Alain Farron

A finite element model of the shoulder: application to the comparison of normal and osteoarthritic joints

OBJECTIVE The objective of the present study was to develop a numerical model of the shoulder able to quantify the influence of the shape of the humeral head on the stress distribution in the scapula. The subsequent objective was to apply the model to the comparison of the biomechanics of a normal shoulder (free of pathologies) and an osteoarthritic shoulder presenting primary degenerative disease that changes its bone shape. DESIGN Since the stability of the glenohumeral joint is mainly provided by soft tissues, the model includes the major rotator cuff muscles in addition to the bones. BACKGROUND No existing numerical model of the shoulder is able to determine the modification of the stress distribution in the scapula due to a change of the shape of the humeral head or to a modification of the glenoid contact shape and orientation. METHODS The finite element method was used. The model includes the three-dimensional computed tomography-reconstructed bone geometry and three-dimensional rotator cuff muscles. Large sliding contacts between the reconstructed muscles and the bone surfaces, which provide the joint stability, were considered. A non-homogenous constitutive law was used for the bone as well as non-linear hyperelastic laws for the muscles and for the cartilage. Muscles were considered as passive structures. Internal and external rotations of the shoulders were achieved by a displacement of the muscle active during the specific rotation (subscapularis for internal and infrapinatus for external rotation). RESULTS The numerical model proposed is able to describe the biomechanics of the shoulder during rotations. The comparison of normal vs. osteoarthritic joints showed a posterior subluxation of the humeral head during external rotation for the osteoarthritic shoulder but no subluxation for the normal shoulder. This leads to important von Mises stress in the posterior part of the glenoid region of the pathologic shoulder while the stress distribution in the normal shoulder is fairly homogeneous. CONCLUSION This study shows that the posterior subluxation observed in clinical situations for osteoarthritic shoulders may also be cause by the altered geometry of the pathological shoulder and not only by a rigidification of the subscapularis muscle as often postulated. This result is only possible with a model including the soft tissues provided stability of the shoulder. RELEVANCE One possible cause of the glenoid loosening is the eccentric loading of the glenoid component due to the translation of the humeral head. The proposed model would be a useful tool for designing new shapes for a humeral head prosthesis that optimizes the glenoid loading, the bone stress around the implant, and the bone/implant micromotions in a way that limits the risks of loosening.

Simulated joint and muscle forces in reversed and anatomic shoulder prostheses.

Reversed shoulder prostheses are increasingly being used for the treatment of glenohumeral arthropathy associated with a deficient rotator cuff. These non-anatomical implants attempt to balance the joint forces by means of a semi-constrained articular surface and a medialised centre of rotation. A finite element model was used to compare a reversed prosthesis with an anatomical implant. Active abduction was simulated from 0 degrees to 150 degrees of elevation. With the anatomical prosthesis, the joint force almost reached the equivalence of body weight. The joint force was half this for the reversed prosthesis. The direction of force was much more vertically aligned for the reverse prosthesis, in the first 90 degrees of abduction. With the reversed prosthesis, abduction was possible without rotator cuff muscles and required 20% less deltoid force to achieve it. This force analysis confirms the potential mechanical advantage of reversed prostheses when rotator cuff muscles are deficient.

Open elbow arthrolysis for posttraumatic elbow stiffness.

