A770. ACCURACY OF CUP CENTER POSITION IN IMAGE-FREE NAVIGATION SYSTEM FOR DYVELOPMENTAL DYSPLASIA OF HIP

Abstract

Nowadays navigation system for THA is widespread and contributes to accurate cup installation as for cup abduction and anteversion angles. On the other hand, cup center position is very important to prevent leg length discrepancy and to acquire appropriate muscle tension especially for DDH cases. However planning and accuracy of cup center position was rarely mentioned when the efficacy of navigation systems were discussed. We therefore examined not only accuracy of cup angles, but also of cup center position in our image-free navigation system for DDH.

One hundred three THA operations were performed with using the image-free OrthoPilot hip navigation system (B. Braun Aesculap, Tuttlingen, Germany) between May 2006 and July 2008 by three experienced surgeons. In this system, we can measure the length between two different points marked by special pointer during surgery. Thus we pointed the upper rim of obturator foramen (this mark was estimated the lower tip of tear drop, and the bottom of reaming hole (this mark was estimated same height from cup center position) before cup installation and measured the vertical length between them(op length). After operation, we measured the vertical length from tear drop to cup center on the x-ray film (xp length), and compared these two values.

The average difference of two values were 6.41±4.17 mm ((op length)-(xp length)). Secondly we divided them into two groups, large error group (> 0.7mm) and small error group (< 0.6mm) and investigated the cause of large error. As result, large error was influenced by difference of surgeons, whereas not influenced by patient’s etiology and BMI.

By using image-free navigation system for DDH, we can plan the cup center position and install it within the error of 6.4mm. This will contribute to avoid a lot of hesitations during surgery. However surgeon’s skill and habitants have influence on this technique. We have to investigate this system and make effort to further improvement continuously.