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. 2013 Dec 10;472(8):2397–2403. doi: 10.1007/s11999-013-3419-3

Effects of External Rotation on Anteroposterior Translations in the Shoulder: A Pilot Study

Andrew J Brown 1, Richard E Debski 1, Carrie A Voycheck 1, Patrick J McMahon 1,2,✉
PMCID: PMC4079883  PMID: 24323688

Abstract

Background

Using physical examination to make the diagnosis of shoulder instability can be difficult, because typical examination maneuvers are qualitative, difficult to standardize, and not reproducible. Measuring shoulder translation is especially difficult, which is a particular problem, because measuring it inaccurately may result in improper treatment of instability.

Questions/purposes

The objective of this study was to use a magnetic motion tracking system to quantify the effects of external rotation of the abducted shoulder on a simulated simple translation test in healthy subjects. Specifically, we hypothesized that (1) increasing external rotation of the abducted shoulder would result in decreasing translation; (2) intraobserver repeatability would be less than 2 mm at all external rotation positions; and (3) mean side-to-side differences would be less than 2 mm at all external rotation positions.

Methods

The intraobserver repeatability and side-to-side differences of AP translation were quantified with a noninvasive magnetic motion tracking system and automated data analysis routine in nine healthy subjects at four positions of external rotation with the arm abducted. A shoulder positioning apparatus was used to maintain the desired arm position.

Results

No differences in translations between the positions of external rotation were found (p = 0.48). Intraobserver repeatability was 1.1 mm (SD, 0.8 mm) and mean side-to-side differences were small: 2.7 mm (SD, 2.8 mm), 2.8 mm (SD, 1.8 mm), 2.5 mm (SD, 1.8 mm), and 4.0 mm (SD, 2.6 mm) at 0°, 20°, 40°, and 60° of external rotation, respectively.

Conclusions

The intraobserver repeatability was strong and the side-to-side differences in translation were small with the magnetic motion tracking system, which is encouraging for development of an improved quantitative test to assess shoulder translation for fast and low-cost diagnosis of shoulder instability.

Clinical Relevance

Clinicians may not have to position the contralateral, normal, abducted shoulder in precisely the same position of external rotation as the injured shoulder while performing side-to-side comparisons.

Introduction

The glenohumeral joint is the most commonly dislocated major joint in the body with approximately 2% of people sustaining a dislocation between the ages of 18 and 70 years [12, 23]. Physical examination has long been used in the diagnosis of shoulder instability [1], but current tests are qualitative and subjective, as evidenced by the numerous different available tests and the absence of consensus on one being most effective. Several tests for shoulder instability depend on the subjective reaction of the patient [7, 16, 22]. Others rely on a qualitative grading of joint translations, but poor intra- and interobserver repeatability hampers diagnosis and comparison of study results [11, 14, 15, 18, 19, 27] and can result in improper treatment of shoulder instability.

Radiography [6, 8, 11], ultrasound [2, 4, 5, 13], or currently available arthrometers for the knee have been used to quantify the loads and translations during physical examination tests for shoulder instability [3, 25]. A magnetic tracking system has also been used that is noninvasive and allows measurement of translations based on a small motion sensor held on the subject’s skin. This method was first used in a study of cadaveric shoulders during a simple translation test and intraobserver repeatability was found to be strong when assessed as the agreement of five successive anterior and posterior translations [24]. Later studies successfully used the same magnetic tracking system to detect differences in shoulder translations between healthy overhead and nonoverhead athletes [26] as well as in unstable shoulders before and after surgical repair [17]. Although clinically relevant differences in shoulder translations have been reported with this system, intraobserver repeatability of the measurements has not been reported with human subjects in vivo.

Integral to such assessments of shoulder instability are the side-to-side differences in translation. Because of variations in shoulder laxity, normal translations vary among individuals [10]. Fortunately for clinicians interested in using the contralateral shoulder as an indicator of normal translations, prior quantitative assessments with a magnetic tracking system have revealed side-to-side differences in shoulder translations to be small [24, 26]. Unfortunately, these prior quantitative studies were performed at a single shoulder position of abduction and neutral rotation. Instead, abduction and external rotation is the important apprehension position for evaluating anterior instability of the shoulder. Patients with instability are usually asymptomatic except when the shoulder is placed into the abduction and external rotation position. Therefore, this position should be the focus when evaluating the shoulder for anterior stability. In addition, clinicians interested in a fast method of assessing shoulder instability such as performing a physical examination of the shoulder of a football player injured on the field of play are often unable to put both shoulders in precisely the same position for side-to-side comparison. Thus, the effects of differences in shoulder position on the translations measured during a physical examination are also needed.

