A computed tomography-based analysis of proximal femoral geometry for lateral impingement with two types of proximal femoral nail anterotation in subtrochanteric fractures

Introduction 

The treatment of subtrochanteric fractures has always been a challenging task and since the introduction of the proximal femoral nails (PFN) for their treatment, peri- or postoperative complications related to the surgical implant were continuously reported, which led to numerous efforts to improve the morphological design of the nail. PFN's design has evolved during the past decades either to reduce the proximal portion diameter10 or to improve the stress concentration and the rotational stability of the distal portion of the nail, thus reducing complications in the femoral diaphysis.1, 2, 6, 17 The PFNA and PFNA-II (Synthes, Solothurn, Switzerland), recent AO/Association for the Study of Internal Fixation (AO/ASIF) intramedullary fixation devices, represent a new generation of nails aimed at treating stable and unstable fractures of proximal femur, and have a helical blade rather than a screw for better purchase in the femoral head. However, problems with them were also recognised related either to mismatch between femoral bowing and the nail geometry during intra-operative reduction manoeuvres or to the penetration of helical blade through the femoral head into the hip joint.7, 9 There is no evidence-based report available studying the mismatch between the proximal femur and nail geometry. In the present study, authors evaluated computed tomography (CT) images from healthy cases of Asian ethnicity and retrospectively compared the anatomical measurements of the proximal femur with dimensions of the currently used PFNA and PFNA-II. In addition, the geometrical discrepancies were analysed in relation to the shape of nail and their implication in lateral cortical impingement during reduction intra-operatively in cases of subtrochanteric fractures.

Materials and methods 

The present study was approved by our institution's scientific research board and a written consent was obtained from each patient.

Unique CT-based measurement method 

A coronal CT section showing the largest diameter of the proximal femur was used for making the anatomical measurements. Two lines1 extending from the inner side of each medial and lateral cortex of the proximal femur were drawn and another line2 was drawn bisecting the area between these two previous lines; this line represents the femoral shaft axis (Fig. 1(A)). Then, a fourth line was drawn6 at 130° to line2 that passed through the most distal part (or the end) of the cervix of the femur and coincides with the insertion angle of the lag screw (Fig. 1(B)). A line7 was obtained by connecting the insertion point of the proximal femur nail, that is, the tip of the proximal greater trochanter with the crossing point made between lines2, 6 (Fig. 1(C)). This line acts as a base line for the centre of the proximal femur while inserting the proximal femoral nail. From the crossing point, a line3 was drawn that was perpendicular to this base line. At this time, two new crossing points are created where this perpendicular line meets the lines drawn from medial and lateral cortex of the proximal femur (Fig. 1(C)). Thus, we obtained a triangle4 that connects these two new crossing points and the tip of the greater trochanter (Fig. 1(D)). The base (b) of this triangle corresponds with the diameter of the proximal femur and the height (a) of this triangle corresponds with the height of the proximal femur. The angle formed by a base line of the proximal femoral medullary cavity or the femoral shaft axis and the mid-line of this triangle, was defined as the bending angle (α) for the proximal femur on the coronal section, and the angle made between the line extending from lateral cortex5 and that of the slope of lateral cortex of the proximal femur,8 was defined as the angle of inclination (β) for the lateral cortex of the proximal femur and the point where these two extended line meet was defined as the inclination/impingement point (Fig. 1(E)). When related to the implant, the α angle represents the antero-posterior bending angle of the nail, while the β angle represents the angle between the proximal portion and narrow portion of the nail. A line9 which is perpendicular to the mid-line of the triangle was drawn from to the tip of the greater trochanter. Then, a line (c) which is parallel to the mid-line of the triangle was drawn from the impingement point. This length that causes collision between the nail and the lateral cortex of femur is defined as the impingement length (Fig. 1(F)).

View Large Image Download to PowerPoint

Fig. 1. (A) The bisecting line between inner cortical extension lines of medial and lateral cortex. (B) 130° neck shaft angle line from inferior margin of femoral neck and the bisecting line make the crossing point. (C) The perpendicular line to the central line between greater trochanteric tip and crossing point meets inner cortical line of medial and lateral cortex. (D) The points of triangle is composed of greater trochanteric tip which is entry point of proximal nail, the points which the perpendicular line to the central line of proximal femur crosses the inner cortical line of medial and lateral cortex. Height of proximal femur (a) and diameter of proximal femur (b). (E) Bending angle in coronal plane (α), Inclination angle of lateral cortex (β). (F) The length between inclination point and the point that the line parallel to the central line through the inclination point meets the line perpendicular to the central line passing through the tip of the greater trochanter (c).

