Abstract
Background
Total hip arthroplasty (THA) is one of the most successful surgical procedures ever performed. Nevertheless if procedure is performed by high or low volume surgeons; more than 50% of cups are still placed out of the safe zone, which is connected to lower survival rate of the prosthesis. The idea was to develop an imageless navigation system for safe and accurate positioning of the cup in THA procedures, without a need of any preoperative computer tomography (CT) or magnetic resonance imagining (MRI).
Methods
The validation of the system was approved by National Ethics Committee. The committee allowed the validation on 10 patients who all signed the agreement for participation in the study. Unselected patients undergoing THA were included. All patients had had performed preoperative x-rays of pelvis and hips for standard preoperative planning. Immediately before skin incision, anterior pelvic plane (APP) was defined with help of specially developed electromagnetic navigation system (Guiding Star, E-Hip module, Ekliptik d.o.o., Ljubljana, Slovenia) and specificaly designed hardware tool which is essential for accurate APP determination [Fig.1]. In all patients THAs were performed through direct lateral approach and all implanted components (Allofit S cup and Alloclassic stem, Zimmer Inc., Warsaw, Indiana, USA) were implanted with freehand technique according to preoperative plan. After placement of the cups their inclination and anteversion angles were determined with aforementioned navigation system [Fig. 2]. The day after surgery, low dose CT scans of pelvises of operated patients were performed and DICOM format files were up-loaded into EBS software (Ekliptik d.o.o., Ljubljana, Slovenia), a multipurpose application for perioperative planning, measuring and constructing where virtual copies of pelvises were generated. On virtual pelvises the position of the cups was measured by independent person [Fig.3]. Measurements were compared, statistically analysed and the deviation calculated with root mean square error (RMSE) method. Afterwards the average error (eaver) and standard deviation (σ) between intraoperatively determined and postoperatively measured angles were calculated.
Results
We included 10 patients in the study, with 6 left and 4 right hips. The maximal and minimal differences between navigation and CT measurements for inclination angles were 5.3° and 0.3° respectively, with calculated eaver of 0.7°, σ of 2.6° and RMSE of 2.6°. The maximal and minimal differences between navigation and CT measurements for anteversion angles were 4.6° and 0.7° respectively, with calculated eaver of −1.9°, σ of 1.8° and RMSE of 2.6°.
Conclusion
We determined that the imageless navigation system we validated is a very accurate tool for cup placement in THA. The accuracy of the system is within 2° which by far exceeds the abilities of the best freehand techniques. In line to the trends, supporting more precise and less invasive surgery, the THA with help of imageless navigation should in our opinion become a golden standard, especially in minimally invasive procedures.
