Translation Of Humeral Head Research Articles

Importance of the subscapularis muscle after total shoulder arthroplasty

BackgroundThe 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. MethodsTo 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. FindingsTo 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. InterpretationThese results confirm the importance of the suscapularis for the long-term stability of total shoulder arthroplasty.

Classic Versus Congruent Coracoid Positioning During the Latarjet Procedure: An In Vitro Biomechanical Comparison

The purpose of this biomechanical study was to compare the classic Latarjet technique and congruent-arc modification with respect to glenohumeral stability, joint stiffness, translation, and range of motion. Eight cadaveric forequarters were tested on a shoulder simulator that applied loads independently to the conjoint tendon, long head of biceps, rotator cuff, and deltoid. The test conditions included: intact, 30% glenoid defect, and reconstruction of the defect with the classic and congruent Latarjets. The Latarjet techniques were randomly ordered, with the outcome variables being anterior dislocation, glenohumeral translation, rotational range of motion, and joint stiffness. All 8 specimens dislocated after creation of a 30% glenoid defect. The classic Latarjet stabilized 7 of 8 specimens, whereas the congruent-arc modification stabilized all specimens (8/8). In abduction neutral rotation, there was no difference in joint translation between techniques (P= .613). In abduction external rotation, there was significantly greater anterior humeral head translation after the congruent technique than after the classic (9.9 and 6.5 mm, respectively, P= .013). Rotational range of motion was significantly reduced after classic (-25.8°) and congruent (-22.2°) transfers as compared with the 30% defect (P ≤ .041). Joint stiffness in the abducted, externally rotated position was significantly reduced in the 30% defect as compared with intact (P= .012), congruent (P= .015), and classic (P < .001) conditions. In all abduction positions, the intact was not significantly different from the Latarjet techniques, and the techniques did not significantly differ from each other (P ≥ .102). The classic and congruent-arc Latarjet techniques restore shoulder stability and motion in cases of considerable bone loss. The techniques do not substantially differ in rotational range of motion or joint stiffness. The congruent-arc technique, however, does result in significantly greater anterior humeral head translation, as compared with the classic technique, before reaching a stable non-dislocated endpoint. On the basis of this biomechanical model, both the classic and congruent-arc Latarjet techniques can be used to stabilize a shoulder with substantial glenoid bone loss. Further clinical and biomechanical studies are required to determine if particular clinical circumstances exist where 1 technique has an advantage over the other.

Conjoined Tendon Transfer vs Modified Bristow in a Glenoid Bone Loss Model: A Biomechanical Study

Objectives:Over the last ten years there has been a resurgent interest in the use of coracoid transfer procedures for the treatment of traumatic, anterior glenohumeral instability of the shoulder. Bankart repair, whether arthroscopic or open, is the gold standard for treatment of the majority of anterior glenohumeral instability cases. Coracoid transfer procedures, on the other hand, are effective for patients with significant bony defects, those with poor quality anterior capsulolabral tissues, and in collision athletes with higher risk of instability after simple soft-tissue repair. Despite high rates of success in most studies involving the Bristow and other coracoid transfer procedures, complications have been reported. Screw breakage, nerve injury, non-union of the coracoid fragment, and arthritis related to improper placement of the bone block have all been documented. Eliminating the bone block would help to decrease the number of potential complications associated with the procedure. For this reason, we sought to evaluate whether the Bristow procedure or the conjoined tendon transfer (CTT) would be more effective in restoring kinematics of the glenohumeral joint in a glenoid bone loss model for anterior shoulder instability. This is the first study to our knowledge comparing a bone block with its attached musculotendinous sling to a sling alone in a glenoid bone loss model.Methods:Utilizing fresh frozen cadavers, biomechanical testing was undertaken comparing anterior glenohumeral translation in two groups of ten specimens each with a 25% anterior glenoid bone defect. Group 1 consisted of a tensioned conjoined tendon transfer (CTT) into the defect. Group 2 consisted of a Modified Bristow bone block (BB) transfer. Cyclic testing was performed with the shoulder at 90 degrees of external rotation in both 60 and 90 degrees of abduction.Results:At 60 degrees of abduction, the conjoined tendon transfer showed a 43% reduction in anterior humeral head translation from the unstable or deficient state, while the bone block transfer showed a 12% reduction. This difference was statistically significant. At 90 degrees of abduction, the conjoined tendon transfer showed a 59% reduction in anterior humeral head translation from the unstable or deficient state, while the bone block transfer showed a 38% reduction. This difference trended towards significance. (Please see table 1. for full details/data)Conclusion:A tensioned conjoined trendon transfer in the setting of 25% anterior glenoid bone loss exhibited a statistically significant reduction in anterior glenohumeral translation at 60 degrees of abduction and a nonsignificant reduction at 90 degrees of abduction versus a Modified Bristow bone block transfer. Biomechanically, we feel the musculotendinous sling of the conjoined tendon functions in place of the anterior and anterior/inferior capsulolabral structures, including the anterior band of the IGHL, which are typically deficient in the setting of recurrent anterior instability. This represents the initial step in determining if this procedure could be a viable alternative to a bone block transfer in the setting of anterior glenohumeral instability with glenoid bone loss.

