Effect of Posture on Acromiohumeral Print

journal of orthopaedic & sports physical therapy | volume 40 | number 10 | october 2010 | 633

[ research report ]

Shoulder pain is the third most common musculoskeletal problem, accounting for at least 21% of all musculoskeletal complaints.36 The most common cause of shoulder pain is rotator cuff disease (RCD), which includes impingement

syndrome, bursitis, tendinitis, tendinosis, and partial- and full-thickness tears.37 Upper quadrant postural deviations have been linked to upper

NitiN Kalra, PT, MS1 Amee L. Seitz, PT, PhD, OCS2 N. DougLAS BoArDmAN iii, MD3 Lori A. micheNer, PT, PhD, ATC, SCS4

1 Orthopaedic Physical Therapist, Select Physical Therapy, Fairfax, VA; Masters student [at time of study], Department of Anatomy and Physical Therapy, Virginia Commonwealth University, Richmond, VA. 2 Research Associate, Department of Physical Therapy, Virginia Commonwealth University, Richmond, VA. 3 Associate Professor, Department of Orthopedic Surgery, Virginia Commonwealth University Health Systems, Richmond, VA. 4 Associate Professor, Department of Physical Therapy, Virginia Commonwealth University, Richmond, VA. This study was completed as partial fulfillment for a Masters Degree in Anatomical Sciences from Virginia Commonwealth University. The study was approved by The Institutional Review Board at Virginia Commonwealth University. Funding for this study was provided partially by the A. D. Williams Fund for Research at Virginia Commonwealth University, the Department of Physical Therapy at Virginia Commonwealth University, and by the Foundation for Physical Therapy. Address correspondence to Dr Lori A. Michener, Department of Physical Therapy, 1200 E Broad St, Richmond, VA, 23298. E-mail: [email protected]

Effect of Posture on Acromiohumeral Distance With Arm Elevation in

Subjects With and Without Rotator Cuff Disease Using Ultrasonography

t StuDY DeSigN: Controlled laboratory study.t oBJectiVeS: To examine the effects of altering posture on the subacromial space (SAS) in sub-jects with rotator cuff disease and subjects without shoulder pain.

t BAcKgrouND: Poor upper quadrant posture has been linked to altered scapular mechanics, which has been theorized to excessively reduce SAS. However, no study has examined the direct effects of altering upper quadrant posture on SAS. We hypothesized that upright posture would increase and slouched posture would decrease the SAS, as compared to a normal posture, when measured both with the shoulder at rest along the side of the trunk and when maintained in 45 of active shoulder abduction.

t methoDS: Participants included 2 groups: the subjects with shoulder pain and rotator cuff disease, as diagnosed via magnetic resonance imaging (n = 31), and control subjects without shoulder pain (n = 29). The SAS was imaged with ultrasound using a 7.5-MHz linear transducer placed in the coronal plane over the posterior to midportion of the acromion. The SAS was mea-sured on ultrasound images using the acromio-humeral distance (AHD), defined as the shortest

distance between the acromion and the humerus. The AHD was measured in 2 trials at 2 arm angles (at rest along the trunk and at 45 of active abduc-tion) and across 3 postures (normal, slouched, and upright), and averaged for data analysis.

t reSuLtS: Two mixed-model analyses of variance, 1 for each arm angle, were used to compare AHD across postures and between groups. There was no interaction between group and posture, and no sig-nificant main effect of group for either arm position. There was no significant main effect of posture for the arm at rest (P = .26); however, there was a significant main effect of posture on AHD at the 45 abduction arm angle (P = .0002), with a significantly greater AHD in upright posture (mean AHD, 9.8 mm), as compared to normal posture (mean AHD, 8.6 mm).

