A Five-Year Comparative Functional and Clinical Evaluation of Two Contemporary Cruciate-Retaining Knee Implants.

BACKGROUND: The purpose of our study was to compare 2 commonly used highly successful cruciate-retaining knee designs on the basis of patient-reported outcome measures (PROMs), range of motion (ROM), and anterior knee pain (AKP) at a minimum follow-up of 5 years.
METHODS: A cohort of 65 patients underwent unilateral total knee arthroplasty, from January 2013 to December 2013, using NexGen Cruciate Retaining (Zimmer Biomet, Warsaw, IN), a nonmorphogenic knee (NMK) system. They were subsequently operated upon for the contralateral knee using Persona Cruciate Retaining (Zimmer Biomet, Warsaw, IN), a morphogenic knee (MK) system between January 2014 and June 2014. Of the 65 patients, 63 participated in this study. All the patients were compared preoperatively and postoperatively on the basis of PROMs, ROM, and AKP.
RESULTS: On the basis of PROMs, ROM, and AKP, there was a statistically significant difference between the 2 groups favoring the MK group, with an ROM of 126.14° in the MK group as compared with 120.76° in the NMK group and P value of <.01.
CONCLUSIONS: PROMs, ROM, and AKP improved significantly over time after total knee arthroplasty using both MK and NMK implants; however, the outcomes of the former were better than those of the latter, although this difference was not clinically significant.

Introduction

Total knee arthroplasty (TKA) is an established treatment option in patients with advanced knee arthritis. Advancements in arthroplasty and increased survival rates have led to an increase in the number of TKAs being performed for knee arthritis. All joint registries have witnessed an increase in the number of TKA. It is being speculated that by year 2040, the demand for primary TKA is projected to increase by more than 401% [1].

Functional recovery after TKA is dependent on factors such as range of motion (ROM), muscle strength, joint stability and pain, as well as the patient’s general health, sense of well-being, and expectations [[2], [3], [4]]. A pain-free and mobile knee increases patient satisfaction as assessed by various patient-reported outcome measures (PROMs) [5,6]. Despite there being greater emphasis on restoring normal function of the knee, a certain percentage of patients undergoing TKA are dissatisfied with the results [[7], [8], [9]]. In an attempt to reduce the percentage of dissatisfied patients after TKA, newer implants have been designed in the hope that they will address the same. These designs are backed by intensive research based on unmet needs of patients.

The purpose of our study was to compare 2 highly successful knee designs used at our center, on the basis of PROMs, ROM, and anterior knee pain (AKP).

Material and methods

The study is a retrospective chart review of prospectively collected data of evidence level 4, wherein 65 patients with advanced degenerative knee arthritis were operated by the senior surgeon for unilateral knee replacement using the NexGen Cruciate Retaining (Zimmer Biomet, Warsaw, IN), a nonmorphogenic knee (NMK) system, between January 2013 and December 2013. These patients were subsequently counseled and operated with the Persona Cruciate Retaining (Zimmer Biomet, Warsaw, IN), a morphogenic knee (MK) system for the contralateral knee, between January 2014 and June 2014. Knees with varus, valgus, and flexion deformity less than 15° were included in the study. Patients with a history of any other lower limb surgeries, spine surgeries or neurological impairments, ankylosed knee joints, a history of septic arthritis, and deep vein thrombosis before the surgery were excluded. Preoperative evaluation was performed using the Kellgren-Lawrence (KL) grading system for osteoarthritis [10]. Patients exhibiting grade II and III changes as per this system were included. Fifty-four of these patients had KL grade III osteoarthritis, and 11 had grade II. The KL grade II patients had spontaneous osteonecrosis of the medial femoral condyle that did not respond to conservative management. Of the 65 patients, 1 had a myocardial infarction and 1 sustained a periprosthetic fracture after trauma in the postoperative period, and both were excluded from the study. A total of 63 patients were subsequently followed up for a minimum period of 5 years (each side). The mean age of the subject group at the start of the study was 61 years, with 42 females (66.6%) and 21 males (33.4%). The body mass index of the subject group was 24.5.

