The IAP of the MCP joint with respect to the atmospheric pressure has been investigated on the MCP joint in a previous study (Gaffney et al., 1995). Their results showed that the IAP was near or below atmospheric pressure in an in vivo MCP joint at rest, which is similar to the results of the present study. A high linear correlation between IAP and load was reported on the shoulder by Itoi et al. in 1993.
However, in this study, a non-linear curve relationship between the load and the IAP
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were found in all specimens, while more linear patterns were presented in the IAP displacement curves. This might be due to the load size as well as to the limited joint volume effect. According to the limited joint volume effect, a relative negative pressure will be induced in the joint cavity by a joint distraction load. The displacement affects the volume of the joint cavity directly. Therefore, in this study, the IAP-displacement curve presented a near linear pattern.
It is clearly shown in Fig 3.5 that the IAP drop before the cross-point exploited only 5% to 10% of the total distraction load, consuming 45% to 80% of the total IAP drop. This matched well with the load-displacement curves, indicating that stiffness increased as load increased, as shown in Fig 2.11. Itoi et al. studied the shoulder joint, and the observed relationship between the load and IAP was found to be almost linear. This might be due to the fact that the applied loading was still before the cross-point. The stiffness of a joint depends on its size and geometry as well as the material properties and the laxity of the capsule. The IAP might not be the major stabilizer in terms of the load it can withstand. This is the reason why we excluded many tissues surrounding the MCP joint that are not directly attached on the bone or capsule of the MCP joint, in order to focus on the IAP response.
However, different tissues and mechanisms play different roles in the stabilization of the MCP joint. The maximum load to bear should not be the only index. Most soft tissues play major mechanical roles in the late stage, that is, after a certain amount of loading or displacement. However, the IAP contributes to joint stability in the early stage of the distraction by reducing the amount of stretching, which might be harmful to other tissues connecting the joint if it is overstretched. This research provided fundamental, if not complete, information about the mechanical role of IAP in the MCP joint.
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Our results indicated that the correlation between the necking volume and IAP (Fig 3.7, r=-0.882) is higher than that between the displacement and IAP (Fig 3.8, r=-0.838). A virtual vented volume was defined in this study to replace the actual vented volume. As the joint is vented, air flows into the joint cavity, and the IAP will equilibrate with the atmospheric pressure. Therefore, the necking phenomenon does not exist in the vented condition. The changes in joint volume before and after vented are the result of losing negative IAP. The space allowed within the Micro CT scanner is too small to allow space for the instruments necessary for the measurement of the IAP. In addition, a metal needle is not suitable for insertion into the joint cavity while scanning to evaluate volume changes in the vented condition due to the metal scattering in CT images. We suppose that the virtual vented volume is similar to the real volume in the vented condition. The necking volume is the difference between the virtual vented volume and total volume, and the necking volume is the effect of the negative IAP on the joint capsule during MCP joint distraction.
The change in necking volume and IAP is highly correlated in this study.
Change in necking volume may be used to predict IAP for the MCP joint in the future. Although change in the necking volume and IAP are highly correlated, it is necessary to provide proof that change in the necking volume is a direct result of the negative IAP. Future efforts should be devoted to measuring the changes in vented volume and to conducting finite element analyses to investigate the effect of IAP on changes in joint volume.
The total volume was the volume of the total soft tissue of the capsule. The results showed the relation between the total volume and IAP in each specimen to be highly linear (r = -0.960 ~ -0.994) although the relation between the total volume
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and IAP in all specimens was lower than the relationships between IAP and other parameters. This might be due to the fact that the limited joint volume effect is a factor that affect IAP in each specimen, as well as the fact that the materials, anatomic morphology, interaction between the synovial fluid and the capsule, and physiological mechanisms also affect the IAP of the MCP joint.
4.5 Finite element analysis
Although significant effort was devoted to formulating the detailed geometry of the joint capsule in the FEA model constructed for the present study, the capsular geometry of the MCP joint adopted was still very different from the actual capsule structure of the MCP joint. The results for the distracted distance and necking phenomena were similar to the FEA results with and without the negative IAP.
Changes in joint volume obtained from FEA were less than those measured from of the Micro CT images. Future studies will improve in regard to the detailed geometry and material properties of different portions of the capsule in FE models in order to present a more realistic model for the MCP joint.