Evaluation of a novel percutaneous pinning guide for femoral head or neck fracture: an ex vivo study

Abstract

Importance

Accurate Kirschner wire placement is essential for safe and effective femoral fracture repair, yet freehand techniques often lead to variability and increased fluoroscopic use.

Objective

To evaluate the precision of a three-dimensional (3D)-printed percutaneous pinning guide (PPG) for Kirschner wire placement in the treatment of femoral fractures using canine cadavers.

Methods

The PPG was designed using 3D computer-aided design software and fabricated using medical-grade resin. In part 1, Kirschner wires were inserted into the intact femurs. In part 2, simulated femoral neck fractures were created using a 3D-printed osteotomy guide. Three wires were inserted into each femur under fluoroscopic guidance. The number of insertion attempts, fluoroscopic images, procedure times, and ease of use were recorded. Post-procedural computed tomography was used to assess angular deviation from ideal trajectories and pin engagement in the proximal femur.

Results

The PPG group required fewer insertion attempts (p < 0.01), fewer fluoroscopic images (p < 0.001), and had higher ease-of-use scores (p < 0.047) than the freehand pinning technique (FHPT) group; angular deviation was significantly smaller in the proximodistal (2.9 ± 6.5° vs. 10.7 ± 5.9°; p = 0.022) and craniocaudal (4.8 ± 3.0° vs. 12.3 ± 8.8°; p < 0.001) directions. In part 2, the PPG group showed lower angular variance and greater proximal pin engagement than the FHPT group (p = 0.011).

Conclusions and Relevance

The PPG showed better pinning precision and procedural efficiency than the FHPT under intact and simulated fracture conditions. The PPG may enhance safety and consistency in the percutaneous pinning of canine femoral fractures.

INTRODUCTION

Femoral fractures typically occur as a result of severe trauma and approximately 25% involve the proximal femur [1,2]. In dogs, 91% of these fractures occur in skeletally immature animals younger than 12 months, with nearly 70% affecting the capital physis [3]. Conservative treatment of femoral head and neck fractures generally results in poor functional outcomes; surgical intervention is therefore the preferred approach [4,5,6].

Open reduction and internal fixation (ORIF) using multiple implants, including Kirschner wires, Steinmann pins, and bone screws, is the standard treatment for femoral head and neck fractures [7,8,9]. ORIF is performed to achieve precise anatomical reduction while ensuring accurate implant placement [6,10,11,12]. However, complications such as premature physeal closure, avascular necrosis of the femoral head, femoral neck resorption, and osteoarthritis can occur [13].

Fluoroscopic-guided percutaneous pinning (FGPP) is a minimally invasive alternative to ORIF for repairing physeal fractures [14]. FGPP has several advantages over ORIF, including a reduced risk of iatrogenic injury to the articular surface and vascular network of the proximal end of the femur [15].

Although previous reports have described the performance of FGPP in the repair of femoral capital physeal or neck fractures in dogs, this technique remains a challenge for inexperienced surgeons. With recent advancements in three-dimensional (3D) printing technology in veterinary medicine, 3D-printed guides for orthopedic surgery are now widely used to improve surgical precision and reduce operating time, thereby overcoming several technical challenges. In this context, the percutaneous pinning guide (PPG) may gain wider acceptance in FGPP of femoral capital physeal or neck fractures among small animal surgeons.

This study aimed to assess the simplicity and precision of pinning through the femoral neck into the head using the PPG to facilitate fluoroscopy-assisted Kirschner wire placement in dog cadavers. We hypothesized that the PPG would facilitate more precise placement of Kirschner wires. We further hypothesized that the use of a 3D-printed guide would simplify Kirschner wire placement, reduce the number of attempts required to accurately insert the wire, reduce procedure time, and minimize the number of fluoroscopic images taken during the procedure.

Building on the results of this initial investigation using anatomically intact femurs, the second part of the study evaluated the usability and feasibility of the guide in a simulated fracture model. This study aimed to simulate real-world surgical scenarios and assess whether the PPG maintains its advantages under realistic clinical conditions.

