Dynamic acromiohumeral interval changes in baseball players during scaption exercises

A Comparison of 2 Exercise Protocols in Athletes With Primary Subacromial Impingement Syndrome: A Randomized Clinical Trial

CONTEXT

To stabilize the humeral head within the glenoid fossa during arm elevation, the rotator cuff muscles may contribute through internal and external rotation. The main purpose of the current study was to compare the acromiohumeral distance between athletes with primary subacromial impingement syndrome who received progressive resistance exercises consisting of either shoulder internal or external rotation.

DESIGN

A randomized and controlled clinical study.

METHODS

Thirty athletes with primary subacromial impingement syndrome were enrolled and randomly divided into 2 experimental groups. The progressive resistance exercise protocol in experimental group I comprised shoulder internal rotation, while in experimental group II consisted of shoulder external rotation. The experimental groups worked out 3 days a week for 6 weeks. The experimental groups were compared with the control group consisting of 15 healthy athletes. The acromiohumeral distance was measured before and after the intervention using an ultrasound machine under the passive and active across no arm elevation and 45°of arm elevation.

RESULTS

The acromiohumeral distance significantly increased in both experimental groups under the passive and active arm positions following the intervention (P < .001), with no significant differences detected between the experimental groups (P > .665). The paired comparisons of the acromiohumeral distance discrepancy indicated a significant difference between the control group and each experimental group under the active and passive arm positions (P < .001), while no significant difference was observed between the experimental groups (P > .999).

CONCLUSIONS

The present study revealed, for the first time, that both progressive resistance exercise protocols involving either the shoulder internal or the external rotation increased the acromiohumeral distance in individuals with primary subacromial impingement syndrome and improved associated pain and disabilities.

Handball-specific loading acutely reduces the acromiohumeral distance in experienced handball players and in non-handball experienced athletes

Context

When playing handball, the preservation of the subacromial space, which can be quantified by the acromiohumeral distance (AHD), plays a crucial role for shoulder health of handball players. Acute effects of handball-specific loading on the subacromial space with consideration of individual adaptions resulting from long-term handball-specific loading experience have yet to be determined in order to prevent injuries such as e. g. an impingement of the supraspinatus tendon.

Objective

To (1) assess the acute effects of handball-specific loading on the AHD in healthy experienced handball players (HB) and non-handball experienced athletes (CG) and (2) to assess the AHD behavior in relation to individual intrinsic factors to identify possible risk factors and the effect of handball-specific experience associated adaptations.

Participants

20 HB (10m; 10f) and 20 CG (10m; 10f); 24 ± 5 years.

Intervention

Handball-specific loading protocol.

Main outcome measures

The AHD was measured by ultrasonography at 0° and 60° abduction pre and post intervention. Isometric shoulder strength was measured with hand-held dynamometry. Shoulder range of motion (ROM) was measured with goniometry.

Results

Handball-specific loading led to significantly reduced AHD in the dominant shoulder in the 60° abducted position in both groups (HB: −1.7 ± 2.0 mm; p = 0.001, d = 0.69; CG: −1.1 ± 2.0 mm; p = 0.024, d = 0.37) and in the non-dominant shoulder in 0° (−0.7 ± 1.5 mm; p = 0.038, d = 0.35) and 60° abducted position (−1.3 ± 1.8 mm; p = 0.004, d = 0.69) in HB only. Handball-specific loading enhanced AHD reduction when elevating the shoulder from 0° to 60° in both groups and arms. Larger shoulder abduction strength affected the maintenance of the AHD positively. HB demonstrated less shoulder strength compared to CG, while ROM did not differ.

Conclusions

Handball-specific loading can affect the ability to preserve the subacromial space which might put handball players at risk for shoulder injuries. Poor shoulder strength can aggravate this mechanism. Therefore, implementation of strengthening exercises of the external rotator and abductor muscles in the training schedule may improve shoulder health of handball players.

Supraspinatus tendon thickness and subacromial impingement characteristics in younger and older adults

Background

Subacromial impingement (SAI) may be a cause of age-related rotator cuff abnormalities; therefore, the purpose of this study was to compare SAI characteristics between younger and older adults. In addition to the fact that thickened supraspinatus tendon (SST) indicates tendon abnormalities, SAI characteristics have been recognized as follows: greater SST thickness, reduced acromiohumeral distance (AHD), greater reduction of AHD (∆AHD) with arm elevation, and a higher percentage of SST within AHD (i.e., occupation ratio: OcAHD). Furthermore, we investigated the relationships between SST thickness and AHD, as well as SST thickness and ∆AHD to clarify the effect of SAI on rotator cuff abnormalities.

