Hypothesis 3 - Neoplasia associated with the stifle joint

	The patient is a large breed middle aged dog, which is a signalment predisposed to synovial sarcomas specifically and neoplasia in general.  In the case of synovial sarcoma, the tumor may invade the joint space or inhabit tissue space surrounding the joint.  These tumors are usually slow growing at first, but they may have a rapid growth phase as they get larger.  They most commonly occur in joints proximal to the carpus and tarsus.  These tumors arrive from synovial tissue precursor cells that undergo neoplastic transformation.  There can be a proliferative periosteal reaction initially.  As the tumor progresses, cortical bone is destroyed and cancellous bone is invaded. These tumors are malignant and highly metastatic, although they are slow to metastisize.  Therefore these tumors commonly present as solitary tumors with no other clinical signs.  Although these tumors are generally considered non-painful, the pressure on the joint capsule due to the distention and bone erosion from the tumor growth could lead to nociceptor stimulation/pain. Also, as the soft tissue is invaded and compressed by the tumor, cell compression and death could lead to the release of inflammatory mediators and pain.  The tumor could also be contributing to lameness by decreasing the range of motion in the joint.  This is supported in the history by difficulty when rising exhibited by the patient.

	Other tumors could also have invaded the joint, causing the clinical signs.  Primary tumors such as osteosarcoma, chondrosarcoma, hemangiosarcoma, and fibrosarcoma  could cause bone tumor resulting in the clinical signs present.  It is less likely for these tumors to invade the joint space, as compared to synovial sarcoma.   Secondary tumors that can metastisize to bone resulting in the clinical signs are mammary carcinomas, primary lung carcinomas, thyroid carcinomas, and hemangiosarcomas.   Other clinical findings would often accompany the secondary tumors.

	The cranial drawer sign present in this patient would not normally be associated with neoplasia in/around the stifle joint.  The neoplasia could have invaded the ligaments of the stifle or could be compressing the ligaments.  This could lead to decreased integrity of the cruciate ligaments and contribute to the cranial drawer motion.