Prospective two-year subsidence analysis of 100 cemented polished straight stems - a short-term clinical and radiological observation

Clinical follow-up included a standardised examination, using the Harris Hip Score (HHS) [16] at all time points.

Standardized digitalized radiographs of the pelvis (patient in supine position, centered on the symphysis, focus film distance 120 cm) were taken at 1 week, 12 weeks, 1 and 2 years postoperatively. The quality of the cement mantle was rated according to Barrack [17]. Varus/valgus alignment of the stem was measured on the postoperative ap radiograph, a deviation of more than 3° was defined as malalignment [18]. Debonding was defined as a radiolucent line at the prosthesis-cement-interface not visible on the first postoperative radiograph [18]. Osteolysis was defined as a progressive, newly developed endosteal bone loss with a diameter greater than 3 mm with an either scalloping or bead-shaped lucency at the cement-bone-interface [3]. Debonding and osteolysis were manually measured on the plain radiographs and reported accoording to their location in the Gruen zones [19]. Subsidence of the stem was measured using the software based EBRA-FCA method. For this method a minimum of 4 comparable digitalized x-rays are needed. An electronic coordinate system is placed on the x-rays to localize cup, stem and certain landmarks (Fig. 1). Compared to e.g. RSA no further markers have to be implanted and routine follow-up radiographs can be used. Therefore it’s an ideal tool to detect early subsidence or migration in huge cohorts [2, 3, 20].

Cup inclination was manually measured on the plain radiographs using the interteardrop line. Osteolysis around the cup was rated according to the zones described by DeLee and Charnley.

A Shapiro-wilk test was used to test for normal distribution of the data. As data were non normal distributed, median and range was used to describe the data.

No patient was lost to follow-up. 17 patients had an incomplete follow-up missing either a clinical or x-ray follow-up, none had been revised. 5 patients died during the first 2 years unrelated to surgery (median 379 days, range 7–684 days). 2 hips had an early infection (<4 weeks postoperative) and were treated with debridement and implant retention (DAIR), both were free of infection at final follow-up. Another 2 patients had a late chronic infection (CNS 4 months, and P. acnes 8 months) and were successfully treated with a one-stage exchange. 1 patient sustained a periprosthetic fracture (Vancouver type B1) 3 months after surgery due to a fall, and was treated with osteosynthesis. 1 female patient had two early dislocations (4 and 5 weeks postoperative, 32 mm head, anterior approach) during night rest and was treated with closed reduction and a brace for 6 weeks. Afterwards she had a uneventful follow-up without further (sub-)luxations. Radiographs showed a normal alignment of the implants, thus the reason for dislocation remains unclear. Exact patient flow is shown in the CONSORT flow chart (Fig. 2).

The KM survival at 2 years for the endpoint reoperation for any reason was 94.9 (95 % confidence interval 90.6–100 %). Survival for the endpoint aseptic loosening at 2 years was 100 %.

The HHS (Harris Hip Score) improved from 56 (14–86) preoperatively to 95 (60–100) 2 years postoperatively (Table 1). There was no difference in HHS scores between the two approaches (p = 0.91).

Seventy-three stems had a complete radiological follow-up consisting of 4 radiographs and were free of complications. 17 patients had an incomplete radiological follow-up. They were doing fine, but refused follow-up examinations or radiographs. 5 patients were revised for the reasons mentioned above and 5 patients deceased. One of these died before having post-op radiographs due to a gastro-intestinal bleeding.

Cementing quality (n = 99) was rated grade A in 47 stems, grade B in 45, grade C in 7 and grade D in 0 stems. There was no influence on cementing quality if the operation was performed by a consultant or a resident in training (p = 0.82).

Osteolysis around the stem was seldom and seen in 2 stems in Gruen zone 7 and was independent from cementing quality or stem alignment. No debonding was noticed.

Fifty-seven of 73 hips (78 %) with a complete radiological follow-up could be analysed with EBRA-FCA. The average subsidence was -0.3 mm (95 % CI -0.5 mm to -0.1 mm). 11 stems showed subsicence above 1 mm (Table 2). The stem with a subsidence of 1.5 mm had a cement mantle Barack grade A, neutral stem alignment and showed no further radiological changes (osteolysis or debonding) at final follow-up. The other patient with a subsidence of 3.1 mm had a cement mantle Barrack grade B, neutral stem alignment and also showed no other radiological abnormalities, but was succesfully treated with DAIR due to an early infection 4 weeks after implantation. There was no correlation between cementing quality and subsidence (p = 0.179) or osteolysis and subsidence (p = 0.634).

