SK™ Linear ESF System FAQs
What pin sizes does each SK™ ESF clamp support?
The mini SK™ clamp is designed to grip pin shaft diameters of 0.035” (0.9mm) to 2.5mm. With current positive-thread pins, the largest pin used is typically the 2.0mm shaft INTERFACE® or CENTERFACE™ pin which features a 2.5mm thread diameter. 2.0mm DuraFace® (2.5mm shaft) and miniature INTERFACE® pins of 0.078” and 0.062” diameters are also popular choices. While the mini SK™ clamp can hold pins as small as 0.035” and is very compact, there will be kittens, toy breeds, and exotics with bones so small they will be unable to wear mechanical frames. Such patients will benefit from acrylic ESF frames constructed using miniature INTERFACE® pins.
The small SK™ clamp is designed to utilize 2.0mm to 4.0mm pin shafts in positive or negative-thread pin designs. This means that 2.0mm shaft, small, small-plus, and medium INTERFACE® half-pins and CENTERFACE® full-pins in standard thread versions can be utilized directly through the clamp. DuraFace® 2.0mm (2.5mm shaft), small, small-plus and medium will also function with small SK™ clamps. Medium, positive-thread pins in cancellous versions (3/16” thread diameter) will not pass directly through the small SK™ clamp, but the shaft can be effectively gripped. Cancellous pins are usually placed as one of the original pins in the frame and the clamp applied to the pin; therefore, use of medium cancellous pins is simple.
The large SK™ clamp is designed to handle 3.0mm shaft, medium, medium-plus, and large ESF pins, both positive- and negative-thread pin designs. These sizes of INTERFACE® and CENTERFACE® fixation pins can be applied directly through the clamp bolt. Medium, medium-plus and large DuraFace® are acceptable. The exception is the large cancellous version of positive-thread pins which are usually placed prior to application of clamp and rod. The maximum pin diameter that can be held by and passed directly through the large SK™ clamp bolt is 3/16” (4.8mm). Any pins in the range of 3.0mm to 4.75mm can also be utilized.
Are through-and-through (full) pins no longer utilized?
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. Sales of full-pins to users of the SK™ ESF System have decreased significantly, while sales of half-pins have increased. This altered use ratio 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, so across-the-board frame recommendations are not a reality. Patients with injuries to multiple limbs or other complicating factors will require increased (initial) frame rigidity.
I never utilize large ESF pins - only small and medium. Why should I consider the large SK™ ESF System?
Frame simplification results in reduced surgery time and hardware cost. The medium 1/8” (3.2mm) pin has certainly been the workhorse over a variety of different patient sizes and readily functions with small and large SK™ ESF clamps. While the medium KE device is capable of supporting fracture healing in large and giant breeds, the resulting frames are typically very complex and hardware-intensive. 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. Staged disassembly or rod downsizing can address potential concerns about excess frame stability.
Tell me more about the economics of simple SK™ frames.
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 8 pin, Type I-b frame will be easy to destabilize to a 4 pin, Type I-a if desired, and the large SK™ Type I-a frame can be easily downsized to a small SK™ rod and clamps if desired. The lack of need for aiming devices, torque wrenches, and rod augmentation further reduces cost of ownership and use.
Why are carbon fiber rods and titanium rods choices with the small SK™ fixator?
The 6.3mm carbon fiber rod is about the strength of the medium KE rod, and the 6.3mm titanium rod is just less than twice the strength of the medium KE rod. Since either rod is much stronger than the small KE rod, the lightweight, radiolucent carbon rod is virtually always used when pin choices are 2.0mm shaft, small, or small-plus diameters. When medium pins are selected, less challenging, lighter weight patients may utilize carbon rods. On lower limbs, Type I-b or Type II minimal frames will extend the upper weight range for carbon rods. For larger patients with medium pins, 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 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.
Why are carbon fiber and aluminum rods available for the large SK™ ESF System?
For the most part, aluminum rods are merely a back-up material for large SK™ rods in the unlikely event that carbon material is temporarily unavailable. Due to the 9.5mm diameter, both are much stronger than corresponding medium and even large KE rods and can serve to simplify the ESF method quite well. Down-staging from 9.5mm carbon to 9.5mm aluminum in the large SK™ is not typical and less likely to be beneficial as compared to downsizing to a small SK™ rod.
How are various SK™ rods cut to length?
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 or an abrasive disc on a Dremel tool. 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. Large SK™ aluminum rods can be cut with a large bolt cutter or hacksaw.
All SK™ ESF System connecting rods are stocked in length increments of 50mm. The variety of lengths available should greatly reduce the need for cutting rods.
Which rod should be used to construct contoured rod frames?
The mini SK™ 3.2mm (1/8”) stainless steel rod is the same rod as the small KE system and is easy to contour.
The small SK™ 6.3mm titanium external rod is the only rod other than the mini stainless steel rod that can be bent for this purpose. 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.
What is a simpler and more stable method of forming an adjustable articulation using SK™ components?
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 (see page 4 for SK™ ESF clamp design): a B-1 body part that is placed on the side closest to the pin-gripping portion of the primary bolt with a threaded hole for the secondary bolt, and a B-2 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 B-2 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 Achilles tendon repairs.
Which size SK™ ESF components should be used on a given patient?
This is the most frequently asked question regarding any ESF device. Bone size should be examined to determine appropriate pin diameter, and pin diameter should be approximately 25% of 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 medium INTERFACE®, CENTERFACE® or DuraFace® 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 a Type II or a Type I-b to a 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:
|Fixation System||Rod Size/Type||Weight/Size|
|Mini SK™||3.2mm Stainless Steel||kittens, small cats/puppies,
miniature breed dogs
|Small SK™||6.3mm Carbon Fiber||large cats, dogs < 20 lbs.|
|Small SK™||6.3mm Titanium||dogs > 20 lbs.|
|Large SK™||9.5mm Carbon Fiber||dogs > 50 lbs.|
These recommendations are general guidelines only. 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.
Is ever-increasing stability necessary?
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.
Tell me more about staged disassembly and rod downsizing.
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. Other examples include conversion of a Type I-b frame to a Type I-a frame or removal of articulations between different components of a bi-planar 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 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 need or 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 at about 6 weeks after surgery.
Overlapping Pin Zones
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. The chart below depicts overlapping pin zones.