There is still controversy regarding the optimal type of cementless femoral and acetabular fixation in primary THA. In this retrospective series, we found that the UNITED hip system had excellent clinical and radiographic results, without revisions of the stem and with only two revisions of the acetabular component due to aseptic loosening at a mean follow-up of 40 months.
Although uncemented, proximally porous-coated tapered stems can be a reliable option for routine THA, they are not without complications. Initial implant stability is essential to avoid early aseptic loosening. In 2011, White et al.  retrospectively analysed 81 proximally-porous tapered Accolade® stems (Stryker Orthopaedics, Mahwah, NJ, USA) for aseptic migration using Ein-Bild-Roentgen-Analysefemoral component analysis (EBRA-FCA). More than 1.5 mm of subsidence was observed in 36% of the stems at 2-years of follow-up. The migration pattern may be a consequence of inadequate initial implant stability. In our series, two patients (1%) showed initial subsidence < 3 mm, without further progression at the latest follow-up. We believe that stem subsidence may not be a major concern with this type of implant design. Of course, further studies with longer follow-up are needed to confirm these preliminary findings.
Trapezoidal wedge-shaped stems are associated with an increased risk of periprosthetic femoral fractures (PFF), being a major concern after cementless THA . We reported 1 case of intraoperative PFF that did not jeopardize stem fixation and was treated with internal fixation. This was probably related to the trapezoidal shape of the implant, which sometimes may not be suitable for all proximal femoral morphologies. In this scenario, more anatomic, rounded stems may be useful to prevent PPFs when compared to single- or double-wedge components . Abdel et al.  reported a 7.7% and 3.5% risk for postoperative PFF after cementless and cemented THA, respectively, at 20 years of follow-up. PPF may become the most frequent cause of failure in the long-term (˃25 years) [3, 6]; thus, we believe that future studies with extended follow-up should be performed to analyse this new femoral stem comparing the long-term incidence of PFF, especially among cementless straight implants.
McLaughlin et al. 's previous long-term studies have shown excellent clinical and functional outcomes with the use of a cementless proximally porous-coated tapered collarless titanium stem . Although we are reporting short-term results, our outcomes were comparable to the Taperloc stem in these long-term studies; the mean MDA scores improved from 13 points preoperatively to 17 at the latest follow-up (p < 0.001).
Porous titanium-coated (PTC) acetabular cups have not proven to perform better than plasma-sprayed (PS) acetabular shells at medium to long-term follow-up . Lindgren et al.  performed a prospective multicentre study comparing PTC and PS cups. The PTC component was associated with a higher risk of pain (OR = 2, p = 0.035) and radiolucency (OR = 5.2, p < 0.001); however, there were no cases of revision surgeries for loosening. In our study, we found 2 (1%) acute aseptic loosening of the acetabular component. Both patients were revised with a primary uncemented cup at 6 and 23 months postoperatively. No further complications were registered in these patients, obtaining good clinical outcomes at the latest follow-up. It is essential to mention that both failures were recorded in the first 20 cases of the series with a PS-cup. We believe that both failures may be related to the learning curve and to the fact that reaming was done line-to-line instead of with a press-fit technique, as it is recommended.
Iliopsoas impingement (IPI) after THA is a possible cause of recurrent groin pain and range of motion restriction with a prevalence of 4.4% . An axial protrusion length ≥ 12 mm, a sagittal protrusion length ≥ 4 mm, a higher native acetabular version, a lower cup anteversion, and inclination have been described as independent predictors of symptomatic IPI . In our series, we had one patient with IPI treated with arthroscopic tendon release. Regarding acetabular cup inclination and anteversion, the patient exhibited normal values for both measures. Although this complication can be seen in sharp acetabular components , we recommend assessing the cup-to-rim ratio in order to discard any prominence over the acetabular rim.
Our study was not without limitations. First, its retrospective nature correlated with the biases exclusive to the study design. The sample size of the series resulted in a small number of cases included, restraining the production of more accurate statistical analyses. However, it is the only study investigating the clinical and radiological outcomes of this cementless hip system. Second, our survival rates should be considered as best-case estimates. Because there was a short-term follow-up period, we expect that some of these patients may yet undergo a revision arthroplasty for any reason at a longer follow-up. Third, our data did not include enough comprehensive information about demographics and specific frailty-comorbidities or perioperative factors that would contribute to developing complications. Hence, our complication outcomes should also be considered as best-case estimates. Finally, the gold standard in analysing initial implant stability is radiostereometric analysis . Another reliable option to evaluate implant migration is EBRA-FCA , which can also measure stem subsidence without the need for tantalum markers using standard pelvic radiographs. Unfortunately, we were not able to perform this kind of evaluation due to the lack of resources in our centre at the moment this study was developed.