Comparing patient reported outcomes in depuy sigma curved versus curved plus polyethylene inserts in cruciate retaining total knee arthroplasty

Abstract

Total knee designs have evolved to allow more motion with compromising stability. The purpose of this study is to compare standard congruency and anterior lipped liners’ effects on patient reported outcomes after TKA. 68 patients were retrospectively reviewed, and their SF-12 and KOOS scores were assessed preoperatively and one year post-operatively. Two-sample t-tests were used to compare differences between groups. The CVD design had a statistically significant greater improvement in KOOS-Pain compared to CVD+. No statistical difference was observed in the outcomes. The results may suggest that higher congruency designs lead to less physiological joint kinematics and pain.

1. Introduction

Total knee arthroplasty has become increasingly more and more prevalent. As of 2010, there were an estimated 4.7 million individuals in the U.S. living with a total knee replacements.¹ According to the National Joint Registry, there were an estimated 108,713 total knee replacements in 2016, which represented an increase of about 3.8% from the previous year.² Since the prevalence of osteoarthritis and subsequent total knee replacements is projected to rapidly rise, it becomes very important to continue to study and optimize current implant designs.

Total knee arthroplasty can vary from one case to another based on the patient's anatomy, degree of arthritis, and deformity. A “one-size fits all” total knee prosthesis does not exist, as companies continue to market to specialized subgroups such as “gender knee,” “active knee,” or “personalized knee”. The market is flooded with different total knee designs based on anatomic and kinematic variables. The goal of knee reconstruction is to build a “stable” knee that allows full motion and strength. Patients complain of instability due to limb malalignment, incompetent soft tissue structures, inadequate balancing, and/or improper component placement. Tibiofemoral instability is a common reason for total knee arthroplasty failure due to pain and overall poor outcomes. To address instability in primary TKA, implant designs have focused on multiple features including constraint, joint congruency, and femoral radius of curvature.³ From low to high constraint, total knees have classically included posterior cruciate-retaining (CR), posterior stabilized (PS), varus-valgus constrained, and rotating hinged implants.^3,4 Some research argues that designs with less constraint may allow patients to have a more normal gait pattern.⁵ Furthermore, the amount of constraint can lead to increased wear, early loosening, and increased risk of implant failure.⁶

Most total knee replacements can be divided simply into CR or PS. There is still much debate over which is better.⁷ PS knees are not without their potential downfalls and complications, including increased post wear, increased femoral bone loss, “jumped post” and potential knee dislocation, and patellar clunk syndrome.8, 9, 10, 11 The CR design can allow more anatomic knee kinematics and potentially reduce the forces on the implant bone interface with less constraint.⁸ For these reasons, some arthroplasty surgeons prefer CR designs, but they can fail due to posterior cruciate ligament (PCL) rupture or mid-flexion instability. To address those failures, CR designs have been incorporating highly conforming, anterior lipped polyethylene bearing surfaces. These inserts exhibit many of the advantages of classic CR design while adding constraint and allowing additional options in certain cases such as incompetent PCL.¹² These anterior-stabilized, ultra-congruent inserts are thought to have overall positive results, even though limited functional outcome data is available.¹²

The Johnson and Johnson DePuy PFC Sigma total knee replacement has been in use since the late 1990s and has had an excellent track record with high satisfaction and performance.¹³ The Sigma system offers multiple options in the cruciate retaining range including ultra-congruent fixed bearing inserts such as the “Curved (CVD)” and “Curved Plus (CVD+)” designs. The CVD + design has an anterior lip that is slightly higher, more abrupt, and more anterior compared to the CVD design (Fig. 1). The designs trade anterior to posterior stability for rotational congruency. The CVD insert allows for about 10° of rotation while the CVD + insert allows 20° of rotation.¹⁴ The aim of this retrospective review was to compare the outcomes of primary TKA patients receiving either PFC Sigma fixed bearing CVD or CVD + polyethylene insert using patient reported outcome scoring systems. We hypothesized that there would be no overall difference in the patient related outcomes improvement from preoperatively to 1-year postoperatively when comparing patients in the CVD and CVD + groups.

Fig. 1.

SIMGA Polyethylene designed described in study Curved (CVD) and Curved Plus (CVD+)¹⁴.

2. Methods

This is a retrospective database study using an institutional registry for patient outcomes. The database was searched for total knee patients over a 2 year period from December 2017 to January 2019. A total of 68 patients were identified who had completed outcome surveys before the surgery and at 1 year follow up, and who underwent primary CR total knee replacement by a single surgeon at the same institution. All patients had a total knee arthroplasty for degenerative osteoarthritis indications. Ethical approval was obtained from Rhode Island Hospital Institutional Review Board. All operations were performed with the patient under either general or spinal anesthesia and using the same technique. Standard total knee arthroplasty consent process was used for each procedure. All surgeries were performed by the senior author (V.A.).

