Calcar referenced tip apex distance (CalTAD) – A better measuring tool to assess lag screw position in pertrochanteric femur fractures treated with twin screw cephalomedullary nails

Harsha K. Kyalakond; Jagadish Laxmansa Katwa; Kiran Kumar K G

doi:10.1016/j.jorep.2024.100530

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Research Article

2025

:4;

100530

doi:

10.1016/j.jorep.2024.100530

Calcar referenced tip apex distance (CalTAD) – A better measuring tool to assess lag screw position in pertrochanteric femur fractures treated with twin screw cephalomedullary nails

Harsha K. Kyalakond , Jagadish Laxmansa Katwa, Kiran Kumar K G

Department of Orthopaedics BVVS S Nijalingappa Medical College and HSK Hospital and Research Centre, Navanagar, Bagalkot, 587102, Karnataka, India

⁎Corresponding author: Harsha K. Kyalakond. drharshakk@gmail.com

Received: 2024-10-2, Accepted: 2024-11-21, Published: 2024-11-23

The Authors

Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Literature comparing TAD and CalTAD values in twin screw cephalomedullary (CM) nails in pertrochanteric femur fractures is sparse. In twin screw CM nails, the lag screw is placed in the inferior half of femoral head to accommodate derotation screw in the upper half. As the tip of the lag screw moves away from apex of femoral head, TAD increases. There is a need for an alternative measurement method for twin screw CM nails.

The aim of this study was to evaluate screw position in twin screw CM nails by measuring TAD and CalTAD and analyse how these two measurements vary.

Postoperative radiographs of patients who underwent fixation of pertrochanteric femur fractures with twin screw CM nails were retrieved from Medsynapse PACS of our institution. TAD and CalTAD were measured in a total of 76 radiographs and the data was analysed.

The mean TAD and CalTAD were 26.7 and 21.8 respectively. TAD was less than 25mm in 32 patients (42 %) and CalTAD was less than 25mm in 54 patients (71 %). Among 44 patients with TAD>25mm, 26 patients had CalTAD<25mm. Although TAD was more than 35mm in 9 patients, none of them had CalTAD more than 35mm.

In twin screw CM nails, the lag screw is placed in the inferior half of femoral head. This results in high TAD values even though screw is placed deep in the subchondral bone. For an inferiorly placed lag screw, CalTAD may be a better measuring tool to assess screw placement.

1 Introduction

Pertrochanteric femur fractures account for high morbidity in geriatric population. Outcome of pertrochanteric femur fractures depends upon bone quality, fragment geometry, quality of reduction, choice of implant and the placement of implant in the femoral head.¹ Reduction, implant selection and implant placement are the factors which a surgeon can control. There is considerable debate about implant placement. The preferred lag screw position in anteroposterior (AP) view is central or inferior, while in lateral view it is central or posterior.²^,³ Inferior lag screw position is proven superior in clinical⁴^,⁵ and biomechanical studies.⁶^,⁷ Baumgaertner et al.⁸ described tip apex distance (TAD) as a predictor of cut out. The lag screw should be placed deep in the femoral head within 10mm of the subchondral bone to achieve a TAD of <25mm.⁸^,⁹ Though the TAD was initially described for Dynamic hip screw (DHS), the same has been used to measure screw placement in cephalomedullary (CM) nails over the years.

The AO/ASIF introduced proximal femur nail (PFN) in the year 1997. Simmermacher et al.¹⁰ conducted a clinical study on this implant and recommended that the load bearing neck screw (lag screw) should be placed in lower half of the femoral head and derotation screw into the upper half. Kuzyk et al.⁶ introduced “Calcar referenced tip apex distance” (CalTAD) in 2012 and recommended inferior lag screw placement. When we use a twin screw CM nail, it is advisable to place lag screw in the lower half of femoral head as it is biomechanically superior and to accommodate derotation screw in the upper half of femur head. As tip of the screw moves away from apex of femur head, TAD increases.

There is dearth of literature comparing TAD and CalTAD values in twin screw CM nails. We aimed at analysing screw position in twin screw CM nails by measuring TAD and CalTAD and assessed how these two measurements vary.

