Introduction

The introduction of two new fully cementless acetabular im- plants in 2008 has revolutionized total hip arthroplasty (THA) as well as THA acetabular revision surgery in the past decade. The use of Trabecular Metal (TM) implants has shown satisfactory re- sults since their introduction in the late twentieth century [1,2]. Despite the development of several different designs and operat- ing techniques, elimination of the most common cause of pros- thesis failure, and subsequently revision acetabular surgery, has not yet been achieved. Risk of a revision significantly increases in the second decade following primary THA and is mostly due to loosening of the acetabular component [3,4]. With the grow- ing demand for THA as well as acetabular revision surgery, use of Trabecular Titanium (TT) has increased significantly [5]. Pure Trabecular Titanium (Delta Trabecular Titanium, Limacorporate, Udine, Italy) is designed as a honeycomb-like structure, with highly porous material consisting of multi-planar hexagonal inter- connected cells [6,7]. In vivo studies have shown that Trabecular Titanium is capable of inducing complete and quick osseo integra- tion by means of its porous, hexagonal cell structure [8,9]. Use of the Electron Beam Melting (EBM) technique produces the mi- crostructure Trabecular Titanium, which provides continuity be- tween the solid part of the acetabular cup cavity and the exter- nal honeycomb-like structure. This one-step production process makes the Trabecular Titanium less susceptible for common risks in coated implants, such as detachment [9,10]. The Delta One TT THA is designed to have a reduced caudal size in order to be ap- plicable in challenging conditions such as hip dysplasia or revision surgery. The Delta Revision TT THA has a cage construct with a hook and three arms to provide the implant with sufficient stabil- ity. In order to further enhance stability, it is possible to insert screws through the holes of the arms, fixating them to the pelvis [1,7]. This makes the Delta One TT and Delta revision TT systems promising implants, especially for complex primary surgery (due to osteonecrosis, dysplasia or otherwise poor bone quality) or challenging revision surgery [11].

Orthopedic surgeons will often opt for placement of a Dual Mobility Cup (DMC) in patients with higher preoperative risk of instability and possible dislocation after THA. A DMC consists of a small prosthetic head inside a larger polyethylene liner. Primary motion occurs between the femoral head of the prosthesis and the inner surface of the polyethylene liner, allowing a secondary motion to occur between the polyethylene liner and the acetabu- lar cup [10,12]. This construction has proven to prevent implant dislocation, improve range of motion, and decrease the risk of dislocation [13,14]. Utilization of the TT scaffold is expected to reduce the incidence of (aseptic) loosening of acetabular compo- nent of the prosthesis. To the best of our knowledge limited data has been published in literature regarding the survival of TT ace- tabular implants, even more so regarding the use of TT implants in complex primary and acetabular revision surgery. The aim of this study was to evaluate five-year survival of the Delta One TT and Delta Revision TT systems in patients undergoing primary com- plex or acetabular revision surgery.

Materials and methods

A retrospective, single-center cohort study was performed using prospectively collected data from our hospital’s electronic medical records. We included all patients who received a Delta One TT or Delta Revision TT implant in acetabular (revision) sur- gery between April 2011 and December 2014.

ASA classification was determined by the anesthesiologist dur- ing routine pre-operative screening [15]. Routine pre-operative antibiotic prophylaxis consisted of two grams of Cephazolin, in- travenously administered at least 30 minutes before the start of the procedure. All patients were operated one by one of four experienced orthopedic surgeons. Surgery was performed under general anesthesia, via the direct lateral approach to the hip as first described by Hardinge except for two cases where an ante- rior approach according to Smith-Petersen was performed based on preference of the surgeon [16,17]. A DMC was implanted in pa- tients with higher risk of instability and possible dislocation after THA. Acetabular defects were evaluated during the surgical pro- cedure, classified according to the Paprosky classification, and ad- dressed if necessary [18]. Defects were primarily corrected by use of autograft, harvested from the patient’s iliac crest, or allograft bone substitutes. When needed, further defects were addressed by use of hemispheric modules or augments.

