- Email: [email protected]
Analysis of glenoid component fixation designs in the rheumatoid scapula
FUNCTIONAL QUALITY ANALYSIS OF THE NEW RADIAL HEAD ENDOPROSTHRSIS IN THE IMPLANTED ELBOW JOINT W. Swieszkowski, K. Skalski, K. Kedxior Warsaw University of Technology, Nowowiejska 24,00-665 Warsaw, Poland INTRODUCTION: The goal of this research is the evaluation of the functioning of a radial bone head endoprosthesis (metal stem with floating polyethylene head) implanted in the elbow joint [l]. Such evaluation involves work describing the kinetics of the elbow joint [2,3].
METHOD: Research on how the endoprosthesis functions in the elbow joint is carried out on the basis of X-ray photographs of the joint with an endoprosthcsis prototype implanted. The polyethylene head of the prototype has circumferential and longitudinal metal markers (Fig. 1). The behaviour of the endoprosthesis when the forearm is placed at different angles and positions (supination, pronation) is analysed. Next are defined and identified the positions of the local system (the endoprosthesis head and the radial bone) within the global system of the humerus, based on an analysis of the shape and positions of the endoprosthesis markers. Fig.1. X-my pictures: Anterio-Posterior (A), Lateral (B) view, with coordinate systems. RESULTS: The presented method is used in analysing the kinetics of the endoprosthesis during changes of the position of the forearm (pronationsupination) at a stable angle of bending ($=90@). The resulting values for the angle of rotation of the local system (at, as, a3), as well as the vector coordinates of the position of the centre of the local system of the endoprosthesis head (Xth, X2”, X3h) within the global system (Tab.l), allow the transformation of the system connected with the endoprosthesis in three-dimensional space to be defined. Tab. 1. Parameters for transformation of system [deg.1 tmml h h Position aI a2 a3 Xlh x2 x3 Pronation 76 -8 - 2.6 -9.1 17.9 7 Supination 7 77 2 - 1.3 -20.7 20.5 DISCUSSION: The presented method, makes possible a quantitative description of the behaviour of the endoprosthesis during limb movement, and this in turn allows a qualitative evaluation of the functioning of the endoprosthesis to be carried out. CONCLUSION: Applying the method herein described to the evaluation of a newly-designed radial bone head endoprosthesis, it is seen that in various movements in the elbow joint, the main axis of the endoprosthesis Xtg is aligned in the direction of the geometric centm of the head of the humerus. This testifies to the automatically correct positioning of the endoprosthesis in relation to its partner, the humerus. REFERENCES: 1. Swieszkowski W. et al., Biology of Sport, Vol.14 Suppl. 7, 271-275, 1997 (in Polish). 2. Money B.F, et al., J. Bone and Joint Surg. Vol.63-A, No.6: 872-87, 1981. 3. Wu G. et al., J. Biomech., 28,1257-1261, 1995. ACKNOWLEDGEMENTS: This research has been accomplished as part of grant no.7 T07A 010 13 sponsored by the State Committee for Scientific Research. CORRESPONDENCE: W. Swieszkowski, Warsaw University of Technology, Nowowiejska 24,00-665 Warsaw, Poland, Tel. 48 22 660 78 14, Fax. 48 22 628 25 87, [email protected]
6
11” Conference
ANALYSIS
OF GLENOID COMPONENT FIXATION DESIGNS IN THE RHEUMATOID SCAPULA T. Kaufler, A.A. Amis, R.J.H. Emery*, Biomechanics Section, Mech.Eng.Dept., Imperial College, London SW7 2BX, UK, *Orthopaedic Surgery Dept., St Mary’s Hospital, London W2 INY. UK.
