ABSTRACT

The iliocapsularis is a relatively unheard-of muscle, located deep in the hip covering the anteromedial capsule of the hip joint. Little is known about this constant muscle despite its clinical relevance. The aims of this scoping review are to collate the various research studies reporting on the detailed anatomy and function of iliocapsularis and to demonstrate how inter-individual differences in iliocapsularis can be used as a clinical adjunct in guiding diagnosis and treatment of certain hip joint pathologies. A computer-assisted literature search was conducted based on Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Our review found 13 studies including 384 cases meeting our inclusion criteria. About 53.8% of the studies involved human cadavers. The current scoping review indicates the relevant anatomy of the iliocapsularis, being a small muscle which arises from the inferior border of the anterior inferior iliac spine and anteromedial capsule of the hip joint, inserting distal to the lesser trochanter. Therefore, based upon these anatomical attachments, iliocapsularis acts as a dynamic stabilizer by tightening the anterior capsule of the hip joint. Implications of this association may be that the muscle is hypertrophied in dysplastic or unstable hips. Determining the size of the iliocapsularis could be of conceivable use in patients with hip symptoms featuring signs of both borderline hip dysplasia and subtle cam-type deformities. Although future research is warranted, this study will aid physicians to understand the clinical importance of the iliocapsularis.

INTRODUCTION

The iliocapsularis muscle, also referred to as the iliacus minor, iliotrochantericus, iliocapsulo trochanteric or iliacus brevis, is a lesser-known but constant muscle located in the anterior aspect of the hip joint. Despite its constant presence, many clinicians are unaware of its anatomy [1]. The muscle lies deep to the rectus femoris and partially covers the anteromedial hip capsule. The first reference to the iliocapsularis appeared in 1843 in the second edition of Cruveilhier’s French anatomy text. The author originally identified this structure as an inconsistent autonomous muscle and referred to it as iliocapsulo trochanteric due to its attachments to the hip capsule and proximal to the lesser trochanter [2].

Ward et al. investigated the anatomy of the iliocapsularis in significant detail, as well as emphasizing its essential role in stabilization of the capsule of the hip joint. In addition, the paper highlighted its application as an important anatomical landmark intra-operatively, being a notable structure which must be identified and elevated during a Bernese periacetabular osteotomy [1]. The anterolateral border of the iliocapsularis has been described as the ideal location for capsulotomy in anterior total hip arthroplasty, and its anteromedial border can be used during a modified Smith-Peterson approach to identify the joint capsule in periacetabular osteotomy [3]. Furthermore, when performing an iliopsoas tenotomy, the iliocapsularis is useful in identifying the iliopsoas tendon during anterolateral or direct lateral approach [4]. The iliocapsularis muscle is also relevant whilst performing a T-capsulotomy during hip arthroscopy, as the desired site for incision is between the fibres of the iliocapsularis and the gluteus minimus to prevent iatrogenic injury to the medial and lateral femoral circumflex arteries [5, 6]. Although the iliocapsularis has been used as a key landmark in multiple surgical procedures including periacetabular osteotomy, anterior hip arthroplasty and hip arthroscopy, there is a paucity in the literature focusing on detailed anatomy, function and clinical relevance of this muscle.

To establish a more complete understanding of the lesser-known iliocapsularis, a scoping review was planned to map the literature on the topic, identifying sources of evidence, key topics and gaps in research. Scoping reviews are beneficial in providing a broad, extensive review of literature on a topic and is advantageous to a systematic review when conducting a comprehensive examination of a topic rather than exploring a specific question [7]. Current and future plausible clinical applications of the muscle have also been documentated and are of great interest to the hip surgeon. The specific aims of this scoping review, therefore, were to collate the various research studies reporting on the anatomy of the iliocapsularis and how inter-individual differences may be used to guide clinical practice bearing in mind the functional relevance of the muscle.

MATERIALS AND METHODS

The methodology for this scoping review was based on the framework outlined by Arksey and O’Malley [8] and ensuing recommendations made by Levac et al [7, 9]. The review included the following five key phases: (i) identifying the research question; (ii) identifying relevant studies; (iii) study selection; (iv) charting the data and (v) collating, summarizing and reporting the results. The optional ‘consultation exercise’ of the framework was not conducted.

Research question

This review was guided by the primary question, ‘What has been currently described in the literature regarding the iliocapsularis and its relevance to hip pathology?’ The secondary questions were set as follows:

What is the detailed anatomy of the iliocapsularis i.e. length, width, depth (thickness), location (proximal and distal attachment), and blood and nerve supply?

