Synovial Fluid Findings in Degenerative Joint Disease

Tricia Hans, DVM; Kenneth S. Latimer, DVM, PhD; Bruce E. LeRoy, DVM, PhD; Perry J. Bain, DVM, PhD; Heather L. Tarpley, DVM; Paul M. Frank, DVM

Class of 2004 (Hans), Department of Pathology (Latimer, LeRoy, Bain, Tarpley), and Department of Anatomy and Radiology (Frank), College of Veterinary Medicine, The University of Georgia, Athens, GA 30605

Introduction

Osteoarthritis or degenerative joint disease (DJD) is a progressive condition in which the articular cartilage is slowly degraded and the surrounding bone reacts by producing osteophytes (Figs. 1 and 2). It is generally distinguished from inflammatory joint disease because, while inflammation may be present, it is not a central feature of the disease. The disease affects many animal species, including 20% of the canine population older than 1 year, and approximately 90% of cats over 12 years of age. Patients often have a history of earlier acute, painful, non-weightbearing lameness, but signalment, medical history, clinical signs, physical examination findings, laboratory data, and radiographic findings should all be taken into account in the diagnosis of DJD.

A patient's signalment can provide valuable information in terms of assessing the relative risk for the development of DJD. Rapidly growing large-breed puppies often are affected with developmental orthopedic conditions that can lead to osteoarthritis. These conditions include hip dysplasia, osteochondrosis, elbow dysplasia, fragmented medial coronoid process, and ununited anconeal process. Obese animals with DJD may be more likely to show clinical signs. Neutered large-breed dogs constitute another group with predisposition to the development DJD. These animals are more susceptible to cranial cruciate rupture and osteoarthritis of the stifle joint (Fig. 3). Small breed dogs often have patellar luxation or aseptic necrosis of the femoral head. Cats also can suffer from hip dysplasia and osteoarthritis.

Figure 1. Diagram of a joint demonstrating the pathologic features of degenerative joint disease (©2003, mydr http://www.mydr.com).	Figure 2. Scanning electron micrograph of human articular cartilage demonstrating free ends of disrupted collagen fiber bundles. (©1997, Microscopy Research and Technique)

Figure 3. Prominent osteophytosis along the trochlear ridge of a dog 12 wks after transection of the anterior cruciate ligament (right-hand side). Note the absence of osteophytes in the stable contralateral knee (left-hand side). (©2002, Biorheology)

Overview of Synovial Fluid

Synovial fluid is a dialysate of plasma. The principal function of synovial fluid is nutritive support, lubrication, and "cushioning" of articular cartilage. Concentrations of synovial fluid electrolytes and nonelectrolytes, such as glucose and urea, are similar to those of blood. It has a relatively low protein concentration. The cells of the synovial membrane add hyaluronate, a glycosaminoglycan that contributes to viscosity of the fluid. Synovial fluid responds in various ways to different types of joint injury.

The volume, color, turbidity, viscosity, mucin (hyaluronic acid) concentration, and protein content of synovial fluid can be useful when attempting to categorize an arthropathy as hemorrhagic, degenerative, or inflammatory. With a degenerative arthropathy such as DJD, joint fluid may provide valuable clues that may be used to distinguish degenerative disease from the other categories. The most important parameters to consider are nucleated and differential cell counts, and protein concentration.

Evaluation of Synovial Fluid

The viscosity of synovial fluid is assessed as the sample is expelled onto a glass side (note that this is a subjective test). Normal synovial fluid is very viscous because of its high concentration of hyaluronic acid. The viscosity of the fluid can be assessed by allowing a drop of synovial fluid to fall slowly from the tip of a needle, or by placing a drop between the thumb and forefinger (of a gloved hand) and slowly separating them. The quantity and quality of mucin (and therefore hyaluronic acid) can be assessed using a mucin clot test. The mucin clot test is a semi-quantitative measure of polymerization that is evaluated after acetic acid precipitation of protein. In general, when a joint is inflamed, hyaluronic acid is depleted and the viscosity of the fluid is reduced. A drop of fluid from an osteoarthritic joint may form a string only 1 to 2 cm long. The reduction in hyaluronic acid may also not be detectable.

Diff-Quik is a rapid and easy stain to evaluate synovial fluid samples cytologically; Wright's stain can be used as well. Nucleated cell counts are variable from joint to joint, even in the same animal. Large mononuclear cells present on cytology may be derived from blood monocytes, tissue macrophages, or synovial membrane cells (Figs. 4 and 5).

