A NOVEL METHOD IN ANTERIOR PELVIC PLANE REGISTRATION IN NAVIGATED HIP ARTHROPLASTY, A CT-SCAN COMPARATIVE STUDY

Abstract

There is a significant variation in registering anterior pelvic plane (APP) among experienced navigated hip surgeons reflecting negatively on the accuracy of determining the inclination and anteversion angles. Registering the APP in a lateral decubitus position is more challenging in obese patients as palpation of pubic tubercle or anterior superior iliac spines (ASIS) is inconsistent. We propose an alternative and easier novel method in which palpation of the posts (pegs) that stabilizes the pelvis will accurately determine the APP plane. The computer data obtained from peg’s palpation was compared to data obtained from post-operative CT scan of the pelvis in determining acetabular and cup version and inclination angles.

The APP was defined and registered in 40 navigated total hip arthroplasty (THA) patients using our novel method. The patient is securely stabilized in a lateral decubitus position as routine with multiple pegs. One peg is positioned against both ASIS with 2 EKG pads placed on the pegs (each represent an ASIS). The other peg supports the pubic symphysis with one EKG pad representing the pubic tubercle. All efforts are made to make sure that the distance between the EKG nipples and the corresponding ASIS or pubic tubercle is equal before scrubbing and draping of the hip. Registration is achieved afterwards, by touching the nipples of the EKG pads placed on the pegs through the drape while the patient is secured in lateral decubitus position. This way sterility is uncompromised. To test the validity of our method of identifying the APP plane, a post-operative CT scan measurements of cup inclination and version angles were independently observed and the data were compared to our navigation registration method using t-student test analysis.(p=0.05 is significant)

The mean CT-scan cup version was 19.4(S.D. ±6.3), and the mean of APP navigated cup version was 14.2(S. D.±3.1). There was no statistical significant difference (p=0.045). Similarly, there was no significant difference between mean CT scan cup inclination angle of 42.3(S. D.±3.7) and the mean navigated cup inclination of 40.9(S.D.± 4.6), (p= 0.69). Therefore, we conclude that the APP plane can be registered reliably and accurately by simply touching the EKG pads on the pegs and through the drapes. Not to mention, both the cup version and inclination angles were within safety zone of Lewinick.

It seems that the accuracy of measuring the inclination angle through our method, although not significant, is better than the accuracy of measuring the cup version. This emphasizes the point that identifying the pubic tubercle is difficult whichever method of registration is used. However, inaccessibility of ASIS or pubic tubercle during manual APP registration leads to great cup orientation inaccuracies. The readily palpable EKG nipples on the pegs, irrespective of patient’s weight or the thickness of surgical draping, makes this novel technique a reliable and an easier alternative registration method than the manual palpation of APP in navigated THA.