Associations between in-vivo glenohumeral joint motion and morphology

Geometric accuracy of low-dose CT scans for use in shoulder musculoskeletal research applications

Computed tomography (CT) imaging is frequently employed in a variety of musculoskeletal research applications. Although research studies often use imaging protocols developed for clinical applications, lower dose protocols are likely possible when the goal is to reconstruct 3D bone models. Our purpose was to describe the dose-accuracy trade-off between incrementally lower-dose CT scans and the geometric reconstruction accuracy of the humerus, scapula, and clavicle. Six shoulder specimens were acquired and scanned using 5 helical CT protocols: 1) 120 kVp, 450 mA (full-dose); 2) 120 kVp, 120 mA; 3) 120 kVp, 100 mA; 4) 100 kVp, 100 mA; 5) 80 kVp, 80 mA. Scans were segmented and reconstructed into 3D surface meshes. Geometric error was assessed by comparing the surfaces of the low-dose meshes to the full-dose (gold standard) mesh and was described using mean absolute error, bias, precision, and worst-case error. All low-dose protocols resulted in a >70 % reduction in the effective dose. Lower dose scans resulted in higher geometric errors; however, error magnitudes were generally <0.5 mm. These data suggest that the effective dose associated with CT imaging can be substantially reduced without a significant loss of geometric reconstruction accuracy.

Scapular morphology does not predict supraspinatus tendon tear propagation following an individualised exercise therapy programme

Purpose

To determine whether scapular morphology could predict isolated supraspinatus tendon tear propagation after exercise therapy. We hypothesised that a larger critical shoulder angle (CSA) and type III acromial morphology predict a positive change in tear size.

Methods

Fifty-nine individuals aged 40-70 years with isolated symptomatic high-grade partial or full-thickness supraspinatus tendon tears were included. Individuals participated in a structured, individualised 12-week exercise therapy programme and underwent ultrasound to measure tear size at baseline and 12 months following therapy. Computed tomography images were segmented to create three-dimensional subject-specific bone models and reviewed by three trained clinicians to measure CSA and to determine acromion morphology based on the Bigliani classification. A binary logistic regression was performed to determine the predictive value of CSA and acromion morphology on tear propagation.

Results

The CSA was 30.0 ± 5.4°. Thirty-one individuals (52.5%) had type II acromial morphology, followed by type III and type I morphologies (25.4% and 22.0%, respectively); 81.4% experienced no change in tear size, four (6.8%) individuals experienced tear propagation and seven (11.9%) individuals had a negative change in tear size. No significant difference in tear propagation rates based on CSA or acromion morphology (not significant [NS]) was observed. The model predicted tear size status in 81.4% of cases but only predicted tear propagation 8.3% of the time. Overall, CSA and acromion morphology only predicted 24.3% (R 2 = 0.243) of variance in tear propagation (NS).

Conclusions

CSA and acromion morphology were NS predictors of tear propagation of the supraspinatus tendon 12 months following an individualised exercise therapy programme.

Level of Evidence

II.

Investigating the multifactorial etiology of supraspinatus tendon tears

The purpose of this study was to develop a multivariable model to determine the extent to which a combination of etiological factors is associated with supraspinatus tendon tears. Fifty-four asymptomatic individuals (55 ± 4 years) underwent testing of their dominant shoulder. Diagnostic ultrasound was used to assess for a supraspinatus tendon tear. The etiological factors investigated included demographics (age and sex), tendon impingement during shoulder motion (via biplane videoradiography), glenohumeral morphology (via computed tomography imaging), family history of a tear (via self-report), occupational shoulder exposure (via shoulder job exposure matrix), and athletic exposure (via self-report). Univariate relationships between etiological predictors and supraspinatus tears were assessed using logistic regression and odds ratios (ORs), while multivariable relationships were assessed using classification and regression tree analysis. Thirteen participants (24.1%) had evidence of a supraspinatus tear. Individuals with a tear had a higher critical shoulder angle (OR 1.2, p = 0.028) and acromial index (OR 1.2, p = 0.016) than individuals without a tear. The multivariable model suggested that a tear in this cohort can be explained with acceptable accuracy (AUROC = 0.731) by the interaction between acromial index and shoulder occupational exposure: a tear is more likely in individuals with a high acromial index (p < 0.001), and in individuals with a low acromial index and high occupational exposure (p < 0.001). The combination of an individual's glenohumeral morphology (acromial index) and occupational shoulder exposure may be important in the development of supraspinatus tears.

