Experimental pre-clinical assessment of the primary stability of two cementless femoral knee components

Optimal intersurface stability for unicompartmental femoral component design with two pegs placed on the distal resection surface: 5 mm peg length increment and 10° peg inclination

PURPOSE

The present study aimed to identify the optimal design of the unicompartmental femoral component through parameter analysis and stability evaluation.

METHODS

A finite element (FE) analysis was applied to analyse and adjust the parameter combinations of the anterior tilt angle of the posterior condyle resection surface, the position of the peg, the length of the peg and the inclination angle of the peg, resulting in 10 different FE models. Setting three knee flexion angles of 8.4° (maximum load state during walking), 40° (maximum load state during stair climbing) and 90° (maximum load state during squatting exercise), quantitatively analysing the micromotion values of the bone-prosthesis interface and defining a weighted scoring formula to evaluate the stability of different FE models. The validity of the FE analysis was verified using the Digital Image Correlation (DIC) device.

RESULTS

The errors between the FE analysis and the DIC test at three flexion angles were 5.6%, 1.7% and 11.1%. The 10 different femoral component design models were measured separately. The FE analysis demonstrated that the design with a 0° anterior tilt angle of the posterior condyle resection surface, both pegs placed on the distal resection surface, lengthened 5 mm pegs and a 10° peg inclination angle provided the best stability.

CONCLUSION

The current study proposed a method for evaluating the stability of the femoral component design. The optimal intersurface stability design of the unicompartmental femoral component was achieved with two pegs placed on the distal resection surface, a 5-mm peg length increment and a 10° peg inclination. These results might provide a reference for the selection of unicompartmental femoral components in clinical practice and therefore improve the survival rate of future unicompartmental knee arthroplasty.

LEVEL OF EVIDENCE

Level III.

Pre-clinical characterization of a novel flexible surface stem design for total knee replacements

Primary stability is crucial for implant osseointegration and the long-term stability of cementless total joint replacements. Biomechanical studies have shown the potential of femoral stems for total knee replacements to reduce micromotions at the bone-implant interface. However, approaches such as focusing on the structural elasticity of the femoral stems are rarely described. Three groups with different femoral stem designs were investigated: group 1: flexible surface stem, group 2: flexible surface stem with open-porous structured lamellas, and group 3: solid stem (reference). The stems were implanted into bone substitute material and dynamically loaded for 1000 cycles. Relative movement and subsidence were measured optically, and axial pull-out forces were determined after dynamic testing. Relative movements increased to 0.10 mm (groups 1 and 2) compared to 0.03 mm (group 3). Subsidence increased to 0.08 mm (group 1) and 0.11 mm (group 2) compared to 0.06 mm (group 3). For each group, subsidence mainly occurred during the first 500 cycles. A similar convergence was observed in the further course. Pull-out forces increased to 1815.0 N (group 1) and 1347.1 N (group 2) compared to 1306.4 N (group 3). The flexible surface stem design resulted in higher relative movements and subsidence, but also exhibited increased pull-out forces. The relative movements were below the critical limit of 0.15 mm and represent a superposition of the elastic deformations of the interacting implant components as well as the micromotion at the bone-implant interface. Therefore, the novel flexible surface stem design appears to offer promising primary implant fixation.

Debonding of coin-shaped osseointegrated implants: Coupling of experimental and numerical approaches

While cementless implants are now widely used clinically, implant debonding still occur and is difficult to anticipate. Assessing the biomechanical strength of the bone-implant interface can help improving the understanding of osseointegration phenomena and thus preventing surgical failures. A dedicated and standardized implant model was considered. The samples were tested using a mode III cleavage device to assess the mechanical strength of the bone-implant interface by combining experimental and numerical approaches. Four rough (Sa = 24.5 μm) osseointegrated coin-shaped implants were left in sheep cortical bone during 15 weeks of healing time. Each sample was experimentally rotated at 0.03°/sec until complete rupture of the interface. The maximum values of the torque were comprised between 0.48 and 0.72 N m, while a significant increase of the normal force from 7-12 N to 31-43 N was observed during the bone-implant interface debonding, suggesting the generation of bone debris at the bone-implant interface. The experimental results were compared to an isogeometric finite element model describing the adhesion and debonding phenomena through a modified Coulomb's law, based on a varying friction coefficient to represent the transition from an unbroken to a broken bone-implant interface. A good agreement was found between numerical and experimental torques, with numerical friction coefficients decreasing from 8.93 to 1.23 during the bone-implant interface rupture, which constitutes a validation of this model to simulate the debonding of an osseointegrated bone-implant interface subjected to torsion.