What are the functions of the muscle?

How may inter-individual differences be used to guide clinical practice, bearing in mind the anatomy and function of the muscle?

Data sources and search strategy

Two reviewers (M.S. and Z.H.K.) searched the online databases [Pubmed (Medline), EMBASE and Cochrane Library] for literature describing the iliocapsularis. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used for designing this study [10]. Database search was conducted on 1 October 2019 and retrieved articles from database inception to the search date. The individual study eligibility criteria were established a priori. We used medical subject headings including the following search terms: iliocapsularis, iliacus minor, iliotrochantericus, iliocapsulo trochanteric and iliacus brevis. Terms were connected by the Boolean operator ‘OR’ ( Appendix A). Results were pooled, and duplicate searches were excluded by having two reviewers (M.S. and Z.H.K.) independently review the titles and abstracts. The remaining search results were divided equally between two reviewers (M.S. and Z.H.K.) and reviewed in duplicate applying the inclusion and exclusion criteria. Any discrepancies at the full-text stage were resolved by consensus between the two reviewers. If a consensus could not be reached, a third more senior reviewer (V.K.) was consulted to resolve the discrepancy.

Study screening

The inclusion and exclusion criteria are shown in Table I. Both reviewers independently abstracted the relevant study data from the final pool of included articles and recorded these data on a spreadsheet designed a priori.

Table I.

Inclusion and exclusion criteria applied to articles identified in the literature

Inclusion criteria     Exclusion criteria     Inclusion criteria     Exclusion criteria     Table I.

Inclusion and exclusion criteria applied to articles identified in the literature

Inclusion criteria     Exclusion criteria     Inclusion criteria     Exclusion criteria      Assessment of level of evidence

The level of evidence was evaluated based on the guidelines by the Oxford Centre for Evidence-Based Medicine [11]. Levels I, II, III, IV and V evidence were elibigible for inclusion in the scoping review.

Date characterization

A proforma was developed by the authors to extract study characteristics such as publication year, the level of evidence, the number of hips, distribution of right and left hips, mean age with range (years), gender distribution, Body Mass Index (BMI) and specific comments. This form was reviewed by the research team and pretested by all reviewers (M.S., Z.H.K., Y.K. and V.K.) before implementation, resulting in minor modifications to the proforma. The characteristics of each full-text article were extracted by two independent reviewers (M.S. and Z.H.K.).

Data summary and synthesis

The data were compiled in a single spreadsheet and imported into Microsoft Excel 2013 (Microsoft Corporation, Redmond, WA) for validation and coding. Statistical analysis in this study focused on descriptive statistics to summarize the data. Frequencies and percentages were utilized to describe nominal data.

RESULTS Search and selection of studies

Flowchart of the literature search using PRISMA guidlines is shown in Fig. 1. The initial search of the online databases resulted in 87 total studies. A systematic screening and assessment of eligibility identified 13 full-text articles including 384 cases that satisfied the inclusion and exclusion criteria.

Fig. 1.

Fig. 1.

General characteristics of included studies

The oldest study included in this review was published in 1950, and approximately 75% of the included studies (10 of 13 studies) were published in or after 2014. The level of evidence in all studies was III, IV or V, and 53.8% of studies (7 of 13 studies) were human cadaveric studies. Details of the 13 studies included are shown in Table II. The studies included in this scoping review are broadly divided into Anatomy, Function and Clinical Relevance, which are described in detail.

