What is swan neck deformity of the cervical spine?

Swan Neck Deformity of the Cervical Spine

Swan neck deformity of the cervical spine is a specific malalignment pattern characterized by simultaneous abnormal kyphosis at the occipitoaxial (C0-C2) segment combined with compensatory hyperlordosis in the subaxial cervical spine (C2-C7). 1

Pathomechanics and Definition

• This deformity represents a complex biomechanical adaptation where primary kyphotic deformity at the upper cervical spine triggers compensatory hyperlordotic changes in the lower cervical segments 1
• The term "swan neck" describes the characteristic S-shaped curve resembling a swan's neck when viewed on lateral radiographs 2, 1
• This pattern most commonly develops in the context of chronic atlantoaxial dislocation, particularly associated with congenital abnormalities such as os odontoideum (present in 8/33 patients in one series) or occipitalization of C1 (19/33 patients) 2

Clinical Significance and Presentation

• Neurological compromise occurs in approximately 76% of patients (25/33 in the largest surgical series), manifesting as myelopathy from spinal cord compression at the craniovertebral junction 2
• The primary pathology is the occipitoaxial kyphosis, which creates mechanical instability and potential for progressive neurological deterioration 2, 1
• The subaxial hyperlordosis represents a compensatory mechanism to maintain horizontal gaze, but this compensation can precipitate degenerative changes in the lower cervical spine over time 2

Radiographic Measurements

• Normal C0-C2 angle averages 7.8° (SD 1.0°) of lordosis, while patients with swan neck deformity present with mean kyphosis of -14.4° (SD 9.5°) 1
• Normal C2-C7 angle averages 18.6° (SD 11.2°), while swan neck patients demonstrate excessive hyperlordosis averaging 43° (SD 2.8°) 1
• A significant correlation exists between upper and lower cervical alignment changes (R = 0.133; P < 0.01), demonstrating the biomechanical interdependence 1

Reversibility and Treatment Implications

• The subaxial hyperlordosis is reversible—when the primary occipitoaxial kyphosis is surgically corrected, the compensatory subaxial lordosis auto-corrects without direct intervention 1
• This reversibility demonstrates that the subaxial deformity is adaptive rather than fixed, eliminating the need for multilevel surgical correction in most cases 1
• Surgical correction focuses on reducing the atlantoaxial dislocation and restoring normal occipitoaxial alignment through anterior release (when needed) and posterior fixation 2, 1

Surgical Approach and Outcomes

• One-stage transoral anterior atlantoaxial release combined with posterior internal fixation achieves anatomic reduction in 76% of cases (25/33 patients) and partial reduction in the remainder 2
• Anterior release can be performed without odontoid resection in most cases, as the irreducibility results from contracted muscles, ligaments, and joint capsules rather than bony obstruction 2
• Posterior fixation options include C1-C2 transarticular screws, C1-C2 pedicle screw-plate constructs, or occipitocervical fixation depending on the specific pathology—occipitocervical fixation was required in 79% of cases (26/33 patients) 2
• Neurological improvement occurs in all but one patient with preoperative deficits, with mean follow-up of 33.7 months demonstrating sustained benefit 2
• Japanese Orthopaedic Society scores improve significantly postoperatively (P < 0.01) with low complication rates 1

Critical Distinction from Hand Deformity

• The term "swan neck deformity" originated in hand surgery to describe hyperextension at the proximal interphalangeal joint with flexion at the distal interphalangeal joint in rheumatoid arthritis 3, 4, 5
• When applied to the cervical spine, the term describes a fundamentally different pathomechanical process involving segmental kyphosis and lordosis rather than individual joint hyperextension 2, 1
• Do not confuse these two distinct entities—cervical swan neck deformity requires spinal imaging and neurosurgical evaluation, while hand swan neck deformity requires hand surgery assessment 3, 1

Common Pitfalls

• Failing to recognize that correction of the primary upper cervical deformity alone is sufficient—attempting to directly address the subaxial hyperlordosis is unnecessary and potentially harmful 1
• Performing odontoid resection when anterior release of contracted soft tissues alone would suffice for reduction 2
• Underestimating the neurological risk—three-quarters of patients present with myelopathy requiring urgent surgical intervention 2