The optional depth of screw for the treatment of tibial plateau defect in total knee arthroplasty: A finite element analysis

doi:10.21203/rs.3.rs-1982645/v1

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The optional depth of screw for the treatment of tibial plateau defect in total knee arthroplasty: A finite element analysis

https://doi.org/10.21203/rs.3.rs-1982645/v1

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Background: The cement-screw technique is an effective method for repairing tibial plateau defects in total knee arthroplasty (TKA). However, it is unknown which depth for the screw is better. This study aimed to perform a finite element analysis (FEA) to determine the advantage of a cement-screw technique and confirm the optimal screw depth.

Results: Four FEA models were set to represent the cement-screw or cement-only techniques. In the cement-screw technique, screws were inserted into the defect area obliquely with an angle of 45 degrees from the mechanical axis with 3 different depths. The FEA models showed that cement-only technique had more stress-shielding areas in the tibial plateau and medullary cervicitis, while higher stress areas were in the defect and medullary cervicitis. For the cement-screw technique, stresses were lower on the surface of cancellous bone around screw when it was inserted below the upper surface of tibia.

Conclusion: From the FEA data, we found that cement-screw technique is superior to cement-only technique for repairing a tibial plateau defect in TKA; for the former, it may be more beneficial to insert the screw below the upper surface of tibia plateau.

cement-screw technique

bone defect

total knee arthroplasty

finite element analysis

Peripheral bone defects of the medial tibia frequently occur after primary total knee arthroplasty (TKA) of varus knees, which requires additional management to ensure implant stabilization [1, 2]. There are several management options for these bone defects; among them are bone cement plus or minus reinforcement with screws, modular metallic augmentations, impacted bone graft, structural homologous graft, or, more recently, metal metaphyseal cones with sleeves [3, 4].

Compared to other techniques, a cement-screw approach is less time-consuming, easier to perform, and less expensive [5, 6]. Successful short-term to long-term follow-up of a cement-screw technique to correct tibial defects have been reported [7–9]. However, screw-insertion depth is based on surgeon’s experiences, with the screw head being placed under the tibial prosthesis slightly, parallel to the upper surface of the tibia, or under the upper surface of the tibia [7, 10, 11].

Currently, there is not a published study that compares the differences between depths of screws with the cement-screw technique. This study aimed to perform a finite element analysis (FEA) to determine the advantage of cement-screw technique, determine the differences among the three insert depths, and identify the optimal screw depth.

The stress at the 6 quartering points of the medial and lateral plateau is shown in Fig. 1. All stresses measured were within the normal range (0.1-2.8MPa) except one point in model 4, and no significant differences were found among the other three models. The stress nephogram showed a maximum area (blue area) less than 0.1MPa in model 4, where bone resorption of cancellous bone may occur. Model 1 had maximum areas with higher stresses (red area), followed by model 4. The other models had similar results either in the high- or low-stress areas (Fig. 2).

As shown in Fig. 3, each of the defects was divided into 4 sections (anteromedial (AM), anterolateral (AL), posteromedial (PM), and posterolateral (PL)), and the stresses at 4 points were measured in each section. The results showed that there was a stress point (PM 4) in the PM area of models 1, 2, and 3, where the stresses were less than 0.1 MPa each, and bone resorption of cancellous bone may occur. Stresses at all other points were within the normal range (0.1–2.8 MPa). Compared to model 4, the other three had significantly lower types of stress distributions, and no significant differences were found among them. The stress nephogram showed that model 2 had the maximum area (blue area), where the stresses were less than 0.1 MPa and bone resorption of cancellous bone may occur. Although model 4 (cement-only) did not have a bone resorption area, the mean stresses were higher than the other models (Fig. 4).

The stresses on the surface of cancellous bone in the medullary cervicitis of all the models are shown in Fig. 5. There was a stress point of less than 0.1 MPa on the anterior surface of cancellous bone in all models, where local stresses shielding may occur. Model 4 had two stress points with stress shielding on the medial and lateral sides; all the other stresses measured were within the normal range (0.1–2.8 MPa). In addition, stresses were significantly higher in model 4 than in the others on most points measured. However, no significant differences were found among models 1, 2, and 3. From the stress nephogram of each model, we found that model 4 had the maximum area (blue area) where the stresses were less than 0.1 MPa and bone resorption of cancellous bone may occur, and maximum areas with high stresses (red area) (Fig. 6).

The stresses on the surface of cancellous bone around the screws in all of the models are shown in Fig. 7. All stresses measured were within the normal range (0.1-2.8MPa). Model 3 had significantly lower stresses on the surface (anterior, posterior, and lateral) of cancellous bone compared with models 1 and 2. From the stress nephogram of each model, we found that model 3 had the maximum area (blue area), followed by model 2 and then model 1, where the stresses were less than 0.1 MPa and bone resorption of cancellous bone may occur. However, model 3 had the minimum area of high stresses (red area). The maximum stress point on the surface of the cancellous bone around the screw was 0.75 MPa in model 1, which was significantly higher than 0.65 MPa in model 2 or 0.63 MPa in model 3 (Fig. 8).

