Pilot comparison of outcome measures across chemical and surgical experimental models of chronic osteoarthritis in the rat (Rattus norvegicus)

Abstract

Relatively little work has evaluated both the disease of osteoarthritis (OA) and clinically-relevant pain outcome measures across different OA models in rats. The objective of this study was to compare sensitivity, pain, and histological disease severity across chemical and surgical models of OA in the rat. Stifle OA was induced in Sprague-Dawley rats via intraarticular injection of monoiodoacetate (MIA) or surgical transection of anterior cruciate ligament and/or destabilization of medial meniscus (ACL+DMM or DMM alone). Reflexive (e.g., mechanical and thermal stimuli) measures of sensitivity and non-reflexive assays (e.g., lameness, static hindlimb weight-bearing asymmetry, dynamic gait analysis) of pain were measured over time. Joint degeneration was assessed histologically. Six-weeks post OA-induction, the ACL+DMM animals had significantly greater visually observed lameness than MIA animals; however, both ACL+DMM and MIA animals showed equal pain as measured by limb use during ambulation and standing. The MIA animals showed increased thermal, but not mechanical, sensitivity compared to ACL+DMM animals. Joint degeneration was significantly more severe in the MIA model at 6 weeks. Our pilot data suggest both the ACL+DMM and MIA models are equal in terms of clinically relevant pain behaviors, but the MIA model is associated with more severe histological changes over time potentially making it more suitable for screening disease modifying agents. Future work should further characterize each model in terms of complex pain behaviors and biochemical, molecular, and imaging analysis of the sensory system and joint tissues, which will allow for more informed decisions associated with model selection and investigative outcomes.

Fig 1. Comparison of lameness scores across OA models.

(a) Overall lameness scores. The mean lameness score at each time point is represented by a single data point, with the bar plot showing the mean score across 8 time points for each group, and these were averaged to calculate an overall lameness score for each group. *p<0.05, **p<0.01, ***p<0.001 shows comparisons between individual experimental groups. (b) Lameness scores at each of the 8 time points post-OA induction (x-axis). Groups represented as: ACL+DMM (n = 6), blue (circle); MIA (n = 22), red (square); Saline control (n = 14), black (triangle), with symbol representing group mean at each time point. Data are expressed as the mean ± SEM. *p<0.05, **p<0.01, ***p<0.001 compared to baseline; ^p<0.05, ^^p<0.01, ^^^p<0.001 compared to saline; ^ϕp<0.05, ^ϕϕp<0.01, ^ϕϕϕp<0.001 for ACL+DMM vs MIA.

Fig 2. Changes on dynamic and static limb use of ipsilateral hindlimb following the induction of OA.

Bar plots show the mean score across 8 time points for each group, which were averaged to calculate an overall (a-e) dynamic gait and (f) static weight distribution on ipsilateral hindlimb following OA-induction shown as overall change for (a) stance phase, (b) print length, (c) print area, (d) swing phase, (e) time to max paw-floor contact, and (f) weight distribution. Groups represented as: ACL+DMM (n = 6), blue (circle); MIA (n = 22), red (square); Saline control (n = 14), black (triangle), with each symbol representing group mean at each time point. Data are expressed as the mean ± SEM. *p<0.05, **p<0.01, ***p<0.001 shows comparisons between individual experimental groups.

Fig 3. Histological scoring of cartilage damage 6 or 14 weeks following OA induction.

Graphical representation of histological grading of ipsilateral medial tibial plateaus at 6 weeks post-OA induction for ACL+DMM, MIA, saline control, and intact control rats and 14 weeks post-OA induction for DMM rats: (a) Pritzker OARSI scores, (b) Modified Mankin score; and (c) Gerwin OARSI scores. Groups represented as: ACL+DMM (n = 5), blue (circle); MIA (n = 21), red (square); Saline control (n = 14), black (triangle); Intact control (n = 6), magenta (octagon); DMM (n = 8), grey (diamond), with each symbol representing an individual animal. Data are expressed as the mean ± SEM. *p<0.05, **p<0.01, ***p<0.001 for group comparisons.

Fig 4. Safranin-O staining of cartilage from medial tibia.

Representative histologic images of the femorotibial joint, focusing on the medial side of the tibial surface. (a) saline controls 6 weeks after intraarticular injection, (b) MIA model 6 weeks after intraarticular injection, (c) intact control 6 weeks after anesthesia, (d) ACL+DMM model 6 weeks post-surgical induction, (e) intact control 14 weeks after anesthesia, and (f) DMM model 14 weeks post-surgical procedure. For panel (b) the loss of Safranin-O staining is observed up until the tidemark. Ulceration of articular cartilage (arrow) and extensive bone remodeling (bracket) are observed.