Finite Element Analysis of the Wrist Arthroplasty in Rheumatoid Arthritis - Computational Biomechanics of the Wrist Joint - page 67

2.49 MPa), followed by the RA model (average 1.99 MPa) and the TWA model
(average 1.29 MPa).
6.4.2 Mechanical Contact Pressure Within the Bones
Contact pressure within the bones has shown that the high magnitude found in the
RA model (average 3.9 MPa) was reduced to almost five times lower after TWA
(average 0.75 MPa). This advantage however was not restoring the pressure of the
healthy model (average 0.4 MPa).
It was also evident that the trapezium could be considered as the most critical bone
as high magnitude of pressure was always found in this bone for all three simulated
cases, with the highest pressure found at the RA model (11 MPa) (Fig. 6.6 ).
6.4.3 Biomechanical Analysis of Different Moduli of Bone Graft
It was observed that the modulus of the bone graft had significant impact on the
stress distribution (Fig. 6.7 a) and contact pressure (Fig. 6.7 b), which were found
significantly at the carpal complex. Increasing the bone graft modulus to 5 GPa
resulted in efficient load transmission. High fluctuation pattern of graphs was
found in the TWA model engaged with bone graft modulus of 0.1 GPa, indicating
inconsistent stress and contact pressure magnitudes.
6.4.4 Biomechanical Assessment of the Total Wrist Arthroplasty
Procedure
As far as treatment is concerned, it is vital to solve two main problems caused by
RA as shown in the previous chapter; which were the effect of high contact
pressure and imbalance load transmission. The ReMotion TM implant used in the
TWA model anticipated two main design concepts—distal radius resurfacing and
intercarpal fusion—to minimize motion that could cause metacarpal perforation
and implant loosening. FEA results have confirmed the ability of this implant
system to reduce high contact pressure in RA bones through intercarpal fusion
approach. This implant system fused the affected bones using two screws, one
central peg and bone grafts, thus has resulted in minimised stress concentration
and therefore reduced the potential problem of wear at the articulation.
In terms of load transmission, the disproportionate pattern of stress distributions
observed in the RA model changed to a more uniform pattern after TWA (Fig. 6.5 ).
This finding complied with the implant design concept of intercarpal fusion to