Knee Surgery, Sports Traumatology, Arthroscopy

, Volume 17, Issue 2, pp 150–156

In vivo flexion and kinematics of the knee after TKA: comparison of a conventional and a high flexion cruciate-retaining TKA design

  • Jeremy F. Suggs
  • Young-Min Kwon
  • Sridhar M. Durbhakula
  • George R. Hanson
  • Guoan Li
Knee

DOI: 10.1007/s00167-008-0637-4

Cite this article as:
Suggs, J.F., Kwon, YM., Durbhakula, S.M. et al. Knee Surg Sports Traumatol Arthrosc (2009) 17: 150. doi:10.1007/s00167-008-0637-4

Abstract

This study investigated the in vivo 6DOF knee kinematics and tibiofemoral contact location after total knee arthroplasty using a conventional and a high flexion cruciate retaining component (15 NexGen CR, 11 NexGen CR-Flex). Each patient performed a single-leg lunge while being imaged by a dual fluoroscopic imaging system. Data were analyzed at hyperextension, 0° to 90° in 15° intervals, and at maximum flexion. The average maximum weight-bearing flexion for all the CR patients was 110.1° ± 13.4°, and for all the CR-Flex patients was 108.2° ± 13.2°. No difference was seen in the maximum flexion achieved by the patients, and the kinematics demonstrated by the groups was similar. However, at high flexion, the tibiofemoral articulating surfaces were more conforming in the CR-Flex design than the CR design, suggesting that the use of the high flexion component improved the tibiofemoral contact environment at high flexion in patients who could achieve high flexion.

Keywords

Total knee arthroplasty High flexion In vivo kinematics Biomechanics Knee 

Introduction

Restoration of the full range of knee flexion after total knee arthroplasty (TKA) is important to patients in maintaining various life style activities, such as sports, gardening, stair ascent/descent, and taking a bath [9, 12, 21]. It is believed that contemporary TKA patients are more active than patients are in the past and have a greater desire to participate in activities that require high flexion. Consequently, many new TKA components have been designed to better accommodate high knee flexion after surgery. It has been suggested that the mechanical environment experienced by the polyethylene insert at high flexion may be highly unfavorable and that participation in high flexion activities could accelerate wear of the polyethylene component [17, 20].

Several studies have evaluated high flexion TKA designs using either clinical examination or single-plane fluoroscopic techniques [2, 6, 7, 10, 11, 22]. These in vivo studies have only dealt with posterior-substituting designs. No study has reported on the biomechanics of high flexion posterior cruciate-retaining TKA designs. Further, no study has compared the in vivo contact biomechanics of high flexion TKAs with those of conventional TKA designs.

The objective of this study was to compare the in vivo kinematics of two cruciate retaining TKA designs, one conventional design (NexGen CR, Zimmer, Warsaw, IN) and one high flexion design (NexGen CR-Flex, Zimmer, Warsaw, IN). We hypothesized that the CR-Flex design would enhance knee flexion compared to the conventional CR design. Six degree-of-freedom kinematics was obtained from patients implanted with either the conventional component or the high flexion component using a dual fluoroscopic imaging system. Information on maximum knee flexion and the contact location between the femoral component and the polyethylene insert were also compared between the two designs.

Materials and methods

In vivo experimental design

Fifteen knees with a NexGen CR implant (11 unilateral patients, 2 bilateral patients, Zimmer, Warsaw, IN) and 11 knees with a NexGen CR-Flex implant (all unilateral patients) were analyzed in this study under IRB approval. Patients were recruited retrospectively from the practice of a single surgeon, and each patient gave informed consent. All patients were undergoing TKA due to osteoarthritis. There was no difference in age, body weight, height, gender, or knee scores between the group with conventional TKA components and the group with high flexion components (Table 1). The patients with CR implants were evaluated 15.6 ± 7.5 months after surgery. The patients with CR-Flex implants were evaluated 10.7 ± 3.3 months after surgery. All patients had their knees replaced at least 6 months prior to participation in this study and demonstrated a passive range of motion (ROM) greater than 90°.
Table 1

Demographics for CR and CR-Flex groups

 

CR

CR-Flex

Age (years)

69.1 ± 10.9

66.6 ± 11.2

Weight (lbs)

195.1 ± 31.0

193.1 ± 41.0

Height (in.)

