The Ceramic Option: Indications, Contraindications, Revision and Surgical Challenges 131
Acetabular Positioning to Maximize Range of Motion
J. A. D'Antonio
Acetabular preparation and component positioning has a direct effect on hip biomechanics, bearing surface w e a r , functional range of motion and stability, and I believe is the greatest challenge for a successful total hip arthroplasty.
Several years ago Dr. Capello and myself with engineers Adam Bastiaan and Mike Cusick developed a computer range of motion model looking at cup position on range of motion of total hip arthroplasty. We found that cup positioning is a three dimensional combination of abduction and version with version being a combination of internal rotation and flexion. Altering the cup position has a direct positive or negative effect on range of motion before impingement in some direction. We standardized the femoral implant with a number eight Omnifit having a 30 head/neck offset with a 28 millimeter head diameter. We then altered the cup position through a multitude of combinations of abduction and version. Using the cup position of 45'' of abduction and ]5° of version assuming a femoral anteversion of 15° as our baseline we kept the femoral anteversion at 15°. Our computer range of motion model demonstrated that the most desirable acetabular position to maximize range of motion before impingement was a combination of 40°-50° of abduction (45°) and 20°-30° of anteversion or flexion (25°). This combination with 15° of femoral anteversion yields a combined femoral and acetabular anteversion of approximately 40°.
The greatest challenge for the surgeon is to consistently achieve this desirable acetabular position at the time of surgery. The pitfalls include anatomic variations, the presence of hypertrophic bone, orientation of the patient on the table (the use of external alignment guides), and visualization of the acetabulum.
To avoid these pitfalls, one must first have an approach permits complete visualization of the acetabulum and includes appropriate soft tissue releases.
Having done that, then the use of anatomic landmarks can lead to correct component orientation to maximize functional range of motion and minimize impingement. I would caution against the use of external alignment guides. They assume the desired position of the pelvis and there is no accounting for the pelvic tilt or pelvic flexion. In short, external alignment guides for placement of the acetabular component ore notoriously inaccurate and should not be used and relied upon at the time of surgery. In preparing the orientation of the acetabular component, the anatomic landmarks that are most useful include the ishium which is nearly flush with the posterior wall, the sciatic notch, the acetabular fossa and inferior rim, and of course the anterior and posterior walls to a lesser degree.
A study of acetabular and femoral morphology published by Maruyama, Capello, D'Antonio and Feinberg in the December issue of Clinical Orthopedics and Related Research validated the accuracy of certain anatomic landmarks.
This study specifically defined the acetabular anteversion angle, acetabular ridge configuration, and defined the notch acetabular angle. The study found the acetabular anteversion angle measured 19.9° ± 6.6° and measured on the average of 21.3° for females and 18.5° for males. It defined the notch acetabular
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a n g l e as the a n g l e a t the intersection of the line from the sciatic n o t c h to the posterior a c e t a b u l a r ridge a n d the line fronn the posterior to anterior a c e t a b u l a r ridge.
Therefore an insertion of an acetabular connponent following appropriate reaming and preparation one can use the ishium which is relatively flush with the posterior wall and the sciatic notch as an excellent indicator of onteversion and to increase the onteversion of the femoral component to 25° or 30° beyond the normal 19°, one would hove to orient the insertional tool towards the top of the sciatic notch as opposed to the center of the wing of the Ileum. Likewise by placing the socket inside the acetabular fossa or inferior rim, one could be relatively certain of not exceeding 40°-45° of abduction.
In conclusion, acetabular component positioning has a major effect on hip stability and hip biomechanics. We believe that the combined femoral and acetabular onteversion should be in the neighborhood of 40° and given the femoral onteversion most often in the neighborhood of 15°, the most desirable acetabular position for functional range of motion is 45° of abduction and 25°-30°
of onteversion. It is important to beware of external alignment guides which ore fraught with error. We strongly recommend the use of anatomic landmarks and advise the use of the sciatic notch as on accurate guide for proper visual placement of the socket.
