Hemivertebra Excision

Daniel J. Hedequist and John B. Emans

DEFINITION

A hemivertebra is a congenital anomaly of the spine that forms during the 8th to 12th weeks of embryologic development. It is characterized by the formation of half of a vertebral body, a corresponding pedicle, and a corresponding hemilamina.
Hemivertebra are classified as a congenital failure of formation.
A hemivertebra may be classified as fully segmented (ie, separated from the bodies above and below by discs); partially segmented (ie, separated from one adjacent body by a disc and fused to the other adjacent body); or unsegmented (ie, fused to the body above and below; FIG 1).3
Progressive curvatures of the spine caused by a hemivertebra result from unbalanced growth. Full-segmented hemivertebra have a much higher rate of progression, because the presence of an intact disc space above and below signifies the presence of growth plates and potential asymmetrical spinal growth.

ANATOMY
The hemivertebra has a partial vertebral body, a pedicle, and a hemilamina.
Anatomically, it may be joined to the level above or below at either the body, the hemilamina, or both. If the hemivertebra is not fused to either adjacent segment, the potential for asymmetric spinal growth is high.
A local kyphotic or lordotic deformity may occur with hemivertebra if the associated failure of formation is greater anteriorly or posteriorly.

PATHOGENESIS
Progressive spinal curvatures due to hemivertebra are a result of disordered growth.

A

B

C

FIG 1 • Schematic of a hemivertebra. A. Fully segmented hemivertebra. B. Partially segmented hemivertebra.

C. Unsegmented hemivertebra.
The hemivertebra is a wedge on the convex side of a curve. In the presence of healthy growth plates above and below (ie, a fully segmented hemivertebra) convex growth is faster than contralateral concave growth, causing a progressive scoliosis.
In cases of hemivertebra, if the vertebral body lies in the pos-terolateral quadrant, a progressive kyphosis may arise with the scoliosis.
The disordered growth eventually may cause curvature to such a degree that normally segmented areas of the spine become involved in the curve, causing deformity and spinal imbalance.

NATURAL HISTORY
The natural history of a hemivertebra depends on its location and the potential for growth and curve progression.
Hemivertebrae that are fully segmented progress at approximately 2 degrees a year and can exceed over 45 degrees at ma-turity.5 These require treatment to prevent deformity and also to prevent adjacent spinal curvature.
Partially segmented hemivertebrae have much less growth potential (less than 1 degree per year), rarely exceeding 40 degrees at maturity. They usually do not require treatment. Unsegmented hemivertebrae require no treatment.
Hemivertebra at the lumbosacral junction almost always require treatment, because the lumbar spine takes off obliquely from the sacrum, causing a long compensatory curve in normally segmented regions of the lumbar spine, with resultant cosmetic deformity and spinal imbalance.

PATIENT HISTORY AND PHYSICAL FINDINGS
Embryologic development of the spine occurs between the 8th and 12th weeks of gestation; hence, other organ systems developing at the same time also may have a congenital anomaly.
A complete musculoskeletal examination looking for diagnoses such as clubfoot, developmental dysplasia of the hip, and limb anomalies is warranted.
A complete neurologic examination should be performed, because as many as 40% of patients with congenital scoliosis have a corresponding spinal dysraphism. This examination includes sensory, motor, and reflex testing.
Occult signs of spinal dysraphism include cutaneous manifestations such as midline spinal hemangiomas, penetrating sacral dimples, or midline hairy patches. Foot anomalies such as vertical talus or asymmetric cavus feet can signify spinal dysraphism.
Cardiac auscultation should be done, because 20% of patients with congenital scoliosis have congenital heart anomalies.
Observe shoulder position, trunk position, and waist symmetry. Truncal imbalance is an indicator of curvature.
Observe the flexibility of the patient’s spine.
Rotation of the spine during the Adams forward bend test is indicative of deformity and points to its location.

