20 Kinematics of Mobile Bearing Total Knee Arthroplasty

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1 2 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D. A. Dennis, R. D. Komistek Summary Review of the kinematic patterns of fixed- vs. mobile TKA has not demonstrated major differences, with the following exceptions. Less (minimal) anteroposterior translation of both the medial and lateral femoral condyles was observed during gait in patients who received mobile- designs than in those implanted with fixed- TKA.This is likely secondary to the increased sagittal femorotibial conformity present in most mobile- designs. This reduces polyethylene shear stresses and should result in lower polyethylene wear rates in mobile- TKA. In rotating-platform mobile- designs,axial rotation occurs primarily on the inferior surface of the polyethylene, as compared with primarily on the superior surface in fixed- TKA. This should reduce shear forces on the superior aspect of the polyethylene, thereby lessening wear. Additionally, while average axial rotational values following TKA were limited (<1 ), a significant number of subjects exhibited higher magnitudes (>2 ) of rotation which exceed the rotational limits of most fixed TKA designs. This may be an advantage of rotating-platform mobile- TKA designs which can accommodate a wider range of axial rotation without creation of excessive polyethylene stresses. Introduction Various methods have been utilized to analyze kinematics of the normal knee and the knee after implantation of total knee arthroplasty (TKA) [1, 11-13, 15, 26-29, 33-36, 4, 41, 46, 49, 5, 59-66]. These have included in vitro cadaveric evaluations [19, 21, 3, 43, 45, 51, 53], gait laboratory motion analysis systems [1-3, 19, 4, 47, 68], roentgen stereophotogrammetric analyses (RSA) [34-36, 49, 5], quasi-dynamic MRI testing [27, 33], and in vivo video fluoroscopy [11-13, 15, 26, 28, 29, 41, 57, 61].Unfortunately, cadaveric studies often do not simulate in vivo conditions since the actuators utilized to apply joint loads are often unable to accurately reproduce in vivo motions. RSA analyses have often been performed under non-weight conditions and are quasi-dynamic [34-36,49,5]. Error analyses of gait laboratory evaluations have suggested that these systems can induce significant out-ofplane rotational and translational error due to motion between skin markers and underlying osseous structures [4, 46]. Murphy [46, 47] determined that the out-ofplane rotational error could be as high as 18 for internalexternal knee rotation, quite unacceptable for analyses of TKA rotation, which is often less than 5 degrees during certain activities [18]. More recently, video fluoroscopy has been utilized to assess kinematics of both non-implanted and implanted knees [11-13, 15, 26, 28, 29, 41, 59-66]. This method has the advantage of testing under in vivo, weight-, and fully dynamic conditions, while subjects perform various activities. Development of automated model-fitting methods allow for three-dimensional (3D) analyses to be conducted from two-dimensional (2D) fluoroscopic images with minimal in-plane or out-of-plane error [29, 41]. Three-dimensional evaluation of sequential fluoroscopic images can then be performed to accurately determine knee kinematic patterns. The objective of the following report is to summarize our various in vivo kinematic analyses of multiple groups of patients implanted with various designs of mobile TKA and to compare their in vivo knee kinematic patterns with those found in studies of fixed- TKA and the normal knee. Methods In vivo knee kinematics (anteroposterior translation, axial rotation, femoral condylar liftoff, and range-of-motion) have been determined in several studies following implantation of multiple designs of fixed- and mobile TKA [4, 7, 11-13, 24, 28, 29, 39, 52, 55, 59, 6, 61, 63, 67]. The material presented is a summation analysis of over 9 individual studies performed in our research facility over the past 1 years. All subjects were analyzed under fluoroscopic surveillance, either during the stance-phase of gait from heel-strike to toe-off or during a deep knee bend maneu-

2 127 2 Chapter 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D.A. Dennis, R.D. Komistek ver from full extension through 9 of flexion. For gait analysis, individual fluoroscopic videoframes at heelstrike, 33% of stance phase, 66% of stance phase, and at toe-off were digitized, whereas videoframes at, 3, 6, and 9 flexion were digitized for deep knee bend analyses. Knees analyzed included those considered normal (nonimplanted) or those implanted with 33 different designs of fixed- and mobile- TKA. All knee replacement subjects analyzed were judged clinically successful (Hospital for Special Surgery Knee Scores [31] rated excellent) without any significant ligamentous laxity or functionally limiting pain. Fluoroscopic images were captured and downloaded to a workstation computer for analysis. Three-dimensional kinematics for each knee were recovered from the 2D fluoroscopic images using a previously described automated model-fitting technique that determines the in vivo orientation of the femoral component relative to the tibial component and,subsequently,the contact positions between the femoral and tibial components [57]. Extensive error analyses of the 3D model fitting technique have shown a translational 3D error of less than.5 [29,41].Sequential fluoroscopic videoframes were then analyzed to determine kinematic patterns.a femorotibial contact position anterior to the midline was denoted as positive and a posterior position was denoted as negative. The magnitudes of anteroposterior translation were then determined for six individual increments of gait and six flexion increments of a deep knee bend ( Table 2-1). The measurements of anteroposterior (AP) translation represented either pure AP linear motion or rotation of the femoral component relative to the tibial component, or a combination of both. The reported measurements comprised the amount of motion each condyle moves along the AP