INTRODUCTION
Sacral stress fractures are divided into fatigue fractures and insufficiency fractures (1 – 3). It is crucial to note the difference between the two, due to the different pathophysiological processes they are based upon and the different approaches to their treatment.
Fatigue fractures occur due to prolonged repetition of abnormal stresses on normal and healthy bone (usually related to sports or one’s profession) or normal stresses in abnormal biomechanics (e.g., coxa vara). In both of the aforementioned cases, bone damage occurs which exceeds the bone’s ability to restore itself. Following that, there is an increased osteoclastic activity and cortical bone resorption and, finally, the occurrence of fracture. Fatigue fractures most often occur in lower extremities (4, 5).
Insufficiency fractures occur under normal stresses, are more common in the elderly, they are usually associated with osteoporosis, and typically occur in and around the pelvic bones (6). In addition to old age and osteoporosis, other risk factors for the occurrence of insufficiency fractures include: long-term glucocorticoid and bisphosphonates therapy, long-term vitamin D insufficiency and osteomalacia, renal osteodystrophy, primary hyperparathyroidism, Paget’s disease, long-term immobilisation and radiotherapy for the treatment of malignant diseases (6, 7).
Sacral insufficiency fractures which occur spontaneously, with no underlying trauma, are often overlooked causes of nonspecific lumbosacral back pain in old age, especially in elderly women with underlying osteoporosis (6). There are no typical clinical signs to suggest sacral stress fracture, which is why the diagnosis is made late, and they are most often revealed as an accidental finding (8). These fractures are often associated with degenerative diseases of the spine and intervertebral disc, and they are often accompanied by pre-existing osteoporotic vertebral compression fractures, which makes the diagnosis even more difficult (9). Plain radiographs of the spine and pelvis, which are the first step in diagnostic imaging, are usually non-diagnostic when it comes to sacral stress fractures (10, 11). Due to radicular pain symptoms caused by degenerative changes, magnetic resonance imaging (MRI) of the lumbosacral (LS) spine is the next step in diagnosis, in which sacral fractures are often revealed as accidental findings. MRI of the pelvis is the most sensitive test in the diagnosis of sacral fractures. Computerised tomography (CT) of the pelvis and bone scintigraphy are additional diagnostic methods.
Treatment is mainly conservative, especially when it comes to fatigue fractures (2). Surgical procedure is indicated if there is severe sacroiliac dysfunction and instability (2).
In recent times, sacroplasty is increasingly being used in the treatment of sacral fractures, as a minimally invasive procedure (12).
CASE REPORT
The patient M. L., a 77-year-old woman, was admitted to Clinical Hospital Centre Rijeka for an examination due to pain in her lower back and hips. The patient’s medical history revealed that 5 months prior to this examination, the patient’s plain radiographs of the LS spine showed vertebral compression fracture of the second lumbar vertebra (L2) without any previous trauma or fall. Due to end-stage renal disease, the patient was treated with chronic (long-term) haemodialysis for 4 years.
During the clinical examination, osteoporotic posture was evident, in the form of a short torso in relation to the extremities and a hunched posture. Kidney percussion came back positive. The patient was walking, by making small steps on a wide surface, without using any aids. The neurological status of the patient showed normal muscle strength in the patient’s legs, with preserved reflexes and superficial sensation. The Oswestry Low Back Pain Disability Questionnaire, used for the measurement of the patient’s functional disability, showed that the patient had severe functional disability, at the score of 57.7% (13).
Bone densitometry (DXA) findings showed osteoporosis with a measured T-value of –5.4 at the spine, at the vertebral level L1 – L4, and –3.0 at the left hip in total, which confirmed the diagnosis of severe osteoporosis with vertebral compression fracture of the second lumbar vertebra (L2).
