Kirschner Wire Fixation in Dorsally Displaced Distal Radius Fractures: A Biomechanical Evaluation

Abstract

Background  There is currently no consensus for the optimum configuration and number of Kirschner wires (K-wires) to use for the stabilization of dorsally displaced distal radius fractures. In this biomechanical study, we compared the load to failure and stiffness of four common K-wire configurations to identify the strongest construct for use in extra-articular dorsally displaced distal radius fractures.

Case Description  We created a standard distal radius fracture model in turkey tarsometatarsi which was stabilized using two or three K-wires (1.6 mm) in four different configurations. Following a power calculation, 10 fracture models of each configuration underwent testing in cantilever bending and axial compression.

Literature Review  Recent randomized trials have shown no evidence that volar locking plates are superior to K-wires for the treatment of dorsally displaced distal radius fractures. This has led to an increase in the popularity of much cheaper K-wires. Several different K-wire techniques have been described but there is no strong evidence to determine which is the optimal configuration and number of wires.

Clinical Relevance  The three-wire interfragmentary configuration was stiffer than the three-wire Kapandji in axial compression and cantilever bending. There was no difference in load to failure in cantilever bending or axial compression. The three-wire interfragmentary technique is the stiffest configuration of K-wires for dorsally displaced distal radius fractures. The two-wire Kapandji technique was significantly weaker than the other configurations, especially in cantilever bending.

Conclusion  The authors recommend to always use three wires for percutaneous pinning and never to use two intrafocal wires alone.

Fractures of the distal radius are the commonest fractures of the upper limb with an incidence in the United Kingdom of 36.8 in 10,000 and 9 in 10,000 person-years for women and men, respectively. ¹ Incidence is rising with each fracture costing around £527 to treat. ^{2 3} The Distal Radius Acute Fracture Fixation Trial (DRAFFT) was a large multicenter, randomized study comparing the effectiveness of Kirschner wire (K-wire) fixation and locking plate fixation for fractures of the distal radius. ⁴ It found that volar locking plates offered no sustained clinically relevant advantage over K-wire fixation which is cheaper and quicker to perform. Other randomized trials have reported similar findings. ⁵

Based on these findings the National Institute for Clinical Excellence (NICE) in the United Kingdom recommended the use of K-wires for percutaneous pinning of dorsally displaced distal radius fractures if the fracture does not involve the articular surface of the radiocarpal joint or if displacement of the radiocarpal joint can be realigned by closed manipulation. ⁶ Despite this, there is little guidance as to how many wires, what size wires, and what configuration of wires should be used. In DRAFFT, the size and number of K-wires used, insertion technique, and configuration of wires were left entirely to the discretion of the surgeon. ⁴ There were 230 patients treated with K-wires. The Kapandji, interfragmentary, and mixed techniques were used in 54 (23%), 78 (34%), and 71 (31%) of the procedures, respectively. Most procedures used either two (42%) or three wires (46%).

The most common techniques for percutaneous pinning using K-wires are interfragmentary and Kapandji's configurations. The simplest method of K-wire insertion is the interfragmentary or transstyloid technique, known eponymously as Willenegger and Guggenbuhl ⁷ which involves the insertion of two or more K-wires into the radial styloid in an oblique plane that crosses the fracture and is buried in the opposite diaphyseal cortex; this technique is often enhanced by the insertion of a dorsoulnar K-wire to create a cross-pin configuration. ⁸

The Kapandji technique was designed to use the K-wires as a buttress to prevent dorsal translation of the distal fragment of the radius by insertion of the K-wire perpendicular to the axis of the radius and directly into the fracture site. ⁹ The wire is then inclined to an oblique plane and drilled into the opposite cortex. Traditionally, this technique used three wires of which one was inserted dorsolateral to the fracture and the remaining two from the dorsal aspect. ¹⁰ The Fritz-modified Kapandji technique involves the insertion of two intrafocal K-wires and a third through the styloid process which acts to hold the distal fragment in place for a more rigid configuration than the buttress wires alone. ¹¹

Currently, there is minimal guidance as to how many wires, what size wires, and what configuration of wires should be used. This study aims to compare the biomechanics of the commonest configurations in bone and provide clinically relevant guidance for the practicing surgeon on the optimum configuration and number of wires.

Model and Method

Fracture Model Creation

An accurate and reproducible fracture model was created using the tarsometatarsus bone or “shank” of a turkey leg. This is the distal third segment of a turkey leg which is specific to birds. It is formed from fused metatarsal and tarsal bones and links the ankle to the toes. Turkey bones have been used as models for human bones and are a readily available long bone. ¹² The articular surface of the tarsometatarsus bone has bony prominences that resemble that of a human distal radius and these can be used as anatomical landmarks for fracture creation ( Figs. 1 and 2 ).

