Impact of patient, surgical, and implant design factors on predicted tray-bone interface micromotions in cementless total knee arthroplasty

Comparison of Artificial Vertebral Body Analogs to Evaluate Initial Stability of Cervical Disc Replacements

Recent studies have raised concerns regarding migration of cervical disc replacements as a significant clinical complication associated with failure. To date, no laboratory models have addressed migration. Bone analog models have been established for fixation studies of large joint replacements. Therefore, this study aimed to develop models to assess micromotions of cervical disc replacements. Five cervical disc replacement designs were biomechanically tested in flexion/extension, lateral bending, and axial rotation. These were selected to represent different clinical outcomes, including some with significant in vivo migration. Each device was tested in a (1) previously validated 3D-printed biomimetic model and (2) commercially available rigid polyurethane foam blocks. Sagittal and coronal plane micromotions were continuously measured throughout testing. Cyclic displacements were compared as a function of device design and bone analog model type. One ball-and-socket cervical device, the PCM, exhibited significantly greater micromotion in the polyurethane foam model than in the 3D-printed biomimetic model during flexion-extension and lateral bending, specifically 25.8 ± 11.4 µM versus 15.0 ± 9.5 µM (p = 0.04) and 122 ± 64 µM versus 14.5 ± 6.4 µM (p = 0.06), respectively. The large amount of micromotion with the PCM device design was consistent with clinical reports of migration leading to failure. In contrast, motions measured in the 3D-printed biomimetic model did not establish the same differences. In summary, the polyurethane foam model indicated differences between devices better in comparison to the 3D-printed biomimetic model. However, the 3D-printed model has greater potential for further material refinements to more precisely predict clinical performance with better simulation of bone mechanical properties.

A Modified Wagner Stem Design Increases the Primary Stability in Cementless Revision Hip Arthroplasty

Background

Primary stability is of great importance for the longevity of the implant in cementless revision total hip arthroplasty, since instability is a major cause of rerevision. The purpose of this study was to evaluate the effect of an additional set of less prominent, wider splines added to an established conical stem design with sharp splines on axial stability in a model with significant proximal bone defects.

Methods

Twenty fresh-frozen human femurs were implanted with either the established or the additional spline design, dynamically loaded and tested in a load-to-failure configuration. Cortical contact in the femoral canal after implantation was evaluated by superimposing computed tomography scans and 3-dimensional laser scans. Stem subsidence and micromotion were evaluated to assess primary stability.

Results

Stems remained stable during cyclic loading of up to 200% body weight, except in bones with cortical bone mineral density below 1000 mgHA/mL. A significant reduction of more than 85% in stem subsidence (P = .040), axial micromotion (P = .007), and rotational micromotion (P = .010) was achieved with the new spline design. Load-to-failure testing exceeded 400% body weight.

Conclusions

The new spline design increased the cortical contact which resulted in increased axial primary stability in this in vitro experiment. Bone mineral density as a measure of bone quality proved to be a decisive factor for achieving immediate postoperative stability. Further variations of the established stem designs could further improve the longevity of artificial joint replacements.

Impact of Surgical Alignment, Bone Properties, Anterior-Posterior Translation, and Implant Design Factors on Fixation in Cementless Unicompartmental Knee Arthroplasty

Micromotion exceeding 150 μm at the implant-bone interface may prevent bone formation and limit fixation after cementless knee arthroplasty. Understanding the critical parameters impacting micromotion is required for optimal implant design and clinical performance. However, few studies have focused on unicompartmental knee arthroplasty (UKA). This study assessed the impacts of alignment, surgical, and design factors on implant-bone micromotions for a novel cementless UKA design during a series of simulated daily activities. Three finite element models that were validated for predicting micromotion of cementless total knee arthroplasty (TKA) were loaded with design-specific kinematics/loading to simulate gait (GT), deep knee bending (DKB), and stair descent (SD). The implant-bone micromotion and the porous surface area ideal for bone ingrowth were estimated and compared to quantify the impact of each factor. Overall, the peak tray-bone micromotions were consistently found at the lateral aspect of the tibial baseplate and were consistently higher than the femoral micromotions. The femoral micromotion was insensitive to almost all the factors studied, and the porous area favorable for bone ingrowth was no less than 93%. For a medial uni, implanting the tray 1 mm medially or the femoral component 1 mm laterally reduced the tibial micromotion by 19.3% and 26.3%, respectively. Differences in tray-bone micromotion due to bone moduli were up to 59.8%. A 5 mm more posterior femoral translation increased the tray-bone micromotion by 35.8%. The presence of the tray keel prevented the spread of the micromotion and increased the overall porous surface area, but also increased peak micromotion. The tray peg and the femoral anterior peg had little impact on the micromotion of their respective implants. In conclusion, centralizing the load transfer to minimize tibial tray applied moment and optimizing the fixation features to minimize micromotion are consistent themes for improving cementless fixation in UKA. Perturbation of femoral-bone alignment may be preferred as it would not create under/overhang on the tibia.