The dimensions of PFNA and PFNA-II were then compared with the anatomical morphology of the proximal femur (Table 1). Diameter of the nails were compared with the base of the triangle that was obtained by CT, height of the nails were compared with the height of the triangle, and each of the antero-posterior (AP) bending angle and the inclination angle on the lateral side of the nails were subsequently compared with the bending angle and the inclination angle of the proximal femur on the coronal plane. All measurements were made by one independent observer who did not participate in any of the operations performed. The femoral neck shaft angles of 130° for PFNA and PFNA-II were used for the calculating the average value of measured data.

Results 

The average height of proximal femur was 61.1±5.2mm (range 51.4–72.5mm), average diameter was 18.1±1.5mm (range 15.3–21.8mm), average bending angle (α) in the coronal plane was 8.4±2.2° (range 4.9–12.5°) and the average inclination angle (β) of the lateral cortex was 11.9±1.1° (range 10.2–13.6°) (Table 1). The dimensions of the PFNA and PFNA-II are summarised in Table 1. Proximal portion diameters were similar for both nails (16.5mm) while the AP bending angle was 6° for PFNA and 5° for PFNA-II.

Table 1. Comparison results of morphological dimensions of proximal femur anatomy with PFNA and PFNA-II.

	Dimensions	Proximal femur Mean±SD	PFNA	PFNA-II
	Proximal length (mm)	61.1mm±5.2	56/61/65	65.1mm (medial), 79.1mm (lateral)
	Proximal diameter (mm)	18.1mm±1.5	16.5mm	16.5mm
	AP bending angle (°)	8.4±2.2	6°	5°
	Lateral inclination angle (°)	11.9±1.1	6°	Flat lateral shape, gradual reduction in diameter
	Lateral impingement length (mm)	54.2mm±4.7	56/61/65	No impingement (flat lateral surface)

A total of 22 cases (44%) had height of their proximal femur shorter than that of the shortest (56mm) proximal PFNA dimension, and there were eight patients (16%) whose proximal femur were longer than the longest PFNA dimension (65mm). The average impingement length of the lateral cortex (c) was 54.2±4.7mm (range 41.4–64.2mm), which was shorter than the proximal dimensions of proximal femur and PFNA both. The PFNA-II has different proximal portion lengths on medial and lateral sides, and the lateral side is much longer and has a flat shape. None of the patients had proximal femur dimension longer than the longest proximal dimensions of PFNA-II (79.1mm). As for the diameter, three patients (6%) had smaller proximal femoral diameter than that of PFNA and PFNA-II (16.5mm). For the bending angle of the proximal femur on the coronal plane, 40 patients (80%) had larger angle than 6°, the largest AP bending angle of both the nails. Only two patients showed smaller angle values. Lastly, for the angle of inclination of the lateral cortex, all the patients (100%) had the angle larger than 6°, the largest angle for the PFNA. The bending angle and angle of inclination of the lateral cortex of proximal femur were found to have an average measured value of 8.4° and 11.9°, respectively, which were greater than the corresponding measurements of both the femoral nails.

Discussion 

Morphological studies for proximal femur have been done in the past using three-dimensional methods14; further, measurements methods for proximal femur of Asian patients using simple X-ray or CT scan have been reported.11, 16 However, these studies considered proximal femoral geometry as related to hip prosthesis design and did not try to relate the intramedullary (IM) implant dimensions with the anatomical measurements. In the present study, the authors designed a unique CT-based measurement method to study the geometry of the proximal femur and then matched these measurements with the dimensions of PFNA and PFNA-II, which are the new generation IM implants for treatment of subtrochanteric fractures.

Complex fractures of the proximal femur involving the subtrochanteric region are challenging injuries for orthopaedic surgeons. The difficulties faced in the treatment of these fractures are related to the anatomic and biomechanical features unique to this area. Anatomically, the subtrochanteric area consists of mostly cortical bone, which often is comminuted and tends to heal more slowly than metaphyseal bone. In the proximal part, the canal widens in the intertrochanteric area, which leads to less optimal fixation with IM devices because of the wide canal and short segment proximally. Biomechanically, the subtrochanteric area is an area of high stress concentration, and the muscle attachments lead to strong deforming forces that can make fracture reduction and maintenance difficult, thus increasing the chances of delayed union/nonunion.8, 19 IM fixation offers mechanical, technical and biologic advantages over other forms of fixation. Statically locked IM nails are probably the most commonly used implant in the treatment of subtrochanteric femoral fractures and considered by many as the implant of choice.18 Their use has led to acceptable union rates with decreased blood loss and short operative times.3, 4, 5, 8, 15

