1.15.1 Insufficiency fractures of the pelvis: metabolic and nonoperative workup

10.1055/b-0035-121639

1.15.1 Insufficiency fractures of the pelvis: metabolic and nonoperative workup

Aasis Unnanuntana, Anas Saleh, Joseph M Lane

1 Introduction

Insufficiency fractures of the pelvis, including the sacrum, are usually present in elderly patients and are the result of normal stress placed on abnormal bone. This condition is distinct from a stress fracture, in which normal bone is subjected to abnormal or unaccustomed stresses, resulting in fracture. Although both insufficiency and pathological fractures occur when normal or physiological stress is applied to weakened bone, the term “insufficiency fracture” is limited to any fracture in bone with deficient elastic resistance by a nontumorous condition [1]. The majority of patients with pelvic and sacral insufficiency fractures are elderly women, presenting with intractable low back pain coupled with a significant reduction in mobility. The fracture occurs either spontaneously or after a trivial episode of trauma [2]. Insufficiency fracture has been shown to be associated with a number of conditions that have lowered fatigue strength of the skeleton such as osteoporosis, osteomalacia, long-term steroid use, or pelvic irradiation [3–7]. Because insufficiency fractures around the pelvis and sacrum are difficult to detect from plain x-rays, the diagnosis is usually delayed. Lack of clinical suspicion delays diagnosis and precludes prompt management.

The incidence of pelvic insufficiency fractures is unknown; however, it is apparent that these fractures are more common than widely assumed and remain largely underdiagnosed. The body of the pubis including the superior and inferior pubic rami is typically involved [8, 9], whereas insufficiency fracture of the iliac bone is less common [10, 11]. Similar to the pelvis, the actual incidence of sacral insufficiency fracture is unknown because recognition and diagnosis have not been frequently emphasized. Finiels et al [12] reported 508 patients with sacral insufficiency fractures: 493 were reported in the literature while 15 came from the authors’ case series. The incidence of sacral insufficiency fracture is, however, suspected to be higher because these fractures are an unrecognized cause of back pain in elderly patients [7, 13]. In addition, sacral fractures usually coexist with pelvic insufficiency fractures but are often missed on pelvic x-rays [14–16]. Sacral fractures may precede or occur simultaneously with pubic fractures, although the chronology may be indeterminable [14]. In a prospective study [15] of women older than 55 years who presented to the authors’ hospital with low back pain, 20 (2.0%) of 1,015 patients were diagnosed with sacral insufficiency fracture. Of these 20 patients with sacral insufficiency fracture, 9 patients (45%) had an associated pubic rami fracture.

Pelvic insufficiency fracture is an important problem. Breuil et al [8] reported that osteoporotic pelvic fractures share most characteristics of hip fractures including a high morbidity and mortality and loss of autonomy. Since the majority of patients with insufficiency fractures are treated by orthopedic surgeons, it is crucial that all orthopedic surgeons have a basic understanding of the principles involved in the care of these patients to provide appropriate management and prevent future fractures. This chapter primarily focuses on the evaluation and nonoperative treatment of patients with insufficiency fractures of the pelvis and sacrum. Subjects for discussion include clinical, radiographical, and laboratory assessments. In addition, medical management is addressed.

2 Etiology and pathogenesis

While the specific etiology of insufficiency fractures is not well delineated, it is believed that the intrinsic factor affecting bone strength plays a major role in the pathogenesis of these fractures. Bone strength reflects the integration of three factors: quantity, quality, and turnover [17]. Bone mineral density (BMD), measured by dual-energy x-ray absorptiometry, reflects bone quantity. Bone quality is a function of the structural and material properties of bone. The structural properties include bone geometry (size and shape of the skeleton) and microarchitecture, whereas the material properties include the organization and composition of the mineral and collagen components of the extracellular matrix, as well as the extent of microdamage within the tissue [18]. In general, bone undergoes continuous renewal by the process of coupled bone resorption and formation, known as bone remodeling or bone turnover. The balance between bone resorption and bone formation allows the bone to remove fatigue damage and replace it with new bone that reinforces the bone integrity. An imbalance between bone resorption and bone formation ultimately results in a net loss or gain of the bone tissue. The process of bone turnover influences both BMD and bone quality and consequently affects the bone strength. Since insufficiency fractures occur in weakened bone, any diseases or conditions that negatively affect bone strength increase the susceptibility of bone to fracture ( Table 1.15.1-1 ).

