Morphological analysis of knee synovial membrane biopsies from a randomized controlled clinical study comparing the effects of sodium hyaluronate (Hyalgan®) and methylprednisolone acetate (Depomedrol®) in osteoarthritis

Objective. The study was part of a randomized open‐label clinical trial designed to evaluate the effects of intra‐articular injections of hyaluronan (Hyalgan®) (HY) in osteoarthritis (OA) of the human knee. Data were compared with those obtained after treatment with methylprednisolone acetate (Depomedrol®) (MP). Methods. Synovial membranes from patients with OA of the knee, primary or secondary to a traumatic event and classified according to the American College of Rheumatology criteria, were examined by arthroscopy and by light and electron microscopy before and 6 months after local injection of HY (2 ml of 500–730 000 MW hyaluronan, 10 mg/ml in saline, one injection per week for 5 weeks) or MP (1 ml of methylprednisolone acetate, 40 mg/ml, one injection per week for 3 weeks). Results. Arthroscopy revealed a significant decrease in inflammatory score after both treatments. Histology showed that HY treatment was effective (P⩽0.05) in reducing the number and aggregation of lining synoviocytes, as well as the number and calibre of the vessels. MP treatment significantly reduced the number of mast cells in primary OA. Both treatments tended to decrease the number of hypertrophic and to increase the number of fibroblast‐like lining cells, to decrease the numbers of macrophages, lymphocytes, mast cells and adipocytes, and to decrease oedema, especially in primary OA, and to increase the number of fibroblasts and the amount of collagen. These phenomena were evident throughout the thickness of the synovial tissue. Conclusion. At least in the medium term, both HY and MP modified a number of structural variables of the synovial membrane of the osteoarthritic human knee towards the appearance of that of normal synovium. The effect was more evident in primary OA than in OA secondary to a traumatic event. This is the first evidence that local hyaluronan injections modify the structural organization of the human knee synovium in OA.