Bilateral ultrasound findings in patients with unilateral subacromial pain syndrome

ABSTRACT

Background

Subacromial pain syndrome is a common musculoskeletal shoulder problem. The accuracy of clinical tests is low and techniques such as ultrasound and magnetic resonance imaging have been added to set up a diagnosis. Previous researchers have usually only examined the symptomatic shoulder. However, there might be similar findings in the asymptomatic shoulder.

Objective

The aim of the present study was to investigate the prevalence of structural abnormalities of both shoulders in patients with clinically diagnosed unilateral subacromial pain syndrome using diagnostic ultrasound.

Methods

Bilateral ultrasound examinations were performed in 115 consecutive patients, 54 men and 61 women. The patients were recruited from primary care centers in the area of Stockholm, Sweden.

Results

Abnormal ultrasound findings were found in both shoulders and increased with age (p = .0004). Bursitis was the most common ultrasound finding and significantly more prevalent in the symptomatic shoulder compared to the asymptomatic shoulder (90%:74%; p = .0021), though 73% of the bursitis were bilateral. Supraspinatus is the most vulnerable tendon of the rotator cuff in both shoulders. In terms of partial-thickness tears and tendinosis, the tendon was affected more commonly in the symptomatic shoulder compared to the asymptomatic shoulder (30%:14%: 14%; p = .0026) and (24%:10%; p = .0054), respectively, but for calcification no significant difference between the shoulders was found (18%:12%; 0.1988).

Conclusion

Ultrasound detected shoulder abnormalities were present in both symptomatic and asymptomatic shoulders. Bursitis and partial-thickness tears were more common in the affected shoulder when compared to the unaffected shoulder. Ultrasound as well as clinical examination findings and patient´s history should be taken into consideration when diagnosing patients with subacromial pain syndrome.

KEYWORDS:

• Imaging techniques
• physiotherapy
• shoulder pain
• ultrasonography

INTRODUCTION

There is still no consensus about the most appropriate terminology when describing disorders of the rotator cuff (Schellingerhout, Verhagen, Thomas, and Koes, 2008). In the present article, we use the term subacromial pain syndrome (SAPS), which is a combination of symptoms and findings, although the pathogenesis still is unknown. SAPS is one of the most common musculoskeletal conditions in patients seeking primary care (Urwin et al., 1998). The etiology is known to be multi-factorial (Seitz et al., 2011) and it includes several rotator cuff pathologies such as increased tendon thickness (e.g. tendinopathies), bursitis, and partial (PTTs´), or full-thickness tears (FTTs´), affecting the rotator cuff or the long head of the biceps tendon (Mitchell, Adebajo, Hay, and Carr, 2005). The pathogenesis of SAPS is still unknown. However, increased age, overload, genetic vulnerability and metabolic disorders such as diabetes mellitus (Abate, Schiavone, Salini, and Andia, 2013) and smoking (Carbone et al., 2012) have been suggested to be risk factors. Also, dysfunction of the scapula or the rotator cuff (Kibler et al., 2013), capsular tightness (Tyler, Nicholas, Roy, and Gleim, 2000) and a poor posture (Lewis, Wright, and Green, 2005) may play a role.

SAPS is a clinical diagnosis and a combination of different tests for physical examination have been suggested (Michener, Walsworth, Doukas, and Murphy, 2009). Due to low accuracy of the clinical tests (May et al., 2010), diagnostic ultrasound (US) has become a popular tool to identify structural changes of the shoulder joint. Diagnostic US has been shown to be equally useful for magnetic resonance imaging (MRI) regarding rotator cuff pathology (Chauhan, Ahluwalia, Sharma, and Thakur, 2016; Vlychou et al., 2009). An increasing number of medical care clinics and sports injury clinics therefore evaluate their patients with US examinations instead of MRI. The US is cost-effective, allows comparisons between both shoulders, can be dynamically executed and is often appreciated by the patients because of its convenience (Naredo et al., 2002).