Gamma nail, one of the first generation of the PFNs, has been reported to cause an impact on the anterolateral cortex of the femoral diaphysis, resulting in a difficult insertion of the lag screw or a fracture on the lateral cortex due to the morphological incompatibility with the diameter and the curvature of the femoral diaphysis; and thus the nail was modified to the present design.12, 13, 14 However, observations of proximal protrusion above the great trochanter are often made while using proximal femoral nail where a deep enough insertion could not be achieved due to the location of the lag screw (Fig. 2). During closed reduction, using C-Arm image intensifier, we observed displacement of proximal fracture fragment as a result of collision between the proximal femoral nail and the lateral cortex without the nail insertion point being lateralised (Fig. 2) and despite the fact that pointed trochars were used to maintain anatomic reduction during nail insertion. This phenomenon was especially observed in case of Russell–Taylor type I or simple subtrochanteric fractures. This finding could not be explained only by the influence of the strong abductor and flexor acting on the fracture fragments. Thus, there must be some other force such as morphological incompatibility of the nail to the anatomical geometry of the proximal femur, which would have led to this loss of reduction. However, such displacement of fracture was not noticed while inserting PFNA-II (Fig. 3), as there occurred no impingement between the lateral femoral cortex and nail due to the flat lateral shape of PFNA-II.

View Large Image Download to PowerPoint

Fig. 2. (A) Preoperative radiograph shows AO type 31A3 subtrochanteric fracture in a 69-year-old male. (B and C) Intra-operative C-arm radiograph shows the impingement of proximal femoral nail with lateral cortex even though pointed trochar is used to push the lateral cortex, maintaining the reduction. (D) Postoperative radiograph shows geometrical mismatch between proximal femoral nail and proximal femur with regards to length of nail (a) and impingement of nail (b).

View Large Image Download to PowerPoint

Fig. 3. (A) Intra-operative C-arm radiograph shows AO type 31A3 subtrochanteric fracture in a 61-year-old female treated with PFNA-II, a pointed trochar is used to push the lateral cortex. (B and C) Intra-operative C-arm radiograph shows no impingement of nail with lateral cortex due to flat lateral shape of the nail. (D) Postoperative radiograph showing satisfactory alignment with no geometrical discrepancies between the nail and proximal femur.

In our study, there were significant differences between the bending angle of the proximal femur and that of the nail as per the coronal view, and also between the inclination angle of the lateral cortex to that of the nails. The average bending angle exceeded the maximum bending angle of the nail by 2.40, which might be accommodated by reaming the cortices. However, the average lateral cortex inclination angle which is 11.90, is 5.90 greater than the maximum inclination angle of the PFNA and contributes to difficulties during insertion and impingement between the nail and the lateral cortex, which could potentially precipitate intra-operative reduction difficulties. However, PFNA-II has a flat lateral surface, which helps in comfortable sliding of the nail past the impingement point and thus, no impingement to lateral cortex was observed.

Our study showed no significant difference between the diameter of the proximal femur, which presents proper match with the proximal diameter of nails. In addition, the PFNA and PFNA-II length was found to be compatible with the average proximal femoral length, with only 6% patients showing length of the proximal femur shorter than that of the nail. There were also some patients with greater diameter or height of the proximal femur compared with the proximal femoral nail, which caused difficulties in acquiring stable reduction and fixation and also some troubles in removing the internal fixation materials. However, on the other hand, this same condition may bring advantages during the insertion procedure, and may lessen the impact on the lateral cortex due to impingement. In such cases of shorter length of the nail with deep insertion, various lengths of end caps can complement the defect.

There exist difficulties in manufacturing an ideal nail that is compatible to the femoral geometry of all patients with wider range of anterolateral curvatures and lateral cortex inclination angles, and such designs may cause a possibility of weakening the fixation by the decrease in the diameter caused by greater lateral inclination. However, the flat lateral shape of PFNA-II with gradual reduction of diameter distally lessens the impingement chances with the lateral femoral cortex and thus solves some of the problems associated with both the differences in the angle of inclinations and other geometrical discrepancies. Further, this modification widens the permissible range of the insertion area at the apex of the greater trochanter both medially and laterally and thus again decreases the chances of fracture malalignment that may be associated with an incorrect entry point.

We used two-dimensional coronal CT images to identify the anatomical mismatch between the proximal femur and nails; however, three-dimensional (3D) CT reconstructions could have provided with a better representation of the actual anatomical configurations of proximal femur. Analysing the difference between the proximal femoral nail and the proximal femur on their length, bending angles and angles of inclination of the lateral cortex was meaningful. Another limitation of this study was inclusion of a small number of cases. The authors also think that some differences in the anatomical configuration definitely exist with respect to gender, age and the race of the population studied. In future multicentre clinical studies, using 3D morphological analysis and multiple demographic variables are required to clarify the clinical influence of problems associated with morphological incompatibility of proximal femoral nails in subtrochanteric fractures.

Conclusion 

The authors identified morphological incompatibility between the proximal femur and PFNA originating from the difference in the bending angle on the coronal section and the angle of inclination of the lateral cortex, which is specially of interest in subtrochanteric fractures, and suggested that the flat lateral shape of PFNA-II lessens the phenomenon of impingement collision between the lateral side of the proximal femoral nail and the lateral cortex of the proximal femur occurring as a result of differences between the bending curvature of the proximal femur and the angle of inclination of the lateral cortex.

Conflict of interest 

None declared.