Background

There is no consensus on which glenoid plane should be used in total shoulder arthroplasty. Nevertheless, anatomical reconstruction of this plane is imperative for the success of a total shoulder arthroplasty.

Lateralization of the reverse arthroplasty may be desirable to more effectively tension the remaining rotator cuff, decrease scapular notching, improve the cosmetic appearance of the shoulder, and improve stability as well as the arc of motion prior to impingement. There are two primary options to lateralise a reverse shoulder arthroplasty: bone graft with a long post (BIO-RSA) vs. using metal. The two metal options generally include a thicker glenosphere or a thicker glenoid baseplate. Potential benefits of a BIO-RSA include lateralization of the glenoid center of rotation but without placing the center of rotation lateral to the prosthetic-bone interface. By maintaining the position of the center of rotation, the shear forces at the prosthesis-bone interface are lessened and are converted to compressive forces which will minimise glenoid failure. Edwards et al. performed a prospective study on a bony increased offset reverse arthroplasty. Among the 18 shoulders in the BIO-RSA group, the incidence of notching was 78% compared to controls 70%. The graft completely incorporated in 12 (67%), partially incorporated in 4 (22%), and failed to incorporate in 2 (11%). Frankle et al. reported on the minimum 5-year follow-up of reverse arthroplasty with a central compression screw and a lateralised glenoid component. The survivorship was 94% at 5 years. There were seven (9%) cases of scapular notching and no patient had glenoid baseplate loosening or baseplate failure. The authors noted that the patients maintained their improved function and radiographic results at a minimum of five years. In summary, lateralisation of the glenosphere is an attractive option to improve the outcome of reverse arthroplasty. Benefits of lateralisation with metal rather than bone graft include elimination of concern over bone graft healing or resorption. In addition, the procedure has the potential to be more precise with the exact offset amount known pre-operatively as well as improved efficiency of the procedure. Preparing the graft takes additional OR time and there is variable quality of the bone graft

Background:. Currently, there are a variety of different reverse shoulder implant designs but few anatomic studies to support the optimal selection of prosthetic size. This study analyzed the glenohumeral relationships of patients who underwent reverse shoulder arthroplasty (RSA). Methods:. Ninety-two shoulders of patients undergoing primary RSA for a massive rotator cuff tear without bony deformity or deficiency and 10 shoulders of healthy volunteers (controls) were evaluated using three-dimensional CT reconstructions and computer aided design (CAD) software. Anatomic landmarks were used to define scapular and humeral planes in addition to articular centers. After aligning the humeral center of rotation with the glenoid center, multiple glenohumeral relationships were measured and evaluated for linearity and size stratification. The correction required to transform the shoulder from its existing state (CT scan) to a realigned image (CAD model) was compared between the RSA and control groups. Size stratification was verified for statistical significance between groups. Generalized linear modeling was used to investigate if glenoid height, coronal humeral head diameter and gender were predictive of greater tuberosity positions. Results:. All 92 shoulders were grouped into three different categories based on glenoid height. The humeral head size, glenoid size, lateral offset, and inferior offset all increased linearly (r. 2. > 0.95), but the rate of increase varied (slopes range from 0.59 to 1.9). Translations required to normalize the shoulder joint were similar between healthy and pathologic cases except for superior migration. Glenoid height, coronal humeral head diameter and gender predicted the greater tuberosity position within 1.09 ± 0.84 mm of actual position in ninety percent of the patient population. Morphometric measurements for each stratified group were all found to be statistically significant between groups (p ≥ 0.05). Conclusion:. Patients who undergo RSA with minimal bony deformity have superior subluxation of the glenohumeral joint. Predicting the anatomic position of the greater tuberosity is dependent on gender, glenoid height and coronal humeral head diameter. This anatomic data provides a guide to avoid inadvertent mismatch of prosthetic and patient shoulder size. If the surgeon is able to measure glenoid height and coronal humeral head diameter preoperatively, accurate planning of the position of the greater tuberosity can be accomplished