Our long-term goal is to improve and further develop noninvasive quantitative translation tests for the fast and low-cost diagnosis of shoulder instability. The objective of this study was to assess the effects of external rotation of the abducted shoulder on translations in healthy subjects during a simple translation test using a magnetic motion tracking system. We hypothesized that (1) increasing external rotation of the abducted shoulder would result in decreasing translation; (2) intraobserver repeatability would be less than 2 mm at all external rotation positions; and (3) mean side-to-side differences would be less than 2 mm at all external rotation positions.

Patients and Methods

Patient recruitment and experimental protocols were approved by the institutional review board at our institution. A total of nine patients were enrolled in the study. There were six women and three men with a mean age of 20 years (SD, 1 year). Seven had right-hand dominance. Mean body mass index was 22 kg/m2 (SD, 3 kg/m2). After informed consent was obtained, patients’ heights, weights, and brief medical histories related to shoulder use and injuries were obtained. Exclusion criteria were a history of shoulder injury, including dislocation, rotator cuff tears, osteoarthritis, or shoulder pain as well as involvement in overhead-throwing activities such as baseball or swimming. All nine patients met the criteria for inclusion in the study and completed the experimental protocol.

Patients were positioned on the examination table in a supine position with their right shoulder positioned at the edge of the table. Initially, each patient’s right shoulder was placed in 90° of humerothoracic abduction and neutral external rotation. Neutral external rotation was defined as the forearm being perpendicular to the patient’s chest with the elbow flexed 90° as measured with a goniometer (Fig. 1A). The elbow and lower arm were then placed in a shoulder-positioning apparatus and secured with nylon cord. The shoulder-positioning apparatus allowed patients to relax and is metal-free for use with a magnetic motion tracking system. The elbow and armrest portion of the shoulder-positioning apparatus were adjustable to allow each shoulder to be consistently positioned in 90° of abduction and in different positions of external rotation.

Fig. 1A–C.

Fig. 1A–C

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(A) A patient’s arm is shown secured in the shoulder positioning apparatus at 90° humerothoracic abduction and neutral external rotation. (B) Placement of a humeral sensor on the patient’s skin over the proximal humerus at the midbicipital groove is highlighted with a circle. The anatomic coordinate system corresponding to the motion tracking system is also displayed. A/P = anteroposterior; M/L = mediolateral S/I = superoinferior. (C) More detailed view of the humeral sensor on the patient’s midbicipital groove.

A magnetic motion tracking system (Flock of Birds®; Ascension Technology Corp, Burlington, VT, USA) was used to noninvasively measure motion. The axes of the system’s transmitter were aligned with the anatomic axes of the subject’s thorax (Fig. 1B). The accuracy of the motion tracking system was quantified in the environment used for testing throughout the study. The motion tracking sensor was moved between known distances, along all three axes, as measured with calipers. The range of the average differences between the known (calipers) and measured (sensor) translations was found to be ± 0.7 mm. The AP translation of the system’s sensor was the primary outcome variable examined in this study.

Initially, the amount of soft tissue deformation required to rigidly hold the sensor against the required anatomic landmarks was determined. An orthopaedic surgeon (PJM), who is a shoulder specialist with more than two decades of clinical experience, placed the motion sensor on the skin overlying the proximal right humerus at the midbicipital groove (Fig. 1C) and underneath his thumb [24]. The sensor was then depressed six times (Fig. 2) into the soft tissues while stabilizing the scapula and the glenohumeral joint. This procedure was then repeated on the left shoulder. The amount of soft tissue deformation on each arm corresponded to the AP translation from the motion tracking system. The side-to-side difference in soft tissue deformation was determined by subtracting the soft tissue deformation of the left arm from the right arm.

Fig. 2A–D.

