Klippel-Feil syndrome

1. High-Yield Summary

• Definition: Klippel-Feil syndrome (KFS) is a congenital musculoskeletal disorder characterized by failure of normal cervical somite segmentation between the 3rd and 8th gestational weeks, resulting in congenital synostosis of two or more cervical vertebrae.
• Classic Clinical Triad: The hallmark triad of a low posterior hairline, short neck (brevicollis), and severely restricted cervical range of motion is clinically evident in only approximately 50% of confirmed cases.
• Mechanical Risk Stratification: Neurological vulnerability is governed by hypermobile open motion segments adjacent to fused blocks (especially Samartzis Type II non-contiguous synostosis patterns and high-cervical C1–C3 fusions), which dramatically accelerate adjacent segment degeneration and heighten the risk of catastrophic cord injury after minor trauma.
• Mandatory Extraspinal Screening: Comprehensive systemic evaluation is mandatory upon initial diagnosis, including renal ultrasound (35% prevalence of silent renal agenesis or ectopia), formal audiometry (30% hearing impairment), and scoliosis/shoulder screening (50% to 60% scoliosis, 25% to 30% Sprengel deformity).
• Critical Airway Safety: Forced cervical hyperextension during endotracheal intubation is strictly contraindicated due to cervical spine rigidity and risk of cord compression; awake fiberoptic intubation or video laryngoscopy with neutral in-line stabilization is mandatory.

2. Anatomy

Congenital cervical synostosis encompasses morphological aberrations spanning the craniocervical junction (occiput, atlas C1, axis C2) and the subaxial cervical spine (C3–C7). The primary pathology is characterized by complete or partial absence of intervertebral discs with continuous osseous bridging across vertebral bodies, laminae, spinous processes, and facet joints. A characteristic developmental waist-like narrowing (the “wasp-waist” sign) is seen at the level of the rudimentary intervertebral disc space on coronal and sagittal views.

Neurovascular Danger Zones

• Vertebral Arteries: As the vertebral artery ascends through the transverse foramina from C6 to C1, congenital segmentation failures frequently alter its course. Patients with Klippel-Feil syndrome demonstrate a high prevalence of anomalous, tortuous, or ectatic vertebral arteries, absent bony septa between adjacent transverse foramina, or unilateral vessel hypoplasia/aplasia. Direct decompression, anterior corpectomy, and posterior lateral mass or pedicle screw placement carry substantial risk of arterial laceration unless mapped with preoperative vascular imaging.
• Cervicomedullary Junction: Basilar invagination, platybasia, and occipital-cervical assimilation (occipitalization of the atlas) frequently coexist with high-cervical synostosis. In this danger zone, the odontoid process projects abnormally superiorly into the foramen magnum, directly compressing the ventral medulla and upper cervical cord.
• Cervical Nerve Roots: While fused motion segments protect exiting nerve roots from acute disc herniation, severe compensatory hypermobility at adjacent open interspaces accelerates facet arthrosis and uncinate spurring, causing progressive neuroforaminal impingement and radiculopathy.

Internervous Planes & Surgical Approaches

• Anterior Smith-Robinson Approach: The standard anterior cervical internervous plane is established between the sternocleidomastoid muscle (innervated by the spinal accessory nerve, cranial nerve XI) laterally, and the strap muscles (sternohyoid, sternothyroid, omohyoid, innervated by the ansa cervicalis) and visceral bundle (esophagus, trachea, recurrent laryngeal nerve) medially. In Klippel-Feil patients, distorted surface landmarks, high clavicles, and elevated scapulae often restrict surgical corridors.
• Posterior Cervical Approach: Utilizes the true avascular midline intermuscular raphe, splitting the ligamentum nuchae and dividing the trapezius, splenius capitis, and semispinalis cervicis to access the posterior elements. Subperiosteal dissection must proceed with extreme caution where posterior elements have failed to fuse (spina bifida occulta).

Critical Bony Landmarks

Essential operative landmarks include the external occipital protuberance (inion), posterior arch of C1, bifid spinous process of C2, and the carotid tubercle of Chassaignac at C6. In fused synostotic segments, standard anatomical landmarks are frequently effaced; fusion of the laminae and spinous processes creates a monolithic bone shield that obscures normal interlaminar spaces and necessitates intraoperative fluoroscopy or computed tomography image guidance.

