Screw placement accuracy in instrumented spinal fusion

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Claudio LaMartina

Screw placement accuracy is a critical point in instrumented spinal fusion, especially in difficult deformity cases There is a real, established risk of pedicle screws malposition and related complications, reported in literature at a range of 8–30%, writes Claudio LaMartina.

When screw malposition is symptomatic, it can cause radicular pain or neurological signs, vascular or visceral injuries. Moreover, the asymptomatic malposition can be detrimental for the stability of the construct, creating the mechanical base for a hardware failure. In order to avoid pedicle screw malposition, some deformity surgeons have adopted intraoperative X-rays, with standard anteroposterior and lateral projection, to check the screw position during the implant. This is not sufficient, however, since a persistent malposition rate is reported, even when the implant is X-ray-guided, compared to the hand-free technique. The intraoperative use of computed tomography (CT) scanning, with the introduction of the O-arm, has demonstrated the ability to reduce malposition rates. At the same time, however, this has increased radiation exposure. This is a problem for both patients and surgeons; idiopathic deformity patients are usually young girls, and the radiation dose repeated for each implanted screw is not suitable for them. In addition, with the routine application of this technique, the radiation exposure for surgeons and operating theatre staff increases dramatically.

Navigation and robotic procedures for spinal deformities have some limitations. The first is related to radiation exposure, as already discussed.

These powerful technological products need a detailed radiological preoperative work-up, thus exposing the patient to the same global amount of radiation as intraoperative control technique. The second limitation is related to the lengthening of the procedure by the set-up of these systems, which require time and staff.

Lastly, deformity surgery is difficult, and its learning curve is long. Mistakes are not tolerated, which produces a growing issue among the junior spinal surgeons. The free-hand technique of an experienced surgeon has been reported as the most reliable method for screw placement, but it is not easily reproducible. The ideal system for placing pedicle screws should guarantee:

  1. Low radiation exposure
  2. Reproducibility
  3. No extra operative time

In my opinion, the Medacta MySpine technology has the potential to address these three areas.

MySpine is a new technology that allows a custom-made guidance for pedicle screw insertion. It consists of 3D-printed guides manufactured according to the spine of each patient, allowing a precise and safe pedicle screw insertion. MySpine is a low-radiation exposure system, made possible by the creation of a low-dose CT scan protocol. This very specific diagnostic tool is the only radiation dose that the patient will receive.

Details of the bony anatomy of the patient are then used to build a 3D model of their vertebrae. Once the 3D anatomy of the patient is created, a dedicated virtual environment can be projected for preoperative planning. Each virtual vertebra is instrumented on this platform. It is possible to choose the best trajectory, entry point, screw length and convergence, taking all the time necessary. When planning is completed, the 3D data from the patient’s anatomy and from our “planned” screws are merged, and a proper guide is created with a 3D printer. Each guide is coupled with the 3D reconstruction of the corresponding vertebra. These components are then sterilised and prepared for intraoperative use. Once the surgeon has accurately dissected the posterior aspect of the spine at the desired levels, a mask is applied on each corresponding vertebra. The two operative tubes on both sides of the mask allow a safe and precise preparation of the pedicle and screw placement. Each passage of pedicle screw insertion (pedicle preparation, pedicle feeler use and screw implant) is addressed by the guides and can be visualised by a window at the bottom of the tubes.

Different cadaveric studies have been published on this system, and a prospective, randomised trial will be soon completed. This trial aims to compare screw placement accuracy in deformity patients operated with standard free-hand technique vs. the MySpine technology. Preliminary data are encouraging, but we are looking for solid evidence, based on large numbers. However, the Medacta MySpine technology appears to be a path towards a more accurate pedicle screw implant and fewer related complications.


Claudio LaMartina is an orthopaedic surgeon at the Galeazzi Orthopedic Institute in Milan, Italy

 

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