Background

Though many advantages of reverse total shoulder arthroplasty (RTSA) have been demonstrated, a variety of complications indicate there is much to learn about how RTSA modifies normal shoulder function. This study assesses how RTSA affects deltoid muscle moment arms post-surgery using a subject-specific computational model driven by in vivo kinematic data.

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.

Reverse total shoulder arthroplasty (RTSA) is increasingly used in the United States since approval by the FDA in 2003. RTSA relieves pain and restores mobility in arthritic rotator cuff deficient shoulders. Though many advantages of RTSA have been demonstrated, there still are a variety of complications (implant loosening, shoulder impingement, infection, frozen shoulder) making apparent much still is to be learned how RTSA modifies normal shoulder function. The goal of this study was to assess how RTSA affects deltoid muscle moment generating capacity post-surgery using a subject-specific computational model driven by in vivo kinematic data.

A subject-specific 12 degree-of-freedom (DOF) musculoskeletal model was used to analyze the shoulders of 27 subjects (14-RTSA, 12-Normal). The model was modified from the work of Holzbaur et al. to directly input 6 DOF humerus and scapula kinematics obtained using fluoroscopy. Model geometry was scaled according to each subject's skeletal dimensions. In vivo abduction kinematics for each subject were input to their subject-specific model and muscle moment arms for the anterior, lateral and posterior aspects of the deltoid were measured over the arc of motion.

Similar patterns of muscle moment arm changes were observed for normal and RTSA shoulders. The moment arm of the anterior deltoid was positive with the arm at the side and decreased monotonically, crossing zero (the point at which the muscle fibers pass across the joint center) between 50°–60° glenohumeral abduction (Figure 1a). The average moment arm of the lateral deltoid was constant and positive in normal shoulders, but showed a decreasing trend with abduction in RTSA shoulders (Figure 1b). The posterior deltoid moment arm was negative with the arm at the side, and increased monotonically to a positive value with increasing glenohumeral abduction (Figure 1c). Subject-specific moment arm values for RTSA shoulders were highly variable compared to normal shoulders. 2-way repeated measures ANOVA showed significant differences between RTSA and normal shoulders for all three aspects of the deltoid moment arm, where the moment arms in RTSA shoulders were smaller in magnitude.

Shoulder functional capacity is a product of the moment generating ability of the shoulder muscles which, in turn, are a function of the muscle moment arms and muscle forces. Placement of implant components during RTSA can directly affect the geometric relationship between the humerus and scapula and, therefore, the muscle moment arms in the RTSA shoulder. Our results show RTSA shoulders maintain the same muscle moment arm patterns as healthy shoulders, but they show much greater inter-subject variation and smaller moment arm magnitudes. These observations show directly how RTSA configuration and implant placement affect deltoid moment arms, and provide an objective basis for determining optimal implant configuration and surgical placement to maximize RTSA function in a patient-specific manner.

Reverse total shoulder arthroplasty (RTSA) is an increasingly common treatment for osteoarthritic shoulders with irreparable rotator cuff tears. Although very successful in alleviating pain and restoring some function there is little objective information relating geometric changes imposed by the reverse shoulder and the moment generating capacity of the shoulder muscles. Recent modeling studies of reverse shoulders have shown significant variation in deltoid muscle moment arms over varied joint centers for shoulders with RTSA. The goal of this study was to investigate the sensitivity of muscle moment arms as a function of varying the joint center in one representative RTSA subject. We hypothesized there may exist a more beneficial joint implant placement, measured by muscle moment arms, compared to the actual surgical implant placement.

A 12 degree of freedom, subject-specific model was used to represent the shoulder of a patient with RTSA for whom fluoroscopic measurements of scapular and humeral kinematics during abduction had been obtained. The computer model used these abduction kinematics and systematically varied joint center locations over 1521 different perturbations from the surgical placement to determine moment arms for the anterior, lateral and posterior aspects of the deltoid muscle. The joint center was varied from its surgical position ±4 mm in the anterior/posterior direction, 0–24 mm in the medial/lateral direction, and −10 mm to 14 mm in the superior/inferior direction.

The anterior deltoid moment arm varied up to 16mm with center of rotations variations, primarily in the medial/lateral and superior/inferior directions (Figure 2, Table 1(Figure 1)). Similarly, the lateral deltoid moment arm demonstrated variations up to 13 mm, primarily with joint center changes in the anterior/posterior and superior/inferior directions. The posterior deltoid moment arm varied up to 10mm, primarily in early abduction, and was most sensitive to changes of the joint center in demonstrated a sensitivity of 6 mm corresponding to variations in the superior/inferior directions (Figure 2).

