1 Introduction

The anterior inferior iliac spine (AIIS) is a frequent site of avulsion fracture in the pelvis representing around one-third of them in most of the series.^1–3 Teenage athletes are more likely than others to have AIIS apophyses avulsion fractures because of abrupt, intense rectus femoris contractions or eccentric muscle lengthening that happens while the growth plate is still open. The hormonally induced muscle strengthening at this age coexists with secondary ossification at the apophyses.⁴

Nowadays, there are two different currents in the literature. On one side, the notion is that avulsion fractures of the inferior iliac spine heal optimally in the absence of surgical treatment in most cases. Numerous studies^5–11 have shown how conservative treatment of these injuries leads to excellent outcomes even when there is radiological evidence of displacement. However, few of them describe clinical and radiological follow-up beyond six months.

On the other side, recently, several studies have been published to demonstrate the efficacy of open^12–16 or arthroscopic^17–20 procedures to solve a frequent cause of extra-articular femur-acetabular impingement (FAI) syndrome associated with previous AIIS avulsion fractures, the so-called sub-spine impingement. In this context, there needs to be more consensus and evidence determining whether acute AIIS avulsion fractures should be treated surgically to avoid later complications.

The purpose of the study was to re-evaluate the acute surgical indications for AIIS avulsion fractures, and consider the three-dimensional anatomy of the supracetabular region, to avoid potential late complications with reduced quality of life, return to sport, and the need for subsequent surgery.

2 Methodology

This review evaluated current evidence of AIIS avulsion fracture treatments and outcomes. A literature search was done in the following databases: PubMed, SCOPUS, Embase, and Cochrane Library. All information relevant to the review was considered. It used the following inclusion criteria: randomized controlled trial (RCT), non-RCT, prospective and retrospective studies published in English, involving humans, and full-text available. Editorials, biochemical and in vitro studies, technical notes, preclinical studies, and studies with a methodology quality lacking were excluded from the research.

3 Historical notes

Historically, a diatribe on treating avulsion fractures of pelvic apophyses in the acute setting did not exist. From a biological and histologic point of view, avulsion fractures of the pelvis have been assimilated to epiphyseal fractures and therefore are naturally prone to bone healing.^21,22

Surgical treatment proposers emphasized those cases in literature with persistent pain and disability after failed conservative treatment due to highly displaced avulsion fractures.²³ The controversy started on which fractures and patients could benefit from operative treatment in the acute setting.

The idea that open reduction and internal fixation (ORIF) should be considered for fractures displaced >2 cm was reported in a series of five pelvic avulsions and one case of bilateral tibial tubercle avulsions in 1965.²⁴ Only 2 of the 5 cases were treated operatively and were only ischial avulsions. From that day on, that notion spreads in almost every article regarding pelvic avulsion fractures.

In 1985 Metzemaker et al.²² did outstanding work describing the clinical, biological, and histological aspects of pelvic avulsion fractures, proposing a rationale for conservative treatment. They were the first authors to describe a five-step conservative protocol. They demonstrated excellent clinical results with a follow-up of at least 18 months for 27 patients, avoiding the risks of open surgery. In that series, however, only four out of 27 cases were AIIS avulsion fractures, and no description of displacement or long-term sport-specific clinical follow-up was explicitly described for those four cases. Only a paper has been published proposing the open reduction and internal fixation in the acute setting for an AIIS avulsion fracture before 2000. In that year, Rajasekhar et al.,²⁵ in their case report, described using a 6.5 mm screw for open reduction and internal fixation in a case of AIIS avulsion fracture with 2.5 cm of displacement with good outcomes at one-year follow-up.

More recently, several case reports and case series have been published focusing on AIIS avulsion fractures. None propose an acute surgical treatment for fractures with less than 2 cm of displacement,^5–11 with conservative treatment that still plays a central role with excellent reported outcomes.

4 Treatment options in the acute settings: current concepts

Nowadays, no evidence-based guideline exists for the ideal treatment of these patients.¹ Since there are no specific indications for the treatment of AIIS avulsion fractures in the literature, there are general indications for the treatment of avulsion fractures of the pelvis.

The main indication for conservative treatment is avulsion fractures with less than 1.5–2 cm^1,4 of displacement on plain radiographs. Most pelvic avulsion fractures, including AIIS, are treated nonoperatively, including analgesics, bed rest, immobilization of the affected muscle group, physical rehabilitation, and crutches for at least 3–6 weeks post-injury.¹

Metzemaker et al.²² first described a standardized 5-phase supervised rehabilitation program generally applied to pelvic avulsion fractures. Different individualized conservative protocols have been proposed in case reports and series that specifically involve the AIIS,^{5,6,8–10,26,27} all of which provide for gradual activity progress until return to sports.

