Jonathon Garrett MS-2¹; Michael McDermott DO²; Joseph Mixa BS¹; Michael Rogers MS-2²; Rebecca Michna PC-A²; Robert Prior DO³; Ashish Patel MD^2
1Rosalind Franklin University
²Duly Health and Care
³Fransiscan Health Olympia Fields

Abstract 

Introduction
Cervical disc replacement (CDR) has quickly gained popularity for treating cervical disc degenerative pathologies, as it can preserve natural function and reduce adjacent segment disease. Numerous implants have been developed to optimize patient outcomes; and while progress has been made, each implant comes with its own nuance for safe placement. Our case describes the post operative discovery of an intraoperative fracture during a multilevel hybrid ACDF-CDR procedure with a keeled implant.

Clinical Presentation
A 53-year-old former smoking female presented with a one-year history of bilateral 8/10 radiating neck pain that began after a fall. Radiographic films demonstrated C4-5 cervical spondylolisthesis and degenerative disc disease with foraminal stenosis at C5-C7. The patient was preoperatively diagnosed with cervical radiculopathy at C4-7 and the consented to CDR from C5-C7 and ACDF at C4-C5. Immediately postoperatively the patient had incomplete loss of motor and sensation below the biceps bilaterally and CT imaging revealed a fracture of the proximal posterior corner of C6 leading to retro-vertebral stenosis of the spinal canal. The patient returned to the OR for revision surgery and was successfully treated with a C6 corpectomy. Immediate post operative symptoms improved mild paresthesias. The patient observed a full recovery of strength and sensation by five months. 

Conclusion
Maintaining endplate integrity is crucial to performing a CDR with a keeled implant and it is important to be gentle when chiseling the end plates. Be patient when progressing the implant and refrain from increasing the pressure to force the implant into position. 

Running Title: Keeled CDR causing intraoperative fracture.

Keywords: Arthroplasty, Cervical, Disc, Fracture, implant, Replacement, Spine 

Abbreviations: ACDF, anterior cervical discectomy and fusion; CDR, cervical disk replacement; CT, computerized tomography; Tc MEPS, transcranial motor evoked potential 

Introduction
Cervical disc replacement (CDR) has become a widely used option for treating cervical disc degenerative pathologies, as it can preserve natural function of the cervical spine and reduce adjacent segment disease. CDR aims to achieve a seamless transition between healthy and instrumented segments by replacing the diseased disc with a mobile implant. Strong bony connection to the implant is crucial to ensure fluidity of the cervical spine, and efforts to achieve this have resulted in numerous implant designs including spikes, wire mesh, keels, increased porosity, screw fixations, and specialized coatings (1-3). Uniquely, each of these design variations require individual insertion techniques, which subsequently creates the opportunity for complications specific to each implant (1-3). 

With the popularity of CDR continually increasing, it is important to examine the possible causes and outcomes of implant specific complications. In cases of device malfunction, vertebral body osteolysis, infection, segmental kyphosis, or lymphocytic reaction; implant explantation and conversion to anterior cervical discectomy and fusion (ACDF) has proven to be a successful treatment option (4). Cases of foraminal stenosis following CDR have been successfully treated with posterior foraminotomies (4) while central stenosis has been treated by explantation, decompression, and then reimplanting the same CDR device or by converting it to ACDF (4-6). There are only a few reported cases of vertebral fractures, and they have been successfully treated nonoperatively, or converted to a fusion with/without a corpectomy (4-8). Case-specific differences in bone loss, bone quality, and fracture location make choosing the appropriate treatment plan following fracture difficult (4). With an increased risk of spinal cord compression and implant migration, understanding how to both resolve and avoid iatrogenic fractures is essential (6). In this report, we describe our clinical decision making of a case with an intraoperative fracture that occurred while placing a CDR, causing retro-vertebral stenosis that was successfully treated with a corpectomy.

Clinical Presentation
A 53-year-old former smoking female presented with a one-year history of bilateral radiating neck pain. The physical examination showed pain with neck flexion/extension and a positive Spurling’s test. Radiographic films demonstrated C4-5 cervical spondylolisthesis and degenerative disc disease with foraminal stenosis at C5-C7 (Figure 1). The patient was diagnosed with cervical radiculopathy at C4-7 and the consented to CDR from C5-C7 and ACDF at C4-C5.

The procedure was performed in a standard right sided Smith-Robinson approach, utilizing keeled CDR implants (Simplify, Nuvasive).  Fluoroscopy showed the implants to be stable and in the desired position (Figure 2). Upon completion, signals from transcranial motor evoked potentials (Tc MEPs) were noted to be absent from the bilateral distal biceps. A wakeup test showed no motion distal to the biceps bilaterally. All the implants were removed, Tc MEP signals returned, with the signals in the left upper extremity weaker than the right. The implants were then replaced sequentially, and the Tc MEP signals remained at their new baseline after each successive implant placed.

