## Clinical Scenario

A trainee is reviewing the anatomy of the atlantoaxial joint before assisting on a rheumatoid atlantoaxial fusion. The consultant asks about the unique biomechanical properties of the C1-C2 articulation. **What percentage of cervical rotation occurs at the atlantoaxial (C1-C2) joint?**

## Answer Options

Select all statements that are **TRUE**. This is an X-Type question where each option is independently True or False.

A 20-25%

B 30-35%

C 50%

D 70-75%

E 90-95%

## Detailed Explanations ### Option A: FALSE ✗

FALSE "20-25%"

**Why this is INCORRECT:** This underestimates atlantoaxial rotation: 1. **Too low**: - Actual is 50% - Significant underestimate - Common misconception - Not supported by literature - Wrong answer **Clinical Pearl**: The atlantoaxial joint (C1-C2) accounts for approximately 50% of total cervical rotation, making it the most mobile segment in the cervical spine. This is why atlantoaxial pathology significantly affects head rotation.

### Option B: FALSE ✗

FALSE "30-35%"

**Why this is INCORRECT:** Still underestimates the contribution: 1. **Below actual value**: - Correct answer is 50% - Closer than Option A - But still incorrect - Not accurate - Wrong choice **Clinical Pearl**: While 30-35% is closer to reality than 20-25%, it still significantly underestimates the true contribution of C1-C2 rotation to overall cervical motion.

### Option C: TRUE ✓

TRUE "50%"

**Why this is CORRECT:** Understanding C1-C2 biomechanics: 1. **50% of cervical rotation**: - **Gold standard answer** - Well-established biomechanics - Most mobile cervical segment - Unique anatomy - Critical knowledge 2. **Anatomical basis**: - Odontoid peg as pivot - No disc at C1-C2 - Synovial joints - Ligamentous stability - Rotational design 3. **Clinical significance**: - Arthritis affects rotation - Fusion reduces rotation 50% - Patient counseling important - Functional impact - Rehabilitation consideration 4. **Remaining rotation**: - Other 50% from C3-C7 - Distributed across levels - Smaller per segment - Compensatory potential - Recovery implications 5. **Pathology effects**: - RA commonly affects C1-C2 - Instability develops - Pain limits rotation - Fusion may be needed - Functional consequence 6. **Post-fusion expectations**: - Lose 50% rotation - Remaining 50% from subaxial - Some adaptation occurs - Counsel patients - Realistic expectations **Clinical Pearl**: The C1-C2 joint accounts for 50% of total cervical rotation due to the unique atlantoaxial articulation around the odontoid peg. C1-C2 fusion therefore reduces cervical rotation by approximately half, which patients must understand during surgical consent.

### Option D: FALSE ✗

FALSE "70-75%"

**Why this is INCORRECT:** This overestimates the contribution: 1. **Too high**: - Actual is 50% - Significant overestimate - Would leave little for subaxial - Biomechanically impossible - Incorrect answer **Clinical Pearl**: While C1-C2 is the most mobile cervical segment, it does not account for 70-75% of rotation. This would leave minimal rotation for the remaining cervical spine, which is not physiologically accurate.

### Option E: FALSE ✗

FALSE "90-95%"

**Why this is INCORRECT:** Extreme overestimate: 1. **Far too high**: - Actual is 50% - Massive overestimate - Biomechanically untenable - Clearly wrong - Distractor option **Clinical Pearl**: 90-95% would mean essentially all cervical rotation occurs at C1-C2, which is obviously incorrect. The subaxial spine (C3-C7) contributes the other 50% of cervical rotation.

--- ### Exam Tip

**High-Yield Points for Atlantoaxial Biomechanics:** - **C1-C2 rotation**: 50% of total cervical rotation - **Odontoid peg**: Acts as pivot point for rotation - **No disc**: C1-C2 is synovial articulation without intervertebral disc - **RA predilection**: Commonly affected in rheumatoid arthritis - **Fusion consequence**: Lose approximately 50% of cervical rotation - **Patient counseling**: Must explain functional impact before surgery - **Key Teaching**: "C1-C2 accounts for half of all cervical rotation"

--- ### References

- Dvorak J, et al. CT-functional diagnostics of the rotatory instability of upper cervical spine. Spine. 1987;12(3):197-205. - Panjabi MM, et al. Three-dimensional movements of the upper cervical spine. Spine. 1988;13(7):726-730. - Oda T, et al. Natural course of cervical spine lesions in rheumatoid arthritis. J Rheumatol. 1995;22(12):2386-2391.