. Author . Year . LOE . Objectives, number of hip . Mean age . Gender (% male) . BMI . Right hip (%) . Comments . 1 Das 1950 5 1 Cadaver − − − 0 Iliocapsularis is a rare muscle in the human body and when present it represents a detached part of the iliacus muscle 2 Ward 2000 4 20 Fresh cadavers No prior hip surgery − 25 − − Iliocapsularis originates in part from the inferior border of the AIIS, but the main origin arises from an elongated attachment to the anteromedial hip capsule and inserts just distal to the lesser trochanter 3 Babst 2011 4 45 Hips with pain Dysplasia 34 ± 9.7 (17–49) 45 25 ± 5 (18–37) 47 Increased thickness, width, circumference, CSA and partial volume of the iliocapsularis, and less fatty infiltration in the patients with dysplasia compared with excessive acetabular coverage     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45  4 Philippon 2014 4 14 Fresh cadavers No prior hip surgery, degenerative change and dysplasia 58 (47–65) 86 24.6 (19.2–32.1) 57 The iliocapsularis originated from the inferior facet of the AIIS. The inferolateral corner of the footprint of the iliocapsularis origin was located 12.5 mm (95% CI, 10.1–15.0 mm) from the acetabular rim 5 Walters 2014 4 11 Fresh cadavers No prior hip surgery 72.3 (67–95) − 24.6 (14.5–36.2) − The iliocapsularis had the most significant capsular contributions and was adherent to the entire length of the anteromedial capsule beginning at its origin at the inferior aspect of the AIIS to its insertion just distal to the lesser trochanter 6 Haefeli 2015 3 45 Hips with pain Dysplasia 34 ± 10 (17–49) 45 25 ± 5 (18–37) 47 The iliocapsularis-to-rectus-femoris ratio for CSA, thickness, width and circumference were increased in hips with radiographic evidence of dysplasia (ratios ranging from 1.31 to 1.35) compared with pincer FAI (ratios ranging from 0.71 to 0.90; P < 0.001) and compared with the control group     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45      30 Asymptomatic hip control 54 ± 12 (29–75) 50 26 ± 8 (14–37) 66  7 Cooper 2015 4 11 Fresh cadavers 79.2 (67–95) − 24.6 (14.5–36.2) − Iliocapsularis had large direct capsular attachments; dimensions defined as being 73.8 mm in length and 16.1 mm in width 8 Wyatt 2016 3 18 Hips with pain Stable dysplasia 32 ± 13 (14–55) 39 − − Iliocapsularis volume did not discriminate between treatment groups (periacetabular osteotomy or FAI surgery) with radiographic evidence of LCEA of 25° or less. However, a larger iliocapsularis volume was associated with greater antetorsion     21 Hips with pain Unstable dysplasia 31 ± 10 (15–46)      20 Asymptomatic hips Age-matched controls 37 ± 11 (15–52)  9 Lawrenson 2017 4 15 Asymptomatic hips No prior hip surgery 22 ± 2 67 − − The greatest muscle activity, which is the highest of electromyographic amplitude, by intramuscular electrode insertion occurred during isometric hip flexion at 90° and the lowest activity during hip extension at 0° 10 Ricci 2019 5 1 Hip with pain 30 100 − 0 Synovial bursitis between the rectus femoris direct tendon and iliocapsularis was likely the cause of anterior hip pain in this case 11 Lawrenson 2019 4 14 Asymptomatic hips 22.4 ± 1.8 71 23.6 ± 3.4 − Iliocapsularis demonstrates a consistent burst of muscle activity around toe-off in natural walking, with inconsistent muscle activity observed in mid-late stance. In shortened strides, the burst of muscle activity in mid to late stance became more consistent and had increased amplitude 12 Elvan 2019 4 21 Formalin-fixed foetuses 29 ±3.9 week (25–36) 43 − − Iliocapsularis is a constant muscle also in the foetal period. Its dimensions, location and course over the anteromedial part of the hip joint capsule suggest its prominent support to hip joint stability 13 Tsutsumi 2019 4 17 Fresh cadavers No prior hip surgery 81 56 − − The origin of the iliocapsularis corresponded with the shallow groove at the anteromedial surface of the AIIS, which was identified by micro-CT  . Author . Year . LOE . Objectives, number of hip . Mean age . Gender (% male) . BMI . Right hip (%) . Comments . 