METHODS

Design and 3D printing of the guide

The PPG was designed using 3D computer-aided design (CAD) software (Autodesk Fusion 360; Autodesk, USA) (Fig. 1). The guide was created in the form of a box with hexagonal top and bottom surfaces, and three guides each for the left and right legs, were fabricated to target femoral anteversion angles of 20°, 25°, and 30°. Each guide was equipped with four target inclination holes at angles of 120°, 125°, 130°, and 135°, designed to accommodate a 15-gauge needle.

Fig. 1. Three-dimensional software rendering of the percutaneous pinning guide designed to facilitate Kirschner wire placement. (A) The body of the guide is marked with the target femoral anteversion angle, with four protrusion feature holes at different angles to set the target inclination. (B) The bottom surface is designed to increase friction with the skin.

A reference pin comprising five Kirschner wires was designed to adjust the position of the guide. Two 1.1 mm Kirschner wires were inserted along the horizontal and vertical axes in the upper-right section of the guide surface, while one 2.0 mm Kirschner wire was inserted perpendicular to the guide surface.

On the bottom surfaces of the guide, thirty cones were formed. This configuration was designed to increase friction against the skin, thereby ensuring stable fixation of the guide. A cylindrical rod was designed and attached to the left side of the guide at a 45° angle. At the end of the rod, a disc-shaped handle was created.

When the design had been completed, the stereolithography file was uploaded to a resin 3D printer (Pixel One; Zerone, Korea) and printed using a medical epoxy resin (ZMD-1000B CLEAR-SG; Zenith, Korea).

Cadavers and preoperative preparation

This study was conducted in accordance with the guidelines and regulations of Jeonbuk National University Institutional Animal Care and Use Committee (JBNU NON2022-001). Sixteen dogs, weighing 5–15 kg (mean ± SD, 9.2 ± 2.7 kg), euthanized for reasons unrelated to this investigation, were used as the study models. The cadavers were stored at −20°C and then thawed for 72 h at 4°C prior to the study. On the day of the procedure, the cadavers were maintained at room temperature for 3 h before surgery. Radiographic imaging of the femur in the craniocaudal and lateral views was performed preoperatively. The anteversion and modified inclination angles of the radiographed femur were calculated. The modified inclination angle was defined as the angle between the femoral neck axis and the line perpendicular to the distal joint orientation (Fig. 2).

Fig. 2. Radiographic image of the femur in the craniocaudal view. The red line represents the femoral neck axis, while the yellow line indicates the axis perpendicular to the distal joint orientation. The angle between these two axes is defined as the modified inclination angle, depicted by the arc of the white dashed line.

Part 1. Intact cadaveric models

Six dogs were included in this part of the study; one femur from each cadaver was assigned to the PPG group, while the contralateral femur was assigned to the freehand pinning technique (FHPT) group. Group assignment was performed randomly using a randomizer software tool, ensuring equal allocation of femurs from the right and left sides. Three Kirschner wires were placed in each femur of each cadaver. The left and right femurs were alternately pinned with one pin each, with a total of three pins inserted into each femur, resulting in 18 sample data points for each group. A single surgeon and assistant performed all procedures.

Part 2. Cadaveric models with a simulated femoral neck fracture

The femurs of 10 dogs were assigned to the PPG and FHPT groups as described in part 1. Simulated fractures were created via osteotomy of the femoral head using a standardized 3D-printed osteotomy guide. The osteotomy line was defined to allow controlled separation of the femoral head without violation of the joint capsule. A medial approach was adopted by incising over the pectineus muscle to expose the proximal femur. The guide was fixed to the medial metaphysis using two Kirschner wires, and the osteotomy was performed using an oscillating saw. Thereafter, the guide was removed and the surgical site was closed in layers. Three Kirschner wires were placed consecutively in one femur and then in the contralateral femur. Data were collected for each femur, yielding 10 samples per group. The same surgical team performed all the procedures.

Cadavers were positioned in lateral recumbency on a surgical table with the femur oriented upward. The amplifier of the C-arm fluoroscopy unit (ZEN-2090; Genoray, Korea) was positioned subjacent to the surgical table and aligned centrally under the femur. Orthogonal fluoroscopic images were obtained before the procedure to ensure accuracy.

