1 Introduction

Total Knee Arthroplasty (TKA) has been widely recognized as the gold standard treatment for end-stage knee osteoarthritis.¹ Pain relief, restoring function, and improvement of quality of life make TKA one of the most successful orthopaedic surgical procedures.² Currently, more than 400 thousand primary TKAs and 90 thousand revision TKAs are performed yearly in the United States.³ With the aging population, TKA demand is projected to see a 670% increase by the year 2030.⁴

Despite the excellent results, with registries reported survivorship greater than 95% at 10 years follow-up,^5,6 not all patients achieve the desired level of improvement regarding knee function and pain relief. In addition, studies reported that up to 20% of the patients are dissatisfied with the final outcomes.^7,8 Multiple factors including prosthesis design, body mass index (BMI), and level of patient educations, are associated with post-operative dissatisfaction and post-operative pain9,10. Patients’ satisfaction may be compromised by altered knee kinematic resulting with the conventional cruciate‐retaining (CR) and posterior‐stabilized (PS) implants. Such abnormalities include altered anteroposterior (AP) translation (“paradoxical motion”),^11-13 mid-flexion instability,^14,15 inadequate or delayed roll-back,^11,16 and reversed axial rotation of the knee.^13,17

In the 1990s, the medial stabilized (MS) TKA design was introduced in order to reproduce the in-vivo kinematics of the native knee by mimicking the asymmetric constraint profile of the knee.¹⁸ The MS design features a deeper, highly conforming medial compartment, similar to a ball-in-socket articulation, and a less congruent (relatively flat) lateral compartment that allows relative natural femoral rollback during knee flexion.

The first MS systems were introduced in 1994 (MRK™; Finsbury Orthopaedics, Leatherhead, Surrey, UK, and currently MatOrtho, Surrey, UK) and 1998 (ADVANCE®, Wright Medical Group Inc, Memphis, TN, USA, and currently MicroPort Orthopaedics Inc, Arlington, TN, USA). Currently, a second generation of MS systems has been introduced in the market (SAIPH® Knee System, MatOrtho, Surrey, UK; Evolution® Medial-Pivot Knee, MicroPort Orthopaedics Inc, Arlington, TN, USA; GMK® Sphere, Medacta International AG, Castel San Pietro, Switzerland; Persona® Medial Congruent Bearing, Zimmer Biomet, Warsaw, IN, USA; Alumina Medial Pivot Knee, Kyocera Corporation, Kyoto, Japan; and K-Mod Dynamic Congruence Implant, Gruppo Bioimpianti, Peschiera Borromeo, Italy).^19-38

Short-term outcomes of the MS systems in primary TKA have been widely reported in the current literature, however, only few studies have described results beyond 5-year follow-up, and thus it seems necessary to analyze the mid-term outcomes of MS systems in order to provide a basis for comparison with standard CR and PS implants.

Therefore, the purpose of this systematic review is to analyze mid-term studies on MS systems in primary TKAs and to evaluate (1) reoperation rate, (2) survivorship, and (3) clinical outcomes in order to compare them with CR and PS implants.

2 Materials and methods

This search was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (PRISMA).¹⁶ A comprehensive search of the PubMed, Medline, Cochrane, Embase, and Ovid databases was performed utilizing various combination of search terms “medial pivot”, “medial stabilized”, “medial rotating”, “medial congruent”, medial ball and socket”, “arthroplasty”, “TKA”, “TKR”, and “knee surgery” in combination with the Boolean operators (AND, OR, *) since inception of database to December 2019. Only abstracts that evaluated the clinical outcomes of patients with MS knee systems following primary TKA were reviewed.

