Effect of single-radius design on in vivo kinematics during stair activities after total knee arthroplasty

Abstract

Background

The single-radius design is one of the major total knee arthroplasty (TKA) designs and widely used all over the world. The objective of this study was to compare in vivo kinematics between the anteroposterior (AP) single-radius design with mediolateral (ML) single-radius (Non Restricted Geometry; NRG) and ML dual-radius (Triathlon) during stair activities.

Methods

A total of 21 knees in 18 patients (NRG group: 10 knees in 7 patients, Triathlon group: 11 knees in 11 patients) with a clinically successful posterior stabilized TKA were examined. Under fluoroscopic surveillance, each patient performed stair ascending and descending motions. In vivo kinematics were analyzed using 2D/3D registration technique. The knee flexion angle, rotation angle, varus-valgus angle, AP translation of the femorotibial contact point for both the medial and lateral sides of the knee, and post-cam engagement were evaluated.

Results

There were no significant differences between the two groups in rotation angle and AP translation at each flexion angle. Examining the varus-valgus angle, the NRG group showed varus position at an early flexion angle during both stair activities.

Post-cam engagement was observed in both groups during both stair activities. The mean flexion angle of engagement in the NRG group, the post of which was located anterior to the Triathlon, was larger than that in the Triathlon group during both stair activities.

Conclusion

Despite the same AP single-radius TKA, ML single-radius might affect varus motion at an early flexion angle.

Keywords

total knee arthroplasty kinematics single-radius stair activity

Introduction

Total knee arthroplasty (TKA) is one of the most common, elective surgical interventions for the treatment of severe knee osteoarthritis and inflammatory arthritis.^1–3 Current literature reports only a 10% dissatisfaction rate. Preoperative factors like less severe arthritis, depression, and pain catastrophizing, and postoperative factors include complications, unmet needs, stiffness, and persistent pain.4 On the other hand, patients suffering from instability after TKA typically complain about problems when performing activities that incur significant transverse or torsion forces in the knee joint, such as stairs activities and walking on sloped or uneven surfaces.^5,6 Evaluation of the kinematics during activities of daily living is important to improve satisfaction after TKA in the future.

Previous studies have evaluated the TKA kinematics of stair motions using video fluoroscopy.^7–11 However, these studies focused on stair-stepping that used a single leg, not swinging through with the opposite leg. Previously, an in vivo three-dimensional (3D) kinematics analysis system for TKA based on a two-to three-dimensional (2D/3D) registration technique was developed,^12,13 and this has enabled patients to be allowed motion flexibility and crossleg motion, as in stair ascending and descending. Investigating the knee kinematics of daily motion in detail can provide useful information about implant design.

The Scorpio Non Restricted Geometry (NRG) (Stryker Orthopaedics, Mahwah, NJ) is a knee joint prosthesis that has high rotatory degree of freedom because of mediolateral (ML) single-radius femoral component design in addition to an anteroposterior (AP) single-radius. Several studies have reported good kinematics and clinical results of AP single-radius implants during squatting and gait activities.^14–17 However, few studies have evaluated the varus-valgus angle to investigate the influence of the ML single-radius design.

Triathlon (Stryker Orthopaedics) is a knee joint prosthesis that has an AP single-radius with ML dual-radius design, and the insert design is different from that of the NRG. Several studies reported that this implant provides good stability in mid-flexion ranges during squatting.^18,19 However, the kinematics during stair activities remains uncertain.

The objective of this study was to compare kinematics between the NRG and the Triathlon during stair activities, and to evaluate the influence of the articular design in AP single-radius TKA (Figure 1).

Figure 1.

Component design. The Scorpio NRG has an AP single-radius design with an ML single-radius. On the other hand, the Triathlon has an AP single-radius with an ML dual-radius.