Fig. 1.

Turkey tarsometatarsus distal articular surface. This was used as the model for a left human radius. (A) Scaphoid fossa. (B) Lunate fossa. (C) Lister's tubercle. (D) Radial styloid process.

Fig. 2.

Anteroposterior turkey tarsometatarsus. This was used as the model for a left human radius. (C) Lister's tubercle. (D) Radial styloid process.

Fracture pattern was replicated following analysis of the fracture pattern seen in 50 consecutive radiographs of patients with extra-articular dorsally displaced distal radius fractures (A2) treated at University Hospitals Leicester.

On the anterior–posterior (AP) view the distance of the fracture line from the apex of the distal articular surface on both the radial (A) and ulnar (B) borders were measured parallel to the long axis of the diaphysis. These measurements showed a mean of 20.6 mm (standard deviation [SD] = 5.15) and 12.6 mm (SD = 4.46) for the radial and ulnar sides, respectively. The lateral radiograph was used to measure the angle of the fracture (C) in relation to the angle of the radiocarpal joint (D). The mean angle of the fracture was 15.6 degrees (SD = 10.6; Fig. 3 ).

Fig. 3.

AP and lateral radiograph of dorsally displaced distal radius fracture, modified with fracture measurements. (A) Radial measurement from distal articular surface to fracture line. (B) Ulnar measurement from distal articular surface to fracture line. (C) The line of the fracture used to calculate the mean angle by comparison to (D) the angle of the radiocarpal joint. Image Courtesy: https://commons.wikimedia.org/wiki/File:Collesfracture.jpg ; author: Lucien Monfils.

The length and diameter of each turkey bone was measured and recorded to provide accurate biomechanical calculations. Soft tissue was maintained as much as possible to replicate the additional effect of soft tissue on fracture stability. Angle calipers and a reciprocating saw were used to cut the bone according to the mean position and angle of the fracture.

Kirschner Wire Techniques

We used 1.6-mm K-wires for all configurations.

The two-wire Kapandji technique: an intrafocal dorsolateral pin was inserted at a point between the dorsal tubercle and styloid process at an angle of between 30 and 45 degrees to the long axis of the bone. ^{13 14} The second intrafocal wire was placed just medially to the dorsal tubercle at a 30- to 45-degree angle and aimed laterally and proximally, so as to just cross the dorsolateral wire. Both wires were then rooted to pierce the cortex but not pass all the way through ( Fig. 4A ).

Fig. 4.

( A ) Two-wire Kapandji configuration. ( B ) Three-wire Kapandji configuration. ( C ) Two-wire interfragmentary configuration. ( D ) Three-wire interfragmentary configuration.

The three-wire Kapandji/mixed method: one further wire was inserted via the styloid process at an angle of 30 degrees to cross the fracture site and pierce the opposite cortex ( Fig. 4B ).

The two-wire interfragmentary technique: two nonparallel wires were inserted into the radial styloid at an acute angle of 30 degrees and then into the cortex proximal to the fracture ( Fig. 4C ).

The three-wire interfragmentary technique: an additional third wire was inserted as a cross wire medial to the dorsal tubercle at an angle of 30 degrees ( Fig. 4D ). Radiographic examples of the clinical use of the techniques are demonstrated in Fig. 5 .

Fig. 5.

Anteroposterior and lateral radiographs demonstrating the clinical use of the four techniques tested: ( A ) two-wire Kapandji's configuration. ( B ) Three-wire Kapandji's configuration. ( C ) Two-wire interfragmentary configuration. ( D ) Three-wire interfragmentary configuration.

Biomechanical Testing

For each configuration, 20 specimens were tested once in either axial compression or cantilever bending using an Instron 5960 Dual Column Testing System (Instron, Norwood, MA). For axial compression, each bone was placed in an upright position with the prepared specimen secured proximally using a grip ( Fig. 6A , B ).

Fig. 6.

( A ) Biomechanical testing of turkey bone in axial compression. ( B ) Biomechanical testing of turkey bone in flexion with a dorsally displacing force.

The flat Instron attachment was moved to touch the most distal point of the bone but without applying a compressive force. The machine was balanced for both load and extension and the axial compression method initiated via Bluehill Software (Instron, Norwood, MA). The three-point cantilever bending set-up used an adjustable two-point stand inferiorly and a triangular-pointed attachment superiorly to produce a dorsally displacing force and replicate extension of the wrist similar to the force seen when falling onto an outstretched hand.