axis fixed on the tibia plateau. If pure linear translation occurred, both condyles moved in the same direction along this axis. If rotation occurred, one condyle moved anteriorly, whereas the other condyle moved in the posterior direction. Table 2-1. Individual increments analyzed during gait and a deep knee bend After completion of the AP femorotibial translation kinematic evaluations, axial rotation patterns were determined.to accomplish this,a line was created from the medial condylar contact point to the lateral condylar contact point. A second line was then constructed, bisecting the center of the tibial plateau in the coronal plane.the angle created between these two lines was measured and denoted as the axial rotation angle. If the lateral condylar contact position was more anterior than the medial condylar contact position, then the axial rotation angle was denoted as negative. If the medial condylar contact point was more anterior than the lateral condylar contact point, the axial rotation angle was denoted as positive. The magnitudes of axial rotation were then determined for the same six individual increments of gait and six flexion increments of a deep knee bend, as had been done in the evaluation of AP translation ( Table 2-1). This was accomplished by subtracting the axial rotation angle at the beginning of the increment to be analyzed from the axial rotation angle at the end of the analyzed increment.if the change in rotation was positive,then it was denoted as a positive (normal) magnitude of axial rotation ( Fig. 2-1) and a negative change was considered a negative magnitude of axial rotation (or reverse axial rotation; Fig. 2-2). In the normal knee, the axial rotation angle is negative at full extension (lateral condylar contact position more anterior than the medial contact Fig Example of normal axial rotation where the lateral femoral condyle rotates more posteriorly than the medial condyle (i.e., internal tibial rotation) as flexion increases Stance phase of gait Deep knee bend Heel-strike to Full extension to 33% of stance phase 3 of knee flexion Heel-strike to Full extension to 66% of stance phase 6 of knee flexion Heel-strike to toe-off Full extension to 9 of knee flexion 33%-66% of stance phase 3-6 of knee flexion 33% of stance phase to toe-off 3-9 of knee flexion 66% of stance phase to toe-off 6-9 of knee flexion Fig Example of a reverse axial rotational pattern where the medial femoral condyle rotates more posteriorly than the lateral condyle (i.e., external tibial rotation) as flexion increases

3 128 III. Kinematics Medial Lateral Lateral Pivot Medial Pivot No Pivot Fig The location of the pivot position was determined from the convergence angle between the lines connecting the medial and lateral condyle contact positions at full extension and at 9 of knee flexion. If the angle between these lines (Q) converged on the medial half of the tibial insert a medial pivot point was denoted, and if this angle converged on the lateral half, a lateral pivot point was denoted. If this angle did not have a definite convergence and tended towards infinity, no pivot position was denoted position) and in deeper flexion, the axial rotation angle is positive (medial contact position more anterior than lateral).therefore,a positive axial rotation pattern occurred when the tibia internally rotated with increasing knee flexion (positive screw-home rotational pattern), and a negative axial rotation pattern (reverse screw-home rotational pattern) occurred when the tibia externally rotated with increasing knee flexion. Next, the condylar pivot position was determined by analysis of medial and lateral condylar contact positions at full extension and 9 flexion. Two lines were constructed between the medial and lateral condylar contact positions at full extension and also at 9 of flexion ( Fig. 2-3). An angle (Q) was then determined between these two lines. If the full extension and 9 flexion lines converged on the medial half of the tibia in the coronal plane (apex of angle Q), it denoted that a medial pivot location occurred. If the lines converged on the lateral half of the tibial insert,it denoted that a lateral pivot location had occurred. If these lines remained parallel to each other and the angle appeared to approach infinity, no convergence was found and it denoted that there was no pivot location. Upon completion of the sagittal plane kinematic analyses, the grouped implants were rotated to a frontal view to assess for femoral condylar liftoff. This was determined by measuring the distances from the medial and lateral femoral condyles to the tibial tray ( Fig. 2-4).If the difference in the medial vs. lateral condylar distance was greater than our error value of.75 mm (3. standard deviations from our true error value of.25 mm), it was determined that femoral condylar liftoff had occurred [42]. Range of motion was determined under weight- conditions by measuring the femorotibial angle in maximum extension and by analyzing the angle at the maximum flexion videoframe. Finally, the existing model-fitting methodology for 3D kinematic analyses of TKA was adapted to determine the 3D kinematics of the normal femorotibial joint [29, 38, 57]. Ten normal knees were evaluated using this methodology. An initial advantage of analyzing TKA kinematics was that the required computer-assisted design (CAD) models could be created from manufacturer blueprints. However, because the skeletal geometry is variable for every person,computer-generated 3D models of the normal femur, tibia, and fibula had to be created for each specific subject. To determine normal knee kinematics, the 3D models of each subject s femur, tibia, and fibula were registered precisely to the 2D fluoroscopic images using an optimization algorithm that automatically adjusts the pose of the model at various knee flexion angles [38]. For each activity, the femorotibial contact patterns of normal knees were determined for the medial and lateral condyles and plotted with respect to knee flexion angle. 2 Fig Method of determination of femoral condylar liftoff in which the distances between the femoral condyles and the tibial tray (arrows) are determined and compared Results and Discussion Anterior-Posterior Translation in Gait In a multicenter analysis [16],AP translation patterns (i.e., posterior femoral roll-back vs. anterior femoral trans-