Based on surgeon preoperative or intraoperative assessment, patients received either a Depuy Synthes PFC Sigma CVD or CVD + polyethylene insert in cruciate retaining TKA. All operations were performed under the direct supervision of one single orthopedic surgeon, who was experienced with the Depuy Synthes PFC Sigma system.

Preoperatively, the patient's demographic data including gender, age, BMI, smoking history, and presence of diabetes were recorded. The 12-item Short Form Survey (SF-12) mental component score (MCS), physical component score (PCS), and the Knee Injury and Osteoarthritis Outcome Score (KOOS) for the pain (KOOS-Pain) and function in daily living (KOOS-ADL) were obtained preoperatively and one year post-operatively.

The groups were matched so that 37 patients received the CVD design, while 31 patients received the CVD + design. The mean age of the patients at the time of surgery was 65 years (range, 51–79 years) in the CVD group and 66 years (range, 46–86 years) in the CVD + group (p = 0.66) (Table 1). At the time of surgery, the average BMI of the patients in the CVD group was 30.26 ± 0.87 and in the CVD + group was 32.6 ± 1.1 (p = 0.13). Overall, there was no association with the type of polyethylene insert (CVD vs. CVD+) and smoking status (p = 0.50), history of DM (p = 0.91) or insurance status (p = 0.78). There was an association with gender and insert design. 79.3% of male patients received the CVD implant, and 64.1% of female patients received the CVD + implant (p < 0.01). (Table 1).

Table 1.

Demographic Information of the CVD and CVD+ Groups.

Demographic	CVD	CVD+
Number of Patients	37	31
Gender Ratio (Male/Female)	23/14	6/25
Mean Age (SD)	65.2 (8.0)	66.2 (9.7)
Mean BMI (SD)	30.2 (6.2)	32.6 (6.1)
Ratio: Cigarette Smoker/Non-Smoker	4/32	2/29
Ratio: Diabetic/Non-Diabetic	5/31	4/27

The primary outcome measures of this study were SF-12 MCS, SF-12 PCS, KOOS-Pain and KOOS-ADL. Absolute improvement (AI) and percentage of maximal possible improvement (%MPI) from preoperative level to 1-year post-operatively were calculated for each primary outcome measure.

Statistical analysis was completed with StataSE software (v14.2; StataCorp, College Station, Texas, USA). Descriptive statistical analysis was completed on all demographic data and primary outcome measures. Two-sample t-tests with equal variances were used to compare difference between CVD and CVD + regarding all primary outcome measures, AI and %MPI. P-values of 0.05 or less were considered to statistically significant and a power of 0.8 was taken for sufficient sample size.

3. Results

Patients in the CVD group had a greater improvement in KOOS-pain score when compared to the CVD + group (Table 2). Preoperatively, the KOOS-pain score averaged 45.57 ± 3.05 in the CVD group and 44.17 ± 2.91 in the CVD + group (p = 0.74). Postoperatively, the KOOS-pain score averaged 85.92 ± 2.44 in the CVD group and 70.13 ± 4.75 in the CVD + group (p < 0.01). There was a statistically significant greater improvement for the patients receiving the CVD implant when compared to those receiving the CVD implant in terms of both AI (p = 0.02) and %MPI (p = 0.03).

Table 2.

Primary Outcome Measurements, Taken Preoperatively and at 1 Year after Surgery, along with Absolute Improvement and Percent Maximal Possible Improvement.

Mean outcome score	Preoperative			One Year Postoperative			Absolute Improvement			Percentage Maximal Possible Improvement
	CVD	CVD+	p-value	CVD	CVD+	p-value	CVD	CVD+	p-value		CVD	CVD+	p-value
SF-12 MCS (SD)	56.45 (1.78)	52.20 (2.58)	0.17	56.95 (1.36)	50.14 (2.02)	<0.01a	0.49 (1.68)	−2.05 (2.26)	0.36		−0.03 (0.04)	−0.09 (0.05)	0.29
SF-12 PCS (SD)	34.53 (1.83)	30.55 (1.76)	0.13	46.33 (1.47)	40.94 (2.01)	0.03a	11.79 (1.99)	10.39 (2.47)	0.66		0.16 (0.03)	0.14 (0.03)	0.60
KOOS-ADL (SD)	52.58 (2.57)	46.67 (3.37)	0.16	80.89 (3.62)	69.34 (5.17)	0.07	28.31 (4.11)	22.66 (4.90)	0.38		0.57 (0.07)	0.42 (0.120)	0.26
KOOS-PAIN (SD)	45.57 (3.05)	44.17 (2.91)	0.74	85.92 (2.44)	70.13 (4.75)	<0.01a	40.35 (3.74)	25.96 (5.06)	0.02a		0.70 (0.06)	0.43 (0.12)	0.03a

Absolute Improvement = Post-Operative Score – Pre-Operative Score.

Maximal Possible Improvement= (Post-Operative Score – Pre-Operative Score)/(100- Pre-Operative Score).

^ap-values below 0.05 are considered to be statistically significant.