2 Methods

Postoperative radiographs of patients who underwent fixation of pertrochanteric femur fractures with twin screw CM nails between April 2022 and August 2024 were retrieved from Medsynapse PACS of our institution. A total of 109 radiographs were available.

3 Exclusion criteria

1.Short screws: Radiographs where distance between tip of lag screw and subchondral bone was more than 10mm (Fig. 1

Short screw (Distance between tip of lag screw and subchondral bone >10mm).

).2.Unacceptable radiographs: Patients with only AP view radiograph and radiographs where tip of screw could not be identified in lateral view.

After exclusion a total of 76 radiographs were analysed (Fig. 2).

Flow chart showing process of inclusion of radiographs in this study.

Various modifications of AO/ASIF PFN are being used in different parts of the world. In our institution, we use twin screw CM nails (TFN/Short PFN/Long PFN) for fixation of pertrochanteric fractures. The proximal part measures 15mm in diameter, lag screw is 8mm, derotation screw is 6.5mm in diameter. It has a valgus angle of 6°.

4 Measurement of TAD and CalTAD

TAD is calculated by measuring the distance between tip of lag screw and apex of femoral head in millimetres (mm) in both AP (TADAP) and lateral (TADLAT) radiographs. The sum of these values gives us the TAD.⁸ The apex of the femoral head is a point on femoral head where the central axis of femoral neck intersects the subchondral bone. CalTAD in AP view is calculated by moving the same line parallel and adjacent to medial cortex of femoral neck (CalTADAP). Sum of CalTADAP and TADLAT gives us the CalTAD.⁵^,⁶^,¹¹ Hence, the difference between these values is only due to measurement on AP radiograph. The measurement on lateral radiograph remains the same (Fig. 3).

Measurement of TAD and CalTAD. (a)TADAP is measured by determining the apex of femoral head using a line, which is in the centre of and parallel to femoral neck. (b)CalTADAP is measured by moving the same line parallel and adjacent to medial cortex of femoral neck. (c) Measurement of TADLAT. TAD = TADAP + TADLAT; CalTAD = CalTADAP + TADLAT (correction has been made for magnification).

5 Statistics

Data was analysed using SPSS software version 19.0 and OpenEpi software version 2.3.1. Descriptive statistics were used for analysing the data. Percentages and proportions were used for qualitative data. Quantitative data was analysed by mean and standard deviation.

6 Results

The mean TAD and CalTAD were 26.7 and 21.8 respectively (Table 1).

Table 1 Table showing the mean and standard deviations of TAD and CalTAD.

Parameters in mm	Minimum	Maximum	Mean	Std. Deviation
TAD	9.98	41.25	26.7	6.78
CalTAD	11.92	30.81	21.8	4.35

TAD was less than 25mm in 32 patients (42 %) and CalTAD was less than 25mm in 54 patients (71 %). Among 44 patients with TAD>25mm, 26 patients had CalTAD<25mm (59 % of patients with TAD more than 25mm, had CalTAD value less than 25mm) (Table 2).

Table 2 Distribution of TAD and CalTAD of patients with 25mm as cut-off value.

Parameters in mm	TAD <25	TAD >25	Total
CalTAD <25	28	26	54 (71 %)
CalTAD >25	4	18	22 (29 %)
Total	32 (42 %)	44 (58 %)	76 (100 %)

We categorized patients into different groups as shown in Table 3. Although TAD was more than 35mm in 9 patients, none of them had CalTAD more than 35mm.

Table 3 Patients categorized based on based on TAD and CalTAD values.

Parameters in mm	TAD	CalTAD
<25	32	54
25–29.9	22	20
30–34.9	13	02
>35	09	00
Total	76	76

7 Discussion

Good reduction¹² and implant placement are key factors to achieve desirable outcomes in pertrochanteric femur fractures. Baumgaertner et al.⁸ described TAD as an important measurement predicting cutout. Initially, TAD was used as a measure to assess placement of screw in DHS and later in various types of CM nails as CM nails became popular.

