Teres minor and quadrilateral space syndrome: A review

Abstract

The teres minor is one of four rotator cuff muscles that is involved in many shoulder pathologies. The integrity of the teres minor can be indicative of treatment success for disorders including rotator cuff tears, impingement syndrome, and quadrangular space syndrome. Quadrangular or quadrilateral space syndrome is a debilitating disorder that may require surgical intervention in chronic cases and can lead to atrophy of the teres minor. A review of the diagnostic techniques and treatment methods for disorders involving teres minor, with a focus on quadrilateral space syndrome, are presented in order to summarize the current understanding of these pathologies.

1. Introduction

The teres minor is a relatively small intrinsic shoulder muscle that acts as a stabilizer of the glenohumeral joint.¹ It is one of the four rotator cuff muscles, which is a collection of the teres minor, supraspinatus, infraspinatus, and subscapularis tendons.² When the arm is in an abducted position, the teres minor is responsible for 45% of the power of external rotation.³

Anatomically, the teres minor sits superior to the teres major and inferior to the infraspinatus. Its fibers move in an oblique orientation, with the upper bundle inserting on the greater tuberosity of the humerus and the lower bundle inserting just below the greater tuberosity. The upper fibers are continuous with the shoulder joint.^1,4 Blood supply to the teres minor is from the subscapular artery and the posterior circumflex humeral artery (PCHA). The PCHA is a branch of the axillary artery and provides blood to two thirds of the humeral head.^1,5 This artery splits into anterior and posterior branches after passing through the quadrilateral space to supply the deltoid, teres minor, and teres major.^1,5 Innervation of the teres minor is from the posterior branch of the axillary nerve. The axillary nerve originates off of the posterior cord of the brachial plexus, consisting of nerve roots C5 and C6. The nerve fibers then cross in front of and below the subscapularis muscle in order to exit through the quadrilateral space. Within the quadrilateral space, the axillary nerve travels superior to the PCHA and splits into an anterior and posterior branch. The anterior branch supplies the anterior portion of the deltoid, while the posterior branch supplies the posterior portion of the deltoid and teres minor. Together, the anterior and posterior branches supply the middle portion of the deltoid.^1,5

There are two important anatomical spaces formed by the teres minor, the quadrangular space and the triangular space. The borders of the quadrangular space include the teres minor superiorly, the teres major inferiorly, the long head of triceps medially, and the surgical neck of the humerus laterally. Important structures that pass through the quadrilateral space include the axillary nerve and the posterior circumflex humeral artery. This anatomy is especially relevant when discussing clinical implications of quadrilateral space syndrome (QSS). The borders of the triangular space include the teres minor superolaterally, teres major inferolaterally, and the long head of the triceps laterally. An important structure that passes through the triangular space is the circumflex scapular artery.^1,4

2. Clinical relevance & treatment

All of the rotator cuff muscles are liable to tearing and the teres minor is no exception. When this occurs, the most reliable treatment to ensure full recovery is surgical intervention. Relatively normal shoulder function can be regained through non-surgical treatment modalities, including physical therapy, anti-inflammatory medications, corticosteroid injections, and activity modification. However, surgery is recommended if pain is persistent and motion is not improved after exhaustive non-surgical treatment. Methods of surgical repair may include subacromial decompression and acromioplasty.^1,2 When considering surgical repair, a physician is able to assess the structural integrity of the teres minor using the Hornblower's Test. This test requires the patient to elevate the arm to 90°, flex the elbow to 90°, and externally rotate the shoulder. Pain and inability to maintain external rotation is a positive Hornblower's sign and may indicate irreparable teres minor damage. According to one study, Hornblower's sign had 100% sensitivity and 93% specificity for identification of irreparable teres minor degeneration.³ The presence of Hornblower's sign should be considered before surgical intervention, as it may help predict surgical outcome. Absence of the sign may signify that the muscle can be successfully repaired.^3,4

Another presentation involving the teres minor is impingement syndrome. This occurs when the rotator cuff tendons are compressed by the acromion of the scapula, and is thought by some to account for 44–65% of all shoulder pain.⁶ This syndrome most commonly presents after overuse with inflammation of the tendons, leading to narrowing of the space between the acromion and rotator cuff muscles. It may be treated with non-surgical techniques consisting of rest, ice, anti-inflammatory medication, and therapeutic exercises. Severe cases may be treated with corticosteroid shots when necessary, while the most extreme cases may require surgical decompression.^1,6

Perhaps the most significant clinical presentation of the teres minor is quadrilateral space syndrome (QSS). This was first described by Cahil and Palmer in 1983 as a chronic compression of the structures that run through the quadrilateral space, specifically, the axillary nerve and/or the PCHA.⁷ They described 4 distinct features of this syndrome: diffuse pain around the shoulder, paresthesias in a nondermatomal distribution, point tenderness above the quadrangular space, and a positive angiogram finding.^7,8 The most commonly cited cause of QSS is a collection of fibrous bands that run from the long head of the triceps brachii through the quadrangular space and attach to the teres major. These fibrous bands effectively decrease the available area within the quadrangular space during shoulder abduction and rotation, leading to compression of its structures. Other notable causes of QSS include muscular hypertrophy, space-occupying lesions, and anatomic variations.^5,9

