Subspine Impingement (SSI)

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Introduction

Subspine impingement is an extra-articular hip impingement syndrome that typically occurs due to abnormal contact between an enlarged or maloriented anterior inferior iliac spine (AIIS) and the distal anterior femoral neck during straight flexion of the hip.

In younger, non-arthritic individuals, intra-articular causes such as femoral acetabular impingement (FAI) and acetabular labral tears have become well-known sources of hip pain and impingement. However, growing experience with hip arthroscopy and an improved understanding of FAI has led to the recognition of many previously undiagnosed extra-articular hip impingement syndromes. These include ischiofemoral impingement, subspine impingement, iliopsoas impingement, and pectineofoveal impingement.

Clinically Relevant Anatomy

anterior inferior iliac spine

The space below the anterior inferior iliac spine (AIIS) and the acetabular rim, known as the "subspine space", serves as a recoil area for anterior hip soft tissue during hip flexion due to its concave and smooth nature.

The anterior inferior iliac spine (AIIS) is a bony eminence located on the anterior and inferior aspect of the ilium, which is usually located proximal to the acetabular rim. The AIIS begins as a separate apophysis and is believed to begin ossification between the ages of 13 to 14 years and fuse with the ilium between the ages of 16 and 18 years. The upper part of the AIIS is where the straight head of the rectus femoris muscle originates.

Mechanism of Injury / Pathological Process

Subspinous impingement can result from various causes. Avulsion fractures of apophyses and spines of the pelvis are usually considered rare and primarily seen in adolescent athletes. This typically occurs due to a sudden, forceful contraction of the attached myotendinous complex.

These apophyseal avulsions are generally associated with the timing of ossification and fusion to the corresponding location on the pelvis. During the acute injury, the athlete may experience sudden pain in the area of the avulsion, as well as loss of function of the affected muscle. These fractures may not be immediately visible on a radiograph. Once the avulsion has occurred, ossification and closure of the physis may cause a distal extension of the AIIS.

Similarly, excessive muscular activity of the rectus femoris during repetitive knee flexion with hip extension, results in an avulsion injury of the AIIS. When healing occurs, the apophysis may be displaced downward, leading to a malunion. This often results in an enlarged bony protrusion at the AIIS that abnormally abuts the femoral neck.

Clinical Presentation

Subspine impingement as one of the extra-articular conditions presents clinical features similar to intra-articular pathologies. Those features include:

• Anterior hip or groin pain is aggravated by active hip flexion and activities such as running or kicking.
• Tenderness on AIIS region.
• Loss of range of movement, sometimes associated with a grinding sensation.
• Patients may report little or no relief with hip flexion after intraarticular injection of a local anesthetic during physical examination.

In the presence of intra-articular femoroacetabular impingement (FAI), if there is minimal relief of groin and/or anterior pain during straight hip flexion after an intra-articular injection of a local anesthetic, it implies the coexistence of extraarticular subspine impingement. It is important to note that patients who have persistent symptoms following surgery for femoroacetabular impingement (FAI) should be evaluated for subspine impingement.

Diagnostic Procedures

The X-ray shows a large bony spur extending inferiorly from the right AIIS. Contact with the greater tuberosity during the frog-leg view.

AIIS can be more accurately visualized using CT or MRI scans, and it often appears as a pointed shape. In certain cases, where calcified deposits are found within the proximal portion of the straight head of the rectus femoris tendon, and impingement cysts are found on the distal femoral neck. To accurately diagnose this condition from a biomechanical perspective, it is beneficial to use three-dimensional CT scans. These scans allow for pre-operative dynamic simulations (motion analysis) to help identify direct bone impingement between the AIIS deformity and the femoral neck during maximum hip flexion.

On radiography, an avulsion fracture of the AIIS can be immediately detected after the injury, but the subtle findings are commonly missed, according to recent literature reports.

Radiographs may also reveal a prominent AIIS deformity extending to the level of the anterior-superior acetabular rim and show evidence of sclerosis at the AIIS (inferior) and distal femoral neck junction. Researchers have also associated the ROM limits with each SSI classification morphology based on the distance between AIIS and the anterosuperior acetabular rim.

Hetsroni et al. classified the morphology of the anterior inferior iliac spine (AIIS) and subspine in relation to the position of the acetabular rim. They suggested subspine variants extending further distally may be linked to extra-articular hip impingement.

Subspine ImpingementClassification

Classification	Description	Range of Motion (Mean) Limitations
Type I	There is a smooth ilium wall between the AIIS and the acetabular rim.	Hip flexion: 120 degrees Hip internal rotation (in 90 degrees of flexion): 21 degrees
Type II	The AIIS is extended to the level of the rim.	Hip flexion: 107 degrees Hip internal rotation (in 90 degrees of flexion): 11 degrees
Type III	Defined as the AIIS extending distally to the acetabular rim.	Hip flexion: 93 degrees Hip internal rotation (in 90 degrees of flexion): 8 degrees

Anterior Inferior Iliac Spine (AIIS) Classification

They reported that Type II and III variants are associated with a decrease in hip flexion and internal rotation.

