The mini SK® clamp is designed to function with pin diameters from 0.9mm (0.035”) to 2.5mm. Representative pins include most miniature Interface® pins, all miniature Centerface® pins, 2.5mm Duraface® pins, 2.0/2.5mm Interface® and Centerface® pins. 1.6mm (0.062”) and 2.0mm (0.078”) miniature Interface® pins are very popular with the mini SK® ESF clamp. While the mini SK® clamp can hold pins as small as 0.9mm and is very compact, there will be kittens, toy breeds, avian patients and exotics that are simply too small for mechanical fixators. Such patients will benefit from Acrylx™ ESF frames constructed using miniature Interface® or Centerface® pins.
The small SK® clamp is designed to utilize pin diameters from 2.0mm to 4.0mm in Duraface® or positive-profile designs. Commonly used Duraface® pins include 2.5mm, 3.2mm, 3.5mm and 4.0mm. Commonly used Interface® and Centerface® pins include 2.0/2.5mm, 2.4/3.2mm, 2.8/3.5mm and 3.2/4.0mm.
The large SK® clamp is designed to handle 3.0mm to 4.8mm pin diameters in either Duraface® or positive-profile pin designs. The maximum pin diameter that can be passed directly through the clamp bolt is 4.8mm (3/16”). Representative pins include 4.0mm, 4.3mm and 4.8mm Duraface® pins. Commonly used Interface® and Centerface® pins include 3.2/4.0mm, 3.5/4.3mm and 4.0/4.8mm.
While dependency on full-pins is greatly reduced, they can occasionally be assets in challenging clinical cases, especially when combined with a strategy for staged disassembly. A review of mechanical testing confirms that simplified full-pin frames (only one full-pin per major fragment) are very powerful with the SK® ESF System. These frames are readily applied to the tibia. In addition, the distal humeral condyle remains a location in which full-pins are frequently used. Due to the anatomy of the radius, full-pins are rarely recommended regardless of the ESF device utilized. Type I-b frames are a very powerful alternative to multiple full-pin frames, providing potential for increased pin number in short fragments while utilizing safe pin corridors. Use of half-pins is supported by mechanical testing and practical results. With any ESF device, mechanical demands for stability must be weighed, and the fixation must be customized to the patient based on a fracture patient assessment; therefore across-the-board frame recommendations are not a reality. Patients with injuries to multiple limbs or other complicating factors will require increased frame rigidity.
Frame simplification results in reduced surgery time and hardware cost. The medium 3.2mm/4.0mm Interface® and 4.0mm Duraface® pins have certainly been the workhorse over a variety of different patient sizes and readily function with small and large SK® ESF clamps. While any device can be pushed to support larger patients, it is easier, quicker, and less expensive to construct a simpler frame of stronger components. This is especially true with fractures of the humerus and femur where Type I-b, Type II, and Type III frames are not possible. Large 4.0mm/4.8mm Interface® and 4.8mm Duraface® pins are reserved for giant breeds.
Full-pins cost more than half-pins and require a clamp on each end. For example, a 6 pin, Type II ESF frame will require 12 clamps and two rods, while an 8 pin, Type I-b frame will require only 8 clamps and two rods. An even larger rod device might function in the same case as a 6 pin, Type I-a frame requiring only 6 clamps and a single rod. The lack of need for aiming devices, torque wrenches, and rod augmentation further reduces cost of ownership and use.
The 6.3mm titanium rod is approximately twice the strength of the 6.3mm carbon fiber rod. On lower limbs, Type I-b or Type II minimal frames will extend the upper weight range for carbon rods. For larger patients, the added strength of titanium rods is advantageous, especially if attempting to simplify frames or in most fractures involving the humerus or femur. At some point, more complex frames need to be built or large SK® ESF rods and clamps should be utilized. Both carbon and titanium rods are in the small SK® starter kit, but customers usually re-order their ultimate preference. Differences in strength can be utilized as part of a plan for staged disassembly or rod downsizing.
Carbon fiber rods are not to be cut with bolt cutters or pin cutters; however, they can readily be sawed with a fine-toothed hacksaw blade. Carbon fiber dust is potentially dangerous, so it is recommended that a damp paper towel be laid over the cutting area to capture any dust. Small SK® titanium rods can be cut with a large bolt cutter but tend to develop a significant burr that must be smoothed with a file or sanding material.