Venous Thromboembolic (VTE) prophylaxis consisted of Na- droparine (low molecular weight heparin) 9500 IE/mL daily. Pa- tients weighing less than 100 kg received 0.3 ml (2850 IE) and those weighing more than 100 kg received 0.6 ml (5700 IE) once daily. This was administered during six weeks starting the first post-operative day. Postoperative x-rays in anteroposterior and lateral views were routinely performed one day after surgery to confirm correct placement of the prosthesis. Patients were in- structed by a physical therapist to follow standard precautions after hip replacement surgery, according to hospital standards. Partial weight-bearing mobilization using a walking aid was al- lowed for six weeks after surgery, followed by full weight-bearing mobilization.

Follow-up occurred according to our hospital standard pro- tocol, i.e. patients were reviewed at our outpatient clinic at two weeks, six weeks, three months, one year, three years and five years postoperatively. X-rays in both anteroposterior and lateral views were routinely performed. Cut-off point for follow-up was determined as December 31st 2019 and maximum follow-up time was set at five years.

Primary outcome consists of survival of the Delta One TT and the Delta Revision TT cup. Also, clinical outcomes were analyzed, and peri- and postoperative complications were registered and graded according to the Clavien-Dindo classification [19]. Baseline characteristics and complications were studied us- ing descriptive statistics, with mean (SD) and numbers (%) shown when appropriate. To study the survival a Kaplan-Meier analysis was performed and to study the effect of Paprosky score and bonegraft on survival, a Pearson Chi-square test was performed on the status at five years. Statistical significance is defined as a p- value <0.05. Analysis was performed using IBM SPSS version 27.0 (IBM corp.).

This research has been approved by the IRB of the authors’ af- filiated institution.

Results

72 patients received a Delta TT implant in complex primary and acetabular revision surgery. The study population consisted of 18 (25%) male and 54 (75%) female patients, with a mean age of 72.4 (SD 12.4) years (Table 1). The prosthesis was implanted on the right side in 29 (40.3%) cases and on the left side in 43 (59.7%) cases. No bilateral procedures were performed.

The Delta One TT cup was placed in 52 cases; 20 patients re- ceived a Delta Revision TT cup. The indication for surgery was aseptic loosening of previous prosthesis in 18 cases (25%), dislo- cation in 13 cases (18%), polyethylene wear in eight cases (11%), Girdlestone in seven cases (9.8%), instability of prior implant in seven cases (9.8%), migration of prior implant component in three cases (4.2%) and osteolysis in three cases (4.2%), or ‘other’ in 13 cases (18%, e.g. secondary osteoarthritis due to hip dysplasia or metallosis and pseudotumor after metal-on-metal prosthesis).

Dual mobility cups were implanted in 51 cases. Patients in our cohort presented with Paprosky scores varying from 1 to 3B (Ta- ble 1). When present, defects were corrected by use of autograft bone substitution, harvested from the pelvic ring, in six cases (8.3%) or allograft bone substitution in 18 cases (25%) in order to provide sufficient stability for the implanted prosthesis. Five cases (6.9%) received both varieties of bone grafts in order to achieve optimal stability.

A total of eight patients required cup revision of their Delta implant resulting in a cumulative survival of the Delta One TT and Delta Revision Cup of 88.9% after five years of follow up (Figure 1).

Indications for revision surgery were prosthetic joint infec- tion (PJI) in four patients (5.6%), aseptic loosening in one patient (1.4%), persistent instability of the affected hip in two patients (2.8%) and migration of the cup in one patient (1.4%). Cumulative survival of the implants for the Paprosky classification groups was 100% in Paprosky 1, 87.0% in Paprosky 2 and 81.8% in Paprosky 3 (Figure 2), but Paprosky score had no significant effect on the survival (p = 0.272).

