Managing large type 2a bone defects from osteoporotic tibial plateau fractures

Abstract

Aims

This computational study evaluated the biomechanical performance of asymmetric metaphyseal cones in managing large type 2a bone defects from osteoporotic tibial plateau fractures.

Methods

Using a pre-existing finite element model of a synthetic medium-sized tibia, six defect patterns with 50 mm cemented stems were analyzed under walking (WA) and stair descending (SD) loads. Models varied in cone unsupported ratios (R_u = 0%, 16%, 32%) and four bone quality levels (stiffness at 100%, 80%, 60%, and 40% of normal values). Gap healing thresholds (0 to 50 μm) were evaluated to assess bone-implant contact areas. Additional long-stem configurations (100 mm) were analyzed for severe osteoporotic conditions.

Results

At 50 μm gap healing threshold, severe defects (R_u = 32%) achieved maximum geometrically possible contact area of 55%, while no-defect bone (R_u = 0%) reached 100%. Maximum micromotion (89 μm) remained below the critical osseointegration threshold of 150 μm, but interface area experiencing elevated micromotion (50 to 90 μm) increased markedly with deteriorating bone quality. High-strain regions formed at the inferior cone surface and stem base, with up to 16% of cancellous bone exceeding yield thresholds in severe osteoporotic models during SD. Long-stem configurations reduced supraphysiological strain volume by 47% compared to the short stem, but induced strain concentrations at the stem terminus. Medial defects generated higher strains than lateral configurations but exhibited more uniform strain distribution patterns.

Conclusion

Metaphyseal cones provide adequate biomechanical stability for tibial plateau fractures in osteoporotic bone. The findings suggest that gap healing enables maximal bone-implant contact within geometrical constraints, supporting the clinical use of these implants with appropriate patient selection and postoperative management.

Cite this article: Bone Joint Res 2026;15(6):662–674.

Article focus

• How did asymmetric metaphyseal cones perform biomechanically in osteoporotic tibial plateau fractures under physiological loading?

• How did bone quality, gap healing threshold, and stability metrics (micromotion, contact area, strain) relate quantitatively?

• How did different stem lengths (50 mm vs 100 mm) affect performance in severe osteoporotic conditions?

Key messages

• Metaphyseal cones provided adequate stability for osteoporotic tibial plateau fractures, with biomechanical stability metrics (micromotion and strain) decreasing predictably as bone quality deteriorated but remaining below critical failure thresholds.

• Gap healing enhanced bone-implant contact, enabling severe defects to achieve 55% contact area, representing a 2.9-fold improvement over initial conditions.

• Long-stem configurations reduced supraphysiological strain by 47% in severe osteoporosis but created strain concentrations requiring careful patient selection.

Strengths and limitations

• This was a systematic analysis of 54 models with variation of bone quality, defect patterns, and loading conditions, which took a conservative approach using established thresholds and worst-case scenarios.

• Linear elastic material properties and static analysis cannot capture time-dependent bone behaviours or the cyclic loading experienced during daily activities.

• Computational predictions provide evidence-based surgical guidance, but require clinical validation through long-term outcome studies.

Introduction

Tibial plateau fractures (TPFs) constitute approximately 8% of fractures in individuals aged over 60 years, with prevalence projected to increase as global life expectancy rises.¹ These fractures present management challenges in elderly patients, particularly those with osteoporosis, due to compromised bone quality and elevated complication rates.^2,3 Conventional fixation methods in osteoporotic bone are associated with higher risks of fixation failure, reduction loss, and post-traumatic osteoarthritis, especially when joint depression exceeds 15 mm.^2,4,5

Acute total knee arthroplasty (TKA) has emerged as a viable alternative for managing severe TPFs in elderly patients.^6-8 However, the proximal tibial bone loss inherent in these fractures creates substantial challenges for achieving implant stability. Metaphyseal cones offer a promising solution by replacing damaged bone tissue and reconstructing the metaphysis, thereby establishing a stable platform for the tibial component.^9,10 Clinical investigations have demonstrated favourable outcomes with porous titanium cones in the treatment of severe bone defects in revision knee arthroplasty.^11-13

Although the biomechanical performance of symmetric cones has been investigated under standard conditions to treat defects up to 10 mm in depth,^14-17 the evidence regarding the efficacy of asymmetric cones in osteoporotic bone, which is characterized by diminished bone quality and altered mechanical properties, has been limited in the literature. Furthermore, current literature lacks systematic finite element (FE) models that accurately represent the sequential relationship between initial bone-cone contact, gap healing, and subsequent osseointegration.

