Gluteus Minimus Anatomy and Tear Patterns

Clinical History: A 70 year-old male presents with chronic lateral hip pain that recently worsened after slipping. Frontal and frog-leg radiographic views of the left hip (Fig 1a, 1b), contiguous axial fat-suppressed proton density-weighted images of the left hip (Fig 1c), and a sagittal proton density-weighted image of the left hip showing the greater trochanter (Fig 1d) are provided. What are the findings? What is your diagnosis?

 

Introduction

Greater trochanteric pain syndrome (GTPS) is a common cause of lateral hip pain in adults. Previously thought to be related to trochanteric bursitis, we now know that tendinopathy or tearing of the gluteus medius (Gmed) and gluteus minimus (Gmin) are a leading cause of GTPS.¹ Patients with lateral hip pain can present acutely, but more commonly with chronic longstanding symptoms of dull/insidious pain along the proximal lateral hip that is worsened when lying on the affected side or with walking/climbing stairs.² On physical exam, patients with GTPS have tenderness to palpation along the anterior aspect of the greater trochanter and reproducible pain with weakness on resisted abduction of the hip.³ Peak prevalence of GTPS and gluteal tendinopathy can be found in female patients over 30 years of age ⁴ Over the past 15 years MRI has emerged as the primary imaging tool for interrogating the gluteal muscles and their tendinous insertions.⁵ This Radsource Web Clinic will focus on gluteus minimus pathology with a review of the relevant anatomy, the MRI appearance of common tear patterns, secondary signs of Gmin pathology, two common pitfalls during MRI interpretation, and a brief review of treatment options.

Relevant Anatomy and Function

The gluteus minimus muscle origin has a broad fan shape, arising from the anterior aspect of the external iliac fossa with a wide muscular attachment. The insertion has two distinct components, a main tendon attaching to the anterior facet of the greater trochanter and a thinner short muscular or capsular attachment along the ventral superior capsule of the hip joint.  Functionally, the Gmin main tendon acts as a hip flexor, an abductor and an internal/external rotator depending on the position of the femur. The capsular attachment helps to stabilize the head of the femur within the acetabulum by tightening the capsule and applying compressive pressure to the femoral head.^6,7 The distal fibers of the main GMin tendon are contiguous with the origin of the vastus lateralis (Vlat). (Fig 3a-d)

The musculotendinous anatomy at the greater trochanter is complex with many different tendons, a multifaceted boney trochanter and several different bursae.  A complete description of the anatomy is beyond the scope of this Web Clinic, but it is important to understand the greater trochanter has 4 distinct facets: the anterior facet serves as the attachment site for the Gmin tendon, and the Gmed tendon attaches to the superoposterior (SP) facet and the lateral facet (LF).⁸ Knowledge of both the Gmed and Gmin tendons is required to accurately interpret MRI of the hip. The gluteus minimus and gluteus medius tendons have been referred to as the “rotator cuff of the hip.”⁹ However, there is a key anatomic difference between the rotator cuff tendons of the shoulder and the gluteal tendons of the hip. In the shoulder, the supraspinatus and infraspinatus tendons are relatively uniform and similar in thickness, with a large area of overlap, and they form a complete arch shaped covering of the humeral head (Fig 4a). In contrast, the Gmin and Gmed tendons are more segmented with a focal bare spot along the lateral facet that can be misinterpreted as a tear (Fig 4b and Fig 5). When evaluating the severity of abductor tears, it is important to not overestimate the size of the tear/tendon detachment by mistaking the normal bare spot as a defect in the insertion of the gluteus medius. This is especially relevant as most Gmed tendon tears begin anteriorly along the lateral facet attachment near the region of the bare spot.¹⁰

Common Gluteus Minimus Pathology

GTPS due to gluteal tendinopathy is a common cause of lateral hip pain; below we will focus on the MRI findings pertaining to the gluteus minimus.

Tendinosis and peritendon edema

The most common pathology of the gluteus minimus tendon seen on MRI is degenerative tendinosis with mild adjacent peritrochanteric edema (Fig 7a-c).

Partial tearing – Chronic

Many patients over 60 years of age have some degree of age-related degenerative tendinosis involving the gluteus minimus tendon with a clear increased prevalence in female patients.  The pathology is thought to be related to repetitive tensile shearing and compressive stress along the deep fibers of the insertional tendon which can ultimately progress towards partial or complete tearing¹¹ (Fig8a,b).

Combined degenerative partial tears of the Gmin and Gmed tendons

Combined degenerative partial tearing of the Gmin and Gmed tendons is a common tear pattern with the tearing typically involving the deep posterior fibers of the Gmin and the anterior Gmed fibers along the lateral facet attachment. (Fig 9a,b)

Complete gluteus minimus insertional tear without retraction

The final common gluteus minimus tear pattern is a complete tear without retraction.  Our experience at Radsource anecdotally indicates this tear pattern is common but underrecognized, often described as partial tearing or severe tendinosis due to the lack of retraction.  White et al. published a similar opinion with supporting data in a 2021 study of gluteus minimus tears.¹² In their study, the vast majority (38/40) of complete tears of the distal insertional gluteus minimus were not retracted. This lack of retraction is another key difference between the rotator cuff of the shoulder and the gluteus minimus tendon of the hip and can be explained anatomically. The distal fibers of the main gluteus minimus tendon are continuous with the proximal vastus lateralis which serves to tether the gluteus minimus in place, limiting retraction. It is also theorized that the small capsular attachment of the gluteus minimus tendon helps to anchor and prevent Gmin retraction (Fig 2c,d and Fig 10a,b). On axial images, the completely torn Gmin tendon often shifts anterior and medial to its native position, a clue the tendon tear is complete (Fig 11a-c).

