‘Whole- body’ radiographs
- Are becoming more widespread
- Advances in detector technology have allowed imaging of a wider field with lower dose.
- 36 inch radiographs including
- C2
- C7
- Whole rib cage
- Pelvis
- Both femoral heads
- AP and lateral
- Ensure knees straight
- Arms up: to prevent anteversion
- Heads of femur visible
Positioning
Static radiograph
- Must be standardized in each unit
- cannot have one xray being done with hands on chest and the other with arms up
- Arms:
- “hands on cheeks” and “fists on clavicles”
- Shoulder flexion at 90° with arm extension creates a misinterpretation of sagittal alignment
- Standing
- Straight
- X-rays in coronal (AP radiograph) AND
- Sagittal plane (lateral radiograph)
- Right and left bending films to
- Assess curve flexibility AND
- Possibility of correction with surgical intervention
- Avoid
- Position of the arms
- Holding supports
- will change the centre of gravity and thus change the appearance of the spine on the radiographs.
Dynamic radiograph
- Supine or prone x-rays
- Bending x-rays → fundamental in:
- Predicting compensatory curve behaviour
- Applying Lenke classification
- Traction x-rays:
- Awake traction
- TRUGA: traction x-ray under general anesthetic
- Flexion/extension xray
- Look at the interspinous space if there is significant spreading of the spinous process during flexion, it is likely that flexion is adequate
- Combine supine and sitting x-rays: In wheelchair-bound patients
Parameters measured
General
- Look for congenital vertebral defects
Sagittal measurements - Kyphosis
Basic measurements
Skull base measurements
- Chamberlain’s line (number 2)
- extends from the hard palate to the opisthion (back of the foramen magnum)
- On xray
- the tip of the dens should be =<3 mm above this line.
- If Dens 6mm above this line = definitely pathology
- On CT and MRI the normal odontoid tip is 1.4mm(± 2.4) below the line
- Seldom used as opisthion is hard to see on Xray and may be invaginated
McGregor’s Slope (MGS)
- Measurement
- is the angle between the posterior edge of the hard palate ↔ the caudal aspect of the occiput
- McGregor’s line (number 1)
- extends from the hard palate to the most caudal point of the occipital curve
- dental tip should by less than 5 mm above this line.
- McGregor has modified chamberlain's line for when the opisthion cannot be identified.
- On CT and MRI the normal odontoid tip is 0.8mm (± 2.4) above the line
- A surrogate for CBVA, which can be diffcult to visualize on standing radiographs.
- Clinical significance
- MGS < 2.16 degrees has been suggested as a goal for for CSD correction,
- MGS of < 0 has been proposed as modier for CSD classication.
- A high MGS is pathologic, as MGS ≥ 20 is an inclusion criteria for cervical deformity in ISSG studies.
- Passias 2021 modifier
- Low: >-9° and <0°
- Moderate: -12° to -9° or 0° to 19°
- Severe: <-12° or >19°
McRae’s line (Number 3)
- from the basion (front of the foramen magnum) to the opisthion.
- Dens should not advance above this line .
- McRae’s line is also used to diagnose Chiari 1 malformations where the cerebellar tonsils, which should normally be above this line, are seen to descend more than 3mm in children and 5 mm in adults.
- The mean position of the odontoid tip below the line is
- 5mm (± 1.8mm SD) on CT
- 4.6mm (± 2.6mm SD) on MRI
Wackenheim’s clivus-canal line (WCCL):
- the odontoid should be tangential to or below the line that extends the course of the clivus (the clivus baseline).
- If the clivus is concave or convex, this baseline is drawn to connect the basion to the base of the posterior clinoids on the clivus26
Klaus index (Number 4)
- distance between tip of dens and the tuberculum–cruciate line between tuberculum (T) and internal occipital protuberance (IP).
- This measures depth of the posterior fossa
- A normal height is 40-41 mm. A decreased Klaus height index is seen in basilar invagination.
- Fischgold’s Digastric line:
- Line joining the digastric notches.
