External fixator pin design.

The integrity of the bone-pin interface is the critical link in the stability of external fixation systems. External fixation pins placed in cancellous metaphyseal bone frequently loosen over time, resulting in fixation failure and an increased risk of infection. To design an external fixation pin with optimal bone-metal interface strength in cancellous bone, a systematic study of various thread design features was performed. Combinations of pitch, tooth profile, and minor diameter in 5 mm self-tapping half pins were evaluated in coaxial pullout testing using a fresh bovine cancellous bone. A significant increase in pullout strength was found with a decrease in minor diameter. No statistical differences were found in pullout strength attributable to thread profile and pitch. There were no significant interactions between minor diameter and tooth profile or minor diameter and pitch. The data obtained suggest significantly greater holding power in cancellous bone can be achieved by using an external fixation pin with a smaller minor diameter or a larger interference. Additional pullout testing of five commercially available external fixator pins was performed. Of these, the two pins with the largest interference demonstrated greater pullout strength. Therefore, within a range of acceptable major diameters and adequate minor diameters for the torsional strength requirements, an optimal interference for cancellous pin application may exist and it may well be larger than that present in currently available external fixation pins.     

The small pin circular fixator for distal tibial pilon fractures with soft tissue compromise

A Monticelli-Spinelli small pin circular external fixator was used in combination with closed reduction or a limited open reduction internal fixation in five cases in an attempt to salvage a satisfactory result in distal tibia pilon fractures when associated soft tissue compromise prevented standard fixation with plates and screws. The small pin fixator enhances the ability to perform a closed reduction through a technique that uses distraction with pins in the tibia and calcaneus, combined with correction of angulation by tensioning wires with a stop nut. Small pin stabilization of these comminuted fractures allows early patient mobilization. The small diameter pins support the soft cancellous bone fragments. This technique attempts to combine the benefits of traction, external fixation, and limited internal fixation. We recommend this technique as a salvage procedure when plates and screws are contraindicated because of poor bone and soft tissue conditions.     

The small pin circular fixator for proximal tibial fractures with soft tissue compromise

We used a Monticelli-Spinelli small pin circular external fixator in five cases, in combination with closed reduction or limited open reduction internal fixation, to salvage a satisfactory result in juxtaarticular, intraarticular fractures of the proximal tibia, when associated soft tissue compromise prevented standard fixation with plates and screws. The small pin circular fixator allows juxtaarticular placement of the small pins, enhancing stabilization of the comminuted fractures, allowing early range of motion of the joint and early patient mobilization. The small diameter pins support the soft cancellous bone fragments. This technique attempts to combine the benefits of traction, external fixation, and limited internal fixation. We recommend this method as a salvage procedure when plates and screws are contraindicated because of poor bone and soft tissue conditions.     

Fixation strength and pin tract infection of hydroxyapatite-coated tapered pins.

In a multicenter, prospective, randomized study, the biomechanical and clinical properties of the bone-pin interface were compared with standard tapered pins and hydroxyapatite-coated tapered pins implanted in patients who underwent femoral and tibial external fixation treatments. The results showed that the hydroxyapatite-coated tapered pins are clinically effective in improving the strength of fixation of the bonepin interface. This improvement corresponded to a lower rate of pin tract infection. In the hydroxyapatite-coated pin group, there were no differences in strength of fixation between the pins removed from the infected and uninfected pin tracts. In this pin group, the mean pin extraction torque was 531 +/- 225 Ncm in the infected pin tracts and 508 +/- 233 Ncm in the uninfected pin tracts. In the standard pin group, the mean pin extraction torque was 73 +/- 142 Ncm in the infected pin tracts and 211 +/- 216 Ncm in the uninfected pin tracts. The advantages provided by the hydroxyapatite-coated pins were higher in cancellous bone than in cortical bone.     

