Bio-medical applications
Project leader
Dr.ir. Harald Bersee
Post Doc.
Dr. Wojciech Swieszkowski
PhD. student
Ir. Rogier Oosterom
Researcher
Drs. Albert van der Pijl
Contactperson
Ir. Rogier Oosterom
Summary
University project:
DIPEX: Development of Improved endo-Prostheses for the upper EXtremities
(http://www.wbmt.tudelft.nl/mms/dipex/)
Faculty of Aerospace Engineeringsub-project:
Design of improved or new endo-prostheses types for the upper extremity
Introduction
Shoulder replacements show bad results in terms of post-operative functionality and long- term fixation of especially the cup- component (glenoid component) to the scapula bone (shoulder blade).
To improve these bad results, a project has been initiated at the Delft University of Technology, entitled DIPEX (Development of Improved endoProstheses for the upper Extremities). This project is divided into six subprojects at several faculties including Aerospace Engineering; the location of the design project. Within this project the focus lies at three design concepts, i.e.
Improvement of the anatomical design,
Development of a conceptually new anatomical shoulder endo-prosthesis,
Development of a conceptually new shoulder endo-prosthesis.
The anatomical design
The anatomical design is the most common used prosthesis for implantation, of which the design goal is to recover the original anatomy of the patient as well as possible. The cup- components is fixed to the scapular bone either by bone cement (PMMA) or by bone-ingrowth fixation in combination with bone screws to assure fixation in the first post-operative weeks. Loosening of cemented components starts both at the bone-cement as well as at the cement-prosthesis interface. The probable mechanism of failure, such as crack origins due to peak stresses or fatigue at locations of tensile stresses, is unknown, as well as the influence of cement thickness and geometrical parameters on the fixation. The aim of this study was to evaluate fixation strength of present used and adjusted glenoid components.
To achieve the prescribed aim, a force controlled test set-up has been developed. Joint loading consists of a joint compression and subluxation (shear) force, in the set-up applied by a pneumatic cylinder (Festo AG & Co) and a hydraulic system (MTS, Eden Prairie, MN), respectively. Both forces are measured by advanced force transducers. Controlling is done by cascade mode, meaning that the actual subluxation force is compared to prescribed values and, if a mismatch occurs, head displacements are corrected to a different pattern. In this way joint compression and subluxation forces with varying amplitudes can be applied, leading to corresponding head displacements.
Synthetic bone substitutes with realistic mechanical properties are used to cement UHMWPE glenoid components with variable, preferred cement thickness. To assure a homogenous cement thickness, components are positioned with a special alignment tool into CNC-milled bone cavities, according to CAD models.
Conceptually new anatomical design
Wear of polyethylene glenoid components is one of the main complications in total shoulder arthroplasty and may be, among others, the result of not properly selected materials for articular surfaces. Therefore, new materials for glenoid component should be developed to reduce generation of wear. The design hypothesis is that a soft porous bearing material, allowing for water in- and outflow and joint conformity, leading to better lubrication properties and lower contact stresses, respectively, leads to reduced wear rates. A conceptually new anatomical design, based on above mentioned design hypothesis, is under development.
The aim of the study is to evaluate wear of present used shoulder prostheses. Therefore a shoulder wear simulator has been developed.
Conceptually new shoulder endo-prosthesis design
End stage Rheumatoid patients show the worst long-term results with respect to joint functionality and component survival, when implanting a conventional shoulder endo-prosthesis. Main characteristics of the disease is the deterioration of both hard and soft tissues in and around the joint. Especially the so-called Rotator-Cuff muscles, an important muscle group for applying a joint compression force and a axial moment to allow for upper arm rotation, are thinned or torn. For this patient group, nothing more than an arthrodesis can be offered, a procedure in which the upper arm is rigidly connected to the scapula by screws. This cannot be seen as the ideal solution with respect to patient comfort and post-operative functionality.
A main cause for the bad results is the decreased joint compression force, required for joint stability, resulting in excessive joint displacements or even a joint dislocation, when applying a joint subluxation force. In combination with bad bone quality, the eccentric loading of the cup component results in early loosening of this component. Very often, surgical treatments are not sufficient in restoring shoulder stability. Future designs must include the stability function of the Rotator Cuff muscles.
Ideas include a 'one-component' design (a flexible connection between upper arm and scapula) and a more conventional design, stabilised with artificial ligaments.
