1 Introduction

While historically considered relatively rare, the diagnosis of iliopsoas impingement after total hip arthroplasty (THA) seems to have been increasing over the past few years. Its current incidence is estimated to be in the neighbourhood of 4%.^1–4 There are multiple potential causes of tendon impingement or irritation, such as anterior osteophytes, over-long screws, excess cement deposits, and collared femoral stems. However, overhanging acetabular components due to a retroverted or oversized cup, excessive offset, or lack of anterior bone coverage appear to be the most frequent causes.

The diagnosis of iliopsoas impingement is clinical, based on a history of continuous groin pain, when ascending stairs, for example, or rising from a chair. Examination may reveal pain with resisted hip flexion, principally while stretching the iliopsoas tendon. Clinical symptoms can be subtle, and a high degree of suspicion is required. Plain radiographs with an anteroposterior view of the pelvis and a cross-table view of the hip are important in ascertaining cup position. Computed tomography (CT) scans can also be used to study the prominence and orientation of the cup. Ultrasound-guided injection of corticoids and local anaesthetic can be a useful tool for confirming the diagnosis; it may also be therapeutic in some patients.

Treatment remains a challenge, and there is a lack of consensus concerning approaches. The majority advocate for conservative management as the first stage, with approximately 50% of patients showing improvement when so treated.^5,6 If non-surgical treatment fails, the most frequent option is endoscopic or arthroscopic iliopsoas tenotomy, because of its low risk and minimal invasiveness.^7,8 Multiple techniques have been described as offering a high degree of safety and good outcomes.^9–11 Acetabular revision is only recommended in cases of severe cup malposition. However, there are no clear indications for this procedure; it presents both surgeons and patients with a complex decision, due to its inherent aggressivity and potential surgical risks.

The purpose of this article is to analyse the role of CT-scan measurement of acetabular cup positioning in an iliopsoas impingement therapeutic algorithm. We hypothesized that greater axial cup overhang is associated with poor clinical improvement, under both conservative treatment and iliopsoas tenotomy. This suggests that in selected cases of very severe acetabular malposition, cup replacement could be considered earlier.

2 Methods

2.2

2.2 Studied parameters

Data regarding demographics, radiological parameters, clinical outcomes, functionality (Harris Hip Score, HHS) and pain (Visual Analogue Scale, VAS) were collected preoperatively, intraoperatively and during follow-up, using digital medical records.

Radiographic measurements were taken using CT-scan images. These CT-scan studies had been performed routinely in the setting of painful THA. A digital templating program — TraumaCad® version 2.0 (BrainLab, Feldkirchen, Germany) — was used for analysis. Each radiological parameter was measured twice by two independent radiologists who were also experts in musculoskeletal imaging.

Radiological parameters analyzed to study acetabular cup positioning were acetabular cup inclination, acetabular cup anteversion, cup medialization, acetabular offset and femoral offset. Acetabular cup overhang was measured in both axial and sagittal planes (Fig. 1).

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Fig. 1 CT-scan images. A) Measure of acetabular cup overhanging in the axial plane. B) Measure of acetabular cup overhanging in the sagittal plane.

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3 Results

We identified 33 patients with iliopsoas impingement following THA, comprising 1.16% (32 of 2750) of all THAs performed at our institution during the study period. Eight of these cases did not meet our inclusion criteria (no post-THA CT-scan available). In consequence, 25 patients were available for the study. Of these, 56% were women (14/25). Mean patient age was 67.4 ± 13.4 years. Mean follow-up was 46.2 ± 20.88 months. Patient baseline characteristics are shown in Table 1. No significant baseline differences existed between groups.

Twelve patients (48%) presented significant clinical improvement under conservative treatment. They showed an HHS of 80.2 ± 5.0 and a VAS of 3.0 ± 1.8. On the other hand, thirteen patients (52%) did not improve with non-surgical treatment, and therefore were considered for endoscopic iliopsoas tenotomy. They presented an HHS of 55.2 ± 19.5 (p-value <0.001) and a VAS of 7.1 ± 2.6 (p-value <0.001). After surgery, 8 patients (61.5%) presented a significant clinical improvement, while 5 (38.5%) did not. Patients with post-tenotomy clinical improvement presented statistically significant pain reduction (1.5 ± 1.69 vs. 8 ± 1.41; p-value<0.01) and higher HHS (87.8 ± 12.8 vs. 53.4 ± 8.68; p-value<0.01). Both groups (patients who improved with surgery and those who did not) presented comparable pre-tenotomy pain (6.13 ± 2.85 vs. 8.6 ± 0.89, p-value 0.09) and HHS (60.3 ± 19.6 vs. 47.2 ± 18.3, p-value 0.257).