Gap healing represents a critical process where bone cells bridge the space between implant and host bone, facilitating subsequent osseointegration and long-term implant stability.^18,19 This cellular bridging can proceed despite imperfect initial contact, provided the interfacial gap remains within biologically acceptable dimensions and interface micromotion stays below the critical threshold of 150 μm.²⁰ When micromotion exceeds this threshold, fibrous tissue formation predominates instead of osseointegration, regardless of whether initial gap healing occurs successfully. Osteoporosis compromises both gap healing and osseointegration through reduced osteoblast activity, impaired bone remodelling capabilities, and diminished structural integrity of the surrounding bone architecture.^2,21-23

This FE study aimed to evaluate the biomechanical performance of asymmetric metaphyseal cones in simulated osteoporotic bone models with medial and lateral TPF defects. By systematically varying bone stiffness parameters to represent different degrees of osteoporosis and modifying contact parameters to reflect bone healing threshold, this investigation provides quantitative estimates of implant stability under physiological loading conditions. The analysis focuses on three primary outcome measures: 1) bone-implant contact area as a function of gap healing threshold; 2) interface micromotion as a predictor of osseointegration success; and 3) bone strain distributions as indicators of load-bearing regions. By establishing biomechanical thresholds for safe clinical application, this research seeks to optimize metaphyseal cone use in osteoporotic patients with TPFs requiring acute TKA.

Methods

Finite element model construction and validation

A pre-existing 3D FE model of a synthetic medium-sized tibia was employed for this study.^24-27 The model was sectioned 9 mm below the lowest point of the lateral tibial plateau along the mechanical axis to simulate the TKA resection plane. A 200 mm tibial segment was selected and isolated for analysis. The tibial baseplate was aligned parallel to the reference line extending from the posterior cruciate ligament (PCL) midpoint to the medial third of the tibial tubercle, achieving 89% baseplate coverage with a maximum peripheral overhang of 1 mm.

The implant system comprised a universal size-five baseplate (5521-B-500; Stryker, USA), an asymmetric size C cone (5549-A-232 for lateral defects and 5549-A-231 for medial defects; Stryker Orthopaedics), and a short cemented stem (10 mm × 50 mm; Stryker Orthopaedics). Bone cement with thickness ranging from 1 to 5 mm filled the interstitial spaces (with the maximum thickness at the stem tip to manage distal strain concentrations), preventing direct bone contact with the baseplate and stem, ensuring that direct osseointegration occurs exclusively at the cone-bone interface.²⁸

The FE mesh consisted of second-order tetrahedral elements (10-node) with an element size ranging from 1 to 3 mm and localized refinement at bone-cone interfaces. Mesh convergence analysis confirmed numerical stability, with displacement variations less than 1% at element counts exceeding 418,734 and 411,501 for the lateral and medial no-defect models, respectively.

Tibial plateau fracture simulation

As fractured areas cannot provide mechanical support, TPFs were modelled as uncontained Anderson Orthopaedic Research Institute (AORI) type 2a defects, corresponding to Schatzker type II and type IV fractures.^8,29 Defect severity was quantified by the unsupported external surface area ratio of the cone (R_u). Six distinct fracture patterns were created with R_u values of 0% (no defect), 16% (partial cone lobe support loss), and 32% (complete cone lobe support loss) for both lateral and medial configurations (Figure 1).

Fig. 1.

Tibial implant configuration and defect modelling. a) 3D bone model with tibial baseplate, asymmetric cone, and cemented stem. b) Lateral (L) and medial (M) defect patterns showing unsupported cone area ratios (R_u) of 0%, 16%, and 32%. c) Comparison between lateral and medial defect volumes in cancellous and cortical bone.