Methods

From November 2003 to November 2010, total of 338 NRG PS TKAs were operated and from December 2009 to December 2014, total of 182 Triathlon PS TKAs were operated by the same surgical team. Among these patients 18 patients agreed to participate in the current investigation with institutional review board approval with documents. Finally 21 knees in 18 patients (NRG group: 10 knees in 7 patients, Triathlon group: 11 knees in 11 patients) with a clinically successful posterior stabilized (PS) TKA resulting in a knee society score (KSS) higher than 90 were examined. The stair activity score in KSS is 4.6 (standard deviation (SD) 0.5) in the NRG group, and 4.4 (SD 1.0) in the Triathlon group. At the time of fluoroscopic analysis, mean duration of postoperative follow-up was 8.1 months (range, 3 to 14 months) in NRG group and 8.4 months (range, 3 to 12 months) in Triathlon group.

Under fluoroscopy, each patient performed stair ascending and descending motions at a natural pace. A two-step staircase that was 15 cm high and 30 cm deep was used, and the first step was imaged. The sequential motion was recorded as digital X-ray images (1024 × 1024 × 12 bits/pixel, 7.5-Hz serial spot images as a DICOM file) using a 17-inch flat panel detector system (C-vision Safire L; Shimadzu, Kyoto, Japan). The patients practiced the motion several times before recording.

Femorotibial motion was analyzed using a 2D/3D registration technique, which uses computer-assisted design (CAD) models to reproduce the spatial position of the femoral and tibial components from single-view fluoroscopic images.^12,13,20 The accuracy of estimating relative motion between metal components was 0.5° or less in rotation and 0.4 mm or less in translation.¹²

The images of the ground touch of the TKA leg on the first step were identified, and the following four phases were selected: (1) at foot strike (FS) (non-weight-bearing); (2) during the stance phase before crossleg motion (full weight-bearing); (3) during the stance phase after crossleg motion (still weight-bearing); and (4) at foot off (FO) (non-weight-bearing).²¹ The knee flexion angle, rotation angle, varus-valgus angle, AP translation of the femorotibial contact point for both the medial and lateral sides of the knee, and post-cam engagement were evaluated. A local coordinate system at the component was constructed according to the previous study.²¹ The knee flexion angle, rotation angle, and varus-valgus angle were described using the joint rotational convention of Grood and Suntay.²² Flexion, external rotation, and valgus of the femoral component relative to the tibial component were denoted as positive. Positive or negative values of AP translation were defined anterior or posterior to the axes of the tibial component. The femorotibial contact was visualized as the region on the insert surface where the proximity was less than the 0.5-mm threshold.¹² Post-cam engagement was determined by the intersection of the CAD model surfaces of the femoral cam and the tibial post. Measurement results were analyzed statistically using the Mann-Whitney U-test. Values of p < .05 were considered significant.

Results

Patient background characteristics

There were no significant differences in the patient background characteristics. All knees had medial osteoarthritis (Kellgren and Lawrence grade Ⅳ) in the NRG group, while 9 knees had medial osteoarthritis (Kellgren and Lawrence grade Ⅳ) and 2 knees had inflammatory arthritis (Larsen grade Ⅲ) in the Triathlon group. (Table 1).

Table 1.

Patient background.

	NRG group	Triathlon group	p-value
Mean age at the time of surgery	75.7 years (71 - 82)	71.2 years (53 - 80)	n. s
Mean body height	159.8 cm (148.0 - 168.0)	154.5 cm (152.8 - 156.0)	n. s
Mean SMD	85.5 cm (79.7 - 95.0)	85.1 cm (81.5 – 90.3)	n. s
Mean body weight	60.5 kg (58.0 - 71.0)	63.7 kg (51.9 - 81.7)	n. s
Male: Female	3:7	0:11	n. s
OA:IA	10:0	9:2	n. s

SMD: spina malleolar distance; OA: osteoarthritis; IA: inflammatory arthritis n. s.: not significan.

X-ray evaluation

The radiographic component position was evaluated following the Knee Society TKA Roentgenographic Evaluation. There were no significant differences in the radiographic component positions. (Table 2).

Table 2.