Magnetic resonance imaging (MRI) can also be used to reveal abnormalities in the anterior inferior iliac spine (AIIS) or subspine area. This area should always be assessed in hip MRI exams, along with the cartilage, labrum, capsule, and surrounding soft tissues like tendons, muscles, and bursae.

The use of advanced dynamic imaging, such as dynamic ultrasound and 3D imaging, can help assess subspine impingement, in addition to radiography, CT, and MRI. A computer software program called A2 Surgical allows for the execution of unique motion paths for each painful hip using CT-based dynamic modeling. This facilitates the reproduction of areas of bone-to-bone contact, providing valuable insights for preoperative planning and tailoring treatment for each patient.

Outcome Measures

Subspine Impingement Test

Hip Outcome Score (HOS)

Modified Harris Hip Score

International Hip Outcome Tool (iHOT)

Management / Interventions

Non-surgical treatments such as activity modification and physiotherapy, therapeutic anesthetic, and steroid injections may be prescribed initially, but their effectiveness has not been extensively studied. If conservative management fails, AIIS decompression is often performed using standard anterolateral and mid-anterior hip arthroscopy portals. Additionally, an arthroscopic procedure may be performed to address any intra-articular pathology.

Treatment: Arthroscopic Approach

The decompression of a symptomatic AIIS prominence has been previously described through the Smith-Petersen approach, either as a single procedure or following arthroscopic exploration of the joint. However, recently, arthroscopic decompression has gained popularity due to several studies demonstrating satisfactory short-term results. It is considered safe, with no reported complications. The arthroscopic approach allows addressing both intra- and extraarticular causes of hip pain in a single procedure. Preoperative planning with 3D CT reconstruction views is crucial to assess the AIIS prominence extension. Advanced arthroscopic techniques have improved access and visualization for evaluating and treating hip joint abnormalities.

Treatment: Surgical Approach

The patient's position depends on the surgeon's preference. For both supine and lateral positions, the feet should be well padded, and a large perineal pad should be used to optimize distraction and minimize complications. Adequate distraction results in approximately 10mm of joint space opening. Traction is gently applied to the leg with specific angles. An extra-wide perineal post is used to prevent neurologic complications such as pudendal nerve injury. Most surgeons use two or three portals, with the anterolateral and true anterior portals being the most widely accepted.

During the procedure, the capsular cut should be limited to the area of labral injury to prevent postoperative capsular instability. The AIIS abnormality and associated capsular-sided labral damage are carefully identified based on preoperative imaging and intraoperative visualization. Following this, subspine decompression and resection of the prominent AIIS can be performed using arthroscopic and fluoroscopic imaging confirmation.

However, over-resection proximally should be avoided to prevent endangering the insertion of the direct head of the rectus femoris. If the prominence of the AIIS extends medially, decompression of the medial border should be performed, especially if there are clinical, radiological, and intraoperative findings of symptomatic psoas impingement against the AIIS. Furthermore, the post-operative administration of non-steroidal anti-inflammatory agents for the first three to four weeks after surgery is advised to prevent heterotopic ossification.

Rehabilitation

After surgery, a postoperative hip brace (Bledsoe Post-Op Hip Brace) and anti-rotational boots are applied to protect the operative site and reduce pain. It is generally recommended to protect weight-bearing with crutches for 4 to 8 weeks and perform a range of motion exercises until basic muscle strength is regained. For example, when performing microfracture, the patient is kept with 20 lbs of weight bearing for at least 7 weeks. This protects the capsular sutures by limiting abduction to 0° to 45° and hip flexion to 0° to 90°, while external rotation and extension are prohibited for the first 3 weeks. This is achieved using the brace.

The rehabilitation period consists of 3 phases that should be adjusted appropriately per each patient’s needs. The first phase lasts 4 to 6 weeks, where mainly passive exercises are performed. Passive rotational movement is initiated immediately after surgery to prevent adhesion formation. Stationary bike exercise and continuous passive motion machine are useful for that purpose. The “strengthening phase” follows during the next 6 to 12 weeks until we finally proceed to the final phase, where sport-specific exercises are emphasized.

These 3 phases usually overlap and vary in duration because of the patient-specific approach. Pain management and patient compliance are important factors to consider before proceeding to the next phase. Progressive functional and sport-specific rehabilitation helps the patient return faster to daily and exercise routines. However, the final “return to sport” decision is based on objective (functional tests) and subjective (physician and patient co-decision) factors.

Resources

Femoral Acetabular Impingement(FAI)