1 Introduction Cervical spondylolysis is a relatively rare anomaly, with only about 100 cases described worldwide^[1]. It is defined as a corticated cleft between the superior and inferior articular facets of the articular pillar,and is considered the cervical equivalent of the pars interarticularis in the lumbar spine^[2]. The anomaly is most commonly seen at C6,followed by C4,and rarely reported to involve C3. To our knowledge,the anomaly has been reported in only one Chinese patient with C6 spondylolysis^[3]. We report the clinical and radiological findings in 4 Chinese patients diagnosed with C3 cervical spondylolysis and managed in our hospital. These reports are presented along with the current literature on cervical spondylolysis,with a focus on clinical diagnosis,treatment,and follow-up.

2 Case reports 2.1 Case 1 A 21‐year‐old woman sustained an extension injury in a traffic accident 1 month before admission. Physical examination revealed a limited range of neck motion due to posterior neck pain. The neurologic examination was normal. Palpation of her neck revealed severe tenderness in the upper cervical spine. Lateral view plain radiography identified a hypoplastic articular mass at C3 (Figure 1). Iodine contrast revealed a narrowed spinal canal at C3 on computed tomography (CT). The lateral mass at C3 was discontinuous and typical features of spina bifida were revealed. The surfaces of the spondylolysis were smooth,suggesting a chronic process. Spinal cord compression was definitive,while spondylolisthesis was not observed. The compression was located mainly at the posterior column and instability was not present; posterior decompression was performed,and the symptoms were relieved after surgery.

2.2 Case 2 A 26‐year‐old man had complained of neck pain for 2 years. Physical examination revealed mild tenderness over his neck. There was no focal neurological sign and he had full range of motion in his neck. Lateral view plain radiography identified a hypoplastic articular mass and spinal process at C2 (Figure 2a). Bilateral clefts between the articular pillar and the facets of C3 as well as spina bifida were seen on iodine contrast CT (Figure 2b). Spinal stenosis was noted. The diagnosis of bilateral cervical spondylolysis was confirmed. The patient was managed with posterior decompression and discharged upon subsequent symptomatic improvement. However,no postoperative image review was available.

2.3 Case 3 A 14‐year‐old boy presented with numbness in both upper extremities and weakness in all 4 extremities. The symptoms began 6 months prior and were recently worse. The weakness manifested worsened with movement. There was no history of significant trauma. Neurologic examination at the time of presentation revealed grade Ⅳ muscle strength in all extremities. There was no neck tenderness. The patient had bladder dysfunction. Radiographic examination of the cervical spine revealed bilateral spondylolysis at C3 (Figure 3). On CT,a lateral mass at C3 was discontinuous at the midportion. In an axial view,the spinous process was divided in the midline,with typical features of spina bifida. This case was also chronic,with smooth spondylolytic surfaces observed on CT and the absence of any soft tissue swelling on magnetic resonance imaging (MRI). Spondylolisthesis was not observed. Spinal cord compression was definitive. A posterior decompression was performed. Muscle strength of the left upper and lower limbs decreased to grade Ⅲ when the patient awoke from anesthesia,but recovered to normal 3 months later and bladder dysfunction were resolved after 6 months.

2.4 Case 4 A 45‐year‐old man had neck discomfort and numbness in the left hand for 2 years. The symptoms gradually worsened. In addition,he had left lower limb weakness,urination difficulty and constipation for 2 weeks. There was no significant history of trauma. Neurologic examination at the time of presentation revealed grade Ⅳ muscle strength in the upper extremities. There was no neck tenderness. Hoffmann’s sign was positive and deep tendon reflexes were slightly increased. On CT (Figure 4),a lateral mass at C3 was discontinuous,with well‐corticated clefts between the bilateral C3 facets. The spondylolytic surface was also smooth on the CT scan,and there was no soft tissue swelling on MRI. Instability was not present. Spinal cord compression was definitive. A posterior decompression was performed,and the symptoms were relieved after surgery.

