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Abstract

Purpose:

The main objective of this study was to compare quality of life and functional outcome in patients who have undergone a single-radius (SR) or multi-radius (MR) total knee arthroplasty (TKA). The secondary objective was to observe changes in knee range of movement (ROM) and standardized knee scores (KSCs) in these patients. The hypothesis was that there would be no statistically significant difference between the two patient groups in quality of life and functional outcome.

Methods:

One hundred three SR TKAs were performed by a single surgeon between August 2008 and December 2012. A propensity score matching algorithm was used to select 103 MR TKAs performed during the same period. Preoperative and postoperative variables such as standardized knee and quality of life scores were captured prospectively and then analyzed via both the Student’s t-test and paired t-test to look for statistically significant differences between the SR and MR patient groups.

Results:

At 2 years postoperatively, there was no statistically significant difference between the SR and MR patient populations in knee extension, Oxford Knee Score, Knee Society Clinical Rating Scores, and the Physical Component Summary of the Short Form 36 Health Survey (SF-36). There was a statistically significant difference between the two patient groups in postoperative knee flexion in favor of the MR design (p = 0.011).

Conclusion:

While an SR femoral implant design has several theoretical biomechanical advantages, postoperative standardized KSCs and quality of life scores in this single-surgeon series do not show a clear advantage of one design over the other.

Level of evidence:

III.

Keywords

functional outcome multi-radius quality of life single radius total knee arthroplasty

Introduction

Total knee arthroplasty (TKA) remains at the forefront of management of degenerative osteoarthritis of the knee, with studies showing a 95% survival rate at 10–15 years postoperatively and a satisfaction rate of up to 90%.^1
–3 With the aim of maximizing patient satisfaction and recreating biomechanics and kinematics of native knees, manufacturers have introduced femoral components of both single-radius (SR) and multi-radius (MR) designs.^4,5

The “radius” is generally defined as “the distance from the flexion/extension axis to the contact point between the femoral and tibial components of the implant.”⁶ Currently, the MR femoral designs provide the J-shaped curve of the normal knee joint, which has a large axis in the front and a decrease in the back at the sagittal plane. This results in MR designs having at least two instantaneous centers of rotation within the functional knee range of motion, driven by the changing radius of curvature of the femoral component.⁷ This is as opposed to the SR femoral design, which has an SR throughout the functional flexion arc.^8
–10

There have been multiple studies discussing the theoretical advantages and disadvantages of SR versus MR designs. The rationale for the MR design was to recreate the geometry of the natural knee joint. It was believed that multiple instantaneous centers of knee flexion/extension rotation existed in normal knee joints; hence, traditional knee replacement designs utilize multiple discrete radii to define the sagittal plane curvature of the femoral component.^7,11 However, due to the differing radii of curvature within an MR design, authors have postulated that abruptly shifting from a longer to shorter radius during knee flexion may create temporary varus–valgus knee instability, whereas the fixed main axis of the SR design theoretically creates consistent collateral ligament tension and muscle moment arms throughout the functional range of motion.^12,13 The concept of “mid-range instability” has been described by several authors examining both the kinematics and stability of SR and MR femoral components, as well as extensor mechanism function post-TKA; the SR femoral design has been reported to require less quadriceps torque for knee extension.^14

–18 To date, no single design has consistently demonstrated statistically significant superiority over the other.^19
–21

Less research, however, has been dedicated to the comparison of clinical outcomes between SR and MR designs. While biomechanical studies have examined quadriceps force and varus–valgus limits of laxity, this article aims to compare outcomes via standardized knee scores (KSCs) as well as quality of life post-TKA in patients who underwent surgery performed by a single high-volume surgeon. The authors hypothesize that there will be no statistically significant differences in quality of life and standardized KSCs of SR and MR femoral implant designs.

Materials and methods

Participants

Between August 2008 and December 2012, 103 unilateral TKAs were performed at a single institution by a single high-volume orthopedic surgeon using SR implants and then prospectively followed up for a minimum of 2 years. The follow-up rate was 100%. A propensity score matching algorithm using age, gender, and preoperative body mass index (BMI) as potential confounders of functional outcome and quality of life after TKA was used to select 103 patients who underwent unilateral TKA with MR implants during the same period.

The study was approved by the local Institution Review Board (CIRB 2016/2645). A retrospective review of the data collected was performed.

Indication

All TKAs were performed for patients with noninflammatory primary osteoarthritis (reported radiographically as Kellgren–Lawrence grades 3–4) of the knee. Exclusion criteria included bilateral TKA, revision knee arthroplasty, inflammatory arthritis, post-traumatic osteoarthritis as well as patients who required constraint implants.

