Collegiate baseball pitchers demonstrate a relationship between ball velocity and elbow varus torque, both within and across pitchers.

High elbow varus torque during baseball pitching has been identified as a potential cause of ulnar collateral ligament injury in baseball pitchers. In general, elbow varus torque increases as ball velocity increases across pitchers. However, studies incorporating within-subject analyses report that not all professional pitchers have a positive relationship between elbow varus torque and ball velocity (T-V relationship). It remains unknown whether collegiate pitchers show the same trend as professionals in their T-V relationships. The current study investigated the T-V relationship of collegiate pitchers focusing on both across and within pitchers. Division 1 collegiate pitchers (n = 81) were assessed for elbow torque and ball velocity during pitching. Both across- and within-pitcher T-V relationships were significant (p < 0.05) using linear regression. However, more variance in elbow varus torque was explained using the within-pitcher relationship (R2 = 0.29) than the across-pitcher relationship (R2 = 0.05). Of the 81 pitchers, nearly half (n = 39) had significant T-V relationships, while the other half (n = 42) did not. Our findings indicate that the T-V relationship should be assessed on an individual basis as T-V is pitcher-specific.

Altered activity of the serratus anterior during unilateral arm elevation in patients with cervical disorders.

Altered activity in the axioscapular muscles is considered to be an important feature in patients with neck pain. The activity of the serratus anterior (SA) and trapezius muscles during arm elevation has not been investigated in these patients. The objectives of this study was to investigate whether there is a pattern of altered activity in the SA and trapezius in patients with insidious onset neck pain (IONP) (n=22) and whiplash associated disorders (WAD) (n=27). An asymptomatic group was selected for baseline measurements (n=23). Surface electromyography was used to measure the onset of muscle activation and duration of muscle activity of the SA as well as the upper, middle, and lower trapezius during unilateral arm elevation in the three subject groups. Both arms were tested. With no interaction, the main effect for the onset of muscle activation and duration of muscle activity for serratus anterior was statistically significant among the groups. Post hoc comparison revealed a significantly delayed onset of muscle activation and less duration of muscle activity in the IONP group, and in the WAD group compared to the asymptomatic group. There were no group main effects or interaction effects for upper, middle and lower trapezius. This finding may have implications for scapular stability in these patients because the altered activity in the SA may reflect inconsistent or poorly coordinated muscle activation that may reduce the quality of neuromuscular performance and induce an increased load on the cervical and the thoracic spine.

The effect of articular malposition after total shoulder arthroplasty on glenohumeral translations, range of motion, and subacromial impingement.

