Unilateral Pedicle Fracture Accompanying Spondylolisthesis and Contralateral Spondylolysis: A Case Report

Study design Case report Objective To report a rare case with unilateral pedicle stress fracture accompanying spondylolytic spondylolisthesis. Summary of Background Data Unilateral pedicle stress fracture accompanying spondylolytic spondylolisthesis is rare. Most are association with major trauma, previous spine surgery, osteopetrosis or stress-related activities. Methods We report a patient with spontaneous pedicle fracture associated with contralateral spondylolysis and spondylolisthesis at the L5 level, complaining severe back pain, radicular lower limb pain and intermittent claudication. The pathophysiological mechanism is discussed, and a review of relevant literature is included. Results This patient was successfully treated by decompression laminectomy with transforaminal lumbar interbody fusion (TLIF) and pedicle screw xation. To our knowledge, cases like this have rarely been reported in the literature. Conclusions Unilateral pedicle stress fracture in patients with spondylolytic spondylolisthesis is rare. It may be related to redistribution of forces in an unstable neural arch resulting from defect in the contralateral pars interarticularis. Our experience suggests that limited decompression and instrumented fusion surgery led to a successful outcome.


Case Report
This 48-year-old woman had suffered no earlier trauma, accidents, or spinal surgery but had mild low back pain for several years. The pain exacerbated progressively without any inducement in resent 3 years, accompanying with left leg pain and intermittent claudication. Bending and twisting commonly aggravated the symptoms, and resting resulted in improvement temporarily. Before surgery, she could only walked fewer than 100 m due to severe lower extremity pain.
A neurological examination revealed grade 4 muscle weakness in the left extensor hallucis longus, and marked tenderness in the low back area with reduced back motion, especially in backward extension. No sensory change, bowel or bladder symptom was found.
The plain radiographs showed grade I spondylolisthesis at the L5 level, and dynamic radiographs illustrated segmental instability (Fig. 1). The computed tomography (CT) demonstrated right side spondylolysis at the L5 vertebra and contralateral pedicle fracture. The callus overgrew along the fracture margin, and bony spur encroached on the spinal canal (Fig. 2). The magnetic resonance imaging reveals herniation of the intervertebral disc and spinal canal stenosis at L5/S1. No edema signal can be seen on the fat-suppression sequence, indicating that the pedicular fracture might be of old (Fig. 3).
This patient was treated with decompression laminectomy with transforaminal lumbar interbody fusion (TLIF) and pedicle screw xation (Fig. 4). At the 9-month follow-up assessment, she had completely returned to normal activities. No residual low back pain or radiating pain was complained. The CT revealed successful fusion of L5/S1 and good maintenance of reduction, along with advanced healing of the pedicle fracture (Fig. 5).

Discussion
Spontaneous pedicle fracture of the lumbar spine without trauma is rare. Its etiology is unknown but may be explicable analogous to stress fracture. Unilateral spondylolysis accompanying sclerosis and hypertrophy of the contralateral neural arch is well described in the literature [13][14]. The sclerosis is believed to occur as a compensatory mechanism secondary to redistribution of forces in an unstable neural arch which is resulted from the defect in the pars interarticularis [15]. Physiological hypertrophy is a response of the pedicle to abnormal stress due to instability of the neural arch, well known as sclerotic pedicle in the literature [16]. One support for such theory comes from the fact that pedicular sclerosis usually decreases after treatment with fusion and xation that supports su cient stabilities [16] . Cadaver studies revealed that the pars interarticularis is most prone to stress fracture, and the pedicle is the second-weakest vertebral area [17]. The pedicle has greater intrinsic strength and shorter moment arm from the vertebral body. Such structure helps resist greater cyclic shear forces, particularly resist shear stress and twisting stress forces [18].
Unilateral spondylolysis occurs in 15% to 30% of spondylolysis cases, L5 is the most commonly affected vertebra [19]. Lowe et al. found that approximately 20% patients with unilateral spondylolysis had structural changes or anomalies including pars hypertrophy, sclerosis and deformity in the opposite pars interarticularis [20]. Finite element analysis also indicated that unilateral pars defect induced greater stress on the contralateral pars, and the opposite pars interarticularis in the unilateral spondylolysis experienced greater stress on torsion [21][22]. As a result, spontaneous lumbar pedicle fracture occurs less often in the neural arch than spondylolysis [23]. This theory has been proved in study using unilateral spondylolysis model, whereby Sairyo et al. demonstrated that the risk of the pedicular fatigue stresses increased in axial rotation to the contralateral side of unilateral pars defect [23]. Therefore, we speculate that unilateral spondylolysis can alter the normal biomechanics of the neural arch, resulting in increased stress on the contralateral pedicle, and led to stress fracture.
The sensitivity of plain radiography is limited in diagnosis of pedicular fractures. Computed tomography is widely regarded as the choice for the diagnosis of such lesions because of clear display of the stress fracture. Bone scintigraphy and MR images are useful in detecting early fractures [24][25][26].
Pedicle fracture with spondylolisthesis can be managed conservatively or surgically depending on the grade of the spondylolisthesis and the impact on the patient's daily activities. Conservative management consists of rest, pain control, and bracing. For patients with incomplete, complete, and juvenile stress fractures of the lumbar pedicle without nerve root irritation, the majority of claims preferred conservative treatment [27]. In most cases, conservative treatment allows for spontaneous healing of the early-stage fracture even in older individuals, providing a satisfactory healing rate [28].
Surgical treatment is recommended for patients who do not respond to conservative treatment [29]. Besides, surgical treatment is considered as rst choice for patients with radicular pain and/or neurogenic claudication that limits the time one can stand and/or the distance one can walk. Especially for patients who present with bowel or bladder dysfunction or with progressive weakness, segmental lumber instability nding in radiological examination, surgical treatment is instant [29].
Surgical management consists of decompression, instrumented and non-instrumented fusion [30]. . As the gold standard for internal xation, pedicle screw technique is preferred because of high success rate, low hardware failure, no postoperative bracing, and su cient maintenance of reduction during exion, extension, torsion, and side bending [30].
In this case, the patient was treated by decompression laminectomy with TLIF and pedicle screw xation. Anterior interbody fusion procedure was performed, using a vertical mesh cage lled with allograft. At the 9-month follow-up, she had completely returned to normal activities. No residual low back pain or radiating pain was complained.
In conclusion, unilateral pedicle stress fracture in patients with spondylolytic spondylolisthesis is rare. It may be related to redistribution of forces in an unstable neural arch resulting from defect in the contralateral pars interarticularis. Our experience suggests that limited decompression and instrumented fusion surgery led to a successful outcome.

Declarations
Ethics approval and consent to participate: The study was approved by the ethical committee of The Friendship Hospital of Yili Kazakh Autonomous Prefecture, and informed consent was obtained from the patient.
Consent for publication: All the authors have approved the nal version and agree to publish.
Availability of data and materials: N/A.
Competing interests: All the authors state that there is no con ict of interest.
Funding: There is no funding source.
Authors' contributions: Xiaofeng Liu and Tao Liu conceived the study. Xiaofeng Liu and Palihati Baiketuerxun completed the collection and assembly of data. Xiaofeng Liu and Junxin Zhang undertook the statistical analysis. Xiaofeng Liu and Palihati Baiketuerxun produced the initial draft manuscript. Junxin Zhang, Tao Liu critically revised the article for important intellectual content. Tao Liu gave nal approval of the article. All authors read the nal manuscript.