Immediate Effects of Soft Tissue Massage on Posterior Shoulder Muscle Tightness: A Preliminary Study

Posterior shoulder muscle tightness is frequently observed in shoulder impingement syndrome because tightness in the posterior portion of the shoulder muscles can cause anterior and superior translation of the humeral head in relation to the glenoid fossa. The purpose of this study was to determine the immediate effects of soft tissue massage on acromiohumeral distance (AHD), anterior translation of the humeral head, and glenohumeral (GH) range of motion (ROM) in subjects with posterior shoulder muscle tightness. Twenty-seven subjects with greater than <TEX>$10^{\circ}$</TEX> difference in the range of GH horizontal adduction between right and left sides were recruited. The range of GH horizontal adduction and internal rotation were measured by a digital inclinometer. The AHD and anterior translation of the humeral head were measured using ultrasonography. A paired t-test was used to compare AHD, anterior translation of the humeral head, and the range of GH horizontal adduction and internal rotation before and after soft tissue massage. The results showed that AHD increased significantly (p<.05) and the anterior translation of humeral head decreased slightly, but not significantly (p=.40) after the soft tissue massage. Furthermore, the ROM of horizontal adduction and internal rotation in the GH joint increased significantly after the soft tissue massage (p<.05). These findings indicate that soft tissue massage on posterior shoulder muscle tightness is an effective method to increase AHD and ROM in the horizontal adduction and internal rotation of the GH joint.

Humeral Head Translation After a Suprascapular Nerve Block

Subacromial impingement syndrome is the most common shoulder disorder. Abnormal superior translation of the humeral head is believed to be a major cause of this pathology. The first purpose of the study was to examine the effects of suprascapular nerve block on superior translation of the humeral head and scapular upward rotation during dynamic shoulder elevation. The secondary purpose was to assess muscle activation patterns during these motions. Twenty healthy subjects participated in the study. Using fluoroscopy and electromyography, humeral head translation and muscle activation were measured before and after a suprascapular nerve block. The humeral head was superiorly located at 60 degrees of humeral elevation, and the scapula was more upwardly rotated from 30 to 90 degrees of humeral elevation after the block. The differences were observed during midrange of motion. In addition, the deltoid muscle group demonstrated increased muscle activation after the nerve block. The study's results showed a compensatory increase in humeral head translation, scapular upward rotation, and deltoid muscle activation due to the nerve block. These outcomes suggest that increasing muscular strength and endurance of the supraspinatus and infraspinatus muscles could prevent any increased superior humeral head translation. This may be beneficial in reducing shoulder impingement or rotator cuff tears over time.

