Athletes may be predisposed to developing scapulothoracic and glenohumeral pathologies resulting from repetitive overhead movements that apply extreme stresses to active and passive structures of the shoulder (Burkhart, Morgan et al. 2003). Therefore, athletes may develop alterations in scapular static positioning, scapular movement
(dyskinesis), or scapular muscle force couple production which eventually may manifest as pain (Fu, Harner et al. 1991; Glousman, Jobe et al. 1998; Kibler, WB, 1991).
Scapular dyskinesis is characterized by alterations of scapular position, at rest or with arm motion, that create clinical dysfunction of the shoulder and that are commonly associated with injuries (Kibler and McMullen, 2003). Scapular dyskinesis may result from excessive thoracic kyphosis and increased cervical lordosis which contribute to excessive scapular protraction and acromial depression (McClure, Michener et al. 2001). A lack of flexibility of the pectoralis minor or short head of the biceps brachii muscles, as well as, a tight posterior glenohumeral joint capsule may contribute to scapular malposition and dyskinesis. (Burkhart, Morgan et al. 2003).
Scapular dysfunction in athletes may be more accurately described as SICK scapula. SICK is an acronym referring to the objective findings common to the scapular syndrome:
Scapular malposition, Inferior medial border prominence, Coracoid pain and malposition, and dysKinesis of scapular movement (Burkhart et al., 2003). The trademark of SICK scapula is malposition characterized by a lowered scapula and prominence of the scapular inferior medial border of the involved shoulder. Coracoid pain and malposition may contribute to dyskinetic alterations in the position and the motion of the scapula relative to the thoracic cage.
The scapula serves as the stable base for glenohumeral motion; providing dynamic stability for the glenohumeral joint, elevating the acromion during throwing motions and serving as a pivotal link in the proximal-to-distal sequencing of velocity, energy, and forces of shoulder function (Kibler and McMullen 2003). Therefore, the presence of SICK scapula may alter scapular kinematics and muscle function; thereby rendering the scapula less effective in contributing to normal shoulder function and predisposing the athlete to glenohumeral joint injury. Abnormal scapular motion in one or multiple planes and associated abnormal muscle function are believed to contribute to shoulder pain and pathology (Fu, Harner et al. 1991; Glousman, Jobe et al. 1998; Kibler, WB, 1991). Furthermore, alterations in scapular positioning and motion occur in 68% to 100% of all patients with shoulder injuries (Warner, Micheli et al. 1992).
Currently, clinicians may utilize manual muscle testing, scapular special tests, soft- tissue mobility assessment and postural assessment in the evaluation of scapular dysfunction. Also, bilateral scapular dynamics may be observed while patients perform multiple
repetitions of shoulder flexion with and without resistance (Sauers 2006). To aid clinicians in the assessment of scapular dysfunction, the SICK Scapula, Static Measurements, 0 to 20 point rating scale was devised in attempts to quantify the degree of scapular dysfunction. This scale contains subjective, objective and scapular measurement sections on which
subjects earn points for reporting symptoms and displaying signs of SICK scapula (Burkhart, Morgan et al. 2003). According to this scale, scores of 10 and above indicate the presence of SICK scapula syndrome. This scale may be considered as an effective clinical tool; however, the scale’s ability to detect scapular malposition and dyskinesis has not been proven
The purpose of this study was to determine the predictive relationship between scapular kinematic data analysis and overhead athlete score on the SICK Scapula, Static Measurements, 0 to 20 point Rating Scale. We hypothesized that subjects who displayed abnormal increases in the degree of scapular anterior tilt and scapular internal rotation would generate higher scores on the SICK Scapula, Static Measurements, 0 to 20 point rating scale. In addition, we predicted that abnormal decreases in scapular upward rotation, elevation and retraction would result in higher scores the SICK Scapula, Static Measurements, 0 to 20 point rating scale.
The results from this research should enhance understanding of the predisposing factors and dyskinetic scapular patterns associated with SICK scapula syndrome. If the SICK Scapula, Static Measurements, 0 to 20 Point Rating Scale (Burkhart, Morgan et al. 2003) was proven to accurately predict scapular dysfunction, sports medicine professionals would be able to better assess scapular dysfunction and to prescribe athletes with more effective treatment and individualized rehabilitation programs to correct scapular malposition and to prevent dyskinesia.