INTRODUCTION: The lack of t&ability of glenoid component fixation limits the use of total shoulder arthroplasty. Reviews have shown that the majority of glenoid components have radiolucent zones around them, and some of these are not seen on x-rays because of malalignment of the scapula. It is surprising that little work has been done to optimise glenoid component fixation. Jn particular, we am not aware of 3D finite element stress analysis. We are also not aware of prior work in which prosthesis fixation has been examined in rheumatoid bone, despite this being the most common situation at shoulder replacement. METHODS: The scapula of a female rheumatoid patient awaiting shoulder arthroplasty was CT scanned in 43 3mm slices. The bone geometry was extracted from the scan data using custom written software and reconstructed as a solid model by defining the outlines as splines in Patran and assembling them in ProEngineer. Three polyethylene glenoid components were also designed in ProEngineer, with a keel, a central cone, and with two diverging pegs for fixation. These could be implanted with bonded interfaces using the ProEngineer assembly command. After meshing a rectangular volume around the glenoid, material properties were assigned on an individual element basis, using CT density data, and the cut surfaces constrained. Loads representing flexion and abduction actions gave joint forces of 1.5BW acting centrally and superiorly, or antero-superiorly. RESULTS: The CT scans showed loss of bone from the glenoid, with large surrounding osteophytes. There were cystic lesions where the prosthesis fixation was expected. Vector plots showed the principal compressive stresses acting medially through the glenoid, then swinging posteriorly into the plane of the blade of the scapula, corresponding with the trabecuhu architecture. The prosthesis had been modelled with a non conforming articulation. This caused relatively localized loading of the lateral face of either the natural glenoid or of the prosthesis. Thus, relatively high stresses were predicted in the bone beneath the area of load application. In general, them was a tendency for stresses to rise as the bone got thinner towards the blade of the scapula. This tendency was increased by the bending effect when the joint force acted on the anterior edge of the prosthesis. We were surprised to find that the three different fixation designs made little difference to the stresses in the bone. DISCUSSION: This work has modelled a scapula in much greater detail than previously, and has included localized property variations caused by rheumatoid disease. Thus, the stresses are more realistic than either those arising from analysis of a simplified geometry or of a normal bone. The mode1 would be more realistic with different interface properties, but the programing work would be great if a number of fixation designs arc to be compared. A change to a metallic implant would reduce the stresses beneath the area of load application, transferring .load to the medial tips of the fixation features. ACKNOWLEDGEMENTS: Funded by the Arthritis & Rheumatism Council. CORRESPONDENCE: Dr Andrew A. Amis Tel: +44-171-594-7062, Fax: +44-171-823-8845, [email protected]
of the ESB, July 8-11 98, Toulouse, France
METHOD: Research on how the endoprosthesis functions in the elbow joint is carried out on the basis of X-ray photographs of the joint with an endoprosthcsis prototype implanted. The polyethylene head of the prototype has circumferential and longitudinal metal markers (Fig. 1). The behaviour of the endoprosthesis when the forearm is placed at different angles and positions (supination, pronation) is analysed. Next are defined and identified the positions of the local system (the endoprosthesis head and the radial bone) within the global system of the humerus, based on an analysis of the shape and positions of the endoprosthesis markers. Fig.1. X-my pictures: Anterio-Posterior (A), Lateral (B) view, with coordinate systems. RESULTS: The presented method is used in analysing the kinetics of the endoprosthesis during changes of the position of the forearm (pronationsupination) at a stable angle of bending ($=90@). The resulting values for the angle of rotation of the local system (at, as, a3), as well as the vector coordinates of the position of the centre of the local system of the endoprosthesis head (Xth, X2”, X3h) within the global system (Tab.l), allow the transformation of the system connected with the endoprosthesis in three-dimensional space to be defined. Tab. 1. Parameters for transformation of system [deg.1 tmml h h Position aI a2 a3 Xlh x2 x3 Pronation 76 -8 - 2.6 -9.1 17.9 7 Supination 7 77 2 - 1.3 -20.7 20.5 DISCUSSION: The presented method, makes possible a quantitative description of the behaviour of the endoprosthesis during limb movement, and this in turn allows a qualitative evaluation of the functioning of the endoprosthesis to be carried out. CONCLUSION: Applying the method herein described to the evaluation of a newly-designed radial bone head endoprosthesis, it is seen that in various movements in the elbow joint, the main axis of the endoprosthesis Xtg is aligned in the direction of the geometric centm of the head of the humerus. This testifies to the automatically correct positioning of the endoprosthesis in relation to its partner, the humerus. REFERENCES: 1. Swieszkowski W. et al., Biology of Sport, Vol.14 Suppl. 7, 271-275, 1997 (in Polish). 2. Money B.F, et al., J. Bone and Joint Surg. Vol.63-A, No.6: 872-87, 1981. 3. Wu G. et al., J. Biomech., 28,1257-1261, 1995. ACKNOWLEDGEMENTS: This research has been accomplished as part of grant no.7 T07A 010 13 sponsored by the State Committee for Scientific Research. CORRESPONDENCE: W. Swieszkowski, Warsaw University of Technology, Nowowiejska 24,00-665 Warsaw, Poland, Tel. 48 22 660 78 14, Fax. 48 22 628 25 87, [email protected]
6
11” Conference
ANALYSIS
OF GLENOID COMPONENT FIXATION DESIGNS IN THE RHEUMATOID SCAPULA T. Kaufler, A.A. Amis, R.J.H. Emery*, Biomechanics Section, Mech.Eng.Dept., Imperial College, London SW7 2BX, UK, *Orthopaedic Surgery Dept., St Mary’s Hospital, London W2 INY. UK.
INTRODUCTION: The lack of t&ability of glenoid component fixation limits the use of total shoulder arthroplasty. Reviews have shown that the majority of glenoid components have radiolucent zones around them, and some of these are not seen on x-rays because of malalignment of the scapula. It is surprising that little work has been done to optimise glenoid component fixation. Jn particular, we am not aware of 3D finite element stress analysis. We are also not aware of prior work in which prosthesis fixation has been examined in rheumatoid bone, despite this being the most common situation at shoulder replacement. METHODS: The scapula of a female rheumatoid patient awaiting shoulder arthroplasty was CT scanned in 43 3mm slices. The bone geometry was extracted from the scan data using custom written software and reconstructed as a solid model by defining the outlines as splines in Patran and assembling them in ProEngineer. Three polyethylene glenoid components were also designed in ProEngineer, with a keel, a central cone, and with two diverging pegs for fixation. These could be implanted with bonded interfaces using the ProEngineer assembly command. After meshing a rectangular volume around the glenoid, material properties were assigned on an individual element basis, using CT density data, and the cut surfaces constrained. Loads representing flexion and abduction actions gave joint forces of 1.5BW acting centrally and superiorly, or antero-superiorly. RESULTS: The CT scans showed loss of bone from the glenoid, with large surrounding osteophytes. There were cystic lesions where the prosthesis fixation was expected. Vector plots showed the principal compressive stresses acting medially through the glenoid, then swinging posteriorly into the plane of the blade of the scapula, corresponding with the trabecuhu architecture. The prosthesis had been modelled with a non conforming articulation. This caused relatively localized loading of the lateral face of either the natural glenoid or of the prosthesis. Thus, relatively high stresses were predicted in the bone beneath the area of load application. In general, them was a tendency for stresses to rise as the bone got thinner towards the blade of the scapula. This tendency was increased by the bending effect when the joint force acted on the anterior edge of the prosthesis. We were surprised to find that the three different fixation designs made little difference to the stresses in the bone. DISCUSSION: This work has modelled a scapula in much greater detail than previously, and has included localized property variations caused by rheumatoid disease. Thus, the stresses are more realistic than either those arising from analysis of a simplified geometry or of a normal bone. The mode1 would be more realistic with different interface properties, but the programing work would be great if a number of fixation designs arc to be compared. A change to a metallic implant would reduce the stresses beneath the area of load application, transferring .load to the medial tips of the fixation features. ACKNOWLEDGEMENTS: Funded by the Arthritis & Rheumatism Council. CORRESPONDENCE: Dr Andrew A. Amis Tel: +44-171-594-7062, Fax: +44-171-823-8845, [email protected]
of the ESB, July 8-11 98, Toulouse, France