Abstract
Bone is a dynamic organ of the endoskeleton, playing an important role in structural integrity, mineral reservoirs, blood production, coagulation, and immunity. Metabolic bone disease encompasses a broad spectrum of inherited and acquired disorders that disrupt the normal homeostasis of bone formation and resorption. For patients affected by these processes, radiologic imaging plays a central role in diagnosis, monitoring treatment, and risk stratification. Radiologists should be familiar with the diseases, intimately aware of the imaging findings, and possessive of multimodality expertise to wisely guide the best practice of medicine. The purpose of this paper is to review the imaging features and characteristics of the most common types of metabolic bone disease with highlights of clinically relevant information so that readers can better generate appropriate differential diagnoses and recommendations. For this review, a thorough literature search for the most up-to-date information was performed on several key types of metabolic bone disease: osteoporosis, osteomalacia, rickets, scurvy, renal osteodystrophy, hyperparathyroidism, Paget’s disease, osteogenesis imperfecta, acromegaly, and osteopetrosis. Although they all affect the bone, these diseases have both shared characteristic features that can be discerned through imaging.
1. Introduction
Bone is composed of extracellular and cellular components and undergoes constant remodeling. The complex regulation of calcium and phosphate involves the intestines, kidneys, hormones, and vitamin D. Metabolic bone disease is a diverse class of acquired and inherited diseases resulting in increased bone matrix, decreased bone matrix, or abnormal bone mineralization. On imaging, the diseases generally result in increased bone density (osteosclerosis) or decreased bone density (osteopenia), which in turn can cause deformity and debilitation through remodeled or severely weakened bone. Because patients at risk for these diseases benefit from early diagnosis and periodic surveillance, a comprehensive understanding of relevant radiological findings is necessary for appropriate prevention and management. In this paper, we discuss the more commonly encountered metabolic bone diseases and, specifically, we review the multimodality imaging features and trends.
2. Osteoporosis
2.1. Introduction
As the most common of the metabolic diseases of the bone, osteoporosis affects one in two women and one in five men over the age of 50 in the western world [1]. Generalized osteoporosis can be categorized into two general types: primary and secondary. Primary osteoporosis can be further categorized into type I (postmenopausal) or type II (senile). Type I osteoporosis predominantly affects women between the ages of 50 and 65 and is a result of estrogen deficiency causing accelerated trabecular bone resorption. This subset of osteoporosis tends to involve the spine and wrist. In contrast, type II (senile) osteoporosis affects both trabecular and cortical bone, resulting in characteristic fractures of the hip, proximal humerus, tibia, and pelvis [2].
The clinical and radiological presentation of osteoporosis is oftentimes similar to other metabolic bone diseases, which are discussed later in detail. However, malignant processes may also result in a similar clinical picture, and it is important to identify the subtle differences. In general, red flags for a malignant fracture include a soft tissue mass and osseous destruction, as well as a location above 7 in the spine, especially in the posterior portion of the vertebral body. Whereas a concave posterior vertebral border is more likely benign, a convex posterior border is more suggestive of malignancy.
2.2. Radiography
Even with the newest advances in quantitative imaging techniques, conventional radiography is still the most commonly used modality when encountering demineralization, although it usually detects pathology only after 30% of bone has been lost [3]. In general, hallmark features of osteoporosis on radiography include thinned cortices, endosteal reabsorption, and a decrease in the number of trabeculae.
Certain bones of the body are preferentially affected by osteoporosis due to relatively larger proportions of trabecular bone. With conventional radiography, these changes are most evident in the axial skeleton along with the ends of long and trabecular bones of the juxtaarticular appendicular skeleton [4].
As the result of osseous resorption in the cortex, cortical thinning occurs and typically involves the endosteal envelope, causing a scalloped inner margin of the cortex. This endosteal scalloping is not specific to osteoporosis and may occur in conditions with high bone turnover, such as hyperparathyroidism or reflex sympathetic dystrophy syndrome. The appearance of intracortical tunneling and subperiosteal resorption is more specific to these rapid bone turnover states that are less apparent with low turnover states such as senile osteoporosis [5].
In the axial skeleton, cortical thinning results in the well-demarcated outline of the vertebral body, giving rise to a “picture frame” or “empty box” appearance (Figure 1) [6]. Another feature of spinal osteoporosis is the verticalization of trabeculae, which is an early sign of the disease that results from the rarefaction of the horizontal trabeculae, leaving the vertical trabeculae relatively accentuated [4].
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A hallmark manifestation of osteoporosis is vertebral compression fracture (Figures 2 and 3). A history of vertebral fracture due to osteoporosis increases an individual’s risk of subsequent vertebral fractures by five and hip fractures by two. 20% of these fractures occur within 12 months of the first. Up to 50% of vertebral fractures are asymptomatic and are found incidentally on imaging studies performed for unrelated reasons [7]. A true vertebral fracture can be diagnosed with anterior vertebral body height loss of greater than 4 mm since the posterior height of the thoracic vertebrae is normally greater than the anterior height by 1–3 mm [8]. An alternative vertebral fraction definition consists of a height loss of more than 20% with an areal reduction of more than 10–20% [9]. When diagnosing osteoporotic vertebral fractures on radiography, it should be noted that they are rarely associated with an apparent breach of the cortex or significant callus formation, unlike with fractures of long bones [10]. The best techniques by which to view vertebral fractures are lateral spinal radiographs and sagittal reformations from multidetector spiral CT since these modalities allow the transverse orientation of the vertebral endplates [1].
