Mobile or fixed unicompartmental knee prostheses? In-vitro wear assessments to solve this dilemma

Preliminary In Vitro Wear Assessment of Ceramic Cemented Femoral Components Coupled with Polyethylene Menisci

Success of total knee replacement (TKR) depends on the prosthetic design and materials. The use of metal components is well established with the disadvantage of allergic reactions. Ceramics have been recently proposed because of high wear resistance, excellent biocompatibility, wettability, and suitable mechanical properties. This study was aimed at investigating in vitro wear resistance of Zirconia Toughened Alumina (ZTA)/Ultra-high-molecular-weight polyethylene (UHMWPE) of TKR femoral components. An in vitro protocol was designed with the application of relevant load profile, 6-degrees-of-freedom knee simulator, and 8 × 10⁵ cycles on the ZTA/UHMWPE configuration under bovine calf serum. Before and after wear test, the femoral components were investigated by using the Scanning Electron Microscope (SEM) and the X-Ray Diffraction (XRD) analyses, and stylus surface roughness measurements. The proposed pre-clinical test yielded repeatable results. In particular, gravimetric results showed that, after 8 × 10⁵ cycles, the mean weight loss of the polyethylene mobile components is 5.3 ± 1.1 mg. The surface roughness measurements (Ra_max) performed after the wear test showed no significant variation on the UHMWPE menisci. A slight increase of roughness has been found on the ZTA (0.02 µm before wear test, 0.28 µm after the test). SEM observations did not show significant modification of the surface morphology. Tetragonal to monoclinic phase ratio was measured by XRD before and after wear test to evaluate stability of tetragonal ZrO₂ phase. Minimal conversion of tetragonal to monoclinic phase was found from 5.4 to 8%. Although this study is a preliminary evaluation limited to in vitro tests, it provides novel pre-clinical indications about the potential of ceramic TKR femoral components.

Wear Behavior Characterization of Hydrogels Constructs for Cartilage Tissue Replacement

This paper aims to characterize the wear behavior of hydrogel constructs designed for human articular cartilage replacement. To this purpose, poly (ethylene glycol) diacrylate (PEGDA) 10% w/v and gellan gum (GG) 1.5% w/v were used to reproduce the superior (SUP) cartilage layer and PEGDA 15% w/v and GG 1.5% w/v were used to reproduce the deep (DEEP) cartilage layer, with or without graphene oxide (GO). These materials (SUP and DEEP) were analyzed alone and in combination to mimic the zonal architecture of human articular cartilage. The developed constructs were tested using a four-station displacement control knee joint simulator under bovine calf serum. Roughness and micro-computer tomography (µ-CT) measurements evidenced that the hydrogels with 10% w/v of PEGDA showed a worse behavior both in terms of roughness increase and loss of uniformly distributed density than 15% w/v of PEGDA. The simultaneous presence of GO and 15% w/v PEGDA contributed to keeping the hydrogel construct's characteristics. The Raman spectra of the control samples showed the presence of unreacted C=C bonds in all the hydrogels. The degree of crosslinking increased along the series SUP < DEEP + SUP < DEEP without GO. The Raman spectra of the tested hydrogels showed the loss of diacrylate groups in all the samples, due to the washout of unreacted PEGDA in bovine calf serum aqueous environment. The loss decreased along the series SUP > DEEP + SUP > DEEP, further confirming that the degree of photo-crosslinking of the starting materials plays a key role in determining their wear behavior. μ-CT and Raman spectroscopy proved to be suitable techniques to characterize the structure and composition of hydrogels.

Retrospective Clinical and Radiological Outcomes after Robotic Assisted Bicompartmental Knee Arthroplasty

Purpose. Bicompartmental knee arthroplasty (BiKA) is a favorable alternative to total knee arthroplasty for degenerative disease limited to two knee compartments. Recently developed robotic-assisted systems improved the clinical efficacy of unicompartmental knee arthroplasty by providing enhanced component positioning with dynamic ligament balancing. The purpose of this study was to evaluate the short-term outcomes of patients, undergoing bicompartmental knee arthroplasty at a single institution by a single surgeon using a robotic-assisted system. It was hypothesized that robotic assisted BiKA is a prevailing choice for degenerative disease limited to two knee compartments with good functional results. Methods. A search of the institution's joint registry was conducted to identify patients that underwent robotic-assisted BiKA of the patellofemoral compartment and the medial or lateral compartment. Results. A total number of 29 patients (30 BiKA) with a mean age of 63.6 years were identified who received a patellofemoral resurfacing in combination with medial or lateral compartment resurfacing. Twenty-four out of 29 patients had good to excellent outcome. Conclusion. Robotic assisted bicompartmental arthroplasty using broad indications and only excluding patients with severe deformity and those that have less than 4 mm of joint space in the surviving compartment demonstrated 83% good to excellent results.

May the surface roughness of the retrieved femoral head influence the wear behavior of the polyethylene liner?

