Microanatomic Structure

The synovium consists of a thin layer of synovial cells or synoviocytes, the intimal layer, above a richly fibrovascular zone, the subintimal layer, which contains arterioles, fat, and other connective tissue cells, such as fibroblasts, histiocytes, and occasionally mast cells (Fig. 72-3). The intimal layer of lining cells is usually one to two cells thick with no discernible differences on light microscopy. The loose connective tissue subintimal zone layer becomes gradually more fibrous at capsular insertions.

Figure 72-3 A, Normal synovium consists of a fine single or double layer of lining intimal cells on top of a subintimal zone of vascular and fatty tissue, fibroblasts, and rare histiocytes and mast cells. Collagen is increased in the vicinity of dense connective tissue structures. B, Hyperplastic synovium showing moderate hypertrophy of cells confined to the surface, with increasing hyperplasia and hyperplastic cells migrating into the subintimal zone with marked hypertrophy. C, Mucinous hyperplasia illustrating the secretory potential of synovium. D, Superficial zone foreign body giant cell reaction to a failed prosthesis.

(From Vigorita VJ: The synovium. In Vigorita VJ [ed]: Orthopaedic pathology, Philadelphia, 2008, Lippincott Williams & Wilkins.)

The villous appearance of synovium is not abnormal but is rather nonspecific and may be seen in a broad range of conditions (Fig. 72-4). In general, traumatic synovitis and degenerative joint disease (DJD) (osteoarthritis) are attended by edematous change and mild villous hypertrophy. Inflammatory arthritis (classically rheumatoid arthritis) shows a dramatically reddish hyperplastic synovium with fibrinous exudation characterized by abundant tan fibrinous loose bodies, called rice bodies, and marked lymphoplasmacytic synovitis. In septic arthritis, leukocytes are seen in tissue. Neuropathic joints and rapidly destructive joint processes are characterized by bone and cartilage debris.

Figure 72-4 Synovium in degenerative joint disease (A), rheumatoid arthritis (B), traumatic synovitis (C), septic arthritis (D), and neuropathic joint (E). Degenerative joint disease is characterized by mild villous hyperplasia; rheumatoid arthritis, by nodular lymphoplasmacytic inflammation; trauma, by fibrinous change; septic arthritis, by leukocytosis; and neuropathic joints, by abundant bone and cartilage detritus.

(From Vigorita VJ: The synovium. In Vigorita VJ [ed]: Orthopaedic pathology, Philadelphia, 2008, Lippincott Williams & Wilkins.)

Ultrastructure

The intimal zone consists of an admixture of cell types often conveniently classified as cells exhibiting macrophage function (synovial A cells) and cells more synthesizing in function (synovial B cells) (Fig. 72-5). Electron microscopy studies and immunophenotyping studies further characterize these cells. Ultrastructural studies show abundant mitochondria, Golgi apparatus, vacuoles, lysosomes, phagosomes, vesicles, and surface undulations (characteristics suited to macrophage activity) in type A cells, and rough endoplasmic reticulum, free ribosomes, and smoother cytoplasmic profiles (characteristics suited to synthetic activity) in type B. As might be expected, synoviocytes may be “intermediate” in nature, featuring organelle functions of types A and B. Some evidence supports the existence of other cells, such as antigen-type cells (HLA-DR, IA-like).

Figure 72-5 Ultrastructure of synovial intimal cells. Type A cells show prominent Golgi cells, vesicles, vacuoles, lysosomes, and mitochondria. Type B cells show abundant rough endoplasmic reticulum. No basal laminae are separating lining cells from subintimal connective tissue and, unless inflamed, rare cell junctions.

(From Vigorita VJ: The synovium. In Vigorita VJ [ed]: Orthopaedic pathology, Philadelphia, 2008, Lippincott Williams & Wilkins.)

The use of special immunologic techniques, including monoclonal antibodies, against a range of antigens can confirm that synovial cells consist of many different cell types. Three distinct populations can be defined immunologically.

Arbitrarily designated type 1 (synovial “A” cells) is a group of cells that seem to be related to mononuclear phagocytes based on their expression of antigen and derivation from cells of the monocyte macrophage cell lineage.²² These cells, which exhibit phagocytosis and contain macrophage markers, abundant HLA-DR IA antigen, and Fc receptors, constitute approximately one third of the synovial cell population.