Tibial bone defects are considered one of the most challenging issues encountered during TKA. There are many types of basic reconstruction methods, depending on the situation. Compared to other methods, cement-screw technique has many advantages, such as being more stable and reinforced than a cement-only technique and a simpler procedure than metal augmentation [5, 6]. Traditionally, bone cement only was used to fill defects less than 5 mm in depth after proximal tibial resection [12]. In contained or uncontained defects that measure 5–10 mm, some have suggested the use of cement-screw technique [13]. Further, studies also suggest that the cement-screw technique may be used in AORI type 1 and 2A bone defects affecting less than 50% of the femoral condylar width and up to 10 mm in depth [14, 15]. Ritter et al. [9] first displayed the efficacy of screws and cement when correcting large tibial defects using in vivo studies on a series of 47 primary TKAs with mild bone loss. With an average of 6.1 years of follow-up, no failures were noted in knees for defects > 5 mm, while nonprogressive radiolucent lines < 1 mm wide were found in 27% of TKAs. Recently, more studies have reported this surgical technique, some describing the screw depths. In a cross-sectional study with 37 knees of 28 patients, Ozcan et al. [11] showed that the cement-screw technique is associated with satisfactory clinical outcomes with a mean follow-up period of 44 months. In that study, the screw heads were inserted lower than the upper surface of the tibia. In a previous study, Liu et al. [10] placed the screw parallel to the upper surface of the tibia; with a mean of 27.5 months of follow-up, they demonstrated that a cement-screw technique was a simple and safe method to repair tibial plateau defects in TKA. In another study with up to 20 years of follow-up, 14,686 primary TKAs were performed, and 256 of them received a cement-screw technique for tibial defects. In this study, the screw head was inserted just below the level of the tibial implant when stabilized on the remaining metaphyseal bone. The results showed that knees with tibial defects and screws performed similarly, if not better than knees without defects, at substantially lower costs than alternatives [7]. Further, the authors used the same technique for a revision TKA and highly recommend the cement-screw technique to correct large defective revision TKAs [16]. However, they also indicated that a cement-screw technique does not have the ability for bone stock restoration, and there is a risk of aseptic loosening with a possibility of cement fracture. Although these studies confirmed the benefits of a cement-screw technique, the depths of the screws inserted seems to depend on the surgeons’ experience, and optimal depth is controversial.

Higher elastic modulus of metallic screws relative to normal bone can lead to "stress shielding" effect that results in localized osteopenia; different depths of screws may lead to different effects [17]. To validate the differences, we set up four FEA models to simulate the cement-only technique and cement-screw technique with three screws in different depths. The load application area used was based on the conditions that occur in the late stance phase of gait, where maximum joint reaction occurs [18]. The fixed application area used was the distal joint surface contacted with the astragalus. Based on the FEA result, we found that stresses were higher on the surface of cancellous bone in the medullary cervicitis and defects in cement-only technique. Meanwhile, more stresses shielding area was also found in cement-only technique, especially the surface of cancellous bone in the medullary cervicitis. For the three models with screws, we found that stresses on the surface of cancellous bone around the screw were significantly lower when the screw was inserted below the upper surface of the tibia. The results indicated that it might be more beneficial to use a cement-screw technique and insert the screw below the upper surface of the tibia for repairing tibial plateau defect in TKA, but possible stress shielding may still be a concern.

There are some limitations associated with our study. Firstly, the model in this study only imitated one prosthesis, and there are other types of prosthesis we did not consider but this study is still a good reference for clinical practice. Secondly, there remains controversy about the optimal screw angle; we inserted the screws obliquely only with an angle of 45 degrees from the mechanical axis. Thirdly, the diameter and length of cancellous screws used were 4 mm and 14 mm, as suggested by our senior surgeons, while different diameters, materials, and lengths of screws may have different effects on the stresses. Fourth, the thickness of cement under the tibial prosthesis was set to 1.5 mm, which may not be a complete representation of real situations. However, we believe this study may provide guidance to those performing TKA on patients with tibial bone defects. Further studies are needed to confirm the optimal angle, diameter, material, and screw length.

From the FEA data, we found that cement-screw technique is superior to cement-only technique for repairing a tibial plateau defect in TKA; for the former, it may be more beneficial to insert the screw below the upper surface of tibia plateau.

Solid model

A three-dimensional model of the tibia was reconstructed from computed tomography (CT) images of a healthy female volunteer (165 cm in height and 50 kg in weight) using Mimics 21.0 (Materialise Company, Leuven, Belgium). We used threshold segmentation and three-dimensional model calculations to build a three-dimensional model of the tibia. Next, the model was imported into the automatic reverse engineering software, Geomagic Studio 12.0 (3D Systems, Inc., North Carolina, USA), for optimization, to obtain a finer bone tissue model. This model was imported into Solidworks 2016 (Dassault Systems SolidWorks Corp., Waltham, Massachusetts, USA) to be assembled with other parts.

