A guide to assessing post-operative complications following en bloc spinal resection

Abstract

En-bloc resection of spinal tumours is a complex procedure with significant morbidity and mortality. The extensive resection leaves a large soft tissue and osseous defect requiring reconstruction. Following en-bloc resection, there may be complications relating to both the removal of the tumour and the subsequent reconstruction. This paper outlines the imaging appearances of the frequently encountered complications in our experience. The primary aim is to improve the confidence of the radiologist when reporting imaging following spinal en-bloc resection, however we believe this is also useful for the spinal and orthopaedic surgeons in assessing the patients following en block resection.

1. Introduction

En bloc resection in the spine is a complex procedure requiring input from an experienced multidisciplinary team. The aim is to resect the entirety of the tumour in one piece along with a margin of healthy tissue. Challenges are faced not only in achieving good oncological margins but also in the complex reconstructive techniques required following resection.¹ En bloc resection for primary spinal tumours is rare, and most studies describe outcomes in terms of local recurrence and disease-free survival. The approach to reporting pre-operative imaging and the normal post-operative appearances have been discussed in previous papers. This paper will discuss the imaging appearances of the frequently encountered post-operative complications in our experience. The primary aim is to improve the radiologist's confidence when assessing imaging post spinal en bloc resection and reconstruction for possible complications. We believe this is also useful for the spinal and orthopaedic surgeons in assessing the patients following en bloc resection.

2. Possible complications and their radiological appearances

Clearly, following such extensive surgery with complex reconstruction, post-operative complications are not uncommon. Imaging may be requested by the operating surgeon for routine post-operative follow-up or due to patient complaints of pain, discomfort or instability. Review of a series of imaging is essential, and comparison with the initial postoperative sequences or images should be considered as changes between consecutive investigations may be minimal and considered within the error of measurement. The following are the most frequent complications in our experience (Table 1).

Table 1.

Post en bloc resection complications: .

3. Immediate complications

3.1. Sub-optimal implant placement

When assessing for complications associated with spinal metalwork, the first consideration should be whether there is correct positioning of the implant. This could first be reviewed on plain radiographs but CT allows for better 3D visualisation of positioning. Ideally, pedicular screws should be along the medial aspect of the pedicle, extending anteriorly and slightly medially. They should not break through the adjacent cortices, endplates or posterior wall.² Excessive medial angulation of the screws and breach of the medial cortex may result in spinal cord injury or spinal canal compromise.^{3, 4, 5} It is important to note that this finding may only be relevant when there are consistent findings in the patient history and examination. This is because although pedicle screw breaches have been reported in up to 5.1% of cases, neurological symptoms occur in less than 0.2%.⁵ (Fig. 1). This re-enforces the idea of using purely descriptive terminology when reporting the position of the implant, rather than describing it as malpositioned or malaligned.⁶ In the thoracic and lumbar spine, screws, which breach the anterior cortex, may damage retroperitoneal structures. A description of proximity to such structures and the preservation or absence of intervening fat planes is advised.⁶ In the cervical spine, screws entering the lateral masses should be angled slightly superiorly and laterally.⁷ Consideration of excessive lateral angulation is important as this risks breaching the transverse foramen and damaging the vertebral artery.⁴

Fig. 1.

Lateral radiograph (A) and axial CT(B) showing anterior displacement of cage (red arrow) which was re sited. Note large pseuodomeningocele (yellow arrow).

3.2. Infection

Post-operative infection is obviously an important complication to identify, which occurs, most commonly in the first month following spinal surgery.⁶ Post-contrast CT may detect a rim enhancing collection in the surgical bed or paraspinal musculature but the infection is better assessed on MRI. At MR, an abscess will appear irregularly shaped and with surrounding oedema. It will be hypointense on T1 weighted images and hyperintense on T2 weighted images. If gadolinium is used, there will be irregular peripheral gadolinium enhancement. Careful consideration should be made to the presence of an epidural component, which may result in spinal cord or nerve root compression and neurological symptoms in the patient.