A second cell population, type 2, is characterized by nonphagocytic activity and strong expression of IA antigen with the absence of immunoglobulin (Ig)G Fc receptors and antigens associated with the monocyte lineage, D and T lymphocytes, and fibroblasts. Type 2 cells include IA antigen–positive dendritic cells. In rheumatoid arthritis, a considerable portion of the cells are type 2 cells.

A third cell population, type 3, is characterized by a nonlymphocytic population with characteristics typical of fibroblasts. These are the cells that usually predominate in tissue culture. They lack phagocytic activity and do not express monocyte antigens or IA antigens.

Cell types as defined by immunologic studies are consistent with previous morphologic observations. Type 1 cells expressing the monocyte macrophage antigens are similar to the cells historically described as characterized by phagocytosis type A and type C cells. Type 1 cells are most likely mononuclear phagocytes of bone marrow origin, as ascertained by their antigenic marking with monocyte and IA antigen. Modifications in antigen expression suggest that they act more like tissue macrophages in the joint. The historically designated “type B” synovial cell that produces glycosaminoglycans is most likely of the type 3 cell variety, probably mesenchymal in origin. These cells lack IA antigens and most of the differentiated antigens of monocyte macrophage lineage.¹¹

The origin of normal synovial lining cells is controversial, but a dual-cell origin remains plausible with bone marrow derivation for the histiocytoid or type A cells and local mesenchymal tissue for fibroblast or type B cells. Many clinicians believe, however, that the heterogeneity of synovial cells is more an expression of functional activity resulting from various factors, and that the cell types are possibly interconvertible.

Although synovial cells lack desmosomes or tight junctions, characteristic of epithelial tissue, the complexity of this cell structure is evident in the changes seen in various pathologic states. Hyperplasia may be limited to a mild increase in intimal cell number, or dramatic change, including large, bizarre cells, such as Grimley-Sokoloff giant cells or even striking mucin-producing cells, may occur. In this latter condition (mucinous hypertrophy of the synovium), the copious amount of material secreted testifies to the potential capacity of this membrane (see Fig. 72-3C). More recently, multipotential mesenchymal stem cells were developed from the adult human synovial membrane and were inducible into chrondrogenesis, myogenesis, osteogenesis, and adipogenesis.¹⁶

Functions of the Synovium

The functions of the synovium are best appreciated by understanding the characteristics of its cellular components and microarchitectural structure. The synovial A cells are suited to phagocytic (or macrophage) activity and ingest native or foreign material, such as hemosiderin in chronic bleeding conditions (hemophilia) or iatrogenically introduced substances (gold in the treatment of rheumatoid arthritis). Most recently, synovial giant cell reactions to viscosupplementation products have been identified (see later). The phagocytic potential of the synovium probably is best illustrated by the marked foreign body giant cell and histiocytic reaction in some cases of loosened prostheses or in the resorption of bone and cartilage debris in rapidly destructive joint disease or neuropathic joints. Absorption of fluid material by synovial intimal cells is shown ultrastructurally by pinocytotic vesicles and vacuoles. Alternatively, the synovial B cell is suited to synthetic function and most characteristically secretes the hyaluronate protein of synovial fluid, with hyaluronate contributing to the lubrication of joint structures. Type A and type B cells seem to have secretory and phagocytic potential, however. Other functions in conjunction with the vascular and lymphatic systems of the synovium include the regulation of movement of physiologically important proteins and electrolytes. The lack of a basal lamina, the presence of gaps between synovial cells, and the lack of a basement membrane in synovial blood vessels facilitate interchange between synovial fluid and blood vessels.

Synovial Fluid

Synovial fluid, a dialysate of plasma, contains small amounts of hyaluronic acid, a copolymer of glucuronic acid, and an acetic glycocyamine, a protein complex hyaluronate synthesized by the cells of the synovial membrane. This material gives synovial fluid its high viscosity. In addition, synovial fluid contains cells, mostly mononuclear phagocytes, and neutrophils. The synovial fluid content of glucose, uric acid, and lactate is similar to that of plasma, but less total protein is present. Albumin constitutes most of the total protein, whereas proteins of higher molecular weight, such as fibrinogen and other globulins, are relatively decreased compared with their plasma concentrations.

