Fig. 4.1
Sequence of locking/‘backslap technique’. Backslap technique for avoiding fragment diastasis. (a) Schematic drawing of a midshaft fracture with a bending wedge. (b) Friction between the nail and the narrow part of the medullary cavity (red lines) frequently causes diastasis of the main fragments, especially when nails are inserted in an unreamed technique and cancellous bone in the distal epiphysis is dense (red dots). This diastasis cannot be prevented or corrected by ‘holding or pushing against it’. (c) This problem is easily addressed by the backslap technique. This includes distal locking first using (preferable) all distal locking options. (d) Then gentle backslapping under fluoroscopic control until the main fragments are adapted or desired length is achieved. (e) Proximal locking is done according to the fracture pattern and localization. In this case, dynamic locking would be sufficient [31, 32]
It is important not to use too long distal interlocking screws, because this frequently causes pain (see chapter femoral nailing).
4.6 Technical Options for the Placement of Locking Screws
Several targeting techniques have been described for distal locking. The described techniques can be categorized as follows: the free-hand techniques, nail mounted tools, guides attached to an image intensifier, techniques based on C-arm image analysis, computer navigation, and self-locking nails.
In retrograde nails, most of the consideration for the distal locking in antegrade nailing apply to the proximal interlocking screws.
Available Techniques for Distal Locking
free-hand techniques
nail mounted tools
guides attached to a C-arm
techniques based on C-arm image analysis
computer navigation
self-locking nails
4.6.1 Nail Mounted Tools
4.6.1.1 Why Nail Mounted Aiming Arms Without Compensation Fail
Slotted Nails
During nail insertion into the medullary cavity the nail can get deformed. In slotted nails, bending forces lead to bending and bending induced implant torsion if the thrust point is not centric (Fig. 4.2). The amount of deformation is dependent on the misfit between nail and medullary cavity, and from the geometric orientation of the slot relative to the bending moment. If the thrust focal point is eccentric with the force vector, this results in bending, a bending induced torque moment and in bending induced implant torsion.
Fig. 4.2
Slotted nails: fixed aiming arms fail because of bending induced implant torsion. During nail insertion, bending forces in slotted nails (straight red arrow) lead to bending and bending induced implant torsion if the thrust point (red/white circle) is not centric. (a) The thrust focal point is opposite the slot and centric with the force vector, which runs exactly through the thrust focal point. Therefore, there is bending, but no bending induced torsion. (b) The thrust focal point is again opposite to the slot but excentric with the force vector. This results in bending and a bending induced torque moment. Bending induced implant torsion results (curved red arrow). (c) Unslotted hollow and (d) solid implants show bending, but no bending induced implant torsion [33]
Unslotted and Solid Nails
Unslotted hollow and solid implants show bending, but no bending induced implant torsion. In an experimental study, the amount and the direction of implant deformation in unslotted stainless steel unreamed tibial nails (DePuy-Synthes) were analyzed. The results showed lateral translations of –4.5 ± 3.5 mm (mean and standard deviation, range 14.3 mm) and dorsal translations of –7.8 ± 5.8 mm (mean and standard deviation, range 19.2 mm). Torsional deformations around the longitudinal axis of the nail were minimal 0.3 ± 0.7° (mean and standard deviation, range 2.4°). These results explain, why a simple aiming arm, mounted on the proximal nail end fails [34]. Experimental investigations on the femur showed similar results with a large range of lateral (48 mm) and sagittal translations (30 mm) with neglectable implant torsion [35] (Fig. 4.3).
Fig. 4.3
Solid nails: why fixed aiming arms fail. Mechanical experiments with unreamed tibial nails in human tibiae have demonstrated, that there is significant bending during nail insertion (range 14.3 mm in frontal plane, and 19.2 mm in sagittal plane). Insertion related implant torsion in solid nails (range 2.4 mm) and changes in length are neglectable [36, 37]
4.6.1.2 Radiation Free Deformity Compensating Nail Mounted Aiming Arms (DAD, MoDAD, Others)
The mentioned investigations led to the development of a nail mounted adjustable targeting device (Distal Aiming Device, DAD, Expert Modular Aiming Device) for solid tibial [38] and femoral nails [39, 40] (Fig. 4.4). A roughly placed working channel (pilot hole) is placed and an asymmetric spacer is inserted through the cortex until it hits the anterior aspect of the nail. Then the spacer is connected to the aiming arm, this compensates for the bending in the sagittal plane. This is followed by transverse drilling; the upper drill hole is slightly overdrilled on the medial side, which allows for compensation of bending in the frontal plane. Clinical studies demonstrated successful applications, especially in countries where fluoroscopy is not widely available in operating rooms [41, 42]. Similar systems which adapt for nail deformation have meanwhile been developed by different companies [43–47]. These devices show good results in studies; in clinical practice however, improved results are less reproducible [47, 48].