3.3. Haematoma

Post-operative haematoma typically presents hours to over a week following surgery. In contrast to most spinal haematoma, which are a result of bleeding from the epidural plexus, post enbloc resection haematoma may be a consequence of bleeding from segmental arteries and veins, iliolumbar and azygos systems or rarely more major vessels locally. The spinal canal is frequently absent at the level of the resection with a large dead space. Therefore, It is expected to have a large fluid filled area in the region of the resection and interpretation of its significance requires careful consideration to differentiate between post-operative seroma, haematoma or other fluid collection. The MRI will demonstrate mixed blood breakdown products. The signal intensity will be variable dependent on the age of the haematoma.⁸ Similar to infection, haematoma may extend into the spinal canal resulting in neurological compromise, and hence mandate a careful assessment. It should be remembered that whilst attention undoubtedly focusses on the level of the resection with regard to haematoma, these can occur due to breach of the medial wall of the pedicle by instrumentation and each level should be carefully assessed. When conservative management is decided upon, MRI may also have a role in the follow up of the haematoma to ensure resolution.

3.4. Pseudomeningocoele

In many cases, the index tumour may have invaded the vertebral canal. In these cases, the tumour will have to be carefully dissected away from the dura, or rarely the dura resected. There may be an inadvertent tear in the dura or difficulty reconstructing the dura after resection resulting in a CSF collection, This may be dorsal, lateral or anterior to the spine. The signal intensities will be that of CSF (Fig. 1, Fig. 2). The T2 images are the most useful to visualise the extent of the collection and to demonstrate communication with the dural sac, and the presence of flow voids should be sought.

Fig. 2.

AP(A), lateral (B) radiographs showing enbloc resection, grafting and posterior spinal instrumentation. Sagittal (C) and axial (D) showing pseudomeningocele (arrow).

3.5. Cord ischemia

En-bloc resection frequently involves the sacrifice of neural elements to allow complete tumour excision. As a consequence, post operative neurological deficits are expected and can be predicted. On occasion an unanticipated deficit may result following surgery. Whilst many may be as a result of implant misplacement, haematoma/abscess or loss of fixation, infrequently no obvious extramedullary structural cause can be identified and in these circumstances, consideration should be given to intrinsic cord ischemia. MRI can provide evidence to support this and help guide prognosis early. Artefact from instrumentation can make interpretation more difficult but careful assessment should be made for cord oedema. T2 sequences are optimal may show oedema extending a considerable distance above/below the injury (Fig. 3). Diffusion images demonstrate restricted diffusion in the involved segments, however acquiring and interpreting diffusion images are challenging in spine due to the requirement of strong gradient, size of the spinal cord and CSF flow artefact.

Fig. 3.

Sagittal T2 showing high signal in the cord post en bloc resection in keeping with anterior cord syndrome.

4. Delayed complications

4.1. Implant failure

Any metalwork present should be assessed carefully to assess for fractures caused by repetitive stress.⁹ This is best achieved using CT with metal artefact reduction and using 3D multiplanar reformats to assess more subtle findings. The metalwork should also be reviewed to ensure that there is an engagement of the screws, bolts and rods and that these have not migrated from their expected location.⁶ (Fig. 4, Fig. 5, Fig. 6). The most common predisposing factor to this is initial suboptimal positioning.¹⁰ These faults may result in instability, neurological symptoms and interfere with overall osseous fusion. Rod failure is more common close to fixation points (pedicle screws, hooks), adjacent to transverse rod connectors and at the transition zone in dual diameter rods. Screw fracture is less common and more likely in thoracic pedicle screws where the screw diameter may be less than rod diameter.

Fig. 4.

Lateral radiograph of spine (A) showing disengagement of the rod, which was revised (B).

Fig. 5.

Sagittal CT (A) and coronal (B) showing fracture (arrow) of the pedicle and pars.

Fig. 6.

AP radiographs (A) showing enbloc resection which required revision and extension of construct (B) due to loosening of the lower screws.

4.2. Peri-implant fracture

In addition to fractures of the implant itself, fractures may also occur in the bones in which the implant is embedded. There may be an underlying pre-disposition such as osteoporosis, or bone resection, which coupled with the altered biomechanical forces through the bone result in fracture. The position of the fracture should be described, along with a description of any displaced fragments and its effect on the spinal alignment. Reporting the extension of the fracture into articular surfaces or foramina is also useful.