Due to chronic (long-term) haemodialysis, which had to be done for the treatment of end-stage renal disease, the osteoporosis treatment in the case of this patient was limited to a replacement treatment with an active form of vitamin D (calcitriol). The patient was put on a physical therapy programme, but there was no improvement with regard to the reduction of pain intensity, nor was there any improvement in the patient’s functional ability, measured by the Oswestry Low Back Pain Disability Questionnaire, so, for the aforementioned reasons, additional radiological tests were performed. Plain radiographs of the thoracic and lumbar spine showed no new vertebral fractures, but they have confirmed the diagnosis of vertebral compression fracture of the second lumbar vertebra (L2) (vertebral body compression corresponding to grade II according to the Genant classification of vertebral fractures) (Figure 1). Plain radiographs of the pelvis have shown degenerative changes in both coxofemoral joints with no other signs of pathological remodelling (Figure 2), and the superposition of bowel loops in the projection of the sacrum made it impossible to give a more precise analysis of the bony structure of the sacrum. Due to persistent pain, during further processing an MRI of the LS spine was performed, which showed advanced degenerative changes with neural foraminal stenosis and, with previously confirmed compression fracture of the lumbar vertebra L2 (grade II according to the Genant classification of vertebral fractures), as well as the fracture of the lumbar vertebra L4 (grade I according to the Genant classification of vertebral fractures) which occurred long before this examination. In addition to that, spondylolisthesis of L5 vertebra (grade I) was observed, caused by hypertrophic osteoarthropathy of the intervertebral joints at the same level. At the L3–L5 level, due to the rotation of the vertebra in the sagittal plane, the results of the plain radiographs gave the impression of spondylolisthesis of the L3 vertebra, but the anterior median line was not disturbed, which was confirmed by the MRI. An unexpected finding during the MRI was an extensive oedema of the spongy bone found in the lateral masses of the sacrum (Figure 3). In addition to the extensive oedema of the surrounding spongy bone (Figure 4), which was shown through the fat suppression technique, bilateral linear zones of low signal intensity were observed on the axial sections of the T1-weighted image. This kind of finding is typical for sacral stress fractures. Due to multiple comorbidities, the patient was treated with conservative methods of treatment, with an emphasis on bed rest. During the following two years, the patient went for regular check-ups. The patient has successfully completed another cycle of physical therapy, which included individual medical exercises, as well as electroanalgesia. The Oswestry Low Back Pain Disability Questionnaire, used for the measurement of the patient’s functional disability showed, that the patient’s functional disability has improved, and that it was now at the score of 48.88%.
DISCUSSION
Sacral insufficiency fractures can cause low back and pelvic pain which contribute to the debilitation of the patient. These fractures can occur spontaneously or due to small, underlying trauma. The occurrence of pain is usually acute. Pain is mechanical in nature, which means that is subsides when the patient is resting, and it becomes more intense when stress and strain are placed on the muscles (such as standing or walking), so in this case moving is usually significantly more difficult. In rare cases, the pain may be clearly localised in the sacral region. At times, patients may experience radicular symptoms in their legs. Therefore, it is difficult to diagnose a patient based on their medical history and clinical examination and the diagnosis is often made late or delayed (9).
Diagnosis is further complicated by the fact that sacral fractures are not usually shown on a plain radiograph of the lumbar spine and pelvis, and the fact that degenerative changes of the spine, osteoporosis and osteoporotic vertebral fractures are common findings, which can lead to misdiagnosis and complicate the diagnosis of sacral insufficiency fractures (14, 15).
When it comes to risk factors, the patient whose case is described in this case report had the factors of old age and severe osteoporosis with osteoporotic vertebral fractures as well as chronic end-stage renal disease due to which she had to be treated with haemodialysis. Low back pain was additionally caused by degenerative changes and instability of the lumbar spine. Considering the fact that the pain did not subside after the applied physical therapy, the patient was subjected to further diagnostic processing. Standard radiographic testing typically did not indicate a sacral fracture. However, what did indicate this was an accidental finding on the MRI of the LS spine. In the study conducted by Cabarrus et al., out of 108 insufficiency fractures in the pelvic region, which were detected with a pelvic MRI or LS spine MRI, only 16 (14.8%) of those fractures were diagnosed by plain radiographs of the pelvis or LS spine, and only 3.8% (2 out of 53) of them were found to be sacral fractures. This fact clearly confirms that, in most cases, it is not possible to diagnose or rule out pelvic fractures, least of all fractures in the sacral area, through the use of plain radiographs. On a plain radiograph, the presence of a sclerotic line in the bone, a cortical rupture, or a visible fracture gap indicate an insufficiency fracture (15).
