Introduction

In 1936 Smith-Petersen described hip pain caused by a bone-to-bone impingement between the femoral neck and the acetabulum [196]. However, it was not until 2003 that the modern concept of femoroacetabular impingement was initiated by Ganz et al. [74].

Femoroacetabular impingement syndrome (FAIS) results from an abnormal morphology of either the femoral head (cam) or the acetabulum (pincer) or a combination of both. This causes an incongruence in the hip joint and is a common source of hip pain, especially in the young active population [216]. Surgical treatment of FAIS aims to restore the normal hip joint morphology and thereby reduce symptoms [154]. Open hip dislocation was initially considered the gold standard for surgical treatment of FAIS, however, the use of a minimally invasive approach with arthroscopy has increased during the 2010′s [46, 154].

With an escalation of the arthroscopic procedures performed, there has been a corresponding increase in the studies published regarding FAIS [106]. Furthermore, several registries have been developed to keep track of performed arthroscopies and evaluate the outcomes after the procedures [93, 126, 185]. Patient-reported outcome measures (PROMs) are commonly used for evaluating the patients’ perspective of outcome of surgical treatment [158]. According to the Warwick Agreement, defined in 2016, the Hip and Groin outcome score (HAGOS) [205], Hip Outcome Score (HOS) [134] and the international Hip Outcome Tool (iHOT) [84, 143] are recommended as preferable PROMs for evaluating the outcome after FAIS surgery [82]. These PROMs are noted to be valid, reliable and responsive after FAIS surgery [170]. Yet, the PROMs used for FAIS have most commonly been developed for an older patient category with osteoarthritis, such as Harris hip score (HHS), while the PROMs recommended for the younger population are gradually being adopted [206]. With the use of PROMs developed for another patient category or condition, there is a risk of ceiling or wash-out effects due to the inclusion of non-relevant items.

The aim of this systematic review was to evaluate the trends in the literature pertaining to FAIS. More specifically, the aim was to present trends for the PROMs used and which surgical approaches have been performed to treat patients with FAIS. The hypothesis was that an increase in the number of studies with arthroscopic procedures performed would be observed with the majority using hip specific PROMs.

Methods

The systematic review was governed in agreement with the Preferred Reporting Items for Systematic Review and Meta-Analysis protocols (PRISMA) [142].

Eligibility criteria

All inclusion and exclusion criteria were prespecified and designed as recommended by PRISMA. The inclusion criteria for this systematic review were clinical studies with patients undergoing surgical treatment for FAIS. Studies defined as prospective, retrospective and randomized controlled trials (RCTs) were included. Only studies comprising PROMs were included. The study could be either therapeutic or prognostic. Therapeutic studies defined as studies exploring the results of FAIS surgery, and, prognostic studies, defined as investigating the effect of a patients’ characteristic on the outcome of FAIS. Only studies with English language in full text were included.

Exclusion criteria were studies including adolescents, children or described as “open physes”. No studies with patients < 18 years were included. Studies with less than 8 patients were deemed not eligible. Studies with primarily patients with slipped capital femoral epiphysis and Leg-Calve-Perthes disease were excluded. Studies with radiographic measurements as only outcomes were also excluded. Conference papers, systematic reviews, commentaries, protocols, narratives and studies validating PROMs were excluded. Studies with primary purpose to evaluate other diagnoses than FAIS and studies with patients undergoing revision surgery were also excluded.

Information sources and search

A systematic literature search was conducted in the online databases PubMed and Embase in September 2020. The searches were performed by a librarian with expertise in electronical searches at the Sahlgrenska University Hospital Library, Gothenburg, Sweden. The search retrieved studies from the period January 1999 until search day 7th of September 2020 to include an interval of over 20 years. The search was performed with controlled terminology and words. Different variations of the terms for “femoroacetabular impingement” OR “FAI” OR “hip impingement” OR “CAM impingement” OR “Pincer Impingement” were used together with different variations of “surgery” OR “operative” OR “arthroscopy” to create the search string. Exact information about the details on the search strategies for the database PubMed is found in Appendix, (Table 2).

Study selection

The studies from the electronic search were systematically evaluated by titles, thereafter abstract and finally their full texts by two reviewers (IL and SN). Both reviewers evaluated all studies from both databases independent of each other. Duplicates were removed manually. If the title or the abstract did not provide enough information regarding inclusion, the study was automatically included to the full-text assessment. The two reviewers were not blinded to the author, year and journal of publication. After all full texts were independently decided by the two reviewers, any disagreements regarding inclusion of studies were solved with discussion between the two reviewers.