Detailed descriptions of abnormalities from both shoulders in patients clinically diagnosed with unilateral SAPS are rare. When bilateral comparison with US has been made, the focus has often been on one or two variables, such as PTTs´ or FTTs´ or bursitis, without giving a more detailed presentation of other possible anomalies (Tsai et al., 2007; Yamaguchi et al., 2006). The samples are often found through mass-screening (Yamamoto et al., 2010), or participants with a defined occupation or sport (Guzowski et al., 2019) or after rotator cuff repair (Ro et al., 2015). This study investigates patients with unilateral pain. In a recent MRI study, a high prevalence of abnormalities in both shoulders was shown in patients with self-reported unilateral shoulder pain (Barreto et al., 2019). In a pilot study, detailed US findings of both shoulders in 20 clinically diagnosed SAPS patients was presented (Lewis et al., 2009). However, to our knowledge, a larger detailed US investigation of both shoulders on clinically diagnosed patients with unilateral SAPS is still lacking. Therefore, the aim of the present study was to use US in order to evaluate the prevalence of shoulder abnormalities of both shoulders in patients with unilateral SAPS. The hypothesis was to find similar abnormalities in both the unaffected shoulder and the affected shoulder, in primary care patients with clinically diagnosed SAPS.

METHODS

Study Design and Patients

This descriptive investigation was approved by the Regional Ethics Committee in Stockholm in 2009 (2009/1197-31/2) and performed according to the Declaration of Helsinki. Patients were consecutively recruited from seven primary care centers in Stockholm between 2012 and 2016. All patients received written information about the study and signed an informed consent to confirm their participation.

One hundredfifteen patients, 54 men and 61 women diagnosed with unilateral SAPS were included in the study. The patients were 20–59 years of age (mean 45, 0 ± 10, 4) with a pain duration of mean 22 ± 14.5 weeks. The right arm was dominant in 108 patients (94%), and the shoulder of the dominant arm was affected in 72 patients (63%). The dominant arm was defined as the arm that the patient chooses naturally when writing. The mean Constant-Murley assessment score (C-M score) of the present cohort was 38 ± 10 points. The recommended combination of clinical tests for evaluating SAPS was used (Michener, Walsworth, Doukas, and Murphy, 2009). All patients underwent bilateral diagnostic US within 5 weeks from the clinical examination. Here, SAPS imply a ruled out presence of restricted passive range of motion (PROM) of the shoulder joint and/or referred pain from the cervical or thoracic spine (Mitchell, Adebajo, Hay, and Carr, 2005). Also, clinical signs of total tears and full thickness tears (FTT´s) were excluded (Hertel, Ballmer, Lombert, and Gerber, 1996).

Inclusion and Exclusion Criteria

The inclusion criterion was patients aged 20–59 years that had suffered from SAPS between 4 weeks and 1 year. We used the recommended combination of clinical tests for evaluating the patients (Michener, Walsworth, Doukas, and Murphy, 2009). For inclusion at least two of these tests had to be positive together with a positive painful arc (Kessel and Watson, 1977).

Exclusion criteria were bilateral shoulder pain, earlier treatment with cortisone injection, diabetes mellitus (Abate, Schiavone, Salini, and Andia, 2013), history of trauma, fractures and dislocations of the shoulder joint, rheumatoid arthritis, severe arthroses, frozen shoulder (loss of passive and active range of motion) (Chiu et al., 2020), a positive drop-arm test (Calis et al., 2000) and other clinical signs of total tears or FTT´s. Fibromyalgia and thoracic or cervical spine syndromes.

Clinical Examination

The examiner followed a study protocol and all the tests followed the same order for each patient. To clinically exclude total tears and FTT´s of the rotator cuff, patients were tested for severe weakness (e.g. total “give-away”) with the following tests: 1) Jobe´s test/empty can test: sensitivity 0.60–0.75, specificity 0.13–0.33 (Jobe and Jobe, 1983; Kelly, Brittle, and Allen, 2010); 2) Drop arm test (lag sign, m. supraspinatus in particular): sensitivity 0.21, specificity 0.92 (Alqunaee, Galvin, and Fahey, 2012; Calis et al., 2000); and 3) Lag signs: sensitivity 0.45–0.56, specificity 0.56–0.98 (Bak et al., 2010; Hertel, Ballmer, Lombert, and Gerber, 1996).