Fracture plane geometries were implemented according to defect location. Lateral models used a sagittal cutting plane. In medial models, the baseline fracture configuration (R_u = 16%) originated at the PCL midpoint and extended at 46° anterolaterally relative to the posterior tibial condylar axis in the axial plane. The fracture line intersected the cortex 40 mm distal to the TKA resection plane. For severe medial defects (R_u = 32%), the fracture plane was rotated laterally by 15° while maintaining both the PCL midpoint and cortical intersection points as fixed references.

Material properties and osteoporotic bone modelling

All materials were characterized as homogeneous, isotropic, and linear elastic (Table I).^26,30-32 The porous surface of the cone was represented as a homogeneous solid layer with reduced effective elastic modulus.²⁵ To simulate varying degrees of bone quality deterioration, four distinct stiffness configurations were implemented by systematically reducing the Young’s modulus of bone (Table II).²⁵ The baseline configuration (E₁₀₀) maintained standard values (cortical: 15,250 MPa; cancellous: 449 MPa), while subsequent configurations reduced cortical and cancellous bone stiffness to 80% and 76% (E₈₀), 60% and 52% (E₆₀), and 40% and 28% (E₄₀) of the baseline values, respectively.²¹

Table I.

Linear elastic material properties of implant components used in the finite element model.^30-32

Tibial component	Young’s modulus (MPa)	Poisson’s ratio
Baseplate (cobalt-chromium alloy)	210,000	0.3
Cone (titanium Ti-6Al-4V)	117,000	0.3
Cone coating	6,200	0.3
Stem (titanium Ti-6Al-4V)	117,000	0.3
Bone cement	2,280	0.3

Table II.

Linear elastic material properties of osteoporotic bone (where E₁₀₀ and E₄₀ represent the healthy bone and the most severe osteoporosis, respectively).^21,25

Bone quality level	Cortical Young’s modulus (MPa)	Cancellous Young’s modulus (MPa)	Poisson’s ratio
E100	15,250	449	0.3
E80	12,200	341	0.3
E60	9,150	233	0.3
E40	6,100	126	0.3

E, Young's modulus.

Physiological loading and boundary conditions

Two physiological loading scenarios were considered (Table III): walking (WA) and stair descending (SD). Each loading scenario incorporated composite forces and moments along the lateral (X), superior (Y), and anterior (Z) directions derived from standardized gait analysis data for a 75 kg individual.³³ The distal end of the tibia model was fully constrained in all degrees of freedom. Applied loads were transmitted through a single reference point kinematically coupled with all nodes on the superior baseplate surface.

Table III.

Physiological forces and moments applied in walking and stair descending loading simulations.³³

Loading scenario	Fx (N)	Fy (N)	Fz (N)	Mx (Nmm)	My (Nmm)	Mz (Nmm)
WA	-13	-1,957	-45	-11,998	6,302	17,571
SD	39	-2,335	-140	-20,424	1,135	18,658

F, force; M, moment; SD, stair descending; WA, walking; X, lateral; Y, superior; Z, anterior.

Bone-implant interface mechanics

The bone-cone interface was modelled using Coulomb friction (μ = 0.35 for the bone-cone body interfaces and μ = 1.01 for bone-coating interfaces).^34-36 The interfaces between cement-implant and cement-bone were defined as bonded contacts (tie constraints). To simulate varying bone healing thresholds, four gap healing thresholds (0 μm, 0.5 μm, 5 μm, and 50 μm), representing increasingly permissive contact criteria, were implemented to evaluate bone-implant contact areas. This parametric approach quantified potential bone ingrowth capability across interfacial gaps of different magnitudes.^18,37 The percentage of available contact area was calculated for each configuration to characterize interfacial stability. All analyses were performed in Abaqus 2021 (Dassault Systèmes Simulia, USA).

Biomechanical stability assessment

In this study, micromotion was evaluated based on the most conservative contact threshold (gap healing potential = 0 μm) to assess implant stability under the worst-case conditions, using an established osseointegration threshold of 150 μm.³⁸ Micromotions at the bone-coating interface were categorized into tangential and normal components. Tangential micromotion refers to movements parallel to the interface, representing shear or sliding displacement.³⁹ Normal micromotion denotes displacement perpendicular to the interface, including separation or compression.³⁹ The distribution of micromotion was quantified by calculating the percentage of interface area within specific magnitude ranges. Bone mechanical response was assessed through maximum (tension) and minimum (compression) principal strain distributions, with yield thresholds of 0.5% for tension and -0.7% for compression applied to cancellous bone.^40,41 The volumetric proportion of bone exceeding these thresholds was calculated for each configuration.