Inclusion criteria were any original study in which a MS system was used in primary TKA, postoperative complications were reported, clinical outcomes were reported using validated patient reported scales, and implant survivorships for aseptic loosening and for any reason were reported. Exclusion criteria were case-reports, surgical technique reports, review articles, expert opinions, letters to editors, biomechanical reports, instructional course lectures, studies on animals, cadaver or in vitro investigations, book chapters, abstracts from scientific meetings, unpublished reports, studies with less than 30 knees, studies with a mean follow-up of less than 5 years, and studies written in non-English language.

Three independent reviewers (G.C., F.M., and F.D.M.) separately conducted the described search by title and abstract. The three authors compiled a list of papers not excluded after application of the inclusion and exclusion criteria. If the title and abstract of each study contained insufficient information to determine its appropriateness for inclusion, the full manuscript was reviewed. If the articles met inclusion criteria following a title and abstract screen, the full text was obtained and reviewed. A cross-reference research of the selected articles was also performed to obtain other relevant articles for the study. If there was disagreement between the 3 reviewers, a fourth reviewer (I.D.M.) was consulted and consensus was reached. During initial review of the data, the following information was collected for each study: title, first author, year of publication, study design, prosthetic knee system, number of patients, patients died and lost at follow-up, age of patients, length of follow-up, complication type, reoperation for any reason, implant loosening requiring reoperation, deep infections, and patient-reported outcomes.

3 Results

3.1

3.1 Study selection

The selection process is illustrated in Fig. 1. The literature search and cross-referencing resulted in a total of 198 references. After excluding duplicate papers, and after applying inclusion and exclusion criteria, a total of 18 articles were included in this systematic review (Table 1).^19-36

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Fig. 1 PRISMA flow diagram outlining the systematic review process.

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Table 1 Study characteristics and patient demographics on medial stabilized system in primary TKA.