Methods

Healthy younger (n = 18, 21–24-year-old) and older (n = 27, 45–80-year-old) adults without any shoulder symptoms participated in this study. We measured their SST thickness and AHD at rest and at arm elevation (30° and 60°) in the scapular plane using ultrasound, and calculated ∆AHD as the relative change expressed as a percentage of the baseline. OcAHD was expressed as the ratio of SST thickness at rest to AHD at rest and in elevated positions.

Results

The older subjects had approximately one mm thicker SST (P = 0.003, 95% Confidence interval [CI] = 0.410 to 1.895) and approximately 1.0 to 1.3 mm greater AHD than the younger subjects (P = 0.011, 95%CI = 0.284 to 2.068 at rest; P = 0.037, 95%CI = 0.082 to 2.609 for 30° of arm elevation; P = 0.032, 95%CI = 0.120 to 2.458 for 60° of arm elevation). However, there were no differences in ΔAHD and OcAHD between the groups.

Conclusion

This study demonstrated that, compared with the younger subjects, the older subjects showed thicker supraspinatus tendon but no other SAI characteristics including decreases in AHD and increases in OcAHD. Thus, this study suggests that older subjects showed age-related SST abnormalities without SAI, although the magnitude of the differences in SST thickness is notably small and the clinical significance of this difference is unclear.

Effects of Modified Posterior Shoulder Stretching Exercises on Shoulder Mobility, Pain, and Dysfunction in Patients With Subacromial Impingement Syndrome

BACKGROUND

Posterior shoulder stretching exercises (PSSEs) aim to reduce posterior shoulder tightness (PST). Position modification of traditional PSSEs has been suggested to minimize inadequate control of scapular and glenohumeral rotation, possibly leading to increased subacromial impingement.

HYPOTHESIS

Modified PSSEs will have positive effects on shoulder mobility, pain, and dysfunction.

STUDY DESIGN

Randomized controlled trial.

LEVEL OF EVIDENCE

Level 1.

METHODS

A total of 67 symptomatic patients with subacromial impingement syndrome (SIS) and shoulder internal rotation asymmetry were randomly assigned to 3 groups: modified cross-body stretch (MCS) (n = 22; treatment program + MCS), modified sleeper stretch (MSS) (n = 22; treatment program + MSS), and a control group (n = 23; treatment program consisting of only modalities, range of motion [ROM], and strength training but no PSSEs) for 4 weeks. Pain, PST, shoulder rotation ROM, and dysfunction were evaluated.

RESULTS

Pain, PST, shoulder rotation ROM, function, and disability improved in all groups (P < 0.05). The MCS and MSS groups had better results compared with the control group with regard to pain with activity, internal rotation ROM, function, and disability (P < 0.05). There was no significant difference between the stretching groups (P > 0.05).

CONCLUSION

All treatments improved pain, shoulder mobility, function, and disability in patients with SIS. However, modified PSSEs in addition to a treatment program was superior to the treatment program alone (without PSSEs) in improving pain with activity, internal rotation ROM, and dysfunction. Moreover, stretching provided clinically significant improvements.

CLINICAL RELEVANCE

Modified PSSEs, in addition to a treatment program, are beneficial for patients with SIS. Both modified cross-body and sleeper stretches are safe and efficacious for improving shoulder mobility, pain, and dysfunction.

Shoulder kinematics impact subacromial proximities: a review of the literature

BACKGROUND

Alterations in glenohumeral and scapulothoracic kinematics have been theorized to contribute to rotator cuff pathology by impacting the magnitude of the subacromial space.

OBJECTIVE

The purpose of this review is to summarize what is currently known about the relationship between shoulder kinematics and subacromial proximities.

CONCLUSIONS

A variety of methods have been used to quantify subacromial proximities including photographs, MR imaging, ultrasonography, and single- and bi-plane radiographs. Changes in glenohumeral and scapulothoracic kinematics are associated with changes in subacromial proximities. However, the magnitude and direction of a particular motion's impact on subacromial proximities often vary between studies, which likely reflects different methodologies and subject populations. Glenohumeral elevation angle has been consistently found to impact subacromial proximities. Plane of humeral elevation also impacts subacromial proximities but to a lesser degree than the elevation angle. The impact of decreased scapulothoracic upward rotation on subacromial proximities is not absolute, but instead depends on the angle of humerothoracic elevation. The effects of scapular dyskinesis and humeral and scapular axial rotations on subacromial proximities are less clear. Future research is needed to further investigate the relationship between kinematics and subacromial proximities using more homogenous groups, determine the extent to which compression and other factors contribute to rotator cuff pathology, and develop accurate and reliable clinical measures of shoulder motion.