Critical Shoulder Angle in Patients With Cuff Tears

OBJECTIVE

The pathogenesis of rotator cuff tears (RCTs) is multifactorial. Critical shoulder angle (CSA), which represents the lateral extension of the acromion over the cuff, has been proposed as an extrinsic risk factor. The aim of the present systematic review and meta-analysis was to analyze the available literature regarding the correlation between RCT and CSA.

METHODS

A review was carried out in accordance with the "Preferred Reporting Items for Systematic reviews and Meta-Analyses" guidelines on July 17, 2023, using the following databases: PubMed, Ovid, and Cochrane Reviews. The following keywords were used: "critical shoulder angle," "rotator cuff tears," and "rotator cuff lesions." The methodological quality of the studies was assessed with the MINORS SCORE.

RESULTS

Twenty-eight studies were included. The average CSA among the 2110 patients with full-thickness RCT was 36.7 degrees, whereas the same value among the 2972 controls was 33.1 degrees. The average CSA in the 348 patients with partial-thickness RCT was 34.6 degrees, whereas it was 38.1 degrees in the 132 patients with massive RCT. The average MINORS score was 15.6.

CONCLUSIONS

CSA values were significantly higher in patients with RCT compared with the asymptomatic population. In addition, it appears that CSA values increase with the severity of rotator cuff involvement.

In Vivo Analysis of Acromioclavicular Kinematics and Distance During Multiplanar Humeral Elevation

BACKGROUND

Knowledge of acromioclavicular (AC) joint kinematics and distance may provide insight into the biomechanical function and development of new treatment methods. However, accurate data on in vivo AC kinematics and distance between the clavicle and acromion remain unknown.

PURPOSE/HYPOTHESIS

The purpose of this study was to investigate 3-dimensional AC kinematics and distance during arm elevation in abduction, scaption, and forward flexion in a healthy population. It was hypothesized that AC kinematics and distance would vary with the elevation angle and plane of the arm.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 19 shoulders of healthy participants were enrolled. AC kinematics and distance were investigated with a combined dual fluoroscopic imaging system and computed tomography. Rotation and translation of the AC joint were calculated. The AC distance was measured as the minimum distance between the medial border of the acromion and the articular surface of the distal clavicle (ASDC). The minimum distance point (MDP) ratio was defined as the length between the MDP and the posterior edge of the ASDC divided by the anterior-posterior length of the ASDC. AC kinematics and distance between different elevation planes and angles were compared.

RESULTS

Progressive internal rotation, upward rotation, and posterior tilt of the AC joint were observed in all elevation planes. The scapula rotated more upward relative to the clavicle in abduction than in scaption (P = .002) and flexion (P = .005). The arm elevation angle significantly affected translation of the AC joint. The acromion translated more laterally and more posteriorly in scaption than in abduction (P < .001). The AC distance decreased from the initial position to 75° in all planes and was significantly greater in flexion (P < .001). The MDP ratio significantly increased with the elevation angle (P < .001).

CONCLUSION

Progressive rotation and significant translation of the AC joint were observed in different elevation planes. The AC distance decreased with the elevation angle from the initial position to 75°. The minimum distance between the ASDC and the medial border of the acromion moved anteriorly as the shoulder elevation angle increased.

CLINICAL RELEVANCE

These results could serve as benchmark data for future studies aiming to improve the surgical treatment of AC joint abnormalities to restore optimal function.

