Frameless image-guided surgical techniques are increasingly being used for functional neurosurgical procedures. Given that several studies have demonstrated equivalent accuracy to a stereotactic frame, and that many younger neurosurgeons have more familiarity with neuro-navigation than with traditional frame-based stereotaxis, frameless techniques may become an increasingly attractive option. However, like frame-based stereotaxis, there are a number of pitfalls that can seriously degrade accuracy, leading to mis-targeting and poor surgical outcomes. Choice of one “mini-frame” targeting platform vs. another is largely a matter of personal preference, although each has its advantages and disadvantages. This brief discussion will focus on the NexFrame system (Medtronic Inc., Minneapolis, MN), although the Starfix platform (FHC, Bowdoin, ME) is also widely used.

 

A number of factors affect the accuracy of a neuro-navigational system, all of which must be optimized in order to achieve true submillimetric stereotactic accuracy. The use of implantable bone fiducial markers is essential, as these provide well-defined fixed landmarks, which maintain a rigid relationship to the targeted anatomy. At least 5 markers should be placed, allowing for redundancy in case one is dislodged. Implanting the markers the day before surgery allows surgical planning to be carried out ahead of time, so that surgery can begin promptly without OR delay. Registration should be performed with an instrument of known geometry, and the geometry errors should be kept below 0.2 mm, and preferably below 0.1 mm. Registration errors of greater than 0.6 mm should not be accepted, and the sphere of estimated 1 mm accuracy should fully encompass the head. The platform must be firmly attached to the skull, checking stability by lifting one side and pushing the other to be sure there is absolutely no movement. The dura mater is opened prior to alignment with the planned trajectory. Perfect alignment of the trajectory with the target should be the goal, since even small deviations will be magnified at the due to the geometry of the aiming platform. Meticulous attention to detail at each step can achieve accuracy equivalent to or better than a stereotactic frame.

 

Frameless techniques are often preferred by patients, who sometimes profess fear of the intimidating and claustrophobic frame. However, in some circumstances a frame may be preferable. Frames are capable of supporting larger instruments, such as injection systems for molecular or cellular therapies. Frames also provide immobilization, which may be important for procedures such as radiosurgery or focused ultrasound treatment. Finally, patients with severe involuntary movements such as ballismus or tremor may require immobilization with a frame. For the majority of DBS cases, however, frameless techniques can provide equivalent or better performance in terms of both patient comfort and placement accuracy.