Arthrocentesis is the procedure whereby the synovial fluid is removed—a procedure that infrequently is complicated by infection. Arthrocentesis may be helpful in detecting crystal-induced synovitis, such as gout, CPPD disease, and hydroxyapatite crystal deposition disease. Blood is a nonspecific finding that may be seen in a broad range of conditions, including trauma-induced hemarthrosis and PVNS, rheumatoid arthritis, and infection. Collection of synovial fluid in heparin or liquid ethylenediaminetetraacetic acid facilitates the identification of crystal deposition disorders. Heparinized tubes should be used for microbial cultures and gram stain. For chemical analyses, observation of fresh synovial fluid in a clean test tube may be beneficial because normal synovial fluid does not clot (low fibrinogen concentration). Subsequent centrifugation can remove cells, fibrin, and other debris for further analysis for lactate, protein, uric acid, glucose, and other substances.

Synovial fluid analysis may be grouped for general classification of types of conditions producing synovial fluid (Table 72-1 and Fig. 72-6).⁷³ Noninflammatory conditions, such as DJD, trauma-induced arthritis, osteochondritis dissecans, and neuropathic arthropathies, or primary synovial metaplastic conditions, such as synovial chondromatosis, are characterized by a yellowish synovial fluid with high viscosity, low leukocyte counts, less than 10 mg/dL of glucose (i.e., below serum level), and low protein. Cultures are negative. This noninflammatory or class I type of synovial fluid group is distinct from inflammatory or crystal-induced arthropathies, which are characterized by a synovial fluid with low viscosity, a yellow or whitish color, and a variable leukocyte count with a greater than 50% neutrophil population. Glucose usually is elevated to more than 25 mg/dL, and protein is elevated to more than 3 g/dL.

Table 72-1 Classification of Synovial Effusions

Figure 72-6 Synovial fluid composite. A, Synovial fluid from a patient with calcium pyrophosphate crystal deposition (cloudy), normal subject (clear amber), patient with rheumatoid arthritis (cloudy yellow), and trauma patient (blood stained). B, Pigmented villonodular synovitis, especially of the diffuse type, can give a bloody or rusty appearance. Septic arthritis yields a turbid yellowish green.

(From Vigorita VJ: The synovium. In Vigorita VJ [ed]: Orthopaedic pathology, Philadelphia, 2008, Lippincott Williams & Wilkins.)

Septic arthritis usually is characterized by a yellowish green fluid of low viscosity with an elevated leukocyte count and greater than 75% neutrophils. Glucose is usually greater than 25 mg/dL with increased protein levels (Figs. 72-7 and 72-8). A wide range of conditions can induce a hemorrhagic arthropathy, including idiopathic hemosideric synovitis, trauma-induced arthritis, hemophilic arthropathy, and PVNS as a secondary change. These synovial disorders are characterized by a reddish brown color with decreased viscosity and a moderate elevation in white blood cell count with greater than 25% neutrophils. Glucose is in the normal range, usually 0 to 10 mg below serum level. Protein may be greater than 3 g/dL.

Figure 72-7 Septic arthritis. Axial fat-suppressed (FSE) (A) and postcontrast fat-suppressed T1-weighted (B) images show thickened synovium (between arrows) with enhancement (e). In septic arthritis, the enhancement goes beyond the confines of the joint (small arrows).

Figure 72-8 Tuberculosis. Sagittal fat-suppressed (A), axial fat-suppressed (FSE) (B), and gradient echo (C) images show synovial thickening without the blooming that would be present in diffuse pigmented villonodular synovitis on the gradient echo image.

Normal synovial fluid is usually less than 3.5 mL and is clear or faintly yellow in color with clear transparency. It has high viscosity as a result of hyaluronic acid and does not clot because of low concentrations of fibrinogen and other clotting factors. In normal synovial fluid, fewer than 200 cells/µL are present, with a predominance of mononuclear phagocytes. The quantity of lymphocytes is variable, although lymphocytes and neutrophils increase in number dramatically in inflammatory arthropathies.

Eosinophilic synovitis has been described and may be associated with mild to marked peripheral eosinophilia. Normal synovial fluid chemistries are typical. The differential diagnosis includes suppurative arthritis, tuberculosis and parasitic infections, allergies, and both degenerative and rheumatoid arthritis. The condition has been noted after arthrography and usually is accompanied by synovial fluid eosinophilia.

