Benefits of an anatomical reconstruction of the humeral head during shoulder arthroplasty: a finite element analysis

OBJECTIVE: To study the influence of the shape of the prosthetic humeral head on shoulder biomechanics and then to evaluate the benefits of an anatomical reconstruction of the humeral head after shoulder arthroplasty. DESIGN: A 3D numerical model of a healthy shoulder was reconstructed. The model included the proximal humerus, the scapula and, for stability purposes, the subscapularis, infraspinatus and supraspinatus rotator cuff muscles. BACKGROUND: Shoulder prostheses used nowadays, called third generation, allow for a better adaptation of the implant to the anatomy of the proximal humerus than previously used implants. However, no biomechanical study has shown the benefits of this anatomical reconstruction of the humeral head. METHODS: The model was used to compare the biomechanics of a shoulder without implant with the biomechanics of the same shoulder after humeral hemiarthroplasty. Two humeral components were tested: a second-generation prosthesis and an implant with an anatomically reconstructed humeral head. RESULTS: The anatomical reconstruction of the humeral head restored the physiological motions and limited eccentric loading of the glenoid. Conversely, the second-generation implant produced contact forces in the superior extremity of the glenoid surface leading to bone stresses up to 8 times higher than for the intact shoulder. CONCLUSIONS: This analysis provided insights into the mechanical effects of different reconstructions of the humeral head and highlighted the advantages of anatomical reconstructions of the humeral head during shoulder arthroplasty.     

Effect of glenoid component inclination on its fixation and humeral head subluxation in total shoulder arthroplasty

Objective. To investigate the effect of glenoid component inclination angle on the fixation of cemented glenoid-bone structures and humeral head subluxation.

Background. Results of shoulder replacements, in terms of glenoid component fixation and joint functionality, are bad and must be improved. Repeated glenoid component tilting, a result of eccentric contact forces harms glenoid component fixation. However, the effect of glenoid component inclination is unknown.

Methods. Keeled glenoid components are cemented into bone substitutes and positioned under inclination angles varying between −4.5° and +4.5°. For each inclination angle 5 glenoid specimens are loaded by a horizontal, constant joint compression force (725 (SD 10) N) and a vertical, superior directed subluxation force (shear force), cyclically varying between 0 and 350 (SD 1) N. After 200,000 load cycles, the upper and lower glenoid component rim-displacements are measured during 1500 additional load cycles by custom made displacement sensors.

Results. The maximal superior rim-displacement significantly increased and the minimal inferior rim-displacement dramatically decreased for increasing glenoid component inclination angles up to +4.5°. Maximal ratio of subluxation force over compression force significantly decreased for increasing glenoid component inclination.

Conclusions. According to this study, decreasing glenoid component inclination angle decreases glenoid component tilting and humeral head subluxation and the results can be used to improve total shoulder replacements.

Relevance Better understanding of the effect of glenoid component inclination on glenoid component tilting and humeral head subluxation may be useful information for the surgeon to improve results of the total shoulder arthroplasty, when replacing the glenoid surface.     

A stochastic analysis of glenoid inclination angle and superior migration of the humeral head

BACKGROUND: Superior glenoid inclination, which is a relatively upward facing of the glenoid in the plane of the scapula, has been associated with rotator cuff pathology. Increased glenoid inclination may cause superior humeral head migration, which can cause impingement of the supraspinatus tendon. The purpose of this study was to test the hypothesis that inclination angle affects the probability of superior humeral head migration. METHODS: A three-dimensional model of the glenohumeral joint was developed in which muscle forces were modeled as random variables. Monte Carlo simulation was used to compute the probability that the glenohumeral reaction force was directed such that superior humeral head migration should occur. An electromyogram-driven model was used to estimate shoulder muscle forces in healthy volunteers performing arm elevation. FINDINGS: The model predicted that the probability of superior humeral head migration increased as glenoid inclination angle was increased. This finding was independent of the assumed shape of the muscle force probability distributions. INTERPRETATION: The results support the theory that glenoid inclination may be a risk factor for rotator cuff pathology.     