69.6 ± 3.0

68.5 ± 3.4

Gender (F/M)

3/12

2/9

Side (L/R)

5/10

4/7

Postop. time (months)

15.6 ± 7.5

10.7 ± 3.3

The passive ROM of each patient was assessed using a goniometer (Table 2). During the experiment, the patient was asked to perform a weight-bearing single-leg lunge while the knee of their forward leg was imaged from full extension to maximum flexion using a dual fluoroscopic imaging system [8, 14]. Pairs of fluoroscopic images were captured simultaneously at intervals of approximately 15° of flexion. Patients were asked to support as much of their body weight as possible with their forward leg, and they were free to use their rearward leg as well as handrails to maintain balance.
Table 2

Clinical results for CR and CR-Flex groups

 

CR

CR-Flex

Passive ROM (degree)

120.6 ± 11.1

117.7 ± 9.7

Max weight-bearing flexion (degree)

110.1 ± 13.4

108.2 ± 13.2

Weight-bearing ROM (degree)

118.5 ± 15.7

115.1 ± 17.0

Knee society knee score

91.4 ± 13.2

89.5 ± 11.5

Knee society functional score

84.9 ± 14.4

84.7 ± 15.0

The positions of the total knee components during the weight-bearing flexion were deduced with the use of a virtual dual fluoroscopic imaging system created in solid modeling software (Rhinoceros®, Robert McNeel and Associates, Seattle, WA), where the image intensifiers were represented by the acquired fluoroscopic images, and the X-ray sources were represented by two virtual cameras [14]. Solid models of the TKA components were imported into the virtual fluoroscopic system. The component models were manipulated in 6DOF until they overlapped their silhouettes on both fluoroscopic images, as seen from their respective cameras. When the models overlapped their silhouettes, the in vivo pose at the time of image acquisition was recreated. By repeating this process for each pair of fluoroscopic images, the in vivo positions of the total knee components along the flexion path were represented by a series of 3D total knee models.

After the in vivo positions were determined, the 6DOF kinematics was calculated relative to a reference pose (Fig. 1). This reference position was defined by orienting the pegs of the femoral component perpendicular to the tibial plate and placing the most distal points of the femoral condyles at the lowest points on the polyethylene articular surface. A fixed coordinate system was created for both the tibial and femoral components at the reference position. Using this coordinate system, we determined anterior–posterior, medial–lateral, and proximal–distal femoral translations as well as internal–external and varus–valgus tibial rotations [8, 13, 15].
Fig. 1

TKA components in their reference position. Femoral translations were measured from the point midway between the peg tips

The tibiofemoral contact location was determined by calculating the centroid of the overlap between the femoral component and the polyethylene surfaces in the medial and lateral compartments [14]. If no overlap was present, the point on the polyethylene surface nearest to the femoral condyle was used as the contact location. A previous study has shown that the imaging system has an accuracy of 0.16 mm for the femoral component and 0.13 mm for the tibial component, so lift-off was defined as the closest distance between the polyethylene and the femoral condyle being greater than 0.29 mm [8].

To quantitatively describe the contact locations, two coordinate systems were created for the articular contact in the medial and lateral compartments. The origins were midway between the anterior–posterior extremes of the polyethylene insert and 25% of the insert’s medial–lateral dimension from the medial–lateral extremes.

Component geometry and surgical technique

The geometry and surgical technique used for the conventional CR component have been discussed previously [14, 16]. The CR-Flex femoral component has a thicker posterior condyle than the conventional CR component (Fig. 2). An additional 2 mm of bone is removed from the posterior condyles to allow this increase in thickness without overstuffing the joint. This modification was made in order to increase the contact area between the femoral component and polyethylene articular surface at high flexion [13, 16]. Both designs were implanted through a medial arthrotomy. The femoral component was placed in 5° of valgus and 3° of external rotation using intramedullary alignment and the epicondylar axis. The posterior condyles and Whiteside’s line were used as secondary references. The tibial component was placed in 7° of posterior slope using an extramedullary guide. The tibial component was also externally rotated with the center of the tibial plateau, the junction of the medial and middle thirds of the tibial turberosity, and the tibial crest as references. Tension in the posterior cruciate ligament was assessed by manual palpation and by flexing the knee while checking for anterior lift-off of the tibial tray. The patella was resurfaced in all the patients, and all the components were cemented. Interrupted absorbable sutures were used to close the extensor mechanism and skin.
Fig. 2

