A Symbiosis of Surgeon and Technology
Total knee replacement is one of the most gratifying and rewarding of all orthopedic surgeries. The success of total knee arthroplasty depends on many factors, including patient selection, prosthesis design, pre-operative condition of the joint, surgical technique including proper soft tissue balancing and most importantly, component and limb alignment. Studies have suggested that the most common cause of premature failure of artificial joint is an error in surgical technique, and small changes in component alignment can make significant impact on the long term performance of the joint. Like a car whose faulty alignment may lead to premature wear of the tyres, malaligned total knees are prone to early failure. Malalignment of more than 3 degrees in the frontal plane can lead to premature loosening of the implant in up to 20% of the cases while the incidence of loosening in cases where alignment is within 3 degrees of neutral is about 3%. Similarly, malalignment of more than 2-5 degrees in other planes has been associated with poor functional outcome due to pain, instability, decreased range of motion and soft tissue irritation.
Conventionally, while doing a knee replacement, surgeon uses manual examination which is aided by the use of jigs and alignment guides which use bony landmarks to align the metallic and plastic components correctly with respect to bone. It has been found that despite the available instrumentation system, at times, human errors and minor toggle in the jigs up to few degrees are unavoidable especially in cases with significant preoperative deformity around the joint. There are other valid limitations like obesity, post-fracture deformity, narrow bone, previous operation around the joint which tend to reduce the surgical accuracy with conventional technique even in the hands of very experienced surgeons. Surgical navigation systems help reduce errors during surgery by providing real time visual information about the accuracy of the bone cuts, implant positioning, ligament balancing, and final alignment of the limb.
The current image-free navigation systems work on the principle of optical tracking in which reference frames with infra-red signal transducers are attached to bone which provide three dimensional data about the anatomy of the joint and limb alignment to the computer which then translates it into visual image on the monitor (Figure 1). Thus, the accuracy of each step of the operation (cutting block placement, saw cuts, seating of the implants) can be verified with the computer, allowing adjustments to be made during surgery.
Various studies have shown that the upper limits of accuracy (within 3 degrees of ideal) in conventional methods varies from 75-90% in the hands of experienced surgeons as opposed to accuracy of more than 96% in the hands of the same surgeon with the use of surgical navigation system. The surgical time with navigation is slightly increased by 10-15 minutes which is actually the time taken to register the individual anatomical data to the computer. The use of navigation also reduces blood loss by up to 100-200 ml since the 
reference frames are placed outside the bone unlike the conventional jigs which need to perforate the whole bony canal for measuring alignment thereby contributing to postoperative blood loss. Surgical navigation, however, is not a substitute for surgical expertise since accurate results with navigation can be delivered only by surgeons who have mastered the technique of conventional knee replacement. Conventionally, knee replacement has been considered as a surgery for the elderly whose activity demands are low anyway, but currently, knee replacement is being performed in younger and more active patients too. Therefore, apart from better quality implant biomaterials, the need of the hour is higher surgical accuracy so as to provide a high performance joint with better long term durability. Computer assisted surgical navigation, in its current form and with the upcoming technological advances, will go a long way in fulfilling this promise of reaching a near ideal goal.
Inputs by Dr. Vivek Logani, MS, DNB, MNAMS, Chief of Joint Replacement Surgery, Paras Hospitals, Gurgaon and Delhi.
