1 Introduction

Internal fixation for tibial shaft fractures is typically accomplished using antegrade intramedullary nails, which are considered the gold standard.^1–3 When employing intramedullary nailing, it is crucial to establish an anatomically correct entry point to achieve proper alignment of the tibia postoperatively. The entry point for tibial intramedullary nailing is typically located on the medial side of the lateral tibial spine in the anteroposterior view, and at the anterior edge of the tibial plateau in the lateral view.⁴

It has become evident that a considerable degree of anatomical variation occurs in the tibia, particularly in the proximal region.^5–7 A patient-specific approach has been recommended for precise implant fit and anatomical reduction based on a principal component analysis.⁵ Previous studies utilizing 3D-CT have suggested that the design of tibial intramedullary nails (TINs) for the tibia affects the medullary canal fit.^8,9 The mismatch between the entry point and the shape of the TIN may potentially lead to malalignment after nail insertion in cases of proximal tibial shaft fractures.¹⁰ The best fit between the tibial intramedullary nail and the shape of the tibia in individual patients is achieved when the TIN aligns with the axis of the tibial shaft, and the proximal shape of the TIN matches the standard entry point position. TINs exhibit proximal bends and shape variation among manufacturers. The greater the proximal bend angle of the TIN and the longer its proximal portion, the larger the anterior offset of the proximal end of the TIN relative to its longitudinal axis. In cases of proximal tibial shaft fractures, where the offset of the entry point from the tibial shaft axis is small, using a nail with a large offset leads to postoperative malalignment due to a mismatch between the shape of the tibial medullary canal and the nail. Even in cases of midshaft and distal tibial shaft fractures, the risks of nail prominence at the entry site and intraoperative fractures increased in the presence of nail incompatibility.

The relationship between the tibial shaft and entry point in the lateral view remains poorly understood. A thorough description of the anatomical variations in the proximal tibia will enhance our understanding of the precise and safe surgical management of tibial fractures using TINs. In clinical practice, lateral radiographs of the tibia are often used to plan for TIN insertion. However, it has been suggested that even a slight rotation of the tibia can significantly change the insertion point, depending on the imaging conditions. Therefore, in the present study, we used transparent 3D-CT images to create an accurate lateral view of the tibia, minimizing the effect of rotation.

The aim of this study is to evaluate the relationship between the standard entry point for the insertion of the TINs and the tibial axis from the accurate lateral view using this method.

2 Materials and methods

2.3

2.3 Measurement

All the patients involved in the present study were scanned in the supine position. Image data were obtained from 0.5 mm slices containing the entire tibia. Transparent 3D images that replicated the radiographic tibia were created using specialized software AquariusNET (TeraRecon, Inc. Tokyo, Japan) was used for data reconstruction. The image was rotated to align with the lateral view of the tibia, where the posterior margins of the medial and lateral femoral condyles were superimposed. The sagittal tibial shaft axis was defined as the best-fit line of the 3 center points measured at the midpoint of the inner cortical boundary. These 3 points consisted of the center point at the middle of the isthmus and the center points 3 cm above and below the isthmus (Fig. 1). The anterior offset of the entry point, that is, the distance from the sagittal tibial shaft axis to the anterior edge of the tibial plateau, and the anterior offset of the tibial tubercle that is, the distance from the sagittal tibial shaft axis to the anterior edge of the tibial tubercle were measured (Fig. 2). The tibial tubercle is placed at the upper margin of the patellar tendon insertion. The longitudinal distance from the entry point to the tibial tubercle on the tibial axis, anterior-posterior length of the tibial plateau, and posterior tilt of the tibial slope were measured in the proximal tibia. Additionally, the tibial length, isthmus diameter, and distance from the center of the isthmus to the tibial end (expressed as the ratio to the tibial length) were evaluated (Fig. 3).

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Fig. 1 The sagittal tibial shaft axis The sagittal tibial shaft axis was defined as the best-fit line of the 3 center points measured at the midpoint of the inner cortical boundary. These 3 points consisted of the center point at the middle of the isthmus and the center points 3 cm above and below the isthmus.

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Fig. 2 The anterior offset of the entry point and the anterior offset of the tibial tubercle. The anterior offset of the entry point, that is, the distance from the sagittal tibial shaft axis to the anterior edge of the tibial plateau, and the anterior offset of the tibial tubercle that is, the distance from the sagittal tibial shaft axis to the anterior edge of the tibial tubercle were measured.

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Fig. 3 The tibial length (a), and the ratio in relation to the tibial length (b/a) The tibial length and distance from the center of the isthmus to the tibial end (expressed as the ratio to the tibial length) were evaluated.

