Comparative Evaluation ofIn VitroMechanical Properties of Different Designs of Epoxy-Pin External Skeletal Fixation Systems

Description of and complications associated with reinforced, free-form external skeletal fixation for treatment of appendicular fractures in cats: 46 cases (2010-2019)

OBJECTIVE

To describe a modified technique for reinforced, free-form external skeletal fixation (rFF-ESF) of appendicular fractures in cats and identify factors associated with development of complications.

ANIMALS

46 cats with fractures repaired with rFF-ESF at Angell Animal Medical Center between 2010 and 2019.

PROCEDURES

Medical records were reviewed for information on signalment, affected bone, fracture location and orientation, degree of comminution, severity (open vs closed), fixator type, number of fixation pins, use of an intramedullary pin (yes vs no), surgeon experience (staff surgeon vs surgical resident), anesthesia time, surgery time, perioperative antimicrobial administration, concurrent surgical procedures, intraoperative complications, postoperative alignment, whether fixator destabilization was performed, and time to complete fixator removal. Postoperative complications were recorded.

RESULTS

43 of the 46 (93%) cats had a successful outcome, with a median time to complete fixator removal of 8 weeks (range, 3 to 61 weeks). Twelve of the 46 (26%) cats had major (n = 3) or minor (9) complications. In univariable analyses, 4 factors were significantly associated with development of postoperative complications: body weight (OR for each 1-kg increase in weight, 1.8), tibial fracture (vs fracture of any other long bone; OR, 16), use of a type 2 fixator (vs a type 1 fixator; OR, 11), and use of destabilization (vs no destabilization; 7).

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that rFF-ESF can be successfully used to stabilize a variety of appendicular fractures in cats. Further studies are required to compare rFF-ESF with other fracture fixation methods.

Clinical outcomes of 119 miniature- and toy-breed dogs with 140 distal radial and ulnar fractures repaired with free-form multiplanar type II external skeletal fixation

OBJECTIVE

To report the clinical outcomes of miniature- and toy-breed dogs with primary and previously treated distal radial and ulnar fractures (RUF) treated with a free-form multiplanar type II external skeletal fixation (ESF).

STUDY DESIGN

Retrospective study.

ANIMALS

One hundred nineteen dogs with 140 distal RUF.

METHODS

Medical records (2003-2017) were reviewed for dogs weighing up to 7 kg that had been treated for distal RUF with a free-form multiplanar type II ESF. Data collected included signalment, fracture location and configuration, fracture reduction, intraoperative realignment, time to bone union and implant removal, duration of destabilization and postoperative splint, complications, and outcomes. Follow-up consisted of clinical and radiographic evaluation or telephone interview.

RESULTS

All fractures healed, with a median time of 60 days, and entire ESF was removed, with a median time of 89 days. Major complications occurred in four fractures: premature pin loosening, elbow subluxation, osteomyelitis, and delayed union. Minor complications occurred in 52 fractures (61 events). All dogs had a successful return to normal limb function at final follow-up (median, seven months; range, 2-97).

CONCLUSION

All primary and previously treated distal RUF in miniature- and toy-breed dogs healed and allowed return to normal function after fixation with a free-form multiplanar type II ESF.

CLINICAL SIGNIFICANCE

The free-form multiplanar type II ESF provides an alternative for minimally invasive fixation of distal RUF in miniature- and toy-breed dogs that facilitates fracture reduction, intraoperative realignment, adequate initial fixation stability, destabilization, and implant removal.

Mechanical testing of a steel-reinforced epoxy resin bar and clamp for external skeletal fixation of long-bone fractures in cats

AIMS

To provide veterinarians with confidence when using a commercially available epoxy resin in external skeletal fixators (ESF), testing was conducted to determine exothermia during curing of the epoxy resin compared to polymethylmethacrylate (PMMA), the hardness of the epoxy resin as a bar over 16 weeks, and the strength of the epoxy resin bar compared with metal clamps in similarly constructed Type 1a ESF constructs simulating the repair of feline long bone fractures.

METHODS

Exothermia of the epoxy resin during curing was tested against PMMA with surface temperatures recorded over the first 15 minutes of curing, using four samples of each product. The hardness of 90 identical epoxy resin bars was tested by subjecting them to cyclic loads (1,000 cycles of 20.5 N, every 7 days) over a 16-week period and impact testing 10 bars every 2 weeks. Ten bars that were not subjected to cyclic loads were impact tested at 0 weeks and another 10 at 16 weeks. Strength of the epoxy resin product, as a bar and clamp composite, was tested against metal SK and Kirschner-Ehmer (KE) clamps and bars in Type 1a, tied-in intramedullary pin, ESF constructs with either 90° or 75° pin placement, subjected to compressive and bending loads to 75 N.

