Impingement / Instability Differentiation: Difference between revisions

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Introduction[edit | edit source]

The differentiation between shoulder impingement and instability refers to the process of distinguishing between these two conditions based on their symptoms, physical examination findings, and diagnostic tests. Both conditions affect the shoulder but have different underlying causes, presentations, and treatment approaches.

Purpose[edit | edit source]

The purpose of this article is to help healthcare providers differentiate between shoulder impingement and instability by utilizing the most current literature.

Shoulder Impingement[edit | edit source]

  • Definition: Shoulder impingement occurs when the tendons of the rotator cuff muscles are compressed or "impinged" between the bones of the shoulder joint, particularly under the acromion .
  • Symptoms: Pain when lifting the arm, especially overhead; pain that worsens at night; limited range of motion; weakness in the shoulder .
  • Diagnosis: Diagnosed through special tests such as the Hawkins-Kennedy, Neer, and Painful Arc tests, which assess pain and range of motion[1] [2] .
  • Cause: Compression of tendons and soft tissues .

Shoulder Instability[edit | edit source]

  • Definition: Shoulder instability occurs when the shoulder joint is loose and slides around too much in the socket, which can lead to partial or complete dislocation .
  • Types: Instability can be anterior (front), posterior (back), or multidirectional .
  • Symptoms: Sensation of the shoulder slipping out of place, pain during specific movements, recurrent dislocations, weakness, and a feeling of looseness in the shoulder .
  • Diagnosis: Diagnosed through special tests such as the Apprehension Test, Relocation Test, and Anterior Drawer Test, which assess the stability and integrity of the shoulder joint  .
  • Cause:  Excessive movement and lack of stability in the joint .

Risk Factors and Contributing Factors[edit | edit source]

Shoulder Impingement[edit | edit source]

  • Repetitive Overhead Activities: Athletes involved in sports such as swimming, tennis, and baseball, as well as individuals with occupations requiring repetitive overhead motions, are at higher risk for shoulder impingement. These activities can cause chronic stress and inflammation in the rotator cuff tendons, leading to impingemen[3]t .
  • Trauma: Acute trauma, such as falls or direct blows to the shoulder, can lead to structural changes or inflammation, resulting in impingement. This is often seen in contact sports or accidents[4] .
  • Age-Related Changes: Degenerative changes in the shoulder joint, such as bone spurs or arthritis, are more common in older adults. These changes can narrow the subacromial space, leading to impingement [5] .
  • Anatomical Variations: Certain anatomical features, such as a curved or hooked acromion, can predispose individuals to impingement by reducing the available space for tendon movement[6] .

Shoulder Instability[edit | edit source]

  • Repetitive Overhead Activities: Similar to impingement, repetitive overhead activities can also contribute to shoulder instability, particularly in athletes. These activities can cause microtrauma to the shoulder ligaments and capsule, leading to laxity and instability [7] .
  • Trauma: Traumatic events, such as dislocations or subluxations, are significant contributors to shoulder instability. These events can stretch or tear the ligaments and capsule of the shoulder, resulting in a loose joint [8].
  • Age-Related Changes: While instability is more common in younger individuals, particularly athletes, age-related changes can also play a role. In older adults, previous injuries or chronic conditions can lead to instability[9] .
  • Anatomical Variations: Anatomical factors such as a shallow glenoid cavity or loose joint capsule can predispose individuals to instability. Genetic factors may also play a role in the inherent laxity of the shoulder joint[10] .

Imaging and Diagnostic Tools[edit | edit source]

  • X-rays: X-rays are often the first imaging modality used for shoulder assessments. They help visualize bone structures and can identify bony abnormalities, such as spurs and fractures, which may contribute to impingement. X-rays can also detect bone lesions, such as Hill-Sachs or Bankart lesions, indicative of shoulder instability .[11]
  • Magnetic Resonance Imaging (MRI): MRI is highly effective for visualizing both bone and soft tissues. It is particularly useful for detecting rotator cuff tears, labral tears, and other soft tissue injuries that are not visible on X-rays. MRI can also provide detailed images of the bony structures and help in assessing the extent of tendon damage in impingement and the degree of instability in the shoulder joint. MRI is considered the gold standard for evaluating soft-tissue injuries in the unstable shoulder .
  • Magnetic Resonance Arthrography (MRA): MRA involves injecting a contrast dye into the shoulder joint before performing an MRI. This technique enhances the visibility of the joint structures, making it particularly useful for diagnosing labral and capsular injuries. MRA has shown high sensitivity (86-91%) and specificity (86-98%) for detecting capsulo-ligamentous complex injuries, making it a preferred method for evaluating shoulder instability .
  • Ultrasound: High-resolution ultrasound is a reliable and non-invasive technique for imaging the rotator cuff, bursae, and long head of the biceps muscle. It is particularly effective in detecting rotator cuff tears and assessing tendon integrity. Ultrasound is operator-dependent but provides dynamic imaging capabilities, which can be useful for diagnosing impingement by visualizing the shoulder structures in motion[12] .
  • Computed Tomography (CT) and CT Arthrography: CT scans offer detailed images of the bone and are useful in cases where MRI is contraindicated. CT arthrography, which involves injecting a contrast dye into the joint, provides detailed images of the bone and soft tissue structures. It is especially beneficial for visualizing small bone fractures and labral tears[13] [14].