The surgical procedure is recommended when the avulsion fractures have a displacement of more than 1.5–2 cm^1,4 on plain radiographs and in highly active patients and athletes who need a faster return to play.¹

ORIF with cortical screws, cannulated screws, and resorbable screws have been described to treat AIIS avulsion fracture.^25,28,29 Endoscopic-assisted reduction and internal fixation have recently been proposed.³⁰

5 Complications

Surgery does not expose significant risks to conservative treatment overall but has been associated with a higher risk of heterotopic ossifications.¹ The use of an anterior surgical approach increases the risk of damage at the lateral femoral cutaneous nerve; the nerve should be carefully localized and protected during the whole procedure; this may be particularly difficult due to the variable anatomy of the nerve.³¹ Painful nonunion and hypertrophic malunion have been classically recognized as the main complications in patients treated conservatively.^4,16,32–35 Endoscopically assisted reduction and internal fixation could potentially minimize complications associated with an open surgical dissection and the risks of later complications. However, further studies are necessary to provide evidence-based guidelines.

6 Bone healing: anatomic and biomechanics consequences

From a biological and histological point of view, apophyseal avulsion fractures are similar to epiphyseal fractures/metaphyseal-physis separation²¹ and, as so, are naturally prone to healing with a solid callus formation when protection and rest are guaranteed.²² Repetitive traumatism, due to muscle traction over the avulsed fragment, often leads to hypertrophic healing, bulking, and appearance that may mimic bone tumors, especially when a delayed diagnosis or no supervised conservative treatment is made.^16,32–34

Femur-acetabular impingement (FAI) syndrome is described as an abnormal dynamic contact of the acetabular rim with the femoral neck, which causes pain and dysfunction and leads to premature arthritis. Pan et al.,³⁶ in their case report, first described impingement between the femoral head-neck junction and an abnormally hypertrophic anterior inferior iliac spine in a 30-year-old man who presented with pain and limitation of movement of the hip. Larson et al.¹⁷ gave the well-known definition of sub-spine impingement, and they reported a case report with three patients successfully treated with arthroscopic decompression. Hetsroni et al.,³⁷ in 2013, in their retrospective studies of 53 patients, showed how the AIIS morphology is a potential mechanical contributor to hip impingement in young patients with hip dysfunction (Fig. 1). They classified three AIIS morphologies and their correlations impingement at different degrees of hip flexion and internal rotation (Table 1) (Fig. 2). Among the three variants, in Type III and to a lesser extent in Type II, the AIIS contributes to hip impingement, as observed by the limitation in flexion and internal rotation and the contact between the AIIS and the femoral neck (Fig. 3). Different authors gave their contributions, with case reports and case series, demonstrating the efficacy of open^12–16 or arthroscopic^17–20 decompression in chronic groin pain syndrome related to the sub-spine impingement.

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Fig. 1 Anterior-posterior radiograph of the pelvis. Arrows indicate left AIIS extending caudad to the superior acetabular rim due to a previous avulsion fracture.

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Fig. 2 3D (three-dimensional) reconstruction of a right hip, view from anterior. Smooth ilium between AIIS and the superior/anterior acetabular labrum. This morphology can be classified as Hetsroni type 1.

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Fig. 3 3D (three-dimensional) reconstruction of a left hip, view from anterior(left) and posterior (right). Arrows indicate AIIS extending caudad to the superior acetabular rim due to a previous avulsion fracture. This morphology can be classified as Hetsroni type 3.

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7 Discussion

Most of the literature still bases the treatment principles of AIIS avulsion fractures on the historical threshold of 2 cm of fracture displacement, a value not supported by solid scientific evidence and indiscriminately applied to all avulsion fractures of the pelvis. Although nonunions of pelvic avulsion fractures are considered rare entities, the assumption that bone healing and excellent results are exclusively associated may be too optimistic when considering AIIS avulsion fractures due to the anatomy of the supracetabular region and hip joint motion. The hypothesis that a bone avulsion may progress an AIIS from grade 2 to grade 3, according to Hetsroni/Larson,³⁷ could be plausible. An overall assessment that considers the fracture displacement relative to the acetabulum's three-dimensional morphology rather than a value calculated on a plain radiograph could be helpful to ensure correct indication in the acute setting. Due to the absence of prospective randomized studies, therapeutic indications are still based on operator sensitivity instead of evidence-based guidelines. AIIS avulsion fractures tolerate a low degree of displacement due to the involvement of these types of fractures in the development of sub-spine FAI.