3. Pathogenesis

Klippel-Feil syndrome arises from a primary disruption of embryonic somitogenesis during the 3rd to 8th weeks of intrauterine development. During normal development, paired paraxial mesoderm somites undergo resegmentation: the caudal half of one sclerotome joins the cranial half of the adjacent sclerotome to form a primitive precartilaginous vertebral body, while the intervening mesenchyme forms the intervertebral disc. Failure of this segmentation process results in uninterrupted chondrification and endochondral ossification across adjacent vertebral segments.

Molecular Genetics

• GDF6 (Growth Differentiation Factor 6): Mutations on chromosome 8q22 are linked to autosomal dominant Klippel-Feil syndrome type 1 (KFS1). GDF6 is an essential bone morphogenetic protein regulatory factor governing vertebral segmentation and chondrogenesis.
• MEOX1 (Mesenchyme Homeobox 1): Homozygous loss-of-function mutations on chromosome 17q21 cause autosomal recessive KFS2, severely impairing sclerotome compartmentalization.
• GDF3 (Growth Differentiation Factor 3): Mutations on chromosome 12p13 result in KFS3, exhibiting both dominant and recessive inheritance patterns with variable clinical expressivity.
• Notch Signaling Disruptions: Disruptions in the oscillating Notch-Hes segmentation clock (involving genes such as HES7 and LFNG) alter somite boundary specification, producing synchronized disruptions of the axial skeleton, ribs, and urinary tract.

Biomechanical Mechanisms of Failure

The biomechanical behavior of the cervical spine is governed by the length and distribution of the fused segments. Congenitally fused vertebrae act as rigid, non-compliant lever arms. Physiological cervical flexion, extension, and axial rotation are concentrated entirely across the remaining open intervertebral spaces. This focal stress concentration produces severe shear stresses, accelerated disc degeneration, facet joint hypertrophy, ligamentum flavum laxity or redundancy, and secondary osteophytosis. The most hazardous biomechanical configuration occurs when an open mobile motion segment is bounded cranially and caudally by fusion blocks (the Samartzis Type II pattern), concentrating angular displacement across a single disc space and predisposing to hypermobility, progressive subluxation, and spinal cord compression.

4. Natural History

Klippel-Feil syndrome occurs in approximately 1 in 40,000 to 42,000 live births, although radiographic evidence of isolated congenital cervical fusion is reported in up to 0.7% of the general population. A slight female predominance exists, with a female-to-male ratio of roughly 1.3:1 to 1.5:1.

The natural history of the condition depends heavily on the anatomical pattern of cervical synostosis. Patients with isolated lower cervical fusions (e.g., C5–C6) frequently remain asymptomatic throughout life, with normal longevity and minimal functional impairment. Conversely, patients harboring extensive multisegmental fusions or high-cervical anomalies (C1–C3 or occipitocervical synostosis) exhibit progressive cervical degenerative disease, early-onset disc herniations, and adjacent segment instability by the third to fifth decades of life. A subset of patients presents with acute catastrophic spinal cord injury or transient quadriplegia following minor, low-velocity falls or athletic impacts due to focal cervical lever-arm mechanics.

Associated Systemic Anomalies

• Spinal Deformity (50% to 60%): Congenital scoliosis, thoracic hemivertebrae, and kyphoscoliosis frequently develop and can produce progressive trunk decompensation requiring corrective stabilization.
• Renal Anomalies (35%): Unilateral renal agenesis is the most prevalent malformation; horseshoe kidneys, duplicate collecting systems, hydronephrosis, and renal ectopia are also common and are often entirely asymptomatic until revealed by screening.
• Sprengel Deformity (25% to 30%): Congenital elevation of the scapula occurs when embryonic scapular caudal migration fails, often accompanied by an omovertebral fibrous, cartilaginous, or bony band connecting the superior medial border of the scapula to the cervical spinous processes.
• Hearing Impairment (30%): Sensorineural, conductive, or mixed hearing loss frequently results from ossicular dysgenesis or inner ear malformations.
• Cardiovascular Anomalies (4% to 14%): Congenital structural cardiac defects, including ventricular septal defects (VSD), patent ductus arteriosus (PDA), and aortic coarctation.
• Neurological Syncopies & Synkinesia (15% to 20%): Mirror movements (involuntary imitation by the contralateral hand during unilateral motor tasks) arise from incomplete decussation of descending corticospinal pathways. Cranial nerve palsies, Chiari I malformation, and syringomyelia (up to 10%) are additional critical associations.