Prosthesis types

The MK system is characterized by the J curve femoral design with a deeper trochlear groove and a shorter, less-thick anterior flange. It has 12 femoral sizes, of which 9 have standard and narrow options, making a total of 21 different sizes. This system also has 9 tibia sizes, 8 different tibial inserts, and 6 patellar sizes. The NMK system has a traditional trochlear notch with 8 different femoral sizes, 10 tibial sizes, 5 tibial inserts, and 4 patellar sizes.

Surgical technique

The surgery was performed by the senior surgeon on both sides, under tourniquet control. An anterior midline incision was used followed by a medial parapatellar approach. In both the groups, the femur was prepared first followed by the tibia. A distal femoral cut of 5° valgus was used in knees with a varus deformity and 3° in those with valgus deformity. An additional distal femoral cut of 2 mm was taken in patients with fixed flexion deformity. The anterior cruciate ligament was excised, and the posterior cruciate ligament was retained in all the knees. After appropriate sizing of the femur, chamfer cuts were taken, which was followed by tibial preparation. Mechanical alignment was achieved by taking tibial cuts at right angles to the mechanical axis of the tibia, and the posterior tibial slope was approximated to the native slope. The cementing technique followed was first-generation digital pressurization in the tibial surface after lavage and drying. Cement was applied to the posterior aspect of the femoral component, and fixation performed. Patellar resurfacing was not performed in both cohorts, but the osteophytes around the patella were removed.

Clinical evaluation

Each of these patients was followed up for a minimum period of 5 years postoperatively as per our standardized treatment protocol at intervals of 6 months, 1 year, 2 years, and 5 years. Preoperatively and at each follow-up, 3 PROM scores (Knee Society Score [KSS], Oxford Knee Score [OKS], and Forgotten Joint Score [FJS]-12) along with ROM and AKP assessment were evaluated separately for each knee. The data were compiled by an experienced physiotherapist who was blinded with regard to the implant allocation. The FJS-12, being a postoperative score, was measured only postoperatively until the latest follow-up. The ROM was measured with the help of a long-arm goniometer, which is a reliable and accurate means of measuring ROM [11]. The recognized method of measuring angles around the knee is to measure the axis of the femur between the center of the greater trochanter and the lateral epicondyle of the femur and between the axis of the tibia between the lateral femoral epicondyle and the center of the lateral malleolus [12]. AKP was evaluated with the help of the Kujala Knee Score questionnaire with emphasis on the site of pain [13]. The Kujala AKP score is a recognized instrument used within the fields of orthopaedics and sports medicine. It is a 13-item screening instrument designed to assess patellofemoral pain in adolescents, young adults, and in patients who underwent TKA with a variable ordinal response format [14,15]. The questionnaires were handed over to the patients and were completed independently by them preoperatively and at each follow-up.

Roentgenographic evaluation

The patients included in the study were assessed for alignment of the limb (tibiofemoral angle), the position of the components, the level of the joint line, posterior condylar offset, and the presence and location of radiolucent lines at the bone-cement interface as per the recommendations of the Knee Society [16]. Anteroposterior radiographs of the lower limbs including the hip, knee, and ankle along with lateral knee and skyline views were taken preoperatively and postoperatively at each follow-up to assess the aforementioned parameters. The postoperative radiographs were analyzed by the second author who could not be blinded to allocation of the implants because of variations in the radiographic appearance.

Statistical analysis

The values of the variables assessed were expressed as mean ± standard deviation. The changes in the Knee Society Clinical Score (KSCS), Knee Society Functional Score (KSFS), OKS, and FJS were assessed using a paired t-test. ROM of the knee in both the groups was similarly assessed using the paired t-test. All the statistical analyses in the study were carried out using a 2-tailed t test, and the level of statistical significance was kept at <0.05. A Kaplan-Meier survivorship analysis was performed to determine the cumulative survival rate during the period of the study, and the result was reported with 95% confidence intervals.

Power analysis was performed for all 6 parameters, namely KSCS, KSFS, OKS, FJS, ROM, and Kujala Knee Score at 5 years.

