Rotating‐platform knee arthroplasty: a review and update

Abstract

Fixed versus rotating‐platform knee arthroplasty for total knee arthroplasty is still a controversial topic. In this article, biomechanical and clinical aspects of rotating‐platform knee arthroplasty are reviewed. In regard to its biomechanical characteristics, the rotating‐platform knee arthroplasty design has been proved to provide less tibiofemoral contact stress under conditions of tibiofemoral malalignment. It also reduces the wear rate. However, in regard to its clinical characteristics, the mid‐term and long‐term survivorship of rotating‐platform knee arthroplasties is not superior to that of fixed‐platform knees. It appears that we are at a crossroads. In this article, progress in biomechanical and clinical aspects of rotating‐platform knee prosthesis is reviewed.

Introduction

Total knee arthroplasty (TKA) has become a standard operative procedure for relieving pain and restoring function in patients with osteoarthritis or rheumatoid arthritis. The rotating‐platform knee prosthetic device is suitable for younger or higher‐demand patients. Even though the rotating‐platform knee design theoretically has favorable features compared to a fixed‐bearing system, neither biomechanical advantages nor increase in implant longevity have been proven. It appears that we are at a crossroads. In this article, we have reviewed biomechanical and clinical aspects of the rotating‐platform knee prosthesis.

Types of knee prosthesis

Current total knee prosthesis (TKP) devices can be subdivided into two groups based on different fundamental design principles: fixed‐bearing knees, the polyethylene tibial inserts of which are locked in respect to the tibial tray; and mobile‐bearing designs which facilitate movement of the inserts relative to the trays. In each group, the knee system can be further subdivided into three groups: posterior cruciate ligament retained, sacrificed or substituted. In the mobile‐bearing group, some designs allow both anterior‐posterior translation and internal‐external rotation at the tray‐insert interface, wereas in others rotation only is facilitated at the tray‐insert counterface ¹ . Currently, rotating‐platform TKPs are widely used due to their mobile‐bearing design.

Advantages of the rotating‐platform knee prosthesis

The kinematics of rotating‐platform knee prostheses

Current fixed‐bearing TKPs are widely used prostheses, it has been reported that the survival rate of the fixed‐bearing TKP is over 90% at 15 years ² . Implant loosening and polyethylene wear are recognized as major causes of late failure in fixed‐bearing knee prostheses ³ , ⁴ , ⁵ . Fixed‐bearing prostheses with high conformity bearing surfaces provide low contact stress but produce high torque at the bone‐implant interface, predisposing to component loosening. Conversely, prostheses with low conformity bearing surfaces produce less constraint force, thus decreasing component loosening, but generate high contact stress, leading to early failure of the polyethylene ⁶ , ⁷ . Furthermore, the kinematic conflict between low stress articulation and free rotation has not been solved by any fixed‐bearing knee design ⁸ . Rotating‐platform knee prostheses were introduced with the aim of reducing polyethylene wear and component loosening. The rotating‐platform design provides both congruity and mobility in the tibiofemoral bearing surface. This allows low contact stress and low constraint force to improve wear resistance and, theoretically, to minimize loosening ⁶ , ⁸ , ⁹ . In addition, the rotating‐platform design offers the advantage over the fixed‐bearing design of self‐adjustment to accommodate surgical malalignment ¹⁰ .

The wear rate of rotating‐platform knee prostheses

One major goal of rotating‐platform knee prostheses is to reduce the overall wear damage by increasing the contact area, while the ability of the polyethylene bearings to move freely on polished plates on the upper tibia minimizes constraint and encourages natural knee motion. McEwen et al. used a physiological knee simulator to compare the wear rate of fixed‐bearing and rotating platform mobile‐bearing TKPs ¹ . The PFC Sigma rotating platform mobile‐bearing knee (Depuy, Warsaw, IN, USA) has a mean wear rate of 5.2 ± 3.8 mm³/million cycles when subjected to standard kinematics with force‐controlled anterior‐posterior translation. This is a two‐fold reduction in volumetric wear rate in comparison to fixed‐bearing PFC Sigma knees subjected to intermediate kinematics (9.8 ± 3.7 mm³/million cycles). Ho et al. examined 51 worn tibial inserts, including 15 from mobile‐bearing, rotating‐platform, posterior‐cruciate‐sacrificing, dished prostheses and 36 from fixed‐bearing, posterior‐cruciate‐retaining, flat prostheses, which were retrieved at revision surgery after an average implantation time of 115 months ¹¹ . They classified types of wear as low‐grade (burnishing, abrasion, and cold flow) and high‐grade (scratching, pitting, metal embedding, and delamination) to assess the degree of wear of polyethylene. To assess symmetry of wear, the insert surface was divided into medial and lateral sides and each side was further divided into three equal zones in the anteroposterior direction. Low‐grade wear was commoner in mobile‐bearing knees, whereas high‐grade wear was commoner in fixed‐bearing knees. They identified no internal/external rotational asymmetric wear or anteroposterior asymmetric wear in mobile‐bearing knees.