Sacral fractures, in addition to insufficiency fractures, are often experienced by athletes, as fatigue fractures that occur due to stress that is placed on normal bone (4, 5, 16). When it comes to insufficiency fractures in the pelvic area, in 70% of patients two or more fractures are present at the same time (15, 17, 18). In 88% of patients, sacral fractures are associated with pubic rami fractures and / or parasymphyseal fractures (17), and somewhat less frequently, with acetabular fractures and fractures in the area of the wing (ala) of the ilium (18).
According to De Smet et al., there are initial changes in the sacrum with consequent mechanical stress on other pelvic bone structures (17).
Sacral fractures are also one of the possible complications after surgical stabilisation of the lumbar spine (19, 20). A sacral fracture associated with grade II spondylolisthesis at the L5–S1 level, caused by spondylolysis, which, most likely, occurred as a consequence of pathological anterior shear force, has also been described (21). Considering that sacral fracture is one of the possible complications after surgical stabilisation of the spine, and due to the low sensitivity of radiography, in elderly patients with spondylolisthesis for whom surgical treatment is planned, it is recommended to perform an MRI of the spine and the sacrum during preoperative preparation in order to exclude or confirm the underlying sacral insufficiency fracture (19, 21).
In everyday clinical work, and due to the often-present radicular symptoms caused by degenerative changes of the spine, the next diagnostic step, after plain radiographs, is the MRI (9, 22). This was also the case with the patient whose case is described in this case report. MRI of the pelvis shows almost 100% sensitivity during the detection of insufficiency fractures in the pelvic area and it showed better results in fracture detection than the CT scan. Cabarrus et al. state that, through the MRI, they have managed to detect 128 out of 129 (99%) fractures in 63 of 64 (98%) patients, while the CT scan revealed only 89 out of 129 (69%) fractures in 34 of 64 (53%) patients (15).
Typical signs of fracture that were visible on the MRI were bone marrow oedema and fracture gaps. Fracture gaps are most visible in sequences in T1 (spin-lattice) relaxation time. Bone marrow oedema is presented as a region of high intensity in sequences in T2 (spin-spin) relaxation time with fat suppression and in STIR sequences, as well as a region of low intensity in sequences in T1 (spin-lattice) relaxation time in relation to the surrounding normal bone. MRI has an almost 100% sensitivity for detecting spongy bone oedema, but it is a relatively nonspecific sign present in various bone-related pathological conditions, ranging from reactive oedema due to mechanical stress, oedema related to postconcussional syndrome, to processes related to inflammation and tumours (15). During the interpretation of MRI findings, it should be taken into account that a fracture gap is not always visible and that diagnostic errors are possible, especially in patients with malignant diseases. However, in almost 90% of insufficiency fractures, bone marrow oedema and fracture gaps are present at the same time (15).
In the differential diagnosis, primary malignant disease or metastatic lesions in malignant disease and osteomyelitis are most often considered.
Sacral insufficiency fracture most commonly affects both sacral alae and is vertical in shape. The horizontal component, which usually passes through the vertebral body of the second sacral (S2) vertebra (22), is also common. Fractures are most visible on oblique coronal sections through the sacrum. The bone scintigraphy which utilizes technetium, shows a typical pattern of pathological accumulation of radiopharmaceuticals (on the posterior scan) in the shape of the letter H (the so-called H sign, the Honda sign or the butterfly sign). In other words, two vertical lines in the area of the sacral alae connected by a horizontal line are shown. It should be noted that, sometimes, the accumulation pattern may be incomplete, e.g., if one vertical component is missing. The sensitivity of scintigraphy in detecting sacral insufficiency fractures is very high (96%), with a positive predictive value of 92% (23).