Psychometric properties[edit | edit source]

Impingement[edit | edit source]

Individual special tests for sub-acromial impingement have varying psychometric properties. For this reason, it is best to utilize special test clusters in order to diagnose this injury. Two special test clusters are provided below that can be used in order to diagnose sub-acromial impingement[15].


Sensitivity Specificity Accuracy% PPV% NPV%
Hawkins-Kennedy .92 .25 72.8 75.2 56.2
Neers .89 .31 72.0 75.9 52.3
Horizontal-adduction .82 .28 66.4 73.7 38.4
Speed .69 .56 64.8 79.2 41.6
Yergason .37 .86 51.2 86.8 35.6
Painful arc .33 .81 46.4 80.5 32.5
Drop arm .08 .97 33.6 87.5 29.9






A total of 5 positive special tests is indicative for subacromial impingement[15].

Positive tests Sensitivity Specificity Accuracy% PPV% NPV%
All positive .04 .97 31.2 80.0 29.1
≥6 positive .30 .89 47.2 87.0 34.0
≥5 positive .38 .86 52.0 87.1 36.0
≥4 positive .70 .67 68.8 83.7 47.0
≥3 positive .84 .44 72.8 72.8 44.4

Accuracy: The percentage of time that the result of the test reflects the true nature of the condition

A second cluster utilizes a slightly different set of special tests[16].


Sensitivity Specificity Positive LR Negative LR
Hawkins-Kennedy .63 .62 1.63 .61
Neers .81 .54 1.17 .35
Painful arc .75 .67 2.25 .38
Empty can .50 .87 3.90 .57
External rotation resistance .56 .87 4.39 .50

Any combination of 3 or more positive tests are indicative of sub-acromial impingement[16]


Sensitivity Specificity Positive LR Negative LR
3+ positive tests .75 .74 2.93 .34

Instability[edit | edit source]

Test Sensitivity Specificity +LR -LR
Apprehension test .72 .96 20.2 .29
Relocation test .81 .92 10.4 .20
Anterior drawer test .53 .85 3.57 .56

Instability can be categorized in a few different ways, including anterior, posterior and multidirectional instability. The 3 tests above focus on the diagnosis for anterior shoulder instability[17].

Positive tests Sensitivity Specificity +LR -LR
Apprehension, relocation and anterior drawer tests .48 .99 39.1 .52
Apprehension and relocation tests .81 .98 39.7 .19
Relocation and anterior drawer tests .48 .96 18.0 .54
Apprehension and anterior drawer tests .46 .98 23.1 .55


The above table represents a cluster to help rule in favor of an anterior shoulder instability diagnosis[17]. With positive results of any 2 tests or more, you can be fairly confident ruling in favor of anterior shoulder instability. It should be noted that the apprehension test is only positive in the presence of apprehension and the relocation test is considered positive with the relief of apprehension. In addition, the anterior drawer test is positive when symptoms of instability are reproduced.

The tests below[18][19] are additional diagnostic tests for instability which may assist you in your examination, however they are not a part of the above mentioned cluster.


