Intra-articular pressure profile of the knee joint in a spectrum of inflammatory arthropathies.

Objectives: The intra-articular pressure (IAP) rises significantly after isometric quadriceps contraction in patients with rheumatoid synovitis, a process that may temporarily impede synovial blood flow and cause oxidative injury. In acute traumatic knee effusions (ATE) pressure rises are trivial. This study compared the IAP profiles of patients with ATE with three different populations--an acute synovitis on the background of a chronic inflammatory arthropathy, a chronic low grade inflammatory arthropathy, and an acute intermittent inflammatory arthropathy. The study objective was to discover if the pressure profiles observed in these groups reflect an influence of the inflammatory process or time or both.

Methods: Thirty three patients were studied. These were divided into four subgroups; group 1: five acute traumatic knee effusions (ATE); group 2: acute effusions on the background of a chronic inflammatory arthropathy: seven rheumatoid arthritis (RA), five psoriatic arthritis (PsA); group 3: seven osteoarthritis (OA) and group 4: acute effusions on the background of an intermittent inflammatory arthropathy: seven pyrophosphate arthropathy (PA), one amyloid (AA), one Behcet's (B). IAP was measured (mm Hg) at rest and during isometric quadriceps contraction using the hand held portable 295-1 intra-compartmental pressure monitor system (Stryker UK). The volume of synovial fluid aspirated was recorded.

Results: Expressed as medians (inter-quartile range). Resting IAP was; ATE 6 (2-12), RA 8 (5-47), PsA 18 (11-31), OA 17 (7-21), PA 25 (9-29), AA 14, and B 12. IAP increased in all subjects during isometric contraction; ATE 9 (7-16), RA 56 (33-150), PsA 52 (43-85), OA 56 (20-116), PA 53 (41-65), AA 47, B 57 and the IAP rise was significant (p < 0.05) in all except the ATE group (p > 0.05). The volume of synovial fluid aspirated in groups 2, 3, and 4 correlated significantly with the magnitude of the IAP change (r = 0.45, p < 0.05).

Conclusion: The IAP rise during isometric quadriceps contraction is a feature of all patients with an inflammatory based effusion irrespective of the duration of the effusion. This is not the case in patients with an ATE. In inflammatory synovitis the rise in intra-articular pressure with isometric quadriceps contraction relates to effusion volume. It is concluded that the inflammatory process prevents reflex muscle inhibition, a locally protective mechanism that minimises the potential for intermittent ischaemia/oxidative injury.