Surgical technique and procedures

The surgical procedures were conducted in two distinct experimental models: part 1, which involved intact cadaveric femurs, and part 2, which involved cadaveric femurs with a simulated femoral neck fracture. Procedural details for each model are described in the following subsections.

Part 1. Intact cadaveric models

In one femur in the FHPT group, FGPP was performed using a previously described technique [14]. Kirschner wires (1.2 mm) were inserted in a normograde manner, using a 15-gauge needle as a drill guide. The pin entry point into the femur was located over the caudolateral aspect of the proximal metaphysis, distal to the greater trochanter. The Kirschner wires were subsequently advanced into the proximal region of the femoral head using intraoperative fluoroscopy to confirm optimization of the insertion site and orientation.

In the PPG group, before the approach, the femoral condyles were manipulated using fluoroscopy to superimpose the two condyles and verify the exact positioning of the femur (Fig. 3). A similar approach to the FHPT was used on the lateral aspect of the proximal femur, using a 15-gauge needle and the PPG to locate the insertion point under fluoroscopy.

Fig. 3. K-wire insertion procedure. (A) Superimposition of the femoral condyles was confirmed using fluoroscopy. (B) The needle was inserted into the skin combined with the guide. (C) The correct position of the guide was verified based on the reference pin in the fluoroscopic image.

Under fluoroscopy, the guide was adjusted in three planes. First, the guide was oriented so that the three reference pins on the horizontal (X) and vertical (Y) axes appeared superimposed in the fluoroscopic view. Meanwhile, the reference pin perpendicular to the guide surface appeared as a dot (Fig. 3). If the target anteversion angle did not exactly match one of the guide’s preset angles, the guide was rotated slightly. At the same time, we made sure that the vertical reference pins were parallel to the femur’s medullary axis in the fluoroscopic image. Once the guide was properly aligned, a Kirschner wire was advanced through the guide’s 15-gauge needle sleeve and into the femoral neck.

At the end of the procedure, the precision of the Kirschner wire placement was verified using fluoroscopy, and in both groups, the femur was moved to check for pin-induced impingement of the hip joint. We documented the number of needle insertions and attempts required to achieve precise Kirschner wire placement as well as the total number of fluoroscopic images taken (from the initial surgical approach to the final image), duration of the procedure (from the start of the surgical approach to the last fluoroscopic image), ease of the procedure (rated on a Likert scale from 1, indicating difficulty, to 5, indicating simplicity), and comments from the surgeon.

Part 2. Cadaveric models with a simulated femoral neck fracture

Initially, closed reduction of the fracture was performed under palpation and fluoroscopic guidance. An assistant secured the femur until the first Kirschner wire had been inserted. The remaining steps followed the protocol described in part 1.

Postsurgical assessment

After the cadaveric surgeries, the femurs and Kirschner wires were imaged using CT (Alexion, TSX-034A; Toshiba Medical System, Japan). Image files of the bones and Kirschner wires were then imported into 3D CAD software (Autodesk Fusion 360; Autodesk) to allow virtual measurement. In the 3D CAD software, the medullary axis of the femur was parallel to the Z-axis, with the YZ plane aligned with the frontal plane, and the XY-plane aligned with the transverse plane.

To facilitate comparison of the optimal virtual and actual pinning tracks created using the Kirschner wires, a 1.2 mm 3D analysis cylinder of the same diameter as the Kirschner wires used in the procedure was employed. An optimal virtual pinning track was established with consideration of the angles of inclination and anteversion. Actual pinning tracks were created aligned with the central longitudinal axis of the images of the Kirschner wires inserted from the 3D reconstruction. The craniocaudal and proximodistal angular deviations of the actual and optimal virtual pinning tracks were measured in the frontal and transverse planes, respectively (Fig. 4). In part 2, the mean angular deviation of the three Kirschner wires and standard deviations of the three pinning track angles for each femur were measured using 3D analysis. Additionally, to assess the pin engagement in the femoral head, the femur was divided into proximal and distal segments using the osteotomy plane. The percentage of the length of the Kirschner wire engaged in the proximal segment relative to the total length within the femur was measured (Fig. 4). All post-procedural data analyses were performed by an investigator blinded to the application method used to create the actual pinning track.