Active Scapular Retraction and Acromiohumeral Distance at Various Degrees of Shoulder Abduction

CONTEXT

  Performing shoulder-abduction exercises with scapular retraction has been theorized to reduce the potential for shoulder impingement. However, objective data to support this premise are lacking.

OBJECTIVE

  To evaluate the influence of active scapular retraction on acromiohumeral distance (AHD) at 4 shoulder-abduction angles using real-time ultrasound.

DESIGN

  Cross-sectional study.

SETTING

  University laboratory.

PATIENTS OR OTHER PARTICIPANTS

  Twenty asymptomatic individuals (10 men, 10 women; age = 22.9 ± 2.8 years, height = 169.3 ± 9.5 cm, mass = 65.5 ± 12.9 kg) were recruited.

MAIN OUTCOME MEASURE(S)

  Real-time ultrasound images of AHD were obtained during nonretracted and retracted scapular conditions at 0°, 45°, 60°, and 90° of shoulder abduction. A 2-factor analysis of variance with repeated measures was used to evaluate the influence of shoulder retraction on AHD across shoulder-abduction angles.

RESULTS

  A scapular-retraction condition × shoulder-abduction-angle interaction for AHD was found ( F_3,57 = 4.56, P = .006). The AHD was smaller at 0° (10.5 versus 11.2 mm, respectively; t₁₉ = 2.22, P = .04) but larger at 90° (9.4 versus 8.7 mm, respectively; t₁₉ = -2.30, P = .04) of shoulder abduction during the retracted than the nonretracted condition. No differences in AHD were observed between conditions at 45° ( t₁₉ = 1.45, P = .16) and 60° ( t₁₉ = 1.17, P = .86) of abduction.

CONCLUSIONS

  The observed differences in AHD at 0° and 90° of shoulder abduction were small and did not exceed the established minimal detectable change for either angle. Our findings suggest that active scapular retraction during shoulder abduction has a minimal influence on AHD at 0° and 90° in healthy individuals. Further investigations are needed to determine whether scapular retraction influences AHD in individuals with subacromial impingement.

Effect of glenohumeral elevation on subacromial supraspinatus compression risk during simulated reaching

Mechanical subacromial rotator cuff compression is one theoretical mechanism in the pathogenesis of rotator cuff disease. However, the relationship between shoulder kinematics and mechanical subacromial rotator cuff compression across the range of humeral elevation motion is not well understood. The purpose of this study was to investigate the effect of humeral elevation on subacromial compression risk of the supraspinatus during a simulated functional reaching task. Three-dimensional anatomical models were reconstructed from shoulder magnetic resonance images acquired from 20 subjects (10 asymptomatic, 10 symptomatic). Standardized glenohumeral kinematics from a simulated reaching task were imposed on the anatomic models and analyzed at 0, 30, 60, and 90° humerothoracic elevation. Five magnitudes of humeral retroversion were also imposed on the models at each angle of humerothoracic elevation to investigate the impact of retroversion on subacromial proximities. The minimum distance between the coracoacromial arch and supraspinatus tendon and footprint were quantified. When contact occurred, the magnitude of the intersecting volume between the supraspinatus tendon and coracoacromial arch was also quantified. The smallest minimum distance from the coracoacromial arch to the supraspinatus footprint occurred between 30 and 90°, while the smallest minimum distance to the supraspinatus tendon occurred between 0 and 60°. The magnitude of humeral retroversion did not significantly affect minimum distance to the supraspinatus tendon except at 60 or 90° humerothoracic elevation. The results of this study provide support for mechanical rotator cuff compression as a potential mechanism for the development of rotator cuff disease. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2329-2337, 2017.

Full can test: Mechanisms of a positive test in patients with shoulder pain

BACKGROUND

The full can test is theorized to produce compressive loads on the supraspinatus tendon within the subacromial space. Characterizing the width of the subacromial outlet, scapular orientation, and shoulder pain during the full can test will improve the mechanistic understanding of the positive full can test.

