In this article, we located the rich bone stock area to study the optimal positions of screw holes. For peripheral screw-hole cluster, five screw holes were evenly distributed between point A and point B in the thicker rim. Three holes were located posterior to the symmetrical axis and two located anterior. For inner screw-hole cluster, screw hole 1 and screw hole 2 are the optimal inner screw holes.
Bone deficiency is a challenge in rTHA. To solve this problem, several reconstruction strategies are performed. In most patients, stability can be achieved using an uncemented normal-sized hemispheric acetabular shell or a Jumbo cup[20–22]. Other strategies include structural allografts, augments, cages and reinforcement rings, oblong acetabular components and custom triflange components. However, graft resorption and nonunion[24, 25], independent preparation for augments, breakage or loosening of cages and rings, absence of biologic fixation, and wide exposure may limit the use of these methods.
Comparing with other methods, jumbo cup has become a preferable way with its unique advantages. First, it has a larger absolute contact area with the host bone, which is basic for long-term biological fixation. Second, the technique of jumbo cup is a relatively easy way (similar to primary THA). Third, jumbo cup provides an alternative to placing a cup into the superior defect or using augments in some certain cases. However, the elevation of hip rotation center has aroused wide concerns using jumbo cups[7, 8]. A vertical hip center shift alters hip biomechanics and potentially causes insufficiency of the abductor muscles, abnormal gait, and increased risk of dislocation from impingement[30–32]. In a computer simulating study of Nwankwo et al., they found that the hip center shifted 0.27 mm superiorly and 0.02 mm anteriorly for every 1 mm increase in reamer diameter using a jumbo cup. Facing with this problem, Ries et al invented an offset COR acetabular shell designed to maintain the center of rotation closer to its anatomic position. Through radiographic evaluation, they reported that the mean vertical COR displacement of the test group was reduced by 3.5 mm. Despite lack of follow-up results, it provides an effective way to address the elevation of hip rotation center. For our eccentric revision cup, in theory, the hip center would reconstruct closer to anatomic COR than using the offset COR cup because of the presence of a 20-degree angle between the planes of inner cup and outer cup. But future studies are needed to confirm this theory.
Limited screw fixation option is another vital problem using jumbo cups. Besides conventional dome screw fixation, the offset COR cup allows peripheral screws to be fixed into the posterior column of the pelvis. However, the distribution of rich bone stock area in revision THA is different from that in primary THA, and the rich bone stock area is asymmetrical along the axis of the cup. The screw-hole design of the offset COR cup failed to reflect the characteristics of bone stock distribution. In our study, the peripheral screw-hole design of eccentric revision cup was based on morphological measurements. We found that most of the bone stock is located between 26.99 degrees anterior to the symmetrical axis and 56.40 degrees posterior to it. After that, we tested the relative location between the screw holes and the “safe zone”. The posterosuperior quadrant and the posteroinferior quadrant of acetabulum are safe for placing long screws[19, 34–36], avoiding injuring vital vessels nor nerves by screw trajectories. All peripheral screw holes were in the “safe zone”. In addition, we selected locking screws as peripheral screws for greater stability. Various mechanical tests have proved that locking screws have significantly greater stiffness and yield strength than non-locking screws (13).
In summary, the eccentric revision cup has inherited the strengths of jumbo cup besides several unique advantages as follows. First, this design decreases the shift of hip rotation center restoring biomechanical function. Second, the peripheral screw holes are designed in the thicker rim of the eccentric revision cup enhancing primary stability of the cup by locking screws. Third, the smaller head-cup differences reduce the risk of dislocation[10, 11]. Fourth, it increases the contact area between the outer cup and the host bone while maintaining a normal inclination of the inner cup. The appropriate inclination angles may reduce the stress on the bearing surface and benefit for long-term results[38, 39].
There are several limitations of this study. First, our study was based on normal pelvic and acetabular anatomy. However, it may not represent the variation in individual anatomy encountered in different revision THA settings. Second, this study was a 3D CT reconstruction morphological study and biomechanical tests are still being further explored. Third, because of individual variation, the peripheral screw holes do not always perfectly match the rich bone stock area in every patient. However, initial stability was achieved with 2 or 3 screw fixations in most jumbo cups. In our design, besides inner screws, one or two peripheral screws may enough for most patients. In the eccentric revision cup, there are 5 screw holes in the thick rim of the cup, which are enough to place screws pursuing the initial stability. Fourth, our study focused on screw-hole design on acetabular cups. However, improved design will not substitute for good surgical techniques. The proper surgical approach, adequate exposure, preparation of the acetabulum, and correct position of the acetabular cup are equally important.