Sagittal profile of NexGen’s conventional (solid) and high flexion (dashed) designs. By removing an additional 2 mm of bone from the posterior cut, the high flexion design maintains a smooth curvature through higher flexion

Data analysis

Patients in each group were averaged at hyperextension, in 15° intervals from 0° to 90° of flexion, and at maximum flexion of the implant [14]. The reported data at hyperextension and maximum flexion only included patients who achieved greater than 3° of hyperextension or 100° of flexion, respectively. A student’s t test with Bonferroni correction was used to compare the maximum flexion, 6DOF kinematics data, and the contact locations in the medial and lateral compartments between the CR group and the CR-Flex group. Differences were considered significant when P < 0.05.

Results

The passive ROM averaged 120.6° ± 11.1° for all the CR patients and 117.7° ± 9.7° for all the CR-Flex patients (Table 2). There was no significant difference between the two patient groups in passive ROM. During weight-bearing flexion, the average maximum flexion for all the CR patients was 110.1° ± 13.4°, and the average maximum flexion for all the CR-Flex patients was 108.2° ± 13.2°. There was no difference in maximum weight-bearing flexion between the two patient groups during the weight-bearing flexion.

6DOF kinematics of CR and CR-Flex TKA patients

The patient groups demonstrated similar posterior femoral translation throughout the flexion range (Fig. 3). The femoral component of the CR patients translated anteriorly from 1.1 ± 1.7 mm at hyperextension to −4.9 ± 2.5 mm at 45° of flexion and then translated posteriorly to 8.5 ± 5.3 mm at maximum flexion. In the CR-Flex group, the femoral component translated anteriorly from 2.7 ± 1.5 mm at hyperextension to −3.4 ± 2.3 mm at 45° of flexion and then translated posteriorly to 9.6 ± 4.3 mm at maximum flexion. No statistical difference was detected between the two patient groups in posterior femoral translation.
Fig. 3

Posterior femoral translation

The patient groups exhibited similar patterns of medial–lateral femoral translation. In the CR patients, the femoral component moved laterally from 0.5 ± 0.7 mm at hyperextension to −0.8 ± 0.9 mm at 45° of flexion and then medially to 0.1 ± 1.7 mm at maximum flexion. In the CR-Flex group, the femoral component moved laterally from −0.1 ± 0.6 mm at full extension to −1.5 ± 0.9 mm at 45° of flexion and then medially to 0.5 ± 1.6 mm at maximum flexion. No statistically significant differences were found between the groups.

The groups also showed similar varus–valgus patterns, starting from around 0.2 ± 0.5° at hyperextension, rotating varus to about 1.6° ± 0.5° at 30° of flexion, and then rotating valgus to 0.0° ± 1.9° at maximum flexion. The two groups also demonstrated similar patterns of internal tibial rotation (Fig. 4). In the CR patients, the tibia rotated internally from −0.2° ± 4.0° at hyperextension to 8.6° ± 5.8° at maximum flexion, and in the CR-Flex patients, the tibia rotated internally from 4.0° ± 4.5° at hyperextension to 11.2° ± 5.6° at maximum flexion. The tibia of the CR-Flex patients was generally more internally rotated compared to that of the CR patients. However, no difference was detected in varus–valgus or internal tibial rotation between the groups.
Fig. 4

Internal tibial rotation

Tibiofemoral contact kinematics of CR and CR-Flex patients

In the lateral compartment of the CR patients, the contact location moved posterior from −2.1 ± 5.4 mm to −8.1 ± 4.4 mm at early flexion and remained constant until maximum flexion, where it moved farther posterior to −15.2 ± 4.0 mm (Fig. 5). In the CR-Flex patients, the contact also moved posteriorly in early flexion, but moved anteriorly through mid-flexion, and then posteriorly again to −14.2 ± 4.0 mm at maximum flexion. No statistical difference was detected between the two patient groups.
Fig. 5

Tibiofemoral contact on the polyethylene for the CR (diamonds) and CR-Flex (crosses) components

In the medial–lateral direction, the lateral compartment contact of the CR group gradually moved laterally from −3.5 ± 3.6 mm at hyperextension to −7.9 ± 6.3 mm at maximum flexion. For the CR-Flex patients, the contact also moved laterally from −3.5 ± 8.1 mm at hyperextension to −5.9 ± 7.9 mm at maximum flexion. There was no difference in lateral compartment contact location between the two patient groups.