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Quantitative vertebral morphometry can be used as an objective way to measure vertebral body heights by tagging landmarks for each vertebra. The most popular of these techniques is six-point digitization, which marks out the four corners of each vertebra from 4 to 4 or 5 with additional points in the middle of the upper and lower end-plates. Software is then used to analyze any anomalous geometry. However, quantitative morphometry cannot be used to identify fractures resulting from vertebral deformities due to pathology such as degenerative spine or disk disease [2].
Osteoporotic pathology in the appendicular skeleton involves the ends of long and tubular bones, especially in the hand, proximal femur, and calcaneus. A semiquantitative measure for grading osteoporosis in the hand is the corticomedullary index, which is based on the evaluation of the cortical thickness at the second metacarpal bone [2].
Radiogrammetry is a quantitative method for measuring the skeleton that was first developed in the 1960s and has returned in 2000 with the advance of digital X-ray radiogrammetry (DXR), which automatically detects the bone edges of the shafts of metacarpals 2 through 4. Due to the greater requirements for precision with imaging of the hand, DXR was developed to automate the calculation of basic geometric measurements of the metacarpal bones in order to estimate bone mineral density. The technique is highly reproducible, predictive of future fractures, and useful in identifying patients at risk who may benefit from further dual energy X-ray absorptiometry (DEXA) evaluation [11]. Osteoporosis of the appendicular skeleton can also be assessed by rating the integrity of the trabecular pattern within the proximal femur or the calcaneus [12]. Because DXR can detect between 0.0012 and 0.0028 g/cm2 of difference, it is useful in following changes in bone mineral density (BMD) over short periods of time [11]. DXR is also especially useful in tracking the effectiveness of bisphosphonate therapy [13]. As of now, however, the technique remains primarily as a research tool rather than a central player in the clinical diagnosis of osteoporosis.
2.3. DEXA
The World Health Organization’s (WHO) definitions for the threshold levels in diagnosing osteopenia and osteoporosis were introduced in 1994 and are currently the standard for the use of bone densitometry in diagnosing and classifying demineralization. DEXA measurements are provided using bone mineral content (in grams) and projected area of the measured site (in square centimeters). Bone mineral density (BMD) is thus calculated by dividing the mineral content by the area and is expressed in terms of standard deviation by comparison to the mean BMD of young adults ( score) or of age-matched patients of the same sex ( score). BMD is classified by the WHO based on score with categories such as normal (≥−1.0), osteopenic (<−1.0 but >−2.5), osteoporotic (≤−2.5), and severely osteoporotic (≤−2.5 with a fragility fracture). scores are adjusted for patients on antiosteoporotic drugs as well as osteoporosis-inducing drugs. Patients on glucocorticoids have a score cutoff of −1.5 while patients on aromatase inhibitors have a cutoff of −2.0 [14, 15].
DEXA has many advantages including its rapidity, low cost, and lower radiation risks, but it does have some limitations as a two-dimensional technique. For example, DEXA does not distinguish between cortical and trabecular bone or changes due to bone geometry and can also result in the overestimation of density in larger bones and the opposite for smaller bones [2]. Since DEXA measures BMD in terms of area, it should be noted that areal BMD accounts for only 60–70% of bone strength [16]. Furthermore, it can take DEXA scanners more than 2 years to detect a statistically significant change in areal BMD [17]. The tendency for DEXA to overestimate areal BMD in large bones is especially problematic in growing children. DEXA also tends to overestimate areal BMD if any high-density object is in the path of the beam, including arterial calcification, fractures, metal implants, and degenerative vertebral body endplate sclerosis. Conversely, laminectomy results in underestimation of BMD. Children under 19 years of age have not yet reached peak bone mass and can only provide reliable data via scores, not scores [16].
Other errors that often occur with interpretation of DEXA images relate to improper positioning, numbering the vertebrae, and detection of bone edges. Anatomic artifacts resulting from degenerative disk disease, compression fractures, postsurgical defects, and overlying atherosclerotic calcifications may also be present as common comorbidities in this aging patient population. Implantable devices such as stents as well as external objects such as buttons can also interfere with interpretation. Finally, it is important to note that the results from different DEXA scanners are not interchangeable due to the use of different algorithms and analytic techniques by various manufacturers [18–20].
Portable DEXA scanners are increasing in availability and can be used to analyze BMD in peripheral sites such as the distal radius and calcaneus. The WHO criterion for diagnosing osteoporosis ( score ≤ −2.5) can be used for the forearm but is not applicable to the calcaneus. The definitive boundaries have not been determined, although scores between −1.0 and −1.5 have been suggested [21].
A significant advantage of DEXA is its unique ability to be used with FRAX (Fracture Risk Assessment Tool), which was introduced in 2008 and is used to identify patients at higher risk for fractures as well as estimate their 10-year probability of a major osteoporotic fracture. In addition to BMD, FRAX utilizes patient demographic data (age, sex, race, and body habitus) in the context of risk factors such as history of previous fractures and alcohol use. The algorithm also incorporates manufacturer of the DEXA scanner as part of its analysis [22].
A growing use for DEXA is with vertebral fracture assessment (VFA), which differs from conventional spinal radiography in that the entire spine is visualized on a single lateral image rather than two separate lateral projections. VFA not only requires less ionizing radiation but also avoids the optical distortion of vertebral endplates often seen with radiography [23].