Bone defect model

Clinically, most defects are located in the posteromedial region. Using a standard tibial model, a posteromedial bone defect (12% of total plateau in area, 8 mm in depth) was made after performing a horizontal resection of 9 mm above the tibial plateau. This type of defect area and depth are usually treated with the cement-screw technique [11, 19].

Tibial component

As our previous study [20], an appropriate modular tibial baseplate made of cobalt-chrome alloy (#2 size, U2™ Knee, United Orthopedic, Taiwan), applied in accordance with the manufacturer’s guidelines, was analyzed. The geometry of the modular tibial component was determined by direct measurement. The posterior-stabilize fixed-bearing ultra-high molecular weight polyethylene (UHMWPE) insert was 9 mm thick and had rounded-top, and flat-inferior surfaces. The tibial component was positioned at the tibial plateau.

Screw and Bone cement models

A 4 mm x 14 mm cancellous screw (Smith and Nephew, Memphis, TN, USA) was inserted into the defect area obliquely with an angle of 45 degrees from the mechanical axis. A 1.5 mm thick cement layer was used to fix the tibial tray underside to the resected surface [21]. For simplification, bone separated from the defect area was redefined as the bone cement model filling the defect area. All parts were finally assembled in SolidWorks software (Fig. 9).

Material properties

The models were imported into ANSYS Workbench 17 (Swanson Analysis Systems, Inc., Houston, Pennsylvania, USA) for meshing and simulation (Fig. 10). The performances of the component materials are shown in Table 1 [19, 22]. The contact behaviors between 1) UHMWPE and tibial prosthesis, 2) tibial prosthesis and cement, 3) cement and bone, 4) screw and cement, and 5) tibial prosthesis and bone were defined as fully bonded. The contact behavior between the screw and cancellous bone was defined as frictional surface-to-surface with a frictional coefficient of 0.25 [23]. All implant components were modeled as linear elastic isotropic materials [24].

Table 1 The performance of component materials in each model.

Material	E (MPa)	v
Cortical bone	17,000	0.3
Cancellous bone	700	0.3
UHMWPE (tibial insert)	2,300	0.25
cobalt-chrome alloy (tibial prothesis)	248,000	0.3
PMMA (cement)	2,270	0.46
Titanium alloy (screw)	110,000	0.3

Boundary and loading conditions

Traditionally, biomechanical loading on the knee joint during walking is estimated to exceed 2–3 times body weight [18]. More recently, somewhat lower levels of loading (2.2 times body weight) have been actually measured in-vivo, which is considered more appropriate [25]. A total load of 1,100 N (representing a 50 kg person) was used and applied in a 1:1 ratio between the medial and lateral plateaus. The inferior surface of the distal tibia was fixed in all directions (Fig. 11). A conservatively low value for damage stress (2.8 MPa) of cancellous bone was adopted while, a resorption threshold of 0.1 MPa was used to assess whether reduced stresses might lead to bone resorption of cancellous bone [26].

Four FEA models were set up to determine the advantage of cement-screw technique and the differences among three different screw depths (Fig. 12): model 1, the superior border of screw was in direct contact with the tibial plant; model 2, the superior border of screw was parallel to the upper surface of the tibia; model 3, the superior border of screw was lower than the upper surface of the tibia and model 4, cement-only without screw. The different stresses were compared using 6 points on the midcourt line of the medial and lateral plateaus, 16 points on the surface of the defects, 16 points on the surface of cancellous bone in the medullary cavity, and 16 points on the surface of cancellous bone around the screws, all of which were measured (Fig. 13).

Ethics approval and consent to participate

All the methods of this study were carried out according to the ethical guidelines of the Helsinki Declaration and were approved by the Human Ethics Committee of Honghui hospital. Written informed consent was obtained from all participants.

Consent for publication

Not applicable

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Funding

Not applicable.

Authors' contributions

JM and SY contributed to the conceptualization and methodology. GZ contributed to the validation and data curation. JW contributed to the modeling and visualization, and was a major contributor in writing the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors thank AiMi Academic Services (www.aimieditor.com) for the English language editing and review services.

Authors' information

All authors come from the Orthopedics of Honghui hospital, Xi’an, Shanxi. JM, director of the department, has finished 3000 total knee arthroplasties in the past 10 years and was very skilled in hip and knee arthroplasties. SY and GZ, associate professors of medicine, are very skilled in the clinical research and writing. JW, visiting staff of the department, are very skilled in the writing and data collecting.

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No competing interests reported.

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The optional depth of screw for the treatment of tibial plateau defect in total knee arthroplasty: A finite element analysis

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Abstract

Figures

Introduction

Results

Discussion

Conclusions

Methods

Solid model

Bone defect model

Tibial component

Screw and Bone cement models

Material properties

Boundary and loading conditions

Declarations

References

Additional Declarations

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