Fig. 2A–D

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A graph shows a typical AP translation curve: AP translation of motion sensor for each trial (A), the motion sensor was depressed to a firm end point in the skin to account for soft tissue deformation before application of six manual maximum AP loads (B), an automated data analysis routine identified 10 end points of the translation curve (C), and five AP translations were calculated based on the 10 end points of translation, the average of which was the translation reported in each trial (D).

The mean deformation of the soft tissues overlying the proximal right humerus at the midbicipital groove was 14 mm (SD, 6 mm; range, 10–36 mm). A squared correlation coefficient of r2 = 0.04 was calculated between the soft tissue deformation and the patient’s body mass index. The mean side-to-side difference in soft tissue deformation was 3.4 mm (SD, 4.2 mm). Soft tissue deformation could also be observed on the AP translation curves from the test as a result of the depression of the motion sensor before loading (Fig. 2A). Soft tissue deformations were similar to translations during simulated simple translation testing. However, by fully depressing the motion sensor into the skin before performing the shoulder translation testing, the soft tissue deformation could be minimized.

The intraobserver repeatability of the translation measurements during application of an AP load was then assessed on each patient’s right arm. The patient’s right shoulder was again positioned in 90° of humerothoracic abduction and neutral external rotation and secured within the shoulder-positioning apparatus. The orthopaedic surgeon placed a motion sensor on the skin overlying the midbicipital groove of the right humerus and stabilized the patient’s scapula. The sensor was then fully depressed into the skin and while attempting to apply forces in only the AP direction. Six continuous cycles of AP loads were applied to the patient’s proximal humerus to simulate a simple translation test for instability. The end points in the anterior and posterior directions were those obtained with a manual maximum force. The sensor was then removed, the patient’s arm was removed from the shoulder-positioning apparatus, and after a 5-minute rest, testing was repeated. This procedure was then repeated two more times. Intraobserver repeatability was defined as the SD of the three translations measured for each patient.

AP translation was defined as the average of the five AP translations for each cycle of loading (Fig. 2). These values were extracted from the AP translation curves obtained during each test. The magnitude of AP translation for each cycle of loading was calculated by subtracting the translational end point during anterior loading from the translational end point during posterior loading as previously reported [17, 24, 26].

The magnitude of translation for the left and right shoulders in multiple external rotation angles was then determined in a similar manner. The left and right shoulders were placed in 0°, 20°, 40°, and 60° of external rotation, as measured with a goniometer from the angle made between the forearm and the patient’s thorax in a randomized order. After pressing the sensor fully into the soft tissues overlying the proximal right humerus at the midbicipital groove, the orthopaedic surgeon applied six cycles of AP loads at each position. The side-to-side differences were computed for each patient at each position by subtracting the left humerus translation from the right humerus translation.

The raw translations measured by the magnetic motion tracking system were input to an automated MATLAB® data analysis routine (The Mathworks Inc, Natick, MA, USA) that calculated the mean AP translation during each trial. Statistical analyses were performed using SPSS® software (IBM Corp, Chicago, IL, USA). Differences between the positions of external rotation and between translations from left and right shoulders were analyzed using a two-way repeated-measures analysis of variance (ANOVA). For intraobserver repeatability, an intraclass correlation coefficient was calculated and a one-way repeated-measures ANOVA was performed to detect any differences in translations among the three trials for each patient. Statistical significance was set at an α value of 0.05 for all analyses.

Post hoc power analyses were performed using G*Power (Universitat Kiel, Kiel, Germany) to calculate statistical power for effect of joint position comparisons and side-to-side difference comparisons because statistical significance was not achieved. An effect size for each experiment was calculated based on experimental data and used to calculated power at a significance level of α = 0.05. The calculated effect size from experimental data was also used to perform an a priori power analysis for each experiment to determine the number of subjects required to achieve statistical significance with α = 0.05 and power = 0.8.

Results

The amount of external rotation did not influence shoulder translation. The mean translations at 0°, 20°, 40°, and 60° of external rotation were 17 mm (SD, 4 mm), 16 mm (SD, 3 mm), 17 mm (SD, 4 mm), and 17 mm (SD, 4 mm), respectively (Fig. 3). There were no significant differences in translations between different positions of external rotation (p = 0.48). Our post hoc power analysis yielded power equal to 0.07 for the comparisons between different positions of external rotation. Using the effect size computed from experimental data (f = 0.11), an a priori power analysis estimated 258 subjects would be required to reach 80% power.