Authors (year of publication)	Study design (level of evidence)	No. of Knees Initial Cohort/Final Cohort	No. of Knees Lost to Follow-Up and/or Died	Female/MaleRatio	Mean Age at Index (y)(Range)	Mean Follow-Up (y)(Range)	System	Indication to TKA (%)	Patellar Resurfacing n°/%
Mannan et al. (2009)19	Retrospective (IV)	208/172	36 (17%)	111/78	68 (28–90)	6 (1–13)	Medial Rotation (Finsbury Orthopaedics)	OA (76.3%), RA (17.1%), PTOA (6.6%)	N/A
Anderson et al. (2009)20	Prospective Non-Comparative (III)	298/238	60 (20%)	165/111	70 (39–84)	5 (N/A)	Advance MP (MicroPort)	OA (95.3%), PTOA (3%),RA (1.3%), Other (0.3%)	256/86%
Fan et al. (2010)21	Prospective Non-Comparative	59/59	0 (0%)	42/13	65 (48–83)	5 (N/A)	Advance MP (MicroPort)	OA (93.2%), PTOA (5.4%), RA (1.7%)	31/52.5%
Vecchini et al. (2012)22	Retrospective (IV)	172/162	10 (6%)	118/42	71 (31–85)	7 (4–10)	Advance MP (MicroPort)	OA (91.3%), RA (5.8%),PTOA (1.7%), Other (1.2%)	0/0%
Brinkman et al. (2014)23	Retrospective (IV)	50/37	13 (26%)	12/35	69 (45–82)	10 (2–14)	Advance MP (MicroPort)	OA (100%)	42/84%
Youm et al. (2014)24	Retrospective (IV)	120/120	0 (0%)	71/9	66 (42–83)	5 (5–7)	Advance MP (MicroPort)	OA (90%), Other (6.7%),RA (3.3%)	104/86.7%
Schmidt et al. (2014)25	Retrospective (IV)	396/365	31 (8%)	258/107	67 (29–86)	5 (2–11)	Advance MP (MicroPort)	N/A	N/A
Chinzei et al. (2014)26	Retrospective (IV)	90/85	5 (6%)	N/A	70 (51–88)	8 (6–11)	Advance MP (MicroPort)	OA (70.6%), RA (25.9%), Other (3.5%)	(0%)
Bae et al. (2015)27	Retrospective comparative (IV)	153/150	3 (2%)	120/4	67(42–83)	5 (2–17)	Advance MP (MicroPort)	OA (93.1%), Other (3.6%)RA (3.3%)	153/100%
Karachalios et al. (2016)28	Retrospective (IV)	285/251	34 (12%)	184/41	71 (52–84)	13 (11–15)	Advance MP (MicroPort)	OA (75%), RA (13%), Other (7%), PTOA (5%)	0/0%
Choi et al. (2017)29	Retrospective comparative (IV)	70/58	12 (17%)	43/6	67 (55–83)	5 (5–6)	Advance MP (MicroPort)	OA (100%)	0/0%
Kim et al. (2017)30	Prospective comparative (IV)	192/182	10 (5%)	52/130	66 (55–79)	12 (11–13)	Advance MP (MicroPort)	OA (100%)	192/100%
Nakamura et al. (2017)31	Retrospective (IV)	107/70	37 (35%)	N/A	72 (45–86)	12 (10–13)	MPK (Kyocera Corporation)	OA (93.9%), RA (6.1%)	107/100%
Macheras et al. (2017)32	Retrospective (IV)	385/347	39 (10%)	215/110	78 (58–86)	15 (15–17)	Advance MP (MicroPort)	OA (78.3%), RA (10.3%),PTOA (10%), Other (1.4%)	0/0%
Dehl et al. (2017)33	Retrospective (IV)	74/50	24 (32%)	35/13	76 (45–93)	10 (8–11)	Advance (MicroPort)	OA (90%), PTOA (4%),Other (4%), RA (2%)	67/90%
Karachalios et al. (2018)34	Prospective Comparative (III)	54/54	0 (0%)	36/18	64 (52–70)	9 (8–9)	Advance (MicroPort)	OA (85.2%), Other (11.1%),PTOA (3.7%)	0/0%
Katchky et al. (2019)35	Retrospective (IV)	100/97	3 (3%)	55/45	68 (46–90)	5 (5–8)	SAIPH (MatOrtho)	OA (78.3%), RA (10.3%),PTOA (10%), Other (1.4%)	0/0%
Cacciola et al. (2020)36	Retrospective (IV)	351/315	36 (10.2%)	225/114	74 (41–89)	5 (5–6)	K-Mod (Gruppo Bioimpianti)	OA (88.6%), RA (6%),Other (5.4%)	0/0%
Total	–	3184/2832	352 (11%)		69 (28–93)	8 (1–15)		OA (87.2%), RA (6.7%),PTOA (3.4%), Other (2.7%)	952/36.9%