The technique is gaining in popularity and guidelines have already been established for its use, although there is currently little data supporting the application for children. An interesting potential adjunct use for VFA is the grading of abdominal aortic calcification, a risk factor associated with cardiovascular disease in elderly women. Thus, VFA allows for this population to undergo evaluation for cardiovascular disease during bone densitometry [24]. Trabecular bone scoring has also emerged in the past decade as a way of measuring trabecular structure, but its reliability and usefulness have yet to be determined in the clinical setting [25].
2.4. Quantitative CT
Another technique that can estimate BMD is quantitative CT, which is different from DEXA in that it provides separate estimates of trabecular and cortical density to assess true volumetric density with macroarchitecture rather than a two-dimensional recreation [26]. This capacity is significant when considering that trabecular bone is eight times more metabolically active than cortical bone and thus more sensitive to change [1].
Because quantitative CT can distinguish trabecular versus cortical bone, it is more sensitive than DEXA in predicting vertebral fractures and tracking chronic bone loss, especially in postmenopausal women. It is also particularly useful for assessing corticosteroid-induced osteoporosis and treatment with parathyroid hormone, since these diseases tend to preferentially affect trabecular bone [13].
Although BMD values from DEXA can be falsely elevated from pathology such as degenerative joint disease and aortic calcification, this is not true for quantitative CT due to its superior spatial resolution [27].
Although literature regarding fracture prediction for QCT is limited compared to that for DEXA, the number of studies is increasing as QCT becomes more popular. QCT is able to better predict spinal fracture by measuring vertebral BMD more accurately compared to areal BMD utilized by DEXA [28]. It is also a good tool for analyzing components and factors regarding hip fracture, but the radiation dose to the gonads with these studies is concerning. It may also be difficult to scan children using QCT since the slow scan-acquisition time can result in many movement artifacts [16].
Accurate measurement with quantitative CT can also be disrupted by the presence of marrow changes within the bone due to an alteration in medullary attenuation. Although the modality tends to become less accurate over time as a result of age-related changes in vertebral bone marrow, this error can be corrected using reference databases and is not usually clinical relevant [29]. Another limitation to QCT’s low spatial resolution is that it has difficulty measuring thin bone cortices [30]. Furthermore, due to the complexity of the hip and femoral region, quantitative CT is only used to assess spinal volumetric BMD. Some of these limitations can be overcome by the use of peripheral quantitative CT, which can make separate assessments of cortical versus trabecular bone as well allow analysis of bone geometry at multiple appendicular sites [2].
It is worthwhile to note that the results from QCT cannot be used for score calculations as defined by the WHO, nor can QCT be used for the FRAX algorithm [1].
Other important restrictions of quantitative CT compared to DEXA include increased radiation dose, high cost, disruption to other diagnostic CT workflow, and space requirements. Compared to conventional CT, however, the dose is decreased in quantitative CT. Because quantitative measurements rather than high-precision images are desired in QCT, the radiation dose can be minimized to around 100 μSv compared to conventional CT, which has doses from 2500 to 4500 μSv [16].
2.5. Quantitative Ultrasound
Quantitative ultrasound was introduced nearly 30 years ago as a cheaper, smaller, faster, safer, and more portable alternative DEXA scanners. While diagnostic ultrasound uses wavelengths of 2–10 MHz, quantitative US stays within a range of less than 1 kHz [1]. It is well-known for its quickness and low risks compared to the other modalities, but it lacks comparable sensitivity. Although it can be used to measure quantitative parameters and assess tissue properties, quantitative US is inadequate for long-term monitoring of osteoporosis and best serves as a screening tool prior to DEXA evaluation [31]. The calcaneus and phalangeal metaphysis may yield the highest benefit when utilizing quantitative US due to their high proportion of trabecular bone [32]. It should be noted that, as of yet, there are no current guidelines for the standardization of these US devices in terms of reliability among manufacturers. Another difficulty is that the WHO criteria cannot be used with quantitative US, relegating the technique as more of a supplemental rather than standalone tool for diagnosing osteoporosis [2]. Although quantitative US cannot utilize the WHO definition to diagnose osteoporosis nor monitor the effectiveness of therapy, calcaneal quantitative US can help predict hip and other osteoporotic fractures in postmenopausal women and elderly men as well as central DEXA scans [33].
2.6. New Advances
Micro-CT is a newer technique that can resolve up to 6 μm for enhanced visualization of trabecular structures. After three-dimensional reconstruction, software can be used to calculate parameters such as bone surface-volume ratio and trabecular parameters like volume, number, sparsity, thickness, and orientation. MR imaging can be used with similar precision except that it cannot directly visualize the trabecular network and must instead be extrapolated as part of a signal void within the fatty bone marrow. Although it is not commonly used clinically and there are no current guidelines, MR imaging shows potential as a technique for monitoring treatment effects, especially as higher field strengths become more widespread [2]. Both of these high-resolution techniques are used on peripheral sites like the radius or tibia but can be technically challenging with regards to fastidious positioning and susceptibility to movement artifacts. These difficulties combined with the availability of these machines and utilization for other diagnostic imaging and intervention have kept them as research tools for the time being [34].