5. Clinical Presentation

History

Clinical evaluation begins with an inquiry into the onset, duration, and progression of cervical symptoms. Patients commonly describe chronic neck stiffness, persistent axial neck pain, and diminished lateral visual fields requiring whole-body turning. Key red-flag historical questions must explore:

• Upper extremity radicular pain, numbness, paresthesias, or hand intrinsic weakness (e.g., difficulty buttoning garments or dropping keys).
• Myelopathic symptoms, including balance unsteadiness, frequent stumbling, spasticity, heavy leg sensations, or bowel and bladder hesitancy.
• History of transient neurological shock, burning dysesthesias, or temporary quadriparesis (“burning hands syndrome”) following minor trauma.
• Family history of congenital skeletal anomalies or developmental auditory and renal impairments.

Physical Examination & Provocative Maneuvers

Physical inspection targets the classic triad: short neck (brevicollis), low posterior hairline extending down to the interscapular region, and severe restriction of cervical range of motion. Shoulder asymmetry, thoracic scoliosis, and elevated scapulae (Sprengel deformity) should be systematically inspected. A complete neurological exam must be performed, including cranial nerves, motor grading, sensory dermatomes, and upper motor neuron reflexes.

• Spurling Maneuver: The cervical spine is positioned in extension, lateral bending, and axial compression toward the affected side -> Positive Finding: Reproduction of ipsilateral radiating radicular dermatomal arm pain [Sensitivity: 30% to 50%, Specificity: 93% to 100%].
• Hoffman Reflex: The examiner stabilizes the patient’s middle finger proximal interphalangeal joint and sharply flicks the dorsal surface of the distal phalanx -> Positive Finding: Rapid involuntary flexion and adduction of the ipsilateral thumb and index finger, indicating upper motor neuron or cervical spinal cord compression [Sensitivity: 58% to 75%, Specificity: 78% to 88%].
• Inverted Radial Reflex: Percussion over the distal styloid process of the radius at the brachioradialis tendon insertion -> Positive Finding: Diminished brachioradialis contraction coupled with paradoxical brisk flexion of the fingers, confirming cervical cord myelopathy at the C5–C6 level [Sensitivity: 61%, Specificity: 85%].
• Babinski Sign: Firm stroking along the lateral aspect of the plantar surface of the foot from the calcaneus forward toward the base of the toes -> Positive Finding: Great toe dorsiflexion with spreading or fanning of the lesser toes [Sensitivity: 53%, Specificity: 99%].
• Tandem Gait & Romberg Testing: Assessment of heel-to-toe walking and balance with eyes closed -> Positive Finding: Truncal ataxia, broad-based unsteadiness, or loss of balance, indicating dorsal column proprioceptive impairment or spastic myelopathy.

6. Imaging and Diagnostic Studies

Accurate diagnosis and risk stratification require a multimodal diagnostic imaging strategy, beginning with dynamic cervical radiographs and proceeding to multiplanar cross-sectional imaging.

Plain Radiography

• Standard Projections: Standing anteroposterior (AP), neutral lateral, and open-mouth odontoid views. Radiographs demonstrate fused vertebral bodies, absence of disc spaces, flattened vertebral bodies with decreased anteroposterior diameter, narrowing at rudimentary intervertebral disc levels (“wasp-waist” sign), and fused posterior neural elements.
• Dynamic Flexion-Extension Lateral Views: Mandatory to identify occult atlantoaxial instability and subaxial adjacent segment hypermobility. Critical parameters include an Anterior Atlanto-Dens Interval (ADI) > 3.5 mm in adults (> 5 mm in children) indicating transverse atlantal ligament compromise, and subaxial translation > 3.5 mm or dynamic sagittal angular displacement > 11° confirming adjacent segment structural instability.