Components and technique

All TKAs were performed by one senior fellowship-trained Adult Reconstruction surgeon (PLC). All TKAs were performed using the standard medial parapatellar quads split approach without patella eversion under tourniquet control at 300 mmHg. The surgical aim was to achieve neutral coronal alignment with a 0° mechanical axis and femoral rotation aligned to the transepicondylar axis and checked using Whiteside’s line.^22,23

The MR knee implants used were the Zimmer Nexgen LPS-Flex implants, while the SR knee implants used were the Stryker Scorpio NRG implants. All implants were of posterior cruciate-substituting and fixed-bearing design. Closure of wounds was performed using subcuticular ETHICON MONOCRYL^™ 3-0 suture (Livingston, UK).

In accordance with our standard institution TKA protocol for postoperative care, all patients underwent physiotherapy with the aim of early mobilization. The general structure of physiotherapy was standardized (spread out over a duration of approximately 12 weeks) with the aim of reducing pain, increasing strength, and gaining independence in ambulation. However, elements of the rehabilitation protocol (e.g. duration) were modified accordingly for individual patients based on their improvement and progress.

Postoperative follow-up and data collection

A trained physiotherapist evaluated the patients preoperatively, at 6 months and at 2 years after TKA. Two-time points were used to measure changes and improvements in surgical outcome.

Preoperative patient-specific variables captured were patient’s age, gender, BMI, and knee range of motion (ROM). Other preoperative variables captured were validated KSCs, such as the Oxford Knee Score (OKS), functional score (FSC), and KSC from the Knee Society Clinical Rating Score (KSS), as well as patients’ quality of life assessed using the Short Form 36 Health Survey (SF-36).

The OKS is a 12-item questionnaire specifically designed and developed to assess function and pain after knee arthroplasty.²⁴ The KSS is a different knee rating system subdivided into a KSC that rates only the knee joint itself and an FSC that rates the patient’s ability to walk and climb stairs. This dual rating system was developed to eliminate the problem of declining KSCs associated with patient infirmity.²⁵

To assess the impact of the procedure on each patient’s quality of life, individual components of the SF-36 were captured, and the composites Physical Component Score (PCS) and Mental Component Score (MCS) were calculated. The SF-36 is a multipurpose short-form health survey which yields physical and mental health summary measures; the PCS correlating well with physical functioning and bodily pain, and the MCS correlating well with mental health and social functioning.²⁶

Postoperative variables captured included the OKS, FSC, KSC, ROM as well as the composites PCS and MCS computed from SF-36 scores.

Statistics

Propensity score matching model (by nearest-neighbor matching) using age, gender, and preoperative BMI as potential confounders of functional outcome and quality of life after TKA was carried out to select for 103 patients with SR implants and 103 patients with MR implants.

The Student’s unpaired t-test was used to compare the two groups for age, BMI, knee ROM, and standardized KSCs both preoperatively and postoperatively. Level of significance was set at p < 0.05.

Power analysis was performed prior to the conduct of this study. The minimal clinically important difference in PCS was found to be 4.5 and 4.8 points for pain relief and function, respectively.²⁷ At 2 years after TKA, to detect a difference of five points in PCS from a baseline score of 49 with SD = 11, a sample size of at least 103 patients in each group would be required to achieve a power of 0.90. This calculation was done for a two-sided test with a type I error of 0.05. After including those patients who underwent a unilateral TKA between August 2008 and December 2012, there were 103 patients in the SR group.

All statistical analyses were carried out with R, version 3.3.1.²⁸

Results

Patients

The mean preoperative BMI, knee extension, knee flexion, and standardized KSCs are reported for patients with implants of SR design as well as those of MR design. Table 1 shows that there are no statistically significant differences in the above-mentioned preoperative variables between the patients with SR implants and those with MR implants.

Table 1.

Patient demographics and clinical parameters at baseline.

	SR design (N =103)		MR design (N = 103)		Comparison
Demographics	Mean	SD	Mean	SD	p-Value for two sample t-test
Age	66	9	66	9	0.828
BMI	28	4	28	5	0.872
Preoperative extension of knee (°)	9	8	9	8	0.913
Preoperative flexion of knee (°)	117	19	116	19	0.876
Preoperative OKS (points)	23	8	25	8	0.168
Preoperative KSC (points)	32	17	36	17	0.123
Preoperative FSC (points)	49	19	53	18	0.144
Preoperative PCS (points)	30	12	32	11	0.268
Preoperative MCS (points)	51	11	52	10	0.535

SD: standard deviation; SR: single radius; MR: multi-radius; BMI: body mass index; OKS: Oxford Knee Score; FSC: functional score of Knee Society Clinical Rating Score; KSC: knee score of Knee Society Clinical Rating Score; PCS: Physical Component Score of Short Form 36 Health Survey Score; MCS: Mental Component Score of Short Form 36 Health Survey Score.