3. Osteomalacia/Rickets
Incomplete mineralization due to lack of calcium salts in osteoid manifests as osteomalacia in adults and rickets in children. While there are various causes such as gastrointestinal absorptive abnormalities and renal tubular lesions, the central abnormality involves calcium, phosphorous, or vitamin D metabolism (Figure 4). General radiographic signs are nonspecific and include decrease in bone density, loss of cortical definition, and coarsening of the trabecular pattern. Commonly noted deformities include protrusio acetabuli, kyphoscoliosis, and increased endplate concavity in the spine [35]. Bony deformities such as bowing can be bilateral and symmetric and are predominantly seen in weight-bearing bones (Figure 5). Looser zones or pseudofractures are insufficiency fractures that appear as linear radiolucent bands perpendicular to the long axis of the bone and are most frequently seen in pubic and ischial rami, medial femoral necks, ribs, scapulae, and long bones (Figures 6 and 7) [35, 36]. They are classically at the concave (compressive) side of a bowing deformity.
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4. Scurvy
4.1. General
Scurvy, also known as Barlow disease, is a manifestation of vitamin C (ascorbic acid) deficiency and is predominantly a disease of the pediatric age group that presents in the first 6–24 months [37]. The adult incidence of scurvy is rare and is primarily due to severe dietary deficiency [38]. Lack of vitamin C results in abnormal collagen formation that results in a variety of clinical signs and symptoms: irritability, defective dentine formation, hemorrhagic tendency, tenderness, and weakness of lower limbs resulting in pseudoparalysis. Musculoskeletal manifestations occur in 80% of patients with scurvy and radiologic findings are characteristic [38].
4.2. Radiographic Manifestations
Radiographic features are most commonly seen at the site of rapid bone growth and include the distal femur, proximal and distal tibia and fibula, radius and ulna, and humerus. Enlargement of the sternal end of the ribs results in a “scorbutic” rosary. Ground glass osteoporosis affecting both the trabecular and cortical bone is highly characteristic [38, 39]. The zone of provisional calcification shows increased density and is known as the white line of Frankel. The Trummerfeld zone is the site of subepiphyseal infraction and seen as a radiolucent zone adjacent to the white line of Frankel [39, 40]. Due to subepiphyseal location, fractures that occur in the Trummerfeld zone heal without affecting bone growth [39, 40]. Other metaphyseal findings include lateral Pelkan spurs that are seen projecting perpendicular to the shaft axis. “Corner sign” occurs due to continued comminution resulting in mushrooming or cupping of epiphysis and can also be seen in syphilis and rickets [39]. Thickening of zone of provisional calcification with sparse trabeculation of the adjacent epiphysis results in a sclerotic epiphyseal ring, also known as Wimberger’s ring [39, 40]. Subperiosteal hemorrhage is a subtle finding that appears as a soft tissue density adjacent to the cortical bone. However, with treatment, calcification of the elevated periosteum becomes apparent and is an early sign of healing [39, 40]. With continued treatment most radiological signs disappear; however, Frankel’s line remains as an indicator of prior vitamin C deficiency.
5. Renal Osteodystrophy/Hyperparathyroidism
5.1. General
Renal osteodystrophy refers to a spectrum of radiographic musculoskeletal abnormalities in the setting of chronic renal failure (CRF). CRF causes secondary hyperparathyroidism, which predominates the radiographic findings; therefore, there is considerable overlap of musculoskeletal findings in these entities. In addition, renal osteodystrophy causes osteomalacia and other treatment related manifestations. In this review, a brief overview of renal osteodystrophy and associated findings along with changes seen in hyperparathyroidism are presented.
Parathyroid hormone plays a crucial role in regulation of serum calcium and phosphate through alteration in osteoclastic activity, intestinal absorption and renal absorption and excretion of calcium and phosphate. Excessive secretion of parathyroid hormone (PTH) results in a spectrum of musculoskeletal, specifically osseous manifestations that most commonly lead to osteomalacia and osteoporosis.
Overproduction of parathyroid hormone may be classified into primary, secondary, and tertiary causes. Primary hyperparathyroidism results from an overproduction of PTH due to adenoma (90%), followed by parathyroid hyperplasia and parathyroid carcinoma. Hypocalcemia in chronic renal failure or due to intestinal malabsorption results in secondary hyperparathyroidism. Tertiary hyperparathyroidism occurs when parathyroid gland acts autonomously and independently of serum calcium in the setting of dialysis.
5.2. Radiographic Features
In the past, a “metabolic bone survey” that included direct magnification radiography, mammographic techniques, and fine-detail radiography techniques were used for early diagnosis of subtle osseous abnormalities. With advances in digital radiography, routine hand radiographs are now considered the most sensitive and cost effective method for early detection [35].
Increased osteoclastic activity results in range of manifestations and the most common nonspecific feature is generalized demineralization. Bony resorption is more specific for hyperparathyroidism and involves periosteal, intracortical (within Haversian system and Volkmann’s canal), subchondral, trabecular, and subligamentous surfaces.
Subperiosteal resorption is the earliest finding, virtually pathognomonic of hyperparathyroidism, and is most commonly seen as fraying and lace-like appearance of external surface of bone along the radial aspects of middle phalanges of second and third digits and phalangeal tufts [35]. Other sites of subperiosteal resorption include the distal end of the clavicles, medial humerus and tibia, femoral neck, and distal ulna.
Intracortical tunneling is a hallmark of rapid bone turnover and manifests as scalloping along the inner cortical surface, most notable in the second metacarpal, as subtle longitudinal striations. Over time, there is loss of cortical definition and corticomedullary differentiation [35].