METHODS
One group of forty overhead athletes (12 female swimmers, 9 male swimmers, 9 female volleyball players, 10 female softball players) participated in this study (Table 1). Subjects were selected from a population of male and female NCAA Division I and/or recreational club athletes, ages 18 to 25 years old, who participated in overhead athletic activity for a minimum duration of 30 minutes, three times a week. Overhead athletic activity was defined as a sport that requires the arm to be above shoulder height on a
repetitive basis during throwing or striking activities (swimming, tennis, volleyball, baseball, softball). Subjects were excluded from this study if they had medical history of shoulder or neck surgery, cervical spine pathology, adhesive capsulitis of the shoulder, rotator cuff lesions, scoliosis, acute acromioclavicular joint injury, or glenohumeral subluxations or dislocations within the past six months.
Instrumentation
The researchers used the SICK Scapula, Static Measurements, 0-20 point Rating Scale to screen overhead athletes for the presence of SICK scapula (Burkhart, Morgan et al. 2003). Prior to the study, reliability and precision of the SICK scapula static measurements point scale were established from a small pilot study by the principal investigators. The inter-session reliability and precision were calculated and yielded an ICC of 0.682 and SEM of 1.44 points, respectively. The inter-tester reliability and precision were calculated and yielded an ICC of 0.684 and SEM of 1.18 points, respectively.
The Motion Star (Ascension Technology Corp, Burlington, VT) electromagnetic tracking device integrated with Motion Monitor (Innovative Sports Training, Inc, Chicago, Ill) motion-capture software was utilized to collect three-dimensional scapular kinematics. The Motion Star tracking system consisted of a transmitter and six miniature receivers, which were all hardwired to the systems main computer unit. The transmitter emitted a low-
frequency electromagnetic field, which was detected by the receivers. Each receiver was able to calculate linear and rotational motion around three axes, thus allowing six degrees of freedom to be measured. The relative orientation and position of the receivers within the
electromagnetic field were relayed to the computer, and were processed and displayed using the Motion Monitor motion-capture software.
The Motion Star system has been shown to be accurate within 1.8mm for linear displacements and 0.5º for angular displacements (Thigpen, Padua et al. 2006). Scapular and glenohumeral kinematics were sampled at a rate of 100 Hz. Three-dimensional scapular kinematic data were recorded in degrees of scapular anterior/posterior tilt, upward/downward rotation and internal/external rotation. Scapular position was recorded as degrees of
elevation/depression and degrees of protraction and retraction.
Procedures
Subjects reported to the university-based laboratory for one session lasting
approximately 90 minutes. Prior to participation, all subjects completed an informed consent form approved by the University Biomedical Institutional Review Board.
Female subjects wore a sports bra and males were shirt-less during testing to allow access to the scapula. Prior to the testing session, subjects were screened for both inclusion and exclusion criteria. The subjects who met the criteria proceeded to the testing procedures. All procedures were performed on the subjects’ testing arm, determined by either the
subjects’ self-reported painful arm, or if no pain was reported, by dominant arm. Subjects were screened by both principal investigators who were blinded to each other’s screening process and to the resulting SICK score until both screenings were completed. The subject’s overall SICK score was taken as the mean of the two scores obtained from the blinded
screenings.
In preparation for the collection of three-dimensional scapular kinematics,
adhesive tape, athletic tape and elastic pre-wrap over the following landmarks: the spinous process of the seventh cervical vertebrae, the flat, broad portion of the acromion process of the scapula (bilaterally), and the posterior aspect of humerus just distal to triceps brachii muscle belly (bilaterally) (Table 2). A fourth receiver was secured to a stylus that was used for digitization of landmarks (Myers, Laudner et al. 2005; Thigpen, Padua et al. 2006).
Once thesensors were secured, subjects stood with their arms hanging naturally beside their body while the investigator digitized the bony landmarks on the thorax, humerus and scapula to allow transformation of the receiver data from the global coordinate system to anatomically based local coordinate systems. The sensor placement and the landmarks used to define the local coordinate system were in accordance to the recommendation of
International Society of Biomechanics Shoulder Group (Table 3).