1 Das 1950 5 1 Cadaver − − − 0 Iliocapsularis is a rare muscle in the human body and when present it represents a detached part of the iliacus muscle 2 Ward 2000 4 20 Fresh cadavers No prior hip surgery − 25 − − Iliocapsularis originates in part from the inferior border of the AIIS, but the main origin arises from an elongated attachment to the anteromedial hip capsule and inserts just distal to the lesser trochanter 3 Babst 2011 4 45 Hips with pain Dysplasia 34 ± 9.7 (17–49) 45 25 ± 5 (18–37) 47 Increased thickness, width, circumference, CSA and partial volume of the iliocapsularis, and less fatty infiltration in the patients with dysplasia compared with excessive acetabular coverage     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45  4 Philippon 2014 4 14 Fresh cadavers No prior hip surgery, degenerative change and dysplasia 58 (47–65) 86 24.6 (19.2–32.1) 57 The iliocapsularis originated from the inferior facet of the AIIS. The inferolateral corner of the footprint of the iliocapsularis origin was located 12.5 mm (95% CI, 10.1–15.0 mm) from the acetabular rim 5 Walters 2014 4 11 Fresh cadavers No prior hip surgery 72.3 (67–95) − 24.6 (14.5–36.2) − The iliocapsularis had the most significant capsular contributions and was adherent to the entire length of the anteromedial capsule beginning at its origin at the inferior aspect of the AIIS to its insertion just distal to the lesser trochanter 6 Haefeli 2015 3 45 Hips with pain Dysplasia 34 ± 10 (17–49) 45 25 ± 5 (18–37) 47 The iliocapsularis-to-rectus-femoris ratio for CSA, thickness, width and circumference were increased in hips with radiographic evidence of dysplasia (ratios ranging from 1.31 to 1.35) compared with pincer FAI (ratios ranging from 0.71 to 0.90; P < 0.001) and compared with the control group     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45      30 Asymptomatic hip control 54 ± 12 (29–75) 50 26 ± 8 (14–37) 66  7 Cooper 2015 4 11 Fresh cadavers 79.2 (67–95) − 24.6 (14.5–36.2) − Iliocapsularis had large direct capsular attachments; dimensions defined as being 73.8 mm in length and 16.1 mm in width 8 Wyatt 2016 3 18 Hips with pain Stable dysplasia 32 ± 13 (14–55) 39 − − Iliocapsularis volume did not discriminate between treatment groups (periacetabular osteotomy or FAI surgery) with radiographic evidence of LCEA of 25° or less. However, a larger iliocapsularis volume was associated with greater antetorsion     21 Hips with pain Unstable dysplasia 31 ± 10 (15–46)      20 Asymptomatic hips Age-matched controls 37 ± 11 (15–52)  9 Lawrenson 2017 4 15 Asymptomatic hips No prior hip surgery 22 ± 2 67 − − The greatest muscle activity, which is the highest of electromyographic amplitude, by intramuscular electrode insertion occurred during isometric hip flexion at 90° and the lowest activity during hip extension at 0° 10 Ricci 2019 5 1 Hip with pain 30 100 − 0 Synovial bursitis between the rectus femoris direct tendon and iliocapsularis was likely the cause of anterior hip pain in this case 11 Lawrenson 2019 4 14 Asymptomatic hips 22.4 ± 1.8 71 23.6 ± 3.4 − Iliocapsularis demonstrates a consistent burst of muscle activity around toe-off in natural walking, with inconsistent muscle activity observed in mid-late stance. In shortened strides, the burst of muscle activity in mid to late stance became more consistent and had increased amplitude 12 Elvan 2019 4 21 Formalin-fixed foetuses 29 ±3.9 week (25–36) 43 − − Iliocapsularis is a constant muscle also in the foetal period. Its dimensions, location and course over the anteromedial part of the hip joint capsule suggest its prominent support to hip joint stability 13 Tsutsumi 2019 4 17 Fresh cadavers No prior hip surgery 81 56 − − The origin of the iliocapsularis corresponded with the shallow groove at the anteromedial surface of the AIIS, which was identified by micro-CT  . Author . Year . LOE . Objectives, number of hip . Mean age . Gender (% male) . BMI . Right hip (%) . Comments . 