Radiological parameters of patients who improved with conservative treatment and of those who did not were compared (Table 2). This comparison revealed that patients who did not improve with conservative treatment presented a greater acetabular cup axial overhang (9.0 ± 5.1 mm vs. 4.4 ± 3.5 mm; p-value = 0.016) and sagittal overhang (10.3 ± 4.9 mm vs. 4.2 ± 4.2 mm; p-value = 0.003). However, no differences were observed in acetabular cup inclination and anteversion or cup medialization and offset, as compared to the patient's contralateral (normal) side.

A radiological comparison was also made between patients who improved with tenotomy and those who did not (Table 2). There were statistically significant degrees of acetabular cup axial overhang between groups. Patients who did not improve after tenotomy had greater axial overhangs (14.1 ± 5.48 mm vs. 5.97 ± 2.14 mm; p-value = 0.005). On the other hand, no difference in acetabular cup sagittal overhang was seen between groups (13.1 ± 5.54 mm vs. 8.6 ± 4.45 mm; p-value = 0.276). Statistically significant differences were also observed when comparing acetabular cup inclination (56 ± 9.56° vs. 39.6 ± 5.05°; p-value = 0.012). However, there were no differences in acetabular cup anteversion, cup medialization or offset as compared to contralateral (normal) side.

The ROC curve showed that setting a cut-off point of 10 mm of axial overhang offered an accuracy of 92.4% in discriminating between patients who improved or did not improve with surgical treatment (sensibility of 74.4%, specificity of 99.8%, p-value = 0.007). Of the 13 patients who underwent endoscopic iliopsoas tenotomy, axial overhang was <10 mm in nine patients and >10 mm in four patients. Significant clinical improvement after the tenotomy was observed in 88.9% (8/9) and 0% (0/4), respectively.

Further, axial overhang proved to be a highly reproducible radiological parameter (correlation coefficient 0.999; p-value<0.001).

4 Discussion

The results of this series of 25 patients with iliopsoas impingement after THA suggest that roughly half of such patients present a clinical improvement and pain relief with conservative treatment. For the rest, CT-scan can be a useful tool for helping the surgeon to decide between iliopsoas tenotomy and more major revision surgery, in selected cases of very severe acetabular malposition.

Optimum management of iliopsoas impingement as a cause of pain after THA is still a matter of debate in our field. In the present study, 80% (20/25) of patients achieved symptom resolution at final follow-up, which is quite higher to other reports: 68.8% (22/32) by Buller et al.¹² or 65% (32/49) by Chalmers et al.⁶ The first line of therapy should always be non-operative treatment. Ultrasound-guided peritendinous injection with corticoids and local anaesthetic is an extended diagnostic technique, but it may also be effective as a non-surgical procedure.^1,5,13 The literature cites clinical improvement after conservative treatment of around 50%,^6,12 which is similar to our own results (48%). In our series, these patients presented a mean HHS of 80.2 and a VAS of 3.0 after a successful non-operative approach. Chalmers et al.⁶ showed a mean HHS of 89. Interestingly, they did not find the extent of acetabular prominence as a risk factor for failure of conservative management (measured on the true lateral hip radiograph). They suggested that the lack of a definitive diagnostic test and the availability of only two radiographic perspectives could explain this fact. In our series, patients who improved with conservative treatment presented lesser acetabular cup axial overhang and sagittal overhang in CT-scan studies. However, no differences were observed in acetabular cup inclination and anteversion or cup medialization.