Once preparation was completed, the subject performed two tasks: glenohumeral elevation in the sagittal plane and glenohumeral elevation in the scapular plane. The researcher implemented counterbalancing of tasks by assigning task order prior to subject testing. Both motions occurred through a range of motion of approximately 0º of humeral elevation to approximately 180 º of humeral elevation in their respective planes of motion. Subjects elevated both arms to the terminal end point in the available range of motion while maintaining a neutral hand position throughout the entire range of motion.
The plane of humeral elevation and the speed of the movement during the two tasks were standardized across subjects with the use of PVC pipes and metronome. Tasks were performed bilaterally, from a standing position, feet at a comfortable width and eyes fixed forward. A pole made of PVC pipe was placed 30º anterior to the frontal plane of the thorax to serve as a guide for subjects performing glenohumeral elevation in the scapular plane.
The pole was placed in the humeral sagittal plane and used as a guide during glenohumeral elevation in the sagittal plane. Subjects were asked to complete their full range of motion bilaterally at a controlled movement velocity by moving in time with a digital metronome set at 1 Hz.
Subjects were allowed three practice trials of each functional task to be tested:
glenohumeral elevation in the sagittal plane and glenohumeral elevation in the scapular plane (30º anterior to the frontal plane of thorax). Each functional task consisted of ten continuous repetitions, with each repetition lasting approximately four seconds (two-second ascending phase, two-second descending phase). After the completion of the task, subjects were allowed a two-minute rest before starting the next task.
Data Reduction
Raw kinematic data were low pass filtered with a fourth-order zero-phase shift at a 6.6 Hz cut off frequency (Ludewig and Cook 2000; Borstad and Ludewig 2002; Myers, Laudner et al. 2005; Thigpen, Padua et al. 2006). Scapular position and orientation were analyzed at 0 º, 30 º, 60 º, 90 º and 120 º of the ascending phase of glenohumeral elevation in the sagittal plane and glenohumeral elevation in the scapular plane.
The position and orientation data of the thoracic, scapular, and humeral receivers were transformed into a local coordinate system for each of the respective segments.
Definitions of the local coordinate systems are presented inTable 3. The coordinate systems used were in accordance with recommendations from the International Shoulder Group of the International Society of Biomechanics (Wu 2005).
Euler-angle decompositions were used to describe humeral and scapular orientation with respect to the thorax. Scapular orientation was defined using three axes with the acromial angle serving as the origin: the z-axis described the vector from thoracic spine to acromial angle; the x-axis described the vector perpendicular to the plane set by the thoracic spine, acromial angle and inferior angle of scapula; and the y-axis described the vector perpendicular to the x and z axes. Orientation of the scapula was described as rotation about the y-axis of scapula (internal/external rotation), rotation about the z-axis of the scapula (upward/downward rotation), and rotation about the x-axis of scapula (anteroposterior tilting). Each of these rotations was chosen based on the recommendations of the International Shoulder Group (Wu 2005).
The scapula’s only point of attachment to the thorax is via the clavicle, a rigid body with a fixed length. As such, the position of the scapula was described by two degrees of freedom as if in spherical space, by both elevation/depression and protraction/retraction. The position of the scapula was calculated by the position vector between the acromioclavicular joint (AC) and incisura jugularis (IJ) points with respect to the coordinate system of the thorax. The angle of this vector relative to the transverse plane that bisects the IJ point represents elevation/depression of the scapula. For protraction/retraction, this vector was projected onto the transverse plane bisecting IJ and was calculated as the angle between this projection and the frontal plane that bisects IJ (Karduna, McClure et al. 2001; McClure, Michener et al. 2001).
Statistical Analysis
Simple linear regression analyses were performed implementing scapular kinematic variables as the predictor of the score on SICK Scapula, Static Measurements rating scale. The analyses were run separately for each scapular kinematic variable at each humeral angle for both tasks. A total of fifty linear regression analyses were performed. An alpha level of .05 was set prior to the study. Due to performing multiple comparisons within the five orthogonal humeral angles of the tasks’ ascending phase, an adjusted alpha level of 0.01 was implemented in order to control for inflation of the type I errors.
