In vivo kinematics of cruciate-retaining and -substituting knee arthroplasties

The Journal of Arthroplasty Vol. 12 No. 3 1997

In vivo Kinematics of Cruciate-retaining

and -substituting Knee Arthroplasties Scott A. Banks, PhD,* George D. Markovich, MD,~- and W. Andrew Hodge, MD:~

Abstract: A fluoroscopic measurement technique has been used to provide detailed three-dimensional kinematic assessment of knee arthroplasty function during a step-up activity. Three groups of knee arthroplasty subjects with excellent clinical outcomes and similar ranges of motion were evaluated. Each group received different prosthetic components and surgical treatments of the posterior cruciate ligament (PCL). Group 1 had relatively fiat articular surfaces with retention of the bony insertion of the PCL, group 2 had similar articular geometry but recessed the PCL without retaining the bony insertion, and group 3 had prostheses with greater sagittal conformity and post/cam substitution of the sacrificed PCL. Although n o n e of the knees exhibited normal knee kinematics, the ranges of axial rotation and condylar translation for group 1 were similar to ranges previously reported for normal and anterior cruciate-deficient knees. Axial rotations and condylar translations decreased w h e n the PCL was surgically recessed or substituted. The smallest kinematic ranges were observed in group 3. The results indicate that both prosthetic component selection and surgical technique have a significant effect on prosthetic knee kinematics during functional activities. K e y w o r d s : total knee arthroplasty, kinematics, fluoroscopic analysis, posterior cruciate-retaining, posterior cruciate-substituting.

There is considerable controversy over w h e t h e r to retain or sacrifice the posterior cruciate ligament (PCL) during primary total knee arthroplasty (TKA) surgery. Proponents of retaining the PCL cite improved knee kinematics and more normal performance of activities of daily living to support their position {1-5]. Those supporting PCL-sacrificing designs cite surgical considerations, more conforming articular surfaces, poorly reproducible PCL tensioning, and reduced tibiofemoral loads as reasons to use substituting prosthetic designs [6-9]. Very little information has been reported on h o w these

devices actually articulate once implanted, however, making it very difficult to evaluate the claims of either side of the argument based on objective in vivo information. No studies to date have reported on the direct, accurate three-dimensional measurem e n t in vivo of prosthetic knee function during dynamic weight-bearing activities. This information is critical to achieve a better understanding of h o w knee arthroplasties function u n d e r actual service conditions, h o w knee kinematics affect patient function, and h o w prosthetic geometries and surgical techniques might be refined to improve patient function and device longevity. N u m e r o u s t e c h n i q u e s have b e e n reported for the d y n a m i c m e a s u r e m e n t of knee motion; h o w ever, most of these t e c h n i q u e s c a n n o t accurately m e a s u r e the subtle rotations and translations occurring w i t h i n the k n e e because the measurem e n t s are p e r f o r m e d with s k i n - m o u n t e d markers or fixtures that are not rigidly affixed to the

From the *Orthopaedic Research Laboratory, Good Samaritan Medical Center, West Palm Beach, Florida; ~Portsmouth Naval Hospital, Portsmouth, Virginia; and ~Palm Beach Orthopaedic Institute, West Palm Beach, Florida.

Supported by The Biomotion Foundation and Good Samaritan Medical Center, West Palm Beach, Florida. Reprint requests: Scott A. Banks, PhD, Orthopaedic Research Laboratory, PO Box 3166, West Palm Beach, FL 33402.

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bones [10-12]. Several techniques that directly quantify bone (prosthesis) m o t i o n using roentgenograms [13] and fluoroscopy [14-171 have been reported, but these techniques either lack truly dynamic capacity or cannot accurately quantify nonsagittal rotations. This paper reports the application of a recently developed three-dimensional kinematic measurem e n t technique [18-20] to the analysis of knee kinematics during a step-up m a n e u v e r in subjects with TKA. The fluoroscopic m e a s u r e m e n t technique provides accurate three-dimensional knee kinematics during dynamic weight-bearing activities, permitting the detailed analysis of prosthetic knee function u n d e r actual service conditions. Three groups of patients were evaluated using this m e a s u r e m e n t technique, each group with a different implant type and surgical t r e a t m e n t of the PCL. The results of this study show that both implant type and surgical technique can have a dramatic effect on the kinematics of prosthetic knees during dynamic activities. This information will be helpful in interpreting functional differences b e t w e e n patient groups with different treatments, and will permit design of improved knee arthroplasties based on objective assessment of function in vivo.