The bone was placed, so that the volar aspect of the distal fragment and a smooth section of the diaphysis were in contact with the stand. A clamp was then firmly attached to the proximal end of the bone, without raising it from the stand, and the superior attachment lowered to the bone 5-mm proximal to the fracture.

A rate of 0.2 mm/s was applied in both tests and after 50 N (in cantilever bending) and 300 N (in axial compression) had been recorded, the test was stopped once a 10% of peak load drop had been measured. A 10% drop in load was the event at which the construct had failed to withstand the applied load and therefore the load at break indicates the strength of the configurations. Strength measures the maximum applied load before a material is permanently deformed. Stiffness is a measure of a material's resistance to displacement by an applied load. Clinically, a stiffer configuration will allow less movement under minor forces (hand movements) and a stronger configuration will withstand greater forces (falling onto an outstretched hand).

Statistical Analysis

A power calculation was performed based on a study that used similar configurations to ours and provided mean values and SDs from which effect sizes could be estimated. ¹⁵ A power calculation with a significance level of 0.05 and power 0.8 suggested that a sample size of 10 would provide sufficient data to determine a significant difference with a small to medium effect size for all configurations.

To compare the four K-wire configurations, unpaired t -tests were performed between two- and three-wire variations of the same configuration and between the different configurations with the same number of wires.

Results

Stiffness

The three-wire interfragmentary configuration is significantly stiffer than all other configurations in axial compression ( Table 1 ). In cantilever bending, the three-wire interfragmentary configuration is significantly stiffer than the three-wire Kapandji configuration ( p  = 0.0018; Table 2 ). The two-wire Kapandji configuration is significantly less stiff than the three-wire Kapandji ( p  < 0.0001) and two-wire interfragmentary ( p  = 0.0001) configurations in cantilever bending.

Table 1. Table showing the mean stiffness of each configuration in axial compression (MPa).

Axial stiffness (MPa)	Kapandji (Ka)	Interfragmentary (IF)	Ka vs. IF p -value
Three wires	63.49 (80.9)	108.2 (142.4)	0.0068
Two wires	67.27 (148.7)	67.73 (94.7)	0.98
Three vs. two p -value	0.82	0.02

Table 2. Table showing the mean stiffness of each configuration in flexion with a dorsally displacing force (MPa).

Flexion stiffness (MPa)	Kapandji (Ka)	Interfragmentary (IF)	Ka vs. IF p -value
Three wires	6.48 (7.3)	12.80 (17.2)	0.0018
Two wires	2.39 (3.0)	9.16 (15.4)	0.0001
three vs. two p -value	<0.0001	0.10

Load at Break

There was no difference in load to break between the three-wire configurations in axial compression or cantilever bending ( Tables 3 and 4 ). The three-wire Kapandji configuration had a significantly higher load at break than the two-wire Kapandji configuration in both axial compression ( p  = 0.01) and cantilever bending ( p  = 0.0034). The two-wire interfragmentary configuration also performed better in cantilever bending ( p  < 0.0001) than the two-wire Kapandji configuration. There were no other significant differences between the mean values.

Table 3. Table showing the mean load at break of each configuration in axial compression (N).

Axial load at break (N)	Kapandji (Ka)	Interfragmentary (IF)	Ka vs. IF p -value
Three wires	2,240 (2,478.4)	2,077 (2,043.8)	0.63
Two wires	1,420 (1,891.4)	1,726 (1,740.8)	0.24
Three vs. two p -value	0.01	0.26

Table 4. Table showing the mean load at break of each configuration in flexion with a dorsally displacing force (N).

Flexion load at break (N)	Kapandji (Ka)	Interfragmentary (IF)	Ka vs. IF p -value
Three wires	311 (414.3)	395.5 (380.9)	0.16
Two wires	136.9 (212.6)	456.7 (528.8)	<0.0001
Three vs. two p -value	0.0034	0.35

Discussion

There is still controversy regarding the best surgical stabilization for dorsally displaced fractures of the distal radius. Volar locking plate use has increased in popularity over the last two decades but K-wire use has increased following recent randomized trials. VLPs do not seem to offer a sustained clinical advantage over K-wires despite their greater expense. ⁴ In patients under the age of 50 years, it is more cost-effective to use K-wires over volar-locking plates and the majority of these fractures reflect our extra-articular model. ¹⁶

Our results have determined that the three-wire interfragmentary technique provided the stiffest construct in both axial compression and cantilever bending. The two-wire Kapandji configuration had the lowest load at break when compared with its three-wire counterpart and two-wire interfragmentary making it the weakest configuration. The addition of a single-interfragmentary wire to form the three-wire Kapandji configuration proves to significantly increase the strength of the intrafocal technique.