Biomechanical analysis of pin placement and pin size for external fixation of distal radius fractures

A series of biomechanical analyses were performed to explain the recent reduction in treatment-related complications of external fixation of distal radius fractures using a limited open approach for pin placement and larger 4-mm self-tapping half pins. A comparison of pull-out strength, stress concentration effect, and inherent bending strength of 3- and 4-mm half pins was performed. The effect of proximal pin placement in the radius or in the ulna and the effect of distal pin placement in four, six, or eight metacarpal cortices were determined. These analyses demonstrate that the 4-mm self-tapping half pins result in a significantly higher pull-out strength and only a small decrease in torsional load strength of the bone. They also demonstrate that proximal pin fixation in the radius produces the most stable fixation and that distal pin fixation into six metacarpal cortices produces a strong configuration that does not violate the interosseous muscles of the second intrinsic compartment. The rate of treatment-related complications in the external fixation of distal radius fractures (specifically, pin loosening, bending and breakage, fracture through pin sites, collapse at the fracture site, and intrinsic contracture) are addressed in this study. Such complications can be minimized by using 4-mm pins after central predrilling, with proximal placement in the radius and distal placement through six cortices of the bases of the second and third metacarpals.     

The effect of radial preload on the implant-bone interface: a cadaveric study

The most common complication of external fixation is pin loosening. Preloading the implant-bone interface is believed to retard this process. Radial preload, in particular, may be useful, as it allows loading in more than one direction. To investigate the effect of varying degrees of radial preload on the pin-bone interface, 30 freshly thawed human cadaveric tibiae were sectioned into 4-cm segments. Uniform drill holes were produced in the anterior tibial ridge of all segments and custom experimental bolts, oversized in diameter by as much as 1 mm, were pressed into each specimen. Macroscopic surface fractures were noted at the time of bolt insertion for misfits greater than 0.2 mm. Following histologic preparation, the implant-bone interface was evaluated microscopically based on the appearance of osteonal compression, lamellar distortion, and microfractures. Insertion of external fixator pins with misfits of greater than 0.4 mm resulted in significant microscopic structural damage to the bone surrounding the pin. High degrees of radial preload, exceeding the elastic limit of cortical bone, may be produced around pin holes by a small misfit. The use of oversized pins or screws must therefore be questioned.     

Effects of low-modulus coatings on pin–bone contact stresses in external fixation

The intent of this study was to investigate the stress distribution in cortical bone around fracture fixation pins and around pins coated with various polymeric and elastomeric materials. Since these interface stresses cannot be measured directly, a photoelastic technique was employed and stresses were measured in two-dimensional bone models fabricated from sheets of epoxy resin. Our results showed that when a fixation pin was loaded in compression, the compressive stress measured in the model was greatest at the pin–model interface. The magnitude of the compressive stress was found to diminish steeply away from the hole in a log decrement distribution which was asymptotic to the value of the average stress in the model. When polymeric and elastomeric materials were applied as pin coatings and the performance of the coated pins was compared to that of uncoated pins of the same overall diameter, a reduction of the maximum stress in the bone model was demonstrated. Among the coatings tested, we found that of the polymeric materials ultrahigh molecular weight polyethylene (UHMWPE) was most effective at reducing the peak cortical stress magnitude. The most effective coating material overall was found to be silicon elastomer. Computation of stress values in models loaded through stainless-steel pins and through pins coated with 1-mm silicon elastomer showed that the presence of the elastomer layer caused a reduction of about 50% in the maximum compressive stress in the model.     

Antimicrobial efficacy of external fixator pins coated with a lipid stabilized hydroxyapatite/chlorhexidine complex to prevent pin tract infection in a goat model.

BACKGROUND: Pin tract infection is a common complication of external fixation. An antiinfective external fixator pin might help to reduce the incidence of pin tract infection and improve pin fixation. METHODS: Stainless steel and titanium external fixator pins, with and without a lipid stabilized hydroxyapatite/chlorhexidine coating, were evaluated in a goat model. Two pins contaminated with an identifiable Staphylococcus aureus strain were inserted into each tibia of 12 goats. The pin sites were examined daily. On day 14, the animals were killed, and the pin tips cultured. Insertion and extraction torques were measured. RESULTS: Infection developed in 100% of uncoated pins, whereas coated pins demonstrated 4.2% infected, 12.5% colonized, and the remainder, 83.3%, had no growth (p < 0.01). Pin coating decreased the percent loss of fixation torque over uncoated pins (p = 0.04). CONCLUSION: These results demonstrate that the lipid stabilized hydroxyapatite/chlorhexidine coating was successful in decreasing infection and improving fixation of external fixator pins.     