In the intrasubject analysis of both NMK and MK cohorts, there was a significant improvement in KSCS, KSFS, OKS, and ROM from preoperative scores to scores obtained at different stages of follow-up postoperatively. The analysis of both the cohorts for different variables revealed that while comparing the NMK and MK groups for KSCS, KSFS, OKS, FJS, ROM, and Kujala Knee Score, there was no significant difference between the 2 groups at the 6-month follow-up. Evaluation at subsequent follow-ups thereafter showed that the MK group performed better than the NMK group with regard to the scores.

Results

Functional outcomes

The KSS (clinical and functional) and OKS did not differ significantly between the 2 groups preoperatively and at 6-month follow-up. In the subsequent follow-ups at 1 year, 2 years, and at 5 years, both the implants showed excellent outcomes; however, the MK group performed better than the NMK group (P value < .05). The Knee Society clinical scores at 5-year follow-up was 93.03 in the MK group and 91.20 in the NMK group, while the functional score was 90.25 and 87.52, respectively (P < .01) (Table 1, Figure 1, Figure 2). The OKS was 41.96 in the MK group and 39.90 in the NMK group (P < .01) (Table 2, Fig. 3). The FJS was 85.67 in the MK group and 81.12 in the NMK group (P < .01) (Table 3, Fig. 4).

Table 1

Intergroup and intragroup comparison of KSCS and KSFS variables.

Time point	NMK group (n = 63)			MK group (n = 63)			Between-group comparison (n = 63)
	Mean ± SD	Δ = (preoperative − follow-up)(Mean ± SD)	P-value	Mean ± SD	Δ = (Preoperative − follow-up)(Mean ± SD)	P-value	Δ =(NMK − MK)(Mean ± SE)	P-value
KSCS
Preoperative	47.60 ± 10.11	-	-	47.43 ± 10.75	-	-	0.17 ± 1.8	.932
6 mo	82.98 ± 9.56	−35.38 ± 6.4	<.0001a	83.13 ± 8.93	−35.70 ± 5.5	<.0001a	−0.15 ± 1.6	.924
12 mo	88.03 ± 8.95	−40.43 ± 7.2	<.0001a	90.10 ± 8.02	−42.67 ± 5.4	<.0001a	−2.07 ± 1.5	.152
24 mo	90.20 ± 6.98	−42.60 ± 7	<.0001a	92.22 ± 6.01	−44.79 ± 6.2	<.0001a	−2.02 ± 1.1	.035a
60 mo	91.20 ± 5.70	−43.60 ± 7	<.0001a	93.03 ± 5.06	−45.60 ± 6.9	<.0001a	−1.83 ± 0.9	.036a
KSFS
Preoperative	52.38 ± 9.04	-	-	52.13 ± 8.99	-	-	0.25 ± 1.6	.870
6 mo	76.63 ± 7.57	−24.25 ± 7.5	<.0001a	77.07 ± 7.16	−24.94 ± 6.7	<.0001a	−0.44 ± 1.3	.690
12 mo	85.70 ± 7.68	−33.32 ± 6.6	<.0001a	88.50 ± 7.76	−36.37 ± 6.5	<.0001a	−2.80 ± 1.4	.039a
24 mo	86.40 ± 6.50	−34.02 ± 6.6	<.0001a	89.80 ± 6.44	−37.67 ± 5.6	<.0001a	−3.40 ± 1.1	.003a
60 mo	87.52 ± 6.16	−35.14 ± 5.9	<.0001a	90.25 ± 5.64	−38.12 ± 5.5	<.0001a	−2.73 ± 1	.001a

SD, standard deviation.

In the NMK group, there was a significant improvement in patients’ KSCS and KSFS scores from preoperative to different stages of follow-up postoperatively, and a similar trend was also observed in the MK group (P-value < .05).

While comparing between the NMK and MK groups, no significant difference was observed in improvement of the KSCS score from preoperative to follow-up at 12 months.

Better improvement was observed in the MK group than in the NMK group at 24 months (NMK vs MK group: 90.20 ± 6.98 vs 92.22 ± 6.01) and 60 months (NMK vs MK group: 91.20 ± 5.70 vs 93.03 ± 5.06) (P-value < .05).

While comparing between the NMK and MK groups, no significant difference was observed in improvement of the KSCS score from preoperatively to follow-up at 6 months.