The articular surface of the normal humeral head has a variable posterior and medial offset with respect to the central axis of the humeral shaft. Recreation of the normal humeral head shaft offset is postulated to be an important consideration during shoulder arthroplasty. However, the effect of humeral head malposition is unknown. The purpose of this study was to determine the effect of articular malposition after total shoulder arthroplasty on glenohumeral translation, range of motion, and subacromial impingement. Twenty-one human cadavers were dissected and tested with the use of an active or passive shoulder model. Range of motion and translation were recorded by means of an electromagnetic tracking device. The experiment was performed in 2 phases. For kinematics study, 11 cadaver shoulders were positioned both passively and actively from maximum internal rotation to maximum external rotation at 90 degrees of total elevation in the scapular plane. Three rotator cuff and 3 deltoid muscle lines of action were simulated for active joint positioning. Passive joint positioning was accomplished with the use of a torque wrench and a nominal centering force. The testing protocol was used for the natural joint as well as for 9 prosthetic head locations: centered and 2- and 4-mm offsets in the anterior, posterior, inferior, and superior directions. Repeated-measures analysis of variance was used to test for significant differences in the range of motion and translation between active and passive positioning of the natural joint as well as all prosthetic head positions. (2) For impingement study, 10 cadaver shoulders were used in a passive model, loading the tendons of the rotator cuff with a 30-N centering force. The humerus was passively rotated from maximum internal rotation (1500 Nmm) to maximum external rotation (1500 Nmm) by means of a continuous-recording digital torque wrench. Trials were performed with the use of centered, 4-, 6-, and 8-mm offset heads in the anterior, posterior, superior, and inferior positions before and after removal of the acromion and coracoacromial ligament. The relation between change in mean peak torque (with and without acromion), passive range of motion, and humeral head offset was analyzed by means of repeated-measures analysis of variance. In the kinematics study, total range of motion and all humeral translations were greater with passive joint positioning than with active positioning (P =.01) except for total superior-inferior translation and superior-inferior translation in external rotation. Anterior to posterior humeral head offset was associated with statistically significant changes in total range of motion (P =.02), range of internal rotation (P =.02), range of external rotation (P =.0001), and total anterior-posterior translation (P =.01). Superior to inferior humeral head offset resulted in statistically significant changes in total range of motion (P =.02), range of internal rotation (P =.0001), anterior-posterior translation during external rotation (P =.01), and total superior-inferior translation (P =.03). In the impingement study, there was a significant increase in torque from centered to 4-mm inferior offset (P =.006), 6-mm inferior offset (P <.001), and 8-mm inferior offset (P <.001). There was no significant increase in torque with superior, anterior, and posterior offsets. Glenohumeral motion significantly decreased from 129 degrees for centered head to 119 degrees for 8-mm superior (P =.002), 119 degrees for 8-mm anterior (P =.014), 118 degrees for 8-mm inferior (P <.001), and 114 degrees for 8-mm posterior (P =.001). Humeral articular malposition of 4 mm or less during prosthetic arthroplasty of the glenohumeral joint may lead to small alterations in humeral translations and range of motion. Inferior malposition of greater than 4 mm can lead to increased subacromial contact; offset of 8 mm in any direction results in significant decreases in passive range of motion. Therefore if subacromial contact is to be minimized and glenohumeral motion maximized after shoulder replacement, anatomic reconstruction of the humeral head-humeral shaft offset to within 4 mm is desirable.

The floating shoulder: a biomechanical basis for classification and management.

BACKGROUND: The floating shoulder (ipsilateral fractures of the clavicular shaft and the scapular neck) is thought to be an unstable injury pattern requiring operative stabilization in most instances. This recommendation has been made with little biomechanical data to support it. The purpose of this study was to determine the osseous and ligamentous contributions to the stability of experimentally created scapular neck fractures in a cadaver model. METHODS: Standardized scapular neck fractures were made in twelve fresh-frozen human cadaveric shoulders. Each specimen was mounted in a specially designed testing apparatus and secured to a standard materials testing device. In group 1 (six shoulders), resistance to medial displacement was determined following sequential creation of an ipsilateral clavicular fracture, coracoacromial ligament disruption, and acromioclavicular capsular disruption. In group 2 (six shoulders), resistance to medial displacement was determined following sequential sectioning of the coracoacromial and coracoclavicular ligaments. RESULTS: The average measured force for all specimens (groups 1 and 2) after scapular neck fracture was 183 +/- 3.3 N (range, 166 to 203 N). The addition of a clavicular fracture (group 1) resulted in an average measured force of 128 +/- 10.5 N (range, 83 to 153 N), which corresponds to only a 30% loss of stability. Subsequent sectioning of the coracoacromial and acromioclavicular capsular ligaments yielded an average force of 126 +/- 9.1 N (range, 114 to 144 N), a 31% loss of stability, and 0 N, a complete loss of stability, respectively. Sectioning of the coracoacromial and coracoclavicular ligaments after scapular neck fracture (group 2) resulted in an average force of 103 +/- 8.4 N (range, 89 to 118 N), a 44% loss of stability, and 0 N, a complete loss of stability, respectively. CONCLUSIONS: Ipsilateral fractures of the scapular neck and the clavicular shaft do not produce a floating shoulder without additional disruption of the coracoacromial and acromioclavicular capsular ligaments. These and other unstable combined injury patterns are likely to be accompanied by substantial medial displacement of the glenoid fragment.

Direct 3-dimensional measurement of scapular kinematics during dynamic movements in vivo.