- The normal distance from this line to the middle of the atlantooccipital joint is 10mm (decreased in BI).
- No part of the odontoid should be above this line.
- More accurate than the bimastoid line (FBML)
- Fischgold’s Bimastoid line:
- Line joining the tips of mastoid processes
- The odontoid tip averages 2mm above this line (range: 3mm below to 10mm above) and this line should cross the atlantooccipital joint
- the tip of the odontoid process projects normally not more than 10 mm above this line
Clival angle:
- Normal clival angle (top) measured by the NTB angle of Welcker joining the nasion (N), tuberculum (T) and basion (B).
- The angle should be < 130°.
- Platybasia (middle) is marked by an increased NTB angle. This raises the basion and forces the foramen magnum plane (dotted line) to tilt upwards. (skull base angle>143°)
- The same upward tilt of this plane also occurs with a short clivus (lower)
Cervicomedullary angle (CMA):
- The angle between a line drawn through the long axis of the medulla on a sagittal MRI and a line drawn through the cervical spinal cord.
- Normal CMA is 135–170°.
- A CMA< 135° correlates with signs of
- Cervicomedullary compression
- Myelopathy
- C2 radiculopathy
- Disease
- Clival axial angle
- Bollo et al. identified radiological parameters that increased the likelihood of occipito-cervical fixation being required.
- descent of the obex as well as the tonsils, below the plane of the foramen magnum (Chiari 1.5)
- basilar invagination,
- A clivo-axial angle < 125 degrees
- Angle determined from the intersection of a line along the slope of the clivus (Wackenheims line) and a line drawn from the posteroinferior point of the C2 vertebral body up along the odontoid (posterior spinal line).
- Ranawat’s line
- Perpendicular distance between the centre of the sclerotic ring of C2 and a line drawn along the axis of the C1 vertebra.
- Normally 17 mm; a distance less than 13 mm would suggest basilar invagination
- Grabb Oakes Measurement
- Midline MRI T2 image used
- Line drawn from Basion to the inferior(bottom), posterior aspect of the C2 bone.
- Perpendicular line drawn to the dura
- Yellow line is the Grab oakes measurement
- A normal Grabb Oakes Measurement is 9mm or less
Cervical Lordosis (CL)
- Normal
- Range 20-40°
- Mean total CL is approximately −40° (neg; Lordosis), with, on average, the occiput-C1 segment being kyphotic.
- The largest percentage (75%–80%) of cervical standing lordosis is at C1–C2
- COG of the head sits almost directly above the centers of the C1 and C2 vertebral bodies there is a positive correlation with CL and increasing age
- Only 6° (15%) of lordosis occurs at the lowest 3 cervical levels (C4–C7).
- Not all flat cervical curvature or a kyphotic cervical spine is abnormal
- C2 Slope
- Passias 2021 modifiers
- Low: <33°
- Moderate: 33° to 49°
- Severe: >49°
- Clinical significance
- Surrogate for T1-CL Mismatch: It acts as a simplified, single-value measure reflecting the degree of T1 slope-cervical lordosis (T1-CL) mismatch.
- Correlation with HQROLs:
- A greater C2 slope (specifically 17–38 degrees) is associated with worse health-related quality of life (HQROLs).
- Classification Modifier:
- It has been proposed as a novel modifier to the Ames classification system for CSD.
- Surgical Target for Correction:
- A goal correction of C2 slope to less than 10 degrees has been proposed as a surgical target.
- Prediction and Prevention of DJK:
- Failure to correct C2 slope to less than 10 degrees is a radiographic predictor of distal junctional kyphosis (DJK).
- Achieving this target is associated with lower rates of DJK.
- Some surgical approaches prioritise correcting C2 slope to less than 10 degrees to achieve optimal clinical outcomes.
- Benzel: when positioning head for CVJ fusion make sure C2 slope is 90 deg to the floor.
- Insight into Deformity Drivers:
- It provides insight into the primary "driver" of the cervical deformity; for example, a low C2 slope with high T1 slope suggests a thoracolumbar issue, while the opposite indicates a primary cervical pathology.
