Where Are We Now?
In their study, Ueno and colleagues  report on planning cup orientation and reducing the potential risk of iliopsoas impingement. The authors used a CT-based simulation to implant virtual THA acetabular components and evaluated the effects of backward tilt of the pelvis and changes in orientation of the natural acetabulum on prosthetic cup coverage. They found that backward pelvic tilt (pelvic extension) increased the risk of psoas tendon impingement on the edge of the component .
The goal of cup orientation is stable fixation, ROM without dislocation, and avoiding iliopsoas impingement caused by protrusion of the acetabular component beyond the edge of the bony acetabulum . Surgeons should consider the size, shape, and orientation of the native acetabulum when planning cup orientation . Variable pelvic tilt due to rigid spinal deformities  and leg-length discrepancies may affect the aimed cup orientation, which can influence both the risk of prosthetic dislocation and of cup protrusion. In patients with developmental dysplasia of the hip, the problem becomes more complex, as there is decreased coverage of the acetabular component in the shallow acetabulum. This is a three-dimensional (3-D) finding that the surgeon may not anticipate if he or she looks only at two-dimensional radiographs prior to surgery . Additionally, surgeons must consider the femoral orientation and offset as the risks of dislocation, hardware impingement, and ROM are affected by the combination of cup and stem orientation .
Implant-related factors like cup sphericity, size, the presence of threads, the macrostructure of the outer surface, and liner elevation might affect the risk of developing symptoms in patients with acetabular protrusion. Implants that cover less than a hemisphere might be useful in complex THAs (particularly those with shallow or dysplastic sockets) as a way to avoid protrusion of the component beyond the bony margin of the acetabulum .
There are several strategies surgeons can use to guide acetabular component orientation during an operation. The anterior acetabular wall, the superior and posterior edge, and the transverse ligament can be used as anatomic landmarks to place the cup in the correct position and orientation. But limited exposure, bony defects, destructive inflammation, and osteophytes can limit the utility of some of these landmarks. The use of an image intensifier may help surgeons obtain greater accuracy and navigation is also an additional option. Finally, although cup protrusion and the risk of psoas impingement are important concerns, stable initial fixation and dislocation prevention are the first priorities during THA.
Where Do We Need To Go?
When treating patients with fixed spinal deformation, a thorough clinical examination is of particular importance. We need additional clinical and radiological investigations into the best ways to anticipate post-operative pelvic orientation .
Regarding imaging, I use pelvic radiographs in the supine position, and false-profile radiographs for “normal” hips. CT scans and 3-D reconstructions, including individual 3-D simulation of movements, are helpful for patients with complex hips . EOS™ (EOS Imaging, Paris, France) can be useful as well, as it provides a reproducible functional assessment of the relationship between the pelvis and the spine . Still, we need to develop criteria to guide which imaging approaches are most useful in which patients.
For patients with dysplasia and high acetabular anteversion, the ideal cup orientation could be difficult to determine, as there can be a discrepancy between orientation of the native acetabulum and the ideal cup orientation. Such incongruence can limit hip extension, cause posterior impingement, and increase the risk of dislocation. Larger femoral heads or dual-mobility cups might reduce these risks, but larger implants are likely to increase the overhang; smaller cups (which are hard to use with larger heads and dual-mobility sockets) may reduce both the amount of overhang and the risk of psoas tendon impingement.
Finally, the role of computer navigation—a controversial topic if ever there was one—needs to be better defined, particularly in more-complex THAs. While navigation might improve the reproducibility of cup orientation, its benefits have not been firmly established in terms of endpoints that matter (dislocation, durability, hip scores, and psoas tendon impingement), and even if one believes it should be used in complex THAs, we need more information about the best reference plane to use and the target cup orientation in patients with pelvic deformities, excessive posterior tilt, or spinal curvature.
How Do We Get There?
Although most THAs go well, complications like psoas impingement can occur and many go unreported in registries and even single-center studies. Careful physical examinations and more-detailed scores like the “forgotten hip score”  are important for obtaining more information on the incidence of those soft-tissue problems.
Although more-complex THAs—like those with severe acetabular deficiencies—typically are treated by experienced surgeons, this is not always the case. Therefore, residencies should train surgeons on how to treat functional problems like psoas impingement and complex geometric inter-relationships . Experimental studies should examine the combined anteversion and the risk of posterior impingement in relation to pelvic tilt as a way to guide surgeons as they make intra-operative decisions.
Prospective studies should evaluate the impact of spinal mobility on pelvic orientation and the effect of improved hip function due to THA on spinal function. These studies should also include clinical assessments of hip and spinal function, as well as radiographic analysis in various positions like sitting and standing or, additionally, with EOS™, both for the hip and the spine. This type of study could provide information for the most suitable pre-operative imaging and for the best reference planes in navigation.
Regardless of how well a surgeon performs pre-operative planning, important decisions will always need to be made during an operation. The usefulness of intra-operative tools, like navigation, can be proven in randomized trials.
Clinical and radiological analyses should identify the implants that could potentially cause overhang. Cups that are less than full hemispheres—perhaps with additional screw fixation—may be better choices in such hips. Rings may be worth considering , and patient-specific devices might be an option to consider in shallow sockets.
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