4 129 2 Chapter 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D.A. Dennis, R.D. Komistek lation) of 261 knees during gait were analyzed: 251 had been implanted with a TKA and ten were judged to be normal knees. Fifteen of the implanted knees were ACLretaining fixed- TKAs. Eighty-three knees had been implanted with a PCL-retaining fixed- TKA, 74 knees received a posterior-stabilized fixed- TKA, ten knees received a PCL-retaining mobile- TKA, 35 knees received a posterior-stabilized mobile TKA, and 34 knees received a posterior cruciatesacrificing mobile- TKA. From heel-strike to toe-off,nine of ten subjects (9%) with a normal knee experienced posterior motion of their lateral femoral condyle, whereas the medial condyle translated posteriorly in five of the ten knees (5%; Fig. 2-5). The average total lateral condyle motion from heelstrike (% of stance phase) to toe-off (1% of stance phase) was -5.8 mm (4.3 to mm; SD=8.1),whereas the medial condylar motion was less, averaging only -.4 mm (1.6 to mm; SD=6.6). As the direction of knee flexion angle changed from extension to flexion, the lateral condyle translated in the posterior direction. When the angle changed from flexion into extension, the lateral condyle translated anteriorly. The average magnitudes of AP translation observed during gait were minimal (typically <2 mm) both medially and laterally for all fixed- and mobile- TKA subjects when compared with normal knees ( Fig. 2-6). Similar patterns were observed in both PCL-retaining and PCL-substituting TKA subjects. This has been attributed to the fact that the cam-and-post mechanism of most PCL-substituting TKA designs does not engage during lesser flexion activities such as gait. While the average magnitudes of AP translation of fixed- versus mobile- TKA designs of similar type (i.e., fixed- PCL-retaining versus mobile- PCL-retaining, etc.) were not statistically different, analy- Lateral 1 Medial Fig Average medial and lateral condylar contact positions for the normal knee from heel-strike to toe-off AP Position (mm) [-posterior, +anterior] Lateral 1 8 Medial Fig Average medial and lateral condyle contact positions for a fixed- posterior cruciate-retaining total knee arthroplasty from heel-strike to toe-off AP Position (mm) [-posterior, +anterior]

5 13 III. Kinematics Lateral 1 8 Medial AP Position (mm) [-posterior, +anterior] Fig Average medial and lateral condylar contact positions for a rotating-platform posterior cruciate-sacrificing total knee arthroplasty from heel-strike to toe-off 2 sis of interval segments of the stance phase of gait demonstrated higher magnitudes of AP translation in fixed than in mobile- designs (p<.5). Additionally, much greater variability in contact position (anterior versus midline versus posterior) on the tibial component was seen in fixed- TKA designs. Typically, minimal AP motion of both the medial and lateral femoral condyles was observed during gait in patients who received mobile- TKAs ( Fig. 2-7). This phenomenon is likely related to the higher sagittal femorotibial conformity present in most mobile- designs. They also demonstrated less paradoxical anterior femoral sliding during the flexion segments of the stance phase of gait. This results in reduced polyethylene shear stresses and may account,at least in part,for the low polyethylene wear rates reported with long-term clinical use of mobile- TKA [8, 48]. Anterior-Posterior Translation in a Deep Knee Bend In the same multicenter study [16], 55 knees were analyzed during a deep knee bend maneuver. Ten were normal knees and ten had been implanted with an ACL-retaining fixed- TKA. Of the remainder, 136 knees had received a PCL-retaining fixed- TKA, 163 knees had a posterior-stabilized fixed- TKA, 69 knees had a PCL-retaining mobile- TKA, 13 knees had a posterior-stabilized mobile- TKA, and 59 knees had been implanted with a posterior cruciate-sacrificing mobile- TKA. The magnitudes of AP translation were substantially higher in deep flexion in normal knees and in all TKA groups tested than in a lesser flexion activity such as gait. All ten (1%) subjects with a normal knee experienced posterior motion (i.e., posterior femoral roll-back) of the lateral femoral condyle from full extension to 9 of knee flexion, whereas nine of ten (9%) subjects experienced posterior motion of the medial condyle ( Fig. 2-8). On average, posterior motion of the lateral femoral condyle was mm (-5.8 to mm; SD=8.4) and medial condylar motion was -3.4 mm (3.3 to mm; SD=4.6) in the posterior direction. All subjects experienced posterior motion of both condyles from full extension to maximum flexion, but three of ten (3%) subjects had greater than 3 mm of medial condyle anterior translation during at least one analyzed increment of flexion. All lateral femoral condyles experienced only posterior motion. Only one of ten (1%) subjects experienced a lateral pivot motion, eight of ten (8%) had a medial pivot motion, and one of ten (1%) did not experience a pivot motion. Similar to the subjects with a normal knee, all ten (1%) patients with an ACL-retaining fixed- TKA experienced posterior motion of their lateral condyle, and nine of ten (9%) patients had posterior motion of their medial condyle ( Fig. 2-9). The average amount of posterior motion of the lateral condyle was mm (-1. to -2.3 mm; SD=3.7) and the medial condylar motion was -6.1 mm (5.6 to -15. mm; SD=6.3), posteriorly. One of ten (1%) patients experienced paradoxical anterior femoral motion of the medial condyle greater than 3 mm from full extension to 9 of knee flexion and six of ten (6%) patients experienced greater than 3 mm of medial condyle anterior translation during any increment of knee flexion. Similar to the normal knee, all lateral femoral condyles had only posterior motion; only one of ten (1%) patients experienced a lateral pivot motion, seven of ten (7%) had a medial pivot mo-