Subchondral resorption is polyarticular and leads to collapse of cortical bone that results in initial “pseudo-widening” of joint space, along the iliac side of the sacroiliac joint and the clavicular side of the acromioclavicular joint and symphysis pubis. With continued erosion, there is damage and collapse of articular cartilage and degenerative changes ensue. Subchondral resorption can sometimes be confused with erosions in an inflammatory arthropathy such as rheumatoid arthritis.
Trabecular resorption is most commonly noted in the calvarium and is seen as spotty deossification, loss of a sharp trabecular pattern, and loss of distinction between inner and outer tables resulting in as a ground glass appearance or “salt and pepper” calvarium (Figures 8 and 9) [36, 41]. Subligamentous and subtendinous changes are nonspecific and can be frequently noted in the inferior calcaneus, coracoclavicular ligaments, greater and lesser trochanters, anterior inferior iliac spine, and humeral and ischial tuberosities.
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Brown tumors or osteoclastomas are cyst-like, well circumscribed lytic lesions that commonly occur in the jaw, pelvis, and metaphyses of long bones (Figure 10). They may be eccentric or central, and associated findings include endosteal scalloping and lack of reactive bone formation distinguishing it from an aggressive entity. As brown tumors heal with treatment, they may have calcifications, dense sclerotic appearance, or persist as a lytic lesion [36]. Histologically, they are composed of highly vascular fibrous tissue (osteitis fibrosa cystica) containing giant cells that often hemorrhage (and imparts a brown color) and undergo liquefactive necrosis giving the cystic appearance. In the past, brown tumors were thought to be specific for primary hyperparathyroidism but can also be seen with secondary hyperparathyroidism due to an overall greater prevalence.
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Osteosclerosis is most commonly seen in secondary hyperparathyroidism, especially during the healing phase and predominantly involves the axial skeleton (due to greater amount of cancellous bone) and less frequently the metaphyses and epiphyses of appendicular skeleton (Figure 11). Exact mechanism of osteosclerosis is unclear and is thought to involve PTH stimulated osteoblastic activity either after treatment or as a counter response to resorption where osteoblasts produce excessive nonhydroxyapatite osteoid [41, 42]. Excess accumulation of osteoid appears more opaque than normal bone and results in the classic “rugger-jersey” spine, alternating sclerotic and lucent bands along the inferior and superior endplates of vertebral bodies similar to the stripes on an English rugby shirt [42–44] (Figure 12). Osteosclerosis may be the sole manifestation of renal osteodystrophy and may regress postrenal transplantation [36].
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Soft tissue and vascular calcifications are also seen more frequently in secondary hyperparathyroidism from chronic renal disease (Figures 13 and 14). Metastatic deposition of calcium hydroxyapatite or amorphous calcium phosphate increases as the calcium-phosphorus product increases. Recently, more complex mechanisms including changes in endogenous calcification inhibitors, deficient clearance of calcified debris, and effects of vitamins K and D have also been implicated in determining extraosseous calcification [45].
While visceral calcifications in heart, lung, and stomach may not be radiographically detectable, periarticular, cartilaginous, and vascular calcifications in the tunica media are frequently seen especially in chronic kidney disease. Periarticular calcifications occur about the large joints (hip, shoulder, elbow, and knee), wrists, ankles, and phalangeal joints. Calcifications can be multifocal and symmetric and appear as discrete, cloud-like densities that may extend to the adjacent joint [36].
6. Dialysis
Patients who progress to end-stage renal disease and are placed on dialysis present with a mixed spectrum of manifestations, where some effects of chronic kidney disease improve, while others persist [46–48].
Aluminum intoxication is a commonly noted condition in patients managed with dialysis and is secondary to aluminum in the dialysate or due to oral ingestion of aluminum hydroxide as a phosphate binder. Radiographic findings are nonspecific and include osteomalacia, fractures of the ribs, cervical spine, hip, and pelvis [49]. Amyloidosis causes extraosseous (tendon and ligament deposition) and osseous deposition that result in periarticular erosive osteoarthropathy, destructive spondyloarthropathy, and carpal tunnel syndrome [50, 51]. Erosive arthropathy has been described of the small joints and mimics degenerative osteoarthritis [52]. Ligamentous laxity with superimposed amyloid deposition and hyperparathyroidism results in destructive spondyloarthropathy, most commonly in the cervical and lumbar spine [53, 54]. Osteonecrosis is another serious complication with a multifactorial etiology that commonly involves the femoral and humeral head and condyles [55]. Tendon rupture and musculoskeletal infections are also commonly seen as dialysis or posttransplant related complications [56, 57].
7. Paget’s Disease
7.1. Introduction
In brief, Paget’s disease is caused by defective osteoclastic bone breakdown with compensatory osteoblastic bone formation, resulting in irregular bone remodeling with a disorganized, structurally weak pattern of woven and lamellar bone [13]. Asymmetrical polyostotic involvement is the most common pattern of bone involvement, especially within the axial skeleton, although the disease may affect any bone [58]. Of the vertebrae, the body is almost always involved with occasionally some posterior elements. The lumbar spine is the most frequently affected, followed by the thoracic and then cervical spine [59]. Paget’s disease in long bones does not spread to adjacent bones but instead tends to envelope the original bone entirely if allowed to progress [60].
7.2. Radiographic Features
Paget’s disease, largely asymptomatic, is often an incidental finding on radiography [59]. Radiography can help distinguish osteoblastic versus osteolytic lesions as well as demonstrate characteristic findings such as cortical thickening and tunneling, trabecular thickening, generalized bone enlargement, and deformities. Of note, Paget’s lesions do not cross normal articular surfaces and rarely involve new bone over long periods of time. Although small fissures may form in pagetic bone that appear similar to the pseudofractures of osteomalacia, the fissures in Paget’s disease occur on the convex portion of the bone versus the concave side in osteomalacia [13].
