Biotribology of a mobile bearing posterior stabilised knee design - Effect of motion restraint on wear, tibio-femoral kinematics and particles

Abstract

The objective of our study was to evaluate the impact of a biphaseal anterior–posterior (AP) and internal–external (IE) motion restraint system on the wear behaviour, tibio-femoral kinematics and particle release of a mobile bearing posterior stabilised knee design in comparison to the widely used linear restraint.

in vitro wear simulation was performed using a posterior stabilised total knee replacement with a mobile rotating platform gliding surface design to compare the standard ISO 14243-1:2002 (E) protocol with a linear AP and IE motion restraint and the new ISO 14243-1:2009 (E) protocol with a biphaseal AP and IE motion restraint.

For the mobile gliding surfaces, an increase in wear rate by more than a magnitude was measured applying the biphaseal protocol (8.5±1.6 mg/million cycles) in a direct comparison to the linear protocol (0.33±0.07 mg/million cycles), with statistically significant difference. The amplitudes of AP displacement were 3.22±0.47 mm for the biphaseal test, compared to 1.97±0.22 mm in the linear test and the amplitudes of the IE rotation angle had mean values of 7.32°±0.91° under the biphaseal setup, compared to 1.97°±0.14° under linear motion restraint test conditions.

From our observations, we conclude that the changes in AP translation and IE rotation motion restraints from ISO linear to ISO biphaseal test conditions highly impact the knee joint kinematics and wear behaviour of a mobile bearing posterior stabilised knee design.

Introduction

Failure of total knee arthroplasty (TKA) is relatively rare and mainly due to aseptic loosening and polyethylene wear as reasons for revision (Robertsson et al., 2001, Sharkey et al., 2002, Gothesen et al., 2013, Sadoghi et al., 2013). In an analysis of more than 40,000 primary knee arthroplasties with a 10-year follow-up captured in the Swedish Knee Arthroplasty Register, Robertsson et al. (2001) identified aseptic loosening to be the key factor for primary revision in 44% of the cases. Sharkey et al. (2002) carried out a single center retrospective review of 212 knee revision surgeries on the basis of pre-operative radiographs, intra-operative findings and subsequent laboratory analysis, and found 25% of the cases to be due to polyethylene wear and 24.1% to aseptic loosening. In a complication-based analysis of 36,307 revised TKA’s entered in the arthroplasty registers in Sweden, Norway, Finland, Denmark, Australia and New Zealand from 1979 to 2009, Sadoghi et al. (2013) reported the most common causes for knee revision to be aseptic loosening in 29.8%, septic loosening in 14.8% and implant wear in 8.3% of the cases.

The inflammatory response to polyethylene wear particles leads to periprosthetic osteolysis and subsequent implant loosening (Amstutz et al., 1992, Algan et al., 1996) as several in vitro and in vivo studies on wear particle-induced osteolysis (Catelas et al., 2011) have shown; material (Petit et al., 2002) dose, size and morphology (Green et al., 1998, Matthews et al., 2000, Illgen et al., 2008) of the particles influence the cellular response.

As total knee arthroplasty today is being increasingly performed on younger, heavier and more active patients (Carr et al., 2012, Pradhan et al., 2006), it appears desirable to reduce wear and improve survival rates in the next decade (Inacio et al., 2013, Ezzet et al., 2012). For demanding patients with good muscular and ligamentous conditions, mobile bearing designs with low intrinsic knee constraint and high articulation congruency may be an appropriate solution. The highly congruent femoral and tibial articulation substantially reduces surface contact stresses, present particularly in demanding activities (Morra and Greenwald, 2005), and may also minimise particulate wear (Lygre et al., 2011). Excellent mid- and longterm results have been reported in arthroplasty registers (Lygre et al., 2011) and clinical follow-up studies (Buechel, 2004, Pradhan et al., 2006, Jenny et al., 2012, Lee et al., 2013, Galasso et al., 2013). However, in two recent systematic reviews and meta-analyses (Oh et al., 2009, Smith et al., 2010), mobile bearing knee designs did not affect the radiographic and clinical outcomes. Furthermore, in vitro wear simulations yielded under human walking gait conditions similar polyethylene wear rates for the mobile (rotating platform) and the fixed bearing design of two implant types with identical femoral articulation (Haider and Garvin, 2008, Grupp et al., 2009). But another study using also an identical femoral component found a 4-fold decreased wear rate for the mobile rotating platform compared to the fixed bearing design (Delport et al., 2010).

These in vitro wear studies have been performed according to ISO 14243-1:2002(E) under force control and a combined linear anterior–posterior (AP) translation and internal–external (IE) rotation motion restraint created by mechanical springs. Pointing to the absence of the ACL after total knee surgery, Kretzer et al. (2010) considered that the comparably high linear motion restraint defined in ISO 14243-1:2002(E) does not represent adequately the in vivo conditions. They introduced an asymmetric non-linear ligament restraint model based on the data of Kanamori et al. (2002) and found an increased AP translation and IE rotation in good agreement with clinical data (DesJardins et al., 2007), resulting in 40% increased wear rates for a fixed bearing design.

Taking this factor into account, the new version of ISO 14243-1:2009 (E) describes a biphaseal spring model with different proportionality constants for AP force and IE torque and foresees a motion restraint system that operates independently for AP translation and IE rotation. In a previous study (Grupp et al., 2013a), examining the influence of linear and biphaseal AP and IE motion restraint on a fixed bearing posterior stabilised (PS) knee design, we found a substantial increase of 41% in AP translation and of 131% in IE rotation, leading to a 3.2-fold higher wear rate under the biphaseal test conditions. Apart from this previous work, there is only one study (Haider et al., 2012) using AP and IE motion restraint as defined in ISO 14243-1:2009 (E) on fixed bearing cruciate-retaining and posterior-stabilised knee designs, but to the best of our knowledge there is no published data about the behaviour of mobile bearing knee designs with intrinsic low knee constraint under biphaseal motion restraint conditions.