4 Discussion

The aim of this study was to analyze the mid-term survivorship of MS systems in primary TKA and to compare it with standard CR and PS implants. The overall survivorship among 2832 MS-TKA was 97.6% at a mean follow-up of 8 years, suggesting that MS systems’ survivorship after primary TKA is in line with standard CR and PS implants, as reported across multiple registries and registry-based studies.^5,6,40,41 According to the 2019 Australian Orthopaedic Association National Joint Replacement Registry (AOANJR),⁵ MS-TKA survivorship at mean follow-up of 5-years was 96.5%, similar to standard CR and PS implants of 96.7% and 95.9%, respectively. In addition, 10-years survivorship of MS-TKA was 94.5% compared with 95.1% and 94% of CR and PS implants, respectively. Moreover, the AOANJR showed a survivorship stratification based on the different MS designs, and the Advance MP TKA seemed to be the one with highest failure rate at mid-term follow-up (5.4%) when compared with other MS designs.⁵ However, since the majority of the studies included in this systematic review (14 out of 18) reported outcomes on the Advance MP TKA, it was not possible to stratify the failure rate and clarify if one specific design was associated with a higher failure rate compared with the others. According to the 16th edition of the NJR for England, Wales, Northern Ireland and the Isle of Man,⁴⁰ it was reported a cumulative revision rate for the MS-TKA systems (26,693 knees) of 2.6% at 5 years, 3.4% at 10 years, and 4.6% at 15 years. In addition, the NJR reported similar revision rates at 15 years for CR (4.38%), PS (5.38%) and MS (4.6%) TKA implants. According to Øhrn et al.,⁴² in a registry-based study from the AOANJR and the Norwegian Arthroplasty Register (NAR) from 2005 to 2017, the authors analyzed the outcomes of 6310 MS-TKA and reported a higher revision risk with MS-TKA systems compared with CR-TKA in Australia (HR 1.4% [95% CI 1.2 to 1.7]; p < 0.001), and no such difference in Norway (HR 1.5 [95% CI 0.9 to 2.4]; p = 0.1). However, the NAR included limited data with a study group of 298 knees and the overall results should be treated with caution because the follow-up period for most of the implants investigated was relatively short. In addition, among the Australian population the Kaplan-Meier for 9-year survivorship considering revision for any cause as endpoint was 94.8% (95% CI 93.4 to 96.3) for the MS-TKA system compared to 96.4% (95% CI 96.2 to 96.6) for the CR implants. In Norway the corresponding survivorship of the MS systems was 92.2% (95% CI 89.0 to 95.4) compared to 94.5% (95% CI 93.8 to 95.2) for the CR.

The overall reoperation rate among the studies included in this review was 2.4% (69 of 2832 knees) at mean 8-years follow-up and PJI was the most common reason of reoperation (25 of 69 knees, 36%), followed by aseptic loosening (12 of 69 knees, 17%). Among most of the available Registries, the aetiologies of MS revisions are not clearly defined, however, the rate of revision due to aseptic loosening was lower than the one reported by Spekenbrink-Spooren et al.⁴³ for CR and PS TKA. In fact, the authors analyzed the 8-year revision rates of all CR and PS TKA implants in the Dutch Arthroplasty Registry between 2007 and 2016 (133,841 cemented fixed bearing primary TKA) and reported that aseptic loosening of the tibial component was the most common reason of revision, happening more frequently on a PS implants (41%) than the CR (27%; p < 0.001). Six of the 18 studies included in this review (33%)^23,28,30-33 reported a follow-up longer than 10 years, however, the majority of them (5 out of 6; 83%) reported outcomes on the Advance MP (Micro Port) implant. Among them, the overall survivorship from reoperation from any cause at mean 12-years follow-up (range, 10–15 years) was 97.3%. Two studies^35,36 described the outcomes of second-generation MS implant, SAIPH (MatOrtho) and K-Mod (Gruppo Bioimpianti), however, they were limited by a short-term follow-up (5 years; range 5–8) reporting overall survivorship of 98.1% (404 out of 412 TKA).Kim et al.³⁰ prospectively evaluated the outcomes of 182 patients that underwent simultaneous sequential bilateral primary TKA with a MS posterior cruciate-substituting knee implant in one knee and a PFC Sigma® (J&J Medical Devices) CR mobile-bearing knee implant in the contralateral at a minimum follow-up of 11 years. The authors reported long-term fixation and survivorship of 99% for the MS knees (95% CI, 0.96–1.00) and 99.5% for the CR knees (95% CI, 0.96–1.00) at final follow-up of 12.6 years. However, the MS systems were associated with worse functional outcomes using the WOMAC score (p = 0.048) and KSS (p = 0.026), and higher complication rate compared to the CR knees (p < 0.05).

