External Fixation

External Fixation. Basic principles.

Biomechanics

Dynamization

Selected constructs External Fixation

Frame biomechanics The required strength of an external frame depends on the intended function of the fixator and the degree of fracture instability. With any frame, following the basic biomechanical principles helps to ensure a stable fixation.1 a b Options to increase frame stability a. Increase the distance between the two outermost Schanz screws in each fragment. c

Note: Innermost Schanz screws should be at least 2 cm from the fracture line. b b. Increase the number of Schanz screws in each fragment. c. Reduce the distance between the rod and bone. d. Add a second rod or tube in the same plane, with the clamps in close contact (i.e., double-stacking). e. Use a larger diameter Schanz screw. Increasing the diameter by 1 mm more than doubles the stiffness and nearly doubles the strength.

In the tibia, since sagittal bending moments are approximately d two to five times greater than those in the coronal plane, placing the frame in the sagittal plane will increase frame stability. To be mechanically effective, the stiffness of the external fixation frame should match the forces and moments at the fracture site. This sagittal-to-coronal stiffness ratio can be achieved if the Schanz screws are oriented in the AP direction.2

Note: The same factors that increase frame stability for unilateral frames apply to modular frames.

1. F. Behrens and K. Searls. “External Fixation of the Tibia.” Journal of Bone and Joint Surgery. 68 (1986): 246–254. 2. Ibid.

Synthes External Fixation Increase load-sharing to stimulate callus formation When the external fixation frame is used as definitive treatment for a fracture, the surgeon may wish to increase load-sharing. Once the soft tissues have healed and the patient has progressed to full weightbearing, the frame may easily be adjusted to transfer load to the fracture site a to stimulate callus formation.3

Options to reduce frame rigidity: a. Decrease the number of Schanz screws in each fragment.

Note: Always maintain at least two Schanz screws in each fragment. a b. Increase the distance between the rod and bone. c. Remove the second rod on a double-stacked unilateral frame. c b

Dynamize the frame Dynamization is the process of altering the frame so that one larger fragment can move axially in relation to the other, while movement in all other planes remains restricted. This allows load transference to the fracture site while maintaining anatomic alignment. This functional loading appears to increase the strength of the callus and decrease the need for immobilization after frame removal.4

In order to create dynamization, the frame must be double- stacked, with rods in the same plane as the axis of the bone. “Crosswise release,” or the loosening of adjacent clamp nuts on a frame, permits dynamization.

3. M.E .Müeller, M. Allgöwer, R. Schneider, and H. Willenegger. Manual of Internal Fixation, 3rd ed. Berlin: Springer-Verlag, 1991. 390–391. 4. Ibid.

Synthes External Fixation continued

Dynamize the frame continued Dynamization with Adjustable Clamps (393.64) Numbers denote nuts that must be loosened. 2 Figure A 1. Loosen the nut on the outer universal joint for two-tubes. 2. Loosen the nuts on the rod side of the inner proximal clamps. or Figure B 1. Loosen the nuts on the rod side of the outer distal clamps. 2 1 2. Loosen the nuts on the rod side of the inner proximal clamps. 1

Figure A Figure B

Dynamization with combination clamps 1. Loosen the bolts on both outer proximal clamps. 2. Insert dynamization clips between the rod vise plates and retighten the bolts. 3. Loosen the bolts on both inner distal clamps. 1 4. Insert dynamization clips between the rod vise plates and tighten the bolts.

The frame will then allow guided collapse of the fragment. 2, 4

Synthes External Fixation Selected External Fixator Constructs and Frame Requirements*

Unilateral frame – Multi-pin clamps, 2 ea. – Schanz screws, 4 ea. – Carbon fiber rod, 1 ea.

Modular frame – Combination clamps, 6 ea. – Schanz screws, 4 ea. – Carbon fiber rods, 3 ea.

* For further information, please refer to the individual frame technique guides.

Synthes Selected External Fixator Constructs and Frame Requirements* continued

Modular bridge frame – Combination clamps, 6 ea. – Schanz screws, 4 ea. – Carbon fiber rods, 3 ea.

Delta bridge frame – Multi-pin clamp, 1 ea. – Rod attachment for multi-pin clamp, 1 ea. – Combination clamp, 2 ea. – Schanz screws, 2 ea. – Steinmann pin with central thread, 1 ea. – Carbon fiber rods, 2 ea.

* For further information, please refer to the individual frame technique guides.

Synthes External Fixation Synthes External Fixation Systems

115.720 Large External Fixator Set, with self-drilling Schanz screws 115.740 Large External Fixator Set, with titanium self-drilling Schanz screws

115.992 Medium External Fixator Set, with self-drilling Schanz screws 115.994 Medium External Fixator Set, with titanium self-drilling Schanz screws

115.982 Small External Fixator Set, with carbon fiber rods 115.760 Non-Spanning Module, for Small External Fixator Set

115.985 Mini External Fixator Instrument and Implant Set

Synthes Synthes (USA) Synthes (Canada) Ltd. 1302 Wrights Lane East 2566 Meadowpine Boulevard West Chester, PA 19380 Mississauga, Ontario L5N 6P9 Telephone: (610) 719-5000 Telephone: (905) 567-0440 To order: (800) 523-0322 To order: (800) 668-1119 Fax: (610) 251-9056 Fax: (905) 567-3185 www.synthes.com