In the medial compartment, the contact location in the anterior–posterior direction remained relatively constant with flexion until maximum flexion for both groups. In the CR group, the medial compartment contact occurred at −2.0 ± 4.0 mm throughout the flexion range until maximum flexion, where the contact moved to −5.4 ± 9.1 mm. The medial compartment contact in the CR-Flex group remained at −2.0 ± 3.5 mm through early and mid-flexion and reached −4.2 ± 6.9 mm at maximum flexion. There was no difference between the groups in the AP location of the medial compartment contact.

The medial compartment contact location was also relatively constant in the medial–lateral direction throughout the entire flexion range for both groups. In the CR group, the contact remained around 4.5 ± 4.5 mm throughout the range of flexion. In the CR-Flex group, the contact was around 2.3 ± 5.0 mm. Again, no difference was found between the CR and CR-Flex knees.

Observation of tibiofemoral contact patterns of CR and CR-Flex patients

Lift-off occurred at maximum flexion in five knees in the CR group and three knees in the CR-Flex group. In the CR group, there were three knees with lift-off only in the lateral compartment, one knee with lift-off only in the medial compartment, and one knee with lift-off in both compartments. In the CR-Flex group, there was one knee with lift-off only in the lateral compartment, one knee with lift-off in the medial compartment, and one knee with lift-off in both compartments. The average maximum flexion for these eight knees with lift-off was 112.8 ± 13.7, which was 5° greater but not statistically different from that of knees with no lift-off (107.7 ± 12.9).

At low flexion angles, the tibiofemoral articulation was similar for both the CR and CR-Flex patients. For example, at 75° of flexion, the articulating surfaces around the contact location were very conforming for both the CR and CR-Flex components. However, at maximum flexion angles, the CR components had a different articulation compared to that of the CR-Flex components. This observation is illustrated in Fig. 6, which shows the articulation of a CR patient and a CR-Flex patient at 131.4° and 131.1°, respectively. In the CR TKA, the femoral condyle tip came into contact with the polyethylene surface. In the CR-Flex TKA, the femoral surface in contact with polyethylene surface is much more conforming than in the conventional design.
Fig. 6

Cross-sections of the tibiofemoral articulation at 130° of flexion in a a CR and b CR-Flex patient. With the CR design, the tip of the femoral component is contacting the polyethylene. With the CR-Flex design, the smooth articular surface of the femoral component remains in contact with the polyethylene

Discussion

Despite the debate over the need and efficacy of high flexion components [17, 18, 19, 20], many new components have been used clinically with the aim of enhancing the flexion capability of the knee after TKA [24]. However, previous studies have only compared high flexion TKA designs to conventional designs in a posterior substituting knee [2, 6, 7, 10, 11, 22]. This study investigated either the 6DOF knee kinematics of patients after TKA using a conventional cruciate retaining component (NexGen, CR) or a high flexion cruciate retaining component (NexGen, CR-Flex).

In this study, patients with specially designed high flexion components behaved similarly kinematically to those with conventional implants. There was no difference in posterior femoral translation throughout the entire flexion range. For the CR patients, the tibiofemoral contact moved 4.5 mm posteriorly in the medial compartment and 13.0 mm in the lateral compartment during the weight-bearing lunge, indicating a “medial pivot” motion of the knee during flexion. For the CR-Flex patients, the tibiofemoral contact moved 5.1 mm posteriorly in the medial compartment and 8.1 mm in the lateral compartment. There were no dramatic differences in the contact positions during knee flexion between the two patient groups. The CR-Flex knees showed approximately 3° greater internal tibial rotation than the CR knees throughout the flexion range, although this difference was not statistically significant.