t coNcLuSioN: The effect of posture on SAS, as measured by the 2-dimensional AHD using ultra-sound of the posterior to middle aspect of the SAS, is small. The AHD increased with upright posture by 1.2 mm compared to normal posture, when the arm was in 45 active abduction. J Orthop Sports Phys Ther 2010;40(10):633-640. doi:10.2519/jospt.2010.3155

t KeY WorDS: impingement, posture, rotator cuff, shoulder, subacromial space

extremity impairments8,20,22 and RCD.16,18,21,25,27 A forward head pos-ture has been linked to pain related to shoulder overuse.16 Adopting a slouched posture has been shown to decrease gle-nohumeral abduction strength,20 while adopting an upright posture resulted in increased glenohumeral elevation.8,22 Slouched posture may limit shoulder motion due to impingement beneath the acromion, creating a mechanical block to shoulder elevation coupled with tissue impingement.11,27,28 Taping applied to the posterior trunk parallel to the thoracic spine and over the scapula of patients with shoulder pain increased thoracic extension, reduced pain with shoulder elevation, and improved resting scapular position.22 Slouched posture is linked to shoulder pain, changes in scapular posi-tion, shoulder strength, and range of mo-tion, which may contribute to disability.

Mechanistically, upper quadrant pos-ture can affect scapular motion or ki-nematics,9,12,20,21 which may reduce the subacromial space (SAS) and contribute to shoulder pain and the development of RCD.6,16,21,23,24,27 Increased thoracic spine kyphosis or slouched thoracic posture has been shown to decrease scapular upward rotation,20 posterior tilting,9,12,20 and ex-ternal rotation.9,20,21 Patients with RCD,

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634 | october 2010 | volume 40 | number 10 | journal of orthopaedic & sports physical therapy

[ research report ]

est distance between the acromion and humerus in the SAS. Studies using ultra-sound imaged the SAS over the middle SAS space,2,3 or did not state the exact lo-cation of the ultrasound transducer dur-ing imaging.14 Ultrasound as an imaging modality is less costly and more practi-cal than MRI, and has established con-current validity with radiographic AHD measures (r = 0.77-0.85).2,3

The link between upper quadrant posture, shoulder pain, and RCD is not well understood. Investigating the effects of posture on the SAS of patients with RCD will expand our understanding of possible mechanisms of posture associ-ated with RCD. Moreover, it may provide mechanistic evidence for postural correc-tion often used to treat these patients in rehabilitation. The purpose of this study was to determine the effect of posture on SAS, using ultrasound images of the SAS to measure the AHD in 2 arm positions across 3 postures, in subjects with and without RCD. We hypothesized that in both groups slouched posture would lead to a reduction in SAS and upright posture would increase the SAS when compared to normal posture, at both arm positions of rest and 45 of active abduction.

methoDS

Subjects

Subjects with shoulder pain and RCD (n = 31) and a control group without shoulder pain (n = 29) were

recruited through the physician offices at the University Health System and with flyers posted at the Virginia Common-

wealth University Health System and Vir-ginia Commonwealth University campus (tABLe 1). The Institutional Review Board at the Virginia Commonwealth Univer-sity approved the study protocol and the informed consent form, which was signed prior to the data collection procedures.

Inclusion criteria for the RCD group were (a) an age of at least 18 years, (b) shoulder pain, (c) MRI confirmation of RCD (tendinosis, bursitis, impinge-ment, or partial- or full-thickness rota-tor cuff tear), and (d) ability to lift the arm up to 90 of elevation, which would ensure their ability to easily obtain and easily maintain the 45 arm abduction test position. If a subject had 2 or more diagnoses, the more severe RCD pathol-ogy diagnosis was used to classify group membership. Inclusion criteria for the control group subjects were an age of at least 18 years and having no pain and no known previous or current shoulder pa-thology. Exclusion criteria for all subjects were (a) a cervical range of motion that reproduced shoulder pain, (b) pain below the elbow indicative of cervical or nerve pathologies, (c) past shoulder surgery, and (d) glenohumeral joint arthritis, as indicated in the MRI report.

Sample size was determined from a pilot study,26 using AHD measures ob-tained from ultrasound images taken on 10 subjects with RCD and 10 subjects without shoulder pain. Two ultrasound images were collected with the arm at rest and 45 active abduction, with the subject in a neutral posture. This proce-dure was subsequently repeated 15 to 30 minutes later. The same methods were

specifically, impingement syndrome, have demonstrated these same altered scapular kinematic patterns.23 These al-tered scapular kinematics may decrease the SAS and clearance of the humeral head beneath the acromion during arm elevation.6,7,13,34,35 Studies provide some support for this relationship. Artificially induced scapular protraction, compared to a position of scapular retraction, de-creased the 2-dimensional (2-D) linear distance between the acromion and hu-merusthe acromiohumeral distance (AHD)as measured on magnetic reso-nance images (MRIs).35 Another study found a reduced AHD measured on ultra-sound images in tennis players without shoulder pain but with scapular dyskine-sis as compared to those without scapular dyskinesis.34

