45.1 Animal Models

To determine the most appropriate animal to use, a number of key factors need to be determined. The first is the model of cartilage repair to be used. The options include heterotopic, cartilage defect, and arthritic or articular fracture, and are described in greater detail below.

After promising in vitro results, a heterotopic model of chondrogenesis should be considered. This would typically be subcutaneous, intramuscular, or intraperitoneal. The rat xiphoid has also been proposed as a potential nonjoint model for cartilage regeneration strategies. Animals used for these studies include nude mice, syngenic mice, rats, and rabbits. Cells can be implanted directly, injected, or introduced within a diffusion chamber. These studies allow researchers to look at the biological effect of the cells being in vivo without the need to use an articular cartilage defect model.

There are two types of cartilage defect models to be considered, partial or full thickness, which relate to whether or not the subchondral plate is breached. 5, 6 The choice depends on the hypothesis to be tested. Breaching the subchondral bone when not intended will release bone marrow (including clotting factors and stem cells), which can invalidate the technique being investigated.

Generalized arthritis can be induced by a number of methods. The most common of these include mechanical resection of ligaments, including the anterior cruciate or the cranial cruciate ligament (the Pond-Nuki model), or menisci, which is resection of the medial meniscus inducing medial compartment arthritis. Other possible methods include treatment of spontaneous disease, obesity models, hormone deficiency (ovariectomy), chemical damage (direct cartilage degradation and indirect metabolism inhibition), limb immobilization, hypermotility, and scoring grooves in the cartilage.

Rodent models have also been used in a model of articular fracture healing.

45.2 Animal Selection

Animals are usually categorized into small or large models. Small animals include mice, rats, rabbits, and guinea pigs, and large animals include dogs, pigs, minipigs, sheep, goats, and horses. The International Society for Cartilage Repair recommends that while small animal models are useful for initial studies and proof-of-concept work, a large animal study is required for pivotal studies.4

Species selection will be affected by the type of model to be used; heterotopic models typically use small animal models. Cartilage defect and arthritis models can use small and large animal models. Small animals are more appropriate for proof-of-concept work or looking at the effect of drugs, compounds, or alterations in genetic expression. Surgical techniques are better assessed with a large animal model.

The advantage of using small animals such as rodents and rabbits includes ease of use, reduced cost, easier husbandry, and the ability to view the entire joint on one histological slide. Their disadvantages include their relatively smaller joints, thinner cartilage, and problems with postoperative care.

Large animals also have their associated benefits and problems. The use of dogs is socially unacceptable in the United Kingdom due to their status as companion animals. Despite having thicker cartilage, pigs are hard to work with and seldom used as the minipig is easier to work with. Goats have been widely used due to their anatomical shape and cartilage thickness; they are, however, hard to work with and may have been affected with prion disease causing early arthritis leading to early termination of experiments. The sheep is easy to handle and relatively amenable. They have thinner cartilage than some of the other animals but have been successfully used for autologous chondrocyte implantation despite this. Horses have the thickest cartilage and the largest joints, making them the best comparison to human joints, 7 and they themselves have problems with osteochondral disease. Horses are, however, very expensive if large numbers are required and they need to be kept for a long time. There are also similar issues as for the dog due to their status as companion animals. Injured horses that are destined for euthanasia may be considered more appropriate for studies of treatment.

As well as the type of model to be used, species selection will be determined by a number of other factors. The literature will suggest if one particular species has been used for a particular area; if this is the case, using the same species will allow results to be comparable. Local expertise, animal availability, and housing facilities also have to be considered. Other factors to be considered include cartilage thickness, skeletal maturity, and the differing in vitro biology of chondrocytes if they are to be used in the repair.

Cartilage thickness varies from study to study. Simon 8 looked at the variation between species and joint (Table 45.1) and observed the stress across the joint; he also related the thickness to the elasticity of the cartilage. 9 Much more recently, Frisbie et al 10 compared the thickness between humans and varying species but also compared various sites within the stifle joint (Table 45.2).

The skeletal maturity of the animals to be used also has to be considered. Immature cartilage in skeletally immature animals has a greater potential to heal and therefore does not represent the condition found in human adults. The age of sheep has been found to affect the behavior of cultured chondrocytes.

Each species has its relative benefits and problems, and no one species is ideal for all stages of research. Our opinion is that mice may be most appropriate for heterotopic models, rabbits for proof-of-concept work, and sheep for large animal studies.