Although the CT scan has a lower sensitivity than the MRI (between 60 and 75%), this radiological technique can also help to distinguish between insufficiency fractures and malignant bone remodelling, due to a better presentation of cortical destruction, and a clearer picture of possible fracture gap spread, as well as the neural foraminal stenosis affected areas, which is especially important when planning treatment with the sacroplasty procedure (15, 24).
On the other hand, in comparison to the CT scan, the MRI is significantly better in detecting lesions of soft tissue structures. In sacral fractures, oedema of the surrounding soft tissue is less common (in 36% of cases) than in pubic bone fractures (65% of cases) or acetabular fractures (64% of cases), as well as the proximal part of the femur (51% of cases) (15).
Cabarrus et al. showed that soft tissue lesions on the MRI were detected in 99% of patients compared to only 12.6% of these lesions that were visible on the CT scan (15). It should be noted that both methods are significantly better at detecting acetabular and sacral fractures compared to plain radiography (15, 25).
Sacral insufficiency fracture treatment is mostly treated with conservative methods of treatment. The usual approach to treatment includes bed rest during the course of 3 to 6 months, which causes accelerated bone mineral density (BMD) loss. Other unwanted complications for people who are on prolonged bed rest (and immobilised) include thromboembolism and the occurrence of decubitus ulcers with secondary infections (26).
Early mobilisation, on the other hand, stimulates osteoblast activity and new bone formation. Weight-bearing exercises and hydrotherapy are standard methods during rehabilitation (27). Pain management is important in the treatment of sacral insufficiency fractures. Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used, but they should be used with caution, as some studies indicate that such drugs may lead to bone mineral density loss and that they can slow down fracture healing by disrupting endochondral ossification (28, 29). Treatment with paravertebral injections with an oxygen-ozone mixture has also shown encouraging results in the treatment of opioid-resistant pain (30).
In recent times, sacroplasty is increasingly being used in the treatment of sacral stress fractures, as a minimally invasive procedure. It is intended to rapidly relieve the symptoms of clinical conditions, primarily pain, which allows early mobilisation of such patients (12). Vertebroplasty is a technique based on injecting polymethyl methacrylate (PMMA) cement into the fracture gap area. The two biggest challenges of this technique are safe needle insertion and safe cement extrusion. Cement injection into the sacrum was originally used to treat painful skeletal metastases, and twenty years ago it was first performed in the treatment of osteoporotic fractures of the sacrum (31).
There are several ways of performing sacroplasty, which differ primarily in terms of the method of approach and the insertion of the needle. A posterior approach is typically used when, through the use of fluoroscopy or, less commonly, a CT scan, an 11- or 13-gauge bone biopsy needle is inserted percutaneously through the sacral cortex, in a plane parallel to the SI (sacroiliac) joint. When using the approach along the longitudinal axis, the needle is inserted in the caudocranial direction, along the sacral Y-axis (the longitudinal axis of the sacrum). The mediosagittal approach requires a postoperative magnetic resonance imaging, due to the fact that the needle passes through the central canal of the sacrum, so it is necessary to confirm that the tip of the needle is located more to the caudal end than the distal end of the thecal sac. The occurrence of complications in sacroplasty are rare. The highest risk is that of cement leakage outside the fracture gap into the sacral foramina and spinal canal. Neuritis of the sacral spinal nerve 1 (S1) has been described as a consequence of cement migration after sacroplasty (32). Cement leakage into the paraspinal soft tissues holds no major clinical significance. As with vertebroplasty, complications of venous thromboembolism and nerve damage are possible (32, 33). Given the importance of fracture gap localisation and extension for a satisfactory sacroplasty outcome, Gesa Bakker, et al. have proposed a classification of sacral fractures, divided into 3 groups: (i) group A includes fractures that have no mechanical importance and fractures with no cortical disruption, (ii) group B is formed by complete fractures of the sacral alae, starting in the anterior cortical bone or fractures which affect the neural foramina and the sacral canal, and (iii) group C includes fractures in the sacral corpus with fracture lines continuing to the anterior or posterior cortical bone or the sacral foramina (24).