Objectives The elbow joint is vulnerable to stiffness, especially after trauma. The aim of this study was to evaluate the results of open arthrolysis for posttraumatic elbow stiffness. Design Cohort retrospective study. Patients Eighteen consecutive patients were evaluated by an independent observer at an average of 16 months (6 to 43) after open elbow arthrolysis was performed for posttraumatic stiffness. Initial traumas were: isolated fractures (11) or dislocation (1) and complex fracture-dislocations (6). Initial treatments were: nonoperative (3), radial head resection (1), and ORIF (14). Patients presented predominantly with mixed contractures (combined extrinsic and intrinsic contractures). Intervention Open elbow arthrolysis. Main Outcome Measurements Elbow function and patient satisfaction were the principal outcome measures. At follow-up European Society for Shoulder and Elbow Surgery (SECEC) elbow scores were calculated. Results and Conclusions Three patients had minor postoperative complications: 1 partial wound dehiscence, 1 subcutaneous infection, and one seroma. None of these complications influenced the final result clinically. The mean total increase in range of motion was 40 degrees (13 to 112 degrees), with a mean gain in flexion of 14 degrees (0 to 45 degrees) and 26 degrees in extension (5 to 67 degrees). No patient showed signs of elbow instability. There was no radiographic evidence of osteoarthritis progression at follow-up. We did not find any correlations between the type of stiffness, the approaches used, and the results. However, patients with the greatest preoperative stiffness had significantly better improvement of mobility (P<0.001). The best results were obtained in patients who had arthrolysis done within 1 year after the initial trauma (P=0.008). The mean SECEC scores were 88 (52 to 100) for the injured elbows, and 96 (88 to 100) for the contralateral elbows. Conclusion Open elbow arthrolysis for patients with posttraumatic stiffness improves joint function and provides patient satisfaction. The best results, in terms of gain of motion and patient satisfaction, were obtained in patients with severe stiffness who had operations within the first year after initial trauma.

Arm position during daily activity

A new method of evaluation for functional assessment of the shoulder during daily activity is presented. An ambulatory system using inertial sensors attached on the humerus was used to detect the ability to work at a specific position of the shoulder. Nine arm positions were defined based on humerus elevation. The method was tested on 31 healthy volunteer subjects. First, we estimated the ability of the system to detect the different elevation angles and arm positions of each subject. Following that, we evaluated their arm positions during approximately 8h of daily activities. Each arm position was recognized with a good sensitivity (range 80-100%) and specificity (range 96-99%). During daily activity, we estimated the frequency (number/h) that the humerus reached each arm position during the periods of 0-1s (period P1), 1-5s (period P2) and 5-30s (period P3). Our data showed that all subjects had 96% of their arm position reached under the 5th level (100-120 degrees ). No significant difference was observed between dominant and non-dominant sides for the frequency and duration of arm positions (p>0.3). Our evaluation was in accordance with the clinical questionnaire (the Constant score) for the P1 duration, but differed for longer periods P2 and P3. By quantifying the arm positions and their durations for both shoulders, we proposed a new score to evaluate the ability to work at a specific level based on the symmetry index of the arms activity. Using this score, we obtained, on average, good symmetry for healthy subjects. This score can be useful in evaluating the asymmetry in arm function in patients with a shoulder disease. The proposed technique could be used in a number of shoulder diseases where problems in performing daily activities should be expressed in terms of objective measure of arm position.

Effect of humeral stem design on humeral position and range of motion in reverse shoulder arthroplasty

PurposeThe impacts of humeral offset and stem design after reverse shoulder arthroplasty (RSA) have not been well-studied, particularly with regard to newer stems which have a lower humeral inclination. The purpose of this study was to analyze the effect of different humeral stem designs on range of motion and humeral position following RSA.MethodsUsing a three-dimensional computer model of RSA, a traditional inlay Grammont stem was compared to a short curved onlay stem with different inclinations (155°, 145°, 135°) and offset (lateralised vs medialised). Humeral offset, the acromiohumeral distance (AHD), and range of motion were evaluated for each configuration.ResultsAltering stem design led to a nearly 7-mm change in humeral offset and 4 mm in the AHD. Different inclinations of the onlay stems had little influence on humeral offset and larger influence on decreasing the AHD. There was a 10° decrease in abduction and a 5° increase in adduction between an inlay Grammont design and an onlay design with the same inclination. Compared to the 155° model, the 135° model improved adduction by 28°, extension by 24° and external rotation of the elbow at the side by 15°, but led to a decrease in abduction of 9°. When the tray was placed medially, on the 145° model, a 9° loss of abduction was observed.ConclusionsWith varus inclination prostheses (135° and 145°), elevation remains unchanged, abduction slightly decreases, but a dramatic improvement in adduction, extension and external rotation with the elbow at the side are observed.

