9 Sacroplasty: Management of Sacral Insufficiency Fractures

10.1055/b-0040-175458

9 Sacroplasty: Management of Sacral Insufficiency Fractures

Kieran Murphy

Summary

Sacral insufficiency fractures are relatively common but are often underdiagnosed and undertreated. These fractures are a frequent cause of low back pain in the elderly patient population but are frequently missed on X-ray and cross sectional evaluation of the lumbosacral spine. The typical sacral insufficiency fractures (SIFs) characteristically involve the sacral ala and the S2 vertebral body. When SIFs are suspected, MR imaging is the single best imaging modality for diagnosing these fractures but a combination of nuclear medicine bone scan and CT can be used in patients who cannot have an magnetic resonance imaging (MRI). The management of SIFs should consist of treatments that allow for early and sustained mobilization of the patient including sacroplasty if necessary. There are various techniques and different ways of performing sacroplasty including the short-axis technique, long-axis technique, lateral approach technique, and the three needle technique. All of these techniques have been shown to be safe and effective and studies of sacroplasty have confirmed its safety and efficacy out to as long as ten years. Providing the option of sacroplasty to those patients who do not or cannot undergo non-surgical management can lead to better mobility, improved function, dramatically decreased pain, and less mortality.

9.1 Introduction

First described by Lourie et al in 1982, ¹ SIFs are a common but often underdiagnosed source of low back pain in the elderly and/or the osteoporotic patient. Insufficiency fractures of the pelvis are a consequence of undue stress onto a weakened bone or of increased stress on a bone with marginally adequate bone mineral density. Risks of SIFs are very similar to that of vertebral compression fractures and include risk factors of osteoporosis, osteopenia, osteomalacia, renal osteodystrophy, prior radiation therapy, Paget’s disease, previous lumbosacral fusion, total hip arthroplasty, and bone metabolic diseases known to weaken the skeletal system. ¹ ^, ² Osteoporosis is the most common cause of fractures of the pelvis and over 25 million people are affected in the United States. There is a strong female predominance for SIFs (10:1), and they can occur without an identifiable injury in someone with low bone mineral density and compromised bone strength. ³ The incidence of SIFs comprises approximately 1 to 2% of the pathologic fractures involving the spine and pelvis. This incidence, however, may be an underestimate as the diagnosis is often delayed or does not happen due to the relatively poor sensitivity of plain radiographs and the lack of adequate recognition of SIFs on cross-sectional imaging including both MR and CT imaging.

9.2 Fractures of the Sacrum: Anatomy

Anatomically, the sacrum is comprised of five fused segments. Weight transfers can occur through the lumbar spine into the sacrum and then through the ilium into the proximal femora with regions of stress in the second sacral segment and in the pubic rami. This stress pattern can produce insufficiency fractures that can have a unique but often characteristic appearance of SIFs. ⁴ In 1988, Denis et al classified the location of sacral fractures into three sacral zones (▶Fig. 9.1). ⁵ Zone 1 fractures are laterally located fractures that involve the sacral ala but do not traverse the foramina or the central sacral canal. Zone 2 fractures involve the sacral foramina but do not involve the central spinal canal. Zone 3 fractures extend into and involve the central spinal canal. A fracture to the central portion of the sacrum is typically associated with a higher energy type injury and patients with zone 3 fractures can present with saddle anesthesia and loss of sphincter tone as a result of cauda equina injury or can present with varying degrees of injuries including neuropraxia or injury to a single nerve root. ⁵ The most common location of SIFs are within zone 1, but a minority of the fractures can involve the sacral foramina. The SIFs typically run parallel to the sacroiliac joint along the entire sacral ala and can have a horizontal component that is usually located at the S2 level of the sacrum when present (▶Fig. 9.2). The most common fracture pattern is that of bilateral sacral alar fractures that primarily involve the S1 and S2 vertebrae. Unilateral fractures can also occur and can be present without a contralateral fracture or can progress to bilateral fractures. The appearance and

**Fig. 9.1** Denis Fracture Classification. Zone 1 fractures are laterally located fractures that involve the sacral ala but do not traverse the foramina or central sacral canal. Zone 2 fractures involve the sacral foramina but do not involve the central spinal canal. Zone 3 fractures extend into and involve the central spinal canal. Image created with BioRender.