An integrated model of active glenohumeral stability

We present the first model of the glenohumeral joint implementing active muscle driven humeral positioning and stabilization without a priori constraints on glenohumeral kinematics. Previously established methods were used to predetermine the path, activation timing and resultant force contribution of 27 individual muscle segments at any given joint position. Artificial boundary conditions were applied in a three-dimensional finite element model of the joint and progressively released until the humeral head was completely free to rotate and translate within the fixed glenoid according to the compressive component of the predetermined resultant force. The shear component was then added such that no boundary conditions other than muscular force were applied. The framework was exploited to simulate elevation as a composite of instantaneous positions and theoretically demonstrate that joint stability can be achieved exclusively through muscular activity. Predicted muscle moment arms, muscle activation timing, humeral head translations, joint contact forces and stability ratio were comparable with existing experimental and in vivo data. This framework could be valuable for subject specific modeling and may be used to address clinical hypotheses related to shoulder joint stability that cannot be pursued using simplified modeling approaches.

EFFECT OF GLENOID IMPLANT DESIGN ON GLENOHUMERAL STABILITY: AN EXPERIMENTAL STUDY

Abstract Background Though several glenoid implants were developed over the past years, a high rate of glenoid loosening remains. This complication is linked to the glenohumeral stability, defined as humeral head translation. In an intact shoulder, this concept is ensured by all active and passive elements, particularly the labrum. Two features of a glenoid implant can be adjusted to improve the stability, or, in other words, to decrease the translations: the first is the mismatch, defined as the difference of curvature between the prosthetic head and glenoid; the second is the shape of the glenoid component. Therefore, the objective of this study was to compare the performance of 2 glenoid components (Ceraver, Roissy, France): (1) a standard design and (2) a design named “labrum design” with a superior part simulating the anatomic labrum. Methods An experimental device was developed to evaluate forces and kinematics. The device simulated active, dynamic and continuous abduction of an entire arm. It reproduced the Scapulo-Humeral Rhythm. The labrum design was installed first. To evaluate the effect of mismatch on the glenohumeral stability, 3 humeral heads were tested, corresponding to the ones recommended by the company. The experiment was repeated for the standard design. Findings The results obtained show a general decrease of the prosthetic head translation with the labrum design compared to the standard design. No noticeable effect of mismatch was found. Interpretation A proof of concept of the interest of the artificial labrum was provided since it improved the glenohumeral stability.

In-vivo glenohumeral translation and ligament elongation during abduction and abduction with internal and external rotation

Study DesignBasic Science. To investigate humeral head translations and glenohumeral ligament elongation with a dual fluoroscopic imaging system.BackgroundThe glenohumeral ligaments are partially responsible for restraining the humeral head during the extremes of shoulder motion. However, in-vivo glenohumeral ligaments elongation patterns have yet to be determined. Therefore, the objectives of this study were to 1) quantify the in-vivo humeral head translations and glenohumeral ligament elongations during functional shoulder positions, 2) compare the inferred glenohumeral ligament functions with previous literature and 3) create a baseline data of healthy adult shoulder glenohumeral ligament lengths as controls for future studies.MethodsFive healthy adult shoulders were studied with a validated dual fluoroscopic imaging system (DFIS) and MR imaging technique. Humeral head translations and the superior, middle and inferior glenohumeral ligaments (SGHL, MGHL, IGHL) elongations were determined.ResultsThe humeral head center on average translated in a range of 6.0mm in the anterior-posterior direction and 2.5mm in the superior-inferior direction. The MGHL showed greater elongation over a broader range of shoulder motion than the SGHL. The anterior-band (AB)-IGHL showed maximum elongation at 90° abduction with maximum external rotation. The posterior-band (PB)-IGHL showed maximum elongation at 90° abduction with maximum internal rotation.DiscussionThe results demonstrated that the humeral head translated statistically more in the anterior-posterior direction than the superior-inferior direction (p = 0.01), which supports the concept that glenohumeral kinematics are not ball-in-socket mechanics. The AB-IGHL elongation pattern makes it an important static structure to restrain anterior subluxation of the humeral head during the externally rotated cocking phase of throwing motion. These data suggest that in healthy adult shoulders the ligamentous structures of the glenohumeral joint are not fully elongated in many shoulder positions, but function as restraints at the extremes of glenohumeral motion. Clinically, these results may be helpful in restoring ligament anatomy during the treatment of anterior instability of the shoulder.