Clinical outcomes

Absolute values for postoperative knee ROM as well as standardized KSCs and SF-36 composite scores at 6 months and 2 years are demonstrated in Table 2. These variables were also compared using a two-sample t-test to look for significant differences in postoperative variables between the two study groups at these two different points in time.

Table 2.

Clinical parameters at 6 months and 2 years postoperatively.

	SR design (N = 103)		MR design (N = 103)		Comparison
Demographics	Mean	SD	Mean	SD	p-Value for two sample t-test
Postoperative extension of knee (°) at 6 months	6	6	4	4	0.037
Postoperative flexion of knee (°)at 6 months	110	17	118	15	<0.001
Postoperative OKS (points) at 6 months	39	6	40	5	0.092
Postoperative KSC (points) at 6 months	79	14	82	11	0.079
Postoperative FSC (points) at 6 months	66	16	69	17	0.216
Postoperative PCS (points) at 6 months	46	11	47	10	0.397
Postoperative MCS (points) at 6 months	55	9	54	11	0.521
Postoperative extension of knee (°) at 2 years	2	5	2	5	0.572
Postoperative flexion of knee (°) at 2 years	115	15	120	15	0.011
Postoperative OKS (points) at 2 years	41	4	41	5	0.709
Postoperative KSC (points) at 2 years	84	12	85	13	0.811
Postoperative FSC (points) at 2 years	72	17	72	19	1.000
Postoperative PCS (points) at 2 years	49	10	47	11	0.174
Postoperative MCS (points) at 2 years	55	8	56	10	0.425

SR: single radius; MR: multi-radius; OKS: Oxford Knee Score; FSC: functional score of Knee Society Clinical Rating Score; KSC: knee score of Knee Society Clinical Rating Score; PCS: Physical Component Score of Short Form 36 Health Survey Score; MCS: Mental Component Score of Short Form 36 Health Survey Score.

At 6 months postoperatively, it was noted that the group of patients with MR implants reported better knee range of motion in both extension and flexion (Table 2). The patients in the MR implant group had postoperative knee flexion of 118 ± 15° as compared to the patients in the SR implant group, who had postoperative knee flexion of 110 ± 17° (p < 0.001).

At 2 years post-TKA, there were no statistically significant differences in knee extension as well as OKS, FSC, KSC, PCS, and MCS between the two groups of patients (SR and MR). In contrast, the difference in postoperative flexion between the two study groups remained statistically significant at 2-year follow-up, with the MR group having 120 ± 14° of postoperative flexion and the SR group having 116 ± 14° of postoperative flexion (p = 0.002).

Five patients from the SR group (0.05%) had major postoperative complications, with two cases of persistent knee pain requiring revision surgery, one case of knee hematoma requiring aspiration, one case of postoperative stiffness requiring manipulation under anesthesia and arthroscopic debridement, and one case of aseptic loosening requiring revision surgery. Three patients from the MR group (0.03%) had major postoperative complications, with one case having postoperative stiffness requiring arthroscopic debridement, one case of wound dehiscence requiring wound exploration and closure, and one case of deep vein thrombosis requiring anticoagulation. The difference in complication rate was not statistically significant (p < 0.05).

Discussion

To date, TKA remains one of the most successful and commonly performed elective procedures in orthopedic surgery. With clear evidence regarding its positive impact on patient satisfaction, an aging population is likely to result in an increase in demand for arthroplasty.^29
–31 Hence, there is a need to continuously evaluate the implications of adjustments in implant design and arthroplasty technique.

The main finding from our study is that despite the purported biomechanical advantages of an SR femoral implant design, there is no statistically significant difference between the two patient populations in chosen standardized KSCs as well as postoperative quality of life. Previously described advantages such as a decrease in patellar load due to increased extensor moment arm, as well as better ligament stability based on maintained isometry throughout knee flexion/extension, do not appear to significantly impact patients in quality of life and physical function at the 2 years postoperative mark.^12

–18

A recent meta-analysis by Liu et al. examined the differences between SR and MR designs with regard to postoperative KSS, ROM, complications as well as survival rate. The meta-analysis concluded that the SR prosthesis in TKA is not significantly different from the MR prosthesis in terms of KSC, FSC, active maximal flexion, passive maximal flexion, complications, and survival rate.³²

Jo et al. assessed postoperative clinical outcome of 58 patients with SR implants and 58 patients with MR implants via passive ROM, Hospital for Special Surgery (HSS) score, Western Ontario McMaster Universities Osteoarthritis Index (WOMAC), and the Visual Analogue Scale (VAS) of anterior knee joint pain during stair climbing. The study found that at the end of a 24–48 month follow-up period, there was no statistically significant difference between the two patient groups in terms of mean HSS score, WOMAC score, VAS score, and ROM.⁸

This study has similarly demonstrated that postoperative standardized KSCs such as the KSC and FSC, as well as knee ROM, are not significantly different between the SR and MR prostheses at 2 years postoperatively. Of note, this study has attempted to expand beyond clinical-reported scores (such as the KSS), capturing patient-reported outcome measures (PROMs) such as OKS, PCS, MCS, and SF-36 scores.

