High risk of hip dislocation following polyethylene liner exchange in total hip arthroplasty—is cup revision necessary?

Polyethylene (PE) wear remains a common reason for revision surgery following total hip arthroplasty (THA) [1, 2]. The results of different arthroplasty registers show that replacing a PE liner because of wear becomes necessary in approximately 10–20% of arthroplasty cups [3, 4]. PE wear in a well-fixed uncemented cup is often treated by exchanging the PE liner and femoral head [1]. Whether to revise a well-fixed and well-positioned acetabular component in the presence of PE wear and osteolysis remains controversial [1]. Choosing the best procedure for revision surgery in patients showing PE wear involves the difficult decision whether to revise only the liner (e.g. lipped liner, constrained liner, increasing femoral head size) and face a high risk of postoperative hip dislocation or to revise both the cup (e.g. reposition of the cup) and the liner and use a dual-mobility component to reduce the risk of hip dislocation [5].

The current literature reports considerable disagreement over the revision method preferred for revising worn-out liners. Previously published studies argue that leaving the well-fixed and well-positioned cup unchanged at revision diminishes the probability of postoperative dislocation [6‐8], whereas other reports claim the opposite [9, 10]. Some authors advocate acetabular cup revision, while others insist on cup preservation with exchange of the liner only [6, 11]. Some studies have shown that leaving a well-fixed acetabular shell in situ may also lead to an increased risk of instability [12].

These considerations need to be balanced against the otherwise low complication rate for revision of the liner [12]. Replacement of a PE liner is considered more benign than full acetabular revision and a relatively simple procedure with less blood loss, shorter operation and hospitalization time [5]. Therefore, isolated exchange of the liner has become an increasingly common indication for revision surgery, even when facing a high rate of dislocation [9].

Among our patients we observed an unexpectedly high incidence of recurrent dislocation after liner exchange followed by revision of the cup. The purpose of this retrospective study was to determine the rate of THA dislocation following liner exchange.

The study was approved by the local ethics committee (Medical University of Innsbruck, Austria). We retrospectively investigated all consecutive patients at our department who between January 2000 and December 2014 underwent isolated PE liner exchange for wear with retention of the acetabular cup and femoral stem. Patients were included if (1) the PE liner was removable, (2) the acetabular shell was stable with acceptable orientation, (3) no osteolysis around the acetabular cup was found, and (4) no dislocation of the THA occurred before revision surgery. Exclusion criteria were revision of either the acetabular or the femoral component at the same intervention or a diagnosis of infection.

We identified a total of 124 patients, 82 of whom fulfilled the inclusion criteria. We then divided our study population of 82 patients into two groups (a) liner exchange without postoperative THA dislocation (control group) and (b) liner exchange with postoperative THA dislocation (study group; Fig. 1). Patient flow chart is shown in Fig. 2.

We also investigated patient medical histories for sociodemographic data, surgical approach, cut to suture time, pre- and postoperative range of motion, Charlson Comorbidity Score [13], body mass index, femoral and acetabular offset, details of revision procedure and blood loss during surgery, Estimated blood loss was calculated using the formula of Meunier [14]. Each substitution of concentrated red blood cells, administered intra- or postoperatively until the fifth day, was included in the calculation with a quantity of 280 ml (ml) and a haematocrit of 0.54.

Prosthetic stability, PE wear and cup migration were retrospectively assessed with EBRA (German: Einzel-Bild-Röntgen-Analyse) [15] from plain x rays. EBRA is a well-established method that evaluates standard anterior–posterior radiographs without requiring additional means at exposure (e.g., ball markers). Simulating the spatial situation, it computes parameters of longitudinal and transverse migration of prosthetic cup, femoral head and wear. The migration of the femoral head, the acetabular cup and wear in the horizontal and vertical directions can be studied. Total wear was calculated from the EBRA wear results in the horizontal and vertical directions by vectorial addition to make the results comparable with those of other methods. Furthermore, total wear was calculated as the differences between migration of the head and cup in the horizontal and vertical directions. A comparability algorithm using a grid of transverse and longitudinal tangents of the pelvis contour divides serial radiographs into sets of comparable ones. Migration is measured only between comparable radiographs. The 95% confidence limits for EBRA results are 1.0 mm for longitudinal and 0.8 mm for transverse migration [15].

In our department, we routinely follow patients with radiographs before discharge, 6 weeks after surgery and 12 month postoperative. We perform additional radiographs if the patient has any complaints with the THA. All radiographs were taken at our Department of Radiology with the same technique (anterior–posterior (AP) radiographs; patient standing in upright position and full weight-bearing). For our EBRA investigation, a minimum of three radiographs per patient and a minimum radiological follow-up of up to 6 months was required for this analysis. Cup migration analysis was done with EBRA by one independent investigator, who was not involved in the surgeries or postoperative treatment of patients.

Additionally, to preoperatively rule out an infection, a preoperative fluoroscopy-guided joint fluid sample was gathered (study group: n = 8/13; control group: n = 38/69). In all cases no infection was found. Unfortunately, no further investigation was performed, e.g., histopathological diagnosis of the synovia for the identification of particles of prosthesis material or polyethylene particles. This would have been of great importance in the evaluation of implant failure. An additional comment will be added in the Limitations section.

The most important findings in our study were the high rate of THA dislocation and the unexpectedly high rate of cup revision after liner exchange in our study group. We found an absolute risk increase of 16% after revision surgery, which results in a number needed to harm of 6. This means that every sixth patient with isolated liner exchange can expect to experience dislocation due to wear.

The purpose of the present study was to determine the rate of THA dislocation after liner exchange because of PE wear in a well-fixed and orientated cup without osteolysis. The most important findings in our study were the high rate of THA dislocation and the unexpectedly high rate of cup revision after liner exchange in our study group.

According to the literature, treating polyethylene wear with isolated liner exchange in a well-fixed acetabular cup has become an increasingly common procedure in revision THA [7]. Changing the PE liner can limit morbidity and avoids bone loss associated with removal of a well-fixed cup [7, 19]. The literature reports that the implant survival rate after liner replacement is comparable with that observed after more complex revisions, but that the risk of THA dislocation is significantly higher [19].

After revision total hip arthroplasty (rTHA), dislocation rates of up to 39% are reported [20] as compared to incidences ranging from 0.5% to 5% after primary THA [12, 21, 22]. The source of this increase in dislocations after revision surgery is still not fully understood and appears to be multifactorial [23]. In our patient series the dislocation rate after PE liner exchange for wear was 15.8%. This rate is consistent and well in line with previously published studies regarding rTHA because of PE wear [1, 5].

Several studies have shown that larger femoral head size directly influences stability [24, 25]. A recent study by Faldini et al. mentioned that it is advisable to use a 36-mm head diameter or larger when performing rTHA [23]. In our study, the head size most commonly used for revision surgery was 32 mm in both groups. Unfortunately, there was insufficient data to analyze our results on head size.

To reduce the risk of dislocation after rTHA some authors advocate using an elevated rim liner; even though a poorly positioned and elevated rim liner can inadvertently result in impingement with the iliopsoas muscle or tendon, leading to anterior hip pain [24, 26]. In addition, the study by Labek et al. demonstrated a very high failure rate for constrained liners [27]. With regard to dislocation rate, we found no advantage in using a 10° lipped or constrained liner, but there was insufficient data to analyze our results considering the above-mentioned liners. Further investigation is needed.

The effect of age on the dislocation rate after rTHA is still the subject of controversy. Wetters et al. [25] found that younger patients have a greater risk of dislocation after rTHA. Jo et al. [28] found dislocation rates to have no significant age-dependency. Yoshimoto et al., on the other hand, identified advanced age as a significant independent risk factor for dislocation. In our study, the two groups showed no significant differences in age composition or comorbidity index. We, therefore, detected no influence of age or comorbidity on the probability of dislocation.

In a study by Kosashvili et al. dislocation rates in patients undergoing first-time revisions were found to be significantly lower than in those undergoing repeat revisions [24, 28‐31]. Some authors suggest combining revision of the femoral and the acetabular side to achieve the best positioning and restoration of correct offset [24, 32]. A previously published study found dislocation rates for acetabulum-only revisions to be significantly higher than those for both components and femur-only reconstructions within the group of first-time revisions [24]. According to the literature, a reduction in rTHA dislocation can be achieved using dual-mobility cups [33‐37]. The risk of dislocation seems to increase with every additional surgical procedure. In our study, we found dislocations when using lipped or constrained liners and when enlarging the head size. We found no difference related to the choice of surgical approach for primary THA of rTHA. Nevertheless, in 7.3% of our study population a revision of the cup in the observed follow-up period became necessary because of recurrent dislocations after liner exchange. Unfortunately, our data are too insufficient to show a statistically significant difference. Neverhteless, a trend can be seen and further investigation is needed.

To the best of our knowledge, we are the first study to date to investigate dislocation rate after liner exchange using EBRA measurements for wear and cup migration. A recently published study by Abrahams et al. reported accuracy of the EBRA cup in uncemented acetabular components [38]. In addition, for EBRA cup and radiostereometric analysis (RSA) the authors reported good agreement on classification of components that migrated proximally 1 mm up or down at two years with 100% sensitivity and 87% specificity [38]. In our study, EBRA analysis showed significantly greater medial and cranial cup migration in the study group. Secondary instability might have occurred after initial subsidence. This may be understood as an early sign of later aseptic loosening and a symptom of osteolytic weakening of the bone stock [39], but this investigation was not part of the present study.

Another very important issue for recurrent dislocations in patients with wear of the polyethylene liner is seen to be the pathophysiological mechanisms of PE wear. In recent studies the pathophysiological mechanisms of PE wear and PE wear-induced osteolysis have been studied extensively [40‐42]. It has been shown that debris particles can induce a cellular response in periprosthetic tissues, with the up-regulation of toll-like receptors (TLRs) on macrophages [43, 44]. TLR signaling leads to up-regulation of many chemokines and cytokines, such as TNF-α, IL-1β, MCP1 and others [43, 44]. The inflammatory response that ensues leads to activation of osteoclasts and induction of local bone resorption [43, 44]. Lachiewicz, Watters and Oral et al. showed improved wear resistance for new generation polyethylene, as highly X-linked polyethylene and vitamin E-doped polyethylene by comparison with conventional liners [43, 44]. Although the description above was not underlying our study results, the wear rate in the study group was twice as high as compared with that of the control group. This could be a weak indication that the PE particle concentration in the surrounding soft tissue have been higher and, therefore, weaken the tightness of the collagen fibers. Nevertheless, our study revealed no significant difference in PE wear in either group. Even though we found no statistical significance with regard to wear rate, but our study may provides further insights into potential risk factors and might add information to the recent debate on dislocation after rTHA, especially concerning cup migration.

The present study has several limitations, such as the retrospective methodology and the selection bias of the study group. Patient follow-up was not blinded or randomized, for which reason bias and confounders are difficult to rule out. Furthermore, we concentrated on measuring cup positioning with EBRA, but the technology does not allow femoral component rotation to be measured to calculate combined anteversion as an important risk factor for prosthetic impingement and dislocation. A preoperative fluoroscopy-guided joint fluid sample was gathered to rule out low-grade infection. Unfortunately, no further investigation was performed, e.g., histopathological diagnosis of the synovia for identification of particles of prosthesis material or polyethylene particles. This would have been of great importance in evaluating implant failure. In addition, we didn't perform a power analysis wether the number of patients has enough volume to compare; instead we calculated a numerative factor with increased risk in percent and the number needed to harm.

Our study shows that exchange of the PE liner for wear entails a high risk of dislocation and an unexpectedly high risk of cup revision. In 46% of our study group patients the cup was revised. It would seem that this risk is increased when PE wear is combined with cup migration, thus potentially leading to instability. Surgeons should address individual risks, e.g., using large head diameters, dual-mobility cups, liners with elevated rim, and should stick to exact positioning, especially in patients with abductor deficiency. Further investigations are needed.

In conclusion, isolated exchange of the polyethylene liner because of wear showed a high risk of dislocation and further cup revision. Further investigations are needed to assess if the threshold for revising well-fixed components in the case of liner wear should be lowered.