Comparison of rotator cuff to glenoid proximity based on scapulothoracic upward rotation classification

BACKGROUND

Recent evidence suggests that internal impingement, or rotator cuff tendon deformation against the glenoid, occurs during overhead motions and may therefore be a mechanism of pathology even in non-athletes. Clinically, knowing how movement impacts potential injury mechanisms would be useful to guide movement-based treatment strategies.

OBJECTIVE

To compare the distance between the glenoid and rotator cuff footprint between two groups classified based on scapulothoracic upward rotation (UR) magnitude (i.e., low, high) at 90° humerothoracic elevation.

METHODS

Shoulder kinematics were quantified during scapular plane abduction in 60 participants using single-plane fluoroscopy. Of these, 40 were subsequently classified as having high or low scapulothoracic UR based on the sample's distribution. The minimum distance between the glenoid and rotator cuff footprint was calculated along with the locations of closest proximity (i.e., proximity centers). Minimum distances and proximity center locations were compared between groups using 2-factor mixed-model ANOVAs. The prevalence of glenoid-to-footprint contact was also compared.

RESULTS

Glenoid-to-footprint distances consistently decreased as humerothoracic elevation angle increased, and the anterior aspect of the footprint was closest to the posterosuperior glenoid. Minimum distances were not significantly different between UR groups (p≥0.16). However, group differences existed in proximity center locations (p<0.01). Glenoid-to-footprint contact was identified in 75.0% of participants at an average (SD) of 133.6° (3.2°) humerothoracic elevation.

CONCLUSION

The results of this study suggest that decreased UR as classified and assessed in this study does not significantly impact glenoid-to-footprint distances but does alter the location of the contact, which occurred in most participants.

In vivo evaluation of rotator cuff internal impingement during scapular plane abduction in asymptomatic individuals

Internal impingement-or entrapment of the undersurface of the rotator cuff tendon against the glenoid during overhead activities-is believed to contribute to articular-sided tears. However, little is known about internal impingement outside athletic populations. Therefore, the objectives of this study were to (1) describe glenoid-to-footprint distances and proximity centers during dynamic, in vivo motion in asymptomatic individuals, and (2) determine the extent to which these measures differed between individuals with and without a rotator cuff tear. Shoulder kinematics were assessed in 37 asymptomatic individuals during scapular plane abduction using a high-speed biplane radiographic system. Glenoid-to-footprint distances and proximity center locations were calculated by combining the kinematics with computerized tomography-derived bone models. Glenoid-to-footprint contact was presumed to occur when the minimum distance was less than the estimated labral thickness. The condition of the supraspinatus tendon (intact, torn) was assessed using ultrasound. Minimum distances and proximity centers were compared over humerothoracic elevation angles (90°, 110°, 130°, 150°) and between supraspinatus pathology groups using two-factor mixed model analysis of variances. Glenoid-to-footprint minimum distances decreased consistently across elevation angles (p < 0.01) without a significant difference between groups. Contact was estimated to occur in all participants. Proximity centers were generally located on the anterior half of the rotator cuff footprint and on the posterosuperior glenoid. Statement of Clinical Significance: Internal impingement during overhead motions may be a prevalent mechanism of rotator cuff pathology as contact appears to be common and involves the region of the rotator cuff footprint where degenerative rotator cuff tears are thought to originate.

Controversy on the Association of the Critical Shoulder Angle and the Development of Degenerative Rotator Cuff Tears: Is There a True Association? A Meta-analytical Approach

BACKGROUND

The role of the critical shoulder angle (CSA) as a risk factor for rotator cuff tear (RCT) remains controversial. Studies on the association between the CSA and RCT show considerable differences in design, and this could be responsible for the variation in study results.

PURPOSE

To (1) describe the reliability of CSA measurement and (2) evaluate the results of the studies reporting the association between the CSA and RCT using meta-analytical techniques to explore potential sources of variation of study results.

STUDY DESIGN

Systematic review and meta-analysis; Level of evidence, 4.

METHODS

MEDLINE, EMBASE, and CINAHL electronic databases were searched through June 30, 2019. Case-control and cross-sectional studies reporting the association of the CSA and RCT were selected. The weighted mean difference in the CSA was estimated using a random-effects model. Prediction interval was computed to better express uncertainties in the effect estimate. Metaregression and subgroup analyses were performed to explore potential sources of heterogeneity.