1 Das 1950 5 1 Cadaver − − − 0 Iliocapsularis is a rare muscle in the human body and when present it represents a detached part of the iliacus muscle 2 Ward 2000 4 20 Fresh cadavers No prior hip surgery − 25 − − Iliocapsularis originates in part from the inferior border of the AIIS, but the main origin arises from an elongated attachment to the anteromedial hip capsule and inserts just distal to the lesser trochanter 3 Babst 2011 4 45 Hips with pain Dysplasia 34 ± 9.7 (17–49) 45 25 ± 5 (18–37) 47 Increased thickness, width, circumference, CSA and partial volume of the iliocapsularis, and less fatty infiltration in the patients with dysplasia compared with excessive acetabular coverage     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45  4 Philippon 2014 4 14 Fresh cadavers No prior hip surgery, degenerative change and dysplasia 58 (47–65) 86 24.6 (19.2–32.1) 57 The iliocapsularis originated from the inferior facet of the AIIS. The inferolateral corner of the footprint of the iliocapsularis origin was located 12.5 mm (95% CI, 10.1–15.0 mm) from the acetabular rim 5 Walters 2014 4 11 Fresh cadavers No prior hip surgery 72.3 (67–95) − 24.6 (14.5–36.2) − The iliocapsularis had the most significant capsular contributions and was adherent to the entire length of the anteromedial capsule beginning at its origin at the inferior aspect of the AIIS to its insertion just distal to the lesser trochanter 6 Haefeli 2015 3 45 Hips with pain Dysplasia 34 ± 10 (17–49) 45 25 ± 5 (18–37) 47 The iliocapsularis-to-rectus-femoris ratio for CSA, thickness, width and circumference were increased in hips with radiographic evidence of dysplasia (ratios ranging from 1.31 to 1.35) compared with pincer FAI (ratios ranging from 0.71 to 0.90; P < 0.001) and compared with the control group     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45      30 Asymptomatic hip control 54 ± 12 (29–75) 50 26 ± 8 (14–37) 66  7 Cooper 2015 4 11 Fresh cadavers 79.2 (67–95) − 24.6 (14.5–36.2) − Iliocapsularis had large direct capsular attachments; dimensions defined as being 73.8 mm in length and 16.1 mm in width 8 Wyatt 2016 3 18 Hips with pain Stable dysplasia 32 ± 13 (14–55) 39 − − Iliocapsularis volume did not discriminate between treatment groups (periacetabular osteotomy or FAI surgery) with radiographic evidence of LCEA of 25° or less. However, a larger iliocapsularis volume was associated with greater antetorsion     21 Hips with pain Unstable dysplasia 31 ± 10 (15–46)      20 Asymptomatic hips Age-matched controls 37 ± 11 (15–52)  9 Lawrenson 2017 4 15 Asymptomatic hips No prior hip surgery 22 ± 2 67 − − The greatest muscle activity, which is the highest of electromyographic amplitude, by intramuscular electrode insertion occurred during isometric hip flexion at 90° and the lowest activity during hip extension at 0° 10 Ricci 2019 5 1 Hip with pain 30 100 − 0 Synovial bursitis between the rectus femoris direct tendon and iliocapsularis was likely the cause of anterior hip pain in this case 11 Lawrenson 2019 4 14 Asymptomatic hips 22.4 ± 1.8 71 23.6 ± 3.4 − Iliocapsularis demonstrates a consistent burst of muscle activity around toe-off in natural walking, with inconsistent muscle activity observed in mid-late stance. In shortened strides, the burst of muscle activity in mid to late stance became more consistent and had increased amplitude 12 Elvan 2019 4 21 Formalin-fixed foetuses 29 ±3.9 week (25–36) 43 − − Iliocapsularis is a constant muscle also in the foetal period. Its dimensions, location and course over the anteromedial part of the hip joint capsule suggest its prominent support to hip joint stability 13 Tsutsumi 2019 4 17 Fresh cadavers No prior hip surgery 81 56 − − The origin of the iliocapsularis corresponded with the shallow groove at the anteromedial surface of the AIIS, which was identified by micro-CT  . Author . Year . LOE . Objectives, number of hip . Mean age . Gender (% male) . BMI . Right hip (%) . Comments . 