Computed Tomography (CT)

• High-Resolution Bone CT: Thin-slice (≤ 1 mm) helical CT with coronal and sagittal 2D multiplanar and 3D surface-rendered reconstructions is the gold standard for defining osseous anatomy. CT precisely delineates posterior arch defects (spina bifida occulta), facet joint synostosis, occipital-cervical assimilation, basilar invagination, and omovertebral bone bridges.
• Computed Tomography Angiography (CTA): Mandatory prior to any planned surgical instrumentation or corpectomy to identify aberrant vertebral artery courses, fenestrations, asymmetric caliber, or anomalous entry into the foramen magnum.

Magnetic Resonance Imaging (MRI)

MRI of the brain and entire spinal axis is essential for soft tissue and neurological evaluation. High-resolution T1-weighted, T2-weighted, and short tau inversion recovery (STIR) sequences are evaluated for:

• Spinal Cord Integrity: Ventral and dorsal spinal cord compression, effacement of surrounding cerebrospinal fluid spaces, and intrinsic T2-weighted hyperintensity (myelomalacia, intramedullary gliosis, or focal edema).
• Craniocervical Malformations: Chiari I malformation (tonsillar herniation > 5 mm below the foramen magnum) and hindbrain crowding.
• Intramedullary Syrinx: Syringomyelia is identified in up to 10% of cases, commonly associated with basilar impression or Chiari malformation.
• Adjacent Disc Degeneration: High-stress adjacent open interspaces frequently show advanced disc herniations, annular tears, and endplate Modic changes.

Multisystem Screening Protocol

• Renal Ultrasonography: Mandatory for all newly diagnosed patients regardless of renal symptomatology to rule out solitary kidney, hydronephrosis, or ectopia.
• Formal Audiological Testing: Baseline pure-tone and speech audiometry to diagnose sensorineural or conductive hearing deficits.
• Transthoracic Echocardiogram & ECG: Cardiac screening for underlying septal defects or coarctation.

7. Classification

Historically, Klippel-Feil syndrome was categorized using the anatomical schema proposed by Maurice Feil in 1919. In modern clinical practice, this has been supplemented by the radiographic classification introduced by D.D. Samartzis and colleagues (2006), which correlates fusion patterns directly with mechanical stability and neurological risk.

8. Treatment

Medical / Nonoperative

Nonoperative management is the primary strategy for asymptomatic patients and those with mild axial neck pain without neurological compromise or structural instability.

• Activity Restrictions: Strict prohibition of collision sports (football, rugby, ice hockey, wrestling, diving, gymnastics, martial arts) is indicated for patients with high-risk anatomical configurations (Samartzis Type II non-contiguous fusions, C1–C3 fusions, occipitalization of the atlas, basilar invagination, or dynamic ADI > 3 mm). Low-contact or non-contact activities (swimming, cycling, low-impact conditioning) are permitted for low-risk single-level subaxial fusions without instability.
• Physical Therapy: Low-resistance isometric cervical strengthening, postural training, and scapular stabilization. High-velocity, passive manipulation of the cervical spine is strictly contraindicated.
• Pharmacotherapy: First-line nonsteroidal anti-inflammatory drugs (NSAIDs), acetaminophen, short-course skeletal muscle relaxants for acute spasms, and neuropathic medications (gabapentin, pregabalin) for radicular symptoms.
• Orthoses: Temporary use of a semi-rigid cervical collar (e.g., Miami J or Aspen) during acute exacerbations; long-term immobilization is discouraged to prevent cervical muscle atrophy.

Operative Management

The primary objectives of surgical intervention are decompression of neural structures (spinal cord and exiting nerve roots), stabilization of hypermobile segments, and reconstruction of cervical alignment.

• Anterior Cervical Decompression and Fusion (ACDF) & Corpectomy: Indicated for ventral spinal cord compression resulting from adjacent segment disc herniations or uncinate/osteophytic spurs. Anterior plate instrumentation and interbody cage/autograft fusion re-establish disc height and achieve solid arthrodesis. In high-cervical or rigid deformities with anterior cord compression, transmandibular or high anterior corpectomy approaches may be required.
• Posterior Cervical Decompression & Instrumented Fusion: Posterior laminectomy alone is contraindicated in the presence of kyphosis or hypermobility due to risk of progressive deformity. Laminectomy or laminoplasty must be coupled with posterior instrumentation (lateral mass screws in the subaxial spine, pedicle screws where anatomy permits) and fusion across hypermobile unstable segments.
• Occipitocervical Arthrodesis: Rigid fixation from the occiput to C2 or the upper subaxial spine using an occipital plate, C2 pedicle/pars/translaminar screws, and connecting rods. Indicated for basilar invagination, irreducible atlantoaxial subluxation, or severe craniocervical pain with cranial settling.
• Sprengel Deformity Correction: In patients with severe cosmetic deformity or shoulder abduction < 90°, surgical relocation of the scapula via the Woodward procedure or Green procedure with omovertebral bone excision provides functional and aesthetic improvement.

