SPECT/CT bone scintigraphy to evaluate low back pain in young athletes: common and uncommon etiologies
© The Author(s). 2016
Received: 15 February 2016
Accepted: 26 May 2016
Published: 7 July 2016
Low back pain of various etiologies is a common clinical presentation in young athletes. In this article, we discuss the utility of SPECT/CT bone scintigraphy for the evaluation of low back pain in young athletes. The spectrum of lower spine lesions caused by sports injuries and identifiable on bone scan is presented along with strategies to avoid unnecessary irradiation of young patients. Also covered are pitfalls in diagnosis due to referred-pain phenomenon and normal skeletal variants specific to this age group.
The etiology of low back pain in young athletes differs from that seen in adults, with bony etiology being more common than disc-related disease [1,2]. Chronic low back pain occurs more often than acute pain and is caused by repetitive microtrauma due to flexion, extension, and rotation movements that increase the risk of injury to the posterior elements of the spine.
The initial evaluation of a young adult with back pain starts with lumbar spine radiograph; however this has limited sensitivity in the detection of pars fractures and stress reactions. MRI is preferred if neurologic symptoms are present or if radicular pain is identified on clinical exam. In the absence of neurologic symptoms, evaluation can proceed by bone scintigraphy with planar and SPECT acquisitions. Bone scintigraphy can differentiate acute spondylolysis from old chronic nonunion fracture, and there is a good correlation between a positive bone scan and painful pars lesion . Bone scan with SPECT is superior to MRI and CT in the detection of spondylolysis . One study comparing SPECT and CT versus MRI showed that only 40 out of 50 lesions seen on SPECT were revealed by MRI and there was no case of positive MRI with negative SPECT .
Besides evaluation for marrow edema at the pars interarticularis (high signal intensity on STIR images and low signal in T1), MRI offers a good evaluation of surrounding soft tissue lesion (like posterior ligamentous complex), spinal cord, and intervertebral disc.
18F NaF PET scan has been also shown to be useful in the detection of a variety of skeletal abnormalities in young patients with back pain with higher resolution and similar radiation dosimetry but a higher cost relative to Tc99m-MDP .
SPECT images have better contrast resolution compared to planar images and detect additional sites of abnormal uptake, in one study performed in patients with low back pain in 24 % of the cases . The large field-of-view surveyed by planar whole-body imaging offers the advantage of identifying additional abnormal bony sites that may trigger referred pain or mimic radicular pain unsuspected upon initial clinical evaluation.
Referred pain in the lower lumbar region originating from the sacroiliac joint and hip can be explained by the common innervation of the hip/sacroiliac joints and intervertebral discs by sacral and lower lumbar nerves [7,8]. Studies have shown that patients with low back pain, with or without leg pain, may have the spine, sacrum/sacroiliac joints, or hip as the cause of their symptoms [7–9]. A study published by Sembrano et al. in patients with low back pain, with or without leg pain, showed that in 65 % of cases, the major pain generator came from the spine only; in 5 %, it originated from the sacroiliac joint only; and in 2.5 % of the cases, the back pain originated from the hip only . Because attention is directed to the referral site of pain, the area that represents the source of pain may be overlooked if the imaging field of view is limited. Therefore, correlation with clinical history and physical exam and consideration of the risk factors for a certain pathology type are important in avoiding a delay in diagnosis. For example, long distance female runners are at slightly higher risk for sacral stress fractures that clinically may manifest as low back pain and buttock pain mimicking radicular pain . Although spinal radicular pain has certain characteristics (dermatomal distribution, extension beyond the knee, and sensory or motor loss), there is a high incidence of nonradicular pain mimicking radicular pain .
Bone scan protocol
Planar (whole body or spot) scintigraphic images are obtained 3–4 h after intravenous injection of 9.3 MBq/kg (0.25 mCi/kg) of Tc99m-MDP (methylene diphosphonate), followed by SPECT acquisition with or without additional CT acquisition . Blood flow and blood pool images may be added if an acute injury is suspected. Planar images include a large area encompassing more than the site of pain to cover areas of potential referred pain.
A standard SPECT acquisition protocol for bone imaging with a dual-head gamma camera consists of 25 s per view, 60 view angles over 180° (3° increments between views), and 128 × 128 binning.
Recent SPECT-CT systems are equipped with fully diagnostic quality CT systems; however, usually, only low-dose CT is obtained for attenuation correction and for visualization of bony details over a limited field of view thereby minimizing the impact of radiation on the patient’s gonads. Our routine low-dose CT consists of a 60-mA tube current and a 0.8-s tube rotation at 120 kVp. In the absence of any abnormality on SPECT images, the CT is not performed, thereby sparing young patients the additional radiation. Rarely, a diagnostic beam CT over a limited filed of view is required for anatomic characterization.