1 Das 1950 5 1 Cadaver − − − 0 Iliocapsularis is a rare muscle in the human body and when present it represents a detached part of the iliacus muscle 2 Ward 2000 4 20 Fresh cadavers No prior hip surgery − 25 − − Iliocapsularis originates in part from the inferior border of the AIIS, but the main origin arises from an elongated attachment to the anteromedial hip capsule and inserts just distal to the lesser trochanter 3 Babst 2011 4 45 Hips with pain Dysplasia 34 ± 9.7 (17–49) 45 25 ± 5 (18–37) 47 Increased thickness, width, circumference, CSA and partial volume of the iliocapsularis, and less fatty infiltration in the patients with dysplasia compared with excessive acetabular coverage     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45  4 Philippon 2014 4 14 Fresh cadavers No prior hip surgery, degenerative change and dysplasia 58 (47–65) 86 24.6 (19.2–32.1) 57 The iliocapsularis originated from the inferior facet of the AIIS. The inferolateral corner of the footprint of the iliocapsularis origin was located 12.5 mm (95% CI, 10.1–15.0 mm) from the acetabular rim 5 Walters 2014 4 11 Fresh cadavers No prior hip surgery 72.3 (67–95) − 24.6 (14.5–36.2) − The iliocapsularis had the most significant capsular contributions and was adherent to the entire length of the anteromedial capsule beginning at its origin at the inferior aspect of the AIIS to its insertion just distal to the lesser trochanter 6 Haefeli 2015 3 45 Hips with pain Dysplasia 34 ± 10 (17–49) 45 25 ± 5 (18–37) 47 The iliocapsularis-to-rectus-femoris ratio for CSA, thickness, width and circumference were increased in hips with radiographic evidence of dysplasia (ratios ranging from 1.31 to 1.35) compared with pincer FAI (ratios ranging from 0.71 to 0.90; P < 0.001) and compared with the control group     40 Hips with pain Pincer FAI 33 ± 11.0 (17–49) 31 23 ± 4 (18–32) 45      30 Asymptomatic hip control 54 ± 12 (29–75) 50 26 ± 8 (14–37) 66  7 Cooper 2015 4 11 Fresh cadavers 79.2 (67–95) − 24.6 (14.5–36.2) − Iliocapsularis had large direct capsular attachments; dimensions defined as being 73.8 mm in length and 16.1 mm in width 8 Wyatt 2016 3 18 Hips with pain Stable dysplasia 32 ± 13 (14–55) 39 − − Iliocapsularis volume did not discriminate between treatment groups (periacetabular osteotomy or FAI surgery) with radiographic evidence of LCEA of 25° or less. However, a larger iliocapsularis volume was associated with greater antetorsion     21 Hips with pain Unstable dysplasia 31 ± 10 (15–46)      20 Asymptomatic hips Age-matched controls 37 ± 11 (15–52)  9 Lawrenson 2017 4 15 Asymptomatic hips No prior hip surgery 22 ± 2 67 − − The greatest muscle activity, which is the highest of electromyographic amplitude, by intramuscular electrode insertion occurred during isometric hip flexion at 90° and the lowest activity during hip extension at 0° 10 Ricci 2019 5 1 Hip with pain 30 100 − 0 Synovial bursitis between the rectus femoris direct tendon and iliocapsularis was likely the cause of anterior hip pain in this case 11 Lawrenson 2019 4 14 Asymptomatic hips 22.4 ± 1.8 71 23.6 ± 3.4 − Iliocapsularis demonstrates a consistent burst of muscle activity around toe-off in natural walking, with inconsistent muscle activity observed in mid-late stance. In shortened strides, the burst of muscle activity in mid to late stance became more consistent and had increased amplitude 12 Elvan 2019 4 21 Formalin-fixed foetuses 29 ±3.9 week (25–36) 43 − − Iliocapsularis is a constant muscle also in the foetal period. Its dimensions, location and course over the anteromedial part of the hip joint capsule suggest its prominent support to hip joint stability 13 Tsutsumi 2019 4 17 Fresh cadavers No prior hip surgery 81 56 − − The origin of the iliocapsularis corresponded with the shallow groove at the anteromedial surface of the AIIS, which was identified by micro-CT  Anatomy

