Investigating the ideal deltoid kinematics and tension in reverse shoulder arthroplasty (RSA).

Abstract

The shoulder is the one of the most active joints within the human body. Recruited in the majority of daily activities either in active use such as moving/carrying of objects or as a source of stability during locomotion. Therefore painful shoulder or its reduced mobility and function can be very debilitating hence affecting the quality of life. While shoulder pain and restricted motion encompasses a diverse array of pathologies, the most common causes are due to infection, arthritis, or trauma. Arthroplasty (the surgical reconstruction or replacement of a joint) of the shoulder has offered the potential for improved function and pain relief where the native structures have been damaged. The conventional total shoulder replacement, however, is not beneficial for all patients and may result in further pain and limited motion in persons with arthritic shoulders with a deficient rotator cuff. For these patients Reverse Shoulder Arthroplasty (RSA), in which anatomic concavities of glenohumeral joint are inverted, is a popular treatment. However, for optimal restoration of motion, the correct positioning of the glenohumeral centre of rotation and initial setting of the deltoid length (Deltoid Tension) must play an important role in the surgery outcome. A study of the key literature has shown that despite common use of RSA, its biomechanical characteristics during motion are not fully understood. This study investigates the influence of some of the key parameters on the intensity of forces and moments in the shoulder joint before, during and after RSA. These parameters include; geometry, kinematics and muscle passive force measurement (deltoid pretension measurement). To investigate the effect of geometrical changes on kinematics of shoulder after RSA, a musculoskeletal model of the shoulder is developed and simulated. Geometrical parameters of the musculoskeletal model are extracted from previous published studies. Results of the simulation enabled the detection of key parameters in reverse shoulder kinematics and its influence on determining the mechanical advantage of the shoulder mechanism. This identified the need for developing an X-ray imaging protocol and image processing tool that enable surgeons to predict optimum implants insertion position and estimate the performance of the shoulder before planning the operation. Subsequently, an assessment tool was proposed to assess shoulder Range of Motion (ROM) and deltoid muscle activity to both quantify and validate the predicted outcome of the surgery. The main purpose of this study is to measure the passive force exerted on the reverse shoulder joint during surgery as a criterion or measure of deltoid pretension. Hence a force sensor is designed, developed and tested in a custom built joint simulator. As part of this research and to allow objective assessment of the joint, a series of tools/hardware/software were proposed, designed and developed, and then tested and evaluated for effectiveness and functionality. The introduction of a system proposed here provides data which could be recorded in a database along with geometrics and kinematics pre and post operatively, residual force in glenohumeral joint intraoperatively and shoulder performance in terms of range of motion and EMG muscle activity of individual patients pre and post operatively. Such a database in time will enable us to find correlations between these parameters and the outcome of surgery in the long term. It is hoped that this will provide a tool for surgeons in future operations to who choose to use a more quantitative and repeatable way of optimizing the implant size and position accordingly.