RESULTS

The maximum temperature during curing of the epoxy resin (min 39.8, max 43.0)°C was less than the PMMA (min 85.2, max 98.5)°C (p<0.001). There was no change in hardness of the epoxy resin bars over the 16 weeks of cyclic loading (p=0.58). There were no differences between the median strength of the epoxy resin, SK or KE ESF constructs in compression or bending when tested to 75 N (p>0.05). Stiffness of constructs with 75° pin placement was greater for SK than epoxy resin constructs in compression (p=0.046), and was greater for KE than epoxy resin constructs in bending (p=0.033).

CONCLUSIONS

The epoxy resin tested was found to be less exothermic than PMMA; bars made from the epoxy resin showed durability over an expected fracture healing timeframe and had mechanical strength characteristics comparable to metal bar and clamp ESF constructs.

CLINICAL RELEVANCE

The epoxy resin ESF construct tested in this study can be considered a suitable replacement for SK or KE ESF constructs in the treatment of feline long-bone fractures, in terms of mechanical strength.

Postoperative complications associated with external skeletal fixators in cats

OBJECTIVES

The objective of this study was to quantify complications associated with external skeletal fixators (ESFs) in cats and to identify potential risk factors.

METHODS

A retrospective review of medical records and radiographs following ESF placement was performed.

RESULTS

Case records of 140 cats were reviewed; fixator-associated complications (FACs) occurred in 19% of cats. The region of ESF placement was significantly associated with complication development. Complications developed most frequently in the femur (50%), tarsus (35%) and radius/ulna (33%). Superficial pin tract infection (SPTI) and implant failure accounted for 45% and 41% of all FACs, respectively. SPTI occurred more frequently in the femur, humerus and tibia, with implant failure more frequent in the tarsus. No association between breed, age, sex, weight, fracture type (open vs closed), ESF classification, number of pins per bone segment, degree of fracture load sharing, and the incidence or type of FAC was identified. No association between region of placement, breed, age, sex, weight, fracture type (open vs closed), ESF classification, number of pins per bone segment, fracture load sharing and the time to complication development was identified.

CONCLUSIONS AND RELEVANCE

Complication development is not uncommon in cats following ESF placement. The higher complication rate in the femur, tarsus and radius/ulna should be considered when reviewing options for fracture management. However, cats appear to have a lower rate of pin tract infections than dogs.

In vitro biomechanical testing of different configurations of acrylic external skeletal fixator constructs

OBJECTIVE

To evaluate the in vitro biomechanical properties of four different configurations of acrylic external skeletal fixator constructs.

MATERIALS AND METHODS

Simulated bone constructs were prepared using two segments of 20 mm ultra-high-density polyethylene rods with a gap of 5 mm. The full pins (1.5 mm) were passed through the proximal and distal segments of ultra-high-density polyethylene rods, in the same plane, parallel to each other in configuration U, and were crossed in the M1, M2 and C configurations at a 90° angle to each other. Configuration U was a single bilateral uniplanar construct, M1 was a double orthogonal bilateral construct, M2 was a double orthogonal bilateral construct with proximal and distal connecting articulations, and C was a double orthogonal bilateral construct with proximal and distal circumferential articulations. Temporary scaffolds of different external skeletal fixator configurations were constructed using commercially available polyvinyl chloride pipes (20 mm) connected and secured to the fixation pins at a fixed distance from the rods. Acrylic powder (polymer) mixed with liquid (monomer) was poured into the pipes and allowed to solidify to form the side bars and rings. The external skeletal fixator constructs were then subjected to axial compression, cranio-caudal three-point bending and torsion (n = 4 each) using a universal testing machine. Mechanical parameters, namely stress, strain, modulus of elasticity, stiffness and bending moment of fixator constructs, were determined from load-displacement curves.

RESULTS

Configuration U was the weakest and configuration C was the strongest under all the testing modes. Under compression, the M1, M2 and C configurations were similar. Under bending, a significant difference was observed among the uniplanar, multiplanar and circular configurations with no difference between M1 and M2. However, under torsion, all the external skeletal fixator configurations differed significantly.

CLINICAL SIGNIFICANCE

The freeform external skeletal fixator using acrylic as a replacement for a metallic bar may be useful to treat bone fractures and luxations in small animals, as it is mechanically strong, lightweight, economical, and pins can be passed from any direction depending upon the clinical situation.