Change in pelvic incidence due to sacral stress fracture following multilevel instrumented fusion

Abstract

Multilevel-instrumented fusion is a common surgical technique used to treat adult spinal deformity (ASD), but it can occasionally lead to rare complications such as sacral insufficiency fractures. The impact of sacral fractures on spinopelvic parameters, particularly pelvic incidence (PI), has not been thoroughly investigated even though they have been documented in the literature. Here, we present a case of a patient who underwent a Th11-sacrum instrumented fusion for ASD. She underwent a revision surgery 18 months after the first procedure to treat proximal junctional pain brought on by a localised kyphosis of the rods. An asymptomatic sacral fracture was discovered during the radiological evaluation: the PI had increased from 71° to 103° between the 2 surgical procedures.

Background

Sacral fracture is a potential complication in patients who underwent thoracolumbosacral instrumented fusion, even if asymptomatic. In this case, the patient did not experience sagittal imbalance or a recurrence of low-back pain, despite the significant increase in Pelvic Incidence (PI)- Lumbar Lordosis (LL) mismatch. This could be due to the fact that a higher pelvic incidence (PI) leads to a greater ability of compensation by pelvic retroversion. Hence, if the patient is asymptomatic and has a good sagittal compensation, surgical correction may not be required.

Case presentation

We present the case of a patient in her 60s reporting problem of chronic low back pain due to lumbar degenerative scoliosis, unresponsive to conservative treatment. She was postmenopausal and a heavy smoker. After she quit smoking and her postmenopausal osteoporosis was treated with vitamin D and calcium supplement, and oral oestrogen, surgery was planned.

The patient underwent a Th11-sacrum instrumented fusion with an L5-S1 anterior interbody fusion. Considering that the risk factors had been addressed, L5-S1 anterior interbody fusion was performed and posterior S1 pedicle screws were inserted bicortically, pelvic fixation was not performed. After surgery, a full standing X-ray showed the following spinopelvic parameters: PI: 71° (the same compared with pre-op values), pelvic tilt (PT): 14°, and lumbar lordosis (LL): 73° (figure 1). The thoracolumbar tract (Th11-L1) was fixed in mild kyphosis (13°). In the early postoperative period, the patient reported problem about pain at the cranial portion of the instrumentation, not responsive to painkillers, injections or facet joint denervation.

Figure 1.

X-ray evaluation of the patients before (A, B) and after (C, D) the first surgery. Spinopelvic parameters were PI: 70°, PT: 25° and LL: 44° pre-op and PI: 71°, PT: 14°, LL: 73° post-op. LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt.

Due to the persistence of the symptomatology and the inefficacy of conservative treatment, the patient underwent revision surgery 18 months after the first procedure to correct the local kyphosis, extending the fusion to Th10.

Preoperative CT scans and X-rays showed a solid bone fusion from Th11 to S1, S1 screw loosening and a change in sacral morphology due to a healed sacral insufficiency fracture. Considering the spinopelvic parameters, PI increased to 103° (32° more than pre-op), and PT to 46°, and LL decreased to 65° (figures 2 and 3). During the whole period, the patient never reported problem of new-onset sacral pain (asymptomatic fracture).

Figure 2.

X-ray evaluation of the patients before (A) and after (B) the second surgery: there was an increase of PI to 103° and PT to 46°, a decrease of LL to 65°; also, the Th11-L1 kyphosis decreased from 13° to 5°. LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt.

Figure 3.

CT scan after the first (A) and after the second surgery (B). It is shown the change in sacral morphology due to the fracture.

Outcome and follow-up

The postoperative long-standing X-rays showed the correction of the local kyphosis (from 13° to 5°) without modification of the other spinopelvic parameters.

At the 6-month, 12-month and 24-month follow-up after revision surgery, the patient did not report problem of either back pain or sagittal imbalance, and she reported a satisfactory return to daily activities.

Discussion

Surgical correction and multilevel instrumented fusion are popular techniques for treating ASD. However, fusion modifies spinal biomechanics and load distribution, which can increase mechanical stresses and lead to adjacent-level degeneration, non-union, implant failure or even sacral fracture.^{1 2}

Diagnosing sacral insufficiency fractures after instrumented fusion is not an easy task. First, the fracture may not be readily detectable as the pain is subtle and could be considered by the patient (and by the surgeon as well) as acceptable postoperative pain. An important aspect in the context of spinal surgeries, especially those involving the lumbar region, is the impact on thigh muscular tightness. Thigh muscles, particularly the hamstrings, can become tight due to altered body mechanics and prolonged bed rest post surgery. This tightness can further exacerbate the stress on the lower back and pelvic region, potentially leading to complications in the healing process. Additionally, such muscular tightness may delay the recognition of postoperative complications, such as stress fractures, by masking their symptoms. Second, in the early stages, radiographic findings are usually inconclusive, leading to a delayed diagnosis. Even CT scans can often be ambiguous in the early stages due to the artefacts caused by instrumentation, which could hide a non-displaced fracture of the sacrum.³

Several cases of insufficiency sacral fractures after multilevel instrumentation and fusion have been presented in the literature,^4–7 mainly presenting risk factors and their treatment without focusing on the change in PI. It is a fracture typically treated with conservative care. Surgery should only be considered in case of persistent pain and/or increasing instability of the fracture itself.

In this case, it is very likely that the long lever arm of the instrumentation increased the shear stresses at the sacrum, leading to a stress fracture. Furthermore, bicortical pedicle S1 screws might act as stress risers through the sacrum, facilitating a potential fracture at this level,⁸ even if this technique is used to increase screw purchase and pullout resistance. In this case, the fracture healed without needing further treatment, but it caused a significant modification of sacral morphology and varying PI. It is important to note that the increase in PI seen in this case is not a common finding after instrumented fusion surgery,^{9 10} and it is likely due to the healed sacral insufficiency fracture rather than the surgical technique itself.^{11 12} However, this case highlights the potential for significant changes in spinopelvic parameters after surgery, which should be considered when planning and performing corrective surgery for adult spinal deformity.

Finally, the case also emphasises a critical aspect to consider in patients undergoing multilevel instrumentation of the lumbar spine: the increased risk of stress fractures, particularly in the sacral region. Key risk factors include the presence of osteoporosis or osteopenia, a history of smoking and postmenopausal status in women. The biomechanical changes induced by the instrumentation, such as altered load distribution and increased mechanical stresses, can predispose these patients to sacral insufficiency fractures.¹³ Additionally, postoperative factors such as physical overwork, inappropriate or intensive physiotherapy and premature return to sporting activities can exacerbate this risk. Therefore, proactive measures like bone density optimisation, smoking cessation, careful surgical planning and a well-tailored postoperative rehabilitation plan are essential to mitigate these risks. Monitoring and early intervention on the first signs of stress fractures can significantly impact patient outcomes.

Learning points.

• This case report presents an interesting example of a huge modification (increase) in pelvic incidence after an asymptomatic sacral insufficiency fracture.

• Sacral insufficiency fractures are increasingly recognised as a complication in thoracolumbar fusion surgery: they can be difficult to diagnose and can alter important spinopelvic parameters.

• Pelvic incidence is thought to be a constant among spinopelvic parameters. In fact, this case showed how it can be altered by a fracture: this variable has to be considered when planning a corrective surgery for spinal deformities in the adult.

Ethics statements

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