Paralabral cyst of the hip: an indirect MRI finding of a tear of the acetabular labrum. Diagnosis and arthroscopic treatment.

A tear of the labrum of the acetabulum is a common cause of pain in the hip. Although new MRI techniques and radiologists' knowledge of this subject have improved in recent years, the diagnosis of this pathological entity continues to be a challenge for the surgeon. Paralabral cysts in the hip have not been adequately described and there are very few reports in the international literature on their arthroscopic treatment, in contrast to the corresponding cysts of the shoulder and knee. It appears that paralabral cysts may be caused by a tear of the labrum. A mechanism has been proposed whereby the increased pressure of the intra-articular fluid passes through the site of the labral tear, and its accumulation in the acetabulum may in the long term cause the formation of the paralabral cyst. The present paper describes the case of a patient with chronic hip pain and MRI findings of a paralabral cyst and a possible labral tear, who underwent hip arthroscopy during which the labral tear was confirmed and treated.
A 39-year-old patient presented with chronic pain in the left hip of two years' duration, which had not settled after conservative treatment with anti-inflammatory and analgesic drugs, together with physiotherapy. No history of traumatic injury was reported, only sporting activity at a gym and aerobic exercise. On clinical examination a normal range of joint movement was found compared with the opposite side, but the impingement sign was positive. Imaging showed a mild dysplasia of the joint; the joint space was nevertheless satisfactory, the contour of the femoral head was smooth, and there were no osteophytes or subchondral cysts. Further investigation with magnetic resonance imaging showed the presence of a paralabral cyst in the anterosuperior region of the acetabulum. A possible labral tear was also diagnosed, without clear imaging findings being present.
Arthroscopy of the left hip was decided upon, during which an extensive longitudinal tear of the acetabular labrum with fibrous degeneration was found. The articular cartilage of the acetabulum and the femoral head was normal, while the remainder of the labrum was intact. Partial resection and smoothing of the labrum was carried out with arthroscopic diathermy and a shaver. An attempt was made to locate the cyst, but because of its position, and in order to avoid further disruption of the anatomy of the joint by an additional capsulotomy, this was abandoned and the cyst was not directly visualised. The peripheral compartment of the joint was then inspected, where the CAM-type femoroacetabular impingement lesion was seen and removed.
The patient's immediate postoperative course in hospital was uneventful and he was discharged the day after the operation. The patient followed a six-week labral tear rehabilitation programme, during which he used elbow crutches and minimal weight-bearing for four weeks, while being supervised throughout by a physiotherapist. Full return to sporting activities (gym and aerobic exercise) was achieved at three months postoperatively. At the patient's last review one year postoperatively an almost complete resolution of symptoms was seen, with very mild residual pain from the hip.
Labral tears are not clearly shown in every case on plain magnetic resonance imaging without a contrast arthrogram, and especially when the radiologist does not have sufficient experience in diagnosing them and in the use of special slices and sequences. In addition, it is not uncommon for the magnetic resonance scan to be reported as showing a possible labral tear, and in these cases the surgeon must rely on the clinical picture in deciding on arthroscopy. By contrast, the presence of a paralabral cyst of the acetabulum appears to be an indirect finding of a labral tear and, in cases where it is not combined with severe osteoarthritic changes, it makes the decision for arthroscopic treatment easier. Patients with similar MRI findings should therefore be considered potential candidates for hip arthroscopy. In such cases the exploration and location of the cyst appears to be unnecessary, especially where its position is difficult to reach arthroscopically, and treating the labral tear alone, which caused its formation, is sufficient for symptomatic relief of the patient.