Rhino3DMedical has been used on a technical note on a study on accuracy and safety of 3D printed surgical guides for placement of cervicothoracic pedicle screws.

The study was led by R.B. Santos, C.M. Ribeiro, D. Grade and A.M. Baptista from the Neurosurgery Department of Centro Hospitalar Vila Nova de Gaia/Espinho, Vila Nova de Gaia, Portugal, and by F. Pagaimo from Pagaimo Medical, Figueira da Foz, Portugal.

Please refer to the technical note for more information:
https://www.sciencedirect.com/science/article/abs/pii/S0028377023000152

In the following we reproduce the highlights of the technical note.
Currently Rhino3DMedical is not certified as a medical device. The intended use of the software is to support research and education purposes.

Abstract

Subaxial cervical pedicle screws provide rigid fixation, but their placement poses an important neurovascular injury risk. 3D printed guides have successfully been used to place pedicle screws, but experience in the subaxial cervical spine is limited. We present a case of cervicothoracic dissociation after a pathological fracture due to tumour involvement of the upper thoracic spine, causing paraparesis and intense pain. The cervicothoracic junction is of difficult visualization on fluoroscopy and the patients’ severe instability made navigation unreliable. 3D printed individualized guidewire guides were used to help place canulated pedicle screws from C4 to T6. We successfully report the use of impedance guidewire monitoring to prevent pedicle violation and improve procedure safety.

Clinical case and surgical technique

3D printed patient-specific screw placement-guides have shown promise in decreasing implant mispositioning, but this technology is not widely used in the subaxial cervical spine. We report the successful use of this technique and propose a technicalnuance to improve its safety.

A 52-year-old previously healthy man presented with a 3-week history of progressive gait dysfunction and lower limbs muscleweakness. He was a heavy smoker and reported a weight loss ofapproximately 10 kg in the previous 2 months.

Physical examination demonstrated a Medical Research Council (MRC) grade I muscle strength in the right lower limb (RLL) and an MRC grade III in the left lower limb (LLL). Hypoesthesia was present from T4 level and down. Proprioception and vibration sensory functions were intact. The patient had reduced dexterity in his hands but no other motor or sensory changes in his upperlimbs.

A thoracic spine contrast tomography (CT) scan disclosed a thoracic tumor involving the lung, pleura and rib cage that invaded the C7, T1, T2 and T3 vertebral bodies. The T2 vertebral body was completely collapsed causing a kyphotic deformity and sagittal and coronal malalignment.

The patient had undergone an emergency laminectomy of T1, T2 and T3 and a tumour specimen was sent to histopathological examination. After the surgery, the patient improved in motor function, but still presented an MRC grade II strength in his RLL, an MRC grade IV strength in his LLL and a severe instability, partially worsened by the surgical procedure. He now suffered from a complete 3 column cervicothoracic dissociation. Histopathological examination showed a squamous cell carcinoma. No other site of disease was detected. In multidisciplinary discussion, the patient was deemed to have a vital prognosis of >1 year and an indication for chemo and radiotherapy. With these premises in mind, a decision for palliative surgery and stabilization of the cervicothoracic junction (CTJ) was made.

Preoperative sagittal and coronal CT-scan showing a paravertebral mass, pathologic fracture with vertebral collapse and 3 column instability.

3D printed model of the patient’s cervicothoracic spine used for iterativetests of the guidewire guides.

Using the Rhino3DMedical plugin on Rhinoceros, a 3D model of the spine was segmented from CT scan images with 1 mm slice thickness and CT gantry tilt equal to zero. Using the obtained 3D model, the optimal trajectory of each pedicle screw was planned. Associated 3D vertebra specific guidewire guides were designed and printed using fused deposition modelling (FDM), an additive manufacturing process that belongs to the material extrusion family where objects are built by selectively depositing melted material (in a filament form) in a pre-determined path layer-by-layer. Polylactic acid filament was used for the spine model while medical grade polyethylene terephthalate glycol (PETG) was used for the surgical guides. Resulting guide parts were sterilized by low-temperature hydrogen peroxide gas plasma. During the design process care was taken to ensure correct adaptability and stability of the guides. The tip of the cervical spinous processes is either covered in the posterior tension band or partially removed during dissection. The lateral aspect of lateral masses is difficult to completely clear of muscle encroachment. We therefore designed each guide to adapt only to the lamina and base of the spinous process of a single vertebra. Having each guide adapt to a single vertebra ensured that intra-operative movement of this patients’ severely unstable spine did not affect the accuracy of the guides.

