1 Introduction

Adolescent idiopathic scoliosis (AIS) is a coronal plane spinal deformity arising in otherwise healthy children between the ages of 10–18.^1,2 In cases where surgery is indicated the goal is to correct the deformity as best possible and prevent further progression from occurring.^1–3

Implant related complications comprise up to 1.1% of all complications as reported in the Scoliosis Research Society database in 2007 and can be associated with significant morbidity.⁴ A recent study by Machida and colleagues found this rate of implant related complications after posterior spinal fusion to be as high as 1.8%.⁵ It has also been noted that early implant complications tend to be due to screw pull out whereas late complications are usually due to pseudarthosis and rod breakage.^5–7

Implant related complications typically occur at the point of anchorage as this implant-vertebrae interface is the most fragile area of the construct.⁵ Much attention over the years has thus gone into the design of this anchor point to minimize the risk of complication and this has amalgamated in the modern pedicle screws we see today.^8,9 Complications involving the rod are more rare and those described in the literature tend to be rod breakage associated with pseudarthrosis.⁵ As such, the design of the rod has received far less attention over the years than that of the screw. Our case describes an implant related complication whereby the rod itself was able to bore through the rib head overtime; something which has yet to be described in literature.

2 Case

A 25 year old female with AIS underwent transthoracic correction surgery in 2010 [Fig. 1, Fig. 2]. She developed chronic pain on the right side at the cranial aspect of her surgery, requiring long-term pain management. She presented to our clinic a decade after her index procedure with persistent pain in the right medial scapular region at the level of T4, particularly with deep breathing and coughing. She denied any neurological symptoms and was otherwise well. She described no changes in her spinal deformity, but described a sensation of something squeaking with spinal and respiratory motion.

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Fig. 1 Pre-operative x-ray imaging in 2010 showing AIS.

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Fig. 2 Post-operative imaging in 2010 showing surgical correction of the lateral curvature.

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Clinical examination revealed a healthy looking 25 year old female who stood with a fixed apex right midthoracic scoliosis with healthy healed previous wounds. She was tender to palpation throughout the thoracic region but predominately at the right T4 rib head region. She had full range of motion of her shoulders and normal scapulothoracic motion but did have a moderate rib hump. Her neurological examination was unremarkable. Deep inspiration and thoracic compression reproduced her pain.

Whole spine alignment x-ray imaging was obtained and this was relatively unremarkable [Fig. 3]. A PET/CT scan was subsequently arranged which showed increased uptake at the T4 rib head level with evidence that the apex of the rod had bored itself into the head of the fourth rib on the right side [Fig. 4].

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Fig. 3 Whole spine alignment x-ray imaging in 2021 showing unremarkable findings.

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Fig. 4 CT/PET imaging in 2021 showing the cranial aspect of the rod boring into the fourth rib head.

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Due to the risks entailed with removal of metalware, she proceeded with a right sided fourth rib head resection. Intraoperatively the prominent rod was seen to have gouged its way through the rib head with marked inflammation and metal debris identified around the rib head defect. The rib head was removed from around the rod achieving complete freeing of the rib from impingement on the rod.

She made an excellent post-operative recovery and was discharged the next day with complete resolution of her previous pain. She ceased all analegesic and pain management inoput within 6 weeks of her operation and has remained asymptomatic beyond one year follow up.

3 Discussion

Our case highlights a rare late complication of correction surgery that has not previously been documented in literature. It is common knowledge that instrumentation may fail overtime due to the biomechanical stressors of everyday life. The lateral approach to spinal fusion undertaken for our patient meant the cranial aspect of the rod was in close proximity to the rib head. It is of our belief that overtime this cranial aspect was able to bore into the head of the fourth rib leading to pain exacerbating by respiratory function that is typical of rib pathology. We undertook resection of the rib head, rather than metalware removal due to the comparative ease and complication profile, and this was met with an excellent clinical outcome and patient satisfaction.

It is worth noting that the rib is a highly mobile structure during respiration whereas the rod itself is a rigid and immobile structure. In our patient this rib motion against an immobile rod led overtime to the rod boring into the rib head. This explains why the patient had pain exacerbated by respiration, and the sensation of a squeaking noise, as the movement of the rib head against the rod was the driving force behind the pathology.

Another important question raised by this case is why did this complication occur ten years after the index procedure? One possible explanation is that the degree of restoration of thoracic kyphosis during surgical correction could affect the proximity of the rod to that of the rib head, particularly so at the junction between where the rod ends and the superior rib head. Rod contouring is an important aspect of correction surgery in determining the degree of restoration and studies have shown increased risk of negative outcomes, such as proximal junction kyphosis (PJK) and rod breakage, associated with over or under contouring of the rod.^10,11 One study by Shardi and colleagues found that without using a template surgeons on average over bend the rod by almost 20° and they hypothesised that this over-contouring is likely associated with increased rates of failure at the proximal junction.¹¹ As such it is imperative that comprehensive preoperative planning occurs to ensure appropriate rod contouring for restoration of sagittal balance. Other studies, such as by Yang and colleagues, have reflected that a mismatch between the proximal junctional angle and the proximal contouring angle is associated with greater rates of failure at this junction, such as PJK.^12–14 There is great variability in reported incidence of PJK ranging from 7% to 46%, in AIS patients treated with posterior fusion, which can be explained by differences in the study designs and characteristics. What is common to all studies however is that patients with severe PJK are more likely to go on to develop further complications such as vertebral collapse and subluxation at the proximal junction.^15,16 In the case of our patient preoperative radiographs show good sagittal alignment and therefore it is unclear if rod contouring or PJK played a role in this presentation. However it is clear from the literature that this proximal junction is an area of high stress and therefore not surprising that this is the area in which a complication occurred.

It is also possible that her chest wall and vertebral column continued to grow whilst the rod itself was fixed in length. This discrepancy would result in a different relationship between rod and rib head than that which was seen at the time of operation. Machida and colleagues commented that late complications tend to be a result of pseudarthosis and/or rod breakage however neither of these were true in our case.⁵ The spinal radiographs taken preoperatively showed good alignment and intraoperative findings showed an adequate relationship between rod, screws and vertebra. As such, it was possible for us to leave the metalware in situ rather than undertaking significant revision surgery. It is worth commenting that similar circumstances could arise in cases where pseudarthrosis or screw loosening occurs if this allows the rod to migrate towards the rib head. Interestingly so, there are several cases in the literature where rod migration occurs in either a cephalic or caudal direction in the setting of screw loosening or pseduarthrosis and in some instances this has been as dramatic as lumbar fixation migrating down to the knee.^17,18

We hope with this case to raise awareness of rib pathology as a possible complication of surgery, particularly in the instance of lateral fusion.

Statements

Statement of Ethics: Written informed consent was obtained from the patient for publication of this case report and any accompanying images. As this was a case report, IRB approval was sought but deemed unnecessary as the single patient consented (and continues to consent) to their involvement, negating the need for review board approval.

Funding

The authors received no funding for this case report.

Author contributions

Conception or design of work: CAF and DCK. Acquisition, analysis, or interpretation of data for the work: CAF and DCK. Drafting of work or revising it critically for important intellectual content: CAF and DCK. Final approval of version to be published: CAF and DCK.