## Clinical Scenario

A 62-year-old woman with a 20-year history of rheumatoid arthritis presents with neck pain, occipital headaches, and recent onset of hand numbness and clumsiness. Flexion-extension cervical radiographs reveal an atlantodental interval (ADI) of 8mm in flexion. **What does this ADI measurement indicate, and what is the normal threshold in adults?**

## Answer Options

Select all statements that are **TRUE**. This is an X-Type question where each option is independently True or False.

A Normal finding; ADI up to 10mm is acceptable in adults

B Borderline instability; ADI over 5mm is concerning, observe and repeat imaging

C Transverse ligament incompetence; ADI over 3.5mm in adults indicates instability

D Alar ligament injury only; transverse ligament is intact up to ADI of 6mm

E Normal variant in rheumatoid patients; surgical intervention only if ADI over 10mm

## Clinical Scenario

## Answer Options

Select all statements that are **TRUE**. This is an X-Type question where each option is independently True or False.

A Normal finding; ADI up to 10mm is acceptable in adults

B Borderline instability; ADI over 5mm is concerning, observe and repeat imaging

C Transverse ligament incompetence; ADI over 3.5mm in adults indicates instability

D Alar ligament injury only; transverse ligament is intact up to ADI of 6mm

E Normal variant in rheumatoid patients; surgical intervention only if ADI over 10mm

## Detailed Explanations ### Option A: FALSE ✗

FALSE "Normal finding; ADI up to 10mm is acceptable in adults"

**Why this is INCORRECT:** This significantly overestimates the normal ADI: 1. **Far too high**: - Normal adult ADI: less than 3.5mm - This patient has 8mm - 10mm threshold incorrect - Major instability present - Urgent intervention needed **Clinical Pearl**: An ADI of 10mm indicates severe atlantoaxial instability with significant risk of spinal cord compression. The normal threshold in adults is less than 3.5mm, and in children less than 5mm due to greater ligamentous laxity.

### Option B: FALSE ✗

FALSE "Borderline instability; ADI over 5mm is concerning, observe and repeat imaging"

**Why this is INCORRECT:** This underestimates the severity: 1. **Too conservative**: - ADI of 8mm is NOT borderline - Observation inappropriate - Active intervention required - Patient has myelopathy symptoms - Surgical candidate **Clinical Pearl**: An ADI greater than 5mm in adults indicates significant instability requiring surgical intervention, especially in the presence of neurological symptoms like hand numbness and clumsiness (myelopathy).

### Option C: TRUE ✓

TRUE "Transverse ligament incompetence; ADI over 3.5mm in adults indicates instability"

**Why this is CORRECT:** Understanding ADI and transverse ligament: 1. **Normal ADI threshold**: - **Adults: less than 3.5mm** - Children: less than 5mm - Well-established criterion - Gold standard measurement - Critical knowledge 2. **Transverse ligament function**: - Primary stabilizer of C1-C2 - Holds odontoid against C1 - Prevents anterior subluxation - Essential for stability - Key anatomical restraint 3. **This patient's ADI**: - **8mm in flexion** - Exceeds normal by greater than 2x - Indicates incompetence - Transverse ligament failure - Requires surgical stabilization 4. **Rheumatoid pathophysiology**: - Pannus formation - Ligamentous erosion - Transverse ligament disruption - Progressive instability - Common presentation 5. **Clinical significance**: - ADI greater than 3.5mm = instability - Greater than 7-8mm = high risk - Myelopathy symptoms present - Urgent surgical evaluation - Fusion indicated 6. **Posterior atlantodental interval**: - PADI more important - Space available for cord - Less than 14mm concerning - Correlates with myelopathy - Assess on imaging **Clinical Pearl**: The transverse ligament is the primary stabilizer preventing anterior atlas translation on the axis. ADI greater than 3.5mm in adults indicates transverse ligament incompetence. ADI of 8mm with myelopathy symptoms is an indication for C1-C2 fusion.