Materials and Methods A total of 20 knees in 14 subjects were studied fluoroscopically during the performance of a step-up maneuver. Subjects were selected for inclusion based on an underlying diagnosis of osteoarthritis, excellent clinical performance at least 1 year after surgery (as assessed by Hospital for Special Surgery and Knee Society rating systems), and availability to participate in the study. The subjects gave informed consent to participate in the institutional review b o a r d - a p p r o v e d study. The subjects constituted three distinct groups, each with a different implant

Table 1. Subject Group Clinical Profiles

Patients No. of TKAs Age (y) Height (cm) Weight (kg) Time f r o m s u r g e r y (ruo) Range of m o t i o n (degrees) Hospital for Special S u r g e r y k n e e score

Group 1

Group 2

Group 3

3 6 66_+ 5 182 _+ 13 92+8 14 _+ 1 107 _+ 12 95 _+ 5

6 9 68_+ 8 176 _+ 9 86_+ 14 13 _+ 5 107 _+ 7 98 _+2

5 5 65_+ 6 164 _+ 14 83_+ 13 20 _+ 6* 110 _+ 12 95 _+ 3

*Significant difference f r o m g r o u p 2 (P < .05).

type and surgical technique (Table 1). Three male patients with bilateral TKAs of the AMK design (DePuy, Warsaw IN), comprised group i. These prostheses were implanted with a tibial insert that is fiat in the sagittal plane with slight coronal plane dishing of each condylar surface. The surgical technique included retention of the entire b o n y tibial insertion of the PCL and posterior sloped tibial resection. Group 2 consisted of one female and five male patients with a total of nine TKAs (ie, 3 males had bilateral knee arthroplasties) of the Series 7000 design (Osteonics, Allendale, N J). These prostheses were implanted with tibial inserts that are substantially fiat in the sagittal and coronal planes with slight dishing at the anterior and posterior borders. The surgical technique included a posteriorly inclined proximal tibial resection and recession of the proximal insertion of the PCL to the level of the cut. Group 3 consisted of two male and three female patients with complete surgical release of the PCL and implantation of the Primary Posterior Stabilized Knee (Osteonics). This prosthesis has femoral condylar geometry identical to that of the Series 7000 knee, with the addition of a posterior intercondylar cam designed to articulate with a tibial post at knee angles greater than 40 °. The fibial condylar surfaces are moderately dished in the sagittal plane. All PCL-retaining TKAs were performed by the senior a u t h o r (W.A.H.), and all PCLsubstituting TKAs were performed by a n o t h e r experienced surgeon, each using consistent and reproducible techniques for balancing flexion and extension gaps and collateral ligament tension. Both surgeons cut the f e m u r with 3 ° of external rotation using the same anatomic landmarks for determining the axial a l i g n m e n t - - t h a t is, transepicondylar axis and medial third tibial t u b e r c l e - - a n d confirmed varus/valgus and anterior/posterior stability and central patellar tracking from 0 ° to 120 ° of flexion. Group 1 and group 2 TKAs were performed approximately 2 years apart, with the difference in c o m p o n e n t s reflecting the type of implant being used in that period. Clinical profiles of each group are summarized in Table 1. Each subject's knee motions were recorded fluoroscopically (10 frames/s) for four trials of a stepup maneuver. The foot was positioned on a 25-cm riser such that the tibial plateau was parallel to the floor, and the proximal shank was positioned against an anterior brace. Subjects were allowed to hold onto an overhead bar for stabilization but were instructed not to use the bar for lifting. Subjects were instructed to rise up the step while holding their torso forward, as if they were going to progress to a n o t h e r step.