- Measured using
- Cobb angles (most common clinically, despite potential over/underestimation)
- Measures the lordosis from either C1–C7 or C2–C7
- First line is drawn either parallel to the inferior endplate of C2 OR extending from the anterior tubercle of C1 to the posterior margin of the spinous process
- Second line drawn parallel to the inferior endplate of C7
- Cobb C1–C7 angle overestimates the CL
- Cobb C2–C7 angle underestimates the CL
- Harrison posterior tangent method
- Drawing parallel lines to the posterior surfaces of all cervical vertebral bodies from C2–C7 and then sums the segmental angles for an overall cervical curvature angle/2
- Harrison method may provide the best estimate of lordosis versus Cobb
- Jackson physiological stress lines
- Drawing a parallel lines to the posterior surface of the C7 and C2 vertebral bodies and measuring the angle between them
C2–T3 SVA Angle
- Measures both cervical and upper-thoracic alignment,
- Unlike the traditional C2-7 SVA
- Higher values corresponding to more lordosis.
- Clinical significance
- Passias 2021
- Low: >-25°
- Moderate: -35° to -25°
- Severe: <-35°
- C2-T3 angle <−25 degrees is associated with more severe myelopathy per the modi ed Japanese Orthopaedic Association (mJOA) scale.
- A high C2-T3 angle may signify compensatory cervical lordosis in the setting of insufficient sagittal correction and may predict DJK.
Thoracic kyphosis
- Average 35°
- Normal range 20-50°
- Measured from T2-T12
- By itself does not tell you the distribution of the magnitude of the curvature of the rest of the spine
T1 Slope
- The T1 slope will determine the amount of subaxial lordosis required to maintain the COG of the head in a balanced position
- Varies depending on Global spinal alignment as measured by
- SVA AND
- Inherent upper thoracic kyphosis (TK).
- In patients with scoliosis, the T1 slope has been shown to correlate directly with SVA measured from the C2 odontoid plumb line to provide a measure of overall sagittal alignment.
- Kim 2013: High preoperative T1 slope were more likely to have postoperative (Laminoplasty for cervical myelopathy) kyphotic changes at 2-years
- Tamai 2018: Not easily visualised on xray so use C7 slope as surrogate
- As only 18% of the T1 slope is visible
- T1 Slope - C2-C7 lordorsis
- Similar to PI - LL
- T1 slope minus Cervical Lordosis (TS-CL)
- Low: <26°
- Moderate: 26° to 45°
- Severe: >45°
- A postoperative T1-CL > 20 degrees correlates with greater postoperative cSVA, indicating higher risk of persistent deformity
- C2–C7 Lordosis
- Low: >3°
- Moderate: -21° to 3°
- Severe: <-21°
Score | TS-CL |
0 | < 15° |
1 | 15°–20° |
2 | > 20° |
Lumbar lordosis
- Average 60°
- Normal range 20-80°
- Measured from superior endplate of L1-S1
- 75% of lumbar lordosis occurs between L4 and S1
- 47% occurring at L5/S1
- by itself does not tell you the distribution of the magnitude of the curvature of the rest of the spine
L4-S1 lordosis
- Used to measure lumbar distribution index (L4-S1 lordosis/L1-S1 lordosis × 100).
- to quantify how much of the lumbar lordosis is low in the whole lumbar lordosis
- L4-S1 lordosis / total lordosis
- Ideally 50-80% of the lumbar lordosis is located in the L4-S1 level
- (which is around 34 deg as the pelvic incidence is on average around 50 deg)
- Correlation between pelvic incidence and lumbar lordosis
- Old patients have OA in hips and cant modify their Pelvic tilt to compensate for their positive sagittal balance
- Common degenerative conditions, such as lumbar disc disease, will result in loss of anterior spinal height, while the height of the posterior elements remains unchanged.
- This leads to loss of lumbar lordosis and a sagittal plane deformity.