Besides the similarity in kinematics, the two patient groups had similar flexion under both passive and weight-bearing conditions. However, the tibiofemoral contact behavior was different between the components at high flexion angles (>120°). Figure 6 showed the tibiofemoral contact patterns of a CR patient and a CR-Flex patient at 130° of flexion. At this flexion angle, the condylar tip of the conventional CR TKA was in contact with the polyethylene surface. This could cause a stress concentration on the polyethylene surface and lead to increased wear in patients who attain high flexion. However, at the same flexion angle, the articulating surface of the CR-Flex component was much more conforming compared to the conventional CR design. The increased conformity would help to reduce any potential high stresses experienced by the polyethylene at high flexion. This improvement in contact can be explained by the thicker posterior femoral condyle of the CR-Flex design (Fig. 2). The increased thickness of the femoral condyle allows for a larger radius of curvature at higher flexion angles, which translates into more conforming surfaces between femoral and polyethylene components at high flexion. Therefore, this high flexion total knee design seems to have improved the articular contact mechanics when the knee is able to achieve high flexion. This observation supports a previous prediction based on radiographs at full flexion, which suggested that the high flex designs had better contact area [11].

It should be noted that the data obtained in the present study for the conventional implant is similar to published data for other cruciate-retaining components. Previous studies have reported passive maximum flexion values between 100° and 120° [1, 3, 4, 5, 25]. The current study found an average weight-bearing maximum flexion of 110° and a mean passive ROM around 120°, which are within the range of the data reported in literature.

In the literature, most studies on high flexion TKA patients consist of Asian cohorts and focus on the passive range of motion of PS TKA designs [6, 7, 10, 11, 22, 23]. There are inconsistent conclusions when comparing the flexion capability of patients with conventional implants and high flexion implants. For example, Bin et al. [6] compared 90 conventional LPS knees to 90 matched LPS-Flex (Zimmer) knees at 1 year postoperatively. They found the LPS-Flex knees to have more ROM (129.8° ± 5.2°) than the conventional knees (124.3° ± 9.2°). Huang et al. [10] also found LPS-Flex knees to have about 10° more flexion than LPS knees at 2 years follow-up. Gupta et al. [7] compared a conventional rotating platform posterior stabilized design (P.F.C. Sigma RP, Depuy) to high flexion version of the same component (P.F.C. Sigma RP-F). They reported that the patients with a high flexion design gained significantly more ROM from preop to postop (110°–125°) than the patients with the conventional design (110°–116°). Kim et al. [11] compared LPS-Flex to LPS in 50 bilateral patients and did not find a difference in ROM between the components (139 vs. 136°). Seon et al. [22] compared LPS-Flex to a mobile bearing CR design (e-motion, B. Braun-Aesculap) and found no difference in maximum flexion (131 vs. 129°). Few studies have reported the maximum flexion of PS TKAs during weight-bearing flexion. In a recent report, we found a maximum weight-bearing flexion of 113° in a group of patients replaced with a LPS-Flex component [23]. The patients with CR or CR Flex components investigated in this study were found to have a similar maximal flexion compared to those patients with LPS Flex components.

One limitation of the current study is that the condition of the polyethylene surface that could not be directly analyzed due to the in vivo nature of the experiment. As contact behavior was revealed different between the two-implant designs at high flexion, it would be clinically interesting to examine the wear modes and patterns of their polyethylene components. In the future, this can be studied using retrieved polyethylene components from revision patients who used one of the two implants.

In conclusion, the kinematics of the CR-Flex patients analyzed in this study was similar to those of the patients with a conventional CR component. No difference was seen in the maximum flexion achieved by the patients, and the kinematics demonstrated by the groups was comparable. Use of this high flexion component did appear to improve tibiofemoral conformity at high flexion in patients that could achieve high flexion. Further analysis is necessary to determine if the longevity of the polyethylene is indeed improved with a high flexion component.

Acknowledgments

This study was supported by a research grant from Zimmer Inc. The guidance of Dr. Harry Rubash and Dr. Andrew Freiberg and the technical assistance of Elizabeth Desouza and Angela Moynihan were greatly appreciated. The experiments performed in the course of this study comply with the laws of the United States.

Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Jeremy F. Suggs
    • 1
    • 2
    • 3
  • Young-Min Kwon
    • 1
  • Sridhar M. Durbhakula
    • 1
  • George R. Hanson
    • 1
  • Guoan Li
    • 1
  1. 1.Bioengineering Laboratory, Department of Orthopaedic SurgeryMassachusetts General Hospital, Harvard Medical SchoolBostonUSA
  2. 2.Department of Mechanical EngineeringMassachusetts Institute of TechnologyCambridgeUSA
  3. 3.Exponent, Inc.PhiladelphiaUSA

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