In vitro kinematics of the shoulder following rotator cuff injury

BACKGROUND: Tears in the rotator cuff may result in altered mechanics of the glenohumeral joint. It is not unusual for some patients with cuff tears to exhibit near normal motion with the injured shoulder, while other patients with the same magnitude of injury are unable to achieve full elevation on the injured side. METHODS: The effect of simulated tears of the rotator cuff on active glenohumeral joint kinematics was investigated by testing eight cadaveric specimens using an in vitro shoulder simulator. Active abduction of the humerus was produced by applying forces to simulate loading of the supraspinatus, subscapularis, infraspinatus/teres minor, and the anterior, middle, and posterior thirds of the deltoid. Three sequential 1cm lesions were created in the rotator cuff, the first two in the supraspinatus tendon and the third in the subscapularis tendon. FINDINGS: The plane of abduction moved posteriorly and became more abnormal throughout abduction with increased tear size. No difference was observed in the internal/external rotation of the humerus or the position of the humeral head on the glenoid during elevation. INTERPRETATION: In order to generate the motions achieved by the intact joint, patients with rotator cuff insufficiency likely employ other muscle groups. Retraining muscle groups surrounding the glenohumeral joint may decrease the need for surgical interventions.     

The relevance of the moment arm of shoulder muscles with respect to axial rotation of the glenohumeral joint in four positions

OBJECTIVE: This study was undertaken to determine the efficiency of the shoulder girdle muscles during axial humeral rotation based on measurements of the moment arms. DESIGN: The instantaneous muscle moment arms of 10 shoulder muscles, including the three portions of the deltoid, the rotator cuff muscles, teres major, and the thoracohumeral muscle group, were measured during four specified glenohumeral rotations. BACKGROUND: Axial humeral rotation is a commonly performed movement during activities of daily living and is a targeted motion of shoulder rehabilitation, particularly in those protocols emphasizing rotator cuff strengthening. An understanding of the function of the movers and stabilizers of the shoulder requires such basic information of muscle moment arms. METHODS: The instantaneous moment arm values of the muscles were derived from the slope of the plot of tendon excursion versus glenohumeral joint rotation angle. Motion studied included axial rotation with the humerus elevated 90 degrees in the coronal, scapular, and sagittal planes, as well as in the neutral position with the arm at the side. RESULTS: Based on the findings, with the humerus in both neutral and elevated positions, the infraspinatus is potentially the most powerful external rotator, followed by teres minor and posterior deltoid. Subscapularis and possibly pectoralis major are the most effective internal rotators in this position. CONCLUSIONS: The moment arm in providing axial humeral rotation of 10 shoulder muscles in four planes were obtained. In general, the teres minor and infraspinatus had the largest moment arms in external rotation, and the subscapularis had the largest moment arm in internal rotation. The muscle function for axial humeral rotation was found to be modified by the plane of arm elevation. RELEVANCE: The data could be used for developing exercise programs in physical therapy.     

Surgical management of the biconcave (B2) glenoid

Glenohumeral osteoarthritis produces a wide spectrum of glenoid pathology. The B2 glenoid is defined by asymmetric posterior bone loss with the development of a biconcavity and posterior translation of the humeral head. Progressive bone loss results in increasing glenoid retroversion, which must be corrected during anatomic shoulder arthroplasty. The goals of arthroplasty should also include centering the humeral head and restoring the normal glenoid joint line. When there is minimal bone loss, this may be accomplished with a standard glenoid component and asymmetric reaming. More significant bone loss requires bone grafting or the use of an augmented glenoid component. Reverse shoulder arthroplasty is also an option for older patients or patients with severe bone loss.     

The variable roles of the upper and lower subscapularis during shoulder motion

Background

The varied roles of the subscapularis muscle as an internal rotator of the humerus, a shoulder abductor, a humeral head depressor and an anterior stabiliser may be a result of differing innervation and lines of torque between its superior and inferior components. The aims of the study were to investigate the differences in the level of muscle activation between the upper and lower subscapularis during abduction, flexion, internal and external rotation movements, and temporal characteristics during abduction and flexion.

Methods

Intramuscular electrodes recorded electromyographic muscle activity from the upper and lower subscapularis muscles of the dominant throwing arm of twenty-four normal subjects. Participants completed ten repetitions of four shoulder movements — abduction, flexion, internal rotation and external rotation. Muscle activity was expressed as a percentage of maximum voluntary isometric contraction.

Findings

The lower subscapularis was found to activate at a higher level than the subscapularis during abduction, flexion and external rotation movements and this was significant during concentric and eccentric phases of abduction and flexion (< 0.001). During internal rotation, upper subscapularis muscle activity mirrored that of lower subscapularis, with a mean difference of 1.14%. Neither upper nor lower subscapularis had onset data commencing prior to the abduction movement; however upper subscapularis activated significantly later than lower subscapularis (P = 0.018).