Direct effects of posture on SAS for individuals with RCD are unknown. Subjects with acquired or idiopathic thoracic hyperkyphosis have a smaller AHD as measured on radiographic images compared to nonkyphotic sub-jects.17 However, this study used subjects without shoulder pain and measured AHD with the arm at rest only. More-over, slouched posture is not analogous to hyperkyphosis, thereby limiting the generalizability of these results. Studies examining the direct effects of posture on SAS in patients with RCD are need-ed to elucidate the relationship between posture and RCD.

Generally, the AHD is smaller in pa-tients with RCD as compared to healthy shoulders.1,3,14,15,19 Studies using MRI re-ported a smaller AHD in patients with impingement syndrome as compared to healthy shoulders.1,15,19 Studies using ultrasound imaging corroborate these findings with a smaller AHD in patients with RCD3,14 and a smaller AHD with in-creased severity of the RCD.3 Although these studies consistently indicated a smaller AHD with RCD, they varied on the location of the measurement of the SAS. MRI studies1,15,19 depicted the loca-tion as the anterior to middle aspect of the SAS, or at the location of the small-

tABLe 1 Descriptive Statistics*

Abbreviation: RCD, rotator cuff disease.* Data presented as mean SD (range).

Subjects With rcD (n = 31) control Subjects (n = 29)

Age (y) 53.5 13.7 (20.0-80.0) 31.9 10.7 (23.0-62.0)

Height (cm) 170.2 9.6 (152.4-187.9) 169.8 8.8 (151.1-1.9)

Body mass (kg) 80.5 13.4 (60.2-115.1) 72.1 13.7 (56.6-113.3)

Pain (mo) 18.2 17.3 (2.0-84.0) 0.0 0.0 (0.0-0.0)

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used as described in this study, using the same investigator to collect data and make AHD measurements. For the rest and 45 arm positions, respectively, the standard error of the measure (SEM) was 2.3 and 2.5 mm. Based on these data, we calculated our desired sample size using a desired minimal difference of 2.5 mm between posture conditions. The average standard deviation was conservatively es-timated at 4.0 mm. For a power of 90% and an alpha of .05, sample size calcu-lations indicated 20 subjects per group. As data collection ensued, the majority of subjects in the RCD group had rota-tor cuff tears; therefore, recruitment was extended to purposefully recruit subjects with impingement to represent the entity of RCD.

ProceduresAfter signing the informed consent form, subjects completed an intake form and the American Shoulder and Elbow Sur-geons patient self-report shoulder score to assess shoulder functional loss and dis-ability (tABLe 2). Next, ultrasound images of the SAS were collected for AHD mea-surement. The examiner was not blinded to group assignment to conduct the ultra-sound imaging, but was blinded for AHD measurements.Subacromial Space Measurement The outlet of the SAS was measured on ul-trasound-generated 2-D images via the AHD. The AHD is a 2-D linear measure defined as the shortest distance between the acromion and the humerus (Figure 1). An ultrasound unit (The Pyramid 764; Pyramid Management LLC, Los Alami-tos, CA) with a 7.5-MHz linear ultrasound transducer was utilized. Placement of the ultrasound transducer was standardized, with its location on the posterior to mid-dle portion of the acromion in the coronal plane, with the transducer placed parallel to the flat superior aspect of the acromion so that both the acromion and humerus were visualized (Figure 2). All ultrasound images were saved on a computer for AHD measurements performed later. Two ultrasound images were taken for

each of 3 postures and 2 arm positions in each posture.Posture and Arm Angle Subjects were tested in 3 sitting postures: (1) normal resting posture, (2) slouched posture, and (3) upright posture with scapular re-traction. For normal posture, the subject was asked to sit in a chair with the back supported, feet flat on the floor, hips and

knees at 90 of flexion, head and shoul-der in their habitual posture, looking straight ahead (Figure 3). Slouched pos-ture was achieved by having each subject move forward in the chair so that the subjects back was a minimum of 15 cm away from the back support, then slump forward and down to attain a flexed tho-racic and lumbar spine, forward head, and rounded shoulder posture (Figure 3). For the slouched posture position, subjects were instructed to slouch down but look straight ahead, to get flexion in the thoracic and lower cervical spine, ex-tension in the upper cervical spine, and

