DuraFace™ ESF Half-pins FAQs
Tell me more about the improvements in stiffness of the DuraFace™ half-pin.
As previously mentioned, DuraFace™ half-pins are on average 55% stiffer than comparable INTERFACE™ positive-profile fixation half-pins. Fifty-five percent is an average of 5 common sizes (small, small-plus, medium, medium-plus and large) based on comparable bone engagement diameters. Test results and methods are being submitted for publication.
Are DuraFace™ half-pins considered positive-profile?
No. DuraFace™ half-pins do not fit the current definition of positive-thread profile. However, based on mechanical performance they out perform currently marketed positive-profile ESF pins of comparable size.
I thought positive-profile threads were responsible for superior fatigue life. How does a negative-thread pin demonstrate increased fatigue life compared to a positive-profile pin of the same thread diameter?
Pin ultimate strength and deflection under load are two key factors involved in determining a pin’s fatigue life. By starting with the geometry of a positive-profile pin and adding shaft diameter while at the same time blending and minimizing the stress riser effect; pin ultimate strength, deformation under load, and fatigue life are all improved. The patent pending TRO (tapered runout) technology from IMEX™ softens the stress riser effect and is a key component of DuraFace™ half-pins mechanical performance.
I am concerned about ESF frame constructs being too stiff. Is this a concern with DuraFace™ half-pins?
Excess frame stiffness is possible with any ESF pin. However, the major cause of excess frame stiffness is the use of multiple full-pins. Full-pins are considered very stiff and with older devices were actually necessary to yield adequate frame stiffness when using weak external bars. IMEX™, with release of the SK™ ESF device, was the first to pioneer the concept of simple ESF frames built primarily with half-pins. Use of these half-pin frames creates excess frame strength much less often than constructs containing multiple full-pins. High frame strength is considered appropriate early in fracture repair. Appropriately constructed frames predominately using half-pins are easier to destabilize while allowing the surgeon to place pins in safe corridors more consistently than those based on full-pins. If you have been hesitant to reduce full-pin use, the DuraFace™ half-pin may be what you have been waiting for.
Will I need to purchase different drill bits to properly pre-drill for DuraFace™ half-pins?
No. A key design feature of these pins was to increase stiffness without a change in the pin tract diameter in the bone. As such, the same drill bits used to pre-drill IMEX™ INTERFACE™ and CENTERFACE™ fixation pins are compatible with DuraFace™ half-pins.
Will DuraFace™ half-pins function in any brand of ESF clamp?
The SK™ ESF clamp from IMEX™ was designed to function over a greater range of pin diameters than any other ESF clamp. The SK™ ESF clamp was also designed to grip any pin style (smooth, positive-profile, and negative profile) effectively. Most other ESF clamps have a much smaller range of pin choices. Clamps that have pin diameter ranges compatible with certain DuraFace™ half-pins will hold these pins; however, it is likely the surgeon using non-SK™ clamps will not have as great a pin diameter choice as with the SK™ ESF clamp by IMEX™. This is also true when standard positive-profile pins are chosen.
When comparing DuraFace™ half-pins to positive-profile pins, why is it important to consider the pin tract diameter in the bone?
Bone size is the first consideration when choosing the appropriate fixation pin diameter. If a pin diameter is selected that is too large compared to the corresponding bone, the risk of iatrogenic fracture increases. In cases where increased pin stiffness is desired the usual approach is to use a larger pin, but again, bone diameter often limits selection of larger ESF pins. So to compare stiffness and fatigue life of pins, it is appropriate to focus on pins that create the same “hole size” in the patient bone.
What are some clinical scenarios where pin mechanical improvements might prove beneficial to my patient?
There are many clinical scenarios where increased pin stiffness and cyclic fatigue life would potentially benefit patient management. Some examples include:
Short Fracture Fragments: Short fragments tend to limit the number of fixation pins that can be placed. If pin number is borderline, pins with increased stiffness offer a greater safety margin by increasing the longevity of the pin-bone interface and increased fatigue life offers longer pin longevity.
Fractures of the Humerus and Femur: The thick soft tissue envelope covering these bones forces the clamp and bar away from the neutral axis of the bone resulting in long pin working lengths. In addition, the femur and humerus are not conducive to use of full-pins. DuraFace™ half-pins are well suited to reduce the impact of long pin working lengths while prolonging the longevity of the pin bone interface. Repair of spinal fractures with ESF presents a similar pin length issue where DuraFace™ half-pins also offer distinct advantages over conventional positive-profile ESF pins.
Highly Comminuted Fractures: These fractures often carry a high degree of soft tissue injury that has potential to prolong the time required for fracture healing while at the same time are non-load sharing scenarios. As a result, the ESF frame and pins must carry high loads for a prolonged period of time. Pins that reduce deformation related end shear and demonstrate prolonged cyclic fatigue life such as DuraFace™ half-pins have potential to support the pin bone interface for a greater period of time.
Transverse Fractures: Unfortunately, these fractures are often considered “easy” but in reality transverse fractures can be very difficult to adequately stabilize. Isolated fracture lines force all movement to be focused in the small fracture gap resulting in high fracture gap strains. High local strains can result in non-union or delayed union. Increased pin stiffness can help reduce fracture gap strains in repair of transverse fractures.
Overly Active or Obese Patients: These scenarios are not uncommon and share the same concern that a recognized need for increased fracture repair strength exists. However, the patient’s bone diameter may not allow for increased fixation pin diameter without undue risk of iatrogenic fracture. In these cases, DuraFace™ half-pins may provide the necessary increased pin stiffness needed to support fracture healing. In many cases, a smaller DuraFace™ half-pin is as stiff as the next larger positive-profile fixation pin, allowing a smaller pin to function in mechanically challenging situations where small bone stock exists.
Multiple Limb Injuries: Any fracture case presenting multiple limb injuries should raise the proverbial “red flag.” Multiple limb injuries typically create a higher number of complications. These include pin loosening, poor limb use, mechanical failure of the fixation device, delayed union, or even non-union due to the high peak stresses placed on the fixation device and pin-bone interface. High peak stresses can be minimized by stiffer pins resulting in prolonged longevity of the pin-bone interface. Pins with increased fatigue life are likely to maintain mechanical integrity for longer periods of time in these challenging scenarios.
In reality, many fracture patients exhibit a combination of the above challenges. Therefore, the improved performance of DuraFace™ half-pins is likely to benefit most patients treated with external skeletal fixation. Use of a fracture patient assessment score will provide more detailed guidelines to assist in pin and frame selection.