- Lumbar spinal fusion and decompression without restoration of the normal lumbar lordosis may improve a patient’s radicular leg pain, but may leave them with a fixed sagittal deformity and a suboptimal surgical outcome.
- Presenti 2018:
- The Amount of Proximal Lumbar Lordosis Is Related to Pelvic Incidence
- PI influences only the proximal part of the lordosis, but not the distal part in an asymptomatic adult population
Lumbar sacral kyphosis
- By Dubousset
- Used in high grade spondylolisthesis due to the fact that the S1 endplate is a dome and not a flat surface
- between L5 ? superior endplate and S1 dorsum
Radiographic risk factor for deformity progression
- Progression of 3°/year
- Curve > 30°
- Asymmetric intervertebral space
- Asymmetric osteophytes > 5 mm
- Lateral slippage > 6 mm
- Apical vertebra rotation > Grade II
- iliac crest line passing through or below L4/5
Compensatory measurements
- Full-body x-rays with lower extremity measurements:
- Sagittal compensatory mechanism
- Spine changes
- Thoracic spine
- Reduction of thoracic kyphosis
- Hyperextension of adjacent segments
- This exposes to the risk of retrolisthesis and may result in accelerated facet joints arthritis, inter-spinous hyperpressure (Baastrup’s disease) and sometimes isthmic lysis.
- Difference between ageing and compensatory hyperextension
- Retrolisthesis
- Retrolisthesis is typically limited to 2–3 mm slippage in the lumbar spine and result in severe foraminal stenosis and more rarely in central stenosis
- Observed at lower or upper adjacent segments of the kyphotic degenerative disease:
- L5-S1 is a common site.
- Retrolisthesis is typically underestimated on lying down radiological imaging techniques (MRI and CT scan).
- They can be suspected on MRI imaging with the presence of loss of coaptation of facet joints with fluid collection and frequent synovial facet cysts
- Lumbar spine
- Lordosis loss
- Common primary disturbance; patients tend to lose normal lumbar curve (hypolordosis or even kyphosis).
- Local extension at segments above the deformity is a spine-based compensation targeted to reduce forward lean.
- Sometimes, the cervical spine develops hyperlordosis (increased backward curve) to maintain horizontal gaze when thoracolumbar kyphosis is pronounced.
- Pelvic Retroversion: The pelvis tilts backward (increases pelvic tilt) as the first and main compensation for loss of lordosis.
- Pelvic Tilt (PT):
- Increases with age
- Compensation of malalignment
- Used as a marker for pelvic retroversion compensation. PT > 25°–30° often indicates overcompensation.
- Pelvic incidence (PI):
- Increases during growth
- Remains constant in adulthood
- Sacral slope (SS):
- Smaller changes over time
- Sacroiliac joint degeneration
- Hip Extension:
- Pelvic retroversion is associated with hip extension to realign the trunk’s center of mass over the hips.
- Knee Flexion:
- When pelvic compensation is insufficient, patients may flex their knees to stay upright. This is typically a late compensation and signals advanced imbalance.
- Progression:
- There is a typical sequence: spine → pelvis → hips → knees.
- Sagittal changes with age:
- Measurements by
- Femoral obliquity angle: (Grey)
- Angle between vertical and femoral axis
- Knee flexion angle: (White)
- Angle between femoral axis and tibial axis
- Global sagittal axis
- Aka Global tilt
- Angle between the line from midst of C7 to midpoint of femoral condyles and the line from this point to the mid point of S1
- Coronal compensatory mechanism
- Limb-length discrepancy
- Pelvic obliquity
- Compensatory lumbar curve
- Flexible deformity that can become structural over time
Sagital instability
- Liu 2015:
- On flexion and extension xrays
- Radiographic Criteria for Instability
- Slippage > 8% of the vertebral body length (change in translation between flexion and extension).
- Disc opening > 8° (angulation change between vertebrae).
- Slippage > 3 mm (absolute anteroposterior translation).