Das and Singh in 1950 described the iliocapsularis as an exceptionally rare muscle in the human body [12]. However, a recent anatomical study involving human formalin-fixed foetuses recorded a prevalence of this muscle at 92% in the foetal period. Elvan et al. found that the iliocapsularis was consistently located lateral to the iliopsoas, deep to the rectus femoris, medial to the gluteus minimus, as well as overlying the anteromedial part of the capsule of the hip joint [13]. As such, it has now been recognized as an individual and constant muscle. The following anatomical features of the iliocapsularis have been reported in detail in the literature.

Proximal attachment of the iliocapsularis

Elvan and colleagues included 21 formalin-fixed foetuses in their study [13], noting three different variations of the proximal attachment of iliocapsularis:

Below the proximal attachment of the rectus femoris muscle found in 54% of cases;

Forming a common tendon with rectus femoris on the anterior inferior iliac spine (AIIS) found in 26% of cases;

Forming an arch along the superior–medial–inferior sides of the proximal attachment of the rectus femoris muscle, found in the remaining 20% of cases.

Muscle fibres originating from the anteromedial part of the capsule of the hip joint were also recognized to be constant in all specimens. However, in adult studies, the proximal attachment of the iliocapsularis still remains a controversial issue. Ward et al. described the iliocapsularis as originating from the anteromedial hip capsule as well as the inferior border of the AIIS [1]. Substantiating this, Philippon et al. described the division of the AIIS into superior and inferior facets, divided by a horizontal osseous ridge (AIIS ridge) [14]. The inferior facet occupied 44% of the total area of the AIIS, and Philippon et al. affirmed the iliocapsularis to originate from the inferior facet of the AIIS in all 14 of their adult cadaveric specimens (Fig. 2) [14]. Similarly, Walters et al. supported the origin to be the inferior aspect of the AIIS in a cadaveric study [15]. On the contrary, a recent cadaveric study showed that the origin of the iliocapsularis corresponded with the shallow groove at the anteromedial surface of the AIIS, which was identified by micro-computed tomography (Fig. 3) [16].

Fig. 2.

An illustration indicating the various anatomical attachments of the iliocapsularis, direct and indirect head of rectus femoris, labrum and capsule of the hip joint to the right hip. Mean measurements of the area of the superior and inferior facets of the AIIS, plus the mean width of the AIIS ridge are indicated with 95% confidence intervals shown in brackets. Illustration retrieved from Phillipon et al. (2014) [14].

Fig. 2.

An illustration indicating the various anatomical attachments of the iliocapsularis, direct and indirect head of rectus femoris, labrum and capsule of the hip joint to the right hip. Mean measurements of the area of the superior and inferior facets of the AIIS, plus the mean width of the AIIS ridge are indicated with 95% confidence intervals shown in brackets. Illustration retrieved from Phillipon et al. (2014) [14].

Fig. 3.

An image derived from micro-computed tomography indicating the shallow groove of the anteromedial surface of the AIIS (indicated by use of arrowheads) where the iliocapsularis was suggested to arise from. The dashed lines correspond roughly to the superior portion of the AIIS and anterolateral wall of the ilium. The star indicates the smooth impression at the inferior portion of the AIIS. Ant= anterior, Med= Medial, Post = posterior, Sup = superior. Image describes the anteromedial (A), anterior (B) and anterolateral (C) aspects of the right hip. Image retrieved from Tsutsumi et al. (2019) [16].

Fig. 3.

An image derived from micro-computed tomography indicating the shallow groove of the anteromedial surface of the AIIS (indicated by use of arrowheads) where the iliocapsularis was suggested to arise from. The dashed lines correspond roughly to the superior portion of the AIIS and anterolateral wall of the ilium. The star indicates the smooth impression at the inferior portion of the AIIS. Ant= anterior, Med= Medial, Post = posterior, Sup = superior. Image describes the anteromedial (A), anterior (B) and anterolateral (C) aspects of the right hip. Image retrieved from Tsutsumi et al. (2019) [16].

Muscle belly

Iliocapsularis muscle length measured between 12 and 13 cm in 20 fresh adult cadavers [1]. Its length was identified to be longer in females than males during the foetal period (P = 0.031) and acknowledged to be wider on the right side (P = 0.029) [13].

Measurements of iliocapsularis width, depth (thickness), circumference and cross-sectional area (CSA) are described in Table III.

Table III.

Measurements of iliocapsularis width, depth (thickness), circumference and CSA in included studies

Subjects . 21 Formalin-fixed foetuses . 20 Cadaveric specimens . 45 Hips with dysplasia . 40 Hips with pincer FAI . P Value (dysplasia versus pincer FAI) . 45 Hips with dyspsia . 40 Hips with pincer FAI . P Value (dysplasia versus pincer FAI) . 30 healthy control . 45 Hips with dysplasia . 40 Hips with pincer FAI . Measurement  location level . its mid-length . 4 cm below the AIIS . The first section inferior to the femoral head . Femoral head centre . Muscle width, cm 1.9 ± 0.4 (1.3–3.3) 1.8–2.5 2.7 ± 0.6 (1.7–4.0) 2.0 ± 0.5 (0.9–3.9) <0.001 2.6 ± 0.5 (1.7–3.7) 2.2 ± 0.5 (1.1–3.1) <0.001 2.1 ± 0.5 (1.2–3.3) 2.3 ± 0.6 (1.4–3.4)*,§ 1.9 ± 0.3 (0.9–2.4)§ Muscle depth (thickness), cm No date 0.4–1.0 1.6 ± 0.4 (1.0–2.8) 1.4 ± 0.4 (0.8–2.4) 0.01 2.1 ± 0.4 (1.2–2.9) 1.7 ± 0.4 (1.0–2.7) <0.001 1.4 ± 0.4 (0.7–2.1) 1.7 ± 0.5 (0.7–2.7)*,