Effect of glenoid implant design on glenohumeral stability: An experimental study

BackgroundThough several glenoid implants were developed over the past years, a high rate of glenoid loosening remains. This complication is linked to the glenohumeral stability, defined as humeral head translation. In an intact shoulder, this concept is ensured by all active and passive elements, particularly the labrum. Two features of a glenoid implant can be adjusted to improve the stability, or, in other words, to decrease the translations: the first is the mismatch, defined as the difference of curvature between the prosthetic head and glenoid; the second is the shape of the glenoid component. Therefore, the objective of this study was to compare the performance of 2 glenoid components (Ceraver, Roissy, France): (1) a standard design and (2) a design named “labrum design” with a superior part simulating the anatomic labrum. MethodsAn experimental device was developed to evaluate forces and kinematics. The device simulated active, dynamic and continuous abduction of an entire arm. It reproduced the Scapulo-Humeral Rhythm. The labrum design was installed first. To evaluate the effect of mismatch on the glenohumeral stability, 3 humeral heads were tested, corresponding to the ones recommended by the company. The experiment was repeated for the standard design. FindingsThe results obtained show a general decrease of the prosthetic head translation with the labrum design compared to the standard design. No noticeable effect of mismatch was found. InterpretationA proof of concept of the interest of the artificial labrum was provided since it improved the glenohumeral stability.

Glenohumeral contact pressure in a simulated active compression test using cadaveric shoulders

The active compression test has been described to detect superior labrum anterior and posterior (SLAP) lesions. Some have speculated that contact between the lesser tuberosity and the superior glenoid in the testing position causes a positive test. This study evaluated the location of peak glenohumeral contact pressures during a simulated active compression test in a cadaveric model with and without SLAP lesions. Six specimens were tested. A pressure sensor was used to record glenohumeral contact, and a motion analysis system was used to measure humeral head translation. Contact pressures and translations were measured during serial deltoid and biceps loads. These were repeated for small and large SLAP lesions. There was a notable shift in the location of peak contact pressure from the anterior-inferior glenoid to the anterior-superior glenoid with increasing deltoid load. Deltoid loading translated the humeral head posteriorly relative to the glenoid. Conversely, biceps loads countered the pull of the deltoid only when the biceps load was greater than the deltoid load. The SLAP tears did not significantly alter the degree of humeral head translation or location of the contact pressures. In the active compression test, the posterior capsule is taut and the anterior capsule is lax, which permits the deltoid to translate the humeral head posteriorly. This shift in the peak contact pressure to the superior glenoid may elicit a positive active compression test. Basic Science Study, Biomechanics, Cadaver Model.

Importance of polyethylene thickness in total shoulder arthroplasty: A finite element analysis

BackgroundArticular surfaces reconstruction is essential in total shoulder arthroplasty. Because of the limited glenoid bone support, thin glenoid component could improve anatomical reconstruction, but adverse mechanical effects might appear. MethodsWith a numerical musculoskeletal shoulder model, we analysed and compared three values of thickness of a typical all-polyethylene glenoid component: 2, 4 (reference) and 6mm. A loaded movement of abduction in the scapular plane was simulated. We evaluated the humeral head translation, the muscle moment arms, the joint force, the articular contact pattern, and the polyethylene and cement stress.FindingsDecreasing polyethylene thickness from 6 to 2mm slightly increased humeral head translation and muscle moment arms. This induced a small decreased of the joint reaction force, but important increase of stress within the polyethylene and the cement mantel.InterpretationThe reference thickness of 4mm seems a good compromise to avoid stress concentration and joint stuffing.