Figure 1. The line represents the acromiohumeral distance (AHD).

tABLe 2 Subject Demographics

Abbreviations: ASES, American Shoulder and Elbow Surgeons Self-Report Form; RCD, rotator cuff disease.* Mean SD (range), 0-50 points, with 50 as no pain. Mean SD (range), 0-50 points, with 50 as no functional loss. Mean SD (range), 0-100 points, with 100 as no pain and functional loss.

Subjects With rcD (n = 31) control Subjects (n = 29)

Gender (n)

Male 11 14

Female 20 15

Dominant shoulder (n)

Right 22 27

Left 5 2

Ambidextrous 4 0

RCD diagnosis (n)

Impingement 15

Partial-thickness tear 9

Full-thickness tear 7

ASES pain score* 31.3 11.8 (10.0-50.0) 50.0 0.0 (50.0-50.0)

Function score 25.2 10.3 (5.0-45.0) 50.0 0.0 (50.0-50.0)

Total score 56.9 17.8 (28.0-95.0) 100.0 0.0 (100.0-100.0)

Figure 2. Ultrasound probe positioning on the acromion.

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[ research report ]

elevation. Moreover, measurement of the SAS at angles greater than 45 may not provide clinically relevant information because, above 35 to 40 of glenohumer-al elevation, the supraspinatus tendon has likely already passed underneath the acromion and may no longer be at risk of impingement in the SAS.4

Each ultrasound image was an indi-vidual measure. Prior to each ultrasound image, the subjects rolled their shoulders and moved about to change their pos-ture, then were asked to assume the des-ignated posture and reposition the arm in the appropriate arm angle. Between measures at the 45 arm angle, subjects took their arm out of the sling and then repositioned the arm for the second ul-trasound image. The sequence of pos-tural alterations was counterbalanced by changing the posture sequence, but the resting arm position was always tested prior to the 45 angle. Images were saved to the computer, and retrieved later for AHD measurement.AHD Measurement The examiner who captured the ultrasound images also measured the AHD. Ultrasound images were randomly retrieved, with the ex-aminer blinded to group, arm angle, and posture. The AHD was measured using the software Universal Desktop Ruler (AVP Inc, Voronezh, Russia) to measure distances on screen. A mark was first placed at the most inferior aspect of the acromion, then a second mark was placed at the humeral head to measure the

shortest distance between the acromion and humerus. The 2 AHD measures at each arm angle for each posture were av-eraged for data analysis.

Data AnalysisTo examine the effect of posture on AHD, 2 separate mixed-model analy-ses of variance (ANOVAs), 1 for each arm position (rest and 45 abduction), were performed. These included effects for group (RCD and control), posture (normal, upright, slouched), and group-posture interaction, with posture as the repeated factor. Post hoc testing was per-formed using contrasts. All analyses used a significance set at = .05. Test-retest intrarater reliability was calculated using an intraclass correlation coefficient (ICC) 2-way random analysis for each arm po-sition. Error was calculated using SEM (SD 1ICC) and minimal detectable change (MDC) (SEM 2) values. The 90% confidence bounds were calculated by multiplying error values by the z score of 1.64.

reSuLtS

the AHD means and standard deviations are reported in tABLe 3. Test-retest reliability analysis on

60 subjects revealed that for the rest and 45 abduction positions, respectively, ICCs (2-way random) were 0.92 (95% CI: 0.87, 0.95) and 0.76 (95% CI: 0.63, 0.86), SEMs were 0.9 and 1.6 mm, and MDCs were 1.3 and 2.2 mm.

There were no significant group-

rounded shoulders. For upright posture, subjects were asked to sit back against the back rest of the chair with a pillow between their back and the back support, then were instructed to sit up straight and pull their shoulders back and look straight ahead, to achieve retracted shoulders and extension in the thoracic and cervical spine (Figure 3). For both upright and slouched postures, subjects altered both the spine and shoulder pos-ture from their normal resting posture. Therefore, changes in AHD between postures can result from changes in spine posture (thoracic or cervical spine) or shoulder posture (humerus or scapula), or a combination of these.

