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.
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.
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 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.
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.