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To confirm the deviation of the entry point owing to tibial rotation on the plane of the tibial plateau, a line was drawn perpendicular to the tangent of the posterior edges of the medial and lateral femoral condyles (the posterior condylar line) and passed through the medial side of the lateral tibial eminence. A line parallel to the posterior condylar line touching the anterior edge of the tibia (anterior edge line), which is the clinical anterior edge of the tibial plateau in the lateral view, was drawn. The anteroposterior and mediolateral distances between the standard entry point and the anterior edge of the tibia were investigated (Fig. 4a and b). If the entry point closely conformed to the point of tangency between the parallel posterior condylar line and the anterior edge of the tibia, the anteroposterior and mediolateral distances would be zero.

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Fig. 4 The deviation of the entry point owing to tibial rotation on the plane of the tibial plateau (a) The upper view of the tibial plateau. The standard entry point (E), the point of tangency between the parallel line of the posterior condylar line and the anterior edge of the tibia (E′), the intersection of the line perpendicular to the posterior condylar line and the anterior edge line (I). The anteroposterior distance was expressed as the length of E-I, and the mediolateral distance was E′-I. (b) The enlarged figure of the rectangle area around the entry point in Fig. 4a.

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3 Results

3.1

3.1 Measurement results

The median anterior offset of the entry point was 9.8 mm (7.2–12.2 mm). Seven patients (7 %) exhibited an anterior offset of less than 5 mm of the anterior offset at the entry point. The minimum of the anterior offset of the entry point was −2.0 mm (Fig. 5a), and the maximum was 18.6 mm (Fig. 5b). The median anterior offset of the tibial tubercle was 16.7 mm (14.6–18.9 mm). The median longitudinal distance from the anterior edge to the tubercle on the tibial axis was 24.5 mm (22.5–26.6 mm). The median anterior-posterior length of the tibial plateau was 44.8 mm (42.7–44.7 mm). The median posterior slope was 7.7° (5.1–10.2°). The median length of the tibia was 330.4 mm (314.3–341.2 mm). The median diameter of the isthmus was 10.4 mm (8.9–11.8 mm). The median ratio of the isthmus in the tibial length was 39.7 % (38.2 %–41.3 %).

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Fig. 5 Transparent 3D-CT images of the smallest anterior offset of the entry point (a) The minimum of the anterior offset of the entry point was −2.0 mm. (b) The maximum of the anterior offset of the entry point was 18.6 mm.

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4 Discussion

The present study revealed that the median anterior offset from the tibial axis to the entry point was approximately 10 mm, and 7 % of cases had an anterior offset of less than 5 mm. Oh et al.¹¹ reported that the distance from the entry point of intramedullary tibia cutting guide in TKA to the anterior margin of the tibial plateau in lateral view was mean 14.1 mm (4.1–21.2 mm), and they advocated that entry points for intramedullary tibia cutting guide should be adjusted for each individual due to significant individual variation in entry points. The distance from the tibial shaft axis to the anterior edge of the tibial plateau was greater than that observed in the present study. However, this may have been influenced by the method used to calculate the bone axis from a lateral radiograph of the proximal half of the tibia. In a study focusing on the entry point of tibial intramedullary nails using anteroposterior radiographs,¹² it was suggested that even slight tibial rotation can significantly alter the insertion point. Therefore, our study utilizes transparent 3D-CT imaging to generate lateral views of the tibia, thereby minimizing the effects of rotation.

In the report using sawbone models of extra-articular proximal tibial fractures, Byun et al. reported that anatomical reduction was not obtained with the standard sagittal entry point and entry angle.¹⁰ Schmutz et al.⁹ reported that the intramedullary fit of two types of nails inserted from the standard entry point of the tibia significantly differed utilizing computer graphic methods. Using a similar research methodology, Amarathunga et al.⁸ reported that the intramedullary fit of the tibial nail through the standard entry point was optimal in 50 % of cases, while in the remaining cases, shifting the insertion point by 5 mm resulted in better fit. These studies suggest that the ideal entry point for a tibial intramedullary nail varies, depending on the shape of the tibial medullary canal. Additionally, they indicated that the nail design that yields the optimal fit may vary individually.

In currently available tibial intramedullary nails, variations exist in the proximal bend angle and length of tibial intramedullary nails. While manufacturers do not publicly disclose the anterior offset from the intramedullary axis, the proximal bend angle ranges from approximately 27 mm–60 mm, with an angle of 10° or 10.5°. When calculating the distance from the proximal bend point of the tibial intramedullary nail to the proximal end, as well as the angle of the bend using trigonometric functions, the anterior offset ranges from 4.7 mm to 11.8 mm. Considering the results of the present study, it is evident that preoperative measurement of the anterior offset of the standard entry point for each patient is crucial for selecting the tibial intramedullary nail that offers optimal fit.