### Option D: FALSE ✗

FALSE "Alar ligament injury only; transverse ligament is intact up to ADI of 6mm"

**Why this is INCORRECT:** This misunderstands atlantoaxial anatomy: 1. **Incorrect threshold**: - Transverse ligament fails at ADI greater than 3.5mm - Not 6mm - Wrong anatomical understanding - Transverse is primary restraint - Alar ligaments secondary **Clinical Pearl**: The alar ligaments provide rotational stability and limit lateral flexion, but the transverse ligament is the primary restraint to anterior translation. ADI greater than 3.5mm indicates transverse ligament failure, not isolated alar ligament injury.

### Option E: FALSE ✗

FALSE "Normal variant in rheumatoid patients; surgical intervention only if ADI over 10mm"

**Why this is INCORRECT:** This is dangerously incorrect: 1. **NOT a normal variant**: - ADI of 8mm pathological - Not acceptable in RA - Requires intervention - Myelopathy present - Surgical urgency **Clinical Pearl**: Rheumatoid arthritis commonly affects the atlantoaxial joint due to synovial inflammation and pannus formation. An ADI of 8mm with myelopathy symptoms is an absolute indication for surgical fusion, not observation.

--- ### Exam Tip

**High-Yield Points for Atlantoaxial Instability:** - **ADI normal threshold**: Less than 3.5mm in adults, less than 5mm in children - **Transverse ligament**: Primary stabilizer preventing anterior subluxation - **PADI (Posterior ADI)**: Space available for cord; less than 14mm concerning - **Rheumatoid arthritis**: Common cause of atlantoaxial instability - **Myelopathy symptoms**: Hand numbness, clumsiness, gait disturbance - **Surgical indications**: ADI greater than 7-8mm, myelopathy, progressive instability - **Key Teaching**: "ADI greater than 3.5mm = transverse ligament incompetence"

--- ### References

- Boden SD, et al. Rheumatoid arthritis of the cervical spine. J Am Acad Orthop Surg. 1994;2(5):275-288. - Ranawat CS, et al. Cervical spine fusion in rheumatoid arthritis. J Bone Joint Surg Am. 1979;61(7):1003-1010. - Dickman CA, et al. Posterior atlantoaxial facet screw fixation. J Neurosurg. 1996;85(2):221-226.

## Clinical Scenario

A surgeon is planning posterior C1-C2 fusion using the Harms technique (C1 lateral mass screws and C2 pedicle screws) for a patient with atlantoaxial instability. **What is the MOST important anatomical structure at risk during C2 pedicle screw placement, and how should it be avoided?**

## Answer Options

Select all statements that are **TRUE**. This is an X-Type question where each option is independently True or False.

A Spinal cord; use lateral fluoroscopy to avoid medial screw breach

B C2 nerve root; sacrifice is acceptable with minimal morbidity

C Vertebral artery; assess for high-riding VA on preoperative CT and use appropriate trajectory

D Internal carotid artery; avoid anterior cortical breach with screw length

E Hypoglossal nerve; use neuromonitoring to avoid injury during C1 lateral mass drilling

## Detailed Explanations ### Option A: FALSE ✗

FALSE "Spinal cord; use lateral fluoroscopy to avoid medial screw breach"

**Why this is INCORRECT:** While spinal cord injury is serious, it's not the most common risk: 1. **Less common risk**: - Medial breach rare - Pedicle trajectory avoids cord - Vertebral artery more at risk - Different primary concern - Wrong answer **Clinical Pearl**: While spinal cord injury is catastrophic, the vertebral artery is the structure most commonly at risk during C2 pedicle screw placement due to anatomical variability, particularly high-riding vertebral arteries.

### Option B: FALSE ✗

FALSE "C2 nerve root; sacrifice is acceptable with minimal morbidity"

**Why this is INCORRECT:** This misrepresents C2 nerve root importance: 1. **Sacrifice has morbidity**: - Occipital neuralgia - Numbness behind ear - Chronic pain possible - Not minimal morbidity - Avoid if possible **Clinical Pearl**: The C2 nerve root (greater occipital nerve) provides sensation to the occiput. While some surgeons sacrifice it for better screw trajectory, it can cause occipital numbness and neuralgia. Most surgeons preserve it when possible.

