Straight stem and threaded cup total hip arthroplasty systems in patients under 60 years of age: outdated or still a benchmark? A report on a cohort of 75 patients after 28.8-30.2 years of follow-up


 Purpose: The goal of this retrospective observational study of one cohort was to evaluate the long-term outcome in patients younger than 60 years after total hip arthroplasty using a straight uncemented stem and an uncemented threaded cup. Methods: Between 1986 and 1987, 75 hips of 75 patients (mean age: 53.35±6.17 years) were consecutively implanted with an Alloclassic Zweymüller/Alloclassic SL stem and an Alloclassic CSF cup. 44 patients had died over the last 30 years. The remaining 31 patients (mean age: 82.9±6.4years) were reinvited for follow-up examinations. Clinical and radiographic evaluations were carried out. Results: At a mean follow-up of 29.5 (28.8-30.2), 4 patients (5.3%) were lost to follow up. For the endpoint aseptic loosening (defined as the removal of stem or the cup for 2 cases), the overall survival rate is 97.3%. For the endpoint revision for any reason (22 patients), the survival rate is 70.6%. Eleven patients needed an exchange of head and liner due to wear. The mean time from implantation until change of head and liner was 21.44 years (SD 5.92). Other reasons for revisons were septic loosenings (3 cases), aseptic loosening of stem and cup (1 case), aseptic loosening of stem (1 case), periprosthetic calcification (2 cases), implant fracture (1 case), periprosthetic fracture (1 case), intraoperative fissure of stem (1 case), total wear of liner including cup (1 case). Conclusion: The combination of a straight stem (Alloclassic) and a screw cup (CSF) shows excellent results in young patients under the age of 60 at ultra-longterm follow up at 30 years. Revisions due to wear of the polyethylene liner are more likely than in the older patients.


Introduction
Various studies and registry data deal with short and midterm results of the cementless implanted Alloclassic Zweymüller hochgezogen, the Alloclassic SL and the Alloclassic CSF screw cup system [1][2][3]. These implants have excellent results in older patients [3]. Some studies describe longterm results up to 20 years[4]. Other well established stem systems are reported to have excellent long term results such as Bicontact [5,6], Taperloc[7], Corail [8,9], CLS[10-12] as well as threaded cups such as T-TAP ST [13].
Recent trends lead to bone and muscle preserving implants such as spherical press-fit cups and bone preserving, metaphyseal fixed short stems, especially in younger and more active patients. According to some authors, available data does not show satisfying evidence regarding the benchmark of 90% survivorship after 10 years, set by the National Institute of Clinical Excellence, NICE [14]. Up to now, only straight stems have an ODEP 13A* rating[http://www.odep.org.uk/products.aspx 15].
Threaded cups and conventional straights stems have proven excellent overall long term-results in various cohorts.
Nevertheless, there is only limited published data for the combination of a straight stem with a screw cup in younger patients, who are mostly more physically active and demanding We already published first results of our cohort 15 years ago with excellent outcome [3]. Up to now, there is no published data reporting clinical and radiological results for a screw cup in combination with a straight stem in patients less than 60 years of age at time of implantation with an ultralongterm follow-up of up to 30 years.