In the present study, we also demonstrated that the median anterior offset of the tibial tubercle was a median of 16.7 mm, and the distance from the entry point to the tibial tubercule on the tibial shaft taxis was a median of 24.5 mm. This result suggests that even TINs with an excessively large proximal offset from the standard entry point may be inserted by shifting the entry point distally. If the entry point was made just above the tibial tubercle, far anterior and distal to the standard entry point, the largest anterior offset nail could be inserted without proximal prominence. However, in this situation, the proximal interlocking screws are located distally and potentially in a suboptimal position, particularly for proximal tibial fractures. Therefore, selecting a nail that closely matches the intramedullary canal morphology is preferable. Another approach to accommodate a TIN in a patient with a relatively large proximal offset is to tilt it posteriorly relative to the tibial shaft axis. This method is feasible in cases where the intramedullary canal is sufficiently spacious compared with the planned nail diameter. However, in cases of narrow intramedullary canals, excessive reaming of the inner cortex is necessary to insert the TINs in a posteriorly tilted position. Given the recognized risk of extensive circular damage associated with excessive reaming, it is advisable to avoid inserting TINs with mismatched anterior offsets. Furthermore, in cases in which the intramedullary canal diameter is significantly narrow (Fig. 6), posterior tilting of the nail is clinically impossible.

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Fig. 6 A case with a significantly narrow medullary canal diameter Transparent 3D-CT image of the patient exhibiting the small anterior offset of the entry point (1.3 mm) and the narrow intramedullary canal (isthmus diameter of 9.3 mm).

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This study also revealed the influence of tibial rotation on the entry point. The median anteroposterior distance from the entry point to the anterior edge of the tibia was a median of 1.2 mm and the median mediolateral distance was a median of 6.5 mm. These results suggested that the difference between the visible anterior edge of the tibial plateau during surgery and the actual distance was approximately 1 mm, which was clinically negligible in most cases.

One limitation of this study was the limited number of cases analyzed; a more extensive dataset would have provided a more accurate assessment. Additionally, the data were predominantly biased towards individuals of Asian descent. Despite these limitations, we believe that the current study offers novel insights into clinical practice, suggesting that there exists a certain proportion of cases in which the anterior offset of the entry point of the tibial intramedullary nail is minimal.

5 Conclusion

The median anterior offset from the tibial axis to the standard entry point was approximately 10 mm, with 7 % of cases having an offset of less than 5 mm, which is too small for most TINs available. Surgeons should be cautious when dealing with cases with a small anterior offset as this may pose potential risks associated with intramedullary nail insertion, including intraoperative fractures, protrusion of the proximal end of the tibial intramedullary nail, and the possibility of post-insertion malalignment.

Disclosure

No conflicts of interest were declared by the authors.

Author contributions

Design of the workShozo Kanezaki, Masashi Miyazaki.

Data acquisitionTaro Ishida, Masahiro Kawagishi, Tomonori Sakamoto, Shozo Kanezaki.

Analysis interpretation of dataAkihiro Hino, Shozo Kanezaki.

Drafting the workShozo Kanezaki.

Critical revision:Masashi Miyazaki, Nobuhiro Kaku.

SupervisionMasashi Miyazaki, Nobuhiro Kaku.

Final approvalShozo Kanezaki, Masashi Miyazaki, Taro Ishida, Akihiro Hino, Masahiro Kawagishi, Tomonori Sakamoto, Nobuhiro Kaku.

Statements and declarations

Conflicts of Interest The authors declare that they have no conflict of interest.

Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

The study protocol was approved by the institutional review board of our hospital.

Consent to participate All study participants or their legal guardians agreed to be part of the database and for the data collected to be used for research on scoliosis.

Credit author statement

Design of the work: Shozo Kanezaki, Masashi Miyazaki.

Data acquisition: Taro Ishida, Masahiro Kawagishi, Tomonori Sakamoto, Shozo Kanezaki.

Analysis interpretation of data: Akihiro Hino, Shozo Kanezaki.

Drafting the work: Shozo Kanezaki.

Critical revision Masashi Miyazaki, Nobuhiro Kaku.

Supervision: Masashi Miyazaki, Nobuhiro Kaku.

Final approval: Shozo Kanezaki, Masashi Miyazaki, Taro Ishida, Akihiro Hino, Masahiro Kawagishi, Tomonori Sakamoto, Nobuhiro Kaku.

Ethical statement

This study was conducted following the ethical principles of the Declaration of Helsinki, and the study protocol for this retrospective and noninvasive study was approved by the institutional review board of Oita University. Informed consent was obtained in the form of an opt-out, and the need to obtain informed consent from individual patients was waived by the ethics committee of Oita University.

Guardian/patient's consent

The study protocol for this retrospective and noninvasive study was approved by the institutional review board of our hospital. Informed consent was obtained in the form of an opt-out, and the need to obtain informed consent from individual patients was waived by the ethics committee of our institution.

Funding statement

This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.