1466

IMAGING AND OTHER DIAGNOSTIC STUDIES
- Standing 36-inch posteroanterior (PA) and lateral radiographs are mandatory to define the deformity and assess the Cobb measurement. Apparent progression may be seen from supine radiographs to standing radiographs (FIG 2A).
  - Bending radiographs, in which the patient is directed to bend in a concave and then in a convex direction, are necessary to assess the flexibility of curves above and below the hemivertebra.
- MRI scanning of the brainstem and spinal cord is mandatory before any surgical intervention, given the height rate (30% to 40%) of congenital scoliosis with spinal dysraphism.3
- CT scans with three-dimensional (3D) reconstructions should be obtained to delineate the anatomy of the anterior and posterior elements as an aid in planning the operation and to avoid intraoperative problems such as unexpected posterior element deficiencies or fusions (FIG 2B).4
  - A pediatric protocol should be used for CT scans to avoid the significant radiation exposure that results when adult protocols are used for children.
- Preoperative evaluation of the genitourinary system with a screening ultrasound and evaluation of the cardiac system with an echocardiogram are necessary if these have not been performed, given the rate of anomalies associated with congenital scoliosis.
  
  DIFFERENTIAL DIAGNOSIS
- Failure of vertebral formation
- Failure of vertebral segmentation
- Sequela of infection causing partial vertebral body destruction
- Tumor
  
  NONOPERATIVE MANAGEMENT
- Nonoperative management is reserved for nonprogressive curves caused by hemivertebra.
- Hemivertebra associated with little or no curve progression (unsegmented or partially segmented) may be followed during

FIG 2 • A. Standing PA radiograph of a 5-year-old patient with a fully segmented hemivertebra at the thoracolumbar junction.

B. A three-dimensional reconstructed CT scan of a fully segmented hemivertebra in a different patient.

growth with radiographs every 6 to 12 months, depending on the degree of deformity and age of the basis.

Bracing has no role in the management of a hemivertebra.

SURGICAL MANAGEMENT
The classic indication for a hemivertebra resection is a patient with a progressive curve secondary to a fully segmented hemivertebra in the thoracolumbar, lumbar, or lumbosacral regions with a resultant deformity.
We have found that excision is best performed between the ages of 18 months and 4 years.
- Patients younger than this may be more difficult to instrument, and waiting until this age rarely has caused irrevocable deformity.
- Excision in older patients is feasible; we have found, however, that if diagnosed early there is no reason to wait past the age of 4 years given the progression of curvature and its effect on normally segmented regions of the spine.
- Instrumentation at these ages is technically feasible.
  
  Preoperative Planning
Review of the preoperative MRI of the spine
- If spinal dysraphism is present, referral to a neurosurgeon is mandatory.
- If the patient requires neurosurgical intervention for dysraphism, that procedure should precede the hemivertebra excision, either at the same setting or in a staged setting, at the discretion of the spine surgeon and neurosurgeon.
Review of the 3D CT scans
- A complete understanding of the anatomy of the hemivertebra is crucial to avoid intraoperative confusion, especially because associated posterior element fusions or absences can make identifying levels difficult.
- Studying the pedicle anatomy (ie, length and diameter) of the levels above and below is efficacious given the smaller size of these patients.
Neurologic monitoring is important and should be done using somatosensory evoked potentials and motor evoked potentials.
- Communication between the monitoring and anesthesia teams should be facilitated to prevent any change in neurologic function brought on by anesthetics, hypotension, or low blood volume.
  
  Positioning
We perform hemivertebra excisions with the patient in the prone position.
- This is done on a radiolucent operating frame with chest and pelvic support, which leaves the abdomen free.
- We also have found it useful to slightly “airplane” the table or bolster the patient so that the convex side is slightly higher than the concave side. This helps with visualization anteriorly, control of bleeding, and retraction of the dura and its contents (FIG 3).
Before draping the patient, we place a marker over the hemivertebra region and obtain a radiograph.
- This both confirms the side of the hemivertebra and helps limit excessive incisions and dissections.
In the past we recommended that hemivertebra excision be performed as a simultaneous anterior-posterior procedure.4
- If the surgeon elects to do this, the patient is placed in the lateral decubitus position with the anterior and posterior fields being prepped into the fields. The patient should be

A B

FIG 3 • Positioning for a hemivertebra resection. A. Prone positioning. Observe the paper clip placed for radiographic marking before incision. B. Positioning for simultaneous AP excision.

placed at the edge of the bed to facilitate retractor placement in the posterior field.