Better improvement was observed in the MK group than in the NMK group at 12 months (NMK vs MK group: 85.70 ± 7.68 vs 88.50 ± 7.76), 24 months (NMK vs MK group: 86.40 ± 6.50 vs 89.80 ± 6.44), and 60 months (NMK vs MK group: 87.52 ± 6.16 vs 90.25 ± 5.64) (P-value < .05).

P-value < .05, statistically significant.

Figure 1

KSS (clinical).

Figure 2

KSS (functional).

Table 2

Intergroup and intragroup comparison of the Oxford Knee Score.

Time point	NMK group (n = 63)			MK group (n = 63)			Between-group comparison (n = 63)
	Mean ± SD	Δ = (Preoperative − follow-up)(Mean ± SD)	P-value	Mean ± SD	Δ = (Preoperative − follow-up)(Mean ± SD)	P-value	Δ = (NMK − MK)(Mean ± SE)	P-value
Preoperative	24.51 ± 6.26	-	-	23.83 ± 6.48	-	-	0.68 ± 1.1	.550
6 mo	33.96 ± 5.24	−9.45 ± 2.3	<.0001a	34.08 ± 5.94	−10.25 ± 3.3	<.0001a	−0.12 ± 0.7	.452
12 mo	38.11 ± 5.05	−13.60 ± 3.6	<.0001a	40.08 ± 4.25	−16.25 ± 4	<.0001a	−1.97 ± 0.8	.016a
24 mo	39.08 ± 4.58	−14.57 ± 3.4	<.0001a	41.40 ± 4.04	−17.57 ± 4.6	<.0001a	−2.32 ± 0.8	.001a
60 mo	39.90 ± 4.13	−15.39 ± 5.2	<.0001a	41.96 ± 3.58	−18.13 ± 5	<.0001a	−2.06 ± 0.7	.001a

In the NMK group, there was a significant improvement in patients’ Oxford Knee Score from preoperative to different stages of follow-up postoperatively, and a similar trend was also observed in the MK group (P-value < .05).

While comparing between the NMK and MK groups, no significant difference was observed in improvement of the OKS from preoperative to follow-up at 6 months.

Better improvement was observed in the MK group than in the NMK group at 12 months (NMK vs MK group: 38.11 ± 5.05 vs 40.08 ± 4.25), 24 months (NMK vs MK group: 39.08 ± 4.58 vs 41.40 ± 4.04), and 60 months (NMK vs MK group: 39.90 ± 4.13 vs 41.96 ± 3.58) (P-value < .05).

P-value < .05, statistically significant.

Figure 3

Oxford Knee Score (OKS).

Table 3

Intergroup and intragroup comparison of the FJS.

Time point	NMK group (n = 63)			MK group (n = 63)			Between-group comparison (n = 63)
	Mean ± SD	Δ = (6 Months − follow-up)(Mean ± SD)	P-value	Mean ± SD	Δ = (6 Months − follow-up)(Mean ± SD)	P-value	Δ =(NMK − MK)(Mean ± SE)	P-value
6 mo	52.43 ± 6.91	-	-	52.45 ± 6.02	-	-	0.02 ± 1.1	.493
12 mo	61.06 ± 6.65	−8.63 ± 3.8	<.0001a	63.75 ± 6.31	−11.30 ± 2.8	<.0001a	−2.69 ± 1.1	.025a
24 mo	80.02 ± 5.75	−27.59 ± 3.4	<.0001a	82.13 ± 5.85	−29.68 ± 2.9	<.0001a	−2.11 ± 1	.042a
60 mo	81.12 ± 5.17	−28.69 ± 4.2	<.0001a	85.67 ± 5.68	−33.22 ± 3.8	<.0001a	−4.55 ± 0.9	<.0001a

While comparing between the NMK and MK groups, no significant difference was observed in improvement of the FJS at 6 months.

Better improvement was observed in the MK group than in the NMK group at 12 months (NMK vs MK group: 61.06 ± 6.65 vs 63.75 ± 6.31), 24 months (NMK vs MK group: 80.02 ± 5.75 vs 82.13 ± 5.85), and 60 months (NMK vs MK group: 81.12 ± 5.17 vs 85.67 ± 5.68) (P-value < .05).

P-value < .05, statistically significant.