Snapshot of parametric planning of pedicle screws trajectory using Rhino3DMedical. In grey the center of the pedicle while in green an optimized surgical trajectory with more favorable insertion angle requiring less soft tissue dissection.

Each guide went through iterative tests with the 3D printed spine model, having its design updated whenever required to assure the proper stability during the drilling process, but also to reduce the required dissection time and exposure.

The full process of design of each guide and production of both spine model and guides took 4 business days, which was enhanced by a parametric design using Grasshopper, a graphical algorithm editor inside Rhinoceros.

A posterior approach was used and the posterior elements of C3 to T7 were exposed. Careful dissection of all soft tissue wasper formed leaving only the bony anatomy. The vertebra-specific guidewire guides were adapted to each vertebra and guidewires were placed bilaterally in the pedicles of C4, C5, C6, C7, T4, T5 and T6 using an electric drill. Such guidewires were originally developed to monitor pedicle screws of some specific spine systems, but here they were used as a drilling guidewire. During their placement, guidewire electrical conductivity monitoring was performedusing Pediguard’s, dynamic surgical guidance (DSG) Technology. DSG Technology is a bipolar sensor placed at the tip of the wire. When used while drilling with the guidewire, DSG Technology gives the surgeon feedback via audible alerts. The alerts indicate a change in impedance at the tip of the guidewire, thus minimizing cortical wall breaches or pedicle compromise, as this monitoring allows to discern if the tip of the guidewire is in contact with cortical bone, cancellous bone, soft tissue, or blood. Intra-operative 3D imaging confirmed correct positioning of all guidewires. Canulated pedicle screws were placed over the guidewires. Cobalt-chrome bars were used to perform an in situ fixation.

Intra-operative photograph depicting the surgical field with a 3D printed guide after guidewire placement at T4.

Surgery was performed with continuous monitoring of evoked motor potentials, which showed a decrease of 20% response inupper limbs and 39% decrease in lower limbs. Despite this, the patient progressed well postoperatively, with no decrease in motor strength, an improvement in pain and a good tolerance for the seated position and motor rehabilitation therapy.

Postoperative CT scan confirmed correct positioning of all pedicle screws. No cortical breaches were identified, and no neurovascular injury was detected.

Discussion and conclusion

This case presented several challenges: severe instability and severe deformity in an area of the spine with poor fluoroscopic visualization.

Intraoperative imaging and navigation are strategies that improve procedure safety and are rapidly becoming the standard of care but are also costly and not universally available. A recent systematic review confirms the scarcity of reliable economic data on 3D printing technology. According to our experience, the cost parcels broken down of such technology can be as follows:

  • Spine model segmentation — this step encompasses the cost of the software, office, technician hours which will vary accordingto the exam quality, software and computing performance and complexity of the case, but as a rule of thumb it can be considered 2 h of labour work.
  • Designing process for custom made surgical guides — it is also variable as it will depend again on the software and computing performance, technician experience (usually a biomedical engineer) and case complexity with associated number of iterations.
  • 3D printed parts production — such process encompasses the cost of required premises, equipment acquisition and maintenance, materials, operator, post-processing, validation process and shipping.
  • Disinfection and sterilization which takes place at the hospital and its cost is like any other device of the same size using the same sterilization technology.

However, it is our understanding that beside the cost of making the 3D printing technology available, further studies should also take into consideration the clinical outcomes compared with existing options and associated risks — a value-based healthcare approach, which is out of the current study scope.

The CTJ is of difficult visualization on fluoroscopy because of the artefact produced by the rib cage and its contents. Furthermore, the patient’s severe instability and mobility of individual vertebrae could potentially make navigation less accurate.

Given these challenges, we decided to use 3D printed vertebra specific guidewire guides. We chose to use these guidewire guides instead of the screw-guides available on the market because, in an area of the spine with so little margin for error, guidewire confirmation with intra-operative DSG conductivity monitoring before screw placement represented a lower risk of neurovascular injury. Although the 3D-printed guides provided an accurate placement of all guidewires in this case, the DSG technology enabled us to be sure of an intra-osseus trajectory and might have prevented a neurovascular injury had one of the guides been flawed or incorrectly adapted to the vertebra.

To the best of our knowledge, experience with guidewire guides in the sub-axial cervical spine is limited. We present this technical report as indication that this procedure is safe and that its safety is further improved by guidewire monitoring.