The anterior approach is on the convex side and should be marked before the patient goes to the operating room.

We still recommend considering an anterior–posterior procedure when medical conditions (eg, congenital heart disease) caution against excessive bleeding, when a lordotic component renders access to the vertebral body difficult, and when the surgeon is unfamiliar with posterior-only approaches to cir-cumferential surgery.

Approach
If an anterior–posterior procedure is being performed, the anterior procedure should be a standard transthoracic, transthoracic-retroperitoneal, or retroperitoneal approach, depending on the location of the hemivertebra. The anterior

approach often can be a limited one, because the only exposure needed is of the hemivertebra and the discs above and below.
The posterior approach is a standard posterior midline incision with subperiosteal dissection out to the tips of the transverse processes.
- Diathermy aids in keeping blood loss to a minimum during dissection.
- Preoperative review of the CT scan should forewarn the surgeon of posterior element fusions and, more importantly, posterior element deficiencies.
- Dissection should proceed with caution over areas of laminar deficiency.
- Once completely dissected, a spot radiograph or fluoroscopic view should be obtained to confirm the appropriate level.
  
  TECHNIQUES
  
  HEMIVERTEBRA EXCISION
  
  Pedicle Screw Placement
  - Implant anchors should be placed before excision, because blood loss at this point should be at a minimum.
    - Where possible, we prefer bilateral pedicle screws as a basis for fixation. Pedicle screws may be placed in patients as young as 1 year of age.
    - Preoperative CT scans can help assess the feasibility of screw placement.
  - Implants should be titanium, and either 3.5- or 4.5-mm rod systems should be used in younger patients.
    - Screw diameter and length can be at least estimated based on the preoperative CT scan.
  - Screws should be placed in a stepwise manner, beginning with obtaining a cancellous blush with a burr at the appropriate starting position.
    - Starting positions in normally segmented areas of the spine are well documented.
A pedicle awl can then be used to obtain access down the pedicle into the vertebral body.
Once the pedicle has been accessed, probing of the four walls of the pedicle and floor of the body is necessary to confirm accurate position. We then use the probe as a depth gauge to determine screw length.
- The hole is then tapped 0.5 mm under the expected screw diameter, and the pedicle walls and floor are reprobed.
- A fixed-angle screw of the appropriate diameter and length is then placed (TECH FIG 1A).
Appropriate screw position is confirmed using triggered EMG stimulation of all screws (TECH FIG 1B) and then checking PA and lateral radiographs and fluoroscopic views (TECH FIG 1C).

TECHNIQUES

A

B C

TECH FIG 1 • Pedicle screw placement. A. Exposure of the spine with placement of screws.

B. Triggered EMG stimulation of the pedicle screws. C. Fluoroscopic view confirming correct screw placement.

Hemivertebra Excision
- The first step in excision is dissecting over the edge of the transverse process and down the lateral wall of the body using a Cobb elevator and curved-tip device, followed by curved retractor placement (TECH FIG 2A).
  - This step aids in protection of structures lateral and anterior to the wall on the hemivertebra. If the hemivertebra is in the thoracic region, it will be necessary to resect the rib head first to obtain access.
- The cartilaginous surfaces of the concave facet should be resected to encourage fusion.
- Resection then begins in the midline with the ligamentum flavum using a Kerrison rongeur (TECH FIG 2B,C), followed by resection of the hemilamina.
  - Resection should extend over to the facet, while the exiting nerve roots above and below the hemivertebra are identified and protected.
  - The transverse process and dorsal cortical bone over the pedicle can be resected in similar fashion until the cancellous bone of the pedicle and cortical outlines of its wall are visualized (TECH FIG 2D,E).
    - Care should be taken to avoid nerve roots, which are present rostral and caudal to the pedicle walls of the hemivertebra.
  - Gelfoam (Pfizer Inc, New York, NY) and cottonoids should be used judiciously to protect the dura and create a space between dura and bone to be resected.
- The subperiosteal plane down the lateral wall of the pedicle and body is then developed, with a Cobb elevator
  