The purpose of this study was to describe 3-dimensional scapular motion patterns during dynamic shoulder movements with the use of a direct technique. Direct measurement of active scapular motion was accomplished by insertion of 2 1.6-mm bone pins into the spine of the scapula in 8 healthy volunteers (5 men, 3 women). A small, 3-dimensional motion sensor was rigidly fixed to the scapular pins. Sensors were also attached to the thoracic spine (T3) with tape and to the humerus with a specially designed cuff. During active scapular plane elevation, the scapula upwardly rotated (mean [SD] = 50 degrees [4.8 degrees ]), tilted posteriorly around a medial-lateral axis (30 degrees [13.0 degrees ]), and externally rotated around a vertical axis (24 degrees [12.8 degrees ]). Lowering of the arm resulted in a reversal of these motions in a slightly different pattern. The mean ratio of glenohumeral to scapulothoracic motion was 1.7:1. Normal scapular motion consists of substantial rotations around 3 axes, not simply upward rotation. Understanding normal scapular motion may assist in the identification of abnormal motion associated with various shoulder disorders.

Dynamic measurements of three-dimensional scapular kinematics: a validation study.

The validation of two noninvasive methods for measuring the dynamic three-dimensional kinematics of the human scapula with a magnetic tracking device is presented. One method consists of simply fixing a sensor directly to the acromion and the other consists of mounting a sensor to an adjustable plastic jig that fits over the scapular spine and acromion. The concurrent validity of both methods was assessed separately by comparison with data collected simultaneously from an invasive approach in which pins were drilled directly into the scapula. The differences between bone and skin based measurements represents an estimation of skin motion artifact. The average motion pattern of each surface method was similar to that measured by the invasive technique, especially below 120 degrees of elevation. These results indicate that with careful consideration, both methods may offer reasonably accurate representations of scapular motion that may be used to study shoulder pathologies and help develop computational models.

New method to assess scapular upward rotation in subjects with shoulder pathology.

STUDY DESIGN: Test-retest repeated measures and correlational design. OBJECTIVES: To examine the reliability and validity of a "modified" digital inclinometer to assess scapular upward rotation during humeral elevation in the scapular plane BACKGROUND: Evidence exists that scapular motion is related to shoulder pathology; however, evaluation and treatment planning for shoulder rehabilitation often fails to include an objective assessment of scapular motion. METHODS AND MEASURES: Two-dimensional measurements by the inclinometer were taken with the arm in a static position. These data were compared to 3-dimensional measurements obtained using a magnetic tracking device with the arm fixed and during arm movement. Both methods were used to assess scapular upward rotation positions with the arm at rest and at 60 degrees , 90 degrees , and 120 degrees of humeral elevation in the scapular plane. Both scapulae were tested on a total of 39 subjects, 16 with shoulder pathology and 23 without. Reliability was assessed using repeated measurements from the inclinometer. Validity was assessed using 2 separate comparisons: inclinometer and magnetic tracking device under static arm conditions and inclinometer and magnetic tracking device during active arm elevation. Reliability and validity were assessed at all 4 arm positions. RESULTS: Intraclass correlation coefficients (ICC [3,1]) varied from 0.89 to 0.96. Pearson Product Moment correlation coefficients, used to assess validity of the static inclinometer, varied from r = 0.74 to 0.92 compared with the static magnetic tracking measures, and from r = 0.59 to 0.73 compared with the active magnetic tracking measures taken during arm elevation. CONCLUSIONS: The "modified" digital inclinometer demonstrated good to excellent intrarater reliability and good to excellent validity when measuring scapular upward rotation during static positions of humeral elevation in the scapular plane.

Scapular kinematics: effects of altering the Euler angle sequence of rotations.

An analysis of Euler angle sequences is presented for the scapula. In vivo kinematics were collected with a magnetic tracking device on eight healthy volunteers. To ensure accurate representation of scapular motion, pins were rigidly drilled into the scapular spine. Three rotations of the scapula with respect to the thorax were recorded during humeral elevation in the scapular plane: posterior (or backward) tilting, upward (or lateral) rotation and external rotation (or retraction). Rotations using all six possible Euler angle sequences were calculated for which each angle was represented only once. The sequence proposed by an International Society of Biomechanics subcommittee on shoulder motion (external rotation, upward rotation, posterior tilting) is consistent with both research- and clinical-based two-dimensional representations of scapular motion. Results from the present study indicate that changing sequence results in significant alterations in the description of motion, with differences up to 50 degrees noted for some angles. Therefore, in order to compare results across different laboratories, it is recommended that the proposed standard sequence be adopted.