Radiographic risk factor for deformity progression
- Progression of 3°/year
- Curve > 30°
- Asymmetric intervertebral space
- Asymmetric osteophytes > 5 mm
- Lateral slippage > 6 mm
- Apical vertebra rotation > Grade II
- iliac crest line passing through or below L4/5
Relative spinopelvic parameters
C2 tilt and C2 pelvic angle (PA)
- Goal C2 Tilt ~ 0
- as C2 is normally is right on top the hips and knees
- C2 PA increases then C2 tilt will be anterior to hips and knees.
- One of the best single measuring tool because
- it is angular
- it looks at the whole spine deformity
Cervical sagittal vertical axis (SVA)
The gravity line (COG SVA)
- Measurements
- Global
- Center of gravity (COG) of the head plumb line ↔ posterior superior corner of sacrum
- Head COG: On lateral radiographs, anterior portion of the external auditory canal
- Regional
- Center of gravity (COG) of the head plumb line ↔ posterior superior corner of C7
C2 SVA
- Measurement
- Global
- Centroid of C2 (or odontoid) plumb line ↔ posterior superior corner of sacrum
- Regional
- C2-7 SVA
- Centroid of C2 (or odontoid) plumb line ↔ posterior superior corner of C7
- Tang 2015: C2–7 SVA threshold of 4 cm was found to correlate with moderate disability based on the NDI
- The T1 slope is a predictor of C2–C7 SVA.
- Has been directly correlated with HRQOL (SF36)
- Larger C2 SVA poorer HRQOL
- Increasing C2 SVA (>40 mm) is correlated with worse outcomes as assessed by the Neck Disability Index (NDI).
- Normal Cervical SVA Values in Asymptomatic Adults
Odontoid marker at C7 | 15.6 ± 11.2 mm |
Odontoid marker at sacrum | 13.2 ± 29.5 mm |
C7 SVA
- Distance from a C7 centroid plumb line ↔ posterior superior corner of sacrum
- Not useful for alignment
- Does not consider compensation
- May be useful to track an individual over time
- This is used since we cannot easily measure the centre of gravity directly, and so substitute the relative positions of the centre of the C7 vertebral body and the posterior limit of the S1 endplate.
- Patients whose overall spinal balance lies outside this range are said to have decompensated sagittal deformity → this group has been shown to have lower quality of life scores.
- Negative sagittal balance
- the axis is posterior to the sacrum and occurs in patients with lumbar hyperlordosis
- Positive sagittal balance
- The axis is anterior to the sacrum and occurs in patients with hip flexion contracture or flat-back syndrome
- Highly correlates with quality of life outcomes
T1 pelvic angle
- T1 pelvic angle = T1 spinopelvic inclination (T1SPi) + pelvic tilt (PT)
- Pros
- accounts for both global mal-alignment and compensation through pelvic retroversion.
- correlates with health-related quality of life in patients with adult spinal deformity.
- unlike sagittal vertical axis, it does not vary on the basis of the extent of pelvic retroversion or patient support in standing.
- Can be useful as a preoperative planning tool, with a target T1 pelvic angle of <14°
- this is however not always true as T1 pelvic angle varies with pelvic tilt
- Measurement
- Angle between
- Line from the femoral head axis ↔ centroid of T1 AND
- Line from the femoral head axis ↔ middle of the S1 superior end plate
Vertebral pelvic angles
- An expansion of the T1 pelvic angle.
- Measurement
- Angle formed between lines drawn from
- The center of the femoral head to the center of the vertebral bodies at (TO)
- T1
- T4
- T9
- L1
- L4
- The midpoint of the superior S1 endplate.
- Describe the shape and the position of the hips
- L1 PA = 50% of PI - 20
- A rule of thumb
- T4 PA in general close proximity to L1
- T4 ventral to L1 if low PI
- T4 dorsal to L1 if high PI
- Global sagittal axis
- Aka Global tilt
- Angle between the line from midst of C7 to midpoint of femoral condyles and the line from this point to the mid point of S1
Odontoid-hip axis:
- Angle between the vertical and the highest point of the dens
- Normal values: -5° to +2°
- If above +2°,predictive of proximal junctional kyphosis (PJK)
Spinopelvic parameters
Pelvic incidence (PI)
- Sacral pedestal
- =Pelvic Tilt + Sacral slope
- Patients with a high PI tend to have a high lumbar lordosis.