RESULTS
Statistical significance was found only with humeral elevation in the sagittal plane task. A simple linear regression indicated that the scapular upward rotation at zero degrees of humeral elevation in sagittal plane significantly predicts the overall SICK score (F(1,37) = 9.812, p = .003, r 2 = .210). The mathematical prediction equation for this regression would be: Overall SICK score = .230 (UR_DR_0) + 4.167.
A simple linear regression indicated that the scapular upward rotation at 30 degrees of humeral elevation in sagittal plane significantly predicts the overall SICK score (F(1,37) = 8.107, p = .007, r 2 = .180). The mathematical prediction equation for this regression would be: Overall SICK score = .198 (UR_DR_30) + 3.015.
A simple linear regression indicated that the scapular elevation at zero degrees of humeral elevation in sagittal plane significantly predicts the overall SICK score (F(1,35) = 8.040, p = .008, r 2 = .187). The mathematical prediction equation for this regression would be: Overall SICK score = .247 (Elv_Dep_0) + 1.820.
Tab le 1. Study Participant Demographics Mean ±SD Mean ±SD Age (years) 19.14 1.07 19.97 1.08 Height (cm) 181.43 2.48 173.79 8.42 Mass (kg) 73.87 3.59 69.00 7.90 SICK Score a 5.29 2.23 4.32 3.44 Subjective b 2.14 1.25 1.66 1.64 Objective c 1.43 0.79 1.45 1.62 Malpositioning d 1.71 0.64 1.23 0.84 (n = 8) (n = 31)
Male Participants Female Participants
a SICK Scapula, Static Measurements, 0 to 20 Point Rating Scale
b Self- reported pain of the coracoid process, AC joint, periscapular soft tissue, proximal lateral arm, and/or elbow (possible 5 points)
c Self- reported tenderness to palpation of the co racoid process, AC joint, superior medial angle; (+) provocative impingement test (Hawkins - Kennedy Impingement Sign), (+) scapular assistance test, and/or (+) thoracic outlet syndrome test (Allen Test) (possible 6 points) d
Scapular malposition based on 1) infera (i.e. the visual appearance of a dropped scapula due to scapular tilting or protraction), 2) lateral displacement, and 3) abduction (possible 9 points)
Table 2. Description of Bony Landmarks
Bony Landmarks Description of Palpation Point
Thorax
8th Thoracic Spinous Process (T8) Most dorsal point
Processus xiphoideus (PX) Most caudal point of sternum 7th Cervical Spinous Process (C7) Most dorsal point
Incisura jugularis (IJ) Most cranial point of the sternum (suprasternal notch)
Scapula
Angulus acromialis (AA) Most lateral-dorsal point of scapula
Trigonum spinae (TS) Midpoint of triangular surface on the medial border of the scapula in line with the scapular spine
Angulus inferior (AI) Most caudal point of scapula
Humerus
Medial epicondyle (ME) Most medial point on the medial epicondyle Lateral epicondyle (LE) Most lateral point on the lateral epicondyle Glenohumeral joint center (GH) ∗∗∗∗
∗ ∗∗
∗ The glenohumeral joint center was not palpated but rather estimated with a least squares algorithm for the point on the humerus which moves the least during several short arc humeral movements.(Harryman, Sidles et al. 1990; Stokdijk, Nagels et al. 2000)
Local Coordinate System Axis Definition
Thorax yt Vector from the midpoint of PX and T8 to
. the midpoint between IJ and C7, pointing upward
zt Vector perpendicular to the plane formed by
IJ, C7, and the midpoint between PX and T8, pointing to the right.
xt Vector perpendicular to zt and yt
Origin IJ
Scapula zs Vector from TS to AA, pointing to AA.
xs Vector perpendicular to the plane formed by
TS, AA, and AI, pointing forward.
ys Vector perpendicular to xs and zs.