6 131 2 Chapter 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D.A. Dennis, R.D. Komistek Lateral 1 Medial Fig Average medial and lateral condyle contact positions during a deep knee bend for the normal knee ( -9 flexion) AP Position (mm) [-posterior, +anterior] Lateral 1 Medial Fig Average medial and lateral condyle contact positions during a deep knee bend for a fixed- anterior cruciate-retaining total knee arthroplasty ( -9 flexion) AP Position (mm) [-posterior, +anterior] tion,and two of ten (2%) did not experience a pivot motion pattern. Analysis of all remaining fixed- and mobile- TKA groups demonstrated reduced magnitudes and percentages of posterior motion of both the medial and lateral femoral condyles, a higher incidence of paradoxical anterior femoral translation, and a reduced percentage of subjects exhibiting a medial pivot pattern ( Table 2-2). No statistical differences in amounts of posterior translation during flexion were observed between fixed- and mobile- posterior-stabilized TKAs or between fixed- and mobile- PCL-retaining TKAs (p>.2). However, subjects with both fixed- and mobile- posterior-stabilized TKA designs exhibited higher magnitudes of posterior femoral translation than either fixedor mobile- designs that lacked a cam-and-post mechanism (p<.1). The magnitudes of posterior translation presented in Table 2-2 may underestimate the maximum amounts of posterior femoral translation occurring during extreme flexion in posterior stabilized TKA designs since analysis was performed only to a maximum of 9 of flexion. Many posterior-stabilized implants are designed to have late cam-post engagement (>7 of knee flexion), which may result in additional posterior translation at flexion increments greater than 9. In the current study, patients with either a fixed- or a mobile- PCL-retaining TKA experienced the highest incidence and magnitude of paradoxical anterior femoral translation of either femoral condyle. During a deep knee bend, 72% of patients with a PCL-retaining fixed- TKA and 6% of patients with a PCLretaining mobile- TKA experienced greater than 3 mm of paradoxical femoral translation during knee

7 132 III. Kinematics Table 2-2. Average for all groups during a deep knee bend from full extension to 9 flexion Implant No. of Percent with Average > 3 mm <3 mm Maximum Standard Pivot type subjects posterior motion -9 any time motion deviation position motion -9 (mm) (%) a (%) b (mm) L M L M M or L M or L L M LP MP NP Normal ACL retaining PCL retaining Posterior stabilized PCL retaining mobile Posterior stabilized mobile PCL , sacrificing mobile Average L lateral condyle, M medial condyle, LP percent of subjects exhibiting a lateral pivot position, MP percent of subjects exhibiting a medial pivot position, NP percent of subjects exhibiting no pivot position a >3. mm anterior motion for either condyle, measuring difference between full extension and 9 flexion positions b >3. mm anterior motion for either condyle, occurring at any time during a deep knee bend 2 flexion. Predominantly during anterior femoral translation, both condyles shift anteriorly. Patients who had either a fixed- or mobile- posterior-stabilized TKA also experienced anterior femoral motion,but it occurred primarily on the medial side after the cam and post had engaged (flexion >6 ).At cam-post engagement,the medial condyle experiences the greatest shear forces and translates anteriorly as the lateral femoral condyle levers posteriorly.this finding was determined by analyzing the data in flexion increments greater than 6.After a patient achieves 6 of knee flexion, the medial femoral condyle typically shifts anteriorly to a similar extent as the lateral condyle translates posteriorly. The anterior translation of the femur on the tibia observed in our investigation has numerous potential negative consequences. First, anterior femoral translation results in a more anterior axis of flexion, lessening maximum knee flexion [11, 14]. Second, the quadriceps moment arm is decreased, resulting in reduced quadriceps efficiency.third,anterior sliding of the femoral component on the tibial polyethylene surface risks accelerated polyethylene wear. In a laboratory evaluation of polyethylene wear, Blunn et al. [7] reported dramatically increased wear with cyclic sliding as compared with compression or rolling,because of increased subsurface shear stresses. While the majority of normal knees demonstrated a medial pivot pattern during a deep knee bend activity, this was much less common in both fixed- and mobile TKA groups. Overall, during a deep knee bend maneuver,313 of 54 subjects (58%) with a TKA had a medial pivot pattern,184 of 54 TKA patients (34%) had a lateral pivot pattern,and 43 of 54 subjects (8%) with a TKA did not exhibit a pivot pattern. When the effects of surgeon variability on AP motion patterns of subjects who received the same TKA design were investigated, no statistical differences among surgeons were noted in mobile- TKA groups (p>.2). In contrast,statistically significant differences among differing surgeons were observed in subjects who received fixed- TKA devices of the same type (p<.5). This may be attributed, at least in part, to the increased sagittal conformity commonly present in mobile- TKA implants, which provides enhanced control of kinematic patterns, therefore lessening the affect of variances in technique among surgeons. There also was great variability among TKA types within each group (posterior-stabilized fixed-,