Data items

The data extracted included the level of evidence, title of the study, authors, year of publication, journal, country where study was performed, type of study (retrospective, prospective, RCT), included number of, and which different PROMs used in the study. The proportion of “hip specific” PROMs in the study was recorded in the extraction sheet. In addition to exploring the development of included PROMs over the years, 2016, when the Warwick agreement was stated, was used as a cut-off to evaluate the adoption of recommended PROMs. It was noted if the study had included any type of “rate of return to sport” (RTS) apart from using a regular PROM and if the study evaluated patient satisfaction. Inclusion of any RTS assessment was in this study defined dichotomously (yes or no). Type of interventions assessed in the study were divided into open, arthroscopic or a combination of arthroscopic/open. Further data as proportion of sex, follow-up time, and number of patients were collected. The number of patients were defined as the patients undergoing surgical intervention, i.e., if the control group consisted of patients without receiving intervention, the control group was not included. Distribution of sex and mean follow-up for the last visit were recorded.

Statistical analyses

Interobserver agreement for full-texts was calculated with the Cohen kappa coefficient (κ) [119]. According to previous recommendations the values of κ were set a priori with a κ of 0–0.2 equals slight agreement, 0.21–0.4 fair agreement, 0.41–0.6 moderate agreement, 0.61–0.8 substantial agreement and > 0.8 equals to near perfect agreement. Descriptive statistics were used to present the data. Mean, standard deviation (SD), median and range values were presented when appropriate. Follow-up period was presented either as average follow-up period, or if not presented in the study, as minimum follow-up period. For studies comparing two or more groups, and no average follow-up period was mentioned for the entire cohort, a combined average follow-up was calculated. The analyses were performed with Microsoft Excel (version 16.40, Microsoft Corporation).

Results

Study identification and characteristics

The first search revealed 2,085 studies in PubMed and 2,218 studies in Embase. After removing duplicates, a total of 2,559 unique studies were eligible for the screening process. Figure 1 displays a flowchart of the screening process in accordance with the PRISMA guidelines. The agreement between the two readers for inclusion of full-text was 97% with a Cohen kappa value of 0.82, considered as near perfect agreement.

Fig. 1
figure 1

Flow chart of the screening process and number of included studies

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There were 6 (3%) RCTs, 55 (28%) prospective studies and 135 (69%) retrospective studies included in this systematic review. There were 6 (3%) Level I studies, 21 (11%) Level II studies, 86 (44%) Level III studies and 83 (42%) Level IV studies (Table 1). The included studies were published between 2004–2020. There was a large increase of published studies in the latter years where 143 (73%) of the studies were published in the last 5 years (2016–2020) compared to 7 (4%) in the first 5 years (2004–2008), an increase of 2,043% (Fig. 2).

Table 1 Included and results of individual studies
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Fig. 2
figure 2

Trend over the years of included studies. *Note the year 2020 only covers studies until search day 7th of September

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More than half of the studies (58%) were conducted in USA. Most studies were published in The American Journal of Sports Medicine (21%), followed by Arthroscopy: The Journal of Arthroscopic and Related Surgery (19%). A total of 32,303 patients were included counting the patients in all studies together, with an average of 165 patients per study (range 8–1,102). The mean follow-up period was 27.0 months (± 17 SD), (range 1.5–120) (Table 1).

Surgical procedure

The majority of the included studies (85%) were evaluating arthroscopic treatment. Only 5% of the included studies were examining solely open dislocation while the remaining 10% discussed either both open and arthroscopic or defined a mini-open technique with arthroscopic assistance. The procedure described in each study is reported in Table 1.

Patient-reported outcome measures

A total of 39 different PROMs were found in the studies, of these, 15 (38%) were hip-specific (Table 3, in Appendix). Between 1–10 PROMs were used in each study with an average of 3 (± 1.8 SD) PROMs per study. Before 2016, the median of included PROMs was two per study, and after 2016 the median had increased to three per study.

The most common used hip-specific PROM was mHHS (used in 120 studies (61%)), followed by HOS (81 studies (41%)) (Fig. 3). An additional question of return to sport/return to activity was seen in 13% of the included studies. Of 196 studies, 40% included a question on satisfaction of which the majority used the visual analog scale.

Fig. 3
figure 3

Trends in the number of recommended PROMs for FAIS and the most commonly used mHHS Abbreviations: HAGOS: The Copenhagen Hip and Groin Outcome Score, HOS: Hip Outcome Score, iHOT: international Hip Outcome Tool, mHHS: modified Harris Hip score, PROM: Patient-reported Outcome Measure. *Note the year 2020 only covers studies until search day 7th of September

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During the first five years (2004–2008), the Merle d’Aubigné and Postel score and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) were equally the most commonly used scores, reported in 3 (43%) of the studies during that period. During the last five years (2016–2020), the mHHS was the most commonly used, in 93 (65%) of the studies.