**Table 1.15.1-1** Diseases and conditions associated with pelvic and sacral insufficiency fractures.
Groups	Diseases/conditions
Imbalance of bone remodeling [9, 12, 15]	Primary osteoporosis
Radiation [19]	Pelvic radiotherapy for cancer of the genitourinary and genitointestinal tract systems
Inflammatory diseases [21]	Rheumatoid arthritis
Disorders of bone mineral homeostasis [8, 24–27]	Rickets and osteomalacia Renal osteodystrophy
Endocrine abnormalities [8]	Hyperparathyroidism Hyperthyroidism Cushing′s syndrome Hypogonadism
Tumor [2]	Multiple myeloma
Drugs [22, 27]	Corticosteroids Antiepileptics
Other [28, 29]	Following total joint arthroplasty

Primary osteoporosis has been reported as the most common risk factor for pelvic insufficiency fractures [8]. Weber et al [15] reported 20 patients with sacral insufficiency fractures; twelve of these patients (60%) had primary osteoporosis. However, osteoporosis was not diagnosed by dual-energy x-ray absorptiometry [15]. Similarly, the literature showed a 32–72% rate of osteoporosis in patients with insufficiency fractures [9, 12, 15]. Pelvic insufficiency fractures are also commonly found in patients receiving radiotherapy, with the fracture rate ranging from 2–89% [19]. Suggested mechanisms include direct effect of radiation on osteoclasts and osteoblasts, as well as compromising the microvascular supply to the bone cells [20]. Rheumatoid arthritis is another risk factor for sacral insufficiency fractures. In one study [21] investigating patients diagnosed with rheumatoid arthritis, 4 (about 1%) of 386 patients developed sacral insufficiency fractures during a period of 6 years follow-up. Although the effect of rheumatoid arthritis on bone is not well understood, it is postulated that these patients represent a unique population with a high risk of insufficiency fractures due to their disease processes or the antirheumatic medications [22]. Since bone mineralization is one of the major determinants in bone quality, diseases that interfere with mineral homeostasis will adversely affect bone quality, leading to fragility fractures. Rickets and osteomalacia from vitamin D deficiency are disorders of bone mineral metabolism characterized by an accumulation of unmineralized osteoid on bone trabeculae as a result of impaired mineral deposition [18]. Low levels of vitamin D result in decreased absorption of calcium across the intestinal tract, causing a decline in the concentration of serum calcium [23]. A retrospective study in patients with osteoporotic pelvic fractures showed that 25 (80.6%) of 31 patients who had serum vitamin D available were diagnosed with vitamin D deficiency (defined as 25-hydroxyvitamin D [25(OH) D] < 20 ng/mL) [8]. In addition, diseases with abnormal bone turnover state, such as renal osteodystrophy or some endocrine abnormalities, are characterized by increased activity of the osteoclasts [24–27] ( Table 1.15.1-1 ). The bone remodeling process is therefore shifted toward bone resorption, resulting in an imbalance of bone turnover that causes fragility fracture. Furthermore, pelvic insufficiency fractures may occur shortly after total joint arthroplasty of the hip or knee [28, 29]. This is possibly due to the limited activity of these patients preoperatively and the subsequent resumption of activity at a much higher level following surgery.

3 Assessment and medical evaluation

A thorough clinical history and physical examination are essential in the diagnosis of pelvic and sacral insufficiency fractures. The majority of patients with these fractures present with no history or trivial history of trauma [2]. If the fracture occurs in the pubis, the presenting symptom is usually a dull aching pain over the groin, perineal region, or thigh [30]. Patients with sacral insufficiency fractures present with low back pain, with or without radiation into the buttock [15, 31, 32]. On physical examination, there is tenderness on direct palpation over the fracture site. Patients with fracture of the pubic rami may have a noticeable limp with walking [33]. The hip range of motion may be normal or decreased. A bilateral examination to compare with the unaffected side is necessary to reveal subtle tenderness [34]. For sacral insufficiency fracture, direct palpation over the sacrum may exacerbate the pain. If the fracture involves sacroiliac joint, there may also be a positive FABER test (flexion, external rotation of the affected hip) or positive flamingo test, which is painful when standing on one leg at the affected side [35]. Pain with axial loading, such as a heel strike maneuver, may signify an anterior fracture [36]. A neurological examination should always be included to identify if there is any injury to the lumbosacral nerve roots. In general, neurological complications after pelvic and sacral insufficiency fractures are rare.

When an insufficiency fracture occurs, this indicates there has been a reduction in bone strength. Therefore, all patients with insufficiency fractures should be evaluated and screened for disorders that can lead to fracture, such as osteoporosis. Initial assessment of patients with insufficiency fractures of the pelvis should include a comprehensive medical history and physical examination of the patient. Identifying risk factors that contribute to fragility fractures should be the primary aim of the history interview. These include the patient′s age, body weight, medical history, history of fractures, medications, family history of fragility fractures/osteoporosis, smoking habits, and alcohol consumption. A history of radiation around the pelvis is also important. The physical examination for patients with pelvic insufficiency fractures should focus on:

Balance and gait disturbances that could increase the risk of falls
Clinical signs of potential collagen disorders, such as joint laxity, skin hyperextensibility, blue sclera, and other skeletal abnormalities.