8 133 2 Chapter 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D.A. Dennis, R.D. Komistek PCL-retaining fixed-, etc.). For example, patients with a PCL-retaining fixed- TKA with asymmetric femoral condyles experienced more than twice the amount of posterior femoral motion and substantially less paradoxical anterior femoral translation when compared with patients who had a PCL-retaining fixed- TKA with symmetric femoral condyles [6]. Although it has often been assumed that all posterior-stabilized fixed- TKA designs have similar kinematic patterns, the opposite is actually true. Differing incidences and magnitudes of posterior femoral roll-back typically were seen when comparing differing fixed- posterior-stabilized TKA designs. One such design, for example, showed an excessively posterior contact position throughout the flexion range, thereby never achieving cam-post engagement. Axial Rotation in Gait In a multicenter evaluation of axial rotation during gait, 267 knees were analyzed [18]. Of these, 252 had been implanted with a TKA, ten were judged to be normal knees, and five were non-implanted, ACL-deficient knees. Fifteen of the implanted knees were ACL-retaining (ACR) fixed- TKAs. Eighty-three knees had been implanted with a PCL-retaining (PCR) fixed- TKA, 74 with a posterior-stabilized (PS) fixed- TKA,ten with a PCL-retaining mobile TKA, 35 with a PS mobile- TKA, and 35 with a posterior cruciatesacrificing (PCS) mobile- TKA. From heel-strike to toe-off,eight of ten subjects (8%) with a normal knee experienced normal axial rotation (tibia internally rotating with increasing knee flexion) and eight of ten subjects (8%) experienced reverse rotation during at least one analyzed increment of the stance phase of gait. The normal knee did not experience progressive increasing axial rotation during stance-phase of gait, rather demonstrating a variable pattern that involves extension, flexion, extension and then flexion before toe-off. It would therefore be expected that subjects experience some reverse rotation during gait.the average amount of axial rotation from heel-strike to toe-off for normal knees was 5.7 and the average maximum axial rotation at any increment during stance-phase increased to The maximum amount of normal rotation observed for any individual normal knee at any analyzed increment was 24. (tibia internal rotation) and the maximum reverse rotation observed was -1.1 (tibia external rotation). Nine of ten subjects (9%) achieved at least 5. of normal rotation, five of ten (5%) greater than 1., and five of ten (5%) demonstrated greater than -5. of reverse rotation. Analysis of all fixed- and mobile- TKA groups demonstrated reduced average rotational values (average 1.2 ) and a reduced incidence of normal axial rotation patterns when compared with the normal knee during gait ( Table 2-3). The axial rotation patterns for the fixed- and mobile- PS TKA groups were similar, as were the axial rotation patterns for the fixed- and mobile- PCL-retaining TKA groups. Although most subjects with a PCL-sacrificing mobile- TKA did experience axial rotation, the average rotation for this group was. Only 146 of 252 knees (58%) experienced a normal axial rotational pattern during the stance phase of gait; this was a decrease from the normal knees,which experienced 8% normal rotation, but was not statistically different (p=.15).also, only 86 of the 252 TKAs (34%) experienced greater than 5. of axial rotation at any analyzed increment, a significant decrease from that observed in normal knees (9%; p<.5). This trend continued for knees experiencing greater than 1. of axial rotation. Only 15 of 252 (6%) TKAs experienced greater than 1. of axial rotation, compared with 5% of the normal knee group (p=.1). Axial Rotation in a Deep Knee Bend In the same multicenter evaluation of axial rotation [18], 76 knees were analyzed during a deep knee bend. Ten were normal knees,five were ACL-deficient knees and ten were ACL-retaining fixed TKAs. There were 183 knees that had been implanted with a PCL-retaining fixed- TKA (163 with a functional PCL and 2 with the PCL sacrificed), 212 knees with a PS fixed- TKA, 17 knees with a PCL-retaining mobile TKA, 157 knees with a PS mobile- TKA, and 76 knees with a PCL-sacrificing mobile- TKA. Increased axial rotation occurred more during deep flexion than during gait in all study groups. From full extension to 9 of knee flexion, all ten normal knees experienced normal axial rotation (tibia internally rotating with increasing knee flexion), and only four of ten subjects (4%) demonstrated reverse rotation during at least one analyzed flexion increment of a deep knee bend. The average amount of axial rotation for normal knees from full extension to 9 of knee flexion was 16.5 and the average maximum amount of axial rotation at any flexion increment was The maximum amounts of normal and reverse rotation of any normal knee at any flexion increment were 27.7 and All ten knees achieved at least 5. of normal rotation,nine of ten (9%) greater than 1., and only one of ten (1%) greater than -5. of reverse rotation. Patients who received an ACL-retaining fixed- TKA also achieved excellent axial rotation patterns, while those with an ACL-deficient knee experienced more variable rotational patterns ( Table 2-4). Seven of ten knees (7%) implanted with an ACL-retaining fixed-

9 134 III. Kinematics 2 Table 2-3. Summation analysis of axial rotation of all knee types during gait Implant No. of Normal Reverse Normal Reverse Average Average Average Maximum Maximum Normal Normal Reverse type knees rotation a rotation b rotation rotation rotation max. rot. max. rev. norm. rot. rev. rot. rot. j rot. k rot. l HS TO HS TO all any HS TO all all all all >5. (%) >1. (%) >-5. (%) (%) (%) incre- incre- (degrees) incre- incre- incre- increments c ments d ments f ments g ments h ments i (%) (%) (degrees) (degrees) (degrees) (degrees) Normal Fixed ACR ACL deficient Fixed PCR (+PCL) Fixed PS Mobile PCR Mobile PS Mobile PCS Average of all knees Average of all TKA a Percentage of knees experiencing normal rotation from heel-strike to toe-off. g Average maximum reverse rotation each knee experienced at any increment of stance phase of gait. b Percentage of knees experiencing reverse rotation from heel-strike to toe-off. h Maximum normal rotation each knee achieved at any increment of stance phase of gait. c Percentage of knees experiencing normal rotation during all increments of stance phase of gait. i Maximum reverse rotation each knee achieved at any increment of stance phase of gait. d Percentage of knees experiencing reverse rotation during at least one increment of stance phase of gait. j Percentage of knees experiencing at least 5. of normal axial rotation at any increment of stance phase of gait. e Average rotation from heel-strike to toe-off. k Percentage of knees experiencing at least 1. of normal axial rotation at any increment of stance phase of gait. f Average maximum normal rotation each knee experienced at any increment of stance phase of gait. l Percentage of knees experiencing at least -5. of reverse axial rotation at any increment of stance phase of