Following the surgery, the patient presented with several deficits including bilateral hand weakness and posterior cervical muscle spasms. Computerized tomography (CT) scan revealed a fracture of the upper posterior corner of the C6 vertebral body causing retro-vertebral stenosis of the central canal (Figure 3:A-B). On post operative day two She was taken back to the operating room and a hooked dissector was used to reach the posterior fragments. This was unsuccessful due to their location relative to the vertebral body. Next, a central 14mm trough was made through the C6 vertebral body to retrieve the fragments. Due to significant bone loss, a single level corpectomy was performed (Figure 3:C-D). For increased stability, a plate and bone graft were applied from C4-C7. Tc MEPs improved demonstrating strong signals globally.

Following the revision, the patient was discharged with minor paresthesia and triceps weakness. At the two-week follow up, the patient had difficulty with fine motor movements/grip strength, and bilateral numbness in the 3rd-5th digits. These symptoms mostly resolved by five months after the surgery, with only minimal hand numbness persisting. At this time radiographic films demonstrated a stable construct and the patient reported feeling fully functional (Figure 4). Further follow up was impossible due to the patient moved and was lost to follow up. 

Discussion
As CDR continues its popularity, it is important to examine each unique intraoperative complication and discuss the different treatment options. This case describes the post operative identification of a fracture during a CDR placement with a keeled implant. This fracture led to retro-vertebral stenosis and was successfully treated with a corpectomy.  

Maintaining endplate integrity while performing CDR is crucial to a successful surgery. The use of a keeled implant requires chiseling an anterior to posterior canal into the caudal end plate of the superior vertebrae and the cranial endplate of the inferior vertebrae. If the canal is not appropriately d to accommodate the keel, increased pressure may be used to force the implant into place. In several reported cases, fractures have occurred in this manner(4-8). Unfortunately, the fracture reported in this case was only noticed upon viewing postoperative imaging; however, we believe that the fracture most likely occurred during this step. It is also possible that the endplate was fractured due to the canal not being large enough for the keel. Currently, no consensus exists on how to resolve fractures of this sort. As such, we have described this case to discuss our experience using single level corpectomy for revision. After discussing this case with the implant manufacturer, a specific back stop is being created to help prevent this issue in the future. 

In cases where substantial bony loss and retro-vertebral stenosis are not present, a corpectomy is not necessary and less invasive methods can be used to successfully treat the fracture. Current reports suggest that the stability of the implant, location of the fragments, and the timing of the fracture discovery are most influential to determining the appropriate revision method (1-15). Indications for terminating CDR intraoperatively have been described for situations where severe disc space narrowing requires massive decompression or when anatomical deformity of the endplates results in inadequate bone to prosthesis contact (6). Over-removal of the endplates and difficult to revise fixation channels also indicate CDR abortion (6). If fractures are discovered intraoperatively, you can continue a CDR procedure if the fragments were either displaced directly posterior to the endplate where they could easily be removed or were contained within the vertebral body without compromising the stability of the implant (5,7). Conversion to a ACDF has also shown successful outcomes in situations where the fragments can easily be removed (4-8). If the fragment cannot be easily removed, and is leading to implant instability or neurologic compromise, we recommend single level corpectomy. There are only two reported cases of a CDR procedure leading to a corpectomy, occurring nine and six years after the initial CDR surgery (16,17). In both cases the patient had an implant failure requiring revision (16,17). 

Conclusion
Intraoperative fractures are a rare complication of cervical disc replacement, and there are no specific guidelines for their treatment. Maintaining endplate integrity is crucial to performing a CDR with a keeled implant and it is important to be gentle when chiseling the end plates. If you do cause an intraoperative fracture during CDR leading to retro-vertebral stenosis, this report demonstrates successful treatment with a corpectomy. 

Figure 1 | Figure 2 | Figure 3 | Figure 4

Figure Legends

Figure 1. A-D: Preoperative AP, Lateral, flexion and extension films of the cervical spine. This figure is original to this submission so no credit or license is needed.

Figure 2.  A-B: Intraoperative Fluoroscopy films demonstrating placement of CDRs and ACDR. This figure is original to this submission so no credit or license is needed.

Figure 3. A-B: Axial and Sagittal images from the Postoperative cervical CT demonstrating a fracture of C6. C-D Intraoperative Fluoroscopy images demonstrating conversion to corpectomy. This figure is original to this submission so no credit or license is needed.

Figure 4. A-B: Five-month postoperative radiographs showing stable corpectomy construct. This figure is original to this submission so no credit or license is needed.

Required Disclosures and Declaration

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