Lumbar spine origin of low back pain
Isthmic spondylolysis represents a pars interarticularis fracture usually associated with repetitive forced hyperextension and rotation. It has a higher incidence during the adolescent growth spurt due to incomplete bone maturation of the neural arch and repetitive stress injury [1,2] but heredity also plays a role . It is common for spondylolysis to be bilateral, and the vast majority of cases occur at L5 with the next more common location being L4 . Spondylolysis usually presents with focal chronic low back pain which is unilateral or bilateral and increases with activity, without radicular symptoms.
Lumbar spine pathology other than spondylolysis
Although isthmic spondylolysis is the most common cause of low back pain in young athletes, other etiologies need to be considered. These include stress reaction or fracture at the pedicle, transitional vertebra, lumbar interspinous bursitis, traction apophysitis (at the iliac crest, spinous process, or anterior vertebral ring apophysis), facet joint disease, facet posterior fracture in the lumbar spine region, avulsion fracture of the secondary ossification centers, endplate degenerative changes, and sacral facet fracture. Ligament injuries cannot be diagnosed on bone scan unless resulting in calcification.
Pedicle stress reaction or fracture
Fractures at the vertebral apophyses
In the adolescent age range, fractures at the secondary centers of ossification of the vertebrae (apophyses) can occur since most of them are not yet fused. There are seven secondary centers of ossification (apophyses) in a lumbar vertebra as described below. On bone scan, only faint uptake is usually seen associated with these apophyses and high and asymmetric uptake at one of the secondary ossification centers is suggestive of an avulsion fracture.
Injury to the ring apophysis caused by disc material protruding through the growth plate of ring apophysis can result in limbus vertebra. This is seen as a small corticated bony fragment matching osseous defect at the superior margin usually in anterosuperior location and on imaging that needs to be differentiated from an acute vertebral fracture, a Schmorl nodule, or calcified disc herniation. Limbus vertebra is usually asymptomatic unless it has a posterior location when it can potentially cause a neurologic symptom .
Fractures of the transverse processes of the lumbar vertebrae
Fractures of the transverse processes of the lumbar vertebrae may result from violent lateral flexion-extension forces (as in football) . Fractures of the L5 transverse process raise the suspicion for sacral fracture.
Lumbosacral transitional vertebrae syndrome (Bertolotti syndrome)
Impingement syndrome of the adjacent spinous processes (Baastrup disease-lumbar interspinous bursitis)
Impingement syndrome of the adjacent spinous processes (Baastrup disease-lumbar interspinous bursitis) can be due to a tight thoracolumbar fascia with accentuated lordosis and worsened by excessive hyperextension and hyperflexion .
Facet joint can be also a source of pain if degenerative changes are present and shows increased uptake on bone scan.
Adolescent disc dysplasia
Referred lower lumbar pain
Referred lower lumbar pain can be triggered by pathologic processes localized in the sacrum (sacral fractures), sacroiliac joints (sacroiliac joint syndrome), or hips [8,9]. The large field of view of planar bone scan helps in identifying these possible etiologies of lower lumbar pain.
Sacral stress fracture
Sacroiliac joint syndrome
Sacroiliac joint syndrome is due to increased sports activities with increased abnormal motion or stress at the sacroiliac joint. This can be difficult to diagnose on bone scintigraphy given the normally increased uptake in this age group patient population . Degenerative changes can be seen on CT: sacroiliac joint irregularity, subchondral cysts, and sclerosis .
Low back pain in young athletes not related to sports activity
Discitis or osteomyelitis can also present with increased uptake on bone scan. In case of discitis, there is radiotracer uptake along the affected facing endplates.
In the first few hours after acute trauma, bone scintigraphy may be falsely negative; however, sensitivity is closed to 100 % at 72 h after fracture onset .
Also, it is important to be familiar with normal skeletal development across different age groups. There are 3 primary ossification centers for a spinal vertebra and a total of 21 for sacrum, and these fuse before puberty (usually by 6–7 years old) and are normally not seen any more in the adolescent age range. However, recognizing confounding variants due to failure of the segmentation process or fusion abnormalities of these centers is important [15,31].
Bone scan is a useful clinical tool to explore the etiology of low back pain like spondylolysis and other less common etiologies in young athletes. It is also particularly important to detect the active source of pain when more than one bony abnormality is seen in anatomical imaging. The addition of SPECT-CT increases the clinical accuracy due to increased contrast resolution and anatomical localization.
MM was responsible for the conception and design of the study. MM, FB, MB, and HV collected and assembled the figures. MM, FB and MB wrote the manuscript. MM, FB, MB, and HV gave the final approval and edited the manuscript.
The authors declare that they have no competing interests.
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