Complications of rotating‐platform knee prostheses

Osteolysis in rotating‐platform knee prostheses

Osteolysis is an important complication associated with TKA ⁴ , ¹² , ¹³ . A lower wear rate does not represent lower risk of osteolysis in vivo because the tissue response is dependent on the size distribution of the wear particles. When the particle sizes from different types of knee prosthesis were measured, it was found that the less conforming the design, the higher the surface damage and the larger the particle size ¹⁴ , ¹⁵ . Sathisivam et al. found that rotating‐platform knees produce smaller particles than fixed‐bearing knees because of their larger contact areas. This hypothesis has been disproved by an in vitro pin‐on‐disk wear test ¹⁶ . Huang et al. performed eighty revision TKAs between 1995 and 1998 ¹⁷ . All had radiographic evidence of advanced polyethylene wear. The mobile‐bearing group consisted of 34 knees with a low contact stress implants, and the fixed‐bearing group included 46 knees. The average time (and standard deviation) from the primary operation to the revision was 102.8 ± 26.5 months in the mobile‐bearing group and 96.0 ± 30.1 months in the fixed‐bearing group. The pre‐revision radiographs and operative findings were reviewed and the prevalence of osteolysis found to significantly higher in the mobile‐bearing group (47%; 16 of 34 knees) than in the fixed‐bearing group (13%; 6 of 46 knees) (P= 0.003). The osteolysis was predominantly on the femoral side, adjacent to the posterior aspect of the condyle. Another study comparing failed mobile‐bearing and fixed‐bearing knees in regard to particle size and morphology of polyethylene wear debris has confirmed this finding ¹⁸ . In that study, tissue specimens from interfacial and lytic regions were excised during revision surgery. Ten mobile bearing knees (all of low contact stress [LCS] design) and 17 fixed bearing knees (10 of porous‐coated anatomic [PCA], Howmedica, Rutherford, CA, USA and 7 of Miller/Galante (M/G) design, Zimmer, Warsaw, IN, USA) were included in this study. Polyethylene particles were isolated from the tissue specimens and examined using both scanning electron microscopy (Hitachi S‐3500 N, Tokyo, Japan) and light‐scattering analyses (Master‐sizer 2000, Malvern, AR, USA). The LCS mobile bearing knees produced smaller particulate debris (mean equivalent spherical diameter: 0.58 µm in LCS, 1.17 µm in PCA and 5.23 µm in M/G) and more granular debris (mean value: 93% in LCS, 77% in PCA and 15% in M/G). Therefore, mobile‐bearing knees may be at increased risk of osteolysis.

Dislocation of rotating‐platform knee prostheses

Despite many studies having demonstrated advantages of using mobile‐bearing knees, especially those of rotating platform design, several concerns have been expressed, including the need for a more precise surgical technique and the occurrence of bearing dislocation ¹⁹ , ²⁰ . In previously reported series, the dislocation rate in LCS rotating‐platform knee systems occurs has been less than 3.5% of cases. All these events occurred at an early stage after knee arthroplasty and were attributed to improper surgical technique. Technical pitfalls predisposing to this complication include malrotation of the tibial base plate and failure to produce properly balanced flexion and extension tension between the femoral and tibial bearing interfaces ²¹ , ²² . Huang et al. also reported five cases with late rotational dislocation of the rotating platform bearing in the LCS rotating‐platform knee system ²⁰ . The prostheses had been functioning well for 8 to 12 years before failure. Pre‐operative radiographs showed asymmetric tibiofemoral joint spaces. Entrapment of dislocated bearings in three patients and spontaneous reduction of dislocated bearings in another two patients were seen at revision. Tibiofemoral ligamentous laxity was found after reduction. The retrieved polyethylene bearings showed advanced wear and cold flow deformities and were of reduced thickness. The degree of rotation of the LCS rotating platform bearing is unrestricted, which can result in late dislocation. To prevent late dislocations, a restraint mechanism to limit rotation of the polyethylene element on the tibial base plate to less than 30° has been suggested.

