Abstract
Introduction
Accurate acetabular position is an important goal during THA. It is also well known that accurate acetabular positioning is very frequently not achieved, even by experienced, high volume surgeons. Problems associated with cup malposition are: dislocation, accelerated poly wear, impingement, ceramic squeaking, metalosis. Murray et al described 3 methods of measurement and assessment of acetabular inclination and anteversion (I&A): anatomic, radiographic and operative. It is the hypothesis of the authors, that the differences and details of these 3 methods are poorly understood by many surgeons and this is contributory to inconsistent cup positioning. Additionally, the radiographic method, which is most commonly used for post op assessment and academic studies, contributes to misunderstanding and error. Modern computer guidance and software assessment of radiographs allows us to easily measure anatomic I&A which should be thought of as “true” I&A.
Methods
The mathematical criteria for radiographic measurement of anatomic I&A are defined as well as the mathematical relationships and discrepancies between anatomic and radiographic I&A for any given cup.
E = Angle of ellipse major diameter to horizontal
E = Radiographic inclination
Tan I = Tan E / Cos A
Tan E = (Tan I) x (Cos A)
Results
Numerical values for radiographic I&A and anatomic I&A coincide for cups placed at 0 degrees anteversion. However, as cup anteversion increases, there is an exponentially increasing discrepancy between anatomic and radiographic inclination values with I always having a higher value than E. Commonly used radiographic inclination values (E) therefore always underestimate anatomic (true) inclination. Additionally, radiographic anteversion, except for 0 degrees anteversion, always underestimates anatomic (true) anteversion.
Wear testing of cups by manufacturers and associated recommendations for cup positioning are based on anatomic measurement of inclination while surgeons now use a different method (radiographic) for measuring position. Axial CT analysis of cup anteversion agrees mathematically with anatomic anteversion and does not mathematically agree with the Murray radiographic criteria.
Conclusions
Surgeons can intuitively understand that accurate radiographic measurement of femoral neck-shaft angle can only be done if the proximal femur is correctly rotated in relation to the x-ray beam, specifically the x-ray beam must be perpendicular to the plane determined by the intersection of the center lines of the neck and shaft. Any other femoral rotation will show a false increase in the neck shaft angle. Though less intuitive, true cup I is only represented by the angle seen on x-ray at only one A value, 0 degrees. Anteverting the cup as is desirable for THA stability creates a discrepancy between the apparent cup angle (E) and true inclination. Since the principles of solid geometry are widely adopted and accepted, the above results and conclusions are based on mathematical proof, not experimental findings.
Erroneous conclusions such as “the cup position is good but the hip still dislocates” can be associated with a surgeon's lack of understanding of true I&A. Surgeons need to understand the differences between what they believe to be represented by x-rays and anatomic or true I&A as represented by the cup's position in relation to the body's transverse, coronal, and sagittal planes and x, y, and z axes. The authors believe that a surgeon's continued lack of understanding of the mathematics can be compensated for by the technologies of computer guidance and/or software analysis of cup x-rays.