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The fluoroscopic images were recorded on video-

post and femoral cam in the PCL-substituting knees

tape, digitized, and processed to identify the outline of the prosthetic components. The analytic technique to derive three-dimensional knee kinematics from the prosthetic outlines has been described in detail elsewhere [20]. Briefly, the technique is based on the principle of matching computer-synthesized images of the prosthetic components with experimentally acquired images of the knee in motion. Manufacturer-supplied geometric (computer-aided design) models of the prosthetic components are used to create a library of computer-generated views of the prosthetic component over a range of known orientations. The in vivo position and orientation of the moving prosthetic components are determined based on comparison with the known position and orientation of the components in the computed library. Precisely controlled calibration studies demonstrated the accuracy of this technique to be approximately 1° for all rotations and 0.5 m m for translations in the plane parallel to the fluoroscope image plane, which corresponds to the sagittal plane for the step-up activity. Knee rotations were determined in the femoral coordinate system using the Cardan angle convention [21]. Detailed articular surface models (computer-aided design models) and the measured three-dimensional positions and orientations of the prosthetic components were used to determine locations of closest proximity, or contact, between the components. The locations of femoral condylar contact on the tibial plateau were determined as the points on the condyles closest to the plane of the tibial articular surface. Contact between the tibial

was determined by evaluating the minimum distance between the two surfaces, with any distance less than 1.0 m m taken as representing contact. An analysis of variance with post hoc multiple comparisons was performed on the subject and kinematic data to determine the significance of differences in mean values (P < .05).

Results Axial Rotations The PCL-substituting group 3 knees exhibited a significantly lower range of tibial internal/external rotation than the PCL-bone block group 1 knees (Table 2). Group 2 knees exhibited axial rotation ranges smaller than group 1 knees and greater than group 3 knees, but neither difference was significant.

Locations of Tibiofemoral Contact Average condylar locations and ranges of translation for step-up/down were determined for the three groups; each parameter was computed for an individual's four-trial average and then pooled for the group average (Table 2). Group 3 knees exhibited significantly lower ranges of condylar translation than either group of PCL-retaining knees. Group 2 knees exhibited a significantly lower range of lateral condylar translation than group 1 knees. Group 2 lateral condylar translations occurred in opposite order compared to groups 1 and 3, with maximum anterior contact occurring

T a b l e 2. A x i a l R o t a t i o n s a n d C o n d y l a r T r a n s l a t i o n s D u r i n g S t e p - u p / - d o w n

Internal/external tibial rotation range (degrees) Medial condylar translations Range (mm) Contact at full flexion (mm) Contact at full extension (mm) M a x i m u m posterior contact (mm) Flexion angle at m a x i m u m posterior contact (degrees) M a x i m u m anterior contact (mm) Flexion angle at m a x i m u m anterior contact (degrees) Lateral condylar translations Range (ram) Contact at full flexion (mm) Contact at full extension (mm) M a x i m u m posterior contact (ram) Flexion angle at m a x i m u m posterior contact (degrees) M a x i m u m anterior contact (mm) Flexion angle at m a x i m u m anterior contact (degrees)

Group 1

Group 2

Group 3

9.6 _+ 3.6

6.5 _+2.6

4.9 _+2.4*

8.9 -2.5 -2.9 -10.0 37.1 -1.1 47.3

_+ 1.6 + 5.2 _+ 5.6 _+4.7 _+7.1 _+ 5.0 _+ 30.6

8.2 -1.4 -7.5 -9.7 28.2 -0.9 56.3

_+4.0 + 3.7 _+ 5.1 _+4.1 ± 11.6 _+ 3.4 _+ 17.9

2.3 -5.4 -5.6 -6.7 39.4 -4.5 38.4

_+0.3" t _+2.0 _+ 1.5 _+ 1.8 _+21.4 ± 1.6 _+23.5

12.2 -6.7 -1.0 -11.7 49.5 0.4 19.0

_+ 3.6 _+4.9 _+ 7.0 _+4.8 _+ 10.5 _+ 5.6 _+22.5

6.l -3.0 -6.9 -9. l 25.0 -2.4 51.5

_+2.9* -+ 4.4 -+ 5.4 _+4.8 _+ 12.2" -+ 3.9 _+ 18.9"