This study was aimed at determining the surface degradation occurred on retrieved ceramic and metallic heads, as well as the influence of the head surface quality on the wear of the polyethylene counterface. To this purpose, 14 ceramic and 14 metallic femoral heads retrieved at revision surgery were examined. Scanning electron microscopic analysis provided visual evidence that some metallic heads presented crescent wear more often than the ceramic ones; the former showed a higher volumetric loss (as determined by Coordinate Measuring Machine) than the latter, but less negative Rsk values. This apparent lack of correlation between volumetric loss (i.e., wear factor) and roughness data may be explained by considering that they are two temporally variant parameters. No significant differences were observed between the Ra values of the two sets of femoral heads. The cups articulating against metal heads were characterized by higher mean wear volumes than those articulating against alumina although this difference was not statistically significant; metal heads displayed significantly higher mean wear volumes than alumina heads. The micro-Raman analysis of the cup articulated against the most worn alumina femoral head showed an orthorhombic into monoclinic phase transformation that was not observed in the cups coupled to metal heads. The obtained results showed that the surface finishing of the femoral head (in terms of Rsk values) determined the morphological changes experienced by the ultra-high-molecular-weight polyethylene crystalline phase at the molecular level. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1374-1385, 2016.

Increase in the tibial slope reduces wear after medial unicompartmental fixed-bearing arthroplasty of the knee

Introduction. Unicompartmental arthroplasty of the knee in patients with isolated medial osteoarthritis gives good results, but survival is inferior to that of total knee prosthesis. Knees may fail because positioning of the prosthesis has been suboptimal. The aim of this study was to investigate the influence of the tibial slope on the rate of wear of a medial fixed-bearing unicompartmental knee arthroplasty. Materials and Methods. We simulated wear on a medial fixed-bearing unicompartmental knee prosthesis (Univation) in vitro with a customised, four-station, and servohydraulic knee wear simulator, which exactly reproduced the walking cycle (International Organisation for Standardisation (ISO) 14243-1:2002(E)). The medial prostheses were inserted with 3 different posterior tibial slopes: 0°, 4°, and 8° (n = 3 in each group). Results. The wear rate decreased significantly between 0° and 4° slope from 10.4 (SD 0.62) mg/million cycles to 3.22 (SD 1.71) mg/million cycles. Increasing the tibial slope to 8° did not significantly change the wear rate. Discussion. As an increase in the tibial slope reduced the wear rate in a fixed-bearing prosthesis, a higher tibial slope should be recommended. However, other factors that are influenced by the tibial slope (e.g., the tension of the ligament) must also be considered.

Toward the interpretation of the combined effect of size and body weight on the tribological performance of total knee prostheses

PURPOSE

The research questions of the present study were: (1) Is total knee prosthesis wear behaviour influenced by implant size, body weight and their combined effect? (2) Are these findings significant and helpful from a clinical point of view?

METHODS

Two very different sizes of the same total knee prosthesis (TKP), previously tested with ISO 14243 parameters, were tested on a knee simulator for a further two million cycles using a modified ISO 14243 load waveform. Roughness examination was performed on the metallic components. Gravimetric and micro-Raman spectroscopic analyses were carried out on the polyethylene inserts.

RESULTS

The average volumetric mass loss was 69 ± 3 mm(3) and 88 ± 4 mm(3) for smaller and bigger size, respectively. Bigger TKPs are little influenced by an increased load, while the wear trend of the smaller TKP showed a redoubled slope, and more significant morphology changes were observed. However, the two sizes seem to behave similarly when subjected to a load increase of 15 %; the slope of the volumetric mass loss trend was comparable for the two sets of inserts, which did not appear significantly different also at the molecular level. Roughness average parameters of the lateral femoral condyle support this evidence.

CONCLUSIONS

It can be asserted that the body weight and implant size are relevant to the understanding of TKP wear behaviour. A post-implantation body weight increase in a patient with smaller knee dimensions could results in more critical effects on prosthesis long-term performance.

Achieving accurate ligament balancing using robotic-assisted unicompartmental knee arthroplasty

Unicompartmental knee arthroplasty (UKA) allows replacement of a single compartment in patients with limited disease. However, UKA is technically challenging and relies on accurate component positioning and restoration of natural knee kinematics. This study examined the accuracy of dynamic, real-time ligament balancing using a robotic-assisted UKA system. Surgical data obtained from the computer system were prospectively collected from 51 patients (52 knees) undergoing robotic-assisted medial UKA by a single surgeon. Dynamic ligament balancing of the knee was obtained under valgus stress prior to component implantation and then compared to final ligament balance with the components in place. Ligament balancing was accurate up to 0.53 mm compared to the preoperative plan, with 83% of cases within 1 mm at 0°, 30°, 60°, 90°, and 110° of flexion. Ligamentous laxity of 1.31 ± 0.13 mm at 30° of flexion was corrected successfully to 0.78 ± 0.17 mm (P < 0.05). Robotic-assisted UKA allows accurate and precise reproduction of a surgical balance plan using dynamic, real-time soft-tissue balancing to help restore natural knee kinematics, potentially improving implant survival and functional outcomes.