The Meniscal "Pseudocyst" - A clinical sign of a torn lateral meniscus
Morgan-Jones FRCS(Tr & Orth) Research Fellow
Institute of Musculo-Skeletal Research
We report a prospective study of 636 patients requiring meniscal knee surgery all of whom underwent detailed pre-operative assessment. 58 patients had a clinical diagnosis of a meniscal cyst. Of these, only 30 patients had a meniscal cyst at surgery. The remaining 28 patients all had meniscal tears, 26 of which were in the lateral meniscus.
From the pre-operative assessment the importance of a specific clinical sign indicating a torn lateral meniscus was realised. This clinical sign has been termed a meniscal ‘pseudocyst’.
On finding a lump in or near the joint line of the knee one must consider a number of differential diagnoses. The pathology may be related or unrelated to the menisci. Pathology unrelated to the menisci include loose bodies, bursae, and exostoses around the joint line. These can be diagnosed readily through the history, examination and use of plain radiographs. Joint line swellings related to menisci have previously been attributed to meniscal cysts (1, 2, 3, 4). We have analysed our pre-operative findings from a large series of patients and have identified a new clinical sign. A joint line swelling related to a meniscal tear, commonly of the lateral meniscus, with no meniscal cyst found at surgery. We have termed this swelling a meniscal ‘pseudocyst’.
Over a period of four years 636 patients with knee pain and a variety of symptoms were operated on by the senior author. Detailed standardised sheets and diagrams were used to record the history and results of clinical examination pre and post-operatively. Operative findings were correlated with the pre-operative diagnosis and examination. Review of the patient data revealed a clinical sign occurring in 28 patients not previously reported before, that of a meniscal ‘pseudocyst’.
Examination for a meniscal ‘pseudocyst’.
The clinical sign of a meniscal ‘pseudocyst’ is a lump protruding in the lateral joint line, tender to palpation and varying in prominence according to the degree of flexion of the knee. The ‘pseudocyst’ appears most prominent at 45 degrees flexion and disappears at 90 degrees flexion and full extension (Figure 1).
The clinical triad of pain in the lateral joint line, radiation proximally into the thigh and distally into the lower leg, and exacerbation on ascending stairs were all associated with a torn lateral meniscus.
Of 636 patients reviewed there were 58 cases in which the clinical findings pointed to a diagnosis of a meniscal cyst. All of these patients presented with symptoms of pain localised to a small area on the joint line, usually worse at night and radiating up and down the ipsilateral aspect of the leg. There was a palpable or visible swelling on the joint line on examination of all patients. At operation all had a meniscal tear. A cyst was present in 30 of these cases but not in the remaining 28. The swelling in these 28 cases were termed a ‘pseudocysts’.
Of these 28 patients, 21 (75%) were male and 7 (25%) were female, with an average age of 27 years. Pseudocysts occurred in 26 patients (93%) on the lateral aspect of the joint and were equally common in both knees. All 28 patients gave a history of trauma to the knee. When asked to name their main symptom pain was noted in 21 (75%) cases, instability in 3 (11%), recurrent effusion in 3, and limited motion in 1(4%). 2 of the patients presenting with instability also had a ruptured anterior cruciate ligament. In all cases the pseudocyst was most prominent at 45 degrees of flexion.
At surgery all 28 patients with pseudocysts were found to have tears of the meniscus warranting partial menisectomy (Figure 2). The majority of the tears, 23 patients (82%) were of a cleavage type. The remaining 18% (5 patients) were classic bucket handle tears. Of the original series of 636 patients, 218 were found to have a torn lateral meniscus at surgery and 26 (12%) presented clinically with a pseudocyst.
It has previously been noted that meniscal cysts vary in prominence with varying positions of the knee joint. It has been stated that meniscal cysts are rendered tight in knee extension and prominent in flexion (5, 6). In this series it was found that all the meniscal cysts became most prominent at 45 degrees of knee flexion.
There are a number of possible explanations for the appearance of joint-line pseudocystic swellings associated with torn menisci, particularly in the lateral compartment. The cleavage tears most commonly associated with this sign allow the inner free edges of the meniscus to overlap. The thick attached border at this site thickens further, and hinges outwards thus producing a "kyphotic" deformity of the meniscus palpable in the joint line as a pseudocyst.
It appears evident that the tibiofemoral contact area changes at about 45 degrees of flexion, causing protrusion of the thickened portion of the meniscus, thus increasing the prominence of the joint line swelling around this point. Both pseudocystic swelling and the swelling caused by small meniscal cysts are less palpable at full flexion and extension. The greater mobility of the lateral meniscus and relative deficiency of the middle third of the lateral capsular ligament, in comparison with the corresponding medial structures, appear to account for the almost complete limitation of this sign to the lateral joint line.
Clinical examination of the symptomatic knee should routinely include inspection and palpation with side to side comparison of the medial and lateral joint lines. This should be performed in varying degrees of flexion, particularly 45 degrees short of full extension. The sign of the pseudocyst elicited in this manner is of considerable clinical importance because of the frequency of its occurrence and complete correlation with meniscal pathology requiring surgical intervention.
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