We theorize that the more bony insertion points per wire, the stiffer the hold. The interfragmentary wires have two holds in bone per wire, whereas the intrafocal wires have only one. This means that the two-wire interfragmentary configuration has four holds; the three-wire interfragmentary configuration has six holds, while the three-wire Kapandji's configuration has four holds and the two-wire Kapandji configuration has only two holds.

Artificial bone can be used in large numbers for biomechanical experiments but is expensive and may not reliably replicate the physiological structure and behavior of bone. ^{13 17} Naidu et al used fresh frozen unembalmed pairs of radii in a biomechanical test of K-wire configurations in both torsion and cantilever bending. ¹⁵ They tested four configurations of K-wires and various wire sizes. When comparing configurations using 1.6-mm wires, they found two transstyloid wires with one ulnar cross wire that was the stiffest in cantilever bending. This is similar to our three-wire interfragmentary configuration. There was no significant difference in torsional stiffness between the configurations. Each configuration was tested six times and the bones were reused meaning that load to failure was unobtainable. There are numerous other biomechanical studies that use fresh frozen human cadaver bone; however, the majority of these are underpowered with a limited number of specimens probably due to ethical restrictions and the requirement of donation. ^{18 19 20 21}

Animal bone has been used extensively as a substitute for human bone in biomechanical testing. Bovine and porcine vertebrae and long bones have been shown to be highly reliable in vitro models and authors have suggested living animal models are not necessary for the most biomechanical studies. ^{22 23 24} Turkeys have powerful, relatively large lower limbs and have been widely used for biomechanical studies of tendon strength and gliding. ^{25 26} We are the first to use turkey long bones for fracture fixation testing but we believe the size, shape, bony properties, and our adequate sample size will produce meaningful results. The use of physiological bone is important as the cortex and medulla play vital roles in the creation and stability of the constructs. The model is reproducible and cheap. Further tests can be performed with different wire configurations, number, size of wire, and angles of entry. Future research could consider whether plaster cast immobilization is necessary with our stiffest construct or whether a removable splint and early movement of the joint could be allowed.

Limitations and Strengths

Limitations of our study include the purely biomechanical study design. Individual patient factors which would be encountered in a clinical setting, such as osteoporosis, are not assessed. Our model replicates an extra-articular fracture in healthy bone which is not the most common circumstance for dorsally displaced distal radius fractures. Ideally an osteoporotic bone model would also be used, but it is difficult to replicate the complex architectural changes involved in osteoporotic bone. This may be possible by use of hydrochloric acid demineralization of bone which has been demonstrated to mimic the effects of osteopenia and osteoporosis depending on acid concentration. ^{22 27} Load at break is a test that shows the amount of load that the K-wire constructs can withstand before absolute failure. The load applied to the fracture via activities of daily living is likely to be much lower than the point of failure; however, our results do show a relatively small axial or bending force that would cause the two-wire Kapandji technique to fail ahead of the other configurations. We are expecting that surgeons would want greater stiffness but the slight movement combined with reasonable strength of the three-wire Kapandji technique may be beneficial to bone healing. The findings from DRAFFT are applicable to distal radius fractures that could be acceptably reduced using a closed technique, those who required open reduction were excluded. ⁴ The greater biomechanical stability of volar locking plates is likely to still have an important role for stabilizing multifragmentary, high-energy fractures or fragility fractures in older patients. Our results take into account the biomechanical stresses exerted on a fractured radius but there is no guaranteed correlation between biomechanical stability and clinical outcome. Future research using patient-reported outcome measures (PROMs) in patients who have undergone treatment with different K-wire configurations is necessary to show how our results relate to retention of reduction and function.

This biomechanical study provides useful information which is likely to be directly transferrable to clinical situations. Our model is cheap, easily available, and reproducible.

Conclusion

We recommend that the three-wire interfragmentary configuration should be used as the first-choice surgical intervention for extra-articular dorsally displaced distal radius fractures in patients under the age of 50 years. The three-wire interfragmentary configuration is the stiffest construct in healthy bone. The two-wire Kapandji configuration is significantly less stiff and weaker than all other configurations tested and should be avoided.

Funding Statement

Funding None.