Influence of Posterior Tibial Slope

Posterior tilting of the tibial tray and insert is frequently used clinically to facilitate a greater range of flexion by promoting natural femoral rollback, and to resist anterior tibial subsidence by optimizing the underlying bone strength following resection. Significant posterior tilt angles may also be included unintentionally. As a result of the altered tibiofemoral constraint and conformity, relative motion and

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Figure 5. Variation in the predicted kinematics and the peak contact pressures as a result of patient specific loading. The thick line represents the mean for the 7 patients and the thin line represents ± one standard deviation of the mean.

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% of gait cycle

Figure 5. Variation in the predicted kinematics and the peak contact pressures as a result of patient specific loading. The thick line represents the mean for the 7 patients and the thin line represents ± one standard deviation of the mean.

insert stresses will also change. Since, as mentioned previously, the joint kinematics and contact mechanics will play a large role in the long-term success of total knee arthroplasty, it is important to quantify the effect of posterior tilting on these measures. Thus, the effect of posterior tilt on joint mechanics during in vitro physiological loading and the sensitivity of various current TKR devices to these changes were evaluated.

The TKR was analyzed during gait simulation at successive, even posterior tilt angles from 0° (no posterior slope) until component dislocation occurred during the gait cycle. In each analysis, the femoral component was loaded at full extension to find a stable initial position before applying the simulated gait loading cycle. During the initial loading cycle, a nominal load was applied to the femoral component while it was unconstrained in the AP and varus-valgus degrees of freedom. This initial loading resulted in settling of the femoral component into the lowest point in the tibial dish. For every tilt angle the simulated soft-tissue constraint remained in the original, 0° plane in order to evaluate changes in joint mechanics as a result of the changing posterior tilt. Again, tibiofemoral AP displacement, IE rotation, insert stresses, and contact area were recorded as a function of the gait cycle for each analysis.

Changes in the posterior tibial slope varied tibiofemoral constraint and hence, relative kinematics. Both changes in relative kinematics and tibiofemoral conformity were factors in increasing surface and subsurface insert stresses. The semi-constrained PCR design showed only small changes in AP motion from 0° to 10° tilt, generally less than 0.5 mm (Fig. 6). From 12° to 16° tilt the peak anterior tibial motion increased by 3.1 mm due to the decrease in anterior constraint. The total range of AP motion was very consistent throughout the range of tilt angles studied (Fig. 6). Femoral component subluxation occurred posteriorly during the gait cycle at 18° tilt. Both the total range and peak internal rotation decreased steadily with increasing tilt. The peak internal rotation decreased from 5.2° with no posterior slope to 2.3° at 16° tilt (Fig. 6). The total IE range of motion decreased nearly linearly from 6.3° to 2.7°, or approximately -0.22° IE/°tilt (Fig. 6). Peak contact pressures during the cycle were steady, then increased linearly after 10° tilt due to variation in kinematics creating posterior edge loading (Figs. 7, 8). Near 55% of the gait cycle the 0°, 2°, and 4° analyses experienced a spike in the contact pressure results due to relatively high IE rotation creating poor-conformity contact. The reduction in IE rotation with posterior tilt angle reduced this peak by approximately 25% for the 8° and 10° analyses (Fig. 7). Although there were only small changes in AP kinematics for the range of tilt studied, the composite von Mises stress distribution (all stress contours added for each increment of the gait cycle) shows a progressive posterior motion of the insert stresses with increasing

Figure 6. Influence of posterior slope on the predicted kinematics.

This particular design of TKR was found to be relatively accommodating to changes in tilt in the range that would be used clinically. Peak contact pressure remained steady until beyond 10° of posterior tilt. Predicted joint kinematics were fairly consistent, again, especially through 10° of tilt. The peak and range of IE rotation decreased consistently with tilt. Increased anterior insert motion marked the rise in contact pressure realized at 16° posterior tilt as a result of posterior edge contact. The single sagittal insert radius was relatively insensitive to changes in tilt that could occur when surgically reproducing the anatomic tibial slope in TKA.

Wasielewski and coauthors [24] found a statistically significant correlation between tibial insert posterior slope and increasingly posterior articular wear track location with an unconstrained insert. This is consistent with the composite von Mises and contact stress distributions.

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