- How deep is your pelvis
- Women will have deeper pelvis = bigger bum!
- Normal value 55 ± 10.6°
- How to draw
- Line1: centre of S1 endplate to centre of femoral head
- Line2: perpendicular to S1 endplate
- This is fixed after skeletal maturity
- PI increases with growth of the pelvis.
- Once skeletal growth is complete, the PI is fixed, and can only be changed by fractures or osteotomies to the pelvis or sacrum.
- Clinical
- Correlates with severity of disease
- PI has direct correlation with the Meyerding–Newman grade
- PI and LL must be within 10 deg
- A PT < 25° and a PI within 9° of the lumbar lordosis are radiographic outcomes that statistically correlate with positive outcomes after spinal deformity surgery.
- Restoring a harmonious sagittal profile should be the surgical goal.
- Some patients are naturally more ‘S shaped’, while others have a flatter profile.
- Presenti 2018:
- The Amount of Proximal Lumbar Lordosis Is Related to Pelvic Incidence
- PI influences only the proximal part of the lordosis, but not the distal part in an asymptomatic adult population
Sacral slope (SS)
- The slope of the S1 endplate relative to the horizontal plane
- Normal value 39 ± 8°
- Sacral slope = pelvic incidence - pelvic tilt
- How to draw:
- Line1: parallel to the S1 endplate
- Line2: horizontal one (edge of film)
- Foundation of the Roussouly classification
Pelvic tilt (PT)
- Pelvic tilt = pelvic incidence - sacral slope
- Pelvic tilt refers to the anteversion and retroversion of the pelvis
- How to draw
- Line1: Center of S1 to center of femoral head
- Line2: vertical line (edge of film)
- We can tolerate pelvic tilt for at most 20 deg
- The angle from the femoral heads to the midpoint of the S1 endplate relative to the vertical plane
- Normal value 13 ± 6°
- A PT less than 25° is one of the desirable outcomes after spinal deformity surgery.
Coronal measurements - Scoliosis
Definitions
- Apical vertebrae (A)
- Is the disk or vertebra deviated farthest from the center of the vertebral column
- Neutral vertebrae (N)
- No rotation
- Spinous process equal distance to pedicles on PA X-ray
- End vertebrae (E)
- Vertebra endplates that is most tilted from the horizontal apical vertebra
- Clavicle angle
- Best predictor of postoperative shoulder balance
- CSVL (dotted line)
- Line that is exactly perpendicular to a tangent drawn across the iliac crests (solid line).
- Stable zone
- Between lines drawn vertically from lumbosacral facet joints
- Stable vertebrae (S)
- Most proximal vertebrae that is most closely bisected by central sacral vertical line
- Last substantially touched vertebra (LSTV)
- Most proximal vertebra where the central sacral vertical line (CSVL) either intersects the pedicle outline or is medial to the pedicle outline
- Last touched vertebra (LTV)
- The most cephalad (uppermost) vertebra that is physically touched by the central sacral vertical line (CSVL) on an upright (standing) posteroanterior radiograph.
Scoliosis is a curve in the coronal plane of >10 degrees.
- Intra-interobserver error of 3-5°
- In EOIS if Cobb angle > 20 degrees associated with progression
Scoliotic curves types
- Structural
- Have a fixed component
- do not fully straighten (i.e. they do not correct to zero degrees).
- Non-structural.
- A curve that is fully correctable.
- due to
- Posture
- Pain
- Paralysis
- Pelvic obliquity
- (e.g. if a curve is the result of a leg length inequality, then the curve will straighten when the patient is sitting).
- Non- structural curves may develop a structural component over time.