3 Discussion Cervical spondylolysis was first reported by Hadley in 1946 as a congenital unilateral absent pedicle from a cervical vertebra^[4]. The anomaly was first described by Perlman and Hawes in 1951 as a defect that allowed forward migration of the superior vertebra^[5]. Cervical spondylolysis with or without spondylolisthesis is a relatively rare anomaly,reported in 5% of the population^[6]. The age at presentation with cervical spondylolysis ranges from 5 to 81. Men and adults are most commonly affected^{[3, 7, 8]}. Of 79 lesions in 71 patients reported by Poggi et al.^[9],53 were bilateral spondylolytic defects,or nearly two‐thirds of all the cases. Ahn et al.^[10] reported bilateral involvement with spondylolisthesis in 100% of cases. We observed the same result (the 4 cases had bilateral involvement).

This condition is rare,and the exact cause of cervical spondylolysis remains unknown. Schwartz et al.^[11] suggested that cervical spondylolysis is caused by posttraumatic nonunion with pseudarthrosis. Morvan et al. and Cautilli et al.^{[12, 13]} believed that cervical spondylolysis arises from repetitive microtrauma resulting in stress fractures to the pars region,which is similar to the process causing lumbar spondylolysis. They also pointed out that spondylolysis has not been reported in an infant and that the transitional vertebra (C6) is the most common level,lending further support for their microtrauma theory. However,many patients do not present with a significant history of trauma. Only one of our patients had a history of acute neck injury. In other reports,the anomaly showed wellcorticated margins,without evidence of fresh fracture on MRI or CT^{[9, 14]}. Moreover,our 4 cases were all diagnosed with bilateral C3 cervical spondylolysis. Hence,many authors contend that congenital or faulty ossification within the single ossification center of the articular mass is the etiology of cervical spondylolysis^{[11, 15, 16, 17]}. Based on observations in our cases,we support congenital theories.

Clinically,the symptoms of cervical spondylolysis vary,ranging from neck stiffness,rigidity,nuchal pain with or without radicular symptoms,and bladder dysfunction. Some authors reported that over 70% of the cases were seen at hospitals because of neck pain with or without injuries^[18]. In this report,2 of the 4 cases had bladder and bowel dysfunction. Some cases are asymptomatic. The diagnosis of cervical spondylolysis mainly depends on the characteristic radiological features. Cervical spondylolysis has characteristic imaging findings. Vertical spondylolytic clefts were present between the superior and inferior facets of the articular pillar of the cervical spine,each of which had a smooth cortical border. Forsberg et al. reported a characteristic “cleft bow tie” or “bat wing” in the sagittal plain^[15]. In addition,three‐dimensional reconstructed CT of the cervical spine showed a triangular shape of the pillar fragments,divided by a cleft^[3]. The presence of these characteristic features on plain films or CT can provide a definitive diagnosis. MRI often cannot identify a spondylolytic defect, with absence of visualization of the spinous process anomaly on sagittal sequences. However,MRI plays an important role in evaluating the spinal cord. Clinically,it is important to differentiate cervical spondylolysis from acute laminal fractures. Acute cervical spine injuries present with low signal change and cord atrophy on T2WI MRI. In our report,the 4 cases were diagnosed with bilateral C3 cervical spondylolysis. They had characteristic bilateral clefts between the articular pillar and the facets of C3,as well as spina bifida on CT.

Operative and nonoperative treatment is reported in the literature. Poggi et al. and Goupille et al. all contended that surgical intervention should be considered when nonoperative therapy fails or when symptoms are present indicating an unstable spondylolytic defect^{[9, 19]}. Ahn et al. concluded that surgical treatment should be the first choice for cases where there is instability of the lamina^[10]. The same study also reported that anterior interbody fusions were performed during surgical treatment. In our cases, the articular defect protruded into the spinal canal and caused cord compression. Hence,all patients underwent surgery. The compression was located mainly at the posterior column and instability was not present. However,C3 is not the transitional vertebra and is subjected to stress less often. Therefore,posterior decompression was performed,and the symptoms were relieved after surgery.

In conclusion,C3 cervical spondylolysis is rare. The exact cause remains controversial and most likely is congenital in origin. Its diagnosis is dependent on characteristic radiological features. It is important to differentiate from acute cervical spine injuries. Surgical treatment is the first choice for cases where there is compression of the spinal cord or instability of the lamina.

Conflict of interests The authors have no financial interest to disclose regarding the article.