Fig. 4.

Fig. 4

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A graph shows intraobserver repeatability for measuring translations from three trials of AP loading of patients’ right side (n = 9). The intraobserver repeatability for all patients was 1.1 mm (SD, 0.8 mm) with the maximum intrasubject repeatability being 2.6 mm. S1 to S9 = Subject 1 to Subject 9. Bar = SD.

Fig. 3.

Fig. 3

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A graph shows the mean translations from AP loading of the right humerus at 0°, 20°, 40°, and 60° external rotation (ER) for all patients (n = 9). The mean translations at 0°, 20°, 40°, and 60° of external rotation were 17 mm (SD, 4 mm), 16 mm (SD, 3 mm), 17 mm (SD, 4 mm), and 17 mm (SD, 4 mm), respectively (Fig. 4) (p = 0.48). S1 to S9 = Subject 1 to Subject 9. Bars = SD.

The intraobserver repeatability was high and the intrasubject variability was low in these tests. The mean translation for all patients’ right sides at 0° was 16 mm (SD, 3 mm; range, 10–22 mm). The intraobserver repeatability for all patients was 1.1 mm (SD, 0.8 mm) with the maximum intrasubject repeatability being 2.6 mm (Fig. 3). The intraclass correlation coefficient was calculated to be 0.89. No significant differences in translations were found among the three trials for each patient (p = 0.97).

Although the mean side-to-side differences were more than 2 mm at all external rotation positions, it did not vary according to the position of external rotation with the numbers available. The mean side-to-side differences at 0°, 20°, 40°, and 60° of external rotation were 2.7 mm (SD, 2.8 mm), 2.8 mm (SD, 1.8 mm), 2.5 mm (SD, 1.8 mm), and 4.0 mm (SD, 2.6 mm), respectively. The side on which the larger translation was measured was evenly distributed between arms (18 left, 18 right) for the 36 trials performed, was not significantly different (p = 0.88), and did not correspond to hand dominance of either the patient or the examiner. Statistical power equal to 0.34 was calculated for the comparisons between the left and right translations. Using the effect size computed from experimental data (f = 0.23), an estimated 24 subjects would be required to reach 80% power.

Discussion

The development of a physical examination for shoulder instability diagnosis is hampered by the qualitative nature, lack of standardization, and low repeatability of our current tests. These problems result in difficulties measuring shoulder translations, which can result in improper treatment of instability. Using a proven magnetic motion tracking system, we quantified the effects of external rotation of the abducted shoulder on a simulated simple translation test of healthy subjects. Intraobserver repeatability was strong and side-to-side differences in translation were small supporting this portion of our hypothesis. Our post hoc power analysis calculated power equal to 0.34, which is influenced by both the small effect size of our experimental data and the number of subjects used. Although additional subjects would improve the power of this analysis, a clinically relevant difference in translation would provide a much larger effect size.

We also found that the measured translations did not decrease as external rotation of the abducted shoulder was increased, disproving this portion of our hypothesis. Again, our post hoc power analysis found our analysis was underpowered as a result of the very small effect size measured with power equal to 0.07. We believe that it is unlikely that increasing the number of subjects would change these conclusions, especially if a more clinically relevant effect size were used. Prior studies have not quantified in vivo shoulder translations at different external rotation positions using noninvasive physical examinations [2–5, 7, 10, 11, 14, 15, 17, 18, 24–26]. Importantly, normal translations were similar in the different joint positions tested, indicating that assessment of in vivo translations may not be sensitive to the differences in joint position studied. This finding supports the conclusion of Krarup et al. [13] who found no differences in anterior translation of normal shoulders among neutral, internal, and 60° external rotation in an ultrasound imaging study. This is promising for improvement and further development of noninvasive quantitative translation testing, because it would not require precise positioning in external rotation of the patient’s abducted shoulder to diagnose side-to-side differences in translation. In other words, a clinician may not have to position the contralateral shoulder in the exact same position as the injured shoulder while performing the test.