There are three phases of Paget’s disease with unique radiological features, although they can occur in synchrony [61]. The first phase is lytic with an advancing edge of resorption to produce a geographic appearance. This phenomenon in long bones begins in the subchondral epiphysis or metaphysis and extends toward the diaphysis. The advancing front results in a “cutting-cone,” “blade of grass,” or “candle flame” sign [62]. In the calvarium, this process is termed “osteoporosis circumscripta” and tends to affect the frontal and occipital bones with a well-demarcated, oval conformation. It can also cross suture lines. Rarely, the lesions may arise in the diaphysis without epiphyseal involvement, but this pattern is unique to the tibia [63].
The second or mixed lytic-sclerotic stage is usually when Paget’s disease is most commonly diagnosed. In addition to cortical thickening, trabecular coarsening, and osseous expansion, endosteal growth can encroach the medullary cavity to produce loss of corticomedullary differentiation (Figure 15). These mixed lesions produce a “cotton-wool” pattern in the skull and a “picture-frame” pattern in the vertebral column [61].
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The final or sclerotic phase of Paget’s disease is the result of rapid new bone formation causing widespread increase in density, medullary sclerosis, and overall bone size. It should be noted that lesions seen on plain radiographs but not scintigraphy are most likely older or “burned out” lesions [61].
Although many other metabolic bone diseases affect the spine, Paget’s disease can form characteristic vertebrae with coarse, dense trabecular structure, and a more dense periphery on lateral spine radiographs. This is termed the “window pane” appearance in the setting of Paget’s disease. These vertebrae have a reduced height but are increased in diameter due to compromised structural integrity; this fact can be extremely helpful in differentiating pagetoid bone from other bone pathology [2]. Sarcomatous degeneration is a significant complication of Paget’s disease that most commonly arises in the pelvis or femur and is rarely multifocal [64]. Sarcomas generally appear osteolytic on radiography and are accompanied by other radiological signs of malignancy. MRI can be useful to discern sarcomatous degeneration by helping to visualize cortical disruption or effacement of normal structures [65].
7.3. Computed Tomography (CT)
Aside from lesions in complex anatomical areas such as the facial bones, neural foramina, and middle ear, CT provides only limited additional information to warrant usage over plain radiography for uncomplicated Paget’s disease. However, CT along with MRI can be utilized to evaluate possible fractures or sarcomatous degeneration. Lytic lesions are enhanced with IV contrast in both modalities [63].
7.4. Magnetic Resonance (MR) Imaging
The disease has a number of characteristic findings on MR imaging. The cortex appears thickened and striated on 1 sequences. Osteolytic foci have low signal intensity on 1 sequences but high intensity on 2. Additionally, the fatty bone marrow signal is preserved in 1-weighted sequences of uncomplicated Paget’s disease, which is significant because this signal is diminished with complications such as acute fracture or sarcoma [66]. MRI is also useful in assessing nerve compression from expanding bone [67].
7.5. Nuclear Imaging
Bone scintigraphy is one of the principal modalities for assessing the extent of Paget’s disease since it allows an overview of the entire skeleton. In the detection of Paget’s lesions, bone scintigraphy is more sensitive than plain radiography due to the increased perfusion in these foci [61]. Paget’s lesions represent metabolically active areas with high bone turnover that are readily visible due to increased tracer uptake (Figure 16) [13]. Osteoporosis circumscripta may show peripherally increased but centrally decreased tracer uptake. Uptake in long bones may abut one joint and extend into the diaphysis [61].
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On the other hand, lesions with low activity are difficult to detect with radioactive tracers and must require radiography for adequate visualization [66]. Bone scans may also provide a window for earlier detection and treatment in that it can reveal lesions even with normal alkaline phosphatase activity [68].
Only one-third of patients with Paget’s disease exhibit increased FDG uptake, which is why the role of PET is still undetermined. However, because lesions that do appear on PET may cause confusion in patients with concurrent skeletal metastases, it is important to utilize CT to evaluate existing lesions in these patients [69].
8. Osteogenesis Imperfecta
8.1. General Radiographic Manifestations
Osteogenesis imperfecta, or “brittle bone disease,” is an inherited disorder caused by the deficiency or defect of collagen type I resulting in increased bone fragility and decreased bone mass [70]. There are several forms of the disorder that can range widely in morbidity and mortality. The milder forms, such as type I, may cause only minimal bone deformities aside from scoliosis resulting from vertebral fractures [70].
In general, OI is often classified as a metabolic bone disease as radiography reveals patterns of both bone destruction and deposition, although similar patterns are seen in other disorders of the bone that result in fractures in various states of healing (Figure 17). Cortical thinning results from abnormal bone structure, lack of lamellar bone, replacement of compact bone by spongiform bone, and defective endochondral and periosteal bone formation [71]. Cortical thinning can also misleadingly cause bones to appear wider in diameter on radiographs than in actuality. Additionally, long bones may be significantly bowed. Like with osteoporosis, trabeculae appear thin, delicate, and widely spaced [72]. Most OI patients develop secondary osteoporosis, and postmenopausal women can be particularly vulnerable [73].