RESULTS

A total of 14 studies, including 1154 cases and 1271 controls, were identified. Of these studies, 79% (11/14) assessed the reliability of the CSA measurement, demonstrating an excellent intraobserver (range, 0.91-0.99) and interobserver (range, 0.87-0.99) reliability. Compared with controls, cases with RCT showed larger measurements of the CSA (3.3° [95% CI, 2.3°- 4.4°]). However, there was a high heterogeneity (I² = 93%), and the 95% prediction interval (-0.4° to 7.1°) included no difference in the CSA. Results of the metaregression analysis showed a significant association of several methodological aspects with the heterogeneity. The difference in the CSA tended to be larger when only full-thickness tears were included, when no specific defined criterion for assessing radiographic viewing perspective was used, in studies with smaller sample sizes, and in studies at higher risk of bias.

CONCLUSION

While the CSA can be reliably measured, the difference in the CSA between cases and controls varied from very large to modest or almost no difference. Several determinants of heterogeneity were determined. Owing to this heterogeneity, it is difficult to gain an insight into the strength and exact nature of the association between the CSA and RCT with the current evidence.

Optimal insertion site of glenoid baseplate in reverse total shoulder arthroplasty: anatomical simulation using three dimensional image processing software

PURPOSE

Conventionally, the central structure of the baseplate is inserted through the point where the vertical and horizontal axes of the glenoid intersect (conventional insertion site (CIS)). However, there is scanty theoretical evidence that CIS has the optimal bone stock. We evaluated the optimal insertion site for the glenoid baseplate through the three-dimensional volumetric measurement of the glenoid bone stock.

METHODS

Pre-operative computed tomography (CT) images of 30 consecutive reverse total shoulder arthroplasty procedures were analyzed. Three-dimensional image processing software was used to reconstruct CT and volumetrically measure the glenoid bone stock according to the simulated central peg. A simulated central peg was inserted to the medial pole of the scapula from 49 points determined along with the intersect point of the vertical and horizontal axes of the glenoid CIS at 2-mm intervals. The overlapped volume between the simulated central peg and glenoid vault, representing the amount of glenoid bone stock along the passage of the central peg, was then automatically calculated.

RESULTS

The depth of the glenoid vault was 25.5 ± 3.0 mm (range, 19.3-31.5), and the mean overlapped volume between the simulated central peg and the glenoid vault was 623.0 ± 185.8 ml. The optimal insertion site for the bony purchase of the central peg was 2 mm inferior and posterior from the CIS (765.3 ± 157.5).

CONCLUSION

The optimal insertion site of the baseplate is located slightly inferiorly and posteriorly to the CIS. This anatomical information may be used as a reference to determine the optimal insertion site of the baseplate according to an implant of a surgeon's choice.

Measuring 3D In-vivo Shoulder Kinematics using Biplanar Videoradiography

The shoulder is one of the human body's most complex joint systems, with motion occurring through the coordinated actions of four individual joints, multiple ligaments, and approximately 20 muscles. Unfortunately, shoulder pathologies (e.g., rotator cuff tears, joint dislocations, arthritis) are common, resulting in substantial pain, disability, and decreased quality of life. The specific etiology for many of these pathologic conditions is not fully understood, but it is generally accepted that shoulder pathology is often associated with altered joint motion. Unfortunately, measuring shoulder motion with the necessary level of accuracy to investigate motion-based hypotheses is not trivial. However, radiographic-based motion measurement techniques have provided the advancement necessary to investigate motion-based hypotheses and provide a mechanistic understanding of shoulder function. Thus, the purpose of this article is to describe the approaches for measuring shoulder motion using a custom biplanar videoradiography system. The specific objectives of this article are to describe the protocols to acquire biplanar videoradiographic images of the shoulder complex, acquire CT scans, develop 3D bone models, locate anatomical landmarks, track the position and orientation of the humerus, scapula, and torso from the biplanar radiographic images, and calculate the kinematic outcome measures. In addition, the article will describe special considerations unique to the shoulder when measuring joint kinematics using this approach.

