A Review of 116 Clinical Cases Treated with External Fixators

Evaluation of a nontoxic rigid polymer as connecting bar in external skeletal fixators

OBJECTIVE

To investigate the mechanical characteristics of a nontoxic, low-cost, rigid polymer (RP) and to compare the structural and mechanical properties of a full-frame external skeletal fixator (ESF) with either RP connecting bars, polymethylmethacrylate (PMMA) connecting bars, or stainless-steel (SS) clamps and connecting bars.

STUDY DESIGN

In vitro mechanical evaluation.

METHODS

Mechanical properties were assessed using an in vitro bone fracture model with a bilateral uniplanar ESF (type II). Identical ESF were built with connecting bars using RP (n = 8), PMMA (n = 8), and SS connecting bars and clamps (System Meynard; n = 3). Nondestructive mechanical tests were performed in uniaxial compression (AC) and craniocaudal (CC) 4-point bending, as well as fatigue AC. Composite stiffness for each specimen and for each loading mode was calculated from 6 replicate measures using the slope of the load displacement curve at small displacements.

RESULTS

RP, PMMA, and SS ESF constructs yielded mean +/- SD composite stiffness values of 227 +/- 15, 381 +/- 30, and 394 +/- 9 N/mm in AC and of 35 +/- 2, 24 +/- 2, and 15 +/- 0 N/mm in CC, respectively.

CONCLUSIONS

Structural and mechanical properties of RP are satisfactorily rigid and fatigue resistant for its use as a connecting bar in ESF.

CLINICAL RELEVANCE

RP connecting bars in an ESF are a reliable, versatile, nontoxic and inexpensive option for the veterinary surgeon.

A comparative mechanical study of 3 external fixator clamps

OBJECTIVE

To compare external fixator clamps from Kirschner-Ehmer (K-E), Synthes, and Meynard with respect to 6 mechanical parameters. Study Design-A bench test of mechanical properties.

METHODS

Specially designed fixtures were used to mechanically test 6 clamps of each type at 2.5, 5.0, and 7.5 Newton-Meters of clamp bolt-tightening torque.

RESULTS

Components slipped axially and torsionally in the K-E clamp at higher forces for all parameters except for clamp bolt axis pivot. No bolt axis pivot occurred with the Synthes clamp. Instead, the clamp plastically deformed at the fixator-pin interface. This failure occurred at a higher applied torque than the pivot torque for other clamps. The Meynard clamp withstood significantly greater force than the K-E clamp when torsion was applied to the clamp bolt axis in the clockwise direction. Pivot forces for the K-E clamp were significantly higher than the Meynard clamp in the counterclockwise direction.

CONCLUSIONS

Overall, the K-E clamp was able to resist higher axial and torsional forces before slipping than the Meynard clamp or the Synthes clamp. The Synthes clamp was best able to resist torsion around the clamp bolt axis. Torsional resistance at the clamp-fixator pin and clamp-connecting bar interface was the weakest parameter of clamp mechanics.

CLINICAL RELEVANCE

The ability to resist motion within a clamp is related to fracture-reduction stability. Knowledge of the mechanical properties of fixator clamps will improve a clinician's ability to apply rigid fixation.

Use of contoured bar transhock external fixators in 17 cats

Seventeen cats with injuries involving the distal tibia, talocrural joints, tarsus and metatarsus were treated with contoured connecting bar transhock external fixators as either primary or secondary fixation. The assemblies stabilised all the injuries successfully. Fracture union was achieved in 11 out of 12 cases. Excellent results were achieved in eight cats and the outcome was satisfactory in a further five. Those cats with injuries involving talocrural joint luxation with displacement and significant periarticular soft tissue disruption had poorer long-term results. Complications occurred in seven cases.

Relative stiffness and stress of type I and type II external fixators: acrylic versus stainless-steel connecting bars--a theoretical approach

OBJECTIVE

To compare the stiffness and pin stresses of three sizes of external fixator systems with stainless-steel and acrylic connecting bars.

STUDY DESIGN

Finite element analysis.

METHODS

Small, medium, and large external fixator systems of type I and type II configurations were modeled for finite element analysis. Each model was evaluated with a standard stainless-steel and three different diameters of acrylic connecting bar. Displacements and stresses were calculated for the loading modes of axial compression, medio-lateral bending, cranio-caudal bending, and torsion. The location of the pin experiencing maximum stress was determined for all configurations and loading modes.

RESULTS

Acrylic column diameters of 9.53 mm for the small external fixator system and 15.9 mm for the medium external fixator system provide equivalent stiffness and maximum pin stresses to those provided by the standard stainless-steel connecting bars (3.2- and 4.8-mm diameter, respectively). The largest diameter acrylic column tested (31.75-mm) produced lower stiffness and higher maximum pin stresses than the standard stainless-steel connecting bar (11.1-mm diameter).

CONCLUSIONS

When applying a small or medium external fixator, an acrylic column of 9.53-mm or 15.9-mm diameter, respectively, can be used. For a large external fixator system, an acrylic column of diameter >31.75 mm is required.

CLINICAL RELEVANCE

The sizes of acrylic connecting bars for use in small and medium external fixator systems have been determined. Large systems should incorporate the standard stainless-steel connecting bar.