Figure 4

Forgotten Joint Score (FJS).

Range of motion

The preoperative ROM of both the groups was comparable with a P value of 0.77. The MK group had a mean ROM of 103.30°, and the NMK group had 102.88°. The ROM increased significantly postoperatively in both the groups. The ROM was 122.11°, 124.31°, and 126.14° at 12, 24, and 60 months, respectively, in the MK group and 117.17°, 119.03°, and 120.76° at the same time intervals in the NMK group (P < .001, P < .001, P < .001) (Table 4, Fig. 5).

Table 4

Intergroup and intragroup comparison of the ROM variable.

Time point	NMK group (n = 63)			MK group (n = 63)			Between-group comparison (n = 63)
	Mean ± SD	Δ = (Preoperative − follow-up)(Mean ± SD)	P-value	Mean ± SD	Δ = (Preoperative − follow-up)(Mean ± SD)	P-value	Δ = (NMK − MK)(Mean ± SE)	P-value
Preoperative	102.88 ± 8.25	-	-	103.30 ± 9.03	-	-	−0.42 ± 1.5	.771
6 mo	115.08 ± 8.95	−12.20 ± 3.6	<.0001a	118.76 ± 7.31	−15.46 ± 2.7	<.0001a	−3.68 ± 1.4	.031a
12 mo	117.17 ± 7.41	−14.3 = 29 ± 3.9	<.0001a	122.11 ± 7.86	−18.81 ± 2.5	<.0001a	−4.94 ± 1.3	<.0001a
24 mo	119.03 ± 5.57	−16.15 ± 4.9	<.0001a	124.31 ± 5.31	−21.01 ± 4.1	<.0001a	−5.28 ± 1	<.0001a
60 mo	120.76 ± 5.01	−17.88 ± 5.2	<.0001a	126.14 ± 5.12	−22.84 ± 4.7	<.0001a	−5.38 ± 0.9	<.0001a

In the NMK group, there was a significant improvement in patients' ROM score from preoperative to ROM at different stages of follow-up, and a similar trend was also observed in the MK group (P-value < .05).

Better improvement was observed in the MK group than in the NMK group at 6 months (NMK vs MK group: 115.08 ± 8.95 vs 118.76 ± 7.31), 12 months (NMK vs MK group: 117.17 ± 7.41 vs 122.11 ± 7.86), 24 months (NMK vs MK group: 119.03 ± 5.57 vs 124.31 ± 5.31), and 60 months (NMK vs MK group: 120.76 ± 5.01 vs 126.14 ± 5.12) (P-value < .05).

P-value < .05, statistically significant.

Figure 5

Range of motion (ROM).

Anterior knee pain

AKP reported in both the groups was assessed by the Kujala Knee Score. The Kujala knee questionnaires were handed over to the patients who filled them independently for each knee preoperatively and at 6 months, 1 year, 2 years, and 5 years of surgery. The preoperative mean score was 38.2. Postoperatively, the mean Kujala score was 68.1 in the MK group and 57.6 in the NMK group at 5-year follow-up; this difference was clinically significant with P < .0001 (Table 5, Fig. 6).

Table 5

Intergroup and intragroup comparison of the Kujala Knee Score.

Time point	NMK group (n = 63)			MK group (n = 63)			Between-group comparison (n = 63)
	Mean ± SD	Δ = (6 Months − follow-up)(Mean ± SD)	P-value	Mean ± SD	Δ = (6 Months − follow-up)(Mean ± SD)	P-value	Δ = (NMK − MK)(Mean ± SE)	P-value
6 mo	44.6 ± 2.6	-	-	43.7 ± 2.2	-	-	0.90 ± 0.4	.026a
12 mo	51.7 ± 2.5	−7.10 ± 1	<.0001a	55.8 ± 2.7	−12.10 ± 1	<.0001a	−4.10 ± 0.5	<.0001a
24 mo	58.4 ± 2.1	−13.80 ± 2.4	<.0001a	64.8 ± 2.8	−21.10 ± 1.4	<.0001a	−6.40 ± 0.4	<.0001a
60 mo	57.6 ± 2.0	−13.0 ± 2.5	<.0001a	68.1 ± 2.2	−24.40 ± 1.1	<.0001a	−10.50 ± 0.4	<.0001a

In the NMK group, there was a significant improvement in patients' Kujala score from preoperative to different stages of follow-up, and a similar trend was also observed in the MK group (P-value < .05).