Figure 4. Lymphocyte with macrophage or synovial cell. (© 2003, The University of Georgia)	Figure 5. Synovial lining cell (© 2003, The University of Georgia)

Reference ranges are variable, but some approximate ranges for nucleated synovial cell counts of healthy animals have been included in table 1 below. Cell counts determined by hemacytometer or automated cell counters are more accurate, but a rough estimate of the nucleated cell count can be calculated from microscopic examination of a stained smear of joint fluid using the following formula:

Average number of cells per field x (objective power)² = number of cells per microliter (µl)

Table 1. Expected synovial fluid parameters from healthy animals and animals with degenerative joint disease.

*Mononuclear cells include monocytes, macrophages, lymphocytes, and synovial lining cells.

Radiographic Changes in the Diagnosis of DJD

Radiography of bones and joints is extremely valuable in diagnosing DJD and is generally regarded as the "gold standard" in the diagnosis of osteoarthritis. It should be noted that clinically detectable radiographic abnormalities may not be present in joints with early osteoarthritis, although effusion may be noted.

Radiographic findings in DJD (Figs 6 and 7): Narrowed joint space Periarticular new bone formation Periosteal proliferation Subchondral bone sclerosis Subchondral cysts
Figure 6. Canine elbow joint demonstrating periarticular new bone formation consistent with osteoarthritis. (© 2003, University of Georgia)	Figure 7. Craniocaudal view of same canine elbow. Joint narrowing is evident on this projection. (© 2003, University of Georgia)

Unfortunately, radiography only presents a record of past destructive events and does not give information on current disease activity. Therefore, it is important to perform additional diagnostic tests, such as cytological evaluation of the synovial fluid, to confirm or exclude the presence of osteoarthritis.

Treatment of DJD

Currently, the treatment for DJD is aimed at palliating clinical signs rather than curing the disease. Nonsteroidal anti-inflammatory drugs are generally the first line of approach. Another approach is to administer compounds that "supplement" cartilage matrix components such as pentosan polysulfate, glucosamine, and chondroitin sulfate. Finally, many arthritic patients may benefit from dietary management, weight loss, controlled exercise, heat and massage therapy, and passive joint manipulation. Surgery is indicated in cases where medical therapy and physical therapy are not providing sufficient relief from lameness and pain. These surgical procedures may include excision arthroplasty, arthrodesis, or total joint replacement.

Promising Techniques for Future Evaluation of DJD

Currently, several promising techniques are being evaluated for diagnosis and assessment of the progression of DJD. These include magnetic resonance imaging (MRI), bone scintigraphy, arthroscopy, and body fluid markers of joint cartilage turnover. Assaying the molecular fragments of cartilage matrix molecules released into joint fluid has been suggested as a means to diagnose, monitor, and provide a prognosis for osteoarthritis.

Various intact, cleaved, and partially degraded matrix molecules are released from articular cartilage in osteoarthritis. Matrix metalloproteinases (MMPs), an example of which is stromelysin, are enzymes involved in cartilage degradation. Glycosaminoglycans (GAGs) are the degradation products of the osteoarthritis process and include keratin sulfate and chondroitin sulfate. Type II collagen is a cartilage component released during the anabolic response to osteoarthritis. All three of these types of matrix components have potential as markers for DJD. GAGs and MMPs have been studied most extensively.

Assays have been developed which recognize keratin sulfate and chondroitin sulfate fragments in synovial fluid. Currently, findings with these assays are not always consistent. Of the two, it has been proposed that chondroitin sulfate shows more promise as a marker of articular cartilage destruction. In humans, stromelysin concentration has been found to be a parameter that distinguishes joints with osteoarthritis from joints without cartilage degradation. Further study is required before these markers may be utilized reliably to provide useful information in animals.

Conclusions

Although synovial fluid analysis alone cannot always differentiate osteoarthritic joints from healthy joints, it is very valuable in excluding sepsis and immune-mediated conditions (which usually have increased fluid color, elevated total nucleated cell counts, and an increase in PMNs). Evaluation of synovial fluid, in conjunction with history, signalment, clinical signs, physical examination findings, laboratory data, and radiographic findings, is still the best method to diagnose and monitor degenerative joint disease.

Currently, evidence for the role of biochemical markers in synovial fluid as useful prognostic tools is still limited, but it is felt that, eventually, markers of bone, cartilage and synovial tissue turnover probably will complement radiographic and MRI evaluations.

Acknowledgments

Tricia Hans would like to thank the staff of the Clinical Pathology Laboratory at the University of Georgia College of Veterinary Medicine for their assistance during her clerkship.

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