The acromioclavicular capsule as a restraint to posterior translation of the clavicle: a biomechanical analysis.

Excessive posterior translation of the residual clavicle after distal clavicle resection can be associated with significant postoperative pain. Although the acromioclavicular capsule has been identified as the primary restraint to translation of the clavicle along this axis, the individual contributions of the anterior, posterior, superior, and inferior components of the capsular ligament have not been established. The purpose of this study was to define the relative roles of the individual acromioclavicular capsular ligaments in preventing posterior translation of the distal clavicle in normal acromioclavicular joints in a human cadaver model. Six fresh-frozen human cadaveric acromioclavicular joints were mounted on a specially designed apparatus which, when attached to a standard servohydraulic materials testing device, allowed translation of the distal clavicle along the anteroposterior axis of the acromioclavicular joint (i.e., parallel to the articular surface). Resistance to posterior displacement was measured for standardized displacements in the normal specimens and after serial sectioning of each of the acromioclavicular ligaments was performed. Sectioning of the anterior and inferior capsular ligaments had no significant effect on posterior translation at the 5% significance level. However, sectioning of the superior and posterior ligaments had statistically significant effects (P < .05). These capsular structures contributed 56% +/- 23% (+/- SEM) and 25% +/- 16%, respectively, of the force required to achieve a given posterior displacement. To avoid excessive posterior translation of the clavicle after distal clavicle excision, surgical techniques that spare the posterior and superior acromioclavicular capsular ligaments should be used.

Total shoulder arthroplasty biomechanics: a study of the forces and strains at the glenoid component.

The objective of this study was to examine how changes in glenohumeral joint conformity and loading patterns affected the forces and strains developed at the glenoid. After removal of soft tissue (muscles, ligaments, and labrum), force-displacement data were collected for both natural and prosthetically reconstructed joints. Joints were shown to develop higher forces for a given translation as joint conformity increased. A rigid body model of joint contact forces was used to determined the so-called effective radial mismatch of each joint. For the purposes of this study, the effective radial mismatch is defined as the mismatch required for a rigid body joint to have the same force-displacement relationship as the joint in question. This parameter is an indication of the deformation at the articular surface. The effective radial mismatch dramatically increased with increasing medial loads, indicating that under physiological loads, the effective radial mismatch of a joint is much greater than its measured mismatch at no load. This increase in effective mismatch as medial loads were increased was found to be threefold greater in cartilaginous joints than in reconstructed joints. Rosette strain gages positioned at the midlevel of the glenoid keel in the reconstructed joints revealed that anterior/posterior component loading leads to fully reversible cyclic keel strains. The highest compressive strains occurred with the head centered in the glenoid, and were larger for nonconforming joints (epsilon = 0.23 percent). These strains became tensile just before rim loading and were greater for conforming joints (epsilon = 0.15 percent). Although recorded peak strains are below the yield point for polyethylene, the fully reversed cyclic loading of the component in this fashion may ultimately lead to component toggling and implant failure.

Experimental and numerical analyses of indentation in finite-sized isotropic and anisotropic rubber-like materials.

Indentation tests perpendicular to the major plane of a material have been proposed as a means to index some of its in-plane mechanical properties. We showed the feasibility of such tests in myocardial tissue and established its theoretical basis with a formulation of small indentation superimposed on a finitely stretched half-space of isotropic materials. The purpose of this study is to better understand the mechanics of indentation with respect to the relative effects of indenter size, indentation depth, and specimen size, as well as the effects of material properties. Accordingly, we performed indentation tests on slabs of silicone rubber fabricated with both isotropic, as well as transversely isotropic, material symmetry. We performed indentation tests in different thickness specimens with varying sizes of indenters, amounts of indentation, and amounts of in-plane stretch. We used finite-element method simulations to supplement the experimental data. The combined experimental and modeling data provide the following useful guidelines for future indentation tests in finite-size specimens: (i) to avoid artifacts from boundary effects, the in-plane specimen dimensions should be at least 15 times the indenter size; (ii) to avoid nonlinearities associated with finite-thickness effects, the thickness-to-radius ratio should be >10 and thickness to indentation depth ratio should be >5; and (iii) we also showed that combined indentation and in-plane stretch could distinguish the stiffer direction of a transversely isotropic material.

