Osteotomies for Ank-spond

Heiko 2017

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Surgical Goals

Halt deformity progression

Restore horizontal

Visual axis:

avoiding over correction of the CBVA

Global balance:

C7 over the sacral endplate
C2 SVA posterior to the hip axis

Reduce pain from muscle fatigue

Improve respiratory function.

Unique Challenges in AS Corrective Surgery

AS patients often have a high medical burden, making them susceptible to complications, similar to other ASD patients.

Surgeries can be co-morbid

Thoracolumbar kyphosis surgeries

1/3 patients have major complications and revision.

Cervical deformity surgery

Major complications: 24–60%

Neurologic complications in 7–25%

Cervicothoracic fusions

Major complications rate 31%

Flexed trunk and neck are difficult to position on the operating table causing increased abdominal pressure and difficult pulmonary ventilation.

Osteoporosis is present in about a third of AS patients, requiring assessment prior to surgery.

Shape of deformity

Bridwell KH (2006)

AS patients usually reveal Type 2 (global [31]) FSI, which is a smooth long- curved kyphosis.

Regarding the cervical spine, AS patients are similar to controls regarding C2–7 lordosis, but signifi- cantly different concerning C2–7 SVA and T1 slope.
That said, the driver for CK usually is the global imbalance, unless fractures of the lower cervical spine have driven the subaxial spine into CK at C2–7 [32] (Fig. 3).

Some AS patients will display Type 1 FSI [31] resembling sharp angular kyphosis as a result from trauma or Anderson lesions in the thoracolumbar spine.

AS kyphosis

Stratified according to the location of the kyphotic apex: (Zhang 2016)

Lumbar (Type I, 7%)
Thoracolumbar (Type II, 69%)
Thoracic (Type III, 19%)
Cervical or cervicothoracic junction (Type IV, 4%)
Mixed types exist

Planning Correction in AS

Preoperative assessments including

full-spine standing radiographs,

CT scans

to include the sacro-iliac joints (SIJ) to screen for mobile levels without evidence of autofusion, fracture sequels, levels with high disease activity as well as scoliosis and rotation

for cervical/cervicothoracic kyphosis

Specialist review

Neurologists: Potential myelopathic changes
ENT: Difficult intubation/dysphagia risk.
Ophthalmologists: Eye stress from prone positioning

Alignment analysis

C2-S1 SVA

C7-S1 SVA

C2-7 SVA

Chin-brow vertical angle (CBVA)

Chin-sternal distance (CSD)

Spinal flexibility assessment

mobile spinal segment

Pros: as it provides motion for a merely stiff AS patient.
Cons: Can be a source of motion → induced pain.

C1/2 motion assessment

Flexion and extension xray
to check degree of residual sagittal rotation and translation
As a mobile craniocervical junction will absorb corrective forces during manual extension of head and neck during CK correction.

Thoracic kyphosis assessment

Standing and lying xray
In Adult spinal deformity:

if thoracic kyphosis is flexible it can allow spontaneous correction post op if lumbar deformity is corrected

In AS

thoracic flexibility will eventually be lost causing hyperkyphosis if not included in the fixation

Compensation mechanisms

such as hip and knee joint contractures or cervical hyperlordosis, are analysed to determine the surgical target angle (STA) needed.
Assess

Pelvic tilt (PT)
Femur flexion angle (FFA),
Translation of C2 SVA and C7 SVA
Cervical lordosis of C2–7 and C1–2

Clinical examination

Hip flexion contractures

In advanced cases, total hip arthroplasty (THA) should be done before spinal osteotomy
Hip involvement in AS is reported in 25–30% and THA is already present in 10% of AS patients undergoing corrective surgery
Extension reserve can be reduced in patients with hip cups implanted in high anteverted position thus reducing the ability of coxofemoral compensation, e.g. after corrective thoracolumbar surgery

Knee joint extension contractures

AS patients with thoracolumbar kyphosis might compensate with cervical hyperlordosis.

Patients with cervical hyperlordosis is shown to experience better HQoL measures than AS patients with kyphotic cervical spines

Classification

In AS with CK + balanced thoracolumbar spine:

Type 1

patients have the head centre of gravity and C2 SVA still balanced over the trunk as per pelvic and lower limb compensation mechanisms.

Type 2

the trunk is still balanced, but C2 SVA translated far anteriorly

In AS with CK:

With C1–2 subluxation (CK+)

Subluxation due to

Chronic compensation at C1–2 to maintain visual axis,
Occult trauma with basilar impression

Without C1–2 subluxation (CK−)

Location of correction (cervical vs. thoracolumbar)

Chosen based on which region contributes most to global imbalance.

This found based on

C2-S1 SVA
C7-S1 SVA
C2-7 SVA

Where to perform osteotomies

At cervical spine

More caudal the better for C2SVA correction

Correction at C1/2

Visual axis restoration can be achieved with the same correction angle at C1–2 or C7–T1.
Cannot improve C2 SVA

Correction at C7/T1

Visual axis restoration can be achieved with the same correction angle at C1–2 or C7–T1.
Osteotomy at C7–T1 has a greater impact on posterior translation of C2 SVA → relocates the head centre of gravity back over the trunk → reducing loads off the cervicothoracic muscles

Effect of location of cervical and cervicothoracic osteotomies on restoration of visual axis and posterior relocation of C2 SVA in relation to C7. The latter is more effective with lower cervical and upper thoracic osteotomies

Best situated below the level of the vertebral artery entrance (C6/7 and below)

At thoracic spine

Midthoracic spine osteotomies NOT recommended

Due to

Rigidity of the costosternal and costovertebral articulations impedes significant correction

T1 or T11–T12 osteotomy recommended

Because

The rib cage can be disconnected laterally from the spine by osseous resection
No costovertebral/costosternal constraints anymore.