The patellar clunk syndrome in rotating‐platform knee prostheses

The patellar clunk syndrome consists of painful catching, grinding or jumping of the patella when the knee is moving from a flexed to an extended position. Posteriorly stabilized total knee prostheses have been reported to have a higher incidence of this syndrome than other types of prosthesis. A mobile‐bearing posteriorly stabilized TKR was recently introduced to improve patellar tracking and reduce patellar‐related problems by using a self‐alignment mechanism ²³ , ²⁴ , ²⁵ . Fukunaga et al. evaluated the patellar clunk syndrome in 133 knees in 93 patients with such a TKR (PFC Sigma rotating‐platform prosthesis) at a mean follow‐up of 2.3 years (2.0–2.3 years) ²⁶ . The syndrome was identified in 15 knees (13.3%). Logistic regression analysis showed that the absolute value of the post‐operative angle of patellar tilt is significantly associated with the occurrence of patellar clunk.

Clinical results in rotating‐platform knee prostheses

Buechel et al. reported a 20 year survival rate for LCS cemented rotating platform prostheses of 97.7% and a 16‐year survival rate for LCS cementless meniscal‐bearing prostheses of 83% ²⁷ . Four hundred and ninety‐five primary LCS TKAs were reviewed by Huang et al. ²⁸ Among these, 228 knees were meniscal‐bearing prostheses and the remaining 267 knees were of the rotating platform type. The mean follow‐up was 12 years (range 10−15 years). The overall survivorship was 88.1% at 15 years using Kaplan‐Meier analysis. The survival rate was 83% for the meniscal‐bearing prostheses and 92.1% for the rotating‐platform prostheses. The mobile‐bearing knee prostheses had no superiority over that of fixed‐bearing knees, especially for those of meniscal bearing design. Ali et al. reported a prospective study of 109 consecutive primary, uncemented, rotating‐platform, low‐contact stress, TKAs in 85 patients who were reviewed 4 to 12 years after the operation ²⁹ . All patients were followed up annually with subjective questionnaires and radiological assessment. At the time of final review, 69 patients with 87 rotating‐platform total knee arthroplasties were still alive. All were reviewed at the final follow‐up using the American Knee Society Score (clinical and radiological). The average knee score was 86.42, and the average functional score was 65.1. No evidence of radiological loosening was observed in any patient. One knee had been revised because of medial collateral ligament laxity. The 10‐year survival rate was 99.08%.

In another study that compared mid‐term follow‐up of rotating‐platform and fixed‐bearing TKAs ³⁰ , no difference between rotating‐platform and fixed‐bearing prostheses was found. Although mid‐term or long‐term survivorship for fixed‐bearing and mobile‐bearing knees has been reported, few studies have compared the performance of fixed‐bearing and mobile‐bearing in patients with bilateral TKAs. Ranawat et al. compared the fixed‐bearing PFC Sigma total knee arthroplasty to the recently introduced rotating‐platform version of the same design in 26 patients ³¹ . At an average follow‐up time of 46 months for the fixed‐bearing type and 16 months for the rotating‐platform type, no significant differences were found in terms of knee preference, knee pain, range of motion, overall satisfaction or Knee Society scores. No revisions, subluxations, dislocations, or infections were seen. In addition, no radiographic evidence of component loosening, osteolysis, or malalignment was found in any knee. So far, any advantages of the mobile‐bearing design in regard to providing long‐term durability are purely theoretical as such advantages have not been demonstrated by any outcome studies.

Summary

The precise indications for using mobile‐bearing knee prostheses are still unclear. Even though the mobile‐bearing design has theoretically favorable features compared with the fixed‐bearing system, these have not been proven either biomechanically or in regard to extending implant longevity. Therefore, fixed‐bearing knee prostheses are recommended for relatively inactive, elderly people. For younger or higher‐demand patients, the mobile‐bearing design is recommended because of its potential for reduced polyethylene wear and more normal kinematics response after joint replacement. For the younger surgeon, use of the fixed‐bearing design is recommended because the surgical technique is less demanding. For the experienced surgeon, one familiar surgical protocol and instrumentation, either fixed‐bearing or mobile‐bearing, is suggested rather than varying the implant design used.

Disclosure

The authors did not receive any outside funding or grants in support of this research for, or preparation of, this work. Neither they nor a member of their immediate families received payments or other benefits, or a commitment or agreement to provide such benefits, from a commercial entity.