Curve location
- The position of the ‘apex’ of the curve is used to describe the position within the spine.
Curve Type | Position of Apex |
Cervical scoliosis | C1 to the C6/C7 disc |
Cervicothoracic scoliosis | C7 vertebra to T1 vertebra |
Thoracic scoliosis | T1/T2 disc to T11/T12 disc |
Thoracolumbar scoliosis | T12 to L1 |
Lumbar scoliosis | L1/L2 disc to L5 |
Curve direction
- The side of the convexity describes the side of a scoliotic curve.
- A curve can be described as having
- Convexity (the ‘outside’ of the curve)
- Concavity (inside of the curve
- The apex is the disc or vertebral body that is furthest from the midline (i.e. the most displaced from the central spinal vertical axis).
Curve magnitude
- Cobb angle
- Finding the angle from the superior endplate of the most tilted vertebra at the cranial end of the curve to the inferior endplate of the most tilted vertebra at the caudal end of the curve.
- The Cobb angle measurement is merely a two- dimensional (2D) approximation of a 3D curve.
- To fully describe a curve, we would need to record the number of spinal segments affected and the rotational deformity as well as the sagittal plane deformity.
- The Cobb angle provides a useful shorthand method for describing a curve, and is the measurement for which we know most about the natural history
Measurements specific for EOIS
- Fulcrum Bending Radiograph (FBR)
- Purpose:
- The FBR was developed to assess the flexibility of the thoracic spine,
- identify any structural changes,
- Determine the selection of fusion levels in AIS patients.
- Fulcrum-Bending flexibility (%) = (Preoperative Angle – Fulcrum Bending Angle) / Preoperative Angle × 100
- Fulcrum Bending Correction Index (FBCI)
- Purpose:
- A preferred method to assess the correction rate because it takes into account the curve’s flexibility.
- It helps compare curve correction and instrumentation systems between different patient series.
- Fulcrum Bending Correction Index (%) (FBCI) = Correction Rate / Fulcrum-Bending Flexibility × 100
- Correction rate (%) = (Preoperative Angle – Postoperative Angle) / Preoperative Angle × 100
- An FBCI close to 100% suggests that the instrumentation has fully utilised the flexibility revealed by the FBR.
- Rib phase
- technique
- convex rib head position with respect to the apical vertebrae
- findings
- phase 1 - no rib overlap
- is associated with resolution in 84– 98% of cases.
- phase 2 - rib overlap with the apical vertebrae
- Associated with progression in 84– 97% of cases.
- high risk for curve progression
- RVAD (rib vertebrae angle difference, Mehta angle)
- Technique
- measure angle between the endplate and rib (line between midpoint of rib head and neck)
- RVAD = difference of 2 rib-vertebral angles
- Findings
- > 20° is linked to high rate of progression
- < 20° is associated with spontaneous recovery
Pelvic Obliquity
- Pelvic coronal reference line (PCRL):
- Tips or sulcus of the sacral ala OR
- Alternatively, the top of the ilium may be used to create the PCRL.
- An angle between PCRL and a horizontal reference line is then measured
- Leg length discrepancy (LLD)
- Measured using PA standing radiographs without blocks under the patient’s feet and with the knees extended.
- A femoral horizontal reference line (FHRL) a horizontal line that is tangent
- to the top of the highest femoral head OR
- to the level of the lesser trochanter.
- The difference between the height of this line and the height of the lower femoral head is then measured as the LLD.
- interpretation
- If the left hip is up, the value is positive (+).
- If the right hip is up, then the value will be negative (−).
- Poorer clinical outcomes have been associated with
- Sagittal (C7-SVA, PT, LL, sagittal LIVDA) and/or
- Coronal imbalance (C7-CSVL, shoulder-tilt, and pelvic obliquity).
T1 Tilt Angle
- Aka:
- Medial shoulder balance (MSB) is defined as the T1 tilt
- Measured between
- A line drawn between a horizontal line AND
- Superior endplate of T1 OR along the zenith of both first ribs if the T1 endplate is not well visualized
- A moderate positive correlation with SVA (dens) and can be used as a good predictor of overall sagittal balance.
