Hypothesis 2- Synovial Cell Sarcoma

There are several factors that lead us to hypothesize that Bonnie is suffering from a synovial cell sarcoma (SCS). Neoplasia is fairly common in middle to old aged dogs.  SCS frequently occurs in medium to large breed dogs 5-8 years of age. The stifle is the most frequently affected joint, although it has been found in the elbow, shoulder, carpal, and phalangeal joints. This lameness has been localized to the left stifle and Bonnie's signalment and chief complaint correlate with the common presentation of SCS.  The biological behavior of SCS can range from slow growth, in which case masses may not be apparent, to aggressive invasion of adjacent tissues. This could explain why we could not palpate a mass in this case.


Pathophysiology

Synovial cell sarcoma arises from the primitive mesenchymal cells of the deep connective tissue around the stifle joint. Although called synovial cell sarcoma, SCS does not originate from the synovial membrane itself but from outside of the joint. Its direct effects on the joint are caused by erosion into the joint capsule and the resulting inflammatory response mounted by the cells comprising the synovial membrane. The neoplastic epithelioid and fibroblastic cells impinge on the synoviocytes, disrupting the cells of the synovial membrane. These damaged cells release inflammatory mediators such as prostaglandins, cytokines, IL-1 beta, and TNF-alpha.  The remaining synoviocytes respond to these mediators by hypertrophy and hyperplasia. The synovial vasculature becomes more permeable in response to the inflammation, which results in an increased synovial fluid volume. This increase is clinically observed as joint effusion.

Pain is caused as the neoplastic cells invade the fibrous joint capsule and the subsynovial layer, both of which are well innervated by nociceptors. Inflammatory mediators further enhance the sensitivity of the pain receptors. Increased joint fluid causes outward pressure on the joint capsule, to which the nociceprtors also respond.  Bonnie's clinical presentation of having difficulty getting up and climbing stairs while still being able to ambulate with partial weight bearing once up correlates with pain response upon flexing and extending the joint. Pain is elicited when the damaged joint capsule is stretched or compressed, such as when the joint goes from being in full flexion while sitting to full extension while standing. Clinical signs of pain are not seen when the dog is walking, because the dog has the benefit of momentum and the joint stays in relative extension.

No clinical signs of progression beyond the events described above have presented themselves yet.  However, due to the neoplastic nature of synovial cell sarcoma, further and accelerated invasion of the joint space, bone, and supporting connective tissue is likely.