Kuzyk et al. conducted biomechanical analysis of 5 different lag screw positions and found that inferior lag screw position produced highest axial and torsional stiffness. This study inferred that inferior placement of lag screw in the AP plane minimises the CalTAD.⁶

“TAD increases as the lag screw is positioned more inferiorly but the implant experiences less deformation”. This revelation was made by finite element analysis study on CM nails by Cheng-Hung-Lee et al.¹³

CalTAD differs from TAD only in AP view. When we use a twin screw CM nail, the lag screw is placed in the inferior half of the femoral head to accommodate derotation screw in the upper half of the head. This may give high TAD value even though the screw placement is accurate. TAD favours central positioning of lag screw and CalTAD favours inferior positioning of lag screw on AP view.

TAD and CalTad have been studied in single screw CM nails extensively.²^,⁵^,¹¹^,¹⁴^,¹⁵ Despite extensive search of literature, there seems to be a huge lacune of studies measuring TAD and CalTAD in twin screw CM nails. To our knowledge, a study published by Puthezhath et al., in 2017, was the first and only study to predict cut out in twin screw CM nails using TAD and CalTAD. They concluded that CalTAD was a better predictor of cut out than TAD.¹⁶

We intended to observe the variation of TAD and CalTAD values in twin screw CM nails. The lag screw should be placed deep in the femoral head within 10mm of the subchondral bone to achieve a TAD of <25mm.⁸^,⁹ In our study, we excluded short screws (distance of screw tips from subchondral bone >10mm in AP views) assuming these cases would invariably have higher TAD and CalTAD. The mean TAD and CalTAD in our study were 26.7 and 21.8 which were comparable with results of Puthezhath et al. (25.19 and 20.5).¹⁶

In few cases where TAD was lesser than CalTAD, the lag screw was placed in the centre of the femoral head. As the tip of the screw moved from the central position to inferior position the TAD increased and the CalTAD decreased (Fig. 4).

Figure showing how TAD and CalTAD change as the tip of lag screw moves from centre(a) to inferior zone(c) of femoral head in AP view of radiographs.

In our study 44 patients (57.9 %) had TAD >25mm, among them 26 patients (59 %) had CalTAD <25mm. Although TAD was more than 35mm in 9 patients, none of them had CalTAD more than 35mm (Table 3). This was difference was probably due to an inferiorly placed lag screw.

M. Güven et al.¹⁷ studied screw position in pertrochanteric femur fractures treated by DHS. Cases where TAD was less than 25mm, lag screws were centrally placed while lag screws were placed more inferiorly when the TAD was more than 25mm. Our findings with twin screw CM nails were in accordance with this study.

Jung et al.¹⁸ compared screw positions in valgus reduction and non valgus reduction groups. Valgus reduction caused inferior positioning of the screw and resulted in lesser CalTAD. This association of lesser CalTAD with inferior positioning of lag screw was concurred by our study.

8 Limitations

We analysed TAD and CalTAD values in our patients, and noted how screw position affects these measurements. However, functional outcome was not assessed. Long term clinical and radiological follow up studies are essential to correlate these values with functional outcome, cut out rates and to define a cut off value of TAD and CalTAD for twin screw CM nails.

9 Conclusion

As the tip of lag screw moves away from apex of femoral head, TAD increases. In twin screw CM nails, the lag screw is placed in the inferior half of femoral head to accommodate derotation screw in the upper half. This may result in high TAD values even though screw is placed deep in the subchondral bone. For an inferiorly placed screw, CalTAD may be a better measuring tool to assess screw placement.

CRediT authorship contribution statement

Harsha K. Kyalakond: Conceptualization, Methodology, Validation, writing the manuscript, and, data analysis. Jagadish Laxmansa Katwa: Supervision, and, Validation. Kiran Kumar K G: Investigation, data collection, and, Formal analysis.

Declaration of competing interest

Nil.

Ethical approval

Not applicable.

Consent for publication

Not applicable.

Ethics approval and consent to participate

Not applicable.

Funding

None.

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