Osteoarthritis

or less stable bony connection. Moreover, if the patient continues to move an OA joint, it will stay rather flexible. This is similar to a fracture that is not fixated in place and might develop into a pseudarthrosis if it is moved constantly.

These are the reasons why it seems possible that OA is a type of bone healing in a misguided context. More precisely, OA might be initiated by the same biochemical reactions as fracture healing because both begin with an inflammation and continue by bone remodeling and formation.

To proof this theory, the biochemical markers sRANKL (soluble receptor activator of nuclear factor-kappaB ligand), chordin and osteocalcin were analyzed in synovial fluid and serum of dogs by enzyme linked immunosorbent assay (ELISA). Their function during fracture repair and bone remodeling is rather clear and they all have been found in healing bone tissue (cf. 2.4), but most of them have not been examined in canine osteoarthritic tissues yet and their roles in osteoarthritis development still have to be elucidated. The marker concentrations were compared to the individual radiographic grades of osteoarthritis. For this purpose, a dog-specific radiographic scoring system for OA in the stifle joint was developed. Other clinical findings like the degree of lameness and duration of orthopedic problems were taken into consideration as well.

The aim of this study was to find out if the markers sRANKL, chordin and osteocalcin can be detected locally in the synovial fluid, how they are concentrated systemically in the blood and how concentrations in both fluids vary among different stages of osteoarthritis. Also, clinical aspects of OA course in dogs should be explored and the diagnostics should be improved.

2. Literature

Arthrosis is defined as a destructive chronic disease of articular cartilage with a primary or secondary inflammation. In case of involvement of bone, it is called osteoarthrosis, but more common is the term osteoarthritis. Histologically, degeneration of cartilage is visible as well as proliferation of synovial membrane and infiltration of inflammatory cells (GRÖNE and BAUMGÄRTNER 2007).

The term osteoarthritis implies that this disease is mainly inflammatory. Still, it is the most common description used for this process in the English language. In German, the word osteoarthrosis is preferred which focuses on the degenerative principle of this disease. Therefore, the German language distinguishs precisely between the degenerative process arthrosis and the pure joint inflammation (arthritis). Many authors believe that the English term osteoarthritis is misleading and wrong as it

“incorrectly implies an inflammatory origin” (CRAIG et al. 2007). But as inflammation plays a pivotal role in OA initiation and progression (see below), the English term is not incorrect. However, in this dissertation the word osteoarthritis is used as it is most common.

The disease OA can be categorized as primary or secondary OA. Primary OA is diagnosed if pathogenetic factors are unknown. In contrast, secondary OA is caused either by congenital disorders or acquired prepositions (like trauma, high weight or aseptic necrosis (GHARBI et al. 2013; MELE 2007)).

For example, congenital disorders like limb malposition and osteochondrosis might lead to a fragmented processus coronoideus (FPC), ununited anconeal process (UAP), joint incongruity and cartilage erosion, all grouped under the term elbow dysplasia (ED). In young dogs (4–8 months) this often results in lameness and can cause elbow OA (KIRBERGER and FOURIE 1998). Hip malpositions like hip dysplasia (HD) can lead to subluxation of the hip. Sooner or later, this constant incongruent workload causes osteoarthritic processes in the hip. Both HD and ED

2.1. Osteoarthritis

14 2.1. Osteoarthritis

are believed to have genetic triggers, but also predispositions like weight, growth, hormones and exercise take part in their genesis (CLEMENTS et al. 2006).

Another joint which is often affected by OA is the knee (or stifle). It is either affected in young dogs due to congenital abnormity or trauma or, in older patients, due to chronic degenerative processes. The luxation of the patella can occur exclusively or in combination with rupture of the cranial cruciate ligament (RCCL). In younger dogs the luxation mostly appears due to hereditary anatomic anomalies like hip malformation and displacement of the quadriceps mechanism. The hind limb develops a malposition and the patellar tendons become lax so that the patella can luxate. This often happens to be at the medial side of the stifle and in rather small dogs. Without the required pressure of the patella during growth the trochlear groove develops a shallow or even plain shape; this causes a lifelong problem.

As an example for acquired prepositions, trauma due to a sudden misload can cause patellar dislocation as well, which is independent of age (MCLAUGHLIN 2001). If tendons become loose and the instability grows, the patella luxates more often. This stimulus can lead to osteoarthritic processes.

RCCL itself is also a common traumatic reason for OA development. In young dogs it mostly occurs as injury because of a sudden hyperextension, whereas in older dogs it is often triggered by degenerative processes and finalized by trauma (MCLAUGHLIN 2001; ROUSH 2001). When the cranial cruciate ligament ruptures partly or completely, the position of the tibial plateau is no longer fixated to the femur in cranial and medial direction. During movement this instability causes cartilage lesions which promote OA, but also the damaged tissue like the ruptured ligament are believed to be initiators of OA as they maintain the inflammatory reactions (RYCHEL 2010; MCLAUGHLIN 2001). The rupture or deliberate transection of the cranial cruciate ligament (ACLT) in dogs and other animals is a frequent model for osteoarthritis research with a benefit for humans as well (LITTLE and SMITH 2008;

GHARBI et al. 2013; BRONNER and FARACH-CARSON 2007).

A frequent acquired risk factor for OA in humans and dogs is overweight. In dogs high weight might occur because puppies are fed excessively and grow too fast, but it is also the breed which predisposes for a certain weight and physique

(MELE 2007). Therefore, some breeds are affected more frequently and in younger ages. For example, the average age of Rottweilers is 3,5 years when OA is diagnosed, whereas it is 9,5 years for Poodles. German Shepherd Dogs, Golden Retrievers, Rottweilers and Labradors have a higher predisposition for OA than other breeds. It is estimated that 45 % of dogs with OA are categorized as “big breeds” and about 50 % among these are even “giant-sized dogs”, but only 27 % of all OA cases count as small breeds (MELE 2007).