Interpretation

The lower subscapularis has significantly higher muscle activity during shoulder elevation and this might reflect its greater role as a humeral head depressor and anterior stabiliser.     

Importance of the subscapularis muscle after total shoulder arthroplasty

Background

The rotator cuff muscles are the main stabilizer of the glenohumeral joint. After total shoulder arthroplasty using anterior approaches, a dysfunction of the subscapularis muscle has been reported. In the present paper we tested the hypothesis that a deficient subscapularis following total shoulder arthroplasty can induce joint instability.

Methods

To test this hypothesis we have developed an EMG-driven musculoskeletal model of the glenohumeral joint. The model was based on an algorithm that minimizes the difference between measured and predicted muscular activities, while satisfying the mechanical equilibrium of the glenohumeral joint. A movement of abduction in the scapular plane was simulated. We compared a normal and deficient subscapularis. Muscle forces, joint force, contact pattern and humeral head translation were evaluated.

Findings

To satisfy the mechanical equilibrium, a deficient subscapularis induced a decrease of the force of the infraspinatus muscle. This force decrease was balanced by an increase of the supraspinatus and middle deltoid. As a consequence, the deficient subscapularis induced an upward migration of the humeral head, an eccentric contact pattern and higher stress within the cement.

Interpretation

These results confirm the importance of the suscapularis for the long-term stability of total shoulder arthroplasty.     

Decreasing glenoid inclination improves function in shoulders with simulated massive rotator cuff tears

BACKGROUND: A massive rotator cuff tear leads to poor shoulder function as evidenced by diminished glenohumeral abduction and superior translation of the humeral head compared to its normal position. The inclination angle of the glenoid has been associated with rotator cuff tears. The objective of this study was to quantify the effect of a decreased glenoid inclination angle on glenohumeral kinematics during active abduction in shoulders with a simulated, massive rotator cuff tear. METHODS: Eight fresh-frozen full upper extremities were tested using a dynamic shoulder testing apparatus. After recording the kinematics of the intact shoulder, a massive rotator cuff tear was surgically simulated. An osteotomy of the glenoid was then performed and the inclination angle was decreased by 30 degrees . The translation of the humeral head during abduction and the maximum abduction angle were recorded. FINDINGS: With an intact rotator cuff minimal humeral head translation on the glenoid occurred and the maximum abduction angle was mean 85.5 degrees (SD 7.4 degrees ). A massive rotator cuff tear resulted in superior translation of the humeral head with impingement on the acromion. The maximum abduction angle was mean 15.5 degrees (SD 9.4 degrees ). Decreasing the inclination angle of the glenoid resulted in a significant reduction of superior humeral head translation during abduction and there was no impingement on the acromion. The maximum abduction achieved was mean 28.5 degrees (SD 17.0 degrees ). INTERPRETATION: From a clinical perspective the reduced superior translation may decrease shoulder pain since the humeral head no longer impinges on the acromion. Further investigations are necessary to assess if the improvement in abduction is clinically significant.     

Rotation of the scapula and shoulder subluxation in hemiplegia

Inferior subluxation of the shoulder in hemiplegia was measured using a tridimensional (3-D) x-ray technique. This technique gave the true vertical distance separating the apex of the humeral head and the inferior margin of the glenoid cavity. Both shoulders of each subject were evaluated and the difference used as a measure of subluxation. This measure was then compared to the orientation of the scapula relative to the vertical, to the abduction, and to the relative abduction of the arm. Relative abduction is defined as the angle between the humerus and the glenoid fossa. It has been suggested that these factors are associated with inferior subluxation in hemiplegia. Results of this study of 50 volunteer stroke patients indicated that the affected and nonaffected shoulders were different (subluxed) in terms of the vertical position of the humerus vis-à-vis the scapula. The orientation of the glenoid cavities was also different, the subluxed one facing less downward. The angle of abduction of the arm of the affected side was significantly greater than on the nonaffected side, but the relative abduction of the arm was on the same order of magnitude for both sides. There was no significant relationship between the orientation of the scapula and the severity of the subluxation. The abduction of the humerus was weakly (r = .24) related to the subluxation, which partly explained the weak association found between the relative abduction of the arm and the subluxation. It was concluded that the position of the scapula and the relative abduction of the arm cannot be considered important factors in the occurrence of inferior subluxation in hemiplegia.     