Several other methods for the treatment of sacral fractures have been described, such as balloon-assisted sacroplasty, kyphoplasty, or transiliosacral screw insertion fluoroscopic techniques, but in clinical practice these methods have not been increasingly implemented so far (34 – 36).
Surgical methods of treatment are indicated in patients in whom conservative methods of treatment have not yielded any positive results. The challenge of surgical treatment is to achieve satisfactory stability in an already structurally weakened bone. By performing stabilisation using transiliac-transsacral screws, mobility is achieved in a significant number of patients with a satisfactory effect on the absence of pain (37). Better interfragmentary compression is achieved through the placement of a transsacral plate (38). Iliosacral screw fixation ((ISS) is a commonly used treatment method, but some studies have shown that through this method satisfactory mechanical stability is not achieved, which leads to displacement and instability of the screws (39). Lumbosacral fixation technique is indicated in patients with neurological complications due to unstable sacral fractures (40).
CONCLUSION
Sacral insufficiency fractures which occur in the structurally weakened bone are often overlooked causes of nonspecific lumbosacral back pain, especially in elderly women with underlying osteoporosis. Plain radiographs of the pelvis are often non-diagnostic when it comes to this type of fractures, and the diagnostic imaging method of choice is MRI, which is also confirmed in our case report. Inflammation or malignant disease must first be excluded through the process of differential diagnosis. Treatment is mainly performed through conservative methods, and in recent times, sacroplasty is increasingly being used in the treatment of sacral fractures, as a minimally invasive procedure.
Conflict of interest statement: Authors declare no conflict of interest.
REFERENCES / LITERATURA
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<jrn>38. Mehling I, Hessmann MH, Rommens PM. Stabilization of fatigue fractures of the dorsal pelvis with a trans-sacral bar. Operative technique and outcome. Injury. 2012;43:446–51.PubMedhttps://doi.org/10.1016/j.injury.2011.08.005</jrn>
<jrn>39. Oberkircher L, Masaeli A, Bliemel C, Debus F, Ruchholtz S, Krüger A. Primary stability of three different iliosacral screw fixation techniques in osteoporotic cadaver specimens – a biomechanical investigation. Spine J. 2016;16:226–32.PubMedhttps://doi.org/10.1016/j.spinee.2015.08.016</jrn>
<jrn>40. Maki S, Nakamura K, Yamauchi T, et al. Lumbopelvic fixation for sacral insufficiency fracture presenting with sphincter dysfunction. Case Rep Orthop. 2019;2019:9097876.PubMedhttps://doi.org/10.1155/2019/9097876</jrn>
Figure 1. Radiographs of the LS spine showed vertebral compression fracture of the second lumbar vertebra (L2) (grade II according to the Genant classification of vertebral fractures) with diffuse degenerative changes, hypertrophic osteoarthropathy of intervertebral joints at the L3–S1 level, and signs of skeletal osteopenia.
Figure 2. Plain radiograph of the pelvis showed signs of femoroacetabular impingement and bone spurs at the muscle attachments to the pelvic girdle.
Figure 3. Pathologically increased signal of lateral masses of the sacrum shown on the MRI of the LS spine. Sagittal sections through the LS spine (STIR image) show a pathological increase in the signal of the lateral masses of the sacrum along with signs of discopathy and foraminal stenosis along the lumbar level of the spine.
Figure 4. On the T1-weighted image, axial sections through the sacrum show linear zones of low signal intensity, which can be seen bilaterally in the areas extending through lateral masses of the sacrum and which correspond to fracture lines. The extensive high signal zones in the STIR image (last image on the right) correspond to the oedema of the surrounding spongy bone. No pathological changes of the SI joints were observed.