3.5 -5.9 ~.4 -6.5 55.1 -3.1 28.8

_+0.8* _+2.4 _+ 1.2 _+ 1.6 _+ 14.7t _+ 1.5 _+ 13.4t

*Significant difference from group 1 (P < .05). ~-Significant difference from group 2 (P < .05).

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in flexion and maximum posterior contact occurring in extension.

Condylar Tracking Patterns Average condylar tracking patterns for step ascent were determined for each group by averaging each knee's average tracking pattern. The tracking patterns were only defined over the flexion range for which at least three individual knee curves were defined. The medial condylar translations of group i were characterized by an initial posterior translation from full flexion to approximately 40 ° of flexion, with a reversal and anterior translation into full knee extension (Fig. 1). The lateral condylar translations were essentially anterior throughout knee extension. The average condylar translations of group 2 were characterized by a posterior translation throughout the motion from flexion to extension of the knee, although some knees did exhibit a small medial reversal as seen with the PCL-bone block group 1. The average condylar translations of the PCL-substituting group 3 were of smaller magnitude than in groups 1 and 2, with the lateral condyle moving less than 1 mm during step-up (Fig. 1): The medial condyle exhibited a small posterior translation (1.2 mm) from full flexion to 35 °, with a small anterior translation (1.3 mm) into knee extension. All three groups exhibited average condylar contact patterns, which were contained entirely on the posterior half of the tibial plateau. The average variability in condylar tracking, as shown by the shaded regions in Figure 1 (± 1 SD), was proportional to the range of condylar translation.

Cam/Post Function For the PCL-substituting knees the calculated minimum distance between the cam and post is shown in Figure 2. Conservative estimates of contact--that is, cam/post distances less than 1.0 mm--indicate the cam and post engaged at approximately 40 ° of knee flexion; less conservative estimates would predict cam/post contact at higher flexion angles.

Discussion The kinematic data presented here represent the first direct measurements of three-dimensional knee arthroplasty kinematics during dynamic weight-bearing activities. Previous reports of dynamic TKA kinematics in vivo relied on indirect measurement [10,11] or made assumptions about knee rotations occurring out of the fluoroscope

image plane [I4-17,22]. The measurement technique used for this study makes no assumptions about component rotations, and provides explicit calculations for all t h r e e - c o m p o n e n t translations and rotations [19,20]. By using three-dimensional surface models of the prosthetic components as an integral part of the m easurem ent process, the detailed interaction between components can be analyzed in clinically relevant frames of reference (eg, condylar contact locations on the tibial plateau). The stair-rise activity was chosen for analysis because it is mechanically more demanding than gait, isolates activity to only the limb of interest, and has been used previously to demonstrate whole-body kinematic differences among subject groups with a variety of knee arthroplasties ]1,3,11,I2,23]. Limitations of this study include the relatively small group of patients examined and the limited sampling rate at which fluoroscopic images were acquired. A larger number of subjects within each of the three groups would have certainly increased the statistical power of the analysis and might have demonstrated additional parameters where kinematic differences exist. The 10-Hz sampling rate was chosen as a compromise between the large number of images (almost 2,000) that required manual outlining of components, and the m i ni m um bandwidth with which to determine knee kinematics. At 10 Hz, an average of 30 frames per step-up/down were acquired, with an average knee flexion increment of 4.4 ° between frames. Stiehl et al. reported acquiring fluoroscopy and then analyzing "still photographs taken at 5 degree increments" [17]. Our approach achieves an average flexion increment slightly smaller than that reported by Stiehl et al. [17] and allows us to more accurately observe the (sometimes large) translations that can occur during small changes in knee flexion. For activities with a higher-frequency content, it is a simple matter to acquire images at higher frame rates using currently available technology. The range of axial rotations in the normal knee during stair-rise [10,111 (measured with skinmounted markers) and gait [24] (using intracortical pins) has been reported to be approximately 10 °. E1 Nahass et al. studied 25 subjects with PCLretaining TKAs using skin-mounted electromagnetic tracking devices and reported an axial rotation range of 7.7 ° during stairclimbing [10]. We observed rotation ranges from 4.9 ° in group 3 to 9.6 ° in group 1, where the larger ranges were observed in knees with fully retained PCLs and unconstrained implants. This observation could

