Animal model of acute gout reproduces the inflammatory and ultrasonographic joint changes of human gout

Abstract

Introduction: Gout is an inflammatory condition induced by the deposition of monosodium urate (MSU) crystals in the joints and soft tissues that can produce acute or chronic arthritis. Several animal models of crystal-induced inflammation have been proposed that involve direct injection of MSU-crystals into different anatomical structures; however, only a few of these models reflect a true diarthrodial joint microenvironment in which an acute gouty attack takes place. The aim of this study was to assess the inflammatory and structural joint changes in a rabbit model of acute gout attack by ultrasound (US), synovial fluid (SF) and histopathological analyses.

Methods: Under US guidance, 42 rabbit knees were randomly injected with a suspension of 50 mg/ml of either MSU or allopurinol synthetic crystals. The control group received intra-articular vehicle of phosphate-buffered saline (PBS). US evaluation, SF and histopathological analyses were performed at days 1, 3, and 7.

Results: A total of 21 rabbit knees were assigned to the control group, 12 to the MSU-crystals group, and 9 to the allopurinol crystals group. By US, the MSU crystals group displayed the double contour sign and bright stippled aggregates in 67% and 75% of joints, respectively. Neither control knees nor allopurinol crystals group displayed these US signs. Power Doppler (PD) signal was moderate to intense in the MSU-crystals group and greater than both the allopurinol crystal and control groups at day 1 (P<0.001) and 3 (P<0.05), with its practical disappearance by day 7. SF leukocyte count was 40,312±6,369 cells/mm3 in the MSU-crystals group, higher than in controls (P=0.004) and allopurinol crystal group (P=0.006). At day 7, SF leukocyte count decreased in both MSU and allopurinol crystal groups reaching the non-inflammatory range. Histologically, at day 3 intense synovial polymorphonuclear cells infiltration and MSU aggregates were identified.

Conclusion: The rabbit model of MSU crystal-induced acute arthritis efficiently reproduces the inflammatory, US, SF and histopathological changes of the human acute gouty attack.

Figure 1

Schematic diagram of the study protocol. MSU, monosodium urate.

Figure 2

Longitudinal ultrasound images of the rabbit knee joints at day 1. (A) Control group showing no abnormalities. (B, C, D) Representative images of the monosodium urate (MSU)-crystal group: (B) double contour sign (DCS) (arrow); (C) bright stippled aggregates (BSA) (arrows); (D) intra-articular power-Doppler signal.

Figure 3

Semiquantitative power-Doppler (PD) score for rabbit knees injected with monosodium urate (MSU), allopurinol and control groups. At days 1 and 3 an increase in PD signal intensity in the MSU crystal group was found when compared to the control group (**P <0.01 and *P <0.05, respectively). Additionally, a significant increase of PD signal was present when comparing the MSU to the allopurinol crystal groups (^# P <0.05).

Figure 4

Synovial fluid leukocyte cell count from rabbit knees in the monosodium urate (MSU), allopurinol and control groups. A significant increased cell count was present at days 1 and 3 in the MSU-crystal group when compared to PBS (**P <0.01) and allopurinol crystal groups (^## P <0.01).

Figure 5

Standard light and compensated polarized light microscopic analysis of synovial fluid (SF) from the monosodium urate (MSU) and allopurinol groups. Observe the presence of intra- (arrows) and extra-cellular negatively birefringent needle-shaped crystals in the MSU-crystal group at day 1. MSU crystals phagocytized by polymorphonuclear cells were seen at days 1 and 3. Absence of MSU and allopurinol crystals was observed at day 7.

Figure 6

Histopathological analysis of rabbit knee joint injected with monosodium urate (MSU), allopurinol and control groups. Hypertrophy, inflammatory cell infiltration and MSU crystal aggregates (arrows) were present in the synovium starting from day 1. No cartilage abnormalities were observed at any time points.