In vivo patellar kinematics during total knee arthroplasty

Figures & data

Figure 1. The patellar marker array was attached in two steps. First, the base was attached with a bone screw to the anterior surface of the patella, mediolaterally, facing towards the optical digitizer. Second, the marker array was screwed onto the base in one of three positions, suiting an upright or everted patella, with either a laterally or medially placed optical digitizer and a medial or lateral incision. [Color version available online.]

Figure 2. The surgeons flexed the leg through 3–7 full range-of-motion cycles, while the system recorded the tibiofemoral and patellofemoral kinematics. This was done before opening the joint capsule and again after completing the arthroplasty. [Color version available online.]

Figure 3. The trajectory point of the patella was defined by digitizing the centre of the median ridge on the posterior surface of the patella. This point also defined the origin of the patellar coordinate system. Although pre-arthroplasty kinematics were recorded before opening the joint capsule, the trajectory information was displayed only after opening the joint capsule and digitizing the posterior point. [Color version available online.]

Figure 4. A sample distoproximal visualization of a pre-arthroplasty femur (left), derived from bone-morphed geometry, is shown beside the corresponding post-arthroplasty femur overlaid with the femoral component position (right). The righthand (green) line shows the trajectory of the centre of the patellar median ridge, while the lefthand (blue) line shows the trajectory of the distal end of the median ridge throughout several cycles of flexion. The custom visualization software also allowed the patellar path to be viewed dynamically. [Color version available online.]

Figure 5. The pre-arthroplasty patellar trajectory (mean and standard deviations) had minimal mediolateral deviation throughout flexion. Geometric data for the femurs were obtained intraoperatively by matching digitized portions of the femoral surface to a statistical shape model. The schematic shows a typical femoral shape. [Color version available online.]

Figure 6. The six degrees of freedom of pre-arthroplasty kinematics are shown relative to tibiofemoral flexion for both the flexion and extension phases (mean and standard deviation). On average, the patella tracked slightly laterally (upper left), with lateral tilt (upper right) and external spin (mid right). The other translations and rotation (AP, PD and flexion) were primarily controlled by the joint geometry (see Figure 5). [Color version available online.]

Figure 7. The post-arthroplasty patellar trajectory (mean and standard deviation) also had minimal mediolateral deviation throughout flexion. All three surgeons resected less from the tibia than the insert size, thus raising the joint line. This resulted in a more proximal femoral component, more distal contact of the patella on the femoral component, and a longer portion of the trajectory within the notch. This is called pseudo patella baja, as the distal patellar contact is not due to shortening of the patellar tendon. Geometric data for the femoral component were obtained from the manufacturer; the schematic shows a typical placement of the component on the femur. [Color version available online.]

Figure 8. The six degrees of freedom of post-arthroplasty kinematics are shown relative to tibiofemoral flexion for both the flexion and extension phases (mean and standard deviation). The overall patterns are similar to the pre-arthroplasty patterns (Figure 6), but with shifts in the absolute values (see Figure 9). [Color version available online.]

Figure 9. The changes between pre-arthroplasty and post-arthroplasty kinematics (mean and standard deviation), for all six degrees of freedom, flexion and extension phases, are shown with respect to the femoral bone reference. Arrows indicate significant differences.The patella was more posterior, more proximal and more flexed after arthroplasty (p < 0.008) due to “rounding the femoral corner” sooner; see the text and Figure 12 for an explanation of these paradoxical results given more distal patellar contact on the femoral component. None of the primary tracking characteristics (ML shift, ML tilt, int/ext spin) showed a significant bias in one direction or the other, although individual changes were potentially relevant (see Figure 10) [Color version available online.].

Figure 10. The absolute change in each degree of freedom shows the effect of arthroplasty regardless of the direction of change (i.e., the absolute value of the data in Figure 9). The histograms sum the results for all four flexion angles studied (15°, 45°, 90° and 120°). The curves show a spline fit to a non-parametric probability density function estimate of the histogram data. The arrow indicates the mean change. At the top of the graph, this mean absolute change is compared to the pre-arthroplasty range for that degree of freedom (see Figures 5 and 6). This shows that mediolateral tilt, shift and internal/external spin had the highest relative change due to arthroplasty. [Color version available online.]

Figure 11. Female and male mediolateral tilt are compared for pre-arthroplasty, post-arthroplasty, and change due to arthroplasty. Female patients in this study had, on average, approximately 10° more lateral tilt than male patients, both pre-arthroplasty and post-arthroplasty; the difference was significant in later flexion (p < 0.004). Arthroplasty had a minimal effect on tilt. [Color version available online.]

Figure 12. When the joint line is raised due to under-resection of the tibia (whether intentional or unintentional) and the femoral component is consequently positioned more proximally, the patella contacts the femoral component more distally (relative to the notch or distal condyles) due to the constant length of the patellar tendon. However, relative to the coordinate frame of the femoral bone (to which the femoral marker array is attached), the position of the patella remains relatively unchanged in extension (blue dot, anteriorly). In flexion, the patella “rounds the corner” sooner than in the intact knee (red dots, distally). Relative to the femoral coordinate frame, this paradoxically causes the patella to be more flexed in early flexion and more proximal and posterior in later flexion relative to the pre-arthroplasty kinematics (Figure 9). [Color version available online.]