While comparing between the NMK and MK groups, no significant difference was observed in improvement of the score at 6 months.

Better improvement was observed in the MK group than in the NMK group at 12 months (NMK vs MK group: 51.7 ± 2.5 vs 55.8 ± 2.7), 24 months (NMK vs MK group: 58.4 ± 2.1 vs 64.8 ± 2.8), and 60 months (NMK vs MK group: 57.6 ± 2.0 vs 68.1 ± 2.2) (P-value < .05).

P-value < .05, statistically significant.

Figure 6

Anterior knee pain (AKP).

Radiological results

The tibiofemoral alignment in both the groups was comparable preoperatively and postoperatively. The mean preoperative malalignment was 12.5° and 11.9° in the MK and NMK groups, respectively, while postoperatively it was 5.4° and 5.5° in the MK and NMK groups, respectively. The postoperative femoral and tibial component position in both the groups showed no statistically different values. The posterior condylar offset in both the groups was 33.53 mm and 33.8 mm, respectively, with no statistical difference. The average posterior tibial slope was 4.8 degrees in the NMK group and 4.44 degree in the MK group. Nonprogressive radiolucent lines were seen in 11 MK and 12 NMK groups (Table 6).

Table 6

Radiological results.

Time point	NMK group (n = 63)			MK group (n = 63)			Between-group comparison (n = 63)
	Mean ± SD	Δ = (Preoperative − postoperative)(Mean ± SD)	P-value	Mean ± SD	Δ = (Preoperative − postoperative)(Mean ± SD)	P-value	Δ = (NMK − MK)(Mean ± SE)	P-value
Malalignment
Preoperative	11.9 ± 5.98	-	-	12.5 ± 6.07	-	-	−0.6 ± 1.1	.780
Postoperative	5.5 ± 2.04	6.4 ± 4.2	<.0001a	5.4 ± 2.1	6.9 ± 4.3	<.0001a	0.1 ± 0.3	.693
Joint line
Postoperative	16.1 ± 2.10	-	-	16.2 ± 2.4	-	-	0.1 ± 0.4	.881
Posterior condylar offset
Postoperative	33.80 ± 2.06	-	-	33.53 ± 2.1	-	-	−0.3 ± 0.4	.714
Radiolucent lines	12	-	-	11	-	-	-	-

P-value < .05, statistically significant.

The mean with standard deviation of the change from baseline to 5 years of these parameters for MK and NMK were used Table 7. The analysis is underpowered in relation to KSCS, KSFS, and OKS as the sample size was small. Based on these results, the implicit power of the study ranges from 20.5% to 100.0%.

Table 7

Power analysis: calculation for power of the study.

Parameters	N	NMK group (n = 63)	MK group (n = 63)	m1 − m2	(m1 − m2)2 ∗n	(σ12 + σ22)	(m1 − m2)2 ∗n}/(σ12 + σ22)]	Square root [{(m1 − m2)2 ∗n}/(σ12 + σ22)]	Zβ = square root [{(m1 − m2)2 ∗n}/(σ12 + σ22)] − Zα	Power
KSCS	63	−43.60 ± 7	−45.60 ± 6.9	−2	252.00	96.61	2.61	1.62	−0.34	20.5%
KSFS	63	−35.14 ± 5.9	−38.12 ± 5.5	−2.98	559.47	65.06	8.60	2.93	0.97	39.3%
OXFORD	63	−15.39 ± 5.2	−18.13 ± 5	−2.74	472.98	52.04	9.09	3.01	1.05	56.2%
FJS	63	−28.69 ± 4.2	−33.22 ± 3.8	−4.53	1292.82	32.08	40.30	6.35	4.39	99.4%
ROM	63	−17.88 ± 5.2	−22.84 ± 4.7	−4.96	1549.90	49.13	31.55	5.62	3.66	97.6%
Kujala score	63	−13.0 ± 2.5	−24.40 ± 1.1	−11.4	8187.48	7.46	1097.52	33.13	31.17	100.0%

Sample size = n = (σ12 + σ22) (Zα + Zβ)2/(m1 − m2)2. Zβ = square root [{(m1 − m2)2 ∗n} / (σ12 + σ22)] − Zα.