METHODS

Cross-sectional repeated measures design. Participants with subacromial pain syndrome (n=30) were compared to a matched control group (n=30) during 2 conditions: passive support, and the full can test. The full can test was performed with the arm elevated to 90° in the scapular plane. In both conditions, measurements were taken of acromiohumeral distance with ultrasonography, scapular position using electromagnetic tracking, shoulder strength using a dynamometer, and shoulder pain with the 11-point rating scale.

FINDINGS

During the full can test, both groups had a decreased acromial humeral distance, scapular upward rotation, posterior tilt, external rotation and clavicular protraction as compared to passive support (p<0.05). The subacromial pain group as compared to the control group reported greater shoulder pain (p<0.001), reduced strength (p=0.002) and greater scapular anterior tilt (p<0.05) during the full can test.

INTERPRETATION

This study indicates the mechanisms of a full can test are a reduction in the acromial humeral distance, accompanied by scapular kinematic changes. A positive test of increased pain and reduced strength in those with subacromial pain syndrome can be explained additionally by an increase in scapular anterior tilt. These mechanistic changes may lead to tendon compression, but this cannot be verified as direct tendon compression was not measured.

Do isometric pull-down exercises increase the acromio-humeral distance?

OBJECTIVES

To evaluate the effect of isometric shoulder extension in 90° shoulder flexion on the acromio-humeral distance, to establish the force required to achieve a clinically important increase in the acromio-humeral distance, and to investigate the practicality and reliability of real-time ultrasound measurement of the acromio-humeral distance in 90° shoulder forward flexion.

DESIGN

Prospective single-group intervention.

SETTING

King's College London, Guy's Campus.

PARTICIPANTS

Twenty healthy volunteers [five males and 15 females (40 shoulders)] with a mean age of 32 (standard deviation 10, range 19 to 55) years were recruited from the faculty and staff at King's College London.

INTERVENTIONS

The acromio-humeral distance in asymptomatic participants was measured using real-time ultrasound in the neutral position at rest, at 90° shoulder flexion at rest, and while performing an isometric pull-down exercise at 100%, 50%, 30% and 10% maximal voluntary isometric contraction.

MAIN OUTCOME MEASURES

Real-time ultrasound measures of the acromio-humeral distance.

RESULTS

Of the 20 participants, 38 shoulders were imaged. In 90° shoulder flexion, pull-down exercises at all levels of force increased the acromio-humeral distance compared with no pull-down (P<0.05), but this was only clinically significant in males. Measures had excellent short-term intra-operator reliability.

CONCLUSIONS

Isometric pull-down exercises lead to an increase in the acromio-humeral distance in asymptomatic males that may be clinically important, and therefore may be an appropriate exercise for patients with shoulder pathology. Ultrasound measurement of the acromio-humeral distance in 90° shoulder flexion is practical and reliable.

Distribution of bone and tissue morphological properties related to subacromial space geometry in a young, healthy male population

PURPOSE

Particular bone and tissue morphological features of the scapula and humerus often exist disproportionately in persons with subacromial impingement syndrome (SAIS) and/or rotator cuff pathology. However, the origins of morphological variation, genetic or mechanistic, remain unclear. This research evaluated the distribution of and correlation between several bone and tissue characteristics associated with these pathologies amongst a baseline cohort population consisting of young, healthy, males. As well, the predisposition to SAIS was estimated by calculating the ratio (occupation ratio) of subacromial tissue thickness to minimum subacromial space width (SAS).

METHODS

Anterior-posterior and trans-scapular radiographs and musculoskeletal ultrasound were used to measure morphological characteristics related to the subacromial space. Each bone morphological characteristic was classified as healthy or unhealthy based on previous definitions. Supraspinatus tendon and subacromial bursa thicknesses were used to calculate the occupation ratio from both radiographic and ultrasonic measures of the SAS.

RESULTS

Each characteristic demonstrated considerable variability, with some participants having 'unhealthy' variants for each bone characteristic examined. The percentage of the population with bone characteristics classified as "unhealthy" ranged from 15 to 55 % across characteristics evaluated. The strongest correlation existed between the acromion index and the minimum subacromial space width (-0.59) suggesting that a larger lateral extension of the acromion may predispose an individual to SAIS. The average occupation ratio was 65.3 % with a 1-99 % confidence interval ranging from 21.6 to 108.9 %.

CONCLUSIONS

The distributions of both morphological characteristics and occupation ratios indicate that individuals within this healthy, baseline population have a highly differential predisposition for subacromial tissue compression solely based on inherent morphological variation. This suggests that while mechanistic and/or age-related degenerative changes may contribute to SAIS and eventual rotator cuff pathology, intrinsic predisposing geometry should not be discounted.