Interface micromotions increase with less-conforming cementless glenoid components

The optimal degree of conformity between the glenoid and humeral components in total shoulder arthroplasty for best performance and durability is still a matter of debate. The main aim of this study is to evaluate the influence of joint conformity on the bone-implant interface micromotions in a cementless glenoid implant. Polyethylene inlays with different degrees of conformity (radial mismatch of 0, 2, 4, and 6 mm) were mounted on a cementless metal back and then implanted in a bone substitute. These glenoid components were loaded by a prosthetic humeral head during a force-controlled experiment. Normal-to-interface micromotions and bone substitute deformations were measured at different points of the interface. Rim displacement and humeral head translation were also measured. A finite element (FE) model of the experiments was implemented to estimate the normal- and tangent-to-interface micromotions in the entire bone-implant interface. All measured variables increased with less-conforming PE inlays. Normal-to-interface micromotions were significantly larger (P < .05) when the radial mismatch was 6 mm compared with the fully conforming inlay. The FE model was in agreement and complemented the experimental results. FE model-predicted interface micromotions were already significantly larger when the radial mismatch was equal to 4 mm. In a force-controlled experiment with a cementless glenoid component, a non-conforming PE inlay allows larger interface micromotions than a conforming inlay, reaching a magnitude that may hamper local bone ingrowth in this type of component. This is mainly because of the larger humeral head translation that boosts the effects of the so-called rocking-horse phenomenon.

The rotator cuff muscles have a direction specific recruitment pattern during shoulder flexion and extension exercises

A recent study has shown that posterior rotator cuff (RC) muscles are recruited at significantly higher levels than the anterior RC during shoulder flexion. It was proposed that the mechanism whereby the posterior RC muscles were providing shoulder stability during flexion was to counterbalance potential anterior humeral head translation caused by flexion torque producing muscles. This hypothesis implies that anterior RC activity should be higher than posterior RC activity during extension to prevent posterior humeral head translation. As the normal recruitment pattern of the RC during extension has not been established, the purpose of this study was to examine this hypothesis by comparing shoulder muscle activation levels and recruitment patterns during flexion and extension exercises. Electromyographic (EMG) activity was recorded from 9 shoulder muscles in 15 volunteers. Flexion and extension exercises were performed in prone at 20%, 50%, and 70% of each participant's maximal load. A repeated measures ANOVA was used to determine differences between exercises, muscles and loads, while Pearson's correlation analysis was used to relate mean EMG patterns. During extension subscapularis and latissimus dorsi were activated at higher levels than during flexion; during flexion, supraspinatus, infraspinatus, deltoid, trapezius, and serratus anterior were more highly activated than during extension. In addition, the pattern of activity in each muscle did not vary with load. These results support the hypothesis that during flexion and extension the RC muscles are recruited in a direction specific manner to prevent potential antero-posterior humeral head translation caused by torque producing muscles.

Biomechanical Evaluation of Margin Convergence

The aim of this study was to examine rotator cuff strain and gap size after margin convergence was performed for a large retracted rotator cuff tear. We tested 20 cadaveric shoulders using a custom shoulder testing system. A large retracted rotator cuff tear was created by removing the supraspinatus muscle-tendon unit to provide a reproducible model. Margin convergence was performed and strain was measured by use of differential variable reluctance transducers in the intact state, after a massive rotator cuff tear was created, and after each of 5 margin convergence sutures were placed. Data were obtained at 0° and 60° of abduction and with internal and external rotational torques applied to the humerus. Gap size was measured before and after margin convergence sutures were placed. Strain was significantly reduced at all degrees of rotation in 0° of abduction after margin convergence sutures were placed (P < .05). There was a significantly significant decrease in gap size with each suture: 50% with the first suture, 60% with the second suture, 67% with the third suture, and 75% with the fourth suture (P < .05). There was only minimal intrinsic rotator cuff tension during knot tying, with each subsequent suture having less of an effect than the previous. Four margin convergence sutures resulted in a mean of 5 mm of anterior humeral head translation. There was a significant decrease in rotator cuff strain and gap size after margin convergence was performed for a large retracted tear. The first margin convergence suture caused the greatest increase in intrinsic rotator cuff tension, with each subsequent suture having a similar but less dramatic effect. Biomechanical rationale exists for the use of margin convergence in large retracted rotator cuff tears.