10 135 2 Chapter 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D.A. Dennis, R.D. Komistek Table 2-4. Summation analysis of axial rotation of all knee types during a deep knee bend Implant No. of Normal Reverse Normal Reverse Average Average Average Maximum Maximum Normal Normal Reverse type knees rotation a rotation b rotation rotation rotation max. rot. max. rev. norm. rot. rev. rot. rot. j rot. k rot. l (9-, (9-, all any (9- e, all all all all >5. (%) >1. (%) >-5. (%) in %) in %) incre- incre- in incre- incre- incre- increments c ments d degrees) ments f ments g ments h ments i (%) (%) (degrees) (degrees) (degrees) (degrees) Normal Fixed ACR ACL deficient Fixed PCR (+PCL) Fixed PCR (-PCL) Fixed PS Mobile PCR Mobile PS Mobile PCS Average all knees Average all TKA a g Percentage of knees experiencing normal rotation from to 9 of knee flexion. Average maximum reverse rotation each knee experienced at any increment of knee flexion. b Percentage of knees experiencing reverse rotation from to 9 of knee flexion. h Maximum normal rotation each knee achieved at any increment of knee flexion. c i Percentage of knees experiencing normal rotation during all increments of knee flexion. Maximum reverse rotation each knee achieved at any increment of knee flexion. d Percentage of knees experiencing reverse rotation during at least one increment of knee flexion. j Percentage of knees experiencing at least 5. of normal axial rotation at any increment of knee flexion. e k Average rotation from to 9 of knee flexion. Percentage of knees experiencing at least 1. of normal axial rotation at any increment of knee flexion. f l Average maximum normal rotation each knee experienced at any increment of knee flexion. Percentage of knees experiencing at least -5. of reverse axial rotation at any increment of knee flexion.

11 136 III. Kinematics 2 TKA experienced normal rotation from full extension to 9 of knee flexion, and six of ten (6%) experienced reverse rotation during at least one flexion increment. The average amount of axial rotation from full extension to 9 of knee flexion was 8.1 and the average maximum amount at any increment of a deep knee bend increased to The maximum amounts of normal and reverse rotation in any ACL-retaining TKA at any flexion increment were 2.9 and Nine of ten (9%) knees achieved at least 5. of normal rotation,four of ten (4%) greater than 1., and six of ten (6%) greater than -5. of reverse rotation. Both the percentage of subjects experiencing normal rotational patterns and the magnitudes of axial rotation decreased in all remaining fixed- and mobile- TKA groups compared with normal knee subjects during a deep knee bend maneuver (p<.1; Table 2-4). The axial rotation patterns for the fixed- and mobile- posterior-stabilized TKA groups were similar,as were the axial rotation patterns for the fixed- and mobile- PCL-retaining TKA groups. No statistically significant difference in average amount of axial rotation was noted between PCL-retaining and posterior-stabilized knee groups (p>.2). As with posterior femoral translation, however, the present data may underestimate the maximum magnitudes of axial rotation occurring during extreme flexion in posterior-stabilized TKA designs, since analysis was performed only to a maximum of 9 of flexion. Since many posterior-stabilized implants have late cam-post engagement (>7 of knee flexion), additional axial rotation at flexion increments greater than 9 may occur. Increased axial rotation was present in deep flexion if the ACL was intact. Average values in normal and ACLretaining TKA subjects (16.5 and 8.1 ) were significantly higher than in TKA study groups in which the ACL was absent (<4. ; p<.1). High variability in axial rotation patterns and magnitudes was found among differing TKA categories,between differing implant designs within the same implant category, and among identical designs implanted by different surgeons. This suggests that axial rotation is determined by many factors, including implant design, individual patient anatomical variances, and the surgical technique of TKA. For example, the range of average axial rotation was for the posterior-stabilized fixed- TKA group. Similarly, the maximum amount of normal rotation ranged from 7. to 35.9 and from 9.5 to 22.4 for subjects in the PCL-retaining and posterior-stabilized mobile- TKA groups, respectively. Considering all TKAs during a deep knee bend, only 551 of 745 knees (74%) experienced a normal axial rotational pattern; this was a significant decrease from the normal knee group, in which 1% (ten of ten) demonstrated a normal axial rotational pattern (p=.1). Also, only 425 of 745 TKAs (57%) experienced greater than 5. of axial rotation at any flexion increment, a significant decrease from the normal knee group (1%, ten of ten; p=.1). This trend continued for the number of knees experiencing greater than 1. of axial rotation. Only 127 of 745 (17%) subjects with a TKA experienced greater than 1. of axial rotation compared with 9% (nine of ten) of normal knees (p=.1). While average magnitudes of axial rotation in fixedvs. mobile- TKA groups were similar, additional analysis suggests the site of rotation differs. Two in vivo, video fluoroscopic analyses of two different designs of rotating-platform TKA have been performed to assess polyethylene mobility [25, 39]. Tantalum beads were embedded within the mobile polyethylene s to determine the amount of polyethylene- mobility during a deep knee bend. In both studies