Power of the study: The power has been calculated for all 6 parameters namely KSCS, KSFS, OXFORD, FJS, ROM, and Kujala score at 5 years. The mean with standard deviation of the change from baseline to 5 years of these parameters for the MK and NMK groups are given in the table. Based on these results, the implicit power of the study ranges from 20.5% to 100.0%. The confidence level is assumed to be 95%.

Survivorship

The Kaplan-Meier survival analysis when the end point was revision for any cause revealed a survival rate of 99% in the MK group because of one revision surgery for periprosthetic joint infection and 100% in the NMK group. When the end point was revision for aseptic causes, the survival rates were 100% in both the groups.

Discussion

The purpose of our study was to determine and compare implant survivorship, radiological outcomes, and patient-reported outcomes of the MK and NMK knee implant systems. Our study documented excellent survivorship of both cohorts where either an MK or an NMK implant was used. Although the radiological outcomes of the 2 design groups showed no significant difference, the PROMs along with the ROM were better in the MK group than in the NMK group.

Pain reduction is an important factor in determining reduction of disability and subsequent patient satisfaction after a joint replacement surgery. AKP is one of the common factors leading to dissatisfaction and revision surgeries [17], others being loosening (39.9%), instability (7.5%), periprosthetic fractures (4.7%), and arthrofibrosis (4.5%) [18]. Patients suffering from AKP experience difficulty in standing up from a chair, ascending and descending stairs, and riding a bicycle [3]. The causes of AKP can be functional (muscle imbalance, dynamic valgus) or mechanical (incorrect positioning of components, patellar fractures, aseptic loosening, etc). However, the prosthesis design can also play a role in the development of patellofemoral problems [18]. In our study, at the latest follow-up, 12 patients had complaints of AKP. The MK group had a less number of patients suffering from AKP than the NMK group.

To allow investigators to assess and quantify preoperative to postoperative improvements in the health status of a patient with TKA, many health-related quality-of-life outcomes have been developed [19]. PROMs improved significantly in both the groups postoperatively. The OKS, KSS (clinical and functional), and FJS improvement plateaued after 1 year of follow-up. At the latest follow-up, all the 3 scores were better in the MK group than in the NMK group. An increase by more than the minimal clinically important difference (MCID) owing to its better correlation with clinical improvement is more important than a significant difference. The MCID of KSS is 34.5 points [20]. The mean improvement of the KSS in the MK group was 46, while it was 43 in the NMK group. The OKS before surgery was 23.83 and 24.51, which improved to 41.96 and 39.9 postoperatively at 5 years, in the MK and NMK groups, respectively. The mean improvement in the OKS was 17 and 15 points in the MK and NMK groups, which was more than the MCID of 9 points needed to detect a change over time in a group of patients as postulated by Beard et al. [21]. The OKS similarly to the KSS was better in the MK group than in the NMK group. The FJS-12 is a postoperative PROM that assesses joint awareness during activities of daily living [22] wherein a high value reflects the ability of the patient to forget about the replaced joint during these activities [23]. In our study, the mean improvement in the FJS was more than the MCID with 33 points in the MK group and 29 points in the NMK group. At the latest follow-up, the FJS of the MK group was better than that in the NMK group with a P value of < .001. While there was a statistically significant difference in all the outcomes favoring the MK group, their clinical relevance needs a longer term evaluation.

ROM is an important factor in determining the success of TKA. To complete normal activities of daily living, such as rising from a chair and ascending and descending stairs, a minimum of 110° of flexion is needed [24]. Bellemans et al. reported that in deep squat, the mean maximal flexion angle achieved was 121.7° using a goniometer [25]. Hancort et al have mentioned that 67° of flexion is required for normal gait, 83° for ascending and 90° for descending stairs, 93° to stand from the sitting position, and 105° to tie shoes [26]. In our study, although the ROM shows remarkable improvement in both types of implants, the MK implants show a statistically significant improvement in ROM as compared with NMK implants. The mean ROM was 126.14° at 60 months in the MK group as compared with 120.76° in the NMK group. This difference may, however, not be clinically significant.