Hydroxyapatite-coated Schanz pins in external fixators used for distraction osteogenesis : a randomized,controlled trial

BACKGROUND: Complications of external fixation include loosening of the fixation pins and pin-track infection. Laboratory studies and clinical trials have suggested that hydroxyapatite coating improves the osteointegration of various orthopaedic implants. The purpose of this study was to determine whether the prevalence of pin-related complications can be reduced by the use of hydroxyapatite-coated pins in fixators applied for distraction osteogenesis. METHODS: Forty-six consecutive patients undergoing segmental transport or lengthening of the tibia were randomized to the use of either standard titanium Schanz pins or hydroxyapatite-coated stainless-steel Schanz pins. The fixators were used for an average of thirty-eight weeks (range, fourteen to seventy-two weeks). All patients were closely monitored for pin loosening and infection, and digitized radiographs were assessed for bone resorption around all pins. When the external fixator was removed, the torque required to extract the pins was determined with use of an electronic torque wrench. RESULTS: In the control group (titanium pins), twenty-two pins (13%) loosened and an infection occurred at the site of twenty pins. An extensive infection of the canal developed in one patient. Twenty-two pins were removed or replaced because of these complications. In the hydroxyapatite group, no clinical or radiographic signs of pin loosening or infection were observed and no pins required early removal or exchange. The mean torque (and standard deviation) required to remove the hydroxyapatite-coated pins was 0.43 0.18 N-m compared with 0.10 0.09 N-m for the uncoated pins (p < 0.001). CONCLUSION: Coating pins with hydroxyapatite increases their fixation to bone and reduces the rate of infection and loosening during external fixation for distraction osteogenesis. Use of hydroxyapatite-coated pins should be considered in clinical situations requiring prolonged external fixation.     

External fixation of the distal radius: to predrill or not to predrill

Using both clinical and laboratory studies we investigated whether predrilling before insertion of external fixation pins is necessary for use in treating distal radius fractures. Our clinical study included 50 consecutive external fixators (4.0- and 2.5-mm pins) using 100 predrilled and 100 direct-drilled pins placed in a randomized manner. There was no increased incidence of pin track infection or other pin problem with the direct-drilled technique. There were, however, significantly elevated temperatures with the direct-drilled technique. We therefore recommend predrilling even though the temperature differences in this bone with this fixator were not clinically evident.     

External fixation of the pelvic ring: an experimental study on the role of pin diameter, pin position, and parasymphyseal fixator pins

Background The mechanical properties of current external fixator systems for unstable (type C) pelvic ring fractures are inferior to internal fixation, and are not optimal for definitive treatment. We explored methods to increase stability of external fixator constructs.

Methods An experimental model was used for load tests. The same pelvic fixator was used while different pin diameters, pin positions, and modes of pubic symphysis fixation were tested.

Results Changing of the pin diameter of the unthreaded part from 6 to 8 mm resulted in an increase in stiffness of 20%. An increase in stiffness by a factor of 1.9 was found by placing a pin on the iliac crest and one supra-acetabular. An additional increase by a factor of 3.6 was obtained by adding pubic symphysis plate fixation. Parasymphyseal pin fixation instead reduced stiffness, but not so much as when parasymphyseal pins were connected to the external fixator of the pelvic ring. The final configuration was at least 6 times stiffer than the initial configuration.

Interpretation The new concept of parasymphyseal pin fixation connected to an external fixator of the pelvic ring produces a considerable increase in stability for the treatment of type C pelvic ring injuries, as does an increase in pin diameter and alternative pin positioning.     

An <Emphasis Type="Italic">in Vivo</Emphasis> Experimental Comparison of Stainless Steel and Titanium Schanz Screws for External Fixation

Abstract Objective: To compare the clinical benefits of stainless steel (SS) to titanium (Ti) on reducing pin track irritation/infection and pin loosening during external fracture fixation. Methods: A tibial gap osteotomy was created in 17 sheep and stabilized with four Schanz screws of either SS or Ti and an external fixation frame. Over the 12 week observation period, pin loosening was assessed by grading the radiolucency around the pins and measuring the extraction torque on pin removal at sacrifice. Irritation/infection was assessed with weekly clinical pin track grading. A histological analysis of the tissue adjacent to the pin site was made to assess biocompatibility. Results: A statistically non-significant trend for less bone resorption around Ti pins was found during the early observation period. However, at sacrifice, there was no difference between the two materials. Also, there was no difference in the extraction torque, and there was similar remodeling and apposition of the bone around the pins. A statistically non-significant trend for more infection about SS pins at sacrifice was found. Histology showed a slightly higher prevalence of reactionary cells in SS samples, but was otherwise not much different than around Ti pins. Conclusions: There is no clinically relevant substantial advantage in using either SS or Ti pins on reducing pin loosening or pin track irritation/infection.     