it was found that the polyethylene typically rotated in conjunction with the femoral component, confirming the presence of mobility. Similar findings were observed by D Lima et al. [1] in a cadaveric analysis of axial rotation following mobile- TKA. Therefore, in rotating-platform TKA designs, axial rotation typically occurs on the inferior surface of the polyethylene, whereas rotation obviously occurs on the superior surface in fixed- TKA. This may play a role in reduction of polyethylene wear following mobile- TKA. Bearing mobility should reduce shear forces on the superior aspect of the polyethylene, thereby lessening wear. Otto et al. [54] have demonstrated that low contact stresses ( 8 MPa) are present on the inferior surface of rotating-platform polyethylene s when articulating against a highly polished cobalt-chromium surface, which should minimize polyethylene wear during activities requiring axial rotation. Reverse axial rotational patterns during individual increments of a deep knee bend were common in all fixed- and mobile- study groups. It was uncommon for subjects in any TKA group to demonstrate a progressive normal rotational pattern (i.e., internal tibial rotation) throughout all increments of deep flexion. Typically, alternating patterns of internal and external tibial rotation were observed as flexion increased. Reverse axial rotation is undesirable, risking patellofemoral instability due to lateralization of the tibial tubercle during deep flexion, as well as lessening maximum knee flexion due to reduced posterior femoral roll-back of the lateral femoral condyle. Finally,while average axial rotational values following TKA were limited,a review of individual TKA subjects revealed a substantial number with high magnitudes (>2 ) of both normal and reverse axial rotation, which exceeds the rotational limits of most fixed- TKA designs. This may be an advantage for rotating-platform mobile TKA designs, which can accommodate a wider

12 137 2 Chapter 2 Kinematics of Mobile Bearing Total Knee Arthroplasty D.A. Dennis, R.D. Komistek range of axial rotation without creation of excessive polyethylene stresses. Femoral Condylar Liftoff The incidence and magnitude of femoral condylar liftoff has been evaluated in numerous studies of both fixedand mobile- TKA [15, 24, 55, 61]. Initially, a series of 2 fixed- PCL-retaining and 2-fixed PCL-substituting TKA were analyzed during a deep knee bend maneuver [15]. All analyzed knees were judged clinically excellent, with no measurable ligamentous laxity or functionally limiting pain. The incidence of femoral condylar liftoff was similar in both fixed- TKA groups, occurring in 7% (14 of 2) of PCL-retaining knees and 8% (16 of 2) of PCL-substituting knees. It occurred with similar frequency medially and laterally in subjects implanted with PCL-substituting knees, but was observed predominantly on the lateral side in those with PCL-retaining knees. Femoral condylar liftoff was most commonly observed between 6 and 9 of flexion in both groups. The average magnitudes of liftoff observed were small (< 2. mm) in these well-functioning knees. Since initial studies to assess for condylar liftoff, additional analyses have produced higher magnitudes of femoral condylar liftoff (as high as 8. mm; Fig. 2-1), particularly in subjects with suboptimal clinical results. A similar incidence (45%-1%) and magnitude ( mm) of femoral condylar liftoff during both gait and a deep knee bend has been observed in various designs of mobile- TKA [24, 55, 61]. Femoral condylar liftoff was again observed more commonly on the lateral side of the joint, particularly if the posterior cruciate ligament was preserved, and has been attributed to an adduction moment occurring during the midstance phase of gait [15]. It was most commonly observed Fig Fluoroscopic image and computer-assisted design overlay of a TKA, demonstrating marked femoral condylar liftoff between 6 and 9 of flexion during a deep knee bend activity and during the mid-stance phase of the gait cycle. In summary,femoral condylar liftoff is a frequent phenomenon which occurs in many TKA types and does not appear to be affected by the presence of mobility. It occurs most commonly on the lateral side of the joint during the deep flexion phase of a deep knee bend activity and during the mid-stance phase of gait. Lift-off occurred both medially and laterally in subjects implanted with fixed- and mobile- TKA designs in which both cruciate ligaments were sacrificed, but was observed predominantly on the lateral side in those with PCL-retaining TKA. This may be related, at least in part, to the presence or absence of the cruciate ligaments. In the knee with intact cruciate ligaments, the ACL originates at the lateral femoral condyle while the femoral attachment of the PCL is medial.we theorize,therefore,that the ACL acts as a checkrein, limiting lateral femoral liftoff, with the PCL similarly resisting femoral liftoff medially. In knees with both cruciate ligaments resected, the incidence of femoral condylar liftoff was similar both medially and laterally; whereas in TKA subjects with the PCL preserved, femoral condylar liftoff occurred predominantly laterally, possibly due to the intact PCL resisting liftoff medially. These findings support the importance of coronal femorotibial curvature and conformity in TKA design. In a laboratory analysis, Miller et al.[44] evaluated peak tibial polyethylene stresses under eccentric loading conditions in both flat-on-flat and more coronally curved femorotibial implant geometries. They observed substantially higher polyethylene stresses in flat-on-flat designs, often exceeding the yield strength of polyethylene, whereas designs with increased coronal curvature and conformity proved less sensitive to eccentric loading conditions with lesser peak polyethylene stresses. Additionally, these studies support the use of metalbacked tibial components with central stems to reduce peak subchondral cancellous bone loads, should femoral condylar liftoff occur. Bartel et al.[5] conducted a finite element analysis of the effect of metal backing on peak subchondral cancellous stresses. They reported reductions in peak cancellous stresses of 16%-39% with metalbacked versus all-polyethylene tibial components when evaluated under eccentric loading conditions.this analysis is supported by clinical studies. Ritter et al. [56] evaluated 21 cases of a metal-backed PCL-retaining TKA with a flat-on-flat coronal plane geometry (Anatomical Graduated Components; Biomet Corporation, Warsaw, IN).A 98% implant survival rate was observed at 1 years. Faris et al. [2] reviewed 536 cases of the same prosthetic design, with the exception that an all-polyethylene tibial component was utilized. At an average follow-up of 1 years, survival of only 68.11% was found. Fifty-eight (73.4%) of the 79 failures observed occurred in association with loosening or collapse of the bone beneath the