If conservative therapy fails, surgical treatment is the recommended choice. Iliopsoas tenotomy or cup revision resolve groin pain in >80% of cases.^1,6,14 Several studies have demonstrated the effectiveness and high success and safety rates of endoscopic or arthroscopic tenotomy. A multicentre study found pain resolution in 92% of cases, with a 3.2% complication rate.⁸ Bell et al. presented 60 patients with endoscopic treatment reporting 93.3% pain resolution, 1.7% complications and 6.7% relapses.¹⁵ Tassinari et al. presented an 81.2% rate of arthroscopic release success with a mean follow-up of 2 years.¹⁰ In our series, after endoscopic tenotomy, 61.5% (8/13) presented clinical improvement with statistically significant pain reduction and higher HHS. On the other hand, five patients (38.5%) did not improve after surgery. Finally, these cases required a revision hip procedure after a careful revaluation.

One of the most important debates in this area is the role of acetabular component prominence. It would be rational to suppose that patients with very high overhang could be candidates for cup revision as the first line of surgical treatment. Chalmers et al. proposed a cut-off point of 8 mm of acetabular prominence, based on true lateral hip radiography.⁶ Their results showed that clinical outcomes after tenotomy were better in patients with <8 mm of acetabular prominence, with 100% pain resolution and a mean HHS of 89. On the other hand, patients with >8 mm of prominence had a high rate of success with acetabular revision, presenting 92% pain resolution and a mean HHS of 82. Other studies have also suggested that cup prominence is a sensitive and specific parameter to take into account in cases of iliopsoas impingement after THA.^1,16 However, Tassinari et al. did not find that such protrusion influences the final outcome after tenotomy.¹⁰ They reported a mean overhang of 13 mm in their cohort, and a high success rate with tenotomy.

The aim of this study was to analyse the role of CT-scan measurements of acetabular cup positioning in a therapeutic algorithm, and to establish a rational method for identifying potential candidates for surgical treatment (tenotomy or cup revision). Therefore, our proposed algorithm (Fig. 2) includes performing a diagnostic injection in all patients with high clinical suspicion of iliopsoas impingement. Other causes of groin pain should also be ruled out. If iliopsoas impingement is finally diagnosed, the first line of therapy should always be conservative treatment (analgesics, physical therapy and infiltrations) for at least six months. In those patients who do not improve with non-operative treatment a CT-scan should be performed to study acetabular prominence. Patients who present <10 mm of axial overhang, endoscopic tenotomy is indicated. On the other hand, for those with >10 mm of axial overhang, an individual decision between tenotomy and acetabular revision must be made, based on patient characteristics and preference. These patients should be informed of the lower possibility of success with tenotomy, but also that it is a less invasive and aggressive procedure than cup revision. Therefore, in selected cases of very severe acetabular malposition, cup replacement could be considered first.

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Fig. 2 Our algorithm proposal in treatment of iliopsoas impingement based on CT scan axial overhanging.

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This study has certain limitations. First, it is a retrospective and uncontrolled study. Second, our patient cohort was relatively small (although comparable to similar studies), which limited its statistical power and the generalizability of its results. Further prospective and matched studies should be performed, analysing the influence of acetabular component overhang in the decision-making process when treating iliopsoas impingement.

In conclusion, conservative management should be the first line of treatment in all cases of iliopsoas impingement after THA. In cases of non-operative treatment failure, axial acetabular cup overhang measurement by CT-scan can be a useful tool in deciding between iliopsoas tenotomy or cup revision surgery in selected cases of very severe acetabular malposition. A cut-off point of 10 mm of axial overhang is a reliable predictor of higher failure risk with iliopsoas tenotomy.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or non-profit sectors.

Informed consent

Informed consent was obtained from all individual participants included in the study.

Institutional ethical committee approval

The study was approved by our Center's Ethics Committee (CEIC). The study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki.

Authors contribution

Carla Carbonell-Rosell: Conceptualization, Data curation, Investigation, Methodology, Project administration, Validation, Visualization and Writing; Diego Soza: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization and Writing; Oriol Pujol: Conceptualization, Data curation, Investigation, Methodology, Project administration, Validation, Visualization, Writing and editing; Matias de Albert de Delás-Vigo: Data curation, Investigation, Methodology, Software, Validation and Writing; Alba Antón: Data curation, Investigation, Methodology, Software, Validation and Writing; Victor Barro: Conceptualization, Investigation, Methodology, Project administration, Supervision Validation, Visualization and Writing.

LA

Local anaesthetics.