Glenohumeral joint translations before and after total shoulder arthroplasty. A study in cadavera.

UNLABELLED: The purpose of the present study was to examine the motions of natural and prosthetically reconstructed glenohumeral joints without capsular contracture in the laboratory to obtain a better understanding of joint motion as it may relate to failure of the implant. Seven joints from fresh-frozen human cadavera were instrumented with a six-degrees-of-freedom magnetic tracking device to study patterns of translation and rotation with and without the application of simulated muscle forces (active and passive models). The specimens were tested before and after reconstruction of the joint with use of modified operative techniques and implants that had from zero to five millimeters of radial mismatch between the humeral head and glenoid components. The natural and reconstructed joints had similar patterns of translation, with larger rotations and translations observed in the passive model. On the average, the active translations of the natural joints were best reproduced by the reconstructed joints with less conforming articulations. The mean active translation in the natural joints was 1.5 millimeters along the anterior-posterior axis and 1.1 millimeters along the superior-inferior axis. The active translations in the reconstructed joints were observed to increase consistently as the conformity of the components decreased: the mean active anterior-posterior translations ranged from 0.3 millimeter for conforming components to 1.7 millimeters for components with a five-millimeter radial mismatch, and the mean active superior-inferior translations ranged from 0.4 to 1.1 millimeters. The patterns of translation during passive motions were less consistent, presumably because of the influence of capsular ligaments and because the translations were large enough for the articular surface of the humeral head to lose contact with the articular surface of the glenoid component. CLINICAL RELEVANCE: Glenohumeral translations during active motions were found to depend on articular conformity, which indicates that the choice of implants may have important consequences for normal motions. Totally conforming designs may impose a degree of restraint to translations that is higher than that in the natural joint. The patterns of translation may influence the longevity of the joint replacement with respect to stability, loosening of the glenoid component, and wear of the components.

Joint stability after total shoulder arthroplasty in a cadaver model.

A cadaver model was used to test the hypothesis that glenohumeral joint stability is independent of articular surface conformity after total shoulder arthroplasty. For the purposes of this study joint stability was defined as the minimum force required for joint dislocation. After arthroplasty components were implanted into fresh-frozen glenohumeral joints, specimens were mounted on a load frame and tested for joint stability. For each specimen the amount of conformity between the articular surfaces was varied from 0 to 5 mm by changing the humeral head radius of curvature. Because the glenoid component was not charged, the wall height, or joint constraint, was maintained constant for a given specimen. Variations in joint conformity changed dislocation forces by an average of only 3%. These small differences are not believed to be clinically relevant, indicating that design changes affecting the joint conformity of a total shoulder arthroplasty system will not significantly affect glenohumeral joint stability, assuming that all other factors remain constant.

Kinematics of the glenohumeral joint: influences of muscle forces, ligamentous constraints, and articular geometry.

Despite recent interest in the study of shoulder kinematics, there is considerable controversy in the literature regarding translations at the glenohumeral joint. The purpose of this study was to investigate the key factors that control shoulder motions, thus leading to a better understanding of joint function. Translation and rotation patterns were studied in fresh-frozen glenohumeral joints of human cadavers with a six-degrees-of-freedom magnetic tracking device. Shoulders were positioned from maximal internal to external rotation at several arm positions (various elevations and planes of motion). In order to determine the effect of muscle forces, joints were positioned both actively and passively. Additionally, articular surface geometry and ligament origin-insertion wrap lengths were measured to assess their influences on joint kinematics. When joints were positioned passively, large translations were observed at the extremes of motion. With active positioning, muscle forces tended to limit humeral head translations, principally by restricting rotational ranges of motion. However, when data from the passive model were reanalyzed by considering only the rotational ranges of motion seen actively, no significant differences in translation were found between the two models. Joint conformity was found to have a significant influence on translations during active positioning but not during passive positioning. Glenohumeral ligament wrap lengths, however, correlated with translations when joints were positioned passively but not when positioned actively. Findings from this study emphasize the importance of muscle forces in keeping the humeral head centered in the glenoid. Although large translations are possible, they can be achieved only with increases in rotational ranges of motion associated with the removal of muscle force. Additionally, joint conformity appears to play a role in controlling translations during active motions, whereas capsular constraints become more important during passive motions.