Finite Element Analysis of the Wrist Joint Affected by Rheumatoid Arthritis - Computational Biomechanics of the Wrist Joint - page 53

Fig. 5.12 Histogram of average EQV stress in each bone of the Healthy and RA model

the RA model as compared to the healthy (Fig. 5.13 ) has vitally conveyed the
significance of having cartilage at the articulation areas. The nature of the carti-
lage—hyper-elastic and frictionless—makes it imperative in preventing high stress
concentration at the underlying bones.
Additionally, the generation of the high contact pressure was also attributed to
the existence of the sharp edges due to bone erosion, as clearly evident in the RA
model (Fig. 5.14 b). In contrast, the healthy model with smoother bone surfaces
(Fig. 5.14 a) showed a well distribution of stress.
Potential eventual deformity of the ligaments of the rheumatic wrist was
attributed to the inflammation of the synovial fluid. The ligaments were stretched
and torn, indicating ligaments laxity [ 2 ]. This condition distorts the constraint
initially provided by the ligaments and led to the random translation, dislocation,
and rotation of the affected bones. As prescribed in this study, the dislocation of
carpus towards ulnar were found to cause biomechanical alteration to the wrist
joint. It was evident that the load was transmitted more through the ulnar side of
the RA model (42 %) as compared to the healthy model (37 %) (Fig. 5.15 ). The
change in the load direction was also due to the hand scoliosis condition, in which
the radially rotated metacarpals caused the direction of the stresses to concentrate
ulnarly. This is the result of having the highest stress at the first metacarpal located
at the lateral side of the wrist joint, which pushed the bones towards ulnar
direction.
Stage 2 of scapholunate advance collapse (SLAC) with scapholunate dissoci-
ation (SLD) may even be the additional causes of the altered loading direction
(Fig. 5.16 ). As the distance between the scaphoid and lunate has increased due to
SLD, it was addressed that the load was dominantly subjected to the lunate. The
simulated collapse had clearly illustrated that the capitolunate joint was unable to