Fig. 4.4
Distal Aiming Device (DAD) adjusts for nail deformation. After the placement of a roughly placed contact hole, a spacer is inserted and gets in contact with the anterior part of the nail. After connection of the spacer with the adjustable aiming arm, the drill sleeve position is centered to the distal locking hole, since the system has adjusted for the sagittal bending nail deformation (Krettek et al. [40])
4.6.1.3 Partial Radiation Free Deformity Compensating Nail Mounted Aiming Arms (Sure Lock, Others)
Another type of aiming devices uses fluoroscopy only for correcting the nail deformation in the anteroposterior plane. This eliminates the need for anterior arm and makes the instrumentation very minimal. These devices are partly radiation dependent. After the nail is inserted, the insertion related deformity is adjusted with a scaled mechanism, based on a few fluoroscopy shots. Since the system does not require perfectly displayed round circled locking holes displayed on the screen, radiation time is significantly less, compared with free hand techniques [49].
4.6.2 Freehand Interlocking and Drill Guides
Freehand interlocking techniques are most commonly used [6, 48]. In the traditional free-hand technique, the C-arm is aligned with the two distal nail holes until the screw hole appears as a perfectly round circle. This indicates a perfect co-axial orientation of the nail hole. After skin incision, the lateral cortex is partially opened with a trocar. The tip of a power drill is centered in the incomplete trocar hole (Fig. 4.5). The drill orientation follows the axis of the transverse nail hole and the drilling is completed. After screw length measurement, the screw is inserted. The process is then repeated for other distal holes [50]. The advantage of this technique is that no special tools are required.
Fig. 4.5
The technique of free-hand interlocking with an image intensifier. In the free-hand locking technique, the C-arm is aligned with the two distal nail holes until the screw hole appears as a perfectly round circle. This indicates a perfect co-axial orientation of the nail hole. The tip of the power drill is centered over the hole and drilling and locking are performed
Several disadvantages have however also been identified: (1) Each step of this technique is dependent on one or several C-arm images and leads to significant radiation exposure of the surgeon and the surgical team. (2) When a standard (non-radiolucent, non-angulated) power drill is in the x-ray beam, the nail hole is not visible and the drill loaded power drive takes significant space between image intensifier and leg. (3) Small but critical changes in orientation might be undetected. (4) Potential nail damage from the drill may result and (5) Defects after meta- or epiphyseal bone manipulation with the trocar or after several unsuccessful drilling attempts can weaken the fixation [50, 51] (Fig. 4.6).
Fig. 4.6
Problems related to the placement of interlocking screws. (a) Divergent screw placement. (b) Screw breakage. (c) Nail damage. (d) Screw damage, for comparison (e) intact screw
Modifications of this technique describe a simple jig and the insertion of the second distal locking screw, using the first drill hole as a pilot. This reduces the degrees of freedom and eases drilling of the second screw hole [52].
Another modification is the ‘Nail over Nail technique’. This concept utilizes a second nail of the same size as a reference, once it is linked to the proximal aiming arm [53].
Creating a large size pilot hole is another concept of locating the distal holes [51].
Salvi has described a creative solution for the targeting problem by temporarily attaching a metallic grid to the area of the locking holes on the lateral side vertical to the insertion handle. After a C-arm shot, the vertical and horizontal distance can be counted from the C-arm shot. The grid then acts as a targeting aid [54].
Several modifications to the free-hand technique have been described, mainly dealing with opening of the cortex (Steinmann pin, K-wire, or guide pin), maintaining correct orientation of the drill, and reducing radiation. Many authors stress the importance of obtaining perfect circles and techniques to aid the alignment of distal hole axis have been described [55, 56]. Although these techniques reduce the problems of radiation and cortical defects, some difficulties remain. There is still radiation exposure; other problems are accidental movements during the change from locating the holes to aiming the drill or wire, and drilling the hole. Additionally, the path of the drill bit is sometimes difficult to monitor [57]. Cortical defects can be produced if a hole is missed and re-drilling is necessary. This can result in bone weakening, or weak fixation. There is also the problem of drill-nail contact.
Table 4.1 gives an overview on the different free-hand locking techniques.