Crystal-induced arthropathy can be diagnosed upon examination of synovial fluid using polarized light microscopy. Crystals other than gout (urate) and pseudogout (calcium pyrophosphate) may be seen. In chronic noninflammatory disorders, cholesterol crystals may be seen; these are square or rectangular with notched corners. Steroid injections may persist and may be detectable as faintly birefringent.

Hemosideric Synovitis

With intra-articular hemorrhage, one may detect fluid–fluid levels depending on the extent and duration of the bleed (Fig. 72-9). In hemorrhages that occur within the synovium, which is a highly vascularized tissue, red blood cells eventually disintegrate, and phagocytosis is begun by histiocytes. Eventually, the hemoglobin is broken down and processed into hemosiderin. Hemosiderin characteristically is seen as a brown-pigmented granular substance in cells of the synovium. Hemosiderin may occur as minute granules or may accumulate into globules 25 µm in diameter. The intracellular deposition of hemosiderin is observed mostly in macrophages or histiocytes, but also may be seen engulfed in various trauma-associated conditions by hypertrophied synovial lining cells. In joints subjected to hemorrhage, hemosiderin may be noted intracellularly or extracellularly. Cells within different compartments or layers of the synovium may be affected, ranging from synovial lining cells to subsynovial proliferating connective tissue cells. The gross appearance of the joint with chronic hemarthrosis includes a rusty pigmentation, although more acute bleeding may be demonstrable as blackish green discoloration.

Figure 72-9 Hemorrhage. Sagittal fat-suppressed (A) and sagittal (B) and axial fat-suppressed (FSE) (C) magnetic resonance images. Multiple fluid levels are seen (black arrows) with intermediate signal material that is a combination of blood and synovitis (white arrows).

The response of the human joint to bleeding is denoted clinically by the formation of a hyperplastic vascular synovium within a few days. Examination of the tissue reveals a proliferation of synovial cells and other subsynovial lining cell connective tissue elements, often including inflammatory cells. Under electron microscope, iron-containing, electron-dense particles that are membrane bound, called siderosomes, are noted within the synovial cells and subsynovial macrophages.

Clinically, hemosideric synovitis may be due to a wide range of conditions, most notably trauma, particularly chronic trauma leading to chronic hemarthrosis. It also may be caused by the use of oral anticoagulant therapies, may result from breakdown of synovial hemangiomas, or may occur as a secondary phenomenon in conditions such as rheumatoid arthritis, PVNS, scurvy, and sickle cell anemia. Radiographic appearances may lead to joint space narrowing and may cause confusion with other ailments destructive to joints. Although most patients with chronic hemarthrosis or with an episode of hemarthrosis recover with no significant sequelae, the potential for joint destruction is present in any patient with bleeding into the joint.

With bleeding into the joint, two pathways may lead to damage. Under one mechanism, the red blood cells may break down, causing macrophage activation. Ensuing inflammation may lead to destructive changes in and of themselves, as seen in rheumatoid arthritis. Intracellular hemosiderin precipitates the release of leukocyte-derived and synovium-derived chondrocytic enzymes.⁶ In excessive cases, iron deposition in organs such as the meniscus may be severe enough to cause mechanical dysfunction and degenerative changes. In a second pathway, bleeding may lead directly to synovial proliferation and the release of an uncontrollable cascade of destructive substances such as proteases.

Hemophilia

Adverse effects on synovium in patients with chronic hemorrhage have been studied most extensively in the joint changes of hemophilic arthropathy of the knee. In tissue removed from hemophiliac patients, the synovium shows marked villous hypertrophy with extensive hypertrophy and hyperplasia of synovial lining cells and with subsynovial connective tissue components with abundant intracytoplasmic hemosiderin granule accumulation (Fig. 72-10). In tissue cultures of synovium in hemophilia, pigment-laden fibroblast cells have been shown to proliferate, and explants of synovial cells have been shown to secrete large quantities of latent collagenase and neutral proteinases, establishing the destructive potential of synovitis in hemophilia. Synovium incited by hemophilia produces enzymes and may do so without coincident inflammation; the latter is a significant component of the destructive arthropathy seen in rheumatoid arthritis.

Figure 72-10 Hyperplastic hemosiderin-laden, rust-brown synovium from hemophilic arthropathy. A, Gross. B and C, Microscopic (C shows Prussian blue stain for iron).