External fixation of the pelvic ring: An experimental study on the role of pin diameter,pin position,and parasymphyseal fixator pins

Background?The mechanical properties of current external fixator systems for unstable (type C) pelvic ring fractures are inferior to internal fixation, and are not optimal for definitive treatment. We explored methods to increase stability of external fixator constructs.

Methods?An experimental model was used for load tests. The same pelvic fixator was used while different pin diameters, pin positions, and modes of pubic symphysis fixation were tested.

Results?Changing of the pin diameter of the unthreaded part from 6 to 8?mm resulted in an increase in stiffness of 20%. An increase in stiffness by a factor of 1.9 was found by placing a pin on the iliac crest and one supra-acetabular. An additional increase by a factor of 3.6 was obtained by adding pubic symphysis plate fixation. Parasymphyseal pin fixation instead reduced stiffness, but not so much as when parasymphyseal pins were connected to the external fixator of the pelvic ring. The final configuration was at least 6 times stiffer than the initial configuration.

Interpretation?The new concept of parasymphyseal pin fixation connected to an external fixator of the pelvic ring produces a considerable increase in stability for the treatment of type C pelvic ring injuries, as does an increase in pin diameter and alternative pin positioning.     

Monoaxial external fixation of the calcaneus: An anatomical study assessing the safety of monoaxial pin insertion

The use of external fixation for intra-articular calcaneal fractures is increasing in popularity. Studies have shown fine wire and monoaxial external fixation to be a viable surgical alternative to more invasive methods of open reduction and internal fixation of the calcaneus. However, there is an absence of literature that quantifies the risk of pin insertion for monoaxial fixation.This study aimed to determine the safety of inserting monoaxial pins within the calcaneus to house the Orthofix Calcaneal Mini-Fixator. Five formalin embalmed cadaveric ankle and lower leg specimens were inserted with six monoaxial pins. Careful dissection then revealed the presence of the tendons of peroneus longus and brevis, the sural nerve and the small saphenous vein in relation to these pins. Measurements from each pin to each of these structures were made as the structures transected lines drawn from each pin to two palpable bony landmarks: the inferior tip of the lateral malleolus and the posterosuperior calcaneus. In doing this, the risk posed by each pin could be evaluated. We found that two particular pins, those used to hold the articular surface of the subtalar joint in a reduced position, posed a larger risk of injury to surrounding structures than the remaining pins. These findings therefore suggest that monoaxial fixation of the calcaneus using a six pin approach is a relatively safe method of rectifying calcaneal fractures and thus may serve as a welcome alternative to other methods of calcaneal fixation.     

Comparative theoretical study of various external fixation devices for the stabilization of distal radius fractures

In the treatment of comminuted Colles' fractures different types of external fixation devices are used. Three main types are compared: (1) Fixation with two pins in each plane: Ace-Colles' type; (2) fixation with four or more parallel pins in one plane: Wagner/Hoffmann type; (3) fixation with four pins in one plane, two pins on each side of the fracture forming an angle of 60 degrees: ASIF-type fixator. These three types are compared with reference to the different forces that have to be neutralized by the seating of the pin in the bone. To minimize these forces with the aim of preventing pin-loosening, the theoretical results are used as the basis of practical surgical advice: (1) The distance between skin and fixator should be as short as possible. (2) The fixator should be fixed as close as possible to the fracture. (3) The diameter of the pins should be as great as possible. (4) If more than two pins per plane are used (Hoffmann/Wagner type), the pins should be wide apart. (5) If only two pins per plane are used (Ace-Colles Type) the bending stress on each pin is high (6) If more than two parallel pins per plane are used (Hoffmann/Wagner type), the axial forces on each pin are high. (7) The pins should be fixed at right angles to the fractured bone. Pins at other angles do more harm than good.     