Sensitivity Specificity +LR -LR
Anterior release test .92 .89 8.6 .09



Sensitivity Specificity +LR -LR
Sulcus Sign .17 .93 2.43 .89


+LR: describes how much the odds of impingement increase with a positive test
-LR: describes how much the odds of impingement decrease with a negative test

References[edit | edit source]

  1. ↑ Calis M, Akgun K, Birtane M, Karacan I, Calis H, Tuzun F. Diagnostic values of clinical diagnostic tests in subacromial impingement syndrome. Ann Rheum Dis. 2000;59(1):44-47.
  2. ↑ Park HB, Yokota A, Gill HS, El Rassi G, McFarland EG. Diagnostic accuracy of clinical tests for the different degrees of subacromial impingement syndrome. J Bone Joint Surg Am. 2005;87(7):1446-1455.
  3. ↑ Braman JP, Zhao KD, Lawrence RL, Harrison AK, Ludewig PM. Shoulder impingement revisited: evolution of diagnostic understanding in orthopedic surgery and physical therapy. *Med Biol Eng Comput*. 2013;51(9):963-71.
  4. ↑ Lewis J. Rotator cuff related shoulder pain: assessment, management and uncertainties. *Man Ther*. 2016;23:57-68.
  5. ↑ Neer CS. Impingement lesions. *Clin Orthop Relat Res*. 1983;(173):70-7.
  6. ↑ Bigliani LU, Levine WN. Subacromial impingement syndrome. *J Bone Joint Surg Am*. 1997;79(12):1854-68.
  7. ↑ Owens BD, Duffey ML, Nelson BJ, DeBerardino TM, Taylor DC, Mountcastle SB. The incidence and characteristics of shoulder instability at the United States Military Academy. *Am J Sports Med*. 2013;35(7):1168-73.
  8. ↑ Arciero RA, Wheeler JH, Ryan JB, McBride JT. Arthroscopic Bankart repair versus nonoperative treatment for acute, initial anterior shoulder dislocations. *Am J Sports Med*. 1994;22(5):589-94.
  9. ↑ Warner JJ, Micheli LJ, Arslanian LE, Kennedy J, Kennedy R. Patterns of flexibility, laxity, and strength in normal shoulders and shoulders with instability and impingement. *Am J Sports Med*. 1990;18(4):366-75.
  10. ↑ Boileau P, Villalba M, Hery JY, Balg F, Ahrens P, Neyton L. Risk factors for recurrence of shoulder instability after arthroscopic Bankart repair. *J Bone Joint Surg Am*. 2005;87(8):1755-63.
  11. ↑ Alzubaidi L, Salhi A, Fadhel MA, Bai J, Hollman F, Italia K, Pareyon R, Albahri AS, Ouyang C, Santamaría J, Cutbush K, Gupta A, Abbosh A, Gu Y. Trustworthy deep learning framework for the detection of abnormalities in X-ray shoulder images. PLoS One. 2024 Mar 11;19(3):e0299545. doi: 10.1371/journal.pone.0299545.
  12. ↑ Daenen B, Houben G, Bauduin E, Lu KV, Meulemans JL. Ultrasound of the shoulder. JBR–BTR. 2007;90:325-37.
  13. ↑ Rainey J, Hameed D, Sodhi N, Malkani AL, Mont MA. Use of Computed Tomography for Shoulder Arthroplasty: A Systematic Review. J Orthop. 2024 May 13. doi: 10.1016/j.jor.2024.05.007.
  14. ↑ Feuerriegel GC, Weiss K, Van AT, Leonhardt Y, Neumann J, Gassert FT, Haas Y, Schwarz M, Makowski MR, Woertler K, Karampinos DC, Gersing AS. Deep-learning-based image quality enhancement of CT-like MR imaging in patients with suspected traumatic shoulder injury. Eur J Radiol. 2024 Jan;170:111246.
  15. ↑ 15.0 15.1 Caliş M, Akgün K, Birtane M, Karacan I, Caliş H, Tüzün F. Diagnostic values of clinical diagnostic tests in subacromial impingement syndrome. Ann Rheum Dis. 2000 Jan;59(1):44-7.
  16. ↑ 16.0 16.1 Michener LA, Walsworth MK, Doukas WC, Murphy KP. Reliability and diagnostic accuracy of 5 physical examination tests and combination of tests for subacromial impingement. Arch Phys Med Rehabil. 2009 Nov;90(11):1898-903.
  17. ↑ 17.0 17.1 Farber AJ, Castillo R, Clough, M, et. al. Clinical assessment of three common tests for traumatic anterior shoulder instability. J Bone Joint Surg Am. 2006;88:1467-1474.
  18. ↑ Gross ML & Distefano MC. Anterior release test. A new test for occult shoulder instability. Clin Orthop Relat Res. 1997;339:105-108.
  19. ↑ Nakagawa S, Yoneda M, Hayashida K, et. al. Forced shoulder abduction and elbow flexion test: a new simple clinical test to detect superior labral injury in the throwing shoulder. J Arthr Relat Surg. 2005;21:1290-1295.
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