There are three different presentations of QSS based on the structures that are being compressed. Neurogenic quadrangular space syndrome (nQSS) presents with compression of the axillary nerve as it travels through the space. Patients suffering from this manifestation will display tenderness, radicular pain, paresthesias, deltoid fasciculations, and atrophy due to denervation of the deltoid and teres minor. Vascular quadrangular space syndrome (vQSS) presents with a compression of the PCHA as it travels through the space. Patients suffering from this manifestation will display ischemia of those structures supplied by the PCHA, cyanosis, or possible thrombi leading to underperfusion. QSS may also present as a combined neurovascular compression of both the axillary nerve and PCHA.^5,9

Diagnosis of QSS involves a large differential diagnosis that must be thoroughly examined. This differential diagnosis includes thoracic outlet syndrome, referred pain from cervical radiculopathy, brachial plexitis, rotator cuff pathology, impingement syndrome, glenohumeral arthritis, adhesive capsulitis, glenohumeral instability, and subscapular nerve injury.⁹

Methods for diagnosing quadrangular space syndrome may include a variety of tests and procedures to rule out other diagnoses. QSS tends to affect otherwise healthy, young patients who have a history of overhead activity and may present with muscle atrophy or weakness.¹⁰ Point tenderness over the QS is almost always present in patients with QSS and may be used as a strong identifier of pathology. Radiographic imaging allows physicians to visualize any fragments creating space-occupying lesions or to rule out any fractures. A subclavian arteriogram can be utilized to assess compression of the PCHA in an abducted and laterally rotated shoulder. Bilateral upper extremity angiography may be employed to compare the patient's abnormal anatomy with their own normal anatomy on the contralateral side. An electromyogram (EMG) may be used to diagnose QSS by demonstrating altered nerve impulses. This is especially prevalent when recording impulses along a pinched axillary nerve as it sends signals to the teres minor/deltoid. Decreased amplitude of the nerve impulses is diagnostic of QSS as it displays the muscle denervation. Ultrasound imaging may show space-occupying lesions and/or swelling of the axillary nerve. Ultrasound also allows comparison of the size of the teres minor on both sides to demonstrate any inflammation. However, it should be noted that ultrasound is not effective in the detection of any fibrous bands. MRI and CT scans are more commonly ordered as they may detect muscle atrophy, fibrous bands, occlusions of the PCHA, and space-occupying lesions. Presently, the gold standard for diagnosis of QSS is a lidocaine block test. This involves injecting 5 mL of 1% lidocaine into the QS and observing for resolution of pain. If the pain is temporarily resolved, this is considered a positive test for QSS.⁹

While our understanding of QSS is growing, the current standards for treatment are not well defined. However, there is wide agreement that treatment should begin with a conservative approach. This should include non-steroidal anti-inflammatory medications, modification of activity, notably avoidance of abduction and external rotation of the shoulder, and/or physical therapy. Activity modification is often met with resistance in young active patients and may be individualized based on what the patient is willing to compromise. Physical therapy modalities may include transverse friction massages, active-release soft tissue massages, posterior rotator cuff strengthening, and scapular stabilization exercises. The conservative approach to treatment is generally attempted for at least a six month period before surgical intervention is considered.⁹

If, after a six month period, the patient displays no progress towards resolution and a lidocaine block test remains positive, surgical techniques are considered. In one study of surgical treatment of QSS, the average amount of time between the onset of symptoms and commencement of surgical treatment was 14.5 months, with a range of 6–24 months.¹⁰ Open surgical decompression is the most commonly utilized procedure to relieve QSS. In this procedure, the patient is placed in the lateral decubitus position and a longitudinal incision is made along the Langer's lines of the shoulder. The posterior border of the deltoid is then reflected to expose the quadrangular space between the teres major and teres minor.⁹ The axillary nerve and PCHA are visualized and dissected freely from compression.¹⁰ Postoperative rehabilitation is recommended after surgical intervention in order to maximize recovery. This includes home exercises and physical therapy immediately after surgery in order to prevent adhesion formation. However, it is recommended to avoid any abduction, external rotation, or hyperextension for four weeks after surgery.⁹

Surgical decompression has led to full recovery in those patients who experience no resolution from conservative therapy. In the same study previously mentioned focusing on surgical treatments of QSS, four athletes were noted to have undergone surgical decompression and all four experienced complete resolution of symptoms.¹⁰

While surgery is capable of completely restoring function, there are associated risks. As with most surgical interventions, there is a risk for damage to nerves and arteries, especially the axillary nerve and PCHA. There is also risk of infection and the possibility of incomplete symptom resolution. To date however, few complications have been reported with QSS surgery.⁹

3. Conclusion

Quadrilateral Space Syndrome is an incompletely understood pathology involving the teres minor and its related structures. While the anatomy and pathology of the syndrome are relatively agreed upon, there is little discussion related to optimal treatment and surgical intervention. As this article has discussed, non-surgical techniques are initially attempted to minimize symptoms. However, if no symptomatic relief is achieved, then surgical techniques are recommended. The most common method utilized is open surgical depression, with dissection of the compressed neurovascular structure(s). Previous studies have shown this to be an effective procedure with nearly full recovery. However, further clinical studies are needed in order to definitively describe the most effective treatment regimen.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or non-for-profit sectors.

Declaration of competing interest

There are no potential conflicts of interest.