This systematic analysis encompassed 48 distinct models: two defect locations (lateral and medial) × three unsupported ratios (R_u = 0%, 16% and 32%) × four bone stiffness values (E₁₀₀, E₈₀, E₆₀, E₄₀) × 2 loading conditions (WA and SD). Additionally, six models with long cemented stems (10 mm × 100 mm) were analyzed for the most severe osteoporotic bone condition (E₄₀) under the most critical loading scenario (SD), bringing the total to 54 models. This matrix provides a systematic assessment of asymmetric metaphyseal cone performance in osteoporotic tibial plateau fractures under physiological conditions with different stem lengths.

Results

Bone-implant contact area

The bone-implant contact area was calculated as the percentage of the cone coating surface within specified gap healing thresholds (0 μm, 0.5 μm, 5 μm, and 50 μm), representing different bone ingrowth capabilities across interfacial gaps. Four configurations were analyzed: lateral defects during WA (L-WA), lateral defects during SD (L-SD), medial defects during WA (M-WA), and medial defects during SD (M-SD). Figure 2 demonstrates that the average potential contact area varies across these configurations with different gap healing thresholds. The results show an inverse relationship with the cone unsupported ratio (R_u) and a positive correlation with bone healing capacity. At the initial implant condition with no gap healing, the contact area was approximately 45% for no-defect bone (R_u = 0%) and 19% for the maximum defect (R_u = 32%) (Figure 2a). For a given R_u, contact area increased as bone stiffness decreased. When the gap healing threshold was raised to 0.5 μm, the contact area stabilized across all configurations (Figure 2b). However, at 5 μm threshold, the trend reversed, with reduced bone stiffness markedly diminishing the contact area (Figure 2c). At a healing threshold of 50 μm, the contact area became largely insensitive to stiffness variations, with no-defect bone and maximum defect reaching peak contact areas of 100% and 55%, respectively (Figure 2d).

Fig. 2.

Average potential contact area between bone and cone coating across four loading-defect configurations after gap healing. The configurations include lateral defects during walking (L-WA), medial defects during walking (M-WA), lateral defects during stair descending (L-SD), and medial defects during stair descending (M-SD), with varying bone stiffness (E₁₀₀ to E₄₀) and cone unsupported ratio (R_u). Results are shown under different gap healing thresholds: a) 0 μm, showing increased contact with reduced stiffness; b) 0.5 μm, with stabilised contact across configurations; c) 5 μm, where contact decreases with lower stiffness; d) 50 μm, approaching maximum contact. E, Young’s modulus

Interface micromotion

Micromotion magnitudes increased with both higher R_u and lower bone stiffness. Tangential (Figure 3) and normal (Figure 4) micromotion displayed opposite spatial patterns across all scenarios. Tangential micromotion mainly occurred in the posterior region and at the lobe base, whereas normal micromotion was concentrated anteriorly. In lateral defect models with R_u = 32%, separation at the posterior superior region produced a peak tangential micromotion of 89 μm in the posterior inferior area during L-SD. Figure 5 and Supplementary Figure a illustrate that elevated micromotion (50 to 90 μm) was especially sensitive to stiffness variations. Higher magnitudes were observed in tangential compared to normal micromotion, SD compared to WA, and medial compared to lateral defects. However, in R_u = 32% configurations, peak lateral tangential micromotion exceeded that of medial defect configurations.

Fig. 3.

Superior view of tangential micromotion contours (0 to 90 μm) at the bone-coating interface for the four scenarios (L-WA, M-WA, L-SD, M-SD), highlighting posterior concentration. E, Young’s modulus; L, lateral; L-SD, lateral defects during stair descending; L-WA, lateral defects during walking; M, medial; M-SD, medial defects during stair descending; M-WA, medial defects during walking.

Fig. 4.

Superior view of normal micromotion contours at the bone-coating interface for four scenarios: L-WA, M-WA, L-SD and M-SD, showing anterior concentration. E, Young’s modulus; L, lateral; L-SD, lateral defects during stair descending; L-WA, lateral defects during walking; M, medial; M-SD, medial defects during stair descending; M-WA, medial defects during walking.

