Jump distance has been related to THA dislocation rates and has commonly been studied in two-dimensional models. However, to the best of our knowledge, there has been little published work on the effect of bearing design, acetabular orientation, or pelvic inclination on jump distance. Because of continued problems with hip dislocation after THA, we created a model that determined that the following factors influenced PHDD: specific acetabular design, acetabular orientation, and pelvic inclination.
There were several limitations of the current study. First, this model was purely based on prosthetic characteristics and designs, and future models could incorporate bony anatomy as well as soft tissues. Despite this limitation, our model has shown that the interplay among acetabular orientation, pelvic position, femoral head size, and prosthetic design is more complex than previously understood. These data further our understanding of the sensitivity of some implant designs to component positioning but assume a purely posterior dislocation and do not account for soft tissue stabilizers. Bench validation could further validate the model, assess the effect of soft tissues and kinetic factors on stability, and determine if the prosthetic head dislocates in a purely posterior fashion (as modeled here, with dislocation modeled at 270° for simplicity of calculation) or if it follows more of a path of least resistance in the generalized posterior direction. However, we believe holding all factors constant, as we did in this model, allows us to identify general trends that can later be refined with more sophisticated cadaveric or dynamic models. Second, this model considers only a translational dislocation mechanism and does not take into account the levering mechanism that occurs from bony or prosthetic impingement. In vivo, dislocation mechanisms are more likely to be a combination of these two mechanisms (levering from impingement with translational movement of the femoral head) with additional stability provided by the soft tissues. The load provided by the patient’s body weight will also affect the in vivo mechanism of dislocation with this load possibly conferring some protective benefits when the pelvis is in certain orientations (eg, standing) and may contribute to amplify the dislocation risk when in other positions (eg, rising from a seated position). All studies such as this one may be limited by application to each patient’s unique anatomy (dysplastic hips, extraarticular femoral deformities, prior acetabular or femoral trauma, etc), which may influence component positioning and cannot be accounted for here.
Many basic science [3
] and clinical studies [8
] support the notion that larger head sizes correlate with increased jump distance and therefore decrease THA dislocation risk. For example, in one of the first cadaver studies on this topic by the Harris Orthopaedic Laboratory, 36-mm heads had decreased dislocation rates compared with 26- and 32-mm heads, which was attributed solely to jump distance [11
]. The present study was consistent with the literature because as head size increased (28, 36, to 48 mm), jump distance correspondingly increased.
The effects of acetabular positioning and pelvic tilt demonstrated more complex, but not unsurprising, interrelationships. For example, PHDD was lower at the more provocative pelvic tilt of 26°; however, as cup inclination angle increased, anteversion played more of a protective role when comparing the two pelvic tilt values. This is in agreement with what should be intuitively concluded in that a more anteverted cup will provide greater stability against a posterior dislocation. This also suggests that, although increased amounts of anteversion and inclination may not always be optimal, they may provide protection against dislocation when performing provocative maneuvers. Interestingly, the dual-mobility design was more stable at all component positions when compared with both hemispherical cups with smaller femoral heads as well as the resurfacing-type implant with a similar, anatomically sized femoral head.
In this study, the dual-mobility hip had the highest PHDD of any design regardless of head size, which held for all acetabular orientations and pelvic inclinations. This correlates with the clinical studies with this type of device that have demonstrated almost no dislocations in the primary or revision hip arthroplasty setting [4
]. For example, in a study by Philippot and coauthors [39
] of 384 primary THAs using a dual-mobility cup, which was similar in design to the one presently evaluated, there was no early or late instability at a mean followup of greater than 15 years (range, 12–20 years). Multiple other reports have confirmed these low dislocation rates with these designs in both the primary and revision setting [9
Multiple studies have reported the theoretical advantages of hip resurfacing for reducing dislocation rates [1
]. However, in a recent systematic review comparing resurfacing versus standard THA in young active patients, the authors found no differences in dislocation rates [22
]. This may be partly the result of the less than hemispherical head coverage and/or because of the low head-neck ratio that might lead to earlier prosthetic and bony impingement [5
]. In the present study, resurfacing showed increased posterior dislocation distances when compared with 28- and 32-mm heads but these were lower than the dual-mobility design. The negative PHDD values indicate that the posterior wall of the resurfacing cup was medial to the head center, implying a risk for edge loading during chair rise with low inclination angles. There have been several reports of high cup inclination angles leading to edge loading and increased wear of metal-on-metal hip arthroplasties [17
]. Based on the present study, it also appears that excessively closed cup positions may also lead to negative wear characteristics.
When evaluating jump distance, most studies have used purely mathematical techniques for assessments in either two or three dimensions coupled with characterization of typically one other factor such as acetabular abduction angle or head size [14
]. Recently, Sariali et al. [41
] evaluated lateral jump distance according to implant characteristics, increasing head size, and head offset as well as a function of cup anteversion and abduction angle. They found that the jump distance decreased as cup abduction angles increased and increased with increasing anteversion angles. Additionally, there were increases in jump distance with increased head diameters, an improvement that decreased with increases in abduction angles. They also found head offset was the most important parameter influencing jump distance (increased offset reduced jump distance). However, similar to the Crowninshield et al. [14
] study, their model ultimately resulted in a vertical dislocation mechanism. One must question the results obtained from these studies because dislocations clinically occur in either the anterior or, more commonly, the posterior direction.
In summary, we have described a new model to assess three-dimensional posterior jump distance (posterior horizontal dislocation distance). We assessed various acetabular designs and found that the dual-mobility socket with anatomic rim had the greatest jump distance. Increasing head size, decreasing acetabular inclination, and decreasing acetabular anteversion had positive influences on increased jump distance. Pelvic orientation of 26° reduced jump distance for all prostheses and acetabular orientations, which supports the hypothesis that chair rise is a high-risk activity for dislocation. The clinical importance of our findings is that newer designs may be useful in preventing hip dislocation and may be appropriate for patients who are at increased risk for dislocation in difficult primary situations and in revision procedures.