Accurate and reproducible measurement of three-dimensional shoulder kinematics would contribute to better understanding shoulder mechanics, and therefore to better diagnosing and treating shoulder pathologies. Current techniques of 3D kinematics analysis use external markers (acromial cluster or scapula locator) or medical imaging (MRI or CT-Scan). However those methods present some drawbacks such as skin movements for external markers or cost and irradiation for imaging techniques. The EOS low dose biplanar X-Rays system can be used to track the scapula, humerus and thorax for different arm elevation positions. The aim of this study is to propose a novel method to study scapulo-thoracic kinematics from biplanar X-rays and to assess its reliability during abduction in the scapular plane. This study is based on the EOS™ system (EOS Imaging, Paris, France), which allows acquisition of 2 calibrated, low dose, orthogonal radiographs with the subject standing at 30 to 40° angle of coronal rotation to the plane of one of the X-ray beams, in order to limit superimposition with the ribcage and spine. Seven abduction positions in the scapular plane were maintained by the subjects for 10 seconds, during X-ray acquisition. Between two positions, the subjects returned at rest position. Arm elevations were approximately 0, 10, 20, 30, 60, 90 and 150° (position 1 to 7). Six subjects were enrolled to perform a reproducibility study based on the 3D reconstructions of 2 experienced observers three times each. For each subject, a personalised 3D reconstruction of the scapula was created. The observer digitises clearly visible anatomical landmarks on both stereoradiographs for each arm position. These landmarks are used to make a first adjustment of a parameterised 3D model of the scapula. This provides a pre-personalised model of the subject's scapula which is then rigidly registered on each pair of X-rays until its retroprojection fits best on the contours that are visible on the X-rays. The thorax coordinate system (CS) was built following the ISB (International Society of Biomechanics) recommendations. The CS associated to the scapula was a glenoid centred CS based on the ellipse which fit on the glenoid rim on the 3D model of scapula. Scapular CS orientation and translation in the thorax CS was calculated following a Y,X,Z angle sequence for each position. Each 3D reconstruction of the scapula was performed in approximately 30 minutes. The most reproducible rotation was upward/downward rotation (along X axis) with a 95% confidence interval (95% CI) from 2.71° to 3.61°. Internal/external rotation and anterior/posterior tilting were comprised respectively between 5.18° to 8.01° and 5.50° to 7.23° (CI 95%). The most reproducible translation was superior-inferior translation (along Y axis) with a 95% CI from 1.22mm to 2.46mm. Translation along X axis (antero-posterior) and Z axis (medio-lateral) were comprised respectively between 2.49mm to 4.26mm and 2.47mm to 3.30mm (CI 95%). We presented a new technique for 3D functional quantitative analysis of the scapulo-thoracic joint. This technique can be used with confidence; uncertainty of the measures seems acceptable compared to the literature. Main advantages of this technique are the very low dose irradiation compared to the CT-Scan and the possibility to study arm elevation above 120°

Reverse total shoulder arthroplasty (RTSA) has had rapidly increasingly utilization since its approval for U.S. use in 2004. RTSA accounted for 11% of extremity market procedure growth in 201. Although RTSA is widely used, there remain significant challenges in determining the location and configuration of implants to achieve optimal clinical and functional results. The goal of this study was to measure the 3D position of the shoulder joint center, relative to the center of the native glenoid face, in 16 subjects with RTSA of three different implant designs, and in 12 healthy young shoulders. CT scans of 12 healthy and 16 pre-operative shoulders were segmented to create 3D models of the scapula and humerus. A standardized bone coordinate system was defined for each bone (Figure 1). For healthy shoulders, the location of the humeral head center was measured relative to the glenoid face center. For the RTSA shoulders, a two-step measurement was required. First, 3D models of the pre-operative bones were reconstructed and oriented in the same manner as for healthy shoulders. Second, 3D model-image registration was used to determine the post-operative implant positioning relative to the bones. The 3D position and orientation of the implants and bones were determined in a sequence of six fluoroscopic images of the arm during abduction, and the mean implant-to-bone relationships were used to determine the surgical positioning of the implants (Figure 2). The RTSA center of rotation was defined as the offset from the center of the implant glenosphere to the center of the native glenoid face. The center of rotation in RTSA shoulders varied over a much greater range than the native shoulders (Table 1 (Figure 3)). Lateral offset of the joint center in RTSA shoulders was at least 6 mm smaller than the smallest joint center offset in the healthy shoulders. The center of rotation in RTSA shoulders was significantly more inferior than in healthy shoulders. The range of anterior/posterior placement of the rotation center for RTSA shoulders was bounded by the range for normal shoulders. How to best position RTSA implants for optimal patient outcomes remains a topic of great debate and research interest. We found that the 3D joint center position can vary over a supraphysiologic range in shoulders with RTSA, and that this variation is primarily in the coronal plane. By relating these geometric variations to muscle, shoulder and clinical function, we hope to establish methods and strategies for predictably obtaining the best clinical and functional outcomes for RTSA patients on a per-subject basis