Origin AA
Humerus yh Vector connecting GH and the midpoint of
. EL and EM, pointing to GH
xh Vector perpendicular to the plane formed by
EL, EM, and GH, pointing forward.
zh Vector perpendicular to yh and xh
Origin GH
Thorax: C7:spinous process of 7th cervical vertebrae; T8:spinous process of 8th thoracic vertebrae; IJ: deepest point of Incisura Jugularis (suprasternal notch); PX: Processus Xiphoideus (xiphoid process), most caudal point on the sternum
Scapula: TS: trigonum spinae scapulae (root of the spine), the midpoint of the triangular surface on the medial border of the scapula in line with the scapular spine; AI: Angulus Inferior (inferior angle), most caudal point of the scapula; AA: Angulus Acromialias (acromial angle), most laterodorsal point of the scapula
Humerus: GH: Glenohumeral rotation center; EL: most caudal point on lateral epicondyle; EM: most caudal point on medial epicondyle
Table 4: Scapular Kinematics Data during Humeral Elevation in Sagittal plane (Flexion)
Mean ± SD r2 p
Scapular upward/downward rotation
0o humeral elevation 1.34 6.53 .210 .003*
30o humeral elevation 7.40 7.09 .180 .007*
60o humeral elevation 22.0 7.66 .088 .067
90o humeral elevation 34.65 10.92 .058 .139
120o humeral elevation 45.72 14.34 .048 .181
Scapular external /internal rotation
0o humeral elevation 25.44 11.38 .004 .713
30o humeral elevation 31.64 11.60 .004 .718
60o humeral elevation 40.26 12.43 .002 .797
90o humeral elevation 42.21 14.63 .001 .882
120o humeral elevation 37.82 20.02 .004 .702
Scapular posterior/anterior tilt
0o humeral elevation 13.33 5.11 .027 .314 30o humeral elevation 10.74 6.01 .049 .177 60o humeral elevation 10.64 6.89 .070 .104 90o humeral elevation 9.05 11.22 .044 .202 120o humeral elevation 2.13 18.92 .007 .600 Scapular protraction/retraction 0o humeral elevation 19.62 12.13 .001 .887 30o humeral elevation 19.26 12.98 .002 .819 60o humeral elevation 20.69 14.30 .046 .203 90o humeral elevation 26.30 15.44 .056 .160 120o humeral elevation 35.07 15.73 .083 .084 Scapular elevation 0o humeral elevation 11.48 5.60 .187 .008* 30o humeral elevation 12.41 5.48 .118 .037 60o humeral elevation 18.76 5.43 .016 .452 90o humeral elevation 24.63 5.72 .000 .912 120o humeral elevation 27.93 5.80 .001 .840
Table 5: Scapular Kinematics Data during Humeral Elevation in Scapular plane (Scaption)
Mean ± SD r2 p
Scapular upward/downward rotation
0o humeral elevation 3.633 9.03 .069 .107
30o humeral elevation 8.26 8.74 .040 .223
60o humeral elevation 21.47 8.13 .072 .099
90o humeral elevation 35.61 11.33 .055 .150
120o humeral elevation 46.75 15.31 .092 .061
Scapular external /internal rotation
0o humeral elevation 25.74 15.06 .032 .273
30o humeral elevation 27.29 18.95 .041 .217
60o humeral elevation 28.54 16.88 .070 .151
90o humeral elevation 32.17 25.51 .044 .258
120o humeral elevation 36.38 25.39 .035 .311
Scapular posterior/anterior tilt
0o humeral elevation 12.53 5.57 .008 .583 30o humeral elevation 10.70 5.27 .013 .482 60o humeral elevation 8.60 5.95 .014 .470 90o humeral elevation 6.52 8.83 .005 .670 120o humeral elevation 4.30 15.54 .001 .870 Scapular protraction/retraction 0o humeral elevation 22.42 13.24 .015 .465 30o humeral elevation 23.20 12.33 .004 .717 60o humeral elevation 26.68 12.99 .008 .592 90o humeral elevation 31.50 13.57 .018 .418 120o humeral elevation 37.99 14.21 .027 .325 Scapular elevation 0o humeral elevation 11.40 6.18 .150 .018 30o humeral elevation 12.91 5.86 .097 .061 60o humeral elevation 19.08 5.67 .044 .214 90o humeral elevation 25.54 5.70 .028 .324 120o humeral elevation 30.60 5.84 .036 .259
Su bje ct # SIC K S co re T ota l Su bje cti ve Pa in Re po rte d Co rac oid AC Jo int Pe ris ca pu lar Pro x L at Arm Ra dic ula r Co