Of the 143 studies published during or after 2016, 67 (47%) studies have included the HOS, 46 (32%) included either iHOT-12 or iHOT-33 and 12 (8%) studies included the HAGOS (Fig. 3). Fifty-two of the 143 studies (36%) did not use any of the three PROMs recommended by the Warwick agreement [82] (Table 1).

Discussion

The most important finding in this systematic review was the expected growth in the number of studies published over the years, where over 70% of the included studies were published between 2016–2020. Although the literature review included studies from 1999–2020, the first study meeting the inclusion criteria was published in 2004.

A total of 39 different PROMs were used among the studies, of which 15 were hip specific. The most common non-hip specific outcome was satisfaction, found in 40% of the studies. Previous studies have reported that satisfaction is the most frequently used non-hip specific outcome tool, although there is a variability how satisfaction is reported [175, 193]. The discrepancy in the use of different PROMs has previously been noted and the reason for this is unknown. The routinely use of a specific PROM, the difficulty in changing PROMs once norms have been established and the inevitable retention of the same PROMs to be able to follow a cohort and evaluate long-term outcomes are possible explanations for the divergence in use of PROMs [175].

After the Warwick agreement in 2016, three patient-reported outcome measures were considered suitable for the target population of FAIS and were recommended to use when evaluating surgery for FAIS [82], 65% of the included studies in this systematic review used at least one of the recommended PROMs (HAGOS, iHOT-12 or iHOT-33 and HOS (ADL + SS)). Nonetheless, the mHHS remains being the most commonly used PROM, even though there is a well-known ceiling effect of mHHS described for young active patients [206]. It could be seen as both surprising and concerning that mHHS still is the most used PROM in studies on FAIS as its outcome’s validity for young and active patients is considered low. Thorborg et al. [206] found HAGOS to be the best suited PROM for patients with FAIS, which only was used in 7% of the studies. This finding can guide future healthcare providers and researchers in using hip specific PROMs valid for the target population and diagnosis. Furthermore, there is a need for adoption of new validated scores, translated into the patients’ native language.

Only 13% of the included studies reported RTS specifically by using a clear definition. There is a current challenge in sports science regarding the definition of RTS, and the most optimal evaluation of RTS has not yet been decided. Activity scores such as the HOS (SS), Tegner activity scale or HSAS, with the purpose to evaluate the patients’ activity level or issues in sport specific activities, are not the best tools to evaluate the RTS. Mainly because these scores do not include training load or performance compared with preinjury status. This could possibly generate a ceiling effect if the patients rate the PROMs higher, yet still not being capable to fully return to their preinjury level of sport. Furthermore, the definition of RTS has been proposed to differ between elite and recreational athletes [42]. Athletes undergoing hip arthroscopic surgery for FAIS usually have a major interest whether they can RTS again, thus, a reliable method to determine RTS is thus needed.

The majority of the studies were published in USA or in Europe. This has previously been reported [106, 213]. Although USA and Europe have been in the front line of hip arthroscopic surgery and research, a small number of studies included in this systematic review were from Korea and China, indirectly indicating an upcoming trend in performed surgeries for FAIS in Asia. Moreover, only studies in the English language were included in this systematic review, which partly might explain the high percentage of studies from USA and Europe.

Although a few RCT:s have been published, retrospective studies are still the most common. Over the years, patient registries have facilitated prospective evaluation of FAIS and yielded important insight on PROMs [126, 185]. Öhlin et al. [155] assessed the methodological quality of prospective studies over a 5-year time period and found no improvement in the quality of the methods despite an increase in the number of published studies. With the dramatic increase seen in the number of published studies in this systematic review, it is of importance to also improve the quality of observational studies. New consensus meetings to enhance adoption of suitable PROMs and education of researchers and clinicians could benefit future research in the outcome of FAIS.

Strengths and limitations

The strength of this study is the methodological rigor using PRISMA guidelines, focus on an important topic and the longitudinal analysis of a 20-year time horizon.

This systematic review is not without limitations. One of the a-priori set exclusion criteria was age, excluding studies with patients < 18 years old, though the focus was on the adult population as validation of PROMS in the pediatric population is still emerging. Moreover, only publications in the English language were included and there is a risk of missing publications in non-English speaking countries. Due to the heterogeneity of the included studies no statistical meta-analysis was conducted.

Conclusion

There has been a continuous increase in the number of published studies regarding FAIS with the majority evaluating arthroscopic surgery. The mHHS remains being the most commonly used PROM.