**Fig 1.15.1-1** Lateral x-ray of the pelvis shows a transverse fracture of the sacrum (white arrows). (Courtesy of Keith Hentel.)

3.1 Radiographic evaluation

For patients with suspected pelvic and sacral insufficiency fractures, plain x-rays of the pelvis including AP, inlet (35–40° of caudal tilt of the x-ray tube) and outlet (45° of cranial tilt of the x-ray tube) views should be obtained [34, 37]. Lateral x-ray can help diagnose transverse fractures of the sacrum ( Fig 1.15.1-1 ). Once a pelvic insufficiency fracture has been identified, it is important to carefully assess the entire pelvic ring for a second lesion as well as for the presence of a concurrent sacral insufficiency fracture [14, 15]. Diagnosis based on plain x-rays is, however, variable and sometimes not appreciated until late in the course when the process of fracture healing has begun. Plain AP and lateral x-rays of the pelvis are usually not helpful in visualizing fractures of the sacrum. Fracture configuration is often overlaid by soft-tissue shadows and bowel gas [38]. In addition, lumbar lordosis and kyphotic sagittal contour of the sacrum make the fracture lines oblique to the plane of the x-ray [37]. Denis et al [39] retrospectively reviewed the x-rays of 236 patients with sacral fractures and found that up to 50% of the patients in their series who were neurologically intact had a delay in diagnosis of the sacral fracture. Therefore, in the settings where there is a high index of suspicion for pelvic and sacral insufficiency fractures, further imaging studies are recommended.

Computed tomography (CT) scanning has become an important adjunct to plain x-ray for evaluation of both pelvic and sacral insufficiency fractures. Many investigators suggest CT scan as a standard tool for diagnosis of pelvic and sacral fractures because areas of sclerosis representing the reparative process are clearly seen and may show occult fracture lines that are not visualized on plain x-rays ( Fig 1.15.1-2 ) [16, 40, 41]. However, it is important to emphasize that proper alignment of the gantry and thin cuts (2 mm) are critical to obtain the maximal amount of data from the study. Due to its parallel to the axial plane of the primary CT scan, transverse sacral fracture is usually difficult to delineate, and thus sagittal and coronal CT scan reconstruction are required to demonstrate the transverse fracture component [42, 43].

Magnetic resonance imaging (MRI) has been shown to be highly sensitive for the detection of insufficiency fractures. Rizzo et al [44] showed that MRI is more sensitive than bone scintigraphy in the early stages of insufficiency fractures. Magnetic resonance imaging patterns of insufficiency fractures have been described as soft-tissue swelling, cortical and medullary bone edema, and the presence or absence of a distinct fracture line [45]. Generally, fracture lines are defined as bands of decreased signal intensity on T1-weighted images. Whereas on T2-weighted and short tau inversion recovery (STIR) images, the lines can be seen as areas of edema ( Fig 1.15.1-3 ). Because of its high sensitivity, MRI should be considered as the primary diagnostic technique when insufficiency pelvic and sacral fractures are suspected [46].

**Fig 1.15.1-2a–b** A 90-year-old woman presented with acute low back pain without history of significant trauma. a AP x-ray of the pelvis shows no evidence of fracture or dislocation. b A xial CT scan image reveals a left sacral fracture involving the anterior aspect of the left sacral ala (white arrow). (Courtesy of Keith Hentel.)

**Fig 1.15.1-3a–e** A 67-year-old woman with a history of multiple sclerosis and long-term corticosteroid use presented with worsening hip and buttock pain after falling from a standing height. No pelvic fractures were identified on the three views of the pelvis. a AP view. b Outlet view. c Inlet view. **d–e** Arrows indicate a vertically oriented area of decreased signal intensity on MRI: axial T1-weighted image and STIR image. (Courtesy of Keith Hentel.)

A technetium-99m bone scan is also a helpful tool to diagnose insufficiency fractures [16, 47, 48]. Scintigraphic images may be obtained at 2 hours postinjection [49]. Multiple views must be obtained to adequately evaluate the affected areas. Standard imaging for a bone scan includes anterior and posterior projections, with the posterior view being more effective in demonstrating activity in these fractures. An outlet view should be considered if there is residual contrast in the bladder [34, 37]. A characteristic H- or butterfly-shaped appearance can be identified in patients with sacral insufficiency fractures ( Fig 1.15.1-4 ). Although a bone scan is highly sensitive, it is not specific, and accurate confirmation of the fracture can be achieved with other imaging studies. Nevertheless, bone scan is a sensitive imaging tool, which is widely available and can be used as an initial study when MRI is contraindicated. Because the bone metabolism of patients who have received chemotherapy or radiation treatment is reduced, the sensitivity of bone scintigraphy to detect the insufficiency lesions in this particular group of patients is limited [44].