  used to facilitate retraction and protection. The dural contents can be protected by a nerve root retractor.
- Bipolar sealing of epidural vessels that lie on the medial aspect of the pedicle and down on the inner wall of the body will aid in controlling blood loss and improving visualization.
- Continued resection down the pedicle and into the hemivertebra body can be done by a diamond-tipped burr, which helps protect against unwanted injury to soft tissue structures.
Working stepwise within the walls of the pedicle and down within the confines of the body helps protect surrounding vital structures and makes removal of the cortical shells easier (TECH FIG 2F). The walls of the pedicle can then be easily resected with a curette or pituitary rongeur, as can the remaining walls of the body of the hemivertebra.
- Protection lateral and anterior to the confines of the hemivertebra wall is necessary to avoid injury to vital structures such as the aorta. Generally, the dorsal cortex of the vertebral body is removed last (TECH FIG 2G).
This resection is a wedge resection, which includes the discs above and below as well as the concave area of the disc.
- The disc material should be removed with a pituitary rongeur and curettes; the dura and its contents are protected with a nerve root retractor.
- If the disc material above and below is not removed, correction will be limited, and anterior fusion will be less reliable.
  
  Ligamentum flavum
  
  B
  
  C
  
  TECHNIQUES
  
  A
  
  Cobb elevator
  
  Burr
  
  Nerve root retractor
  
  F
  
  D E
  
  TECH FIG 2 • Hemivertebra excision. A. Placement of Cobb elevator at lateral border of hemivertebra (arrow). B. Resection of the posterior hemilamina using a Kerrison rongeur.
  
  C. Rongeur resecting down the pedicle with Gelfoam protecting the dura. D. Further resection down the pedicle (arrow), with lateral structures protected. E. Complete visualization of the vertebral body (arrow) with anterolateral protection. F. Axial schematic illustration of working down the pedicle with medial and lateral protection. G. Arrow points to the
  
  G area of complete resection.
  
  Closure of Wedge Resection
  - We place resected vertebral cancellous bone as well as allograft clips into the wedge resection site anteriorly.
  - We have found that it is beneficial to compress and close the resection site with laminar hooks and by external three-point pressure on the body (TECH FIG 3A).
    - We place a downgoing supralaminar hook at the superior level and an upgoing infralaminar hook on the inferior level.
    - We place a rod and compress with closure of the resection site and correction of the deformity. Using this rod avoids having to place large compression forces across pedicle screws. This allows the screws to
  maintain correction without possible plowing of the screws into the immature bone or pedicles.
The compression should be slow and controlled, with the dura under visualization so that it is not caught in the closure of the posterior elements (TECH FIG 3B,C).
- If insufficient correction is achieved or if the adjacent laminae abut prematurely, it may be necessary to resect further along the edges of the laminae.
Two additional rods are then placed, one on either side of the spine, connected to the corresponding screws. A crosslink should be applied if at all possible (TECH FIG 3D).
The spine is then decorticated. We prefer to place cortic-ocancellous allograft, because it is effective and avoids harvesting the iliac crest.

TECHNIQUES

A B

C D

TECH FIG 3 • Closure of wedge resection. A. Laminar hooks in place. Note the spacing between pedicle screws. B. Compression of laminar hooks with closure of the excision site. C. Complete closure of the excision site. The convex screws have now come together, representing wedge closure.

D. The three-rod system in place, plus the crosslink that should be applied if technically feasible.

Anteroposterior Excision
- We routinely place our posterior implant anchors before performing any resection. Once complete exposure (both anterior and posterior) has been performed (TECH FIG 4A), posterior screws are placed.
- Anterior resection begins by creating a full-thickness subperiosteal flap over the hemivertebra after localization is confirmed (TECH FIG 4B).
- Starting at the inferior endplate of the adjacent superior body and the superior endplate of the adjacent inferior body, we create longitudinal full-thickness cuts in the periosteum.
  - At the endplate region, we make anteroposterior cuts in the periosteum and start a full-thickness periosteal flap, working anteriorly to the contralateral side.
  - We move posteriorly until we can visualize the hemivertebra pedicle.
- The discs above and below the hemivertebra are resected all the way posteriorly to the posterior longitudinal ligament.
We then start resection of the hemivertebra vertebral body back to the posterior cortical wall of the body with rongeurs and a diamond-tipped burr.
- The posterior wall can be resected and peeled off the posterior longitudinal ligament with a rongeur, obtaining access by starting at the level of the disc resections.
- Part of the visualized pedicle can be resected.
Posterior resection can then begin, starting with the hemilamina and proceeding to the pedicle (TECH FIG 4C).
With both incisions open and fields exposed, resection of the pedicle can be done by working through both regions (TECH FIG 4D). This allows complete visualization and maximum control of the surgical field.
Once the hemivertebra has been resected, correction of the deformity is the same as described earlier, using a three-rod technique if possible (TECH FIG 4E).
- With this technique, correction is aided by unbreak-ing the table (or removing any lateral bolster) and pushing down on the convex spine to facilitate closure of the wedge resection.
  