Materials And Methods
We established a retrospective observational study of one cohort. Between 1986 and 1987 seventyfive hips in 75 consecutive patients 60 years of age or younger at time of surgery were treated in our institution with a cementless total hip arthoplasty system. All patients received a straight uncemented stem (n=46, Alloclassic Zweymueller/ n=29, Alloclassic SL) and a uncemented threaded cup (n=75; Alloclassic CSF (AlloPro/Sulzer Medica; Centerpulse; Winterthur, Switzerland). In all cases, a standard non highly cross-linked PE liner was used with 28mm ceramic heads with 12/14 trunnion (Cerasul, AlloPro/SulzerMedica). The group included 38 women and 37 men. Mean age at surgery was 53.35 years (32.5 -60.5; standard-deviation 6.17; 95%-Confidence Interval 51.9 -54.7). 44 patients (44 hips) had died over the past 30 years. Using data from a previous study, we know for 42 of them if the arthroplasty system was in situ at death or revised for any reason. The patients were interviewed by telephone if the prostheses were still in situ and if there was any revision surgery or any problems over the past years, concerning the prostheses. If feasible, the patients were invited to come to our outpatient clinic for a routine clinical and radiological check-up.
We were able to assess 16 patients clinically and radiologically. All other 11 patients refused to come to the hospital but data from a structured telephone-interview was available.
The surgical indication (Table 1) for total hip arthroplasty was primary osteoarthritis in 55 cases (73 percent) and secondary osteoarthritis in 8 cases (16 percent). Others were revision surgeries in 12 cases (10.6 percent).
We calculated the survival rate of the implanted system including graphs for survival curves according to Kaplan-Meier, the adverse events correlating with the implants over the past 30 years, the occurrence of radiolucent lines and osteolytic zones in the recent weight bearing x-rays according to the classification of DeLee and Charnley [16] and Gruen[17] on the a-p view and the patients' satisfaction with the result after 30 years. Confidence-intervals are given for calculated values, if applicable. Patients without follow-up were excluded from further analysis.
Survival curves were calculated for all implanted stems for aseptic loosening as well as revision for any reason. The software used was Microsoft Excel 12.2.6 and jamovi 0.9.5.12 for detailed statistical analysis.

Survival rate:
If the endpoint is defined as the removal of the stem or cup for aseptic loosening (2 cases), the overall calculated survival rate is 97.33% ( Figure 2). If the endpoint is revision for any reason (22 cases), the calculated survival rate is 70.6% (Figure 3). Eleven patients needed revision surgery for exchange of head and liner, one of them was operated twice on one hip due to wear. The mean time from implantation until change of head and liner was 21.44 years (SD 5.92). Those patients, who died over the last 30 years, have lived with the arthroplasty system for an average of 17.68 years (3.7 -28.3; SD 7.3; 95%-CI 15.9 -19.9).

Radiological:
Most of the radiolucent lines and osteolytic zones were found in the proximal Gruen-zones 1 and 7 (94.7 percent; n=19) and in the lateral DeLee and Charnley zone 1 (87.5 percent; n=8). None of the implanted stems and cups were considered to be at risk.

Clinical:
Overall satisfaction of our patients with the implanted total hip arthoplasty system was very good ( Figure 4). None of the 27 patients described pain.

Adverse events resulting in further surgery (Table 2):
We had two cases of aseptic loosening as well as three cases of septic loosening. Aseptic loosening of the acetabular component was found two times, whereof one was seen a case of a loose stem (wear and osteolysis). One totally worn-out liner leaded to revision surgery of the acetabular component due to destruction of the acetabular component ( Figures 5, 6 and 7).
Two patients had to be revised because of a periprosthetic fracture. Major calcification occurred in two cases.
We had one case of implant-fracture, which lead to revision-surgery of the femoral component.
One case periprosthetic fracture resulted in a Girdlestone-procedure.
In one patient a fissure of the proximal femur was seen intraoperatively, so no second surgical procedure was needed.