In all 3 postures, ultrasound images were obtained at 2 arm angles: at rest with arm at the side and in 45 actively maintained coronal plane shoulder ab-duction. For the 45 position, the arm was initially suspended in an adjustable sling, but the arm was held actively above the sling for imaging (Figure 4). This sling al-lowed the patient to rest the arm between measurements and during setup. The subject was instructed to hold the arm up and off the sling for a few seconds, while the arm angle was verified with a bubble inclinometer. Then, ultrasound images were captured with active arm elevation. An elevated arm angle was used because patients with RCD complain of pain dur-ing shoulder elevation. In pilot testing, it was technically difficult to obtain ad-equate ultrasound images above 45 of

Figure 4. Subject with arm supported at 45 abduction.

Figure 3. (A) Normal posture. (B) Slouched posture. (C) Upright posture.

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0.7, 1.7) with the arm at rest and 0.1 mm (95% CI: 0.8, 0.9) for the 45 abduc-tion position. There was no significant main effect of posture with the arm at rest (F2,116 = 1.4, P = .26); however, there was a statistically significant main effect of posture at 45 of abduction (F2,115.4 = 9.1, P = .0002). Post hoc testing, using contrasts for the 45 arm abducted po-sition, revealed a statistically significant difference (t = 3.1, P = .002) of a greater AHD in the upright posture (mean SD AHD, 9.8 2.0 mm) compared to nor-mal posture (mean SD AHD, 8.6 1.9 mm), with a mean difference of 1.2 mm (95% CI: 0.3, 2.0). However, there was no significant difference (t = 1.6, P = .106) between slouched posture (mean SD AHD, 9.2 1.9 mm) and normal posture at the 45 abduction position.

DiScuSSioN

the posture impairment theory links postural deviations with ana-tomical changes, impairments, and

pain of the shoulder.6 Slouched posture can alter scapular kinematics9,12,20,21; but this posture is only theoretically linked to a change in the outlet of the SAS.7,13,35 This study has demonstrated a link between upper quadrant posture and SAS, with a small change in the AHD

posture interactions with the arm at rest (F2,116 = 0.4, P = .658) and 45 abduction (F2,115.4 = 1.0, P = .364). Therefore, the ef-fect of posture was determined to be in-dependent of group classification. Means and standard deviations of all subjects are reported in tABLe 3, and 95% CIs are

represented in Figure 5. There were no statistically significant main effects of group classification for either the rest (F1,58 = 0.6, P = .431) or 45 abduction (F1,58.1 = 0.04, P = .839) positions. There-fore, the difference between the control and RCD groups was 0.5 mm (95% CI:

Slouch Normal Upright

7

8

9

10

11

12

13

14

Ac

rom

iohu

mer

al D

ista

nce

(mm

)

At rest (0) 45 abduction

Figure 5. Acromiohumeral distance in millimeters for 3 postures and for 2 arm positions (at rest and 45 abduction). Data represent combined results for the control and patients with rotator cuff disease. Vertical bars represent 95% confidence intervals.

tABLe 3 Acromiohumeral Distance*

Abbreviations: Abd, shoulder abduction; RCD, rotator cuff disease.* Data presented as mean SD mm. Subgroup definition based on magnetic resonance imaging. Significantly different from normal posture (P = .002).

rest 45 Abd rest 45 Abd rest 45 Abd

All subjects 12.1 2.6 8.6 1.9 12.5 3.1 9.2 1.9 12.6 2.5 9.8 2.0

Groups

Control 11.8 2.5 8.7 1.9 12.2 2.5 9.4 2.0 12.5 2.3 9.6 1.9

RCD 12.5 2.6 8.5 2.0 12.8 3.6 9.0 2.0 12.7 2.6 9.9 2.1

Subgroups of RCD

Impingement 12.8 2.5 8.6 2.4 12.9 4.0 8.3 1.7 12.8 2.4 9.8 1.7

Partial-thickness tear 12.0 2.1 8.6 1.8 13.6 2.6 9.4 1.8 12.8 2.2 11.1 2.5

Full-thickness tear 12.3 3.8 8.4 1.8 11.6 4.2 9.8 2.4 11.8 3.6 8.9 1.7

Normal Posture Slouched Posture upright Posture

Posture and Arm elevation Angle

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[ research report ]2-D measure in the coronal plane of the posterior to middle portion of the SAS. Upright posture increased the AHD, as compared to normal posture, when the arm was actively held at 45 abduc-tion but not when the arm was at rest. Slouched posture did not change AHD as compared to normal posture.

