After THA, hips with less JD are theoretically more susceptible to dislocate than hips with more JD. Our findings confirmed that conventional DM cups provide better joint stability achieving higher JD in comparison with modDM systems, at same cup size and same cup position. JD with DM linearly increased with increasing of cup size, similarly to the results reported by Sariali. Conversely, JD with modDM was lower in comparison of DM and maintained almost constant as size increased.
Thus, using larger DM cups, it is possible to guarantee higher JD, so, better hip stability. However, this finding is not replicable with modDM systems which keep JD constantly lower, whatever size of cup is used. The reason could be found looking at the formula by Sariali.
The equation highlights that JD depends not only on femoral head size, but also on orientation of the implanted cup and the femoral head offset. JD is mainly affected by the cup abduction angle than anteversion angle [16, 18]. With constant cup abduction and anteversion angles, JD is directly related to femoral head size and inversely related to head offset, which is the geometrical factor that has the highest influence on JD [16]. Offset can have a positive value or a negative value (inset). In case of negative sign, as with DM, an inset increase, in absolute value, leads to JD increase. Conversely, in case of positive sign, as with modDM, an offset increasing leads to JD decreasing and, no matter how R increases, JD remains low. So, in our simulation, offset progressively increased per cup size increase with modDM and keep JD low in comparison with DM where, instead, offset progressively decreased but remaining always negative (inset), thus increasing JD.
Regarding to polyethylene FB cup with 28mm, 32mm, 36mm and 40mm femoral head diameters, JD changed according to Sariali, increasing with femoral head size increase, because femoral head offset was set constant for each size as being a design parameter (Table 2).
JD with DM cup resulted higher than JD with FB coupled with all femoral head diameters per each cup size. Interestingly, JD with modDM resulted very similar to JD with polyethylene FB cup coupled with Ø36mm femoral head for sizes ≥Ø56mm but, on the contrary, JD with modDM resulted lower for sizes smaller than Ø56mm (Figure 4).
But with ceramic liners is inverted with larger sizes. JD with ceramic FB coupled with 36mm and 40mm femoral head diameters decrease for sizes ≥ Ø56mm in comparison with modDM (Figure 5). The use of large heads requires an offset increase that reduce JD [18]. For a 1-mm increase in head offset, JD is decreased by 0.92mm. This is why use of very large femoral heads leads to a moderate JD increase than expected and this could be one possible reason to explain high dislocation rates reported in revision THA with femoral heads larger than 36-mm diameter [19]. Recently, in a study of patients undergoing revision THA for a variety of reasons, Hartzler et al. found a lower rate of patients who dislocated postoperatively when revised to a modular dual-mobility construct compared with those revised with a large 40-mm femoral [20].
Our findings showed how the use of modDM system leads to a lateral deviation of the CR in comparison with DM cup and this CR lateralization is more or less evident depending on cup size. The dimensions of both modDM liner and acetabular cup for every size largely affected CD distance. As showed in Table 1, modDM liner sizes are matched with cup sizes in a 1:2 or 1:3 ratio, explaining the CD variability with modDM system with size increasing.
To date, the use of modDM provided excellent results in terms of dislocation incidence. A large matched cohort single-center study comparing modDM and standard DM reported for both groups 0% of dislocation after primary THA at a mean follow-up of 2.8 years [11].
A retrospective case-series study of modDM cups used in revision THA found a dislocation prevalence of 3.1% after 3-year average follow-up [21]. Another recent multicenter retrospective study reported a similar dislocation rate (2.9%) after revision THA in a large cohort of patients treated with modDM [12].
The use of modDM is not risk-free but, conversely, involves more potential complications than conventional DM. modDM is a prosthetic construct which adds one more modular cobalt-chromium liner. The fretting and crevice corrosion at the non-articulating metal-on-metal interface between the modular liner and the titanium socket cause an extra metal release in comparison with conventional DM [22–24].
In literature several studies reported uniformly low blood metal ions concentrations in patients undergone modDM primary or revision THA, which were found to be acceptable for the safety of patients [13–15]. However, all these studies reported short follow-ups and it is unknown to date the possible adverse biological effects of metal release in the long-term. Metal release from metal modularity thus still remain a cause for concern that need to be continuously surveilled.
Use of modDM implicates also the risk of modular metal liner malseating which is reported with an incidence up to 5.8%. Liner malseating may lead to increased fretting corrosion and metal related issues, component dissociation and reduced stability [25, 26].
Thus, the use of the modDM should be indicated in complex primary THA and revision THA and should be limited to those high-risk patients where the use of conventional DM cup is not recommended or even not feasible. Typical cases indicated to use of modDM are, for instance, severe hip dysplasia or high hip dislocation, patients at high risk of dislocation with poor pelvic bone quality that requires a further cup stabilization with additional bone screws into shell holes, revision THA for recurrent instability when it is required to replace the acetabular liner with a modDM liner in a well-osseointegrated cup.
Facing these considerations modDM should not be used as the first choice instead of a DM but rather when required or when modDM can solve intraoperatively a problem.
Study limitations.
The major limitation of the present study was that the analysis performed for JD and lateralization of the CR were strictly dependent on the design technical specifications of the studied prosthetic components. Even if the take-home message from the present study is suitable for DM and modDM THA, the authors did not exclude design-related differences between different DM cups and modDM systems currently available on the market. Therefore, the results from the present study were valid for the studied components but may change with other devices.
The focus of this investigation was on hemispherical or cylindrical-extended hemispherical cups which are both designed for standard and DM THA. In the past JD changes were studied according to femoral head offset, head size and cup position [16]. However, these studies evaluated only standard implants and sub-hemispherical cups for large head diameters (above 38mm), specifically designed for metal-on-metal implants, which have a negative effect on JD, due to their smaller coverage angle and higher head offset [27, 28].