The median cup inclination was 38° (range 11°–55°, 10–20°: n = 1, 21–30°: n = 9, 31–40°: n = 46, 41–50°, n = 38, 51–60°: n = 5). No progessive osteolysis were noted around the cup. No cup was revised for aseptic loosening or malpositioning.

Our clinical results are comparable to normal short-term results of other successful implants. The increase of the functional status combined with a high rate of subjective satisfaction of patients receiving THA is a normal development for the postoperative course and a main reason for the high success rate and patient satisfaction for this operative procedure being the operation of the 20^th century [22]. A limitation of the study is that 22 patients were operated with a lateral transgluteal approach which was than changed to a direct anterior minimal invasive approach during the study period [23], which showed no statistical difference. There might be some patient bias in this study, since the elder and rather frail patients received the cemented twinSys stem including also femoral neck fractures. The healthier and younger patients (n = 185) during the observed time period were operated with the uncemented twinSys stem. Furthermore, as we are a teaching hospital, a substantial number of operations have been performed by residents what might, in parts, explain our higher rate of infection and death during the course of follow-up [24‐27].

As expected radiological changes on plain radiographs were scarce concerning osteolysis and debonding, which is the expected course of well cemented stems with short-term follow-up. Cementing quality has been shown to be crucial for long-term survival of cemented implants [28]. The overall cementing quality in our series was good especially in the setting of a teaching hospital with residents performing 36 % percent of the surgery in this series. Anyhow we did not reach the cemeting quality of highly specialised centers [29].

A limitation of our radiological analysis is that 27 stems had no complete radiological follow-up. With a mean age of 79 years at the time of surgery the study group was rather old and patients unwilling to come in case of a symptom free follow-up. 57 out of 73 stems with a complete follow-up the number of radiographs suitable for an EBRA-FCA analysis is a bit higher than reported in the literature [3].

In the literature differerent cut-off values for early subsidence being indicative for later aseptic loosening are described. Freeman et al. reported a later failure of the femoral component if early migration was above 1.2 mm/year in the first 2 years with a sensitivity of 78 % measured on plain radiographs [1]. Similar values on plain radiographs were supported by Walker et al. stating a long-term survival rate of less than 5 % for Charnley and Stanmore stems, that showed a subsidence of <2.6 mm in the first 2 years [4]. Measurements for cemented shape-closed stems (Lubinius and Spectron SP1) on normal radiographs has a lower accuracy compared to software based methods [30]. Kärrholm et al. [2] defined 1.2 mm in the first 2 years as the cut-off value for later aseptic loosening of the stem (Lubinius SP1, shape-closed) using the more accurate RSA method (accuracy 0.2 mm) while De Vries et al. analysed 15 different stem designes and reported 1.24 mm for straight polished cemented stems as the cut-off for later aseptic loosening [31]. RSA is the method with the highest precision but as a short-coming needs special radiographs, implants (markers) and is therefore not suitable for larger series [32]. The EBRA-FCA method used in this study has the advantage that routine clinical radiographs can be used to analyse large series [33] but lacks the accuracy of RSA with a detection limit for subsidence of 0,2 mm. Krismer et al. [3] reported a subsidence >1.5 mm in the first 2 years as the cut-off value for later aseptic loosening for cemented Müller straight stems. The Müller straight stem is a shape closed stem [34], which by design should not subside. In contrast to shape-closed stems with a force-closed philosophy as the twinSys are intended to show some subsidence [13, 35]. We measured a mean subsidence of 0.33 mm which is clearly below all publishedcut-off values independent from the respective method.

Anyhow 9 stems in our series showed an early subsidence between 1–1.5 mm which is of concern. These stems have to be closely monitored, whether this is the final position or if further migration will occur. We found only two stems with a subsidence > 1.5 mm indicative for later aseptic loosening. One of these two just subsided 1.5 mm lacking any other radiological abnormalities. The other one subsided 3.1 mm after an early infection 4 weeks after surgery and was treated with DAIR. It is rather unlikely that subsidence was caused by a persisting chronical infection as the patient was free of symptoms of infection during further follow-up, but intraoperative manipulation of the stem during DAIR might be a potential explanation for the excessive subsidence.