Also, the use of autograft, allograft, both types of bone substi- tution, or no bone substitution did not have an effect on risk of revision of the implant (p=0.211).

Eight patients (11.1%) died during follow-up. Of these eight, one patient deceased on the third postoperative day from the ef- fects of abdominal ischemia and aspiration. The remaining seven patients died of unrelated causes during the follow-up period.

Three of the included patients underwent revision surgery af- ter placement of a Delta TT prosthesis; however, the implanted cups were not revised during these procedures. For this reason, these three patients were classified as non-failure. One patient needed revision due to failure of the dual-mobility liner, therefore the cup required no revision and was left in situ. The second pa- tient was in need of revision for complaints of instability due to a stem implanted with too much anteversion. The stem was revised and a 20° liner was placed. The third patient needed revision for a dislocation which could not be treated by closed reduction. The liner was exchanged for a dual-mobility liner.

Table 1: Baseline patient characteristics and surgery outcome.

Discussion

Our study found a cumulative survival of the Delta One TT and Revision TT cups of 88.9% after five years. Compared to the lit- erature this is somewhat lower. Perticarini et al. report a survival of the Delta TT cup after five years 99.3% [3]. This difference in survival may be due to the fact that in our study Delta One TT and Revision TT cups were used for complex primary and acetabular revision surgery, as opposed to the Delta TT used for primary ac- etabular surgery performed by Perticarini et al.

Whereas several studies have been conducted on short-to mid-term clinical outcomes of Trabecular titanium cups, such as Delta TT, Delta One TT and Revision TT cups, to our knowledge, few studies have been conducted on the use of these cups in com- plex primary and revision surgery. Munegato et al. report survival of the Revision TT cup in 37 acetabular revisions of 91.7% when using the cup for revision surgery with an indication other than aseptic loosening. In this study mean follow-up was 3.3 years (range 1.0-7.6 years) [20].

Perticarini et al. assessed the mid-term outcomes of Trabecu- lar Titanium cups in revision surgery in 104 cases with a mean follow-up of 7.6 years (range 2-12.2 years), reporting a survival of 88.54% [21]. This is similar to the mid-term survival outcomes of Delta TT systems, both Delta One TT and Revision TT cups in complex primary or acetabular revision arthroplasty found in our study.

With one case of aseptic loosening, our results are comparable to those published by Gallart et al. [1] and Perticarini et al. [3,21] on the use of Trabecular Titanium components, and to the results published for the Trabecular Metal™ (TM) system by Zimmer [22].

Although recent literature suggests higher Paprosky bone de- fect scores are associated with higher risk for implant failure our study found no significant difference in revision rates when con- sidering Paprosky bone defect classification [1,23,24].

Cumulative survival of the Delta One TT and Revision TT cups as found in this study are in line with recent literature on use of trabecular metal cups and are therefore regarded by the authors as a reliable representation of implant survival in primary complex and acetabular revision surgery.

It should be noted that our study was focused on complex primary acetabular surgery and need for revision of previous im- plants as indication for surgery. For this reason, the outcomes of this study should be interpreted as such and not as outcomes of the use of implants in simple primary acetabular surgery. Further- more, outcome data for Delta One TT and Revision TT was pooled.

Limitations

This study has two limitations. Firstly, the retrospective design with its inherent limitations. We expect that for the execution of the study a prospective design would not have yielded different results as all parameters were collected from the hospital infor- mation system and patient selection would not have been differ- ent. Secondly, our study presents mid-term follow-up results and further long-term follow-up is needed in order to gain valuable in- sight in the long-term reliability of this particular type of implant. However, the current results are valuable information concerning these systems.

Conclusion

Five-year survival of the Delta One TT and Delta Revision TT ac- etabular systems shows promising and encouraging results in pa- tients undergoing primary complex or acetabular revision surgery.

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