Cementless, modular, distally fixed stem in hip revision arthroplasty: a single-center study of 132 consecutive hips

This retrospective study showed very good results of the MP prosthesis with reasonable complication and re-operation rates and negligible extraction rate. The commonest indication for MP prosthesis was aseptic loosening, the commonest complication was instability and the commonest re-operation type was soft tissue debridement due to peri-prosthetic infection. These results resemble previous studies [5‐7] and indicate the good performance of this implant even when used in the setting of a county hospital with limited caseload.

The philosophy of MP prosthesis relies on prosthetic bony fixation distal to the isthmus of the proximal femoral diaphysis. A minimal of 4-cm press-fit fixation between the stem and diaphysis is required to achieve axial and rotational stability. By this way, surgeons can use this prosthesis even in the presence of bony defects in the proximal femur. Osteo-integration in the proximal femur with some bony growth at the site of defects can occur. In the present study, 70% of MP prostheses were implanted in patients with aseptic loosening of cemented stem. In these patients, extraction of old cement gives bony defects and weakness and therefore compromises the possibility of using conventional stems. The same problem can be encountered in patients with septic loosening and fractures of the proximal femur with distortion of the calcar.

The fluted distally fixed stems can subside in the femoral canal a few millimeters during the first weeks–months postoperatively, if the initial press-fit fixation is inadequate. This can compromise the long-term survival [6, 8, 9]. In the present study, we did not include the postoperative radiographs to evaluate this parameter or because of the lack of uniform routine for timing and standardization of the radiographic evaluation in our patients. Another important aspect is the risk for intra-operative peri-prosthetic fractures. This happens when the pressure applied on the diaphysis is increased by reaming or prosthetic insertion. In many cases, surgeons choose to do prophylactic wiring to give additional stability to the diaphysis. In this study, we had six intra-operative peri-prosthetic fractures. The treatment of these fractures is usually technically demanding with high rate of mechanical failure and prosthetic subsidence. Therefore, every effort should be made to prevent this complication. Another complication is instability. Indeed, this complication is one of the commonest complications in many reports [5‐7, 10] including the present one. The soft tissue insufficiency, mainly defective gluteal muscles, and inadequate restoration of soft tissue tension are predisposing factors to instability. Preoperative planning is essential to choose the right stem position and version and stem neck length and angle. When instability takes place, a detailed workup is necessary to choose the right treatment. The modularity of the stem allows changing the version, angle, and length of the stem neck to improve the prosthetic stability. Other methods may involve cup revision to dual-mobility cup or cup with constrained liner. The latter implies coupling of the head of the prosthesis to the cup to prevent the head from dislocating from the cup. This coupling is associated with high mechanical loading, which can give rise to cup dislodgment. We have limited experience with this type of coupling. Another drastic complication is peri-prosthetic infection. Many patients are elderly and with co-morbidities (36% of patients had ASA class 3–5). Furthermore, local factors such as excessive fibrous tissue and soft tissue and bony defects increase the risk of superficial and deep infection. Early infections should be treated with aggressive debridement and antibiotics [11, 12]. When the infection is more than 4–8 weeks, a biofilm is formed. This provides a suitable media for the bacteria where antibiotics cannot work. Fortunately, we had only one patient who sustained such an infection and who needed prosthetic extraction and resection arthroplasty. We succeeded to treat our infected prostheses with debridement and antibiotics. Measures such as prophylactic peri-operative antibiotics, careful surgical technique, and postoperative wound care are minimal requirements in this regard.

The overall survival of MP prosthesis in this study was 99%, while the re-operation rate was 22%. These very good results concur with those reported by others. Weiss et al. [5] showed a 98% survival of the MP prosthesis at 5-year and 10% re-operation rate. Implant failure (modular junction) was no problem in our series, which is in accordance with other studies. Tamvakopoulos et al. [13] reported comparable findings with an overall survival of 92.5% of the MP system at an average follow-up of 5.6 years in 40 cases. Kwong et al. [8] evaluated 143 patients at an average of 3.3 years with 97% component survival using the same stem. Wirtz et al. [14] reported on the results of 142 hip revisions using the distally fixated MRP-Titan stem. The survival was 95% at an average of 2.3 years of follow-up.

Previous studies have reported pertinent information related to the results of the present study. Rodriguez et al., for instance, observed 97 hips for an average of 45 months, with clinical improvement of Harris hip score from 36 to 84 (range 54–99). Radiographically, 93 hips were considered stable, with no circumferential lucencies at the distal fixation surface. Three hips migrated and required revision, along with one peri-prosthetic fracture. Five other hips had nonprogressive migration of 1–2 mm. Also, McIinns reported on 70 MP stems with a mean follow-up of 47 months. Combined metaphyseal/diaphyseal bone loss was present preoperatively in 36 (51%) of 70 hips. Three hips (4.3%) were re-revised or in need of re-revision, and worst-case survival was 87%. The mean postoperative patient-assessed Oxford Hip Score was 21.1. Restoration of proximal bone was noted in 56%. Complications included mean subsidence of 9.9 mm, dislocation in 7 (10%) of 70 hips, and fracture or cortical perforation in 17 (24.2%) of 70. Furthermore, Amanatullah et al. used MP prosthesis in 92 patients with a mean clinical follow-up of 6.4 years. A total of 47 patients had peri-operative complications, 27 of which required further surgery. Most of these further operations involved retention of a well-fixed femoral stem, and 88/92 femoral components (97%) remained in situ. Of the four components requiring revision, three were revised for infection and were well fixed at the time of revision; only one (1%) was revised for aseptic loosening. The most common complications were postoperative instability (17 hips, 19%) and intra-operative femoral fracture during insertion of the stem (11 hips, 12%). Diaphyseal stress shielding was noted in 20 hips (22%). At the final follow-up 78% of patients had minimal or no pain. The mean HHS, which was available for 88 patients, at final follow-up was 69 points. There were 26 with good or excellent HHSs (30%), 24 with fair (27%), and 38 with poor scores (43%).

The present study has a few limitations owing to its retrospective design. The lack of functional and radiological measures makes the evaluation of study outcome results incomplete. We chose not to include these parameters because of insufficiency of data documentation in some patient records. The strength of this study is the relatively large sample size and the independency of the observer who collected the data. The authors think that the results of the present study reflect the actual everyday practice and results more than registry data, which can be jeopardized by missing data and underreporting.

In conclusion, our results indicate a very good performance of the MP prosthesis with comparable survival and complication rates compared with other reports. This motivates us to continue using this implant for our patients, even with if our caseload is relatively low. Further prospective studies are warranted to address the clinical and radiological outcome.