An evidence-based review of current perceptions with regard to the subacromial space in shoulder impingement syndromes: Is it important and what influences it?

BACKGROUND

Reduction of the subacromial space as a mechanism in the etiology of shoulder impingement syndromes is debated. Although a reduction in this space is associated with shoulder impingement syndromes, it is unclear if this observation is cause or consequence.

METHOD

The purposes of this descriptive review are to provide a broad perspective on the current perceptions with regard to the pathology and pathomechanics of subacromial and internal impingement syndromes, consider the role of the subacromial space in impingement syndromes, describe the intrinsic and extrinsic mechanisms considered to influence the subacromial space, and critique the level of evidence supporting these concepts.

FINDING

Based on the current evidence, the hypothesis that a reduction in subacromial space is an extrinsic cause of impingement syndromes is not conclusively established and the evidence permits no conclusion.

INTERPRETATION

If maintenance of the subacromial space is important in impingement syndromes regardless of whether it is a cause or consequence, research exploring the correlation between biomechanical factors and the subacromial space, using the later as the outcome measure, would be beneficial.

Supraspinatus tendon and subacromial space parameters measured on ultrasonographic imaging in subacromial impingement syndrome

PURPOSE

To characterize the supraspinatus tendon thickness, subacromial space, and the relationship between tendon thickness and subacromial space to further elucidate the mechanisms of subacromial impingement syndrome.

METHODS

In a single-blind cross-sectional study, subjects were recruited with subacromial impingement syndrome (n = 20) and asymptomatic controls (n = 20) matched for age, gender, and hand dominance. Ultrasound images were collected using a 4-12-MHz linear transducer in B-mode of the supraspinatus tendon in the transverse (short axis) and the anterior aspect of the subacromial space outlet. Using image callipers, measurements of tendon thickness were taken at 3 points along the tendon and averaged for a single thickness measure. The subacromial space outlet was measured via the acromiohumeral distance (AHD) defined by the inferior acromion and superior humeral head. The occupation ratio was calculated as the tendon thickness as a percentage of AHD.

RESULTS

The subacromial impingement syndrome group had a significantly thicker tendon (mean difference = 0.6 mm, p = 0.048) and a greater tendon occupation ratio (mean difference = 7.5 %, p = 0.014) compared to matched controls. There were no AHD group differences.

CONCLUSIONS

The supraspinatus tendon was thicker and occupied a greater percentage of AHD, supporting an intrinsic mechanism. An extrinsic mechanism of tendon compression is theoretically supported, but future imaging studies need to confirm direct compression with elevation. Treatment to reduce tendon thickness may reduce symptoms, and surgical intervention to increase subacromial space may be considered if tendon compression can be verified.

Dynamic comparison of segmentary scapulohumeral rhythm between athletes with and without impingement syndrome

Background:

Patients who have shoulder pain usually have compensatory or contributory deviation of shoulder motion during arm elevation. In the traditional scapulohumeral rhythm, the share of the acromioclavicular (AC) and the sternoclavicular (SC) joint movements and also the role of AC internal rotation angle are unknown.

Objectives:

The main purpose of this study was to measure and compare the segmentary scapulohumeral rhythm (SSHR) during scapular arm elevation at a steady rotational speed in athletes with and without impingement syndrome.

Patients and Methods:

Using a speedometer, the maximum speed of arm elevation was measured in 21 men in each of the involved and uninvolved groups. Using fluoroscopy on the dominant side, SSHR during scapular arm elevation at a rotational speed equal to 1/30 of maximum speed was compared between the two groups. The ratio of glenohumeral (GH) elevation angle to AC rotation angle in the scapular plane was considered as SSHR.

Results:

The maximum speed of arm elevation between the two groups was significantly different (P < 0.001). The rhythm of the involved group significantly exceeded the rhythm of the uninvolved group in a part of the first quarter range of the arm elevation. SSHR during arm elevation in the uninvolved group did not change significantly (P = 0.845); however, it decreased significantly in the involved group (P = 0.024).

Conclusions:

Speed differences between the two groups were probably due to the pain in some ranges of arm elevation. SSHR in the involved group probably changed in order to compensate downward rotation of the scapula in the resting position. Study of the AC upward rotation range can be misleading; therefore, the study of scapulohumeral rhythm is recommended.