  TECHNIQUES
  
  Far side annulus
  
  Posterior cortex of hemivertebra
  
  A B C
  
  Pedicle Nerve
  
  root
  
  TECH FIG 4 • AP excision.
  
  A. Hemivertebra isolated anteriorly with removal of discs above and below. B. Anterior resection. C. Resection back to the pedicle. D. Photograph of surgeon working through both operative sites simultaneously.
  
  E. Compression to close wedge
  
  D E resection site.
  
  PEARLS AND PITFALLS
  
  Localization of the hemivertebra ■ The intraoperative anatomy can be confusing. A thorough understanding of the
  
  patient’s anatomy can be gained by studying the preoperative 3D CT scans.
  
  Implant placement ■ It is useful to place implant anchors first, because the resection may make this difficult
  
  owing to blood loss and possible instability of the spine following resection.
  
  Blood loss ■ Blood loss may be minimized by sealing the epidural veins on the inner aspect of the hemivertebral wall and pedicle with a bipolar cautery before resecting these areas.
  
  Inadequate correction ■ Can be avoided by resection of the far-side concave disc and complete resection of the
  
  hemivertebra.
  
  POSTOPERATIVE CARE
The immediate postoperative hospitalization and care are similar to that for most patients being treated for spinal deformity.
When fixation is adequate, we place the patients in a custom-molded thoracolumbosacral orthosis for 3 months.
In patients who are younger than 2 years of age, or in cases where fixation is not adequate, we recommend a Risser-type cast, to include a shoulder or both thighs for 2 months, followed by a brace for up to a total of 6 months postoperatively.
The removal of spinal implants is not mandatory; however, given the young age of the patients and individual body habitus, occasionally it is necessary to remove the implants after a year secondary to prominence.

OUTCOMES
Hemivertebra excision may be performed as a posterior-only technique or as a combined anterior–posterior technique, with excellent curve correction of approximately 70% (FIG 4).6
The rate of union for this procedure is near 100% in pediatric patients.
The procedure may be performed safely using either technique with no neurologic complications.

COMPLICATIONS
Inadequate correction
Dural injury
Neurologic injury

FIG 4 • Correction of deformity. A. Postoperative standing radiograph after excision and curve correction of patient shown in Figure 2A. B. Standing lateral radiograph of the same patient showing excellent sagittal balance after

A B excision.
- Loss of fixation
- Implant failure
- Excessive blood loss
- Nonunion
- Infection

REFERENCES

Belmont P, Kuklo T, et al. Intraspinal anomalies associated with isolated congenital hemivertebra; the role of routine magnetic resonance imaging. J Bone Joint Surg Am 2004;86A:1704–1710.
Hedequist DJ, Emans JB. The correlation of preoperative three-dimensional computed tomography reconstructions with operative findings in congenital scoliosis. Spine 2003;28:2531–2544.
Hedequist D, Emans J. Congenital scoliosis. J Am Acad Orthop Surg 2004;12:266–275.
Hedequist DJ, Hall JE, et al. Hemivertebra excision in children via simultaneous anterior and posterior exposures. J Pediatr Orthop 2005;25:60–63.
McMaster MJ, David CV. Hemivertebra as a cause of scoliosis: A study of 104 patients. J Bone Joint Surg Br 1986;68B:588–595.
Ruf M, Harms J. Hemivertebra excision by a posterior approach: innovative operative technique and first results. Spine 2002;27:1116–1123.