Friday, August 10, 2018

Fat Pad Most Painful In The Knee?


I had a patient who came to our clinic recently complaining that his MRI showed that his patella (knee cap) cartilage had "worn out" completely but he didn't have any pain prior to that. He had actually gone to do his MRI under his doctor's insistence for investigating something else.

His  MRI results was like in his words "opening a can of worms" telling him what's wrong with his knees and perhaps that's why he started having pain after that.

After his ranting, I had to explain very thoroughly about the structures in our knees that cause the most pain. The information I gave him was derived from an article published quite a while ago in the American Journal of Sports Medicine but still very relevant today.

The doctors in that study came up with a simple method to document the various sensations felt inside a single subject's knees one week apart. Right knee first, followed by the left a week later. (Note that the subject had no prior knee pain).

They would arthroscopically poke/ palpate (using a specially built spring loaded device) different structures inside the knee while video recording the procedure and record what the subject's response was. Force used was between 0 to 500 grams. All this done without intra articular anesthesia. Ouch! That must really hurt.

The doctors only injected local anesthesia at the portal site (incision). The first author inspected both knees arthroscopically. He asked the patient when he poked at different structures and graded the sensation as follows (0) no sensation; (1) was non painful awareness; (2) slight discomfort; (3) moderate discomfort and (4) severe pain. This was done with with a modifier of either accurate spatial localization (A) or poor spatial localization (B).

Ready for the results? They were exactly the same for both knees. Even though it was done one week apart.

Palpation of the patellar articular cartilage in the three under surfaces (central ridge, medial and lateral facets) resulted in no sensation, or a 0 score, even with a strongest force of 500 grams. Palpation of the odd facets elicited a score of 1B. Asymptomatic grade II or III chondromalacia (wearing out) of the central ridge was identified on both patellas of the subject!

Palpation of the articular cartilage surfaces of the femoral condyles, trochlea, and tibial plateaus at 500 g of force universally produced a sensation of 1B to 2B.

The sensation from the meniscus ranged from 1B on the inner rim of the meniscus to to 3B near the capsular margin.

Sensation from the  cruciate liagaments (Anterior, posterior cruciate ligaments) range from 1-2B in the mid-portion of the ligaments and 3-4B at the insertion sites.

Palpation of the suprapatellar pouch, capsule, and the medial and lateral retinacula produced a score of 3A to 4A (moderate to severe localized pain) at relatively low levels of force (about 100 g).


The most painful structures were the anterior synovium of the knee, the fat pad and the joint capsule - 4A.

The human knee can be very complex, especially our patellofemoral joint (patella and the femur). The three asymnetrical surfaces on the underside of the patella (or knee cap) has to work together with the femur as it accepts, transfers and dissipates loads between the bones.

We know from previous research that various structures in the knee send neurosensory signals (or messages) to the brain. It is theses signals that result in us feeling pain.

Even though my patient's patella cartilage had worn out (just like the subject) there shouldn't be any pain there as articular cartilage doesn't have any nerve supply. No nerve endings means it is unable to detect pain.

Even the ACL and meniscus wasn't really that sensitive to the poking. This observation may provide an explanation for the often poor localization of structural damage that many patients experience with a cruciate ligament or meniscal injury.

Now you know, worn out articular cartilage doesn't cause you pain. The pain you have is likely to come from other structures.



Reference

SF Dye, GL Vaupel and CC Dye (1998). Conscious Neurosensory Mapping Of The Internal Structures Of The Human Knee Without Intraarticular Anesthesia. AM J Sp Med. 26(6): 773-777. DOI: 10.1177/03635465980260060601.

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