Scapular kinematics during shoulder elevation performed with and without elastic resistance in men without shoulder pathologies

STUDY DESIGN

Controlled laboratory study using within-group comparisons. Objectives To compare scapular kinematics between active and resisted shoulder elevation performed in the sagittal (flexion), frontal (abduction), and scapular (scapular abduction) planes.

BACKGROUND

Several studies have documented scapular kinematics during arm elevation against an external load; however, there is little information on how scapular kinematics change with loading provided by elastic bands, an exercise approach often used in the clinic.

METHODS

Thirty-two men without shoulder pathology participated in the study. The level of resistance to be used for each individual was determined prior to data collection and standardized by perceived effort on a Borg scale. Three-dimensional scapular kinematics were recorded with an electromagnetic tracking device in all 3 planes of shoulder elevation for both the unloaded (active) and loaded (resisted) conditions. Data for scapular kinematics were analyzed at 30°, 60°, 90°, and 120° of humerothoracic elevation and lowering. Comparisons between loading conditions were made using analysis-of-variance models.

RESULTS

In general, for all 3 planes of movement, the scapula was more downwardly rotated and anteriorly tilted during the elevation phase and more so during the lowering phase of shoulder elevation when performed against elastic resistance. While some of the statistically significant differences might not have been large enough to be considered clinically meaningful, some values were of a magnitude similar to previously reported differences between healthy and symptomatic individuals.

CONCLUSIONS

The changes in scapular motion during the loaded condition were relatively small in this population with normal scapular motion, but they were in a direction that would be considered to have potential to lead to injuries, suggesting caution when using these exercises in individuals with poor scapular control.

Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests

BACKGROUND

The empty and full can arm positions are used as diagnostic tests and in therapeutic exercise programs for patients with subacromial impingement syndrome. The adverse effects of these arm positions on the rotator cuff have not been fully described. The purpose of this investigation was to compare the acromio-humeral distance, three-dimensional scapular position, and shoulder pain during maximum isometric contractions in the empty and full can arm positions.

METHODS

Subjects with subacromial impingement syndrome (n=28) and a matched control group without shoulder pain (n=28) participated. Acromio-humeral distance, scapular/clavicular positions and shoulder pain were measured during maximal isometric contractions in each position.

FINDINGS

No difference was found in acromio-humeral distance (P=0.314) between the arm positions or between the groups (P=0.598). The empty can position resulted in greater scapular upward rotation (P<0.001, difference=4.9°), clavicular elevation (P<0.001, difference=2.7°), clavicular protraction (P=0.001, difference=2.5°) and less posterior tilt (P<0.001, difference=3.8°) than the full can position. No differences in the scapular positions were found between the groups. Positive correlations were seen between the scapular positions in the control and not in the subacromial impingement group.

INTERPRETATION

Our results did not show a difference in acromio-humeral distance between the arm positions or groups, indicating that the kinematic differences between the positions are not associated with altered acromio-humeral distance. The increased pain in the EC position might be due to the lack of an association amongst the scapular positions rather than the deficiency of a single scapular motion.

The effects of arm elevation on the 3-dimensional acromiohumeral distance: a biplane fluoroscopy study with normative data

HYPOTHESIS AND BACKGROUND

Narrowing of the subacromial space has been implicated in several shoulder pathologies. However, the location of the minimum distance points during clinical testing has not been defined. We sought to measure the in vivo minimum distance and location of the minimum distance points on the acromion and proximal humerus during arm elevation.

METHODS

Eight healthy male subjects (mean age, 30 years) underwent a dynamic in vivo biplane fluoroscopy assessment of scaption and forward elevation. For each frame, the 3-dimensional position and orientation of the humerus and scapula were determined, and the acromiohumeral distance (AHD) was measured as the shortest distance between the acromion and proximal humerus.

RESULTS

The minimum AHD was 2.6 ± 0.8 mm during scaption and 1.8 ± 1.2 mm during forward flexion at elevation angles of 83° ± 13° and 97° ± 23°, respectively. The minimum distance point was located on the articular surface of the humeral head from the neutral arm position until 34° ± 8° for scaption and 36° ± 6° for forward flexion. Upon further elevation, the minimum distance point was located within the footprint of the supraspinatus muscle until 72° ± 12° for scaption and 65° ± 8° for forward flexion. At greater elevation angles, the minimum distance points were between the acromion and the proximal humeral shaft, distal from the greater tuberosity.

CONCLUSIONS

The shortest AHD was at approximately 90° of arm elevation. The AHD was no longer measured intra-articularly or within the supraspinatus footprint above approximately 70° of arm elevation.