A randomized controlled trial comparing two-year postoperative femoral and tibial migration of a new and an established cementless rotating platform total knee arthroplasty

AIMS

The primary aim of this study was to compare the migration of the femoral and tibial components of the cementless rotating platform Attune and Low Contact Stress (LCS) total knee arthroplasty (TKA) designs, two years postoperatively, using radiostereometric analysis (RSA) in order to assess the risk of the development of aseptic loosening. A secondary aim was to compare clinical and patient-reported outcome measures (PROMs) between the designs.

METHODS

A total of 61 TKAs were analyzed in this randomized clinical RSA trial. RSA examinations were performed one day and three, six, 12, and 24 months postoperatively. The maximal total point motion (MPTM), translations, and rotations of the components were analyzed. PROMs and clinical data were collected preoperatively and at six weeks and three, six, 12, and 24 months postoperatively. Linear mixed effect modelling was used for statistical analyses.

RESULTS

The mean MTPM two years postoperatively (95% confidence interval (CI)) of the Attune femoral component (0.92 mm (0.75 to 1.11)) differed significantly from that of the LCS TKA (1.72 mm (1.47 to 2.00), p < 0.001). The Attune femoral component subsided, tilted (anteroposteriorly), and rotated (internal-external) significantly less. The mean tibial MTPM two years postoperatively did not differ significantly, being 1.11 mm (0.94 to 1.30) and 1.17 mm (0.99 to 1.36, p = 0.447) for the Attune and LCS components, respectively. The rate of migration in the second postoperative year was negligible for the femoral and tibial components of both designs. The mean pain-at-rest (numerical rating scale (NRS)-rest) in the Attune group was significantly less compared with that in the LCS group during the entire follow-up period. At three months postoperatively, the Knee injury and Osteoarthritis Outcome Physical Function Shortform score, the Oxford Knee Score, and the NRS-activity scores were significantly better in the Attune group.

CONCLUSION

The mean MTPM of the femoral components of the cementless rotating platform Attune was significantly less compared with that of the LCS design. This was reflected mainly in significantly less subsidence, posterior tilting, and internal rotation. The mean tibial MTPMs were not significantly different. During the second postoperative year, the components of both designs stabilized and low risks for the development of aseptic loosening are expected.Cite this article: Bone Joint J 2023;105-B(2):148-157.

Migration of the femoral component and clinical outcomes after total knee replacement: a narrative review

Loosening is considered as a main cause of implant failure in total knee replacement (TKR). Among the predictive signs of loosening, migration is the most investigated quantitative parameter. Several studies focused on the migration of the tibial component in TKR, while no reviews have been focused on the migration of the femoral component and its influence on patients' clinical outcomes. The aim of this narrative review was (1) to provide information about of the influence of migration in femoral component of TKR prostheses, (2) to assess how migration may affect patient clinical outcomes and (3) to present alternative solution to the standard cobalt-chrome prostheses. A database search was performed on PubMed Central® according to the PRISMA guidelines for studies about Cobalt-Chrome femoral component migration in people that underwent primary TKR published until May 2020. Overall, 18 articles matched the selection criteria and were included in the study. Few studies investigated the femoral component through the migration, and no clear migration causes emerged. The Roentgen Stereophotogrammetric Analysis has been mostly used to assess the migration for prognostic predictions. An annual migration of 0.10 mm seems compatible with good long-term performance and good clinical and functional outcomes. An alternative solution to cobalt-chrome prostheses is represented by femoral component in PEEK material, although no clinical evaluations have been carried out on humans yet. Further studies are needed to investigate the migration of the femoral component in relation to clinical outcomes and material used.

No effect in primary stability after increasing interference fit in cementless TKA tibial components

Cementless total knee arthroplasty (TKA) implants rely on interference fit to achieve initial stability. However, the optimal interference fit is unknown. This study investigates the effect of using different interference fit on the initial stability of tibial TKA implants. Experiments were performed on human cadaveric tibias using a low interference fit of 350 μm of a clinically established cementless porous-coated tibial implant and a high interference fit of 700 μm. The Orthoload peak loads of gait and squat were applied to the specimens with a custom-made load applicator. Micromotions and gaps opening/closing were measured at the bone-implant interface using Digital Image Correlation (DIC) in 6 regions of interest (ROIs). Two multilevel linear mixed-effect models were created with micromotions and gaps as dependent variables. The results revealed no significant differences for micromotions between the two interference fits (gait p = 0.755, squat p = 0.232), nor for gaps opening/closing (gait p = 0.474, squat p = 0.269). In contrast, significant differences were found for the ROIs in the two dependent variables (p < 0.001), where more gap closing was seen in the posterior ROIs than in the anterior ROIs during both loading configurations. This study showed that increasing the interference fit from 350 to 700 μm did not influence initial stability.