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In addition, diffuse osteopenia, sclerosis of the vertebral endplates, and biconcave vertebral bodies may result [71]. Vertebral disease often causes severe kyphoscoliosis and platyspondyly or decreased height in the thoracic vertebrae [74]. Similar to other metabolic bone diseases, OI can produce “fish mouth” vertebrae on lateral radiographs due to deformity in vertebral endplates, which causes intervertebral disc spaces to appear as biconvex or ellipsoid areas. Patients with type II OI can develop generalized osteoporosis with minimal vertebral ossification [71]. A small proportion of patients (5%) can also suffer from spondylolisthesis [75]. Milder forms of OI may exhibit only subtle radiographic changes [73].
In some forms of osteogenesis imperfecta, radiographic examination may reveal a dense band adjacent to the growth plate of long bones [73]. Additionally, the interosseous membrane of the forearm may undergo marked calcification, which can restrict forearm rotation in these patients [76]. Abnormal calcification within the middle ear as well as persistent cartilage often causes hearing difficulty and can appear similar to otosclerosis on CT scans [73].
Around one-third to one-half of OI patients may develop dentinogenesis imperfecta, which the teeth appear misshapen and brown or opalescent on exam. Radiographically, the teeth may have bulbous crowns, narrow roots, and narrow or obliterated pulp chambers [73].
8.2. Types of Osteogenesis Imperfecta
There are multiple types of osteogenesis imperfecta. In general, type I is the most common type and is mild without associated deformity, while type II is the least common type and is fatal in the perinatal period. Type III is severely deforming while type IV is deforming but less so than type III [71]. In type I or mild OI, fractures are less common after puberty due to completion of ossification and adult patients may indeed have normal bone density [71]. Type I represents around half of all OI cases and is different from the other phenotypes of the disease in that long bone fractures in these patients tend to heal without major deformity. Although osteopenia is present, long bones in OI type I generally do not bow. However, platyspondyly and scoliosis of approximately 20 degrees can be expected in the vertebral column. Type I patients may have a characteristic triangular face shape resulting from frontal bossing accompanied by a narrowed mandible. DEXA scans can reveal decreased bone mineral density in even mild cases of the disease [73].
On the other hand, type II or lethal OI is characterized by in utero fractures of the skull, vertebrae, and long bones, as well as by decreased vertebral ossification, beaded ribs, and a small chest cage. These patients usually die within one year of birth due to pulmonary hypoplasia or cerebral hemorrhage [71]. Although birth weight and length are small for gestational age, the skull tends to be disproportionately large. Limbs can be shortened and bowed, with legs often abducted perpendicular to the body in a “frog leg” position [73]. Additionally, the thorax can take on a characteristic conical or “beehive” shape in these patients [77]. Type III OI is similar to type II OI in that it is evident at birth, although more severe. Like with type II OI, significant deformity throughout the skeleton may occur. However, unlike type II patients, type III patients lack the beaded appearance of ribs and crumpling of long bones. Type III patients also share the triangular facies seen in type I patients. However, an additional deformity called the “helmet deformity” may arise as a result of cranial molding causing an occipital protrusion [73].
It should be noted that up to 5% patients with OI type V can develop a hyperplastic callus that can mimic osteosarcoma unless further distinguished by MRI or CT [78, 79].
8.3. DEXA and OI
Although it can be difficult to clinically differentiate mild osteogenesis imperfecta from child abuse, dual energy X-ray absorptiometry can be used to aid diagnosis in conjunction with radiography. DEXA of the anteroposterior spine has a sensitivity of 91.7% and DEXA of the whole body has a specificity of 100% in detecting osteogenesis imperfecta [80]. Children with OI may receive annual DEXA scans to assess development and predict fractures, but there is little data on predicted values of bone mineral density in infants with the disease [70]. DEXA is also useful in tracking the effectiveness of therapy such as bisphosphonates [73]. Compared to plain radiography, DEXA has a much lower radiation dose at around 10% of that required for a chest radiography [81]. DEXA and other techniques used to monitor BMD in osteoporosis can be applied to OI patients. It should be noted that the short stature of many OI patients can result in artificially low BMD readings, making them especially difficult to interpret [81].
8.4. Cranial Findings
OI patients may present with a triangular face shape and frontal bossing, which may be accompanied by mandibular malformation, especially prognathism. Accessory or “Wormian” bones along the sutures is caused by impaired membranous and endochondral bone formation. Sutures may also be prematurely closed and may result in brachycephaly (widened frontal fontanel) if the coronal suture fuses prematurely with associated overgrowth of the sagittal and lambdoid sutures [71].
8.5. Neurological Findings
An interesting manifestation of OI is the development of moyamoya disease, which can result in subarachnoid hemorrhage. Additional vascular disease associated with OI includes carotid cavernous fistulas as well as cervical artery dissection and cerebral vessel aneurysms [82, 83]. In utero vascular compression due to poor positioning may result in unilateral cerebellar hypoplasia [84]. OI patients can also have diffusely atrophied cerebral hemispheres and/or hydrocephalus [85, 86]. In fetuses with a familial history of OI, prenatal ultrasound screening has been recommended in the second trimester to detect manifestations such as ventriculomegaly, rhizomelia, and femoral deformity [87]. Intrauterine fractures for the most severe forms of OI can be detected as early as 14 weeks’ gestation while some are not apparent until 20 weeks. However, type I OI fractures can be mild enough to escape detection in utero [73].