Table 4.1
Distal locking options: free hand techniques
Author | Tools | Description |
|---|---|---|
Kelley et al. [58] | Cone | 3 points in space must be identified for the successful screw placement: (1) the center of the nail hole, (2) the entry hole in the bone and (3) the butt end of the drill bit. With the drill tip at the entry point on the bone and the butt end of the drill aligned, the center of the screw hole will be penetrated |
Kelley et al. also suggested to place a plastic cone on the fluoroscopic emitter to coincide with the center of the beam. When the drill tip is correctly aligned on the bone and the end of the drill is aligned with the tip of the cone, the hole should be drilled in the correct axis | ||
Knudsen et al. [50] | K-wire | The C-arm is positioned until the nail hole is aligned as a perfect circle |
The tip of a wire is positioned until located in the center of one of the two holes and the wire is then inserted. After the orientation is checked with the C-arm, the wire is driven through the bone. The same procedure is used for the other hole. The first wire is then removed and a hole is drilled using the other wire as a reference | ||
Noordeen et al. [59] | Cannulated drill | Owen and Coorsh tested this method in a study and found that the free hand technique is more difficult and more time consuming compared to the cannulated screws technique |
Barrick [60] | ||
Owen and Coorsh [61] | ||
Reynders et al. [62] | ||
MacMillan and Grosse [51] | Steinmann pin + handle of disposable suction unit | A 2 mm Steinmann pin is attached to the handle of a disposable suction unit to increase the distance of the surgeon to the radiation source. With the distal hole perfectly round in the C-arm image the pin is aligned, driven through the bone and fixed in the opposite cortex. After confirmation of the correct position with the C-arm, a 6.0 mm cannulated reamer is placed over the Steinmann pin and the lateral cortex is reamed up to the nail. With the guide in place the guide pin is then removed, followed by drilling and screw insertion |
Harrington and Howell [63] | Cannulated screws | Cannulated screws are introduced over the guide wire which was inserted using a modified free-hand fluoroscopic technique. The placement of the cannulated screws over the wire is easier and radiation exposure to the surgeon is reduced |
Pennig et al. [57] | Handle with a radiolucent cylinder containing a metal ring at each end and a central hole | The instrument is a handle with a radiolucent cylinder containing a metal ring at each end and a central hole for a 4 mm Steinmann pin. The position of the nail hole is displayed with the C-arm. The device is moved until the two metal rings are perfectly superimposed on each other before the Steinmann pin is inserted in the lateral cortex and through the nail. This reduces the radiation exposure rates up to 80 % when compared with conventional free-hand techniques |
Rahman et al. [64] | Metallic washer | After a perfectly round nail hole is visible, a forceps is used to position a 13/6.7 mm diameter AO metallic washer overlying the hole. The washer is held in place with a sterile transparent adhesive dressing. The holes are drilled through the washer with the drill bit perpendicular to the bone. The method relies on a short distance between the skin and bone. The technique is recommended for the tibia |
Hashemi-Nejad et al. [52] | jig that aligns the second distal hole once the first has been obtained by the free-hand method | The jig is positioned in the first hole, obtained by free-hand technique |
The jig simplifies the procedure by limiting the alignment of the jig to the rotational plane. The jig guides the drill and screw into the second hole. The jig has a long handle to increase the distance to the radiation source | ||
In a clinical study, the screening time was reduced to 6 s, compared with 24 s when using the free-hand technique | ||
Rao et al. [65] | Nail used as a guide | After placement of a K-wire into the distal hole, another nail is held to the limb with the K-wire passing through the corresponding hole. The nail is used as a guide; a wire is passed through the more proximal of the distal holes. A cannulated drill opens the near cortex before the wires are removed |
Graham and Mackie [66] | Modified AO aiming device for the AO interlocking femoral nail | The nail holes are displayed as perfect circles. The aiming sleeve is positioned over one of the holes and tapped into the bone to prevent its slipping. The trocar and parallax aiming device are removed and the handle and guide sleeve are left |
The device requires one surgeon to steady the device, while another drills and taps the hole, and then inserts the screw | ||
Hudson [67] | Pair of drill guides with long handles. The inner guide, fits inside the outer guide | The device consists of a pair of drill guides with long handles. The inner guide, with an external diameter of 5 mm, fits inside the outer guide |
The nail holes in the C-arm are displayed perfectly round, the drill guides are aligned with the holes and the bone is drilled with a 3.5 mm drill bit. The inner guide is removed and then the 5 mm drill is passed through the outer guide to drill the near cortex. The screw is inserted once the outer guide is removed | ||
Reynders et al. [62] | Radiolucent block with 2 concentric metal rings around a central perforation and a long radiolucent handle | A radiolucent plastic block contains two metal rings, 1 cm apart around a central hole for the Steinmann pin and a fixation hole for a K-wire. The block is placed over the Steinmann pin and positioned, until the tip of the Steinmann Pin is in the middle of the nail hole and the rings are superimposed in the C-arm image |