Comparative Biomechanical In Vitro Study of Different Modular Total Knee Arthroplasty Revision Stems With Bone Defects

BACKGROUND

The aim of this study is to investigate the effects of different stem lengths and types including cones on primary stability in revision total knee arthroplasty with different femoral bone defects and fixation methods in order to maximize bone preservation. It is hypothesized that longer stems provide little additional mechanical stability.

METHODS

Thirty-five human femurs were investigated. A distal bone defect, Anderson Orthopedic Research Institute classification (s. 33) type-F2a, was created in group 1-3 and type-F3 in group 4-6. A cemented, rotating hinge femoral component was combined with different stems (100 and 160 mm total or hybrid cemented cones, or a 100-mm custom-made anatomical cone stem). The femora were loaded according to in vivo loading during gait. Relative movements were measured to investigate primary stability. Pull-out testing was used to obtain a parameter for the primary stability of the construct.

RESULTS

Relative movements were small and similar in all groups (<40 μm). For small defect, the pull-out forces of cemented long (4583 N) and short stems (4650 N) were similar and about twice as high as those of uncemented stems (2221 N). For large defects, short cemented stems with cones showed the highest pull-out forces (5500 N). Long uncemented stems (3324 N) and anatomical cone stems (3990 N) showed similar pull-out forces.

CONCLUSION

All tested stems showed small relative movements. Long cemented stems show no advantages to short cemented stems in small bone defects. The use of cones or an anatomical cone stem with hybrid cementation seems to offer good stability even for larger bone defects. The use of a short cemented stem (with or without cone) may be a suitable choice with a high potential for bone preservation in total knee arthroplasty revision with respective bone defects.

The effect of different interference fits on the primary fixation of a cementless femoral component during experimental testing

Cementless femoral total knee arthroplasty (TKA) components use a press-fit (referred to as interference fit) to achieve initial fixation. A higher interference fit could lead to a superior fixation, but it could also introduce more damage to the bone during implantation. The purpose of the current study was to investigate the effect of interference fit on the micromotions and gap opening/closing at the bone-implant interface. Experimental tests were performed in six pairs of cadaveric femurs implanted with femoral components using a low interference fit of 350 μm and a high interference fit of 700 μm. The specimens were subjected to the peak loads of gait and squat, based on the Orthoload dataset. Digital Image Correlation (DIC) was used to measure the micromotions and opening/closing in different regions of interest (ROIs). Two linear mixed-effect statistical models were created with micromotions and gap opening/closing as dependent variables. ROIs, loading conditions, and implant designs as independent variables, and cadaver specimens as random intercepts. The results revealed no significant difference between the two interference fit implants for micromotions (p = 0.837 for gait and p = 0.065 for squat), nor for the gap opening/closing (p = 0.748 for gait and p = 0.561 for squat). In contrast, significant differences were found between loading and most of the ROIs in both dependent variables (p < 0.0001). Additionally, no difference in bone deformation was found between low and high interference fit. Changing interference between either 350 μm or 700 μm did not affect the primary stability of a femoral TKA component. There could be an interference fit threshold beyond which fixation does not further improve.

Influence of extension stem length and diameter on clinical and radiographic outcomes of revision total knee arthroplasty

BACKGROUND

Extension stems provide stability to revision total knee arthroplasty (RTKA). Little is known regarding the relationship between stem characteristics and RTKA stability. We aimed to identify the relationship between canal filling ratio (CFR) and aseptic loosening following RTKA.

METHODS

We retrospectively reviewed demographics, radiographic parameters, and outcomes associated with RTKA performed between 2008 and 2013 in a tertiary hospital. The inclusion criteria were: revision for aseptic loosening, hybrid fixation, minor bone defect, Zimmer® LCCK prosthesis, and follow-up > 24 months. Using the modified Knee Society radiographic scoring system, radiographic prosthesis loosening was defined as a radiolucent line (RLL) score ≥ 9 on the femoral side or ≥ 10 on the tibial side. We utilized receiver operating characteristic (ROC) curve analysis to evaluate the cutoff value for stem length and diameter in terms of prosthesis loosening or not. Furthermore, CFR-related parameters were analyzed with logistic regression to clarify their relationships with prosthesis loosening.

RESULTS

Prosthesis loosening was detected in 17 of 65 patients included. On logistic regression analysis, male sex and severity of the tibial bone defect were associated with loosening. On multivariate analysis, male sex and bone defect severity were associated with loosening on the femoral side, while malalignment was associated with loosening on the tibial side. Protective factors included femoral CFR > 0.85, CFR > 0.7 for > 2 cm, and CFR > 0.7 for > 4 cm, as well as tibial CFR > 0.85.

CONCLUSIONS

To minimize loosening post-RTKA, femoral CFR > 0.7 for > 2 cm and tibial CFR > 0.85 are recommended. Risk factors may include male sex, bone defect severity, and malalignment.