4.3. Peri-implant osteolysis

Loosening is one of the most common post-operative complications.^{11, 12, 13} It occurs over months to years after the surgery and is apparent on radiographic or CT follow-up. Osteolysis may be secondary to micromotion, infection or foreign body reaction.^7,14 If the osteolysis is circumferential and larger than 2 mm, or progressive, loosening is suspected. Whilst change in the position of the implant confirms loosening,^15, it is not essential. Comparison of alignment between weight bearing X-rays and CT scans may help identify loosening. The osteolysis may be secondary to mechanical loosening or due to infection. Radiologically, the osteolysis associated with mechanical loosening tends to be more pronounced at the pivot point at which most movement would be expected (eg: the tip of the screw). Early loosening like this does not imply instability since maximum purchase is found within the pedicle of an appropriately sited screw (Fig. 6). The osteolysis associated with infection is more diffuse. Clearly, correlation with presentation, blood tests and possibly even PET scans is required.^16,17

4.4. Adjacent segment disease

When spinal fixation has taken place, there is a development of a junction between the immobilised spinal segment and the rest of the mobile spine. This can accelerate degenerative change at the level above or below the fixed segment.⁴ This is especially common in the lumbar spine and may become apparent as early as 3 months post-operatively to 13 years.^{18, 19, 20, 21, 22, 23} Radiological assessment of disc degeneration and facet joint arthropathy should be made, which may eventually result in spinal stenosis.^4,6,9

4.5. Local recurrence

The imaging appearance of local recurrence will be variable and depends on the initial index tumour. For this reason, a comparison with pre-operative imaging is beneficial. Local recurrence will appear as either a bony or soft tissue mass with similar signal characteristics to the initial tumour and be located at the operative site (Fig. 7). The patient is frequently asymptomatic initially although dependent on the site of recurrence may develop symptoms earlier if there are residual bony structures in the vicinity restricting the space available to the roots and spinal cord.

Fig. 7.

Axial T1(A) and T2(B) showing recurrence of chondrosarcoma around the cage (arrow).

4.6. Non-union/Delayed union

There is a high incidence of non-union in en-bloc resection due to the large defects left after tumour excision. Assessment should be made of bony integration both across the level of the resection, but also across the instrumented levels proximally and distally as non-union here increases the risk of loosening and pull out with subsequent construct failure. Union is best assessed on multiplanar CT and can take a considerable period, much longer than routine bone healing (Fig. 8). Bony resection of 2 or more levels has significantly higher non-union rates.

Fig. 8.

AP radiograph (A), coronal CT (B) (C- 2 years later) of the pelvis post enbloc resection and lumbosacral fixation with fibular graft showing delayed union of the fibular strut graft.

4.7. Spinal deformity

Typically, a later complication of en-bloc resection, loss of sagittal or coronal plane balance may be encountered and slow progression may be identified relatively early on serial imaging. This may be as a consequence of mechanical instability in the construct, or due to loss of tone within the supporting paraspinal muscles consequent to resection or denervation (Fig. 9). This is less likely to be appreciated on cross-sectional, non-weight bearing imaging.

Fig. 9.

AP thoracic spine post en bloc resection (A) showing development of scoliosis (B) 2 years later.

4.8. Visceral pain

The radical resection of vertebral column and the surrounding soft tissues can result in significant defects, particularly in chest or abdominal wall. These may result in pseudo-hernia or at times visceral displacement resulting in proximity of other organs to the spine (Fig. 10). Cord tethering to the graft is an uncommon complications but one needs to be aware of this (Fig. 11). The reporting radiologist should be aware of these possibilities and the potential for this to cause symptoms such as back pain that may otherwise be assumed to be spinal.

Fig. 10.

Axial T2 showing abutment of left kidney with the rod causing left upper lumbar pain.

Fig. 11.

Axial (A) and sagittal (B) T2 showing cord tethering with the anterior graft post enbloc resection (arrow).

5. Conclusion

En-bloc resection invariably results in a large soft tissue or osseous defect requiring reconstruction. In the post-operative period complications may arise related either to the resection itself or the reconstruction. By describing the imaging appearances of the most commonly encountered complications in our experience we hope to have improved the radiologist's and clinician's knowledge and confidence in imaging following en-bloc resection.

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Declaration of competing interest

No conflicts of interest.