Navigation in orthopaedic surgery is becoming more accepted as a means of improving and documenting accuracy. Yet, despite the improvements made, the road towards recognition of the capability and versatility of computer-assisted surgery has not always been smooth. Initially, navigation involved imagery, which utilized a localizer to calculate the geometry of the infrared reflective surfaces and light emitting diodes or tracking grids . Image guided procedures utilized patient X-rays and fluoro-imaging obtained prior to or during a procedure as a guide for the physician. However, this was constrained by the necessity of direct, unobstructed line of sight. Both image-guided and imageless navigation systems require trackers to establish the position of the patient as movement occurs, whereas traditional imageless infrared (IR) navigation eliminates the need for fluoro-imaging while still requiring line-of-sight. Constraints on signal strength as well as a limited arc or azimuth of signal reception hinder its use. Additionally, IR accuracy is proportional to the separation between reflector markers, requiring expansive arrays, which are obtrusive to the surgical field and may injure soft tissue as movement of the extremity is performed. Due to problems including fixation, signal acquisition, soft tissue trauma, and operative constraints, a technology using a signal that does not require line-of-sight reception was desirable. An electromagnetic computer-assisted surgery (EM-CAS) system has this capability. Some of the first reports of the use of EM in surgery were in neurosurgical and ENT applications for cranial surgery [2,3]. Other applications of EM technology arose in cardiovascular surgery, where an electromagnetic field was used to guide ferrous-tipped catheters into tortuous vessels that were non-navigable by traditional catheterization techniques. These were tracked by fluoro-imaging yet had the distinction of reversing the role of EM to steer them rather than track them into extremely tight, convoluted vessels . Part of the suspicion about EM technology for orthopaedic applications arises from concerns over the stability of the signal around metallic objects. However, with the advent of multiplex magnetic generators, known as localizers and receiver coils or trackers called dynamic reference frames (DRFs), much of the instability and signal inaccuracy is removed. Precision and accuracy is vastly improved to well beyond the industry standard of ±2 mm and ±2° of angulation. Achieving this level of accuracy allows EM tracking to have equivalent status as traditional line-of-sight IR navigation systems with the added benefit of soft tissue penetration.
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