All SK® ESF System connecting rods are stocked in several length choices. The variety of lengths available should greatly reduce the need for cutting rods.
The mini SK® 3.2mm (1/8”) stainless steel rod is easy to contour.
The small SK® 6.3mm titanium external rod is the only rod other than the mini SK® stainless steel rod that can be bent for this purpose. However, it is quite difficult to bend and is best bent prior to sterilization using a vise and two hand chucks. Plate benders can also be utilized. A different strategy must be used with large SK® clamps because carbon fiber composite rods cannot be contoured. Readily constructed circular/SK® hybrid frames, stable double clamps, and other alternatives to rod bending usually result in fixation superior to contoured rod frames. Contouring rods is rarely performed with modern external fixators.
A popular technique that has been successful for spanning joints with all sizes of SK® frames is the utilization of modified single SK® clamps to form an adjustable articulation that is quite secure. The SK® single clamp body is comprised of two different parts: a B1 body part that is placed on the side closest to the pin-gripping portion of the pin-gripping bolt with a threaded hole for the secondary bolt, and a B2 body part that is placed on the opposite side with a smooth hole for the secondary bolt. Modified clamps for transarticular articulations are built with two B2 body parts, two primary pin-gripping bolts, and two nuts. The articulation is built with two of these modified clamps and two short pieces of appropriate size IM pin stock or KE rod. The angle of this articulation is adjustable and can be used to the surgeon’s advantage when a transarticular fixator is employed in the management of tendon and ligament injuries.
This is the most frequently asked question regarding any ESF device. Bone size should be examined to determine appropriate pin diameter. This diameter should be approximately 25-30% of the bone diameter. Pin choice is then considered with the overall clinical demands of the patient, other injuries, and many other factors to determine not only device size, but the overall frame geometry. As previously discussed, undersized devices can be constructed to have great strength, but often become cumbersome and expensive. The better alternative is to increase inherent device strength to maintain a simpler, less expensive frame construct.
For a highly comminuted, unstable fracture in a medium/large dog, large SK® frame components and 4.0mm Duraface®, 3.2/4.0mm Interface® or Centerface® pins might be used to construct the initial frame. At approximately six weeks after surgery when staged disassembly is appropriate, small SK® components can be substituted for large SK® components as one method of reducing the support provided by the frame.
In addition to removing frame elements (i.e. conversion of Type II or Type I-b to Type I-a), carbon fiber composite rods can be substituted for titanium rods as part of the staged disassembly of a small SK® fixator. If one considers only Type I-a frames, the following very general guidelines can serve as a starting point:
- Mini SK® ESF System (3.2mm stainless steel rod) — kittens, small cats and puppies, miniature breed dogs
- Small SK® ESF System (6.3mm carbon fiber rod) — larges cats, dogs less than 20 lbs.
- Small SK® ESF System (6.3mm titanium rod) — dogs greater than 30 lbs.
- Large SK® ESF System (9.5mm carbon fiber rod) — dogs greater than 50 lbs.
These recommendations are general guidelines only and based on the simplest Type I-a frame. On radius/ulna fractures, it is often prudent to consider the use of a Type I-b frame when unsure. With tibial fractures, also consider the use of a Type I-b frame or a minimal Type II. With fractures of the humerus/femur, it is best to use larger frame elements when unsure. These more aggressive strategies will increase the potential need for staged disassembly but will support early weight bearing and function.
No, we are not always looking to build the strongest ESF frame, but desire a clinically appropriate fixator that maintains a simple frame geometry that is economical, biologically friendly, and requires minimal instrumentation.
Historically, as veterinary surgeons evolved from using simple KE frames to frames utilizing multiple full-pins, the axial stiffness of the frames increased dramatically. Since this high level of axial stiffness sometimes slowed bone healing, it became popular to convert complex frames to less stiff frames as early stages of healing occurred. This sequential frame disassembly may be done in one step or in several and is termed staged disassembly. The planned reduction of fixator rigidity transfers more of the load-bearing forces across the bone, stimulating callus maturation and the later stages of bone healing. Common examples of converting a complex frame to a simpler, less rigid frame include: conversion of a Type III frame to a Type II frame, conversion of a Type II frame to a less complex Type II frame, or conversion of a Type II frame to a Type I frame. The simplest and most common conversion is a Type I-b frame to a Type I-a frame.
