Hypothesis:  Inflammation

Infection-bacterial, fungal, viral, protozoal organisms produce toxins or have other 
chemotactic factors which attract WBC such as neutrophils and macrophages.  Chemicals 
that are produced by infectious organisms or WBC are factors that result in the 
production of pain, fever, and inflammation.

Immune-mediated-The host immune system attacks the cells of the synovium/joint 
components and damages them.  These damaged cells and immune cells release 
cytokines, interleukins, and other factors.  These result in signs of inflammation.

Trauma-Trauma damages cartilage or other parts of the joint capsule.  These damaged 
cells release proteins and chemotactic factors/chemicals.  These attract WBC to clean up 
the cellular debris.  The WBCs release cytokines that result in further signs of 
inflammation.

Neoplasia-A Neoplasm (a space occupying lesion) causes pressure necrosis of the joint 
structures.  Dying cells release factors that attract WBC resulting in inflammation.  
Neoplasia is not a very likely cause in this case.  

Inflammation and Actual Mechanisms

Infectious-bacterial products act as chemoattractants.  Inflammatory mediators in the 
plasma in cells are released. These mediators damage the cells of the synovial joint.  
Bacterial endotoxins cause the release of complements, which cause an aggregation of 
clotting factors leading to intravascular coagulation.  This further leads to prostaglandin 
formation, which is involved in the production of pain.  Infectious causes can include 
bacteria, viruses, parasites, fungi, rickettsia, mycoplasma, and spirochetes. 

 Immune-mediated-There can be immune-complex deposition in the synovium.  In 
rheumatoid arthritis, there can also be T-Cells, local inflammatory mediators, and 
cytokines attracted to the joint.  Lysosomal enzymes are released in response to the 
inflammatory response eliciting degradation of the cartilaginous matrix.   Deposition of 
fibrin impairs nutrient metabolite exchange.   Mechanical damage adds to the problem.  

Trauma-A traumatic incident involving crushing or penetration damages the blood 
vessels and other cells.  The damaged cells release factors/chemicals that act as 
chemoattractants for WBCs.  The blood released from the damaged blood vessels also 
attracts WBCs.  

Neoplasia-Although this is a very unlikely cause of the symptoms in this case, a space 
occupying neoplasm of the joint causes pressure necrosis of the joint/joint structures.  
Dying cells in the joint, release factors that attract WBCs resulting in inflammation.

Degenerative Joint Disease-Chondrocytes undergo biochemical and metabolic changes.  
They release Il-1 and TNF-A which stimulate catabolic metalloproteinases and inhibit 
collagen and proteoglycan synthesis.  Prostaglandins and Il-6 are also produced which 
lead to matrix degradation and inflammation.