While the SR design has been reported to have biomechanical benefits advantageous in day-to-day activities,^8,17 with improved stability, walking, and stair climbing demonstrated by Cook et al.,³³ this study did not find any statistically significant difference in PCS or MCS between the two study groups.

This study did, however, note that there was a statistically significant difference in postoperative knee flexion at 2 years, with the MR design (Zimmer Nexgen LPS-Flex), achieving greater knee flexion (mean of 4°) as compared to the SR design. The Zimmer Nexgen LPS-Flex implant has been shown to allow greater degrees of postoperative flexion compared to other mobile-bearing designs, with the posterior condyle of the femoral component made thicker and more rounded to allow the tibia to roll back further in deep flexion.³⁴ However, in agreement with findings from Tarabichi et al., this finding had no impact on postoperative KSC or FSC. Other studies examining postoperative knee flexion have also revealed that improved knee flexion does not necessarily correlate with improved clinical outcomes.^35
–37 However, the caveat is that standardized tools such as the KSS may not be adequately sensitive in detecting minute changes in patients’ functional outcomes.³⁸

To the best of our knowledge, this is the first single-surgeon study that has compared both postoperative physical and mental component scores of the SF-36 questionnaire between the two patient populations. This element is important as the literature has demonstrated a discrepancy between clinician and patient ratings of quality of life, with satisfaction relying heavily on fulfillment of expectations rather than absolute function.^39
–41 At the same time, it is important to note that current standardized KSCs may not accurately capture minute differences in knee extensor function, knee stability, or gait cycle.^9,19

The significant strength of this study would be that all procedures were performed by a single high-volume surgeon within a tertiary institution, reducing heterogeneity in surgical technique and postoperative care. Use of consistent SR and MR implants, as mentioned above, allowed the authors to compare SR and MR patient groups directly.

Furthermore, this study examined the impact of implant type on PROMs. With a shift of focus from revision surgery to patient satisfaction as an end point for arthroplasty, measuring patient-centered outcomes provides vital information with regard to outcomes that matter to patients.⁴² The Outcome Measures in Rheumatology conference (OMERACT 3) stated that patient reported measures of functional outcomes are recommended for all Randomized Controlled Trials (RCTs) of interventions for patients with knee Osteoarthritis (OA).⁴³ As the differences between SR and MR femoral implants are mainly theoretical/biomechanical, there have been several basic science studies performed to compare the two types of implants.^6,12,44,45 However, theoretical superiority may not translate directly to improved clinical outcomes. Hence, an analysis of PROMs is vital to bridge the gap between theoretical and clinical outcomes.

A limitation of this study would be the exclusive use of implants from two different companies (i.e. SR implants from Stryker and MR implants from Zimmer). Minor differences in implant design may have introduced confounding factors into our data analysis. Furthermore, the comparison of only two implants may result in reduced generalizability of our results to larger groups of patients with implants from other companies. However, the authors chose a single-surgeon series (who used implants from these two companies) to reduce bias in surgical experience, technique, and postoperative rehabilitation protocol.

A further limitation of this study was that the authors were not able to report on the rate of minor complications, as the patients were typically managed in the outpatient setting and, hence, details of their management were not captured in the central database. On the other hand, major complications (requiring inpatient admission) have been reported within the section on clinical outcomes. Future studies could improve on this by capturing minor complications such as superficial skin infections and reporting it accordingly to provide more in-depth comparisons between the two patient groups.

Conclusion

This study showed that despite the theoretical advantages SR femoral implants hold over MR femoral implants, postoperative outcomes at 2 years show no significant differences between the two patient groups in most clinical parameters, including standardized KSCs. Hence, there is no clear evidence to suggest superiority of one design over the other.

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.

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Quality of life and functional outcome after single-radius and multi-radius total knee arthroplasty

Abstract

Purpose:

Methods:

Results:

Conclusion:

Level of evidence:

Keywords

Introduction

Materials and methods

Participants

Indication

Components and technique

Postoperative follow-up and data collection

Statistics

Results

Patients

Clinical outcomes

Discussion

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References