Caudal osteotomies generally have a greater impact on posterior translation of C2 SVA and C7 SVA.

At thoracolumbar spine

Osteotomy level shall be at the position with lowest neurologic risk AND
At or close to the apex of kyphosis to succeed in a harmonious postoperative alignment

Sequencing of corrections

Thoracolumbar correction is done first before cervical correction

Because

It can significantly improve global balance and visual axis
Might not need cervical correction potentially negating the need for cervical correction.
Coronal trunk imbalance can be address with TL correction

Cervical correction only

Indicated

Severe cervical kyphosis (CK) AND
Balanced trunks

Definition of surgical target angle

Restore

ideal sagittal alignment (e.g., C7 SVA < 50 mm, PT < 25°, PI-LL mismatch < 10°).
Follow Roussouly shapes

Overcorrection of the CBVA (ideally 10°-20°) must be avoided in AS to prevent loss of downward gaze.

can increase risk of falls, and in ability to perform daily task (cooking)

Computer software (e.g., Surgimap)

aids in simulating osteotomies and predicting changes in alignment, although predicting individual reciprocal changes in non-instrumented mobile segments remains challenging.

Predicting Postoperative Alignment and Complications

Residual post op imbalance

Sagittal imbalance: 17–39%
Coronal imbalance:18–32%

Significantly increasing the risk of

Construct failure
Delayed union
Loss of correction

Prediction accuracy is suboptimal due to

Variable postoperative responses of compensatory mechanisms,
Incomplete correction
Factors like BMI and preoperative pelvic tilt.

Complication rates:

Up to 34% for perioperative complications (e.g., DVT, infection, pneumonia, pulmonary embolism, dysphagia)

Major medical complications rates

3.1% to 44.4%

Mortality rates

3.1% and 7.3%

Neurological injury

13.5% across studies, especially with dorsal osteotomies at the cervicothoracic interface

There is a greater risk of complications with the degree of kyphosis correction

TL Osteotomy Techniques

Use the following osteotomies classification

Cervical spine osteotomy

SRS-Schwab osteotomy classification

Thoracolumbar Osteotomies:

Closing-wedge osteotomies (CWO):

Mainly made up of

Involve posterior shortening and typically yield 25–35° of correction.

Risks

Nerve injury

Spinal cord dysfunction due to shortening
Nerve root entrapment.

Opening-wedge osteotomies (OWO)

Mainly made up of

Grade 1 osteotomy

Smith-Petersen Osteotomy (SPO)

Polysegmental Wedge Osteotomies (PWO)

involve anterior opening, rely on mobile discs

Closing–Opening-Wedge Osteotomy (COWO), or Y-Type:

Technique

A combination of a Grade 1 and Grade 3 osteotomy
Posterior column resection is similar to CWO, with the tip of the wedge at the vertebral mid-sagittal point.
A plane is osteotomized anteriorly from this point parallel to the endplates, or the anterior cortex is fractured and the anterior column opened during osteotomy closure.
Shortening/distraction is balanced between the posterior and anterior column.

Aims to balance anterior and posterior changes, allowing large corrections while mitigating some drawbacks of CWO/OWO.

Illustration of Y-Type opening–closing wedge osteotomy.
a Concept.
b Preop CT scan in AS patient.
c Postop CT scan shows anterior opening and endplates gapping with posterior osteotomy closure and osseous approximation of endplates C7–T1

Combined/Multiple Osteotomies:

In complex cases, combinations like multiple SPOs or dual non-contiguous PSOs can be used to achieve greater correction, but they increase destabilisation and require robust instrumentation.
Overall, there's no significant difference in correction of C7 SVA or LL between SPO and PSO, but PSO may have increased blood loss. However, the number of mechanical complications is increased with SPO vs. PSO.

Combined Sagittal–Coronal Correction (Asymmetric Osteotomy):

Used for complex curves, but asymmetric closure can lead to issues like osseous offset and nerve root impingement.

Graphs illustrate concept and implications of asymmetric closing-wedge-type osteotomies.
Resection gap for asymmetric osteotomy in a sagittal plane and b coronal plane, also shown in patients with deformed vertebrae at osteotomy level:
c sagittal plane and d coronal plane

Correction Techniques

Instrumented correction involves specialised tables, temporary rods, screw-rod cantilever manoeuvres, and osteotomy pliers to provide controlled closure and prevent sudden sagittal translation (ST).

Sagittal Translation (ST), or unintended vertebral displacement, is a significant risk (14–27% in thoracolumbar, 27–44% in cervical osteotomies) that can lead to neurological injury. It occurs when forces are not precisely centred at the osteotomy's theoretical centre of rotation (COR).

Cervical Osteotomies for AS Kyphosis

3 column osteotomies:

These are particularly high-risk procedures, with complication rates up to 60% and severe neurological deficits in 17–19% of cases.

Cervical OWO (SPO/Mehdian-Type):

Can correct 3D deformity, with average corrections of 20–25° (or 37–47° CBVA improvement).
Risk of prolonged dysphagia with excessive anterior gapping.

Cervical CWO (PSO):

Typically performed at C7 or T1, with an average correction of 36–57°.
Requires careful attention to avoid vertebral artery injury and nerve root irritation.

Cervical COWO (Y-Type):

Balances anterior/posterior changes and is beneficial around pre-existing implants.

Intraoperative monitoring (e.g., neuromonitoring, CSD increase) and radiographic checks are crucial.

Indices to stop further correction include

Excessive posterior wall resection
ST
MEP drop
Foraminal stenosis
Dural buckling
Bleeding