The goal of this study was to assess the sensitivity of the deltoid muscle moment arms as a function of joint configuration for an existing RTSA subject. High variations were found for all three deltoid components. Variation over the entire abduction arc was greatest in the anterior and lateral deltoid, while the posterior deltoid moment arm was mostly sensitive to joint center changes early in the abduction arc. Moment arm changes of 10–16mm represent a significant amount of the total deltoid moment arm. This means there is an opportunity to dramatically change the deltoid moments arms through surgical placement of the joint center of rotation. Computational models of the shoulder may help surgeons optimize subject-specific placement of RTSA implants to provide the best possible muscle function, and assist implant designers to configure devices for the best overall performance.

Background:

The use of reverse total shoulder arthroplasty (RTSA) has been increasing around the world. However, because of concerns over lack of internal rotation with the reverse prosthesis and the resultant difficulties with activities of daily living (ADLs), many have recommended against performing bilateral RTSA.

Purpose:

While the use of press-fit humeral components has been accepted in total shoulder arthroplasty, few studies focus on the outcomes after uncemented reverse total shoulder arthroplasty. The purpose of this study is to compare the radiographic and functional results of uncemented and cemented humeral fixation in reverse total shoulder arthroplasty.

Background:

An upper extremity model of the shoulder was developed from the Stanford upper extremity model (Holzbaur 2005) in this study to assess the muscle lengthening changes that occur as a function of kinematics for reverse total shoulder athroplasty (RTSA). This study assesses muscle moment arm changes as a function of scapulohumeral rhythm (SHR) during abduction for RTSA subjects. The purpose of the study was to calculate the effect of RTSA SHR on the deltoid moment arm over the abduction activity.

Background:

Little is known about scapular kinematics in patients with reverse total shoulder arthroplasty (RTSA). Understanding how RTSA affects shoulder function may help refine its design, use, and rehabilitation strategies. The purpose of this study was to quantify motion in the reverse shoulder. The scapulohumeral rhythm (SHR) of the RTSA shoulder was calculated using 3d-2d image registration techniques. SHR was compared to normal subjects in literature to asses kinematic changes post RTSA.

Introduction

Persistent problems and relatively high complication rates with reverse total shoulder arthroplasty (RTSA) are reported (1, 2). It is assumed that some of these complications are affected by improper intraoperative soft tissue tension. Achieving proper intraoperative soft tissue tension is an obvious surgical goal. However, intraoperative soft tissue tension measurements and methods for RTSA have not been reported. One way to quantify soft tissue tension is to measure intraoperative joint forces using an instrumented prosthesis. Hence, we have developed an instrumented RTSA to measure shoulder joint forces intraoperatively. The goal of this study was to measure intraoperative shoulder joint forces during RTSA.

Introduction:

The subscapularis muscle experiences significant strain as it accommodates common movements of the shoulder. Little is known about what happens with this obligatory strain once the subscapularis insertion is disrupted and repaired in the course of shoulder arthroplasty. Subscapularis failure is a serious known complication after shoulder arthroplasty. It is not known what the effect of increasing the thickness of the shoulder head will have on subscapularis strain. It is our hypothesis that the use of large or expanded humeral heads during shoulder replacement will cause increased tension in the repaired subscapularis. The primary purpose of this study was to identify the optimal manner to perform a passive range of motion (PROM) program without invoking a significant increase in strain in the repaired subscapularis. The secondary purpose was to determine the impact of varying the thickness of the humeral head on subscapularis strain using the same PROM protocol.

Background:

Little validation has been done to compare the principle of using the contralateral side as compared to and age and gender matched control. This study seeks to assess the validity of using the contralateral shoulder as the control as opposed to an age- gender- matched control. This study will give insight as to whether the contralateral side is a viable control as compared to a normal age and gender matched control. The study showed that the use of the contralateral shoulder was not a viable normal control.

Introduction:

External rotation (ER) of the shoulder is a commonly used clinical measurement to assess the glenohumeral joint; however, the position in which these measurements are obtained varies between clinicians. The purpose of this study was to compare the following: ER in the upright & supine positions, motion capture & goniometric values of ER, active & passive ER, ER in the right & left shoulders, and ER in male & female subjects.