X-ray evaluation.

	NRG group	Triathlon group	p-value
α angle	95.2 (SD 1.8)°	95.3 (SD 1.3)°	n. s
β angle	89.7 (SD 1.1)°	89.7 (SD 1.3)°	n. s
γ angle	5.3 (SD 4.8)°	3.0 (SD 1.0)°	n. s
Tibial posterior slope	3.7 (SD 1.5)°	3.2 (SD 1.9)°	n. s

SD: standard deviation, n. s.: not significant.

Knee flexion, rotation, and varus-valgus angle

During stair descending, the knees were gradually flexed from FS to FO. The mean flexion angle at FS was significantly smaller in the NRG group than in the Triathlon group. During stair ascending, (Figure 2) the knees were gradually extended from FS to FO. The mean flexion angle at FS was significantly smaller and the mean flexion angle during the stance phase before crossleg motion was significantly larger in the NRG group (Figure 2(a)).

Figure 2.

Flexion, rotation, and varus-valgus angles (a) Flexion angle. The asterisk indicates p < .05. (b) Rotation angle. There is no significant difference between the two groups during stair motion. (c) Varus-valgus angle. In the NRG group, varus position is observed at FS during stair descending and from the stance phase after crossleg motion to FO during stair ascending (p < .05). (d) Varus-valgus angle at each flexion angle. There are significant differences at a shallow flexion angle (p < .05).

Regarding rotation angle, there were no significant differences between the two groups (Figure 2(b)). However, the mean flexion angle was different between the two groups, so further investigation was done at each flexion angle, but there were no significant differences.

In the NRG group, varus position was observed at FS during stair descending and from the stance phase after crossleg motion to FO during stair ascending (Figure 2(c)). In the investigation at each flexion angle, there were significant differences at the early flexion angle (Figure 2(d)). The differences between the maximum varus-valgus angle and the minimum varus-valgus angle during stair descending were 2.1 (SD 2.4) ° in the NRG group and 1.5 (SD 1.8) ° in the Triathlon group. There was no significant difference between the two groups. The differences between the maximum varus-valgus angle and the minimum varus-valgus angle during stair ascending were 3.4 (SD 3.5) ° in the NRG group and 1.2 (SD 0.52) ° in the Triathlon group; both were significant differences.

AP translation

There was no significant difference between the two groups during stair descending. On the other hand, the medial contact point in the Triathlon group was significantly posterior compared to the NRG group at FS during stair ascending. However, in the investigation at each flexion angle, there were no significant differences between the two groups (Figure 3).

Figure 3.

AP translation at each flexion angle. There are no significant differences between the two groups.

Post-cam engagement

During stair descending, post-cam engagement was observed in 2 knees (20.0%) from the stance phase after crossleg motion in the NRG group. On the other hand, 7 knees (63.6%) in the Triathlon group were engaged. The mean flexion angle of the engagement was 62.1 (SD 2.4) ° in the NRG group and 37.9 (SD 10.0) ° in the Triathlon group. During stair ascending, post-cam engagement was observed in all knees from FS. The mean flexion angle of the engagement was 58.7 (SD 10.3) ° in the NRG group and 34.2 (SD 16.9) ° in the Triathlon group. The mean flexion angle of the engagement in the NRG group was larger than that in the Triathlon group significantly during both stair activities. (Figure 4).

Figure 4.

Post-cam engagement. The upper row shows the NRG group, and the lower row shows the Triathlon group.

Discussion

In vivo 3D kinematics during stair activities were investigated among AP single-radius designs. NRG has an ML single-radius design, while Triathlon has an ML dual-radius design. Since stair activities are necessary in daily living, it is important to evaluate how an ML single-radius design that has a high rotatory degree of freedom influences the kinematics during stair activities.

