Achieving target acetabular orientation

INTRODUCTION: Optimal cup orientation is determined by correlating inclination and anteversion, measured on postoperative radiographs, with outcome. The orientation at which the surgeon implants the cup is often very different from that seen on postoperative X-ray. There are two reasons for this: Firstly, the angles of inclination and anteversion measured relative to the pelvis are defined differently in various situations (i.e. Murray’s operative Vs radiographic; definition difference) and secondly, the positions of the pelvis relative to the measurement reference frame are distinct (pelvic orientation difference).
OBJECTIVES: The primary aim of this prospective, in vivo study was to measure the intra-operative cup orientation and determine how this relates to radiographic orientation. Secondary aim was to identify which factors influence this relationship.
METHODS: 71 hip arthroplasties (55 THAs and 16 resurfacings) by 5 surgeons, were studied. Intra-operatively, using a validated stereo-photogrammetric technique, of 2° accuracy, the 3-D orientation of the cup introducer at implantation was measured relative to the theatre table. This allowed for calculation of the intra-operative cup inclination (IOI)/ anteversion (IOA) and simulated radiographic cup inclination (SRI)/ anteversion (SRA). IOI and IOA are the angles of the cup relative to the operating table in the same reference planes as Murray’s operative orientation. SRI and SRA are the angles that would have been measured from a radiograph had it been taken during the operation, with the film placed perpendicular to the table. These measurements were compared with the radiographic inclination (RI) and anteversion (RA) measured post-operatively. The difference between RI/RA and IOI/IOA (inclination/anteversion) is due to both definition and pelvic orientation differences. The difference between intra-operative and simulated radiographic (table: T) is a measure of definition difference. The difference between the simulated radiographic and the post-operative radiographic (radiographic: R) is a measure of pelvic orientation difference.
RESULTS: inclination was 5° (SD:5°) and anteversion was 8° (SD:8°) (p<0.001). Tinclination was 4° (SD:2°) and T anteversion was 6° (SD: 2°). Rinclination was 1° (SD:5°), Ranteversion was 2° (SD:7°). Surgeons implanted the components with different IOI and IOA and hence had a resultant different RI and RA (p<0.001). This difference was due to both definition (p<0.001) and pelvic orientation (p=0.02) differences. Procedure influenced anteversion, resurfacing has greater anteversion [11° (SD:7°)], compared to THAs [5° (SD:8°)]. This was mostly due to Ranteversion (p=0.04) than Tanteversion (p=0.08).
CONCLUSION: The variability in radiographic cup orientations detected stems from two equally contributing factors; the variability of cup impaction angles and the variability in pelvic orientation at impaction compared to X-ray. In order to reduce cup orientation scatter both factors need addressing. On average, surgeons should impact the cup with an IOI 5° smaller than their desired RI target and an IOA 8° greater than their desired RA. For resurfacings, the cup should be impacted with greater intra-operative anteversion compared to THA in order to counter-act increased pelvic movement during surgery.