Although encouraging in improvement of a quantitative translation test of the shoulder, prior biomechanical studies [9, 10, 20] had led us to hypothesize that translations of the shoulder in the abduction and external rotation position would be less than those in the abducted and neutral rotation position that were tested previously. Moore et al. [20] used a magnetic motion tracking system to assess AP glenohumeral translations in cadaveric shoulders during a simulated simple translation test. They found that anterior translation decreased with the abducted shoulder positioned in 30° of external rotation compared with neutral rotation and also decreased at 60° of external rotation. Different motions of the scapula in different positions of external rotation in vivo that were not assessed in the cadaveric study, as the scapula was fixed, may have accounted for this, or differences in soft tissue deformation when the shoulder is externally rotated may have prevented us from finding such a difference.

The translation at neutral rotation among our nine healthy subjects was 16.6 mm, which is similar to that reported by Tibone et al. who found translations of 10.1 and 13.0 mm in healthy soccer players and swimmers, respectively [28], and Sethi et al. who reported translations of 29.3 mm [26]. Our side-to-side difference of 2.7 mm at neutral rotation agrees well with the side-to-side difference in healthy subjects of 2.1 mm reported by Reis et al. [24] and 0.2 mm reported by Sethi et al. [26].

A limitation of our study was that we did not quantify the force applied during the quantitative translation test. We attempted to standardize the force applied by using a manual maximum force until a firm end point was reached and the intraobserver repeatability of these measurements in healthy subjects was acceptable. However, Musahl et al. [21] found that force feedback affected the translations assessed by clinicians and could subsequently affect diagnosis. Standardization of applied force, in the same manner the KT1000™ is used for diagnosis of anterior cruciate ligament rupture, may improve repeatability or affect translations measured in subjects with shoulder instability. Another limitation was that only one examiner performed the translations.

Another limitation was the motion sensors that were placed on the skin, rather than sensors fixed rigidly into bone, are affected by the soft tissue and differences in the amount of soft tissue between subjects and may have affected measured translations. Although firmly depressing the sensor prior to loading the joint and keeping the sensor firmly depressed during cycles of loading minimized the effect of the soft tissue, it did not eliminate it. Scapular motion was not accounted for during testing and may have affected the measured translations because the scapula cannot be easily fixed during typical physical examinations in a clinical setting. We examined shoulder translations at external rotation up to 60° only. Different parts of the shoulder capsule may be strained at higher external rotations as well as at higher degrees of abduction; thus, translations may differ at these positions, particular in patients with shoulder instability.

We also did not study patients with shoulder instability. Although a prior study has quantified in vivo shoulder translations in subjects diagnosed with shoulder instability [17], none studied different positions of external rotation that simulate a shoulder instability test. Performing the shoulder translation testing at different positions of external rotation on patients with shoulder instability would place different regions of the injured capsule under strain.

In the future, we will validate the conclusions of this study by measuring intraobserver and interobserver repeatability, translations at different positions of external rotation, and side-to-side differences of individuals with shoulder instability. These future studies will determine whether there is an optimal joint position that results in side-to-side differences in joint translation that are larger than those in healthy individuals and would allow for a fast, effective, and low-cost diagnosis of shoulder instability. We suspect there will be a range of translations suggestive of shoulder instability and a value that will be diagnostic, similar to use of KT1000™ in standardizing the Lachman test of the knee for aid in diagnosis of anterior cruciate ligament rupture.

The results of this study suggest that it may be possible to improve on current physical examinations and further develop a fast, effective, inexpensive quantitative translation test for diagnosis of shoulder instability. The good intraobserver repeatability and small side-to-side differences that we found in this study are promising in this regard. Lastly, we found translations to be similar in healthy subjects in different positions of external rotation so clinicians may not have to position the contralateral, normal, abducted shoulder in precisely the same position of external rotation as the injured shoulder while performing side-to-side comparisons.

Footnotes

One of the authors (RED) certifies that he or she, or a member of his or her immediate family, received funding, during the study period, from the National Institutes of Health (Bethesda, MD, USA) (R01-AR050218) and the Orthopaedic Research Laboratory Alumni Council (Pittsburgh, PA, USA).

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Reated Research editors and board members are on file with the publication and can be viewed on request.

Each author certifies that his or her institution approved the human protocol for this investigation, that all investigations were conducted in conformity with ethical principles of research, and that informed consent for participation in the study was obtained.

This work was performed at the University of Pittsburgh, Pittsburgh, PA, USA.

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