Thinking outside the glenohumeral box: Hierarchical shape variation of the periarticular anatomy of the scapula using statistical shape modeling

Variation in the shape of the glenoid and periarticular anatomy of the scapula has been associated with shoulder pathology. The goal of this study was to identify the modes of shape variation of periarticular scapular anatomy in relation to the glenoid in nonpathologic shoulders. Computed tomography scans of 31 cadaveric scapulae, verified to be free of pathology, were three-dimensionally reconstructed. Statistical shape modeling and principal component analysis identified the modes of shape variation across the population. Corresponding linear and angular measurements quantified the morphometric variance identified by the modes. Linear measures were normalized to the radius of the inferior glenoid to account for differences in the scaling of the bones. Five modes captured 89.7% of total shape variation of the glenoid and periarticular anatomy. Apart from size differences (mode 1: 33.0%), acromial anatomy accounted for the largest variation (mode 2: 32.0%). Further modes described variation in glenoid inclination (mode 3: 11.8%), coracoid orientation and size (mode 4: 9.0%), and variation in coracoacromial (CA) morphology (mode 5: 3.1%). The average scapula had a mean acromial tilt of 49 ± 7°, scapular spine angle of 61 ± 6°, the glenoid inclination of 84 ± 4°, coracoid deviation angle of 26 ± 4°, coracoid length of 3.7 ± 0.3 glenoid radii, and a CA base length of 5.6 ± 0.5 radii. In this study, the identified shape modes explain almost all of the variance in scapular anatomy. The acromion exhibited the highest variance of all periarticular anatomic structures of the scapula in relation to the glenoid, which may play a role in many shoulder pathologies.

Anatomic shoulder parameters and their relationship to the presence of degenerative rotator cuff tears and glenohumeral osteoarthritis: a systematic review and meta-analysis

BACKGROUND

Scapular anatomy, as measured by the acromial index (AI), critical shoulder angle (CSA), lateral acromial angle (LAA), and glenoid inclination (GI), has emerged as a possible contributor to the development of degenerative shoulder conditions such as rotator cuff tears and glenohumeral osteoarthritis. The purpose of this study was to investigate the published literature on influences of scapular morphology on the development of degenerative shoulder conditions.

METHODS

A systematic review of the Embase and PubMed databases was performed to identify published studies on the potential influence of scapular bony morphology on the development of degenerative rotator cuff tears and glenohumeral osteoarthritis. The studies were reviewed by 2 authors. The findings were summarized for various anatomic parameters. A meta-analysis was completed for parameters reported in more than 5 related publications.

RESULTS

A total of 660 unique titles and 55 potentially relevant abstracts were reviewed with 30 published articles identified for inclusion. The AI, CSA, LAA, and GI were the most commonly reported bony measurements. Increased CSA and AI correlated with rotator cuff tears, whereas lower CSA appeared to be related to the presence of glenohumeral osteoarthritis. Decreased LAA correlated with degenerative rotator cuff tears. Five articles reported on the GI with mixed results on shoulder pathology.

DISCUSSION

Degenerative rotator cuff tears appear to be significantly associated with the AI, CSA, and LAA. There does not appear to be a significant relationship between the included shoulder parameters and the development of osteoarthritis.

The Relationship Between the Critical Shoulder Angle and the Incidence of Chronic, Full-Thickness Rotator Cuff Tears and Outcomes After Rotator Cuff Repair: A Systematic Review

PURPOSE

To summarize the available evidence and examine the relationship between the critical shoulder angle (CSA) and (1) the incidence of chronic full-thickness rotator cuff tears (RCTs) and (2) outcomes after rotator cuff repair (RCR).

METHODS

A comprehensive search of MEDLINE, Embase, and CINAHL was completed. Comparative studies were included and the influence of the CSA on either the incidence of chronic, full-thickness RCTs, or outcomes following RCR was evaluated. Demographic variables and outcomes were collected.

