Due to its superficial position direct trauma to the gluteal region may lead to GM injuries. The most frequent type of lesion found in this context is a muscle contusion, which is characterized by a hyperintensity of the muscle fibers on T2-weighted sequences with irregular margins. The deep muscle fibers are preferentially injured, which helps in the differential diagnosis with muscle sprains which tend to be superficial [6]. GM sprains are rare and the literature is scarce on this subject.

Gluteal region compartment syndromes have also been described following trauma or surgery. Patients usually present with buttock swelling and tenderness. Sciatica is described in about half of the patients. Gluteal compartment syndrome can be caused by intra muscular hematomas or any space-occupying mass originating in any of the gluteal compartments (Fig. 19.2). Additionally, lesions to the superior gluteal artery secondary to acetabular fracture or hip dislocation can also cause a gluteal compartment syndrome [7].

Sustained compression to the gluteal muscles can lead to compressive myopathy, probably related to ischemia [8]. This type of injury can be seen in patients with prolonged immobilization and presents as a signal hyperintensity of the muscular fibers that does not respect anatomic boundaries. It can be seen close to the bony prominences of the hip, such as adjacent to the greater trochanter, where compressive forces are maximal (Fig. 19.3). Compressive myopathy usually causes local pain and discomfort but regresses spontaneously. Compartment syndrome has been described in extensive cases [9].

The iliac crest is the most frequent site of avulsion injuries at the gluteal region. This type of lesion usually occurs in children and young adults. Although it can be secondary to acute trauma it is most commonly seen in association with micro-trauma and overuse. The anterior portion of the iliac crest adjacent to the insertion of the tensor of the fascia lata insertion and the ITB is the more frequently injured. An adapted MRI protocol with a field of view depicting the whole extension of the iliac crest is important for the diagnosis of these lesions. In acute injuries MRI shows typically edema and inflammatory changes at the tendinous insertions to the iliac crest, associated with reactive bone marrow edema-like changes to the iliac bone (Fig. 19.4). In some patients imaging findings can be discrete and a correlation with clinical findings is crucial. It is important to access the displacement of the iliac crest ossification center, for large displacements bear a worse prognosis. Sub-acute or chronic avulsions may have a bizarre appearance, with prominent bone proliferation, which should not be mistaken for bone or peri-osseous tumors. Finally, GM avulsions have been described after direct trauma but are exceedingly rare and are usually associated with other serious injuries such as GM degloving [10].

GM contractures can occur after direct trauma, repetitive gluteal injections or can be idiopathic. They are related to fibrous scarring of the muscle fibers and may lead to gait disturbances, pain, stiffness and anatomical deformity. MRI demonstrates a heterogeneous fibrous mass, with hypointense areas on T1- and T2-weighted images that may be associated with various degrees of muscle atrophy. Long standing contractures may be associated with bone remodeling at the posterior ilium, which can be identified on cross sectional imaging methods or conventional radiographs as an iliac hyperdense line parallel to the sacro-iliac joint [11]. Surgical release is the treatment of choice with very good results [12].

Morell Lavallée lesions are characterized by a sero-hematic collection at the interface between the deep lower limb fascia and the hypodermis after traumatic injuries with shear stress to the skin. This type of lesion has been termed a closed degloving injury and is frequent at the gluteal region where the vascularization of the hypodermis is particularly rich [13]. The site of the lesion is the most important diagnostic clue on MRI, which usually demonstrates a well-defined fluid collection adjacent to the fascia lata. The signal intensity of the fluid varies depending on the age of the lesion and the stage of hemoglobin degradation. One interesting finding is a rim of signal hypointensity at the periphery of the lesion related to hemosiderin deposits, which is particularly evocative in association with a hypersignal intensity of the fluid in T1-weighted images (methemoglobin) (Fig. 19.5). Morell Lavallée lesions regress spontaneously in about 50 % of cases. Chronic lesions can be heterogenous with internal calcifications. Care should be taken not to confound these lesions with soft tissue masses of other origins, and an active search for an ancient trauma is warranted [14].

The treatment of Morel Lavallée effusions can be conservative or surgical. Outcomes are better with surgical treatment but there is no consensus in the literature on which is the best technique. Surgical interventions with dead space closure techniques are preferred. In patients with chronic lesions with a well formed fibrous capsule around the lesion a complete resection of the lesion can be attempted. Morel Lavallée lesions can recur and serious surgical complications have been reported (infection, skin necrosis, delayed wound closure).

Snapping hip syndrome (coxa saltans) is characterized by a painful audible or palpable snap or popping at the peri-trochanteric region during hip flexion-extension. Snapping hip syndrome has multiple possible etiologies both intra and extra articular. Lateral snapping is probably the most frequent type [15]. The passage of the ITB and the myotendinous junction of the GM over the greater trochanter creates this phenomenon, which can be completely asymptomatic or can cause pain, pseudo-instability and locking sensations leading to significant disability [16].