Biomechanical consequences of humeral component malpositioning after anatomical total shoulder arthroplasty

We hypothesized that the malpositioning of the humeral component can preclude the long-term success of anatomical total shoulder arthroplasty. The goal of this study was to evaluate the mechanical consequences of superior and inferior malpositioning of the humeral head. A numerical musculoskeletal model of the shoulder joint allowing natural humeral head translation was used to simulate a loaded abduction movement controlled by muscular activation. An inferior and superior malpositioning of 5 mm were compared to an optimal positioning. Impingements, articular contact pattern, and cement stress were evaluated. Inferior malpositioning of the humeral head induced impingement and limited the abduction level, while superior malpositioning increased the subluxation risk. Both inferior and superior malpositioning increased the stress level within the cement mantle. This numerical study highlights the importance of an anatomical reconstruction of the glenohumeral surfaces for the success rate of anatomical total shoulder arthroplasty.

The specificity of fatiguing protocols affects scapular orientation: Implications for subacromial impingement

Background Shoulder impairments are often associated with altered scapular kinematics. As muscles control scapular movement, functionally altering muscle performance through fatigue may produce scapular kinematics that mimic those of injured patients. The aim of this study was to examine if changes in scapular tilt, rotation and pro/retraction following two different upper extremity fatiguing protocols have any implications with respect to subacromial impingement. Methods Scapular orientation was monitored during posturally constrained static holds (at 0°, 45° and 90° of humeral elevation) before and after two fatiguing protocols, one global and one local. Both protocols are associated with producing changes in shoulder skeletal arrangement. Findings Following the global fatiguing protocol, there was significantly more scapular posterior tilt ( P < 0.01) and upward rotation ( P < 0.02), particularly at 90° humeral elevation. No changes in scapular orientation occurred following the local fatiguing protocol. Interpretation Scapular orientation changes following muscle fatigue acted to increase the subacromial space. Thus, the rotator cuff muscles, not the scapular stabilizers, have more influence on actively preventing mechanical subacromial impingement. The lack of evidence of reduction of the subacromial space thus implicates superior humeral head translation as a more likely primary mechanism of the initiation of subacromial impingement.

A biomechanical study of posterior glenoid bone loss and humeral head translation

Results of shoulder arthroplasty have been reported to be inferior with posterior glenoid wear and accompanying subluxation of the humeral head. The purpose of this study was to examine the effect of posterior glenoid wear on posterior subluxation of the humeral head. A custom loading device was used to simulate physiologic loading conditions in 8cadaver shoulders with the humerus positioned at neutral, forward flexion, and extension. Three-dimensional motion analysis recorded humeral head translation with respect to the scapula at each humerusposition after removing posterior glenoid bone in 5° increments. The magnitude of humeral head translation was analyzed with 2-way ANOVA to determine the effects of arm position and glenoid condition. Glenoid condition and arm position in the transverse plane significantly influenced head translation (P < .0001). With the humerus at neutral, posterior translation became significant after 20° of posterior bone removal (P < .05). However, with the humerus in forward flexion, posterior translation became significant at only 5° of posterior bone removal (P < .001). No significant differences in translation were detected for posterior defects up to 25° with the arm in extension. Posterior humeral head translation increased significantly with 5° of posterior glenoid bone loss, which equates to approximately 2.5° of glenoid retroversion. Awareness that humeral head translation may be seen with small amounts of retroversion should be recognized during preoperative planning for shoulder arthroplasty and when counseling the patient with regard to expected outcomes.