Introduction:. The complex 3D geometry of the scapula and the variability among individuals makes it difficult to precisely quantify its morphometric features. Recently, the scapular neck has been recognized as an important morphometric parameter particularly due to the role it plays in scapular notching, which occurs when the humeral component of a reverse shoulder arthroplasty (RSA) prosthesis engages the posterior column of the scapula causing mechanical impingement and osseous wear. Prosthetic design and positioning of the glenoid component have been accepted as two major factors associated with the onset of notching in the RSA patient population. The present image-based study aimed to develop an objective 3D approach of measuring scapular neck, which when measured pre-operatively, may identify individuals at risk for notching. Materials and Methods:. A group of 81 subjects (41 M, 69.7 ± 8.9 yrs.; 40 F, 70.9 ± 8.1 yrs.) treated with RSA were evaluated in this study. The 3D point-cloud of the scapular geometry was obtained from pre-operative computed tomography (CT) scans and rendered in Mimics. Subsequently, a subject-specific glenoid coordinate system was established, using the extracted glenoid surface of each scapula as a coordinate reference. The principal component analysis approach was used to establish three orthogonal coordinate axes in the geometric center of the glenoid. Utilization of glenoid-specific reference planes (glenoid, major axis, and minor axis plane) were selected in order to remove subjectivity in assessing “true” anterior/posterior and profile views of the scapula. The scapular neck length was defined as the orthogonal distance between the glenoid surface and the point on the posterior column with the significant change of curvature (Fig. 1). In addition, the angle between the glenoid plane, area center of the glenoid, and the point of significant change of the curvature were assessed (Fig. 2). This new parameter was developed to serve as a predictive critical value for the occurrence of notching. The incidence of notching increases as the value of the notching angle decreases. In order to evaluate relationships between glenoid and scapular neck, the glenoid width and height was also measured at the glenoid plane. Results:. Glenoid neck length and notching angle within the population were normally distributed with mean values of 7.8 ± 2.3 mm and 19.6 ± 4.8°, respectively (Fig. 3). No gender difference was found (p = 0.676). In one subject, a glenoid neck length of less than 1 mm was measured with the notching angle less than 2.5°. No association between glenoid neck length and glenoid size were identified (vs. glen. height r. 2. = 0.001, and vs. glen. width r. 2. = 0.05). Conclusion:. The present study reported on the scapular neck length and notching angle as measureable morphometric parameters that follow a normal distribution throughout the population and that are not correlated to the subject's glenoid size. Pre-operative acquisition of these novel and unique CT-based measurements may promote more appropriate RSA prosthesis selection to account for subject-specific anatomy in an effort to avoid scapular notching. Inferior placement of a baseplate or lateralization of glenoid component center of rotation (either biologically or mechanically) both serves to theoretically increase the notching angle

Lateralizing the center of rotation in reverse shoulder arthroplasty has been the subject of renewed interest due to complications associated with medialized center of rotation implants. Benefits of lateralization include: increased joint stability, decreased incidence of scapular notching, increased range of motion, and cosmetic appeal. However, lateralization may be associated with increased risk of glenoid loosening, which may result from the increased shear forces and the bending stresses that manifest at the bone-implant interface. To address glenoid loosening in reverse implants with lateralized joint centers, recent studies have focused on testing and improving implant fixation. However, these studies use loads derived from literature specific to subjects with normal anatomy. The aim of this study is to characterize how joint center lateralization affects the loading in reverse shoulder arthroplasty. Using an established computational shoulder model that describes the geometry of a commercial reverse prosthesis (DELTA® III, DePuy), motion in abduction, scapular plane elevation, and forward flexion was simulated. The simulations were run for five progressively lateralized centers of rotation: −5, 0, +5, +10, and +15 mm (Figure 1). The model was modified to simulate a full thickness rotator cuff tear, where all cuff musculature except Teres Minor were excluded, to reflect the clinical indication for reverse shoulder arthroplasty on cuff tear arthropathy patients. To analyze the joint contact forces, the resultant glenohumeral force was decomposed into compression, anterior-posterior shear, and superior-inferior shear on the glenoid. Joint center lateralization was found to affect the glenohumeral joint contact forces and glenoid loads increased by up to 18% when the center was lateralized from −5 mm to +15 mm. Compressive forces were found to be more sensitive to lateralization in abduction, while changes in shear forces were more affected in forward flexion and scapular plane abduction. On average, the superior shear component showed the largest increases due to lateralization (up to a 21% increase), while the anterior-posterior shear component showed larger changes than those of compression, except in the most lateralized center position (Figure 2). The higher joint loads in the lateralized joint centers reflect a shortening of the Deltoid muscle moment arms (Figure 3), since the muscle needs to exert more force to provide the desired motions. The additional shear forces generated by the lateralization may increase the risk of the ‘rocking-horse’ effect. Together with the lateralized joint center, this creates an additional bending stress at the bone-implant interface that puts the implant at further risk of loosening (Figure 1). Current studies on implant fixation tend to use loads in compression and superior shear that exceed the forces seen in this study but have not investigated anterior-posterior shear loads. Our data support that loading in anterior-posterior direction can be significant. Using inappropriate loads to design fixation may result in excessive loss of bone stock and/or unforeseen implant loosening. The implication is that future studies may be performed using this more relevant data set to navigate the tradeoff between fixation and bone conservation