We examined the direct effect of posture on the AHD linear measure in patients with RCD and control subjects at 2 arm angles. At 45 active abduc-tion, AHD increased by a mean of 1.2 mm with a change from normal to the upright posture. This supports the hy-pothesis that upright posture increases AHD. This increase in AHD may have the effect of relieving the symptoms of compression of the SAS structures. The meaningful change in AHD, the amount needed to change patient symptoms and shoulder function is unknown. Subjects without shoulder pain but with thoracic hyperkyphosis had 1.4 to 1.7 mm smaller mean AHD than those without hyper-kyphosis.17 A study of 4 healthy subjects using MRI images of the midcoronal plane, which approximated the middle SAS, noted a mean AHD increase of 0.5 mm (range, 0.3-1.5 mm) with scapular retraction as compared to protraction.35 In our study, the mean change of 1.2 mm in AHD with upright posture was great-er than that of the scapular protraction-retraction study, but smaller than the study of thoracic hyperkyphosis. The MDC, the distribution-based error for our AHD measure, was 2.2 mm. A change in AHD of 2.2 mm from normal to upright posture was experienced by 17 of 60 subjects (28%) in this study. Although the AHD change with up-right posture is statistically significant, it was less than the MDC in 72% of the subjects. The relationship of a 1.2-mm change in AHD to patient symptoms is unknown. Research is needed to deter-mine the meaningful amount of change in AHD.

Slouched posture was expected to de-crease AHD when compared to normal posture in both subjects with RCD and

control. However, our results did not confirm this hypothesis. What may par-tially explain this lack of difference is the subject report of difficulty and pain while maintaining their arm at 45 of abduc-tion when in the slouched posture. Sub-jects might have elevated their scapula to relieve pain, and this substitution move-ment might have prevented a reduction in the AHD. Also, scapular muscle ac-tivity might have been altered with the postures, and this might have an effect on AHD. We did not monitor scapular motion or muscle activity.

Upper quadrant posture is a com-bination of thoracic and cervical spine posture, and shoulder posture of the humerus and scapula. The com-ponents of upper quadrant posture were not measured in the 3 postures. Changes in shoulder and spine pos-ture were inferred with the postures. However, changes in components of the upper quadrant posture could have been inconsistent across subjects and, therefore, would explain the lack of dif-ferences across postures.

The posterior to middle aspect of the SAS was the best position to obtain the landmarks for AHD measures with ultrasound images. Prior studies using ultrasound to measure the AHD in indi-viduals with RCD imaged the SAS over the middle2,3 or anterior10 aspect of the SAS space, or did not describe location14 of the ultrasound probe. Findings from these ultrasound studies were generally consistent with those using MRI to im-age the SAS,1,15,19 which depicted the AHD measure of the anterior to middle aspect of the SAS or described it as the smallest distance between the acromion and hu-merus regardless of location in the SAS. There is evidence of greater humeral contact on the anterior aspect of the ac-romion13 and a decrease of the anterior aspect of the SAS with clinical impinge-ment maneuvers30; however, the purpose of this study was not to look at contact or absolute values but to examine the ef-fects of change in AHD with change in posture. Additionally, no evidence indi-

cates that the anterior aspect differs from the posterior aspect with respect to AHD change during arm movement or altered postures. A recent study33 comparing changes in AHD with arm elevation be-tween the anterior and posterior aspect of SAS indicated that changes in AHD with arm elevation were not significantly dif-ferent when AHD was measured at the anterior versus the posterior aspect of the SAS. AHD was significantly smaller in the anterior aspect of SAS compared to posterior aspect, but the change in AHD with arm elevation was not significantly different. However, this does not exclude the possibility that different effects of posture on AHD might have occurred if the anterior SAS was imaged. Measure-ment of the anterior SAS is indicated in future studies.

