, this radioactively marked sugar (is taken up by cells with high metabolism), the tracer will concentrate in the body where there is a lot of cellular activity (due to inflammation. Another PET/CT method uses a radioactive diphosphonate tracer that is taken up by bone cells involved in bone remodeling and is also used to look for loosening – however, both FDG and diphosphonate tracers detect inflammation and don’t specifically indicate either loosening or infection).

And this can happen when the cells are trying to heal around the implant because it's not bonded to the bone. There is a loosening. They image the exam immediately after injection, then again after an hour and after 4 hours to see different timing of tracer uptake. And if there's a clear concentration of these radioactively marked cells around the implant, unfortunately you could have a loosening. 

If there's an infection though, or you suspect an infection, the PET/CT scan to call for is the white blood cells scan. Because what we mark now with radioactivity is the white blood cells. They go where the infection is and we want to see if that's around the implant (it’s more specific for infection vs inflammation). Unfortunately, if there is a positive signal it’s likely you have an infection and a revision of the total replacement may be necessary, along with other treatment.

Clinical Evaluation. Also, let's talk about the clinical evaluation because quite often imaging is only telling us a part of the story. If we have patella mobility that's very reduced in all the directions, we have a problem and that's indicative of an onset of stiffening, potentially arthrofibrosis. When palpating the anterior interval, the fullness, the stiffness we feel might lead us to suspect that. And in general, there is a loss of range of motion in terms of flexion or extension, but also tibial femoral rotation. We want to see if this area around the fat pad is still soft or if becomes became thicker and harder, with the sense of fullness. Sometimes it also bulges outward and these patients are completely over this. The majority of knee arthrofibrosis patients tell you, I have something in this area. It is all full and I feel stuck. It's like a tight bend. I feel a lot of compression and pressure in there and it's not a good sign when you feel that instead of being soft. It means it's going through a fibrotic process and with a consequent reduction in mobility and ability to mobilize the area.

Sometimes the patient barely can walk with trousers on because just the movement of the fabric on the kneecap is intolerable. There might be an over representation of pain. There's also sometimes nerve trapping, entrapment of the nerve in the scar tissues (which makes movement extremely painful). So, how do we spot arthrofibrosis on imaging? Well, we have X-rays (and CT), MRI. We have one projection, one X-ray view we cannot miss, which is the bilateral (both knees) lateral (side) view inflection (bend) at 30°. It's the most reliable way in which you can evaluate the Patella Baja, which means lowering of the patella.