Neurologic testing of the upper extremities was always performed. If a patient presented with significant upper extremity weakness and active ROM deficits, revealed reduced or altered sensation or reduced tendon reflexes, they were excluded from the trial. A traumatic history, together with the positive tests above, led to the subjective conclusion that the patient had a total rupture or FTT (larger injury), which led to exclusion. Tests to further identify pain or weakness during resisted flexion, abduction, external and internal rotations were performed (Park et al., 2005) as well as the lift off test (Gerber and Krushell, 1991), the Palm-up test (Hegedus et al., 2008). The Cross- body adduction test (Park et al., 2005) and the Hawkin-Kennedy test were used to identify possible impingement of the shoulder (Hawkins and Kennedy, 1980). We also measured ROM and evaluated joint-play in order to detect loss of active and passive range of motion (i.e., frozen shoulder, severe cases of arthritis, arthroses) was conducted. The clinical examination was carried out by the same physical therapist, with more than 20 years of clinical experience.

Diagnostic Ultrasound

For diagnostic purposes, the reliability and quality of US findings is highly dependent on the experience and skill of the investigator (Teefey et al., 2004). Therefore, a biomedical technician with a PhD in musculoskeletal US and 20 years of experience of US, performed all US examinations. Two different US machines were used (ACUSON Antares^TM and ACUSON S1000^TM; Siemens Medical Solutions, USA), both equipped with high resolution Multi-D matrix transducers – WFX 13–5 (5–13 MHz) or 14L5 (5–14 MHz). The sonographic evaluation of the rotator cuff was performed in all patients according to a standardized protocol, the same that was thoroughly described by Vlychou et al. (2009). A total tear was defined as a complete release of the tendon, a FTT as a tear that extends from the bursal to the articular surface and a partial-thickness tear (PTT) as a defect within the tendon, in either the longitudinal or the transverse plane.

During the US examinations the patients were seated on a swivel chair with a low backboard, and the examiner was standing behind the patient. The tendons of supraspinatus, subscapularis, infraspinatus and the long head of biceps were visualized in both a longitudinal and transversal plane. Both the tendons of subscapularis and infraspinatus were evaluated dynamically during internal and external rotation with the forearm in supination and the elbow in 90° of flexion. The acromioclavicular joint was evaluated in terms of possible sprains and osteoarthritis. The supraspinatus tendon and the subdeltoid/subacromial bursae were evaluated dynamically during abduction with the arm slightly internally rotated. The supraspinatus tendon was examined with the patient’s palm placed on the posterior region of the iliac wing with the elbow flexed and directed posteriorly. The US examiner was blinded to arm dominance, the patient´s symptom as well as to the purpose of the study.

Statistical Analysis

Statistical analyses were conducted using TIBCO Statistica® 13.4.0 and Excel 2016. All variables were summarized using standard descriptive statistics such as frequency, percent, mean and standard deviation. Chi² (with Yates corrected Chi² when appropriate) or Fisher's exact test, was used comparing the symptomatic and asymptomatic shoulder. The total number of US findings and bilateral findings in the age intervals; 20–29 years, 30–39 years, 40–49 years and 50–59 years, were presented and compared using Kruskal Wallis ANOVA. If differences were found, Mann Whitney U-test and Chi² or Fisher's exact test was used to find out where the differences were. The significance level was set at p ≤ 0.05 (two-tailed).

RESULTS

Structural Abnormalities

US findings of structural abnormalities were present in both shoulders in patients with unilateral SAPS (Table 1). Bursitis was the most common US finding in all age groups, 90% in the symptomatic shoulder and 74% in the asymptomatic shoulder (p = .0021) (Tables 1 and 2). Partial-thickness tears of the rotator cuff was found in 40% of the symptomatic shoulder and 17% of the asymptomatic shoulder (p = .0001) (Table 1). Twelve percent of the patients had ruptures in more than one tendon of the symptomatic shoulder when compared to 3.5% of the asymptomatic shoulder (p = .0247) (Table 1). Partial-thickness tears of the supraspinatus tendon was twice as common in the symptomatic shoulder when compared to the asymptomatic shoulder, 30% and 14%, respectively (p = .0026) (Table 3). Tendinosis of the rotator cuff was present in 29% of the symptomatic shoulder and 12% of the asymptomatic shoulder (p = .0019) (Table 1). Supraspinatus was found to be the most common muscle in terms of tendinosis, 24% of the symptomatic shoulder and 10% of the asymptomatic shoulder (p = .0054) (Table 3). Synovitis of the symptomatic shoulder (long head of biceps) was more common than of the asymptomatic shoulder, 16.5%, and 6%, respectively (p = .0220) (Table 1). There were fewer total structural abnormalities in the age group 20–29 years (A), compared to the other age groups (B, C, and D) (p = . 0004) (Table 2).