RESULTS

Seven comparative studies analyzed the influence of the CSA on the incidence of chronic, full-thickness RCTs (the control group constituted patients with a normal rotator cuff). High heterogeneity limited pooling of studies, but the majority concluded that a greater CSA significantly increased the likelihood of a chronic, full-thickness RCT. Conversely, 5 comparative studies analyzed the influence of CSA on outcomes following RCR, and although a greater CSA was associated with a greater re-tear rate, the majority reported that CSA did not significantly influence postoperative functional outcomes, including patient-reported outcome measures (PROMs), range of motion (ROM), and strength.

CONCLUSIONS

Based on the available evidence, there appears to be a relationship between a greater CSA and the presence of a chronic, full-thickness RCT. Furthermore, a greater CSA may be associated with a greater re-tear rate following RCR; however, CSA does not appear to influence functional outcomes following RCR. Despite these observations, the available evidence is of poor quality, and the clinical utility and role of the CSA in the diagnosis and surgical management of a chronic, full-thickness RCT remains in question.

LEVEL OF EVIDENCE

Level IV: Systematic review of Level II-IV studies.

Effect of different geometrical structure of scapula on functional recovery after shoulder arthroscopy operation

Background

There are no published studies of the influence of geometry of the scapula on the postoperative recovery of rotator cuff injuries. Our aim was to explore the relationship between the critical shoulder angle (CSA), acromion index (AI), glenoid inclination (GI), and postoperative repair outcomes in shoulder joints after arthroscopic supraspinatus tendon repair.

Methods

Sixty two patients suffering a supraspinatus tear were analyzed retrospectively following failure of conservative treatment and subsequent shoulder arthroscopy in our hospital. Standard anterior and posterior X-rays of the injured shoulder had been performed prior to surgery, with follow ups for at least 2 years (24–43 months). Magnetic resonance imaging (MRI) was performed 2 years after surgery to assess repair of the supraspinatus tendon. Patients were divided into either the intact or re-tear group, according to the MRI results. In addition, assessments using the Constant Shoulder Score (CSS), the American Shoulder and Elbow Surgeon (ASES) Shoulder Assessment Form, the University of California at Los Angeles (UCLA) score and visual analog scale (VAS) score were performed to establish shoulder function at the 2-year evaluation for each patient.

Results

The mean CSA of all patients was 35.79° ± 3.59°, mean AI was 0.72 ± 0.05, and mean GI was 15.87° ± 3.62°. The CSA, AI, and GI in the intact group were statistically significantly different than the re-tear group (p < 0.05). There was no correlation between the magnitude of the CSA, AI, or GI and any shoulder function score (p > 0.05).

Conclusions

The geometry of the scapula had no significant effect on the recovery of postoperative function of patients with rotator cuff injury, but the value of the CSA, AI, and GI affected the risk of rotator cuff re-tear.

In Vivo Assessment of Exercise-Induced Glenohumeral Cartilage Strain

BACKGROUND

The human shoulder joint is the most mobile joint in the body. While in vivo shoulder kinematics under minimally loaded conditions have been studied, it is unclear how glenohumeral cartilage responds to high-demand loaded exercise.

HYPOTHESIS

A high-demand upper extremity exercise, push-ups, will induce compressive strain in the glenohumeral articular cartilage, which can be measured with validated magnetic resonance imaging (MRI)-based techniques.

STUDY DESIGN

Descriptive laboratory study.

METHODS

High-resolution MRI was used to measure in vivo glenohumeral cartilage thickness before and after exercise among 8 study participants with no history of upper extremity injury or disease. Manual MRI segmentation and 3-dimensional modeling techniques were used to generate pre- and postexercise thickness maps of the humeral head and glenoid cartilage. Strain was calculated as the difference between pre- and postexercise cartilage thickness, normalized to the pre-exercise cartilage thickness.