Quantitative analysis of traction in the glenohumeral joint. In vivo radiographic measurements

PURPOSE: To analyse change in distance between the humeral head and the glenoid fossa during traction in the maximally loose-packed position (MLPP) and the maximally closed-packed position (MCPP) under standardized conditions. SUBJECTS: Six healthy subjects (three male and three female) with a mean age of 40.5 years, volunteered to participate in this study. MATERIALS AND METHODS: Subjects were placed with the right shoulder in a modified shoulder brace (Otto Bock Armabduktions-Orthese in Modular Bauweise in 45 degrees abduction in the plane of the scapula with neutral rotation (MLPP). A standard anterior-posterior radiograph of the glenohumeral joint was made. A 14 kg traction force was applied for 40s, and a second radiograph was made. The same procedure was repeated with the shoulder placed in the MCPP, which was 90 degrees abduction and 90 degrees external rotation. A radiologist, blinded for the variable traction or no traction, performed all radiographic measurements. Measurements were made on the same radiographs on two separate occasions (O1 and O2) with a 2-month interval. RESULTS: No significant differences were found in mean distance between the humeral head and the glenoid fossa during traction in the MLPP compared to traction in the MCPP (O1: P=1.00) and (O2: P=0.63). CONCLUSIONS: Application of a 14 kg force does not result in a significant increase of distance between the humeral head and the glenoid fossa. No significant difference was found between the change in distance of the humeral head and the glenoid fossa after traction in the MLPP compared to traction in the MCPP.     

Arthroskopisch relevante Anatomie der Schulterinstabilität

For the diagnosis and therapy of shoulder diseases a detailed knowledge of the anatomical structures and their functional importance is fundamental. Shoulder stability is maintained through a complex interplay between dynamic and static mechanisms. The static mechanisms include the bony configuration of the glenoid and humeral head, the glenoid labrum, the capsule and ligaments. The dynamic stabilization mechanisms are the muscles of the rotator cuff, the long head of biceps muscle and partly the deltoid muscle. The main advantage of shoulder arthroscopy over open procedures is due to the fact that anatomical structures can be also evaluated during dynamic examinations. In the context of shoulder instability, injuries of the glenoid labrum superiorly (SLAP) or anterior-inferior (Bankart, ALPSA, GLAD), capsular lesions (HAGL), accompanying cartilage lesions (Hill-Sachs), rotator interval tears and pulley lesions can be exactly diagnosed and accordingly treated.     

Three-dimensional in vivo comparative analysis of the kinematics of normal shoulders and shoulders with massive rotator cuff tears with successful conservative treatment

BackgroundThis study used in vivo three-dimensional to two-dimensional image registration techniques to compare the glenohumeral kinematics of shoulders with massive rotator cuff tears that were successfully treated conservatively and those of normal shoulders.MethodsTen patients (age, 67.4 ± 3.63 years) with massive rotator cuff tears on one side and without contralateral tears were enrolled. We performed computed tomography and fluoroscopy on both shoulder joints and created three-dimensional bone models of the humerus and scapula using image registration techniques. We measured the humeral superoinferior translation, angle of humeral external rotation, scapular upward rotation, scapular anteroposterior tilt, and scapular external rotation of the torn shoulders with good range of motion after effective conservative treatment and compared these measurements to those of the contralateral normal shoulders.FindingsThere was a significant difference in the initial position of the humeral head relative to the glenoid in the tear group; it was 2.0 mm higher than that in the normal group (p < .05). This difference disappeared in the range from 40° to full elevation. The scapular motion of the tear group was significantly more upwardly rotated than that of the normal group: by 9.9° at rest (p < .05) and by 11.6° at terminal elevation (p < .05). No significant differences were detected for humeral head external rotation, scapular anteroposterior tilt, and scapular external rotation between the two groups.InterpretationKinematics of shoulders with massive cuff tears could not be recovered completely even though the patients had no significant symptoms after successful conservative treatment.     