Table 1. Ultrasound findings of structural abnormalities in 115 patients with unilateral subacromial pain syndrome.

US structural abnormalities	Symptomatic shoulder n (%)	Asymptomatic shoulder n (%)	P-values
Total rupture RC	1(1)	4(3.5)	0.3695
FTT RC	1(1)	7(6)	0.0656
PTT RC	46(40)	19(17)	0.0001
PTT LHB	1(0.9)	1(1)	1.0
Tendinosis RC	33(29)	14(12)	0.0019
Tendinosis LHB	1(0.9)	0	1.0
Calcifications	30(26)	20(17)	0.1099
AC joint pathology	23(20)	15(13)	0.1555
Fluid/Synovitis LHB	19(16.5)	7(6)	0.0220
Bursitis SD/SA	103(90)	85(74)	0.0021
No abnormality	3(2.6)	11(9.6)	0.0500
Ruptures, more than one (total, FTT, PTT)	14(12)	4(3.5)	0.0247

RC = Rotator cuff, FTT = Full thickness tear, PTT = Partial thickness tear, LHB = Long head of biceps, SD/SA = subdeltoid/subacromial

Table 2. Ultrasound findings by age group.

	20–29 years n = 14 (A)		30–39 years n = 19 (B)		40–49 years n = 35 (C)		50–59 years n = 47 (D)		Comparing all age groups; p-values	Differences between age groups
US structural abnormalities	Symp	Asymp	Symp	Asymp	Symp	Asymp	Symp	Asymp	Symp + Asymp	Symp + Asymp
Total rupture RC	0	0	0	0	0	1	1	3	0.3063
FTT RC	0	0	0	1	1	2	0	3	0.8232
PTT RC	1	1	8	2	13	7	38	20	0.0170	Btw A and C^1, A and D^2
Tendinosis RC	3	0	5	2	10	5	14	6	0.9919
Calcification	1	0	5	2	8	7	15	11	0.0937
AC joint pathology	0	0	4	4	9	6	10	5	0.0789
Fluid/Synovitis LHB	0	0	1	0	6	1	16	6	0.0062	n.s. comparing each age group
Bursitis SD/SA	8	5	19	17	30	25	45	37	0.0074	Btw A and B^3, A and D^4
Total findings	13	6	42	28	77	54	139	91	0.0004	Btw A and B^5, A and C^6, A and D^7

Symp = symptomatic shoulder, Asymp = asymptomatic shoulder, RC = Rotator cuff, FTT = Full thickness tear, PTT = Partial thickness tear, LHB = Long head of Biceps muscle, SD/SA = subdeltoid /subacromial, Btw = Between, n.s. = non significant. *significant differences between age groups: ¹ p = 0.0494, ² p = 0.0059, ³ p = 0.0139, ⁴ p = 0.0199, ⁵ p = 0.0002, ⁶ p = 0.0020, ⁷ p = 0.0000

Table 3.. Detailed ultrasound findings of structural abnormalities in 115 patients with unilateral subacromial pain syndrome..

US structural abnormalities	Symptomatic shoulders n (%)	Asymptomatic shoulders n (%)	P-values
Total rupture
Supraspinatus tendon	1(0.9)	2(2)	1.0
Infraspinatus tendon	0	1(0.9)	1.0
Subscapularis tendon	0	1(0.9)	1.0
Long head of biceps	0	0	N/A
FTT
Supraspinatus tendon	0	0	N/A
Infraspinatus tendon	0	0	N/A
Subscapularis tendon	1(0.9)	7(6)	0.0656
PTT
Supraspinatus tendon	35(30)	16(14)	0.0026
Infraspinatus tendon	8(7)	1(0.9)	0.0354
Subscapularis tendon	11(10)	4(4)	0.1064
Long head of biceps	5(4)	1(0.9)	0.2126
Tendinosis
Supraspinatus tendon	28(24)	12(10)	0.0054
Infraspinatus tendon	3(3)	1(0.9)	0.6217
Subscapularis tendon	3(3)	1(0.9)	0.6217
Long head of biceps	1(0.9)	0	1.0
Calcification
Supraspinatus tendon	21(18)	14(12)	0.1988
Infraspinatus tendon	2(2)	2(2)	1.0
Subscapularis tendon	9(8)	9(8)	0.8061
Long head of biceps	0	0	N/A