**Fig. 9.2** The sacral insufficiency fractures typically run parallel to the sacroiliac joint along the entire sacral ala and can have a horizontal component that is usually located at the S2 level of the sacrum when present.

9.3 Sacral Insufficiency Fractures: Causes and Natural History

There are some studies that have examined the morbidity and mortality associated with SIFs. Park et al recently published their experience, looking at 325 patients with a mean follow-up of 51.5 months. ⁶ The mean age at time of diagnosis was 69.4 years. There was a history of malignancy in 43.1% of patients and 21.8% had undergone pelvic irradiation prior to the fracture. The 6-month mortality rate was 9.8% and the 1-year mortality rate was 17.5%, while mortality increased to 25.5% at 3 years. The sex- and age-adjusted mortality ratio increased after these fractures, and the overall 3-month standardized mortality ratio (SMR) was 8.9.

The major cause of SIFs is normal stress on an osteoporotic or otherwise weakened bone. Traumas, both minor and severe, are also causes of sacral fractures, but SIFs are a low-velocity, low-energy type of fractures that develop due to normal or slightly increased stress on a weak bone. When sacral fractures are associated with some type of inciting event, it is typically a low-velocity injury such as a fall on the sacrum and coccyx from a standing height or a minimal axial load impact like stepping off a curb. Patients will often notice a severe and immediate pain in their back or buttock. ⁷ The pain is often not severe and can be described as a dull ache that increases in severity over time. Other patients will feel pain primarily when sitting and will shift in their seats because their back and buttock hurts. Patients often develop an antalgic gait pattern due to the forces transmitted across the sacral ala that increase when the patient is walking. This pain may be quite debilitating and the patient may require a cane or walker to ambulate or may even be confined to a stretcher. Other patients with SIFs have severe localized pain in the low back or sacrum with a sciatica-type pain, usually in an S1 distribution.

9.4 History and Examination

Determining the cause and contributing factors to the SIFs is important and appropriate history taking is an essential element in identifying the risk factors as well as the inciting event. Important factors are whether the patient is on medications for osteoporosis, ⁸ whether they have had a recent dual energy X-ray absorptiometry (DEXA) scan or quantitative CT (QCT) to determine bone density, and if they have had previous fractures of the wrist, hip, or spine. Appropriate imaging studies are also necessary to evaluate the fracture pattern and surrounding anatomy. ⁴ A thorough laboratory evaluation should include a complete blood count and an extended blood chemistry along with measurements of ionized calcium, parathyroid hormone (PTH), and 1, 25-hydroxyvitamin D levels, alkaline phosphatase, albumin, free testosterone, thyroid-stimulating hormone, and serum protein electrophoresis. Other laboratory work for an osteoporosis should include urinary measurements of calcium and cortisol and a urinary protein electrophoresis. Obtaining a history of malignancy is also important and consideration should be given to whether the SIF may be related to a neoplastic process. If this is a concern, performing a biopsy during the process to treat the sacral fracture is warranted.

Physical examination techniques are also important in the diagnostic process and include the standing leg test and the presence of tenderness of the sacrum with compression of the pelvis. Typically, there is no muscle weakness or reflex changes and normal sphincter tone is most often present, but abnormalities of any of these must be documented. Patients with SIFs may be able to walk into the physician’s clinic independently or with the aid of a walker or wheelchair and on a stretcher in some cases. If they are walking without assist devices, they most often have a slow antalgic gait. It is not unusual for older patients or patients with more severe SIFs to be unable to walk. location of the fractures must be noted as well as the anatomic regions affected, as these factors can have significant implications in the repair of these fractures.

9.5 Imaging

If an SIF is suspected by history and/or physical examination, then the next step is to obtain the appropriate imaging. Spinal and/or pelvic radiographs have poor sensitivity and are of questionable value in the diagnosis of SIFs. The best imaging modality that yields the highest sensitivity and specificity is MR imaging ⁹ . Fractures of the sacrum are best shown on dedicated studies of the sacrum and on the axial and coronal images. Traditionally, MR imaging examinations of the lumbar spine may miss a fracture of the sacrum as the axial images may not extend far enough inferiorly to see the fractures in this plane and the sagittal images may not optimally show the fractures or the fractures may be confused with the sacroiliac joint. They can be seen on lumbar imaging, however, if close attention is paid to the symptoms and the findings. ⁹ Short tau inversion recovery (STIR) and non–fat saturated T1-weighted MR imaging sequences are the best imaging to diagnose these fractures as edema from the fractures are well seen on the STIR images and the fracture lines are most optimally demonstrated on the T1-weighted images (▶Fig. 9.3).