(From Vigorita VJ: The synovium. In Vigorita VJ [ed]: Orthopaedic pathology, Philadelphia, 2008, Lippincott Williams & Wilkins.)

Iron accumulation may lead directly to chondrocyte destruction. In experiments aimed at developing a model for PVNS, a condition related to expanding nodules of proliferative fibroblasts and histiocytes, blood was experimentally injected into various animal models. The resultant proliferative synovitis was characterized by hyperplasia of fibroblasts, lipid-laden macrophages and giant cells, and extensive hemosiderin accumulation similar to that seen in reactive hemosideric synovitis or hemophilic arthropathy.

Although it is less visible, iron is known to accumulate in cartilage as well. Extensive hemosiderin deposition leading to grossly visible brown menisci has been observed clinically.⁶ In the study of hemophilic joints, iron has been localized to superficial chondrocytes, suggesting chondrocytic phagocytic uptake triggering a degradative enzyme release similar to that described in synovial cells. Iron has been localized histochemically to the tidemark in lupus and in hemochromatosis. Hemophilia A and hemophilia B are inherited X-linked deficiencies of factor VIII and factor X.

Hemophilia A is the most common hereditary coagulation disorder, affecting 1 per 5000 live male births as the result of deficiency, absence, or malfunction of coagulation factor VIII.⁶⁵ Roughly one third of patients have no history of this disease. In normal patients, a very low concentration of factor VIII (0.2 mg/mL plasma) is sufficient for adequate coagulation. Bleeding leading to clinically perceptible illness requires a reduction of at least 75%.

Hemophilia A is a clinically heterogeneous disorder ranging from mild disease (1% to 4% deficiency) to severe disease. Patients with mild or moderate disease may not be recognized unless a significant traumatic event precipitates abnormal bleeding. Excessive bleeding during surgery may be the first clue. Therapy is centered on replacement of factor VIII.²⁵ Replacement traditionally was done with shotgun frozen plasma, and cryoprecipitate became available in the 1960s. More recently, purified concentrates have been used. Current treatment consists of regularly scheduled replacement of deficient factor with recombinant proteins synthesized in tissue culture. In general, 1 U of factor VIII increases plasma activity by 0.024 U/mL. Because 0.3 U/mL usually is needed to treat a mild bleeding episode, clinical goals should strive for more than this amount. Therapy has significantly improved morbidity.

Clinically, bleeding is the key, especially bleeding into joints. Most commonly involved, in order of frequency, are the knees and elbows, followed by the ankles, shoulders, and hips. Not usually evident in infancy, childhood signs include mild discomfort and joint limitation. Pain and swelling ensue. Numerous damaging microhemarthroses have transpired before the initial arousal of clinical suspicion, thereby delaying diagnosis. Hemosiderin deposition and synovial hypertrophy with joint destruction may occur.

Radiographically, joint space narrowing, synovial proliferation, loss of articular cartilage, cystic remodeling of bone, and hemophilic pseudotumors characterize the illness (Fig. 72-11). Strict adherence to maintenance of factor VIII levels to prevent spontaneous hemorrhage has been associated with significantly decreased morbidity and has halted radiographic progression of disease.

Figure 72-11 Hemophilia. Coronal (A) and axial fat-suppressed (FSE) (B) magnetic resonance images show extensive masslike synovial proliferation (S) with low signal (dark areas) from hemosiderin deposition. Note the erosions (E) and the chondral loss (long arrow).

Surgical joint reconstruction in hemophiliac patients has been attempted in an effort to preserve function, but radioactive synoviorthesis and arthroscopic synovectomy have been used.⁶ Total knee arthroplasty performed for hemophilic arthropathy has a high risk of failure as a result of infection, which develops late and often is caused by Staphylococcus epidermidis.⁵⁹

Intra-articular, intrabursal, and soft tissue bleeding in hemophilia may result in painless masses that clinically and radiographically mimic a tumor, called pseudotumors of hemophilia (Fig. 72-12).²⁴ These masses consist of spongy coagula of partially clotted blood encapsulated by thick fibrous membranes. Complications of these so-called hemophilic pseudotumors include muscle and bone damage, infection, and neuropathies. Surgical removal is not undertaken without danger. Pseudotumors occur in 1% to 2% of hemophiliac individuals,⁶ mostly in the lower extremity and pelvis. Bleeding in the vicinity of the periosteum has been implicated in peculiar juxtaosseous changes, among them cystlike bone changes and large soft tissue masses that erode bone. MRI studies indicate low signal on T1-weighted imaging and high signal on T2-weighted imaging.

