Abstract
Introduction
Conventional methods of aligning the acetabular component during hip arthroplasty and hip resurfacing often rely upon anatomic information available to the surgeon. Such anatomical information includes the transverse acetabular ligament and the locations of the pubis, ischium and ilium. The current study assesses the variation in orientation of the plane defined by the pubis, ischium and ilium on a patient-specific basis as measured by CT.
Methods
To assess the reliability of anatomical landmarks in surgery, we assessed 54 hips in 51 patients (32 male, 22 female) who presented for CT-based surgical navigation of total hip arthroplasty. From a 3D model of each patient, standardised points for the anterior pelvic plane and landmarks on the ilium, ischium, and pubis were entered. The plane defined by the anatomical landmarks was calculated in degrees of operative anteversion and operative inclination according to the definitions of Murray.
Results
The plane representing cup position defined by the anatomical landmarks ranged from 7.8° to 64.6° in operative anteversion (mean = 32.1°, SD = 15.0°) and 37.6° to 68.2° in operative inclination (mean = 53.2, SD = 7.1°). If a safe zone of 27 degrees of operative anteversion (± 10°) and 42 degrees of operative inclination (± 10°) is selected, 50.0% of hips are out of the safe zone in operative anteversion, and 57.4% of hips are out of the safe zone in operative inclination.
Discussion and Conclusion
Surgeons have very specific and limited anatomical information available at the time of surgery to assist in determining optimal component orientation. Alignment relative to the operating table and intraoperative signs such as the co-planar test are unreliable due to the wide variation of position of the pelvis during surgery. This leaves anatomical landmarks that can be palpated during surgery as one remaining method upon which component orientation may be based. Unfortunately, these anatomical landmarks vary quite widely on an individual patient basis, with 83.3% of hips out of the a safe zone in this study of 27° of operative anteversion and 42° of operative inclination and 77.8% our of a safe zone of 20 degrees of operative anteversion and 45 degrees of operative inclination. As such, internal anatomical landmarks are likely to lead to systematically high incidences of component malposition such as those repeatedly documented in the literature. Based on the current study we conclude that, unless the orientation of the palpable anatomical landmarks is assessed in three-dimensions pre-operatively, these anatomical landmarks provide poor and sometimes dangerously misleading information.