Fig. 4. Objective three-dimensional analysis to measure the angular deviation and K-wire engagement. The 1.2 mm red cylinders represent the ideal virtual Kirschner wire placement. The 1.2 mm blue cylinders represent the actual Kirschner wire placement. (A) An angular deviation in the frontal plane, and (B) an angular deviation in the transverse plane is shown. (C) The percentage of the Kirschner wire’s length engaged in the proximal segment is calculated from the total length within the femur.

Statistical analysis

All statistical analyses were performed using SPSS (version 26.0; IBM, USA). The normality of the data was assessed using the Shapiro–Wilk test. Descriptive statistics were used to present all the results as means and standard deviations. The FHPT and PPG groups were compared using independent t-tests for normally distributed variables. The Mann–Whitney U test was used to compare non-normally distributed data between groups. Statistical significance was set at p < 0.05.

RESULTS

On radiography, the mean ± SD modified inclination angle was 125.1 ± 5.1° (range, 117°–132°) and the anteversion angle was 24.7 ± 2.5° (range, 22°–31°). On postprocedural CT-based 3D-reconstructed images, the mean ± SD modified inclination angle was 125.3 ± 5.3° (range, 117°–134°), while the anteversion angle was 26.4 ± 2.7° (range, 24°–33°).

Part 1. Intact cadaveric models

The number of attempts to achieve a precise Kirschner wire was significantly higher in the FHPT group than in the PPG group (p = 0.01), as was the number of total fluoroscopy images (p < 0.001). However, the number of needle insertions and procedure times did not differ significantly (p > 0.5) between the groups. The PPG group also had significantly higher Likert scores for ease of the procedure (p = 0.047) (Table 1).

Table 1. Overview of the data of cadaveric models in the FHPT and PPG groups.

Part	Factors	Group	Mean ± SD	p value
Part 1	Number of needle insertions	FHPT	1.7 ± 0.7	0.563
		PPG	1.6 ± 0.5
	Number of attempts to achieve precise Kirschner wire placement	FHPT	2.1 ± 0.9*	0.01
		PPG	1.3 ± 0.5*
	Total number of fluoroscopy images	FHPT	21.4 ± 3.2*	< 0.001
		PPG	16.9 ± 1.0*
	Procedure time (sec)	FHPT	227.7 ± 95.3	0.815
		PPG	208.6 ± 61.5
	Ease of the procedure	FHPT	3.0 ± 1.2*	0.047
		PPG	3.8 ± 0.9*
Part 2	Number of needle insertions	FHPT	5.4 ± 0.84	0.136
		PPG	4.8 ± 0.79
	Number of attempts to achieve precise Kirschner wire placement	FHPT	4.9 ± 0.99*	0.008
		PPG	3.6 ± 0.7*
	Total number of fluoroscopy images	FHPT	33.1 ± 5.4*	< 0.001
		PPG	20.9 ± 2.96*
	Procedure time (min)	FHPT	44.8 ± 5.7*	0.009
		PPG	34.7 ± 7.9*
	Ease of the procedure	FHPT	3.5 ± 0.5*	0.011
		PPG	4.3 ± 0.46*
	Mean proximodistal absolute angular deviation	FHPT	7.6 ± 1.5*	< 0.001
		PPG	3.5 ± 1.2*
	Mean craniocaudal absolute angular deviation	FHPT	4.8 ± 2.0*	< 0.001
		PPG	1.7 ± 1.6*
	Standard deviation of three proximodistal angulation	FHPT	2.2 ± 1.8	0.2413
		PPG	1.6 ± 1.3
	Standard deviation of three craniocaudal angulation	FHPT	2.8 ± 1.1*	0.0113
		PPG	1.5 ± 1.0*
	Pin engagement in femoral head	FHPT	0.21 ± 0.02*	0.0106
		PPG	0.23 ± 0.01*

FHPT, freehand pinning technique; PPG, percutaneous pinning guide.

^*Significant difference (p < 0.05) between group.