FTT = Full thickness tear, PTT = Partial thickness tear, N/A = Not Applicable

Bilateral Ultrasound Findings

Out of the 115 patients, 84 patients (73%) had bilateral bursitis and 18 patients (15, 6%) presented with bilateral partial-thickness tears of the rotator cuff, 12 patients (10.4%) with bilateral calcification and 10 patients (8, 7%) with bilateral tendinosis (Table 4). In patients 30 years and older 73% of the bursitis, 39% of the partial-thickness tears, 10.4% of the calcifications, and 8.7% of the tendinosis were bilateral (Table 4). Three patients (2.6%) did not have any abnormalities in any of their shoulders. Bilateral abnormal US findings in five “typical” patients with unilateral SAPS are shown in Figures 1a-5b.

Table 4. Bilateral ultrasound findings of structural abnormalities in 115 patients with unilateral subacromial pain syndrome, presented in age intervals.

US structural abnormalities	20–29 years(A) n = 14 n(%)	30–39 years(B) n = 19 n(%)	40–49 years(C) n = 35 n(%)	50–59 years(D) n = 47 n(%)	Differences between age groups, p-values
Total rupture RC	0	0	0	0	N/A
FTT RC	0	0	0	0	N/A
PTT RC	1(7.1)	2(10.5)	4(11.4)	11(23.4)	0.2927
Tendinosis RC	0(0)	1(5.3)	4(11.4)	5(10.6)	0.8736
Calcification	0(0)	2(10.5)	4(11.4)	6(12.8)	0.5345
AC joint pathology	0(0)	1(5.3)	3(8.6)	2(4.2)	0.6502
Fluid/Synovitis LHB	0(0)	0(0)	1(2.9)	6(12.8)	0.0941
Bursitis SD/SA	5(35.7)	17(89.5)	25(71.4)	37(78.7)	0.0188*

RC = Rotator cuff, FTT = Full thickness tear, PTT = Partial thickness tear, LHB = Long head of Biceps muscle, SD/SA = subdeltoid /subacromial. N/A = Not Applicable, *significant differences between A and B (p = 0.0023), A and C (p = 0.0462), A, and D (p = 0.0065)

Figure 1. A and b. Ultrasound of both shoulders in a 43 year old female patient with unilateral subacromial pain syndrome.

Right shoulder (symptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Intact rotator cuff. The supraspinatus tendon shows a slightly increased width and a somewhat irregular structure as in tendinosis. Minimal neovascularity. Increased fluid content in the subdeltoid bursa. No pathological findings of the acromio-clavicular joint. An area filled with fluid is found within the muscle belly of the infraspinatus, ganglion? Left shoulder (asymptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Within the middle superficial circumference of the infraspinatus tendon, a probable partial rupture is found. No pathological findings of the acromio-clavicular joint. Both muscle bellies of the supraspinatus and infraspinatus are well-developed.

Figure 2. A and b. Ultrasound of both shoulders in a 48 year old female patient with unilateral subacromial pain syndrome.

Right shoulder (symptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Around the tendon there is a small increase in fluid content just below the sulcus like in synovitis. The supraspinatus tendon shows an increased width and hypo echogenic areas similar to in tendinosis. No neovascularity. In the superior distal part of the subscapularis tendon there might be a partial rupture. Some increased content of fluid in the subdeltoid bursa. High echogenic changes like in calcification or osteophytes are present in the acromio-clavicular joint. Both muscle bellies of the supraspinatus and infraspinatus are well-developed. Negative abduction test. Left shoulder (asymptomatic). Intact the rotator cuff. The supraspinatus tendon has an increased width and hypo echogenic areas similar to those in tendinosis. No neovascularity. Some increased content of fluid in the subdeltoid bursa. No pathological findings were present in the acromio-clavicular joint. Negative abduction test

Figure 3. A and b. Ultrasound of both shoulders in a 33 year old male patient with unilateral subacromial pain syndrome.

Right shoulder (asymptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Intact rotator cuff. No neovascularity. Some swelling is present in the subdeltoid/subacromial bursae. No pathological findings of the acromio-clavicular joint were seen from a superficial view. Both muscle bellies of the supraspinatus and infraspinatus are well developed. Left shoulder (symptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Intact rotator cuff. No neovascularity. Some swelling is present in the subdeltoid/subacromial bursae. No pathological findings of the acromio-clavicular joint were seen from a superficial view. Both muscle bellies of the supraspinatus and infraspinatus are well-developed.