D y n a m i c T K A Kinematics in viva

B a n k s et al.

301

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have several important implications. First, it has been observed [25] that axial rotation of the knee during flexion and extension allows the patella to smoothly follow the helicoid path of the femoral trochlea and that restrictions in axial rotation will result in abnormal stresses on the patella. Knees that demonstrate (or accommodate) little axial rotation m a y therefore exhibit patellar function that is more sensitive to axial alignment of the knee components. Indeed, the incidences of anterior knee pain in two groups of patients with cruciate-substituting knee arthroplasties of the IB2 and PFC designs have recently been reported to be 30% and 28%, respectively [26]. Unfortunately, we are u n a w a r e of any similar studies of patients with cruciate-retaining knees that would permit a correlation b e t w e e n our axial rotation results and the reported incidence of anterior knee pain. Second, although it is unlikely that the PCL can operate normally in the absence of the anterior cruciate ligament (ACL) [27], our data show that greater axial rotations were associated with complete preservation of the PCL insertion. W h e n maintained, the PCL m a y act as an eccentric tether or tension band that constrains the axial rotations of the tibia with respect to the f e m u r or, likewise, the relative medial and lateral condylar translations of the f e m u r on the tibia. Group 1 exhibited larger lateral than medial condylar translations, whereas group 2 exhibited larger medial than lateral translations, which is consistent with the PCL exerting greater medial constraint w h e n fully maintained.

Several studies have reported on condylar translations in vitro during simulated stairclimbing with intact, cruciate-sacrificed and implanted knees. Draganich et al. showed translations of approximately 10 and 12 m m for the medial and lateral condyles of ACL-deficient meniscectomized knees during 0 ° to 90 ° of flexion, and 9 - m m translations for both condyles in knees with both cruciates and the menisci r e m o v e d [281. Grady-Benson et al. reported femoral rollback of 11.3 m m in cadaver specimens with PCL-retaining TKAs [29,30]. M a h o n e y et al. reported femoral rollback of 12.3 m m in ACL-deficient knees, 8.2 m m in knees with cruciate-retaining TKAs, and 11.0 m m in knees with cruciate-substituting TKAs [9]. Our in viva data for the PCL-bone block group show condylar translations that are most similar, in magnitude and direction, to the translations in vitro of ACLdeficient knees and knees with PCL-retaining TKAs. Our data for the PCL recessed group show translations of smaller magnitude, but opposite direction, w h e n compared with translations reported for similar specimens in vitro ]9,29,30], and our data for the PCL-substituting knees show m u c h smaller condylar translations than have been reported for in vitro studies of similar knees [9]. Some of the differences b e t w e e n our in viva data and reported in vitro data can be explained on the basis of differences in the mechanics of the m e a s u r e d activity. First, our observations indicate condylar translation ranges equal to, or smaller than, those reported in vitro. Our data were acquired during weight-bearing dynamic motions with the muscles active, and the soft tissues had not b e e n subjected to f r e e z e - t h a w - i n d u c e d degradation. These greater passive and active constraints could result in reduced ranges of condylar translation. In the PCL-substituting knees, the anterior/posterior conformity of the tibial insert m a y have also contributed to the very small translations observed u n d e r dynamic loading. Second, during initial stair ascent, we observed posterior translations of both condyles in the PCL recessed knees and in the medial condyle of the PCL-bone block and PCL-substituting knees, a pattern that has been reported for only one dynamic in vitro study [31]. During stair rise, our subjects' tibial plateaus remained nearly horizontal t h r o u g h o u t the activity, whereas the tibia is anteriorly flexed t h r o u g h o u t the flexion range for m a n y of the reported in vitro studies ]9,29,30]. For a flexed tibia, the hip load causes an anterior shear of the f e m u r that counteracts the posterior femoral shear force induced by the patella and quadriceps. For a horizontal tibial plateau, the hip load does not pro-

