At the last follow-up, we identified no implant fractures. Therefore, the modularity of the revision stems is not synonymous with implant fracture. Several medium-term studies are already pointing to this conclusion [20,21]. The relative fragility of the metaphyseal-stem junction of revision implants has been well-examined and is due to several factors [22-24]. 1/ The design of the implant: the biomechanical study by Krull et al [25] highlights the importance of the balance between the diameter of the "male taper" and therefore the diameter of the stem and that of the female metaphyseal part. This stress distribution prevents overloading of the taper and therefore metal fatigue. 2 / The placement method and the choice of the implant size. After an analysis of 24 cases of implant fracture at this level, B. Fink [26] stressed the need to distalize the metaphyseal-stem junction in the femur (usually below the lesser trochanter). He recommends using shorter diaphyseal implants that follow the anatomical curvature of the femur to achieve a more distal press-fit, in combination with long metaphyses. Moreover, there should be close contact between the proximal metaphyseal implant and the medial femur (autograft, allograft, osteotomies). This was already addressed by Lakstein et al [14] who, after studying 6 ZMR implant fractures in a population of 165 patients, found a lack of bone support at the metaphyseal-stem junction in all cases of implant fracture. In all these cases, the stems were well sealed distally.
At the 5-year follow-up, the survival of the femoral stem without revision regardless of the cause was 99% [95% CI = 91.5 to 99.8%].
Few studies have analyzed a single type of implant in a heterogeneous patient and surgical population. The literature reports heterogeneous results with limited samples for MFT stems. Hashem et al [27] and Weiss et al [28] reported respective survival rates of 99% for 132 revisions in 4 years and 98% for 90 revisions in 5 years for the MP - Link prosthesis. For the same prosthesis, Rodriguez et al [29] and Amanatullah et al [4] respectively reported a survival rate of 95.6% for 71 revisions in 10 years and a survival rate of 97% for 92 revisions in 6 years. While, Ovesen et al [30] and Van Houwelingen et al [6] respectively reported a survival rate of 94% for 125 revisions in 4 years and 90% for 65 revisions in 7 years for the ZMR - Zimmer prosthesis. Finally, for the Revitan prosthesis, Zimmer Fink et al [31] reported a survival rate of 95.5% for 116 revisions in 7.5 years. The results of our series in terms of survival, regardless of the cause, were excellent and are consistent with other studies in both primary and revision surgery. Survival of different titanium MFTs varies minimally according to the type of implant used. On the other hand, while survival is often satisfactory, implant fractures may have negatively impacted the reputation of these modular implants. In the series that reported the lowest survival rate, Van Houwelingen et al [6] identified 5 implant fractures at the metaphyseal-stem junction.
In spite of satisfactory clinical results consistent with previous studies [7,28,32,33], we identified some complications. Instability was reported in 6% of the cases. The acetabular implants were single-mobility in 4 cases and dual-mobility in 2 cases. These early postoperative dislocations (< 3 months) did not require revision or repeat operations except in 1 case. In addition to the correct positioning of the cup, ensuring the right length of the metaphysis is a crucial element in the use of MFT stems. The patient who underwent revision was stabilized by the use of a longer metaphysis and a dual-mobility cup. It is important to endeavor to correct or maintain the levelness of the lower limbs. Systematic preoperative planning makes it possible to mark reference points, which are particularly useful in major bone lesions. However, we found that the incidence of instability is particularly low in the literature, including when the indications for revisions alone were isolated. In fact, Weiss et al [28] and Amanatullah et al [4], for example, found 19% instability. We believe this is due to the consistent use of dual-mobility acetabular cups in our department to mitigate the risk of dislocation when the tissue environment may be wanting [34,35]. In a review of the literature, Reina et al [34] reported an Odds Ratio of 3.59 for dislocations in revision total hip arthroplasty with a single-mobility acetabulum compared to those with a dual-mobility acetabulum.
Our radiographic study showed a high rate of subsidence. However, the average distance was favorable compared to the results in the literature. Abdel et al [8] reported 2.4% subsidence of an average of 16 mm, Rodriguez et al [29] 2.8% and 8 mm, Van Houwelingen et al [6] 12.5% and 12 mm and Parry et al [9] 13% and 18 mm. In our series, 89% of the cases of subsidence were early and none of the stems were unsealed. This is specific to the press-fit technique and to implant wedging [36]. This should be taken into account in the preoperative planning and in the clinical (length of the lower limbs) and radiographic follow-up of the patient.
In addition, our radiographic study highlighted a proximal osseointegration defect. Although not associated with revision, we found a significant amount of clear lines and bone resorption mainly at the metaphyseal level and in the body of the prosthesis. Similar findings were made by Rodiguez et al [29] and Amanatullah et al [4], with respectively 38% and 42% of lines at this level and 68% and 50% restoration of proximal bone stock. This is contradictory to the principle of bone remodeling and reconstitution of the proximal femur described above [37], particularly with the Wagner prosthesis [38,39]. Metaphyseal bone is often sclerotic during revisions and the operating procedure for MFT stems tends to bypass the stresses directly to the metaphysis. Therefore, it seems difficult to achieve better osseointegration at this level with the type of implant under study from a biomechanical point of view and also because of the bone lesions of the proximal femur. On the other hand, these radiographic signs are absent in the distal diaphyseal zone, which presents optimal osseointegration.
This study has certain limitations. The rationale for the study stemmed from the publication on implant fractures by several authors which justified an evaluation of our practice. Therefore, there is no control group. This evaluation of our practice of using an implant with a specific design made it difficult to conduct a comparative study in this heterogeneous population of patients, some of whom were operated on in an emergency. The objective was to study the survival of the implant and especially the risk of implant failure.
Another limitation of this work is the inclusion of indications for primary surgery. The rationale is related to the specific contexts in which these implants have been used. MFT stems had similar behaviors and homogeneous results in our entire sample population. These were primary but complex THAs and therefore comparable to revision surgery. Finally, our sample was an elderly population and mortality is high among these patients. Several patients died during follow-up, but none in the operating room. We found no indication that the implant was related to mortality which was primarily due to cardiac causes. The predisposition of these fragile patients makes them candidates for revision arthroplasty, and they require a reliable long-term implant and a reproducible technique.