Figure 4. A and b. Ultrasound of both shoulders in a 42 year old male patient with unilateral subacromial pain syndrome.

Right shoulder (symptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. There are signs of a small earlier partial rupture within the anterior deep circumference of the supraspinatus tendon at the insertion of the tuberculum majus. A few small minimal calcifications are also seen here. The tendon has an increased width and hypo echogenic areas with moderate neovascularity similar to in tendinosis. Intact rotator cuff. Some increased the content of fluid in the subdeltoid bursa. No pathological findings of the acromio-clavicular joint were seen from a superficial view. Both muscle bellies of the supraspinatus and infraspinatus are well-developed. Negative abduction test. Left shoulder (asymptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Intact rotator cuff. The supraspinatus tendon presents with increased width and hypo-echogenic areas similar to those in tendinosis. Minimal neovascularity at the insertion of tuberculum majus. Some increased the content of fluid in the subdeltoid bursa. No neovascularity. Some swelling is present in the subdeltoid/subacromial bursae. No pathological findings of the acromio-clavicular joint were seen from a superficial point of view. Both muscle bellies of the supraspinatus and infraspinatus are well-developed. Negative abduction test.

Figure 5. A and b. Ultrasound of both shoulders in a 53 year old male patient with unilateral subacromial pain syndrome.

Right shoulder (symptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Around the tendon there is a somewhat increase content of fluid just below the sulcus, which may be a sign of peritendinitis or synovitis. Intact the rotator cuff. No neovascularity. Increased content of fluid in the subdeltoid bursa. The joint capsule of the acromio-clavicular joint is somewhat curved. Both muscle bellies of the supraspinatus and infraspinatus are well-developed. Negative abduction test. Left shoulder (asymptomatic). No pathological findings of the long biceps tendon, which is correctly positioned in its sulcus. Around the tendon there is a somewhat increased content of fluid just below the sulcus, which might be a sign of peritendinitis or synovitis. Intact rotator cuff. No neovascularity. Some increased the content of fluid in the subdeltoid bursa. No pathological findings were seen in the acromio-clavicular joint. Both muscle bellies of the supraspinatus and infraspinatus are well-developed. Negative abduction test. A = acromion, CH = caput humeri, B = bursa, D = deltoideus muscle, SSP = supraspinatus tendon.

DISCUSSION

The most important finding of the present study was the amount of bilateral findings in patients diagnosed with unilateral subacromial pain syndrome (SAPS). The findings were more prevalent in the symptomatic shoulder compared to the asymptomatic shoulder and this was particularly evident regarding bursitis and partial thickness tears. More than half of the patients in the present study (age group 30–59 years) presented with PTTs´ in their symptomatic shoulder. From a clinical perspective, this is important knowledge, especially knowing that patients seeking primary care frequently are offered corticosteroid injections as a treatment option. A tear of the rotator cuff is considered to be a contraindication for corticosteroid injections (Dean et al., 2014; Puzzitiello et al., 2020), and thus, when a corticosteroid injection is the planned treatment, a diagnostic US is a valuable recommendation.

Despite the fact that bursitis and PTTs were more evident in the affected shoulder, abnormalities were found in almost 90% of the patients´ asymptomatic shoulder. This high rate is in concert with a detailed description of US findings in asymptomatic shoulders in males (Girish et al., 2011). They reported abnormalities in 96% of their patients with the most common finding being bursal thickening. This is similar to the present study, where bursitis was shown in 90% of the symptomatic shoulder and 74% of the asymptomatic shoulder. The subacromial bursa has been suggested to play a key role when it comes to generate pain in patients with SAPS (Chillemi et al., 2016; Rahme, Nordgren, Hamberg, and Westerberg, 1993). Although, yet not proven, presence of neovascularity has been mentioned as another theory of pain (Lewis et al., 2009).

In the present study, 51.3% of the patients aged 30–59 years presented with partial-thickness tears of their symptomatic shoulder and 25.2% of their asymptomatic shoulder. This is in line with two recent publications reporting a rotator cuff tear in one shoulder to be a risk factor for the presence of a rotator cuff tear in the contralateral shoulder (Ranebo, Bjornsson Hallgren, and Adolfsson, 2018; Ro et al., 2015).