All patients included in this review showed considerable improvement in a variety of functional scores. Specifically, MS-TKA was associated with an average of a 49.1 kKSS increase and an average of 38.1 fKSS increase between preoperative and final follow-up. On average, patients reported ΔROM of 10.8° and improved WOMAC score by mean 36 points between preoperative and postoperative period. In addition, favourable functional outcomes were reported using OKS, SF-12, FJS, KOOS and UCLA score when preoperative score were compared to mean follow-up evaluation (Table 4). According to the current available literature, there are limited studies that compares the outcomes of MS systems with other implant designs.^{18,27,29,38,44–49} Samy et al.,³⁸ retrospectively compared two groups (76 MS-TKA vs 88 PS-TKA) and reported no difference in ΔROM compared to the preoperative value at 6 weeks (MS = −12.4° vs PS = −3.8°; p = 0.066), 6 months (MS = −4.3° vs PS = 2.7°; p = 0.182), or 1 year (MS = 0.2° vs PS = 3.31; p = 0.499). However, patients who underwent the MS-TKA scored significantly better than the PS-TKA on the FJS (MS = 59.7 vs PS = 44.8; p = 0.007), particularly regarding the deep knee flexion and implant's stability. Jones et al.⁵⁰ evaluated the sagittal stability using a KT-1000 arthrometer, Lachman's test, and the anterior drawer test in a matched-cohort cross-sectional study on 60 patients after TKA at 1-year follow-up. The authors reported that MS-TKA had significantly decreased translation on KT-1000, improved stability, and improved patient reported outcome measures (PROMs) compared with non-MS TKA (p < 0.05). Hossain et al.,¹⁸ prospectively randomized 82 patients in a single-center single-blinded controlled trial and compared the functional outcomes at 1 and 2-years follow-up of MS-TKA standard PS-TKA. The authors reported that patients who underwent MS-TKA had improved ROM at 2 years after surgery compared to preoperatively (p = 0.003), whilst no difference was detected in the PS-TKA group at 2-years. In addition, the authors detected higher physical component of the SF-36 scores (p < 0.05), all components of the Total Knee Function Quesionnaire (TKFQ; p < 0.05), and less pain as measured on the WOMAC subscale (p < 0.05) in the MS-TKA group compared to the PS-TKA group both at 1 and 2-year follow-up, suggesting that the MS system provide better high-end function. Despite that, the two knee systems reported similar survivorship at 2 years follow-up indicating that the improved stability and functional outcomes have not translated yet into better survivorship when compared to traditional CR and PS implants. Kulshrestha et al.,⁵¹ prospectively analyzed the outcomes at 2-years follow-up of 80 randomized patients (40 MS vs 40 PS) and reported similar patient-reported outcomes assessed by new KSS (satisfaction, expectation, and activity scales) and FJS. MS knee patients had better performance in the timed up and go test (p = 0.026) and self-paced walk test (p = 0.002) of Delaware Osteoarthritis Profile Score (DOPS) but the gain in knee flexion (9.3° ± 14°) compared to baseline was significantly greater in the PS group (p = 0.013). Finally, the authors reported overall similar outcomes among the two TKA systems with better function in daily activities in the MS group and better knee flexion in the PS group, however, the patients did not perceive the relative superiority of the MS system and were equally satisfied with both designs.

There were a variety of limitations in this study. First, we were limited by the quality and descriptive nature of the original studies, the variability in inclusion criteria, the limited number of prospective studies, the limited number of studies reporting on the modern design, and the relatively low number of patients included. Second, our methodology did not allow for identification of unpublished literature on MS-TKA and it was limited by potential publication bias. Third, multiple different scores were used across the included studies in order to assess the functional outcomes. Larger multicenter studies that use similar scores to evaluate the outcomes should be considered necessary to better compare results of MS-TKA with standard CR and PS implants.⁵²

Finally, according to the findings of this systematic review, MS systems in primary TKA provide overall mid-term survivorship comparable to standard CR and PS implants, however, according to the available data MS system is associated with better high-end function compared to standard implants. Results of second-generation MS-systems are promising but still lacking at a long-term follow-up. Further RCT are necessary in order to evaluate if the increased sagittal stability, better functional outcomes, and more natural kinematics provided by the MS system can result in an increased long-term survivorship when compared to traditional TKA designs.