- When T1 tilt was > 25°,
- all patients had at least 10 cm of positive sagittal imbalance
- When T1 tilt < 13° of angulation,
- Patients had negative sagittal balance
- Used as a measure intraop during AIS surgery
- If there is no T1 Tilt then the shoulder would be balanced.
Clavicle angle
- Angle between
- Line connecting a the highest points of the clavicles
- drawn perpendicular to the lateral edge of the radiograph and touches the most cephalad portion of the elevated clavicle and a line which touches the most cephalad aspect of both the right and left clavicles (clavicle reference line)
- Horizontal plane
- Angles in which the left shoulder up are positive
- Angles with the right shoulder up are negative.
- This directionality is consistent with that of the T1 tilt angle described earlier.
- Clavicle tilt has been shown in some studies to have an association with postoperative shoulder imbalance
Shoulder tilt
- Radiographic shoulder height is defined as the distance measured in millimeters between the superior horizontal reference line (SHRL) and the inferior horizontal reference line (IHRL)
- The SHRL passes through the intersection of the soft tissue shadow of the shoulder and a line drawn vertically up from the acromial clavicular joint of the cephalad shoulder.
- The IHRL is a similar line drawn over the caudal acromial clavicular joint.
- The distance the linear distance between these two reference lines is measured in millimeters
- Line is positive if the left shoulder is up
- Line is negative if the right shoulder is up.
- Again, by convention, left up is always positive and right up is always negative
- Medial vs. Lateral Shoulder Tilt
- Ono 2012
- Shoulder height difference” that patients notice may involve medial trapezial asymmetry as much as, or more than, lateral clavicle tilt
Aspect | Medial Shoulder Tilt | Lateral Shoulder Tilt |
Primary feature | Trapezius prominence due to T1 tilt and proximal rib deformity | Clavicle angle affecting outer shoulder height |
Main drivers | Proximal thoracic curve, T1 tilt, first rib angle | Clavicle orientation and position |
Correlation with T1 tilt | Correlates with T1 tilt, and correlates | Correlates poorly with T1 tilt |
Clinical appearance | More “inner”/neck–trapezial asymmetry | More “outer”/acromial and shoulder cap asymmetry |
Surgical implication | -Better corrected by addressing proximal thoracic/T1 alignment -Hard to correct as it requires to derotate the T1 which is difficult | Less predictable; depends on clavicle-related balance |
Lateral instability (Coronal subluxation)
- Wang 2022
- Lateral flexion and extension x-rays
- Definition of lateral instability
- Lateral translation > 5 mm OR
- Lateral disc wedging > 5° in the coronal plane
- If lateral instability present
- More back pain
- Poorer HRQOL scores
Thoracic trunk shift
- Measured on standing PA or AP full-length spine films.
- Vertical trunk reference line (VTRL).
- Identify apical thoracic vertebra
- a horizontal reference line is drawn through the center of the vertebra and extending to the right and left rib cages.
- The horizontal midpoint of the reference line is marked and a perpendicular line is dropped
- Trunk shift is then calculated by measuring the linear distance in millimeters between the VTRL and the CSVL
- Trunk shift is a spinal imbalance in which the thorax is not centered over the pelvis.
- A trunk shift:
- to the right of the CSVL is a positive value
- to the left of the CSVL a negative value.
- LIV (lowest instrumented vertebrae) selection and ratio of MT to TL/L curve are highly correlated with the onset of postoperative trunk shift in Lenke Type 1C curves.
Rotational deformity
- Apical vertebral rotation: Nash and moe classification
Bi-planar slot scanner imaging (EOS)
- low-dose biplanar digital radiographic imaging system with two linear x-ray radiographic sources and two gaseous detector arrays moving together to scan the patient
- Pros:
- Low-radiation dose
- Less image distortion than conventional x-ray
- Possibility of 3D reconstruction
- Cons:
- High costs
- Low availability
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