Comparison of polyethylene wear in anatomical and reversed shoulder prostheses

Wear of polyethylene is associated with aseptic loosening of orthopaedic implants and has been observed in hip and knee prostheses and anatomical implants for the shoulder. The reversed shoulder prostheses have not been assessed as yet. We investigated the volumetric polyethylene wear of the reversed and anatomical Aequalis shoulder prostheses using a mathematical musculoskeletal model. Movement and joint stability were achieved by EMG-controlled activation of the muscles. A non-constant wear factor was considered. Simulated activities of daily living were estimated from in vivo recorded data. After one year of use, the volumetric wear was 8.4 mm(3) for the anatomical prosthesis, but 44.6 mm(3) for the reversed version. For the anatomical prosthesis the predictions for contact pressure and wear were consistent with biomechanical and clinical data. The abrasive wear of the polyethylene in reversed prostheses should not be underestimated, and further analysis, both experimental and clinical, is required.

A musculoskeletal shoulder model based on pseudo-inverse and null-space optimization

The goal of the present work was assess the feasibility of using a pseudo-inverse and null-space optimization approach in the modeling of the shoulder biomechanics. The method was applied to a simplified musculoskeletal shoulder model. The mechanical system consisted in the arm, and the external forces were the arm weight, 6 scapulo-humeral muscles and the reaction at the glenohumeral joint, which was considered as a spherical joint. The muscle wrapping was considered around the humeral head assumed spherical. The dynamical equations were solved in a Lagrangian approach. The mathematical redundancy of the mechanical system was solved in two steps: a pseudo-inverse optimization to minimize the square of the muscle stress and a null-space optimization to restrict the muscle force to physiological limits. Several movements were simulated. The mathematical and numerical aspects of the constrained redundancy problem were efficiently solved by the proposed method. The prediction of muscle moment arms was consistent with cadaveric measurements and the joint reaction force was consistent with in vivo measurements. This preliminary work demonstrated that the developed algorithm has a great potential for more complex musculoskeletal modeling of the shoulder joint. In particular it could be further applied to a non-spherical joint model, allowing for the natural translation of the humeral head in the glenoid fossa.

Importance of the subscapularis muscle after total shoulder arthroplasty

BACKGROUND The 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. METHODS To 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. FINDINGS To 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. INTERPRETATION These results confirm the importance of the suscapularis for the long-term stability of total shoulder arthroplasty.

Measurements of three-dimensional glenoid erosion when planning the prosthetic replacement of osteoarthritic shoulders

The three-dimensional (3D) correction of glenoid erosion is critical to the long-term success of total shoulder replacement (TSR). In order to characterise the 3D morphology of eroded glenoid surfaces, we looked for a set of morphological parameters useful for TSR planning. We defined a scapular coordinates system based on non-eroded bony landmarks. The maximum glenoid version was measured and specified in 3D by its orientation angle. Medialisation was considered relative to the spino-glenoid notch. We analysed regular CT scans of 19 normal (N) and 86 osteoarthritic (OA) scapulae. When the maximum version of OA shoulders was higher than 10°, the orientation was not only posterior, but extended in postero-superior (35%), postero-inferior (6%) and anterior sectors (4%). The medialisation of the glenoid was higher in OA than normal shoulders. The orientation angle of maximum version appeared as a critical parameter to specify the glenoid shape in 3D. It will be very useful in planning the best position for the glenoid in TSR.

Total shoulder arthroplasty: Downward inclination of the glenoid component to balance supraspinatus deficiency

HYPOTHESIS Supraspinatus deficiency associated with total shoulder arthroplasty (TSA) provokes eccentric loading and may induce loosening of the glenoid component. A downward inclination of the glenoid component has been proposed to balance supraspinatus deficiency. METHODS This hypothesis was assessed by a numeric musculoskeletal model of the glenohumeral joint during active abduction. Three cases were compared: TSA with normal muscular function, TSA with supraspinatus deficiency, and TSA with supraspinatus deficiency and downward inclination of the glenoid. RESULTS Supraspinatus deficiency increased humeral migration and eccentric loading. A downward inclination of the glenoid partly balanced the loss of stability, but this potential advantage was counterbalanced by an important stress increase within the glenoid cement. The additional subchondral bone reaming required to incline the glenoid component indeed reduced the bone support, increasing cement deformation and stress. CONCLUSION Glenoid inclination should not be obtained at the expense of subchondral bone support.