During stair descending, the knees were gradually flexed from FS to FO in both groups. On the other hand, during stair ascending, the knees were gradually extended from FS to FO in both groups (Figure 2(a)). The previous study that investigated another implant during stair activities reported similar bending motions.²¹ The mean flexion angle at FS was significantly smaller in the NRG group than in the Triathlon group during both stair motions. The patients’ mean height and spina malleolar distance (SMD) were not significantly different between the two groups, so this suggested that the Triathlon group might accommodate a higher staircase.

There was no significant difference between the two groups in the rotation angle. However, the standard deviation was large in the NRG group. This suggested that the ML single-radius design had a high rotatory degree of freedom (Figure 2(b)).

In the NRG group, varus position was observed at phases without post-cam engagement, which were influenced mainly by the articular surface (Figure 2(d)). This suggests that ML single-radius TKA influenced the kinematics during stair activities. However, the differences between the two groups were less than 1 degree, so that this might not be clinically useful. Jo et al. reported that a single-radius implant was more stable than a multi-radius implant in the varus-valgus angle during mid-flexion under anesthesia, but the differences were less than 1 degree, so it might not be clinically useful, as in the present study.²³ The previous study that analyzed in vitro kinematics using cadavers reported that instability in mid-flexion was caused not by the implant design, but by the ligament laxity.²⁴ However, the present study analyzed in vivo kinematics, so it might reflect daily activities more accurately.

Regarding AP translation at each flexion angle, there was no significant difference between the two groups. This suggested that the AP single-radius influenced both stair activities. Some previous studies reported that AP single-radius TKA displayed few AP translations during stair activities,^15,25 and this might also be caused by the AP single-radius. However, in the previous study of Tamaki et al, AP single-radius TKA showed a pivot pattern in mid-flexion during squatting.¹⁴ Therefore, despite the same mid-flexion, the kinematics might be different with different kinds of daily activities.

The mean flexion angle of post-cam engagement was larger in the NRG group than in the Triathlon group. The predicted post-cam engagement angles as the implant designs are 65 degrees in the NRG and 45 degrees in the Triathlon. The results of this study were close to the predicted post-cam engagement angles. However, regarding AP translation, there was no significant difference between the two groups around each post-cam engagement angle. We have reported that femoral posterior sliding has been observed during stair ascending and descending in AP multi-radius designs.²¹ These facts suggested that the post-cam engagement didn’t affect AP translation among the AP single-radius designs during stair activities.

During stair ascending, the differences between the maximum varus-valgus angle and the minimum varus-valgus angle were significantly smaller in the Triathlon group than in the NRG group. In addition, the mean flexion angle of post-cam engagement was smaller in the Triathlon group than in the NRG group. This suggested that the Triathlon is a more kinematically stable design than the NRG during stair ascending.

Previous studies reported that gradual-radius TKA, ultra-congruent TKA, and medial-pivot TKA provides stable AP kinematics.^26–29 In contrast, rotating platform mobile-bearing TKAs showed paradoxical anterior motion at early-flexion although they showed stable AP translation at mid-flexion.³⁰ Regarding anatomical articular surface design, bicruciate stabilized TKA (BCS TKA) showed more anteriorly located in the medial contact point, and the femoral component was more externally rotated in comparison to multi-radius PS TKA.³¹ In addition, a previous cadaveric study demonstrated that bicruciate retaining TKA with anatomical articular surface showed a better approximation of native kinematics compared with BCS TKA. Further studies that evaluate various designs might be needed.

There are several limitations to this study. First, only limited cases were analyzed, and therefore, the motions of implants might not be completely reproduced. Second, PS designs were investigated, so the difference between the positions of the posts might affect the condylar radius. Third, this study did not evaluate stress radiographs. The NRG group showed a large varus/valgus angle and the difference, it might be possible that the NRG group had more coronal laxity. Fourth, only stair activities were analyzed in this study. Further kinematic analyses during various activities of daily living might be needed for further improvement of the clinical outcome.

Conclusion

Despite the same AP single-radius TKA, an ML single-radius design might influence varus motion at an early flexion angle. Additionally, ML single-radius design is capable of having large coronal laxity.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Kenichi Kono

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