RESULTS

Significant compressive cartilage strains of 17% ± 6% and 15% ± 7% (mean ± 95% CI) were detected in the humeral head and glenoid cartilage, respectively. The anterior region of the glenoid cartilage experienced a significantly higher mean strain (19% ± 6%) than the posterior region of the glenoid cartilage (12% ± 8%). No significant regional differences in postexercise humeral head cartilage strain were observed.

CONCLUSION

Push-ups induce compressive strain on the glenohumeral joint articular cartilage, particularly at the anterior glenoid. This MRI-based methodology can be applied to further the understanding of chondral changes in the shoulder under high-demand loading conditions.

CLINICAL RELEVANCE

These results improve the understanding of healthy glenohumeral cartilage mechanics in response to loaded upper extremity exercise. In the future, these methods can be applied to identify which activities induce high glenohumeral cartilage strains and deviations from normal shoulder function.

The critical acromial point: the anatomic location of the lateral acromion in the critical shoulder angle

BACKGROUND

Acromioplasty has been proposed as a means of altering elevated critical shoulder angles (CSAs). We aimed to localize the critical acromion point (CAP) responsible for the acromial contribution of the CSA and determine whether resection of the CAP can alter the CSA to a normal range.

METHODS

The CAP and 3-dimensional (3D) CSAs were determined on 3D computed tomography reconstructions of 88 cadaveric shoulders and compared with corresponding CSAs on digitally reconstructed radiographs. The position of the CAP was fluoroscopically isolated in 20 of these specimens and the resulting fluoroscopic CSA compared with the corresponding 3D CAP and 3D CSA. We investigated the CSA before and after a virtual acromioplasty of 2.5 and 5 mm at the CAP in specimens with a CSA greater than 35°.

RESULTS

The mean CAP was 21% ± 10% of the acromial anterior-posterior length from the anterolateral corner. There was no difference between the mean 3D CSA and the CSA on digitally reconstructed radiographs (32° vs 32°, P = .096). No difference between the mean fluoroscopic CSA and 3D CSA was found (31° vs 31°, P = .296). A 2.5-mm acromial resection failed to reduce the CSA to 35° or less in 7 of 13 shoulders, whereas a 5-mm resection reduced the CSA to 35° or less in 12 of 13.

CONCLUSION

The CAP was localized to the anterolateral acromial edge and was easily identified fluoroscopically. A 5-mm acromial resection was effective in reducing the CSA to 35° or less. These data can guide surgeons in where and how to alter the CSA if future studies demonstrate a clinical benefit to surgically modifying this radiographic parameter.

Effects of glenoid inclination and acromion index on humeral head translation and glenoid articular cartilage strain

BACKGROUND

Previous clinical studies have reported associations between glenoid inclination (GI), the acromion index (AI), and the critical shoulder angle (CSA) on the one hand and the occurrence of glenohumeral osteoarthritis and supraspinatus tendon tears on the other hand. The objective of this work was to analyze the correlations and relative importance of these different anatomic parameters.

METHODS

Using a musculoskeletal shoulder model developed from magnetic resonance imaging scans of 1 healthy volunteer, we varied independently GI from 0° to 15° and AI from 0.5 to 0.8. The corresponding CSA varied from 20.9° to 44.1°. We then evaluated humeral head translation and critical strain volume in the glenoid articular cartilage at 60° of abduction in the scapular plane. These values were correlated with GI, AI, and CSA.

RESULTS

Humeral head translation was positively correlated with GI (R = 0.828, P < .0001), AI (R = 0.539, P < .0001), and CSA (R = 0.964, P < .0001). Glenoid articular cartilage strain was also positively correlated with GI (R = 0.489, P = .0004) but negatively with AI (R = -0.860, P < .0001) and CSA (R = -0.285, P < .0473).

CONCLUSIONS

The biomechanical shoulder model is consistent with clinical observations. The prediction strength of CSA is confirmed for humeral head translation and thus presumably for rotator cuff tendon tears, whereas the AI seems more appropriate to evaluate the risk of glenohumeral osteoarthritis caused by excessive articular cartilage strain. As a next step, we should corroborate these theoretical findings with clinical data.