Indications & Contraindications

Definitive Surgical Indications:

• Progressive degenerative cervical myelopathy (mJOA score ≤ 14, Nurick grade ≥ 2, or objective gait impairment).
• Radiographic cervical instability on dynamic imaging (ADI ≥ 3.5 mm in adults or > 4.5 mm in children; subaxial translation > 3.5 mm or dynamic angular motion > 11°).
• Persistent, debilitating radiculopathy refractory to ≥ 6 to 12 weeks of structured conservative therapy with concordant nerve root compression.
• Acute traumatic spinal cord injury with cord compression or unstable fracture-dislocation.

Contraindications:

• Asymptomatic congenital fusion without dynamic instability or canal stenosis.
• Isolated neck stiffness without neurological deficits or focal mechanical hypermobility.
• Active systemic sepsis or severe uncorrectable coagulopathy.
• Failure to establish a definitive airway plan with awake fiberoptic or video-laryngoscopic capabilities.

9. Outcome

Surgical decompression and stabilization halt neurological deterioration and achieve meaningful neurological recovery in approximately 85% to 90% of myelopathic patients. Postoperative functional recovery is quantified using validated patient-reported outcome measures, including the modified Japanese Orthopaedic Association (mJOA) score, the Nurick grade, the Neck Disability Index (NDI), and the SF-36 physical component score.

The predominant long-term threat to functional survivorship is accelerated adjacent segment disease (ASD). Biomechanical studies demonstrate that fusing segments creates abnormal lever arms that subject neighboring open disc spaces to heightened stress concentration. Longitudinal studies show that patients with congenital cervical fusions experience symptomatic adjacent segment degeneration at an annualized rate of 2.9% to 5.8%, frequently requiring extension of instrumentation and secondary decompression within 5 to 10 years after index surgery.

10. Complications

Due to anatomical distortions, altered vascular corridors, and localized biomechanical lever arms, surgical intervention in Klippel-Feil syndrome carries elevated complication rates compared to routine degenerative spine surgery.

11. Pearls and Pitfalls

• The Anesthesia Airway Pearl: Always notify the operative anesthesiologist well in advance that the patient has Klippel-Feil syndrome. Direct rigid laryngoscopy with neck hyperextension can precipitate irreversible quadriparesis; awake fiberoptic intubation or video-guided intubation in neutral position is mandatory.
• The Screening Pearl: Never evaluate a child or adult with Klippel-Feil syndrome as a purely orthopaedic spine patient. Order a baseline renal ultrasound and audiological assessment at the initial encounter to identify silent renal agenesis or hearing loss before complications arise.
• The Craniocervical Vigilance Pearl: Carefully scrutinize the odontoid process and the C1–C2 articulation. Basilar invagination and occipitalization of C1 often mimic simple degenerative spondylosis but carry immense risk of brainstem compression.
• The Sandwich Interspace Pitfall: Never overlook an isolated open disc space situated between two congenitally fused segments (Samartzis Type II). This motion segment absorbs almost all cervical kinematics and is prone to rapid occult translation and traumatic failure.
• The Sports Clearance Pitfall: Clearing an adolescent with multiple cervical fusions or high-cervical (C1–C3) involvement for collision sports is a catastrophic judgment error. High-impact collisions generate extreme shear across open segments, carrying risk of spinal cord transection.
• The Torticollis Misdiagnosis Pitfall: Avoid diagnosing a child presenting with neck tilt and asymmetry as simple congenital muscular torticollis without obtaining complete cervical radiographs. Physical therapy or sternocleidomastoid release in unrecognized Klippel-Feil syndrome can be disastrous.

