Iatrogenic instability of the lateral meniscus after partial meniscectomy

Demange MK, Von Keudell A, Gomoll AH. The Knee 20 (2013) 360–363.

This is the editor's interpretation of a paper published in the orthopaedic literature in 2013 - our attempt to make relevant medical articles accessible to lay readers. If you wish to read the original it is easy to ask your librarian to obtain a reprint for you from any medical library.

The authors discuss their findings in two puzzling cases who had some time previously had partial lateral meniscectomies, and who returned complaining of their knee intermittently locking in flexion. One of the patients could demonstrate a lump that appeared during the episode at the side of the knee and which disappeared when the knee was able to extend again. The other did not have such a lump, but described a popping sensation.


The first patient

The first patient was 63-year-old man at the time that he presented to the surgeon with painful locking that could be triggered by hyperflexing (bending as much as possible) his left knee. When he did this a palpable mass appeared in the joint line at the side of the knee. A long time before he had had a partial lateral meniscectomy, and he had experienced locking symptoms ever since, but it was becoming more frequent. When the doctor examined him the patient was able to elicit the locking and the lump, which the doctor could feel too. As the patient slowly straightened the leg, and reached about 30 degrees of flexion, the tibia (shin bone) visibly shifted and the doctor could hear a 'clunk' at which point the mass disappeared. MRI scan was largely unhelpful, confirming only the reduced bit of meniscus in the anterior horn where the previous surgery had taken place. There was no mass on MRI.

The surgeon and patient decided to do a more radical meniscectomy, removing most of the meniscus but leaving the posterior horn behind. But instead of the patient's symptoms improving, it was found that, although indeed the lump no longer appeared, the locking had become worse.

A new MRI shed no light on the situation, so the surgeons decided to go back into the knee via an arthroscopy and see if they could reproduce the locking. They found that they could sublux the posterior horn remnant anterior to (in front of) the femoral condyle when they hyperflexed the knee. This reduced spontaneously at about 30 degrees when the knee was straightened again, and they could hear at this point a 'snap' and could feel the lateral tibia moving back into its proper position.

They decided to proceed with completely removing the remaining meniscus, and after this surgery the patient's symptoms totally resolved.


The second patient

The second patient was 43-year-old man who had had a twisting injury at work some time before and who had been found at that time to have a tear in the lateral meniscus - he was treated at that time with a partial lateral meniscectomy. This improved the lateral joint line pain that he had been experiencing with walking, but caused new locking symptoms of the knee with flexion.

The story then becomes rather similar to the first patient because he also underwent another arthroscopy with further trimming of the lateral meniscus so that only the posterior horm was left. Also it was found that this procedure had not helped him - symptoms persisted with a sensation of 'popping' and sometimes the knee got stuck in a bent position. Further MRI showed evidence of the previous surgery but did not throw light onto the cause of the continuing symptoms. He was getting into problems because it was thought that he was putting this on to avoid returning to work.

The surgeon could find nothing significant on examination, and he was unable to reproduce the locking, which only occurred with certain combinations of hyperflexion of the knee and tibial rotation. The surgeons decided - as in the first patient - to do a diagnostic arthroscopy, and again they found that the posterior horn could be subluxed anterior to the femoral condyle, resulting in locking of the knee. Again the patient's symptoms completely resolved after completion of the meniscectomy.



The lateral meniscus is more mobile than the medial meiscus, because it has fewer and flimsier attachments to the capsul and it also has no attachment at all at the point where the popliteus tendon passes alongside the meniscus. In full flexion, the posterior horn of the lateral meniscus actually rotates around the posterior border of the tibia, and movement of up to 1 cm from anterior to posterior (front to back) can be observed normally during most arthroscopies. This makes it difficult for the surgeon to assess the whether the posterior horn is abnormally unstable or not.

Normally a knee surgeon would try to preserve the outer rim of the lateral meniscus, as well as the posterior horn, but the authors point out that in the presence of continuing symptoms - such as clunking, popping or locking in flexion -  the situation demands that a  "high index of suspicion is required since these unstable lateral meniscus posterior horn lesions can appear benign on MRI" when in fact doing a total meniscectomy can resolve the symptoms.

After such a procedure there is  always, of course, the residual risk of osteoarthritis from the lack of the shock-absorbing meniscus, but the authors point out that lateral meniscus transplantation may be considered in some young patients after subtotal meniscectomy, especially in the lateral compartment.

In order to follow the discussion, readers must appreciate that the two menisci differ from one another in their capsular attachment around the outer rim. The lateral meniscus is more mobile as its capsular attachment is looser, but also the lateral side has the 'hiatus' that allows the popliteal tendon to pass up alongside the joint to the femur bone - here there is no capsular attachment at all.