Fig. 5.

Area percentage distributions of tangential micromotion (20 to 90 μm) across L-WA, M-WA, L-SD and M-SD, illustrating higher magnitudes during stair descending conditions and in most medial defect configurations. E, Young’s modulus; L, lateral; L-SD, lateral defects during stair descending; L-WA, lateral defects during walking; M, medial; M-SD, medial defects during stair descending; M-WA, medial defects during walking.

Principal strain

Figure 6 and Figure 7 illustrate maximum principal strain contour plots for lateral and medial defects under SD loads, with corresponding minimum principal strain contours provided in Supplementary Figures b and c. High-strain regions were consistently located at the inferior cone surface and the stem (cement) base, particularly near geometrical discontinuities. Following complete lobe support loss (R_u = 32%), strains intensified at the stem base region. The posterior bone-cone interface contact area exhibited elevated compressive strains. M-SD configurations generated higher strains compared to L-SD configurations. However, the increased bone-implant contact area characteristic of medial defects facilitated more homogeneous strain distribution.

Fig. 6.

Superior and coronal views of maximum principal strain contours for lateral defects during stair descending, showing strain concentration at the cone inferior surface and stem base with varying stiffness (E₁₀₀ to E₄₀). E, Young’s modulus; L, lateral.

Fig. 7.

Superior and coronal views of maximum principal strain contours for medial defects during stair descending, with more uniform distribution due to increased contact area. E, Young’s modulus; M, medial.

Bone stiffness was the primary factor affecting the volume of bone exceeding yield strain thresholds (Figure 8). In M-SD configurations, the severely osteoporotic model (M₃₂E₄₀) had approximately five times the yielding volume of the moderately compromised model (M₃₂E₆₀), reaching 16%. Overall, the volume of yielding bone followed an ascending order: L-WA < M-WA < L-SD < M-SD. The yielding pattern demonstrated a clear dependence on both defect location and loading condition, with strain concentrations most prominent during SD activities in medial defects with severe osteoporosis.

Fig. 8.

Percentage of cancellous bone volume exceeding yield strain thresholds (0.5% tension, -0.7% compression) for: a) L-WA; b) M-WA; c) L-SD; d) M-SD, with exponential increases in severe osteoporosis (E₄₀). E, Young’s modulus; L, lateral; M, medial.

Biomechanical performance of the long stem

Comparative analysis of the long-stem configurations in the most severe osteoporotic bone (E_₄₀) during SD demonstrated marked improvements in biomechanical stability compared to the short-stem configurations. Long-stem configurations reduced both tangential and normal micromotion compared to short-stem models across all fracture configurations (Figure 9). The area percentage experiencing elevated micromotion (50 to 90 μm range) was diminished in all defect patterns, indicating enhanced interfacial stability.

Fig. 9.

Micromotion analysis for long-stem lateral and medial fracture models with the most severe osteoporosis (E₄₀) during stair descending: a) Superior view of tangential and normal micromotion contours at the bone-coating interface. b) Area percentage distributions of tangential and normal micromotions (20 to 90 μm). L, lateral; M, medial.

Principal strain analysis demonstrated that long-stem configurations effectively redistributed mechanical loads away from the metaphyseal region, resulting in reductions in bone volume exceeding yield thresholds (Figure 10). In the most severe medial defect configuration (M₃₂E₄₀) during SD, long-stem configurations reduced the percentage of cancellous bone experiencing supraphysiological strain from 16% to 8.5%, representing a 47% improvement. However, localized strain concentrations were observed at the stem terminus in long-stem configurations.

Fig. 10.

Strain analysis for long-stem lateral and medial fracture models with the most severe osteoporosis (E₄₀) during stair descending: a) Superior and coronal views of maximum and minimum principal strain contours. b) Percentage of cancellous bone volume exceeding the yield strain thresholds. L, lateral; M, medial.

Discussion

This FE analysis quantitatively estimated the biomechanical performance of asymmetric metaphyseal cones implanted with 50 mm cemented stems in managing large uncontained defects in osteoporotic bone with TPFs during acute TKA. Through assessment using varying bone stiffness parameters and gap healing thresholds, the investigation elucidated complex relationships among diminished bone quality, interface contact mechanics, and osseointegration potential.