Is Tibial Bone Mineral Density Related to Sex, Age, Preoperative Alignment, or Fixation Method in Primary Total Knee Arthroplasty?

BACKGROUND

Cementless total knee arthroplasty (TKA) has regained interest for its potential for long-term biologic fixation. The density of the bone is related to its ability to resist static and cyclic loading and can affect long-term implant fixation; however, little is known about the density distribution of periarticular bone in TKA patients. Thus, we sought to characterize the bone mineral density (BMD) of the proximal tibia in TKA patients.

METHODS

We included 42 women and 50 men (mean age 63 years, range: 50 to 87; mean body mass index 31.6, range: 20.5 to 49.1) who underwent robotic-assisted TKA and had preoperative computed tomography scans with a BMD calibration phantom. Using the robotic surgical plan, we computed the BMD distribution at 1 mm-spaced cross-sections parallel to the tibial cut from 2 mm above the cut to 10 mm below. The BMD was analyzed with respect to patient sex, age, preoperative alignment, and type of fixation.

RESULTS

The BMD decreased from proximal to distal. The greatest changes occurred within ± 2 mm of the tibial cut. Age did not affect BMD for men; however, women between 60 and 70 years had higher BMD than women ≥ 70 years for the total cut (P = .03) and the medial half of the cut (P = .03). Cemented implants were used in 1 86-year-old man and 18 women (seven < 60 years, seven 60 to 70 years, and four ≥ 70 year old). We found only BMD differences between cemented or cementless fixation for women < 60 years.

CONCLUSIONS

To our knowledge, this is the first study to characterize the preoperative BMD distribution in TKA patients relative to the intraoperative tibial cut. Our results indicate that while sex and age may be useful surrogates of BMD, the clinically relevant thresholds for cementless knees remain unclear, offering an area for future studies.

Finite element analysis in the optimization of posterior-stabilized total knee arthroplasty

Posterior-stabilized total knee arthroplasty (PS-TKA) is associated with high rates of satisfaction and functional recovery. This is notably attributed to implant optimization in terms of design, choice of materials, positioning and understanding of biomechanics. Finite elements analysis (FEA) is an assessment technique that contributed to this optimization by ensuring mechanical results based on numerical simulation. By close teamwork between surgeons, researchers and engineers, FEA enabled testing of certain clinical impressions. However, the methodological features of the technique led to wide variations in the presentation and interpretation of results, requiring a certain understanding of numerical and biomechanical fields by the orthopedic community. The present study provides an up-to-date review, aiming to address the following questions: what are the principles of FEA? What is the role of FEA in studying PS design in TKA? What are the key elements in the literature for understanding the role of FEA in PS-TKA? What is the contribution of FEA for understanding of tibiofemoral and patellofemoral biomechanical behavior? What are the limitations and perspectives of digital simulation and FEA in routine practice, with a particular emphasis on the "digital twin" concept? LEVEL OF EVIDENCE: V, expert opinion.

A multimodal assessment of cementless tibial baseplate fixation using radiography, radiostereometric analysis, and magnetic resonance imaging

Fixation in cementless total knee arthroplasty is provided by osseous integration. Radiography, radiostereometric analysis (RSA), and magnetic resonance imaging (MRI) were used simultaneously to investigate fixation. Relationships between RSA-measured implant micromotions and MRI-evaluated osseous integration at the component-bone interface were assessed in 10 patients up to 6 months postoperation. Supine MRI (using multispectral imaging sequences) and RSA exams were performed to evaluate osseous integration and measure longitudinal migration, respectively. Inducible displacement was measured from standing RSA exams. Radiolucent lines were detected on conventional radiographs. Of 10 patients, 6 had fibrous membranes detected on MRI. No fluid or osteolytic interfaces were found, and no components were scored loose. Of 10 patients, 6 had radiolucent lines detected. Average maximum total point motion (MTPM) for longitudinal migration at 6 months was 0.816 mm (range 0.344-1.462 mm). Average MTPM for inducible displacement at 6 months was 1.083 mm (range 0.553-1.780 mm). Fictive points located in fibrous-classified baseplate quadrants had greater longitudinal migration than fictive points located in baseplate quadrants with normal interfaces at 2 weeks (p = 0.031), 6 weeks (p = 0.046), and 3 months (p = 0.047), and greater inducible displacements at 3 months (p = 0.011) and 6 months (p = 0.045). Greater early micromotion may be associated with the presence of fibrous membranes at the component-bone interface. Clinical significance: This multimodal imaging study contributes knowledge of the fixation of modern cementless TKA, supporting the notion that osseous integration is important for optimal implant fixation.