The most important risk factor for OA in humans is the age (GLYN-JONES et al. 2015), and as it is believed that degenerative processes are the reason, it seems likely that it has high importance for dogs as well because aging of dogs is similar to aging of humans.

Lastly, fractures and other instabilities that lead to imbalanced workload are OA-facilitating factors as they cause inadequate movement of the joint (RYCHEL 2010; BRONNER et al. 2007).

As previously mentioned, OA is a disease that not only affects cartilage, but also the entire joint, meaning bone, synovial membrane, muscles, tendons and ligaments (EGLOFF et al. 2012). Whichever activator is responsible for the onset of OA, the disease always develops by synovial inflammation, cartilage degradation, subchondral bone remodeling and osteophyte formation in areas with the greatest movement. This process continues until the joint stiffens progressively and ankylosis begins (HUNTER 2011; NEUMANN 2015). Today mostly the tissues cartilage, synovial membrane and subchondral bone are considered to play key roles in the pathogenesis of OA. There are a lot of cytokines and growth factors which originate from various tissues and are involved in disease progression:

In cartilage dysregulation of repair is thought to be the initiating process of OA.

Presumably activated by fissures which occur during aging and mechanical stress, chondrocytes secrete cytokines like interleukin-1β (IL-1β), IL-6, tumor-necrosis-factor α (TNF-α) and proteinases (mainly matrix metalloproteinases (MMPs) like MMP-13 and aggrecanases) which are leading to matrix degradation.

Instead of collagen II and aggrecan, more type X collagen is produced. Additionally,

16 2.1. Osteoarthritis

pathologic calcification takes place. These factors are responsible for degradation of cartilage resistance and buffering ability. Augmented toll-like receptors are expressed, and the complement system is activated by damage-associated triggers like extracellular matrix molecules, so the innate immune system partakes as well.

Eicosanoids (like prostaglandins) and reactive oxygen species are also thought to contribute to cartilage degradation. Consequently, chondrocytes become hypertrophic and decline. Cartilage volume initially increases while its surface becomes erosive. Beginning with microcracks, lesions are growing towards subchondral bone until vertical clefts have destroyed the protective function of the subchondral bone cartilage. Fragments of cartilage can even detach and move into articular cavity (KLIPPEL et al. 2008, GLYN-JONES et al. 2015, HAQ et al. 2003).

In subchondral bone mechanisms of endochondral ossification take place.

Osteoblasts and osteoclasts are activated and start enhanced bone turnover. This bone turnover leads to thickened subchondral bone mass of inferior quality, as the new bone is less mineralized (BRONNER and FARACH-CARSON 2007). Growth factors like vascular endothelial growth factor (VEGF) initiate vascular infiltration and neo-angiogenesis towards cartilage. Additionally, osteophytes are built at predisposed positions and subchondral cysts develop. Histologically, microfractures can be seen in areas with the greatest cartilage damage. Because subchondral bone is highly innervated an important generation of pain in OA might be localized here (GLYN-JONES et al. 2015, HAQ et al. 2003).

Synovial inflammation plays a pivotal role in OA. Synovial lining cells activate macrophages that secrete proinflammatory cytokines like IL-1β, IL-6, IL-17, TNF-α, TGF-β (transforming growth factor-β), IGF-1 (insulin-like growth factor-1) and LIF (leukemia inhibitory factor). Subsequently, acute-phase proteins like the C-reactive protein are produced (HASSANALI and OYOO 2011; HAQ et al. 2003).

Synovitis supports joint degeneration in a positive feedback circle and makes its contribution to OA symptoms. The synovial membrane thickens and the produced lubricants for synovial fluid display functional loss so that deficiency of lubrication takes its part in joint space narrowing and cartilage friction (BRONNER et al. 2007).

In final stage OA the most common sign is cartilage degeneration. If the innervated bones get into contact, movement becomes painful. Damaged structures can cause crepitus. Osteophytes grow and play their part in joint immobilization, supported by pain and loss of gliding function. Other tissues like the cranial cruciate ligament can be affected as well, although a rupture mostly precedes OA (ROUSH 2001).

If the inflammation, caused by any reason, activates the chondrocytes to start OA or if degradation of cartilage is the beginning of OA procedure remains unproven. Both pathways are plausible. But in recent studies the idea emerged that subchondral bone metabolism might start and/or promote OA (LAJEUNESSE and REBOUL 2003).

Fracture healing or bone healing describes all pathophysiological and biochemical processes which occur after a disconnection of a bone. A fracture is always accompanied by damage of the surrounding soft tissue. The aim of the healing is to heal the soft tissues and to connect the fracture ends in order to obtain the former physiologic stability (THOMAS and ADLER 1996; HIRNER and WEISE 2008).

After a bone has broken, the tissues surrounding it immediately start inflammation and repairing processes with the aim of reconnecting severed fragments with solid bone. Fracture healing can occur in two forms: primary bone healing is rare and in need of perfect conditions like absolute immobility of the concerned limb and direct contact of fracture ends. In this case bone tissue can be built directly without inflammation and cartilage formation. If these perfect conditions are not given, secondary bone healing takes place. It can be described in a 4-stage model in which inflammation, proliferation, remodeling and occurring biochemical factors are considered (LIEBERMAN and FRIEDLAENDER 2005; SCHINDELER et al. 2008).

Naturally, stages are partly overlapping:

The first stage is called inflammation. It starts immediately after injury and lasts about 1 week. Initiated by a trauma that has torn a bone into two parts, haematoma fills the damaged tissue. Platelets secrete chemoattractants and induce chemotaxis