Dynamic TKA Kinematics in vivo

duce an anterior femoral shear force, so that the patellofemoral contact force contributes to posterior femoral translations as the knee begins to extend. This posterior femoral shear is further increased by the forward acceleration of the body during stair rise, which does not occur in vitro u n d e r static conditions. Observations of posterior condylar translations during knee extension have been reported previously by our group [14,19,22,32,33[ and others [16,17,27,34]. Although the absence of the ACL is generally acknowledged to contribute to this p h e n o m e n o n [271, PCL tension, articular geometry, and hamstring activity [35,36] may also have important influences. Our data indicate that, in two similar patient groups with unconstrained PCL-retaining TKAs, the surgical handling of the PCL can influence condylar translations and may have been the difference that resulted in net anterior femoral translations in group 1 and net posterior femoral translations in group 2. Net anterior condylar translations during knee extension (group 1) indicate a greater c o m p o n e n t of rolling contact than the pure sliding contact that occurs during posterior translations (group 2). Blunn et al. have demonstrated that rolling contact produces less aggressive polyethylene (ultrahigh-molecular-weight polyethylene) wear than sliding contact with incongruent surfaces {37] and therefore is the preferred kinematic condition for unconstrained TKA. It is difficult to assess the function of the cam/post mechanism in the PCL-substituting knees from our stair-rise data. Although the cam and post were estimated to have engaged at approximately 40 ° of knee flexion, there were not corresponding posterior translations of the condyles to confirm proper function of the rollback mechanism. Limited stair height, which dictates flexion range, and finite m e a s u r e m e n t resolution make it difficult to determine if the angle at which the cam and post engaged was underestimated, or if in fact the m e c h a n i s m was not providing condylar rollback. Studies using chair-rise, higher step heights, or squatting activities (ie, more knee flexion) should provide the data to better address this issue. After TKA, prosthetic articular geometries and residual ligamentous constraint can vary substantially from the normal knee. Therefore, it is not surprising that our data exhibit considerable differences from the kinematics reported for normal knees [28]. These differences reflect the mechanical e n v i r o n m e n t in which a successful TKA must function, and provide the basis for optimizing TKA designs lo satisfy a variety of surgical preferences. For example, the cruciate-substituting knees

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exhibited lower ranges of axial rotation and condylar translation than the implant was designed to accommodate. A more conforming articulation could a c c o m m o d a t e the same kinematic ranges, while possibly decreasing contact stresses and articular wear. Similarly, if patellar problems result from reduced axial rotations, the tibiofemoral articulation or patellar articulation could be modified to reduce the sensitivity of patellar tracking to implant axial alignment. Furthermore, it is not always possible, or even desirable, to maintain the PCL insertion with a bone block. In these cases, the prosthetic articulations could be modified to control the posterior femoral translations during extension and provide a m o r e conforming articulation. This study provides a detailed kinematic assessm e n t of dynamic prosthetic knee function in three subject groups with excellent clinical and functional outcomes. By comparing the kinematic results between the three groups, it is clear that both implant geometry and surgical technique play important roles in the mechanics of the prosthetically replaced knee. This type of kinematic information will permit the closed-loop design of improved knee arthroplasties, w h e r e both articular details and surgical considerations are incorporated into an objective approach to optimized in vivo function and device longevity.

Acknowledgments We would like to acknowledge Osteonics for providing surface models for the Series 7000 and PPSK components, DePuy for providing surface models for the AMK components, and James A. D'Antonio, MD, for participating in and referring patients to the study.

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