Figure 72-12 Radiograph of hemophilic pseudotumor.

(From Vigorita VJ: The synovium. In Vigorita VJ [ed]: Orthopaedic pathology, Philadelphia, 2008, Lippincott Williams & Wilkins.)

Before 1970, hemophilia A was associated with significant severe disability and death at a young age. Median life expectancy grew significantly throughout the 20th century, however, from 11.4 years to 68 years in carefully monitored populations.⁴⁷ Transfusion of blood products, including factor VIII concentrates, has led to the well-recognized modern complication of hemophilia—transfusion-related acquired immunodeficiency syndrome (AIDS). Life expectancy now has reversed after considerable gains. Large numbers of hemophiliac individuals developed serologically detectable antibodies to human immunodeficiency virus (HIV) beginning around 1979. Approximately two thirds of HIV-positive hemophiliac patients have eventually died of AIDS. Risk to the surgeon is minimal if proper precautions are taken.³⁰ More recent therapy has used factor VIII concentrates exposed to vigorous virus-killing heat treatment or solvent cleaning. Genetically engineered products now available are being tested for complications. Expense is a significant consideration.

Another condition leading to a congenital deficiency is von Willebrand’s disease. von Willebrand’s factor is a protein that promotes the secretion of factor VIII and protects it from proteolytic inactivation. Mutation can lead to cases similar to mild to moderate hemophilia. Pedigree analysis, use of specific assays identifying factor VIII bonding to von Willebrand’s factor, and specific molecular defect analysis are clinical and laboratory approaches to the diagnosis.

Hemochromatosis

Much current knowledge about the effects of iron on tissue comes from studies of hemochromatosis, the systemic disorder in which iron deposition is associated with tissue dysfunction. In general, iron overload has been studied in clinical situations involving hyperplastic refractory anemia, excessive blood transfusions for underlying hematopoietic abnormalities (thalassemia), and hereditary forms of hemochromatosis. In the United States and northern Europe, hereditary hemochromatosis is a common cause of iron overload; 15% of the population exhibits the gene in its heterozygous state. The abnormal gene, located on the short arm of chromosome 6, is closely linked to the HLAA locus. In hereditary hemochromatosis, increased absorption of dietary iron results in excess iron deposition in parenchymal tissues of endocrine organs, liver, and heart; death often results from heart failure or chronic liver disease.

The cause of secondary hemochromatosis is ineffective erythropoiesis, as seen in β-thalassemia and sideroblastic or aplastic anemia, for which treatment may include repeated transfusions. Secondary hemochromatosis also may be caused by chronic liver disease, and in certain populations, such as within sub-Saharan Africa, hemochromatosis has been attributed to increased intake of dietary iron, such as from large amounts of iron in beer brewed in steel drums. Individuals at greatest risk probably have a genetic predisposition.²⁷

The deposition of iron in bone, particularly at the osteoblast–osteoid interface or at the mineralization front, or as hemosiderosis within hematopoietic cells of the marrow, is a relatively common sequela to primary and transfusional hemosiderosis. Although the primary organs involved are the heart and the liver with eventual development of cirrhosis and hepatocellular carcinoma and heart failure, endocrine organs are particularly involved. Joint pain in the clinical presentation of arthritis has been noted in 11% of patients carefully analyzed clinically; bone pain is a less frequent presentation.³

First recognized in 1964 as a feature of hemochromatosis, arthropathy now is generally believed to be a common clinical symptom. In hemochromatosis, joint changes may mimic DJD. Less than 50% of patients seem to have clinically osteoarticular problems. The metacarpophalangeal joints and the interphalangeal joints are reported most frequently. Wrists, elbows, shoulders, hips, and knees are less frequently affected. Iron may inhibit pyrophosphatase activity in cartilage tissue, leading to precipitation of calcium pyrophosphate crystals, an association between the two that is well recognized.

The osteopathy seen in iron overload has been associated with various concurrent metabolic abnormalities, including hypogonadism, hyperparathyroidism, diabetes, and both vitamin C and D deficiencies.