Effect of supraspinatus deficiency on humerus translation and glenohumeral contact force during abduction

BACKGROUND: Supraspinatus deficiency is the most frequent and important problem associated to rotator cuff pathologies. It reduces shoulder stability and can lead to osteoarthritis. The goal of this study was to develop a numerical model of the shoulder to analyse the biomechanical consequences of this pathology. METHODS: A 3D finite element model of the shoulder was developed from a normal cadaver specimen. It included the scapula, the humerus and the major abduction muscles. Instead of the usual ball-socket assumption, which prevents the natural translation of the humerus, shoulder stability was actively achieved by muscles. A feedback algorithm was developed to synchronise muscle forces during abduction. The numerical algorithm was validated against an algebraic model, and the calculated muscle moment arms were compared to the literature. Two cases were considered: a normal shoulder and the same one without supraspinatus. FINDINGS: For the normal shoulder, the model predicted the initial upward migration of the humeral head. The maximal humerus translation occurred at 30 degrees of abduction and was 0.75 mm above its ideal centered position. Without supraspinatus, it was 1.6 times higher and the contact point in the glenoid fossa was more eccentric. For the normal shoulder, the maximal glenohumeral force was 81% of the body weight, at 82 degrees of abduction. Without supraspinatus, it increased by 8%, while the increase of muscle forces was 30%. INTERPRETATION: Supraspinatus deficiency increased the upward migration of the humerus, the eccentric loading, and the joint and muscle forces, which may cause a limitation of active abduction and degenerative glenohumeral changes (osteoarthritis and the rotator cuff tear).     

The stabilizing role of the rotator cuff at the shoulder--responses to external perturbations

Background

The rotator cuff has been hypothesized as a dynamic stabilizer at the shoulder joint yet evidence supporting this role remains inconclusive. We aimed to investigate the activity levels and recruitment patterns between the rotator cuff and superficial shoulder muscles in response to external perturbations to provide insight into the stabilizing role of the rotator cuff.

Methods

Surface and intramuscular electromyography (EMG) were used to measure timing of onset and level of activation (EMG amplitude as a percentage of maximum voluntary isometric contraction, % MVIC) of rotator cuff (supraspinatus, infraspinatus and subscapularis) and superficial muscles (anterior and posterior deltoid) on 19 healthy participants. Participants received expected and unexpected externally applied perturbations in directions of internal and external rotation at the glenohumeral joint.

Findings

All three rotator cuff muscles demonstrated pre-activation in anticipation of the perturbation prior to their representative global synergists, anterior and posterior deltoid (P < 0.05). Subscapularis and infraspinatus were activated prior to all other muscles during external rotation and internal rotation perturbation trials respectively (P < 0.01). Direction specific activation levels were observed; subscapularis was moderately strongly active (37% MVIC) in response to an external rotation perturbation and infraspinatus was moderately active (28% MVIC) in response to an internal rotation perturbation. No muscle was activated > 10% MVIC when not acting as the main muscle opposing the movement.

Interpretation

The rotator cuff may function in part as a dynamic stabilizing unit of the shoulder demonstrating a feedforward muscle activation pattern. These results may assist in improving assessment and treatment of shoulder dysfunction.     

人工肱骨头置换中使用胸骨针行肱骨近端骨折肩袖重建

背景：越来越多的粉碎、移位严重而无法重建的肱骨近端骨折患者需要行人工肱骨头置换,但此类患者肩袖的重建直接影响治疗效果,置换过程中往往需要良好的肩袖重建。目的：探讨胸骨针在肱骨近端骨折人工肱骨头置换肩袖重建中的应用体会。方法：34例肱骨近端四部分骨折患者行人工肱骨头置换时使用胸骨针修复肩袖,年龄67—78岁。人工肱骨头置换时未过分剥离骨折块与肩袖组织,保留肩袖组织与骨块相连,将胸骨针沿着肩袖大小结节表面肌腱一骨结合部环形缝合备用,可用多根,假体置入后,将肱骨大、小结节及碎骨块解剖复位,收紧胸骨针,大小结节及肩袖附着的碎骨块均原位贴在人工肱骨头下方。术中应尽可能将残余的肩袖和肌肉组织损伤进行缝合修复,并要注意缝合后的动力平衡。采用Neer标准对人工肩关节功能的恢复情况进行评价。结果与结论：34例患者均获随访,随访时间1-3年,24例优,10例良,2例可;无关节脱位、半脱位等关节不稳情况,未见感染、神经损伤及假体松动病例。提示人工肱骨头置换过程中使用胸骨针进行肩袖修复及大小结节固定能使肩袖和大小结节接近解剖位置,并且比常规的涤纶线强度高,固定牢靠,能满足人工肱骨头置换后康复训练的需要,对肩关节的稳定性及功能恢复有重要作用。     