The mean±SD proximodistal pinning track angles for the freehand and PPG groups, measured using 3D CT analysis, were 10.7 ± 5.9° and 2.9 ± 6.5°, respectively, and the corresponding craniocaudal pinning track angles were 12.3 ± 8.8° and 4.8 ± 3.0°, respectively. Statistically significant differences were observed in the mean craniocaudal (p < 0.001) and proximodistal (p = 0.022) pinning track angulations between the application groups as measured using 3D analysis.

In the Freehand group, the pinning trajectories deviated cranioproximally in eight pins, caudodistally in four, craniodistally in three, and caudoproximally in three. Conversely, in the PPG group, the cranial pinning trajectories deviated cranioproximally in eight pins, craniodistally in seven, caudoproximally in two, and caudodistally in one (Fig. 5).

Fig. 5. Direction and magnitude of the angular deviation of the placed Kirschner wire (blue circles) compared with the planned Kirschner wire placement (green diamond) in (A) freehand pinning technique group and (B) percutaneous pinning guide group. The average angular deviation is indicated by the yellow square. Four wires with angular deviations exceeding 16° were omitted.

Part 2. Cadaveric models with a simulated femoral neck fracture

The number of attempts, total number of fluoroscopic images, and procedure time were significantly lower in the PPG group than in the FHPT group (p < 0.05). No significant differences were observed in the number of needle insertions between the groups. The Likert scores were significantly higher in the PPG group than in the FHPT group (p = 0.011) (Table 1).

The mean proximodistal and craniocaudal pinning track angles of the pins in each femur were significantly smaller in the PPG group (p < 0.001). The PPG group had significantly smaller mean proximodistal and craniocaudal angular deviations per femur than the FHPT group (p < 0.001 for both). The standard deviations of the three pinning track angles in a single femur differed significantly in the craniocaudal direction (p = 0.011) but not in the proximodistal direction (p = 0.241) (Table 1). The variability in pin angle within each femur (i.e., the standard deviation of the three pin angles) was significantly lower in the PPG group than in the FHPT group for craniocaudal angles (p = 0.011). However, there was no significant difference between the groups in variability of the proximodistal angle (p = 0.241).

The mean percentage of pin length engaged in the proximal segment of the femur was 21.3 ± 2.0% in the FHPT group and 23.4 ± 1.1% in the PPG group, showing a significant difference (p = 0.011) (Table 1).

DISCUSSION

The aim of this study was to assess the precision and efficacy of PPG vs. FHPT for Kirschner wire placement. Overall, we found that the use of PPG resulted in higher precision in pinning at all angles than the use of FHPT, which supports our primary hypothesis. Our secondary hypothesis was also partially accepted, as the procedure time was significantly reduced only in the simulated fracture model.

Minimally invasive techniques have transformed surgery, leading to significantly lower postoperative morbidity and shorter hospitalization periods [14,16,17,18,19]. These techniques allow for a quicker return to function than open approaches, minimizing tissue dissection. Despite these advantages, they have not become popular among less-experienced surgeons because they are technically challenging. PPG overcomes the anatomical complexities of the proximal femoral region by achieving precise Kirschner wire placement with minimal soft tissue disturbance.

In this study, the number of fluoroscopic images obtained in the PPG group was significantly lower than that in the FHPT group. PPG reduced the number of intraoperative fluoroscopic images required for Kirschner wire assessment. Minimization of the imaging requirements can reduce radiation exposure and anesthesia time, and may also lower the risk of perioperative complications such as surgical site infections, patient comorbidities, and financial costs for both medical staff and owners [20,21,22]. Despite the lower number of fluoroscopic images in the PPG group, we did not observe a significant difference in procedure time between the two groups in part 1. This could have been influenced by the superimposing of the femoral condyle before attempting Kirschner wire insertion in PPG, unlike in FHPT. However, in part 2, the procedure time was significantly shorter in the PPG group than in the FHPT group. Unlike in part 1, three pins were inserted without the changing of the limb to the contralateral side, and this condition required superimposition of the femoral condyle only once, which may have resulted in a shorter procedural time than expected in part 1. In clinical settings, the time difference is likely to be more pronounced owing to the constrained space and additional personnel required to manage fluoroscopic examinations during surgery.