Improvement of the bone-pin interface strength in osteoporotic bone with use of hydroxyapatite-coated tapered external-fixation pins. A prospective, randomized clinical study of wrist fractures

BACKGROUND: Achieving adequate fixation strength in osteoporotic bone is a challenge. In this study, we examined the use of hydroxyapatite-coated tapered external-fixation pins for the fixation of wrist fractures in patients with osteoporosis. METHODS: Twenty female patients with osteoporosis and a fracture of the wrist were divided into two paired groups and randomized to receive either standard tapered pins or hydroxyapatite-coated tapered pins. Two pins were inserted in the distal part of the radius, two pins were inserted in the second metacarpal, and an external fixation device was mounted. All fixation devices were removed six weeks after surgery. RESULTS: The mean pin-insertion torque (and standard deviation) was 461 +/- 254 Nmm in the group managed with standard pins and 332 +/- 176 Nmm in the group managed with hydroxyapatite-coated pins (p = 0.01). The mean pin-extraction torque was 191 +/- 155 Nmm in the group managed with standard pins and 600 +/- 214 Nmm in the group managed with hydroxyapatite-coated pins (p < 0.0001, power 95%). The mean extraction torque was lower than the corresponding insertion torque at each pin position in the group managed with standard pins (p < 0.05), whereas the mean extraction torque was higher than the corresponding insertion torque at each pin position in the group managed with hydroxyapatite-coated pins (p = 0.001). Two patients managed with standard pins and no patient managed with hydroxyapatite-coated pins had a pin-track infection. Pain during pin removal did not differ between the two groups. CONCLUSIONS: The present study showed that hydroxyapatite-coated tapered external-fixation pins provided improved fixation in the treatment of wrist fractures in patients with osteoporosis.     

A study of external skeletal fixation systems for unstable pelvic fractures

This study was undertaken to determine the feasibility of constructing an anterior pelvic external fixator capable of resisting displacement of vertical shear fractures. Newly designed tapered thread pins for cancellous bone had better bone pin fixation and their greater 6-mm diameter provided more than twice the stiffness of Hoffmann 4-mm pins when tested in clusters. Using these pins and a more rigid anterior frame as one model, various pelvic fixators were tested to determine their resistance to vertical shear forces. These tests determined that this new fixator was 16 times stiffer than a Bonnel single anterior frame and five times stiffer than a double anterior frame (both constructed of Hoffmann components). Extrapolation from the data shows that 13 mm of posterior fracture displacement would occur with loads of one half body weight using the new fixator. Additionally it was noted that Hoffmann frames constructed with two 5-mm pins performed as well as those using three 4-mm pins.     

Hydroxyapatite-coated external-fixation pins. The effect on pin loosening and pin-track infection in leg lengthening for short stature

Pin loosening and infection are inherent complications of external fixation. This study deals with their effects of using either hydroxyapatite (HA)-coated or uncoated external fixation pins in leg-lengthening procedures on patients of short stature. We used HA-coated pins on one side and uncoated pins on the other (randomly determined) in 28 bilateral lengthenings undertaken in 23 patients. A total of 322 pins was used. The mean implantation time was 530 days and the mean lengthening achieved was 78% of initial bone length. Mean extraction torque was 7611.6 Nmm degree(-1) for HA-coated and 85.4 Nmm degree(-1) for uncoated pins (p < 0.001). The rate of pin loosening was 4% (7/161) for HA-coated and 80% (129/161) for uncoated pins (p < 0.001). There was no statistically significant difference in the incidence of pin-track infection between the two groups. The use of HA coating appears to be an effective method of reducing the incidence of pin loosening in external fixation with a long implantation time and for mechanically highly stressed procedures such as leg lengthening for short stature.     

Management of acute distal radioulnar dislocations associated with radial shaft substance loss

In patients with distal radioulnar dislocations that are associated with a loss of radial shaft substance, the use of an external distractor that is applied to the distal radial fragment and the proximal ulnar shaft allows proper reduction of distal radioulnar articulation. The pins should be placed close to a joint so that the holes left after the pins are removed do not act as diaphyseal stress risers. The position is maintained by appropriate internal fixation and bone grafting of the radius. In cases where the bone is severely crushed and internal fixation cannot be applied, the external distractor may be converted to an external fixator.