The presence of peritrochanteric edema and gluteal tendinopathy is almost always present in patients with GTPS; however, these MRI findings are not always symptomatic.  Blakenbaker et al. demonstrated in a series of 256 hips, all patients with GTPS had peritrochanteric T2 signal abnormalities and 88% had gluteal tendinopathy, but up to 50% of the hips without symptoms of GTPS had similar findings.¹³ Therefore, it is important to recognize that lateral hip pain has a broad differential diagnosis including spinal abnormalities, bursitis, arthritis, and femoral nerve irritation.¹⁴ Identifying gluteal tendinopathy should not terminate the search for other causes of lateral hip pain.

Partial tearing – Acute/Subacute

Acute gluteus minimus tearing is less common and can be isolated muscle injury or involve both the muscle and insertional tendon. (Figs 13a,b)

Isolated Gluteus Minimus muscle injury

Rarely, trauma can cause isolated muscle injury to the gluteus minimus (Fig 14).

Enthesopathy is commonly associated with Gluteal tendon tear.

Several studies have shown that trochanteric enthesopathy or bone spurs are associated with tendinopathy of the gluteal tendons. In a study by Steinert et al., trochanteric enthesophytes of greater than or equal to 2mm had a positive predictive value of >90% for underlying gluteal tendinopathy.¹⁵ In our index case, Fig 2b, enthesophytes are present along the anterior facet of the greater trochanter. Below, Fig 15a,b demonstrates large enthesophytes on radiographs and underlying gluteal tendinopathy. Radiographically, the enthesophytes along the anterior facet are best seen on the frog-leg lateral view.

Fat stripe sign

Similar to other skeletal muscles, atrophy of the gluteal muscles can occur secondary to tendon tears. This atrophy can serve as a diagnostic clue for underlying tendinopathy. Atrophic gluteal muscles combined with increased intergluteal fatty tissue has been termed the “intergluteal fat stripe” sign in a study published by Beicker et al.¹⁶ Of note, atrophy of the anterior gluteus minimus is relatively non-specific, found in symptomatic and asymptomatic patients, while fatty atrophy of the posterior gluteus minimus and gluteus medius muscles tends to be more suggestive of a symptomatic condition¹⁷(Fig 16a,b).

Treatment options

Treatment for isolated gluteus minimus pathology is typically conservative.  Surgery is uncommon and reserved for cases that do not respond to lifestyle modifications, time for healing, physical therapy and/or PRP injections. Rarely is an isolated gluteus minimus tear treated surgically if there isn’t an associated gluteus medius tendon tear. Part of the reason for this conservative treatment strategy is dictated by the anatomy of the Gmed muscle. As we saw back in figures 3c and 6a, the muscle of the Gmed covers the majority of the Gmin insertion. The Gmed muscle effectively blocks access to the Gmin tendon. If a patient is undergoing surgery for a gluteus medius tear, the gluteus minimus tear will often be addressed at the same time.  Open or endoscopic approaches are used with similar success and recovery periods.  The degenerated tendons are often reattached to the greater trochanter using tendon-bone anchors. Recovery from gluteal surgery is often long and challenging, with expected recovery periods of greater than 6 months and extensive physical therapy.

Summary and Key Learning Points

1. Gluteal tendinopathy is almost always present in patients with GTPS; but gluteal tendinopathy is also commonly present in patients without lateral hip pain. It is important to evaluate for other causes of lateral hip pain on MRI even if gluteal tendinopathy is present.
2. The bare/bald spot along the superior aspect of the lateral trochanteric facet between the insertion of the gluteus minimus tendon and the anterior fibers of the gluteus medius tendon insertion should not be mistaken for a tear. This distinction can be made difficult in cases with peritrochanteric edema and bursitis. Understanding the anatomy of the insertional tendons will help avoid this pitfall.
3. The gluteus minimus tendon will commonly tear/strip completely off the anterior facet of the greater trochanter but will not retract due to distal tethering by the vastus lateralis. A completely torn gluteus minimus tendon will often shift anteriorly and medially relative to the anterior facet and the contralateral side can often be used as a reference to compare.
4. Enthesophytes measuring greater than 2mm on radiographs or MRI are commonly associated with gluteal tendinopathy.
5. Atrophy of the anterior gluteus minimus muscle is a nonspecific finding, while atrophy of the posterior gluteus minimus and gluteus medius muscles is more commonly associated with symptomatic gluteal pathology.

References