Anderson–D’Alonzo — Odontoid (Dens) Fractures

Fractures of the odontoid process (dens) of the axis (C2 vertebra) are the most common cervical spine fractures in the elderly, accounting for approximately 10–15% of all cervical fractures. They present a unique management challenge because of their anatomical location at the craniocervical junction — where the dens articulates with the anterior arch of the atlas (C1) and is constrained by the transverse ligament — and because of the profound variability in the natural history of different fracture subtypes. The Anderson-D`Alonzo classification (1974) remains the universally used system for describing odontoid fractures and directly guides management: Type I and III fractures generally heal with immobilisation; Type II fractures (the most common and most controversial) are associated with high non-union rates and frequently require surgical stabilisation.

• Anatomy: the odontoid process is the superiorly projecting peg of the axis (C2) that articulates with the anterior arch of C1 and is held in position by the transverse ligament of the atlas (the strongest ligament of the C1-C2 complex); the alar ligaments (from the dens to the occipital condyles) and the apical ligament (from the tip of the dens to the anterior foramen magnum) also provide stability; the blood supply to the dens is from branches of the vertebral arteries entering at the base — the tip of the dens (Type I) is relatively avascular; the waist of the dens at the base (Type II) is the most vulnerable to non-union from poor vascularity
• Bimodal age distribution: a young, high-energy peak (road traffic accidents, sports — typically Type II displaced) and an elderly, low-energy peak (falls from standing — the most common cause of odontoid fracture in the over-65 population; a fall directly onto the back of the head is the classic mechanism); the elderly osteoporotic patient with a Type II odontoid fracture represents one of the most challenging clinical decisions in spine surgery

• Risk factors for non-union in Type II (the `Rule of 5s`): age >50 years; displacement >5 mm; posterior displacement; comminution; delay in presentation (>6 weeks); osteoporosis; any single factor increases non-union risk — multiple factors = very high non-union rate with conservative management; if more than one risk factor is present, surgical stabilisation is generally recommended
• Anterior odontoid screw fixation (AOSF): indications — acute (<6 weeks) Type II fracture with anterior or horizontal displacement; no comminution at the fracture; young patient; adequate bone stock; normal cervical lordosis (allows screw trajectory); contraindications — posterior displacement (screw placement would distract rather than compress the fracture), comminution, delayed presentation (>6 weeks — fibrous tissue in the fracture gap prevents compression), osteoporosis (poor screw purchase), barrel-chest anatomy (limits fluoroscopic trajectory), pre-existing cervical kyphosis
• Posterior C1-C2 fusion — Harms technique: the modern standard; C1 lateral mass screws + C2 pedicle/isthmic screws connected by rods; the Harms technique provides excellent biomechanical stability with rigid segmental fixation; its primary disadvantage is the loss of approximately 50° of cervical rotation (the C1-C2 joint contributes ~50% of total cervical rotation — the `yes` movement); bone graft is packed between C1 and C2; union rates >95%; the Brooks fusion (wiring of C1 to C2 with bone graft) and the Magerl transarticular screw technique are older alternatives; the Harms technique has superseded these at most centres due to better fixation in osteoporotic bone
• Conservative management (halo vest or rigid collar): the halo vest provides the most rigid cervical immobilisation available non-surgically; it consists of a halo ring (4 pins anchored in the outer table of the skull) connected to a rigid vest by 4 upright rods; it immobilises the upper cervical spine; halo vest complications in the elderly: pin-site infections (most common), pin loosening, respiratory restriction, pressure sores, dysphagia, and a paradoxical `snaking motion` at C1-C2 despite apparently rigid external fixation; rigid collar (Philadelphia or Miami J collar): provides less rigid immobilisation than halo; appropriate for lower-risk Type II fractures and most Type III fractures

• Anderson-D`Alonzo: Type I (dens tip — rare, stable, non-op, but check for craniocervical instability); Type II (dens base/waist — most common 60%, high non-union risk, controversial management); Type III (C2 body — good vascularity, low non-union, non-op in most)
• Type II non-union risk factors (`Rule of 5s`): age >50, displacement >5 mm, posterior displacement, comminution, delayed presentation (>6 weeks); any one factor increases risk; multiple factors = surgical indication
• Anterior odontoid screw: preserves C1-C2 rotation; for acute anterior/horizontal displacement; contraindicated for posterior displacement (screw distracts the fracture), comminution, and delayed presentation; requires intact transverse band to guide screw trajectory
• Posterior C1-C2 fusion (Harms): most reliable fixation; union >95%; loses ~50% of cervical rotation; C1 lateral mass + C2 pedicle screws; preferred for elderly, osteoporosis, posterior displacement, comminution; the modern standard for surgical C1-C2 stabilisation
• Type I significance: stable in isolation BUT may indicate craniocervical instability; assess the occipitoatlantal interval (Powers ratio, atlantodental interval); a Type I fracture with craniocervical instability = surgical occipitoaxial fusion (occipito-C2 fusion)
• Elderly Type II dilemma: very high non-union with conservative management but surgical risk is also high; halo vest in elderly has its own significant morbidity (pin problems, respiratory restriction); the optimal management remains debated; most spine surgeons in the UK favour surgical stabilisation (Harms C1-C2 fusion) for displaced Type II in elderly patients fit for surgery
• Neurological deficit with odontoid fracture: the `atlas clamp` — the spinal cord is compressed between the displaced dens/C1 and the posterior C1 arch; presents as myelopathy (central cord syndrome most common); urgent reduction (traction) and stabilisation; incomplete deficit = best chance of recovery with decompression