Once the first pin is inserted through the nail, the radiolucent block is rotated until it fits the other hole. The block is then fixed with a K-wire to the bone | ||
A Steinmann pin can then be introduced through the other hole, without the use of the image intensifier | ||
Mahaisavariya et al. [68] | In patients with tibial traction, a Steinmann pin and a long tubular rod are connected with a clamp to the Steinmann pin of the traction system which acts as fulcrum | The device consists of a long tubular rod, a universal adjustable clamp to act as a fulcrum and a single clamp, all obtained from the AO external fixator device. The clamp connects to the Steinmann pin of the skeletal traction. This enables the rod to pivot around the universal clamp and position the target guide located at the end of the rod. Once perfectly round nail holes and the correct position of the targeting device are obtained, the universal clamp is locked in position to aid the procedure of distal locking |
Saw [69] | Modification to the image intensifier | Two wires are taped to the image intensifier receiver. One is aligned with the axis of the C-arm and the other at right angles to it. The C-arm is adjusted to make sure that one of the wires is parallel to the long axis of the nail and the other perpendicular to that axis, while passing across the center of the hole in the ap-plane. With a 90°-rotation of the C-arm and adjusting the height, both wires pass through the center of the hole and the hole appears perfectly round |
Granhed [56] | Double C-arm image intensifier | A double C-arm image intensifier is used to produce perfect circles in a lateral view of the nail in femoral locking. When the lateral C-arm is positioned 20°–45° oblique to the femur, the fluoroscope can be used during drilling with the hands out of the beam |
Ohe et al. [70] | Portable stereoscopic fluoroscope (prototype) | The system consists of a C-arm, an image processing unit and a stereo monitor. It provides a real time 3D view, allowing the perception of the 3D relationship between the drill bit and the nail hole. Due to the lower pulse rate of the X-ray, the radiation dose is supposed to be 50 % of a typical fluoroscope |
The AO radiolucent drill guide has a concentric ring and is connected in 90° angle to a powerdrive | The radiolucent drill guide has a concentric ring and is connected in 90° angle to a powertool. All components including case, gear and fixation for the drill are radiolucent. The metal drill and a concentric ring allow the control of the drilling process. A big advantage is that the drilling can be done in even narrow space between C-arm and leg, since the drill guide is connected in 90° to the powertool | |
Lim and coworker described a much cheaper version of it using a modified suction device |
For the confirmation of the correct screw placement inside the nail, a torque test can be done. In cannulated nails, the position of the screw inside the nail can be assessed with a long K-wire (deep stick technique).
4.6.3 Guides Attached to an C-Arm
Another early solution for targeting of locking screws includes the attachment of the drill guide to the image intensifier [2]. Difficulties in the management and handling of a heavy and bulky C-arm probably prevented a wider use.
A more recent concept is the use of a laser beam identifying the location for the skin incision and the orientation of the screw hole, if centered orthogonally to the nail hole [73]. This concept was further improved using two perpendicular laser beams allowing a cross hair marking of the center of the beam [74, 75].
4.6.4 Computer Navigation
Locating objects or part of them in space is a central part of computer navigation in orthopaedic surgery. Various prototypes have been developed but so far none of them is widely used [76–83].
Windolf and coworkers developed an interesting solution based on a ‘single’ image from a C-arm [84]. A computer program plans the correct drilling trajectory by processing the lens-shaped projections of the interlocking holes from a single image. Holes can be drilled by visually aligning the drill to the planned trajectory. Besides a conventional C-arm, no additional tracking or navigation equipment is required.
Recently, robotic applications combined with image analysis and navigation were developed. These techniques however are still far from routine clinical application [82, 85–87].
While image based computer navigation has not yet reached broad clinical acceptance, an electromagnetic field for the orientation of the drill with the use of a probe inside the cannulated nail is successfully used in clinical practice (Smith & Nephew, Memphis, Tennessee) [88–92]. However, the system is limited by interference with pacemakers and metal bars, for example from operating tables. Other disadvantages are that the probes are single use and since the system is not generic, it works only with the producers nail line.
4.6.5 Self Locking Nails
A variety of implants have been developed with a built-in locking mechanism. The Brooker-Wills nail (Biomet Inc., Warsaw, In) [93] interfered with fins, the claw interlocking nail system (Mathys, Bettlach, Switzerland) [33, 94] interfered with claws with the predominately cancellous bone of the distal femur. Self-locking nails have not become widely popular, because of their limited mechanical performance in unstable fracture patterns or outside the center of diaphysis [6] when compared to conventional interlocking nails. For the Brooker-Wills nail, a relatively high complication rate has been reported [95–97].
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