Good morphological fit between TKA components and the resected knee anatomy is an important factor for success in TKA [27,28]. Availability of multiple sizing options in the MK system eliminates the problem of overhang and soft-tissue impingement, resulting in decreased postoperative pain, thereby improving overall function and flexion [27]. These factors of less AKP and increased ROM work very well in favor of the Asian community as our culture demands high-flex activities [29]. The lower percentage of patients with residual AKP in the MK group can possibly be attributed to the design modifications of MK implants, namely J curve, deeper femoral groove, and a shorter less-thick anterior flange. These features in the MK knee system result in an increased area of contact in deep flexion, while the curved arcuate bearing pathway on the lateral surface accommodates external femoral rotation in deep flexion. In the MK group, there was no incidence of implant overhang as compared with the NMK group, wherein 7.9% (5 knees) had a 1-2 mm of overhang [28]. The MK implant has an anatomical tibial baseplate as opposed to a symmetrical baseplate in the NMK implant with a separate side specificity [27].The configuration of the tibial baseplate with its anatomical characteristics lends itself to a considerably reduced incidence of tibial malrotation [30]. The differential conformity in articular surfaces in the MK implant has decreasing sagittal radius medially and increasing sagittal radius laterally, whereas the polyethylene liner in the MK knee system has asymmetric condyles based on the “Big Wheel/Little Wheel” principle. The increase in the ROM can possibly be attributed to these factors along with the fact that the posteromedial edge of the tibial baseplate tapers in the MK implant as compared with the NMK implant.

In our study, the 2 implant designs showed excellent survivorship at 5-year follow-up. One knee in the MK group had a periprosthetic joint infection for which debridement and tibial insert exchange were performed. The survival estimate was 0.99 in the MK group and 1 in the NMK group. The survival estimate of revision for aseptic causes was 1.00 in both the groups. Various joint registries have experienced a significant improvement in the survival rates of TKAs because of better implant designs, patient selection, improved surgical techniques, and patient education. The Norwegian arthroplasty registry recorded an improvement of 10-year survival rates to 94% in the period 2005-2015. The Swedish registry recorded an improvement from 89% for TKAs operated during 1985-1994 to 96% for TKAs operated during 2005-2014 [31].

A similar study by Ranawat et al shows that incidence of AKP was less (12.5%) in Attune (which is a subsequent generation to the P.F.C. SIGMA) knee system than in the P.F.C. SIGMA (25.8%) because of an anatomic trochlear groove with a medialized dome patellar component in the Attune design as compared with a single-radius trochlear groove with a dome-shaped patella in P.F.C. SIGMA [32]. Although our study deals with different implant designs, it does show that subsequent generation knee design leads to better results in the patient population.

This study is not without its limitations. One of the limitations of this study is that because of the short-term follow-up and a small sample size, it is underpowered to detect the differences in the KSCS, KSFS, and the OKS. We believe that this is an early review of outcomes, and a longer term study would be desirable. The other limitation may be the method of measuring the ROM using a goniometer. Radiographs may be a more reliable way of measuring ROM but are more time-consuming [33]. The placebo effect influences results after orthopaedic surgery, stating that patients may score better at an earlier stage when they are more enthusiastic about the procedure, whereas at a later stage, the scores are more realistic [34]. Not resurfacing the patella in this study may be another limitation; however, this may possibly be nullified by the fact that in both cohorts the patella was not resurfaced, thereby offering a comparable option.

Conclusions

While PROMs, ROM, and AKP improved significantly over time after TKA using both MK and NMK implants, our study demonstrated minimal differences in the functional outcomes between the MK and the NMK cohorts. The MK cohort demonstrated a greater reduction in the incidence of AKP than the NMK cohort, although this was not clinically significant. A longer follow-up and a larger sample size may be desirable to evaluate long-term outcomes and function.

Conflict of interest

A.R. received royalty from Zimmer and serves as a consultant to Zimmer and Smith & Nephew.

For full disclosure statements refer to https://doi.org/10.1016/j.artd.2020.05.009.