12. Controversy

• Athletic Participation Guidelines: Determining athletic clearance in asymptomatic patients remains contentious. While traditional guidelines by Watkins strictly disqualify individuals with multi-segment fusions, high-cervical synostosis (C1–C3), or occipital assimilation from all collision sports, debates persist regarding whether patients with a single, stable lower-cervical fusion (C5–C6) and normal range of motion may safely participate in non-collision or limited-contact athletics.
• Cervical Disc Arthroplasty (CDA) vs. Arthrodesis: While motion-preserving cervical disc replacement is popular in routine degenerative spine surgery to mitigate adjacent segment pathology, its role in Klippel-Feil syndrome is highly controversial. Most spine surgeons consider congenital multi-segment fusion an absolute contraindication to disc arthroplasty due to abnormal facet joint orientation, severe hypermobility vectors, and unconstrained kinetic forces that precipitate early implant subsidence or mechanical dislocation.
• Prophylactic Fusion in Asymptomatic Hypermobility: Whether to perform prophylactic instrumented fusion in neurologically intact patients harboring dangerous anatomical patterns (such as severe non-contiguous Samartzis Type II fusions with adjacent translational laxity) remains unresolved. Proponents advocate early stabilization to prevent catastrophic spinal cord trauma, whereas opponents point to surgical morbidity, loss of remaining cervical motion, and lack of randomized prospective data.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Nouri A, Martin AR, Lange SF et al. Congenital Cervical Fusion as a Risk Factor for Development of Degenerative Cervical Myelopathy. World Neurosurg. 2017. doi:10.1016/j.wneu.2017.01.048 [Level I: Prospective Multicenter Comparative Cohort (AOSpine)]
• Nie JW, Sadeh M, Almadidy Z et al. Transmandibular Cervical Corpectomy for Persistent Spinal Cord Compression in a Patient With Klippel-Feil Syndrome: A Technical Note and Systematic Review. Oper Neurosurg. 2023. doi:10.1227/ons.0000000000000754 [Level I: Systematic Review & Meta-Analysis]
• Fisher M, Simonsen A, Stewart C et al. Comparing intubation techniques of Klippel–Feil syndrome patients in the last 10 years: a systematic review. J Osteopath Med. 2025. doi:10.1515/jom-2024-0136 [Level I: Systematic Review & Meta-Analysis]
• Hensinger RN, Lang JE, MacEwen GD. Klippel-Feil Syndrome. J Bone Joint Surg Am. 1974. doi:10.2106/00004623-197456060-00018 [Level IV: Retrospective Cohort Series]
• Samartzis DD, Shen FH, Herman J et al. Classification of Congenitally Fused Cervical Patterns in Klippel-Feil Patients. Spine. 2006. doi:10.1097/01.brs.0000239222.36505.46 [Level IV: Retrospective Radiographic Cohort & Classification]
• Pizzutillo PD, Woods M, Nicholson L, Cowell HR. Risk Factors in Klippel-Feil Syndrome. Spine. 1994. doi:10.1097/00007632-199409150-00020 [Level IV: Retrospective Cohort Risk Stratification]
• Tracy MR, Dormans JP, Kusumi K. Klippel-Feil Syndrome. Clin Orthop Relat Res. 2004. doi:10.1097/01.blo.0000130267.49895.20 [Level V: Expert Clinical & Molecular Genetic Review]
• Clarke RA, Catalan G, Diwan AD, Kere J. Heterogeneity in Klippel-Feil syndrome: a new classification. Pediatr Radiol. 1998. doi:10.1007/s002470050511 [Level IV: Radiological & Pedigree Genetic Cohort]

15. Master Class Pro-Tip

When operating on patients with congenital cervical synostosis, the primary surgical hazard is not the fused block itself, but the hidden mechanical hypermobility of the open interspaces and the aberrant trajectory of the vertebral arteries. Never position the patient’s head using manual traction or extreme extension; always pin and position in a rigid three-pin head holder under neutral cervical alignment with continuous somatosensory and motor evoked potential monitoring. Review preoperative thin-cut CT angiography meticulously to map the vertebral arteries through every transverse foramen: anomalous intra-foraminal loops or bone-deficient foramina are ubiquitous and forgive zero navigational errors. If anterior corpectomy or decompression is planned, stay strictly in the midline and confirm landmarks with intraoperative fluoroscopy or navigation before initiating burring.

Master Class Surgical Pearl: Congenital Cervical Synostosis & Craniovertebral Instability