Development and Validation of a Finite Element Model of the Superior Glenoid Labrum

Pathology of the superior glenoid labrum is a common source of musculoskeletal pain and disability. One of the proposed mechanisms of injury to the labrum is superior humeral head migration, which can be seen with rotator cuff insufficiency. Due to the size, anatomical location, and complex composition of the labrum, laboratory experiments have many methodological difficulties. The purpose of this study was to develop and validate a finite element model of the glenoid labrum. The model developed includes the glenoid labrum, glenoid cartilage, glenoid bone, and the humeral head cartilage. Labral displacements derived from the finite element model were compared to those measured during a controlled validation experiment simulating superior humeral head translations of 1, 2, and 3 mm. The results of the finite element model compared well to experimental measurements, falling within one standard deviation of the experimental data in most cases. The model predicted maximum average strains in the superior labrum of 7.9, 10.1, and 11.9%, for 1, 2, and 3 mm of humeral translation, respectively. The correspondence between the finite element model and the validation experiment supports the use of this model to better understand the pathomechanics of the superior labrum.

Effect of Humeral Head Defect Size on Glenohumeral Stability

Background Hill-Sachs lesions are often present with recurrent shoulder instability and may be a cause of failed Bankart repair. Hypothesis Glenohumeral joint stability decreases with increasingly larger humeral head defects. Study Design Descriptive laboratory study. Methods Humeral head defects, 1/8, 3/8, 5/8, and 7/8 of the humeral head radius, were created in 8 human cadaveric shoulders, simulating Hill-Sachs defects. Testing positions included 45° and 90° of abduction and 40° of internal rotation, neutral, and 40° of external rotation. Testing occurred at each defect size sequentially from smallest to largest for all abduction and rotation combinations. The humeral head was translated at 0.5 mm/s 45° anteroinferiorly to the horizontal glenoid axis until dislocation. Distance to dislocation, defined as humeral head translation until it began to subluxate, was the primary outcome measure. Results Significant factors by ANOVA were rotation (P < .001) and defect size (P < .001). There was no difference for the 2 abduction angles. External rotation of 40° significantly reduced distance to dislocation compared with neutral and 40° internal rotation (P < .001). Osteotomies of 5/8 and 7/8 radius significantly decreased distance to dislocation over the intact state (P = .009 and P < .001, respectively). Post hoc analysis determined significant differences for the rotational positions. Decreased distance to dislocation occurred at 5/8 radius osteotomy at 40° external rotation with 90° of abduction (P = .008). For the 7/8 radius osteotomy at 90° abduction, there was a decreased distance to dislocation for neutral and 40° external rotation (P < .001); at 45° abduction, there was a decreased distance to dislocation at 40° external rotation (P < .001). With the humerus internally rotated, there was no significant change in distance to dislocation. Conclusion Glenohumeral stability decreases at a 5/8 radius defect in external rotation and abduction. At 7/8 radius, there was a further decrease in stability at neutral and external rotation. Clinical Relevance Defects of 5/8 the humeral head radius may require treatment to decrease the failure rate of shoulder instability repair.

Measuring humeral head translation using fluoroscopy: A validation study

Numerous techniques have been employed to monitor humeral head translation due to its involvement with several shoulder pathologies. However, most of the techniques were not validated. The objective of this study is to compare the accuracy of manual digitization and contour registration in measuring superior translation of the humeral head. Eight pairs of cadaver scapulae and humerii bones were harvested for this study. Each scapula and humerus was secured in a customized jig that allowed for control of humeral head translations and a vise that permitted rotations of the scapula about three axes. Fluoroscopy was used to take images of the shoulder bones. Scapular orientation was manipulated in different positions while the humerus was at 90° of humeral elevation in the scapular plane. Humeral head translation was measured using the two methods and was compared to the known translation. Additionally, accuracy of the contour registration method to measure 2-D scapular rotations was assessed. The range for the root mean square (RMS) error for manual digitization method was 0.27mm - 0.43mm and the contour registration method had a RMS error ranging from 0.18mm - 0.40mm. In addition, the RMS error for the scapular angle rotation using the contour registration method was 2.4°. Both methods showed acceptable errors. However, on average, the contour registration method showed lesser measurement error compared to the manual digitization method. In addition, the contour registration method was able to show good accuracy in measuring rotation that is useful in 2-D image analysis.