US anatomy of the shoulder: Pictorial essay

A thorough knowledge of the anatomy of the shoulder is essential for the assessment of its condition. The purpose of this article is to provide a useful tool for the ultrasound (US) study of this joint. The shoulder girdle and upper arm are made up of a number of muscles and tendons: rotator cuff (supraspinatus, infraspinatus, teres minor and subscapularis), humeral biceps, deltoid and pectoral muscles, which can all be evaluated at US examination. Various and complex capsular ligamentous structures contribute to the stability of the shoulder, but only a few can be adequately evaluated by US and will therefore receive particular attention. Numerous serous bursae are situated among muscles, skin, subcutaneous tissues, joint capsule structures and bones to prevent friction and they can be evaluated by US only in the presence of pathologies. Subacromial-subdeltoid and subcoracoid bursa are most frequently involved and will therefore be described in detail. There are furthermore nerves and vessels providing the various components of the shoulder with innervation and vascularization, and they can also be studied by US. The shoulder girdle (humerus, scapula, clavicle and sternal manubrium) is situated in the deep layers; only the cortex of the bone can be seen at US as a continuous hyperechoic line. For a better understanding of the location and relationship between the structures which can be studied by US, magnetic resonance imaging (MRI) can be carried out as this method provides a wider and more complete view of the structures.     

Glenohumeral kinematics and capsulo-ligamentous strain resulting from laxity exams

Objective. Identification and quantification of strain in shoulder capsular-ligamentous structures during clinical exams and validation of this testing on cadavers.

Methods. Mercury strain gauges were sutured in seven locations on shoulders from cadavers. An electromagnetic tracker quantified humeral head translations during laxity exams. Strain and humeral position were acquired during performance of Sulcus, Feagin, Apprehension, Load and Shift, Drawer, and Hawkins tests.

Results. Anterior humeral head translation in neutral position was primarily constrained by the coracohumeral ligament. With the arm abducted, anterior middle and inferior ligaments also became active. External rotation and abduction activated inferior and middle capsules. Posterior capsule constrained motion for posterior tests in neutral and abduction. Superior and inferior capsular ligaments were active during inferior tests in neutral position. With abduction, inferior ligaments provided primary translation constraint.

Conclusion. Study of kinematics and strain evaluation on cadavers can yield useful information on mechanisms of glenohumeral instability.Relevance

This study clarifies the contribution of specific structures of the shoulder to strain in the joint capsule. It also identifies which structures are challenged by provocative laxity exams commonly used by orthopaedic physicians.     

The three-dimensional motions of glenohumeral joint under semi-loaded condition during arm abduction using vertically open MRI

BACKGROUND: Magnetic resonance imaging is an accurate non-invasive tool for visualizing muscles, tendons, and bones. It also provides 3D coordinate values. The purpose of the present study was to visualize and quantify the 3D positions of the glenohumeral joint during isometric abduction of the arm using vertically open magnetic resonance imaging. METHODS: We examined 14 shoulders of seven healthy volunteers. Magnetic resonance images were obtained in a seated position and in seven static positions of the arm from 0 degrees to maximum abduction using vertically open magnetic resonance imaging. 3D surface models were created and 3D movements of each bone in the glenohumeral joint were calculated using a computer algorithm. We analyzed the translation and contact pattern of the glenohumeral joint. FINDINGS: In supero-inferior direction, the humeral head translated slight inferiorly from +1.9 (SD 1.0) mm at 0 degrees to +0.8 (SD 1.8) mm at the maximum abduction. In antero-posterior direction, the humeral head translated anteriorly from 0 degrees to 90 degrees (mean +2.4, SD 2.6 mm) and posteriorly from 90 degrees to 150 degrees of abduction (mean -1.4, SD 2.7 mm). Furthermore, the humeral head had a unique contact patterns with the glenoid; the contact part of the humeral head with the glenoid changed from the central part to the posterior in the midrange of abduction. INTERPRETATION: The humeral head showed a small translation in the antero-posterior direction between 90 degrees and 150 degrees of abduction. In addition, the posterior part of the humeral head contacted the glenoid in this range of abduction. These findings of motion patterns in asymptomatic subjects will be necessary when comparing the kinematics with pathologic condition such as the glenohumeral instability and rotator cuff tear.