8.6. Craniocervical Junction Findings
The softening of the bones in OI patients can result in a number of abnormalities in the craniocervical junction. One manifestation is basilar invagination, which refers to the upward protrusion of the odontoid process into the foramen magnum [87]. Although conventional lateral skull radiography can detect basilar invagination, MRI is the best modality for depicting craniospinal manifestations of this disease, especially in the evaluation of tonsillar herniation and posterior fossa abnormalities [88].
Another deformity, called platybasia or basilar impression, results from the upward infolding of the foramen magnum into the skull, which can cause displacement of the brainstem [71]. It can affect up to 25% of OI patients [73]. Either basilar invagination or basilar impression can result in aqueductal stenosis, hydrocephalus, cord edema, or syrinx [86].
9. Acromegaly
9.1. General
Acromegaly is characterized by excessive growth hormone secretion from a pituitary adenoma and, occasionally, hyperplasia of eosinophilic cells. Growth hormone (GH) produces several systemic manifestations and progressive somatic disfigurement. Main clinical features are prominent forehead, mandibular hyperplasia, thickened tongue and broadening of extremities, and soft tissue thickening. Gigantism results if GH production occurs prior to closure of long bone growth centers. Bitemporal hemianopsia and chronic headaches are effects of progressive growth of adenoma.
Skeletal effect of growth hormone is to stimulate chondrocytes, fibroblasts, and osteocytes which results in new bone formation and consequent irregular thickening of the cortex [35, 47]. Musculotendinous insertions develop bony excrescences and bony spurs which indicate an overall increase in ectopic bone formation. Carpal tunnel syndrome may be a combination of soft tissue hypertrophy and bony overgrowth with edema within the nerve itself [35, 47].
9.2. Radiographic Features
Soft tissue changes are most notable in the hands and feet and heel pad thickness of >20 mm is suggestive, although not pathognomonic of acromegaly [89]. Bony changes include widening of phalangeal shafts and metacarpals, prominent ungual tufts, and bony protuberances [35]. Initially, there is generalized increase in joint space due to cartilage overgrowth, but there is fissuring and ulceration of the cartilage over time and results in degenerative joint disease with bony remodeling, osteophytes, and enthesopathic changes. Joints affected at an early stage include knees, shoulders, elbows, and hips [47].
Skull changes include enlargement and erosion of the sella, widened mandibular angle, hyperplasia of sinuses, occipital protuberance overgrowth, and malocclusion [35, 90]. In the spine, sagittal and transverse dimensions of vertebral bodies increase, whereas vertebral body heights remain unchanged, resulting in platyspondyly [35]. Widened disc heights, widened atlantodental interspace, posterior body scalloping, premature degeneration, and exuberant osteophytes are other classic findings. Thickening of lamina, spinal ligaments, and articular processes may rarely result in spinal stenosis [35].
10. Osteopetrosis
10.1. General
Osteopetrosis, also known as marble bone disease or Albers-Schönberg disease, is a rare hereditary disorder that should be considered in the differential for sclerotic bones. Because it is characterized by defective osteoclastic activity, the disorder is often classified as a metabolic bone disease [91]. Genetic mutations have been identified to be responsible and while several heterogeneous forms of osteopetrosis have been recognized, there are two main forms: infantile autosomal recessive form that is severe, and the benign adult autosomal dominant osteopetrosis [92–94]. Underlying pathogenesis for both entities involves defective osteoclast function that disrupts the normal equilibrium between bone formation and resorption. Continued production of bone results in remodeling of the primary spongiosa and leads to dense, sclerotic bones with weak structural integrity. Defect in remodeling bone at the endosteum results in failure to create normal marrow spaces that clinically manifests as anemia with extramedullary hematopoiesis, or in severe cases, leukemia [94, 95]. Despite the increased density of the bones, decrease in osteoclastic activity leads to lack of structural integrity that predisposes to fractures at sites of stress and heals with abundant callus formation [92, 94].
10.2. Radiologic Features
Radiologic manifestations in the infantile form are severe dental caries, fractures from minor trauma, cranial vault and skull base thickening, diffuse involvement of the axial and appendicular skeleton, and splayed metaphyses that are commonly seen as transverse lucencies [93, 94]. There is loss of corticomedullary differentiation and bones are uniformly dense with smooth margins (Figure 18). Crowding and underdevelopment of marrow spaces are most prominent in the distal femur [94].
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In the benign adult types, two different phenotypes exist [96]. In Type 1 phenotype, there is cranial vault osteosclerosis without involvement of the skull base and limited involvement of the appendicular skeleton and spine [96]. In Type 2, diffuse osteosclerosis with involvement of the skull base is noted. Due to lack of remodeling, continued deposition of bony matrix results in “bone within bone” appearance that is most evident in the spine and ilium [96]. Dense sclerotic appearance of the endplates causes “sandwich” appearance of vertebrae that can be confused with “rugger jersey” spine appearance [96] (Figure 19). Although the two entities are difficult to differentiate, osteopetrosis generally causes a more defined and dense appearance of the endplates. Treatment is focused on symptomatic relief, with stem cell transplantation reserved for the more severe forms [93].
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11. Conclusion
Metabolic bone disease is a diverse spectrum of pathology affecting bone homeostasis. Imaging is essential for the diagnosis, evaluation, and treatment of these patients. Knowledge of the diseases, recognition of the imaging findings, and expertise to appropriately guide the imaging evaluation and treatment are fundamental to the role of the radiologist.
Conflict of Interests
The authors declare that they have no conflict of interests regarding the publication of this paper.