If one begins with a simple Type I-a frame, as is frequently possible using the SK® ESF System, the previously listed options for staged disassembly are not applicable; however, two alternate strategies can be utilized. If reduction in pin number might jeopardize adequate pin bone interfaces; substituting a smaller, more flexible external rod for a larger, stronger one becomes a very attractive option to decrease the stiffness of a Type I-a frame (e.g. removal of large SK® clamps and a 9.5mm carbon fiber composite rod and replacing them with small SK® clamps and a 6.3mm titanium rod). While not truly a disassembly, rod downsizing does achieve the purpose of transferring a greater percentage of the load-bearing forces back to the bone and across the healing callus. A variation of this concept, when utilizing the small SK® device with 6.3mm titanium rods, is to replace the titanium rod with the less rigid carbon fiber composite rod of the same diameter. While not “downsizing” the connecting rod, this method does achieve a similar planned decrease in rigidity and might be useful in dogs when initial construction utilized medium fixation pins.
Since each SK® clamp is designed to grip a wide range of pin diameters, and there is an overlap zone between the different sizes of fixation pins gripped by the different SK® clamp sizes, it is frequently possible to construct the initial fixator with the larger clamps and rods and replace these components at about six weeks with those one size smaller. This wide range of pin shaft diameters that can effectively be gripped with the SK® clamp makes utilization of “overlapping pin zones” with the SK® device particularly beneficial.
Not all fixator frames will require staged disassembly. In particular, young patients tend to produce bony callus rapidly and benefit less often from staged disassembly. All patients will benefit from early fracture stability which promotes fracture zone debridement, revascularization, and early callus formation. Only after these stages occur will the potential benefits of decreased rigidity become pertinent. With several options for converting more complex frames to less complex frames, or downstaging larger stronger rods to smaller less rigid ones, it is prudent to initially use the stronger choice with a staged exit strategy available. In skeletally mature canine patients, the optimal time period for initiating staged disassembly appears to be about 6 weeks after surgery.
After approximate reduction of the fracture, a single pin is placed in the major proximal fragment and a single pin is placed into the major distal fragment. All fixator pins are placed through liberal release incisions. A connecting rod is secured to these first two pins using SK® single clamps. One can pre-place empty clamps onto the rod (faster) or place them one at a time later (less crowded). Accuracy of reduction is rechecked and adjusted if necessary. Some difficulty in maintaining reduction may occur until additional pins are added to stabilize the environment.
Typically, the third and fourth pins placed into an ESF frame are the two pins on each side and closest to the fracture. This is not mandatory; however, it is easier to verify accuracy of reduction and ensures good pin placement. These pins are placed via liberal release incisions and through pre-drilled holes. Placing the SK® clamp over the area of soft tissue release, and using the secondary bolt to secure it in perfect alignment for the desired drill hole and pin position, ensures correct targeting and pin placement. Next, the appropriate drill sleeve is passed through the primary pin-gripping bolt to protect the soft tissue as the bone is pre-drilled. One must be careful not to crush the thin walled drill sleeve by tightening the primary pin-gripping bolt more than just enough to hold the sleeve in position. Pre-drilling is performed through the drill sleeve while exercising care not to exert undue pressure on the drill bit. Excess pressure results in drill bit flexing and a sudden “push through” of the drill bit into the opposite soft tissues as it breaks through the bone. The drill sleeve is then removed and low speed insertion of the fixator pin is performed. After placing pins three and four, recheck reduction and alignment before placing additional pins. Repeat this procedure for as many additional pins as desired.
Tightening torque has the potential to disrupt fracture reduction or alignment – especially early in frame construction when pin numbers are minimal. Applying minimal torque to the pin bolts early in frame design followed by increases as subsequent pins are placed can minimize this potentially disruptive force. Tightening torque can be neutralized during clamp tightening with use of the two-wrench technique. An open-end wrench placed on the flat surfaces of the head of the primary bolt, or on the flats of the clamp body, is used to neutralize torque force while a second wrench is used to tighten the clamp. After complete frame construction and verification of reduction accuracy, apply final torque to all bolts in an alternating fashion.