At the 0 to 0.5 μm gap healing thresholds, an inverse relationship was observed between bone stiffness and potential implant contact area: a biomechanical phenomenon attributable to the enhanced deformability of low-density cancellous bone, which more effectively conformed to implant geometry under compressive loading. Nevertheless, this increased contact area did not necessarily translate to improved mechanical stability, as reduced bone stiffness simultaneously generated elevated micromotion and strain distributions. When the gap healing threshold was elevated to 5 μm and beyond, contact area demonstrated a strong positive correlation with bone stiffness. At the 50 μm threshold, representing maximum biological compensatory mechanisms, all configurations approached their maximum theoretical contact area based on geometrical constraints irrespective of bone stiffness values. With the contribution of bone gap healing, the no-defect (R_u = 0%) and the largest defect (R_u = 32%) models could achieve the maximum potential contact area of 100% and 55%, respectively, which was 2.2 and 2.9 times greater than the bone-implant contact area at the initial implantation stage. This threshold-dependent reversal underscored the critical importance of considering both initial mechanical conditions and biological healing capacity when formulating surgical interventions. Bone stiffness reduction created a complex contact pattern: more areas with perfect contact, but also more regions with larger gaps. This phenomenon may explain why osteoporotic patients may show adequate initial implant seating but higher long-term failure rates.^2,23

Interface micromotion analysis demonstrated distinct biomechanical patterns that varied systematically with defect configuration and bone quality. The mechanical differential between posterior (primarily tangential micromotion) and anterior regions (primarily normal micromotion) intensified proportionally with decreasing bone stiffness, particularly when the supporting lobe was completely compromised. Notably, while medial defects generally produced higher tangential and normal micromotions than lateral defects, this pattern changed in severely osteoporotic bone (E₄₀) with maximum unsupported ratio (R_u = 32%). The separation at the posterior superior surface in lateral defects reduced bone-implant contact area and consequently concentrated tangential micromotion at the posterior inferior surface.

Although maximum recorded micromotion (89 μm) remained below the established critical threshold for osseointegration (150 μm),³⁸ the area proportion of interface experiencing elevated micromotion (50 to 90 μm) increased markedly with diminishing bone stiffness, suggesting substantially reduced biomechanical safety margins in severe osteoporotic conditions. These quantitative estimates indicate that while metaphyseal cones maintain theoretical osseointegration potential in osteoporotic bone, the interfacial biomechanical environment becomes progressively less favourable as bone quality deteriorates.

Principal strain analysis demonstrated that regions of elevated strain are consistently observed at the geometrical interfaces, particularly at the cone’s inferior surface and stem base. The volume of bone exceeding yield strain thresholds increased markedly as bone stiffness declined. In the most severe osteoporotic model (M₃₂E₄₀) during SD, up to 16% of cancellous bone exceeded critical yield thresholds, indicating regions susceptible to irreversible plastic deformation or microfracture propagation. The predominant moment about the lateral axis (M_x) governed the principal strain distributions in the coronal plane. Elevated tensile strains at the implant base may predispose to interfacial debonding or progressive implant loosening, while concentrated compressive strains at the posterior interface may contribute to secondary fractures. Furthermore, anterior interface separation altered load transfer mechanisms, potentially inducing stress shielding phenomena.^42-44 With progressive reduction in lobe support, strain concentrations intensified at the stem terminus, a biomechanical factor strongly associated with the clinically documented ‘end-of-stem pain’ phenomenon.^45,46

Bone volume experiencing supraphysiological strain increased markedly with decreasing bone stiffness. This finding indicated a substantially elevated risk for maladaptive bone remodelling processes in osteoporotic patients. Although physiologically normal bone demonstrates the capacity to accommodate localized yielding through microdamage repair mechanisms,⁴⁷ osteoporotic bone exhibiting compromised regenerative capacity may be unable to withstand the elevated strain environment, potentially leading to progressive bone loss and implant subsidence.²³ Longer stems may be needed in conjunction with cones to manage these phenomena in osteoporotic patients.³⁹ The comparative analysis of long-stem configurations confirmed this hypothesis, demonstrating considerable improvements in metaphyseal stability through enhanced diaphyseal fixation. Extended stems demonstrated reductions in micromotion across all fracture configurations. Additionally, supraphysiological strain volume decreased by 47% in severely osteoporotic bone (E₄₀), indicating that extended stems can effectively bypass compromised metaphyseal bone quality. However, this enhanced proximal stability was achieved at the expense of increased strain concentration at the stem terminus, creating potential risk zones for end-of-stem complications.^45,46 These findings support the clinical rationale for longer stems in severely osteoporotic patients while highlighting the importance of careful patient selection to minimize distal complications.