13 138 III. Kinematics 2 medial tibial plateau. We postulate that lateral femoral condylar liftoff may have played a role in the premature failure of these all-polyethylene, flat-on-flat tibial components. If lateral liftoff occurs, excessive medial loads are created. In the absence of tibial component metal backing, these high loads become transmitted to the underlying medial subchondral bone, resulting in osseous collapse and eventual tibial component loosening. Range of Motion Table 2-5. Average range of motion for various types of TKA Knee type Passive Weight- (degrees) (degrees) Normal [14] Fixed- TKA PFC PS [14] PFC PCR [14] PFC Sigma PS [9] Mobile- TKA LCS RP [63] LCS MB a [6] LCS RP PS [22] LCS AP Glide [52] PFC Sigma PS RP [17] a Meniscal Numerous analyses have been performed to assess magnitudes of range of motion of various designs of TKA [14, 52, 6, 63]. On average, subjects with a normal knee experienced 139 and 135 of knee flexion under passive, nonweight- and weight- conditions, respectively [14]. Under passive conditions, subjects with fixed PCL-retaining (123 ) or -substituting TKAs (127 ) experienced similar magnitudes of knee flexion. When tested under weight- conditions, however, subjects with a PCL-substituting TKA experienced statistically more flexion (113 ) than those implanted with a PCLretaining TKA (13 ; p<.24). The reduction of motion observed under weight- conditions in the PCLretaining TKA group is likely related to paradoxical anterior femoral translation during progressive knee flexion, which is commonly noted in these designs. This kinematic abnormality may limit maximum flexion due to anteriorization of the axis of flexion, earlier impingement of the posterior soft-tissue structures, and tightening of the extensor mechanism (from anterior femoral displacement). Alternatively, patients implanted with PCLsubstituting TKAs routinely demonstrate posterior femoral roll-back, dictated by interaction of the femoral cam and tibial post mechanism of the PCL-substituting design, regardless of weight- status [11]. Similar studies have subsequently been completed to assess whether mobility influences range of motion [52, 6, 63]. These are summarized in Table 2-5. In summary, when tested under weight- conditions,subjects implanted with mobile- TKA did not have superior range of motion compared with those who had fixed- designs (range, ). In most mobile- TKA designs, a reduced range of motion was observed. This may be related to the increased sagittal conformity typically seen in most mobile- designs, which limits posterior femoral roll-back due to the increased prominence of the posterior lip of mobile polyethylene s. Additionally, the sagittal dwell point (the point where the polyethylene is the thinnest) of the mobile- TKA designs tested is positioned more anteriorly than in most fixed- TKA designs [14]. This may position the axis of flexion more anteriorly and limit maximum flexion. In summary, knee range of motion is determined more by implant condylar geometry than by mobility. References 1. Andriacchi TP, Stanwyck TS, Galante JO (1986) Knee biomechanics and total knee replacement. J Arthroplasty 1: Andriacchi TP (1993) Functional analysis of pre- and post-knee surgery. Total knee arthroplasty and ACL reconstruction. J Biomech Eng 115: Andriacchi TP, Galante JO, Fermier RS (1994) Patient outcomes following tricompartmental total knee replacement. JAMA 271: Argenson JN, Komistek RD, Aubaniac JM, Dennis DA, Northcut EJ, Anderson DT, Agostini S (22) In vivo determination of knee kinematics for subjects implanted with a unicompartmental arthroplasty. J Arthroplasty 17: Bartel DL, Burstein AH, Santavicca EA, Insall JN (1982) Performance of the tibial component in total knee replacement. J Bone Joint Surg [Am] 64: Bertin KC, Komistek RD, Dennis, DA, Hoff WA, Anderson DT, Langer T (22) In vivo determination of posterior femoral rollback for subjects having a NexGen posterior cruciate retaining total knee arthroplasty. J Arthroplasty 17: Blunn GW, Walker PS, Joshi A, Hardinge K (1991) The dominance of cyclic sliding in producing wear in total knee replacements. Clin Orthop 273: Buechel FF, Buechel FF, Pappas MJ, Dalessio J (22) Twenty-year evaluation of the New Jersey LCS rotating platform knee replacement. J Knee Surg 15: Callaghan JJ, Insall JN, Greenwald AS, Dennis DA, Komistek RD, et al (2) Mobile knee replacement. J Bone Joint Surg [Am] 82: D Lima DD, Trice M, Urquhart AG, Colwell CW Jr (21) Tibiofemoral conformity and kinematics of rotating- knee prostheses. Clin Orthop 386: Dennis DA, Komistek RD, Hoff WA, Gabriel SM (1996) In vivo knee kinematics derived using an inverse perspective technique. Clin Orthop 331: Dennis DA, Komistek RD, Cheal EJ, Stiehl JB, Walker SA (1997) In vivo femoral condylar lift-off in total knee arthroplasty. Orthop Trans 21: Dennis DA, Komistek RD, Colwell CE, Ranawat SC, Scott RD, Thornhill TS, Lapp MA (1998) In vivo anteroposterior femorotibial translation of total knee arthroplasty: a multicenter analysis. Clin Orthop 356: Dennis DA, Komistek RD, Stiehl JB, Walker SA, Dennis K (1998) Range of motion following total knee arthroplasty: The effect of implant design and weight- conditions. J Arthroplasty 13: