Joints and Synovium
Peter A. Humphrey
I. NORMAL ANATOMY. Joints are composed of the ends of contiguous bones and the associated soft tissue elements, including cartilage, ligaments, tendons, and synovium. Diarthrodial movable joints, which are the most common type, are usually covered by hyaline cartilage. Histologically, this articular cartilage is hypocellular with a glassy extracellular matrix composed mainly of collagen, proteoglycan, and water. Embedded within the matrix are chondrocytes within surrounding spaces (lacunae). There are four zones in articular cartilage—superficial, intermediate, deep, and calcified (e-Fig. 50.1)* and chondrocytes have a different appearance depending on their location. Those near the surface of the articular cartilage are small and flattened; in the middle zones, the chondrocytes are more rounded and arranged in columns. The deep and calcified layers of articular cartilage are separated by a thin, basophilic line known as the tidemark, which represents the mineralized front. The calcified cartilage base interdigitates with underlying subchondral bone.
Ligaments, which join two adjacent bones, are formed mainly of collagen. At the insertion site onto bone the ligamentous tissue is calcified. Tendons are connective tissue structures connecting muscle to bone. Microscopically, scant fibroblasts are found within parallel collagen bundles.
Synovium is a glistening white membrane with delicate villous projections that lines the inner surface of the joint capsule (Am J Clin Pathol. 2000;14:773) (e-Fig. 50.2). The inner lining surface is created by synovial cells, including fibroblastlike cells and histiocytes, which are arranged as a thin two- to three-cell layer of closely packed cells with elliptical nuclei and abundant cytoplasm. A fibrous or fibroadipose supporting layer lies beneath the synovial cell layer. Synovium also lines the flexor tendons of the hand and bursae (subcutaneous and subtendinous sacs).
II. GROSS EXAMINATION AND TISSUE SAMPLING. Joint or synovial soft tissue is received as needle core tissue, as fragments from arthroscopic or open synovectomy, as fragments from revised total joint arthroplasty, or as excisions of soft tissue tendon sheath or extra-articular masses. If fragments are received, the number, color, shape, and aggregate size of the fragments should be recorded. If meniscus tissue is submitted, fibrillations or tears should be noted. If chalky white deposits are identified, some of the tissue should be placed into absolute (100%) alcohol to preserve the crystals. Decalcification may be required for calcified cartilage, bone, or soft tissue. Fragments that appear different from normal should be selected for embedding, along with representative unremarkable-appearing fragments.
A. Revision arthroplasties. The presence or absence of necrosis, purulent exudate, and foreign material should be recorded. The explanted prosthesis should be described, including any identification numbers or defects.
B. Soft tissue tumors. For excised soft tissue tendon sheath or juxta-articular masses size, color, consistency, shape, and nodularity (single vs. multiple) should be provided. The outer surface of the specimen should then be inked, and cut sections should be characterized as to color, presence or absence of hemorrhage and necrosis, and distance of tumor to inked margin. One section per centimeter
of tumor is a useful guide for section submission. Demonstration of tumor in relation to the closest inked margin(s) and to any recognizable normal tissue is important.
C. Joint replacement surgeries. Joint tissue may also be removed along with bone in orthopedic joint replacement surgeries, such as knee and total hip replacement procedures, which are most often performed for osteoarthritis. In such cases, where there is identifiable bone and attached articular cartilage, gross examination is particularly important. The overall dimensions and shape of the submitted bone and soft tissue should be recorded. Articular cartilage presence or absence, color, thickness, and abnormalities such as loss, cleft, or tuft formation, crystalline deposits, and bony and cartilaginous overgrowths (osteophytes or exostosis) should be documented. Synovium color, thickness, and consistency, and any nodules or villous projections should be described. One or several sections of synovium should be submitted for formalin fixation. Any associated gross bone defects, such as subchondral cyst formation and superficial bony necrosis, should be noted. Again, if chalky white deposits are identified, tissue should be placed into absolute (100%) alcohol to preserve the crystals. Sections of macroscopically abnormal areas should be submitted for histologic examination as follows. The bone and overlying cartilage should be fixed overnight in formalin, decalcified, and sectioned into 3- to 5-mm slices; one or two sections are usually sufficient to document cartilaginous and associated bone abnormalities. Junctions of normal and abnormal cartilage with underlying bone should be demonstrated by the sections.
D. Synovial fluid. Examination can be extremely useful in the diagnosis of different types of arthritis, especially infectious arthritis.
III. DIAGNOSTIC FEATURES OF COMMON DISEASES OF JOINTS AND SYNOVIUM
A. Osteoarthritis (also known as degenerative joint disease) is defined by the American College of Rheumatology as a “heterogeneous group of conditions that leads to joint symptoms and signs which are associated with defective integrity of articular cartilage, in addition to related changes in the underlying bone at the joint margins” (Semin Arthritis Rheum. 2005;3:1). It is the most common disease of the joints after the age of 65 years, with a prevalence of about 60% in men and 70% in women. The etiology of osteoarthritis is multifactorial, with inflammatory, metabolic, and mechanical causes. Major trauma, repetitive joint use, chronic inflammatory arthritis, and congenital malformations are major risk factors. The diagnosis is usually based on clinical and radiographic features. Pathologically, although the name osteoarthritis indicates an inflammatory condition, disruption of the articular cartilage is the fundamental finding. The joints most commonly affected are the distal and proximal interphalangeal joints of the hands, the hips and knees, and the cervical and lumbar spine.
Grossly, cartilaginous thinning, disruption, and fibrillation can be seen. In areas of complete cartilage loss, the underlying bone is exposed; this bone has a dense polished appearance like marble (known as eburnation). Microscopically, vertical clefts in the cartilage are characteristic (e-Fig. 50.3). There may be associated villous hyperplasia and mild chronic inflammation of the synovium (which should not be confused with rheumatoid arthritis). Papillary masses of metaplastic cartilage, bone, or adipose tissue may form in the synovial membrane; detachment of the masses results in intra-articular loose bodies (known as rice bodies). In eburnated areas, the bone may show sclerotic thickened bony trabeculae, cysts with fluid and fibromyxoid tissue, and superficial bony necrosis. It should be noted that in the vast majority of cases, the pathologic findings are confirmatory of the clinical diagnosis, so it has been suggested that routine pathologic examination in uncomplicated total hip and total knee arthroplasties may not be necessary. In only a small percentage of cases is the pathologic diagnosis different from the clinical impression; in these discrepant cases, the
pathologic diagnosis in most cases is avascular necrosis, rheumatoid arthritis, pseudogout, or pigmented villonodular synovitis (J Arthroplasty. 2000;15:69; Bone Joint Surg Am. 2000;82:1531). Although rare, these cases with unexpected findings argue that microscopic examination of all cases of osteoarthritis is warranted.
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