5.3 Biomechanical Behaviours of the Rheumatic Wrist Joint
5.3.2 The Biomechanical Effect of Symptoms
The worsened mechanical properties of the associated tissues and physiological functions of the wrist affected by RA [2] were simulated. Discussion on the effect of the pathophysiology will be presented separately. As one of the characteristics, the osteoporotic RA bones analysed here were found to have 20 % lower stress in comparison with the healthy bones (Fig.5.12). This condition was significantly mimicking the condition where the rheumatic patients tend to underutilise their hands. As the condition progresses, there is high tendency to be affected by bone atrophy as a result of slow generation of the bone cells [34].
Soft tissues are destructed at the early stage of the RA mechanism [7]. This study has significantly evident the effect of this situation towards biomechanical behaviour of the joint. The absence of the articular cartilage resulted in unphys- iological changes to the contact pressures within bones and compressive stress within the cartilages layers, which were addressed in this study and further sup- ported by work done by Carrigan et al. [19]. Ten times higher contact pressure in Fig. 5.11 Equivalent von Mises (EQV) stress plot for the healthy and RA model
52 5 Finite Element Analysis
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.5.14b). In contrast, the healthy model with smoother bone surfaces (Fig.5.14a) 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 Fig. 5.12 Histogram of average EQV stress in each bone of the Healthy and RA model 5.3 Biomechanical Behaviours of the Rheumatic Wrist Joint 53
Fig. 5.13 The average contact pressure in each bone of the healthy and RA model
Fig. 5.14 Stress distribution in the forearm (the radius and ulna) for the healthy (a) and RA model (b). Two critical articulations were identified; radioscaphoid and radiolunate. It was also observed that existence of sharp edges at the RA model resulted in high stress concentration
54 5 Finite Element Analysis
physiologically sustain the applied load, thus resulted in high stress concentration at this destructed articulation.
It was noteworthy to highlight that the SLAC and SLD in rheumatic wrist were also led to high contact pressure found at the radiocarpal articulation with a max- imum pressure of 116 MPa at the radiolunate joint. In comparison with the healthy Fig. 5.15 Von Mises stress
distribution for the palmar aspect of the healthy and the RA models
5.3 Biomechanical Behaviours of the Rheumatic Wrist Joint 55
model, a very less pressure was generated with a maximum pressure of 21 MPa found at the radioscaphoid joint (Fig.5.17). Structure based argument was made, whereby the scaphoid is naturally at risk as its articulation was found to have elliptical shape. Therefore, axial rotation of the scaphoid as the diseases progresses has significantly generating high contact pressure at the articulation due to reduction of the contacted area subjected with loads [5].
All findings presented and discussed in this chapter have brought to a con- clusion, which was biomechanical alteration of the healthy joint has significantly occurred in the rheumatic wrist joint. We believed that the outcomes presented here could notably provide insights to better understand the disease, and thus come with much more encouraging treatments for future benefits.
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58 5 Finite Element Analysis
Chapter 6
Finite Element Analysis of the Wrist Arthroplasty in Rheumatoid Arthritis
Abstract This chapter presents information on finite element analyses (FEA) of stress distribution and contact pressure with the carpal articulation following total wrist arthroplasty (TWA) for rheumatoid arthritis (RA) of the wrist. Results from the previous analyses on the healthy and rheumatic wrist were used for comparison. A TWA model was developed based on parameters of a wrist implant named ReMotionTM total wrist system, and was then applied with the same boundary condition (static hand grip action) as the other two models. FEA has revealed that the contact pressure for the TWA model was five times lower than the RA model. Despite this encouraging finding, small variations in the amount of stress distribution were still present when compared to the healthy model. This comes to a conclusion that the used of TWA could reduces the high contact pressure induced in the RA model thus improving the diseased condition, however, there are rooms for improvement for TWA procedure to restore the biomechanical behaviour of the healthy wrist joint.
Keywords Wrist joint
Rheumatoid arthritisTotal wrist arthroplastyContactpressure
StressFinite element analysis