**Fig. 9.3** Axial T1-weighted **(a)** and short tau inversion recovery (STIR; b) MR images show bilateral fracture lines (*white arrows* in a) that are well demonstrated on this T1-weighted image. The bilateral sacral insufficiency fractures are seen to be associated with considerable edema (*white arrowheads* in b) on the coronal STIR MR image.

Traditionally, the most sensitive but not specific imaging modality is a nuclear medicine bone scan ¹⁰ . A bone scan has increased activity of the radiotracer within the fracture and the pattern has classically been described as Honda’s sign, a typical H-shaped pattern on bone scan that is pathognomonic for SIFs (▶Fig. 9.4). The pattern of increased uptake on the bone scan represents increased activity that is vertically oriented in the sacral ala and is joined by horizontal activity in the mid to upper portion of the sacrum usually located at the S2 level of the sacrum. ¹⁰ The H shape with the vertical components narrower at the inferior portion and wider at the superior portion resembles the shape of the H on the front of Honda automobiles (▶Fig. 9.4). A cross-sectional evaluation of a bone scan or a single-photon emission computed tomography (SPECT) scan may also be helpful in identifying and localizing SIFs. Despite the sensitivity of bone scans in detecting SIFs, MRI has replaced this modality in many departments, due largely to better availability of MR imaging.

**Fig. 9.4** Nuclear medicine bone scan showing increase uptake of radiotracer in an H pattern (*white lines*) that resembles the shape of the H on the front of Honda automobiles.

If a patient has a pacemaker, cochlear implant, or something else that precludes from them getting an MRI, a CT scan is the necessary examination to compliment the bone scan. Any cross-sectional imaging examination may not highlight sacral fractures optimally and although CT scans can be more sensitive in detecting cortical breaks associated with SIFs, nondisplaced fractures without obvious reactive sclerosis may be missed ¹¹ (▶Fig. 9.5).

**Fig. 9.5** Axial CT image shows bilateral sacral fractures with fracture lines (*white arrows*) causing cortical and cancellous bone disruption. This disruption is seen without much surrounding sclerosis, thereby making these fractures more subtle than those with a large amount of sclerosis surrounding the fractures.

9.6 Treatment

9.6.1 Conservative Treatment

The treatment of SIFs either can be interventional with a percutaneous approach, surgical, or can consist of nonsurgical management (NSM). Unstable fractures, especially high-velocity injuries with associated cauda equina syndrome, may require open reduction and internal fixation. As opposed to closed or percutaneous procedures, open procedures are associated with an increased risk of surgical and postsurgical complications including wound problems and infection. ¹²

Historically, treatment has been limited to bed rest, oral narcotic medications, lumbosacral or pelvic corsets, and protected weight bearing with the patient using a walker in order to accomplish early mobilization. ⁴ ^, ¹³ If early mobilization is not possible, prolonged bed rest can lead to additional morbidities including deep venous thromboses, pulmonary emboli, reduced muscle strength, postural hypotension, impaired cardiac function, atelectasis, pneumonia, pressure ulcers, constipation, fecal impaction, depression, and side effects from opioids including altered sensorium, additional constipation, memory loss, and putting the patient at an increased risk for falling. ¹⁴ These morbidities are profoundly impactful and are known complications of periods of inactivity. In addition to the substantial impact of the morbidities associated with SIFs, the associated mortality is also significant, with studies estimating the 1- to 3-year mortality at 20 to 25%. ⁶ ^, ¹⁵ When patients respond promptly to NSM, the initial clinical improvement may occur quickly, but it is important to know that compete resolution of symptoms may not occur for up to 9 to 12 months. All these factors contribute to a clinical scenario that can be arduous for the patient to tolerate with the goal being to prevent or limit the spiral of immobility, followed by bed rest, deconditioning, secondary events (e.g., infections, venous thromboembolism), and other sequelae of the initial event. ¹⁶ In patients for whom the symptoms are sufficiently severe or debilitating, invasive treatments may benefit them and prevent the incapacitation that is often seen in patients with SIFs.