Discussion
From 1986 on, our institution has been implanting the described Alloclassic SL and CSF screw cup total hip arthroplasty system in primary and secondary hip surgery. This is the first study to provide a 30-year follow-up for young patients under 60 years of age treated with an uncemented total hip arthroplasty system consisting of a porous coated straight stem and threaded.
Pursuing our published data for the Alloclassic stem system after 30 years of follow-up, our evaluation in combination with the cementless implanted Alloclassic CSF cup system is the first ultra-longtermstudy in young and demanding patients with a mean follow up of 29.5 years [18,19]. During this timespan, a broad variety of complications occurred in our cohort. Reasons for revisions were: wear and change of head and liner (11 patients), septic loosenings (3), aseptic loosening of stem and cup (1), aseptic loosening of stem (1), periprosthetic calcification (2), implant fracture (1), periprosthetic fracture (1), intraoperative fissure of stem (1), total wear of liner including cup (1).
Nevertheless, for a majority of 70.6%, the implantation of the evaluated THA-system was the last surgery needed involving the replaced joint.
Previous data shows, that youth as well as higher activity levels may increase the incidence of mechanical failure of total hip prostheses. Wangen et al. showed a failure rate of 49 percent for the acetabular components (press fit and screw cups) due to mechanical failure in patients younger than 30 years after a mean follow-up of 16 years [20]. The main type of failure in our study did not occur in the interface between cup and bone, but wear of the polyethylene liner was the main reason for revision.
In our population group 29.6% needed revision surgery for any reason, which seems to be on a high level compared to other published cohorts with a long-term follow-up in the third decade. It has to be noted, that in those cases, which needed revision surgery to exchange head and liner, a regular PEliner had been implanted in primary surgery. As soon as ultra-high cross-linked UHMWPE liners have entered the market in 1998, secondary surgery was performed using this long material, which has proven about 10-times less wear in long-term follow-up cohorts. As a result, a far smaller revision rate due to wear can be expected. [21][22][23].
It can be assumed, that the known cost-effectiveness of THA gets stronger when the revision rate decreases [24,25].
Regarding the low wear rates of modern materials, the fear of wear-associated complications in younger individual over decades does not seem to be justified.
Most of the radiolucent lines and osteolytic zones were seen in the lateral zone I according to DeLee and Charnley describing changes at the cup. Radiolucent lines around the stem were commonly seen in Gruen-zones 1 and 7, but none of the implanted systems was considered to be at risk [3,26]. The authors were able to show in a previous study, that there may be a positive effect for the reduction of osteolytic zones around the stem caused by wear when performing an exchange of head and liner [27].
The results of this study group are comparable to to previous published cohorts as shown by Delauny, giving survival rates of 99% for aseptic loosening after a timespan of 7 to 8 years [1]. years [29].
Other stems with excellent long term results in younger patients such as CLS Spotorno (72.8 any reason after 23.8 years)[10, 12,11], Bicontact (93.57% at 12 years)[6,5] and Corail (95% at 12 years) [8,9] show good results up to 24 years of follow-up, but none of the mentioned cohorts give results for a period up to the end of the third decade as we do. Published data does sometimes not focus on the long-term problems such as wear of early polyethylene liners, therefore the reason for aseptic loosenings stays unclear in most cases. We observed a high number of worn-out liners, so we assume that the reason for our aseptic loosenings may be the result of wear after a long timespan.
It has to be stated, that the criteria ‚revision for any reason' increases with the years and usage because of progressive wear and the need for replacing head and liner [30]. Comparison with data from our previous publications shows, that all of the patients, who needed an exchange of head and liner, were not older than 60 years at primary surgery.
Knowing the excellent results for aseptic loosening of the discussed system a successing arthroplasty system needs further benchmarks to prove superiority. Criteria such as bone loss in revision surgery and reproducibility of surgical results are arguments to pursue the development and evaluation of bone sparing, and more anatomically shaped arthroplasty systems [31][32][33].
Our evaluation is limited by the small number of patients being alive after 30 years. Furthermore, most of the surviving patients are in a bad general medical condition or have moved far away from our institution, which leads to a very small number of clinical evaluations. Nevertheless, our primary aim was not the collection of clinical scores, but the survival rate of the implant and the resulting complications during a 30-year-timespan in younger, more demanding patients. This objective was achieved satisfactorily.
The evaluated system is very reliable in primary and secondary THA, as shown in previous studies as well as in this one. In our institution, it has been the basic implant for many years. It has been our backup-system in cases of failure of spherical pressfit cups for a long period of time.   Radiograph showing excellent results after 30 years in a highly active 73-year old male (l) Figure 5 Radiograph showing totally worn-out liner after 25 years Radiograph showing implant fracture after 20 years