1300 THE JOURNAL OF BONE AND JOINT SURGERY  CASE REPORT Congenital absence of the posterior arch of the atlas associated with a fracture of the anterior arch L. Corominas, K. Z. Masrouha From University of Iowa Hospitals and Clinics, Iowa City, Iowa, United States  L. Corominas, MD, Orthopaedic Surgeon Department of Orthopaedics and Rehabilitation Hospital Universitario Central de Asturias, Celestino Villamil s/n, 33006 Oviedo, Asturias, Spain.  K. Z. Masrouha, MD, Research Fellow Department of Orthopaedics and Rehabilitation University of Iowa Hospitals and Clinics, 01006 JPP, 200 Hawkins Drive, Iowa City, Iowa 52242, USA. Correspondence should be sent to Dr L. Corominas; e-mail: lauracorominas1979@hotmail. com ©2010 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.92B9. 24071 $2.00 J Bone Joint Surg [Br] 2010;92-B:1300-2. Received 29 November 2009; Accepted after revision 27 April 2010 Structural defects of the posterior arch of the atlas are rare, and range from clefts of variable location and size to more extensive defects such as complete agenesis. These abnormalities are usually incidental radiological findings. We present a case of a fracture of the anterior arch of the atlas associated with a congenital abnormality of the posterior arch. Congenital partial or complete aplasia of the posterior arch of the atlas is usually an inciden- tal finding 1,2 during the investigation of neck pain, radiculopathy, swelling in the neck or trauma. Careful differentiation between an acute burst fracture and a congenital defect is essential. A CT scan is helpful in evaluating the integrity of the atlas and differentiating an acute injury from a developmental cleft. 3 It is unclear as to whether this is a hereditary defect, although two reports have documented the existence of cases involving a mother and son. 1,4 The incidence between genders is simi- lar and the clinical presentation is variable. Patients are most commonly asymptomatic, although the defect can cause chronic cervical pain, headache and L’Hermitte’s sign. 5 Despite being well-documented, the rarity of the condition leads to errors in diagnosis, and to the lack of confirmation of stability which can result in increased neurological morbidity. 6 Iden- tifying the transverse atlantoaxial ligament between C1-2 in cases of complete agenesis of the posterior arch is important, since its absence can lead to neurological sequelae, such as atlanto- axial instability or transient quadriparesis. 7 We present a patient with a previously unre- ported combination of a fracture of the anterior arch and complete absence of the posterior arch. Case report A previously healthy, right-handed, nine-year- old boy presented with a history of occipital headache for two months after axial compres- sion of his neck while performing somersaults. He reported that the pain increased with hyperextension of the neck. He had no other symptoms. On examination, there was tenderness and localised swelling at the occip- itocervical junction and slight restriction of flexion of the neck. Neurological examination was normal and there were no other musculo- skeletal injuries. Radiographs demonstrated a defect in the posterior arch of the atlas, with no evidence of atlantoaxial instability as assessed by flexion and extension views (Fig. 1). A helical CT scan confirmed complete absence of ossification of the posterior arch of C1, corresponding with type E of the Curriano classification. 1,8 A three-dimensional reconstruction gave an additional perspective (Fig. 2). The CT scan also showed appearances suggestive of a verti- cal fracture through the anterior arch of C1, with early callus formation. It appeared as a cleft with sclerotic margins. MRI of the cervi- cal spine confirmed the existence of the poste- rior ligament between C1 and C2, providing evidence for the stability of the congenital aplasia (Fig. 3). The patient was treated conservatively, with non-steroidal anti-inflammatory drugs and a cervical collar for three months. He was referred for physiotherapy and was asked to avoid contact sports and athletic activities. On his last follow-up after 18 months, he was asymptomatic and fully active. Discussion The atlas can be divided anatomically into three parts: the anterior arch, the lateral masses, and the posterior arch. The anterior arch ossifies from one or two centres or, in the absense of a separate centre of ossification, by extension of the lateral masses. Ossification is usually complete by ten years of age. It begins in the posterior arch during the seventh week of intrauterine life, proceeding perichondrally from two centres located in the lateral masses. 9 The laminae arise from buds in these growth