Biomechanically, the posteromedial tibial plateau functions as the primary load-bearing region during physiological gait cycles.⁴⁸ In this study, medial TPFs generated more micromotion and strain compared to the lateral fractures under most conditions. However, contact alterations due to osteoporosis presented unique challenges for implant stability in lateral defects, particularly with severe bone quality deterioration (E₄₀). In these cases, separation at the posterior superior interface resulted in concentrated tangential micromotion that exceeded values observed in medial defects. This finding highlights how defect location and bone quality interact in complex ways that cannot be predicted by considering either factor in isolation.

SD consistently produced the most adverse biomechanical environment across all tested parameters.^14,26,49 This finding was consistent with the greater forces and moments associated with SD activity, underscoring the importance of evaluating metaphyseal cone performance under diverse physiological loading conditions rather than relying solely on simplified axial loading.

The markedly elevated strain and micromotion values during SD in osteoporotic models suggest that activity modification during the early healing phase may be advantageous. Specifically, temporary restriction of SD activities could be particularly beneficial for osteoporotic patients with TPFs treated with metaphyseal cones, potentially enhancing primary stability and reducing the risk of early fixation failure or delayed osseointegration.

Primary limitations of this study included the implementation of simplified material properties and the exclusion of time-dependent mechanical behaviours. Specifically, while cortical bone is known to be orthotropic,⁵⁰ the isotropic approach used in this study is justified as the analysis primarily focuses on cone-cancellous bone interactions, with cortical bone playing a secondary mechanical role. Similarly, although trabecular bone is naturally anisotropic, heterogeneous, and time-dependent,⁵¹ it was modelled here as homogeneous. The construction of heterogeneous models typically relies on subject-specific scans with varying geometry and properties. However, as the aim of this study was to compare distinct fracture scenarios while preserving geometrical and material consistency to establish trends, a generic geometry with homogeneous properties was preferred. This approach prevented results from becoming subject-specific and maintained simulation transparency, consistent with previous methodologies.⁵²

Although bone exhibits complex viscoelastic properties and strain-rate dependency, the use of linear elastic models was justified by the relatively small deformations observed under physiological loading conditions. Previous studies have demonstrated that material nonlinearity minimally influences interface micromotion predictions, and the bone volume exceeding strain thresholds remains a reliable indicator of potential mechanical failure.^30,41 Additionally, static analysis cannot fully capture the cyclic and time-dependent loads experienced during daily activities. However, the applied loading parameters were derived from peak values observed in in vivo dynamic experiments, representing worst-case scenarios for each activity type. This approach ensured conservative estimations of implant stability and bone response. Furthermore, direct experimental validation of internal strain fields for these specific multivariable defect configurations is currently not feasible for whole-bone specimens, limiting our ability to directly corroborate the predicted strain distributions with in vitro data.

This research provides biomechanical evidence supporting the use of metaphyseal cones for managing large bone defects resulting from TPFs in osteoporotic patients requiring acute TKA. Additionally, the study provides quantitative data that can inform critical thresholds for surgical decision-making. The data suggest that with appropriate patient selection and postoperative management, these implants can achieve satisfactory stability even in compromised bone conditions. The comparative analysis of long-stem configurations demonstrated substantial biomechanical advantages in severely osteoporotic bone, with reductions in micromotion and a 47% decrease in supraphysiological strain volume. However, these benefits came at the expense of increased strain concentration at the stem terminus.

The quantitative relationship between bone quality and biomechanical performance identified in this study establishes a foundation for patient-specific preoperative planning and follow-up care. Future clinical investigations should focus on validating these biomechanical findings through long-term outcome studies of patients with varying degrees of osteoporosis treated with metaphyseal cones for TPFs.

Data Availability

All data generated or analyzed during this study are included in the published article and/or in the supplementary material.