### Option C: TRUE ✓

TRUE "Vertebral artery; assess for high-riding VA on preoperative CT and use appropriate trajectory"

**Why this is CORRECT:** Understanding vertebral artery anatomy at C2: 1. **Most important risk**: - **Vertebral artery injury** - Most common complication - Can be catastrophic - Stroke risk - Critical structure 2. **Anatomical variability**: - **High-riding vertebral artery** - Courses through C2 pedicle - Up to 20% of patients - Cannot place screw safely - Must identify preoperatively 3. **Preoperative assessment**: - **CT angiography or CT with reconstruction** - Assess VA course - Measure pedicle dimensions - Identify anomalies - Plan screw trajectory 4. **CT findings to assess**: - Pedicle width - VA position - Foramen transversarium location - Bone quality - Anatomical variations 5. **Surgical technique**: - Entry point: medial-superior C2 facet - Trajectory: 20-25° medial, 25-30° cephalad - Palpate medial wall - Use image guidance - Avoid lateral breach 6. **Alternative if unsafe**: - C2 pars screw - C2 laminar screw - Skip C2, fuse C1-C3 - Transarticular screw - Patient-specific decision **Clinical Pearl**: High-riding vertebral artery (VA within or near the C2 pedicle) occurs in up to 20% of patients. Preoperative CT with thin cuts through C2 is essential to assess pedicle anatomy and VA course. If pedicle width is less than 4mm or VA is high-riding, consider alternative fixation (pars screw, laminar screw, or transarticular screw).

### Option D: FALSE ✗

FALSE "Internal carotid artery; avoid anterior cortical breach with screw length"

**Why this is INCORRECT:** This misidentifies the anatomical structure at risk: 1. **Wrong structure**: - Internal carotid not at risk - Too anterior and lateral - Not near screw trajectory - Vertebral artery is concern - Incorrect answer **Clinical Pearl**: The internal carotid artery is not at risk during C2 pedicle screw placement. The vertebral artery, which runs through the foramen transversarium at C2, is the vascular structure at risk with lateral pedicle breach.

### Option E: FALSE ✗

FALSE "Hypoglossal nerve; use neuromonitoring to avoid injury during C1 lateral mass drilling"

**Why this is INCORRECT:** This misidentifies the anatomical structure: 1. **Wrong structure**: - Hypoglossal not at risk at C1 - Courses more caudally - C2 nerve root at risk - Not primary concern - Incorrect answer **Clinical Pearl**: The hypoglossal nerve (CN XII) exits the skull at the hypoglossal canal and courses anteriorly to innervate the tongue. It is not at risk during C1 lateral mass screw placement. The C2 nerve root is the neural structure at risk during C1 lateral mass screw placement.

--- ### Exam Tip

**High-Yield Points for Harms Technique (C1-C2 Fusion):** - **C2 pedicle screw**: MOST at risk = vertebral artery - **High-riding VA**: 20% of patients, assess on preoperative CT - **C2 entry point**: Medial-superior C2 facet - **C2 trajectory**: 20-25° medial, 25-30° cephalad - **Unsafe pedicle**: Width less than 4mm or high-riding VA - **Alternatives**: C2 pars screw, C2 laminar screw, transarticular screw - **C1 lateral mass**: 3-4mm lateral to midline, 10mm above C2 facet - **Key Teaching**: "Always assess C2 pedicle anatomy and VA course on preoperative CT"

--- ### References

- Harms J, Melcher RP. Posterior C1-C2 fusion with polyaxial screw and rod fixation. Spine. 2001;26(22):2467-2471. - Wright NM, Lauryssen C. Vertebral artery injury in C1-2 transarticular screw fixation: results of a survey of the AANS/CNS section on disorders of the spine and peripheral nerves. J Neurosurg. 1998;88(4):634-640. - Yeom JS, et al. Risk of vertebral artery injury: comparison between C1-C2 transarticular and C2 pedicle screws. Spine J. 2013;13(7):775-785.

Atlantoaxial Arthritis MCQ

Atlantoaxial Arthritis MCQ