In the 3D analysis, the PPG group exhibited significantly less angular deviation across all angles than the FHPT group. Due to the limited bone stock available proximal to the physis, we hypothesized that using PPG instead of FHPT would allow the placement of pins parallel to the femoral neck, potentially maximizing the fixation strength of the implant. In part 1, 15/18 patients in the PPG group showed angular deviation directed cranially. This was likely due to the positioning of the femur during the surgical procedure, in which the alignment was based on the superimposition of femoral condyles rather than the true mechanical axis. In particular, because a femoral varus of approximately 5° is common in healthy dogs (as reported in various studies), superimposing the distal ends of the condyles in the craniocaudal view could have introduced errors in the mechanical axis, likely causing a greater angular deviation in the cranial direction.

To stabilize capital physeal fractures in dogs, Kirschner wires should be placed in parallel, because diverging placement might cause compression of the physis. In part 2, the angular deviation and variance of the inserted pins were significantly higher in the FHPT group than in the PPG group. Moreover, a larger number of implants was induced in the femoral head of the PPG group than in that of the FHPT group. These results suggest that PPG may contribute to more stable fixation of fractures and favorable surgical outcomes.

In the FHPT group, there were four cases of pin penetration into the trochanteric fossa, and two case of penetration into the joint space. Our FHPT results are difficult to compare due to the paucity of similar studies. In a retrospective case series investigating the stabilization of femoral capital physeal or neck fractures in 11 dogs, postoperative radiographs revealed no evidence of penetration into the trochanteric fossa. However, one dog underwent intra-articular pin placement [19]. A limitation of this report [19] is that CT, which can be much more informative for postoperative assessment than conventional radiography, was not performed. In clinical settings in which postoperative CT scans are not routinely performed, instances of poor pin orientation may remain undetected. The hemispheric contour of the epiphysis creates a challenging environment for precise pin advancement because the depth of penetration varies with the location of the implant [15]. Overreliance on intraoperative fluoroscopy without a thorough understanding of the 3D relationship between the implants and subchondral bone may result in violation of the articular surface. To avoid iatrogenic joint lesions, we suggest performing more than one fluoroscopic projection in different planes to confirm proper pin placement.

Higher Likert scores in the PPG group support our belief that the guide is effective in enabling straightforward and precise Kirschner wire placement. In fluoroscopy, superimposition of the two z-axis Kirschner wires helps to establish the target inclination angle and superimposes the two x-axis Kirschner wires to set the target anteversion angle. If necessary, the angle of the guide relative to the femur was adjusted to alter the trajectory of the Kirschner wires. The base of the guide incorporated approximately 30 cones to create friction when pressed against the skin, facilitating a stable attachment between the guide and the skin. Stable fixation allows more reliable percutaneous pinning. Additionally, an angulated rod with a circular handle was adopted from the design of a 3D printed drill guide documented in a previous report [23]. This feature provided considerable maneuverability within the confined space.

The PPG provided four target inclination angles: 120°, 125°, 130°, and 135°. These values were based on the existing literature [24,25,26]. Similarly, the target anteversion angles provided by the guide were also established using measurements obtained from radiographs of the experimental subjects, as well as by referencing data from previous studies [25,27]. The guide used in this study was generic and not patient-specific. If manufactured as a patient-specific guide, it could provide the precise target inclinations and anteversion angles. However, patient-specific 3D-printed guides require a more extensive presurgical process that includes cost, design, and 3D printing. However, generic guides are readily available, which eliminates the need for guide fabrication.

The limitations of the current study include its cadaveric nature. Furthermore, we evaluated the procedure only in canine cadavers weighing 5–15 kg using a single size of 1.2 mm Kirschner wire. Therefore, further research is required to assess the applicability of the guide for dogs outside this weight range and for Kirschner wires of different sizes.

The results of this study demonstrate that the PPG is a promising surgical instrument in the treatment of femoral head and neck fractures in dogs, offering good precision and efficiency. Further clinical studies employing objective evaluation metrics are required to evaluate the long-term outcomes of PPG in fluoroscopy-guided percutaneous pinning of femoral head and neck fractures in dogs.