Hallgren et al. (2014) reported the percentage of patients with partial-thickness tears and full-thickness tears of the rotator cuff to be 22% and 9%, respectively in the symptomatic shoulder. In the present study, almost twice as many patients (40%) presented with partial-thickness tears of the rotator cuff when compared to solely 1.7% with full-thickness tears and total ruptures (Table 1). The low prevalence of full-thickness tears and total ruptures in the present study might be due to the fact that patients with lag signs were excluded from the study. Furthermore, the patient´s in the present study also had a lower mean age and were not on a waiting list for surgery. Non-traumatic partial-thickness tears have been suggested to be a natural part of aging (Milgrom, Schaffler, Gilbert, and van Holsbeeck, 1995; Moosmayer, Smith, Tariq, and Larmo, 2009).

The supraspinatus muscle was the most vulnerable tendon of the rotator cuff in the symptomatic shoulder in terms of partial-thickness tears, tendinosis, as well as calcification. This was also the case when it comes to the asymptomatic shoulder although not of the same magnitude. The vulnerability of the supraspinatus muscle in patients with SAPS, found in the present study has been known for long and also recently reported in other US publications (Jobe and Moynes, 1982; Rodríguez-Piñero Durán, Vidal Vargas, and Castro Agudo, 2019).

Today, US has become increasingly common within physical therapy clinics (Kooijman et al., 2020) in diagnosing shoulder pain. However, several authors are questioning the patho-anatomical diagnostic approach when diagnosing SAPS (Cook, Rio, Purdam, and Docking, 2016; Lewis, 2009). Other authors still suggest that an US examination of patients with shoulder pain might be a useful tool to distinguish between the pathologic patterns in order to choose the optimal treatment (Tran et al., 2017). There are poor associations between symptoms in patients with SAPS and pathological US or MRI findings (Guzowski et al., 2019; Lewis, 2009). This might be due to the fact that imaginary findings are somewhat hard to interpret clinically and many individuals in the general population also present with abnormal findings of their asymptomatic shoulder, increasing with age (Barreto et al., 2019; Milgrom, Schaffler, Gilbert, and van Holsbeeck, 1995; Tempelhof, Rupp, and Seil, 1999). Approximately two-thirds of patients with rotator cuff tears do not complain of any symptoms (Yamamoto et al., 2011). Due to the fact that several findings are bilateral, it might be a good idea to include exercises bilaterally; however, this is yet to be studied in a clinical trial.

Considering the current knowledge, it is of utmost importance to explain to our patients that patho-anatomical changes does not always lead to symptoms, and that structural changes, including tears may be part of expected degeneration. Today, if a rupture of any sort is revealed, it is not uncommon for practitioners and patients to warrant a referral to an orthopedic clinic, instead of starting graded exercise, which is consensus for the physiotherapy treatment (Klintberg et al., 2015). For this group of patients surgical intervention is unnecessary and also correlated to significantly greater costs for the society than physiotherapy (Ketola et al., 2013). In concert with previous studies, the present study showed that US findings in terms of diagnosing patients with non-traumatic shoulder pain should not be used as the only diagnostic tool (Guzowski et al., 2019; Moosmayer, Smith, Tariq, and Larmo, 2009).

Study Limitations and Strengths

A limitation of the present investigation was lack of grading the pathology, which would have been valuable in order to compare the symptomatic and asymptomatic shoulder. The strength of the present study was the large sample size. Another strength was the thorough clinical examination performed before the US examination in order to rule out total rotator cuff tears and other symptoms not compatible with SAPS. A further strength was the blinded diagnostic US examination, following a standardized protocol, and performed by one expert within the field of musculoskeletal US. This minimizes the risk of misinterpretation of the diagnostic US.

CONCLUSION

Patients with unilateral SAPS presented with a number of abnormal US findings in both their symptomatic and asymptomatic shoulder. Most abnormalities were present in the symptomatic shoulder, which might explain the pain reported by the patients in solely their symptomatic shoulder. It is important to design treatment not only based on the imaginary findings, but also to take into account the patient´s history, complaints, and the results of a clinical examination. However, before the decision to treat the patient with a corticosteroid injection, a diagnostic US could be valuable in order to rule out partially thick tears.

Acknowledgments

The authors are grateful for the participating primary care centres and their SAPS patients, without whom this study could not have been conducted.

Disclosure Statement

No competing interest.