The AHD measure does not represent the entire SAS, rather, only a 2-D lin-ear distance of a portion of the outlet of the SAS. AHD measured on ultrasound images are reliable2,3,10,38 and have dem-onstrated concurrent validity with ra-diographs (r = 0.77-0.85),2,3 and a high correlation has been demonstrated be-tween AHD measures taken with radio-graphs and those with MRI (r = 0.81).32 With 2-D imaging, significant projection variations31 can be avoided with stan-dardization of subject and transducer position. The reliability of our AHD measures were excellent, with all ICC values greater than 0.75. Our results are comparable to a recent reliability study of AHD measured on ultrasound images of patients with RCD, with reported reli-ability of 0.92 and 0.90 at rest and 60 abduction, respectively.29

There was no effect of the presence or absence of RCD on AHD across pos-tures and arm angles. Subjects in the RCD group represented the broad spectrum of the disease as identified by their MRI, ranging from impingement syndrome to partial-thickness and full-thickness rota-tor cuff tear. This supports external validi-ty. However, this heterogeneity might have limited the internal validity and, therefore, the ability to detect differences between

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coNcLuSioN

We examined the direct effects of upper quadrant posture on the SAS, using ultrasound to image

the SAS, performing a linear AHD mea-sure in patients with RCD and control subjects without shoulder pain. Upright posture increased AHD in both groups of subjects when tested in 45 actively held abduction position. The AHD did not differ between slouched posture as com-pared to normal posture, and between those with or without the presence of RCD in the 2 arm positions. The AHD measure in this study represents only the posterior to midportion of the SAS in the coronal plane; therefore, results may dif-fer for the anterior SAS. t

KeY PoiNtSFiNDiNgS: Upright posture resulted in a 1.2-mm increase in SAS, as measured by AHD, with the arm at 45 abduc-tion, when compared to normal posture. Change in AHD with upright posture was not dependent on the presence or absence of RCD. Slouched posture did not induce a change in AHD, as com-pared to normal posture.imPLicAtioN: Upright posture can in-crease the SAS, as measured by the AHD. However, the magnitude of the change is small and within the range of measurement error for the majority of subjects.cAutioN: Posture was artificially in-duced. Furthermore, the SAS was as-sessed using the AHD, which is a 2-D measure and taken from ultrasound images of the posterior to middle aspect of the SAS.

groups. In a prior study that measured the AHD from ultrasound images taken over the middle acromion,3 RCD severity was linked to AHD, with a smaller AHD in those with more severe RCD. Our RCD subgroup data do not descriptively sup-port this premise, which may be due to the lower number of subjects or differences in methods, as we generated our ultrasound images from the middle to posterior as-pect of the acromion.

Patients with RCD have pain with shoulder elevation, suggesting that the SAS should be measured at various po-sitions of shoulder elevation. However, evidence suggests that clinical relevance of measuring SAS at elevation angles greater than 35 to 40 of glenohumeral elevation may not be important, as the supraspinatus tendon has likely already passed underneath the acromion and thus may no longer be at risk of impinge-ment in the SAS.4 Measurements of the SAS and orientation of supraspinatus tendon found that, anatomically, the su-praspinatus tendon was at greatest risk of impingement between the acromion and greater tuberosity of humerus between 27.7 to 36.1.4,5

The healthy group was not age and gender matched. Age of control subjects (mean SD, 31.9 10.7 years) was sig-nificantly less (P.001) than the age of the RCD group (mean SD, 53.5 13.7 years), with a mean difference of 21.8 years (95% CI: 15.2, 27.9). With aging, there is a potential of osteophyte forma-tion on the inferior acromion that could affect the AHD measure. Gender was not matched; however, there were no signifi-cant differences in distribution between groups (P.05).

Postures were artificially induced in a laboratory setting, therefore they may not represent faulty postures seen in cli-ents without shoulder pain or in patients with shoulder pain. No intervention for posture correction was given, so results cannot be applied to the effects of a pos-tural treatment. Lastly, the tester was not blinded to group membership during ul-trasound imaging of the SAS.

reFereNceS

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2. Azzoni R, Cabitza P. Sonographic versus radio-graphic measurement of the subacromial space width. Chir Organi Mov. 2004;89:143-150.

3. Azzoni R, Cabitza P, Parrini M. Sonographic

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