Most pelvic avulsion fractures, including AIIS, have been treated nonoperatively, with excellent results reported.^1,21,22 However, most studies^1–4,22,24 generalize pelvic avulsion fracture as one category, not focusing on a specific fracture location. Recently, several case reports and case series^{5–11,26,27,38,39} have been published, focusing on AIIS avulsion fractures reporting excellent outcomes of conservative treatment even when there is radiological evidence of gross displacement. Most have a clinical and radiological follow-up of fewer than six months^5,6,9,11 or are not reported.^8,10 Two of them^7,39 have methodological limits and data acquisition bias. Few studies describe medium or long-term clinical and radiological follow-up. Kishta et al.,³⁸ in their case report, described subsequent AIIS and ASIS avulsion fractures in a thirteen-year-old patient with excellent results thirty-six months after the first diagnosis for a minimal displaced AIIS avulsion fracture. In their case report, Oldenburg et al.²⁷ reported simultaneous AIIS and ASIS avulsion fractures in a 16-year-old male adolescent with radiologic and clinical healing at a 10-month follow-up. Weel et al.²⁶ were the first authors to evaluate long-term patient-reported outcomes (PROMs), The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and Tegner score in adolescents with AIIS avulsion fractures. They reported a case series of seven conservatively treated patients with an AIIS displacement of less than 1.5 cm. Of the seven patients with a median follow-up of 33 months, one required a surgical resection following a hypertrophic malunion (initial 1 cm displacement). The patient with the most significant displacement (1.5 cm) scored the worst physical functioning scores compared to the other patients, although he still had good results overall. Looking at the time elapsed between the initial trauma and subsequent decompression surgery in the context of sub-spine impingement in the last ten years (Table 2), it appears that a long-time interval may divide the initial trauma into the subsequent secondary intervention. In their recent retrospective review of 228 apophyseal avulsion fractures of the pelvis, Schuett et al.,² demonstrate how AIIS is almost five times more prone to develop chronic pain than other apophyses and how all patients with hip pain that last more than three months from their initial injury were unable to return to their preinjury sports. Consequently, delayed surgery is more common for AIIS avulsion fracture than other avulsion fracture sites in the pelvis.⁴⁰ Current evidence suggests that patients treated surgically return to sport more than those receiving conservative treatment (92% vs 80%), with a higher rate of excellent outcomes when displacement on plain radiographs is major than 1.5 cm.¹

The present study represents the first comprehensive review that analyzes and synthesizes the current literature regarding conservative and surgical treatment following an AIIS avulsion fracture, providing relevant insights into the proper management and treatment of this injury in the acute setting.

This study has several limitations. First, the literature on this topic is poor, with a low LoE. Second, most of the analyzed studies were case series or case reports with very small sample sizes of patients. Third, many examined papers had very short follow-ups, some less than six months. Fourth, studies from the literature show variability in terms of fracture displacement and indication for conservative and surgical treatment. Fifth, the acute surgical treatment of AIIS avulsion fractures appears anecdotal in the literature. Therefore, it is yet to be possible to make a comprehensive critical analysis of the therapeutic indication. Further studies with more extensive case series and medium-to long-term follow-ups would be needed to better understand the treatment in the acute setting when dealing with an AIIS avulsion fracture, especially in the young sports patient.

8 Conclusions

The acute surgical indication in anterior inferior iliac spine avulsion fractures should consider the three-dimensional anatomy of the supracetabular region, especially in patients with high functional demands. Three-dimensional evaluation would allow accurate assessment of the position and dislocation of the fragment, guiding toward surgical indication when appropriate, reducing the risk of complications such as sub-spine impingement and femur-acetabular impingement related to conservative treatment in selected cases.

Funding/sponsorship

This study did not receive any grants.

Authors' contributions

Michele Reboli: Conceptualization, Original draft preparation, Methodology. Alessandro Aprato: Supervision. Jacopo Vittori: Original draft preparation, Supervision. Matteo Olivero: Visualization, Supervision, Francesco Bosco: Investigation, Visualization, Supervision. Alessandro Massè: Supervision.