Due to the association between gender and type of polyethylene implant, we further analyzed the AI and %MPI of KOOS-pain Scores by gender and type of implant. Overall, there is no statistically significant difference between men and women with regards to AI and %MPI of KOOS-Pain Score. Among men, those receiving has CVD implants had better AI than CVD + implants (p < 0.01). Similarly, women in the CVD group had better AI than those with CVD + implants (p < 0.01). When stratified by gender, there is no statistically significant difference between CVD or CVD+ with regards to %MPI of KOOS-Pain Scores.

There were no significant differences between patients in the CVD group and in the CVD + group with respect to the KOOS-ADL score (Table 2). Preoperatively, the KOOS-ADL score averaged 52.58 ± 2.57 in the CVD group and 46.67 ± 3.37 in the CVD + group (p = 0.16). Postoperatively, the KOOS-ADL score averaged 80.89 ± 3.62 in the CVD group and 69.34 ± 5.17 in the CVD + group (p = 0.07). There was no significant difference in improvement for the patients receiving the CVD implant when compared to those receiving the CVD implant in terms of both AI (p = 0.38) and %MPI (p = 0.26).

There was a significant difference between patients in the CVD group and in the CVD + group with respect to postoperative SF-12 MCS and SF-12 PCS scores, but no difference in preoperative levels or improvement. Preoperatively, the SF-12 MCS score averaged 56.45 ± 1.78 in the CVD group and 52.20 ± 2.28 in the CVD + group (p = 0.17), while the SF-12 PCS score averaged 34.53 ± 1.83 in the CVD group and 30.55 ± 1.76 in the CVD + group (p = 0.13). Postoperatively, the SF-12 MCS score averaged 56.95 ± 1.36 in the CVD group and 50.14 ± 2.02 in the CVD + group (p < 0.01), while SF-12 PCS averaged 46.33 in the CVD group and 40.94 ± 2.01 in the CVD + group (p = 0.03). However, there was no significant difference in the AI (p = 0.36) and %MPI (p = 0.29) of SF-12 MCS scores between the patients receiving CVD and CVD + implants, likewise there was no significant difference in the AI (p = 0.66) and %MPI (p = 0.60) of SF-12 PCS scores between the two groups.

4. Discussion

The use of an anterior lipped or stabilized polyethylene insert (such as the DePuy CVD or CVD + design in the PFC sigma TKA) has become a more recent addition to the cruciate retaining total knee arthroplasty design. The purposed advantage of the anterior stabilized knee are increased stability without having to convert to a posterior stabilized system, as well as increased contact area, theoretically leading to decreased contact forces, and improved wear.¹² However, some argue that this design may lead to less normal knee kinematics.¹⁵ Previous investigations has shown that the anterior lipped prosthesis have favorable outcomes, clinically and radiographically, compared to both ordinary CR TKA polyethylene inserts and PS TKA designs²². Although similar to the previous literature, our study looked at only the curved polyethylene insert and curved plus design with a larger anterior lip in the PFC sigma TKA design.

Our original hypothesis was that there would be no overall difference in patient reported outcome improvements between the two inserts. Interestingly, in our patient cohort, the curved insert design had a statistically significant increase in KOOS-pain improvement scores when compared to the curved plus designed polyethylene. The KOOS-ADL improvement scores were also higher in the curved group compared to the curved plus group; however, this was not statically significant. This differs slightly from the previous literature in that previously, patient reported outcomes were the same between groups. One possible explanation for these results may be that the potential increased constraint in the curved plus design, with a larger anterior lip, could cause less normal joint mechanics compared to the curved design which could lead to a potential increase in pain.¹⁵ Another potential explanation could be that the curved plus design was chosen by the surgeon when they were not as confident in the overall balance intraoperatively and choose the curved plus design for some increased constraint. This potentially less ideally balance knee could lead to increased pain. However, we cannot say with certainty why each design was chosen intra- or preoperatively.

There are several limitations of this study. There are always inherent limitations in retrospective reviews, which can increase bias. The main bias is selection bias. The patients were not randomized to either the CVD or CVD + insert, rather they were chosen intraoperatively by the senior surgeon. This decision was usually made on multiple criteria including but not limited to: overall balance, PCL competence, and patient demographic factors. It should be noted that statistically, more men did receive curved designs and more females received more curved plus designs. This is a source of potential confounding bias. However, when looking at each gender's pain improvements score independent of design there was no statistically significant difference. Potentially showing that the difference in outcomes measures were due to the implant design rather than the gender. Finally, this is an overall smaller sample size which can affect the overall power of the study.

5. Conclusion

As more and more different polyethylene designs, and overall total knee designs are put on the market it is important to study their outcomes. We report here that in the PFC sigma CR total knee system, the CVD polyethylene insert had slightly better pain reported outcome improvements than CVD + insert; however, both were similar on other subjective related outcome measures. It will be important to continue to study the effect of an anterior lipped polyethylene insert in cruciate-retaining designs, especially prospectively to help control bias.